6.5 Issues for Other Transport Protocols

6.5 Issues for Other Transport Protocols

Some transport protocols (such as ISO TP4 [3]) are not allowed to repacketize when doing a retransmission. That is, once an attempt is made to transmit a datagram of a certain size, its contents cannot be split into smaller datagrams for retransmission. In such a case, the original datagram should be retransmitted without the DF bit set, allowing it to be fragmented as necessary to reach its destination. Subsequent datagrams, when transmitted for the first time, should be no larger than allowed by the Path MTU, and should have the DF bit set.

The Sun Network File System (NFS) uses a Remote Procedure Call (RPC) protocol [11] that, in many cases, sends datagrams that must be fragmented even for the first-hop link. This might improve performance in certain cases, but it is known to cause reliability and performance problems, especially when the client and server are separated by routers.

We recommend that NFS implementations use PMTU Discovery whenever routers are involved. Most NFS implementations allow the RPC datagram size to be changed at mount-time (indirectly, by changing the effective file system block size), but might require some modification to support changes later on.

Also, since a single NFS operation cannot be split across several UDP datagrams, certain operations (primarily, those operating on file names and directories) require a minimum datagram size that may be larger than the PMTU. NFS implementations should not reduce the datagram size below this threshold, even if PMTU Discovery suggests a lower value. (Of course, in this case datagrams should not be sent with DF set.)