 are standard- ized in the future. During a session, a mobile device may also change IP address as it switches between one wireless network and another (the Mobile IP protocol is not assumed-it will be discussed in the next section). Basic SIP supports this sce- nario as well, since a re-INVITE in a dialog can be used to update the Con- tact URI and change media information in the SDP. This is shown in the call flow of Figure 9.3. Here, Bowditch detects a new wireless network, uses DHCP to acquire a new IP address, then performs a re-INVITE to make the signaling and media flow to the new IP address. If the user agent momentarily is able to Visited Bowditch registrar Laplace Media session DHCP request DHCP ACK REGISTER 200 OK INVITE 200 OK New media session Figure 9.3 Midcall mobility using a re-INVITE. SIP: Understanding the Session Initiation Protocol receive media from both networks, the interruption can be almost negligible. If this is not the case, a few RTP packets may be lost as the media catches up with the signaling, resulting in a slight interruption to the call. The re-INVITE would appear as follows: INVITE sip: laplace@client.mathematica.org SIP/2.0 Via: SIP/2.0/UDI 65.32.21.2:5060; branch=z9hG4bK34213 Max-Forwards: 70 To: Marquis de Laplace 0:laplace@mathematica.org>;tag=90210 From: Nathaniel Bowditch <sip:n.bowditch@salem.ma.us>; Call-ID: 413830e4leoi34ed4223123343ed2 CSeq: 5 INVITE Contact: <sip:nat@65.43.21.2> Content-Type: application/sdp Content-Length: 143 o=bowditch 2590844326 2590944533 IN IP4 65.32.21.2 s=Bearing C=IN IP4 65.32.21.2 t=0 0 m=audio 32852 RTP/AVP 0 a=rtpmap: PCMU/8000 which contains Bowditch's new IP address in the Via, Contact header fields and SDP media information. Note that both of the mobility scenarios in Figures 9.2 and 9.3 do not require cooperation between the two wireless networks. As such, this is a useful scenario in which a user agent can hand off a call between, for example, a com- mercial wireless network and a home or office 802.11 wireless network. For midcall m