6.1 Layering

6.1 Layering

In the IP architecture, the choice of what size datagram to send is made by a protocol at a layer above IP. We refer to such a protocol as a "packetization protocol". Packetization protocols are usually transport protocols (for example, TCP) but can also be higher-layer protocols (for example, protocols built on top of UDP).

Implementing PMTU Discovery in the packetization layers simplifies some of the inter-layer issues, but has several drawbacks: the implementation may have to be redone for each packetization protocol, it becomes hard to share PMTU information between different packetization layers, and the connection-oriented state maintained by some packetization layers may not easily extend to save PMTU information for long periods.

We therefore believe that the IP layer should store PMTU information and that the ICMP layer should process received Datagram Too Big messages. The packetization layers must still be able to respond to changes in the Path MTU, by changing the size of the datagrams they send, and must also be able to specify that datagrams are sent with the DF bit set. We do not want the IP layer to simply set the DF bit in every packet, since it is possible that a packetization layer, perhaps a UDP application outside the kernel, is unable to change its datagram size. Protocols involving intentional fragmentation, while inelegant, are sometimes successful (NFS being the primary example), and we do not want to break such protocols.

To support this layering, packetization layers require an extension of the IP service interface defined in [1]:

A way to learn of changes in the value of MMS_S, the "maximum send transport-message size", which is derived from the Path MTU by subtracting the minimum IP header size.