Section: 11.2.1.5 IP Fragmentation Across Gi/Sgi
Content: 3GPP and non-3GPP accesses provide IP services for a MS/UE. The GGSN/P-GW endpoint is a GTPv1-U tunnel (controlled by GTP Gn/Gp or S5/S8/S2a/S2b) or IP tunnel (controlled by S5/S8/S2a/S2b PMIPv6 or employed by a UE with MIPv4 or DSMIPv6). The MTU of the IP tunnel on the MS/UE side of the IP link may be different than the MTU of the IP link connecting the GGSN/P-GW to the PDN. As a result IP packets crossing the Gi/Sgi interface may need to be fragmented. Unnecessary fragmentation should be avoided when possible due to the following; - Fragmentation is bandwidth inefficient, since the complete IP header is duplicated in each fragment. - Fragmentation is CPU intensive since more fragments require more processing at IP endpoints and IP routers. It also requires additional memory at the receiver. - If one fragment is lost, the complete IP packet has to be discarded. The reason is there is no selective retransmission of IP fragments provided in IPv4 or IPv6. To avoid unnecessary fragmenting of IP packets the MS/UE, or a server in an external IP network, may find out the end-to-end MTU by path MTU discovery and hence fragment correctly at the source. IP Fragmentation on Gi/Sgi shall be handled according to IETF RFC 791 [16] and IETF RFC 2460 [49]. The GGSN/P-GW shall enforce the MTU of IP packets to/from the MS/UE based on IETF RFC 791 [16] and IETF RFC 2460 [49].