path: root/doc/rfc/rfc5925.txt

Internet Engineering Task Force (IETF)                          J. Touch
Request for Comments: 5925                                       USC/ISI
Obsoletes: 2385                                                A. Mankin
Category: Standards Track                            Johns Hopkins Univ.
ISSN: 2070-1721                                                R. Bonica
                                                        Juniper Networks
                                                               June 2010


                     The TCP Authentication Option

Abstract

   This document specifies the TCP Authentication Option (TCP-AO), which
   obsoletes the TCP MD5 Signature option of RFC 2385 (TCP MD5).  TCP-AO
   specifies the use of stronger Message Authentication Codes (MACs),
   protects against replays even for long-lived TCP connections, and
   provides more details on the association of security with TCP
   connections than TCP MD5.  TCP-AO is compatible with either a static
   Master Key Tuple (MKT) configuration or an external, out-of-band MKT
   management mechanism; in either case, TCP-AO also protects
   connections when using the same MKT across repeated instances of a
   connection, using traffic keys derived from the MKT, and coordinates
   MKT changes between endpoints.  The result is intended to support
   current infrastructure uses of TCP MD5, such as to protect long-lived
   connections (as used, e.g., in BGP and LDP), and to support a larger
   set of MACs with minimal other system and operational changes.  TCP-
   AO uses a different option identifier than TCP MD5, even though TCP-
   AO and TCP MD5 are never permitted to be used simultaneously.  TCP-AO
   supports IPv6, and is fully compatible with the proposed requirements
   for the replacement of TCP MD5.

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/rfc5925.






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Copyright Notice

   Copyright (c) 2010 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.





































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Table of Contents

   1. Introduction ....................................................4
      1.1. Conventions Used in This Document ..........................4
      1.2. Applicability Statement ....................................5
      1.3. Executive Summary ..........................................6
   2. The TCP Authentication Option ...................................7
      2.1. Review of TCP MD5 Option ...................................7
      2.2. The TCP Authentication Option Format .......................8
   3. TCP-AO Keys and Their Properties ...............................10
      3.1. Master Key Tuple ..........................................10
      3.2. Traffic Keys ..............................................12
      3.3. MKT Properties ............................................13
   4. Per-Connection TCP-AO Parameters ...............................14
   5. Cryptographic Algorithms .......................................15
      5.1. MAC Algorithms ............................................15
      5.2. Traffic Key Derivation Functions ..........................18
      5.3. Traffic Key Establishment and Duration Issues .............22
           5.3.1. MKT Reuse Across Socket Pairs ......................22
           5.3.2. MKTs Use within a Long-Lived Connection ............23
   6. Additional Security Mechanisms .................................23
      6.1. Coordinating Use of New MKTs ..............................23
      6.2. Preventing Replay Attacks within Long-Lived Connections ...24
   7. TCP-AO Interaction with TCP ....................................26
      7.1. TCP User Interface ........................................27
      7.2. TCP States and Transitions ................................28
      7.3. TCP Segments ..............................................28
      7.4. Sending TCP Segments ......................................29
      7.5. Receiving TCP Segments ....................................30
      7.6. Impact on TCP Header Size .................................32
      7.7. Connectionless Resets .....................................33
      7.8. ICMP Handling .............................................34
   8. Obsoleting TCP MD5 and Legacy Interactions .....................35
   9. Interactions with Middleboxes ..................................35
      9.1. Interactions with Non-NAT/NAPT Middleboxes ................36
      9.2. Interactions with NAT/NAPT Devices ........................36
   10. Evaluation of Requirements Satisfaction .......................36
   11. Security Considerations .......................................42
   12. IANA Considerations ...........................................43
   13. References ....................................................44
      13.1. Normative References .....................................44
      13.2. Informative References ...................................45
   14. Acknowledgments ...............................................47








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1.  Introduction

   The TCP MD5 Signature (TCP MD5) is a TCP option that authenticates
   TCP segments, including the TCP IPv4 pseudoheader, TCP header, and
   TCP data.  It was developed to protect BGP sessions from spoofed TCP
   segments, which could affect BGP data or the robustness of the TCP
   connection itself [RFC2385][RFC4953].

   There have been many recent concerns about TCP MD5.  Its use of a
   simple keyed hash for authentication is problematic because there
   have been escalating attacks on the algorithm itself [Wa05].  TCP MD5
   also lacks both key-management and algorithm agility.  This document
   adds the latter, and provides a simple key coordination mechanism
   giving the ability to move from one key to another within the same
   connection.  It does not however provide for complete cryptographic
   key management to be handled in band of TCP, because TCP SYN segments
   lack sufficient remaining space to handle such a negotiation (see
   Section 7.6).  This document obsoletes the TCP MD5 option with a more
   general TCP Authentication Option (TCP-AO).  This new option supports
   the use of other, stronger hash functions, provides replay protection
   for long-lived connections and across repeated instances of a single
   connection, coordinates key changes between endpoints, and provides a
   more explicit recommendation for external key management.  The result
   is compatible with IPv6, and is fully compatible with proposed
   requirements for a replacement for TCP MD5 [Ed07].

   TCP-AO obsoletes TCP MD5, although a particular implementation may
   support both mechanisms for backward compatibility.  For a given
   connection, only one can be in use.  TCP MD5-protected connections
   cannot be migrated to TCP-AO because TCP MD5 does not support any
   changes to a connection's security algorithm once established.

1.1.  Conventions Used in This Document

   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].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS.  Lowercase uses of these words are not to be
   interpreted as carrying RFC 2119 significance.

   In this document, the characters ">>" preceeding an indented line(s)
   indicates a compliance requirement statement using the key words
   listed above.  This convention aids reviewers in quickly identifying
   or finding the explicit compliance requirements of this RFC.





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1.2.  Applicability Statement

   TCP-AO is intended to support current uses of TCP MD5, such as to
   protect long-lived connections for routing protocols, such as BGP and
   LDP.  It is also intended to provide similar protection to any long-
   lived TCP connection, as might be used between proxy caches, for
   example, and is not designed solely or primarily for routing protocol
   uses.

   TCP-AO is intended to replace (and thus obsolete) the use of TCP MD5.
   TCP-AO enhances the capabilities of TCP MD5 as summarized in Section
   1.3.  This document recommends overall that:

   >> TCP implementations that support TCP MD5 MUST support TCP-AO.

   >> TCP-AO SHOULD be implemented where the protection afforded by TCP
   authentication is needed, because either IPsec is not supported or
   TCP-AO's particular properties are needed (e.g., per-connection
   keys).

   >> TCP-AO MAY be implemented elsewhere.

   TCP-AO is not intended to replace the use of the IPsec suite (IPsec
   and Internet Key Exchange Protocol (IKE)) to protect TCP connections
   [RFC4301][RFC4306].  Specific differences are noted in Section 1.3.
   In fact, we recommend the use of IPsec and IKE, especially where
   IKE's level of existing support for parameter negotiation, session
   key negotiation, or rekeying are desired.  TCP-AO is intended for use
   only where the IPsec suite would not be feasible, e.g., as has been
   suggested is the case to support some routing protocols [RFC4953], or
   in cases where keys need to be tightly coordinated with individual
   transport sessions [Ed07].

   TCP-AO is not intended to replace the use of Transport Layer Security
   (TLS) [RFC5246], Secure BGP (sBGP) or Secure Origin BGP (soBGP)
   [Le09], or any other mechanisms that protect only the TCP data
   stream.  TCP-AO protects the transport layer, preventing attacks from
   disabling the TCP connection itself [RFC4953].  Data stream
   mechanisms protect only the contents of the TCP segments, and can be
   disrupted when the connection is affected.  Some of these data
   protection protocols -- notably TLS -- offer a richer set of key
   management and authentication mechanisms than TCP-AO, and thus
   protect the data stream in a different way.  TCP-AO may be used
   together with these data stream protections to complement each
   other's strengths.






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1.3.  Executive Summary

   This document replaces TCP MD5 as follows [RFC2385]:

   o  TCP-AO uses a separate option Kind (29).

   o  TCP-AO allows TCP MD5 to continue to be used concurrently for
      legacy connections.

   o  TCP-AO replaces TCP MD5's single MAC algorithm with MACs specified
      in a separate document and can be extended to include other MACs.

   o  TCP-AO allows rekeying during a TCP connection, assuming that an
      out-of-band protocol or manual mechanism provides the new keys.
      The option includes a 'key ID', which allows the efficient
      concurrent use of multiple keys, and a key coordination mechanism
      using a 'receive next key ID' manages the key change within a
      connection.  Note that TCP MD5 does not preclude rekeying during a
      connection, but does not require its support either.  Further,
      TCP-AO supports key changes with zero segment loss, whereas key
      changes in TCP MD5 can lose segments in transit during the
      changeover or require trying multiple keys on each received
      segment during key use overlap because it lacks an explicit key
      ID.  Although TCP recovers lost segments through retransmission,
      loss can have a substantial impact on performance.

   o  TCP-AO provides automatic replay protection for long-lived
      connections using sequence number extensions.

   o  TCP-AO ensures per-connection traffic keys as unique as the TCP
      connection itself, using TCP's Initial Sequence Numbers (ISNs) for
      differentiation, even when static master key tuples are used
      across repeated instances of connections on a single socket pair.

   o  TCP-AO specifies the details of how this option interacts with
      TCP's states, event processing, and user interface.

   o  TCP-AO is 2 bytes shorter than TCP MD5 (16 bytes overall, rather
      than 18) in the initially specified default case (using a 96-bit
      MAC).

   TCP-AO differs from an IPsec/IKE solution as follows
   [RFC4301][RFC4306]:

   o  TCP-AO does not support dynamic parameter negotiation.






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   o  TCP-AO includes TCP's socket pair (source address, destination
      address, source port, destination port) as a security parameter
      index (together with the KeyID), rather than using a separate
      field as an index (IPsec's Security Parameter Index (SPI)).

   o  TCP-AO forces a change of computed MACs when a connection
      restarts, even when reusing a TCP socket pair (IP addresses and
      port numbers) [Ed07].

   o  TCP-AO does not support encryption.

   o  TCP-AO does not authenticate ICMP messages (some ICMP messages may
      be authenticated when using IPsec, depending on the
      configuration).

2.  The TCP Authentication Option

   The TCP Authentication Option (TCP-AO) uses a TCP option Kind value
   of 29.  The following sections describe TCP-AO and provide a review
   of TCP MD5 for comparison.

2.1.  Review of TCP MD5 Option

   For review, the TCP MD5 option is shown in Figure 1.

               +---------+---------+-------------------+
               | Kind=19 |Length=18|   MD5 digest...   |
               +---------+---------+-------------------+
               |          ...digest (con't)...         |
               +---------------------------------------+
               |                  ...                  |
               +---------------------------------------+
               |                  ...                  |
               +-------------------+-------------------+
               | ...digest (con't) |
               +-------------------+

                  Figure 1: The TCP MD5 Option [RFC2385]

   In the TCP MD5 option, the length is fixed, and the MD5 digest
   occupies 16 bytes following the Kind and Length fields (each one
   byte), using the full MD5 digest of 128 bits [RFC1321].

   The TCP MD5 option specifies the use of the MD5 digest calculation
   over the following values in the following order:

   1. The IP pseudoheader (IP source and destination addresses, protocol
      number, and segment length).



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   2. The TCP header excluding options and checksum.

   3. The TCP data payload.

   4. A key.

2.2.  The TCP Authentication Option Format

   TCP-AO provides a superset of the capabilities of TCP MD5, and is
   minimal in the spirit of SP4 [SDNS88].  TCP-AO uses a new Kind field,
   and similar Length field to TCP MD5, a KeyID field, and a RNextKeyID
   field as shown in Figure 2.

            +------------+------------+------------+------------+
            |  Kind=29   |   Length   |   KeyID    | RNextKeyID |
            +------------+------------+------------+------------+
            |                     MAC           ...
            +-----------------------------------...

               ...-----------------+
               ...  MAC (con't)    |
               ...-----------------+

             Figure 2: The TCP Authentication Option (TCP-AO)

   TCP-AO defines these fields as follows:

   o  Kind: An unsigned 1-byte field indicating TCP-AO.  TCP-AO uses a
      new Kind value of 29.

      >> An endpoint MUST NOT use TCP-AO for the same connection in
      which TCP MD5 is used.  When both options appear, TCP MUST
      silently discard the segment.

      >> A single TCP segment MUST NOT have more than one TCP-AO in its
      options sequence.  When multiple TCP-AOs appear, TCP MUST discard
      the segment.

   o  Length: An unsigned 1-byte field indicating the length of the
      option in bytes, including the Kind, Length, KeyID, RNextKeyID,
      and MAC fields.

      >> The Length value MUST be greater than or equal to 4.  When the
      Length value is less than 4, TCP MUST discard the segment.

      >> The Length value MUST be consistent with the TCP header length.
      When the Length value is invalid, TCP MUST discard the segment.




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      This Length check implies that the sum of the sizes of all
      options, when added to the size of the base TCP header (5 words),
      matches the TCP Offset field exactly.  This full verification can
      be computed because RFC 793 specifies the size of the required
      options, and RFC 1122 requires that all new options follow a
      common format with a fixed-length field location
      [RFC793][RFC1122].  A partial verification can be limited to check
      only TCP-AO, so that the TCP-AO length, when added to the TCP-AO
      offset from the start of the TCP header, does not exceed the TCP
      header size as indicated in the TCP header Offset field.

      Values of 4 and other small values larger than 4 (e.g., indicating
      MAC fields of very short length) are of dubious utility but are
      not specifically prohibited.

   o  KeyID: An unsigned 1-byte field indicating the Master Key Tuple
      (MKT, as defined in Section 3.1) used to generate the traffic keys
      that were used to generate the MAC that authenticates this
      segment.

      It supports efficient key changes during a connection and/or to
      help with key coordination during connection establishment, to be
      discussed further in Section 6.1.  Note that the KeyID has no
      cryptographic properties -- it need not be random, nor are there
      any reserved values.

      >> KeyID values MAY be the same in both directions of a
      connection, but do not have to be and there is no special meaning
      when they are.

      This allows MKTs to be installed on a set of devices without
      coordinating the KeyIDs across that entire set in advance, and
      allows new devices to be added to that set using a group of MKTs
      later without requiring renumbering of KeyIDs.  These two
      capabilities are particularly important when used with wildcards
      in the TCP socket pair of the MKT, i.e., when an MKT is used among
      a set of devices specified by a pattern (as noted in Section 3.1).

   o  RNextKeyID: An unsigned 1-byte field indicating the MKT that is
      ready at the sender to be used to authenticate received segments,
      i.e., the desired 'receive next' key ID.

      It supports efficient key change coordination, to be discussed
      further in Section 6.1.  Note that the RNextKeyID has no
      cryptographic properties -- it need not be random, nor are there
      any reserved values.





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   o  MAC: Message Authentication Code.  Its contents are determined by
      the particulars of the security association.  Typical MACs are
      96-128 bits (12-16 bytes), but any length that fits in the header
      of the segment being authenticated is allowed.  The MAC
      computation is described further in Section 5.1.

      >> Required support for TCP-AO MACs is defined in [RFC5926]; other
      MACs MAY be supported.

   TCP-AO fields do not indicate the MAC algorithm either implicitly (as
   with TCP MD5) or explicitly.  The particular algorithm used is
   considered part of the configuration state of the connection's
   security and is managed separately (see Section 3).

   Please note that the use of TCP-AO does not affect TCP's advertised
   Maximum Segment Size (MSS), as is the case for all TCP options
   [Bo09].

   The remainder of this document explains how TCP-AO is handled and its
   relationship to TCP.

3.  TCP-AO Keys and Their Properties

   TCP-AO relies on two sets of keys to authenticate incoming and
   outgoing segments: Master Key Tuples (MKTs) and traffic keys.  MKTs
   are used to derive unique traffic keys, and include the keying
   material used to generate those traffic keys, as well as indicating
   the associated parameters under which traffic keys are used.  Such
   parameters include whether TCP options are authenticated, and
   indicators of the algorithms used for traffic key derivation and MAC
   calculation.  Traffic keys are the keying material used to compute
   the MAC of individual TCP segments.

3.1.  Master Key Tuple

   A Master Key Tuple (MKT) describes TCP-AO properties to be associated
   with one or more connections.  It is composed of the following:

   o  TCP connection identifier.  A TCP socket pair, i.e., a local IP
      address, a remote IP address, a TCP local port, and a TCP remote
      port.  Values can be partially specified using ranges (e.g.,
      2-30), masks (e.g., 0xF0), wildcards (e.g., "*"), or any other
      suitable indication.

   o  TCP option flag.  This flag indicates whether TCP options other
      than TCP-AO are included in the MAC calculation.  When options are
      included, the content of all options, in the order present, is
      included in the MAC, with TCP-AO's MAC field zeroed out.  When the



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      options are not included, all options other than TCP-AO are
      excluded from all MAC calculations (skipped over, not zeroed).
      Note that TCP-AO, with its MAC field zeroed out, is always
      included in the MAC calculation, regardless of the setting of this
      flag; this protects the indication of the MAC length as well as
      the key ID fields (KeyID, RNextKeyID).  The option flag applies to
      TCP options in both directions (incoming and outgoing segments).

   o  IDs.  The values used in the KeyID or RNextKeyID of TCP-AO; used
      to differentiate MKTs in concurrent use (KeyID), as well as to
      indicate when MKTs are ready for use in the opposite direction
      (RNextKeyID).

      Each MKT has two IDs - -- a SendID and a RecvID.  The SendID is
      inserted as the KeyID of the TCP-AO option of outgoing segments,
      and the RecvID is matched against the TCP-AO KeyID of incoming
      segments.  These and other uses of these two IDs are described
      further in Sections 7.4 and 7.5.

      >> MKT IDs MUST support any value, 0-255 inclusive.  There are no
      reserved ID values.

      ID values are assigned arbitrarily, i.e., the values are not
      monotonically increasing, have no reserved values, and are
      otherwise not meaningful.  They can be assigned in sequence, or
      based on any method mutually agreed by the connection endpoints
      (e.g., using an external MKT management mechanism).

      >> IDs MUST NOT be assumed to be randomly assigned.

   o  Master key.  A byte sequence used for generating traffic keys,
      this may be derived from a separate shared key by an external
      protocol over a separate channel.  This sequence is used in the
      traffic key generation algorithm described in Section 5.2.

      Implementations are advised to keep master key values in a
      private, protected area of memory or other storage.

   o  Key Derivation Function (KDF).  Indicates the key derivation
      function and its parameters, as used to generate traffic keys from
      master keys.  It is explained further in Section 5.2 of this
      document and specified in detail in [RFC5926].

   o  Message Authentication Code (MAC) algorithm.  Indicates the MAC
      algorithm and its parameters as used for this connection.  It is
      explained further in Section 5.1 of this document and specified in
      detail in [RFC5926].




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   >> Components of an MKT MUST NOT change during a connection.

   MKT component values cannot change during a connection because TCP
   state is coordinated during connection establishment.  TCP lacks a
   handshake for modifying that state after a connection has been
   established.

   >> The set of MKTs MAY change during a connection.

   MKT parameters are not changed.  Instead, new MKTs can be installed,
   and a connection can change which MKT it uses.

   >> The IDs of MKTs MUST NOT overlap where their TCP connection
   identifiers overlap.

   This document does not address how MKTs are created by users or
   processes.  It is presumed that an MKT affecting a particular
   connection cannot be destroyed during an active connection -- or,
   equivalently, that its parameters are copied to an area local to the
   connection (i.e., instantiated) and so changes would affect only new
   connections.  The MKTs can be managed by a separate application
   protocol.

3.2.  Traffic Keys

   A traffic key is a key derived from the MKT and the local and remote
   IP address pairs and TCP port numbers, and, for established
   connections, the TCP Initial Sequence Numbers (ISNs) in each
   direction.  Segments exchanged before a connection is established use
   the same information, substituting zero for unknown values (e.g.,
   ISNs not yet coordinated).

   A single MKT can be used to derive any of four different traffic
   keys:

   o  Send_SYN_traffic_key

   o  Receive_SYN_traffic_key

   o  Send_other_traffic_key

   o  Receive_other_traffic_key

   Note that the keys are unidirectional.  A given connection typically
   uses only three of these keys, because only one of the SYN keys is
   typically used.  All four are used only when a connection goes
   through 'simultaneous open' [RFC793].




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   The relationship between MKTs and traffic keys is shown in Figure 3.
   Traffic keys are indicated with a "*".  Note that every MKT can be
   used to derive any of the four traffic keys, but only the keys
   actually needed to handle the segments of a connection need to be
   computed.  Section 5.2 provides further details on how traffic keys
   are derived.

                     MKT-A                            MKT-B
            +---------------------+        +------------------------+
            | SendID = 1          |        | SendID = 5             |
            | RecvID = 2          |        | RecvID = 6             |
            | MAC = HMAC-SHA1     |        | MAC = AES-CMAC         |
            | KDF = KDF-HMAC-SHA1 |        | KDF = KDF-AES-128-CMAC |
            +---------------------+        +------------------------+
                       |                                |
            +----------+----------+                     |
            |                     |                     |
            v                     v                     v
       Connection 1          Connection 2          Connection 3
   +------------------+  +------------------+  +------------------+
   | * Send_SYN_key   |  | * Send_SYN_key   |  | * Send_SYN_key   |
   | * Recv_SYN_key   |  | * Recv_SYN_key   |  | * Recv_SYN_key   |
   | * Send_Other_key |  | * Send_Other_key |  | * Send_Other_key |
   | * Recv_Other_key |  | * Recv_Other_key |  | * Recv_Other_key |
   +------------------+  +------------------+  +------------------+

           Figure 3: Relationship between MKTs and Traffic Keys

3.3.  MKT Properties

   TCP-AO requires that every protected TCP segment match exactly one
   MKT.  When an outgoing segment matches an MKT, TCP-AO is used.  When
   no match occurs, TCP-AO is not used.  Multiple MKTs may match a
   single outgoing segment, e.g., when MKTs are being changed.  Those
   MKTs cannot have conflicting IDs (as noted elsewhere), and some
   mechanism must determine which MKT to use for each given outgoing
   segment.

   >> An outgoing TCP segment MUST match at most one desired MKT,
   indicated by the segment's socket pair.  The segment MAY match
   multiple MKTs, provided that exactly one MKT is indicated as desired.
   Other information in the segment MAY be used to determine the desired
   MKT when multiple MKTs match; such information MUST NOT include
   values in any TCP option fields.







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   We recommend that the mechanism used to select from among multiple
   MKTs use only information that TCP-AO would authenticate.  Because
   MKTs may indicate that options other than TCP-AO are ignored in the
   MAC calculation, we recommend that TCP options should not be used to
   determine MKTs.

   >> An incoming TCP segment including TCP-AO MUST match exactly one
   MKT, indicated solely by the segment's socket pair and its TCP-AO
   KeyID.

   Incoming segments include an indicator inside TCP-AO to select from
   among multiple matching MKTs -- the KeyID field.  TCP-AO requires
   that the KeyID alone be used to differentiate multiple matching MKTs,
   so that MKT changes can be coordinated using the TCP-AO key change
   coordination mechanism.

   >> When an outgoing TCP segment matches no MKTs, TCP-AO is not used.

   TCP-AO is always used when outgoing segments match an MKT, and is not
   used otherwise.

4.  Per-Connection TCP-AO Parameters

   TCP-AO uses a small number of parameters associated with each
   connection that uses TCP-AO, once instantiated.  These values can be
   stored in the Transport Control Block (TCB) [RFC793].  These values
   are explained in subsequent sections of this document as noted; they
   include:

   1. Current_key - the MKT currently used to authenticate outgoing
      segments, whose SendID is inserted in outgoing segments as KeyID
      (see Section 7.4, step 2.f).  Incoming segments are authenticated
      using the MKT corresponding to the segment and its TCP-AO KeyID
      (see Section 7.5, step 2.c), as matched against the MKT TCP
      connection identifier and the MKT RecvID.  There is only one
      current_key at any given time on a particular connection.

      >> Every TCP connection in a non-IDLE state MUST have at most one
      current_key specified.

   2. Rnext_key - the MKT currently preferred for incoming (received)
      segments, whose RecvID is inserted in outgoing segments as
      RNextKeyID (see Section 7.4, step 2.d).

      >> Each TCP connection in a non-IDLE state MUST have at most one
      rnext_key specified.





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   3. A pair of Sequence Number Extensions (SNEs).  SNEs are used to
      prevent replay attacks, as described in Section 6.2.  Each SNE is
      initialized to zero upon connection establishment.  Its use in the
      MAC calculation is described in Section 5.1.

   4. One or more MKTs.  These are the MKTs that match this connection's
      socket pair.

   MKTs are used, together with other parameters of a connection, to
   create traffic keys unique to each connection, as described in
   Section 5.2.  These traffic keys can be cached after computation, and
   can be stored in the TCB with the corresponding MKT information.
   They can be considered part of the per-connection parameters.

5.  Cryptographic Algorithms

   TCP-AO uses cryptographic algorithms to compute the MAC (Message
   Authentication Code) that is used to authenticate a segment and its
   headers; these are called MAC algorithms and are specified in a
   separate document to facilitate updating the algorithm requirements
   independently from the protocol [RFC5926].  TCP-AO also uses
   cryptographic algorithms to convert MKTs, which can be shared across
   connections, into unique traffic keys for each connection.  These are
   called Key Derivation Functions (KDFs) and are specified [RFC5926].
   This section describes how these algorithms are used by TCP-AO.

5.1.  MAC Algorithms

   MAC algorithms take a variable-length input and a key and output a
   fixed-length number.  This number is used to determine whether the
   input comes from a source with that same key, and whether the input
   has been tampered with in transit.  MACs for TCP-AO have the
   following interface:

      MAC = MAC_alg(traffic_key, message)

      INPUT: MAC_alg, traffic_key, message

      OUTPUT: MAC

   where:

   o  MAC_alg - the specific MAC algorithm used for this computation.
      The MAC algorithm specifies the output length, so no separate
      output length parameter is required.  This is specified as
      described in [RFC5926].





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   o  Traffic_key - traffic key used for this computation.  This is
      computed from the connection's current MKT as described in Section
      5.2.

   o  Message - input data over which the MAC is computed.  In TCP-AO,
      this is the TCP segment prepended by the IP pseudoheader and TCP
      header options, as described in Section 5.1.

   o  MAC - the fixed-length output of the MAC algorithm, given the
      parameters provided.

   At the time of this writing, the algorithms' definitions for use in
   TCP-AO, as described in [RFC5926], are each truncated to 96 bits.
   Though the algorithms each output a larger MAC, 96 bits provides a
   reasonable trade-off between security and message size.  However,
   this could change in the future, so TCP-AO size should not be assumed
   as fixed length.

   The MAC algorithm employed for the MAC computation on a connection is
   done so by definition in the MKT, per the definition in [RFC5926].

   The mandatory-to-implement MAC algorithms for use with TCP-AO are
   described in a separate RFC [RFC5926].  This allows the TCP-AO
   specification to proceed along the IETF Standards Track even if
   changes are needed to its associated algorithms and their labels (as
   might be used in a user interface or automated MKT management
   protocol) as a result of the ever evolving world of cryptography.

   >> Additional algorithms, beyond those mandated for TCP-AO, MAY be
   supported.

   The data input to the MAC is in the following fields in the following
   sequence, interpreted in network-standard byte order:

   1. The Sequence Number Extension (SNE), in network-standard byte
      order, as follows (described further in Section 6.2):

                  +--------+--------+--------+--------+
                  |                SNE                |
                  +--------+--------+--------+--------+

                    Figure 4: Sequence Number Extension

      The SNE for transmitted segments is maintained locally in the
      SND.SNE value; for received segments, a local RCV.SNE value is
      used.  The details of how these values are maintained and used are
      in Sections 6.2, 7.4, and 7.5.




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   2. The IP pseudoheader: IP source and destination addresses, protocol
      number, and segment length, all in network byte order, prepended
      to the TCP header below.  The IP pseudoheader is exactly as used
      for the TCP checksum in either IPv4 or IPv6 [RFC793][RFC2460]:

               +--------+--------+--------+--------+
               |           Source Address          |
               +--------+--------+--------+--------+
               |         Destination Address       |
               +--------+--------+--------+--------+
               |  Zero  | Proto  |    TCP Length   |
               +--------+--------+--------+--------+

                 Figure 5: TCP IPv4 Pseudoheader [RFC793]

               +--------+--------+--------+--------+
               |                                   |
               +                                   +
               |                                   |
               +           Source Address          +
               |                                   |
               +                                   +
               |                                   |
               +                                   +
               +--------+--------+--------+--------+
               |                                   |
               +                                   +
               |                                   |
               +         Destination Address       +
               |                                   |
               +                                   +
               |                                   |
               +--------+--------+--------+--------+
               |     Upper-Layer Payload Length    |
               +--------+--------+--------+--------+
               |      Zero       |   Next Header   |
               +--------+--------+--------+--------+

                 Figure 6: TCP IPv6 Pseudoheader [RFC2460]

   3. The TCP header, by default including options, and where the TCP
      checksum and TCP-AO MAC fields are set to zero, all in network-
      byte order.

      The TCP option flag of the MKT indicates whether the TCP options
      are included in the MAC.  When included, only the TCP-AO MAC field
      is zeroed.




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      When TCP options are not included, all TCP options except for TCP-
      AO are omitted from MAC processing.  Again, the TCP-AO MAC field
      is zeroed for the MAC processing.

   4. The TCP data, i.e., the payload of the TCP segment.

      Note that the traffic key is not included as part of the data; the
      MAC algorithm indicates how to use the traffic key, for example,
      as HMACs do [RFC2104][RFC2403].  The traffic key is derived from
      the current MKT as described in Section 5.2.

5.2.  Traffic Key Derivation Functions

   TCP-AO's traffic keys are derived from the MKTs using Key Derivation
   Functions (KDFs).  The KDFs used in TCP-AO have the following
   interface:

      traffic_key = KDF_alg(master_key, context, output_length)

      INPUT: KDF_alg, master_key, context, output_length

      OUTPUT: traffic_key

   where:

   o  KDF_alg - The specific Key Derivation Function (KDF) that is the
      basic building block used in constructing the traffic key, as
      indicated in the MKT.  This is specified as described in
      [RFC5926].

   o  Master_key - The master_key string, as will be stored into the
      associated MKT.

   o  Context - The context used as input in constructing the
      traffic_key, as specified in [RFC5926].  The specific way this
      context is used, in conjunction with other information, to create
      the raw input to the KDF is also explained further in [RFC5926].

   o  Output_length - The desired output length of the KDF, i.e., the
      length to which the KDF's output will be truncated.  This is
      specified as described in [RFC5926].

   o  Traffic_key - The desired output of the KDF, of length
      output_length, to be used as input to the MAC algorithm, as
      described in Section 5.1.






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   The context used as input to the KDF combines the TCP socket pair
   with the endpoint Initial Sequence Numbers (ISNs) of a connection.
   This data is unique to each TCP connection instance, which enables
   TCP-AO to generate unique traffic keys for that connection, even from
   an MKT used across many different connections or across repeated
   connections that share a socket pair.  Unique traffic keys are
   generated without relying on external key management properties.  The
   KDF context is defined in Figures 7 and 8.

               +--------+--------+--------+--------+
               |           Source Address          |
               +--------+--------+--------+--------+
               |         Destination Address       |
               +--------+--------+--------+--------+
               |   Source Port   |    Dest. Port   |
               +--------+--------+--------+--------+
               |            Source ISN             |
               +--------+--------+--------+--------+
               |             Dest. ISN             |
               +--------+--------+--------+--------+

               Figure 7: KDF Context for an IPv4 Connection





























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               +--------+--------+--------+--------+
               |                                   |
               +                                   +
               |                                   |
               +           Source Address          +
               |                                   |
               +                                   +
               |                                   |
               +                                   +
               +--------+--------+--------+--------+
               |                                   |
               +                                   +
               |                                   |
               +         Destination Address       +
               |                                   |
               +                                   +
               |                                   |
               +--------+--------+--------+--------+
               |   Source Port   |    Dest. Port   |
               +--------+--------+--------+--------+
               |            Source ISN             |
               +--------+--------+--------+--------+
               |             Dest. ISN             |
               +--------+--------+--------+--------+

               Figure 8: KDF Context for an IPv6 Connection

   Traffic keys are directional, so "source" and "destination" are
   interpreted differently for incoming and outgoing segments.  For
   incoming segments, source is the remote side; whereas for outgoing
   segments, source is the local side.  This further ensures that
   connection keys generated for each direction are unique.

   For SYN segments (segments with the SYN set, but the ACK not set),
   the destination ISN is not known.  For these segments, the connection
   key is computed using the context shown above, in which the
   destination ISN value is zero.  For all other segments, the ISN pair
   is used when known.  If the ISN pair is not known, e.g., when sending
   a reset (RST) after a reboot, the segment should be sent without
   authentication; if authentication was required, the segment cannot
   have been MAC'd properly anyway and would have been dropped on
   receipt.

   >> TCP-AO SYN segments (SYN set, no ACK set) MUST use a destination
   ISN of zero (whether sent or received); all other segments use the
   known ISN pair.





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   Overall, this means that each connection will use up to four distinct
   traffic keys for each MKT:

   o  Send_SYN_traffic_key - the traffic key used to authenticate
      outgoing SYNs.  The source ISN is known (the TCP connection's
      local ISN), and the destination (remote) ISN is unknown (and so
      the value 0 is used).

   o  Receive_SYN_traffic_key - the traffic key used to authenticate
      incoming SYNs.  The source ISN is known (the TCP connection's
      remote ISN), and the destination (remote) ISN is unknown (and so
      the value 0 is used).

   o  Send_other_traffic_key - the traffic key used to authenticate all
      other outgoing TCP segments.

   o  Receive_other_traffic_key - the traffic key used to authenticate
      all other incoming TCP segments.

   The following table describes how each of these traffic keys is
   computed, where the TCP-AO algorithms refer to source (S) and
   destination (D) values of the IP address, TCP port, and ISN, and each
   segment (incoming or outgoing) has a value that refers to the local
   side of the connection (l) and remote side (r):

                               S-IP S-port S-ISN D-IP D-port D-ISN
   ----------------------------------------------------------------
    Send_SYN_traffic_key       l-IP l-port l-ISN r-IP r-port 0
    Receive_SYN_traffic_key    r-IP r-port r-ISN l-IP l-port 0
    Send_other_traffic_key     l-IP l-port l-ISN r-IP r-port r-ISN
    Receive_other_traffic_key  r-IP r-port r-ISN l-IP l-port l-ISN

   The use of both ISNs in the traffic key computations ensures that
   segments cannot be replayed across repeated connections reusing the
   same socket; their 32-bit space avoids repeated use except under
   reboot, and reuse assumes both sides repeat their use on the same
   connection.  We do expect that:

   >> Endpoints should select ISNs pseudorandomly, e.g., as in
   [RFC1948].

   A SYN is authenticated using a destination ISN of zero (whether sent
   or received), and all other segments would be authenticated using the
   ISN pair for the connection.  There are other cases in which the
   destination ISN is not known, but segments are emitted, such as after
   an endpoint reboots, when it is possible that the two endpoints would
   not have enough information to authenticate segments.  This is
   addressed further in Section 7.7.



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5.3.  Traffic Key Establishment and Duration Issues

   TCP-AO does not provide a mechanism for traffic key negotiation or
   parameter negotiation (MAC algorithm, length, or use of TCP-AO on a
   connection), or for coordinating rekeying during a connection.  We
   assume out-of-band mechanisms for MKT establishment, parameter
   negotiation, and rekeying.  This separation of MKT use from MKT
   management is similar to that in the IPsec suite [RFC4301][RFC4306].

   We encourage users of TCP-AO to apply known techniques for generating
   appropriate MKTs, including the use of reasonable master key lengths,
   limited traffic key sharing, and limiting the duration of MKT use
   [RFC3562].  This also includes the use of per-connection nonces, as
   suggested in Section 5.2.

   TCP-AO supports rekeying in which new MKTs are negotiated and
   coordinated out of band, either via a protocol or a manual procedure
   [RFC4808].  New MKT use is coordinated using the out-of-band
   mechanism to update both TCP endpoints.  When only a single MKT is
   used at a time, the temporary use of invalid MKTs could result in
   segments being dropped; although TCP is already robust to such drops,
   TCP-AO uses the KeyID field to avoid such drops.  A given connection
   can have multiple matching MKTs, where the KeyID field is used to
   identify the MKT that corresponds to the traffic key used for a
   segment, to avoid the need for expensive trial-and-error testing of
   MKTs in sequence.

   TCP-AO provides an explicit MKT coordination mechanism, described in
   Section 6.1.  Such a mechanism is useful when new MKTs are installed,
   or when MKTs are changed, to determine when to commence using
   installed MKTs.

   Users are advised to manage MKTs following the spirit of the advice
   for key management when using TCP MD5 [RFC3562], notably to use
   appropriate key lengths (12-24 bytes) and to avoid sharing MKTs among
   multiple BGP peering arrangements.

5.3.1.  MKT Reuse Across Socket Pairs

   MKTs can be reused across different socket pairs within a host, or
   across different instances of a socket pair within a host.  In either
   case, replay protection is maintained.

   MKTs reused across different socket pairs cannot enable replay
   attacks because the TCP socket pair is included in the MAC, as well
   as in the generation of the traffic key.  MKTs reused across repeated





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   instances of a given socket pair cannot enable replay attacks because
   the connection ISNs are included in the traffic key generation
   algorithm, and ISN pairs are unlikely to repeat over useful periods.

5.3.2.  MKTs Use within a Long-Lived Connection

   TCP-AO uses Sequence Number Extensions (SNEs) to prevent replay
   attacks within long-lived connections.  Explicit MKT rollover,
   accomplished by external means and indexed using the KeyID field, can
   be used to change keying material for various reasons (e.g.,
   personnel turnover), but is not required to support long-lived
   connections.

6.  Additional Security Mechanisms

   TCP-AO adds mechanisms to support efficient use, especially in
   environments where only manual keying is available.  These include
   the previously described mechanisms for supporting multiple
   concurrent MKTs (via the KeyID field) and for generating unique per-
   connection traffic keys (via the KDF).  This section describes
   additional mechanisms to coordinate MKT changes and to prevent replay
   attacks when a traffic key is not changed for long periods of time.

6.1.  Coordinating Use of New MKTs

   At any given time, a single TCP connection may have multiple MKTs
   specified for each segment direction (incoming, outgoing).  TCP-AO
   provides a mechanism to indicate when a new MKT is ready, which
   allows the sender to commence use of that new MKT.  This mechanism
   allows new MKT use to be coordinated, to avoid unnecessary loss due
   to sender authentication using an MKT not yet ready at the receiver.

   Note that this is intended as an optimization.  Deciding when to
   start using a key is a performance issue.  Deciding when to remove an
   MKT is a security issue.  Invalid MKTs are expected to be removed.
   TCP-AO provides no mechanism to coordinate their removal, as we
   consider this a key management operation.

   New MKT use is coordinated through two ID fields in the header:

   o  KeyID

   o  RNextKeyID








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   KeyID represents the outgoing MKT information used by the segment
   sender to create the segment's MAC (outgoing), and the corresponding
   incoming keying information used by the segment receiver to validate
   that MAC.  It contains the SendID of the MKT in active use in that
   direction.

   RNextKeyID represents the preferred MKT information to be used for
   subsequent received segments ('receive next').  That is, it is a way
   for the segment sender to indicate a ready incoming MKT for future
   segments it receives, so that the segment receiver can know when to
   switch MKTs (and thus their KeyIDs and associated traffic keys).  It
   indicates the RecvID of the MKT desired for incoming segments.

   There are two pointers kept by each side of a connection, as noted in
   the per-connection information (see Section 4):

   o  Currently active outgoing MKT (current_key)

   o  Current preference for incoming MKT (rnext_key)

   Current_key indicates an MKT that is used to authenticate outgoing
   segments.  Upon connection establishment, it points to the first MKT
   selected for use.

   Rnext_key points to an incoming MKT that is ready and preferred for
   use.  Upon connection establishment, this points to the currently
   active incoming MKT.  It can be changed when new MKTs are installed
   (e.g., by either automatic MKT management protocol operation or user
   manual selection).

   Rnext_key is changed only by manual user intervention or MKT
   management protocol operation.  It is not manipulated by TCP-AO.
   Current_key is updated by TCP-AO when processing received TCP
   segments as discussed in the segment processing description in
   Section 7.5.  Note that the algorithm allows the current_key to
   change to a new MKT, then change back to a previously used MKT (known
   as "backing up").  This can occur during an MKT change when segments
   are received out of order, and is considered a feature of TCP-AO,
   because reordering does not result in drops.  The only way to avoid
   reuse of previously used MKTs is to remove the MKT when it is no
   longer considered permitted.

6.2.  Preventing Replay Attacks within Long-Lived Connections

   TCP uses a 32-bit sequence number, which may, for long-lived
   connections, roll over and repeat.  This could result in TCP segments
   being intentionally and legitimately replayed within a connection.
   TCP-AO prevents replay attacks, and thus requires a way to



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   differentiate these legitimate replays from each other, and so it
   adds a 32-bit Sequence Number Extension (SNE) for transmitted and
   received segments.

   The SNE extends the TCP sequence number so that segments within a
   single connection are always unique.  When the TCP's sequence number
   rolls over, there is a chance that a segment could be repeated in
   total; using an SNE differentiates even identical segments sent with
   identical sequence numbers at different times in a connection.  TCP-
   AO emulates a 64-bit sequence number space by inferring when to
   increment the high-order 32-bit portion (the SNE) based on
   transitions in the low-order portion (the TCP sequence number).

   TCP-AO thus maintains SND.SNE for transmitted segments, and RCV.SNE
   for received segments, both initialized as zero when a connection
   begins.  The intent of these SNEs is, together with TCP's 32-bit
   sequence numbers, to provide a 64-bit overall sequence number space.

   For transmitted segments, SND.SNE can be implemented by extending
   TCP's sequence number to 64 bits; SND.SNE would be the top (high-
   order) 32 bits of that number.  For received segments, TCP-AO needs
   to emulate the use of a 64-bit number space and correctly infer the
   appropriate high-order 32-bits of that number as RCV.SNE from the
   received 32-bit sequence number and the current connection context.

   The implementation of SNEs is not specified in this document, but one
   possible way is described here that can be used for either RCV.SNE,
   SND.SNE, or both.

   Consider an implementation with two SNEs as required (SND.SNE, RCV.
   SNE), and additional variables as listed below, all initialized to
   zero, as well as a current TCP segment field (SEG.SEQ):

   o  SND.PREV_SEQ, needed to detect rollover of SND.SEQ

   o  RCV.PREV_SEQ, needed to detect rollover of RCV.SEQ

   o  SND.SNE_FLAG, which indicates when to increment the SND.SNE

   o  RCV.SNE_FLAG, which indicates when to increment the RCV.SNE

   When a segment is received, the following algorithm (in C-like
   pseudocode) computes the SNE used in the MAC; this is the "RCV" side,
   and an equivalent algorithm can be applied to the "SND" side:







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      /* set the flag when the SEG.SEQ first rolls over */
      if ((RCV.SNE_FLAG == 0)
         && (RCV.PREV_SEQ > 0x7fff) && (SEG.SEQ < 0x7fff)) {
            RCV.SNE = RCV.SNE + 1;
            RCV.SNE_FLAG = 1;
      }
      /* decide which SNE to use after incremented */
      if ((RCV.SNE_FLAG == 1) && (SEG.SEQ > 0x7fff)) {
         SNE = RCV.SNE - 1; # use the pre-increment value
      } else {
         SNE = RCV.SNE; # use the current value
      }
      /* reset the flag in the *middle* of the window */
      if ((RCV.PREV_SEQ < 0x7fff) && (SEG.SEQ > 0x7fff)) {
         RCV.SNE_FLAG = 0;
      }
      /* save the current SEQ for the next time through the code */
      RCV.PREV_SEQ = SEG.SEQ;

   In the above code, the first time the sequence number rolls over,
   i.e., when the new number is low (in the bottom half of the number
   space) and the old number is high (in the top half of the number
   space), the SNE is incremented and a flag is set.

   If the flag is set and a high number is seen, it must be a reordered
   segment, so use the pre-increment SNE; otherwise, use the current
   SNE.

   The flag will be cleared by the time the number rolls all the way
   around.

   The flag prevents the SNE from being incremented again until the flag
   is reset, which happens in the middle of the window (when the old
   number is in the bottom half and the new is in the top half).
   Because the receive window is never larger than half of the number
   space, it is impossible to both set and reset the flag at the same
   time -- outstanding segments, regardless of reordering, cannot
   straddle both regions simultaneously.

7.  TCP-AO Interaction with TCP

   The following is a description of how TCP-AO affects various TCP
   states, segments, events, and interfaces.  This description is
   intended to augment the description of TCP as provided in RFC 793,
   and its presentation mirrors that of RFC 793 as a result [RFC793].






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7.1.  TCP User Interface

   The TCP user interface supports active and passive OPEN, SEND,
   RECEIVE, CLOSE, STATUS, and ABORT commands.  TCP-AO does not alter
   this interface as it applies to TCP, but some commands or command
   sequences of the interface need to be modified to support TCP-AO.
   TCP-AO does not specify the details of how this is achieved.

   TCP-AO requires that the TCP user interface be extended to allow the
   MKTs to be configured, as well as to allow an ongoing connection to
   manage which MKTs are active.  The MKTs need to be configured prior
   to connection establishment, and the set of MKTs may change during a
   connection:

   >> TCP OPEN, or the sequence of commands that configure a connection
   to be in the active or passive OPEN state, MUST be augmented so that
   an MKT can be configured.

   >> A TCP-AO implementation MUST allow the set of MKTs for ongoing TCP
   connections (i.e., not in the CLOSED state) to be modified.

   The MKTs associated with a connection need to be available for
   confirmation; this includes the ability to read the MKTs:

   >> TCP STATUS SHOULD be augmented to allow the MKTs of a current or
   pending connection to be read (for confirmation).

   Senders may need to be able to determine when the outgoing MKT
   changes (KeyID) or when a new preferred MKT (RNextKeyID) is
   indicated; these changes immediately affect all subsequent outgoing
   segments:

   >> TCP SEND, or a sequence of commands resulting in a SEND, MUST be
   augmented so that the preferred outgoing MKT (current_key) and/or the
   preferred incoming MKT (rnext_key) of a connection can be indicated.

   It may be useful to change the outgoing active MKT (current_key) even
   when no data is being sent, which can be achieved by sending a zero-
   length buffer or by using a non-send interface (e.g., socket options
   in Unix), depending on the implementation.

   It is also useful to indicate recent segment KeyID and RNextKeyID
   values received; although there could be a number of such values,
   they are not expected to change quickly, so any recent sample should
   be sufficient:






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   >> TCP RECEIVE, or the sequence of commands resulting in a RECEIVE,
   MUST be augmented so that the KeyID and RNextKeyID of a recently
   received segment is available to the user out of band (e.g., as an
   additional parameter to RECEIVE or via a STATUS call).

7.2.  TCP States and Transitions

   TCP includes the states LISTEN, SYN-SENT, SYN-RECEIVED, ESTABLISHED,
   FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and
   CLOSED.

   >> An MKT MAY be associated with any TCP state.

7.3.  TCP Segments

   TCP includes control (at least one of SYN, FIN, RST flags set) and
   data (none of SYN, FIN, or RST flags set) segments.  Note that some
   control segments can include data (e.g., SYN).

   >> All TCP segments MUST be checked against the set of MKTs for
   matching TCP connection identifiers.

   >> TCP segments whose TCP-AO does not validate MUST be silently
   discarded.

   >> A TCP-AO implementation MUST allow for configuration of the
   behavior of segments with TCP-AO but that do not match an MKT.  The
   initial default of this configuration SHOULD be to silently accept
   such connections.  If this is not the desired case, an MKT can be
   included to match such connections, or the connection can indicate
   that TCP-AO is required.  Alternately, the configuration can be
   changed to discard segments with the AO option not matching an MKT.

   >> Silent discard events SHOULD be signaled to the user as a warning,
   and silent accept events MAY be signaled to the user as a warning.
   Both warnings, if available, MUST be accessible via the STATUS
   interface.  Either signal MAY be asynchronous, but if so, they MUST
   be rate-limited.  Either signal MAY be logged; logging SHOULD allow
   rate-limiting as well.

   All TCP-AO processing occurs between the interface of TCP and IP; for
   incoming segments, this occurs after validation of the TCP checksum.
   For outgoing segments, this occurs before computation of the TCP
   checksum.







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   Note that use of TCP-AO on a connection is not negotiated within TCP.
   It is the responsibility of the receiver to determine when TCP-AO is
   required via other means (e.g., out of band, manually or with a key
   management protocol) and to enforce that requirement.

7.4.  Sending TCP Segments

   The following procedure describes the modifications to TCP to support
   inserting TCP-AO when a segment departs.

   >> Note that TCP-AO MUST be the last TCP option processed on outgoing
   segments, because its MAC calculation may include the values of other
   TCP options.

   1. Find the per-connection parameters for the segment:

       a. If the segment is a SYN, then this is the first segment of a
          new connection.  Find the matching MKT for this segment based
          on the segment's socket pair.

          i. If there is no matching MKT, omit TCP-AO.  Proceed with
             transmitting the segment.

         ii. If there is a matching MKT, then set the per-connection
             parameters as needed (see Section 4).  Proceed with the
             step 2.

       b. If the segment is not a SYN, then determine whether TCP-AO is
          being used for the connection and use the MKT as indicated by
          the current_key value from the per-connection parameters (see
          Section 4) and proceed with the step 2.

   2. Using the per-connection parameters:

       a. Augment the TCP header with TCP-AO, inserting the appropriate
          Length and KeyID based on the MKT indicated by current_key
          (using the current_key MKT's SendID as the TCP-AO KeyID).
          Update the TCP header length accordingly.

       b. Determine SND.SNE as described in Section 6.2.

       c. Determine the appropriate traffic key, i.e., as pointed to by
          the current_key (as noted in Section 6.1, and as probably
          cached in the TCB).  That is, use the send_SYN_traffic_key for
          SYN segments and the send_other_traffic_key for other
          segments.





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       d. Determine the RNextKeyID as indicated by the rnext_key
          pointer, and insert it in the TCP-AO RNextKeyID field (using
          the rnext_key MKT's RecvID as the TCP-AO KeyID) (as noted in
          Section 6.1).

       e. Compute the MAC using the MKT (and cached traffic key) and
          data from the segment as specified in Section 5.1.

       f. Insert the MAC in the TCP-AO MAC field.

       g. Proceed with transmitting the segment.

7.5.  Receiving TCP Segments

   The following procedure describes the modifications to TCP to support
   TCP-AO when a segment arrives.

   >> Note that TCP-AO MUST be the first TCP option processed on
   incoming segments, because its MAC calculation may include the values
   of other TCP options that could change during TCP option processing.
   This also protects the behavior of all other TCP options from the
   impact of spoofed segments or modified header information.

   >> Note that TCP-AO checks MUST be performed for all incoming SYNs to
   avoid accepting SYNs lacking TCP-AO where required.  Other segments
   can cache whether TCP-AO is needed in the TCB.

   1. Find the per-connection parameters for the segment:

       a. If the segment is a SYN, then this is the first segment of a
          new connection.  Find the matching MKT for this segment, using
          the segment's socket pair and its TCP-AO KeyID, matched
          against the MKT's TCP connection identifier and the MKT's
          RecvID.

          i. If there is no matching MKT, remove TCP-AO from the
             segment.  Proceed with further TCP handling of the segment.

             NOTE: this presumes that connections that do not match any
             MKT should be silently accepted, as noted in Section 7.3.

         ii. If there is a matching MKT, then set the per-connection
             parameters as needed (see Section 4).  Proceed with step 2.








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   2. Using the per-connection parameters:

       a. Check that the segment's TCP-AO Length matches the length
          indicated by the MKT.

          i. If the lengths differ, silently discard the segment.  Log
             and/or signal the event as indicated in Section 7.3.

       b. Determine the segment's RCV.SNE as described in Section 6.2.

       c. Determine the segment's traffic key from the MKT as described
          in Section 5.1 (and as likely cached in the TCB).  That is,
          use the receive_SYN_traffic_key for SYN segments and the
          receive_other_traffic_key for other segments.

       d. Compute the segment's MAC using the MKT (and its derived
          traffic key) and portions of the segment as indicated in
          Section 5.1.

          i. If the computed MAC differs from the TCP-AO MAC field
             value, silently discard the segment.  Log and/or signal the
             event as indicated in Section 7.3.

       e. Compare the received RNextKeyID value to the currently active
          outgoing KeyID value (current_key MKT's SendID).

          i. If they match, no further action is required.

         ii. If they differ, determine whether the RNextKeyID MKT is
             ready.

             1. If the MKT corresponding to the segment's socket pair
                and RNextKeyID is not available, no action is required
                (RNextKeyID of a received segment needs to match the
                MKT's SendID).

             2. If the matching MKT corresponding to the segment's
                socket pair and RNextKeyID is available:

                a. Set current_key to the RNextKeyID MKT.

       f. Proceed with TCP processing of the segment.

   It is suggested that TCP-AO implementations validate a segment's
   Length field before computing a MAC to reduce the overhead incurred
   by spoofed segments with invalid TCP-AO fields.





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   Additional reductions in MAC validation overhead can be supported in
   the MAC algorithms, e.g., by using a computation algorithm that
   prepends a fixed value to the computed portion and a corresponding
   validation algorithm that verifies the fixed value before investing
   in the computed portion.  Such optimizations would be contained in
   the MAC algorithm specification, and thus are not specified in TCP-AO
   explicitly.  Note that the KeyID cannot be used for connection
   validation per se, because it is not assumed random.

7.6.  Impact on TCP Header Size

   TCP-AO, using the initially required 96-bit MACs, uses a total of 16
   bytes of TCP header space [RFC5926].  TCP-AO is thus 2 bytes smaller
   than the TCP MD5 option (18 bytes).

   Note that the TCP option space is most critical in SYN segments,
   because flags in those segments could potentially increase the option
   space area in other segments.  Because TCP ignores unknown segments,
   however, it is not possible to extend the option space of SYNs
   without breaking backward compatibility.

   TCP's 4-bit data offset requires that the options end 60 bytes (15
   32-bit words) after the header begins, including the 20-byte header.
   This leaves 40 bytes for options, of which 15 are expected in current
   implementations (listed below), leaving at most 25 for other uses.
   TCP-AO consumes 16 bytes, leaving 9 bytes for additional SYN options
   (depending on implementation dependant alignment padding, which could
   consume another 2 bytes at most).

   o  SACK permitted (2 bytes) [RFC2018][RFC3517]

   o  Timestamps (10 bytes) [RFC1323]

   o  Window scale (3 bytes) [RFC1323]

   After a SYN, the following options are expected in current
   implementations of TCP:

   o  SACK (10bytes) [RFC2018][RFC3517] (18 bytes if D-SACK [RFC2883])

   o  Timestamps (10 bytes) [RFC1323]

   TCP-AO continues to consume 16 bytes in non-SYN segments, leaving a
   total of 24 bytes for other options, of which the timestamp consumes
   10.  This leaves 14 bytes, of which 10 are used for a single SACK
   block.  When two SACK blocks are used, such as to handle D-SACK, a
   smaller TCP-AO MAC would be required to make room for the additional
   SACK block (i.e., to leave 18 bytes for the D-SACK variant of the



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   SACK option) [RFC2883].  Note that D-SACK is not supportable in TCP
   MD5 in the presence of timestamps, because TCP MD5's MAC length is
   fixed and too large to leave sufficient option space.

   Although TCP option space is limited, we believe TCP-AO is consistent
   with the desire to authenticate TCP at the connection level for
   similar uses as were intended by TCP MD5.

7.7.  Connectionless Resets

   TCP-AO allows TCP resets (RSTs) to be exchanged provided both sides
   have established valid connection state.  After such state is
   established, if one side reboots, TCP-AO prevents TCP's RST mechanism
   from clearing out old state on the side that did not reboot.  This
   happens because the rebooting side has lost its connection state, and
   thus its traffic keys.

   It is important that implementations are capable of detecting
   excesses of TCP connections in such a configuration and can clear
   them out if needed to protect its memory usage [Ba10].  To protect
   against such state from accumulating and not being cleared out, a
   number of recommendations are made:

   >> Connections using TCP-AO SHOULD also use TCP keepalives [RFC1122].

   The use of TCP keepalives ensures that connections whose keys are
   lost are terminated after a finite time; a similar effect can be
   achieved at the application layer, e.g., with BGP keepalives
   [RFC4271].  Either kind of keepalive helps ensure the TCP state is
   cleared out in such a case; the alternative, of allowing
   unauthenticated RSTs to be received, would allow one of the primary
   vulnerabilities that TCP-AO is intended to prevent.

   Keepalives ensure that connections are dropped across reboots, but
   this can have a detrimental effect on some protocols.  Specifically,
   BGP reacts poorly to such connection drops, even if caused by the use
   of BGP keepalives; "graceful restart" was introduced to address this
   effect [RFC4724], and extended to support BGP with MPLS [RFC4781].
   As a result:

   >> BGP connections SHOULD require support for graceful restart when
   using TCP-AO.









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   We recognize that support for graceful restart is not always
   feasible.  As a result:

   >> When BGP without graceful restart is used with TCP-AO, both sides
   of the connection SHOULD save traffic keys in storage that persists
   across reboots and restore them after a reboot, and SHOULD limit any
   performance impacts that result from this storage/restoration.

7.8.  ICMP Handling

   TCP can be attacked both in band, using TCP segments, or out of band
   using ICMP.  ICMP packets cannot be protected using TCP-AO
   mechanisms; however, in this way, both TCP-AO and IPsec do not
   directly solve the need for protected ICMP signaling.  TCP-AO does
   make specific recommendations on how to handle certain ICMPs, beyond
   what IPsec requires, and these are made possible because TCP-AO
   operates inside the context of a TCP connection.

   IPsec makes recommendations regarding dropping ICMPs in certain
   contexts or requiring that they are endpoint authenticated in others
   [RFC4301].  There are other mechanisms proposed to reduce the impact
   of ICMP attacks by further validating ICMP contents and changing the
   effect of some messages based on TCP state, but these do not provide
   the level of authentication for ICMP that TCP-AO provides for TCP
   [Go10].  As a result, we recommend a conservative approach to
   accepting ICMP messages as summarized in [Go10]:

   >> A TCP-AO implementation MUST default to ignore incoming ICMPv4
   messages of Type 3 (destination unreachable), Codes 2-4 (protocol
   unreachable, port unreachable, and fragmentation needed -- 'hard
   errors'), and ICMPv6 Type 1 (destination unreachable), Code 1
   (administratively prohibited) and Code 4 (port unreachable) intended
   for connections in synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-
   WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT) that match MKTs.

   >> A TCP-AO implementation SHOULD allow whether such ICMPs are
   ignored to be configured on a per-connection basis.

   >> A TCP-AO implementation SHOULD implement measures to protect ICMP
   "packet too big" messages, some examples of which are discussed in
   [Go10].

   >> An implementation SHOULD allow ignored ICMPs to be logged.

   This control affects only ICMPs that currently require 'hard errors',
   which would abort the TCP connection [RFC1122].  This recommendation
   is intended to be similar to how IPsec would handle those messages,
   with an additional default assumed [RFC4301].



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8.  Obsoleting TCP MD5 and Legacy Interactions

   TCP-AO obsoletes TCP MD5.  As we have noted earlier:

   >> TCP implementations that support TCP MD5 MUST support TCP-AO.

   Systems implementing TCP MD5 only are considered legacy, and ought to
   be upgraded when possible.  In order to support interoperation with
   such legacy systems until upgrades are available:

   >> TCP MD5 SHOULD be supported where interactions with legacy systems
   are needed.

   >> A system that supports both TCP-AO and TCP MD5 MUST use TCP-AO for
   connections unless not supported by its peer, at which point it MAY
   use TCP MD5 instead.

   >> A TCP implementation MUST NOT use both TCP-AO and TCP MD5 for a
   particular TCP connection, but MAY support TCP-AO and TCP MD5
   simultaneously for different connections (notably to support legacy
   use of TCP MD5).

   The Kind value explicitly indicates whether TCP-AO or TCP MD5 is used
   for a particular connection in TCP segments.

   It is possible that MKTs could be augmented to support TCP MD5,
   although use of MKTs is not described in RFC 2385.

   It is possible to require TCP-AO for a connection or TCP MD5, but it
   is not possible to require 'either'.  When an endpoint is configured
   to require TCP MD5 for a connection, it must be added to all outgoing
   segments and validated on all incoming segments [RFC2385].  TCP MD5's
   requirements prohibit the speculative use of both options for a given
   connection, e.g., to be decided by the other end of the connection.

9.  Interactions with Middleboxes

   TCP-AO may interact with middleboxes, depending on their behavior
   [RFC3234].  Some middleboxes either alter TCP options (such as TCP-
   AO) directly or alter the information TCP-AO includes in its MAC
   calculation.  TCP-AO may interfere with these devices, exactly where
   the device modifies information TCP-AO is designed to protect.









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9.1.  Interactions with Non-NAT/NAPT Middleboxes

   TCP-AO supports middleboxes that do not change the IP addresses or
   ports of segments.  Such middleboxes may modify some TCP options, in
   which case TCP-AO would need to be configured to ignore all options
   in the MAC calculation on connections traversing that element.

   Note that ignoring TCP options may provide less protection, i.e., TCP
   options could be modified in transit, and such modifications could be
   used by an attacker.  Depending on the modifications, TCP could have
   compromised efficiency (e.g., timestamp changes), or could cease
   correct operation (e.g., window scale changes).  These
   vulnerabilities affect only the TCP connections for which TCP-AO is
   configured to ignore TCP options.

9.2.  Interactions with NAT/NAPT Devices

   TCP-AO cannot interoperate natively across NAT/NAPT (Network Address
   Port Translation) devices, which modify the IP addresses and/or port
   numbers.  We anticipate that traversing such devices may require
   variants of existing NAT/NAPT traversal mechanisms, e.g.,
   encapsulation of the TCP-AO-protected segment in another transport
   segment (e.g., UDP), as is done in IPsec [RFC2663][RFC3947].  Such
   variants can be adapted for use with TCP-AO, or IPsec with NAT
   traversal can be used instead of TCP-AO in such cases [RFC3947].

   An alternate proposal for accommodating NATs extends TCP-AO
   independently of this specification [To10].

10.  Evaluation of Requirements Satisfaction

   TCP-AO satisfies all the current requirements for a revision to TCP
   MD5, as summarized below [Ed07].

   1. Protected Elements

      A solution to revising TCP MD5 should protect (authenticate) the
      following elements.

      This is supported -- see Section 5.1.

      a. IP pseudoheader, including IPv4 and IPv6 versions.

         Note that optional coverage is not allowed because IP addresses
         define a connection.  If they can be coordinated across a
         NAT/NAPT, the sender can compute the MAC based on the received
         values; if not, a tunnel is required, as noted in Section 9.2.




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      b. TCP header.

         Note that optional port coverage is not allowed because ports
         define a connection.  If they can be coordinated across a
         NAT/NAPT, the sender can compute the MAC based on the received
         values; if not, a tunnel is required, as noted in Section 9.2.

      c. TCP options.

         Note that TCP-AO allows the exclusion of TCP options from
         coverage, to enable use with middleboxes that modify options
         (except when they modify TCP-AO itself).  See Section 9.

      d. TCP payload data.

   2. Option Structure Requirements

      A solution to revising TCP MD5 should use an option with the
      following structural requirements.

      This is supported -- see Section 5.1.

      a. Privacy.

         The option should not unnecessarily expose information about
         the TCP-AO mechanism.  The additional protection afforded by
         keeping this information private may be of little value, but
         also helps keep the option size small.

         TCP-AO exposes only the MKT IDs, MAC, and overall option length
         on the wire.  Note that short MACs could be obscured by using
         longer option lengths but specifying a short MAC length (this
         is equivalent to a different MAC algorithm, and is specified in
         the MKT).  See Section 2.2.

      b. Allow optional per connection.

         The option should not be required on every connection; it
         should be optional on a per-connection basis.

         This is supported because the set of MKTs can be installed to
         match some connections and not others.  Connections not
         matching any MKT do not require TCP-AO.  Further, incoming
         segments with TCP-AO are not discarded solely because they
         include the option, provided they do not match any MKT.






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      c. Require non-optional.

         The option should be able to be specified as required for a
         given connection.

         This is supported because the set of MKTs can be installed to
         match some connections and not others.  Connections matching
         any MKT require TCP-AO.

      d. Standard parsing.

         The option should be easily parseable, i.e., without
         conditional parsing, and follow the standard RFC 793 option
         format.

         This is supported -- see Section 2.2.

      e. Compatible with Large Windows and SACK.

         The option should be compatible with the use of the Large
         Windows and SACK options.

         This is supported -- see Section 7.6.  The size of the option
         is intended to allow use with Large Windows and SACK.  See also
         Section 1.3, which indicates that TCP-AO is 2 bytes shorter
         than TCP MD5 in the default case, assuming a 96-bit MAC.

   3. Cryptography requirements

      A solution to revising TCP MD5 should support modern cryptography
      capabilities.

      a. Baseline defaults.

         The option should have a default that is required in all
         implementations.

         TCP-AO uses a default required algorithm as specified in
         [RFC5926] and as noted in Section 5.1 of this document.

      b. Good algorithms.

         The option should use algorithms considered accepted by the
         security community, which are considered appropriately safe.
         The use of non-standard or unpublished algorithms should be
         avoided.





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         TCP-AO uses MACs as indicated in [RFC5926].  The KDF is also
         specified in [RFC5926].  The KDF input string follows the
         typical design (see [RFC5926]).

      c. Algorithm agility.

         The option should support algorithms other than the default, to
         allow agility over time.

         TCP-AO allows any desired algorithm, subject to TCP option
         space limitations, as noted in Section 2.2.  The use of a set
         of MKTs allows separate connections to use different
         algorithms, both for the MAC and the KDF.

      d. Order-independent processing.

         The option should be processed independently of the proper
         order, i.e., they should allow processing of TCP segments in
         the order received, without requiring reordering.  This avoids
         the need for reordering prior to processing, and avoids the
         impact of misordered segments on the option.

         This is supported -- see Sections 7.3, 7.4, and 7.5.  Note that
         pre-TCP processing is further required, because TCP segments
         cannot be discarded solely based on a combination of connection
         state and out-of-window checks; many such segments, although
         discarded, cause a host to respond with a replay of the last
         valid ACK, e.g., [RFC793].  See also the derivation of the SNE,
         which is reconstituted at the receiver using a demonstration
         algorithm that avoids the need for reordering (in Section 6.2).

      e. Security parameter changes require key changes.

         The option should require that the MKT change whenever the
         security parameters change.  This avoids the need for
         coordinating option state during a connection, which is typical
         for TCP options.  This also helps allow "bump in the stack"
         implementations that are not integrated with endpoint TCP
         implementations.

         Parameters change only when a new MKT is used.  See Section 3.

   4. Keying requirements.

      A solution to revising TCP MD5 should support manual keying, and
      should support the use of an external automated key management
      system (e.g., a protocol or other mechanism).




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      Note that TCP-AO does not specify an MKT management system.

      a. Intraconnection rekeying.

         The option should support rekeying during a connection, to
         avoid the impact of long-duration connections.

         This is supported by the use of IDs and multiple MKTs; see
         Section 3.

      b. Efficient rekeying.

         The option should support rekeying during a connection without
         the need to expend undue computational resources.  In
         particular, the options should avoid the need to try multiple
         keys on a given segment.

         This is supported by the use of the KeyID.  See Section 6.1.

      c. Automated and manual keying.

         The option should support both automated and manual keying.

         The use of MKTs allows external automated and manual keying.
         See Section 3.  This capability is enhanced by the generation
         of unique per-connection keys, which enables use of manual MKTs
         with automatically generated traffic keys as noted in Section
         5.2.

      d. Key management agnostic.

         The option should not assume or require a particular key
         management solution.

         This is supported by use of a set of MKTs.  See Section 3.

   5. Expected Constraints

      A solution to revising TCP MD5 should also abide by typical safe
      security practices.

      a. Silent failure.

         Receipt of segments failing authentication must result in no
         visible external action and must not modify internal state, and
         those events should be logged.

         This is supported - see Sections 7.3, 7.4, and 7.5.



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      b. At most one such option per segment.

         Only one authentication option can be permitted per segment.

         This is supported by the protocol requirements - see Section
         2.2.

      c. Outgoing all or none.

         Segments out of a TCP connection are either all authenticated
         or all not authenticated.

         This is supported - see Section 7.4.

      d. Incoming all checked.

         Segments into a TCP connection are always checked to determine
         whether their authentication should be present and valid.

         This is supported - see Section 7.5.

      e. Non-interaction with TCP MD5.

         The use of this option for a given connection should not
         preclude the use of TCP MD5, e.g., for legacy use, for other
         connections.

         This is supported - see Section 8.

      f. "Hard" ICMP discard.

         The option should allow certain ICMPs to be discarded, notably
         Type 3 (destination unreachable), Codes 2-4 (transport protocol
         unreachable, port unreachable, or fragmentation needed and IP
         DF field set), i.e., the ones indicating the failure of the
         endpoint to communicate.

         This is supported - see Section 7.8.

      g. Maintain TCP connection semantics, in which the socket pair
         alone defines a TCP association and all its security
         parameters.

         This is supported - see Sections 3 and 9.







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11.  Security Considerations

   Use of TCP-AO, like the use of TCP MD5 or IPsec, will impact host
   performance.  Connections that are known to use TCP-AO can be
   attacked by transmitting segments with invalid MACs.  Attackers would
   need to know only the TCP connection ID and TCP-AO Length value to
   substantially impact the host's processing capacity.  This is similar
   to the susceptibility of IPsec to on-path attacks, where the IP
   addresses and SPI would be visible.  For IPsec, the entire SPI space
   (32 bits) is arbitrary, whereas for routing protocols typically only
   the source port (16 bits) is arbitrary (typically with less than 16
   bits of randomness [La10]).  As a result, it would be easier for an
   off-path attacker to spoof a TCP-AO segment that could cause receiver
   validation effort.  However, we note that between Internet routers,
   both ports could be arbitrary (i.e., determined a priori out of
   band), which would constitute roughly the same off-path antispoofing
   protection of an arbitrary SPI.

   TCP-AO, like TCP MD5, may inhibit connectionless resets.  Such resets
   typically occur after peer crashes, either in response to new
   connection attempts or when data is sent on stale connections; in
   either case, the recovering endpoint may lack the connection key
   required (e.g., if lost during the crash).  This may result in
   timeouts, rather than a more responsive recovery after such a crash.
   Recommendations for mitigating this effect are discussed in Section
   7.7.

   TCP-AO does not include a fast decline capability, e.g., where a SYN-
   ACK is received without an expected TCP-AO and the connection is
   quickly reset or aborted.  Normal TCP operation will retry and
   timeout, which is what should be expected when the intended receiver
   is not capable of the TCP variant required anyway.  Backoff is not
   optimized because it would present an opportunity for attackers on
   the wire to abort authenticated connection attempts by sending
   spoofed SYN-ACKs without TCP-AO.

   TCP-AO is intended to provide similar protections to IPsec, but is
   not intended to replace the use of IPsec or IKE either for more
   robust security or more sophisticated security management.  TCP-AO is
   intended to protect the TCP protocol itself from attacks that TLS,
   sBGP/soBGP, and other data stream protection mechanisms cannot.  Like
   IPsec, TCP-AO does not address the overall issue of ICMP attacks on
   TCP, but does limit the impact of ICMPs, as noted in Section 7.8.

   TCP-AO includes the TCP connection ID (the socket pair) in the MAC
   calculation.  This prevents different concurrent connections using
   the same MKT (for whatever reason) from potentially enabling a
   traffic-crossing attack, in which segments to one socket pair are



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   diverted to attack a different socket pair.  When multiple
   connections use the same MKT, it would be useful to know that
   segments intended for one ID could not be (maliciously or otherwise)
   modified in transit and end up being authenticated for the other ID.
   That requirement would place an additional burden of uniqueness on
   MKTs within endsystems, and potentially across endsystems.  Although
   the resulting attack is low probability, the protection afforded by
   including the received ID warrants its inclusion in the MAC, and does
   not unduly increase the MAC calculation or MKT management.

   The use of any security algorithm can present an opportunity for a
   CPU Denial-of-Service (DoS) attack, where the attacker sends false,
   random segments that the receiver under attack expends substantial
   CPU effort to reject.  In IPsec, such attacks are reduced by the use
   of a large Security Parameter Index (SPI) and Sequence Number fields
   to partly validate segments before CPU cycles are invested validated
   the Integrity Check Value (ICV).  In TCP-AO, the socket pair performs
   most of the function of IPsec's SPI, and IPsec's Sequence Number,
   used to avoid replay attacks, isn't needed due to TCP's Sequence
   Number, which is used to reorder received segments (provided the
   sequence number doesn't wrap around, which is why TCP-AO adds the SNE
   in Section 6.2).  TCP already protects itself from replays of
   authentic segment data as well as authentic explicit TCP control
   (e.g., SYN, FIN, ACK bits) but even authentic replays could affect
   TCP congestion control [Sa99].  TCP-AO does not protect TCP
   congestion control from this last form of attack due to the
   cumbersome nature of layering a windowed security sequence number
   within TCP in addition to TCP's own sequence number; when such
   protection is desired, users are encouraged to apply IPsec instead.

   Further, it is not useful to validate TCP's Sequence Number before
   performing a TCP-AO authentication calculation, because out-of-window
   segments can still cause valid TCP protocol actions (e.g., ACK
   retransmission) [RFC793].  It is similarly not useful to add a
   separate Sequence Number field to TCP-AO, because doing so could
   cause a change in TCP's behavior even when segments are valid.

12.  IANA Considerations

   The TCP Authentication Option (TCP-AO) was assigned TCP option 29 by
   IANA action.

   This document defines no new namespaces.

   To specify MAC and KDF algorithms, TCP-AO refers to a separate
   document [RFC5926].





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13.  References

13.1.  Normative References

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
             Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
             Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
             Signature Option", RFC 2385, August 1998.

   [RFC2403] Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within ESP
             and AH", RFC 2403, November 1998.

   [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.

   [RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
             Extension to the Selective Acknowledgement (SACK) Option
             for TCP", RFC 2883, July 2000.

   [RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
             Conservative Selective Acknowledgment (SACK)-based Loss
             Recovery Algorithm for TCP", RFC 3517, April 2003.

   [RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol",
             RFC 4306, December 2005.

   [RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
             Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
             January 2007.

   [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
             Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4781] Rekhter, Y. and R. Aggarwal, "Graceful Restart Mechanism
             for BGP with MPLS", RFC 4781, January 2007.






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   [RFC5926] Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms for
             the TCP Authentication Option (TCP-AO)", RFC 5926, June
             2010.

13.2.  Informative References

   [Ba10]    Bashyam, M., Jethanandani, M., and A. Ramaiah
             "Clarification of sender behaviour in persist condition",
             Work in Progress, January 2010.

   [Bo07]    Bonica, R., Weis, B., Viswanathan, S., Lange, A., and O.
             Wheeler, "Authentication for TCP-based Routing and
             Management Protocols", Work in Progress, February 2007.

   [Bo09]    Borman, D., "TCP Options and MSS", Work in Progress, July
             2009.

   [Ed07]    Eddy, W., Ed., Bellovin, S., Touch, J., and R. Bonica,
             "Problem Statement and Requirements for a TCP
             Authentication Option", Work in Progress, July 2007.

   [Go10]    Gont, F., "ICMP Attacks against TCP", Work in Progress,
             March 2010.

   [La10]    Larsen, M. and F. Gont, "Transport Protocol Port
             Randomization Recommendations", Work in Progress, April
             2010.

   [Le09]    Lepinski, M. and S. Kent, "An Infrastructure to Support
             Secure Internet Routing", Work in Progress, October 2009.

   [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             April 1992.

   [RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
             for High Performance", RFC 1323, May 1992.

   [RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
             RFC 1948, May 1996.

   [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
             Hashing for Message Authentication", RFC 2104, February
             1997.

   [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
             Translator (NAT) Terminology and Considerations", RFC 2663,
             August 1999.




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   [RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
             Issues", RFC 3234, February 2002.

   [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
             Signature Option", RFC 3562, July 2003.

   [RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
             "Negotiation of NAT-Traversal in the IKE", RFC 3947,
             January 2005.

   [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
             Internet Protocol", RFC 4301, December 2005.

   [RFC4808] Bellovin, S., "Key Change Strategies for TCP-MD5", RFC
             4808, March 2007.

   [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", RFC
             4953, July 2007.

   [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [Sa99]    Savage, S., N. Cardwell, D. Wetherall, T. Anderson, "TCP
             Congestion Control with a Misbehaving Receiver", ACM
             Computer Communications Review, V29, N5, pp71-78, October
             1999.

   [SDNS88]  Secure Data Network Systems, "Security Protocol 4 (SP4)",
             Specification SDN.401, Revision 1.2, July 12, 1988.

   [To07]    Touch, J. and A. Mankin, "The TCP Simple Authentication
             Option", Work in Progress, July 2007.

   [To10]    Touch, J., "A TCP Authentication Option NAT Extension",
             Work in Progress, January 2010.

   [Wa05]    Wang, X., H. Yu, "How to break MD5 and other hash
             functions", Proc. IACR Eurocrypt 2005, Denmark, pp.19-35.

   [We05]    Weis, B., Appanna, C., McGrew, D., and A. Ramaiah, "TCP
             Message Authentication Code Option", Work in Progress,
             December 2005.









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14.  Acknowledgments

   This document evolved as the result of collaboration of the TCP
   Authentication Design team (tcp-auth-dt), whose members were
   (alphabetically): Mark Allman, Steve Bellovin, Ron Bonica, Wes Eddy,
   Lars Eggert, Charlie Kaufman, Andrew Lange, Allison Mankin, Sandy
   Murphy, Joe Touch, Sriram Viswanathan, Brian Weis, and Magnus
   Westerlund.  The text of this document is derived from a proposal by
   Joe Touch and Allison Mankin [To07] (originally from June 2006),
   which was both inspired by and intended as a counterproposal to the
   revisions to TCP MD5 suggested in a document by Ron Bonica, Brian
   Weis, Sriran Viswanathan, Andrew Lange, and Owen Wheeler [Bo07]
   (originally from September 2005) and in a document by Brian Weis
   [We05].

   Russ Housley suggested L4/application layer management of the master
   key tuples.  Steve Bellovin motivated the KeyID field.  Eric Rescorla
   suggested the use of TCP's Initial Sequence Numbers (ISNs) in the
   traffic key computation and SNEs to avoid replay attacks, and Brian
   Weis extended the computation to incorporate the entire connection ID
   and provided the details of the traffic key computation.  Mark
   Allman, Wes Eddy, Lars Eggert, Ted Faber, Russ Housley, Gregory
   Lebovitz, Tim Polk, Eric Rescorla, Joe Touch, and Brian Weis
   developed the master key coordination mechanism.

   Alfred Hoenes, Charlie Kaufman, Adam Langley, and numerous other
   members of the TCPM WG also provided substantial feedback on this
   document.

   This document was originally prepared using 2-Word-v2.0.template.dot.





















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Authors' Addresses

   Joe Touch
   USC/ISI
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695
   U.S.A.

   Phone: +1 (310) 448-9151
   EMail: touch@isi.edu
   URL:   http://www.isi.edu/touch

   Allison Mankin
   Johns Hopkins Univ.
   Baltimore, MD
   U.S.A.

   Phone: 1 301 728 7199
   EMail: mankin@psg.com
   URL:   http://www.psg.com/~mankin/

   Ronald P. Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, VA  20171
   U.S.A.

   EMail: rbonica@juniper.net























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