path: root/doc/rfc/rfc7574.txt

Internet Engineering Task Force (IETF)                         A. Bakker
Request for Comments: 7574                  Vrije Universiteit Amsterdam
Category: Standards Track                                    R. Petrocco
ISSN: 2070-1721                                           V. Grishchenko
                                           Technische Universiteit Delft
                                                               July 2015


              Peer-to-Peer Streaming Peer Protocol (PPSPP)

Abstract

   The Peer-to-Peer Streaming Peer Protocol (PPSPP) is a protocol for
   disseminating the same content to a group of interested parties in a
   streaming fashion.  PPSPP supports streaming of both prerecorded (on-
   demand) and live audio/video content.  It is based on the peer-to-
   peer paradigm, where clients consuming the content are put on equal
   footing with the servers initially providing the content, to create a
   system where everyone can potentially provide upload bandwidth.  It
   has been designed to provide short time-till-playback for the end
   user and to prevent disruption of the streams by malicious peers.
   PPSPP has also been designed to be flexible and extensible.  It can
   use different mechanisms to optimize peer uploading, prevent
   freeriding, and work with different peer discovery schemes
   (centralized trackers or Distributed Hash Tables).  It supports
   multiple methods for content integrity protection and chunk
   addressing.  Designed as a generic protocol that can run on top of
   various transport protocols, it currently runs on top of UDP using
   Low Extra Delay Background Transport (LEDBAT) for congestion control.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7574.








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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................5
      1.1. Purpose ....................................................5
      1.2. Requirements Language ......................................6
      1.3. Terminology ................................................6
   2. Overall Operation ...............................................9
      2.1. Example: Joining a Swarm ...................................9
      2.2. Example: Exchanging Chunks ................................10
      2.3. Example: Leaving a Swarm ..................................10
   3. Messages .......................................................11
      3.1. HANDSHAKE .................................................11
           3.1.1. Handshake Procedure ................................12
      3.2. HAVE ......................................................14
      3.3. DATA ......................................................15
      3.4. ACK .......................................................15
      3.5. INTEGRITY .................................................15
      3.6. SIGNED_INTEGRITY ..........................................16
      3.7. REQUEST ...................................................16
      3.8. CANCEL ....................................................16
      3.9. CHOKE and UNCHOKE .........................................17
      3.10. Peer Address Exchange ....................................17
           3.10.1. PEX_REQ and PEX_RES Messages ......................17
      3.11. Channels .................................................19
      3.12. Keep Alive Signaling .....................................20
   4. Chunk Addressing Schemes .......................................21
      4.1. Start-End Ranges ..........................................21
           4.1.1. Chunk Ranges .......................................21
           4.1.2. Byte Ranges ........................................21
      4.2. Bin Numbers ...............................................22
      4.3. In Messages ...............................................23
           4.3.1. In HAVE Messages ...................................23
           4.3.2. In ACK Messages ....................................24



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   5. Content Integrity Protection ...................................24
      5.1. Merkle Hash Tree Scheme ...................................25
      5.2. Content Integrity Verification ............................26
      5.3. The Atomic Datagram Principle .............................27
      5.4. INTEGRITY Messages ........................................28
      5.5. Discussion and Overhead ...................................28
      5.6. Automatic Detection of Content Size .......................29
           5.6.1. Peak Hashes ........................................29
           5.6.2. Procedure ..........................................31
   6. Live Streaming .................................................32
      6.1. Content Authentication ....................................32
           6.1.1. Sign All ...........................................33
           6.1.2. Unified Merkle Tree ................................33
                  6.1.2.1. Signed Munro Hashes .......................34
                  6.1.2.2. Munro Signature Calculation ...............36
                  6.1.2.3. Procedure .................................37
                  6.1.2.4. Secure Tune In ............................37
      6.2. Forgetting Chunks .........................................38
   7. Protocol Options ...............................................38
      7.1. End Option ................................................39
      7.2. Version ...................................................39
      7.3. Minimum Version ...........................................40
      7.4. Swarm Identifier ..........................................40
      7.5. Content Integrity Protection Method .......................41
      7.6. Merkle Tree Hash Function .................................41
      7.7. Live Signature Algorithm ..................................42
      7.8. Chunk Addressing Method ...................................42
      7.9. Live Discard Window .......................................43
      7.10. Supported Messages .......................................44
      7.11. Chunk Size ...............................................44
   8. UDP Encapsulation ..............................................45
      8.1. Chunk Size ................................................45
      8.2. Datagrams and Messages ....................................46
      8.3. Channels ..................................................47
      8.4. HANDSHAKE .................................................47
      8.5. HAVE ......................................................48
      8.6. DATA ......................................................48
      8.7. ACK .......................................................49
      8.8. INTEGRITY .................................................50
      8.9. SIGNED_INTEGRITY ..........................................51
      8.10. REQUEST ..................................................52
      8.11. CANCEL ...................................................52
      8.12. CHOKE and UNCHOKE ........................................53
      8.13. PEX_REQ, PEX_RESv4, PEX_RESv6, and PEX_REScert ...........53
      8.14. KEEPALIVE ................................................55
      8.15. Flow and Congestion Control ..............................56
      8.16. Example of Operation .....................................57
   9. Extensibility ..................................................61



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      9.1. Chunk Picking Algorithms ..................................61
      9.2. Reciprocity Algorithms ....................................62
   10. IANA Considerations ...........................................62
      10.1. PPSPP Message Type Registry ..............................62
      10.2. PPSPP Option Registry ....................................62
      10.3. PPSPP Version Number Registry ............................62
      10.4. PPSPP Content Integrity Protection Method Registry .......62
      10.5. PPSPP Merkle Hash Tree Function Registry .................63
      10.6. PPSPP Chunk Addressing Method Registry ...................63
   11. Manageability Considerations ..................................63
      11.1. Operations ...............................................63
           11.1.1. Installation and Initial Setup ....................63
           11.1.2. Migration Path ....................................64
           11.1.3. Requirements on Other Protocols and
                   Functional Components .............................64
           11.1.4. Impact on Network Operation .......................64
           11.1.5. Verifying Correct Operation .......................65
           11.1.6. Configuration .....................................65
      11.2. Management Considerations ................................66
           11.2.1. Management Interoperability and Information .......67
           11.2.2. Fault Management ..................................67
           11.2.3. Configuration Management ..........................67
           11.2.4. Accounting Management .............................68
           11.2.5. Performance Management ............................68
           11.2.6. Security Management ...............................68
   12. Security Considerations .......................................68
      12.1. Security of the Handshake Procedure ......................68
           12.1.1. Protection against Attack 1 .......................69
           12.1.2. Protection against Attack 2 .......................70
           12.1.3. Protection against Attack 3 .......................70
      12.2. Secure Peer Address Exchange .............................71
           12.2.1. Protection against the Amplification Attack .......71
           12.2.2. Example: Tracker as Certification Authority .......72
           12.2.3. Protection against Eclipse Attacks ................73
      12.3. Support for Closed Swarms ................................73
      12.4. Confidentiality of Streamed Content ......................74
      12.5. Strength of the Hash Function for Merkle Hash Trees ......74
      12.6. Limit Potential Damage and Resource Exhaustion by
            Bad or Broken Peers ......................................74
           12.6.1. HANDSHAKE .........................................75
           12.6.2. HAVE ..............................................75
           12.6.3. DATA ..............................................75
           12.6.4. ACK ...............................................75
           12.6.5. INTEGRITY and SIGNED_INTEGRITY ....................76
           12.6.6. REQUEST ...........................................76
           12.6.7. CANCEL ............................................76
           12.6.8. CHOKE .............................................77
           12.6.9. UNCHOKE ...........................................77



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           12.6.10. PEX_RES ..........................................77
           12.6.11. Unsolicited Messages in General ..................77
      12.7. Exclude Bad or Broken Peers ..............................77
   13. References ....................................................78
      13.1. Normative References .....................................78
      13.2. Informative References ...................................79
   Acknowledgements ..................................................84
   Authors' Addresses ................................................85

1.  Introduction

1.1.  Purpose

   This document describes the Peer-to-Peer Streaming Peer Protocol
   (PPSPP), designed for disseminating the same content to a group of
   interested parties in a streaming fashion.  PPSPP supports streaming
   of both prerecorded (on-demand) and live audio/video content.  It is
   based on the peer-to-peer paradigm where clients consuming the
   content are put on equal footing with the servers initially providing
   the content, to create a system where everyone can potentially
   provide upload bandwidth.

   PPSPP has been designed to provide short time-till-playback for the
   end user and to prevent disruption of the streams by malicious peers.
   Central in this design is a simple method of identifying content
   based on self-certification.  In particular, content in PPSPP is
   identified by a single cryptographic hash that is the root hash in a
   Merkle hash tree calculated recursively from the content [MERKLE]
   [ABMRKL].  This self-certifying hash tree allows every peer to
   directly detect when a malicious peer tries to distribute fake
   content.  The tree can be used for both static and live content.
   Moreover, it ensures only a small amount of information is needed to
   start a download and to verify incoming chunks of content, thus
   ensuring short start-up times.

   PPSPP has also been designed to be extensible for different
   transports and use cases.  Hence, PPSPP is a generic protocol that
   can run directly on top of UDP, TCP, or other protocols.  As such,
   PPSPP defines a common set of messages that make up the protocol,
   which can have different representations on the wire depending on the
   lower-level protocol used.  When the lower-level transport allows,
   PPSPP can also use different congestion control algorithms.

   At present, PPSPP is set to run on top of UDP using LEDBAT for
   congestion control [RFC6817].  Using LEDBAT enables PPSPP to serve
   the content after playback (seeding) without disrupting the user who
   may have moved to different tasks that use its network connection.




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   PPSPP is also flexible and extensible in the mechanisms it uses to
   promote client contribution and prevent freeriding, that is, how to
   deal with peers that only download content but never upload to
   others.  It also allows different schemes for chunk addressing and
   content integrity protection, if the defaults are not fit for a
   particular use case.  In addition, it can work with different peer
   discovery schemes, such as centralized trackers or fast Distributed
   Hash Tables [JIM11].  Finally, in this default setup, PPSPP maintains
   only a small amount of state per peer.  A reference implementation of
   PPSPP over UDP is available [SWIFTIMPL].

   The protocol defined in this document assumes that a peer has already
   discovered a list of (initial) peers using, for example, a
   centralized tracker [PPSP-TP].  Once a peer has this list of peers,
   PPSPP allows the peer to connect to other peers, request chunks of
   content, and discover other peers disseminating the same content.

   The design of PPSPP is based on our research into making BitTorrent
   [BITTORRENT] suitable for streaming content [P2PWIKI].  Most PPSPP
   messages have corresponding BitTorrent messages and vice versa.
   However, PPSPP is specifically targeted towards streaming audio/video
   content and optimizes time-till-playback.  It was also designed to be
   more flexible and extensible.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.3.  Terminology

   message
       The basic unit of PPSPP communication.  A message will have
       different representations on the wire depending on the transport
       protocol used.  Messages are typically multiplexed into a
       datagram for transmission.

   datagram
       A sequence of messages that is offered as a unit to the
       underlying transport protocol (UDP, etc.).  The datagram is
       PPSPP's Protocol Data Unit (PDU).

   content
       Either a live transmission or a prerecorded multimedia file.






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   chunk
       The basic unit in which the content is divided.  For example, a
       block of N kilobytes.  A chunk may be of variable size.

   chunk ID
       Unique identifier for a chunk of content (e.g., an integer).  Its
       type depends on the chunk addressing scheme used.

   chunk specification
       An expression that denotes one or more chunk IDs.

   chunk addressing scheme
       Scheme for identifying chunks and expressing the chunk
       availability map of a peer in a compact fashion.

   chunk availability map
       The set of chunks a peer has successfully downloaded and checked
       the integrity of.

   bin
       A number denoting a specific binary interval of the content
       (i.e., one or more consecutive chunks) in the bin numbers chunk
       addressing scheme (see Section 4).

   content integrity protection scheme
       Scheme for protecting the integrity of the content while it is
       being distributed via the peer-to-peer network.  That is, methods
       for receiving peers to detect whether a requested chunk has been
       modified, either maliciously by the sending peer or accidentally
       in transit.

   hash
       The result of applying a cryptographic hash function, more
       specifically a Modification Detection Code (MDC) [HAC01], such as
       SHA-256 [FIPS180-4], to a piece of data.

   Merkle hash tree
       A tree of hashes whose base is formed by the hashes of the chunks
       of content, and its higher nodes are calculated by recursively
       computing the hash of the concatenation of the two child hashes
       (see Section 5.1).

   root hash
       The root in a Merkle hash tree calculated recursively from the
       content (see Section 5.1).






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   munro hash
       The hash of a subtree that is the unit of signing in the Unified
       Merkle Tree content authentication scheme for live streaming (see
       Section 6.1.2.1).

   swarm
       A group of peers participating in the distribution of the same
       content.

   swarm ID
       Unique identifier for a swarm of peers, in PPSPP a sequence of
       bytes.  For video on demand with content integrity protection
       enabled, the identifier is the so-called root hash of a Merkle
       hash tree over the content.  For live streaming, the swarm ID is
       a public key.

   tracker
       An entity that records the addresses of peers participating in a
       swarm, usually for a set of swarms, and makes this membership
       information available to other peers on request.

   choking
       When Peer A is choking Peer B, it means that A is currently not
       willing to accept requests for content from B.

   seeding
       Peer A is said to be seeding when A has downloaded a static
       content file completely and is now offering it for others to
       download.

   leeching
       Peer A is said to be leeching when A has not completely
       downloaded a static content file yet or is not offering to upload
       it to others.

   channel
       A logical connection between two peers.  The channel concept
       allows peers to use the same transport address for communicating
       with different peers.

   channel ID
       Unique, randomly chosen identifier for a channel, local to each
       peer.  So the two peers logically connected by a channel each
       have a different channel ID for that channel.

   heavy payload
       A datagram has a heavy payload when it contains DATA messages,
       SIGNED_INTEGRITY messages, or a large number of smaller messages.



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   In this document the prefixes kilo-, mega-, etc., denote base 1024.

2.  Overall Operation

   The basic unit of communication in PPSPP is the message.  Multiple
   messages are multiplexed into a single datagram for transmission.  A
   datagram (and hence the messages it contains) will have different
   representations on the wire depending on the transport protocol used
   (see Section 8).

   The overall operation of PPSPP is illustrated in the following
   examples.  The examples assume that the content distributed is
   static, UDP is used for transport, the Merkle Hash Tree scheme is
   used for content integrity protection, and that a specific policy is
   used for selecting which chunks to download.

2.1.  Example: Joining a Swarm

   Consider a user who wants to watch a video.  To play the video, the
   user clicks on the play button of a HTML5 <video> element shown in
   his PPSPP-enabled browser.  Imagine this element has a PPSPP URL (to
   be defined elsewhere) identifying the video as its source.  The
   browser passes this URL to its peer-to-peer streaming protocol
   handler.  Let's call this protocol handler Peer A.  Peer A parses the
   URL to retrieve the transport address of a peer-to-peer streaming
   protocol tracker and swarm metadata of the content.  The tracker
   address may be optional in the presence of a decentralized tracking
   mechanism.  The mechanisms for tracking peers are outside of the
   scope of this document.

   Peer A now registers with the tracker following the peer-to-peer
   streaming protocol tracker specification [PPSP-TP] and receives the
   IP address and port of peers already in the swarm, say, Peers B, C,
   and D.  At this point, the PPSPP starts operating.  Peer A now sends
   a datagram containing a PPSPP HANDSHAKE message to Peers B, C, and D.
   This message conveys protocol options.  In particular, Peer A
   includes the ID of the swarm (part of the swarm metadata) as a
   protocol option because the destination peers can listen for multiple
   swarms on the same transport address.

   Peers B and C respond with datagrams containing a PPSPP HANDSHAKE
   message and one or more HAVE messages.  A HAVE message conveys (part
   of) the chunk availability of a peer; thus, it contains a chunk
   specification that denotes what chunks of the content Peers B and C
   have, respectively.  Peer D sends a datagram with a HANDSHAKE and
   HAVE messages, but also with a CHOKE message.  The latter indicates
   that Peer D is not willing to upload chunks to Peer A at present.




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2.2.  Example: Exchanging Chunks

   In response to Peers B and C, Peer A sends new datagrams to Peers B
   and C containing REQUEST messages.  A REQUEST message indicates the
   chunks that a peer wants to download; thus, it contains a chunk
   specification.  The REQUEST messages to Peers B and C refer to
   disjoint sets of chunks.  Peers B and C respond with datagrams
   containing HAVE, DATA, and, in this example, INTEGRITY messages.  In
   the Merkle hash tree content protection scheme (see Section 5.1), the
   INTEGRITY messages contain all cryptographic hashes that Peer A needs
   to verify the integrity of the content chunk sent in the DATA
   message.  Using these hashes, Peer A verifies that the chunks
   received from Peers B and C are correct against the trusted swarm ID.
   Peer A also updates the chunk availability of Peers B and C using the
   information in the received HAVE messages.  In addition, it passes
   the chunks of video to the user's browser for rendering.

   After processing, Peer A sends a datagram containing HAVE messages
   for the chunks it just received to all its peers.  In the datagram to
   Peers B and C, it includes an ACK message acknowledging the receipt
   of the chunks and adds REQUEST messages for new chunks.  ACK messages
   are not used when a reliable transport protocol is used.  When, for
   example, Peer C finds that Peer A obtained a chunk (from Peer B) that
   Peer C did not yet have, Peer C's next datagram includes a REQUEST
   for that chunk.

   Peer D also sends HAVE messages to Peer A when it downloads chunks
   from other peers.  When Peer D is willing to accept REQUESTs from
   Peer A, Peer D sends a datagram with an UNCHOKE message to inform
   Peer A.  If Peer B or C decides to choke Peer A, they send a CHOKE
   message and Peer A should then re-request from other peers.  Peers B
   and C may continue to send HAVE, REQUEST, or periodic keep-alive
   messages such that Peer A keeps sending them HAVE messages.

   Once Peer A has received all content (video-on-demand use case), it
   stops sending messages to all other peers that have all content
   (a.k.a. seeders).  Peer A can also contact the tracker or another
   source again to obtain more peer addresses.

2.3.  Example: Leaving a Swarm

   To leave a swarm in a graceful way, Peer A sends a specific HANDSHAKE
   message to all its peers (see Section 8.4) and deregisters from the
   tracker following the tracker specification [PPSP-TP].  Peers
   receiving the datagram should remove Peer A from their current peer
   list.  If Peer A crashes ungracefully, peers should remove Peer A
   from their peer list when they detect it no longer sends messages
   (see Section 3.12).



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3.  Messages

   No error codes or responses are used in the protocol; absence of any
   response indicates an error.  Invalid messages are discarded, and
   further communication with the peer SHOULD be stopped.  The rationale
   is that it is sufficient to classify peers as either good or bad and
   only use the good ones.  A good peer is a peer that responds with
   chunks; a peer that does not respond, or does not respond in time is
   classified as bad.  The idea is that, in PPSPP, the content is
   available from multiple sources (unlike HTTP), so a peer should not
   invest too much effort in trying to obtain it from a particular
   source.  This classification in good or bad allows a peer to deal
   with slow, crashed, and (silent) malicious peers.

   Multiple messages MUST be multiplexed into a single datagram for
   transmission.  Messages in a single datagram MUST be processed in the
   strict order in which they appear in the datagram.  If an invalid
   message is found in a datagram, the remaining messages MUST be
   discarded.

   For the sake of simplicity, one swarm of peers deals with one content
   file or stream only.  There is a single division of the content into
   chunks that all peers in the swarm adhere to, determined by the
   content publisher.  Distribution of a collection of files can be done
   either by using multiple swarms or by using an external storage
   mapping from the linear byte space of a single swarm to different
   files, transparent to the protocol.  In other words, the audio/video
   container format used is outside the scope of this document.

3.1.  HANDSHAKE

   For Peer P to establish communication with Peer Q in Swarm S, the
   peers must first exchange HANDSHAKE messages by means of a handshake
   procedure.  The initiating Peer P needs to know the metadata of Swarm
   S, which consists of:

   (a)  the swarm ID of the content (see Sections 5.1 and 6),

   (b)  the chunk size used,

   (c)  the chunk addressing method used,

   (d)  the content integrity protection method used, and

   (e)  the Merkle hash tree function used (if applicable).






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   (f)  If automatic content size detection (see Section 5.6) is not
        used, the content length is also part of the metadata (for
        static content.)

   This document assumes the swarm metadata is obtained from a trusted
   source.  In addition, Peer P needs to know a transport address for
   Peer Q, obtained from a peer discovery/tracking protocol.

   The payload of the HANDSHAKE message contains a sequence of protocol
   options.  The protocol options encode the swarm metadata just
   described to enable an end-to-end check to see whether the peers are
   in the right swarm.  Additionally, the options encode a number of
   per-peer configuration parameters.  The complete set of protocol
   options are specified in Section 7.  The HANDSHAKE message also
   contains a channel ID for multiplexing communication and security
   (see Sections 3.11 and 12.1).  A HANDSHAKE message MUST always be the
   first message in a datagram.

3.1.1.  Handshake Procedure

   The handshake procedure for a peer, Peer P, to start communication
   with another peer, Peer Q, in Swarm S is now as follows.

   1.  The first datagram the initiating Peer P sends to Peer Q MUST
       start with a HANDSHAKE message.  This HANDSHAKE message MUST
       contain:

       *  A channel ID, chanP, randomly chosen as specified in
          Section 12.1.

       *  The metadata of Swarm S, encoded as protocol options, as
          specified in Section 7.  In particular, the initiating Peer P
          MUST include the swarm ID.

       *  The capabilities of Peer P, in particular, its supported
          protocol versions, "Live Discard Window" (in case of a live
          swarm) and "Supported Messages", encoded as protocol options.

       This first datagram MUST be prefixed with the (destination)
       channel ID 0; see Section 3.11.  Hence, the datagram contains two
       channel IDs: the destination channel ID prefixed to the datagram
       and the channel ID chanP included in the HANDSHAKE message inside
       the datagram.  This datagram MAY also contain some minor
       additional payload, e.g., HAVE messages to indicate Peer P's
       current progress, but it MUST NOT include any heavy payload
       (defined in Section 1.3), such as a DATA message.  Allowing minor





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       payload minimizes the number of initialization round trips, thus
       improving time-till-playback.  Forbidding heavy payload prevents
       an amplification attack (see Section 12.1).

   2.  The receiving Peer Q checks the HANDSHAKE message from Peer P.
       If any check by Peer Q fails, or if Peers P and Q are not in the
       same swarm, Peer Q MUST NOT send a HANDSHAKE (or any other)
       message back, as the message from Peer P may have been spoofed
       (see Section 12.1).  Otherwise, if Peer Q is interested in
       communicating with Peer P, Peer Q MUST send a datagram to Peer P
       that starts with a HANDSHAKE message.  This reply HANDSHAKE MUST
       contain:

       *  A channel ID, chanQ, randomly chosen as specified in
          Section 12.1.

       *  The metadata of Swarm S, encoded as protocol options, as
          specified in Section 7.  In particular, the responding Peer Q
          MAY include the swarm ID.

       *  The capabilities of Peer Q, in particular, its supported
          protocol versions, its "Live Discard Window" (in case of a
          live swarm) and "Supported Messages", encoded as protocol
          options.

       This reply datagram MUST be prefixed with the channel ID chanP
       sent by Peer P in the first HANDSHAKE message (see Section 3.11).
       This reply datagram MAY also contain some minor additional
       payload, e.g., HAVE messages to indicate Peer Q's current
       progress, or REQUEST messages (see Section 3.7), but it MUST NOT
       include any heavy payload.

   3.  The initiating Peer P checks the reply datagram from Peer Q.  If
       the reply datagram is not prefixed with (destination) channel ID
       chanP, Peer P MUST discard the datagram.  Peer P SHOULD continue
       to process datagrams from Peer Q that do meet this requirement.
       This check prevents interference by spoofing, see Section 12.1.
       If Peer P's channel ID is echoed correctly, the initiator Peer P
       knows that the addressed Peer Q really responds.

   4.  Next, Peer P checks the HANDSHAKE message in the datagram from
       Peer Q.  If any check by Peer P fails, or Peer P is no longer
       interested in communicating with Peer Q, Peer P MAY send a
       HANDSHAKE message to inform Peer Q it will cease communication.
       This closing HANDSHAKE message MUST contain an all zeros channel
       ID and a list of protocol options.  The list MUST either be empty
       or contain the maximum version number Peer P supports, following
       the min/max versioning scheme defined in [RFC6709], Section 4.1.



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       The datagram containing this closing HANDSHAKE message MUST be
       prefixed with the (destination) channel ID chanQ.  Peer P MAY
       also simply cease communication.

   5.  If the addressed peer, Peer Q, does not respond to initiating
       Peer P's first datagram, Peer P MAY resend that datagram until
       Peer Q is considered dead, according to the rules specified in
       Section 3.12.

   6.  If the reply datagram by Peer Q does pass the checks by Peer P,
       and Peer P wants to continue interacting with Peer Q, Peer P can
       now send REQUEST, PEX_REQ, and other messages to Peer Q.
       Datagrams carrying these messages MUST be prefixed with the
       channel ID chanQ sent by Peer Q.  More specifically, because Peer
       P knows that Peer Q really responds, Peer P MAY start sending
       Peer Q messages with heavy payload.  That means that Peer P MAY
       start responding to any REQUEST messages that Peer Q may have
       sent in this first reply datagram with DATA messages.  Hence,
       transfer of chunks can start soon in PPSPP.

   7.  If Peer Q receives any datagram (apparently) from Peer P that
       does not contain channel ID chanQ, Peer Q MUST discard the
       datagram but SHOULD continue to process datagrams from Peer P
       that do meet this requirement.  Once Peer Q receives a datagram
       from Peer P that does contain the channel ID chanQ, Peer Q knows
       that Peer P really received its reply datagram, and the three-way
       handshake and channel establishment is complete.  Peer Q MAY now
       also start sending messages with heavy payload to Peer P.

   8.  If Peer P decides it no longer wants to communicate with Peer Q,
       or vice versa, the peer SHOULD send a closing HANDSHAKE message
       to the other, as described above.

3.2.  HAVE

   The HAVE message is used to convey which chunks a peer has available
   for download.  The set of chunks it has available may be expressed
   using different chunk addressing and availability map compression
   schemes, described in Section 4.  HAVE messages can be used both for
   sending a complete overview of a peer's chunk availability as well as
   for updates to that set.

   In particular, whenever a receiving Peer P has successfully checked
   the integrity of a chunk, or interval of chunks, it MUST send a HAVE
   message to all peers Q1..Qn it wants to allow to download those
   chunks.  A policy in Peer P determines when the HAVE is sent.  Peer P
   may send it directly, or Peer P may wait either until it has other
   data to send to Peer Qi or until it has received and checked multiple



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   chunks.  The policy will depend on how urgent it is to distribute
   this information to the other peers.  This urgency is generally
   determined in turn by the chunk picking policy (see Section 9.1).  In
   general, the HAVE messages can be piggybacked onto other messages.
   Peers that do not receive HAVE messages are effectively prevented
   from downloading the newly available chunks; hence, the HAVE message
   can be used as a method of choking.

   The HAVE message MUST contain the chunk specification of the received
   and verified chunks.  A receiving peer MUST NOT send a HAVE message
   to peers for which the handshake procedure is still incomplete, see
   Section 12.1.  A peer SHOULD NOT send a HAVE message to peers that
   have the complete content already (e.g., in video-on-demand
   scenarios).

3.3.  DATA

   The DATA message is used to transfer chunks of content.  The DATA
   message MUST contain the chunk ID of the chunk and chunk itself.  A
   peer MAY send the DATA messages for multiple chunks in the same
   datagram.  The DATA message MAY contain additional information if
   needed by the specific congestion control mechanism used.  At
   present, PPSPP uses LEDBAT [RFC6817] for congestion control, which
   requires the current system time to be sent along with the DATA
   message, so the current system time MUST be included.

3.4.  ACK

   ACK messages MUST be sent to acknowledge received chunks if PPSPP is
   run over an unreliable transport protocol.  ACK messages MAY be sent
   if a reliable transport protocol is used.  In the former case, a
   receiving peer that has successfully checked the integrity of a
   chunk, or interval of chunks C, MUST send an ACK message containing a
   chunk specification for C.  As LEDBAT is used, an ACK message MUST
   contain the one-way delay, computed from the peer's current system
   time received in the DATA message.  A peer MAY delay sending ACK
   messages as defined in the LEDBAT specification [RFC6817].

3.5.  INTEGRITY

   The INTEGRITY message carries information required by the receiver to
   verify the integrity of a chunk.  Its payload depends on the content
   integrity protection scheme used.  When the Merkle Hash Tree scheme
   is used, an INTEGRITY message MUST contain a cryptographic hash of a
   subtree of the Merkle hash tree and the chunk specification that
   identifies the subtree.





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   As a typical example, when a peer wants to send a chunk and Merkle
   hash trees are used, it creates a datagram that consists of several
   INTEGRITY messages containing the hashes the receiver needs to verify
   the chunk and the actual chunk itself encoded in a DATA message.
   What are the necessary hashes and the exact rules for encoding them
   into datagrams is specified in Sections 5.3, and 5.4, respectively.

3.6.  SIGNED_INTEGRITY

   The SIGNED_INTEGRITY message carries digitally signed information
   required by the receiver to verify the integrity of a chunk in live
   streaming.  It logically contains a chunk specification, a timestamp,
   and a digital signature.  Its exact payload depends on the live
   content integrity protection scheme used, see Section 6.1.

3.7.  REQUEST

   While bulk download protocols normally do explicit requests for
   certain ranges of data (i.e., use a pull model, for example,
   BitTorrent [BITTORRENT]), live streaming protocols quite often use a
   push model without requests to save round trips.  PPSPP supports both
   models of operation.

   The REQUEST message is used to request one or more chunks from
   another peer.  A REQUEST message MUST contain the specification of
   the chunks the requester wants to download.  A peer receiving a
   REQUEST message MAY send out the requested chunks (by means of DATA
   messages).  When Peer Q receives multiple REQUESTs from the same Peer
   P, Peer Q SHOULD process the REQUESTs in the order received.
   Multiple REQUEST messages MAY be sent in one datagram, for example,
   when a peer wants to request several rare chunks at once.

   When live streaming via a push model, a peer receiving REQUESTs also
   MAY send some other chunks in case it runs out of requests or for
   some other reason.  In that case, the only purpose of REQUEST
   messages is to provide hints and coordinate peers to avoid
   unnecessary data retransmission.

3.8.  CANCEL

   When downloading on-demand or live streaming content, a peer can
   request urgent data from multiple peers to increase the probability
   of it being delivered on time.  In particular, when the specific
   chunk picking algorithm (see Section 9.1), detects that a request for
   urgent data might not be served on time, a request for the same data
   can be sent to a different peer.  When a Peer P decides to request
   urgent data from a Peer Q, Peer P SHOULD send a CANCEL message to all
   the peers to which the data has been previously requested.  The



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   CANCEL message contains the specification of the chunks Peer P no
   longer wants to request.  In addition, when Peer Q receives a HAVE
   message for the urgent data from Peer P, Peer Q MUST also cancel the
   previous REQUEST(s) from Peer P.  In other words, the HAVE message
   acts as an implicit CANCEL.

3.9.  CHOKE and UNCHOKE

   Peer A can send a CHOKE message to Peer B to signal it will no longer
   be responding to REQUEST messages from Peer B, for example, because
   Peer A's upload capacity is exhausted.  Peer A MAY send a subsequent
   UNCHOKE message to signal that it will respond to new REQUESTs from
   Peer B again (Peer A SHOULD discard old requests).  When Peer B
   receives a CHOKE message from Peer A, it MUST NOT send new REQUEST
   messages and it cannot expect answers to any outstanding ones, as the
   transfer of chunks is choked.  When Peer B is choked but receives a
   HAVE message from Peer A, it is not automatically unchoked and MUST
   NOT send any new REQUEST messages.  The CHOKE and UNCHOKE messages
   are informational as responding to REQUESTs is OPTIONAL, see
   Section 3.7.

3.10.  Peer Address Exchange

3.10.1.  PEX_REQ and PEX_RES Messages

   Peer Exchange (PEX) messages are common in many peer-to-peer
   protocols.  They allow peers to exchange the transport addresses of
   the peers they are currently interacting with, thereby reducing the
   need to contact a central tracker (or Distributed Hash Table) to
   discovery new peers.  The strength of this mechanism is therefore
   that it enables decentralized peer discovery: after an initial
   bootstrap, a central tracker is no longer needed.  Its weakness is
   that it enables a number of attacks, so it should not be used on the
   Internet unless extra security measures are in place.

   PPSPP supports peer-address exchange on the Internet and in benign
   private networks as an OPTIONAL feature (not mandatory to implement)
   under certain conditions.  The general mechanism works as follows.
   To obtain some peer addresses, a Peer A MAY send a PEX_REQ message to
   Peer B.  Peer B MAY respond with one or more PEX_REScert messages.
   Logically, a PEX_REScert reply message contains the address of a
   single peer Ci.  Peer B MUST have exchanged messages with Peer Ci in
   the last 60 seconds to guarantee liveliness.  Upon receipt, Peer A
   may contact any or none of the returned peers Ci.  Alternatively,
   peers MAY ignore PEX_REQ and PEX_REScert messages if uninterested in
   obtaining new peers or because of security considerations (rate
   limiting) or any other reason.  The PEX messages can be used to
   construct a dedicated tracker peer.



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   To use PEX in PPSPP on the Internet, two conditions must be met:

   1.  Peer transport addresses must be relatively stable.

   2.  A peer must not obtain all its peer addresses through PEX.

   The full security analysis for PEX messages can be found in
   Section 12.2.  Physically, a PEX_REScert message carries a swarm-
   membership certificate rather than an IP address and port.  A
   membership certificate for Peer C states that Peer C at address
   (ipC,portC) is part of Swarm S at Time T and is cryptographically
   signed by an issuer.  The receiver Peer A can check the certificate
   for a valid signature by a trusted issuer, the right swarm, and
   liveliness and only then consider contacting C.  These swarm-
   membership certificates correspond to signed node descriptors in
   secure decentralized peer sampling services [SPS].

   Several designs are possible for the security environment for these
   membership certificates.  That is, there are different designs
   possible for who signs the membership certificates and how public
   keys are distributed.  Section 12.2.2 describes an example where a
   central tracker acts as the Certification Authority.

   In a hostile environment, such as the Internet, peers must also
   ensure that they do not end up interacting only with malicious peers
   when using the peer-address exchange feature.  To this extent, peers
   MUST ensure that part of their connections are to peers whose
   addresses came from a trusted and secured tracker (see
   Section 12.2.3).

   In addition to the PEX_REScert, there are two other PEX reply
   messages.  The PEX_RESv4 message contains a single IPv4 address and
   port.  The PEX_RESv6 message contains a single IPv6 address and port.
   They MUST only be used in a benign environment, such as a private
   network, as they provide no guarantees that the host addressed
   actually participates in a PPSPP swarm.

   Once a PPSPP implementation has obtained a list of peers (either via
   PEX, from a central tracker, or via a Distributed Hash Table (DHT)),
   it has to determine which peers to actually contact.  In this
   process, a PPSPP implementation can benefit from information by
   network or content providers to help improve network usage and boost
   PPSPP performance.  How a peer-to-peer (P2P) system like PPSPP can
   perform these optimizations using the Application-Layer Traffic
   Optimization (ALTO) protocol is described in detail in [RFC7285],
   Section 7.





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3.11.  Channels

   It is increasingly complex for peers to enable communication between
   each other due to NATs and firewalls.  Therefore, PPSPP uses a
   multiplexing scheme, called channels, to allow multiple swarms to use
   the same transport address.  Channels loosely correspond to TCP
   connections and each channel belongs to a single swarm, as
   illustrated in Figure 1.  As with TCP connections, a channel is
   identified by a unique identifier local to the peer at each end of
   the connection (cf.  TCP port), which MUST be randomly chosen.  In
   other words, the two peers connected by a channel use different IDs
   to denote the same channel.  The IDs are different and random for
   security reasons, see Section 12.1.

   In the PPSP-over-UDP encapsulation (Section 8.3), when a Channel C
   has been established between Peer A and Peer B, the datagrams
   containing messages from Peer A to Peer B are prefixed with the four-
   byte channel ID allocated by Peer B, and vice versa for datagrams
   from Peer B to A.  The channel IDs used are exchanged as part of the
   handshake procedure, see Section 8.4.  In that procedure, the channel
   ID with value 0 is used for the datagram that initiates the
   handshake.  PPSPP can be used in combination with Session Traversal
   Utilities for NAT (STUN) [RFC5389].




























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               _________    _________          _________
               |       |    |       |          |       |
               | Swarm |    | Swarm |          | Swarm |
               |  Mgr  |    |   A   |          |   B   |
               |_______|    |_______|          |_______|
                   |            |                /   \
                   |            |               /     \
               ____|____    ____|____    ______/__    _\_______
               |       |    |       |    |       |    |       |
               | Chan  |    | Chan  |    | Chan  |    | Chan  |
               |   0   |    |  481  |    |  836  |    |  372  |
               |_______|    |_______|    |_______|    |_______|
                   |            |            |            |
                   |            |            |            |
               ____|____________|____________|____________|____
               |                                              |
               |                      UDP                     |
               |                   port 6778                  |
               |______________________________________________|


   Network stack of a PPSPP peer that is reachable on UDP port 6778 and
   is connected via channel 481 to one peer in Swarm A and two peers in
     Swarm B via channels 836 and 372, respectively.  Channel ID 0 is
                   special and is used for handshaking.

                                 Figure 1

3.12.  Keep Alive Signaling

   A peer SHOULD send a "keep alive" message periodically to each peer
   it is interested in, but has no other messages to send to them at
   present.  The goal of the keep alives is to keep a signaling channel
   open to peers that are of interest.  Which peers those are is
   determined by a policy that decides which peers are of interest now
   and in the near future.  This document does not prescribe a policy,
   but examples of interesting peers are (a) peers that have chunks on
   offer that this client needs or (b) peers that currently do not have
   interesting chunks on offer (because they are still downloading
   themselves, or in live streaming) but gave good performance in the
   past.  When these peers have new chunks to offer, the peer that kept
   a signaling channel open can use them again.  Periodically sending
   "keep alive" messages prevents other peers declaring the peer dead.
   A guideline for declaring a peer dead when using UDP consists of a
   three minute delay since that last packet has been received from that
   peer and at least three datagrams having been sent to that peer
   during the same period.  When a peer is declared dead, the channel to
   it is closed, no more messages will be sent to that peer and the



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   local administration about the peer is discarded.  Busy servers can
   force idle clients to disconnect by not sending keep alives.  PPSPP
   does not define an explicit message type for "keep alive" messages.
   In the PPSP-over-UDP encapsulation they are implemented as simple
   datagrams consisting of a four-byte channel ID only, see Sections 8.3
   and 8.4.

4.  Chunk Addressing Schemes

   PPSPP can use different methods of chunk addressing, that is, support
   different ways of identifying chunks and different ways of expressing
   the chunk availability map of a peer in a compact fashion.

   All peers in a swarm MUST use the same chunk addressing method.

4.1.  Start-End Ranges

   A chunk specification consists of a single (start specification,end
   specification) pair that identifies a range of chunks (end
   inclusive).  The start and end specifications can use one of multiple
   addressing schemes.  Two schemes are currently defined: chunk ranges
   and byte ranges.

4.1.1.  Chunk Ranges

   The start and end specification are both chunk identifiers.  Chunk
   identifiers are 32-bit or 64-bit unsigned integers.  A PPSPP peer
   MUST support this scheme.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                    Start chunk (32 or 64)                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                    End chunk (32 or 64)                       ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.1.2.  Byte Ranges

   The start and end specification are 64-bit byte offsets in the
   content.  The support for this scheme is OPTIONAL.










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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Start byte offset (64)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    End byte offset (64)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.2.  Bin Numbers

   PPSPP introduces a novel method of addressing chunks of content
   called "bin numbers" (or "bins" for short).  Bin numbers allow the
   addressing of a binary interval of data using a single integer.  This
   reduces the amount of state that needs to be recorded per peer and
   the space needed to denote intervals on the wire, making the protocol
   lightweight.  In general, this numbering system allows PPSPP to work
   with simpler data structures, e.g., to use arrays instead of binary
   trees, thus reducing complexity.  The support for this scheme is
   OPTIONAL.

   In bin addressing, the smallest binary interval is a single chunk
   (e.g., a block of bytes that may be of variable size), the largest
   interval is a complete range of 2**63 chunks.  In a novel addition to
   the classical scheme, these intervals are numbered in a way that lays
   them out into a vector nicely, which is called bin numbering, as
   follows.  Consider a chunk interval of width W.  To derive the bin
   numbers of the complete interval and the subintervals, a minimal
   balanced binary tree is built that is at least W chunks wide at the
   base.  The leaves from left-to-right correspond to the chunks 0..W-1
   in the interval, and have bin number I*2 where I is the index of the
   chunk (counting beyond W-1 to balance the tree).  The bin number of
   higher-level node P in the tree is calculated as follows:

       binP = (binL + binR) / 2

   where binL is the bin of node P's left-hand child and binR is the bin
   of node P's right-hand child.  Given that each node in the tree
   represents a subinterval of the original interval, each such
   subinterval now is addressable by a bin number, a single integer.
   The bin number tree of an interval of width W=8 looks like this:







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                                   7
                                  / \
                                /     \
                              /         \
                            /             \
                           3                11
                          / \              / \
                         /   \            /   \
                        /     \          /     \
                       1       5        9       13
                      / \     / \      / \      / \
                     0   2   4   6    8   10  12   14

                     C0  C1  C2  C3   C4  C5  C6   C7

              The bin number tree of an interval of width W=8

                                 Figure 2

   So bin 7 represents the complete interval, bin 3 represents the
   interval of chunk C0..C3, bin 1 represents the interval of chunks C0
   and C1, and bin 2 represents chunk C1.  The special numbers
   0xFFFFFFFF (32-bit) or 0xFFFFFFFFFFFFFFFF (64-bit) stands for an
   empty interval, and 0x7FFF...FFF stands for "everything".

   When bin numbering is used, the ID of a chunk is its corresponding
   (leaf) bin number in the tree, and the chunk specification in HAVE
   and ACK messages is equal to a single bin number (32-bit or 64-bit),
   as follows.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                    Bin number (32 or 64)                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3.  In Messages

4.3.1.  In HAVE Messages

   When a receiving peer has successfully checked the integrity of a
   chunk or interval of chunks, it MUST send a HAVE message to all peers
   it wants to allow download of those chunk(s) from.  The ability to
   withhold HAVE messages allows them to be used as a method of choking.
   The HAVE message MUST contain the chunk specification of the biggest
   complete interval of all chunks the receiver has received and checked
   so far that fully includes the interval of chunks just received.  So




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   the chunk specification MUST denote at least the interval received,
   but the receiver is supposed to aggregate and acknowledge bigger
   intervals, when possible.

   As a result, every single chunk is acknowledged a logarithmic number
   of times.  That provides some necessary redundancy of
   acknowledgements and sufficiently compensates for unreliable
   transport protocols.

   Implementation note:

       To record which chunks a peer has in the state that an
       implementation keeps for each peer, an implementation MAY use the
       efficient "binmap" data structure, which is a hybrid of a bitmap
       and a binary tree, discussed in detail in [BINMAP].

4.3.2.  In ACK Messages

   PPSPP peers MUST use ACK messages to acknowledge received chunks if
   an unreliable transport protocol is used.  When a receiving peer has
   successfully checked the integrity of a chunk or interval of chunks
   C, it MUST send an ACK message containing the chunk specification of
   its biggest, complete interval covering C to the sending peer (see
   HAVE).

5.  Content Integrity Protection

   PPSPP can use different methods for protecting the integrity of the
   content while it is being distributed via the peer-to-peer network.
   More specifically, PPSPP can use different methods for receiving
   peers to detect whether a requested chunk has been maliciously
   modified by the sending peer.  In benign environments, content
   integrity protection can be disabled.

   For static content, PPSPP currently defines one method for protecting
   integrity, called the Merkle Hash Tree scheme.  If PPSPP operates
   over the Internet, this scheme MUST be used.  If PPSPP operates in a
   benign environment, this scheme MAY be used.  So the scheme is
   mandatory to implement, to satisfy the requirement of strong security
   for an IETF protocol [RFC3365].  An extended version of the scheme is
   used to efficiently protect dynamically generated content (live
   streams), as explained below and in Section 6.1.

   The Merkle Hash Tree scheme can work with different chunk addressing
   schemes.  All it requires is the ability to address a range of
   chunks.  In the following description abstract node IDs are used to
   identify nodes in the tree.  On the wire, these are translated to the
   corresponding range of chunks in the chosen chunk addressing scheme.



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5.1.  Merkle Hash Tree Scheme

   PPSPP uses a method of naming content based on self-certification.
   In particular, content in PPSPP is identified by a single
   cryptographic hash that is the root hash in a Merkle hash tree
   calculated recursively from the content [ABMRKL].  This self-
   certifying hash tree allows every peer to directly detect when a
   malicious peer tries to distribute fake content.  It also ensures
   only a small the amount of information is needed to start a download
   (the root hash and some peer addresses).  For live streaming, a
   dynamic tree and a public key are used, see below.

   The Merkle hash tree of a content file that is divided into N chunks
   is constructed as follows.  Note the construction does not assume
   chunks of content to be of a fixed size.  Given a cryptographic hash
   function, more specifically an MDC [HAC01], such as SHA-256, the
   hashes of all the chunks of the content are calculated.  Next, a
   binary tree of sufficient height is created.  Sufficient height means
   that the lowest level in the tree has enough nodes to hold all chunk
   hashes in the set, as with bin numbering.  The figure below shows the
   tree for a content file consisting of 7 chunks.  As with the content
   addressing scheme, the leaves of the tree correspond to a chunk and,
   in this case, are assigned the hash of that chunk, starting at the
   leftmost leaf.  As the base of the tree may be wider than the number
   of chunks, any remaining leaves in the tree are assigned an empty
   hash value of all zeros.  Finally, the hash values of the higher
   levels in the tree are calculated, by concatenating the hash values
   of the two children (again left to right) and computing the hash of
   that aggregate.  If the two children are empty hashes, the parent is
   an empty all-zeros hash as well (to save computation).  This process
   ends in a hash value for the root node, which is called the "root
   hash".  Note the root hash only depends on the content and any
   modification of the content will result in a different root hash.


















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                               7 = root hash
                              / \
                            /     \
                          /         \
                        /             \
                      3*               11
                     / \              / \
                    /   \            /   \
                   /     \          /     \
                  1       5        9       13* = uncle hash
                 / \     / \      / \      / \
                0   2   4   6    8   10* 12   14

                C0  C1  C2  C3   C4  C5  C6   E
                =chunk index     ^^           = empty hash

            Merkle hash tree of a content file with N=7 chunks

                                 Figure 3

5.2.  Content Integrity Verification

   Assuming a peer receives the root hash of the content it wants to
   download from a trusted source, it can check the integrity of any
   chunk of that content it receives as follows.  It first calculates
   the hash of the chunk it received, for example, chunk C4 in the
   previous figure.  Along with this chunk, it MUST receive the hashes
   required to check the integrity of that chunk.  In principle, these
   are the hash of the chunk's sibling (C5) and that of its "uncles".  A
   chunk's uncles are the sibling Y of its parent X, and the uncle of
   that Y, recursively until the root is reached.  For chunk C4, the
   uncles are nodes 13 and 3 and the sibling is 10; all marked with a *
   in the figure.  Using this information, the peer recalculates the
   root hash of the tree and compares it to the root hash it received
   from the trusted source.  If they match, the chunk of content has
   been positively verified to be the requested part of the content.
   Otherwise, the sending peer sent either the wrong content or the
   wrong sibling or uncle hashes.  For simplicity, the set of sibling
   and uncle hashes is collectively referred to as the "uncle hashes".

   In the case of live streaming, the tree of chunks grows dynamically
   and the root hash is undefined or, more precisely, transient, as long
   as new data is generated by the live source.  Section 6.1.2 defines a
   method for content integrity verification for live streams that works
   with such a dynamic tree.  Although the tree is dynamic, content
   verification works the same for both live and predefined content,
   resulting in a unified method for both types of streaming.




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5.3.  The Atomic Datagram Principle

   As explained above, a datagram consists of a sequence of messages.
   Ideally, every datagram sent must be independent of other datagrams:
   each datagram SHOULD be processed separately, and a loss of one
   datagram must not disrupt the flow of datagrams between two peers.
   Thus, as a datagram carries zero or more messages, both messages and
   message interdependencies SHOULD NOT span over multiple datagrams.

   This principle implies that as any chunk is verified using its uncle
   hashes, the necessary hashes SHOULD be put into the same datagram as
   the chunk's data.  If this is not possible because of a limitation on
   datagram size, the necessary hashes MUST be sent first in one or more
   datagrams.  As a general rule, if some additional data is still
   missing to process a message within a datagram, the message SHOULD be
   dropped.

   The hashes necessary to verify a chunk are, in principle, its
   sibling's hash and all its uncle hashes, but the set of hashes to
   send can be optimized.  Before sending a packet of data to the
   receiver, the sender inspects the receiver's previous
   acknowledgements (HAVE or ACK) to derive which hashes the receiver
   already has for sure.  Suppose the receiver had acknowledged chunks
   C0 and C1 (the first two chunks of the file), then it must already
   have uncle hashes 5, 11, and so on.  That is because those hashes are
   necessary to check C0 and C1 against the root hash.  Then, hashes 3,
   7, and so on must also be known as they are calculated in the process
   of checking the uncle hash chain.  Hence, to send chunk C7, the
   sender needs to include just the hashes for nodes 14 and 9, which let
   the data be checked against hash 11, which is already known to the
   receiver.

   The sender MAY optimistically skip hashes that were sent out in
   previous, still-unacknowledged datagrams.  It is an optimization
   trade-off between redundant hash transmission and the possibility of
   collateral data loss in the case in which some necessary hashes were
   lost in the network so some delivered data cannot be verified and
   thus had to be dropped.  In either case, the receiver builds the
   Merkle hash tree on-demand, incrementally, starting from the root
   hash, and uses it for data validation.

   In short, the sender MUST put into the datagram the hashes he
   believes are necessary for the receiver to verify the chunk.  The
   receiver MUST remember all the hashes it needs to verify missing
   chunks that it still wants to download.  Note that the latter implies
   that a hardware-limited receiver MAY forget some hashes if it does
   not plan to announce possession of these chunks to others (i.e., does
   not plan to send HAVE messages.)



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5.4.  INTEGRITY Messages

   Concretely, a peer that wants to send a chunk of content creates a
   datagram that MUST consist of a list of INTEGRITY messages followed
   by a DATA message.  If the INTEGRITY messages and DATA message cannot
   be put into a single datagram because of a limitation on datagram
   size, the INTEGRITY messages MUST be sent first in one or more
   datagrams.  The list of INTEGRITY messages sent MUST contain an
   INTEGRITY message for each hash the receiver misses for integrity
   checking.  An INTEGRITY message for a hash MUST contain the chunk
   specification corresponding to the node ID of the hash and the hash
   data itself.  The chunk specification corresponding to a node ID is
   defined as the range of chunks formed by the leaves of the subtree
   rooted at the node.  For example, node 3 in Figure 3 denotes chunks
   0, 2, 4, and 6, so the chunk specification should denote that
   interval.  The list of INTEGRITY messages MUST be sorted in order of
   the tree height of the nodes, descending (the leaves are at height
   0).  The DATA message MUST contain the chunk specification of the
   chunk and the chunk itself.  A peer MAY send the required messages
   for multiple chunks in the same datagram, depending on the
   encapsulation.

5.5.  Discussion and Overhead

   The current method for protecting content integrity in BitTorrent
   [BITTORRENT] is not suited for streaming.  It involves providing
   clients with the hashes of the content's chunks before the download
   commences by means of metadata files (called .torrent files in
   BitTorrent.)  However, when chunks are small, as in the current UDP
   encapsulation of PPSPP, this implies having to download a large
   number of hashes before content download can begin.  This, in turn,
   increases time-till-playback for end users, making this method
   unsuited for streaming.

   The overhead of using Merkle hash trees is limited.  The size of the
   hash tree expressed as the total number of nodes depends on the
   number of chunks the content is divided (and hence the size of
   chunks) following this formula:

       nnodes = math.pow(2,math.log(nchunks,2)+1)

   In principle, the hash values of all these nodes will have to be sent
   to a peer once for it to verify all of the chunks.  Hence, the
   maximum on-the-wire overhead is hashsize * nnodes.  However, the
   actual number of hashes transmitted can be optimized as described in
   Section 5.3.





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   To see a peer can verify all chunks whilst receiving not all hashes,
   consider the example tree in Section 5.1.  In the case of a simple
   progressive download, of chunks 0, 2, 4, 6, etc., the sending peer
   will send the following hashes:

          +-------+---------------------------------------------+
          | Chunk | Node IDs of hashes sent                     |
          +-------+---------------------------------------------+
          |   0   | 2,5,11                                      |
          |   2   | - (receiver already knows all)              |
          |   4   | 6                                           |
          |   6   | -                                           |
          |   8   | 10,13 (hash 3 can be calculated from 0,2,5) |
          |   10  | -                                           |
          |   12  | 14                                          |
          |   14  | -                                           |
          | Total | # hashes        7                           |
          +-------+---------------------------------------------+

                  Table 1: Overhead for the Example Tree

   So the number of hashes sent in total (7) is less than the total
   number of hashes in the tree (16), as a peer does not need to send
   hashes that are calculated and verified as part of earlier chunks.

5.6.  Automatic Detection of Content Size

   In PPSPP, the size of a static content file, such as a video file,
   can be reliably and automatically derived from information received
   from the network when fixed-size chunks are used.  As a result, it is
   not necessary to include the size of the content file as the metadata
   of the content for such files.  Implementations of PPSPP MAY use this
   automatic detection feature.  Note this feature is the only feature
   of PPSPP that requires that a fixed-size chunk is used.  This feature
   builds on the Merkle hash tree and the trusted root hash as swarm ID
   as follows.

5.6.1.  Peak Hashes

   The ability for a newcomer peer to detect the size of the content
   depends heavily on the concept of peak hashes.  The concept of peak
   hashes depends on the concepts of filled and incomplete nodes.
   Recall that when constructing the binary trees for content
   verification and addressing the base of the tree may have more leaves
   than the number of chunks in the content.  In the Merkle hash tree,
   these leaves were assigned empty all-zero hashes to be able to
   calculate the higher-level hashes.  A filled node is now defined as a
   node that corresponds to an interval of leaves that consists only of



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   hashes of content chunks, not empty hashes.  Reversely, an incomplete
   (not filled) node corresponds to an interval that also contains empty
   hashes, typically, an interval that extends past the end of the file.
   In the following figure, nodes 7, 11, 13, and 14 are incomplete: the
   rest is filled.

   Formally, a peak hash is the hash of a filled node in the Merkle hash
   tree, whose sibling is an incomplete node.  Practically, suppose a
   file is 7162 bytes long and a chunk is 1 kilobyte.  That file fits
   into 7 chunks, the tail chunk being 1018 bytes long.  The Merkle hash
   tree for that file is shown in Figure 4.  Following the definition,
   the peak hashes of this file are in nodes 3, 9, and 12, denoted with
   an *. E denotes an empty hash.

                                  7
                                 / \
                               /     \
                             /         \
                           /             \
                         3*               11
                        / \              / \
                       /   \            /   \
                      /     \          /     \
                     1       5        9*      13
                    / \     / \      / \      / \
                   0   2   4   6    8   10  12*  14

                   C0  C1  C2  C3   C4  C5  C6   E
                                            = 1018 bytes

                     Peak hashes in a Merkle hash tree

                                 Figure 4

   Peak hashes can be explained by the binary representation of the
   number of chunks the file occupies.  The binary representation for 7
   is 111.  Every "1" in binary representation of the file's packet
   length corresponds to a peak hash.  For this particular file, there
   are indeed three peaks: nodes 3, 9, and 12.  Therefore, the number of
   peak hashes for a file is also, at most, logarithmic with its size.

   A peer knowing which nodes contain the peak hashes for the file can
   therefore calculate the number of chunks it consists of; thus, it
   gets an estimate of the file size (given all chunks but the last are
   of a fixed size).  Which nodes are the peaks can be securely
   communicated from one (untrusted) peer, Peer A, to another peer, Peer
   B, by letting Peer A send the peak hashes and their node IDs to Peer




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   B.  It can be shown that the root hash that Peer B obtained from a
   trusted source is sufficient to verify that these are indeed the
   right peak hashes, as follows.

   Lemma: Peak hashes can be checked against the root hash.

   Proof: (a) Any peak hash is always the left sibling.  Otherwise, if
   it is the right sibling, its left neighbor/sibling must also be a
   filled node, because of the way chunks are laid out in the leaves,
   which contradicts the definition of a peak hash. (b) For the
   rightmost peak hash, its right sibling is zero. (c) For any peak
   hash, the right sibling might be calculated using peak hashes to the
   left and zeros for empty nodes. (d) Once the right sibling of the
   leftmost peak hash is calculated, its parent might be calculated. (e)
   Once that parent is calculated, we might trivially get to the root
   hash by concatenating the hash with zeros and hashing it repeatedly.

   Informally, the Lemma might be expressed as follows: peak hashes
   cover all data, so the remaining hashes are either trivial (zeros) or
   might be calculated from peak hashes and zero hashes.

   Finally, once Peer B has obtained the number of chunks in the
   content, it can determine the exact file size as follows.  Given that
   all chunks except the last are of a fixed size, Peer B just needs to
   know the size of the last chunk.  Knowing the number of chunks, Peer
   B can calculate the node ID of the last chunk and download it.  As
   always, Peer B verifies the integrity of this chunk against the
   trusted root hash.  As there is only one chunk of data that leads to
   a successful verification, the size of this chunk must be correct.
   Peer B can then determine the exact file size as:

       (number of chunks -1) * fixed chunk size + size of last chunk

5.6.2.  Procedure

   A PPSPP implementation that wants to use automatic size detection
   MUST operate as follows.  When Peer A sends a DATA message for the
   first time to Peer B, Peer A MUST first send all the peak hashes for
   the content, in INTEGRITY messages, unless Peer B has already
   signaled that it knows the peak hashes by having acknowledged any
   chunk.  If they are needed, the peak hashes MUST be sent as an extra
   list of uncle hashes for the chunk, before the list of actual uncle
   hashes of the chunk as described in Section 5.3.  The receiver, Peer
   B, MUST check the peak hashes against the root hash to determine the
   approximate content size.  To obtain the definite content size, Peer
   B MUST download the last chunk of the content from any peer that
   offers it.




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   As an example, let's consider a 7162-byte file, which fits in 7
   chunks of 1 kilobyte, distributed by Peer A.  Figure 4 shows the
   relevant Merkle hash tree.  Peer B, which only knows the root hash of
   the file after successfully connecting to Peer A, requests the first
   chunk of data, C0 in Figure 4.  Peer A replies to Peer B by including
   in the datagram the following messages in this specific order: first,
   the three peak hashes of this particular file, the hashes of nodes 3,
   9, and 12; second, the uncle hashes of C0, followed by the DATA
   message containing the actual content of C0.  Upon receiving the peak
   hashes, Peer B checks them against the root hash determining that the
   file is 7 chunks long.  To establish the exact size of the file, Peer
   B needs to request and retrieve the last chunk containing data, C6 in
   Figure 4.  Once the last chunk has been retrieved and verified, Peer
   B concludes that it is 1018 bytes long, hence determining that the
   file is exactly 7162 bytes long.

6.  Live Streaming

   The set of messages defined above can be used for live streaming as
   well.  In a pull-based model, a live streaming injector can announce
   the chunks it generates via HAVE messages, and peers can retrieve
   them via REQUEST messages.  Areas that need special attention are
   content authentication and chunk addressing (to achieve an infinite
   stream of chunks).

6.1.  Content Authentication

   For live streaming, PPSPP supports two methods for a peer to
   authenticate the content it receives from another peer, called "Sign
   All" and "Unified Merkle Tree".

   In the "Sign All" method, the live injector signs each chunk of
   content using a private key.  Upon receiving the chunk, peers check
   the signature using the corresponding public key obtained from a
   trusted source.  Support for this method is OPTIONAL.

   In the "Unified Merkle Tree" method, PPSPP combines the Merkle Hash
   Tree scheme for static content with signatures to unify the video-on-
   demand and live streaming scenarios.  The use of Merkle hash trees
   reduces the number of signing and verification operations, hence
   providing a similar signature amortization to the approach described
   in [SIGMCAST].  If PPSPP operates over the Internet, the "Unified
   Merkle Tree" method MUST be used.  If the protocol operates in a
   benign environment, the "Unified Merkle Tree" method MAY be used.  So
   this method is mandatory to implement.

   In both methods, the swarm ID consists of a public key encoded as in
   a DNSSEC DNSKEY resource record without Base64 encoding [RFC4034].



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   In particular, the swarm ID consists of a 1-byte Algorithm field that
   identifies the public key's cryptographic algorithm and determines
   the format of the Public Key field that follows.  The value of this
   Algorithm field is one of the values in the "Domain Name System
   Security (DNSSEC) Algorithm Numbers" registry [IANADNSSECALGNUM].
   The RSASHA1 [RFC4034], RSASHA256 [RFC5702], ECDSAP256SHA256 and
   ECDSAP384SHA384 [RFC6605] algorithms are mandatory to implement.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Algo Number(8)|                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                DNSSEC Public Key (variable)                   ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.1.1.  Sign All

   In the "Sign All" method, the live injector signs each chunk of
   content using a private key and peers, upon receiving the chunk,
   check the signature using the corresponding public key obtained from
   a trusted source.  In particular, in PPSPP, the swarm ID of the live
   stream is that public key.

   A peer that wants to send a chunk of content creates a datagram that
   MUST contain a SIGNED_INTEGRITY message with the chunk's signature,
   followed by a DATA message with the actual chunk.  If the
   SIGNED_INTEGRITY message and DATA message cannot be contained into a
   single datagram, because of a limitation on datagram size, the
   SIGNED_INTEGRITY message MUST be sent first in a separate datagram.
   The SIGNED_INTEGRITY message consists of the chunk specification, the
   timestamp, and the digital signature.

   The digital signature algorithm that is used, is determined by the
   Live Signature Algorithm protocol option, see Section 7.7.  The
   signature is computed over a concatenation of the on-the-wire
   representation of the chunk specification, a 64-bit timestamp in NTP
   Timestamp format [RFC5905], and the chunk, in that order.  The
   timestamp is the time signature that was made at the injector in UTC.

6.1.2.  Unified Merkle Tree

   In this method, the chunks of content are used as the basis for a
   Merkle hash tree as for static content.  However, because chunks are
   continuously generated, this tree is not static, but dynamic.  As a
   result, the tree does not have a root hash, or, more precisely, it
   has a transient root hash.  Therefore, a public key serves as swarm




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   ID of the content.  It is used to digitally sign updates to the tree
   allowing peers to expand it based on trusted information using the
   following process.

6.1.2.1.  Signed Munro Hashes

   The live injector generates a number of chunks, denoted
   NCHUNKS_PER_SIG, corresponding to fixed power of 2
   (NCHUNKS_PER_SIG>=2), which are added as new leaves to the existing
   hash tree.  As a result of this expansion, the hash tree contains a
   new subtree that is NCHUNKS_PER_SIG chunks wide at the base.  The
   root of this new subtree is referred to as the munro of that subtree,
   and its hash as the munro hash of the subtree, illustrated in
   Figure 5.  In this figure, node 5 is the new munro, labeled with a $
   sign.

                                     3
                                    / \
                                   /   \
                                  /     \
                                 1       5$
                                / \     / \
                               0   2   4   6

   Expanded live tree.  With NCHUNKS_PER_SIG=2, node 5 is the munro for
      the new subtree spanning 4 and 6.  Node 1 is the munro for the
    subtree spanning chunks 0 and 2, created in the previous iteration.

                                 Figure 5

   Informally, the process now proceeds as follows.  The injector signs
   only the munro hash of the new subtree using its private key.  Next,
   the injector announces the existence of the new subtree to its peers
   using HAVE messages.  When a peer, in response to the HAVE messages,
   requests a chunk from the new subtree, the injector first sends the
   signed munro hash corresponding to the requested chunk.  Afterwards,
   similar to static content, the injector sends the uncle hashes
   necessary to verify that chunk, as in Section 5.1.  In particular,
   the injector sends the uncle hashes necessary to verify the requested
   chunk against the munro hash.  This differs from static content,
   where the verification takes places against the root hash.  Finally,
   the injector sends the actual chunk.









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   The receiving peer verifies the signature on the signed munro using
   the swarm ID (a public key) and updates its hash tree.  As the peer
   now knows the munro hash is trusted, it can verify all chunks in the
   subtree against this munro hash, using the accompanying uncle hashes
   as in Section 5.1.

   To illustrate this procedure, lets consider the next iteration in the
   process.  The injector has generated the current tree shown in
   Figure 5, and it is connected to several peers that currently have
   the same tree and all posses chunks 0, 2, 4, and 6.  When the
   injector generates two new chunks, NCHUNKS_PER_SIG=2, the hash tree
   expands as shown in Figure 6.  The two new chunks, 8 and 10, extend
   the tree on the right side, and to accommodate them, a new root is
   created: node 7.  As this tree is wider at the base than the actual
   number of chunks, there are currently two empty leaves.  The munro
   node for the new subtree is 9, labeled with a $ sign.

                                     7
                                    / \
                                  /     \
                                /         \
                              /             \
                            3               11
                           / \              / \
                          /   \            /   \
                         /     \          /     \
                        1       5        9$      13
                       / \     / \      / \      / \
                      0   2   4   6    8   10   E   E

    Expanded live tree.  With NCHUNKS_PER_SIG=2, node 9 is the munro of
             the newly added subtree spanning chunks 8 and 10.

                                 Figure 6

   The injector now needs to inform its peers of the updated tree,
   communicating the addition of the new munro hash 9.  Hence, it sends
   a HAVE message with a chunk specification for nodes 8 + 10 to its
   peers.  As a response, Peer P requests the newly created chunk, e.g.,
   chunk 8, from the injector by sending a REQUEST message.  In reply,
   the injector sends the signed munro hash of node 9 as an INTEGRITY
   message with the hash of node 9, and a SIGNED_INTEGRITY message with
   the signature of the hash of node 9.  These messages are followed by
   an INTEGRITY message with the hash of node 10 and a DATA message with
   chunk 8.






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   Upon receipt, Peer P verifies the signature of the munro and expands
   its view of the tree.  Next, the peer computes the hash of chunk 8
   and combines it with the received hash of node 10, computing the
   expected hash of node 9.  He can then verify the content of chunk 8
   by comparing the computed hash of node 9 with the munro hash of the
   same node he just received; hence, Peer P has successfully verified
   the integrity of chunk 8.

   This procedure requires just one signing operation for every
   NCHUNKS_PER_SIG chunks created, and one verification operation for
   every NCHUNKS_PER_SIG received, making it much cheaper than "Sign
   All".  A receiving peer does additionally need to check one or more
   hashes per chunk via the Merkle Hash Tree scheme, but this has less
   hardware requirements than a signature verification for every chunk.
   This approach is similar to signature amortization via Merkle Tree
   Chaining [SIGMCAST].  The downside of this scheme is in an increased
   latency.  A peer cannot download the new chunks until the injector
   has computed the signature and announced the subtree.  A peer MUST
   check the signature before forwarding the chunks to other peers
   [POLLIVE].

   The number of chunks per signature NCHUNKS_PER_SIG MUST be a fixed
   power of 2 for simplicity.  NCHUNKS_PER_SIG MUST be larger than 1 for
   performance reasons.  There are two related factors to consider when
   choosing a value for NCHUNKS_PER_SIG.  First, the allowed CPU load on
   clients due to signature verifications, given the expected bitrate of
   the stream.  To achieve a low CPU load in a high bitrate stream,
   NCHUNKS_PER_SIG should be high.  Second, the effect on latency, which
   increases when NCHUNKS_PER_SIG gets higher, as just discussed.  Note
   how the procedure does not preclude the use of variable-size chunks.

   This method of integrity verification provides an additional benefit.
   If the system includes some peers that saved the complete broadcast,
   as soon as the broadcast ends, the content is available as a video-
   on-demand download using the now stabilized tree and the final root
   hash as swarm identifier.  Peers that saved all the chunks, can now
   announce the root hash to the tracking infrastructure and instantly
   seed the content.

6.1.2.2.  Munro Signature Calculation

   The digital signature algorithm used is determined by the Live
   Signature Algorithm protocol option, see Section 7.7.  The signature
   is computed over a concatenation of the on-the-wire representation of
   the chunk specification of the munro node (see Section 6.1.2.1), a
   timestamp in 64-bit NTP Timestamp format [RFC5905], and the hash
   associated with the munro node, in that order.  The timestamp is the
   time signature that was made at the injector in UTC.



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6.1.2.3.  Procedure

   Formally, the injector MUST NOT send a HAVE message for chunks in the
   new subtree until it has computed the signed munro hash for that
   subtree.

   When Peer B requests a chunk C from Peer A (either the injector or
   another peer), and Peer A decides to reply, it must do so as follows.
   First, Peer A MUST send an INTEGRITY message with the chunk
   specification for the munro of chunk C and the munro's hash, followed
   by a SIGNED_INTEGRITY message with the chunk specification for the
   munro, timestamp, and its signature in a single datagram, unless Peer
   B indicated earlier in the exchange that it already possess a chunk
   with the same corresponding munro (by means of HAVE or ACK messages).
   Following these two messages (if any), Peer A MUST send the necessary
   missing uncles hashes needed for verifying the chunk against its
   munro hash, and the chunk itself, as described in Section 5.4,
   sharing datagrams if possible.

6.1.2.4.  Secure Tune In

   When a peer tunes in to a live stream, it has to determine what is
   the last chunk the injector has generated.  To facilitate this
   process in the Unified Merkle Tree scheme, each peer shares its
   knowledge about the injector's chunks with the others by exchanging
   their latest signed munro hashes, as follows.

   Recall that, in PPSPP, when Peer A initiates a channel with Peer B,
   Peer A sends a first datagram with a HANDSHAKE message, and Peer B
   responds with a second datagram also containing a HANDSHAKE message
   (see Section 3.1).  When Peer A sends a third datagram to Peer B, and
   it is received by Peer B, both peers know that the other is listening
   on its stated transport address.  Peer B is then allowed to send
   heavy payload like DATA messages in the fourth datagram.  Peer A can
   already safely do that in the third datagram.

   In the Unified Merkle Tree scheme, Peer A MUST send its rightmost
   signed munro hash to Peer B in the third datagram, and in any
   subsequent datagrams to Peer B, until Peer B indicates that it
   possess a chunk with the same corresponding munro or a more recent
   munro (by means of a HAVE or ACK message).  Peer B may already have
   indicated this fact by means of HAVE messages in the second datagram.
   Conversely, when Peer B sends the fourth datagram or any subsequent
   datagram to Peer A, Peer B MUST send its rightmost signed munro hash,
   unless Peer A indicated knowledge of it or more recent munros.  The
   rightmost signed munro hash of a peer is defined as the munro hash
   signed by the injector of the rightmost subtree of width
   NCHUNKS_PER_SIG chunks in the peer's Merkle hash tree.  Peer A MUST



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   NOT send the signed munro hash in the first datagram of the HANDSHAKE
   procedure and Peer B MUST NOT send it in the second datagram as it is
   considered heavy payload.

   When a peer receives a SIGNED_INTEGRITY message with a signed munro
   hash but the timestamp is too old, the peer MUST discard the message.
   Otherwise, it SHOULD use the signed munro to update its hash tree and
   pick a tune-in in the live stream.  A peer may use the information
   from multiple peers to pick the tune-in point.

6.2.  Forgetting Chunks

   As a live broadcast progresses, a peer may want to discard the chunks
   that it already played out.  Ideally, other peers should be aware of
   this fact so that they will not try to request these chunks from this
   peer.  This could happen in scenarios where live streams may be
   paused by viewers, or viewers are allowed to start late in a live
   broadcast (e.g., start watching a broadcast at 20:35 when it actually
   began at 20:30).

   PPSPP provides a simple solution for peers to stay up to date with
   the chunk availability of a discarding peer.  A discarding peer in a
   live stream MUST enable the Live Discard Window protocol option,
   specifying how many chunks/bytes it caches before the last chunk/byte
   it advertised as being available (see Section 7.9).  Its peers SHOULD
   apply this number as a sliding window filter over the peer's chunk
   availability as conveyed via its HAVE messages.

   Three factors are important when deciding for an appropriate value
   for this option: the desired amount of playback buffer for peers, the
   bitrate of the stream, and the available resources of the peer.
   Consider the case of a fresh peer joining the stream.  The size of
   the discard window of the peers it connects to influences how much
   data it can directly download to establish its prebuffer.  If the
   window is smaller than the desired buffer, the fresh peer has to wait
   until the peers downloaded more of the stream before it can start
   playback.  As media buffers are generally specified in terms of a
   number of seconds, the size of the discard window is also related to
   the (average) bitrate of the stream.  Finally, if a peer has few
   resources to store chunks and metadata, it should choose a small
   discard window.

7.  Protocol Options

   The HANDSHAKE message in PPSPP can contain the following protocol
   options.  Unless stated otherwise, a protocol option consists of an
   8-bit code followed by an 8-bit value.  Larger values are all encoded




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   big-endian.  Each protocol option is explained in the following
   subsections.  The list of protocol options MUST be sorted on code
   value (ascending) in a HANDSHAKE message.

             +--------+-------------------------------------+
             | Code   | Description                         |
             +--------+-------------------------------------+
             | 0      | Version                             |
             | 1      | Minimum Version                     |
             | 2      | Swarm Identifier                    |
             | 3      | Content Integrity Protection Method |
             | 4      | Merkle Hash Tree Function           |
             | 5      | Live Signature Algorithm            |
             | 6      | Chunk Addressing Method             |
             | 7      | Live Discard Window                 |
             | 8      | Supported Messages                  |
             | 9      | Chunk Size                          |
             | 10-254 | Unassigned                          |
             | 255    | End Option                          |
             +--------+-------------------------------------+

                          Table 2: PPSPP Options

7.1.  End Option

   A peer MUST conclude the list of protocol options with the end
   option.  Subsequent octets should be considered protocol messages.
   The code for the end option is 255, and unlike others, it has no
   value octet, so the option's length is 1 octet.

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |1 1 1 1 1 1 1 1|
   +-+-+-+-+-+-+-+-+

7.2.  Version

   A peer MUST include the maximum version of the PPSPP it supports as
   the first protocol option in the list.  The code for this option is
   0.  Defined values are listed in Table 3.











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           +---------+----------------------------------------+
           | Version | Description                            |
           +---------+----------------------------------------+
           | 0       | Reserved                               |
           | 1       | Protocol as described in this document |
           | 2-255   | Unassigned                             |
           +---------+----------------------------------------+

                      Table 3: PPSPP Version Numbers

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|  Version (8)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.3.  Minimum Version

   When a peer initiates the handshake, it MUST include the minimum
   version of the PPSPP it supports in the list of protocol options,
   following the min/max versioning scheme defined in [RFC6709],
   Section 4.1, strategy 5.  The code for this option is 1.  Defined
   values are listed in Table 3.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 1| Min. Ver. (8) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.4.  Swarm Identifier

   When a peer initiates the handshake, it MUST include a single swarm
   identifier option.  If the peer is not the initiator, it MAY include
   a swarm identifier option, as an end-to-end check.  This option has
   the following structure:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 1 0|     Swarm ID Length (16)      |               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                       Swarm Identifier (variable)             ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Swarm ID Length field contains the length of the single Swarm
   Identifier that follows in bytes.  The Length field is 16 bits wide
   to allow for large public keys as identifiers in live streaming.



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   Each PPSPP peer knows the IDs of the swarms it joins, so this
   information can be immediately verified upon receipt.

7.5.  Content Integrity Protection Method

   A peer MUST include the content integrity method used by a swarm.
   The code for this option is 3.  Defined values are listed in Table 4.

                   +--------+-------------------------+
                   | Method | Description             |
                   +--------+-------------------------+
                   | 0      | No integrity protection |
                   | 1      | Merkle Hash Tree        |
                   | 2      | Sign All                |
                   | 3      | Unified Merkle Tree     |
                   | 4-255  | Unassigned              |
                   +--------+-------------------------+

            Table 4: PPSPP Content Integrity Protection Methods

   The "Merkle Hash Tree" method is the default for static content, see
   Section 5.1.  "Sign All", and "Unified Merkle Tree" are for live
   content, see Section 6.1, with "Unified Merkle Tree" being the
   default.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 1 1|   CIPM (8)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.6.  Merkle Tree Hash Function

   When the content integrity protection method is "Merkle Hash Tree",
   this option defining which hash function is used for the tree MUST be
   included.  The code for this option is 4.  Defined values are listed
   in Table 5 (see [FIPS180-4] for the function semantics).














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                        +----------+-------------+
                        | Function | Description |
                        +----------+-------------+
                        | 0        | SHA-1       |
                        | 1        | SHA-224     |
                        | 2        | SHA-256     |
                        | 3        | SHA-384     |
                        | 4        | SHA-512     |
                        | 5-255    | Unassigned  |
                        +----------+-------------+

                   Table 5: PPSPP Merkle Hash Functions

   Implementations MUST support SHA-1 (see Section 12.5) and SHA-256.
   SHA-256 is the default.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 0 0|    MHF (8)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.7.  Live Signature Algorithm

   When the content integrity protection method is "Sign All" or
   "Unified Merkle Tree", this option MUST be defined.  The code for
   this option is 5.  The 8-bit value of this option is one of the
   values listed in the "Domain Name System Security (DNSSEC) Algorithm
   Numbers" registry [IANADNSSECALGNUM].  The RSASHA1 [RFC4034],
   RSASHA256 [RFC5702], ECDSAP256SHA256 and ECDSAP384SHA384 [RFC6605]
   algorithms are mandatory to implement.  Default is ECDSAP256SHA256.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 0 1|    LSA (8)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.8.  Chunk Addressing Method

   A peer MUST include the chunk addressing method it uses.  The code
   for this option is 6.  Defined values are listed in Table 6.









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                     +--------+---------------------+
                     | Method | Description         |
                     +--------+---------------------+
                     | 0      | 32-bit bins         |
                     | 1      | 64-bit byte ranges  |
                     | 2      | 32-bit chunk ranges |
                     | 3      | 64-bit bins         |
                     | 4      | 64-bit chunk ranges |
                     | 5-255  | Unassigned          |
                     +--------+---------------------+

                  Table 6: PPSPP Chunk Addressing Methods

   Implementations MUST support "32-bit chunk ranges" and "64-bit chunk
   ranges".  Default is "32-bit chunk ranges".

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 1 0|    CAM (8)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.9.  Live Discard Window

   A peer in a live swarm MUST include the discard window it uses.  The
   code for this option is 7.  The unit of the discard window depends on
   the chunk addressing method used, see Table 6.  For bins and chunk
   ranges, it is a number of chunks; for byte ranges, it is a number of
   bytes.  Its data type is the same as for a bin, or one value in a
   range specification.  In other words, its value is a 32-bit or 64-bit
   integer in big-endian format.  If this option is used, the Chunk
   Addressing Method MUST appear before it in the list.  This option has
   the following structure:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 1 1|       Live Discard Window (32 or 64)          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A peer that does not, under normal circumstances, discard chunks MUST
   set this option to the special value 0xFFFFFFFF (32-bit) or
   0xFFFFFFFFFFFFFFFF (64-bit).  For example, peers that record a
   complete broadcast to offer it directly as a static file after the
   broadcast ends use these values (see Section 6.1.2).  Section 6.2
   explains how to determine a value for this option.



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7.10.  Supported Messages

   Peers may support just a subset of the PPSPP messages.  For example,
   peers running over TCP may not accept ACK messages or peers used with
   a centralized tracking infrastructure may not accept PEX messages.
   For these reasons, peers who support only a proper subset of the
   PPSPP messages MUST signal which subset they support by means of this
   protocol option.  The code for this option is 8.  The value of this
   option is a length octet (SupMsgLen) indicating the length, in bytes,
   of the compressed bitmap that follows.

   The set of messages supported can be derived from the compressed
   bitmap by padding it with bytes of value 0 until it is 256 bits in
   length.  Then, a 1 bit in the resulting bitmap at position X
   (numbering left to right) corresponds to support for message type X,
   see Table 7.  In other words, to construct the compressed bitmap,
   create a bitmap with a 1 for each message type supported and a 0 for
   a message type that is not, store it as an array of bytes, and
   truncate it to the last non-zero byte.  An example of the first 16
   bits of the compressed bitmap for a peer supporting every message
   except ACKs and PEXs is 11011001 11110000.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 0 0 0| SupMsgLen (8) |                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~            Supported Messages Bitmap (variable, max 256)      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.11.  Chunk Size

   A peer in a swarm MUST include the chunk size the swarm uses.  The
   code for this option is 9.  Its value is a 32-bit integer denoting
   the size of the chunks in bytes in big-endian format.  When variable
   chunk sizes are used, this option MUST be set to the special value
   0xFFFFFFFF.  Section 8.1 explains how content publishers can
   determine a value for this option.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 0 0 1|       Chunk Size (32)                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |
   +-+-+-+-+-+-+-+-+





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8.  UDP Encapsulation

   PPSPP implementations MUST use UDP as transport protocol and MUST use
   LEDBAT for congestion control [RFC6817].  Using LEDBAT enables PPSPP
   to serve the content after playback (seeding) without disrupting the
   user who may have moved to different tasks that use its network
   connection.  Future PPSPP versions can also run over other transport
   protocols or use different congestion control algorithms.

8.1.  Chunk Size

   In general, a UDP datagram containing PPSPP messages SHOULD fit
   inside a single IP packet, so its maximum size depends on the MTU of
   the network.  If the UDP datagram does not fit, its chance of getting
   lost in the network increases as the loss of a single fragment of the
   datagram causes the loss of the complete datagram.

   The largest message in a PPSPP datagram is the DATA message carrying
   a chunk of content.  So the (maximum) size of a chunk to choose for a
   particular swarm depends primarily on the expected MTU.  The chunk
   size should be chosen such that a chunk and its required INTEGRITY
   messages can generally be carried inside a single datagram, following
   the Atomic Datagram Principle (Section 5.3).  Other considerations
   are the hardware capabilities of the peers.  Having large chunks and
   therefore less chunks per megabyte of content reduces processing
   costs.  The chunk addressing schemes can all work with different
   chunk sizes, see Section 4.

   The RECOMMENDED approach is to use fixed-size chunks of 1024 bytes,
   as this size has a high likelihood of traveling end-to-end across the
   Internet without any fragmentation.  In particular, with this size, a
   UDP datagram with a DATA message can be transmitted as a single IP
   packet over an Ethernet network with 1500-byte frames.

   A PPSPP implementation MAY use a variant of the Packetization Layer
   Path MTU Discovery (PLPMTUD), described in [RFC4821], for discovering
   the optimal MTU between sender and destination.  As in PLPMTUD,
   progressively larger probing packets are used to detect the optimal
   MTU for a given path.  However, in PPSPP, probe packets SHOULD
   contain actual messages, in particular, multiple DATA messages.  By
   using actual DATA messages as probe packets, the returning ACK
   messages will confirm the probe delivery, effectively updating the
   MTU estimate on both ends of the link.  To be able to scale up probe
   packets with sensible increments, a minimum chunk size of 512 bytes
   SHOULD be used.  Smaller chunk sizes lead to an inefficient protocol.
   An implication is that PPSPP supports datagrams over IPv4 of 576
   bytes or more only.  This variant is not mandatory to implement.




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   The chunk size used for a particular swarm, or the fact that it is
   variable, MUST be part of the swarm's metadata (which then minimally
   consists of the swarm ID and the chunk nature and size).

8.2.  Datagrams and Messages

   When using UDP, the abstract datagram described above corresponds
   directly to a UDP datagram.  Most messages within a datagram have a
   fixed length, which generally depends on the type of the message.
   The first byte of a message denotes its type.  The currently defined
   types are:

                      +----------+------------------+
                      | Msg Type | Description      |
                      +----------+------------------+
                      | 0        | HANDSHAKE        |
                      | 1        | DATA             |
                      | 2        | ACK              |
                      | 3        | HAVE             |
                      | 4        | INTEGRITY        |
                      | 5        | PEX_RESv4        |
                      | 6        | PEX_REQ          |
                      | 7        | SIGNED_INTEGRITY |
                      | 8        | REQUEST          |
                      | 9        | CANCEL           |
                      | 10       | CHOKE            |
                      | 11       | UNCHOKE          |
                      | 12       | PEX_RESv6        |
                      | 13       | PEX_REScert      |
                      | 14-254   | Unassigned       |
                      | 255      | Reserved         |
                      +----------+------------------+

                       Table 7: PPSPP Message Types

   Furthermore, integers are serialized in network (big-endian) byte
   order.  So, consider the example of a HAVE message (Section 3.2)
   using bin chunk addressing.  It has a message type of 0x03 and a
   payload of a bin number, a 4-byte integer (say, 1); hence, its on-
   the-wire representation for UDP can be written in hex as
   "0300000001".

   All messages are idempotent or recognizable as duplicates.
   Idempotent means that processing a message more than once does not
   lead to a different state from if it was processed just once.  In
   particular, a peer MAY resend DATA, ACK, HAVE, INTEGRITY, PEX_*,
   SIGNED_INTEGRITY, REQUEST, CANCEL, CHOKE, and UNCHOKE messages
   without problems when loss is suspected.  When a peer resends a



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   HANDSHAKE message, it can be recognized as duplicate by the receiver,
   because it already recorded the first connection attempt, and be
   dealt with.

8.3.  Channels

   As described in Section 3.11, PPSPP uses a multiplexing scheme,
   called channels, to allow multiple swarms to use the same UDP port.
   In the UDP encapsulation, each datagram from Peer A to Peer B is
   prefixed with the channel ID allocated by Peer B.  The peers learn
   about each other's channel ID during the handshake as explained in
   Section 3.1.1.  A channel ID consists of 4 bytes and MUST be
   generated following the requirements in [RFC4960] (Section 5.1.3).

8.4.  HANDSHAKE

   A channel is established with a handshake.  To start a handshake, the
   initiating peer needs to know the swarm metadata, defined in
   Section 3.1 and the IP address and UDP port of a peer.  A datagram
   containing a HANDSHAKE message then looks as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Destination Channel ID (32)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|            Source Channel ID (32)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                     Protocol Options                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Destination Channel ID:

         If the datagram is sent by the initiating peer, then it MUST be
         an all-zeros channel ID.

         If the datagram is sent by the responding peer, then it MUST
         consist of the Source Channel ID from the sender's HANDSHAKE
         message.

      The octet 0x00: The HANDSHAKE message type




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      Source Channel ID: A locally unused channel ID

      Protocol Options: A list of protocol options encoding the swarm's
      metadata, as defined in Section 7.

   A peer SHOULD explicitly close a channel by sending a HANDSHAKE
   message that MUST contain an all zeros Source Channel ID and a list
   of protocol options.  The list MUST either be empty or contain the
   maximum version number the sender supports, following the min/max
   versioning scheme defined in [RFC6709], Section 4.1.

8.5.  HAVE

   A HAVE message (type 0x03) consists of a single chunk specification
   that states that the sending peer has those chunks and successfully
   checked their integrity.  The single chunk specification represents a
   consecutive range of verified chunks.  A bin consists of a single
   integer, and a chunk or byte range of two integers, of the width
   specified by the Chunk Addressing protocol options, encoded big-
   endian.

   A HAVE message using 32-bit chunk ranges as Chunk Addressing method:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 1 1|                 Start chunk (32)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                  End chunk (32)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |
   +-+-+-+-+-+-+-+-+

   where the first octet is the HAVE message (0x03) followed by the
   start chunk and the end chunk describing the chunk range.  Note this
   diagram shows a message and not a datagram, so it is not prefixed by
   the destination Channel ID.  This holds for all subsequent message
   diagrams.

8.6.  DATA

   A DATA message (type 0x01) consists of a chunk specification, a
   timestamp, and the actual chunk.  In case a datagram contains one
   DATA message, a sender MUST always put the DATA message in the tail
   of the datagram.  A datagram MAY contain multiple DATA messages when
   the chunk size is fixed and when none of the DATA messages carry the
   last chunk, if that is smaller than the chunk size.  As LEDBAT
   congestion control is used, a sender MUST include a timestamp, in



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   particular, a 64-bit integer representing the current system time
   with microsecond accuracy.  The timestamp MUST be included between
   chunk specification and the actual chunk.

   A DATA message using 32-bit chunk ranges as Chunk Addressing method:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 1|                 Start chunk (32)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                  End chunk (32)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Timestamp (64)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                            Data                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where the first octet is the DATA message (0x01) followed by the
   start chunk and the end chunk describing the single chunk, the
   timestamp, and the actual data.

8.7.  ACK

   An ACK message (type 0x02) acknowledges data that was received from
   its addressee; to comply with the LEDBAT delay-based congestion
   control, an ACK message consists of a chunk specification and a
   timestamp representing a one-way delay sample.  The one-way delay
   sample is a 64-bit integer with microsecond accuracy, and it is
   computed from the timestamp received from the previous DATA message
   containing the chunk being acknowledged following the LEDBAT
   specification.













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   An ACK message using 32-bit chunk ranges as Chunk Addressing method:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 1 0|                 Start chunk (32)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                  End chunk (32)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  One-way delay sample (64)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |
   +-+-+-+-+-+-+-+-+

   where the first octet is the ACK message (0x02) followed by the start
   chunk and the end chunk describing the chunk range and the one-way
   delay sample.

8.8.  INTEGRITY

   An INTEGRITY message (type 0x04) consists of a chunk specification
   and the cryptographic hash for the specified chunk or node.  The type
   and format of the hash depends on the protocol options.

   An INTEGRITY message using 32-bit chunk ranges as Chunk Addressing
   method and a SHA-256 hash:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 0 0|                 Start chunk (32)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                  End chunk (32)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                            Hash (256)                         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |
   +-+-+-+-+-+-+-+-+







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   where the first octet is the INTEGRITY message (0x04) followed by the
   start chunk and the end chunk describing the chunk range and the
   hash.

8.9.  SIGNED_INTEGRITY

   A SIGNED_INTEGRITY message (type 0x07) consists of a chunk
   specification, a 64-bit timestamp in NTP Timestamp format [RFC5905]
   and a digital signature encoded as a Signature field would be in an
   RRSIG record in DNSSEC without the Base64 encoding [RFC4034].  The
   signature algorithm is defined by the Live Signature Algorithm
   protocol option, see Section 7.7.  The plaintext over which the
   signature is taken depends on the content integrity protection method
   used, see Section 6.1.

   A SIGNED_INTEGRITY message using 32-bit chunk ranges as Chunk
   Addressing method:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 1 1|                 Start chunk (32)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                  End chunk (32)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Timestamp (64)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Signature                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where the first octet is the SIGNED_INTEGRITY message (0x07) followed
   by the start chunk and the end chunk describing the chunk range, the
   timestamp, and the Signature.

   The length of the digital signature can be derived from the Live
   Signature Algorithm protocol option and the swarm ID as follows.  The
   first mandatory algorithms are RSASHA1 and RSASHA256.  For those
   algorithms, the swarm ID consists of a 1-byte Algorithm field
   followed by an RSA public key stored as a tuple (exponent length,
   exponent, modulus) [RFC3110].  Given the exponent length and the
   length of the public key tuple in the swarm ID, the length of the
   modulus in bytes can be calculated.  This yields the length of the



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   signature, as in RSA this is the length of the modulus [HAC01].  The
   other mandatory algorithms are ECDSAP256SHA256 and ECDSAP384SHA384
   [RFC6605].  For these algorithms, the length of the digital signature
   is 64 and 96 bytes, respectively.

8.10.  REQUEST

   A REQUEST message (type 0x08) consists of a chunk specification for
   the chunks the requester wants to download.

   A REQUEST message using 32-bit chunk ranges as Chunk Addressing
   method:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 0 0 0|                 Start chunk (32)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                  End chunk (32)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |
   +-+-+-+-+-+-+-+-+

   where the first octet is the REQUEST message (0x08) followed by the
   start chunk and the end chunk describing the chunk range.

8.11.  CANCEL

   A CANCEL message (type 0x09) consists of a chunk specification for
   the chunks the requester no longer is interested in.

   A CANCEL message using 32-bit chunk ranges as Chunk Addressing
   method:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 0 0 1|                 Start chunk (32)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                  End chunk (32)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |
   +-+-+-+-+-+-+-+-+

   where the first octet is the CANCEL message (0x09) followed by the
   start chunk and the end chunk describing the chunk range.





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8.12.  CHOKE and UNCHOKE

   Both CHOKE and UNCHOKE messages (types 0x0a and 0x0b, respectively)
   carry no payload.

   A CHOKE message:

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |0 0 0 0 1 0 1 0|
   +-+-+-+-+-+-+-+-+

   where the first octet is the CHOKE message (0x0a).

   An UNCHOKE message:

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |0 0 0 0 1 0 1 1|
   +-+-+-+-+-+-+-+-+

   where the first octet is the UNCHOKE message (0x0b).

8.13.  PEX_REQ, PEX_RESv4, PEX_RESv6, and PEX_REScert

   A PEX_REQ (0x06) message has no payload.  A PEX_RESv4 (0x05) message
   consists of an IPv4 address in big-endian format followed by a UDP
   port number in big-endian format.  A PEX_RESv6 (0x0c) message
   contains a 128-bit IPv6 address instead of an IPv4 one.  If a PEX_REQ
   message does not originate from a private, unique-local, link-local,
   or multicast address [RFC1918] [RFC4193] [RFC4291], then the PEX_RES*
   messages sent in reply MUST NOT contain such addresses.  This is to
   prevent leaking of internal addresses to external peers.

   A PEX_REQ message:

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 1 0|
   +-+-+-+-+-+-+-+-+

   where the first octet is the PEX_REQ message (0x06).






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   A PEX_RESv4 message:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 1 0 1|              IPv4 Address (32)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |             Port (16)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where the first octet is the PEX_RESv4 message (0x05) followed by the
   IPv4 address and the port number.

   A PEX_RESv6 message:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 1 0 0|                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   IPv6 Address (128)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |             Port (16)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where the first octet is the PEX_RESv6 message (0x0c), followed by
   the IPv6 address and the port number.

   A PEX_REScert (0x0d) message consists of a 16-bit integer in big-
   endian specifying the size of the membership certificate that
   follows, see Section 12.2.1.  This membership certificate states that
   Peer P at Time T is a member of Swarm S and is a X.509v3 certificate
   [RFC5280] that is encoded using the ASN.1 distinguished encoding
   rules (DER) [CCITT.X690.2002].  The certificate MUST contain a
   "Subject Alternative Name" extension, marked as critical, of type
   uniformResourceIdentifier.











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   A PEX_REScert message:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 1 0 1|   Size of Memb. Cert. (16)    |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                    Membership Certificate                     ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where the first octet is the PEX_REScert message (0x0d) followed by
   the size of the membership certificate and the membership
   certificate.

   The URL contained in the name extension MUST follow the generic
   syntax for URLs [RFC3986], where its scheme component is "file", the
   host in the authority component is the DNS name or IP address of Peer
   P, the port in the authority component is the port of Peer P, and the
   path contains the swarm identifier for Swarm S, in hexadecimal form.
   In particular, the preferred form of the swarm identifier is
   xxyyzz..., where the 'x's, 'y's, and 'z's are 2 hexadecimal digits of
   the 8-bit pieces of the identifier.  The validity time of the
   certificate is set with notBefore UTCTime set to T and notAfter
   UTCTime set to T plus some expiry time defined by the issuer.  An
   example URL:

       file://192.0.2.0:6778/e5a12c7ad2d8fab33c699d1e198d66f79fa610c3

8.14.  KEEPALIVE

   Keep alives do not have a message type on UDP.  They are just simple
   datagrams consisting of the 4-byte channel ID of the destination
   only.

   A keep-alive datagram:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Channel ID (32)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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8.15.  Flow and Congestion Control

   Explicit flow control is not required for PPSPP over UDP.  In the
   case of video on demand, the receiver explicitly requests the content
   from peers, and is therefore in control of how much data is coming
   towards it.  In the case of live streaming, where a push model may be
   used, the amount of data incoming is limited to the stream bitrate,
   which the receiver must be able to process for a continuous playback.
   Should, for any reason, the receiver get saturated with data, the
   congestion control at the sender side will detect the situation and
   adjust the sending rate accordingly.

   PPSPP over UDP can support different congestion control algorithms.
   At present, it uses the LEDBAT congestion control algorithm
   [RFC6817].  LEDBAT is a delay-based congestion control algorithm that
   is used every day by millions of users as part of the uTP
   transmission protocol of BitTorrent [LBT] [LCOMPL] and is suitable
   for P2P streaming [PPSPPERF].

   LEDBAT monitors the delay of the packets on the data path.  It uses
   the one-way delay variations to react early and limit the congestion
   that the stream may induce in the network [RFC6817].  Using LEDBAT
   enables PPSPP to serve the content to other interested peers after
   the playback has finished (seeding), without disrupting the user.
   After the playback, the user might move to different tasks that use
   its network link, which are prioritized over PPSPP traffic.  Hence,
   the user does not notice the background PPSPP traffic, which in turn
   increases the chances of seeding the content for a longer period of
   time.

   The property of reacting early is not a problem in a peer-to-peer
   system where multiple sources offer the content.  Considering the
   case of congestion near the sender, LEDBAT's early reaction impacts
   the transmission of chunks to the receiver.  However, for the
   receiver, it is actually beneficial to learn early that the
   transmission from a particular source is impacted.  The receiver can
   then choose to download time-critical chunks from other sources
   during its chunk picking phase.

   If the bottleneck is near the receiver, the receiver is indeed
   unlucky that transmissions from any source that runs through this
   bottleneck will back off quite fast due to LEDBAT.  However, for the
   rest of the network (and the network operator), this is beneficial as
   the video-streaming system will back off early enough and not
   contribute too much to the congestion.






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   The power of LEDBAT is that its behavior can be configured.  In the
   case of live streaming, a PPSPP deployer may want a more aggressive
   behavior to ensure quality of service.  In that case, LEDBAT can be
   configured to be more aggressive.  In particular, LEDBAT's queuing
   target delay value (TARGET in [RFC6817]) and other parameters can be
   adjusted such that it acts as aggressive as TCP (or even more).
   Hence, LEDBAT is an algorithm that works for many scenarios in a
   peer-to-peer context.

8.16.  Example of Operation

   We present a small example of communication between a leecher and a
   seeder.  The example presents the transmission of the file "Hello
   World!", which fits within a 1024-byte chunk.  For an easy
   understanding, we use the message description names, as listed in
   Table 7, and the protocol option names as listed in Table 2, rather
   than the actual binary value.

   To do the handshake, the initiating peer sends a datagram that MUST
   start with an all-zeros channel ID (0x00000000); followed by a
   HANDSHAKE message, whose payload is a locally unused; a random
   channel ID (in this case 0x00000001); and a list of protocol options.
   Channel IDs MUST be randomly chosen, as described in Section 12.1.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   HANDSHAKE   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 1|    Version    |0 0 0 0 0 0 0 1|  Min Version  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 1|   Swarm ID    |0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 0 0 0 1 1 1 1 0 1 0 0 0 0 0 0 0 0 1 0 0 1 1 1 1 1 0 0 1 1 0|
   ~                             .....                             ~
   |1 0 0 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Cont. Int.  |0 0 0 0 0 0 0 1| Mer.H.Tree F. |0 0 0 0 0 0 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Chunk Add.  |0 0 0 0 0 0 1 0|   Chunk Size  |0 0 0 0 0 0 0 0~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0|      End      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   The protocol options are:

      Version: 1

      Minimum supported Version: 1

      Swarm Identifier: A 32-byte root hash (47a0...b03b) identifying
      the content

      Content Integrity Protection Method: Merkle Hash Tree

      Merkle Tree Hash Function: SHA-256

      Chunk Addressing Method: 32-bit chunk ranges

      Chunk Size: 1024

   The receiving peer MAY respond, in which case the returned datagram
   MUST consist of the channel ID from the sender's HANDSHAKE message
   (0x00000001); a HANDSHAKE message, whose payload is a locally unused;
   a random channel ID (0x00000008); and a list of protocol options;
   followed by any other messages it wants to send.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   HANDSHAKE   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 1 0 0 0|    Version    |0 0 0 0 0 0 0 1|   Cont. Int.  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 1| Mer.H.Tree F. |0 0 0 0 0 0 1 0|   Chunk Add.  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 1 0|  Chunk Size   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0|      End      |      HAVE     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   With the protocol options, the receiving peer agrees on speaking
   protocol version 1, on using the Merkle Hash Tree as the Content
   Integrity Protection Method, SHA-256 hash as the Merkle Tree Hash
   Function, 32-bit chunk ranges as the Chunk Addressing Method, and




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   Chunk Size 1024.  Furthermore, it sends a HAVE message within the
   same datagram, announcing that it has locally available the first
   chunk of content.

   At this point, the initiator knows that the peer really responds; for
   that purpose, channel IDs MUST be random enough to prevent easy
   guessing.  So, the third datagram of a handshake MAY already contain
   some heavy payload.  To minimize the number of initialization round
   trips, the first two datagrams MAY also contain some minor payload,
   e.g., the HAVE message.

   The initiating peer MAY send a request for the chunks of content it
   wants to retrieve from the receiving peer, e.g., the first chunk
   announced during the handshake.  It always precedes the message with
   the channel ID of the peer it is communicating with (0x00000008 in
   our example), as described in Section 3.11.  Furthermore, it MAY add
   additional messages such as a PEX_REQ.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    REQUEST    |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|    PEX_REQ    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   When receiving the third datagram, both peers have proof that they
   really talk to each other; the three-way handshake is complete.  The
   receiving peer responds to the request by sending a DATA message
   containing the requested content.

















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     DATA      |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 1 0 1 0 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 1 1 0 1 1 1 1 1 0 1 1 0 1 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 0 0 0 1 0 0|0 1 0 0 1 0 0 0 0 1 1 0 0 1 0 1 0 1 1 0 1 1 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                           .....                               ~
   |0 1 1 0 1 1 0 0 0 1 1 0 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 0 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The DATA message consists of:

      The 32-bit chunk range: 0,0 (the first chunk)

      The timestamp value: 0004e94180b7db44

      The data: 48656c6c6f20776f726c6421 (the "Hello world!" file)

   Note that the above datagram does not include the INTEGRITY message,
   as the entire content can fit into a single message; hence, the
   initiating peer is able to verify it against the root hash.  Also, in
   this example, the peer does not respond to the PEX_REQ as it does not
   know any third peer participating in the swarm.

   Upon receiving the requested data, the initiating peer responds with
   an ACK message for the first chunk, containing a one-way delay sample
   (100 ms).  Furthermore, it also adds a HAVE message for the chunk.















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ACK      |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 1 0 0 1 0 0|      HAVE     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   At this point, the initiating peer has successfully retrieved the
   entire file.  Then, it explicitly closes the connection by sending a
   HANDSHAKE message that contains an all-zeros Source Channel ID.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   HANDSHAKE   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0|      End      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9.  Extensibility

9.1.  Chunk Picking Algorithms

   Chunk (or piece) picking entirely depends on the receiving peer.  The
   sending peer is made aware of preferred chunks by the means of
   REQUEST messages.  In some (live) scenarios, it may be beneficial to
   allow the sender to ignore those hints and send unrequested data.

   The chunk picking algorithm is external to the PPSPP and will
   generally be a pluggable policy that uses the mechanisms provided by
   PPSPP.  The algorithm will handle the choices made by the user





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   consuming the content, such as seeking or switching audio tracks or
   subtitles.  Example policies for P2P streaming can be found in
   [BITOS], and [EPLIVEPERF].

9.2.  Reciprocity Algorithms

   The role of reciprocity algorithms in peer-to-peer systems is to
   promote client contribution and prevent freeriding.  A peer is said
   to be freeriding if it only downloads content but never uploads to
   others.  Examples of reciprocity algorithms are tit-for-tat as used
   in BitTorrent [TIT4TAT] and Give-to-Get [GIVE2GET].  In PPSPP,
   reciprocity enforcement is the sole responsibility of the sending
   peer.

10.  IANA Considerations

   IANA has created a new top-level registry called "Peer-to-Peer
   Streaming Peer Protocol (PPSPP)", which hosts the six new sub-
   registries defined below for the extensibility of the protocol.  For
   all registries, assignments consist of a name and its associated
   value.  Also, for all registries, the "Unassigned" ranges designated
   are governed by the policy "IETF Review" as described in [RFC5226].

10.1.  PPSPP Message Type Registry

   The registry name is "PPSPP Message Type Registry".  Values are
   integers in the range 0-255, with initial assignments and
   reservations given in Table 7.

10.2.  PPSPP Option Registry

   The registry name is "PPSPP Option Registry".  Values are integers in
   the range 0-255, with initial assignments and reservations given in
   Table 2.

10.3.  PPSPP Version Number Registry

   The registry name is "PPSPP Version Number Registry".  Values are
   integers in the range 0-255, with initial assignments and
   reservations given in Table 3.

10.4.  PPSPP Content Integrity Protection Method Registry

   The registry name is "PPSPP Content Integrity Protection Method
   Registry".  Values are integers in the range 0-255, with initial
   assignments and reservations given in Table 4.





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10.5.  PPSPP Merkle Hash Tree Function Registry

   The registry name is "PPSPP Merkle Hash Tree Function Registry".
   Values are integers in the range 0-255, with initial assignments and
   reservations given in Table 5.

10.6.  PPSPP Chunk Addressing Method Registry

   The registry name is "PPSPP Chunk Addressing Method Registry".
   Values are integers in the range 0-255, with initial assignments and
   reservations given in Table 6.

11.  Manageability Considerations

   This section presents operations and management considerations
   following the checklist in [RFC5706], Appendix A.

   In this section, "PPSPP client" is defined as a PPSPP peer acting on
   behalf of an end user which may not yet have a copy of the content,
   and "PPSPP server" as a PPSPP peer that provides the initial copies
   of the content to the swarm on behalf of a content provider.

11.1.  Operations

11.1.1.  Installation and Initial Setup

   A content provider wishing to use PPSPP to distribute content should
   set up at least one PPSPP server.  PPSPP servers need to have access
   to either some static content or some live audio/video sources.  To
   provide flexibility for implementors, this configuration process is
   not standardized.  The output of this process will be a list of
   metadata records, one for each swarm.  A metadata record consists of
   the swarm ID, the chunk size used, the chunk addressing method used,
   the content integrity protection method used, and the Merkle hash
   tree function used (if applicable).  If automatic content size
   detection (see Section 5.6) is not used, the content length is also
   part of the metadata record for static content.  Note the swarm ID
   already contains the Live Signature Algorithm used, in case of a live
   stream.

   In addition, a content provider should set up a tracking facility for
   the content by configuring, for example, a peer-to-peer streaming
   protocol tracker [PPSP-TP] or a Distributed Hash Table.  The output
   of the latter process is a list of transport addresses for the
   tracking facility.






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   The list of metadata records of available content, and transport
   address for the tracking facility, can be distributed to users in
   various ways.  Typically, they will be published on a website as
   links.  When a user clicks such a link, the PPSPP client is launched,
   either as a standalone application or by invoking the browser's
   internal PPSPP protocol handler, as exemplified in Section 2.  The
   clients use the tracking facility to obtain the transport address of
   the PPSPP server(s) and other peers from the swarm, executing the
   peer protocol to retrieve and redistribute the content.  The format
   of the PPSPP URLs should be defined in an extension document.  The
   default protocol options should be exploited to keep the URLs small.

   The minimal information a tracking facility must return when queried
   for a list of peers for a swarm is as follows.  Assuming the
   communication between tracking facility and requester is protected,
   the facility must at least return for each peer in the list its IP
   address, transport protocol identifier (i.e., UDP), and transport
   protocol port number.

11.1.2.  Migration Path

   This document does not detail a migration path since there is no
   previous standard protocol providing similar functionality.

11.1.3.  Requirements on Other Protocols and Functional Components

   When using the peer-to-peer streaming protocol tracker, PPSPP
   requires a specific behavior from this protocol for security reasons,
   as detailed in Section 12.2.

11.1.4.  Impact on Network Operation

   PPSPP is a peer-to-peer protocol that takes advantage of the fact
   that content is available from multiple sources to improve
   robustness, scalability, and performance.  At the same time, poor
   choices in determining which exact sources to use can lead to bad
   experience for the end user and high costs for network operators.
   Hence, PPSPP can benefit from the ALTO protocol to steer peer
   selection, as described in Section 3.10.1.












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11.1.5.  Verifying Correct Operation

   PPSPP is operating correctly when all peers obtain the desired
   content on time.  Therefore, the PPSPP client is the ideal location
   to verify the protocol's correct operation.  However, it is not
   feasible to mandate logging the behavior of PPSPP peers in all
   implementations and deployments, for example, due to privacy reasons.
   There are two alternative options:

   o  Monitoring the PPSPP servers initially providing the content,
      using standard metrics such as bandwidth usage, peer connections,
      and activity, can help identify trouble, see next section and
      [RFC2564].

   o  The tracker protocol [PPSP-TP] may be used to gather information
      about all peers in a swarm, to obtain a global view of operation,
      according to PPSP.OAM.REQ-3 in [RFC6972].

   Basic operation of the protocol can be easily verified when a tracker
   and swarm metadata are known by starting a PPSPP download.  Deep
   packet inspection for DATA and ACK messages help to establish that
   actual content transfer is happening and that the chunk availability
   signaling and integrity checking are working.

11.1.6.  Configuration

   Table 8 shows the PPSPP parameters, their defaults, and where the
   parameter is defined.  For parameters that have no default, the table
   row contains the word "var" and refers to the section discussing the
   considerations to make when choosing a value.





















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   +-------------------------+-----------------------+-----------------+
   | Name                    | Default               | Definition      |
   +-------------------------+-----------------------+-----------------+
   | Chunk Size              | var, 1024 bytes       | Section 8.1     |
   |                         | recommended           |                 |
   |                         |                       |                 |
   | Static Content          | 1 (Merkle Hash Tree)  | Section 7.5     |
   | Integrity Protection    |                       |                 |
   | Method                  |                       |                 |
   |                         |                       |                 |
   | Live Content Integrity  | 3 (Unified Merkle     | Section 7.5     |
   | Protection Method       | Tree)                 |                 |
   |                         |                       |                 |
   | Merkle Hash Tree        | 2 (SHA-256)           | Section 7.6     |
   | Function                |                       |                 |
   |                         |                       |                 |
   | Live Signature          | 13 (ECDSAP256SHA256)  | Section 7.7     |
   | Algorithm               |                       |                 |
   |                         |                       |                 |
   | Chunk Addressing Method | 2 (32-bit chunk       | Section 7.8     |
   |                         | ranges)               |                 |
   |                         |                       |                 |
   | Live Discard Window     | var                   | Section 6.2,    |
   |                         |                       | Section 7.9     |
   |                         |                       |                 |
   | NCHUNKS_PER_SIG         | var                   | Section 6.1.2.1 |
   |                         |                       |                 |
   | Dead peer detection     | No reply in 3 minutes | Section 3.12    |
   |                         | + 3 datagrams         |                 |
   +-------------------------+-----------------------+-----------------+

                          Table 8: PPSPP Defaults

11.2.  Management Considerations

   The management considerations for PPSPP are very similar to other
   protocols that are used for large-scale content distribution, in
   particular HTTP.  How does one manage large numbers of servers?  How
   does one push new content out to a server farm and allows staged
   releases?  How are faults detected and how are servers and end-user
   performance measured?  As standard solutions to these challenges are
   still being developed, this section cannot provide a definitive
   recommendation on how PPSPP should be managed.  Hence, it describes
   the standard solutions available at this time and assumes a future
   extension document will provide more complete guidelines.






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11.2.1.  Management Interoperability and Information

   As just stated, PPSPP servers providing initial copies of the content
   are akin to WWW and FTP servers.  They can also be deployed in large
   numbers and thus can benefit from standard management facilities.
   Therefore, PPSPP servers may implement an SNMP management interface
   based on the APPLICATION-MIB [RFC2564], where the file object can be
   used to report on swarms.

   What is missing is the ability to remove or rate limit specific PPSPP
   swarms on a server.  This corresponds to removing or limiting
   specific virtual servers on a web server.  In other words, as
   multiple pieces of content (swarms, virtual WWW servers) are
   multiplexed onto a single server process, more fine-grained
   management of that process is required.  This functionality is
   currently missing.

   Logging is an important functionality for PPSPP servers and,
   depending on the deployment, PPSPP clients.  Logging should be done
   via syslog [RFC5424].

11.2.2.  Fault Management

   The facilities for verifying correct operation and server management
   (just discussed) appear sufficient for PPSPP fault monitoring.  This
   can be supplemented with host resource [RFC2790] and UDP/IP network
   monitoring [RFC4113], as PPSPP server failures can generally be
   attributed directly to conditions on the host or network.

   Since PPSPP has been designed to work in a hostile environment, many
   benign faults will be handled by the mechanisms used for managing
   attacks.  For example, when a malfunctioning peer starts sending the
   wrong chunks, this is detected by the content integrity protection
   mechanism and another source is sought.

11.2.3.  Configuration Management

   Large-scale deployments may benefit from a standard way of
   replicating a new piece of content on a set of initial PPSPP servers.
   This functionality may need to include controlled releasing, such
   that content becomes available only at a specific point in time
   (e.g., the release of a movie trailer).  This functionality could be
   provided via NETCONF [RFC6241], to enable atomic configuration
   updates over a set of servers.  Uploading the new content could be
   one configuration change, making the content available for download
   by the public another.





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11.2.4.  Accounting Management

   Content providers may offer PPSPP hosting for different customers and
   will want to bill these customers, for example, based on bandwidth
   usage.  This situation is a common accounting scenario, similar to
   billing per virtual server for web servers.  PPSPP can therefore
   benefit from general standardization efforts in this area [RFC2975]
   when they come to fruition.

11.2.5.  Performance Management

   Depending on the deployment scenarios, the application performance
   measurement facilities of [RFC3729] and associated [RFC4150] can be
   used with PPSPP.

   In addition, when the PPSPP tracker protocol is used, it provides a
   built-in, application-level, performance measurement infrastructure
   for different metrics.  See PPSP.OAM.REQ-3 in [RFC6972].

11.2.6.  Security Management

   Malicious peers should ideally be locked out long term.  This is
   primarily for performance reasons, as the protocol is robust against
   attacks (see next section).  Section 12.7 describes a procedure for
   long-term exclusion.

12.  Security Considerations

   As any other network protocol, PPSPP faces a common set of security
   challenges.  An implementation must consider the possibility of
   buffer overruns, DoS attacks and manipulation (i.e., reflection
   attacks).  Any guarantee of privacy seems unlikely, as the user is
   exposing its IP address to the peers.  A probable exception is the
   case of the user being hidden behind a public NAT or proxy.  This
   section discusses the protocol's security considerations in detail.

12.1.  Security of the Handshake Procedure

   Borrowing from the analysis in [RFC5971], the PPSPP may be attacked
   with three types of denial-of-service attacks:

   1.  DoS amplification attack: attackers try to use a PPSPP peer to
       generate more traffic to a victim.

   2.  DoS flood attack: attackers try to deny service to other peers by
       allocating lots of state at a PPSPP peer.





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   3.  Disrupt service to an individual peer: attackers send bogus,
       e.g., REQUEST and HAVE messages appearing to come from victim
       Peer A to the Peers B1..Bn serving that peer.  This causes Peer A
       to receive chunks it did not request or to not receive the chunks
       it requested.

   The basic scheme to protect against these attacks is the use of a
   secure handshake procedure.  In the UDP encapsulation, the handshake
   procedure is secured by the use of randomly chosen channel IDs as
   follows.  The channel IDs must be generated following the
   requirements in [RFC4960] (Section 5.1.3).

   When UDP is used, all datagrams carrying PPSPP messages are prefixed
   with a 4-byte channel ID.  These channel IDs are random numbers,
   established during the handshake phase as follows.  Peer A initiates
   an exchange with Peer B by sending a datagram containing a HANDSHAKE
   message prefixed with the channel ID consisting of all zeros.  Peer
   A's HANDSHAKE contains a randomly chosen channel ID, chanA:

   A->B: chan0 + HANDSHAKE(chanA) + ...

   When Peer B receives this datagram, it creates some state for Peer A,
   that at least contains the channel ID chanA.  Next, Peer B sends a
   response to Peer A, consisting of a datagram containing a HANDSHAKE
   message prefixed with the chanA channel ID.  Peer B's HANDSHAKE
   contains a randomly chosen channel ID, chanB.

   B->A: chanA + HANDSHAKE(chanB) + ...

   Peer A now knows that Peer B really responds, as it echoed chanA.  So
   the next datagram that Peer A sends may already contain heavy
   payload, i.e., a chunk.  This next datagram to Peer B will be
   prefixed with the chanB channel ID.  When Peer B receives this
   datagram, both peers have the proof they are really talking to each
   other, the three-way handshake is complete.  In other words, the
   randomly chosen channel IDs act as tags (cf.  [RFC4960]
   (Section 5.1)).

   A->B: chanB + HAVE + DATA + ...

12.1.1.  Protection against Attack 1

   In short, PPSPP does a so-called return routability check before
   heavy payload is sent.  This means that attack 1 is fended off: PPSPP
   does not send back much more data than it received, unless it knows
   it is talking to a live peer.  Attackers sending a spoofed HANDSHAKE
   to Peer B pretending to be Peer A now need to intercept the message




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   from Peer B to Peer A to get Peer B to send heavy payload, and ensure
   that that heavy payload goes to the victim, something assumed too
   hard to be a practical attack.

   Note the rule is that no heavy payload may be sent until the third
   datagram.  This has implications for PPSPP implementations that use
   chunk addressing schemes that are verbose.  If a PPSPP implementation
   uses large bitmaps to convey chunk availability, these may not be
   sent by Peer B in the second datagram.

12.1.2.  Protection against Attack 2

   On receiving the first datagram Peer B will record some state about
   Peer A.  At present, this state consists of the chanA channel ID, and
   the results of processing the other messages in the first datagram.
   In particular, if Peer A included some HAVE messages, Peer B may add
   a chunk availability map to Peer A's state.  In addition, Peer B may
   request some chunks from Peer A in the second datagram, and Peer B
   will maintain state about these outgoing requests.

   So presently, PPSPP is somewhat vulnerable to attack 2.  An attacker
   could send many datagrams with HANDSHAKEs and HAVEs and thus allocate
   state at the PPSPP peer.  Therefore, Peer A MUST respond immediately
   to the second datagram, if it is still interested in Peer B.

   The reason for using this slightly vulnerable three-way handshake
   instead of the safer handshake procedure of Stream Control
   Transmission Protocol (SCTP) [RFC4960] (Section 5.1) is quicker
   response time for the user.  In the SCTP procedure, Peers A and B
   cannot request chunks until datagrams 3 and 4 respectively, as
   opposed to 2 and 1 in the proposed procedure.  This means that the
   user has to wait less time in PPSPP between starting the video stream
   and seeing the first images.

12.1.3.  Protection against Attack 3

   In general, channel IDs serve to authenticate a peer.  Hence, to
   attack, a malicious Peer T would need to be able to eavesdrop on
   conversations between victim A and a benign Peer B to obtain the
   channel ID Peer B assigned to Peer A, chanB.  Furthermore, attacker
   Peer T would need to be able to spoof, e.g., REQUEST and HAVE
   messages from Peer A to cause Peer B to send heavy DATA messages to
   Peer A, or prevent Peer B from sending them, respectively.

   The capability to eavesdrop is not common, so the protection afforded
   by channel IDs will be sufficient in most cases.  If not, point-to-
   point encryption of traffic should be used, see below.




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12.2.  Secure Peer Address Exchange

   As described in Section 3.10, Peer A can send Peer-Exchange messages
   PEX_RES to Peer B, which contain the IP address and port of other
   peers that are supposedly also in the current swarm.  The strength of
   this mechanism is that it allows decentralized tracking: after an
   initial bootstrap, no central tracker is needed.  The vulnerability
   of this mechanism (and DHTs) is that malicious peers can use it for
   an Amplification attack.

   In particular, a malicious Peer T could send PEX_RES messages to
   well-behaved Peer A with addresses of Peers B1..Bn; on receipt, Peer
   A could send a HANDSHAKE to all these peers.  So, in the worst case,
   a single datagram results in N datagrams.  The actual damage depends
   on Peer A's behavior.  For example, when Peer A already has
   sufficient connections, it may not connect to the offered ones at
   all; but if it is a fresh peer, it may connect to all directly.

   In addition, PEX can be used in Eclipse attacks [ECLIPSE] where
   malicious peers try to isolate a particular peer such that it only
   interacts with malicious peers.  Let us distinguish two specific
   attacks:

      E1.   Malicious peers try to eclipse the single injector in live
            streaming.

      E2.   Malicious peers try to eclipse a specific consumer peer.

   Attack E1 has the most impact on the system as it would disrupt all
   peers.

12.2.1.  Protection against the Amplification Attack

   If peer addresses are relatively stable, strong protection against
   the attack can be provided by using public key cryptography and
   certification.  In particular, a PEX_REScert message will carry
   swarm-membership certificates rather than IP address and port.  A
   membership certificate for Peer B states that Peer B at address
   (ipB,portB) is part of Swarm S at Time T and is cryptographically
   signed.  The receiver Peer A can check the certificate for a valid
   signature, the right swarm and liveliness, and only then consider
   contacting Peer B.  These swarm-membership certificates correspond to
   signed node descriptors in secure decentralized peer sampling
   services [SPS].







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   Several designs are possible for the security environment for these
   membership certificates.  That is, there are different designs
   possible for who signs the membership certificates and how public
   keys are distributed.  As an example, we describe a design where the
   peer-to-peer streaming protocol tracker acts as certification
   authority.

12.2.2.  Example: Tracker as Certification Authority

   Peer A wanting to join Swarm S sends a certificate request message to
   a Tracker X for that swarm.  Upon receipt, the tracker creates a
   membership certificate from the request with Swarm ID S, a Timestamp
   T, and the external IP and port it received the message from, signed
   with the tracker's private key.  This certificate is returned to Peer
   A.

   Peer A then includes this certificate when it sends a PEX_REScert to
   Peer B.  Receiver Peer B verifies it against the tracker public key.
   This tracker public key should be part of the swarm's metadata, which
   Peer B received from a trusted source.  Subsequently, Peer B can send
   the member certificate of Peer A to other peers in PEX_REScert
   messages.

   Peer A can send the certification request when it first contacts the
   tracker or at a later time.  Furthermore, the responses the tracker
   sends could contain membership certificates instead of plain
   addresses, such that they can be gossiped securely as well.

   We assume the tracker is protected against attacks and does a return
   routability check.  The latter ensures that malicious peers cannot
   obtain a certificate for a random host, just for hosts where they can
   eavesdrop on incoming traffic.

   The load generated on the tracker depends on churn and the lifetime
   of a certificate.  Certificates can be fairly long lived, given that
   the main goal of the membership certificates is to prevent that
   malicious Peer T can cause good Peer A to contact *random* hosts.
   The freshness of the timestamp just adds extra protection in addition
   to achieving that goal.  It protects against malicious hosts causing
   a good Peer A to contact hosts that previously participated in the
   swarm.

   The membership certificate mechanism itself can be used for a kind of
   amplification attack against good peers.  Malicious Peer T can cause
   Peer A to spend some CPU to verify the signatures on the membership
   certificates that Peer T sends.  To counter this, Peer A SHOULD check
   a few of the certificates sent and discard the rest if they are
   defective.



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   The same membership certificates described above can be registered in
   a Distributed Hash Table that has been secured against the well-known
   DHT specific attacks [SECDHTS].

   Note that this scheme does not work for peers behind a symmetric
   Network Address Translator, but neither does normal tracker
   registration.

12.2.3.  Protection against Eclipse Attacks

   Before we can discuss Eclipse attacks, we first need to establish the
   security properties of the central tracker.  A tracker is vulnerable
   to Amplification attacks, too.  A malicious Peer T could register a
   victim Peer B with the tracker, and many peers joining the swarm will
   contact Peer B.  Trackers can also be used in Eclipse attacks.  If
   many malicious peers register themselves at the tracker, the
   percentage of bad peers in the returned address list may become high.
   Leaving the protection of the tracker to the peer-to-peer streaming
   protocol tracker specification [PPSP-TP], we assume for the following
   discussion that it returns a true random sample of the actual swarm
   membership (achieved via Sybil attack protection).  This means that
   if 50% of the peers are bad, you'll still get 50% good addresses from
   the tracker.

   Attack E1 on PEX can be fended off by letting live injectors disable
   PEX -- or at least, letting live injectors ensure that part of their
   connections are to peers whose addresses came from the trusted
   tracker.

   The same measures defend against attack E2 on PEX.  They can also be
   employed dynamically.  When the current set of Peers B that Peer A is
   connected to doesn't provide good quality of service, Peer A can
   contact the tracker to find new candidates.

12.3.  Support for Closed Swarms

   Regarding PPSP.SEC.REQ-1 in [RFC6972], the Closed Swarms [CLOSED] and
   Enhanced Closed Swarms [ECS] mechanisms provide swarm-level access
   control.  The basic idea is that a peer cannot download from another
   peer unless it shows a Proof-of-Access.  Enhanced Closed Swarms
   improve on the original Closed Swarms by adding on-the-wire
   encryption against man-in-the-middle attacks and more flexible access
   control rules.

   The exact mapping of ECS to PPSPP is defined in [ECS-protocol].






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12.4.  Confidentiality of Streamed Content

   Regarding PPSP.SEC.REQ-1 in [RFC6972], no extra mechanism is needed
   to support confidentiality in PPSPP.  A content publisher wishing
   confidentiality should just distribute content in ciphertext and/or
   in a format to which Digital Rights Management (DRM) techniques have
   been applied.  In that case, it is assumed a higher layer handles key
   management out-of-band.  Alternatively, pure point-to-point
   encryption of content and traffic can be provided by the proposed
   Closed Swarms access control mechanism, by DTLS [RFC6347], or by
   IPsec [RFC4301].

   When transmitting over DTLS, PPSPP can obtain the PMTU estimate
   maintained by the IP layer to determine how much payload can be put
   in a single datagram without fragmentation ([RFC6347],
   Section 4.1.1.1).  If PMTU changes and the chunk size becomes too
   large to fit into a single datagram, PPSPP can choose to allow
   fragmentation by clearing the Don't Fragment (DF) bit.
   Alternatively, the content publisher can decide to use smaller chunks
   and transmit multiple in the same datagram when the MTU allows.

12.5.  Strength of the Hash Function for Merkle Hash Trees

   Implementations MUST support SHA-1 as the hash function for content
   integrity protection via Merkle hash trees.  SHA-1 may be preferred
   over stronger hash functions by content providers because it reduces
   on-the-wire overhead.  As such, it presents a trade-off between
   performance and security.  The security considerations for SHA-1 are
   discussed in [RFC6194].

   In general, note that the hash function is used in a hash tree, which
   makes it more complex to create collisions.  In particular, if
   attackers manage to find a collision for a hash, it can replace just
   one chunk, so the impact is limited.  If fixed-size chunks are used,
   the collision even has to be of the same size as the original chunk.
   For hashes higher up in the hash tree, a collision must be a
   concatenation of two hashes.  In sum, finding collisions that fit
   with the hash tree are generally harder to find than regular
   collisions.

12.6.  Limit Potential Damage and Resource Exhaustion by Bad or Broken
       Peers

   Regarding PPSP.SEC.REQ-2 in [RFC6972], this section provides an
   analysis of the potential damage a malicious peer can do with each
   message in the protocol, and how it is prevented by the protocol
   (implementation).




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12.6.1.  HANDSHAKE

   o  Secured against DoS Amplification attacks as described in
      Section 12.1.

   o  Threat HS.1: An Eclipse attack where Peers T1..Tn fill all
      connection slots of Peer A by initiating the connection to Peer A.

      Solution: Peer A must not let other peers fill all its available
      connection slots, i.e., Peer A must initiate connections itself
      too, to prevent isolation.

12.6.2.  HAVE

   o  Threat HAVE.1: Malicious Peer T can claim to have content that it
      does not.  Subsequently, Peer T won't respond to requests.

      Solution: Peer A will consider Peer T to be a slow peer and not
      ask it again.

   o  Threat HAVE.2: Malicious Peer T can claim not to have content.
      Hence, it won't contribute.

      Solution: Peer and chunk selection algorithms external to the
      protocol will implement fairness and provide sharing incentives.

12.6.3.  DATA

   o  Threat DATA.1: Peer T sending bogus chunks.

      Solution: The content integrity protection schemes defend against
      this.

   o  Threat DATA.2: Peer T sends Peer A unrequested chunks.

      To protect against this threat we need network-level DoS
      prevention.

12.6.4.  ACK

   o  Threat ACK.1: Peer T acknowledges wrong chunks.

      Solution: Peer A will detect inconsistencies with the data it sent
      to Peer T.

   o  Threat ACK.2: Peer T modifies timestamp in ACK to Peer A used for
      time-based congestion control.




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      Solution: In theory, by decreasing the timestamp, Peer T could
      fake that there is no congestion when in fact there is, causing
      Peer A to send more data than it should.  [RFC6817] does not list
      this as a security consideration.  Possibly, this attack can be
      detected by the large resulting asymmetry between round-trip time
      and measured one-way delay.

12.6.5.  INTEGRITY and SIGNED_INTEGRITY

   o  Threat INTEGRITY.1: An amplification attack where Peer T sends
      bogus INTEGRITY or SIGNED_INTEGRITY messages, causing Peer A to
      checks hashes or signatures, thus spending CPU unnecessarily.

      Solution: If the hashes/signatures don't check out, Peer A will
      stop asking Peer T because of the atomic datagram principle and
      the content integrity protection.  Subsequent unsolicited traffic
      from Peer T will be ignored.

   o  Threat INTEGRITY.2: An attack where Peer T sends old
      SIGNED_INTEGRITY messages in the Unified Merkle Tree scheme,
      trying to make Peer A tune in at a past point in the live stream.

      Solution: The timestamp in the SIGNED_INTEGRITY message protects
      against such replays.  Subsequent traffic from Peer T will be
      ignored.

12.6.6.  REQUEST

   o  Threat REQUEST.1: Peer T could request lots from Peer A, leaving
      Peer A without resources for others.

      Solution: A limit is imposed on the upload capacity a single peer
      can consume, for example, by using an upload bandwidth scheduler
      that takes into account the need of multiple peers.  A natural
      upper limit of this upload quotum is the bitrate of the content,
      taking into account that this may be variable.

12.6.7.  CANCEL

   o  Threat CANCEL.1: Peer T sends CANCEL messages for content it never
      requested to Peer A.

      Solution: Peer A will detect the inconsistency of the messages and
      ignore them.  Note that CANCEL messages may be received
      unexpectedly when a transport is used where REQUEST messages may
      be lost or reordered with respect to the subsequent CANCELs.





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12.6.8.  CHOKE

   o  Threat CHOKE.1: Peer T sends REQUEST messages after Peer A sent
      Peer B a CHOKE message.

      Solution: Peer A will just discard the unwanted REQUESTs and
      resend the CHOKE, assuming it got lost.

12.6.9.  UNCHOKE

   o  Threat UNCHOKE.1: Peer T sends an UNCHOKE message to Peer A
      without having sent a CHOKE message before.

      Solution: Peer A can easily detect this violation of protocol
      state, and ignore it.  Note this can also happen due to loss of a
      CHOKE message sent by a benign peer.

   o  Threat UNCHOKE.2: Peer T sends an UNCHOKE message to Peer A, but
      subsequently does not respond to its REQUESTs.

      Solution: Peer A will consider Peer T to be a slow peer and not
      ask it again.

12.6.10.  PEX_RES

   o  Secured against amplification and Eclipse attacks as described in
      Section 12.2.

12.6.11.  Unsolicited Messages in General

   o  Threat: Peer T could send a spoofed PEX_REQ or REQUEST from Peer B
      to Peer A, causing Peer A to send a PEX_RES/DATA to Peer B.

      Solution: the message from Peer T won't be accepted unless Peer T
      does a handshake first, in which case the reply goes to Peer T,
      not victim Peer B.

12.7.  Exclude Bad or Broken Peers

   This section is regarding PPSP.SEC.REQ-2 in [RFC6972].  A receiving
   peer can detect malicious or faulty senders as just described, which
   it can then subsequently ignore.  However, excluding such a bad peer
   from the system completely is complex.  Random monitoring by trusted
   peers that would blacklist bad peers as described in [DETMAL] is one
   option.  This mechanism does require extra capacity to run such
   trusted peers, which must be indistinguishable from regular peers,
   and requires a solution for the timely distribution of this blacklist
   to peers in a scalable manner.



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13.  References

13.1.  Normative References

   [CCITT.X690.2002]
              International Telephone and Telegraph Consultative
              Committee, "ASN.1 encoding rules: Specification of basic
              encoding Rules (BER), Canonical encoding rules (CER) and
              Distinguished encoding rules (DER)", CCITT Recommendation
              X.690, July 2002.

   [FIPS180-4]
              National Institute of Standards and Technology,
              Information Technology Laboratory, "Federal Information
              Processing Standards: Secure Hash Standard (SHS)", FIPS
              PUB 180-4, March 2012.

   [IANADNSSECALGNUM]
              IANA, "Domain Name System Security (DNSSEC) Algorithm
              Numbers", March 2014,
              <http://www.iana.org/assignments/dns-sec-alg-numbers>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., J. de Groot,
              G., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <http://www.rfc-editor.org/info/rfc1918>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3110]  Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
              Domain Name System (DNS)", RFC 3110, DOI 10.17487/RFC3110,
              May 2001, <http://www.rfc-editor.org/info/rfc3110>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <http://www.rfc-editor.org/info/rfc4034>.






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   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5702]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
              and RRSIG Resource Records for DNSSEC", RFC 5702,
              DOI 10.17487/RFC5702, October 2009,
              <http://www.rfc-editor.org/info/rfc5702>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

   [RFC6605]  Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
              Signature Algorithm (DSA) for DNSSEC", RFC 6605,
              DOI 10.17487/RFC6605, April 2012,
              <http://www.rfc-editor.org/info/rfc6605>.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,
              <http://www.rfc-editor.org/info/rfc6817>.

13.2.  Informative References

   [ABMRKL]   Bakker, A., "Merkle hash torrent extension", BitTorrent
              Enhancement Proposal 30, March 2009,
              <http://bittorrent.org/beps/bep_0030.html>.

   [BINMAP]   Grishchenko, V. and J. Pouwelse, "Binmaps: Hybridizing
              Bitmaps and Binary Trees", Delft University of Technology
              Parallel and Distributed Systems Report Series, Report
              number PDS-2011-005, ISSN 1387-2109, April 2009.

   [BITOS]    Vlavianos, A., Iliofotou, M., Mathieu, F., and M.
              Faloutsos, "BiToS: Enhancing BitTorrent for Supporting
              Streaming Applications", IEEE INFOCOM Global Internet
              Symposium, Barcelona, Spain, April 2006.






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   [BITTORRENT]
              Cohen, B., "The BitTorrent Protocol Specification",
              BitTorrent Enhancement Proposal 3, February 2008,
              <http://bittorrent.org/beps/bep_0003.html>.

   [CLOSED]   Borch, N., Mitchell, K., Arntzen, I., and D. Gabrijelcic,
              "Access Control to BitTorrent Swarms Using Closed Swarms",
              ACM workshop on Advanced Video Streaming Techniques for
              Peer-to-Peer Networks and Social Networking (AVSTP2P '10),
              Florence, Italy, October 2010,
              <http://doi.acm.org/10.1145/1877891.1877898>.

   [DETMAL]   Shetty, S., Galdames, P., Tavanapong, W., and Ying. Cai,
              "Detecting Malicious Peers in Overlay Multicast
              Streaming", IEEE Conference on Local Computer Networks,
              (LCN'06), Tampa, FL, USA, November 2006.

   [ECLIPSE]  Sit, E. and R. Morris, "Security Considerations for Peer-
              to-Peer Distributed Hash Tables", IPTPS '01: Revised
              Papers from the First International Workshop on Peer-to-
              Peer Systems, pp. 261-269, Springer-Verlag, 2002.

   [ECS]      Jovanovikj, V., Gabrijelcic, D., and T. Klobucar, "Access
              Control in BitTorrent P2P Networks Using the Enhanced
              Closed Swarms Protocol", International Conference on
              Emerging Security Information, Systems and Technologies
              (SECURWARE 2011), pp. 97-102, Nice, France, August 2011.

   [ECS-protocol]
              Gabrijelcic, D., "Enhanced Closed Swarm protocol", Work in
              Progress, draft-ppsp-gabrijelcic-ecs-01, June 2013.

   [EPLIVEPERF]
              Bonald, T., Massoulie, L., Mathieu, F., Perino, D., and A.
              Twigg, "Epidemic live streaming: optimal performance
              trade-offs", Proceedings of the 2008 ACM SIGMETRICS
              International Conference on Measurement and Modeling of
              Computer Systems, Annapolis, MD, USA, June 2008.

   [GIVE2GET] Mol, J., Pouwelse, J., Meulpolder, M., Epema, D., and H.
              Sips, "Give-to-Get: Free-riding-resilient Video-on-Demand
              in P2P Systems", Proceedings Multimedia Computing and
              Networking conference (Proceedings of SPIE, Vol. 6818),
              San Jose, CA, USA, January 2008.







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   [HAC01]    Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
              of Applied Cryptography", CRC Press, (Fifth Printing,
              August 2001), October 1996.

   [JIM11]    Jimenez, R., Osmani, F., and B. Knutsson, "Sub-Second
              Lookups on a Large-Scale Kademlia-Based Overlay", IEEE
              International Conference on Peer-to-Peer Computing
              (P2P'11), Kyoto, Japan, August 2011.

   [LBT]      Rossi, D., Testa, C., Valenti, S., and L. Muscariello,
              "LEDBAT: the new BitTorrent congestion control protocol",
              Computer Communications and Networks (ICCCN), Zurich,
              Switzerland, August 2010.

   [LCOMPL]   Testa, C. and D. Rossi, "On the impact of uTP on
              BitTorrent completion time", IEEE International Conference
              on Peer-to-Peer Computing (P2P'11), Kyoto, Japan, August
              2011.

   [MERKLE]   Merkle, R., "Secrecy, Authentication, and Public Key
              Systems", Ph.D. thesis, Dept. of Electrical Engineering,
              Stanford University, CA, USA, pp 40-45, 1979.

   [P2PWIKI]  Bakker, A., Petrocco, R., Dale, M., Gerber, J.,
              Grishchenko, V., Rabaioli, D., and J. Pouwelse, "Online
              video using BitTorrent and HTML5 applied to Wikipedia",
              IEEE International Conference on Peer-to-Peer Computing
              (P2P'10), Delft, The Netherlands, August 2010.

   [POLLIVE]  Dhungel, P., Hei, Xiaojun., Ross, K., and N. Saxena,
              "Pollution in P2P Live Video Streaming", International
              Journal of Computer Networks & Communications (IJCNC) Vol.
              1, No. 2, Jul 2009.

   [PPSP-TP]  Cruz, R., Nunes, M., Yingjie, G., Xia, J., Huang, R.,
              Taveira, J., and D. Lingli, "PPSP Tracker Protocol-Base
              Protocol (PPSP-TP/1.0)", Work in Progress,
              draft-ietf-ppsp-base-tracker-protocol-09, March 2015.

   [PPSPPERF] Petrocco, R., Pouwelse, J., and D. Epema, "Performance
              Analysis of the Libswift P2P Streaming Protocol", IEEE
              International Conference on Peer-to-Peer Computing
              (P2P'12), Tarragona, Spain, September 2012.








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   [RFC2564]  Kalbfleisch, C., Krupczak, C., Presuhn, R., and J.
              Saperia, "Application Management MIB", RFC 2564,
              DOI 10.17487/RFC2564, May 1999,
              <http://www.rfc-editor.org/info/rfc2564>.

   [RFC2790]  Waldbusser, S. and P. Grillo, "Host Resources MIB", RFC
              2790, DOI 10.17487/RFC2790, March 2000,
              <http://www.rfc-editor.org/info/rfc2790>.

   [RFC2975]  Aboba, B., Arkko, J., and D. Harrington, "Introduction to
              Accounting Management", RFC 2975, DOI 10.17487/RFC2975,
              October 2000, <http://www.rfc-editor.org/info/rfc2975>.

   [RFC3365]  Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61, RFC
              3365, DOI 10.17487/RFC3365, August 2002,
              <http://www.rfc-editor.org/info/rfc3365>.

   [RFC3729]  Waldbusser, S., "Application Performance Measurement MIB",
              RFC 3729, DOI 10.17487/RFC3729, March 2004,
              <http://www.rfc-editor.org/info/rfc3729>.

   [RFC4113]  Fenner, B. and J. Flick, "Management Information Base for
              the User Datagram Protocol (UDP)", RFC 4113,
              DOI 10.17487/RFC4113, June 2005,
              <http://www.rfc-editor.org/info/rfc4113>.

   [RFC4150]  Dietz, R. and R. Cole, "Transport Performance Metrics
              MIB", RFC 4150, DOI 10.17487/RFC4150, August 2005,
              <http://www.rfc-editor.org/info/rfc4150>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <http://www.rfc-editor.org/info/rfc4193>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <http://www.rfc-editor.org/info/rfc4960>.





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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <http://www.rfc-editor.org/info/rfc5389>.

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424,
              DOI 10.17487/RFC5424, March 2009,
              <http://www.rfc-editor.org/info/rfc5424>.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions", RFC
              5706, DOI 10.17487/RFC5706, November 2009,
              <http://www.rfc-editor.org/info/rfc5706>.

   [RFC5971]  Schulzrinne, H. and R. Hancock, "GIST: General Internet
              Signalling Transport", RFC 5971, DOI 10.17487/RFC5971,
              October 2010, <http://www.rfc-editor.org/info/rfc5971>.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <http://www.rfc-editor.org/info/rfc6194>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <http://www.rfc-editor.org/info/rfc6241>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6709]  Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              DOI 10.17487/RFC6709, September 2012,
              <http://www.rfc-editor.org/info/rfc6709>.

   [RFC6972]  Zhang, Y. and N. Zong, "Problem Statement and Requirements
              of the Peer-to-Peer Streaming Protocol (PPSP)", RFC 6972,
              DOI 10.17487/RFC6972, July 2013,
              <http://www.rfc-editor.org/info/rfc6972>.





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   [RFC7285]  Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
              Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
              "Application-Layer Traffic Optimization (ALTO) Protocol",
              RFC 7285, DOI 10.17487/RFC7285, September 2014,
              <http://www.rfc-editor.org/info/rfc7285>.

   [SECDHTS]  Urdaneta, G., Pierre, G., and M. van Steen, "A Survey of
              DHT Security Techniques", ACM Computing Surveys,
              vol. 43(2), January 2011.

   [SIGMCAST]
              Wong, C. and S. Lam, "Digital Signatures for Flows and
              Multicasts", IEEE/ACM Transactions on Networking 7(4),
              pp. 502-513, August 1999.

   [SPS]      Jesi, G., Montresor, A., and M. van Steen, "Secure Peer
              Sampling", Computer Networks vol. 54(12), pp. 2086-2098,
              Elsevier, August 2010.

   [SWIFTIMPL]
              Grishchenko, V., Paananen, J., Pronchenkov, A., Bakker,
              A., and R. Petrocco, "Swift reference implementation",
              2015, <https://github.com/libswift/libswift>.

   [TIT4TAT]  Cohen, B., "Incentives Build Robustness in BitTorrent",
              1st Workshop on Economics of Peer-to-Peer Systems,
              Berkeley, CA, USA, May 2003.

Acknowledgements

   Arno Bakker, Riccardo Petrocco, and Victor Grishchenko are partially
   supported by the P2P-Next project <http://www.p2p-next.org/>, a
   research project supported by the European Community under its 7th
   Framework Programme (grant agreement no. 216217).  The views and
   conclusions contained herein are those of the authors and should not
   be interpreted as necessarily representing the official policies or
   endorsements, either expressed or implied, of the P2P-Next project or
   the European Commission.

   PPSPP was designed by Victor Grishchenko at Technische Universiteit
   Delft under supervision of Johan Pouwelse.  The authors would like to
   thank the following people for their contributions to this document:
   the chairs (Martin Stiemerling, Yunfei Zhang, Stefano Previdi, and
   Ning Zong) and members of the IETF PPSP working group, and Mihai
   Capota, Raul Jimenez, Flutra Osmani, and Raynor Vliegendhart.






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Authors' Addresses

   Arno Bakker
   Vrije Universiteit Amsterdam
   De Boelelaan 1081
   Amsterdam  1081HV
   The Netherlands

   Email: arno@cs.vu.nl


   Riccardo Petrocco
   Technische Universiteit Delft
   Mekelweg 4
   Delft  2628CD
   The Netherlands

   Email: r.petrocco@gmail.com


   Victor Grishchenko
   Technische Universiteit Delft
   Mekelweg 4
   Delft  2628CD
   The Netherlands

   Email: victor.grishchenko@gmail.com
























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