7.6 Overview of SIP

7.6 Overview of SIP

SIP (Session Initiation Protocol) is an application layer protocol used for setting up and tearing down VoIP sessions. The major difference between SIP and H.323 is SIP is fully based on Internet context and thus does not support the Q.931 or ISUP messages that are currently used for telephony networks. But SIP extends the functionality of telephony signaling and supports mobility, and it is part of the overall IETF multimedia architecture framework that includes protocols such as RTP, RTSP (Real-Time Streaming Protocol), SDP (Session Description Protocol), SAP (Session Announcement Protocol), and others. SIP uses a text message format with an encoding scheme very similar to HTTP. Currently, SIP uses SDP [24] to establish the media session and the terminal capabilities, as H.245 in H.323. SDP messages are carried as the message body of a SIP message. A complete VoIP session includes a number of SDP messages for resource reservation, connection, and ringing in addition to SIP INVITE and BYE messages, as shown in Figure 7.6. All SIP messages are transported at the RTP layer, whereas H.323 control messages use TCP. SIP is based on client/server architecture with a SIP user agent and a SIP proxy server. The SIP user agent has two important functions: (1) it listens to the incoming SIP messages and (2) it sends SIP messages on receipt of an incoming SIP message or on user actions. The SIP proxy server relays SIP messages, so that it is possible to use a domain name to find a user. This simplifies the user location determination and allows scalability. The SIP server can be used as a redirect server, in which case it will provide the host location information without relaying the SIP messages, and the SIP user client will set up the session directly with the user.

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Figure 7.6: SIP signaling for VoIP.

The SIP mobility architecture components for VoIP are shown in Figure 7.7, where we assumed the mobile host and foreign network use DHCP (Dynamic Host Configuration Protocol) [25] or one of its variations for subnetwork configuration. An SIP-capable mobile host uses DHCP to register in the network. A mobile host broadcasts a DHCP_DISCOVER message to register to a network. Multiple DHCP servers will respond to this request with the IP address of the server and default gateway in the DHCP_OFFER message. The mobile host selects the DHCP server and sends the DHCP_REQUEST message to register. The registration is confirmed by DHCP_ACK at the DHCP server. As previously mentioned, SIP includes the mobility function, and the mobile host then uses its temporary IP address to register to the visiting register of the foreign network. The registration in a foreign network includes the authentication function, which uses AAA (authentication, accounting, and administration) [26] servers. The foreign AAA server communicates with the home AAA server to get the confirmation from the home register about customer authenticity. Ultimately, the visiting register receives the authentication response message and, if it is accepted, it sends the 200 OK messages to the mobile host. If authentication fails, it sends 401 messages indicating unauthorized request for registration. After this registration, the mobile host initiates SIP registration for session start by sending an INVITE message to the SIP proxy server. In the case of micro-mobility, authentication with the AAA server is not necessary. The visiting register can authenticate the mobile host (expedited registration). The complete SIP registration sequence is required for macro-mobility. To reduce the macro-mobility registration delay of SIP, a quasiregistration concept is proposed in Schulzrinne. [27] In quasiregistration, whenever the mobile host hands off from an old visiting register to a new visiting register, it informs its home register of its location by sending a REGISTER message. When the home register replies its OK message, it will include the old visiting register's IP address along with the response. Thus, visiting registers will know the adjacent visiting register's address to use for fast registration.

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Figure 7.7: SIP mobility architecture components.

[24]Song, J. et al., MIPv6 User Authentication Support through AAA, Internet draft, draft-song-mobileip-mipv6-user-authentication-00.txt, Nov. 2001.

[25]Schulzrinne, H., DHCP Option for SIP Servers, Internet draft, draft-ietf-sip-dhep-05.txt, Nov. 2001.

[26]Schulzrinne, H. et al., RTP: A Transport Protocol for Real-Time Applications, RFC 1889, IETF, Jan. 1996.

[27]Schulzrinne, H., DHCP Option for SIP Servers, Internet draft, draft-ietf-sip-dhep-05.txt, Nov. 2001.

Wireless Internet Handbook. Technologies, Standards and Applications
Wireless Internet Handbook: Technologies, Standards, and Applications (Internet and Communications)
ISBN: 0849315026
EAN: 2147483647
Year: 2003
Pages: 239

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