case due to the existence of ruser and the SIP server on the same local network. Besides, note here that the server will issue one handle resolution and one update for the first REGISTER request only. Additionally, the first handle resolution is always cached internally on the server. Unless ruser moves to another domain or un- registers, no handle resolution/update is required. This means that subsequent REGISTER requests will read the cached value and thus, for i > 1, x = U = 0. This also means that X and U can be discarded for N sufficiently large. Here, t A and tc are measured by r-user as the average time between sending the REGISTER request and receiving the 200 OK response. In Figure 10.5, ruser performs a DNS lookup or a Handle-DNS lookup in scenarios A and C, respectively. These lookups are excluded from the performance metrics, i.e., t1 and t2 do not include the DNS lookups for the SIP servers. However, we closely examine and compare the two lookup times and we show the perfor- mance results in Section 10.4.3. 10.4.2.2 Call Establishment Figure 10.6 is a magnified image of Figure 10.2 that focuses on the call establishment process according to the test-bed, where c_user will try to establish a call with the roaming user ruser. For this section, cdomain is ece.unrn.edu [New Mexico]. The hdomain is istec.or [Panama] and the fdo- main is cnri.reston.va.ua [Virginia]. For brevity, the 100 TRYING messages are excluded from Figure 10.6. We focus on the INVITE message flow and the servers require no authentication. We denote by rt(m,n) the round-trip C user cdomain hdomain fdomain r user Handle 129.24.27.139 129.24.24.106 168.77.202.59 132.151.9.104 132.151.9.101 system INVITE <r_user@hdomain> INVITE INVITE RINGING RINGING RINGING 200 OK 200 OK 200 OK INVITE <2118~r_user> Lookup <2118~r_user> Invite <2118.r_user@fdomain> INVITE RINGING RINGING RINGING 200 OK 200 OK 200 OK FIGURE 10.6 INVITE message flow A. With no roaming logic C. With proposed roaming logic H-SIP. C 2009 by Taylor & Francis Group, LLC SIP Han