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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: April 2010                                           R. Bonica
                                                       Juniper Networks
                                                       October 27, 2009



                       The TCP Authentication Option
                    draft-ietf-tcpm-tcp-auth-opt-08.txt


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on April 27, 2010.

Copyright Notice

   Copyright (c) 2009 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
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   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document. Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

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 tuple (MKT) configuration or an external, out-of-band MKT
   management mechanism; in either case, TCP-AO also protects
   connections when using the same MKT across repeated instances of a
   connection, using traffic keys derived from the MKT, and coordinates
   MKT changes between endpoints. The result is intended to support
   current infrastructure uses of TCP MD5, such as to protect long-lived
   connections (as used, e.g., in BGP and LDP), and to support a larger
   set of MACs with minimal other system and operational changes. TCP-AO
   uses 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 proposed requirements for the replacement
   of TCP MD5.

Table of Contents


   1. Contributors...................................................3
   2. Introduction...................................................4
      2.1. Executive Summary.........................................4
   3. Conventions used in this document..............................6
   4. The TCP Authentication Option..................................6
      4.1. Review of TCP MD5 Option..................................6
      4.2. The TCP-AO Option.........................................7
   5. TCP-AO Keys and Their Properties...............................9
      5.1. Master Key Tuple..........................................9
      5.2. Traffic Keys.............................................11
      5.3. MKT Properties...........................................12
   6. Per-Connection TCP-AO Parameters..............................13
   7. Cryptographic Algorithms......................................14
      7.1. MAC Algorithms...........................................14
      7.2. Key Derivation Functions.................................17
      7.3. Traffic Key Establishment and Duration Issues............21
         7.3.1. MKT Reuse Across Socket Pairs.......................21
         7.3.2. MKTs Use Within a Long-lived Connection.............22
   8. Additional Security Mechanisms................................22


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      8.1. Coordinating Use of New MKTs.............................22
      8.2. Preventing replay attacks within long-lived connections..23
   9. TCP-AO Interaction with TCP...................................25
      9.1. TCP User Interface.......................................26
      9.2. TCP States and Transitions...............................27
      9.3. TCP Segments.............................................27
      9.4. Sending TCP Segments.....................................28
      9.5. Receiving TCP Segments...................................29
      9.6. Impact on TCP Header Size................................31
   10. Obsoleting TCP MD5 and Legacy Interactions...................32
   11. Interactions with Middleboxes................................32
      11.1. Interactions with non-NAT/NAPT Middleboxes..............33
      11.2. Interactions with NAT/NAPT Devices......................33
   12. Evaluation of Requirements Satisfaction......................33
   13. Security Considerations......................................39
   14. IANA Considerations..........................................41
   15. References...................................................42
      15.1. Normative References....................................42
      15.2. Informative References..................................43
   16. Acknowledgments..............................................44

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





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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
   have been escalating attacks on the algorithm itself [Wa05]. TCP MD5
   also lacks both key management and algorithm agility. This document
   adds the latter, and provides a simple key coordination mechanism
   giving the ability to move from one key to another within the same
   connection. It does not however provide for complete cryptographic
   key management to be handled in-band of TCP, because TCP SYN segments
   lack sufficient remaining space to handle such a negotiation (see
   Section 9.6). 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, coordinate key changes between endpoints, and to provide
   a more structured recommendation on external key management. The
   result is compatible with IPv6, and is fully compatible with proposed
   requirements 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 mechanisms for backward
   compatibility. For a given connection, only one can be in use. TCP
   MD5-protected connections cannot be migrated to TCP-AO because TCP
   MD5 does not support any changes to a connection's security algorithm
   once established.

2.1. Executive Summary

   This document replaces TCP MD5 as follows [RFC2385]:



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   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 can be extended to include other MACs.

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

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

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

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

   o  The TCP-AO option is 2 bytes shorter than TCP MD5 (16 bytes
      overall, rather than 18) in the initially specified 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 an index (IPsec's
      SPI).



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   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 when using IPsec, depending on the
      configuration).

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.

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

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]


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

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

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

   2. The TCP header excluding options and checksum.

   3. The TCP data payload.

   4. A key.

4.2. The 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, a KeyID field, and a
   RNextKeyID field as shown in Figure 2.

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

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

                        Figure 2 The TCP-AO Option

   The TCP-AO defines the fields as follows:

   o  Kind: An unsigned 1-byte field indicating the TCP-AO Option. TCP-
      AO uses a new Kind value of TBD-IANA-KIND.

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

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



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

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

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

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

   o  KeyID: An unsigned 1-byte field indicating the MKT used to
      generate the traffic keys which were used to generate the MAC that
      authenticates this segment.

      It supports efficient key changes during a connection and/or to
      help with key coordination during connection establishment, to be
      discussed further in Section 8.1. Note that the KeyID has no
      cryptographic properties - it need not be random, nor are there
      any reserved values.

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

   o  RNextKeyID: An unsigned 1-byte field indicating the MKT that the
      sender is ready use to receive authenticated segments, i.e., the
      desired 'receive next' keyID.

      It supports efficient key change coordination, to be discussed
      further in Section 8.1. Note that the RNextKeyID has no
      cryptographic properties - it need not be random, nor are there
      any reserved values.

   o  MAC: Message Authentication Code. Its contents are determined by
      the particulars of the security association. Typical MACs are 96-
      128 bits (12-16 bytes), but any length that fits in the header of
      the segment being authenticated is allowed. The MAC computation is
      described further in Section 7.1.

      >> Required support for TCP-AO MACs are defined in [ao-crypto];
      other MACs MAY be supported.



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   The TCP-AO option fields do not indicate the MAC algorithm either
   implicitly (as with TCP MD5) or explicitly. The particular algorithm
   used is considered part of the configuration state of the
   connection's security and is managed separately (see Section 5).

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

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

5. TCP-AO Keys and Their Properties

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

5.1. Master Key Tuple

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

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

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



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

      Each MKT has two IDs - a SendID and a RecvID. The SendID is
      inserted as the KeyID of the TCP-OP option of outgoing segments,
      and the RecvID is matched against the KeyID of the TCP-AO option
      of incoming segments. These and other uses of these two IDs are
      described further in Section 9.4 and 9.5.

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

      ID 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 MKT management mechanism).

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

   o  Master key. A byte sequence used for generating traffic keys, this
      may be derived from a separate shared key by an external protocol
      over a separate channel. This sequence is used in the traffic key
      generation algorithm described in Section 7.2.

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

   o  Key Derivation Function (KDF). Indicates the key derivation
      function and its parameters, as used to generate traffic keys from
      master keys. Explained further in Section 7.1 of this document and
      specified in detail in [ao-crypto].

   o  Message Authentication Code (MAC) algorithm. Indicates the MAC
      algorithm and its parameters as used for this connection,
      explained further in Section 7.1 of this document and specified in
      detail in [ao-crypto].

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

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

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



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

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

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

5.2. Traffic Keys

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

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

   o  Send_SYN_traffic_key

   o  Receive_SYN_traffic_key

   o  Send_other_traffic_key

   o  Receive_other_traffic_key

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

   The relationship between MKTs and traffic keys is shown in Figure
   Figure 3. Traffic keys are indicated with a "*". Note that every MKT
   can be used to derive any of the four traffic keys, but only the keys
   actually needed to handle the segments of a connection need to be
   computed. Section 7.2 provides further details on how traffic keys
   are derived.




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

            Figure 3 Relationship between MKTs and traffic keys

5.3. MKT Properties

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

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

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

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



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

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

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

6. Per-Connection TCP-AO Parameters

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

   1. Current_key - the MKT currently used to authenticate outgoing
      segments, whose SendID is inserted in outgoing segments as KeyID
      (see Section 9.4, step 5). Incoming segments are authenticated
      using the MKT corresponding to the segment and the KeyID in its
      TCP-AO header (see Section 9.5, step 5), as matched against the
      MKT TCP connection identifier and the MKT RecvID. There is only
      one current_key at any given time on a particular connection.

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

   2. Rnext_key -the MKT currently preferred for incoming (received)
      segments, whose RecvID is inserted in outgoing segments as
      RNextKeyID (see Section 9.5, step 5).

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

   3. A pair of Sequence Numbers Extensions (SNEs). SNEs are used to
      prevent replay attacks, as described in Section 8.2. Each SNE is
      initialized to zero upon connection establishment. Its use in the
      MAC calculation is described in Section 7.1.

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

   MKTs are used, together with other parameters of a connection, to
   create traffic keys unique to each connection, as described in


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   Section 7.2. These traffic keys can be cached after computation, and
   can be stored in the TCB with the corresponding MKT information. They
   can be considered part of the per-connection parameters.

7. Cryptographic Algorithms

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

7.1. MAC Algorithms

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

      MAC = MAC_alg(traffic_key, message)

      INPUT: MAC_alg, traffic_key, message

      OUTPUT: MAC

   where:

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

   o  Traffic_key - traffic key used for this computation. This is
      computed from the connection's current MKT as described in Section
      7.2.

   o  Message - input data over which the MAC is computed. In TCP-AO,
      this is the TCP segment prepended by the IP pseudoheader and TCP
      header options, as described in Section 7.1.




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

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

   The MAC algorithm employed for the MAC computation on a connection is
   done so by definition in the MKT, per [ao-crypto]'s definitions.

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

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

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

   1. The sequence number extension (SNE), in network-standard byte
      order, as follows (described further in Section 8.2):

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

                    Figure 4 Sequence number extension

      The SNE for transmitted segments is maintained locally in the
      SND.SNE value; for received segments, a local RCV.SNE value is
      used. The details of how these values are maintained and used is
      described in Sections 8.2, 9.4, and 9.5.

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





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

                  Figure 5 TCP IPv4 pseudoheader [RFC793]



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

                 Figure 6 TCP IPv6 pseudoheader [RFC2460]













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

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

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

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

   Note that the traffic key is not included as part of the data; the
   MAC algorithm indicates how to use the traffic key, e.g., as HMACs do
   [RFC2104][RFC2403]. The traffic key is derived from the current MKT
   as described in Sections 7.2.

7.2. Key Derivation Functions

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

      traffic_key = KDF_alg(master_key, context, output_length)

      INPUT: KDF_alg, master_key, context, output_length

      OUTPUT: traffic_key

   where:

   o  KDF_alg - the specific key derivation function (KDF) that is the
      basic building block used in constructing the traffic key, as
      indicated in the MKT. This is specified as described in [ao-
      crypto].

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

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




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

   o  Traffic_key - The desired output of the KDF, of length
      output_length, to be used as input to the MAC algorithm, as
      described in Section 7.1.

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

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

                Figure 7 KDF Context for an IPv4 connection



















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

                Figure 8 KDF Context for an IPv6 connection

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

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

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



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

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

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

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

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

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

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

   The use of both ISNs in the traffic key computations ensures that
   segments cannot be replayed across repeated connections reusing the
   same socket 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).

   A SYN is authenticated using a destination ISN of zero (whether sent
   or received), and all other segments would be authenticated using the
   ISN pair for the connection. There are other cases in which the
   destination ISN is not known, but segments are emitted, such as after
   an endpoint reboots, when 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


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   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 [Ba09].

7.3. Traffic Key Establishment and Duration Issues

   The TCP-AO option does not provide a mechanism for traffic 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 MKT establishment,
   parameter negotiation, and rekeying. This separation of MKT use from
   MKT 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 MKTs, including the use of reasonable master key lengths,
   limited traffic key sharing, and limiting the duration of MKT use
   [RFC3562]. This also includes the use of per-connection nonces, as
   suggested in Section 7.2.

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

   TCP-AO provides an explicit MKT coordination mechanism, described in
   Section 8.1. Such a mechanism is useful when new MKTs are installed,
   or when MKTs are changed, to determine when to commence using
   installed MKTs.

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

7.3.1. MKT Reuse Across Socket Pairs

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


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   MKTs reused across different socket pairs cannot enable replay
   attacks because the TCP socket pair is included in the MAC, as well
   as in the generation of the traffic key. MKTs reused across repeated
   instances of a given socket pair cannot enable replay attacks because
   the connection ISNs are included in the traffic key generation
   algorithm, and ISN pairs are unlikely to repeat over useful periods.

7.3.2. MKTs Use Within a Long-lived Connection

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

8. Additional Security Mechanisms

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

8.1. Coordinating Use of New MKTs

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

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

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

   o  KeyID

   o  RNextKeyID



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

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

   There are two pointers kept by each side of a connection, as noted in
   the per-connection information (see Section 6):

   o  Currently active outgoing MKT (Current_key)

   o  Current preference for incoming MKT (rnext_key)

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

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

   Rnext_key is changed only by manual user intervention or MKT
   management protocol operation. It is not manipulated by TCP-AO.
   Current_key is updated by TCP-AO when processing received TCP
   segments as discussed in the segment processing description in
   Section 9.5. Note that the algorithm allows the current_key to change
   to a new MKT, then change back to a previously used MKT (known as
   "backing up"). This can occur during a MKT change when segments are
   received out of order, and is considered a feature of TCP-AO, because
   reordering does not result in drops. The only way to avoid reuse of
   previously used MKTs is to remove the MKT when it is no longer
   considered permitted.

8.2. Preventing replay attacks within long-lived connections

   TCP uses a 32-bit sequence number which may, for long-lived
   connections, roll over and repeat. This could result in TCP segments
   being intentionally and legitimately replayed within a connection.


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   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 sequence number extension (SNE) for transmitted and
   received segments.

   The SNE 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 SNE differentiates even identical segments sent with
   identical sequence numbers at different times in a connection. TCP-AO
   emulates a 64-bit sequence number space by inferring when to
   increment the high-order 32-bit portion (the SNE) based on
   transitions in the low-order portion (the TCP sequence number).

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

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

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

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

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

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

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

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

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




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

   In the above code, the first line when the sequence number first
   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 SNE is incremented
   and a flag is set.

   If the flag is set and a high number is seen, it must be a reordered
   segment, so use the pre-increment SNE, otherwise use the current SNE.
   The flag will be cleared by the time the number rolls all the way
   around.

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

9. TCP-AO Interaction with TCP

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





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9.1. 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 MKTs
   to be configured, as well as to allow an ongoing connection to manage
   which MKTs are active. The MKTs need to be configured prior to
   connection establishment, and the set of MKTs may change during a
   connection:

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

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

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

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

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

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

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

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




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

9.2. 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 MKT MAY be associated with any TCP state.

9.3. TCP Segments

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

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

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

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

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

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

   Note that use of the TCP-AO option is not negotiated within TCP. It
   is the responsibility of the receiver to determine when TCP-AO is


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   required via other means (e.g., out of band, manually or with an key
   management protocol) and to enforce that requirement.

9.4. Sending TCP Segments

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

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

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

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

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

          ii. If there is a matching MKT, then set the per-connection
               parameters as needed (see Section 6). Proceed with the
               step 2.

       b. If the segment is not a SYN, then determine whether TCP-AO is
          being used for the connection and use the MKT as indicated by
          the current_key value from the per-connection parameters (see
          Section 6) and proceed with the step 2.

   2. Using the per-connection parameters:

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

       b. Determine SND.SNE as described in Section 8.2.

       c. Determine the appropriate traffic key, i.e., as pointed to by
          current_key (as noted in Section 8.1, and as probably cached
          in the TCB). I.e., use the send_SYN_traffic_key for SYN
          segments, and the send_other_traffic_key for other segments.






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       d. Determine the RNextKeyID as indicated by the rnext_key
          pointer, and insert it in the TCP-AO option (using the
          rnext_key MKT's RecvID as the TCP-AO KeyID) (as noted in
          Section 8.1).

       e. Compute the MAC using the MKT (and cached traffic key) and
          data from the segment as specified in Section 7.1.

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

       g. Proceed with transmitting the segment.

9.5. Receiving TCP Segments

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

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

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

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

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

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

               NOTE: this presumes that connections that do not match
               any MKT should be silently accepted, as noted in Sec 9.3.

          ii. If there is a matching MKT, then set the per-connection
               parameters as needed (see Section 6). Proceed with the
               step 2.

   2. Using the per-connection parameters:


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

           i. If lengths differ, silently discard the segment. Log
               and/or signal the event as indicated in Section 9.3.

       b. Determine the segment's RCV.SNE as described in Section 8.2.

       c. Determine the segment's traffic key from the MKT as described
          in Section 7.1 (and as likely cached in the TCB). I.e., use
          the receive_SYN_traffic_key for SYN segments, and the
          receive_other_traffic_key for other segments.

       d. Compute the segment's MAC using the MKT (and its derived
          traffic key) and portions of the segment as indicated in
          Section 7.1.

           i. If the computed MAC differs from the TCP-AO MAC field
               value, silently discard the segment. Log and/or signal
               the event as indicated in Section 9.3.

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

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

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

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

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

                     a. Set Current_key to the RNextKeyID MKT.

       f. Proceed with TCP processing of the segment.

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

   Additional reductions in MAC validation overhead can be supported in
   the MAC algorithms, e.g., by using a computation algorithm that


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   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.

9.6. Impact on TCP Header Size

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

   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.

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

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

   o  Timestamps (10 bytes) [RFC1323]

   o  Window scale (3 bytes) [RFC1323]

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

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

   o  Timestamps (10 bytes) [RFC1323]

   TCP-AO continues to consume 16 bytes in non-SYN segments, leaving a
   total of 24 bytes for other options, of which the timestamp consumes
   10. This leaves 14 bytes, of which 10 are used for a single SACK
   block. When two SACK blocks are used, such as to handle D-SACK, a
   smaller TCP-AO MAC would be required to make room for the additional
   SACK block (i.e., to leave 18 bytes for the D-SACK variant of the
   SACK option) [RFC2883]. Note that D-SACK is not supportable in TCP


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   MD5 in the presence of timestamps, because TCP MD5's MAC length is
   fixed and too large to leave sufficient option space.

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

10. Obsoleting TCP MD5 and Legacy Interactions

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

   >> 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 MKTs could be augmented to support TCP MD5,
   although use of MKTs 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


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   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.

   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 may require variants of existing NAT/NAPT
   traversal mechanisms, e.g., encapsulation of the TCP-AO-protected
   segment in another transport segment (e.g., UDP), as is done in IPsec
   [RFC2663][RFC3947]. Such variants can be adapted for use with TCP-AO,
   or IPsec NAT traversal can be used instead in such cases [RFC3947].

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

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 7.1.







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       a. IP 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.

       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 7.1.

       a. Privacy.

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

          TCP-AO exposes only the MKT IDs, MAC, and overall option
          length on the wire. Note that short MACs could be obscured by
          using longer option lengths but specifying a short MAC length
          (this is equivalent to a different MAC algorithm, and is
          specified in the MKT). See Section 4.2.






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       b. Allow optional per connection.

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

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

       c. Require non-optional.

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

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

       d. Standard parsing.

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

          This is supported - see Section 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 9.6. 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 2 bytes shorter
          than TCP MD5 in the default case, assuming a 96-bit MAC.

   3. Cryptography requirements

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






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       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 [ao-
          crypto], as noted in Section 7.1.

       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 [ao-crypto]. The KDF is also
          specified in [ao-crypto]. The KDF input string follows the
          typical design (see [ao-crypto]).

       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 use of set of
          MKTs allows separate connections to use different algorithms,
          both for the MAC and the KDF.

       d. Order-independent processing.

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

          This is supported - see Sections 9.3, 9.4, and 9.5. Note that
          pre-TCP processing is further required, because TCP segments
          cannot be discarded solely based on a combination of
          connection state and out-of-window checks; many such segments,
          although discarded, cause a host to respond with a replay of
          the last valid ACK, e.g. [RFC793]. See also the derivation of
          the SNE, which is reconstituted at the receiver using a
          demonstration algorithm that avoids the need for reordering
          (in Section 8.2).



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       e. Security parameter changes require key changes.

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

          Parameters change only when a new MKT is used. See Section 5.

   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 MKT management system.

       a. Intraconnection rekeying.

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

          This is supported by the use of IDs and multiple MKTs; see
          Section 5.

       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 8.1.

       c. Automated and manual keying.

          The option should support both automated and manual keying.

          The use of MKTs allows external automated and manual keying.
          See Section 5. This capability is enhanced by the generation
          of unique per-connection keys, which enables use of manual
          MKTs with automatically generated traffic keys as noted in
          Section 7.2.




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       d. Key management agnostic.

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

          This is supported by use of a set of MKTs. See Section 5.

   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 9.3, 9.4, and 9.5.

       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 9.4.

       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 9.5.









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       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.

       f. Optional ICMP discard.

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

          This is supported - see Section 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 5 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


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   noted in Section 7.2, 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 [Ba09].

   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
   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 [Go09].

   >> 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 MKTs. 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 MKT (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 MKT, it would be useful to know that segments intended for one
   ID could not be (maliciously or otherwise) modified in transit and
   end up being authenticated for the other ID. That requirement would


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   place an additional burden of uniqueness on MKTs within endsystems,
   and potentially across endsystems. Although the resulting attack is
   low probability, the protection afforded by including the received ID
   warrants its inclusion in the MAC, and does not unduly increase the
   MAC calculation or MKT management.

   The use of any security algorithm can present an opportunity for a
   CPU DOS attack, where the attacker sends false, random segments that
   the receiver under attack expends substantial CPU effort to reject.
   In IPsec, such attacks are reduced by the use of a large Security
   Parameter Index (SPI) and Sequence Number fields to partly validate
   segments before CPU cycles are invested validated the Integrity Check
   Value (ICV). In TCP-AO, the socket pair performs most of the function
   of IPsec's SPI, and IPsec's Sequence Number, used to avoid replay
   attacks, isn't needed due to TCP's Sequence Number, which is used to
   reorder received segments (provided the sequence number doesn't wrap
   around, which is why TCP-AO adds the SNE in Section 8.2). TCP already
   protects itself from replays of authentic segment data as well as
   authentic explicit TCP control (e.g., SYN, FIN, ACK bits, but even
   authentic replays could affect TCP congestion control [Sa99]. TCP-AO
   does not protect TCP congestion control from this last form of attack
   due to the cumbersome nature of layering a windowed security sequence
   number within TCP in addition to TCP's own sequence number; when such
   protection is desired, users are encouraged to apply IPsec instead.

   Further, it is not useful to validate TCP's Sequence Number before
   performing a TCP-AO authentication calculation, because out-of-window
   segments can still cause valid TCP protocol actions (e.g., ACK
   retransmission) [RFC793]. It is similarly not useful to add a
   separate Sequence Number field to 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 KDF algorithms, TCP-AO refers to a separate
   document that may involve IANA actions [ao-crypto].





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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.

   [RFC2018] Mathis, M., J. Mahdavi, S. Floyd, 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., R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification," RFC-2460, Proposed Standard, Dec.
             1998.

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

   [RFC3517] Blanton, E., M. Allman, K. Fall, 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.

   [ao-crypto] Lebovitz, G., E. Rescorla, "Cryptographic Algorithms for
             TCP's Authentication Option, TCP-AO", draft-ietf-tcpm-tcp-
             ao-crypto-02, Oct. 2009.






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15.2. Informative References

   [ao-nat]  Touch, J., "A TCP Authentication Option NAT Extension,"
             draft-touch-tcp-ao-nat-00, Oct. 2009.

   [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.

   [Bo09]    Borman, D., "TCP Options and MSS," draft-ietf-tcpm-tcpmss-
             02, Jul. 2009.

   [Go09]    Gont, F., "ICMP attacks against TCP," draft-ietf-tcpm-icmp-
             attacks-06, (work in progress), Aug. 2009.

   [Ba09]    Bashyam, M., M. Jethanandani,, A. Ramaiah "Clarification of
             sender behaviour in persist condition," draft-ananth-tcpm-
             persist-01, (work in progress), Jul. 2009.

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

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

   [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.


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   [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.

   [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.











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Authors' Addresses

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

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


   Allison Mankin
   Johns Hopkins Univ.
   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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