Network Working Group                                             L. Ong
Request for Comments: 3286                             Ciena Corporation
Category: Informational                                        J. Yoakum
                                                         Nortel Networks
                                                                May 2002


   An Introduction to the Stream Control Transmission Protocol (SCTP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document provides a high level introduction to the capabilities
   supported by the Stream Control Transmission Protocol (SCTP).  It is
   intended as a guide for potential users of SCTP as a general purpose
   transport protocol.

1. Introduction

   The Stream Control Transmission Protocol (SCTP) is a new IP transport
   protocol, existing at an equivalent level with UDP (User Datagram
   Protocol) and TCP (Transmission Control Protocol), which provide
   transport layer functions to many Internet applications.  SCTP has
   been approved by the IETF as a Proposed Standard [1].  The error
   check algorithm has since been modified [2].  Future changes and
   updates will be reflected in the IETF RFC index.

   Like TCP, SCTP provides a reliable transport service, ensuring that
   data is transported across the network without error and in sequence.
   Like TCP, SCTP is a session-oriented mechanism, meaning that a
   relationship is created between the endpoints of an SCTP association
   prior to data being transmitted, and this relationship is maintained
   until all data transmission has been successfully completed.

   Unlike TCP, SCTP provides a number of functions that are critical for
   telephony signaling transport, and at the same time can potentially
   benefit other applications needing transport with additional
   performance and reliability.  The original framework for the SCTP
   definition is described in [3].

2. Basic SCTP Features

   SCTP is a unicast protocol, and supports data exchange between
   exactly 2 endpoints, although these may be represented by multiple IP
   addresses.

   SCTP provides reliable transmission, detecting when data is
   discarded, reordered, duplicated or corrupted, and retransmitting
   damaged data as necessary.  SCTP transmission is full duplex.

   SCTP is message oriented and supports framing of individual message
   boundaries.  In comparison, TCP is byte oriented and does not
   preserve any implicit structure within a transmitted byte stream
   without enhancement.

   SCTP is rate adaptive similar to TCP, and will scale back data
   transfer to the prevailing load conditions in the network.  It is
   designed to behave cooperatively with TCP sessions attempting to use
   the same bandwidth.

3. SCTP Multi-Streaming Feature

   The name Stream Control Transmission Protocol is derived from the
   multi-streaming function provided by SCTP.  This feature allows data
   to be partitioned into multiple streams that have the property of
   independently sequenced delivery, so that message loss in any one
   stream will only initially affect delivery within that stream, and
   not delivery in other streams.

   In contrast, TCP assumes a single stream of data and ensures that
   delivery of that stream takes place with byte sequence preservation.
   While this is desirable for delivery of a file or record, it causes
   additional delay when message loss or sequence error occurs within
   the network.  When this happens, TCP must delay delivery of data
   until the correct sequencing is restored, either by receipt of an
   out-of-sequence message, or by retransmission of a lost message.

   For a number of applications, the characteristic of strict sequence
   preservation is not truly necessary.  In telephony signaling, it is
   only necessary to maintain sequencing of messages that affect the
   same resource (e.g., the same call, or the same channel).  Other
   messages are only loosely correlated and can be delivered without
   having to maintain overall sequence integrity.

   Another example of possible use of multi-streaming is the delivery of
   multimedia documents, such as a web page, when done over a single
   session.  Since multimedia documents consist of objects of different
   sizes and types, multi-streaming allows transport of these components

   to be partially ordered rather than strictly ordered, and may result
   in improved user perception of transport.

   At the same time, transport is done within a single SCTP association,
   so that all streams are subjected to a common flow and congestion
   control mechanism, reducing the overhead required at the transport
   level.

   SCTP accomplishes multi-streaming by creating independence between
   data transmission and data delivery.  In particular, each payload
   DATA "chunk" in the protocol uses two sets of sequence numbers, a
   Transmission Sequence Number that governs the transmission of
   messages and the detection of message loss, and the Stream ID/Stream
   Sequence Number pair, which is used to determine the sequence of
   delivery of received data.

   This independence of mechanisms allows the receiver to determine
   immediately when a gap in the transmission sequence occurs (e.g., due
   to message loss), and also whether or not messages received following
   the gap are within an affected stream.  If a message is received
   within the affected stream, there will be a corresponding gap in the
   Stream Sequence Number, while messages from other streams will not
   show a gap.  The receiver can therefore continue to deliver messages
   to the unaffected streams while buffering messages in the affected
   stream until retransmission occurs.

4. SCTP Multi-Homing Feature

   Another core feature of SCTP is multi-homing, or the ability for a
   single SCTP endpoint to support multiple IP addresses.  The benefit
   of multi-homing is potentially greater survivability of the session
   in the presence of network failures.  In a conventional single-homed
   session, the failure of a local LAN access can isolate the end
   system, while failures within the core network can cause temporary
   unavailability of transport until the IP routing protocols can
   reconverge around the point of failure.  Using multi-homed SCTP,
   redundant LANs can be used to reinforce the local access, while
   various options are possible in the core network to reduce the
   dependency of failures for different addresses.  Use of addresses
   with different prefixes can force routing to go through different
   carriers, for example, route-pinning techniques or even redundant
   core networks can also be used if there is control over the network
   architecture and protocols.

   In its current form, SCTP does not do load sharing, that is, multi-
   homing is used for redundancy purposes only.  A single address is
   chosen as the "primary" address and is used as the destination for
   all DATA chunks for normal transmission.  Retransmitted DATA chunks

   use the alternate address(es) to improve the probability of reaching
   the remote endpoint, while continued failure to send to the primary
   address ultimately results in the decision to transmit all DATA
   chunks to the alternate until heartbeats can reestablish the
   reachability of the primary.

   To support multi-homing, SCTP endpoints exchange lists of addresses
   during initiation of the association.  Each endpoint must be able to
   receive messages from any of the addresses associated with the remote
   endpoint; in practice, certain operating systems may utilize
   available source addresses in round robin fashion, in which case
   receipt of messages from different source addresses will be the
   normal case.  A single port number is used across the entire address
   list at an endpoint for a specific session.

   In order to reduce the potential for security issues, it is required
   that some response messages be sent specifically to the source
   address in the message that caused the response.  For example, when
   the server receives an INIT chunk from a client to initiate an SCTP
   association, the server always sends the response INIT ACK chunk to
   the source address that was in the IP header of the INIT.

5. Features of the SCTP Initiation Procedure

   The SCTP Initiation Procedure relies on a 4-message sequence, where
   DATA can be included on the 3rd and 4th messages of the sequence, as
   these messages are sent when the association has already been
   validated.  A "cookie" mechanism has been incorporated into the
   sequence to guard against some types of denial of service attacks.

5.1 Cookie Mechanism

   The "cookie" mechanism guards specifically against a blind attacker
   generating INIT chunks to try to overload the resources of an SCTP
   server by causing it to use up memory and resources handling new INIT
   requests.  Rather than allocating memory for a Transmission Control
   Block (TCB), the server instead creates a Cookie parameter with the
   TCB information, together with a valid lifetime and a signature for
   authentication, and sends this back in the INIT ACK.  Since the INIT
   ACK always goes back to the source address of the INIT, the blind
   attacker will not get the Cookie.  A valid SCTP client will get the
   Cookie and return it in the COOKIE ECHO chunk, where the SCTP server
   can validate the Cookie and use it to rebuild the TCB.  Since the
   server creates the Cookie, only it needs to know the format and
   secret key, this is not exchanged with the client.

   Otherwise, the SCTP Initiation Procedure follows many TCP
   conventions, so that the endpoints exchange receiver windows, initial
   sequence numbers, etc.  In addition to this, the endpoints may
   exchange address lists as discussed above, and also mutually confirm
   the number of streams to be opened on each side.

5.2 INIT Collision Resolution

   Multi-homing adds to the potential that messages will be received out
   of sequence or with different address pairs.  This is a particular
   concern during initiation of the association, where without
   procedures for resolving the collision of messages, you may easily
   end up with multiple parallel associations between the same
   endpoints.  To avoid this, SCTP incorporates a number of procedures
   to resolve parallel initiation attempts into a single association.

6. SCTP DATA Exchange Features

   DATA chunk exchange in SCTP follows TCP's Selective ACK procedure.
   Receipt of DATA chunks is acknowledged by sending SACK chunks, which
   indicate not only the cumulative Transmission Sequence Number (TSN)
   range received, but also any non-cumulative TSNs, implying gaps in
   the received TSN sequence.  Following TCP procedures, SACKs are sent
   using the "delayed ack" method, normally one SACK per every other
   received packet, but with an upper limit on the delay between SACKs
   and an increase to once per received packet when there are gaps
   detected.

   Flow and Congestion Control follow TCP algorithms.  The advertised
   receive window indicates buffer occupancy at the receiver, while a
   per-path congestion window is maintained to manage the packets in
   flight.  Slow start, Congestion avoidance, Fast recovery and Fast
   retransmit are incorporated into the procedures as described in RFC
   2581, with the one change being that the endpoints must manage the
   conversion between bytes sent and received and TSNs sent and
   received, since TSN is per chunk rather than per byte.

   The application can specify a lifetime for data to be transmitted, so
   that if the lifetime has expired and the data has not yet been
   transmitted, it can be discarded (e.g., time-sensitive signaling
   messages).  If the data has been transmitted, it must continue to be
   delivered to avoid creating a hole in the TSN sequence.

7. SCTP Shutdown Features

   SCTP Shutdown uses a 3-message procedure to allow graceful shutdown,
   where each endpoint has confirmation of the DATA chunks received by
   the remote endpoint prior to completion of the shutdown.  An Abort
   procedure is also provided for error cases when an immediate shutdown
   must take place.

   Note that SCTP does not support the function of a "half-open"
   connection as can occur in TCP, when one side indicates that it has
   no more data to send, but the other side can continue to send data
   indefinitely.  SCTP assumes that once the shutdown procedure begins,
   both sides will stop sending new data across the association, and
   only need to clear up acknowledgements of previously sent data.

8. SCTP Message Format

   The SCTP Message includes a common header plus one or more chunks,
   which can be control or data.  The common header has source and
   destination port numbers to allow multiplexing of different SCTP
   associations at the same address, a 32-bit verification tag that
   guards against insertion of an out-of-date or false message into the
   SCTP association, and a 32-bit checksum (this has been modified to
   use the CRC-32c polynomial [2]) for error detection.

   Each chunk includes chunk type, flag field, length and value.
   Control chunks incorporate different flags and parameters depending
   on the chunk type.  DATA chunks in particular incorporate flags for
   control of segmentation and reassembly, and parameters for the TSN,
   Stream ID and Stream Sequence Number, and a Payload Protocol
   Identifier.

   The Payload Protocol ID has been included for future flexibility.  It
   is envisioned that the functions of protocol identification and port
   number multiplexing will not be as closely linked in the future as
   they are in current usage.  Payload Protocol ID will allow the
   protocol being carried by SCTP to be identified independent of the
   port numbers being used.

   The SCTP message format naturally allows support of bundling of
   multiple DATA and control chunks in a single message, to improve
   transport efficiency.  Use of bundling is controllable by the
   application, so that bundling of initial transmission can be
   prohibited.  Bundling will naturally occur on retransmission of DATA
   chunks, to further reduce any chance of congestion.

9. Error Handling

9.1 Retransmission

   Retransmission of DATA chunks occurs from either (a) timeout of the
   retransmission timer; or (b) receipt of SACKs indicating the DATA
   chunk has not been received.  To reduce the potential for congestion,
   the rate of retransmission of DATA chunks is limited.  The
   retransmission timeout (RTO) is adjusted based on estimates of the
   round trip delay and backs off exponentially as message loss
   increases.

   In an active association with fairly constant DATA transmission,
   SACKs are more likely to cause retransmission than the timeout.  To
   reduce the chance of an unnecessary retransmission, a 4 SACK rule is
   used, so that retransmission only occurs on receipt of the 4th SACK
   that indicates that the chunk is missing.  This is intended to avoid
   retransmits due to normal occurrences such as packets received out of
   sequence.

9.2 Path Failure

   A count is maintained of the number of retransmissions to a
   particular destination address without successful acknowledgement.
   When this count exceeds a configured maximum, the address is declared
   inactive, notification is given to the application, and the SCTP
   begins to use an alternate address for the sending of DATA chunks.

   Also, Heartbeat chunks are sent periodically to all idle destinations
   (i.e., alternate addresses), and a counter is maintained on the
   number of Heartbeats sent to an inactive destination without receipt
   of a corresponding Heartbeat Ack.  When this counter exceeds a
   configured maximum, that destination address is also declared
   inactive.

   Heartbeats continue to be sent to inactive destination addresses
   until an Ack is received, at which point the address can be made
   active again.  The rate of sending Heartbeats is tied to the RTO
   estimation plus an additional delay parameter that allows Heartbeat
   traffic to be tailored according to the needs of the user
   application.

9.3 Endpoint Failure

   A count is maintained across all destination addresses on the number
   of retransmits or Heartbeats sent to the remote endpoint without a
   successful Ack.  When this exceeds a configured maximum, the endpoint
   is declared unreachable, and the SCTP association is closed.

10. API

   The specification includes a model of the primitives exchanged
   between the application and the SCTP layer, intended as informational
   material rather than a formal API statement.  A socket-based API is
   being defined to simplify migration of TCP or UDP applications to the
   use of SCTP.

11. Security Considerations

   In addition to the verification tag and cookie mechanisms, SCTP
   specifies the use of IPSec if strong security and integrity
   protection is required.  The SCTP specification does not itself
   define any new security protocols or procedures.

   Extensions to IPSec are under discussion to reduce the overhead
   required to support multi-homing.  Also, work is in progress on the
   use of Transport Layer Security (TLS) over SCTP [4].

12. Extensions

   The SCTP format allows new chunk types, flags and parameter fields to
   be defined as extensions to the protocol.  Any extensions must be
   based on standard agreements within the IETF, as no vendor-specific
   extensions are supported in the protocol.

   Chunk Type values are organized into four ranges to allow extensions
   to be made with a pre-defined procedure for responding if a new Chunk
   Type is not recognized at the remote endpoint.  Responses include:
   whole packet discard; packet discard with reporting; ignoring the
   chunk; ignoring with reporting.  Similar pre-defined responses are
   specified for unrecognized Parameter Type values.

   Chunk Parameter Type values are in principle independent ranges for
   each Chunk Type.  In practice, the values defined in the SCTP
   specification have been coordinated so that a particular parameter
   type will have the same Chunk Parameter Type value across all Chunk
   Types.  Further experience will determine if this alignment needs to
   be maintained or formalized.

13. Informative References

   [1] 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.

   [2] Stewart, Sharp, et. al., "SCTP Checksum Change", Work in
       Progress.

   [3] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
       Lin, H., Juhasz, I., Holdrege, M. and C. Sharp, "Framework
       Architecture for Signaling Transport", RFC 2719, October 1999.

   [4] Jungmeier, Rescorla and Tuexen, "TLS Over SCTP", Work in
       Progress.

14. Authors' Addresses

   Lyndon Ong
   Ciena Corporation
   10480 Ridgeview Drive
   Cupertino, CA 95014

   EMail: lyong@ciena.com


   John Yoakum
   Emerging Opportunities
   Nortel Networks

   EMail: yoakum@nortelnetworks.com

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Acknowledgement

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