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EXPERIMENTAL
Internet Engineering Task Force (IETF) X. Fu
Request for Comments: 6084 C. Dickmann
Category: Experimental University of Goettingen
ISSN: 2070-1721 J. Crowcroft
University of Cambridge
January 2011
General Internet Signaling Transport (GIST)
over Stream Control Transmission Protocol (SCTP)
and Datagram Transport Layer Security (DTLS)
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).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6084.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Fu, et al. Experimental [Page 1]
RFC 6084 GIST over SCTP and DTLS January 2011
to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Abbreviations . . . . . . . . . . . . . . . . 4
3. GIST over SCTP . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Message Association Setup . . . . . . . . . . . . . . . . 5
3.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Protocol-Definition: Forwards-SCTP . . . . . . . . . . 5
3.2. Effect on GIST State Maintenance . . . . . . . . . . . . . 6
3.3. PR-SCTP Support . . . . . . . . . . . . . . . . . . . . . 6
3.4. API between GIST and NSLP . . . . . . . . . . . . . . . . 7
4. Bit-Level Formats . . . . . . . . . . . . . . . . . . . . . . 7
4.1. MA-Protocol-Options . . . . . . . . . . . . . . . . . . . 7
5. Application of GIST over SCTP . . . . . . . . . . . . . . . . 8
5.1. Multihoming Support of SCTP . . . . . . . . . . . . . . . 8
5.2. Streaming Support in SCTP . . . . . . . . . . . . . . . . 8
6. NAT Traversal Issue . . . . . . . . . . . . . . . . . . . . . 8
7. Use of DTLS with GIST . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 11
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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].
GIST, in its initial specification for Connection mode (C-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 the framing
mechanisms must be provided separately. In addition, its Partial
Reliability extension (PR-SCTP) [3] supports partial retransmission
based on a programmable retransmission timer. Furthermore, DTLS
provides a viable solution for securing SCTP [4], which allows SCTP
to use almost all of 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 multihoming support of SCTP.
GIST over SCTP as described in this document does 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
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transport when appropriate. In addition, SCTP implementations to
transport GIST MUST support the optional feature of fragmentation of
SCTP user messages.
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 [5]. Other
terminologies and abbreviations used in this document are taken from
related specifications ([1], [2], [3], [6]):
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
signaling 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 bidirectional: 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.
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3. GIST over SCTP
This section defines a new MA-Protocol-ID type, "Forwards-SCTP", for
using SCTP as the GIST transport protocol. The use of DTLS in GIST
is defined in Section 7.
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 information is a main part of the Stack Configuration Data (SCD)
[1]. This section adds Forwards-SCTP (value 3) 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 in 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 multihoming 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 the possible change of the primary path.
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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 unidirectional streams, for the purpose of this
document, the concept of a bidirectional stream is used.
Implementations MUST establish both downstream and upstream
(unidirectional) SCTP streams and use the same stream identifier in
both directions. Thus, the two unidirectional streams (in opposite
directions) form a bidirectional 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 [3] 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
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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
The 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 interpretations
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 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 multihoming
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.
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5. Application of GIST over SCTP
5.1. Multihoming Support of SCTP
In general, the multihoming 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 failing 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 multihomed 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 multihoming 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
multihoming 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 the head-of-line
blocking issue in TCP. Since a single GIST MA may be reused by
multiple sessions, using TCP as the 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 procedures for GIST over
SCTP, although an approach for SCTP NAT traversal is described in
[13].
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RFC 6084 GIST over SCTP and DTLS January 2011
7. Use of DTLS with GIST
This section specifies a new MA-Protocol-ID "DTLS" (value 4) 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
integrity (as long as the NULL ciphersuite is not selected during
ciphersuite 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. NULL authentication and integrity ciphers
MUST NOT be negotiated for GIST nodes supporting DTLS. For
confidentiality ciphers, nodes can negotiate the NULL ciphersuites.
The same rules for negotiating TLS ciphersuites 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], [6], and [2] apply.
Additionally, although [4] does not support replay detection in DTLS
over SCTP, the SCTP replay protection mechanisms [6] [8] should be
able to protect NSIS messages transported using GIST over (DTLS over)
SCTP from replay attacks.
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RFC 6084 GIST over SCTP and DTLS January 2011
9. IANA Considerations
According to this specification, IANA has registered the following
codepoints (MA-Protocol-IDs) in a registry created by [1]:
+---------------------+------------------------------------------+
| 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 R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, October 2010.
[2] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[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] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control Transmission
Protocol (SCTP)", RFC 6083, January 2011.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Stewart, R., "Stream Control Transmission Protocol", RFC 4960,
September 2007.
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RFC 6084 GIST over SCTP and DTLS January 2011
[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)", Work in Progress, January 2011.
[12] Manner, J., Bless, R., Loughney, J., and E. Davies, "Using and
Extending the NSIS Protocol Family", RFC 5978, October 2010.
[13] Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation", Work
in Progress, December 2010.
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Authors' Addresses
Xiaoming Fu
University of Goettingen
Institute of Computer Science
Goldschmidtstr. 7
Goettingen 37077
Germany
EMail: fu@cs.uni-goettingen.de
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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