MMUSIC S. Loreto
Internet-Draft G. Camarillo
Intended status: Standards Track Ericsson
Expires: July 25, 2013 January 21, 2013
Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
Session Description Protocol (SDP)
draft-ietf-mmusic-sctp-sdp-03
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 25, 2013.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. datachannels and m-line . . . . . . . . . . . . . . . . . 5
4.2. Media Descriptions . . . . . . . . . . . . . . . . . . . . 5
4.3. Predefinition of Data Channels . . . . . . . . . . . . . . 6
5. Media attributes . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. sctpmap Attribute . . . . . . . . . . . . . . . . . . . . 7
5.2. stream Attribute . . . . . . . . . . . . . . . . . . . . . 7
5.3. label Attribute . . . . . . . . . . . . . . . . . . . . . 8
5.4. subprotocol Attribute . . . . . . . . . . . . . . . . . . 8
5.5. max_retr Attribute . . . . . . . . . . . . . . . . . . . . 8
5.6. max_time Attribute . . . . . . . . . . . . . . . . . . . . 8
5.7. unordered Attribute . . . . . . . . . . . . . . . . . . . 9
6. The Setup and Connection Attributes and Association
Management . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Multihoming . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Network Address Translation (NAT) Considerations . . . . . . . 10
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 10
9.2. Existing Connection Reuse . . . . . . . . . . . . . . . . 11
9.3. SDP description for DTLS Connection . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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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 (Transmission Control Protocol)
streams. RFC 4572 [RFC4572] extends RFC4145 [RFC4145] for describing
TCP-based media streams that are protected using TLS (Transport Layer
Security) [RFC5246].
This document defines 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.
Additionally this document defines two new protocol identifiers:
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].
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
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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].
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].
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
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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. datachannels and m-line
SCTP defines a stream as an unidirectional logical channel existing
within an SCTP association one to another SCTP endpoint. The streams
are used to provide the notion of in-sequence delivery. Each user
message is sent on a particular stream, either order or unordered.
Ordering is preserved only for all ordered messages sent on the same
stream.
A Data Channel may be:
unidirectional: formed by one single incoming or outgoing SCTP
stream
bidirectional: a pair of one incoming stream and one outgoing SCTP
stream
Using the format part of the m line for negotiating datachannel
allows the intermediaries nodes to become aware of the kind of
traffic actually exchanged on the datachannels, as well as the
initial set of datachannels established within the association.
This document allows only one media format on top of an association.
That means that all the datachannels must use the same media format.
[NOTE] The exact definition of datachannel depends on the actual
media type using it.
4.2. 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.
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m=application 54111 SCTP/DTLS 54111
a=sctpmap:54111 t38 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.
When a list of port number identifiers is given, this implies that
all of these associations MUST run on top of the same DTLS
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
4.3. Predefinition of Data Channels
An 'm' line that specifies 'SCTP' or 'SCTP/DTLS' or 'DTLS/SCTP' MAY
further provide the definition of channels running within the
association. When provided the channel definition MUST include the
"a=stream:" attribute (see Section 5.2) indicating the stream number
used to form the channel, the "a=label:" attribute (see Section 5.3)
indicating the name of the channel and the "a=subprotocol:" attribute
(see Section 5.4) indicating which protocol the client would like to
speak on the channel.
By default a channel is defined as 'reliable', however it is possible
specify a channel as Partial Reliable indicating that the messages
will not be retransmitted more times than specified in the
"a=max_retr:" attribute (see Section 5.5) or indicating messages
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might not be transmitted or retransmitted after a specified life-time
given in milli-seconds in the "a=max_time:" attribute (see
Section 5.6).
By default a channel is defined as 'ordered' (i.e. within a stream,
an endpoint MUST deliver DATA chunks received to the upper layer
according to the order of their Stream Sequence Number), however it
is possible specify a channel as Unordered using the "a=unordered"
attribute (see Section 5.7).
m=application 54111 DTLS/SCTP 5000 5001 5002
c=IN IP4 79.97.215.79
a=sctpmap:5000 webrtc-DataChannel 2
a=sctpmap:5001 bfcp 1
a=sctpmap:5002 t38 1
a=webrtc-DataChannel:5000 stream=1;label="channel 1";subprotocol="chat";
a=webrtc-DataChannel:5000 stream=2;label="channel 2";subprotocol="file transfer";max_retr=3
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 streams to be supported by 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 protocol [streams]
sctpmap-number = 1*DIGIT
protocol = labelstring
labelstring = text
text = byte-string
streams = 1*DIGIT
5.2. stream Attribute
The 'stream' attribute indicates the actual stream number within the
association used to form the channel.
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stream-attr = "a=stream=" streamnumber
streamnumber = 1*DIGIT
5.3. label Attribute
The 'label' attribute indicates the name of the channel. It
represents a label that can be used to distinguish, in the context of
the WebRTC API, an RTCDataChannel object from other RTCDataChannel
objects.
label-attr = "a=label=" labelstring
labelstring = text
text = byte-string
5.4. subprotocol Attribute
The 'subprotocol' attribute indicates which protocol the client would
like to speak on the channel.
subprotocol-attr = "a=subprotocol=" labelstring
labelstring = text
text = byte-string
5.5. max_retr Attribute
The 'max_retr' attribute indicates the max times an user message will
be retransmitted.
maxretr-attr = "a=maxretr=" maxretrvalue
maxretrvalue = 1*DIGIT
5.6. max_time Attribute
An user messages might not be transmitted or retransmitted after a
specified life-time given in milli-seconds in the 'max_time'
attribute.
maxtime-attr = "a=maxtime=" maxtimevalue
maxtimevalue = 1*DIGIT
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5.7. unordered Attribute
The 'unordered' attribute indicates that DATA chunks in the channel
MUST be dispatched to the upper layer by the receiver without any
attempt to reorder.
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
[I-D.tuexen-sctp-udp-encaps] 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=image 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 9 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 DTLS Connection
An offerer at 192.0.2.2 signals the availability of a T.38 fax
session over SCTP/DTLS.
m=image 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
a=webrtc-DataChannel:5000 stream=1;label="channel 1";subprotocol="chat";
a=webrtc-DataChannel:5000 stream=2;label="channel 2";subprotocol="file transfer"
Figure 5
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.
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 two SDP session and media-level attributes:
'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)".
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12. References
12.1. 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", BCP 13, 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.
[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.
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12.2. 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.
[I-D.ietf-behave-sctpnat]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation",
draft-ietf-behave-sctpnat-04 (work in progress),
December 2010.
[I-D.tuexen-sctp-udp-encaps]
Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP
Packets", draft-tuexen-sctp-udp-encaps-06 (work in
progress), January 2011.
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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