Each peer assigns a Call ID value to each user session it requests or accepts. This Call ID value MUST be unique for the tunnel between the PNS and PAC to which it belongs. Tunnels to other peers can use the same Call ID number so the receiver of a packet on a tunnel needs to associate a user session with a particular tunnel and Call ID. It is suggested that the number of potential Call ID values for each tunnel be at least twice as large as the maximum number of calls expected on a given tunnel.
A session is defined by the triple (PAC, PNS, Call ID).