CEO Commentary

RFC3840 For FoIP Routing?


In our testing, we’ve found that FoIP with T.38 over the open Internet – no carriers involved – is even more reliable than PSTN fax for just about any call. The big difference? No carrier routing and no TDM-IP conversion. It’s T.38 end-to-end.


So, how do we approach that ideal with carriers responsible for the routing? You might know that the FoIP Task Groups of the SIP and i3 Forums have been performing extensive testing of FoIP in international calls. And it hasn’t been pretty. In most cases, it was the carrier’s first exposure to FoIP qualification, leading to significant problems with equipment and network interoperability and configuration.

These problems have been addressed one-by-one. But the remaining problems caused by least-cost routing-as-usual for FoIP calls are significant enough to slow the industry’s move to an all-IP global network. FoIP-aware routing, which would be IP everywhere but to and from the TDM-connected fax terminals, is usually required for success.


A SIP call switches from voice to FoIP (which usually means T.38) when the called fax endpoint answers. Of course, by that time the call has already been routed, since routing decisions begin when the on-ramp proxy or SIP server receives the first SIP Invite. At that point, the provider’s routing algorithm should, if possible, route the call over qualified FoIP routes.


Ten months ago the two task groups proposed that the SIP servers routing a fax call would set the user= parameter to user=fax in the header of the SIP Invite (see RFC 3261, section 25.1). But then just last month we found a better answer: RFC 3840, “Indicating User-Agent Capabilities in SIP”, which is specific to the task. According to RFC 3840, setting the header fields to indicate the capabilities of a terminal (user agent) can be used to “express a preference for routing.”


The SIP and i3 Forum FoIP task groups are now moving ahead to standardize “SIP.FAX=”. It is interesting to note that RFC3840 defines 18 different feature tags in its section10, with none of them being fax, so we are setting out to make that 19 tag definitions.


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Mike Coffee
Mike Coffee
CEO, Commetrex