Description: GSN peer table. The SGSN-GGSN peer relationship is established as given in the following series of steps: 1. When Mobile System (MS) wants service, it sends packets to a SGSN with specific APN. 2. SGSN uses the DNS to resolve this APN to the IP address of the GGSN which is the designated as the gateway to this APN. 3. SGSN establishes a path to the GGSN using GTP protocol. 4. The SGSN and GGSN peer maintains path by sending echo request message to each other. If one side fails in echo reply for certain times, the other side will send a notification to NMS. A SGSN can have multiple GGSN as peers, while a GGSN can have multiple SGSN peers, depending on routing path.