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A proxy receives SIP requests and responses and forwards them through the network on behalf of the user agent. Think of proxies as routers that are also capable of generating requests and responses. You will learn later about how proxies will send certain responses backward to the UAC, for example, to notify the UAC that it is forwarding a request on to the recipient UAS. The proxy is also responsible for determining how to route requests and responses. This can be done through two methods: loose routing and strict routing. We won t go into the specifics of either one, as they are described in detail in 5. Next we will discuss several different types of proxies: stateful proxies, stateless proxies, and forking proxies. Proxies should be considered as routers in the network. But routers are only concerned about forwarding packets to the next hop in the path. They only know about the other routers in the network and do not concern themselves with the status of the session itself. This is where SIP proxies are different. The SIP proxy must be able to resolve URIs like a router, but the SIP proxy must also be able to send responses when a request is received. The SIP proxy is also equipped as a user agent, capable of requests and responses.
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Stateful Proxies A stateful proxy must maintain the state of each and every dialog that it is part of. As a request is sent to the proxy, it saves the call identifier for the session, as well as other pertinent identification data. This means that in order for the stateful proxy to know when a session has changed status or has ended, it must receive all responses. This is why networks that use stateful proxies must route all messages for a session through the same route. This ensures that all SIP responses use the same path as the request, so that the stateful proxy will receive all of the responses and any additional requests for a session. Each stateful proxy adds a new VIA header with its own address as the topmost VIA header to maintain routing order. These are then used in reverse order for routing responses. The VIA header is used to ensure that the responses are sent through the same proxies as the request, or routing can be forced through the same path using strict
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routing methods. In addition to maintaining the state of the session, there are several other functions of the stateful proxy. A stateful proxy must process all routing data to determine the target(s) for the request. It must then determine how to route the request to the destination and process all responses from the recipient(s). It will also send a 1xx response to the UAC to prevent retransmission of the request and to let the UAC know that it has received the request and is forwarding the request to the next hop. This is where the proxy behaves as a UAS. The stateful proxy cannot send 2xx responses. The stateful proxy must also perform a series of checks on requests and responses to ensure they meet the proper criteria. This includes checking the syntax of a message to ensure it is correct prior to forwarding to the next hop. If the syntax is not correct, the UAS of the stateful proxy will return an error response. Likewise, the stateful proxy will also check the URI provided in the request to ensure that the URI format can be understood by the proxy. If the URI is not of the correct format, the message cannot be forwarded, because the proxy does not know where to send the message. Again, the proxy will send an error response back to the UAC. The proxy also looks at the MAX-FORWARDS value to ensure that the request has not already been through too many hops. This would indicate that the message has been through too many networks or is possibly being sent in circular fashion. The proxy will also look to see if its own address has been included in the VIA header, indicating that the request has already been through the proxy and the request is being circular-routed. When a proxy adds its own address to a VIA header, it computes a BRANCH value by taking the tags from the TO and FROM headers, along with the values from the CALL-ID and REQUEST-URI headers, the topmost VIA header, and the CSeq header and creating a cryptographic hash from all of these values. This then becomes the latter portion of the BRANCH value. The stateful proxy then checks for this value whenever a request is received and the proxy s own address is already present in a VIA header. If there are headers in the request or response that the proxy does not understand, it will not reject the message. This is because there are many extensions to the SIP protocol that have not been defined by the IETF but are proprietary extensions added by vendors. This makes them proprietary and therefore unknown to the stateful proxy. For this reason they are ignored to ensure interoperability between all SIP entities. A stateful proxy can terminate a session by using the CANCEL method if no 2xx response is received. Each stateful proxy possesses a timer that is set upon the receipt of a request. If that timer expires prior to the 2xx being received (or any other final response), then the stateful proxy will respond to the UAC with the CANCEL method to terminate the session.
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Stateless Proxies A stateless proxy does not have any concept of a dialog or any knowledge of the state for any dialog or session. The stateless proxy is nothing more than a simple router responsible for nothing more than the forwarding of a request or response. It does not maintain any knowledge of previous requests or responses, and it does not maintain any routing data other than a routing table of its own used to determine how to route messages based on the URI provided in the SIP message.
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