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According to RFC 3261, a SIP spiral is "a SIP request that is routed to a proxy, forwarded onwards, and arrives once again at that proxy, but this time differs in a way that will result in a different processing decision than the original request. Typically, this means that the request's Request-URI differs from its previous arrival. A spiral is not an error condition, unlike a loop. A typical cause for this is call forwarding. A user calls joe@example.com. The example.com proxy forwards it to Joe's PC, which in turn, forwards it to bob@example.com. This request is proxied back to the example.com proxy. However, this is not a loop. Since the request is targeted at a different user, it is considered a spiral, and is a valid condition."

SBC is enhanced with addition of a proprietary parameter, "call-info", to the Record-Route header of outgoing requests to avoid such a spiral condition. This parameter received in subsequent Route headers is used to perform call association for responses and OOD requests (such as SUBSCRIBE, REFER, OPTIONS, NOTIFY, INFO, MESSAGE, and so on) for both dialog transparency and non-dialog transparency call flows. This necessitates having the SIPFE pre-process Route headers in the context of the Dialog Transparency functionality. 

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