Network Working Group X. Fu
Internet-Draft C. Dickmann
Intended status: Standards Track University of Goettingen
Expires: August 17, 2008 J. Crowcroft
University of Cambridge
February 14, 2008
General Internet Signaling Transport (GIST) over SCTP
draft-ietf-nsis-ntlp-sctp-03.txt
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Abstract
The General Internet Signaling Transport (GIST) protocol currently
uses TCP or TLS over TCP for connection mode operation. This
document describes the usage of GIST over the Stream Control
Transmission Protocol (SCTP). The use of SCTP can take advantage of
features provided by SCTP, namely streaming-based transport, support
of multiple streams to avoid head of line blocking, and the support
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of multi-homing to provide network level fault tolerance.
Additionally, the support for the Partial Reliability Extension of
SCTP is discussed.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Abbreviations . . . . . . . . . . . . . . . . 3
3. GIST Over SCTP . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Message Association Setup . . . . . . . . . . . . . . . . 4
3.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. Protocol-Definition: Forwards-SCTP . . . . . . . . . . 4
3.2. Effect on GIST State Maintenance . . . . . . . . . . . . . 5
3.3. PR-SCTP Support . . . . . . . . . . . . . . . . . . . . . 6
3.4. API between GIST and NSLP . . . . . . . . . . . . . . . . 6
3.4.1. SendMessage . . . . . . . . . . . . . . . . . . . . . 6
3.4.2. NetworkNotification . . . . . . . . . . . . . . . . . 6
4. Bit-Level Formats . . . . . . . . . . . . . . . . . . . . . . 7
4.1. MA-Protocol-Options . . . . . . . . . . . . . . . . . . . 7
5. Application of GIST over SCTP . . . . . . . . . . . . . . . . 7
5.1. Multi-homing support of SCTP . . . . . . . . . . . . . . . 7
5.2. Streaming support in SCTP . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
This document describes the usage of the General Internet Signaling
Transport (GIST) protocol [1] over the Stream Control Transmission
Protocol (SCTP) [2].
GIST, in its initial specification for connection mode operation,
runs on top of a byte-stream oriented transport protocol providing a
reliable, in-sequence delivery, i.e., using the Transmission Control
Protocol (TCP) [5] for signaling message transport. However, some
NSLP context information has a definite lifetime, therefore, the GIST
transport protocol could benefit from flexible retransmission, so
stale NSLP messages that are held up by congestion can be dropped.
Together with the head-of-line blocking issue and other issues with
TCP, these considerations argue that implementations of GIST should
support the Stream Control Transport Protocol (SCTP)[2] as an
optional transport protocol for GIST, especially if deployment over
the public Internet is contemplated. Like TCP, SCTP supports
reliability, congestion control and fragmentation. Unlike TCP, SCTP
provides a number of functions that are desirable for signaling
transport, such as multiple streams and multiple IP addresses for
path failure recovery. In addition, its Partial Reliability
extension (PR-SCTP) [3] supports partial retransmission based on a
programmable retransmission timer.
This document defines the use of SCTP as a transport protocol for
GIST Messaging Associations and discusses the implications on GIST
State Maintenance and API between GIST and NSLPs. Furturemore, this
document shows how GIST SHOULD be used to provide the additional
features offered by SCTP to deliver the GIST C-mode messages (which
can in turn carry NSIS Signaling Layer Protocol (NSLP) [6] messages
as payload). More specifically:
o How to use the multiple streams feature of SCTP.
o How to use the PR-SCTP extention of SCTP.
o How to take advantage of the multi-homing support of SCTP.
The method described in this document does not require any changes of
GIST or SCTP. However, SCTP implementations MUST support the
optional feature of fragmentation of SCTP user messages.
2. Terminology and Abbreviations
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [4]. Other
terminologies and abbreviations used in this document are taken from
related specifications (e.g., [1] and [2]) as follows:
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o SCTP - Stream Control Transmission Protocol
o PR-SCTP - SCTP Partial Reliability Extension
o MRM - Message Routing Method
o MRI - Message Routing Information
o MRS - Message Routing State
o MA - A GIST Messaging Association is a single connection between
two explicitly identified GIST adjacent peers on the data path. A
messaging association may use a specific transport protocol and
known ports. If security protection is required, it may use a
specific network layer security association, or use a transport
layer security association internally. A messaging association is
bidirectional; signaling messages can be sent over it in either
direction, and can refer to flows of either direction.
o SCTP Association - A protocol relationship between SCTP endpoints,
composed of the two SCTP endpoints and protocol state information.
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 Stream - A sequence of user messages that are to be delivered to
the upper-layer protocol in order with respect to other messages
within the same stream.
3. GIST Over SCTP
3.1. Message Association Setup
3.1.1. Overview
The basic GIST protocol specification defines two possible protocols
to be used in Messaging Associations, namely Forwards-TCP and TLS.
This document adds Forwards-SCTP as another possible protocol. In
Forwards-SCTP, analog to Forwards-TCP, connections between peers are
opened in the forwards direction, from the querying node, towards the
responder.
A new MA-Protocol-ID type, "Forwards-SCTP", is defined in this
document for using SCTP as GIST transport protocol. A formal
definition of Forwards-SCTP is given in the following section.
3.1.2. Protocol-Definition: Forwards-SCTP
This MA-Protocol-ID denotes a basic use of SCTP between peers.
Support for this protocol is OPTIONAL. If this protocol is offered,
MA-protocol-options data MUST also be carried in the SCD object. The
MA-protocol-options field formats are:
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o in a Query: no information apart from the field header.
o in a Response: 2 byte port number at which the connection will be
accepted, followed by 2 pad bytes.
The connection is opened in the forwards direction, from the querying
node towards the responder. The querying node MAY use any source
address and source port. The destination information MUST be derived
from information in the Response: the address from the interface-
address from the Network-Layer-Information object and the port from
the SCD object as described above.
Associations using Forwards-SCTP can carry messages with the transfer
attribute Reliable=True. If an error occurs on the SCTP connection
such as a reset, as can be detected for example by a socket exception
condition, GIST MUST report this to NSLPs as discussed in Section
4.1.2 of [1].
3.2. Effect on GIST State Maintenance
This document defines the use of SCTP as a transport protocol for
GIST Messaging Associations. As SCTP provides additional
functionality over TCP, this section dicusses the implications of
using GIST over SCTP on GIST State Maintenance.
While SCTP defines uni-directional streams, for the purpose of this
document, the concept of a bi-direction stream is used.
Implementations MUST establish downstream and upstream (uni-
directional) SCTP streams always together and use the same stream
identifier in both directions. Thus, the two uni-directional streams
(in opposite directions) form a bi-directional stream.
Due to the multi-streaming support of SCTP, it is possible to use
different SCTP streams for different resources (e.g., different NSLP
sessions), rather than maintaining all messages along the same
transport connection/association in a correlated fashion as TCP
(which imposes strict (re)ordering and reliability per transport
level). However, there are limitations to the use of multi-
streaming. All GIST messages for a particular session MUST be sent
over the same SCTP stream to assure the NSLP assumption of in-order
delivery. Multiple sessions MAY share the same SCTP stream based on
local policy.
The GIST concept of Messaging Association re-use is not affected by
this document or the use of SCTP. All rules defined in the GIST
specification remain valid in the context of GIST over SCTP.
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3.3. PR-SCTP Support
A variant of SCTP, PR-SCTP [3] provides a "timed reliability"
service. It allows the user to specify, on a per message basis, the
rules governing how persistent the transport service should be in
attempting to send the message to the receiver. Because of the chunk
bundling function of SCTP, reliable and partial reliable messages can
be multiplexed over a single PR-SCTP association. Therefore, a GIST
over SCTP implementation SHOULD attempt to establish a PR-SCTP
association instead of a standard SCTP association, if available, to
support more flexible transport features for potential needs of
different NSLPs.
3.4. API between GIST and NSLP
GIST specification defines an abstract API between GIST and NSLPs.
While this document does not change the API itself, the semantics of
some parameters have slightly different interpretation in the context
of SCTP. This section only lists those primitives and parameters,
that need special consideration when used in the context of SCTP.
The relevant primitives are repeatet from [1] to improve readability,
but [1] remains authoritative.
3.4.1. SendMessage
The SendMessage primitive is used by the NSLP to initiate sending of
messages.
SendMessage ( NSLP-Data, NSLP-Data-Size, NSLP-Message-Handle,
NSLP-Id, Session-ID, MRI,
SSI-Handle, Transfer-Attributes, Timeout, IP-TTL, GHC )
The following parameter has changed semantics:
Timeout: According to [1] this parameter represents the "length of
time GIST should attempt to send this message before indicating an
error". When used with SCTP, this parameter is also used as the
timeout for the "timed reliability" service of PR-SCTP.
3.4.2. NetworkNotification
The NetworkNotification primitive is passed from GIST to an NSLP. It
indicates that a network event of possible interest to the NSLP
occurred.
NetworkNotification ( MRI, Network-Notification-Type )
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If SCTP detects a failure of the primary path, GIST SHOULD indicate
this event to the NSLP by calling the NetworkNotification primitive
with Network-Notification-Type "Routing Status Change". This
notification should be done even if SCTP was able to remain an open
connection to the peer due to its multi-homing capabilities.
4. Bit-Level Formats
4.1. MA-Protocol-Options
This section provides the bit-level format for the MA-protocol-
options field that is used for SCTP protocol in the Stack-
Configuration-Data object of GIST.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: SCTP port number | Reserved :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SCTP port number = Port number at which the responder will accept
SCTP connections
The SCTP port number is only supplied if sent by the responder.
5. Application of GIST over SCTP
5.1. Multi-homing support of SCTP
In general, the multi-homing support of SCTP can be used to improve
fault-tolerance in case of a path- or link-failure. Thus, GIST over
SCTP would be able to deliver NSLP messages between peers even if the
primary path is not working anymore. However, for the Message
Routing Methods (MRMs) defined in the basic GIST specification such a
feature is only of limited use. The default MRM is path-coupled,
which means, that if the primary path is failing for the SCTP
association, it most likely is also for the IP traffic that is
signaled for. Thus, GIST would need to perform a refresh anyway to
cope with the route change. Nevertheless, the use of the multi-
homing support of SCTP provides GIST and the NSLP with another source
to detect route changes. Furthermore, for the time between detection
of the route change and recovering from it, the alternative path
offered by SCTP can be used by the NSLP to make the transition more
smoothly. Finally, future MRMs might have different properties and
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therefore benefit from multi-homing more broadly.
5.2. Streaming support in SCTP
Streaming support in SCTP is advantageous for GIST. It allows better
parallel processing, in particular by avoiding head of line blocking
issue in TCP. Since a same GIST MA may be reused by multiple
sessions, using TCP as transport GIST signaling messages belonging to
different sessions may be blocked if another message is dropped. In
the case of SCTP, this can be avoided as different sessions having
different requirements can belong to different streams, thus a
message loss or reordering in a stream will only affect the delivery
of messages within that particular stream, and not any other streams.
6. Security Considerations
The security considerations of both [1] and [2] apply. For securing
GIST over SCTP channel, it is recommended to use DTLS [7], to take
the advantage of all the features provided by SCTP and its
extensions. DTLS over SCTP is currently being specified in [8]. The
usage of DTLS for GIST is similar to TLS for GIST as specified in
[1], and a MA-protocol-ID for DTLS is yet to be defined in another
document.
7. IANA Considerations
A new MA-Protocol-ID (Forwards-SCTP) needs to be assigned, with a
recommended value of 3.
8. Acknowledgments
The authors would like to thank John Loughney, Robert Hancock, Andrew
McDonald, Fang-Chun Kuo and Jan Demter for their helpful suggestions.
9. References
9.1. Normative References
[1] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", draft-ietf-nsis-ntlp-15 (work in
progress), February 2008.
[2] Stewart, R., "Stream Control Transmission Protocol", RFC 4960,
September 2007.
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[3] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad,
"Stream Control Transmission Protocol (SCTP) Partial Reliability
Extension", RFC 3758, May 2004.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[5] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[6] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
June 2005.
[7] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[8] Tuexen, M. and E. Rescorla, "Datagram Transport Layer Security
for Stream Control Transmission Protocol",
draft-tuexen-dtls-for-sctp-02 (work in progress), November 2007.
Authors' Addresses
Xiaoming Fu
University of Goettingen
Institute of Computer Science
Lotzestr. 16-18
Goettingen 37083
Germany
Email: fu@cs.uni-goettingen.de
Christian Dickmann
University of Goettingen
Institute of Computer Science
Lotzestr. 16-18
Goettingen 37083
Germany
Email: mail@christian-dickmann.de
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Jon Crowcroft
University of Cambridge
Computer Laboratory
William Gates Building
15 JJ Thomson Avenue
Cambridge CB3 0FD
UK
Email: jon.crowcroft@cl.cam.ac.uk
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