Network Working Group X. Fu
Internet-Draft C. Dickmann
Intended status: Experimental University of Goettingen
Expires: December 26, 2010 J. Crowcroft
University of Cambridge
June 24, 2010
General Internet Signaling Transport (GIST) over Stream Control
Transmission Protocol (SCTP) and Datagram Transport Layer Security
(DTLS)
draft-ietf-nsis-ntlp-sctp-13.txt
Abstract
The General Internet Signaling Transport (GIST) protocol currently
uses TCP or Transport Layer Security (TLS) over TCP for connection
mode operation. This document describes the usage of GIST over the
Stream Control Transmission Protocol (SCTP) and Datagram Transport
Layer Security (DTLS). It discusses how the use of SCTP can take
advantage of features provided by SCTP, as well as how to establish
GIST security over datagram transport protocols with DTLS.
Status of this Memo
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This Internet-Draft will expire on December 26, 2010.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and Abbreviations . . . . . . . . . . . . . . . . 5
3. GIST Over SCTP . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Message Association Setup . . . . . . . . . . . . . . . . 5
3.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.2. Protocol-Definition: Forwards-SCTP . . . . . . . . . . 6
3.2. Effect on GIST State Maintenance . . . . . . . . . . . . . 6
3.3. PR-SCTP Support . . . . . . . . . . . . . . . . . . . . . 7
3.4. API between GIST and NSLP . . . . . . . . . . . . . . . . 7
4. Bit-Level Formats . . . . . . . . . . . . . . . . . . . . . . 8
4.1. MA-Protocol-Options . . . . . . . . . . . . . . . . . . . 8
5. Application of GIST over SCTP . . . . . . . . . . . . . . . . 8
5.1. Multi-homing support of SCTP . . . . . . . . . . . . . . . 8
5.2. Streaming support in SCTP . . . . . . . . . . . . . . . . 9
6. NAT Traversal Issue . . . . . . . . . . . . . . . . . . . . . 9
7. Use of DTLS with GIST . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
This document describes the usage of the General Internet Signaling
Transport (GIST) protocol [1] and Datagram Transport Layer Security
(DTLS) [2] over the Stream Control Transmission Protocol (SCTP) [3].
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) [9] for signaling message transport. However, some
Next Steps in Signaling (NSIS) Signaling Layer Protocol (NSLP) [10]
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 and multihoming issues with
TCP, these considerations argue that implementations of GIST should
support SCTP as an optional transport protocol for GIST. 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. Furthermore, SCTP offers an
advantage of message-oriented transport instead of using the byte
stream oriented TCP where one has to provide its own framing
mechanisms. In addition, its Partial Reliability extension (PR-SCTP)
[4] supports partial retransmission based on a programmable
retransmission timer. Furthermore, DTLS provides a viable solution
for securing SCTP [5], which allows SCTP to use almost all its
transport features and its extensions.
This document defines the use of SCTP as the underlying transport
protocol for GIST and the use of DTLS as a security mechanism for
protecting GIST Messaging Associations and discusses the implications
on GIST state maintenance and API between GIST and NSLPs.
Furthermore, this document describes how GIST is transported over
SCTP and used by NSLPs in order to exploit the additional
capabilities offered by SCTP to deliver GIST C-mode messages more
effectively. More specifically:
o How to use the multiple streams feature of SCTP.
o How to use the PR-SCTP extension of SCTP.
o How to take advantage of the multi-homing support of SCTP.
GIST over SCTP described in this document do not require any changes
to the high level operation and structure of GIST. However, adding
new transport options requires additional interface code and
configuration support to allow applications to exploit the additional
transport when appropriate. In addition, SCTP implementions to
transport GIST MUST support the optional feature of fragmentation of
SCTP user messages.
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Additionally, this document also specifies how to establish GIST
security using DTLS for use in combination with e.g., SCTP and UDP.
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 [6]. Other
terminologies and abbreviations used in this document are taken from
related specifications ([1], [2], [3], [4]):
o SCTP - Stream Control Transmission Protocol
o PR-SCTP - SCTP Partial Reliability Extension
o MRM - Message Routing Method
o MRI - Message Routing Information
o SCD - Stack-Configuration-Data
o Messaging Association (MA) - a single connection between two
explicitly identified GIST adjacent peers, i.e. between a given
signalling source and destination address. A messaging
association may use a transport protocol; 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 bi-directional; signaling
messages can be sent over it in either direction, referring 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 unidirectional 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.
3. GIST Over SCTP
This section defines a new MA-Protocol-ID type, "Forwards-SCTP", for
using SCTP as GIST transport protocol. The use of DTLS in GIST is
defined in Section 7.
3.1. Message Association Setup
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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 information is a main part of the Stack Configuration Data (SCD)
[1]. This section adds "Forwards-SCTP" as another possible protocol
option. In Forwards-SCTP, analog to Forwards-TCP, connections
between peers are opened in the forwards direction, from the querying
node, towards the responder.
3.1.2. Protocol-Definition: Forwards-SCTP
The MA-Protocol-ID "Forwards-SCTP" 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:
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 for establishing the
message association 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 reported by an SCTP socket API
notification[11], GIST MUST report this to NSLPs as discussed in
Section 4.1.2 of [1]. For the multi-homing scenario, when a
destination address of a GIST over SCTP peer encounters a change, the
SCTP API will notify GIST about the availability of different SCTP
endpoint addresses and possible change of the primary path.
3.2. Effect on GIST State Maintenance
As SCTP provides additional functionality over TCP, this section
discusses 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-directional 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
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(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. When an SCTP implementation is used for GIST transport,
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.
3.3. PR-SCTP Support
A variant of SCTP, PR-SCTP [4] provides a "timed reliability"
service, which would be particularly useful for delivering GIST
Connection mode messages. 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
partially reliable messages can be multiplexed over a single PR-SCTP
association. Therefore, an SCTP implementation for GIST transport
SHOULD attempt to establish a PR-SCTP association using "timed
reliability" service instead of a standard SCTP association, if
available, to support more flexible transport features for potential
needs of different NSLPs.
When using a normally reliable session (as opposed to a partially
reliable session), if a node has sent the first transmission before
the lifetime expires, then the message MUST be sent as a normal
reliable message. During episodes of congestion this is particularly
unfortunate, as retransmission wastes bandwidth that could have been
used for other (non-lifetime expired) messages. The "timed
reliability" service in PR-SCTP eliminates this issue and is hence
RECOMMENDED to be used for GIST over PR-SCTP.
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,
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that need special consideration when used in the context of SCTP.
The relevant primitives from [1] are as follows:
o The Timeout parameter in API "SendMessage": According to [1], this
parameter represents the "length of time GIST should attempt to
send this message before indicating an error." When used with PR-
SCTP, this parameter is used as the timeout for the "timed
reliability" service of PR-SCTP.
o "NetworkNotification": According to [1], this primitive "is passed
from GIST to a signalling application. It indicates that a
network event of possible interest to the signalling application
occurred." Here, if SCTP detects a failure of the primary path,
GIST SHOULD also indicate this event to the NSLP by calling this
primitive with Network-Notification-Type "Routing Status Change".
This notification should be done even if SCTP was able to retain
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
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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 to the NSIS
nodes to the alternative path anyway to cope with the route change.
When the two endpoints of a multi-homed SCTP association (but none of
the intermediate nodes between them) support NSIS, GIST over SCTP
provides a robust means for GIST to deliver NSLP messages even when
the primary path fails but at least one alternative path between
these (NSIS-enabled) endpoints of the multihomed path is available.
Additionally, 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 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 for 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. NAT Traversal Issue
NAT traversal for GIST over SCTP will follow Section 7.2 of [1] and
the GIST extensibility capabilities defined in [12]. This
specification does not define NAT traversal procedure for GIST over
SCTP, although an approach for SCTP NAT traversal is described in
[13].
7. Use of DTLS with GIST
This section specifies a new MA-Protocol-ID "DTLS" for the use of
DTLS in GIST, which denotes a basic use of datagram transport layer
channel security, initially in conjunction with GIST over SCTP. It
provides server (i.e., GIST transport receiver) authentication and
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integrity (as long as the NULL cipher suite is not selected during
cipher suite negotiation), as well as optionally replay protection
for control packets. The use of DTLS for securing GIST over SCTP
allows GIST to take the advantage of features provided by SCTP and
its extensions. The usage of DTLS for GIST over SCTP is similar to
TLS for GIST as specified in [1], where a stack-proposal containing
both MA-Protocol-IDs for SCTP and DTLS during the GIST handshake
phase.
The usage of DTLS [2] for securing GIST over datagram transport
protocols MUST be implemented and SHOULD be used.
GIST message associations using DTLS may carry messages with transfer
attributes requesting confidentiality or integrity protection. The
specific DTLS version will be negotiated within the DTLS layer
itself, but implementations MUST NOT negotiate to protocol versions
prior to DTLS v1.0 and MUST use the highest protocol version
supported by both peers. GIST nodes supporting DTLS MUST be able to
negotiate the DTLS NULL and block ciphers and SHOULD be able to
negotiate the new cipher suites. They MAY negotiate any mutually
acceptable ciphersuite that provides authentication, integrity, and
confidentiality. The same rules for negotiating TLS cipher suites as
specified in Section 5.7.3 of [1] apply.
DTLS renegotiation [7] may cause problems for applications such that
connection security parameters can change without the application
knowing it. Hence, it is RECOMMENDED that renegotiation be disabled
for GIST over DTLS.
No MA-protocol-options field is required for DTLS. The configuration
information for the transport protocol over which DTLS is running
(e.g. SCTP port number) is provided by the MA-protocol-options for
that protocol.
8. Security Considerations
The security considerations of [1], [3] and [2] apply. Additionally,
although [5] does not support replay detection in the DTLS over SCTP,
the SCTP replay protection mechanisms [3] [8] should be able to
protect NSIS messages transported using GIST over (DTLS over) SCTP
from replay attacks.
9. IANA Considerations
This specification requests the following codepoints (MA-Protocol-
IDs) be assigned in a registry created by [1]:
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+---------------------+------------------------------------------+
| MA-Protocol-ID | Protocol |
+---------------------+------------------------------------------+
| 3 | SCTP opened in the forwards direction |
| | |
| 4 | DTLS initiated in the forwards direction |
+---------------------+------------------------------------------+
Note that MA-Protocol-ID "DTLS" is never used alone but always
coupled with a transport protocol specified in the stack proposal.
10. Acknowledgments
The authors would like to thank John Loughney, Jukka Manner, Magnus
Westerlund, Sean Turner, Lars Eggert, Jeffrey Hutzelman, Robert
Hancock, Andrew McDonald, Martin Stiemerling, Fang-Chun Kuo, Jan
Demter, Lauri Liuhto, Michael Tuexen, and Roland Bless for their
helpful suggestions.
11. References
11.1. Normative References
[1] Schulzrinne, H. and M. Stiemerling, "GIST: General Internet
Signalling Transport", draft-ietf-nsis-ntlp-20 (work in
progress), June 2009.
[2] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[3] Stewart, R., "Stream Control Transmission Protocol", RFC 4960,
September 2007.
[4] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad,
"Stream Control Transmission Protocol (SCTP) Partial
Reliability Extension", RFC 3758, May 2004.
[5] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control Transmission
Protocol (SCTP)", draft-ietf-tsvwg-dtls-for-sctp-05 (work in
progress), March 2010.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[7] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, "Transport
Layer Security (TLS) Renegotiation Indication Extension",
RFC 5746, February 2010.
[8] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, August 2007.
11.2. Informative References
[9] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[10] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
June 2005.
[11] Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P. Lei,
"Sockets API Extensions for Stream Control Transmission
Protocol (SCTP)", draft-ietf-tsvwg-sctpsocket-22 (work in
progress), March 2010.
[12] Manner, J., Bless, R., Loughney, J., and E. Davies, "Using and
Extending the NSIS Protocol Family", draft-ietf-nsis-ext-07
(work in progress), April 2010.
[13] Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation",
draft-ietf-behave-sctpnat-02 (work in progress), December 2009.
Authors' Addresses
Xiaoming Fu
University of Goettingen
Institute of Computer Science
Goldschmidtstr. 7
Goettingen 37077
Germany
Email: fu@cs.uni-goettingen.de
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Christian Dickmann
University of Goettingen
Institute of Computer Science
Goldschmidtstr. 7
Goettingen 37077
Germany
Email: mail@christian-dickmann.de
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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