Path MTU Discovery Support
The following RFCs for handling large packets over a GRE tunnel are supported:
RFC 1191 describes a method for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. When a RUCKUS ICX device receives an IP packet that has its Do not Fragment (DF) bit set, and the packet size is greater than the MTU value of the outbound interface, then the RUCKUS ICX device returns an ICMP Destination Unreachable message to the source of the packet, with the code indicating "fragmentation needed and DF set". The ICMP Destination Unreachable message includes the MTU of the outbound interface. The source host can use this information to help determine the minimum MTU of a path to a destination.
RFC 4459 describes solutions for issues with large packets over a tunnel. The following methods, from RFC 4459, are supported:
- If a source attempts to send packets that are larger than the lowest MTU value along the path, Path MTU Discovery (PMTUD) can signal to the source to send smaller packets. This method is described in Section 3.2 of RFC 4459.
- Inner packets can be fragmented before encapsulation, in such a manner that the encapsulated packet fits in the tunnel path MTU, which is discovered using PMTUD. This method is described in Section 3.4 of RFC 4459.
By default, PMTUD is enabled.