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Chapter 22. NAT Traversal > RTP Traversal through NAT - Pg. 479

22.4 RTP traversal through NAT A problem associated with the mechanism described before refers to the fact that the mappings in the NAT expire after periods of inactivity. An incoming request might come a long time after the registration is done, so there is a need for a refresh mechanism that keeps the pinhole open. This might be done by hav- ing the client issue REGISTER requests after some timer expires or by sending other protocol messages (e.g., STUN messages 4 ). A best current practices (BCP) document for NAT traversal for SIP is actually in progress in the IETF [draft-ietf-sipping-nat-scenarios]. 479 22.4 RTP Traversal through NAT 22.4.1 Issues Let us consider that both John and Alice are located in private network domains behind respective NAT devices. Both domains are connected to the public Internet. The scenario is depicted in Figure 22.13. UA John private network NAT public network NAT private network UA Alice FIGURE 22.13 Now John wants to call Alice. We will assume that the SIP signaling traverses NAT without problems, based on the solutions discussed before. During the SDP offer/response exchange, an IP address and port combination are specified by each UA for sending and receiving RTP media. Given that the endpoints are located in private domains, the IP addresses specified in the SDP are also private. Therefore, once the SDP exchange is completed, John and Alice will try to send packets to private addresses that are not resolvable in the Internet; thus, the sub- missions will fail. This situation is depicted in Figure 22.14. 22.4.2 Proposed Solutions Though the RTP problem when traversing NAT is quite obvious, the solutions to overcome it are not so simple. Furthermore, different NAT topologies and differ- ent NAT behaviors increase the difficulty of proposing a single solution. We will now examine some NAT scenarios and their solutions. The focus of the scenarios is on the RTP traffic, so we will assume that the SIP signaling traversal of NAT has already been resolved. 4 More specifically, the STUN binding keep-alive usage could be used for this purpose. STUN will be presented in subsequent sections.