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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 5925

TCPM WG                                                        J. Touch
Internet Draft                                                  USC/ISI
Obsoletes: 2385                                               A. Mankin
Intended status: Proposed Standard                  Johns Hopkins Univ.
Expires: August 2009                                          R. Bonica
                                                       Juniper Networks
                                                      February 16, 2009



                       The TCP Authentication Option
                    draft-ietf-tcpm-tcp-auth-opt-03.txt


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 16, 2009.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   Please review these documents carefully, as they describe your rights
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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 static
   master key configuration or an external, out-of-band master key
   management mechanism; in either case, TCP-AO also protects
   connections when using the same master key across repeated instances
   of a connection, using connection keys derived from the master key.
   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 its own option
   identifier, even though used mutually exclusive of TCP MD5 on a given
   TCP connection. TCP-AO supports IPv6, and is fully compatible with
   the requirements for the replacement of TCP MD5.

Table of Contents


   1. Contributors...................................................3
   2. Introduction...................................................3
      2.1. Executive Summary.........................................4
      2.2. Changes from Previous Versions............................5
         2.2.1. New in draft-ietf-tcp-auth-opt-03....................6
         2.2.2. New in draft-ietf-tcp-auth-opt-02....................6
         2.2.3. New in draft-ietf-tcp-auth-opt-01....................7
         2.2.4. New in draft-ietf-tcp-auth-opt-00....................8
         2.2.5. New in draft-touch-tcp-simple-auth-03................9
         2.2.6. New in draft-touch-tcp-simple-auth-02................9
         2.2.7. New in draft-touch-tcp-simple-auth-01................9
   3. Conventions used in this document.............................10
   4. The TCP Authentication Option.................................10
      4.1. Review of TCP MD5 Option.................................10
      4.2. TCP-AO Option............................................11
   5. Preventing replay attacks within long-lived connections.......14
   6. Computing connection keys from TSAD entries...................16
   7. Security Association Management...............................17
   8. TCP-AO Interaction with TCP...................................21
      8.1. TCP User Interface.......................................21
      8.2. TCP States and Transitions...............................22
      8.3. TCP Segments.............................................22
      8.4. Sending TCP Segments.....................................23
      8.5. Receiving TCP Segments...................................24


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      8.6. Impact on TCP Header Size................................25
   9. Connection Key Establishment and Duration Issues..............26
      9.1. Master Key Reuse Across Socket Pairs.....................27
      9.2. Master Key Use Within a Long-lived Connection............27
   10. Obsoleting TCP MD5 and Legacy Interactions...................27
   11. Interactions with Middleboxes................................28
      11.1. Interactions with non-NAT/NAPT Middleboxes..............28
      11.2. Interactions with NAT/NAPT Devices......................29
   12. Evaluation of Requirements Satisfaction......................29
   13. Security Considerations......................................35
   14. IANA Considerations..........................................37
   15. References...................................................37
      15.1. Normative References....................................37
      15.2. Informative References..................................38
   16. Acknowledgments..............................................40

1. Contributors

   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 [To06] (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 Sept. 2005) and in a document by Brian Weis [We05].

   Russ Housley suggested L4/application layer management of the TSAD.
   Steve Bellovin motivated the KeyID field. Eric Rescorla suggested the
   use of ISNs in the connection key computation and ESNs to avoid
   replay attacks, and Brian Weis extended the computation to
   incorporate the entire connection ID and provided the details of the
   connection key computation.

2. 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


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   have been escalating attacks on the algorithm itself [Wa05]. TCP MD5
   also lacks both key management and algorithm agility. This document
   adds the latter, but notes that TCP does not provide a sufficient
   framework for cryptographic key management, because SYN segments lack
   sufficient remaining space to support key coordination in-band (see
   Section 8.6). This document obsoletes the TCP MD5 option with a more
   general TCP Authentication Option (TCP-AO), to support the use of
   other, stronger hash functions, provide replay protection for long-
   lived connections and across repeated instances of a single
   connection, and to provide a more structured recommendation on
   external key management. The result is compatible with IPv6, and is
   fully compatible with requirements under development for a
   replacement for TCP MD5 [Be07].

   This document is not intended to replace the use of the IPsec suite
   (IPsec and IKE) to protect TCP connections [RFC4301][RFC4306]. 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, or in cases
   where keys need to be tightly coordinated with individual transport
   sessions [Be07].

   Note that TCP-AO obsoletes TCP MD5, although a particular
   implementation may support both 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.

2.1. Executive Summary

   This document replaces TCP MD5 as follows [RFC2385]:

   o  TCP-AO uses a separate option Kind for TCP-AO (TBD-IANA-KIND).

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

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







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   o  TCP-AO allows rekeying during a TCP connection, assuming that an
      out-of-band protocol or manual mechanism coordinates the key
      change. In such cases, a key ID allows the efficient concurrent
      use of multiple keys. Note that TCP MD5 does not preclude rekeying
      during a connection, but does not require its support either.
      Further, TCP-AO supports rekeying with zero packet loss, whereas
      rekeying in TCP MD5 can lose packets 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.

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

   o  TCP-AO ensures per-connection keys as unique as the TCP connection
      itself, using TCP's ISNs for differentiation, even when static
      master keys are used across repeated instances of a socket pair.

   o  This document provides detail in how this option interacts with
      TCP's states, event processing, and user interface.

   o  The TCP-AO option is 3 bytes shorter than TCP MD5 (15 bytes
      overall, rather than 18) in the default case (using a 96-bit MAC).

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

   o  TCP-AO does not support dynamic parameter negotiation.

   o  TCP-AO uses TCP's socket pair (source address, destination
      address, source port, destination port) as a security parameter
      index, rather than using a separate field as a primary index
      (IPsec's 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) [Be07].

   o  TCP-AO does not support encryption.

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

2.2. Changes from Previous Versions

   [NOTE: to be omitted upon final publication as RFC]



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2.2.1. New in draft-ietf-tcp-auth-opt-03

   o  Added a placeholder to discuss key change coordination in Section
      9.

   o  Moved discussion of required MAC algorithms and PRF to a separate
      document, indicated as RFC-TBD until assigned. Included the PRF in
      the TSAD master key tuple so that TCP-AO is PRF algorithm agile,
      and updated general PRF input format.

   o  Revised the description the TSAD and impact to the TCP user
      interface. Removed the description of the TSAD API. Access to the
      API is assumed specific to the implementation, and not part of the
      protocol specification.

   o  Clarified the different uses of the term key; includes master key
      (from the TSAD) and connection key (per-connection key, derived
      from the master via the PRF).

   o  Explained the ESN pseudocode operation in detail.

   o  Added a contributors section up front.

   o  Update discussion of requirements to be sufficiently stand-alone;
      update list to correlate more directly to Be07 (so that Be07 can
      be dropped from consideration for publication).

   o  Provided detail on size of typical options (motivating a small
      option).

   o  Confirmed WG consensus on IETF-72 topic - no algorithm ID and T-
      bit (options excluded) locations in the header.

   o  Confirmed WG consensus on IETF-72 topic - no additional header
      bits for in-band key change signaling (the "K" bit from [Bo07]).

2.2.2. New in draft-ietf-tcp-auth-opt-02

   o  List issue - Replay Protection: incorporated extended sequence
      number space, not using KeyID space.

   o  List issue - Unique Connection Keys: ISNs are used to generate
      unique connection keys even when static keys used for repeated
      instances of a socket pair.

   o  List issue - Header Format and Alignment: Moved KeyID to front.



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   o  List issue - Reserved KeyID Value: Suggestion to reserve a single
      KeyID value for implementation optimization received no support on
      the WG list, so this was not changed.

   o  List issue - KeyID Randomness: KeyIDs are not assumed random; a
      note was added that nonce-based filtering should be done on a
      portion of the MAC (incorporated into the algorithm), and that
      header fields should not be assumed to have cryptographic
      properties (e.g., randomness).

   o  List issue - Support for NATs: preliminary rough consensus
      suggests that TCP-AO should not be augmented to support NAT
      traversal. Existing mechanisms for such traversal (UDP support)
      can be applied, or IPsec NAT traversal is recommended in such
      cases instead.

   o  IETF-72 topic - providing algorithm ID and T-bit (options
      excluded) locations in the header: (No current consensus was
      reached on this topic, so no change was made.)

   o  IETF-72 topic - providing additional header bits for in-band key
      change signaling (draft-bonica's "K" bit): (No current consensus
      was reached on this topic, so no change was made.)

   o  Clarified TCP-AO as obsoleting TCP MD5.

   o  Clarified the MAC Type as referring to the IANA registry of IKEv2
      transforms, not the RFC establishing that registry.

   o  Added citation to the Wang/Yu paper regarding attacks on MD5 Wa05
      to replace reports in Be05 and Bu06.

   o  Explained why option exclusion can't be changed during a
      connection.

   o  Clarified that AO explicitly allows rekeying during a TCP
      connection, without impacting packet loss.

   o  Described TCP-AO's interaction with reboots more clearly, and
      explained the need to clear out old state that persists
      indefinitely.

2.2.3. New in draft-ietf-tcp-auth-opt-01

   o  Require KeyID in all versions. Remove odd/even indicator of KeyID
      usage.



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   o  Relax restrictions on key reuse: requiring an algorithm for nonce
      introduction based on ISNs, and suggest key rollover every 2^31
      bytes (rather than using an extended sequence number, which
      introduces new state to the TCP connection).

   o  Clarify NAT interaction; currently does not support omitting the
      IP addresses or TCP ports, both of which would be required to
      support NATs without any coordination. This appears to present a
      problem for key management - if the key manager knows the received
      addrs and ports, it should coordinate them (as indicated in Sec
      8).

   o  Options are included or excluded all-or-none. Excluded options are
      deleted, not just zeroed, to avoid the impact of reordering or
      length changes of such options.

   o  Augment replay discussion in security considerations.

   o  Revise discussion of IKEv2 MAC algorithm names.

   o  Remove executive summary comparison to expired documents.

   o  Clarified key words to exclude lower case usage.

2.2.4. New in draft-ietf-tcp-auth-opt-00

   o  List of TBD values, and indication of how each is determined.

   o  Changed TCP-SA to TCP-AO (removed 'simple' throughout).

   o  Removed proposed NAT mechanism; cited RFC-3947 NAT-T as
      appropriate approach instead.

   o  Made several changes coordinated in the TCP-AUTH-DT as follow:

   o  Added R. Bonica as co-author.

   o  Use new TCP option Kind in the core doc.

   o  Addresses the impact of explicit declines on security.

   o  Add limits to TSAD size (2 <= TSAD <= 256).

   o  Allow 0 as a legitimate KeyID.

   o  Allow the WG to determine the two appropriate required MAC
      algorithms.


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   o  Add TO-DO items.

   o  Added discussion at end of Introduction as to why TCP MD5
      connections cannot be upgraded to TCP-AO.

2.2.5. New in draft-touch-tcp-simple-auth-03

   o  Added support for NAT/NAPT.

   o  Added support for IPv6.

   o  Added discussion of how this proposal satisfies requirements under
      development, including those indicated in [Be07].

   o  Clarified the byte order of all data used in the MAC.

   o  Changed the TCP option exclusion bit from a bit to a list.

2.2.6. New in draft-touch-tcp-simple-auth-02

   o  Add reference to Bellovin's need-for-TCP-auth doc [Be07].

   o  Add reference to SP4 [SDNS88].

   o  Added notes that TSAD to be externally implemented; this was
      compatible with the TSAD described in the previous version.

   o  Augmented the protocol to allow a KeyID, required to support
      efficient overlapping keys during rekeying, and potentially useful
      during connection establishment. Accommodated by redesigning the
      TSAD.

   o  Added the odd/even indicator for the KeyID.

   o  Allow for the exclusion of all TCP options in the MAC calculation.

2.2.7. New in draft-touch-tcp-simple-auth-01

   o  Allows intra-session rekeying, assuming out-of-band coordination.

   o  MUST allow TSAD entries to change, enabling rekeying within a TCP
      connection.

   o  Omits discussion of the impact of connection reestablishment on
      BGP, because added support for rekeying renders this point moot.

   o  Adds further discussion on the need for rekeying.


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3. 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. Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

4. The TCP Authentication Option

   The TCP Authentication Option (TCP-AO) uses a TCP option Kind value
   of TBD-IANA-KIND.

4.1. Review of TCP MD5 Option

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

                +---------+---------+-------------------+
                | Kind=19 |Length=18|   MD5 digest...   |
                +---------+---------+-------------------+
                |                                       |
                +---------------------------------------+
                |                                       |
                +---------------------------------------+
                |                                       |
                +-------------------+-------------------+
                |                   |
                +-------------------+

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

   2. The TCP header excluding options and checksum.

   3. The TCP data payload.



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   4. The connection key.

4.2. TCP-AO Option

   The new TCP-AO option 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, as well as a KeyID
   field as shown in Figure 2.

            +----------+----------+----------+----------+
            |   Kind   |  Length  |  KeyID   |   MAC    |
            +----------+----------+----------+----------+
            |                 MAC (con't)      ...
            +----------------------------------...

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

                        Figure 2 The TCP-AO Option

   The TCP-AO defines the following fields:

   o  Kind: An unsigned 1-byte field indicating the TCP-AO Option. TCP-
      AO uses a new Kind value of TBD-IANA-KIND. Because of how keys are
      managed (see Section 7), an endpoint will not use TCP-AO for the
      same connection in which TCP MD5 is used.

      >> A single TCP segment MUST NOT have more than one TCP-AO option.

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

      >> The Length value MUST be greater than or equal to 3.

      >> The Length value MUST be consistent with the TCP header length;
      this is a consistency check and avoids overrun/underrun abuse.

      Values of 3 and other small values are of dubious utility (e.g.,
      for MAC=NONE, or small values for very short MACs) but not
      specifically prohibited.







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   o  KeyID: An unsigned 1-byte field is used to support efficient key
      changes during a connection and/or to help with key coordination
      during connection establishment, and will be discussed further in
      Section 4. Note that the KeyID has no cryptographic properties -
      it need not be random, nor are there any reserved values.

   o  MAC: Message Authentication Field. 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.

      >> Required support for TCP-AO MACs as defined in RFC-TBD; other
      MACs MAY be supported [RFC2403].

   The MAC is computed over the following fields in the following order:

   1. The extended sequence number (ESN), in network-standard byte
      order, as follows (described further in Section 5):

                   +--------+--------+--------+--------+
                   |                ESN                |
                   +--------+--------+--------+--------+

                     Figure 3 Extended sequence number

      The ESN for transmitted segments is locally maintained from a
      locally maintained SND.ESN value, for received segments, a local
      RCV.ESN value is used. The details of how these values are
      maintained and used is described in Sections 5, 8.4, and 8.5.

   2. The TCP pseudoheader: IP source and destination addresses,
      protocol number and segment length, all in network byte order,
      prepended to the TCP header below. The 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 4 TCP IPv4 pseudoheader [RFC793]




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                   +--------+--------+--------+--------+
                   |                                   |
                   +                                   +
                   |                                   |
                   +           Source Address          +
                   |                                   |
                   +                                   +
                   |                                   |
                   +                                   +
                   +--------+--------+--------+--------+
                   |                                   |
                   +                                   +
                   |                                   |
                   +         Destination Address       +
                   |                                   |
                   +                                   +
                   |                                   |
                   +--------+--------+--------+--------+
                   |      Upper-Layer Packet Length    |
                   +--------+--------+--------+--------+
                   |      zero       |   Next Header   |
                   +--------+--------+--------+--------+

                 Figure 5 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

   4. TCP data, in network byte order

   Note that the connection key is not included here; the MAC algorithm
   indicates how to use the connection key, e.g., as HMACs do in general
   [RFC2104][RFC2403]. The connection key is derived from the TSAD
   entry's master key as described in Sections 7, 8.4, and 8.5.

   By default, TCP-AO includes the TCP options in the MAC calculation
   because these options are intended to be end-to-end and some are
   required for proper TCP operation (e.g., SACK, timestamp, large
   windows). Middleboxes that alter TCP options en-route are a kind of
   attack and would be successfully detected by TCP-AO. In cases where
   the configuration of the connection's security association state
   indicates otherwise, the TCP options can be excluded from the MAC
   calculation. When options are excluded, all options - including TCP-
   AO - are skipped over during the MAC calculation (rather than being
   zeroed).



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   The TCP-AO option does 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 association and is managed separately (see
   Section 7).

5. 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
   differentiate these legitimate replays from each other, and so it
   adds a 32-bit extended sequence number (ESN) for transmitted and
   received segments.

   The ESN extends TCP's sequence number so that segments within a
   single connection are always unique. When TCP's sequence number rolls
   over, there is a chance that a segment could be repeated in total;
   using an ESN 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 ESN) based on
   transitions in the low-order portion (the TCP sequence number).

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

   For transmitted segments SND.ESN can be implemented by extending
   TCP's sequence number to 64-bits; SND.ESN 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.ESN from the
   received 32-bit sequence number and the current connection context.

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

   Consider an implementation with two ESNs as required (SND.ESN,
   RCV.ESN), 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



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

   o  SND.ESN_FLAG, which indicates when to increment the SND.ESN

   o  RCV.ESN_FLAG, which indicates when to increment the RCV.ESN

   When a segment is received, the following algorithm (written in C)
   computes the ESN used in the MAC; an equivalent algorithm can be
   applied to the "SND" side:

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

   In the above code, ROLL is true in the first line when 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 first time this happens, the ESN is
   incremented and a flag is set. The flag prevents the ESN 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 packets, regardless
   of reordering, cannot straddle both regions simultaneously.



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6. Computing connection keys from TSAD entries

   TSAD entries, described in Section 7, include master keys which are
   used in conjunction with a TCP's connection ISNs to generate unique
   connection keys. This allows a static master key to be reused across
   different connections, or across different instances of connections
   within a socket pair, while maintaining unique connection keys.
   Unique connection keys are generated without relying on external key
   management properties.

   Given a master key tuple, the TCP socket pair, and the connection
   ISNs, the connection key used in the MAC algorithm is computed as
   follows, truncated to the same length as the master key, using a
   pseudorandom function (PRF):

      Conn_key = PRF(TSAD_master_key, input)
      where
         input = 0 + "TCP-AO" + connblock + TSAD_master_key_len


   The components of the input are concatenated as a single byte string
   (the string concatenation operator is shown here as "+"). The initial
   zero of the input is a single byte, "TCP-AO" is a null-terminated
   string, connblock is defined below, and TSAD_master_key_len is the
   length of the TSAD master key in bytes, as stored in the TSAD entry.
   The PRF to be used for a given master key is indicated in the TDAD
   master key tuple, and details of the PRF are provided in [RFC-TBD].

   The connection block (connblock) is defined as follows (IP addresses
   are correspondingly longer for IPv6 addresses):

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

       Figure 6 Connection block used for connection key generation

   "Source" and "destination" are defined by the direction of the
   segment being MAC'd; for incoming packets, source is the remote side,


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   whereas for outgoing packets 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 connection block 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 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.

   >> Segments sent in response to connections for which the ISNs are
   not known SHOULD NOT use TCP-AO.

   Once a connection is established, a connection key would typically be
   cached to avoid recomputing it on a per-segment basis (e.g., in the
   TCP Transmission Control Block, i.e, the TCB [RFC793]). The use of
   both ISNs in the connection key computation ensures that segments
   cannot be replayed across repeated connections reusing the same
   socket pair (provided the ISN pair does not repeat, which is unlikely
   because both endpoints should select ISNs pseudorandomly [RFC1948],
   their 32-bit space avoids repeated use except under reboot, and reuse
   assumes both sides repeat their use on the same connection).

   In general, a SYN would be MAC'd using a destination ISN of zero
   (whether sent or received), and all other segments would be MAC'd
   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 is possible that the two endpoints
   would not have enough information to authenticate segments. In such
   cases, TCP's timeout mechanism will allow old state to be cleared to
   enable new connections, except where the user timeout is disabled; 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 [Je07].

7. Security Association Management

   TCP-AO relies on a TCP Security Association Database (TSAD), which
   indicates whether a TCP connection requires TCP-AO, and its
   parameters when so. The TSAD is described as an explicit component of


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   TCP-AO to enable external (master) key management mechanisms -
   automatic or manual - to interact with TCP-AO as needed.

   TSAD entries are assumed to exist at the endpoints where TCP-AO is
   used, in advance of the connection:

   1. TCP connection identifier (ID), i.e., socket pair - IP source
      address, IP destination address, TCP source port, and TCP
      destination port [RFC793]. TSAD entries are uniquely determined by
      their TCP connection ID, which is used to index those entries. A
      TSAD entry may allow wildcards, notably in the source port value.

      >> There MUST be no more than one matching TSAD entry per
      direction for a fully-instantiated (no wildcards) TCP connection
      ID.

   2. For each of inbound (for received TCP segments) and outbound (for
      sent TCP segments) directions for this connection (except as
      noted):

       a. TCP option flag. When 0, this flag allows default operation,
          i.e., TCP options are included in the MAC calculation, with
          TCP-AO's MAC field zeroed out.  When 1, all options (including
          TCP-AO) are excluded from all MAC calculations (skipped over,
          not simply zeroed).

          >> The TCP option flag MUST default to 0 (i.e., options not
          excluded).

          >> The TCP option flag MUST NOT change during a TCP
          connection.

          The TCP option flag 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.

       b. An extended sequence number (ESN). The ESN enables each
          segment's MAC calculation to have unique input data, even when
          payload data is retransmitted and the TCP sequence number
          repeats due to wraparound. The ESN is initialized to zero upon
          connection establishment. Its use in the MAC calculation is
          described in Section 4.2, and its management is described in
          Section 5.





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       c. An ordered list of zero or more master key tuples. Each tuple
          is defined as the set <KeyID, MAC type, master key length,
          master key, PRF> as follows:

          >> Components of a TSAD master key tuple MUST NOT change
          during a connection.

          Keeping the tuple components static ensures that the KeyID
          uniquely determines the properties of a packet; this supports
          use of the KeyID to determine the packet properties.

          >> The set of TSAD master key tuples MAY change during a
          connection, but KeyIDs of those tuples MUST NOT overlap. I.e.,
          tuple parameter changes MUST be accompanied by master key
          changes.

           i. KeyID. The value as used in the TCP-AO option; used to
               differentiate connection keys in concurrent use that are
               derived from different master keys.

               >> A TSAD implementation MUST support at least two KeyIDs
               per connection per direction, and MAY support up to 256.

               >> A KeyID MUST support any value, 0-255 inclusive. There
               are no reserved KeyID values.

               KeyID values are assigned arbitrarily. They can be
               assigned in sequence, or based on any method mutually
               agreed by the connection endpoints (e.g., using an
               external master key management mechanism).

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

















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          ii. MAC type. Indicates the MAC used for this connection, as
               defined in [RFC-TBD]. This includes the MAC algorithm
               (e.g., HMAC-SHA1, AES-CMAC, etc.) and the length of the
               MAC as truncated to (e.g., 96, 128, etc.).

               >> A MAC type of "NONE" MUST be supported, to indicate
               that authentication is not used in this direction; this
               allows asymmetric use of TCP-AO.

               >> At least one direction (inbound/outbound) SHOULD have
               a non-"NONE" MAC in practice, but this MUST NOT be
               strictly required by an implementation.

               >> When the outbound MAC is set to values other than
               "NONE", TCP-AO MUST occur in every outbound TCP segment
               for that connection; when set to NONE or when no tuple
               exists, TCP-AO MUST NOT occur in those segments.

               >> When the inbound MAC is set to values other than
               "NONE", TCP-AO MUST occur in every inbound TCP segment
               for that connection; when set to "NONE" or when no tuple
               exists, TCP-AO SHOULD NOT be added to those segments, but
               MAY occur and MUST be ignored.

         iii. Master key length. Indicates the length of the master key
               in bytes.

          iv. Master key. A byte sequence used for generating
               connection keys, this may be derived from a separate
               shared key by an external protocol over a separate
               channel. This sequence is used in network-standard byte
               order in the connection key generation algorithm
               described in Section 6.

           v. PRF. A pseudorandom function used for the geneation of a
               connection key from the master key tuple, as described in
               Section 6. The specific functions used are described in
               [RFC-TBD].

   It is anticipated that TSAD entries for TCP connections in states
   other than CLOSED can be indexed in the TCP TCB for convenience, but
   that the index would reference a separate database with entries for
   all connections to an IP address (see Section 9.1 for notes on the
   latter. This means that for a particular endpoint (i.e., IP address)
   there would be exactly one database that is consulted by all pending
   connections, the same way that there is only one table of TCBs (a
   database can support multiple endpoints, but an endpoint is


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   represented in only one database). Multiple databases could be used
   to support virtual hosts, i.e., groups of interfaces.

   Note that the TCP-AO fields omit an explicit algorithm ID; that
   algorithm is already specified by the TCP connection ID and stored in
   the TSAD.

   Also note that this document does not address how TSAD entries are
   created by users/processes; it specifies how they must be destroyed
   corresponding to connection states, but users/processes may destroy
   entries as well. It is presumed that a TSAD entry affecting a
   particular connection cannot be destroyed during an active connection
   - or, equivalently, that its parameters are copied to TSAD entries
   local to the connection (i.e., instantiated) and so changes would
   affect only new connections. The TSAD could be managed by a separate
   application protocol.

8. 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].

8.1. 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 the TCP user interface be extended to allow the TSAD
   to be configured, as well as to allow an ongoing connection to manage
   which KeyID tuples are active. The TSAD needs to be configured prior
   to connection establishment, and possibly changed 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
   a TSAD entry can be configured.

   >> A TCP-AO implmentation MUST allow TSAD entries for ongoing TCP
   connections (i.e., not in the CLOSED state) to be modified.
   Parameters not used to index a connection MAY be modified; parameters
   used to index a connection MUST NOT be modified.



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   The TSAD information of a connection needs to be available for
   confirmation; this includes the ability to read the connection key:

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

   Senders need to be able to determine when the outgoing KeyID changes;
   this change immediately affects all subsequent outgoing segments
   (i.e., it need not be synchronized with the data of the SEND call, if
   indicated therein):

   >> TCP SEND, or a sequence of commands resulting in a SEND, MUST be
   augmented so that the KeyID of a TSAD entry can be indicated.

   It may be useful to change the sender-side active KeyID 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 for the receive side to indicate the recent KeyID
   received; although there could be a number of such KeyIDs, the KeyIDs
   are not expected to change quickly so any recent sample of a received
   KeyID is sufficient:

   >> TCP RECEIVE, or the sequence of commands resulting in a RECEIVE,
   MUST be augmented so that the KeyID 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).

8.2. 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.

   >> A TSAD entry MAY be associated with any TCP state.

   >> A TSAD entry MAY underspecify the TCP connection for the LISTEN
   state. Such an entry MUST NOT be used for more than one connection
   progressing out of the LISTEN state.

8.3. 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).



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   >> All TCP segments MUST be checked against the TSAD for matching TCP
   connection IDs.

   >> TCP segments matching TSAD entries with non-NULL MACs without TCP-
   AO, or with TCP-AO and whose MACs and KeyIDs do not validate MUST be
   silently discarded.

   >> TCP segments with TCP-AO but not matching TSAD entries MUST be
   silently accepted; this is required for equivalent function with TCPs
   not implementing TCP-AO.

   >> 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.

   Note that the TCP-AO option is not negotiated. It is the
   responsibility of the receiver to determine when TCP-AO is required
   and to enforce that requirement.

8.4. Sending TCP Segments

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

   1. Check the segment's TCP connection ID against the TSAD

   2. If there is NO TSAD entry, omit the TCP-AO option. Proceed with
      computing the TCP checksum and transmit the segment.

   3. If there is a TSAD entry with zero master key tuples, omit the
      TCP-AO option. Proceed with computing the TCP checksum and
      transmit the segment.

   4. If there is a TSAD entry and a master key tuple and the outgoing
      MAC is NONE, omit the TCP-AO option. Proceed with computing the
      TCP checksum and transmit the segment.

   5. If there is a TSAD entry and a master key tuple and the outgoing
      MAC is not NONE:


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       a. Augment the TCP header with the TCP-AO, inserting the
          appropriate Length and KeyID based on the indexed TSAD entry.
          Update the TCP header length accordingly.

       b. Determine SND.ESN as described in Section 5.

       c. Determine the connection key from the indexed TSAD entry as
          described in Section 6.

       d. Compute the MAC using the indexed TSAD entry and data from the
          segment as specified in Section 4.2, including the TCP
          pseudoheader and TCP header. Include or exclude the options as
          indicated by the TSAD entry's TCP option exclusion flag.

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

       f. Proceed with computing the TCP checksum on the outgoing packet
          and transmit the segment.

8.5. Receiving TCP Segments

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

   1. Check the segment's TCP connection ID against the TSAD.

   2. If there is NO TSAD entry, proceed with TCP processing.

   3. If there is a TSAD entry with zero master key tuples, proceed with
      TCP processing.

   4. If there is a TSAD entry with a master key tuple and the incoming
      MAC is NONE, proceed with TCP processing.

   5. If there is a TSAD entry with a master key tuple and the incoming
      MAC is not NONE:

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

           i. If Lengths differ, silently discard the segment. Log
               and/or signal the event as indicated in Section 8.3.

       b. Use the KeyID value to index the appropriate connection key
          for this connection.




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           i. If the TSAD has no entry corresponding to the segment's
               KeyID, silently discard the segment.

       c. Determine the segment's RCV.ESN as described in Section 5.

       d. Determine the segment's connection key from the indexed TSAD
          entry as described in Section 6.

       e. Compute the segment's MAC using the indexed TSAD entry and
          portions of the segment as indicated in Section 4.2.

          Again, if options are excluded (as per the TCP option
          exclusion flag), they are skipped over (rather than zeroed)
          when used as input to the MAC calculation.

           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 8.3.

       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.

   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.

8.6. Impact on TCP Header Size

   The TCP-AO option typically uses a total of 17-19 bytes of TCP header
   space. TCP-AO is no larger than and typically 3 bytes smaller than
   the TCP MD5 option (assuming a 96-bit MAC).

   Note that 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.




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   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 20 for TCP-AO.
   Assuming a 96-bit MAC, TCP-AO consumes 15 bytes, leaving up to 10
   bytes for other 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]

   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.

9. Connection Key Establishment and Duration Issues

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

   We encourage users of TCP-AO to apply known techniques for generating
   appropriate master keys, including the use of reasonable master key
   lengths, limited connection key sharing, and limiting the duration of
   master key use [RFC3562]. This also includes the use of per-
   connection nonces, as suggested in Section 4.2.

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



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   TCP-AO does not currently provide an explicit key coordination
   mechanism. Such a mechanism is useful when new keys are installed, or
   when keys are changed, to determine when to commence using installed
   keys. Note that because TCP-AO uses directional keys, the receive-
   side keys can be installed in advance of the send side, avoiding the
   need for tight coordination between endpoints.

   The KeyID field is also useful in coordinating master keys used for
   new connections. A TSAD entry may be configured that matches the
   unbound source port, which would return a set of possible master
   keys. The KeyID would then indicate the specific master key, allowing
   more efficient connection establishment; otherwise, the master keys
   could have been tried in sequence. See also Section 9.1.

   Users are advised to manage master keys following the spirit of the
   advice for key management when using TCP MD5 [RFC3562], notably to
   use appropriate key lengths (12-24 bytes), to avoid sharing master
   keys among multiple BGP peering arrangements, and to change master
   keys every 90 days. This requires that the TSAD support monitoring
   and modification.

9.1. Master Key Reuse Across Socket Pairs

   Master keys 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.

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

9.2. Master Key Use Within a Long-lived Connection

   TCP-AO uses extended sequence numbers (ESNs) to prevent replay
   attacks within long-lived connections. Explicit master key 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.

10. Obsoleting TCP MD5 and Legacy Interactions

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


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   >> TCP implementations 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
   is 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 the TSAD could be augmented to support TCP MD5,
   although use of a TSAD-like system is not described in RFC2385.

   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.

11. 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.

11.1. 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.




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   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.

11.2. Interactions with NAT/NAPT Devices

   TCP-AO cannot interoperate natively across NAT/NAPT devices, which
   modify the IP addresses and/or port numbers. We anticipate that
   traversing such devices will 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
   [RFC2766][RFC3947]. Such variants can be adapted for use with TCP-AO,
   or IPsec NAT traversal can be used instead in such cases [RFC3947].

12. Evaluation of Requirements Satisfaction

   TCP-AO satisfies all the current requirements for a revision to TCP
   MD5, as summarized below [Be07].

   1. Protected Elements

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

      This is supported - see Section 4.2.

       a. TCP pseudoheader, including IPv4 and IPv6 versions.

          Note that we do not allow optional coverage 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 11.2.

       b. TCP header.

          Note that we do not allow optional port coverage 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
          11.2.




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       c. TCP options.

          Note that TCP-AO allows exclusion of TCP options from
          coverage, to enable use with middleboxes that modify options
          (except when they modify TCP-AO itself). See Section 11.

       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 4.2.

       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 master key index, 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 TSAD entry). See Section 4.2.

       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 - see Sections 8.3, 8.4, and 8.5.

       c. Require non-optional.

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

          This is supported - see Sections 8.3, 8.4, and 8.5.








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       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 4.2.

       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 8.6. The size of the option is
          intended to allow use with Large Windows and SACK. See also
          Section 2.1, which indicates that TCP-AO is 3 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 [RFC-
          TBD], as noted in Section 4.2.

       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.

          TCP-AO uses MACs as indicated in [RFC-TBD]. The PRF is also
          specified in [RFC-TBD]. The PRF input string follows the
          typical design (in Section 6).








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       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 4.2. The TSAD allows
          separate connections to use different algorithms, both for the
          MAC and the PRF.

       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 8.3, 8.4, and 8.5. 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 ESN, which is reconstituted at the receiver using a
          demonstration algorithm that avoids the need for reordering
          (in Section 5).

       e. Security parameter changes require key changes.

          The option should require that the key 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.

          TSAD parameters that should not change during a connection (by
          defininition, e.g., TCP connection ID, receiver TCP connection
          ID, TCP option exclusion list) cannot change. Other parameters
          change only when a master key is changed, using the master key
          tuple mechanism in the TSAD. See Section 7.







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   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).

      Note that TCP-AO does not specify a master key management system,
      but does indicate a proposed interface to the TSAD, allowing a
      completely separate master key system, as noted in Section 7.

       a. Intraconnection rekeying.

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

          This is supported by the KeyID and multiple master key tuples
          in a TSAD entry; see Section 7.

       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 9.

       c. Automated and manual keying.

          The option should support both automated and manual keying.

          The use of a separate TSAD allows external automated and
          manual keying. See Section 9. This capability is enhanced by
          the generation of unique per-connection keys, which enables
          use of manual master keys with automatically generated
          connection keys as noted in Section 6.

       d. Key management agnostic.

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

          This is supported by use of a separate TSAD. See Section 9.1.






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   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 8.3, 8.4, and 8.5.

       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
          4.2.

       c. Outgoing all or none.

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

          This is supported - see Section 8.4.

       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 8.5.

       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 10.








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       f. Optional ICMP discard.

          The option should allow certain ICMPs to be discarded, notably
          Type 3, Codes 2-4.

          This is supported - see Section 13.

       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 7 and 11.

13. Security Considerations

   Use of TCP-AO, like 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. 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 time-
   outs, rather than more responsive recovery after such a crash. As
   noted in Section 6, such cases may also result in persistent TCP
   state for old connections that cannot be cleared, and so
   implementations should be capable of detecting an excess of such
   connections and clearing their state if needed to protect memory
   utilization [Je07].

   TCP-AO does not include a fast decline capability, e.g., where a SYN-
   ACK is received without an expected TCP-AO option 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


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   optimized because it would present an opportunity for attackers on
   the wire to abort authenticated connection attempts by sending
   spoofed SYN-ACKs without the TCP-AO option.

   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 does not address the issue of ICMP attacks on TCP. 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 [Go07].

   >> A TCP-AO implementation MUST allow the system administrator to
   configure whether TCP will ignore incoming ICMP messages of Type 3
   (destination unreachable) Codes 2-4 (protocol unreachable, port
   unreachable, and fragmentation needed - 'hard errors') intended for
   connections that match TSAD entries with non-NONE inbound MACs. 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
   [RFC4301].

   TCP-AO includes the TCP connection ID (the socket pair) in the MAC
   calculation. This prevents different concurrent connections using the
   same connection key (for whatever reason) from potentially enabling a
   traffic-crossing attack, in which segments to one socket pair are
   diverted to attack a different socket pair. When multiple connections
   use the same master key, it would be useful to know that packets
   intended for one ID could not be (maliciously or otherwise) modified
   in transit and end up being authenticated for the other ID. The ID
   cannot be zeroed, because to do so would require that the TSAD index
   was unique in both directions (ID->key and key->ID). That requirement
   would place an additional burden of uniqueness on master keys 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 master key management system.

   The use of any security algorithm can present an opportunity for a
   CPU DOS attack, where the attacker sends false, random segments that
   the receiver under attack expends substantial CPU effort to reject.


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   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 in all cases due to TCP's Sequence Number,
   which is used to reorder received segments. 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 such attacks 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 the TCP-AO option, because doing so
   could cause a change in TCP's behavior even when segments are valid.

14. IANA Considerations

   [NOTE: This section be removed prior to publication as an RFC]

   The TCP-AO option defines no new namespaces.

   The TCP-AO option requires that IANA allocate a value from the TCP
   option Kind namespace, to be replaced for TCP-IANA-KIND throughout
   this document.

   To specify MAC and PRF algorithms, TCP-AO refers to a separate
   document that may involve IANA actions [RFC-TBD].

15. References

15.1. Normative References

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

   [RFC1122] Braden, R., "Requirements for Internet Hosts --
             Communication Layers," RFC-1122, Oct. 1989.





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   [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
             Selective Acknowledgement Options", RFC-2018, Proposed
             Standard, April 1996.

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

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

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

   [RFC2460] Deering, S., Hinden, R., "Internet Protocol, Version 6
             (IPv6) Specification," RFC-2460, Proposed Standard, Dec.
             1998.

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

   [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol,"
             RFC-4306, Proposed Standard, Dec. 2005.

   [RFC-TBD] Lebovitz, G., "MAC Algorithms for TCP-AO," RFC-TBD, date
             TBD.

15.2. Informative References

   [Be07]    Eddy, W., (ed), S. Bellovin, J. Touch, R. Bonica, "Problem
             Statement and Requirements for a TCP Authentication
             Option," draft-bellovin-tcpsec-01, (work in progress), Jul.
             2007.

   [Bo07]    Bonica, R., B. Weis, S. Viswanathan, A. Lange, O. Wheeler,
             "Authentication for TCP-based Routing and Management
             Protocols," draft-bonica-tcp-auth-06, (work in progress),
             Feb. 2007.

   [Go07]    Gont, F., "ICMP attacks against TCP," draft-ietf-tcpm-icmp-
             attacks-04, (work in progress), Oct. 2008.

   [Je07]    Jethanandani, M., and M. Bashyam, "TCP Robustness in
             Persist Condition," draft-mahesh-persist-timeout-02, (work
             in progress), Oct. 2007.


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

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

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

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

   [RFC2766] Tsirtsis, G., Srisuresh, P., "Network Address Translation -
             Protocol Translation (NAT-PT)," RFC-2766, Proposed
             Standard, Feb. 2000.

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

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

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

   [RFC4301] Kent, S., K. Seo, "Security Architecture for the Internet
             Protocol," RFC-4301, Proposed Standard, Dec. 2005.

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

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

   [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.

   [To06]    Touch, J., A. Mankin, "The TCP Simple Authentication
             Option," draft-touch-tcpm-tcp-simple-auth-03, (expired work
             in progress), Oct. 2006.


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

   [We05]    Weis, B., "TCP Message Authentication Code Option," draft-
             weis-tcp-mac-option-00, (expired work in progress), Dec.
             2005.

16. Acknowledgments

   Alfred Hoenes, Charlie Kaufman, and Adam Langley provided substantial
   feedback on this document.

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

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.
   Washington, DC
   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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