MMUSIC S. Loreto
Internet-Draft G. Camarillo
Intended status: Standards Track Ericsson
Expires: May 7, 2009 November 3, 2008
Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
Session Description Protocol (SDP)
draft-loreto-mmusic-sctp-sdp-02
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Abstract
SCTP (Stream Control Transmission Protocol) is a transport protocol
used to establish associations between two endpoints. This document
describes how to express media transport over SCTP in SDP (Session
Description Protocol). This document defines the 'SCTP' protocol
identifier for SDP.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . . 3
4. The Setup and Connection Attributes and Association
Management . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Multihoming . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Actpass/Passive . . . . . . . . . . . . . . . . . . . . . . 5
6.2. Existing Connection Reuse . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
9. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
SDP (Session Description Protocol) [RFC4566] provides a general-
purpose format for describing multimedia sessions in announcements or
invitations. RFC4145 [RFC4145] specifies a general mechanism for
describing and establishing TCP streams. RFC 4572 [RFC4572] extends
RFC4145 [RFC4145] for describing TCP-based media streams that are
protected using TLS [RFC4346].
This document defines the a new protocol identifier, 'SCTP', to
describe SCTP-based [RFC4960] media streams. Additionally, this
document specifies the use of the 'setup' and 'connection' SDP
attributes to establish SCTP associations. These attributes were
defined in RFC4145 [RFC4145] for TCP. This document discusses their
use with SCTP.
OPEN ISSUE: we have to specify how to establish secure SCTP-based
media streams over Datagram Transport Layer Security (DTLS)
[I-D.ietf-tsvwg-dtls-for-sctp] using the Session Description Protocol
(SDP). We have also to define a new SDP protocol identifier, 'SCTP/
TLS'.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [RFC2119] and indicate requirement
levels for compliant implementations.
3. Protocol Identifier
The following is the format for an 'm' line, as specified in RFC4566
[RFC4566]:
m=<media> <port> <proto> <fmt> ...
This document defines a new value for the 'proto' field: 'SCTP'.
The 'SCTP' protocol identifier is similar to both the 'UDP' and 'TCP'
protocol identifiers in that it only describes the transport protocol
and not the upper-layer protocol. An 'm' line that specifies 'SCTP'
MUST further qualify the application-layer protocol using an fmt
identifier. Media described using an 'm' line containing the 'SCTP'
protocol identifier are carried using SCTP [RFC4960].
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4. The Setup and Connection Attributes and Association Management
The use of the 'setup' and 'connection' attributes in the context of
an SCTP association is identical to the use of these attributes in
the context of a TCP connection. That is, SCTP endpoints MUST follow
the rules in Sections 4 and 5 of RFC 4145 [RFC4145] when it comes to
the use of the 'setup' and 'connection' attributes in offer/answer
[RFC3264] exchanges.
The management of an SCTP association is identical to the management
of a TCP connection. That is, SCTP endpoints MUST follow the rules
in Section 6 of RFC 4145 [RFC4145] to manage SCTP associations.
Whether to use the SCTP ordered or unordered delivery service is up
to the applications using the SCTP association.
5. Multihoming
An SCTP endpoint, unlike a TCP endpoint, can be multihomed. An SCTP
endpoint is considered to be multihomed if it has more than one IP
address. A multihomed SCTP endpoint informs a remote SCTP endpoint
about all its IP addresses using the address parameters of the INIT
or the INIT-ACK chunk (depending on whether or not the multihomed
endpoint is the one initiating the establishment of the association).
Therefore, once the address provided in the 'c' line has been used to
establish the SCTP association (i.e., to send the INIT chunk),
address management is performed using SCTP. This means that two SCTP
endpoints can use addresses that were not listed in the 'c' line but
that were negotiated using SCTP mechanisms.
OPEN ISSUE: that intermediaries such as SBCs will not be aware of
some of the IP addresses used for media because they will not appear
in the SDP. We can RECOMMEND that SCTP endpoints use a main address
all the time (e.g., not to retransmit to a backup address) and that
they send a re-INVITE every time they change that address.
Alternatively (or additionally), we could add SDP attributes with all
the IP addresses that can be used by the association.
6. Examples
The following examples show the use of the 'setup' and 'connection'
SDP attributes. As discussed in Section 4, the use of this
attributes with an SCTP association is identical to their use with a
TCP connection. For the purpose of brevity, the main portion of the
session description is omitted in the examples, which only show 'm'
lines and their attributes (including 'c' lines).
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6.1. Actpass/Passive
An offerer at 192.0.2.2 signals its availability for an SCTP
association at SCTP port 54111. Additionally, this offerer is also
willing to initiate the SCTP association:
m=image 54111 SCTP *
c=IN IP4 192.0.2.2
a=setup:actpass
a=connection:new
The endpoint at 192.0.2.1 responds with the following description:
m=image 54321 SCTP *
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:new
This will cause the offerer (at 192.0.2.2) to initiate an SCTP
association to port 54321 at 192.0.2.1.
6.2. Existing Connection Reuse
Subsequent to the exchange in Section 6.1, another offer/answer
exchange is initiated in the opposite direction. The endpoint at
192.0.2.1, which now acts as the offerer, wishes to continue using
the existing association:
m=application 54321 SCTP *
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:new
Figure 1
The endpoint at 192.0.2.2 also wishes to use the existing SCTP
association and responds with the following description:
m=application 9 SCTP *
c=IN IP4 192.0.2.2
a=setup:active
a=connection:new
Figure 2
The existing SCTP association between 192.0.2.2 and 192.0.2.1 will be
reused.
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7. Security Considerations
See RFC 4566 [RFC4566] for security considerations on the use of SDP
in general. See RFC 3264 [RFC3264] and RFC 4145 [RFC4145] for
security considerations on establishing media streams using offer/
answer exchanges. See RFC 4960 [RFC4960] for security considerations
on SCTP in general. This specification does not introduce any new
security consideration in addition to the ones discussed in those
specifications.
8. IANA Considerations
This document defines a new proto value: SCTP. Its format is defined
in Section 3. This proto value should be registered by the IANA
under "Session Description Protocol (SDP) Parameters" under "proto".
The SDP specification, [RFC4566], states that specifications defining
new proto values, like the SCTP proto value defined in this RFC, must
define the rules by which their media format (fmt) namespace is
managed. For the SCTP protocol, new formats SHOULD have an
associated MIME registration. Use of an existing MIME subtype for
the format is encouraged. If no MIME subtype exists, it is
RECOMMENDED that a suitable one is registered through the IETF
process [RFC2048] by production of, or reference to, a standards-
track RFC that defines the transport protocol for the format.
9. Normative References
[RFC2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
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[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[I-D.ietf-tsvwg-dtls-for-sctp]
Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security for Stream Control Transmission
Protocol", draft-ietf-tsvwg-dtls-for-sctp-00 (work in
progress), October 2008.
Authors' Addresses
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Salvatore.Loreto@ericsson.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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