By default, when there is a message body, the Content-Type is "application/sdp".
domain has been renamed "ex.com" and the "client" subdomain for
"atlanta.ex.com", "biloxi.ex.com" and "chicago.ex.com" has been
replaced respectively by "u1", "u2" and "u3".
In the context of these illustrations, mnemonics are used for To/From's tag parameter and Call-ID values,
according to the following rules:
a tag is under the form: <x>-tag, where x is typically a letter identifying the UA
(a for Alice, b for Bob, c for Carol). When other tags for the same UA are required
in an example, a digit is added (e.g. a2, b2).
the Call-ID for a Session is under the form: <xy>-cid, where x is the
tag identifier of the session's initiator and y the tag identifier of the recipient: e.g. ab-cid
when Alice calls Bob.