Issues with NAT/Routers

Lets use this page to document peculiar things that NAT/routers do when it comes to processing SIP/STUN messages.

Behavior Classification

Inspection and Modification of SIP/SDP Messages

Some routers inspect and modify SIP/SDP message content in its naive attempt to assist NAT traversal. Parts of the SIP/SDP message elements that the router modifies include the Via and Contact header, and SDP c= and m= lines. Some router even goes further with doing what seems to be simple text search/replace, replacing all occurrences of private IP address/port in SIP/SDP message with the public IP address/port.

This behavior may break ICE offer/answer negotiation. If the router replaces the default candidate with IP address/port which is not listed in the candidate list in SDP, remote callee will reject ICE offer with ice-mismatch, and ICE negotiation will not take place.

Known Router Brands

Belkin Wireless ADSL Router

Reported in:	UK
Version:	Firmware: 6.01.06 (Jun 7 2006 20:25:29), boot version: 0.70.2v6, hardware: 01

Inspects and modifies SIP/SDP messages

This router inspects and modifies SIP/SDP messages if the outer address is port 5060.