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SIP: Alternative Softswitch Architecture
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SIP: Alternative Softswitch Architecture
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Table 5-3 SIP response codes
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100 180 181 Trying Ringing Call is being forwarded Queued Session progress OK 413 414 415
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Number code Description Request, entity too large Request, URI too large Unsupported media type
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182 183 200
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420 480 481
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Bad extension Temporarily not available Call leg/transaction does not exist Loop detected Too many hops Address incomplete Ambiguous Busy here Request cancelled Not acceptable here Internal severe error Not implemented Bad gateway Service unavailable Gateway timeout SIP version not supported Busy everywhere Decline Does not exist anywhere Not acceptable
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202 300 301 302 305 380 400 401 402 403 404 405 406 407 408 409 410 411
Accepted Multiple choices Moved permanently Moved temporarily Use proxy Alternative service Bad request Unauthorized Payment required Forbidden Not found Method not allowed Not acceptable Proxy authentication Request timeout Conflict Gone Length required
482 483 484 485 486 487 488 500 501 502 503 504 505 600 603 604 606
Source: Camarillo
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
SIP: Alternative Softswitch Architecture
5
Stateless proxies keep with no state. They receive a request, forward it to the next hop, and immediately delete all states related to that request. When a stateless proxy receives a response, it determines routing based solely on the Via header and it does not maintain a state for it.1 An SIP call using a proxy server is a little more complicated than the simple SIP call model described previously (see Figure 5-3). In this call, the caller is configured with the called party s SIP server. An INVITE is sent to the called party s SIP server with the called party s text address in the To field. The called party s server determines if the called party is registered in that server. If the called party is registered on that server, it then determines the called party s current location on the network. This is called the mobility feature. Once the called party is located via the mobility feature, the proxy server generates an INVITE request with no alteration of the headers of the request, except to add its own name in the Via field. Multiple servers may be involved in tracking down the called party. Next the server must retain state information on the call. The server does this by correlating Cseq numbers, call IDs, and other elements of the headers as they pass through the proxy server. The server sends a TRYING message back to the calling party s agent. When the called party answers at the new location, a RINGING response is sent to the proxy server via the remote server (the called party s server). Both servers have Via entries in the response message to the calling party. Finally, ACK messages are
Figure 5-3 SIP call using a proxy server
Calller (1) Invite (4) 200 OK (5) ACK
SIP PROXY (2) Invite (3) 200 OK (6) ACK
Calllee
Conversation (7) BYE (10) 200 OK (8) BYE (9) 200 OK
Camarillo, Gonzalo. SIP Demystified. New York: McGraw-Hill, 2002, p. 126 129.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
SIP: Alternative Softswitch Architecture
SIP: Alternative Softswitch Architecture
exchanged, the call is established, and the media flow over RTP can begin. The call is terminated via a BYE request. What makes the proxy server marketable is its user mobility feature. The called party can be logged in at multiple locations at once. This results in the proxy server generating the INVITE to all names on the list until the called party is found (preferably RINGING, but also TRYING or OK).2 A redirect server accepts SIP requests, maps the destination address to zero or more new addresses, and returns the translated address to the originator of the request. After that, the originator of the request can send requests to the addresses returned by the redirect server. The redirect server originates no requests of its own. Redirect servers pose an alternative means of call forwarding and follow-me services. What differentiates the redirect server from a proxy server is that the originating client redirects the call. The redirect server provides the intelligence to enable the originating client to perform this task as the redirect server is no longer involved. The redirect server call model is a mix of the two previously described call models. Here a proxy model reverts to the direct call model once the called party is located. The redirect server returns a redirection response to the INVITE with code 301 or 302 indicating the called party is at the location listed in the Contact field of the message body. The calling party s Media Gateway Controller (MGC) closes its signaling with the redirect server and initiates another INVITE to the located returned in the redirect response. After that, the call flow is that of the direct model. If the called party is registered at a number of locations, the redirect server will return a list of names (URI) to be contacted. The calling party can then contact those addresses directly. Registrar A registrar is a server that accepts SIP REGISTER requests. SIP includes the concept of user registration where a user tells the network that he is available at a given address. This registration occurs by issuing a REGISTER request by the user to the registrar. A registrar is often combined with a proxy or redirect server. Practical implementations often combine the UAC and UAS with registrars with either proxy servers or redirection servers. This can result in a network having only UAs and redirection or proxy servers.3
Douskalis, Bill. IP Telephony: The Integration of Robust VoIP Services. New York: Prentice Hall, 2000, pg. 74.
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