Mapping of Media Streams to Resource Reservation Flows
Extracted elements (10)
The SRF mechanism is useful when the remote party needs to be involved in resource reservation. When a resource reservation protocol allows the SDP generator to unilaterally allocate resources in both directions (sendrecv), the generator can choose any mapping and the SRF mechanism is unnecessary.
Some resource reservation protocols allow the entity generating the SDP session description to allocate resources in both directions for the session, making the SRF grouping mechanism unnecessary in those scenarios. SRF is specifically needed when the remote party participates in reservation.
An SRF group MAY consist of a single media line, in which case that media line is mapped into one reservation flow that carries traffic only from that media line.
It is STRONGLY RECOMMENDED that integrity protection be applied to SDP session descriptions to prevent tampering with SRF group lines. For SIP-transported SDP, S/MIME is the natural choice for end-to-end integrity protection. Other applications MAY use a different form of integrity protection.
Media lines grouped using SRF semantics SHOULD be mapped into the same resource reservation flow.
Media lines that do not belong to a particular SRF group SHOULD NOT be mapped into the reservation flow used for that SRF group.
Single Reservation Flow (SRF) is a new 'semantics' attribute defined within the SDP grouping framework. It allows a group of media streams to be expressed as requiring mapping into a single resource reservation flow.
The SRF group line in SDP uses the syntax 'a=group:SRF <mid1> <mid2> ...', combining with per-media 'a=mid:' identifiers from the SDP grouping framework. An example groups audio (mid:1) and video (mid:2) into a single RSVP session.
IANA registered the 'SRF' token as a new semantics value for the 'group' SDP attribute in the SDP parameters registry under 'Semantics for the group SDP Attribute', referencing RFC 3524.
An attacker adding SRF group lines to an SDP session description could force a user agent to establish an incorrect number of resource reservation flows, consuming extra endpoint resources or degrading QoS for the session.