MMUSIC S. Loreto
Internet-Draft G. Camarillo
Intended status: Standards Track Ericsson
Expires: April 24, 2014 October 21, 2013
Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
Session Description Protocol (SDP)
draft-ietf-mmusic-sctp-sdp-05
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', 'SCTP/DTLS'
and 'DTLS/SCTP' protocol identifiers for SDP.
Status of this Memo
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This Internet-Draft will expire on April 24, 2014.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 4
4. Media Formats . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Media Descriptions . . . . . . . . . . . . . . . . . . . . 5
5. Media attributes . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. sctpmap Attribute . . . . . . . . . . . . . . . . . . . . 6
6. The Setup and Connection Attributes and Association
Management . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Multihoming . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Network Address Translation (NAT) Considerations . . . . . . . 8
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 8
9.2. Existing Connection Reuse . . . . . . . . . . . . . . . . 9
9.3. SDP description for SCTP over DTLS Connection . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
13.1. Normative References . . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
SDP (Session Description Protocol) [RFC4566] provides a general-
purpose format for describing multimedia sessions in announcements or
invitations. TCP-Based Media Transport in the Session Description
Protocol (SDP) [RFC4145] specifies a general mechanism for describing
and establishing TCP (Transmission Control Protocol) streams.
Connection-Oriented Media Transport over the Transport Layer Security
(TLS) Protocol in the Session Description Protocol (SDP) [RFC4572]
extends RFC4145 [RFC4145] for describing TCP-based media streams that
are protected using TLS (Transport Layer Security) [RFC5246].
This document defines three new protocol identifiers:
SCTP : to describe SCTP-based [RFC4960] media streams.
SCTP/DTLS : to allow the usage of the Datagram Transport Layer
Security (DTLS) [RFC4347] protocol over SCTP, as specified in
[RFC6083], using SDP. DTLS over SCTP provides communications
privacy for applications that use SCTP as their transport
protocol.
DTLS/SCTP : to allow the usage of SCTP on top of the Datagram
Transport Layer Security (DTLS) protocol, as defined in
[I-D.tuexen-tsvwg-sctp-dtls-encaps], using SDP. SCTP over DTLS is
used by the RTCWeb protocol suite for transporting non-media data
between browsers.
The authentication certificates are interpreted and validated as
defined in RFC4572 [RFC4572]. Self-signed certificates can be used
securely, provided that the integrity of the SDP description is
assured as defined in RFC4572 [RFC4572].
TLS is designed to run on top of a byte-stream oriented transport
protocol providing a reliable, in-sequence delivery like TCP. Since
no-one so far has implemented SCTP over TLS, due to some serious
limitations described in [RFC6083], this document does not make use
of TLS over SCTP as described in RFC3436 [RFC3436].
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.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
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"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 three new values for the 'proto' field: 'SCTP',
'SCTP/DTLS' and 'DTLS/SCTP'.
The 'SCTP', 'SCTP/DTLS' and 'DTLS/SCTP' protocol identifiers are
similar to both the 'UDP' and 'TCP' protocol identifiers in that they
only describe the transport protocol and not the upper-layer
protocol.
Media described using an 'm' line containing the 'SCTP' protocol
identifier are carried using SCTP [RFC4960].
The 'SCTP/DTLS' protocol identifier indicates that the media
described will use the Datagram Transport Layer Security (DTLS)
[RFC4347] over SCTP as specified in [RFC6083].
The 'DTLS/SCTP' protocol identifier indicates that the media
described will use SCTP on top of the Datagram Transport Layer
Security (DTLS) protocol as specified in
[I-D.tuexen-tsvwg-sctp-dtls-encaps]. The actual layer below DTLS can
be plain UDP or what ICE agrees on (in the case ICE is used to
negotiate the actual transport flow). The lower layer used is
identified from the elements present inside the m= line block.
An 'm' line that specifies 'SCTP' or 'SCTP/DTLS' or 'DTLS/SCTP' MUST
further qualify the application-layer protocol using an fmt
identifier.
An 'm' line that specifies 'SCTP/DTLS' or 'DTLS/SCTP' MUST further
provide a certificate fingerprint. An SDP attribute (an 'a' line) is
used to transport and exchange end point certificate. The
authentication certificates are interpreted and validated as defined
in [RFC4572].
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4. Media Formats
The SDP specification, [RFC4566], states that specifications defining
new proto values, like the SCTP, SCTP/DTLS and DTLS/SCTP proto values
defined in this RFC, must define the rules by which their media
format (fmt) namespace is managed. 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 [RFC4288] [RFC4289] by production of, or reference to, a
standards-track RFC that defines the transport protocol for the
format.
4.1. Media Descriptions
The media description change slightly depending on the actual
<proto>.
If the <proto> sub-field is 'SCTP' or 'SCTP/DTLS'
the <port> is the SCTP transport port and follows the same active/
passive offer/answer model described in Section 4.1 of [RFC4145];
the <fmt> sub-field carries the same port number value specified
in the <port> and the mandatory "a=sctpmap:" attribute contains
the actual media format within the protocol parameter.
m=application 54111 SCTP/DTLS 54111
a=sctpmap:54111 webrtc-datachannel 1
Running SCTP over DTLS make possible to have multiple SCTP
associations on top of the same DTLS connection; each SCTP
association make use of a distinct port number that is mainly used to
demultiplex the associations.
If the <proto> sub-field is 'DTLS/SCTP'
the <port> is the UDP transport port;
the <fmt> sub-field carries the SCTP port number and the mandatory
"a=sctpmap:" attribute contains the actual media format within the
protocol parameter. The SCTP port number is the UA chosen port to
use on the DTLS channel.
When a list of SCTP port number identifiers is given, this implies
that all of these associations MUST run on top of the same DTLS
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connection. For the payload type assignments the "a=sctpmap:"
attribute (see Section 5.1) SHOULD be used to map from a port number
to a media encoding name that identifies the payload format
transported by the association or the actual application protocol
running on top of it.
m=application 54111 DTLS/SCTP 5000 5001 5002
c=IN IP4 79.97.215.79
a=sctpmap:5000 webrtc-datachannel 16
a=sctpmap:5001 bfcp 2
a=sctpmap:5002 t38 1
5. Media attributes
5.1. sctpmap Attribute
The sctpmap attribute maps from a port number (as used in an "m="
line) to an encoding name denoting the payload format to be used on
top of the SCTP association or the actual protocol running on top of
it. It also can provide the number of incoming streams to be
supported by each side of the association. If this attribute is not
present, the implementation should provide a default, with a
suggested value of 16.
sctpmap-attr = "a=sctpmap:" sctpmap-number media-subtypes [streams]
sctpmap-number = 1*DIGIT
protocol = labelstring
labelstring = text
text = byte-string
streams = 1*DIGIT
For each "a=sctpmap:" attribute line in the offer, there MUST be a
corresponding "a=sctpmap:" attribute line in the answer. The answer
MUST contain exactly the same number of "a=sctpmap:" attribute lines
as the offer. This allows for SCTP associations to be matched up
based on their order.
Any offered association MAY be rejected in the answer, for any
reason. If an association offer is rejected, the offerer and
answerer MUST NOT establish an SCTP association for it. To reject an
SCTP association, the SCTP port number in the corresponding
"a=sctpmap:" attribute line in the answer MUST be set to zero.
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Any offered association with an "a=sctpmap:" attribute line providing
an incoming stream number of zero or larger than 65535 MUST be
rejected in the answer. An offered association answered with an
"a=sctpmap:" attribute line providing an incoming stream number of
zero or larger than 65535 MUST NOT be established.
6. 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.
7. 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 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.
During the lifetime of an SCTP association, the endpoints can add and
remove new addresses from the association at any point [RFC5061]. If
an endpoint removes the IP address listed in its 'c' line from the
SCTP association, the endpoint SHOULD update the 'c' line (e.g., by
sending a re-INVITE with a new offer) so that it contains an IP
address that is valid within the SCTP association.
In some environments, intermediaries performing firewall control use
the addresses in offer/answer exchanges to perform media
authorization. That is, policy-enforcement network elements do not
let media through unless it is sent to the address in the 'c' line.
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In such network environments, the SCTP endpoints can only exchange
media using the IP addresses listed in their 'c' lines. In these
environments, an endpoint wishing to use a different address needs to
update its 'c' line (e.g., by sending a re-INVITE with a new offer)
so that it contains the new IP address.
It is worth to underline that when using SCTP on top of DTLS, only
single homed SCTP associations can be used, since DTLS does not
expose any address management to its upper layer.
8. Network Address Translation (NAT) Considerations
SCTP specific features (not present in UDP/TCP), such as the checksum
(CRC32c) value calculated on the whole packet (not just the header)
or its multihoming capabilities, present new challenges for NAT
traversal. [I-D.ietf-behave-sctpnat] describes an SCTP specific
variant of NAT, which provides similar features of Network Address
and Port Translation (NAPT).
Current NATs do not typically support SCTP. As an alternative to
design SCTP specific NATs, Encapsulating SCTP into UDP [RFC6951]
makes it possible to use SCTP in networks with legacy NAT and
firewalls not supporting SCTP.
At the time of writing, the work on NAT traversal for SCTP is still
work in progress. Additionally, no extension has been defined to
integrate ICE (Interactive Connectivity Establishment) [RFC5768] with
SCTP and its multihoming capabilities either. Therefore, this
specification does not define how to describe SCTP-over-UDP streams
in SDP or how to establish and maintain SCTP associations using ICE.
Should these features be specified for SCTP in the future, there will
be a need to specify how to use them in an SDP environment as well.
9. Examples
The following examples show the use of the 'setup' and 'connection'
SDP attributes. As discussed in Section 6, the use of these
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).
9.1. 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
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willing to initiate the SCTP association:
m=application 54111 SCTP 54111
c=IN IP4 192.0.2.2
a=setup:actpass
a=connection:new
a=sctpmap:54111 t38 1
Figure 1
The endpoint at 192.0.2.1 responds with the following description:
m=application 54321 SCTP 54321
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:new
a=sctpmap:t54321 t38 1
Figure 2
This will cause the offerer (at 192.0.2.2) to initiate an SCTP
association to port 54321 at 192.0.2.1.
9.2. Existing Connection Reuse
Subsequent to the exchange in Section 9.1, 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:existing
Figure 3
The endpoint at 192.0.2.2 also wishes to use the existing SCTP
association and responds with the following description:
m=application 54111 SCTP *
c=IN IP4 192.0.2.2
a=setup:active
a=connection:existing
Figure 4
The existing SCTP association between 192.0.2.2 and 192.0.2.1 will be
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reused.
9.3. SDP description for SCTP over DTLS Connection
This example shows the usage of SCTP over DTLS.
An offerer at 192.0.2.2 signals the availability of a webrtc-
DataChannel session over SCTP/DTLS. The DTLS connection runs on top
of port 54111.
m=application 54111 DTLS/SCTP 5000
c=IN IP4 192.0.2.2
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=sctpmap:5000 webrtc-DataChannel 16
Figure 5
The endpoint at 192.0.2.1 responds with the following description:
m=application 62442 DTLS/SCTP 5001
c=IN IP4 192.0.2.1
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=sctpmap:5001 webrtc-DataChannel 16
Figure 6
10. Security Considerations
See RFC 4566 [RFC4566] for security considerations on the use of SDP
in general. See RFC 3264 [RFC3264], RFC 4145 [RFC4145] and RFC 4572
[RFC4572] for security considerations on establishing media streams
using offer/answer exchanges. See RFC 4960 [RFC4960] for security
considerations on SCTP in general and [RFC6083] for security
consideration using DTLS on top of SCTP. This specification does not
introduce any new security consideration in addition to the ones
discussed in those specifications.
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11. IANA Considerations
This document defines three new proto values: 'SCTP', 'SCTP/DTLS' and
'DTLS/SCTP'. Their formats are defined in Section 3. These proto
values should be registered by the IANA under "Session Description
Protocol (SDP) Parameters" under "proto".
This document defines a new SDP session and media-level attribute:
'sctpmap'. Its format is defined in Section 5.1. This attribute
should be registered by IANA under "Session Description Protocol
(SDP) Parameters" under "att-field" (both session and media
level)".
12. Acknowledgments
The authors wish to thank Harald Alvestrand, Randell Jesup, Paul
Kyzivat, Michael Tuexen for their comments and useful feedback.
13. References
13.1. Normative References
[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.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", RFC 4288, December 2005.
[RFC4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 4289, December 2005.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
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[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.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
September 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[I-D.tuexen-tsvwg-sctp-dtls-encaps]
Jesup, R., Loreto, S., Stewart, R., and M. Tuexen, "DTLS
Encapsulation of SCTP Packets for RTCWEB",
draft-tuexen-tsvwg-sctp-dtls-encaps-01 (work in progress),
July 2012.
13.2. Informative References
[RFC3436] Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
Layer Security over Stream Control Transmission Protocol",
RFC 3436, December 2002.
[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083, January 2011.
[RFC5768] Rosenberg, J., "Indicating Support for Interactive
Connectivity Establishment (ICE) in the Session Initiation
Protocol (SIP)", RFC 5768, April 2010.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951, May 2013.
[I-D.ietf-behave-sctpnat]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation",
draft-ietf-behave-sctpnat-08 (work in progress),
February 2013.
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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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