IP Fragmentation and Reassembly on L2GRE and GMIP Tunnels

Starting in software version 4.6, L2GRE and GMIP tunnels support IP fragmentation and reassembly of packets. IP fragmentation occurs with encapsulation. Fragmented packets are sent on the tool port at the sending end of the tunnel (for example, at a remote site). IP reassembly occurs with decapsulation. Fragmented packets reaching the network port at the receiving end of the tunnel (for example, at a main office site), are decapsulated and reassembled before being sent to the destination.

The tool port at the remote site is configured with a tunnel MTU. If a packet exceeds the tunnel MTU, the packet will be fragmented, and the fragmented packets will be sent out the IP interface.

Note:  The first fragmented packet contains the tunnel header (Eth+IP+GRE). The rest of the fragments have the Ethernet and IP headers.

The packet size plus the tunnel header size is calculated and checked against the tunnel MTU. For example, if the tunnel MTU is 1518 and the packet is 1526, the packet exceeds the tunnel MTU. If the tunnel MTU is 1518 and the packet is 1518, the packet will also exceed the tunnel MTU due to the addition of the tunnel header.

IPv6 tunnel supports Path MTU. When a GigaSMART (GSOP) is associated to a map, Path MTU discovery message is sent to the tunnel destination and the Path MTU learnt is used for IPv6 fragmentation.

The network port at the main office site receives the fragmented packets sent from the remote site. The tunnel header is removed from all fragmented packets, and they are buffered in memory. After all the fragmented packets are available, they are reassembled. The reassembled packet is then sent to the tool.

Take into account the following notes and considerations:

Feature

Description

IPv4 and IPv6 Support

IPv4 and IPv6 packets are supported.

Note: To avoid the overhead and improve the performance, existing GMIP IPv4 tunneling does not calculate the UDP Checksum on the Encapsulated Packets. The same will be adopted for IPv6 and IPv6 RFCs, where the checksum value will be set to 0. UDP checksum of the out header is not mandatory in the IPv6 tunneling.

Always Enabled

IP fragmentation and reassembly are always enabled. No configuration is required.

Tunnel MTU

The tunnel MTU is configured using the MTU field on the IP Interfaces configuration page. (Select Ports > IP Interfaces, and then click New to open the page.)

The MTU is fixed at 9400 for all network/tool ports on the following platforms:

o GigaVUE‑HC2 and GigaVUE‑HC2 equipped with Control Card version 2 (HC2 CCv2)
o GigaVUE‑HC1
o GigaVUE‑HC3
o GigaVUE-TA100

Encapsulation Statistics

The encapsulation statistics count the number of fragmented packets. Refer to Display GMIP Tunnel Encapsulation Statistics and Display L2GRE Tunnel Encapsulation Statistics. For definitions, refer to Tunnel Encapsulation Statistics Definitions.

Decapsulation Statistics

The decapsulation statistics count the number of reassembled packets. Refer to Display GMIP Tunnel Decapsulation Statistics and Display L2GRE Tunnel Decapsulation Statistics. For definitions, refer to Tunnel Decapsulation Statistics Definitions.

GigaSMART Engine Ports

GigaSMART operations with a tunnel component can be assigned to GigaSMART groups consisting of multiple GigaSMART engine ports. Refer to Groups of GigaSMART Engine Ports for details.