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Network Working Group                                       C. Hohendorf
Internet-Draft                              University of Duisburg-Essen
Expires: January 12, 2006                                   E. Unurkhaan
                                                   Mongolian Science and
                                                Technological University
                                                            T. Dreibholz
                                            University of Duisburg-Essen
                                                           July 11, 2005


                              Secure SCTP
                   draft-hohendorf-secure-sctp-00.txt

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
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   This Internet-Draft will expire on January 12, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document explains the reason for the integration of security
   functionality into SCTP, and gives a short description of S-SCTP and
   its services.  S-SCTP is fully compatible with SCTP defined in
   RFC2960, it is designed to integrate cryptographic functions into



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   SCTP.


















































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1.  Introduction

   SCTP is a message oriented reliable transmission protocol which works
   on top of the IP-based network.  It provides several advantages over
   other transmission protocols, such as TCP and UDP over IP.  One of
   the advantages is multistreaming -- user data transported by
   individual streams.  When multistreaming is used, network blocking
   can be avoided in certain cases (e.g. network loss).  Also, SCTP
   supports multihoming -- the endpoints support multiple IP addresses.
   SCTP provides unordered delivery, so that a receiver immediately
   delivers user data to the upper layers upon receipt.  For more
   details, see RFC2960 [4].







































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2.  A brief description of S-SCTP

   S-SCTP provides security functionalities in the transport layer
   without the need for any other security protocols (e.g.  TLS or IP-
   sec).  Normally, a data transport over SCTP can either be secured
   with TLS or can be protected by IPsec.  As both TLS over SCTP and
   SCTP over IPsec have disadvantages in certain scenarios, it is
   preferable to integrate cryptographic functions into SCTP.

   The main issues for the security solutions TLS over SCTP RFC3436 [5]
   and SCTP over IPSec RFC3554 [6] is scalability with the number of
   streams.  For N secure streams, N TLS connections have to be created,
   and N handshakes have to be performed.  If N is small, this is not a
   big issue, but as N grows larger, it becomes a problem because a
   handshake is a slow and expensive process.  So, when an application
   performs N handshakes, the load in terms of memory use, CPU use etc.
   increases linearly over time.  This problem could be overcome by
   using IPsec.  However, IPsec is not so flexible and on the other hand
   SCTP over IPsec has to establish new security associations (SA) for
   newly added IP addresses in dynamic address reconfiguration scenario.
   In this case, the application has to configure a new SA and to
   negotiate a new key exchange.





























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3.  Key terms

   This part gives the definitions of the key terms, which are used in
   this draft:

   o  Secure session: This is the session, which provides the security
      functionalities for an established SCTP association.

   o  Master secret key: S-SCTP uses two kinds of secret keys.  One is
      the secret key for the S-SCTP packet authentication, and the other
      is the secret key for the data encryption and decryption.

   o  Cipher suite: This is the suite of cryptographic algorithms, which
      are used for key exchange, data encryption/decryption and the
      packet authentication.

   o  Pre-enc-data: This is the collection of the data chunks, which
      requires encryption.  The data chunks are concatenated together
      and create pre-enc-data.  Pre-enc-data may include the padding
      chunk.

   o  Cipher suite sequence: This is the bundle of cipher suites chosen
      by an endpoint from the supported cipher suites.




























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4.  Additional chunks and parameters

   Several new chunks and parameters are defined in S-SCTP.  This
   section explains those chunks and parameters.  All new chunks can be
   bundled with other chunks.  The new parameters follow the Type-
   Length-Value format as defined in section 3.2.1 of RFC2960.

4.1  New type chunks and definitions

   The following table shows the new chunks.  All new chunks, except for
   the Encrypted Data (EncData) chunk, Authentication (AUTH) chunk and
   Padding (PADDING) chunk, are used for building the secure session and
   to update the master secret key.  The new chunks are to be
   interpreted as described in Section 3.2 of RFC 2960, by the receiver.

    Chunktype     Chunk name
    ---------     ---------------------
      0xD0        Secure Session Open Request Chunk
      0xD1        Secure Session Certificate Chunk
      0xD2        Secure Session Acknowledge Chunk
      0xD3        Secure Session Server Key Chunk
      0xD4        Secure Session Client Key Chunk
      0xD5        Secure Session Open Complete Chunk
      0xD6        Secure Session Close Chunk
      0xD7        Secure Session Close Acknowledge Chunk
      0xD8        Security Level Change Chunk
      0xD9        Security Level Change Acknowledge Chunk
      0x10        Encrypted Data Chunk
      0x11        Authentication Chunk
      0x12        Padding Chunk

   The new parameters are defined in this section.

4.1.1  Secure Session Open request chunk (SSOpReq)

    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=0xD0   | Reserved=0  |CF|           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Version             |     Key material length       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                                \
    /                     Optional parameters                        /
    \                                                                \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   CF: Certificate flag: 1 bit



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   This flag indicates whether or not the client will send a
   certificate.  It is set to 1 when the client sends a certificate.  If
   a receiver receives SSOpReq chunk with CF=1 and does not receive a
   certificate it raises an error and terminates the secure session
   initialisation.

   Length: 16 bits unsigned integer

   The length field contains the size of the chunk in bytes, including
   the chunk header, version, random number length and optional
   parameter(s).

   Version: 16 bits unsigned integer

   This field indicates the S-SCTP version 1.0.  The high eight bits
   indicate the major version, the low eight bits indicate minor
   version.

   Key material length: 16 bits unsigned integer

   This number has two meanings:

   o  when the handshake is made using the RSA key exchange protocol,
      the key material length defines the random number length, which is
      generated by the server and client to calculate a master secret
      key (see RSA parameters of the SSSerKey and SSCliKey chunks)

   o  when the handshake is made using the DH key exchange protocol, the
      key material length defines the DH prime number length (see DH
      parameters of the SSSerKey and SSCliKey chunks).  For security
      reasons, the key material length MUST be 512 bits (default) or
      longer when the key exchange mechanism uses RSA, and 1024 bits
      (default) or longer when the key exchange mechanism uses DH.  The
      key material length is increased in steps of 64 bits.  If a user
      defines the key material length to be shorter than the default
      value, S-SCTP automatically sets it to the default.

   Parameter(S):

   SSOpReq chunk includes the cipher suite and compression method
   parameters, which are described below:

   Cipher suite parameter:

   This parameter contains the cipher suites, which are chosen from all
   supported cipher suites by the client.  One of them is used for the
   secure session.  The user can choose certain cipher suites from the
   cipher suites supported by the client.



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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=30               |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Cipher suite 1           |      Cipher suite 2           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      ..............           |      ..............           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Cipher suite N-1         |      Cipher suite N           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Cipher suite: 16 bits unsigned integer:

   This field indicates a cipher suite, which is supported by the
   client.  The next table includes cipher suites supported in S-SCTP.
   Additional cipher suites can be specified.

   Value  Cipher suite               Key exchange  Encryption    Hash
                                     algorithm     algorithm   algorithm
   -----  -------------------------  ------------  ----------  ---------
     1    RSA_with_DES_CBC_MD5       RSA           DES_CBC     MD5
     2    RSA_with_DES_CBC_SHA-1     RSA           DES_CBC     SHA-1
     3    RSA_with_3DES_CBC_MD5      RSA           3DES_CBC    MD5
     4    RSA_with_3DES_CBC_SHA-1    RSA           3DES_CBC    SHA-1
     5    RSA_with_AES-128_CBC_MD5   RSA           AES-128     MD5
     6    RSA_with_AES-128_CBC_SHA-1 RSA           AES-128     SHA-1
     7    DH_with_DES_CBC_MD5        DH            DES_CBC     MD5
     8    DH_with_DES_CBC_SHA-1      DH            DES_CBC     SHA-1
     9    DH_with_3DES_CBC_MD5       DH            3DES_CBC    MD5
    10    DH_with_3DES_CBC_SHA-1     DH            3DES_CBC    SHA-1
    11    DH_with_AES-128_CBC_MD5    DH            AES-128     MD5
    12    DH_with_AES-128_CBC_SHA-1  DH            AES-128     SHA-1
    13    RSA_with_NULL_MD5          RSA           NULL        MD5
    14    RSA_with_NULL_SHA-1        RSA           NULL        SHA-1
    15    DH_with_NULL_MD5           DH            NULL        MD5
    16    DH_with_NULL_SHA-1         DH            NULL        SHA-1
    17    RSA_with_AES-192_CBC_MD5   RSA           AES-192     MD5
    18    RSA_with_AES-192_CBC_SHA-1 RSA           AES-192     SHA-1
    19    RSA_with_AES-256_CBC_MD5   RSA           AES-256     MD5
    20    RSA_with_AES-256_CBC_SHA-1 RSA           AES-256     SHA-1
    21    DH_with_AES-192_CBC_MD5    DH            AES-192     MD5
    22    DH_with_AES-192_CBC_SHA-1  DH            AES-192     SHA-1
    23    DH_with_AES-256_CBC_MD5    DH            AES-256     MD5
    24    DH_with_AES-256_CBC_SHA-1  DH            AES-256     SHA-1

   The hash algorithms, defined in cipher suites, are used only for the
   S-SCTP packet authentication, and not for the signature of the



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   handshake messages.  An S-SCTP implementation MUST at least support
   the default cipher suite, DH_with_3DES_CBC_SHA-1 (value=0).  If the
   SSOpReq chunk does not contain a cipher suite parameter, then:

   a.)  S-SCTP will use the default, or:

   b.)  S-SCTP will use the old cipher suite.

   The case "a" will be used at the beginning of the secure session.
   The case "b" will be used when the SSOpReq chunk is created after a
   secure session establishment.  The signature schemes are derived from
   both the client and server certificates, and may be different.

   Compression method parameter

   This parameter contains compression methods, which are used for data
   compression.  The data compression uses lossless compression methods.
   The user chooses several compression methods and sends it to the
   receiver.  The receiver chooses one compression method.

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Compression   | Compression   | Compression   | Compression   |
    |  method 1     |  method 2     |  method 3     |  method 4     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    ....       |    ....       | Compression   | Compression   |
    |               |               | method N-1    |  method N     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Compression method: 8 bits unsigned char

   This field indicates a compression method, which is supported by the
   client.  The next table includes compression methods supported in
   S-SCTP.  Additional compression methods can be specified.

    Value  Compression method
    -----  ---------------------
       1   Huffman Code
       2   Lempel-Ziv Code

   The secure session uses null compression when the SSOpReq chunk
   contains no compression parameters.

4.1.2  Secure Session Certificate chunk: (SSCert)

   This chunk can be sent by both endpoints.  The certificate helps to
   authenticate the endpoint, that establishes a S-SCTP session.  This



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   chunk contains only one parameter, the certificate parameter.  The
   SSCert chunk is optional.  For security reasons, both endpoints
   SHOULD use a certificate to authenticate each other.

    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=0xD1   | Reserved=0    |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                           Certificate                         /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                        Optional parameter                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bits unsigned integer

   The length field contains the size of the chunk in bytes, including
   the chunk header and parameter.

   Certificate: Variable length

   The certificate field contains the certificate of the endpoint.
   S-SCTP uses the X.509v3 certificate which is defined in RFC2459 [3].

   Optional parameter

   SSCert chunk has only one optional parameter.

   Certificate parameter

   The SSCert chunk uses the certificate parameter for additional
   certificates, when the endpoint has two or more certificates.

    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=33               |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                          Certificate                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Certificate: Variable length



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   The endpoint can send two or more certificates.  In this case the
   certificate field contains the endpoint's additional certificate.
   S-SCTP uses the X.509v3 certificate, which is defined in RFC2459 [3].

4.1.3  Secure Session Open Acknowledge chunk (SSOpReq_Ack)

   The Secure Session Open Acknowledge chunk is sent by the server to
   tell the client that the secure session open request is accepted.

    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=0xD2   | Reserved=0  |CF|           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Version            |        Cipher suite           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Compression method          |        Reserved = 0           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   CF: Certificate flag: 1 bit

   This flag indicates whether or not the server has a certificate.
   This flag is set to 1 when the server has a certificate, else it is
   zero.

   Length: 16 bits unsigned integer

   The chunk length is 8 bytes, including the chunk header, version and
   cipher suite field.

   Version: 16 bits unsigned integer

   This field indicates the S-SCTP version 1.0.  The high eight bits
   indicate the major version, the low eight bits indicate the minor
   version.

   Cipher suite: 16 bits unsigned integer

   This field indicates the cipher suite, which is used for a secure
   session.  The cipher suite includes necessary information for the key
   derivation, message encoding and MAC computation.  The server uses
   the following rules to choose the cipher suite:

      Client and Server do not have a certificate:  Use DH key exchange.

      Client or Server has a certificate:  Use DH key exchange.





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      Client and Server have a RSA certificate:  Use RSA key exchange.

      Client and Server have a DSS certificate:  Use DH key exchange.

   Compression method: 16 bits unsigned char

   This field indicates the compression method, which is used for data
   compression in the secure session.

4.1.4  Secure Session Server Key chunk (SSSerKey)

   This chunk includes the parameter which is used for the key exchange
   algorithm.  The Server Key Exchange chunk consists of the chunk
   header and one parameter.  The parameter type depends on the key
   exchange algorithm.

    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=0xD3   | Reserved=0  |SL|           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                                \
    /                           Parameter                            /
    \                                                                \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Security level (SL): 2 bits

   This 2-bit value indicates a server's security level of the reserved
   flags.

   Length: 16 bit unsigned integer

   The length field contains the size of the chunk in bytes, including
   the chunk header and parameter.

   Parameters:

   The following two parameters define the key exchange protocols.
   Their parameter formats are shown in the next two tables.  When a
   user specifies a new cipher suite with a new key exchange algorithm,
   then they must define a new parameter.

   Diffie-Hellman parameter

   The SSSerKey chunk uses this parameter when the handshake is done via
   the DH key exchange algorithm.




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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=0xD001             |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Length of DH prime number, P | Length of DH prmitive root, R |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Length of DH public key, Y  |         Reserved=0            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                      DH prime number, P                       /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                     DH primitive root, R                      /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                         DH value, Y                           /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                          Signature                            /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bit unsigned integer

   The length field contains the size of the parameter in bytes,
   including the parameter header, length of DH prime number, length of
   DH primitive root, length of DH public key, reserved, DH prime
   number, DH primitive root, DH public key and signature.

   Length of DH prime number, P: 16 bits unsigned integer

   This field contains the size of the DH prime number.

   Length of DH primitive root, Q: 16 bits unsigned integer

   This field contains the size of the DH primitive root.  The size of
   the prime number is equal R, where R is a random number defined in
   the SSOpReq chunk.

   Length of DH value, Y: 16 bits unsigned integer

   This field contains the size of the DH public key.

   DH value, P: Variable length



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   This is the prime number of the DH key exchange protocol.

   DH value, Q: Variable length

   This is the primitive root of the prime number P.

   DH value, Y: Variable length

   This is the public key of the DH key exchange protocol.

   Signature: Variable length

   The field contains the signature which is derived from the chunk
   header and the whole parameter except the signature field.  The
   signature computation uses the signature algorithm which is defined
   in the server certificate.  If the server does not have a
   certificate, this field does not exist in the parameter.

   RSA parameter

   The SSSerKey chunk uses this parameter when the handshake uses the
   RSA key exchange algorithm.

    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=0xD002             |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                 Encrypted (random number, R)                  /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                          Signature                            /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bits unsigned integer

   The length field contains the size of the parameter in bytes,
   including the parameter header, the encrypted random number and the
   signature.

   Encrypted (Random number, R): Variable length

   The random number is used to generate the secret keys for user data
   encryption and authentication.  The random number encryption uses the
   client public key.



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   Signature: Variable length

   The field contains the signature, which is derived from the chunk
   header and the whole parameter except the signature field.  The
   signature computation uses the signature algorithm which is defined
   in the server certificate.

4.1.5  Secure Session Client Key chunk (SSCliKey)

   This chunk includes the parameters which are used for the key
   exchange algorithm.  The Secure Session Client Key Exchange chunk
   consists of the chunk header and one parameter.  The parameter format
   depends on the key exchange algorithm.

    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=0xD4   | Reserved=0  |SL|           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                                \
    /                           Parameter                            /
    \                                                                \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Security level (SL): 2 bits

   This 2-bit value indicates a client's security level.

   Length: 16 bit unsigned integer

   The length field contains the size of the chunk in bytes, including
   the chunk header and parameter.

   Parameters:

   Two new parameters are defined here that can appear in the SSCliKey
   chunk.  Their parameter formats are shown in the next two tables.

   Diffie-Hellman parameter

   The SSCliKey chunk uses this parameter when the handshake uses the DH
   key exchange algorithm.









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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=0xD003             |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                         DH value, Y                           /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                          Signature                            /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bit unsigned integer

   The length field contains the size of the parameter in bytes,
   including the parameter header, the DH public key and the signature.

   DH value, Y: Variable length

   This field contains the public key of the DH key exchange protocol.

   Signature: Variable length

   The field contains a signature which is derived from the chunk header
   and the whole parameter except the signature field.  The signature
   computation uses the signature algorithm defined in the client
   certificate.  If the client does not have a certificate, then this
   field does not exist in the parameter.

   RSA parameter

   The SSCliKey chunk uses this parameter when the handshake uses RSA
   key exchange algorithm.

    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=0xD003             |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                 Encrypted (random number, R)                  /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                          Signature                            /
    \                                                               \



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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bits unsigned integer

   The length field contains the size of the parameter in bytes,
   including the parameter header, the encrypted random number and a
   signature.

   Encrypted (Random number): Variable length

   This field contains the random number, encrypted by the server's
   public key, which is used to generate the master secret key for
   encryption and authentication.

   Signature: Variable length

   The field contains the signature which is derived from the chunk
   header and the whole parameter except the signature field.  The
   signature computation uses the signature algorithm defined in the
   server certificate.

4.1.6  Secure Session Open Complete chunk (SSOpCom)

   This chunk is the last chunk of the handshake and it indicates the
   completion of the secure session establishment.  After receiving this
   chunk the endpoint verifies the verification data which is contained
   in the chunk.  The secure session procedure is complete when the
   verification is successful.  Otherwise the secure session will be
   closed.

    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=0xD5   | Reserved=0    |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                       Verification data                       /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bits unsigned integer

   The length field contains the size of the chunk in bytes, including
   the chunk header and verification data.

   Verification data: Variable length

   The verification data contains a hashed value which is an output of



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   the HMAC function.  The HMAC uses the authentication secret key,
   which is individually generated by the endpoints.  The HMAC input
   contains  all received secure session handshake chunks of the current
   endpoint.  Both endpoints compute the hash value individually and
   exchange it using the SSOpCom chunk.  The receiver computes the hash
   value using the same algorithm as the sender, and compares it with
   the verification data.  If the receiver's computed value is the same
   as the sender's verification data, then the receiver assures that the
   secure session open is successfully completed.  If it is not the
   same, then the receiver sends an ERROR message to the sender, and
   immediately closes the secure session.

4.1.7  Secure Session Close chunk (SSClose)

   This chunk indicates a request to close the current secure session.
   The sender MUST NOT send any encrypted or authenticated chunks after
   it has sent this chunk.

    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=0xD6   | Reserved=0  |OF|           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               TSN                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Outstanding flag (OF): 1 bit

   This flag indicates that the endpoint has sent the SSClose chunk and
   has no outstanding secured data.

   Length: 16 bits unsigned integer

   The length field contains the size of the chunk in bytes, including
   the chunk header and TSN.

   Transmission sequence number (TSN): 16 bits unsigned integer

   This is the transmission sequence number of the data chunk that was
   last encrypted and sent.  The TSN helps the server to detect
   outstanding EncData chunks.

4.1.8  Secure Session Close Acknowledge chunk (SSClose_Ack)

   This chunk is used to acknowledge the receipt of the secure session
   close chunk.  When the endpoint receives the secure session close
   chunk, it immediately stops sending encrypted or authenticated
   chunks.  The Secure Session Close Acknowledge chunk only consists of



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   the chunk header.

    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=0xD7   |   Reserved=0  |          Length=4             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.1.9  Security Level Changed chunk (SecLevCHD)

   This chunk is used to convey the other associated endpoint of the
   endpoint's new security level.  The endpoint sends SecLevCHD chunk
   every time it selects a new security level.  The endpoint uses the
   new selected security level when it receives the Security Level
   Changed Acknowledged chunk.  The sender MUST not send a new SecLevCHD
   chunk when an unacknowledged SecLevCHD chunk exists.

    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=0xD8   | Reserved=0  |SL|          Length=4             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Security level (SL): 2 bits

   This 2-bit value indicates the sender's new security level.

4.1.10  Security Level Changed Acknowledged chunk (SecLevCHD_Ack)

   This chunk is used to acknowledge the receipt of the SecLevCHD chunk.
   When the endpoint receives the SecLevCHD chunk, it immediately sends
   the SecLevCHD_Ack chunk.

    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=0xD9   |   Reserved=0  |          Length=4             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.1.11  Encrypted Data Chunk (EncData)

   Each S-SCTP packet includes at most one encrypted data chunk, and the
   packet could also include several (normal, unencrypted) data chunks.
   The encrypted data chunk may contain one or several data chunks.  The
   EncData chunk includes a padding chunk when it is needed.




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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=0x10   | Reserved=0    |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Master secret key reference # |     Random number length      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                         Random number                         /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                         Encrypted data                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bits unsigned integer

   The length field contains the size of the chunk in bytes,including
   the chunk header and encrypted data.

   Master secret key reference number: 16 bits unsigned integer

   The association can be updated by changing the master secret key
   several times during the association lifetime.  The association keeps
   old secret keys.  The number of the kept old secret keys depends on
   the implementation.  This field helps to identify  which key (old or
   new) has been used for encryption.  That means the endpoint is able
   to receive messages, which were encrypted with an old key, after the
   update of a master secret key.

   Random number length: 16 bits unsigned integer

   This field contains the size of the random number which is defined
   below.

   Random number: Variable length

   This field indicates the random number which is used as
   initialisation vector of the cipher block chaining (CBC) mode for
   encryption.

   Encrypted data: Variable length

   Contains a byte vector that consists of the encrypted data chunks.
   Before encryption, the chunk(s) MUST fulfill the following
   conditions.  If encryption is performed using the DES or 3DES
   algorithm, the total size of the chunk(s) MUST be a multiple of 8



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   bytes.  If encryption is performed using the AES algorithm, the total
   size of the chunk(s) MUST be a multiple of 16 bytes.  If the total
   size of the chunk(s) is not a multiple of 8 bytes or 16 bytes, the
   sender MUST add appropriate padding at the chunk's end.

4.1.12  Padding chunk (PADDING)

   This padding chunk is used with an EncData chunk.  The symmetric
   encryption algorithms use a block oriented encryption of the user
   data.  For example DES uses 64 bit blocks, and AES uses 128 bit
   blocks.  Before encryption, the user data which has to be encrypted
   has to be formatted according to the required block size.  If the
   last block is not completely full, padding has to be added.  If less
   than 4 bytes padding are required, the block is filled up will all
   zeros.  The receiver can calculate the number of padding bytes based
   on the length field of the original data chunks.  If 4 bytes or more
   are required, a padding chunk of appropriate length is added.

   The algorithms split user data into blocks when the data length is
   greater  than the block size.  The blocks MUST be full.  If 104 bits
   are to be encrypted using DES algorithm with 64 bit block size, user
   data is split into two blocks of 64 and 40 bits.  The second block
   must be padded with 24 bits up to the full block size of 64 bits.

    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=0x12   | Reserved=0    |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Padding                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: Variable length

   This field indicates the padding size.  The padding follows the
   padding chunk.  The length includes the padding chunk and padding.

   Padding: Variable length

   The padding is a random number.  The random number generation is
   implementation specific.

4.1.13  Authentication chunk (AUTH)

   This chunk is dedicated to the authentication of an S-SCTP packet.
   S-SCTP inserts this chunk into the packet when the security level
   requires authentication.




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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=0x11   | Reserved=0    |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Master secret key reference # |          Reserved=0           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                              HMAC                             /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bits unsigned integer

   The length field contains the size of the chunk in bytes, including
   the chunk header, master secret key reference, reserved field and
   MAC.

   Master secret key reference number: 16 bits unsigned integer

   The association can update the secret keys several times during the
   association lifetime.  The association keeps old secret keys.  The
   number of the kept old secret keys depends on the implementation.
   This field identifies the key which is used for authentication.  If
   the endpoint receives a message which was authenticated by an old
   key, this message can still be authenticated after an update of the
   master secret key.

   HMAC: Variable length

   This field contains the authentication code for the SCTP packet.  The
   message authentication uses the HMAC algorithm defined in RFC 2104.
   The hash function used in the HMAC algorithm is derived from the
   negotiated cipher suite, which was chosen by the server.

















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5.  New Error Cause

   This part explains the new error causes defined for S-SCTP.  When a
   secure session or certificate failure is detected in the secure
   session open process, the S-SCTP association immediately stops the
   process.  However, the association continues (without any security
   functionality).  When the secure session failure happens during an
   update of the master secret key the association stops the update
   operation and closes the secure session.  The following table shows
   four general failure groups.

    Cause Code Value  Cause Code
    ----------------  ---------------------------------------
          0x20        Secure Session failure
          0x21        Secure Session Certificate failure
          0x22        Secure Session Decryption failure
          0x23        Secure Session Authentication failure
          0x24        Secure Session Decompression failure


5.1  Secure Session failure

    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=0x20         |     Cause length = 8          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Error Code           |          Reserved=0           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If any error happens in the secure session open and update process,
   endpoints alert their peers with these error codes.  The next table
   shows error codes for what can happen.

    Error Code Value  Error Code
    ----------------  -------------------------------------
         0            Timer expired
         1            Signature failure
         2            Secure Session Open Complete failure

      Timer expired: The sender starts a timer when it sends the secure
      session message.  When the sender receives no response from the
      receiver before the timer expires, it sends this error code.

      Signature failure: Some secure session chunks include a signature,
      which identifies and protects the secure session message.  If the
      receiver checks the signature and cannot identify the chunk, this
      error code is used in the error chunk.



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      Secure Session Open Complete failure: This chunk is a very
      important part of the secure session.  Both server and client
      individually compute the master secret and HMAC secret keys.  Both
      sides check these secret keys and parameters (i.e. secure session
      chunks exchanged before, source and destination ports).  If these
      keys are not identical,  an error chunk is sent containing this
      error code.


5.2  Secure Session Certificate failure

    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=0x21         |     Cause length = 8          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Error Code           |          Reserved=0           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The certificate failure signals that an error has occurred in
   processing the certificates.  The next table shows error codes for
   what can happen.

    Error Code Value  Error Code
    ----------------  -------------------------------------
           0          No certificate
           1          Bad certificate
           2          Certificate expired
           3          Unknown certificate

      No certificate: This error happens when the sender sets the CF
      flag and the receiver does not receive the certificate.

      Bad certificate: The signature of the certificate is bad and the
      certificate could not be verified.

      Certificate expired: The certificate is no longer valid.

      Unknown certificate: The received certificate a X.509v3
      certificate.


5.3  Decryption failure

   This error happens when the EncData chunk cannot be decrypted or the
   data chunk(s) cannot be identified after decryption.  The receiver
   discards the EncData and increases a counter by 1.  This counter
   counts errors.  If the number of errors reaches a limit, the secure



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   session is terminated.  The limit of the errors depends on the
   implementation.

    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=0x22         |     Cause length = 4          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


5.4  Authentication failure

   In the event of a HMAC error, the packet is discarded by the
   receiver.  To check for an error, the receiver computes the HMAC and
   compares it to the HMAC field of the packet.  If they do not match,
   an error is reported back.

    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=0x23         |     Cause length = 4          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


5.5  Decompression failure

   This error happens when the compressed chunk(s) cannot be
   decompressed or the data chunk(s) cannot be identified after
   decompression.  The receiver discards the decompressed chunk(s).

    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=0x24         |     Cause length = 4          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
















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6.  S-SCTP packet format and security levels

   S-SCTP has four different security levels, which cover privacy
   settings of an S-SCTP association.  The S-SCTP application can change
   the security levels at any time during the security session lifetime.

      Security level 0: This is the null security level.  S-SCTP  does
      use neither data chunk encryption  nor authentication.  The S-SCTP
      packet is the same as the SCTP packet (this level is fully
      compatible to SCTP).

      Security level 1: This security level requires packet
      authentication but does not use encryption.  Every outgoing packet
      (including the SCTP common header) is authenticated.

      Security level 2: In this security level, data chunks may be
      encrypted.  When an S-SCTP packet contains an encrypted data
      chunk, it MUST include an AUTH chunk as well.  That means every
      chunk and the packet header are authenticated.  When a packet
      includes only unencrypted data chunks or control chunks or both
      unencrypted data chunks and control chunks, the packet will not be
      authenticated.

      Security level 3: This is the highest security level.  S-SCTP
      requires both encryption and authentication.  Every outgoing chunk
      is encrypted and the packet is authenticated.

   Both  endpoints  can use different security levels, e.g. the
   association can use security functions only for one direction, e.g.
   from server to client.  In this case the server uses security level 3
   and the client uses security level 0.  The transmission control block
   (TCB) of the association includes the security level as a new
   parameter.


















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7.  S-SCTP data format

   S-SCTP sorts data chunks before bundling them into the outgoing SCTP
   packet.  The data chunks are sorted according to whether they have to
   be encrypted or not.  The chunks belonging to the encryption group
   are concatenated and encrypted into an EncData chunk.  May be a
   PADDING chunk is inserted into the encryption group.  Insertion of a
   PADDING chunk is done depending on data length and encryption block
   size.

   An assortment of encrypted and non-encrypted chunks are bundled in
   the packet.  The control chunk(s) are placed first in the packet when
   bundled with other chunks.  Finally, an AUTH chunk may be added to
   the packet.

   HMAC computation is performed over all chunks and the SCTP common
   header with a 0 checksum.  The checksum is then computed over the
   complete packet (including AUTH chunk).  The HMAC length depends on
   the hash function in the cipher suite.  In every security level, the
   SCTP packet construction is slightly different.  In security level 0
   the packet format is same as the SCTP packet format.






























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8.  Procedures

   In this section an explanation of the procedures of secure session:
   initialisation, termination, update and etc., is given.

8.1  Establishment of a secure session

   The following process is used to establish the S-SCTP secure session.
   The handshake process runs in parallel with the data transmission.
   The secure session start and close is controlled by the user.  The
   user can establish and close a secure session at any time during the
   association lifetime.  Each time a secure session is established, a
   new set of keys is generated.  It is not possible to create a new
   secure session when a secure session already exists.  The following
   describes secure session establishment, which makes use of a
   handshake timer and retransmissions in case packets are lost during
   transmission.  S-SCTP uses a four-way handshake.  After all messages
   of one of the connection "legs" have been sent, client or server
   starts a RTO.hand (handshake retransmission time out) timer.  For
   example, the secure session certificate is the last handshake message
   of the first leg.  The sender waits for a response from the receiver
   until the RTO.hand timer expires.  The sender stops the RTO.hand
   timer when it receives the expected message(s).  If the RTO.hand
   timer expires before all expected messages have been received, the
   sender retransmits the handshake message(s).

   The retransmission uses the following algorithm.  The RTO.hand timer
   gets a value from RTO of the path where the message is sent to, which
   is defined in RFC2960.  Before a retransmission, the sender checks
   RTN.hand.max (handshake maximum retransmission number).  This initial
   value is dependent upon specific implementations.  The suggested
   value for RTN.hand.max is Path.Max.Retrans (see RFC 2960).

   RTN.hand.max should be a constant parameter.  We introduce a counter
   for the number of retransmissions, and if that counter exceeds the
   parameter RTN.hand.max, the timer expired error message is sent to
   the peer.  If a retransmission is required then S-SCTP uses the same
   retransmission rules as defined in RFC2960.  If the receiver receives
   a retransmission of a handshake message that was already received,
   the message last received MUST be dropped.  The endpoint discards the
   message(s) when they are unexpected.  A secure session initialisation
   begins when one of the associated endpoints sets the security level
   to a value higher than 0.  The endpoint starting a secure session
   initialisation is called client and the other associated endpoint is
   called server.

      The client sends the SSOpReq chunk to the server.  If the client
      has a certificate, it  sets the CF flag of the SSOPReq chunk to 1.



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      The client sends the SSCert chunk immediately after the SSOpReq
      chunk.  The SSCert chunk can be bundled with the SSOpReq chunk or
      with other chunk(s).  When the CF flag is set to 0, the client
      sends only the SSOpReq chunk.

      The server receives a SSOpReq chunk and checks the CF flag.  If
      the CF flag is set to 1, the server waits for the SSCert chunk.
      Upon receipt, the server checks the certificate and if there is a
      problem with it, the server stops the handshake and goes to an
      error state, aborts secure session setup and reports the cause to
      its peer.  It there is no error, the server chooses the cipher
      suite and sends the SSOpReq_Ack chunk with CF=1 flag to the client
      when the server has a certificate.  The server immediately sends
      the certificate and the SSSerKey chunks after the SSOpReq_Ack
      chunk.  All three chunks may be bundled together or with other
      chunks.  The server sends only the SSOpReq_Ack chunk with the
      SSSerKey chunk if CF=0.  Before sending the server key exchange
      chunk, the server generates key material.  The server starts the
      update master secret key operation when it receives the SSOpReq
      chunk after secure session establishment.  If the server receives
      the SSCert chunk before the SSOpReq chunk, it stores the SSCert
      chunk and waits until it receives the SSOpReq chunk.  But the
      server drops a second SSCert chunk.

      The client receives the handshake messages and checks the
      certificate in the SSSerKey chunk.  If the client detects any
      errors, it stops the handshake and goes to an error state, aborts
      secure session setup and reports the cause to its peer.  The
      client generates key material and sends the SSCliKey chunk to the
      server.  The client sends the SSOpCom chunk immediately after the
      client key exchange chunk.  Before sending the handshake-finished
      chunk, the client computes the encryption secret and MAC secret
      keys.

      The server receives the SSCliKey chunk and computes the master
      secret and the MAC secret keys.  It then computes the SSOpCom
      chunk and sends it to the client.  Finally, the server checks the
      SSOpCom chunk of the client.  If the server detects any error, it
      reports a secure session open complete error and closes the
      handshake.  The secure session is established only when both sides
      detect no errors.  The server is ready for secure transmission
      when it detects no errors, but the client must wait for the
      SSOpCom chunk of the server.  When this is received, the client
      checks it and reports to the peer a secure session open complete
      error if any error is detected before aborting secure session
      setup.  The handshake may run simultaneously with normal SCTP data
      transmission.  If the client receives  encrypted or authenticated
      data chunks before it receives the server's SSOpCom chunk, then



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      those chunks MUST be discarded.

   When both associated endpoints request the initialisation of a secure
   session  simultaneously (both endpoints send an SSOpReq message),
   both ignore the received SSOpReq message and wait a random time
   before resending the SSOpReq message.  Each endpoint generates the
   random time independently.  The random number must be small, e.g. 120
   seconds maximum.

8.2  Choice of cipher suite and compression method

   This section explains how to choose the cipher suite and compression
   method which are used for the secure session.  Each endpoint
   maintains an ordered list of supported cipher suites (cipher suite
   list).  The ordering in the list indicates the preference with which
   a cipher suite should be used (first in the list have higher
   preference).  The order in the list is defined by the retrospective
   S-SCTP user.

   S-SCTP users on both sides can allow all cipher suited in the list
   when establishing a secure session or limit the allowed cipher suites
   to a subset.  The complete list or the selected subset can be
   indicated to the server in the SSOpReq.  If the complete list is
   sent, the default cipher suite list must be located first in the
   list.  The server uses the following rules to choose the cipher suite
   to be used for the secure session:

   The server chooses the default cipher suite, if the SSOpReq chunk
   does not contain any cipher suite.

   The server gets the first cipher suites from SSOpReq chunk and
   server's cipher suite sequence.  When both cipher suites are
   identical the server chooses this cipher suite for the secure
   session.  Otherwise, the server takes its first cipher suite and
   looks for a match in the cipher suite sequence of the client.  When
   there is no matche, the server takes the client's first cipher suite
   and searches for match in its cipher suite sequence.  S-SCTP checks
   the first  cipher suite in the SSOpReq chunk against all cipher
   suites in the cipher suite list of the server.  If no match is found,
   all subsequent cipher suites in the SSOpReq are checked sequentially
   in the order they appear in the SSOPReq until a match is found.  The
   first cipher suite supported by both endpoints is chosen.  When two
   cipher suites match each other then this cipher suite is selected for
   the secure session.  If not, the server looks, its second cipher
   suite, for a match in the cipher suite sequence of the client.
   Furthermore, the server uses the same mechanism to look a cipher
   suite for the secure session.




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   The server chooses the default cipher suite, when the cipher suites
   in the SSOpReq chunk are not supported by the server.

   Both client and server also maintain a list of compression methods.
   The choice of the compression mechanism works similarly to the cipher
   suite selection mechanism described above.  S-SCTP uses a NULL
   compression method as default compression method.

8.3  Data transfer

   Before transporting the packet over the network, S-SCTP takes the
   following steps.  First, it checks the security level.  If the
   security level is:

      0, jump to step "d"

      1, jump to step "c"

      2, check the user data.  If the user data requires encryption,
      jump to step "a" .  If the user data does not require encryption,
      jump to step "c"

      3, jump to step "a"

      a) S-SCTP sorts data chunks in two groups, which are encrypted and
      unencrypted.  The encrypted group consists of those data chunks
      requiring encryption.  The unencrypted group consists of those
      data chunks not requiring encryption.  If the secure session's
      security level is set to 3, all chunks are sorted into the
      encrypted group.

      b) The data chunks in the encrypted group are concatenated.  After
      this, S-SCTP calculates the padding chunk and inserts the padding
      chunk on the last position into pre-enc-data if necessary.  The
      Pre-enc-data size MUST be smaller than the current MTU.  If the
      pre-enc-data is bigger than the current MTU, S-SCTP must create
      two pre-enc-datas.  Every pre-enc-data is encrypted and stored in
      the encryption data field of the EncData chunk.

      c) SCTP builds the packet according to the security level and
      inserts the AUTH chunk in the last position in the packet.

      d) S-SCTP sends the packet.


8.4  Closing of a secure session

   The termination of a secure session begins when one of the endpoints



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   sends the secure session close chunk.  This chunk includes the last
   encrypted data TSN and OF.  The endpoint (sender) stops the
   encryption or authentication of all chunks or packets after it has
   sent the secure session close chunk.  But normal (unsecured) data
   transfer will continue.  The endpoint then waits until it receives
   the SSClose_Ack chunk.  After receiving the SSClose_Ack chunk, the
   association clears the TCB parameters belonging to the secure
   session.  The receiver (other endpoint) immediately stops encryption
   and authentication of all chunks or packets after it receives the
   secure session close chunk.  Before sending the SSClose_Ack, the
   receiver waits for outstanding data (encrypted or authenticated
   data), which are the receiver's unacknowledged data chunks and
   sender's data chunks that have a TSN less than the last encrypted
   data TSN in the SSClose chunk.  If the receiver does not receive the
   outstanding data chunks before RTO.hand timer expires, the S-SCTP
   association closes the secure session and outstanding data chunks
   will be dropped.  The receiver ignores the last TSN of SSClose chunk
   and waits only for the receiver's unacknowledged data chunks when
   SSClose chunk's OF=1.  The SSClose and SSClose_Ack chunks may be
   bundled with other chunks.  If the sender does not receive the
   acknowledge chunk, the client follows the standard retransmission
   rule for messages.  After the termination of the secure session, the
   TCB parameters belonging to the secure session MUST be set to zero.
   If the SCTP association begins to close the current association, the
   SSClose chunk is sent.  If the SCTP association creates an ABORT
   chunk, the secure session closes immediately and the TCB parameters
   belonging to the secure session MUST be set to zero.

8.5  Generation of the Master secret key

   Secret key generation uses the 3DES_CBC algorithm.  Both server and
   client compute the master secret key separately.  The key material is
   split into 64 bit blocks.  Every block will be input to the 3DES_CBC
   encryption.  The key material is as follows:

      If the secure session key exchange algorithm uses DH, the key
      material consists of the DH's secret key.

      If the secure session key exchange algorithm uses RSA, the key
      material consists of random numbers of both client and server.


8.6  Update of the master secret key

   A secure update mechanism of the secret keys is a very important
   requirement for a secure session.  The secret keys consist of the
   master secret key, which is used for data chunk encryption, and the
   HMAC secret key, which is used for packet authentication.  If an



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   association exists for a long time, the S-SCTP association needs to
   update the secret keys.  Both the client and the server can request
   an update of the secret keys.  A three way handshake, called an
   abbreviated handshake, is used to update the master secret keys.  All
   actions of the handshake are encrypted by the current master secret
   key.  The current security level does not affect the packets, which
   contain the handshake messages.  The key update handshake works
   similar to the first establishment handshake (e.g. the endpoints
   start an RTO.hand timer when sending  handshake chunks).  Format and
   function of the chunks used to update keys are the same as for the
   handshake.  When an endpoint receives a SSOpReq chunk (after a secure
   session establishment) it begins to update secret keys.  Both the
   server and client key exchange chunks always use the RSA key exchange
   algorithm.  The random numbers in SSSerKey and SSCliKey chunks are
   encrypted by the current master secret key.  The following describes
   the method used to update the master secret key:

   The client generates a random number and sends the SSopReq chunk with
   the SSCliKey chunk.  The key material length in the handshake request
   chunk may be equal to 0.  If  not, the number indicates the size of
   the new key material.  If 0, both sides will use the key material
   length which was used in the last handshake.  The server  sends the
   SSop_Ack, the SSSerKey and the SSOpCom chunks immediately after
   receiving the SSOpReq and the SSCliKey chunks.  After receiving the
   handshake messages from the server, the client computes a new master
   secret key and checks the  SSOpCom chunk of the server.  If it
   detects any error, the client closes the secure session and reports
   an error to the peer.  The client computes the SSOpCom chunk and
   sends it to the server.  After sending the SSOpCom chunk the client
   is ready to use the new master secret key.  The server receives the
   SSOpCom chunk of the client and checks the new keys.  If it detects
   any error, the server closes the secure session and reports an error
   to the peer.  Before receiving the client's SSOpCom chunk, the server
   discards any encrypted or authenticated chunk that make use of the
   new master secret key.

   The encrypted and unencrypted user data transmission works in
   parallel with the update operation.  After the update operation, the
   new master secret key is used for data encryption and authentication.
   When both client and server receive an SSOpReq chunk simultaneously,
   the client ignores the server's SSopReq chunk and the server accepts
   the client's SSOpReq chunk.  The next steps are the same as for the
   secure session initialisation.

   The new master secret key generation uses the same algorithm as
   described above.  The secure session includes one parameter which is
   called secure session lifetime.  This parameter is used to initialise
   a timer which indicates the secure session secret key's lifetime in



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   seconds.  When the timer expires, the association automatically
   updates the secret keys.  The user can define this parameter.  If the
   user does not define it, the parameter assumes a default value.  This
   default value depends on the implementation.  The implementation MUST
   define secure session's lifetime initial value.  We suggest a value
   of 600 seconds for the lifetime as a compromise between security and
   overhead.

8.7  Random number generation

   As the security of S-SCTP depends on the quality of the random number
   generator, we suggest to use one according to RFC1750 [1].

8.8  HMAC algorithm

   S-SCTP uses the HMAC algorithm which is defined in RFC2104 [2] for
   the packet authentication.


































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9.  HMAC algorithm

   ULP-to-SCTP primitives deliver upper layer requests to S-SCTP.  The
   following part describes new ULP-to-SCTP primitives and thus enhances
   the section 10 of RFC2960.  All new ULP-to-SCTP primitives described
   below are defined in the ssctp.h header file.

   INITSECSESS: This primitive initialises a new secure session.

   Format: {initSecSess(secure session ID, key material length, cipher
   suites list, compression methods list, certiticate(s)  ) --> result}

      secure session ID: This parameter identifies a secure session.

      key material length: This defines the key material length which is
      used in the SSOPReq chunk.

      cipher suite list: Eligible cipher suites for a new secure
      session.

      compression method list: Eligible compression methods for a new
      secure session.

      certificate(s): Local endpoint certificate(s).

   SETSECLEVEL: This primitive sets a new security level for an existing
   secure session.

   Format: {setSecLevel(secure session ID, security level) --> result}

      secure session ID: local handle to the secure session

      security level: This parameter indicates the new security level

   GETSECLEVEL: This pritimive gets the current security level of a
   secure session.

   Format: {getSecLevel(secure session ID) --> security level}

      secure session ID: local handle to the secure session

   SENDSEC: This primitve sends secure data via S-SCTP.

   Format: {sctp_send_enc(association id, buffer address, byte count,
   context, stream id, life time, destination transport address, unorder
   flag, no-bundle flag, payload protocol-id, encryption flag,
   compression flag) --> result}




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   Every parameter, except the encryption and compression flags, defined
   in this function is the same as the corresponding parameter defined
   in the SEND function of RFC2960 section 10.

      encryption flag: This flag defines if a current user data message
      needs encryption or not.

      compression flag: This flag defines if a current user data message
      needs compression or not.

   GETSECSTATUS: This primitive gets the security status of an
   association.  The security status indicates if the SCTP association
   is using a secure session or not.

   Format: {setSecStatus(association ID) -->  status}

      association ID: local handle to the SCTP association.

   SETSECSESSTTL: This primitive sets a new lifetime for a secure
   session.

   Format: {setSecSessTTL(secure session ID, Time) --> result}

      secure session ID: local handle to the secure session.

      time: The new lifetime in seconds.

   SHUTSECSESS: This primivite deletes a secure session.

   Format: {shutSecSess(secure session ID) --> result}

      secure session ID: local handle to the secure session.

      security level: This parameter indicates the new security level.

















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10.  S-SCTP to ULP

   S-SCTP defines new notifications to deliver information to the upper
   layer.  The notifications extend the section 10.2 of RFC2960 [4].
   All new notifications are defined in the ssctp.h header file.

   SECSESSUP:

   This notification indicates that S-SCTP is ready to send or receive
   secure data ({secsessUpNotif()}).

   SECSESSDOWN:

   This notification indicates that an association has lost a secure
   session ({secsessdownNotif()}).

   SECSESSREKEY:

   This notification indicates that a secure session updated the secret
   keys ({secsessrekeyNotif()}).

   Additional changes had to be made in the socket API implementation to
   access the new sctplib functions described above.  A user calls the
   same socket API functions as in standard SCTP to send and receive
   user data, but has to set an additional encryption flag (MSG_ENC) to
   request encryption of user data.  Also, a compression flag (MSG_COMP)
   has to be set in ext_send, ext_sendto, ext_sendmsg to request
   compression of user data.  S-SCTP compression performs per user
   message not per chunk or per packet.  In the SCTP DATA chunk, a new
   flag is defined, which indicates if the data is compressed or not.
   On the receiver side there are no changes.




















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11.  Transmission Control Block (TCB) extension

   A SCTP TCB contains parameters which are related to an association
   (e.g. an association id, port number, IP address list...).  S-SCTP
   defines several parameters which are related to a secure session and
   it extends the TCB defined in section 12 of RFC2960.

   Security level:

   This parameter contains the association's current security level.

   Second security level:

   This is the security level of the associated second endpoint.

   Key material length:

   The size of the key material, which was last used for key generation.

   Secure session status:

   This parameter indicates whether the association is using a secure
   session or not.

   Secure session lifetime:

   This parameter indicates the lifetime of the secret keys of a secure
   session.

   Server indication:

   This parameter indicates if an endpoint is server or client.  If the
   parameter is equal to 1 then it is a server, otherwise it is a
   client.

   Secure session ID:

   This parameter indicates the local secure session ID.

   Master secret key reference:

   This is an "array of secret data" collection and every array element
   includes the following parameters.

   o  Selected cipher suite: This parameter indicates the encryption and
      authentication algorithms that are used in a secure session.





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   o  Selected compression: This parameter indicates the compression
      method that is used in a secure session.

   o  Encryption key: This is a secret key which is used for encryption.

   o  Authentication key: This is a secret key which is used for
      authentication.

   This information is used in EncData and AUTH chunks.










































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12.  Socket API extensions for Secure SCTP

   S-SCTP defines new socket options for the ext_setsockopt() and
   ext_getsockopt() socket functions to initialise, delete and rekey a
   secure session.  A user calls the ext_setsockopt or ext_getsockopt
   functions with a new option.  It is not necessary to define new
   socket API functions, as this is a more standard socket API fashion.
   The following paragraphs describe the new socket options.

   SSCTP-INIT:

   This socket option is used to initialise or update a secure session.
   The following structure is used to access these parameters.

   struct ssctp_init {
          uint16_t  secsessID;
          uint16_t  key_length;
          uint8_t   num_cipher;
          uint8_t   *cipher_suites;
          uint8_t   num_comp;
          uint8_t   *comp_methods;
          uint8_t   *certificate;
   };

   o  secsessID: This parameter indicates a current secure session ID.

   o  key_length: This parameter defines the length of a key material.

   o  num_cipher: This parameter defines the number of cipher suites.

   o  cipher_suites: This parameter includes a list of cipher suites.

   o  num_comp: This parameter defines the number of compression
      methods.

   o  comp_methods: This parameter includes a list of compression
      methods.

   o  certificate: This parameter includes a certificate of the
      endpoint.

   SSCTP-SECLEVEL:

   This socket option is used to set and get a secure session security
   level.  The following structure is used to access and modify these
   parameters.





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   struct ssctp_seclevel {
          uint16_t  secsessID;
          uint8_t  seclevel;
   };

   o  secsessID: This parameter indicates a current secure session ID.
      This parameter MUST be zero when  beginning a secure session
      initialisation.

   o  seclevel: This parameter contains a new security level before
      socket write access or contains the current security level after
      socket read access.

   SSCTP-SECSTATUS:

   This socket option is used to get the secure session status and
   secure session ID when a secure session exists.  The following
   structure is used to access these parameters.

   struct ssctp_secstatus {
          uint16_t secsessID;
          uint8_t sec_status;
   };

   o  secsessID: This parameter contains the current secure session ID.
      This parameter MUST be zero when a secure session does not exist.

   o  sec_status: This parameter contains a security status.  This
      parameter MUST be zero when a secure session does not exist.  This
      parameter is equal to 1 when a secure session exists.

   SSCTP-SECSESSTTL:

   This socket option is used to set and get the secure session
   lifetime.  The following structure is used to access and modify these
   parameters.

   struct ssctp_secsessTTL {
          uint16_t secsessID;
          uint16_t secsessTTL;
   };

   o  secsessID: This parameter indicates the current secure session ID.

   o  secsessTTL (seconds): This parameter contains a new secure session
      lifetime before socket write access or contains a current secure
      session lifetime  after socket read access.




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   SSCTP-CLOSE:

   This socket option is used to close an existing secure session.  The
   following structure is used to access these parameters.

   struct ssctp_secclose {
          uint16_t  secsessID;
   };

   o  secsessID: This parameter contains the current secure session ID.


13.  Normative References

   [1]  Eastlake, D., Crocker, S., and J. Schiller, "Randomness
        Recommendations for Security", RFC 1750, December 1994.

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

   [3]  Housley, R., Ford, W., Polk, W., and D. Solo, "Internet X.509
        Public Key Infrastructure Certificate and CRL Profile",
        RFC 2459, January 1999.

   [4]  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.

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

   [6]  Bellovin, S., Keromytis, J., and R. Stewart, "On the Use of
        Stream Control Transmission Protocol (SCTP) with IPsec",
        RFC 3554, July 2003.
















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Authors' Addresses

   Carsten Hohendorf
   University of Duisburg-Essen, Institute for Experimental Mathematics
   Ellernstrasse 29
   45326 Essen, Nordrhein-Westfalen
   Germany

   Phone: +49 201 183-7637
   Fax:   +49 201 183-7673
   Email: hohend@iem.uni-due.de
   URI:   http://www.exp-math.uni-essen.de


   Esbold Unurkhaan
   Mongolian Science and Technological University
   Bayanzurkh duureg, 2-nd khoroo
   313/49 Ulaanbaatar
   Mongolia

   Email: esbold@csms.edu.mn


   Thomas Dreibholz
   University of Duisburg-Essen, Institute for Experimental Mathematics
   Ellernstrasse 29
   45326 Essen, Nordrhein-Westfalen
   Germany

   Phone: +49 201 183-7637
   Fax:   +49 201 183-7673
   Email: dreibh@exp-math.uni-essen.de
   URI:   http://www.exp-math.uni-essen.de/~dreibh/


















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