8. Host Fragmentation

Packetization Layers SHOULD avoid sending messages that will require fragmentation [Kent87] [frag-errors]. However, entirely preventing fragmentation is not always possible. Some Packetization Layers, such as a UDP application outside the kernel, may be unable to change the size of messages it sends, resulting in datagram sizes that exceed the Path MTU.

IPv4 permitted such applications to send packets without the DF bit set. Oversized packets without the DF bit set would be fragmented in the network or sending host when they encountered a link with an MTU smaller than the packet. In some case, packets could be fragmented more than once if there were cascaded links with progressively smaller MTUs. This approach is NOT RECOMMENDED.

It is RECOMMENDED that IPv4 implementations use a strategy that mimics IPv6 functionality. When an application sends datagrams that are larger than the effective Path MTU, they SHOULD be fragmented to the Path MTU in the host IP layer even if they are smaller than the MTU of the first link, directly attached to the host. The DF bit SHOULD be set on the fragments, so they will not be fragmented again in the network. This technique will minimize the likelihood that applications will rely on IPv4 fragmentation in a way that cannot be implemented in IPv6. At least one major operating system already uses this strategy. Section 9 describes some exceptions to this rule when the application is sending oversized packets for probing or diagnostic purposes.

Since protocols that do not implement PLPMTUD are still subject to problems due to ICMP black holes, it may be desirable to limit to these protocols to "safe" MTUs likely to work on any path (e.g., 1280 bytes). Allow any protocol implementing PLPMTUD to operate over the full range supported by the lower layer.

Note that IP fragmentation divides data into packets, so it is minimally a Packetization Layer. However, it does not have a mechanism to detect lost packets, so it cannot support a native implementation of PLPMTUD. Fragmentation-based PLPMTUD requires an adjunct protocol as described in Section 10.3.