Stream Control Transmission Protocol (SCTP)-Based Media Transport in the Session Description Protocol (SDP)

SDP (Session Description Protocol) provides a general-purpose format for describing multimedia sessions in announcements or invitations. TCP-Based Media Transport in the Session Description Protocol (SDP) specifies a general mechanism for describing and establishing TCP streams. Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in SDP extends RFC4145 for describing TCP-based media streams that are protected using TLS. The Stream Control Transmission Protocol (SCTP) is a transport protocol used to establish associations between two endpoints. This specification defines how to describe SCTP associations using the Session Description Protocol (SDP) , and defines the following new SDP Media Description protocol identifiers (proto values):'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'. The specification also describes how to use the new proto values together with the SDP Offer/Answer mechanism in order to negotiate and establish SCTP associations, and how to indicate the SCTP application usage. NOTE: TLS is designed to run on top of a byte-stream oriented transport protocol providing a reliable, in-sequence delivery like TCP. presents serious limitations with transporting TLS on top of SCTP. Therefore, defining a mechanism to negotiate media streams transported using TLS on top of SCTP, i.e. 'SCTP/TLS', is outside the scope of this specification.

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 and indicate requirement levels for compliant implementations.

SCTP Association: A protocol relationship between SCTP endpoints, composed of the two SCTP endpoints and protocol state information including Verification Tags and the currently active set of Transmission Sequence Numbers (TSNs), etc. An association can be uniquely identified by the transport addresses used by the endpoints in the association. SCTP 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. SCTP Transport address: A transport address is traditionally defined by a network-layer address, a transport-layer protocol, and a transport-layer port number. In the case of SCTP running over IP, a transport address is defined by the combination of an IP address and an SCTP port number (where SCTP is the transport protocol).

This section defines the following new SDP Media Description (m- line) protocol identifiers (proto values) for describing an SCTP association: 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'. The section also describes how an m- line, associated with the proto values, is created. The following is the format for an 'm' line, as specified in RFC4566 :

... ]]> The 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto values are similar to both the 'UDP' and 'TCP' proto values in that they only describe the transport-layer protocol and not the upper-layer protocol. NOTE: When the 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto values are used, the underlying transport protocol is respectively UDP and TCP; SCTP is carried on top of DTLS which is on top of those transport-layer protocols. The m- line fmt value, identifying the application-layer protocol, MUST be registered by IANA.

The new proto values are defined as below: The 'SCTP' proto value describes an SCTP association, as defined in . The 'SCTP/DTLS' proto value describes a Datagram Transport Layer Security (DTLS) connection on top of an SCTP association, as defined in . The 'UDP/DTLS/SCTP' proto value describes an SCTP association on top of a DTLS connection on top of UDP, as defined in . The 'TCP/DTLS/SCTP' proto value describes an SCTP association on top of a DTLS connection on top of TCP, as defined in .

defines that specifications defining new proto values 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 by production of, or reference to, a standards-track RFC that defines the transport protocol for the format. An m- line with a proto value of 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP' always describe a single SCTP association. In addition, such m- line MUST further indicate the application-layer protocol using an 'fmt' identifier. There MUST be exactly one 'fmt' value per m- line associated with the proto values defined in this specification. The "fmt" namespace associated with those proto values describes the generic application usage of the entire SCTP association, including the associated SCTP streams. NOTE: A mechanism on how to describe, and manage, individual SCTP streams within an SCTP association, is outside the scope of this specification.

This section defines the ABNF for the SDP media description when associated with any of the proto values defined in this document. This specification creates an IANA registry for 'association-usage' values.

This section defines a new SDP media-level attribute, 'sctp-port'. The attribute can be associated with an SDP media description (m- line) with a 'UDP/DTLS/SCTP' or a 'TCP/DTLS/SCTP' proto value. In that case the m- line port value indicates the port of the underlying transport layer protocol (UDP or TCP), and the 'sctp-port' value indicates the SCTP port. No default value is defined for the SDP sctp-port attribute. Therefore, if the attribute is not present, the associated m- line MUST be considered invalid. Usage of the SDP sctp-port attribute with other proto values is not specified, and MUST be discarded if received.

The ABNF for the SDP 'sctp-port' attribute is:

The mux category for the SDP sctp-port' attribute is SPECIAL. Usage of the attribute is only applicable when associated with 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto value m- lines. As the usage of multiple SCTP associations on top of a single DTLS connection is outside the scope of this specification, no mux rules are specified for the 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto values. Future extensions, that define how to negotiate multiplexing of multiple SCTP associations of top of a single DTLS connection, need to also define the mux rules for the attribute.

This section defines a new SDP media-level attribute, 'max-message-size'. The attribute can be associated with an m- line to indicate the maximum message size (indicated in bytes) that an SCTP endpoint is willing to receive on the SCTP association associated with the m- line. Different attribute values can be used in each direction. The remote peer MUST assume that larger messages will be rejected by the SCTP endpoint. SCTP endpoints need to decide on appropriate behavior in case a message that exceeds the maximum size needs to be sent. If the SDP 'max-message-size' attribute contains a maximum message size value of zero, it indicates the SCTP endpoint will handle messages of any size, subject to memory capacity etc. If the SDP 'max-message-size' attribute is not present, the default value is 64K. NOTE: This specification only defines the usage of the SDP 'max-message-size' attribute when associated with an m- line containing one of the following proto values: 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP'. Usage of the attribute with other proto values needs to be defined in a separate specification.

The ABNF for the SDP 'max-message-size' attribute is:

The mux category for the SDP 'max-message-size' attribute is SPECIAL. The mux rules depends on the proto value of the associated m- line. If the proto value is 'SCTP' or 'SCTP/DTLS' the rules are identical to the rules associated with the TRANSPORT mux category. As the usage of multiple SCTP associations on top of a single DTLS connection is outside the scope of this specification, no mux rules are specified for the 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto values.

The UDP/DTLS/SCTP transport is realized as described below: SCTP on top of DTLS is realized according to the procedures defined in ; and DTLS on top of UDP is realized according to the procedures in defined in . NOTE: While allows multiple SCTP associations on top of a single DTLS connection, the procedures in this specification only supports the negotiation of a single SCTP association on top of any given DTLS connection.

The TCP/DTLS/SCTP transport is realized as described below: SCTP on top of DTLS is realized according to the procedures defined in ; and DTLS on top of TCP is realized using the framing method defined in , with DTLS packets being sent instead of RTP/RTCP packets, and SDP signaling according to the procedures defined in this specification. NOTE: DTLS on top of TCP, without using the framing method defined in is outside the scope of this specification. A separate proto value would need to be registered for such transport realization.

The management of an SCTP association is identical to the management of a TCP connection. An SCTP endpoint MUST follow the rules in Section 6 of to manage SCTP associations. Whether to use the SCTP ordered or unordered delivery service is up to the applications using the SCTP association, and this specification does not define a mechanism to indicate the type of delivery service using SDP.

This specification does not define semantics for the SDP direction attributes . Unless semantics of these attributes for an SCTP association usage have been defined, SDP direction attributes MUST be discarded if present.

The SDP setup attribute is used to determine the 'active/passive' status of the endpoints, following the procedures for TCP in .

Both the 'active' and 'passive' endpoint MUST initiate the SCTP association, and MUST use the same SCTP port as client port and server port (in order to prevent two separate SCTP associations from being established). NOTE: The procedure above is different from TCP, where only the 'active' endpoint initiates the TCP connection . NOTE: If the underlying transport protocol is UDP or TCP (e.g. if the m- line proto value is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP'), when the SCTP association is established it is assumed that any NAT traversal procedures for the underlying transport protocol has successfully been performed. If the m- line proto value is 'TCP/DTLS/SCTP', the 'active' endpoint only MUST initiate the TCP connection, following the procedures in . Both endpoints MUST still initiate the SCTP association on top of the TCP connection.

If the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', the 'active/passive' status is used to determine the (D)TLS roles of the endpoints. Following the procedures in , the 'active' endpoint will take the (D)TLS client role. Once the DTLS connection has been established, the endpoints MUST NOT modify (as result of an offer/answer exchange) the TLS roles, or the 'active/passive' status, of the endpoints, unless the underlying transport protocol is also modified (e.g. if an IP address- or port value associated with the transport protocol is modified). If the underlying transport protocol is modified, the endpoints MUST establish a new DTLS connection. In such case the 'active/passive' status of the endpoints will again be determined following the procedures in , and the new status will be used to determine the (D)TLS roles of the endpoints associated with the new DTLS connection. NOTE: The procedure above is identical to the one defined for SRTP-DTLS in . NOTE: As described in [add-reference], if the Interactive Connectivity Establishment (ICE) mechanism is used, all ICE candidates associated with an SCTP association on top of a DTLS connection as part of the same DTLS connection. Thus, a switch from one candidate pair to another candidate pair will not trigger the establishment of a new DTLS connection.

The SDP connection attribute is used following the procedures in , with the additional SCTP specific considerations described in this section. If the m- line proto value is 'TCP/DTLS/SCTP', an SDP connection attribute associated with that m- line applies to both the SCTP association and the TCP connection. Therefore, an attribute 'new' value indicates that both a new SCTP association and new TCP connection have to be established, following the procedures in . NOTE: This specification does not define a mechanism which allows re-establishing of a new SCTP association, while maintaining the underlying TCP connection. The SDP connection attribute value does not automatically impact an existing DTLS connection. describes in which cases a new DTLS connections will have to be re-established.

This section defines the SDP Offer/Answer procedures for negotiating and establishing an SCTP association. Unless explicitly stated, the procedures apply to all m- line proto values ('SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP') defined in this specification. If the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', each endpoint MUST provide a certificate fingerprint, using the SDP 'fingerprint' attribute , if the endpoint supports, and is willing to use, a cipher suite with an associated certificate. The authentication certificates are interpreted and validated as defined in . Self-signed certificates can be used securely, provided that the integrity of the SDP description is assured as defined in . NOTE: The procedures apply to a specific m- line describing an SCTP association. If an offer or answer contains multiple m- lines describing SCTP associations, the procedures are applied separately to each m- line.

When the offerer creates an initial offer, the offerer: MUST, if the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', associate an SDP setup attribute, with an 'actpass' value, with the m- line (see ); MUST, if the m- line proto is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', associate an SDP 'sctp-port' attribute with the m- line (see ); MUST associate an SDP 'connection' attribute, with a 'new' value, with the m- line (see ); and MAY associate an SDP 'max-message-size' attribute with the m- line (see ).

When the answerer receives an offer, which contains an m- line describing an SCTP association, if the answerer accepts the m- line it: MUST insert a corresponding m- line in the answer, with an identical m- line proto value ; MUST, if the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', associate an SDP 'setup' attribute, with an 'active' or 'passive' value, with the m- line (see ); MUST, if the m- line proto is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', associate an SDP 'sctp-port' attribute with the m- line (see ); and MAY associate an SDP 'max-message-size' attribute with the m- line (see ). The attribute value in the answer is independent from the value (if present) in the corresponding m- line of the offer. Once the answerer has sent the answer, the answerer: MUST, if an SCTP association associated with the m- line has yet not been established, or if an existing SCTP association is to be re-established, initiate the establishing of the SCTP association; and MUST, if the answerer is the 'active' endpoint, and if an DTLS connection associated with the m- line is to be established (or re-established), initiate the establishing of the DTLS connection (by sending a ClientHello message). If the answerer does not accept the m- line in the offer, it MUST assign a zero port value to the corresponding m- line in the answer. In addition, the answerer MUST NOT establish an SCTP association, or a DTLS connection, associated with the m- line.

When the offerer receives an answer, which contains an m- line with a non-zero port value, describing an SCTP association, the offerer: MUST, if the offerer is the 'active' endpoint, if the m- line proto value is 'TCP/DTLS/SCTP', and if a TCP connection used to carry the SCTP association has not yet been established (or if an existing TCP connection is to be re-established), initiate the establishing of the TCP connection; MUST, if an SCTP association associated with the m- line has not yet been established (or if an existing SCTP association is to be re-established), initiate the establishing of the SCTP association; and MUST, if the offerer is the 'active' endpoint, and if a DTLS connection associated with the m- line is to be established (or if an existing DTLS connection is to be re-established), initiate the establishing of the DTLS connection (by sending a ClientHello message). NOTE: If the m- line proto value is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', the underlying DTLS connection needs to be established before the SCTP association can be established. If the m- line in the answer contains a zero port value, the offerer MUST NOT establish a TCP connection, an SCTP association, or a DTLS connection, associated with the m- line.

When an offerer sends an updated offer, in order to modify a previously established SCTP association, it follows the procedures in , with the following exceptions: Unless the offerer wants to re-establish an existing SCTP association, the offerer MUST associate an SDP connection attribute, with an 'existing' value, with the m- line; and If the offerer wants to disable a previously established SCTP association, it MUST assign a zero port value to the m- line associated with the SCTP association, following the procedures in .

SCTP supports multihoming. An SCTP endpoint is considered multihomed if it has more than one IP address on which SCTP can be used. An SCTP endpoint inform the remote peer about its IP addresses using the address parameters in the INIT/INIT-ACK chunk. Therefore, when SDP is used to describe an SCTP association, while the "c=" line contains the address which was used to negotiate the SCTP association, multihomed SCTP endpoints might end up using other IP addresses. If an endpoint removes the IP address that it offered in the SDP "c=" line associated with the SCTP association, it MUST send a new Offer, in which the "c=" line contains an IP address which is valid within the SCTP association. NOTE: In some network environments, intermediaries performing gate- and firewall control using the address information in the SDP "c=" and "m=" lines to authorize media, and will not pass media sent using other addresses. In such network environments, if an SCTP endpoints wants to change the address information on which media is sent and received, it needs to send an updated Offer, in which the SDP "c=" and "m=" lines contain the new address information. Multihoming is not supported when sending SCTP on top of DTLS, as DTLS does not expose address management of the underlying transport protocols (UDP or TCP) to its upper layer.

SCTP features not present in UDP or TCP, including the checksum (CRC32c) value calculated on the whole packet (rather than just the header), and multihoming, introduce new challenges for NAT traversal. defines an SCTP specific variant of NAT, which provides similar features of Network Address and Port Translation (NAPT). Current NATs typically do not support SCTP. defines a mechanism for sending SCTP on top of UDP, which makes it possible to use SCTP with NATs and firewalls that do not support SCTP.

At the time of writing this specification, no procedures have been defined for using ICE together with SCTP as transport layer protocol. Such procedures, including the associated SDP Offer/Answer procedures, are outside the scope of this specification, and might be defined in a future specification. When the transport layer protocol is UDP (in case of an SCTP association on top of a DTLS connection on top of UDP), if ICE is used, the ICE procedures defined in are used. When the transport layer protocol is TCP (in case of an SCTP association on top of a DTLS connection on top of TCP), if ICE is used, the ICE procedures defined in are used. Implementations MUST treat all ICE candidate pairs associated with a an SCTP association on top of a DTLS connection as part of the same DTLS connection. Thus, there will only be one DTLS handshake even if there are multiple valid candidate pairs, and shifting from one candidate pair to another will not impact the DTLS connection. If new candidates are added, they will also be part of the same DTLS connection.

defines general SDP security considerations, while , and define security considerations when using the SDP offer/answer mechanism to negotiate media streams. defines general SCTP security considerations, while defines security considerations when using DTLS on top of SCTP, and defines security considerations when using SCTP on top of DTLS. This specification does not introduce new security considerations in addition to those defined in the specifications listed above.

[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.] This document updates the "Session Description Protocol (SDP) Parameters" registry, following the procedures in , by adding the following values to the table in the SDP "proto" field registry: Type SDP Name Reference proto SCTP [RFCXXXX] proto SCTP/DTLS [RFCXXXX] proto UDP/DTLS/SCTP [RFCXXXX] proto TCP/DTLS/SCTP [RFCXXXX]

[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.] This document defines a new SDP media-level attribute,'sctp-port', as follows:

[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.] This document defines a new SDP media-level attribute,'max-message-size', as follows:

[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.] This specification creates a new IANA registry, following the procedures in , for the "fmt" namespace associated with the 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' protocol identifiers. Each "fmt" value describes the usage of an entire SCTP association, including all SCTP streams associated with the SCTP association. NOTE: Usage indication of individual SCTP streams is outside the scope of this specification. The "fmt" value, "association-usage", used with these "proto" is required. It is defined in []. As part of this registry, IANA maintains the following information: The identifier of the subprotocol, as will be used as the "fmt" value. A reference to the document in which the association-usage is defined. association-usage names are to be subject to the "First Come First Served" IANA registration policy [RFC5226]. IANA is asked to add initial values to the registry.

The authors wish to thank Harald Alvestrand, Randell Jesup, Paul Kyzivat, Michael Tuexen, Juergen Stoetzer-Bradler, Flemming Andreasen and Ari Keranen for their comments and useful feedback.

[RFC EDITOR NOTE: Please remove this section when publishing] Changes from draft-ietf-mmusic-sctp-sdp-15 Re-submission due to expiration of previous version. Reference updates. Changes from draft-ietf-mmusic-sctp-sdp-14 Changes based on WGLC comments from Magnus Westerlund. - ABNF clarification that token and port are defined in RFC4566. - Specify 40 as maximum digit character length for the SDP max-message-size value. - Editorial clarification. Changes based on discussions at IETF#92. - Specify that all ICE candidate pairs belong to the same DTLS connection. Changes from draft-ietf-mmusic-sctp-sdp-13 Changes based on comments from Paul Kyzivat. - Text preventing usage of well-known ports removed. - Editorial clarification. Changes from draft-ietf-mmusic-sctp-sdp-12 Mux category rules added for new SDP attributes. Reference to draft-ietf-mmusic-sdp-mux-attributes added. Changes based on comments from Roman Shpount: - Specify that fingerprint or setup roles must not be modified, unless underlying transport protocol is also modified. Changes based on comments from Ari Keranen: - Editorial corrections. Changes based on comments from Flemming Andreasen: - Clarify that, if UDP/DTLS/SCTP or TCP/DTLS/SCTP is used, the DTLS connection is established before the SCTP association. - Clarify that max-message-size value is given in bytes, and that different values can be used per direction. - Section on fmtp attribute removed. - Editorial corrections. Changes from draft-ietf-mmusic-sctp-sdp-11 Example added. Changes from draft-ietf-mmusic-sctp-sdp-10 SDP max-message-size attribute added to IANA considerations. Changes based on comments from Paul Kyzivat: - Text about max message size removed from fmtp attribute section. Changes from draft-ietf-mmusic-sctp-sdp-09 'DTLS/SCTP' split into 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' Procedures for realizing UDP/DTLS/SCTP- and TCP/DTLS/SCTP transports added. Changes from draft-ietf-mmusic-sctp-sdp-08 Default SCTP port removed: - Usage of SDP sctp-port attribute mandatory. SDP max-message-size attribute defined: - Attribute definition. - SDP Offer/Answer procedures. Text about SDP direction attributes added. Text about TLS role determination added.