This Recommendation contains functionality to describe the use of the RTP Control Protocol
(RTCP) in H.248 controlled media gateways. RTCP is used for instance to monitor the quality of
service and to convey information about the participants in an on-going RTP session.
H.248.1 assumes that when a RTP stream is specified in a Local or Remote descriptor that a RTCP
flow may be established according to the rules defined in [IETF RFC 3550] clause 11. Additional
H.248/RTCP behaviour is defined in H.248.1 clause E.12.5. However some media gateways may
not instantiate an RTCP flow. For interoperability and for some applications such as firewall and
Network Address Translation (NAT) it is important for the MGC to be certain of the media gateway
behaviour with respect to port allocation for RTCP. The “RTCP Handling Package” defined in this
Recommendation provides a property to control this RTCP flow allocation.
1.1 Overview
RTCP handling may comprise the following functions:
1. Indication of the existence of an RTCP flow of an RTP session (RTP with or without RTCP).
2. Indication of endpoint identifiers on RTP level that are relevant for both RTP and RTCP (e.g.
SSRC).
In addition, if an RTCP flow exists:
3. Resource management of endpoint identifiers on layer 4 (UDP ports; i.e. reservation, allocation
and indication of a specific allocation rule) for RTCP flows.
4. Resource management of endpoint identifiers on RTP level (e.g. CNAME) for RTCP flows.
5. Resource management of other resources required for RTCP flows (e.g. reservation and
allocation of transport capacity).
6. Indication of specific modes of operation for RTCP (e.g. layer 4 multiplexing of RTP and RTCP
flows, or multiplexing mode of RTCP reports in a single RTCP packet).
Orthogonal to above functions:
7. Mapping scheme of RTP and RTCP flows on H.248 Streams.
This Recommendation deals with functions (1) and (3) only. Other functions are out of the scope of
this Recommendation.
In addition, the Recommendation assumes that regarding (7), RTP and RTCP flows are always
mapped into a single H.248 Stream. Other mapping schemes are out of the scope of this
Recommendation and may make this Recommendation’s procedures irrelevant.