path: root/doc/rfc/rfc6013.txt

Independent Submission                                        W. Simpson
Request for Comments: 6013                                    DayDreamer
Category: Experimental                                      January 2011
ISSN: 2070-1721


                    TCP Cookie Transactions (TCPCT)

Abstract

   TCP Cookie Transactions (TCPCT) deter spoofing of connections and
   prevent resource exhaustion, eliminating Responder (server) state
   during the initial handshake.  The Initiator (client) has sole
   responsibility for ensuring required delays between connections.  The
   cookie exchange may carry data, limited to inhibit amplification and
   reflection denial of service attacks.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This is a contribution to the RFC Series, independently
   of any other RFC stream.  The RFC Editor has chosen to publish this
   document at its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see 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/rfc6013.

















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Copyright Notice

   Copyright (c) 2011 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.

   This document may not be modified, and derivative works of it may not
   be created, except to format it for publication as an RFC or to
   translate it into languages other than English.




































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Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................4
   2. Protocol Overview ...............................................4
      2.1. Message Summary (Simplified) ...............................6
      2.2. Compatibility and Transparency .............................7
      2.3. Fully Loaded Cookies .......................................7
      2.4. TCP Header Extension .......................................8
      2.5. <SYN> Option Handling ......................................9
   3. Protocol Details ................................................9
      3.1. TCP Cookie Option .........................................10
      3.2. TCP Cookie-Pair Standard Option ...........................10
      3.3. TCP Cookie-less Option ....................................11
      3.4. TCP Timestamps Extended Option ............................11
      3.5. Cookie Generation .........................................13
   4. Cookie Exchange ................................................16
      4.1. Initiator <SYN> ...........................................16
      4.2. Responder <SYN,ACK(SYN)> ..................................17
      4.3. Initiator <ACK(SYN)> ......................................17
      4.4. Responder <ACK> ...........................................18
      4.5. Simultaneous Open .........................................18
   5. Accelerated Close ..............................................19
      5.1. Initiator Close ...........................................20
      5.2. Responder Close ...........................................20
   6. Accelerated Open ...............................................21
      6.1. Initiator <SYN> Data ......................................21
      6.2. Responder <SYN,ACK(SYN)> Data .............................22
      6.3. Initiator <ACK(SYN)> Data .................................23
      6.4. Responder <ACK> Data ......................................24
   7. Advisory Reset .................................................24
   8. Interactions with Other Options ................................24
      8.1. TCP Selective Acknowledgment ..............................25
      8.2. TCP Timestamps ............................................25
      8.3. TCP Extensions for Transactions ...........................25
      8.4. TCP MD5 Signature .........................................25
      8.5. TCP Authentication ........................................25
   9. History ........................................................26
   10. Acknowledgments ...............................................27
   11. IESG Considerations ...........................................27
   12. Operational Considerations ....................................28
   13. Security Considerations .......................................28
   Appendix A. Example Headers .......................................30
      A.1. Example <SYN> Options .....................................30
      A.2. Example <ACK(SYN)> with Sack ..............................31
      A.3. Example <ACK(SYN)> with 64-bit Timestamps .................32
   Normative References ..............................................33
   Informative References ............................................34



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1.  Introduction

   TCP Cookie Transactions (TCPCT) provide a cryptologically secure
   mechanism to guard against simple flooding attacks sent with bogus IP
   [RFC791] Sources or TCP [RFC793] Ports.  The initial TCP <SYN>
   exchange is vulnerable to forged IP Addresses, predictable Ports, and
   discoverable Sequence Numbers [Morris1985] [Gont2009].  (See also
   [RFC2827], [RFC3704], and [RFC4953].)

   During connection establishment, the cookie (nonce) exchange
   negotiates elimination of Responder (server) state.  These cookies
   are later used to inhibit premature closing of connections, and
   reduce retention of state after the connection has terminated.

   The cookie pair is much too large to fit with the other recommended
   options in the maximal 60 byte TCP header (40 bytes of option space).
   A successful option exchange signals availability of the TCP header
   extension, adding space for additional options.

   Also, implementations may optionally exchange limited amounts of
   transaction data during the initial cookie exchange, reducing network
   latency and host task context switching.

   Finally, implementations may optionally rapidly recycle prior
   connections.  For otherwise stateless applications, this
   transparently facilitates persistent connections and pipelining of
   requests over each connection.

   Many of these ideas have been previously proposed in one form or
   another (see History and Acknowledgments sections).  This
   specification integrates these improvements into a coherent whole.
   Further motivation and rationale were detailed in [MSV2009].

1.1.  Terminology

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

   byte     An 8-bit quantity; also known as "octet" in standardese.

2.  Protocol Overview

   The TCPCT extensions consist of several simple phases:

   1. Each party passes a "cookie" to the other.  Due to limited space,
      only the most basic options are included.




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      The Cookie option also indicates that optional <SYN> data is
      acceptable.  This data MAY be ignored by either party.

      A Responder that understands the Cookie option remains stateless.

   2. During the remainder of the standard TCP three-way handshake, the
      Timestamps and Cookie-Pair options guard the exchange.

      Other options present in the original <SYN> that were successfully
      returned in the <SYN,ACK(SYN)> MUST be included with the
      <ACK(SYN)>.  Additional options MAY also be included as desired.

      As there is no Responder state, it has no record of acknowledging
      previous data.  Any optional <SYN> data MUST be retransmitted.

      Upon verification of the Timestamps and Cookie-Pair, the Responder
      creates its Transport Control Block (TCB) [RFC793].

      Note that the Responder returns the Cookie-Pair with its initial
      data, but subsequent data segments need only the Timestamps.

   3. During close (or reset) of the TCP connection, the Timestamps and
      Cookie-Pair options guard the exchange.

      Upon verification of the Timestamps and Cookie-Pair, the Responder
      removes its TCB.

   The sequence of messages is summarized in the diagram below.























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2.1.  Message Summary (Simplified)

   Initiator                            Responder
   =========                            =========
   <SYN>                          ->
   base options
   Timestamps
   Cookie
   [request data]
                                   <-   <SYN,ACK(SYN)>
                                        base options
                                        Timestamps
                                        Cookie
                                        [response data]
                                        (stateless)

   <ACK(SYN)>                     ->
   full options
   Timestamps
   Cookie-Pair
   [Sack(response)]
   data
                                   <-   <ACK>
                                        full options
                                        Timestamps
                                        Cookie-Pair
                                        data
                                        (TCB state created)
                                   <-   <ACK>
                                        Timestamps
                                        data

                                   <-   <FIN,ACK>
                                        Timestamps
                                        Cookie-Pair
   <FIN,ACK(FIN)>                 ->
   Timestamps
   Cookie-Pair
                                   <-   <ACK(FIN)>
                                        Timestamps
                                        Cookie-Pair
                                        (TCB state removed)
   TIME-WAIT








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2.2.  Compatibility and Transparency

      It is usually better that data arrive slowly, than not at all.

   Many/most unmanaged middleboxes [RFC3234] (such as stateless
   firewalls, load balancers, intrusion detection systems, or network
   address translators [RFC3022]) cannot carry transport traffic other
   than TCP and UDP.

   Every TCP implementation MUST ignore without error any TCP option it
   does not implement ([RFC1122] section 4.2.2.5).  In a study of the
   effects of middleboxes on transport protocols [MAF2004], the vast
   majority of modern TCP stacks correctly handle unknown TCP options.
   But it is still prudent to follow the [RFC793] "general principle of
   robustness: be conservative in what you do, be liberal in what you
   accept from others."

   Therefore, for each of the extensions defined here, an extension
   option will be sent in a <SYN,ACK(SYN)> segment only after the
   corresponding option was received in the original <SYN> segment.

   Furthermore, TCP options will be sent on later segments only after an
   exchange of options has indicated that both parties understand the
   extension (see [RFC1323] [rfc1323bis] and its antecedents).

   Unfortunately, not all middleware adheres to these long-standing
   requirements.  Instead, unknown <SYN> options are copied to the
   <SYN,ACK(SYN)>.  This is indistinguishable from a Monkey in the
   Middle (MITM) reflection attack.

2.3.  Fully Loaded Cookies

             One Kind to aid them all, One Kind to find them,
          One Kind to hold them all and in the header bind them.

   The cookie exchange provides a singular opportunity to extend TCP
   with backward compatibility.  Semantics for the option have been
   "overloaded" with a baker's dozen of capabilities and facilities.

   A. First and foremost, the cookie exchange improves operational
      security for vulnerable servers against flooding attacks.  The
      cookie exchange indicates that the Responder (server) will discard
      its initial state.  All other semantics are subordinate.

   B. Together with Sequence and Timestamp values, Cookie values protect
      against insertion and reflection attacks.

   C. Cookie values allow applications to detect replay attacks.



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   D. Cookie values MAY be used as an index or nonce for application
      security protocols.  This facility is beyond the scope of this
      specification.

   E. The <SYN> and <SYN,ACK(SYN)> MAY carry application data.  This
      feature is entirely optional, and data is not guaranteed to pass
      successfully through middleware.  Nor are the parties guaranteed
      to process this data without changes to the Application Program
      Interface (API).  Such changes are beyond the scope of this
      specification.

   F. The size of the cookies precludes most other options in the
      standard TCP header space.  The cookie exchange negotiates TCP
      header extension.

   G. The cookie exchange and resulting TCP header extension permit
      negotiation of larger 64-bit (or 128-bit) Timestamps for paths
      with large bandwidth-delay products.

   H. TCP header extension frees some space for additional options.

   I. Previously SYN-only options can be updated.

   J. The cookie exchange indicates agreement to use accelerated close.

   K. The cookie exchange indicates agreement that only the Initiator
      (client) handles TIME-WAIT state.

   L. The Timestamps and Cookie-Pair combination inhibits third parties
      from disrupting communications with <FIN> and <RST>.

   M. The Timestamps and Cookie-Pair combination facilitates rapid reuse
      of the TCP Source Port with a common destination.

2.4.  TCP Header Extension

   Once the Cookie option has been successfully exchanged, TCP header
   extension is permitted.  The Timestamps extended option (defined
   below) indicates the presence of the header extension.

   Validation of known timestamp values protects against data corruption
   by misbehaving middleboxes.









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2.5.  <SYN> Option Handling

   As the Responder retains no TCB state after the initial TCP <SYN>
   exchange, all options present in the original <SYN> MUST be repeated.

   For example, an option defined in the [RFC793] original specification
   -- Maximum Segment Size (MSS) -- previously appeared only in a <SYN>
   bearing segment (including <SYN,ACK(SYN)>).  If present, MSS will be
   repeated in the Initiator <ACK(SYN)>, together with any additional
   options.

   Generally, the Initiator MAY propose SYN-only options -- such as MSS
   -- anytime both Timestamps and Cookie-Pair options are present.
   These options are treated the same as with an original <SYN>.  The
   Responder acknowledges using a subsequent <ACK> segment containing
   both Timestamps and Cookie-Pair options (similar to <SYN,ACK(SYN)>
   processing).

   This facility allows previously SYN-only options to be updated from
   time to time.  They take effect upon receipt.

   However, <ACK> segments without data will not be delivered reliably.
   Any otherwise SYN-only options sent without data MUST be
   retransmitted with successive segments until sent with data (or
   <FIN>), and an <ACK> is received.

3.  Protocol Details

   Another solution [RFC5452] describes use of an unpredictable Source
   Port.  That is RECOMMENDED by this specification.  See [RFC6056] for
   further information.

   An earlier solution [RFC1948] describes an unpredictable Initial
   Sequence Number (ISN).  That is REQUIRED by this specification.

   Support for the (32-bit) TCP Timestamps Option [RFC1323] is REQUIRED.
   A TSoffset SHOULD be generated per connection [GO2010].  The Don't
   Fragment (DF) bit MUST be set in the IP (v4) header.

   The TCP User Timeout Option [RFC5482] is RECOMMENDED.

   Only one instance is permitted of any of the Cookie, Cookie-less, or
   Cookie-Pair option(s).  Segments with duplicative or mutually
   exclusive options MUST be silently discarded.

   For examples, see Appendix A.





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3.1.  TCP Cookie Option

                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |      Kind     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                            Cookie                             ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Kind             1 byte: constant 253 (experimental).

   Length           1 byte: range 10 to 18 (bytes); limited by remaining
                    space in the options field.  The number MUST be
                    even; the cookie is a multiple of 16 bits.

   Cookie           8 to 16 bytes (Length - 2): an unpredictable value.

   Options with invalid Length values MUST be ignored.  The minimum
   Cookie size is 64 bits.  If there is not sufficient space for a
   64-bit cookie, this option MUST NOT be used.

   The Responder Cookie MUST be the same size as the Initiator Cookie.
   The cookie pair is a multiple of 32 bits.

   Although the diagram shows a cookie aligned on 32-bit boundaries,
   that is not required.

3.2.  TCP Cookie-Pair Standard Option

                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |      Kind     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Kind             1 byte: constant 253 (experimental).

   Length           1 byte: range 18 to 34 (bytes).  The number MUST be
                    even; the cookie pair is a multiple of 32 bits.

   Initiator-Cookie 8 to 16 bytes, from the original <SYN>.



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   Responder-Cookie 8 to 16 bytes, from the <SYN,ACK(SYN)>.

   The Cookie-Pair standard option only appears after the Timestamps
   extended option (below).

   Options with invalid Length values MUST be ignored.  As the minimum
   Initiator-Cookie size is 64 bits, the minimum cookie pair is 128 bits
   (64 bits followed by 64 bits), while the maximum is 256 bits (128
   bits followed by 128 bits).

3.3.  TCP Cookie-less Option

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Kind     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Kind             1 byte: constant 253 (experimental).

   Length           1 byte: constant 2 (bytes).  This distinguishes the
                    option from other Cookie options.

   Although no cookie is attached, this indicates that other features of
   this specification are available, including TCP header extension,
   Accelerated Close, Accelerated Open, and Advisory Reset.  This is
   intended for use with TCP authentication options, beyond the scope of
   this specification.

3.4.  TCP Timestamps Extended Option

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Kind     |    Length     |    Extend     |    R    |  S  |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   ~                           TS Value                            ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                         TS Echo Reply                         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Kind             1 byte: constant 254 (experimental).

   Length           1 byte: constant 4 (bytes).







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   Extend           1 byte: range 9 to 255; the data offset (in 32-bit
                    words) following the standard TCP header.  Note this
                    value MUST include the timestamp pair indicated by
                    (S)ize.

   (R)eserved       5 bits: default zero.  Reserved for future use.

   (S)ize           3 bits:

                    1. 32-bit timestamps.

                    2. 64-bit timestamps.

                    4. 128-bit timestamps.

                    Other values are beyond the scope of this
                    specification.

   TS Value         4, 8, or 16 bytes.  The current value of the
                    timestamp for the sender.

   TS Echo Reply    4, 8, or 16 bytes.  A copy of the most recently
                    received TS Value.

   The full timestamp pair follows the TCP header (indicated by +=+
   delimiters) and maintains 32-bit alignment.

   This TCP header extension is ignored for sequence number
   computations.  The Sequence Number of the first byte of segment data
   will be the Initial Sequence Number (ISN) plus one (1) for the <SYN>.

   Every TCPCT implementation MUST recognize a Timestamps extended
   option.  The larger 64-bit (or 128-bit) timestamps only appear in an
   extended option.

   Segments with invalid Extend values MUST be silently discarded.

   Only one instance is permitted of either the (32-bit) Timestamps
   standard option or this Timestamps extended option.  Segments with
   duplicative or mutually exclusive options MUST be silently discarded.

   Implementation Notes:

      Serendipitous alignment allows simple loads and stores, instead of
      slower byte by byte iterations.






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      When the TCP header is aligned on a 32-bit boundary and this is
      the only option, the timestamps in the extended header SHOULD be
      aligned on a 64-bit boundary.  For both 32-bit and 64-bit
      timestamps, any data following the extended header will be aligned
      on a 64-bit boundary.

      However, the 128-bit timestamps are not 128-bit aligned.

3.5.  Cookie Generation

   The technique by which a party generates a cookie is implementation
   dependent.  The method chosen must satisfy some basic requirements:

   1. The cookie MUST depend on the specific parties.  This prevents an
      attacker from obtaining a cookie using a real IP address and TCP
      port, and then using it to swamp the victim with requests from
      randomly chosen IP addresses or ports.

   2. It MUST NOT be possible for anyone other than the issuing entity
      to generate cookies that will be accepted by that entity.  This
      implies that the issuing entity will use local secret information
      in the generation and subsequent verification of a cookie.  It
      must not be possible to deduce this secret information from any
      particular cookie.

   3. The cookie generation and verification methods MUST be fast to
      thwart attacks intended to sabotage CPU resources.

   A recommended technique is to use a cryptographic hashing function.

   An incoming cookie can be verified at any time by regenerating it
   locally from values contained in the incoming datagram and the local
   secret random value.

3.5.1.  Initiator Cookie

   The Initiator secret value that affects its cookie SHOULD change for
   each new exchange, and is thereafter internally cached per TCB.  This
   provides improved synchronization and protection against replay
   attacks.

   An alternative is to cache the cookie instead of the secret value.
   Incoming cookies can be compared directly without the computational
   cost of regeneration.







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   It is RECOMMENDED that the cookie be calculated over the secret
   value, the IP Source and Destination addresses, the TCP Source and
   Destination ports, and any (optional) Initiator <SYN> segment data.

   Implementation Notes:

      Although the recommendation includes the TCP Source Port, this is
      very implementation specific.  For example, it might not be
      included when the value is constant or unknown.

      Likewise, segment data might not be included directly.  For
      example, a pointer to the data could be included instead, with
      care taken to ensure the pointer changes anytime the data changes.

      However, it is important that the implementation protect mutually
      suspicious users of the same system from generating the same
      cookie.

3.5.2.  Responder Cookie

   The Responder secret value that affects its cookies remains the same
   for many different Initiators.  However, this secret SHOULD be
   changed periodically to limit the time for use of its cookies
   (typically each 600 seconds).

   The Responder-Cookie calculation MUST include its own TCP Sequence
   and Acknowledgment Numbers (after updating values), its own TCP
   Timestamps value, and the Initiator-Cookie value.  This provides
   improved synchronization and protection against replay attacks.

   It is RECOMMENDED that the cookie be calculated over the secret
   value, the IP Source and Destination addresses, its own TCP
   Destination Port (that is, the incoming Source Port), and the
   required values (above), followed by the secret value again.

   The cookie is not cached per Initiator to avoid saving state during
   the initial TCP <SYN> exchange.  On receipt of a TCP <ACK(SYN)>, the
   Responder regenerates its cookie for validation.

   Implementation Notes:

      Although the recommendation does not include the TCP Source Port,
      this is very implementation specific.  It might be successfully
      included in some variants.

      The Responder Cookie depends on the TCP Sequence and
      Acknowledgment Numbers as they will appear for future
      verification.  The Sequence Number will be the Initial Sequence



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      Number (ISN) plus one (1) for its <SYN> that will be acknowledged.
      The Acknowledgment Number will be the Initial Sequence Number
      (ISN) plus one (1) for the <SYN> that it is now acknowledging.

      The (32-bit) TCP Timestamps standard option MAY change to the
      larger 64-bit (or 128-bit) extended form; only the least
      significant 32 bits are included.  The Initiator Timestamp field
      value MAY increment during the exchange; it MUST NOT be included.

      The secret value is included twice to better protect against pre-
      calculated attacks using substitutions for variable length data.
      Some examples using this technique are IP-MAC and H-MAC, and it is
      likely that existing code could be shared.

      The Responder SHOULD designate a (fixed or randomly selected) bit
      of its cookie to distinguish each changed secret value.  The bit
      is set to a (fixed or randomly selected) constant 0 or 1, and
      checked upon receipt before further verification.  This ensures
      that only one verification calculation is necessary (on average)
      during Denial of Service (DoS) attacks.

      If a Responder Cookie is identical to the Initiator Cookie, the
      Responder SHOULD change one or more bits of its cookie to prevent
      its accidental appearance as a reflection attack.

3.5.3.  Responder Secret Value

   Each Responder maintains up to two secret values concurrently for
   efficient secret rollover.  Each secret value has 4 states:

   Generating
      Generates new Responder-Cookies, but not yet used for primary
      verification.  This is a short-term state, typically lasting only
      one Round Trip Time (RTT).

   Primary
      Used both for generation and primary verification.

   Retiring
      Used for verification, until the first failure that can be
      verified by the newer Generating secret.  At that time, this
      cookie's state is changed to Secondary, and the Generating
      cookie's state is changed to Primary.  This is a short-term state,
      typically lasting only one RTT.







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   Secondary
      Used for secondary verification, after primary verification
      failures.  This state lasts no more than twice the Maximum Segment
      Lifetime (2MSL).  Then, the secret is discarded.

   Implementation Notes:

      Care MUST be taken to ensure that any expired secrets are promptly
      wiped from memory, and secrets are never saved to external
      storage.

      The first secret after initialization begins in Primary state.
      The system might have shutdown and restarted rapidly during the
      previous first secret.  Thus, the first secret MUST be partially
      time dependent, to ensure that it differs from previous first
      secrets, usually by appending a time to lengthen the first secret.
      Those that are not the first secret SHOULD NOT include the time.

      At the same time, there is no TCP TIME-WAIT requirement before
      accepting connections, and there may be pent up demand for a busy
      service.  Also, there may be outstanding datagrams attempting to
      complete an earlier cookie exchange.  The first secret is likely
      to be the weakest, as no recent entropy has been included.

      Therefore, while terminating outstanding exchanges with the first
      secret, a new Generating secret SHOULD be created after no more
      than one Maximum Segment Lifetime (1MSL).  Subsequent secrets
      SHOULD be generated at the usual rate (typically 600 seconds).

      The implementation SHOULD continually gather additional entropy
      from checksums, cookies, timestamps, and packet arrival timing.

4.  Cookie Exchange

   A successful option exchange signals availability of additional
   features.

4.1.  Initiator <SYN>

   The Cookie exchange MAY be initiated at any time, limited only by the
   frequency of the timestamp clock.

   If the TCB exists from a prior (or ongoing) connection, the timestamp
   MUST be incremented in the option.

   The Initiator generates its unpredictable cookie value, and includes
   the Cookie option.




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   During the initial exchange, the Initiator is solely responsible for
   retransmission.  Although the cookie and sequence have not changed,
   each retransmission appears to the Responder as another original
   <SYN>.

   Implementation Notes:

      Sending the <SYN> SHOULD NOT affect any existing TCB.  This allows
      an additional RTT for duplicate or out-of-sequence segments to
      drain.

      The new TCB information SHOULD be temporarily cached until a valid
      matching <SYN,ACK(SYN)> arrives.  Then, any old TCB values are
      replaced.

4.2.  Responder <SYN,ACK(SYN)>

   Upon receipt of the <SYN> with a Cookie option, the Responder
   determines whether there are sufficient resources to begin another
   connection.

   If the TCB exists from a prior (or ongoing) connection, the timestamp
   MUST be incremented in the option.

   Each Sequence Number MUST be randomized [RFC1948].

   The Responder generates its unpredictable cookie value, and includes
   the Cookie option.

   As the Responder retains no TCB state, retransmission timers are not
   available.  Arrival of an Initiator's retransmission appears to be an
   original <SYN> transmission.  There are no differences in processing.

   Implementation Notes:

      Sending the <SYN,ACK(SYN)> MUST NOT affect any existing TCB.  This
      allows an additional RTT for duplicate or out-of-sequence segments
      to drain.

      This also inhibits third parties from disrupting communications.

4.3.  Initiator <ACK(SYN)>

   Upon receipt of the <SYN,ACK(SYN)> with a Cookie option, the
   Initiator validates its cookie, timestamp, and corresponding
   Acknowledgment Number.  The existing TCB is updated as necessary.





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   All Initiator <SYN> options are always retransmitted on this first
   <ACK(SYN)>, allowing the Responder to validate its cookie and
   establish its state.

   This segment contains both Timestamps and Cookie-Pair options.

   The Initiator sends the Timestamps extended option with an
   appropriate Size -- chosen by a configurable parameter, or
   automatically based on its analysis of the bandwidth-delay product
   discovered through the RTT of its <SYN> timestamp.  When the chosen
   Size is greater than 32 bits, the Initiator adds a random prefix to
   its own timestamp, and a random prefix to the Responder timestamp
   echo reply.

   Implementation Notes:

      A Responder Cookie identical to the Initiator Cookie MUST be
      discarded.  This is usually an indication of a Monkey in the
      Middle (MITM) reflection attack or a seriously misconfigured
      network, and SHOULD be logged.

4.4.  Responder <ACK>

   Upon receipt of the <ACK(SYN)> with a Cookie-Pair option, the
   Responder validates its cookie, timestamp, and corresponding
   Acknowledgment Number, and establishes state for the connection.  Any
   existing TCB is updated as necessary.

   This segment contains both Timestamps and Cookie-Pair options.

   However, the Responder MAY refuse to negotiate the larger 64-bit (or
   128-bit) Timestamps extended option by returning the least
   significant bits in a smaller Timestamps extended option.

   Implementation Notes:

      An <ACK(SYN)> that fails to validate MUST be discarded, and SHOULD
      be logged.

4.5.  Simultaneous Open

   TCP allows two parties to simultaneously initiate the connection.
   Both parties send and receive an original <SYN> without an
   intervening <SYN,ACK(SYN)> (see [RFC793] section 3.4 and Figure 8).
   Each party receives a Cookie for a <Source Address, Source Port,
   Destination Address, Destination Port> connection that has also
   issued a Cookie.




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   This condition will be unusual.  The Source Port SHOULD be randomized
   [RFC5452], and SHOULD be chosen to differ from the Destination Port.
   In particular, the Source Port SHOULD be greater than 1024,
   preventing intervening network equipment from incorrectly classifying
   the return traffic.  The Destination Port is most likely to be a
   well-known port less than 1024 [RFC3232].

   In the event that these protections are insufficient, the conflict is
   resolved in an orderly fashion:

   a. The lesser TCP Port number becomes the Responder;

   b. The lesser IP Address becomes the Responder;

   c. The lesser Cookie becomes the Responder;

   d. All of the above being equal, there is an egregiously insufficient
      source of randomness, but both Initiators are probably present on
      the same host: the lesser TCB memory address becomes the
      Responder.

   The Initiator silently discards the simultaneous <SYN>.  The
   Responder revises its Cookie option, and sends the <SYN,ACK(SYN)> as
   usual, but without removing its existing TCB.

   Implementation Notes:

      This is usually an indication of a Monkey in the Middle (MITM)
      reflection attack or a seriously misconfigured network, and SHOULD
      be logged.

5.  Accelerated Close

   Support for accelerated close is REQUIRED.  Accelerated close relies
   on the presence of cookies and timestamps.  This provides improved
   synchronization and protection against replay attacks.

   Either party MAY close with <FIN> at any time.  This <FIN> SHOULD be
   sent with the final data segment.

   This segment contains both Timestamps and Cookie-Pair options.

   When all segments preceding the <FIN> have been processed and
   acknowledged, each party SHOULD acknowledge the <FIN>.

   In general, <FIN> is treated as advisory.  A persistent connection
   can be rapidly re-established.  This also inhibits third parties from
   disrupting communications.



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   Rapidly closing the connection expedites removing Responder state.
   Any <FIN> bearing segment SHOULD terminate delayed <ACK> [RFC5681].
   Retransmit at the latest Timestamps estimated Smoothed Round Trip
   Time (SRTT).  Backoff SHOULD NOT be used for <FIN> bearing
   retransmissions [RFC2988].

   As the Responder retains no TCB state after closing, a successful
   option exchange signals the Initiator will be responsible for
   handling TIME-WAIT state.  (For previous proposal and rationale, see
   [FTY1999] section 3.)

   A new Cookie exchange MAY be initiated at any time.  This facilitates
   persistent connections through intervening network equipment.

5.1.  Initiator Close

   Upon receipt of the Initiator <FIN> (and verification of the
   Timestamps and Cookie-Pair options), the Responder sends its
   <FIN,ACK(FIN)> unless there is additional data pending.  In the
   latter case, the <FIN> is ignored until the data has been processed
   and acknowledged.

   Upon receipt of the Responder <FIN,ACK(FIN)> (and verification of the
   Timestamps and Cookie-Pair options), the Initiator sends its final
   <ACK(FIN)> unless there is additional data pending.  The Initiator
   enters TIME-WAIT state.

   This segment contains both Timestamps and Cookie-Pair options.

   Upon receipt of the Initiator <ACK(FIN)> (and verification of the
   Timestamps and Cookie-Pair options), the Responder removes its TCB.

   Upon arrival of more data prompting a new Cookie exchange, the
   Initiator SHOULD NOT send a final <ACK(FIN)> and/or SHOULD NOT wait
   the remaining TIME-WAIT interval.  Any existing TSoffset SHOULD be
   incremented.  TSoffset will be removed (with the TCB itself) at the
   conclusion of a future TIME-WAIT state.

5.2.  Responder Close

   Upon receipt of the Responder <FIN> (and verification of the
   Timestamps and Cookie-Pair options), the Initiator sends its
   <FIN,ACK(FIN)> unless there is additional data pending.  In the
   latter case, the <FIN> is ignored until the data has been processed
   and acknowledged.






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   Upon receipt of the Initiator <FIN,ACK(FIN)> (and verification of the
   Timestamps and Cookie-Pair options), the Responder sends its final
   <ACK(FIN)> and removes its TCB.

   This segment contains both Timestamps and Cookie-Pair options.

   If the Responder's final <ACK(FIN)> is lost, the Responder is likely
   to send a <RST> (as the Responder retains no TCB state).  This
   distinguished <RST> SHOULD copy both Timestamps and Cookie-Pair
   options.

   Upon receipt of the Responder's final <ACK(FIN)> (and verification of
   the Timestamps and Cookie-Pair options), the Initiator enters TIME-
   WAIT state.

   Upon arrival of more data prompting a new Cookie exchange, the
   Initiator SHOULD NOT send a <FIN,ACK(FIN)> and/or SHOULD NOT wait the
   remaining TIME-WAIT interval.  Any existing TSoffset SHOULD be
   incremented.  TSoffset will be removed (with the TCB itself) at the
   conclusion of a future TIME-WAIT state.

6.  Accelerated Open

   Support for accelerated open is OPTIONAL.

   When an application is capable of idempotent transactions (such as a
   query that returns a consistent result or service response heading),
   the application sets the appropriate limit separately for each port
   or connection.  Applications are responsible for ensuring that
   retransmissions do not cause duplication of data.

   This facility allows single data segment transactions without
   establishing TCB state at the Responder (server).  For longer
   transactions, a short look-ahead of upcoming data allows the
   Initiator (client) to select alternatives for further processing.

6.1.  Initiator <SYN> Data

   By default, the Initiator <SYN> does not contain data.  The
   application sets the TCP_SYN_DATA_LIMIT to indicate that the <SYN>
   MAY be sent with data.

   The Responder Maximum Segment Size (MSS) is unknown, and the default
   MSS (536 bytes) MUST be used instead ([RFC1122] section 4.2.2.6).
   This is further reduced by the total length of the TCP options (in
   this case, commonly 496 bytes).  Applications MAY specify a shorter
   limit.




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   If the data will not entirely fit within the initial segment, data
   MUST NOT be sent until after the Responder's <SYN,ACK(SYN)> is
   received.

   Unlike T/TCP [RFC1644], <FIN> SHOULD NOT be sent with <SYN> data.
   This facilitates persistent connections.

   Likewise, <PSH> SHOULD NOT be set.  Although the application might
   use push to indicate that its data is ready to send, the push is
   implied for <SYN> data segments.

   During the initial exchange, the Initiator is solely responsible for
   retransmission.  Although the cookie and sequence have not changed,
   each retransmission appears to the Responder as another original
   <SYN>.

   Implementation Notes:

      Initiator <SYN,FIN> with the Cookie option and no segment data is
      permitted in a test environment.  This combination SHOULD be
      silently discarded.

      Initiator <SYN,FIN> with both the Cookie option and segment data
      is similar to T/TCP [RFC1644].  However, whenever the Responder
      <SYN,ACK(SYN),FIN> has been sent with data (there is no further
      data expected), TCB state has not been saved at the Responder.
      There is no need to send <FIN> to close the connection.

6.2.  Responder <SYN,ACK(SYN)> Data

   By default, the Responder <SYN,ACK(SYN)> does not contain data.  The
   application sets the TCP_SYN_ACK_DATA_LIMIT to indicate that the
   <SYN,ACK(SYN)> MAY be sent with data.

   Segment data is limited to the Maximum Transmission Unit (MTU).
   Applications MAY specify a shorter limit to prevent spoofed
   amplification and reflection attacks [RFC5358].

   Upon receipt of the <SYN> with a Cookie option, the Responder MAY
   process any data present.  If the initial data is not accepted, the
   Acknowledgment Number will be the received Sequence Number plus one
   (1) for the <SYN>.

   If the segment data is the entire response (there is no further data
   expected), <FIN> MAY be set.






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   However, <PSH> SHOULD NOT be set.  Although the application might use
   push to indicate that its data is ready to send, the push is implied
   for <FIN> data segments (see [RFC793] section 3.7, page 41).

   As the Responder retains no TCB state, retransmission timers are not
   available.  Arrival of an Initiator's retransmission appears to be an
   original <SYN> transmission.  There are no differences in processing.

   Implementation Notes:

      The Responder Cookie depends on the TCP Sequence and
      Acknowledgment Numbers after processing <SYN>.  Therefore, neither
      will include data.

6.3.  Initiator <ACK(SYN)> Data

   Upon receipt of the <SYN,ACK(SYN)> with a Cookie option, the
   Initiator MAY process any data present.  In this case, the internal
   RCV.NXT is advanced to provide at-most-once semantics.

   If the segment data is the entire response (there is no further data
   expected), the Initiator enters TIME-WAIT state.

   Otherwise, original <SYN> data is retransmitted in <ACK(SYN)>, as its
   processing is optional.  The Acknowledgment Number will be the
   received Sequence Number plus one (1) for the <SYN>.  The Sequence
   Number will be the Initial Sequence Number (ISN) plus one (1) for the
   <SYN>.

   Unlike T/TCP [RFC1644], there is no implicit acknowledgment.

   If the Selective Acknowledgment (Sack) option [RFC2018] has been
   successfully negotiated, a short Sack acknowledging the response data
   MAY be sent following the Cookie-Pair in the extended header.

   At this time, any second segment may be sent without awaiting an
   <ACK>, according to the usual [RFC5681] TCP congestion control
   process.

   Implementation Notes:

      Upon arrival of more data prompting a new Cookie exchange, there
      is no need to increment the previous timestamp; TCB state has not
      been saved at the Responder.  Instead, use the saved RCV.NXT, plus
      one (1) for the (actual or implied) <FIN>.






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      Initiator <ACK(SYN),FIN> with the Cookie-Pair option and no
      segment data is never required; TCB state has not been saved at
      the Responder.  This combination MUST be silently discarded.

6.4.  Responder <ACK> Data

   Upon receipt of the <ACK(SYN)> with a Cookie-Pair option (and
   verification of the Timestamps and Cookie-Pair options), the
   Responder SHOULD process any data present.

   Since the TCP Sequence and Acknowledgment Numbers have not advanced,
   the Responder will process the same incoming data, and transmit the
   same response.

   If the Selective Acknowledgment (Sack) option [RFC2018] has been
   successfully negotiated, with a short Sack covering earlier response
   data, only additional unacknowledged response data is sent.

   At this time, any second segment may be sent without awaiting an
   <ACK>, according to the usual [RFC5681] TCP congestion control
   process.

7.  Advisory Reset

   When a TCB with matching Addresses and Ports is found, but the
   Cookie-Pair fails to verify, the datagram MUST be silently discarded.

   When no TCB with matching Addresses and Ports is found, a <RST> is
   sent as usual.  The Timestamps option SHOULD be copied [RFC1323].  A
   Cookie-Pair option MUST also be copied.  The Cookie option (or
   Cookie-less option) MUST NOT be copied.

   Any <RST> is always treated as advisory.  A <RST> without a matching
   Cookie-Pair option could be caused by antique duplicates.  Receipt
   has no effect on the operation of the protocol.  The implementation
   SHOULD continue until a USER TIMEOUT expires.  (See [RFC5482] for
   additional information.)

   This also inhibits third parties from disrupting communications.

8.  Interactions with Other Options

   A successful Cookie (or Cookie-less) option exchange signals
   availability of the TCP header extension.  Other options with large
   data portions MAY also use this feature.  The extended option data is
   processed in the order that the options appear.





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8.1.  TCP Selective Acknowledgment

   (Kind 5 [RFC2018].)  The pairs of 32-bit fields are well suited to
   the header extension.  Because of its variable size, this is
   RECOMMENDED as the final extended option.

   During the cookie exchange, the <ACK(SYN)> MAY include this option to
   acknowledge any optional transaction response data.

8.2.  TCP Timestamps

   (Kind 8 [RFC1323].)  Support is REQUIRED.  See also section 3.

   When a segment needs no header extension, and 32-bit timestamps have
   been negotiated, this option MUST be sent.

8.3.  TCP Extensions for Transactions

   (Kinds 11-13 [RFC1644].)  Incompatible with this specification, and
   MUST be ignored on receipt.

8.4.  TCP MD5 Signature

   (Kind 19 [RFC2385].)  This option is beyond the scope of this
   specification.  Because specific configuration is required, sending
   is under the complete control of the operator.  Segments lacking this
   option will be silently discarded.

   The size of the option itself precludes use with the Cookie option in
   the <SYN>.  Regardless of the system default, the Cookie option MUST
   NOT be sent, and MUST be ignored on receipt.  Instead, the Cookie-
   less extension option indicates that other features of this
   specification are available.

8.5.  TCP Authentication

   (Kind 29 [RFC5925].)  This option is beyond the scope of this
   specification.  Because specific configuration is required, sending
   is under the complete control of the operator.  Segments lacking this
   option will be silently discarded.

   The size of the option itself precludes use with the Cookie option in
   the <SYN>.  Regardless of the system default, the Cookie option MUST
   NOT be sent, and MUST be ignored on receipt.  Instead, the Cookie-
   less extension option indicates that other features of this
   specification are available.





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9.  History

   T/TCP [RFC1379] [RFC1644] permits lightweight TCP transactions for
   applications that traditionally have used UDP.  However, T/TCP has
   unacceptable security issues [Hannum1996] [Phrack1998].

   The initial specification [KS1995] of Photuris [RFC2522], now called
   version 1 (December 1994 to March 1995), was based on a short list of
   design requirements, and simple experimental code by Phil Karn.  A
   "Cookie" Exchange guards against simple flooding attacks sent with
   bogus IP Sources or UDP Ports.

   During 1995, the Photuris efficient secret rollover and many other
   extensions were specified.  Multiple interoperable implementations
   were produced.

   By September 1996, the long anticipated Denial of Service (DoS)
   attacks in the form of TCP SYN floods were devastating popular (and
   unpopular) servers and sites.  Phil Karn informally mentioned
   adapting anti-clogging cookies to TCP.  Perry Metzger proposed adding
   Karn's cookies as part of a "TCP++" effort [Metzger1996].

   Later in 1996, Daniel J. Bernstein implemented "SYN cookies", small
   cookies embedded in the TCP SYN Initial Sequence Number (ISN).  This
   technique was exceptionally clever, because it did not require
   cooperation of the remote party and could be deployed unilaterally.
   However, SYN cookies can only be used in emergencies; they are
   incompatible with most TCP options.  As there is insufficient space
   in the Sequence Number, the cookie is not considered cryptologically
   secure.  Therefore, the mechanism remains inactive until the system
   is under attack, and thus is not well tested in operation.  SYN
   cookies were not accepted for publication until recently [RFC4987].

   In 1998, Perry Metzger proposed adding Karn's cookies as part of a
   "TCPng" discussion [Metzger1998].

   In 1999, Faber, Touch, and Yue [FTY1999] proposed using an option to
   negotiate the party that would maintain TIME-WAIT state.  This
   permits a server to entirely eliminate state after closing a
   connection.

   In 2000, the Stream Control Transmission Protocol (SCTP) [RFC2960]
   was published with an inadequate partial cookie mechanism claiming to
   be based upon Photuris.  It featured a deficient checksum (replaced
   in 2002 by [RFC3309] without graceful transition), and has undergone
   subsequent revisions [RFC4960].





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   In 2006, the Datagram Congestion Control Protocol (DCCP) [RFC4340]
   was published with a mechanism analogous to SYN cookies.

10.  Acknowledgments

   Andre Broido informally described utilizing cookies for Transport
   Layer Security (TLS) session identifiers, in place of the [RFC5077]
   ticket.  Rapid TLS session resumption would improve both latency and
   privacy, but is beyond the scope of this specification.  Also, he
   provided numerous helpful comments and additional references, such as
   [KBC2005].

   H. K. Jerry Chu and Arvind Jain informally described retaining
   existing cookies for accelerated open on subsequent connections.
   That feature was subsumed by this specification.

   Wesley M. Eddy and Adam Langley previously proposed another pair of
   options [EL2008] extending the TCP header option space.

   Adam Langley previously proposed another option [Langley2008]
   permitting <SYN,ACK(SYN)> constant payload data.  His (August 2008)
   code was a base for the initial TCPCT implementation.

   Joe Touch postulated a (hopefully hypothetical) failure mode: options
   re-ordered by middleware.  This caused a change in specifications,
   and has considerably complicated option interactions and processing.
   His helpful comments were appreciated.

   Many thanks to Fernando Gont for suggestions, and Rick Jones for
   performance testing.

11.  IESG Considerations

   Two TCP Option numbers are reserved for general experimental use
   under the rules laid out in [RFC4727] and [RFC3692] section 1.  Such
   values reserved for experimental use are never to be made permanent;
   permanent assignments should be obtained through standard processes.
   Experimental numbers are intended for experimentation and testing and
   are not intended for wide or general deployments.

   For further information, contact the author.










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12.  Operational Considerations

   Any implementation of this specification SHOULD be configurable,
   separately for each port or connection.

   TCPCT_COOKIE_DESIRED
      Values: 0 (disabled), 8, 10, 12, 14, 16.  Default: 16.  Send the
      Cookie option with the <SYN>.

   TCPCT_EXTEND_TS[32|64|128]
      Default: off.  If defined, may designate 32-bit, 64-bit, or
      128-bit timestamps extension.

   TCPCT_IN_ALWAYS
      Default: off.  Silently discard any incoming <SYN> that is missing
      the Cookie option.

   TCPCT_OUT_NEVER
      Default: off.  Refuse to send (override) the Cookie option.

   TCP_SYN_DATA_LIMIT
      Default: 0.  Maximum: 496.  The maximum amount of data transmitted
      with the <SYN>.  Wait for data before sending.

   TCP_SYN_ACK_DATA_LIMIT
      Default: 0.  Maximum: 1220.  The maximum amount of data
      transmitted with the <SYN,ACK(SYN)>.  Wait for data before
      sending.

13.  Security Considerations

   TCPCT was based on currently available tools, by experienced network
   protocol designers with an interest in cryptography, rather than by
   cryptographers with an interest in network protocols.  This
   specification is intended to be readily implementable without
   requiring an extensive background in cryptology.

   Therefore, only minimal background cryptologic discussion and
   rationale is included in this document.  Although some review has
   been provided by the general cryptologic community, it is anticipated
   that design decisions and tradeoffs will be thoroughly analysed in
   subsequent dissertations and debated for many years to come.
   Cryptologic details are reserved for separate documents that may be
   more readily and timely updated with new analysis.







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   The security depends on the quality of the random numbers generated
   by each party.  Generating cryptographic quality random numbers on a
   general purpose computer without hardware assistance is a very tricky
   problem (see [RFC4086] for discussion).

   TCPCT is not intended to prevent or recover from all possible
   security threats.  Rather, it is designed to inhibit inadvertent
   middlebox interference, while protecting against Denial of Service
   (DoS) attacks.  (See [RFC4732], and [RFC3552] section 4.6.3 et seq.)

   The cookie exchange does not protect against an interloper that can
   race to substitute another value, nor an interceptor that can modify
   and/or replace a value.  These attacks are considerably more
   difficult than passive vacuum-cleaner monitoring.

   Note that each incoming <SYN,ACK(SYN)> replaces the Responder cookie.
   The initial exchange is most fragile, as protection against spoofing
   relies entirely upon the sequence and timestamp.  This replacement
   strategy allows the correct pair to pass through, while any others
   will be filtered via Responder verification later.































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Appendix A. Example Headers

A.1.  Example <SYN>

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=MSS      | Length=4      |            (value)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=UTO      | Length=4      |           (timeout)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=SackOK   | Length=2      | Kind=TS       | Length=10     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           TS Value                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         TS Echo Reply                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=Cookie   | Length=16     |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                            Cookie                             +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=wscale   | Length=3      |    (value)    | Kind=EOL      |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

   A 14 byte (112-bit) Cookie barely fits with the other recommended
   options in the maximal 60 byte TCP header (40 bytes of option space).

   Since the cookies are required to be the same size and meet a 32-bit
   alignment requirement, the implementor recognizes that this order
   provides optimal packing.

   The UserTimeOut (UTO) option can appear in other locations instead,
   such as following the Cookie option.  Because some middleboxes are
   sensitive to the order of options, UTO should not appear before MSS
   nor between the TS and Cookie.














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A.2.  Example <ACK(SYN)> with Sack

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=TSX      | Length=4      | Extend=16     |    0    | S=1 |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                           TS Value                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         TS Echo Reply                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=nop      | Kind=nop      | Kind=Cookie   | Length=30     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                       Initiator-Cookie                        +
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                       Responder-Cookie                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=MSS      | Length=4      |            (value)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=UTO      | Length=4      |           (timeout)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=nop      | Kind=nop      | Kind=Sack     | Length=10     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Starting Value                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Ending Value                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=wscale   | Length=3      |    (value)    | Kind=EOL      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sack implies SackOK.












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A.3.  Example <ACK(SYN)> with 64-bit Timestamps

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=TSX      | Length=4      | Extend=15     |    0    | S=2 |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   +                           TS Value                            +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         TS Echo Reply                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=SackOK   | Length=2      | Kind=Cookie   | Length=30     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                       Initiator-Cookie                        +
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                       Responder-Cookie                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=MSS      | Length=4      |            (value)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=UTO      | Length=4      |           (timeout)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Kind=wscale   | Length=3      |    (value)    | Kind=EOL      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The larger 64-bit (or 128-bit) Timestamps extended option MUST be
   recognized, although the Responder MAY return a smaller Timestamps
   extended option.












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Normative References

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [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.

   [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.

   [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.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

   [RFC3232]  Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced
              by an On-line Database", RFC 3232, January 2002.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452, January 2009.

   [RFC5482]  Eggert, L. and F. Gont, "TCP User Timeout Option", RFC
              5482, March 2009.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.














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Informative References

   [EL2008]   Eddy, W. and A. Langley, "Extending the Space Available
              for TCP Options", Work in Progress, July 2008.

   [FTY1999]  Faber, T., Touch, J., and W. Yue, "The TIME-WAIT state in
              TCP and Its Effect on Busy Servers", IEEE INFOCOM 99, pp.
              1573-1584.

   [Gont2009] Gont, F., "Security assessment of the Transmission Control
              Protocol (TCP)", February 2009.
              https://www.cpni.gov.uk/Docs/tn-03-09-security-
              assessment-TCP.pdf

   [GO2010]   Gont, F. and A. Oppermann, "On the generation of TCP
              timestamps", Work in Progress, June 2010.

   [Hannum1996]
              Hannum, C., "Security Problems Associated With T/TCP",
              unpublished work in progress, September 1996.
              http://www.mid-way.org/doc/ttcp-sec.txt

   [KBC2005]  Kohno, T., Broido, A., and K. C. Claffy, "Remote physical
              device fingerprinting", IEEE Symposium on Security and
              Privacy, May 2005.  http://www.caida.org/
              outreach/papers/2005/fingerprinting/
              KohnoBroidoClaffy05-devicefingerprinting.pdf

   [KS1995]   Karn, P. and W. Simpson, "The Photuris Session Key
              Management Protocol", March 1995.

              Published as: "Photuris: Design Criteria", Proceedings of
              Sixth Annual Workshop on Selected Areas in Cryptography,
              LNCS 1758, Springer-Verlag.  August 1999.

   [Langley2008]
              Langley, A., "Faster application handshakes with SYN/ACK
              payloads", Work in Progress, August 2008.

   [MAF2004]  Medina, A., Allman, M., and S. Floyd, "Measuring
              Interactions Between Transport Protocols and Middleboxes",
              Proceedings 4th ACM SIGCOMM/USENIX Conference on Internet
              Measurement, October 2004.
              http://www.icsi.berkeley.edu/pubs/networking/tbit-
              Aug2004.pdf






Simpson                       Experimental                     [Page 34]
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RFC 6013                 TCP Cookie Transactions            January 2011


   [Metzger1996]
              Metzger, P., "Re: SYN floods (was: does history repeat
              itself?)", September 9, 1996.
              http://www.merit.net/mail.archives/nanog/
              1996-09/msg00235.html

   [Metzger1998]
              Metzger, P., "Re: what a new TCP header might look like",
              May 12, 1998.  ftp://ftp.isi.edu/end2end/end2end-
              interest-1998.mail

   [Morris1985]
              Morris, R., "A Weakness in the 4.2BSD Unix TCP/IP
              Software", Technical Report CSTR-117, AT&T Bell
              Laboratories, February 1985.
              http://pdos.csail.mit.edu/~rtm/papers/117.pdf

   [MSV2009]  Metzger, P., Simpson, W., and P. Vixie, "Improving TCP
              Security With Robust Cookies", Usenix ;login:, December
              2009.  http://www.usenix.org/publications/login/
              2009-12/openpdfs/metzger.pdf

   [Phrack1998]
              route|daemon9, "T/TCP vulnerabilities", Phrack Magazine,
              Volume 8, Issue 53, July 8, 1998.
              http://www.phrack.org/issues.html?issue=53&id=6

   [RFC1379]  Braden, R., "Extending TCP for Transactions -- Concepts",
              RFC 1379, November 1992.

   [RFC1644]  Braden, R., "T/TCP -- TCP Extensions for Transactions
              Functional Specification", RFC 1644, July 1994.

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

   [RFC2522]  Karn, P. and W. Simpson, "Photuris: Session-Key Management
              Protocol", RFC 2522, March 1999.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.




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   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001.

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

   [RFC3309]  Stone, J., Stewart, R., and D. Otis, "Stream Control
              Transmission Protocol (SCTP) Checksum Change", RFC 3309,
              September 2002.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552, July
              2003.

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692, January 2004.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              June 2005.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
              ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.

   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and Internet
              Architecture Board, "Internet Denial-of-Service
              Considerations", RFC 4732, November 2006.

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

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.




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   [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive
              Nameservers in Reflector Attacks", BCP 140, RFC 5358,
              October 2008.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

   [RFC6056]  Larson, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056, January
              2011.

   [rfc1323bis]
              Borman, D., Braden, R., and V. Jacobson., "TCP Extensions
              for High Performance", Work in Progress, March 2009.

Author's Address

   Questions about this document can be directed to:

   William Allen Simpson
   DayDreamer
   Computer Systems Consulting Services
   1384 Fontaine
   Madison Heights, Michigan 48071

   EMail: William.Allen.Simpson@Gmail.com

























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