





Network Working Group                                          M. Tuexen
Request for Comments: 4895            Muenster Univ. of Applied Sciences
Category: Standards Track                                     R. Stewart
                                                                  P. Lei
                                                     Cisco Systems, Inc.
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                             August 2007


                       Authenticated Chunks for
            the Stream Control Transmission Protocol (SCTP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes a new chunk type, several parameters, and
   procedures for the Stream Control Transmission Protocol (SCTP).  This
   new chunk type can be used to authenticate SCTP chunks by using
   shared keys between the sender and receiver.  The new parameters are
   used to establish the shared keys.























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  New Parameter Types  . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Random Parameter (RANDOM)  . . . . . . . . . . . . . . . .  4
     3.2.  Chunk List Parameter (CHUNKS)  . . . . . . . . . . . . . .  5
     3.3.  Requested HMAC Algorithm Parameter (HMAC-ALGO) . . . . . .  6
   4.  New Error Cause  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Unsupported HMAC Identifier Error Cause  . . . . . . . . .  7
   5.  New Chunk Type . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Authentication Chunk (AUTH)  . . . . . . . . . . . . . . .  8
   6.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Establishment of an Association Shared Key . . . . . . . . 10
     6.2.  Sending Authenticated Chunks . . . . . . . . . . . . . . . 11
     6.3.  Receiving Authenticated Chunks . . . . . . . . . . . . . . 12
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  A New Chunk Type . . . . . . . . . . . . . . . . . . . . . 15
     8.2.  Three New Parameter Types  . . . . . . . . . . . . . . . . 15
     8.3.  A New Error Cause  . . . . . . . . . . . . . . . . . . . . 15
     8.4.  A New Table for HMAC Identifiers . . . . . . . . . . . . . 16
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   11. Normative References . . . . . . . . . . . . . . . . . . . . . 17


























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1.  Introduction

   SCTP uses 32-bit verification tags to protect itself against blind
   attackers.  These values are not changed during the lifetime of an
   SCTP association.

   Looking at new SCTP extensions, there is the need to have a method of
   proving that an SCTP chunk(s) was really sent by the original peer
   that started the association and not by a malicious attacker.

   Using Transport Layer Security (TLS), as defined in RFC 3436 [6],
   does not help because it only secures SCTP user data.

   Therefore, an SCTP extension that provides a mechanism for deriving
   shared keys for each association is presented.  These association
   shared keys are derived from endpoint pair shared keys, which are
   configured and might be empty, and data that is exchanged during the
   SCTP association setup.

   The extension presented in this document allows an SCTP sender to
   authenticate chunks using shared keys between the sender and
   receiver.  The receiver can then verify that the chunks are sent from
   the sender and not from a malicious attacker (as long as the attacker
   does not know an association shared key).

   The extension described in this document places the result of a
   Hashed Message Authentication Code (HMAC) computation before the data
   covered by that computation.  Placing it at the end of the packet
   would have required placing a control chunk after DATA chunks in case
   of authenticating DATA chunks.  This would break the rule that
   control chunks occur before DATA chunks in SCTP packets.  It should
   also be noted that putting the result of the HMAC computation after
   the data being covered would not allow sending the packet during the
   computation of the HMAC because the result of the HMAC computation is
   needed to compute the CRC32C checksum of the SCTP packet, which is
   placed in the common header of the SCTP packet.

   The SCTP extension for Dynamic Address Reconfiguration (ADD-IP)
   requires the usage of the extension described in this document.  The
   SCTP Partial Reliability Extension (PR-SCTP) can be used in
   conjunction with the extension described in this document.

2.  Conventions

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL", when they appear in this document, are to be interpreted
   as described in RFC 2119 [3].



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3.  New Parameter Types

   This section defines the new parameter types that will be used to
   negotiate the authentication during association setup.  Table 1
   illustrates the new parameter types.

    +----------------+------------------------------------------------+
    | Parameter Type | Parameter Name                                 |
    +----------------+------------------------------------------------+
    | 0x8002         | Random Parameter (RANDOM)                      |
    | 0x8003         | Chunk List Parameter (CHUNKS)                  |
    | 0x8004         | Requested HMAC Algorithm Parameter (HMAC-ALGO) |
    +----------------+------------------------------------------------+

                                  Table 1

   Note that the parameter format requires the receiver to ignore the
   parameter and continue processing if the parameter is not understood.
   This is accomplished (as described in RFC 2960 [5], Section 3.2.1.)
   by the use of the upper bits of the parameter type.

3.1.  Random Parameter (RANDOM)

   This parameter is used to carry a random number of an arbitrary
   length.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter Type = 0x8002   |       Parameter Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   \                          Random Number                        /
   /                               +-------------------------------\
   |                               |           Padding             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

   Parameter Type: 2 bytes (unsigned integer)
      This value MUST be set to 0x8002.

   Parameter Length: 2 bytes (unsigned integer)
      This value is the length of the Random Number in bytes plus 4.







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   Random Number: n bytes (unsigned integer)
      This value represents an arbitrary Random Number in network byte
      order.

   Padding: 0, 1, 2, or 3 bytes (unsigned integer)
      If the length of the Random Number is not a multiple of 4 bytes,
      the sender MUST pad the parameter with all zero bytes to make the
      parameter 32-bit aligned.  The Padding MUST NOT be longer than 3
      bytes and it MUST be ignored by the receiver.

   The RANDOM parameter MUST be included once in the INIT or INIT-ACK
   chunk, if the sender wants to send or receive authenticated chunks,
   to provide a 32-byte Random Number.  For 32-byte Random Numbers, the
   Padding is empty.

3.2.  Chunk List Parameter (CHUNKS)

   This parameter is used to specify which chunk types are required to
   be authenticated before being sent by the peer.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter Type = 0x8003   |       Parameter Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Chunk Type 1  | Chunk Type 2  | Chunk Type 3  | Chunk Type 4  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               /
   \                              ...                              \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Chunk Type n  |                   Padding                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

   Parameter Type: 2 bytes (unsigned integer)
      This value MUST be set to 0x8003.

   Parameter Length: 2 bytes (unsigned integer)
      This value is the number of listed Chunk Types plus 4.

   Chunk Type n: 1 byte (unsigned integer)
      Each Chunk Type listed is required to be authenticated when sent
      by the peer.






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   Padding: 0, 1, 2, or 3 bytes (unsigned integer)
      If the number of Chunk Types is not a multiple of 4, the sender
      MUST pad the parameter with all zero bytes to make the parameter
      32-bit aligned.  The Padding MUST NOT be longer than 3 bytes and
      it MUST be ignored by the receiver.

   The CHUNKS parameter MUST be included once in the INIT or INIT-ACK
   chunk if the sender wants to receive authenticated chunks.  Its
   maximum length is 260 bytes.

   The chunk types for INIT, INIT-ACK, SHUTDOWN-COMPLETE, and AUTH
   chunks MUST NOT be listed in the CHUNKS parameter.  However, if a
   CHUNKS parameter is received then the types for INIT, INIT-ACK,
   SHUTDOWN-COMPLETE, and AUTH chunks MUST be ignored.

3.3.  Requested HMAC Algorithm Parameter (HMAC-ALGO)

   This parameter is used to list the HMAC Identifiers the peer MUST
   use.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter Type = 0x8004   |       Parameter Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          HMAC Identifier 1    |      HMAC Identifier 2        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               /
   \                              ...                              \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        HMAC Identifier n      |           Padding             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 3

   Parameter Type: 2 bytes (unsigned integer)
      This value MUST be set to 0x8004.

   Parameter Length: 2 bytes (unsigned integer)
      This value is the number of HMAC Identifiers multiplied by 2, plus
      4.

   HMAC Identifier n: 2 bytes (unsigned integer)
      The values expressed are a list of HMAC Identifiers that may be
      used by the peer.  The values are listed by preference, with
      respect to the sender, where the first HMAC Identifier listed is
      the one most preferable to the sender.



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   Padding: 0 or 2 bytes (unsigned integer)
      If the number of HMAC Identifiers is not even, the sender MUST pad
      the parameter with all zero bytes to make the parameter 32-bit
      aligned.  The Padding MUST be 0 or 2 bytes long and it MUST be
      ignored by the receiver.

   The HMAC-ALGO parameter MUST be included once in the INIT or INIT-ACK
   chunk if the sender wants to send or receive authenticated chunks.

   Table 2 shows the currently defined values for HMAC Identifiers.

              +-----------------+--------------------------+
              | HMAC Identifier | Message Digest Algorithm |
              +-----------------+--------------------------+
              | 0               | Reserved                 |
              | 1               | SHA-1 defined in [8]     |
              | 2               | Reserved                 |
              | 3               | SHA-256 defined in [8]   |
              +-----------------+--------------------------+

                                  Table 2

   Every endpoint supporting SCTP chunk authentication MUST support the
   HMAC based on the SHA-1 algorithm.

4.  New Error Cause

   This section defines a new error cause that will be sent if an AUTH
   chunk is received with an unsupported HMAC Identifier.  Table 3
   illustrates the new error cause.

               +------------+-----------------------------+
               | Cause Code | Error Cause Name            |
               +------------+-----------------------------+
               | 0x0105     | Unsupported HMAC Identifier |
               +------------+-----------------------------+

                                  Table 3

4.1.  Unsupported HMAC Identifier Error Cause

   This error cause is used to indicate that an AUTH chunk has been
   received with an unsupported HMAC Identifier.








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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Cause Code = 0x0105      |       Cause Length = 6        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         HMAC Identifier       |            Padding            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 4

   Cause Code: 2 bytes (unsigned integer)
      This value MUST be set to 0x0105.

   Cause Length: 2 bytes (unsigned integer)
      This value MUST be set to 6.

   HMAC Identifier: 2 bytes (unsigned integer)
      This value is the HMAC Identifier which is not supported.

   Padding: 2 bytes (unsigned integer)
      The sender MUST pad the error cause with all zero bytes to make
      the cause 32-bit aligned.  The Padding MUST be 2 bytes long and it
      MUST be ignored by the receiver.

5.  New Chunk Type

   This section defines the new chunk type that will be used to
   authenticate chunks.  Table 4 illustrates the new chunk type.

               +------------+-----------------------------+
               | Chunk Type | Chunk Name                  |
               +------------+-----------------------------+
               | 0x0F       | Authentication Chunk (AUTH) |
               +------------+-----------------------------+

                                  Table 4

   It should be noted that the AUTH-chunk format requires the receiver
   to ignore the chunk if it is not understood and silently discard all
   chunks that follow.  This is accomplished (as described in RFC 2960
   [5], Section 3.2.) by the use of the upper bits of the chunk type.

5.1.  Authentication Chunk (AUTH)

   This chunk is used to hold the result of the HMAC calculation.






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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type = 0x0F   |   Flags=0     |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Shared Key Identifier      |        HMAC Identifier        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   \                             HMAC                              /
   /                                                               \
   /                               +-------------------------------\
   |                               |           Padding             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 5

   Type: 1 byte (unsigned integer)
      This value MUST be set to 0x0F for all AUTH-chunks.

   Flags: 1 byte (unsigned integer)
      SHOULD be set to zero on transmit and MUST be ignored on receipt.

   Length: 2 bytes (unsigned integer)
      This value holds the length of the HMAC in bytes plus 8.

   Shared Key Identifier: 2 bytes (unsigned integer)
      This value describes which endpoint pair shared key is used.

   HMAC Identifier: 2 bytes (unsigned integer)
      This value describes which message digest is being used.  Table 2
      shows the currently defined values.

   HMAC: n bytes (unsigned integer)
      This holds the result of the HMAC calculation.

   Padding: 0, 1, 2, or 3 bytes (unsigned integer)
      If the length of the HMAC is not a multiple of 4 bytes, the sender
      MUST pad the chunk with all zero bytes to make the chunk 32-bit
      aligned.  The Padding MUST NOT be longer than 3 bytes and it MUST
      be ignored by the receiver.

   The control chunk AUTH MUST NOT appear more than once in an SCTP
   packet.  All control and data chunks that are placed after the AUTH
   chunk in the packet are sent in an authenticated way.  Those chunks
   placed in a packet before the AUTH chunk are not authenticated.
   Please note that DATA chunks can not appear before control chunks in
   an SCTP packet.




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6.  Procedures

6.1.  Establishment of an Association Shared Key

   An SCTP endpoint willing to receive or send authenticated chunks MUST
   send one RANDOM parameter in its INIT or INIT-ACK chunk.  The RANDOM
   parameter MUST contain a 32-byte Random Number.  The Random Number
   should be generated in accordance with RFC 4086 [7].  If the Random
   Number is not 32 bytes, the association MUST be aborted.  The ABORT
   chunk SHOULD contain the error cause 'Protocol Violation'.  In case
   of INIT collision, the rules governing the handling of this Random
   Number follow the same pattern as those for the Verification Tag, as
   explained in Section 5.2.4 of RFC 2960 [5].  Therefore, each endpoint
   knows its own Random Number and the peer's Random Number after the
   association has been established.

   An SCTP endpoint has a list of chunks it only accepts if they are
   received in an authenticated way.  This list is included in the INIT
   and INIT-ACK, and MAY be omitted if it is empty.  Since this list
   does not change during the lifetime of the SCTP endpoint there is no
   problem in case of INIT collision.

   Each SCTP endpoint MUST include in the INIT and INIT-ACK a HMAC-ALGO
   parameter containing a list of HMAC Identifiers it requests the peer
   to use.  The receiver of an HMAC-ALGO parameter SHOULD use the first
   listed algorithm it supports.  The HMAC algorithm based on SHA-1 MUST
   be supported and included in the HMAC-ALGO parameter.  An SCTP
   endpoint MUST NOT change the parameters listed in the HMAC-ALGO
   parameter during the lifetime of the endpoint.

   Both endpoints of an association MAY have endpoint pair shared keys
   that are byte vectors and pre-configured or established by another
   mechanism.  They are identified by the Shared Key Identifier.  For
   each endpoint pair shared key, an association shared key is computed.
   If there is no endpoint pair shared key, only one association shared
   key is computed by using an empty byte vector as the endpoint pair
   shared key.

   The RANDOM parameter, the CHUNKS parameter, and the HMAC-ALGO
   parameter sent by each endpoint are concatenated as byte vectors.
   These parameters include the parameter type, parameter length, and
   the parameter value, but padding is omitted; all padding MUST be
   removed from this concatenation before proceeding with further
   computation of keys.  Parameters that were not sent are simply
   omitted from the concatenation process.  The resulting two vectors
   are called the two key vectors.





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   From the endpoint pair shared keys and the key vectors, the
   association shared keys are computed.  This is performed by selecting
   the numerically smaller key vector and concatenating it to the
   endpoint pair shared key, and then concatenating the numerically
   larger key vector to that.  If the key vectors are equal as numbers
   but differ in length, then the concatenation order is the endpoint
   shared key, followed by the shorter key vector, followed by the
   longer key vector.  Otherwise, the key vectors are identical, and may
   be concatenated to the endpoint pair key in any order.  The
   concatenation is performed on byte vectors, and all numerical
   comparisons use network byte order to convert the key vectors to a
   number.  The result of the concatenation is the association shared
   key.

6.2.  Sending Authenticated Chunks

   Endpoints MUST send all requested chunks that have been authenticated
   where this has been requested by the peer.  The other chunks MAY be
   sent whether or not they have been authenticated.  If endpoint pair
   shared keys are used, one of them MUST be selected for
   authentication.

   To send chunks in an authenticated way, the sender MUST include these
   chunks after an AUTH chunk.  This means that a sender MUST bundle
   chunks in order to authenticate them.

   If the endpoint has no endpoint pair shared key for the peer, it MUST
   use Shared Key Identifier zero with an empty endpoint pair shared
   key.  If there are multiple endpoint shared keys the sender selects
   one and uses the corresponding Shared Key Identifier.

   The sender MUST calculate the Message Authentication Code (MAC) (as
   described in RFC 2104 [2]) using the hash function H as described by
   the HMAC Identifier and the shared association key K based on the
   endpoint pair shared key described by the Shared Key Identifier.  The
   'data' used for the computation of the AUTH-chunk is given by the
   AUTH chunk with its HMAC field set to zero (as shown in Figure 6)
   followed by all the chunks that are placed after the AUTH chunk in
   the SCTP packet.












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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type = 0x0F   |   Flags=0     |         Chunk Length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Shared Key Identifier      |        HMAC Identifier        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   \                               0                               /
   /                               +-------------------------------\
   |                               |           Padding             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 6

   Please note that all fields are in network byte order and that the
   field that will contain the complete HMAC is filled with zeroes.  The
   length of the field shown as zero is the length of the HMAC described
   by the HMAC Identifier.  The padding of all chunks being
   authenticated MUST be included in the HMAC computation.

   The sender fills the HMAC into the HMAC field and sends the packet.

6.3.  Receiving Authenticated Chunks

   The receiver has a list of chunk types that it expects to be received
   only after an AUTH-chunk.  This list has been sent to the peer during
   the association setup.  It MUST silently discard these chunks if they
   are not placed after an AUTH chunk in the packet.

   The receiver MUST use the HMAC algorithm indicated in the HMAC
   Identifier field.  If this algorithm was not specified by the
   receiver in the HMAC-ALGO parameter in the INIT or INIT-ACK chunk
   during association setup, the AUTH chunk and all the chunks after it
   MUST be discarded and an ERROR chunk SHOULD be sent with the error
   cause defined in Section 4.1.

   If an endpoint with no shared key receives a Shared Key Identifier
   other than 0, it MUST silently discard all authenticated chunks.  If
   the endpoint has at least one endpoint pair shared key for the peer,
   it MUST use the key specified by the Shared Key Identifier if a key
   has been configured for that Shared Key Identifier.  If no endpoint
   pair shared key has been configured for that Shared Key Identifier,
   all authenticated chunks MUST be silently discarded.

   The receiver now performs the same calculation as described for the
   sender based on Figure 6.  If the result of the calculation is the






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   same as given in the HMAC field, all the chunks following the AUTH
   chunk are processed.  If the field does not match the result of the
   calculation, all the chunks following the AUTH chunk MUST be silently
   discarded.

   It should be noted that if the receiver wants to tear down an
   association in an authenticated way only, the handling of malformed
   packets should not result in tearing down the association.

   An SCTP implementation has to maintain state for each SCTP
   association.  In the following, we call this data structure the SCTP
   transmission control block (STCB).

   When an endpoint requires COOKIE-ECHO chunks to be authenticated,
   some special procedures have to be followed because the reception of
   a COOKIE-ECHO chunk might result in the creation of an SCTP
   association.  If a packet arrives containing an AUTH chunk as a first
   chunk, a COOKIE-ECHO chunk as the second chunk, and possibly more
   chunks after them, and the receiver does not have an STCB for that
   packet, then authentication is based on the contents of the COOKIE-
   ECHO chunk.  In this situation, the receiver MUST authenticate the
   chunks in the packet by using the RANDOM parameters, CHUNKS
   parameters and HMAC_ALGO parameters obtained from the COOKIE-ECHO
   chunk, and possibly a local shared secret as inputs to the
   authentication procedure specified in Section 6.3.  If authentication
   fails, then the packet is discarded.  If the authentication is
   successful, the COOKIE-ECHO and all the chunks after the COOKIE-ECHO
   MUST be processed.  If the receiver has an STCB, it MUST process the
   AUTH chunk as described above using the STCB from the existing
   association to authenticate the COOKIE-ECHO chunk and all the chunks
   after it.

   If the receiver does not find an STCB for a packet containing an AUTH
   chunk as the first chunk and does not find a COOKIE-ECHO chunk as the
   second chunk, it MUST use the chunks after the AUTH chunk to look up
   an existing association.  If no association is found, the packet MUST
   be considered as out of the blue.  The out of the blue handling MUST
   be based on the packet without taking the AUTH chunk into account.
   If an association is found, it MUST process the AUTH chunk using the
   STCB from the existing association as described earlier.

   Requiring ABORT chunks and COOKIE-ECHO chunks to be authenticated
   makes it impossible for an attacker to bring down or restart an
   association as long as the attacker does not know the association
   shared key.  But it should also be noted that if an endpoint accepts
   ABORT chunks only in an authenticated way, it may take longer to
   detect that the peer is no longer available.  If an endpoint accepts
   COOKIE-ECHO chunks only in an authenticated way, the restart



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   procedure does not work, because the restarting endpoint most likely
   does not know the association shared key of the old association to be
   restarted.  However, if the restarting endpoint does know the old
   association shared key, he can successfully send the COOKIE-ECHO
   chunk in a way that it is accepted by the peer by using this old
   association shared key for the packet containing the AUTH chunk.
   After this operation, both endpoints have to use the new association
   shared key.

   If a server has an endpoint pair shared key with some clients, it can
   request the COOKIE_ECHO chunk to be authenticated and can ensure that
   only associations from clients with a correct endpoint pair shared
   key are accepted.

   Furthermore, it is important that the cookie contained in an INIT-ACK
   chunk and in a COOKIE-ECHO chunk MUST NOT contain any endpoint pair
   shared keys.

7.  Examples

   This section gives examples of message exchanges for association
   setup.

   The simplest way of using the extension described in this document is
   given by the following message exchange.

       ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
       <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
       -------------------- COOKIE-ECHO -------------------->
       <-------------------- COOKIE-ACK ---------------------

   Please note that the CHUNKS parameter is optional in the INIT and
   INIT-ACK.

   If the server wants to receive DATA chunks in an authenticated way,
   the following message exchange is possible:

       ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
       <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
       --------------- COOKIE-ECHO; AUTH; DATA ------------->
       <----------------- COOKIE-ACK; SACK ------------------

   Please note that if the endpoint pair shared key depends on the
   client and the server, and is only known by the upper layer, this
   message exchange requires an upper layer intervention between the
   processing of the COOKIE-ECHO chunk and the processing of the AUTH
   and DATA chunk at the server side.  This intervention may be realized
   by a COMMUNICATION-UP notification followed by the presentation of



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   the endpoint pair shared key by the upper layer to the SCTP stack,
   see for example Section 10 of RFC 2960 [5].  If this intervention is
   not possible due to limitations of the API (for example, the socket
   API), the server might discard the AUTH and DATA chunk, making a
   retransmission of the DATA chunk necessary.  If the same endpoint
   pair shared key is used for multiple endpoints and does not depend on
   the client, this intervention might not be necessary.

8.  IANA Considerations

   This document (RFC 4895) is the reference for all registrations
   described in this section.  All registrations need to be listed in
   the document available at SCTP-parameters [9].  The changes are
   described below.

8.1.  A New Chunk Type

   A chunk type for the AUTH chunk has been assigned by IANA.  IANA has
   assigned the value (15), as given in Table 4.  An additional line has
   been added in the "CHUNK TYPES" table of SCTP-parameters [9]:

   CHUNK TYPES

   ID Value    Chunk Type                                     Reference
   -----       ----------                                     ---------
   15          Authentication Chunk (AUTH)                    [RFC4895]

8.2.  Three New Parameter Types

   Parameter types have been assigned for the RANDOM, CHUNKS, and HMAC-
   ALGO parameter by IANA.  The values are as given in Table 1.  This
   required two modifications to the "CHUNK PARAMETER TYPES" tables in
   SCTP-parameters [9]: the first is the addition of three new lines to
   the "INIT Chunk Parameter Types" table:

   Chunk Parameter Type                       Value
   --------------------                       -----
   Random                             32770 (0x8002)
   Chunk List                         32771 (0x8003)
   Requested HMAC Algorithm Parameter 32772 (0x8004)

   The second required change is the addition of the same three lines to
   the to the "INIT ACK Chunk Parameter Types" table.

8.3.  A New Error Cause

   An error cause for the Unsupported HMAC Identifier error cause has
   been assigned.  The value (261) has been assigned as in Table 3.



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   This requires an additional line of the "CAUSE CODES" table in SCTP-
   parameters [9]:

   VALUE            CAUSE CODE                               REFERENCE
   -----            ----------------                         ---------
   261 (0x0105)     Unsupported HMAC Identifier              [RFC4895]

8.4.  A New Table for HMAC Identifiers

   HMAC Identifiers have to be maintained by IANA.  Four initial values
   have been assigned by IANA as described in Table 2.  This required a
   new table "HMAC IDENTIFIERS" in SCTP-parameters [9]:

   HMAC Identifier      Message Digest Algorithm             REFERENCE
   ---------------      ------------------------             ---------
   0                    Reserved                             [RFC4895]
   1                    SHA-1                                [RFC4895]
   2                    Reserved                             [RFC4895]
   3                    SHA-256                              [RFC4895]

   For registering a new HMAC Identifier with IANA, in this table, a
   request has to be made to assign such a number.  This number must be
   unique and a message digest algorithm usable with the HMAC defined in
   RFC 2104 [2] MUST be specified.  The "Specification Required" policy
   of RFC 2434 [4] MUST be applied.

9.  Security Considerations

   Without using endpoint shared keys, this extension only protects
   against modification or injection of authenticated chunks by
   attackers who did not capture the initial handshake setting up the
   SCTP association.

   If an endpoint pair shared key is used, even a true man in the middle
   cannot inject chunks, which are required to be authenticated, even if
   he intercepts the initial message exchange.  The endpoint also knows
   that it is accepting authenticated chunks from a peer who knows the
   endpoint pair shared key.

   The establishment of endpoint pair shared keys is out of the scope of
   this document.  Other mechanisms can be used, like using TLS or
   manual configuration.

   When an endpoint accepts COOKIE-ECHO chunks only in an authenticated
   way the restart procedure does not work.  Neither an attacker nor a
   restarted endpoint not knowing the association shared key can perform
   an restart.  However, if the association shared key is known, it is
   possible to restart the association.



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   Because SCTP already has a built-in mechanism that handles the
   reception of duplicated chunks, the presented solution makes use of
   this functionality and does not provide a method to avoid replay
   attacks by itself.  Of course, this only works within each SCTP
   association.  Therefore, a separate shared key is used for each SCTP
   association to handle replay attacks covering multiple SCTP
   associations.

   Each endpoint presenting a list of more than one element in the HMAC-
   ALGO parameter must be prepared for the peer using the weakest
   algorithm listed.

   When an endpoint pair uses non-NULL endpoint pair shared keys and one
   of the endpoints still accepts a NULL key, an attacker who captured
   the initial handshake can still inject or modify authenticated chunks
   by using the NULL key.

10.  Acknowledgments

   The authors wish to thank David Black, Sascha Grau, Russ Housley,
   Ivan Arias Rodriguez, Irene Ruengeler, and Magnus Westerlund for
   their invaluable comments.

11.  Normative References

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

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

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

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

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

   [6]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport Layer
        Security over Stream Control Transmission Protocol", RFC 3436,
        December 2002.

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




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   [8]  National Institute of Standards and Technology, "Secure Hash
        Standard", FIPS PUB 180-2, August 2002,
        <http://csrc.nist.gov/publications/fips/fips180-2/
        fips180-2.pdf>.

   [9]  <http://www.iana.org/assignments/sctp-parameters>

Authors' Addresses

   Michael Tuexen
   Muenster Univ. of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt
   Germany

   EMail: tuexen@fh-muenster.de


   Randall R. Stewart
   Cisco Systems, Inc.
   4875 Forest Drive
   Suite 200
   Columbia, SC  29206
   USA

   EMail: rrs@cisco.com


   Peter Lei
   Cisco Systems, Inc.
   8735 West Higgins Road
   Suite 300
   Chicago, IL  60631
   USA

   Phone:
   EMail: peterlei@cisco.com


   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA 94303
   USA

   Phone: +1 650-320-8549
   EMail: ekr@rtfm.com




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