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Understanding the SIP Protocol
November 2009 253
could proxy the INVITE to Caller B's voicemail server. A proxy
server can also send an INVITE to a number of locations at the same
time. This type of parallel search is known as forking (see below).
In this case, the 200 (OK) is routed back through the two proxies and
is received by Caller A's softphone, which then stops the ringback
tone and indicates that the call has been answered. Finally,
Caller A's softphone sends an acknowledgement message, ACK, to
Caller B's SIP phone to confirm the reception of the final response
(200 (OK)). In this example, the ACK is sent directly from Caller A's
softphone to Caller B's SIP phone, bypassing the two proxies. This
occurs because the endpoints have learned each other's address from
the Contact header fields through the INVITE/200 (OK) exchange,
which was not known when the initial INVITE was sent. The
lookups performed by the two proxies are no longer needed, so the
proxies drop out of the call flow. This completes the
INVITE/200/ACK three-way handshake used to establish SIP
sessions.
Caller A and Caller B's media session has now begun, and they send
media packets using the format to which they agreed in the
exchange of SDP. In general, the end-to-end media packets take a
different path from the SIP signaling messages.
During the session, either Caller A or Caller B may decide to change
the characteristics of the media session. This is accomplished by
sending a re-INVITE containing a new media description. This
re-INVITE references the existing dialog so that the other party
knows that it is to modify an existing session instead of establishing
a new session. The other party sends a 200 (OK) to accept the
change. The requestor responds to the 200 (OK) with an ACK. If the
other party does not accept the change, he sends an error response
such as 488 (Not Acceptable Here), which also receives an ACK.
However, the failure of the re-INVITE does not cause the existing
call to fail - the session continues using the previously negotiated
characteristics.
At the end of the call, Caller B disconnects (hangs up) first and
generates a BYE message. This BYE is routed directly to Caller A's
softphone, again bypassing the proxies. Caller A confirms receipt of
the BYE with a 200 (OK) response, which terminates the session and
the BYE transaction. No ACK is sent - an ACK is only sent in
response to an INVITE request. The reasons for this special
handling for INVITE relate to the reliability mechanisms in SIP, the
length of time it can take for a ringing phone to be answered, and