This document discusses a number of considerations related to the
topologies of Real-Time Transport Protocol (RTP) sessions initated
using the Session Description (SDP) unicast Offer/Answer Model,
especially as applied to source-multiplexed sessions.
The primary observation is that certain topologies cannot be created
by unicast SDP Offer/Answer. Notably, the it is not possible to
negotiate the topology that RFC 5117 calls Topo-Transport-Translator
As a consequence of this limitation, certain topological assumptions
can safely be made for RTP sessions initiated using unicast SDP
Offer/Answer; and therefore, certain optimizations to RTP are
possible in such sessions. This document also describes the
optimizations that these assumptions make possible.