icates that a response referencing an existing call or transaction has been received for which the server has no records or state information. 5.4.25 482 Loop Detected This response indicates that the request has been looped and has been routed back to a proxy that previously forwarded the request. Each server that forwards a request adds a Via header with its address to the top of the request. A branch parameter is added to the Via header, which is a hash function of the Request-URI, and the To, From, Call - ID, and CSeq number. A second part is added to the branch parameter if the request is being forked. The rea- son the branch parameter must be checked is to allow a request to be routed back to a proxy, provided that the Request-URI has changed. This could hap- pen with a call forwarding feature. In this case, the Via headers would differ by having different branch parameters. 5.4.26 483 Too Many Hops This response indicates that the request has been forwarded the maximum number of times as set by the Max-Forwards header in the request. This is SIP: Understanding the Session Initiation Protocol indicated by the receipt of a Max-Forwards: 0 header in a request. In the following example, the UAC included a Max-Forwards: 4 header in the REGISTER request. A proxy receiving this request five hops later generates a 483 response: REGISTER sip:registrar.timbuktu.tu SIP/2.0 Via: SIP/2.0/UDP 01.202.203.204:5060;branch=z9hG4bK45347.1 Via: SIP/2.0/UDP 198.20.2.4:6128;branch=z9hG4bk917a4d4.1 Via: SIP/2.0/UDP 18.56.3.1:5060;branch=z9hG4bK7154.1 Via: SIP/2.0/TCP 101.102.103.104:5060;branch=z9hG4bKa5ff4d3.1 Via: SIP/2.0/UDP 168.4.3.1:5060;branch=z9hG4bK676746 To: ip:explorer@geographic.org From: sip:explorer@geographic.org>;tag=341323 Call-ID: 67483010384@168.4.3.1 CSeq: 1 REGISTER Max-Forwards: 0 Contact: sip:explorer@national.geographic.org Content-Length: 0 SIP/2.0 483 Too Many Hops Via: SIP/2.0/UDP 201.202.203.204:5060;branch=z9hG4bK45347.1 Via: SIP/2.0/UDP 98.20.2.4:6128;branch=z9hG4bk917a4d4.1 Via: SIP/2.0/UDP 18.56.3.