Hi firat, I didn't understand fully the configuration you are describing. Something like this? Host1 --SGSN1--\GGSN--Server Host2 --SGSN2--/ Where Host1 has been assigned IP1 and Host2 has been assigned IP2, both assigned by GGSN where IP1 != IP2. Let's assume the server IP is IPs and the GGSN public uplink (non-GTP) IP is IPg. As far as I understand, it works as follow: - Case without NAT between GGSN and Server: Host1 sends ICMP packet with saddr=IP1 daddr=IPs, which gets encapsulated through GTP and GGSN decapsulates it. Same for Host2 but in this case the packet will have saddr=IP2. As there's no NAT (eg. host clients are assigned a public IP), the server receives 2 ICMP packets with different saddr, and when answering back using the original saddr now as daddr. As GGSN keeps track of the saddr assigned to each pdp context, when it receives a packet from the uplink (non-GTP side), it matches the daddr of the packet against the saddr of the active pdp ctx to find to which pdp ctx should forward the packet. - Case with NAT between GGSN and Server: Almost the same but with extra steps done by the NAT. When the GGSN sends the packet saddr=IP1 daddr=IPs to the server, the NAT changes saddr=IP1->IPg. It does the same for saddr=IP2, but the NAT keeps track of the binding. When the response is received from the server, the NAT converts back IPg->IP1 and GGSN can again track the pdp ctx as described in the previous case. -- - Pau Espin Pedrol <pespin(a)sysmocom.de> http://www.sysmocom.de/ ======================================================================= * sysmocom - systems for mobile communications GmbH * Alt-Moabit 93 * 10559 Berlin, Germany * Sitz / Registered office: Berlin, HRB 134158 B * Geschaeftsfuehrer / Managing Director: Harald Welte