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TCPM Working Group                                             R. Bonica
Internet-Draft                                          Juniper Networks
Expires: August 17, 2007                                         B. Weis
                                                          S. Viswanathan
                                                           Cisco Systems
                                                                A. Lange
                                                                 Alcatel
                                                              O. Wheeler
                                                                      BT
                                                       February 13, 2007


     Authentication for TCP-based Routing and Management Protocols
                        draft-bonica-tcp-auth-06

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 17, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo describes a TCP extension that enhances security for BGP,
   LDP and other TCP-based protocols.  It is intended for applications



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   where secure administrative access to both the end-points of the TCP
   connection is normally available.  TCP peers can use this extension
   to authenticate messages passed between one another.

   The strategy described herein improves upon current practice, which
   is described in RFC 2385.  Using this new strategy, TCP peers can
   update authentication keys during the lifetime of a TCP connection.
   TCP peers can also use stronger authentication algorithms to
   authenticate routing messages.


Table of Contents

   1.  Conventions Used In This Document  . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  TCP Enhanced Authentication Option . . . . . . . . . . . . . .  6
   7.  Key Attributes . . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  MAC Calculation  . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  Authentication Algorithms  . . . . . . . . . . . . . . . . . .  9
   10. Migration Issues . . . . . . . . . . . . . . . . . . . . . . . 10
   11. Future Enhancements  . . . . . . . . . . . . . . . . . . . . . 10
   12. Implications . . . . . . . . . . . . . . . . . . . . . . . . . 11
     12.1.  Connectionless Resets . . . . . . . . . . . . . . . . . . 11
     12.2.  Performance . . . . . . . . . . . . . . . . . . . . . . . 11
     12.3.  TCP Header Size . . . . . . . . . . . . . . . . . . . . . 11
     12.4.  Backwards Compatibility . . . . . . . . . . . . . . . . . 12
     12.5.  ICMP-based attacks  . . . . . . . . . . . . . . . . . . . 12
     12.6.  Relationship With TLS . . . . . . . . . . . . . . . . . . 12
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
   14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     17.1.  Normative References  . . . . . . . . . . . . . . . . . . 14
     17.2.  Informative References  . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17











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1.   Conventions Used In This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC2119 [1].


2.  Terminology

   The following terms are used in this document:

      key - A data structure used to authenticate TCP segments.  One or
      more keys can be associated with a TCP connection.  Each key
      contains an identifier, a shared secret, an algorithm identifier,
      an "active flag" and an "eligible flag".

      key set - A set of keys that is associated with a TCP connection.
      A single key set can be associated with multiple TCP connections.
      Each key within a key set contains an identifier that is unique
      within the key set.

      active key - Each key set contains exactly one active key.  The
      sending TCP station uses the shared secret from its active key to
      generate a Message Authentication Code (MAC) for outgoing TCP
      segments.  The "active flag" on a key indicates whether a
      particular key is active.

      eligible key - Each key set contains zero or more eligible keys.
      The receiving TCP station uses the shared secret from a key to
      authenticate an incoming TCP segment only if that key is eligible.
      The "eligible flag" on a key indicates whether a particular key is
      eligible.


3.  Introduction

   RFC 2385 [8] proposes a mechanism that authenticates TCP [2] sessions
   by including a message authentication code (MAC) in each TCP header.
   Authentication coverage includes the following fields:

      - the TCP pseudo-header

      - the TCP header, excluding options, and assuming a checksum of
      zero







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      - the TCP segment data (if any)

   To spoof a connection using the scheme described above, an attacker
   would not only have to guess TCP sequence numbers, but would also
   have to obtain the key that was used to calculate the MAC.  This key
   never appears in the connection stream.

   RFC 3562 [9] addresses key management considerations regarding the
   TCP MD5 Signature Option.  Based upon the strength of the MD5 [10]
   hashing algorithm, RFC 3562 recommends that keys be changed at least
   every 90 days.

   Unfortunately, the strategy described in RFC 2385 permits keys to be
   changed during the lifetime of a TCP connection only so long as the
   change is synchronized at both ends.  This limitation has proven to
   be a significant deterrent to the effective deployment of the TCP MD5
   Signature Option.  This memo addresses that limitation.

   Also, the MD5 algorithm does not now provide a sufficient level of
   security, and recently published attacks motivate its replacement.
   In addition, the keyed hash MAC construction used by RFC 2385 has
   serious cryptographic weaknesses.  An attacker who can find a
   collision in the underlying hash function can forge a MAC using a
   simple chosen-message attack [11].  This memo makes use of MAC
   algorithms that do not have these weaknesses.  It also provides a
   mechanism to add additional algorithms as the state-of-the-art in
   cryptography progresses.


4.  Proposal

   This memo proposes a TCP Enhanced Authentication Option that is used
   as follows:

   Network operators associate a set of keys with each protected TCP
   connection.  Each key contains an identifier that is unique to the
   key set, a shared secret, an algorithm identifier, an "active flag"
   and an "eligible flag".

   Whenever TCP generates a segment, it searches the associated key set
   for its active key.  Each key set MUST have exactly one active key
   that is identified as such by having its "active flag" set.

   Having identified the active key, TCP executes the following
   sequence:






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      - append the TCP Enhanced Authentication Option to the TCP header.
      (See Section 6 of this document for details regarding the TCP
      Enhanced Authentication Option.)

      - update the TCP Enhanced Authentication Option to include the
      active key's unique identifier

      - calculate a Message Authentication Code (MAC) using the shared
      secret from the active key.  (See Section 8 of this document for
      MAC calculation details.)

      - update the TCP Enhanced Authentication Option to include the MAC
      that was calculated above

      - calculate and update the TCP checksum

      - forward the segment to a TCP peer.

   The receiving TCP associates the inbound TCP segment with a local key
   set based upon source IP address, destination IP address, source port
   and destination port.  It then searches the associated key set for a
   key whose identifier matches that which was specified by the incoming
   segment option.

   If TCP finds such a key and if that key's "eligible flag" is set, TCP
   continues processing.  If no matching eligible key is found then TCP
   MUST declare an authentication failure and discard the segment.

   TCP verifies that the algorithm used to produce the MAC is correct.
   The verification is done by comparing the algorithm attribute
   associated with the key with the algorithm id listed in the segment
   option.  If the algorithm identifiers do not match, then the MAC
   calculation will fail.  TCP MUST declare an authentication error and
   discard the segment.

   TCP uses the shared secret from the key to calculate a MAC.  TCP will
   accept the segment if the calculated MAC matches the MAC specified by
   the inbound segment.  Otherwise, TCP MUST declare an authentication
   failure and discard the segment.

   TCP MUST also declare an authentication failure and discard a segment
   if the segment is received from a connection that is associated with
   a key set and the segment does not include the TCP Enhanced
   Authentication Option.

   To help protect against denial of service attacks it is RECOMMENDED
   that the inbound TCP segment is validated against the normal TCP
   criteria (e.g. that the segment sequence number is within the current



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   receive window) prior to the MAC being calculated.  Inbound TCP
   segments on connections requiring the Enhanced Authentication Option
   MUST NOT cause the current TCP state variables for that connection to
   be updated unless the MAC has been verified as correct.

   An authentication failure MUST NOT produce any response back to the
   sender.  Routers SHOULD log authentication failures.

   Unlike other TCP extensions (e.g., the Window Scale option [12]), the
   absence of the option in the SYN,ACK segment must not cause the
   sender to disable its sending of authentication data.  This
   negotiation is typically done to prevent some TCP implementations
   from misbehaving upon receiving options in non-SYN segments.  This is
   not a problem for this option, since the SYN,ACK sent during
   connection negotiation will not be signed and will thus be ignored.
   The connection will never be made, and non-SYN segments with options
   will never be sent.  More importantly, the sending of authentication
   data must be under the complete control of the application, not at
   the mercy of the remote host not understanding the option.


5.  Applications

   The mechanisms described in this memo are intended for use with
   applications that manipulate key set contents.  An application might
   toggle the active and eligible flags based upons system time,
   connection duration or any other external event.


6.  TCP Enhanced Authentication Option

   Figure 1 depicts the TCP Enhanced Authentication Option format.

         0                   1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     Kind      |     Length    |T|K|   Alg ID  |Res|  Key ID   |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                    Authentication Data                        |
        |                            //                                 |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 1: Option Syntax

   Kind: 8 bits

   The Kind field identifies the TCP Enhanced Authentication Option.
   This value will be assigned by IANA.



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   Length: 8 bits

   The Length field specifies the length of the TCP Enhanced
   Authentication Option, in octets.  This count includes two octets
   representing the Kind and Length fields.

   The valid range for this field is from 4 to 40 octets, inclusive.
   For all algorithms specified in this memo the value will be 16
   octets.

   T-Bit: 1 bit

   The T-bit specifies whether TCP Options were omitted from the TCP
   header for the purpose of MAC calculation.  A value of 1 indicates
   that all TCP options other than the Extended Authentication Option
   were omitted.  A value of 0 indicates that TCP options were included.
   The default value is 0.  (See Section 8 of this document for MAC
   calculation details.)

   K-Bit: 1 bit

   This bit is reserved for future enhancement.  Its value MUST be equal
   to zero.  See Section 11 for details.

   Alg ID: 6 bits

   The Alg ID field identifies the MAC algorithm.  See Section 9 for
   permissible values.

   Res: 2 bits

   These bits are reserved.  They MUST be set to zero.

   Key ID: 6 bits

   The Key ID field identifies the key that was used to generate the
   message digest.

   Authentication Data: Variable length

   The Authentication Data field contains data that is used to
   authenticate the TCP segment.  This data includes, but need not be
   restricted to, a MAC.  The length and format of the Authentication
   Data Field can be derived from the Alg ID.  See Section 9 for
   details.






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7.  Key Attributes

   A key set is a set of keys, where each key is {L[i], S[i], A[i],
   E[i]}:

    i     Key identifier, integer (0...63)
    L[i]  Authentication algorithm to use with key[i].
    S[i]  Shared secret to use with key[i].
    A[i]  Active flag to use with key[i]
    E[i]  Eligible flag to use with key[i]

   For the purpose of this document, key[i] is defined as the key whose
   identifer is equal to i.

   A list of values for L[i] is provided in Section 9 of this document.
   The format of S[i] depends upon L[i].  Also see Section 9 for
   details.

   L[i] and S[i] MUST be configured symmetrically on TCP peers.  That
   is, if key[i] is configured on two peer systems, L[i] and S[i] must
   be configured identically on each system.

   For each key set, exactly one element MUST have A[i] set.  Zero or
   more elements MAY have E[i] set.

   In general, network operators should avoid reusing shared secrets.
   The degree to which an operator can reuse keys is defined by local
   security policy.

   During the lifetime of a TCP connection, network operators may add
   and delete keys from the key set.  However, the network operator must
   ensure that the active key is always configured on both TCP
   endpoints.


8.  MAC Calculation

   The sending TCP calculates a MAC by applying the authentication
   algorithm from the active key to the following items in the order
   that they are listed:

      - the TCP pseudo-header

      - the TCP header, assuming a checksum of zero.  (See below for a
      discussion of TCP Options within the the TCP header.)






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      - the TCP segment data (if any)

   For IPv4, the pseudo-header is described in RFC 793 [2].  It includes
   the 32-bit source IP address, the 32-bit destination IP address, the
   zero-extended protocol number (to form 16 bits), and the 16-bit
   segment length.  Note that this includes use of IPv4 via IPv4-mapped
   IPv6 addresses, in which case the source and destination IP addresses
   are from the IPv4 portions of the IPv6 source and destination
   addresses, respectively.

   For IPv6, the pseudo-header is described in RFC 2460 [3].  It
   includes the 128-bit source IPv6 address, the 128-bit destination
   IPv6 address, the zero-extended next header value (to form 32 bits),
   and the 32-bit segment length.

   The header and pseudo-header are in network byte order.

   By default, for the purpose of MAC calculation, the TCP header
   includes all TCP options, including the TCP Enhanced Authentication
   Option with its Authentication Data Field (i.e., MAC) set to zero.
   However, TCP implementations MAY omit all other TCP options from the
   MAC calculation; the TCP Enhanced Authentication Option itself must
   still be included in the calculation, as described above.  When
   implementation do so, they MUST set the T-bit in the TCP Enhanced
   Authentication Option.

   The receiving TCP calculates the MAC in a manner identical to the
   sending TCP.  However, it MUST examine the T-bit from the incoming
   TCP Enhanced Authentication Option to determine whether incoming TCP
   Options should be included in the MAC calculation.


9.  Authentication Algorithms

   The following MAC Algorithms are suitable for use with this option.

      - AES-128-CMAC-96.  AES with a 128-bit key in the CMAC mode of
      operation [4] [5].  When this algorithm is used, implementations
      MUST specify a value of 1 (in binary, 000001) in the TCP Enhanced
      Authentication Option Alg ID field.  Also, the Authentication Data
      field must contain exactly 96 bits representing the MAC, truncated
      to that length, with the high-order bit first.  The
      AES-128-CMAC-96 algorithm MUST be implemented for an
      implementation to conform to this specification.







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      - HMAC-SHA-1-96.  SHA-1 with a 160-bit key in the HMAC mode of
      operation [6].  When this algorithm is used, implementations MUST
      specify a value of 2 (in binary, 000010) in the TCP Enhanced
      Authentication Option ALG ID field.  Also, the Authentication Data
      field must contain exactly 96 bits representing the MAC, truncated
      to that length, with the high-order bit first.  This algorithm MAY
      be implemented for an implementation to conform to this
      specification.

   The above algorithms are expected to safe to use in this application
   for many years.  New algorithms may be added to this list as
   necessary, but it is important that they be properly vetted by the
   cryptographic community.  To this end, the above algorithms are
   described in a list maintained by IANA, and the algorithm identifier
   associated with the algorithm is placed in the TCP Authentication
   Option header.  Other algorithms may also be defined by an
   implementation using a Private Use identifier, but the suitability of
   those algorithms when used with the TCP Extended Authentication
   option is not assured.

   Implementations MUST present a management interface through which the
   user can specify any member of the key space.  For example, if the
   key contains 128 bits, the command line interface might accept this
   value as a string of exactly 32 hexadecimal digits, with each
   hexadecimal digit representing 4 bits of the shared secret.

   Implementations MAY employ authentication algorithms not listed
   above.


10.  Migration Issues

   Either the TCP Enhanced Authentication Option or RFC 2385 may be
   applied to a TCP segment, but the two options SHOULD NOT be present
   in the same TCP segment.  An implementation MAY support both options
   for a particular TCP session during migration from RFC 2385, but they
   MUST use different keys so as not to weaken the security of the TCP
   Enhanced Authentication Option (see the Security Considerations
   section for details).  A receiver SHOULD accept either option.  A
   sender MAY choose to continue sending RFC 2385 options until it has
   evidence that the other TCP endpoint shows use of the TCP Enhanced
   Authentication Option, in which case it migrates to the TCP Enhanced
   Authentication Option.


11.  Future Enhancements

   In the future, the TCP Enhanced Authentication Option will be



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   enhanced to support automated session key distribution.  The K-bit is
   reserved for the purpose of indicating that session key distribution
   extensions are present.  These extensions are beyond the scope of
   this memo.


12.  Implications

12.1.  Connectionless Resets

   A connectionless reset will be ignored by the receiver of the reset
   if the originator of that reset does not know the key and therefore
   cannot generate the proper authentication data for the segment.  This
   means, for example, that connection attempts by a TCP which is
   generating authentication data to a port with no listener will time
   out instead of being refused.  Similarly, resets generated by a TCP
   in response to segments sent on a stale connection will also be
   ignored.  Operationally this can be a problem since resets help some
   protocols recover quickly from peer crashes.

12.2.  Performance

   The performance hit in calculating digests may inhibit the use of
   this option.  Performance will vary depending upon processor type,
   authentication algorithm, packet size and number of MAC calculations
   per second.

12.3.  TCP Header Size

   As with other options that are added to every segment, the size of
   the TCP Enhanced Authentication Option must be factored into the MSS
   offered to the other side during connection negotiation.
   Specifically, the size of the header to subtract from the MTU
   (whether it is the MTU of the outgoing interface or IP's minimal MTU
   of 576 octets) is now increased by the size of the TCP Enhanced
   Authentication Option.

   The total header size is also an issue.  The TCP header specifies
   where segment data starts with a 4-bit field which gives the total
   size of the header (including options) in 32-byte words.  This means
   that the total size of the header plus options must be less than or
   equal to 60 octets.  This leaves 40 octets for options.

   As a concrete example, assume that an implementation defaults to
   sending window-scaling and timestamp information for connections it
   initiates.  The most loaded segment will be the initial SYN packet to
   start the connection.  With the TCP Enhanced Authentication Option
   using AES-128-CMAC-96, the SYN packet will contain the following:



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      -- 4 octets MSS option

      -- 4 octets window scale option (3 octets padded to 4 in many
      implementations)

      -- 12 octets for timestamp

      -- 16 octets for the TCP Enhanced Authentication Option

   This sums to 36 octets, leaving only four octets for future
   expansion.

12.4.  Backwards Compatibility

   On any particular TCP connection, use of the TCP Enhanced
   Authentication Option precludes use of the TCP MD5 Signature Option.
   However, use of the TCP Enhanced Authentication Option on one
   connection does not preclude the use of the TCP MD5 Signature Option
   on another connection by the same system.

12.5.  ICMP-based attacks

   The mechanism described in this document does not provide significant
   protection against ICMP-based attacks [13].

12.6.  Relationship With TLS

   The Transport Layer Security protocol (TLS) [14] provides
   confidentiality and message authentication to TCP connections.
   However, TLS works above the TCP layer, and does not protect the TCP
   connection itself.  In contrast, the TCP Authentication Option
   provides protection against attacks on the TCP layer, such as
   connection reset attacks.


13.  Contributors

   The following individuals contributed to this document:

      Chandrashekhar Appanna (achandra@cisco.com)

      Andy Heffernan (ahh@juniper.net)

      Kapil Jain (kapil@juniper.net)







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      David McGrew (mcgrew@cisco.com)

      Satish Mynam (mynam@cisco.com)

      Anantha Ramaiah (ananth@cisco.com)


14.  Acknowledgments

   Thanks to Steve Bellovin, Ted Faber, Ross Callon, Joe Touch and Ran
   Atkinson for their comments regarding this draft.


15.  Security Considerations

   This proposal describes a strong authentication method for
   authenticating TCP segments.  It defines the use of cryptographic MAC
   algorithms, which are considered state-of-the-art.  As such, their
   expected lifetime of usefulness extends for several years.  But
   cryptographic algorithms have an effective lifetime, depending on
   advancing processor speed and cryptographic research.  This proposal
   provides for the future addition of new MAC algorithms as they are
   needed.

   Management of RFC 2385 keys has been a significant operational
   problem, both in terms of key synchronization and key selection.
   Current guidance [9] warns against sharing RFC 2385 keys between
   systems, and recommends changing keys according to a schedule.  The
   same general operational issues are relevant for the management of
   MAC keys.

   Because the TCP Authentication Option relies on manual configuration,
   it is possible that misconfigurations will occur.  We review the
   scenarios and describe their impact on security.

   When multiple devices are configured with the same key, it is
   possible that one or more of the devices is configured to use the
   wrong MAC algorithm.  If the misconfigured device is using a MAC that
   is significantly weaker than that used by the correctly configured
   devices, where the weakness allows an attacker to recover the MAC
   key, the misconfiguration reduces the security of the properly
   configured devices.  An attacker who can recover the key through
   cryptanalysis of the weaker algorithm can use that information to
   attack the stronger algorithm.  For this reason, implementations
   SHOULD verify the length of the keys entered into the system, and
   reject keys that are too short.  The extent of vulnerability will
   also be reduced when the receiver discards TCP segments due to a MAC
   Algorithm ID mismatch (i.e., the MAC Algorithm ID field in the TCP



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   segment and the MAC Algorithm ID associated with the key do not
   match).  When this event is detected, it SHOULD provide that
   information to an administrator, e.g. through logging or a management
   interface.  This attack is not applicable to AES-CMAC or HMAC, since
   neither of those MACs is vulnerable to a key recovery attack.

   When one or more devices are configured with a particular key, it is
   possible that another device is configured with a slightly different
   key, due to a typographical error.  For example, the two keys might
   differ only in a single hexadecimal digit.  Message authentication
   codes that are vulnerable whenever two related keys are used could be
   vulnerable in this scenario.  In order to protect against this
   potential vulnerability, it is RECOMMENDED that no MACs with such
   vulnerabilities be used.  Neither AES-CMAC nor HMAC have such a
   vulnerability.


16.  IANA Considerations

   The terms "Standards Action" and "Private Use" in this section
   indicate the polices described for these terms in [7].

   A new TCP Option Kind value must be defined in the IANA TCP
   Parameters registry.

   The option header contains an 8-bit ALG ID, for which IANA is to
   create and maintain a registry entitled "MAC Algorithm IDs".  This
   document defines the following message authentication code types:

             MAC Algorithm ID     Value
             ----------------     -----
             RESERVED             0
             AES-128-CMAC-96      1
             HMAC-SHA-1-96        2
             Standards Action     3-47
             Private Use          48-63

   Note to RFC Editor: this section may be removed on publication as an
   RFC.


17.  References

17.1.  Normative References

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




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

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

   [4]   National Institute of Standards and Technology, "Recommendation
         for Block Cipher Modes of Operation: The CMAC Mode for
         Authentication", FIPS PUB 800-38B, May 2005, <http://
         csrc.nist.gov/publications/nistpubs/800-38B/SP_800-38B.pdf>.

   [5]   Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The AES-CMAC
         Algorithm", RFC 4493, June 2006.

   [6]   National Institute of Standards and Technology, "The Keyed-Hash
         Message Authentication Code (HMAC)", FIPS PUB 198, March 2002,
         <http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf>.

   [7]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.

17.2.  Informative References

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

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

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

   [11]  Bellare, M., Canetti, R., and H. Krawczyk, "Keying Hash
         Functions for Message Authentication", Proceedings of
         Crypto'96 , LNCS 1109, pp. 1-15., June 1996, <An extended
         version of this paper is available at
         http://www.research.ibm.com/security/bck2.ps>.

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

   [13]  Gont, F., "ICMP attacks against TCP",
         draft-ietf-tcpm-icmp-attacks-01 (work in progress),
         October 2006.

   [14]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
         RFC 2246, January 1999.



Bonica, et al.           Expires August 17, 2007               [Page 15]

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Authors' Addresses

   Ronald P. Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, VA  20171
   US

   Email: rbonica@juniper.net


   Brian Weis
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134-1706
   US

   Email: bew@cisco.com


   Sriram Viswanathan
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   US

   Email: sriram_v@cisco.com


   Andrew Lange
   Alcatel
   710 E. Middlefield Road
   Mountain View, CA  94043
   US

   Email: andrew.lange@alcatel.com


   Owen N. Wheeler
   British Telecommunications plc
   Adastral Park
   Martlesham Heath
   IPSWICH, Suffolk  IP5 3RE
   GB

   Email: owen.wheeler@bt.com





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