rfc2960.txt

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Stewart, et al.             Standards Track                    [Page 11]

RFC 2960          Stream Control Transmission Protocol      October 2000


   o  Cumulative TSN Ack Point: The TSN of the last DATA chunk
      acknowledged via the Cumulative TSN Ack field of a SACK.

   o  Idle destination address: An address that has not had user
      messages sent to it within some length of time, normally the
      HEARTBEAT interval or greater.

   o  Inactive destination transport address: An address which is
      considered inactive due to errors and unavailable to transport
      user messages.

   o  Message = user message:  Data submitted to SCTP by the Upper Layer
      Protocol (ULP).

   o  Message Authentication Code (MAC):  An integrity check mechanism
      based on cryptographic hash functions using a secret key.
      Typically, message authentication codes are used between two
      parties that share a secret key in order to validate information
      transmitted between these parties.  In SCTP it is used by an
      endpoint to validate the State Cookie information that is returned
      from the peer in the COOKIE ECHO chunk.  The term "MAC" has
      different meanings in different contexts.  SCTP uses this term
      with the same meaning as in [RFC2104].

   o  Network Byte Order: Most significant byte first, a.k.a., Big
      Endian.

   o  Ordered Message: A user message that is delivered in order with
      respect to all previous user messages sent within the stream the
      message was sent on.

   o  Outstanding TSN (at an SCTP endpoint): A TSN (and the associated
      DATA chunk) that has been sent by the endpoint but for which it
      has not yet received an acknowledgement.

   o  Path: The route taken by the SCTP packets sent by one SCTP
      endpoint to a specific destination transport address of its peer
      SCTP endpoint.  Sending to different destination transport
      addresses does not necessarily guarantee getting separate paths.

   o  Primary Path: The primary path is the destination and source
      address that will be put into a packet outbound to the peer
      endpoint by default.  The definition includes the source address
      since an implementation MAY wish to specify both destination and
      source address to better control the return path taken by reply
      chunks and on which interface the packet is transmitted when the
      data sender is multi-homed.




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RFC 2960          Stream Control Transmission Protocol      October 2000


   o  Receiver Window (rwnd): An SCTP variable a data sender uses to
      store the most recently calculated receiver window of its peer, in
      number of bytes.  This gives the sender an indication of the space
      available in the receiver's inbound buffer.

   o  SCTP association: A protocol relationship between SCTP endpoints,
      composed of the two SCTP endpoints and protocol state information
      including Verification Tags and the currently active set of
      Transmission Sequence Numbers (TSNs), etc.  An association can be
      uniquely identified by the transport addresses used by the
      endpoints in the association.  Two SCTP endpoints MUST NOT have
      more than one SCTP association between them at any given time.

   o  SCTP endpoint: The logical sender/receiver of SCTP packets.  On a
      multi-homed host, an SCTP endpoint is represented to its peers as
      a combination of a set of eligible destination transport addresses
      to which SCTP packets can be sent and a set of eligible source
      transport addresses from which SCTP packets can be received.  All
      transport addresses used by an SCTP endpoint must use the same
      port number, but can use multiple IP addresses.  A transport
      address used by an SCTP endpoint must not be used by another SCTP
      endpoint.  In other words, a transport address is unique to an
      SCTP endpoint.

   o  SCTP packet (or packet): The unit of data delivery across the
      interface between SCTP and the connectionless packet network
      (e.g., IP).  An SCTP packet includes the common SCTP header,
      possible SCTP control chunks, and user data encapsulated within
      SCTP DATA chunks.

   o  SCTP user application (SCTP user): The logical higher-layer
      application entity which uses the services of SCTP, also called
      the Upper-layer Protocol (ULP).

   o  Slow Start Threshold (ssthresh): An SCTP variable.  This is the
      threshold which the endpoint will use to determine whether to
      perform slow start or congestion avoidance on a particular
      destination transport address.  Ssthresh is in number of bytes.

   o  Stream: A uni-directional logical channel established from one to
      another associated SCTP endpoint, within which all user messages
      are delivered in sequence except for those submitted to the
      unordered delivery service.

   Note: The relationship between stream numbers in opposite directions
   is strictly a matter of how the applications use them.  It is the
   responsibility of the SCTP user to create and manage these
   correlations if they are so desired.



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RFC 2960          Stream Control Transmission Protocol      October 2000


   o  Stream Sequence Number: A 16-bit sequence number used internally
      by SCTP to assure sequenced delivery of the user messages within a
      given stream.  One stream sequence number is attached to each user
      message.

   o  Tie-Tags: Verification Tags from a previous association.  These
      Tags are used within a State Cookie so that the newly restarting
      association can be linked to the original association within the
      endpoint that did not restart.

   o  Transmission Control Block (TCB): An internal data structure
      created by an SCTP endpoint for each of its existing SCTP
      associations to other SCTP endpoints.  TCB contains all the status
      and operational information for the endpoint to maintain and
      manage the corresponding association.

   o  Transmission Sequence Number (TSN): A 32-bit sequence number used
      internally by SCTP.  One TSN is attached to each chunk containing
      user data to permit the receiving SCTP endpoint to acknowledge its
      receipt and detect duplicate deliveries.

   o  Transport address:  A Transport Address is traditionally defined
      by Network Layer address, Transport Layer protocol and Transport
      Layer port number.  In the case of SCTP running over IP, a
      transport address is defined by the combination of an IP address
      and an SCTP port number (where SCTP is the Transport protocol).

   o Unacknowledged TSN (at an SCTP endpoint): A TSN (and the associated
      DATA chunk) which has been received by the endpoint but for which
      an acknowledgement has not yet been sent. Or in the opposite case,
      for a packet that has been sent but no acknowledgement has been
      received.

   o  Unordered Message: Unordered messages are "unordered" with respect
      to any other message, this includes both other unordered messages
      as well as other ordered messages.  Unordered message might be
      delivered prior to or later than ordered messages sent on the same
      stream.

   o  User message: The unit of data delivery across the interface
      between SCTP and its user.

   o  Verification Tag: A 32 bit unsigned integer that is randomly
      generated.  The Verification Tag provides a key that allows a
      receiver to verify that the SCTP packet belongs to the current
      association and is not an old or stale packet from a previous
      association.




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RFC 2960          Stream Control Transmission Protocol      October 2000


1.5. Abbreviations

   MAC    - Message Authentication Code [RFC2104]

   RTO    - Retransmission Time-out

   RTT    - Round-trip Time

   RTTVAR - Round-trip Time Variation

   SCTP   - Stream Control Transmission Protocol

   SRTT   - Smoothed RTT

   TCB    - Transmission Control Block

   TLV    - Type-Length-Value Coding Format

   TSN    - Transmission Sequence Number

   ULP    - Upper-layer Protocol

1.6 Serial Number Arithmetic

   It is essential to remember that the actual Transmission Sequence
   Number space is finite, though very large.  This space ranges from 0
   to 2**32 - 1. Since the space is finite, all arithmetic dealing with
   Transmission Sequence Numbers must be performed modulo 2**32.  This
   unsigned arithmetic preserves the relationship of sequence numbers as
   they cycle from 2**32 - 1 to 0 again.  There are some subtleties to
   computer modulo arithmetic, so great care should be taken in
   programming the comparison of such values.  When referring to TSNs,
   the symbol "=<" means "less than or equal"(modulo 2**32).

   Comparisons and arithmetic on TSNs in this document SHOULD use Serial
   Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 32.

   An endpoint SHOULD NOT transmit a DATA chunk with a TSN that is more
   than 2**31 - 1 above the beginning TSN of its current send window.
   Doing so will cause problems in comparing TSNs.

   Transmission Sequence Numbers wrap around when they reach 2**32 - 1.
   That is, the next TSN a DATA chunk MUST use after transmitting TSN =
   2*32 - 1 is TSN = 0.

   Any arithmetic done on Stream Sequence Numbers SHOULD use Serial
   Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 16.




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RFC 2960          Stream Control Transmission Protocol      October 2000


   All other arithmetic and comparisons in this document uses normal
   arithmetic.

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
   [RFC2119].

3.  SCTP packet Format

   An SCTP packet is composed of a common header and chunks. A chunk
   contains either control information or user data.

   The SCTP packet format is shown below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Common Header                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Chunk #1                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           ...                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Chunk #n                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Multiple chunks can be bundled into one SCTP packet up to the MTU
   size, except for the INIT, INIT ACK, and SHUTDOWN COMPLETE chunks.
   These chunks MUST NOT be bundled with any other chunk in a packet.
   See Section 6.10 for more details on chunk bundling.

   If a user data message doesn't fit into one SCTP packet it can be
   fragmented into multiple chunks using the procedure defined in
   Section 6.9.

   All integer fields in an SCTP packet MUST be transmitted in network
   byte order, unless otherwise stated.











Stewart, et al.             Standards Track                    [Page 16]

RFC 2960          Stream Control Transmission Protocol      October 2000


3.1 SCTP Common Header Field Descriptions

                         SCTP Common Header Format

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Source Port Number        |     Destination Port Number   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Verification Tag                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Checksum                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Source Port Number: 16 bits (unsigned integer)

      This is the SCTP sender's port number.  It can be used by the
      receiver in combination with the source IP address, the SCTP
      destination port and possibly the destination IP address to