tributes. Some of these attributes describe location, permissions, adminis- trators, and state. The fact that handles are defined independently of any of the attributes or public keys of the underlying objects makes them persis- tent identifiers [34]. These identifiers are managed and resolved using a secure global name service that guarantees the association of the identifier with its respective attributes over distributed communication. Security is a crucial property of the Handle System. The system acts as a certification authority, assuring that attributes of the name/reference are securely transferred between the communicating ends. Hence, the Handle Sys- tem allows for secure name resolution and administration in a distributed fashion, making it highly scalable and suitable to operate in mobile envi- ronments. In our approach, elements of the SIP architecture, SIP users and proxy servers, are identified with handles abstracting any domain binding. Users will identify each other, as well as the SIP servers they associate with using handles instead of URIs and domain names, respectively. In Figure 10.1, the roaming user ruser will have his own handle 2118/r_user with the neces- sary administrative privileges over the handle. Additionally, the home proxy server has a handle 2118/hproxy, and the foreign proxy server has a han- dle 10.200/fproxy. Note that a handle has the form prefix/suffix. The prefix represents the naming authority (NA) while the suffix represents a unique Please note that a direct mapping between domains and handles exists and is enabled by a handle/DNS proxy approach. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Handle Field type:index Value 2118/r_user HS_ADMIN:100 rwr:0.NA/2118:300 HS_ADMIN:101 rwr:2118/r_user:200 HS_VLIST:200 rwr:2118/r_user:300 rwr:2118/hproxy:300 rwr:10.200/fproxy:300 SIP_URL:250 sip:2118.r_user@x.y.z.w SIP_PWD:251 password HS_PUBKEY:300 00BE0034 FIGURE 10.3 Sample user handle structure. local name under the NA namespace [12]