2. Overview of operation

   There are two types of SIP messages; SIP requests and SIP responses.
   Clients send SIP requests to the host part of a URI and servers send
   responses to the host in the sent-by parameter of the Via header
   field.

   We define two parameters, one for SIP URIs and the other for the Via
   header field.  The format of both parameters is the same, as shown in
   the examples below:

   sip:alice@atlanta.com;comp=sigcomp
   Via: SIP/2.0/UDP server1.foo.com:5060;branch=z9hG4bK87a7;comp=sigcomp

   The presence of this parameter (comp=sigcomp) in a URI indicates that
   the request has to be compressed using SigComp, as defined in [2].
   The presence of comp=sigcomp in a Via header field indicates that the
   response has to be compressed using SigComp.

   Therefore, the presence of comp=sigcomp indicates that the SIP entity
   identified by the URI or by the Via header field supports SigComp and
   is willing to receive compressed messages.  Having comp=sigcomp mean
   "willingness" as well as "support" allows the receiver of a SIP
   message to influence the decision of whether or not to use SigComp at
   a given time.