MMUSIC S. Loreto
Internet-Draft G. Camarillo
Intended status: Standards Track Ericsson
Expires: September 15, 2011 March 14, 2011
Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
Session Description Protocol (SDP)
draft-loreto-mmusic-sctp-sdp-07
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' and 'SCTP/
DTLS' protocol identifiers for SDP.
Status of this Memo
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described in the Simplified BSD License.
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Actpass/Passive . . . . . . . . . . . . . . . . . . . . . . 5
6.2. Existing Connection Reuse . . . . . . . . . . . . . . . . . 6
6.3. SDP description for DTLS Connection . . . . . . . . . . . . 6
7. Network Address Translators (NAT) Considerations . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
10. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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 a new protocol identifier, 'SCTP/
DTLS', to establish secure SCTP-based media streams over DTLS
(Datagram Transport Layer Security) [RFC4347], as specified in
[RFC6083], using SDP. 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 realible, 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
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> ...
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This document defines two new values for the 'proto' field: 'SCTP'
and 'SCTP/DTLS'.
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. 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].
An 'm' line that specifies 'SCTP' or 'SCTP/DTLS' MUST further qualify
the application-layer protocol using an fmt identifier.
An 'm' line that specifies 'SCTP/DTLS' 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. 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),
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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.
Some intermediaries performing firewall control use the addresses in
offer/answer exchanges to perform media authorization. That is, they
will not let media through unless it is sent to the address in the
'c' line.
The SCTP endpoints MAY choose to use the main address all the time
(e.g., they do not send retransmissions to a backup address) and to
send a re-INVITE every time they change that address.
However not using non-primary paths for retransmission means not to
utilize the multi-homing feature of SCTP for resilience. Therefore,
the SCTP endpoints MAY use the 'candidate' SDP attribute to
disclosure, to intermediaries performing firewall control, all its
alternative IP addresses; this will avoid the need for the SCTP
endpoints to send a re-INVITE every time they change the primary
path.
6. Examples
The following examples show the use of the 'setup' and 'connection'
SDP attributes. As discussed in Section 4, 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).
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
Figure 1
The endpoint at 192.0.2.1 responds with the following description:
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m=image 54321 SCTP *
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:new
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.
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 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:new
Figure 4
The existing SCTP association between 192.0.2.2 and 192.0.2.1 will be
reused.
6.3. SDP description for DTLS Connection
An offerer at 192.0.2.2 signals the availability of a T.38 fax
session over SCTP/DTLS.
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m=image 54111 SCTP/DTLS t38
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
Figure 5
7. Network Address Translators (NAT) Considerations
SCTP specific features (not present in UDP/TCP), as the checksum
(CRC32c) value calculated on the whole packet (not just the header)
or multihoming, present new challenges for a NAT.
[I-D.ietf-behave-sctpnat] describes an SCTP specific variant of NAT
which provides similar features of Network Address and Port
Translation (NAPT).
As an alternative to design SCTP specific NAT, SCTP-over-UDP
[I-D.tuexen-sctp-udp-encaps] makes possible, encapsulating SCTP
packets into UDP packets, to use SCTP in networks with legacy NAT and
firewalls not supporting SCTP.
OPEN ISSUE: do we want to include in SDP the ability to signal SCTP-
over-UDP ?
TBD: How to use Interactive Connectivity Establishment (ICE) to
establish SCTP streams.
OPENT ISSUE: Do we want ICE to only use SCTP over IP candidates, or
we ant ICE to use SCTP over UDP candidates as well?
8. 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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9. IANA Considerations
This document defines two new proto values: 'SCTP' and 'SCTP/DTLS'.
Their formats are defined in Section 3. These proto values 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 and SCTP/DTLS proto values 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 [RFC4288] [RFC4289] by production of, or reference to, a
standards-track RFC that defines the transport protocol for the
format.
10. 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.
[RFC3436] Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
Layer Security over Stream Control Transmission Protocol",
RFC 3436, December 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.
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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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083, January 2011.
[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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