Figure 15.1 Basic scenario for roaming users: (a) DHCP network acquisition and (b) SIP
registration
The terminal mobility scenario before starting a SIP session is shown in Fig-
ure 15.2. The roaming user belongs to a home network domain company.com
and has a registered IP address in the home network domain. While traveling
and roaming in a visited network, the mobile host will acquire an IP address
using DHCP as previously shown, and register this new address with the
home SIP registrar. The diagram shows what happens when a corresponding
host elsewhere on the Internet tries to communicate with the mobile host. The
initial INVITE(Message 1) will get a 302 response moved temporarilywith
the new IP address of the roaming user. In Message 4, the corresponding host
will send another INVITEto the new IP address.
DHCPDISCOVERF1 SIP REGISTER
REGISTER sip:reg.home.com SIP/2.0
Via: SIP/2.0/UDP venus.home.com:5060
Max-Forwards: 70
From: Alice <sip:[email protected]>;tag=4
To: Alice <sip:[email protected]>
Call-ID: [email protected]
CSeq: 1 REGISTER
Contact: sip:[email protected]
Content Length: 0F2 200 OK
SIP/2.0 200 OK
Via: SIP/2.0/UDP venus.home.com
From: Alice <sip:[email protected]>;tag=5
To: Alice <sip:[email protected]>;tag=3
Call-ID: [email protected]
CSeq: 1 REGISTER
Contact: sip:[email protected]
Content Length: 0
Ref: DHCP option for SIP (RFC 3361)DHCPOFFERDHCPREQUESTDHCPACKMS DHCPMSSIP Registrar
(home network)aF1 SIP REGISTERF2 200 OKbSIP Application Level Mobility 257