On Thu, 2012-12-06 at 16:06 +0100, Olle E. Johansson wrote:

The flow token points to a flow saying that Kamailio should NOT parse the Contact URI and find a destination. It should find the proper connection.

In the specific case of a single server (no edge proxies) the flow token will indicate exactly what the source of the REGISTER was (assuming the example algorithm from section 5.2 of RFC 5626 is used) - which may have no connection because it could have arrived over UDP. Assuming that any binding within the NAT in-front of the client is still live this allows us to route back to client without any of the contact aliasing/received stuff. In actual fact, the information in the flow token should allow both $du to be correctly set and force_send_socket() to be called correctly.

      Example Algorithm: When the proxy boots, it selects a 20-octet
      crypto random key called K that only the edge proxy knows. A byte
      array, called S, is formed that contains the following information
      about the flow the request was received on: an enumeration
      indicating the protocol, the local IP address and port, the remote
      IP address and port. The HMAC of S is computed using the key K
      and the HMAC-SHA1-80 algorithm, as defined in [RFC2104]. The
      concatenation of the HMAC and S are base64 encoded, as defined in
      [RFC4648], and used as the flow identifier. When using IPv4
      addresses, this will result in a 32-octet identifier.
However, this strikes me as something that does need to be handled within C code when setting the destination because as sophisticated as Kamailio configuration can be, performing base64 decoding followed by HMAC-SHA1-80 followed by complex string parsing may be a bit too much :-)

-- 
Peter Dunkley
Technical Director
Crocodile RCS Ltd