RFC 3486:Compressing the Session Initiation Protoc...
RFC-Ref

Layer


Click on the red underlined text to get to the source

... Since it is foreseen that the number of transport protocols supported by a particular application layer protocol is not going to grow dramatically, having a DNS entry per transport ...
... However, sometimes it is necessary to include new layers between the transport protocol and the application layer protocol. Examples of these layers are transport layer security and compression ...
... transport protocol and the application layer protocol. Examples of these layers are transport layer security and compression. If DNS ...
... It is clear that this way of using DNS is not scalable. Therefore, an application layer mechanism to express support of signalling compression is needed. ...


... SIPS URI, the request SHOULD be compressed before it is passed to the TLS layer. A client ...



Google
Web
RFC-Ref