3. Host Specification
3. Host Specification
When a host receives a Datagram Too Big message, it MUST reduce its estimate of the PMTU for the relevant path, based on the value of the Next-Hop MTU field in the message (see section 4). We do not specify the precise behavior of a host in this circumstance, since different applications may have different requirements, and since different implementation architectures may favor different strategies.
We do require that after receiving a Datagram Too Big message, a host MUST attempt to avoid eliciting more such messages in the near future. The host may either reduce the size of the datagrams it is sending along the path, or cease setting the Don't Fragment bit in the headers of those datagrams. Clearly, the former strategy may continue to elicit Datagram Too Big messages for a while, but since each of these messages (and the dropped datagrams they respond to) consume Internet resources, the host MUST force the PMTU Discovery process to converge.
Hosts using PMTU Discovery MUST detect decreases in Path MTU as fast as possible. Hosts MAY detect increases in Path MTU, but because doing so requires sending datagrams larger than the current estimated PMTU, and because the likelihood is that the PMTU will not have increased, this MUST be done at infrequent intervals. An attempt to detect an increase (by sending a datagram larger than the current estimate) MUST NOT be done less than 5 minutes after a Datagram Too Big message has been received for the given destination, or less than 1 minute after a previous, successful attempted increase. We recommend setting these timers at twice their minimum values (10 minutes and 2 minutes, respectively).
Hosts MUST be able to deal with Datagram Too Big messages that do not include the next-hop MTU, since it is not feasible to upgrade all the routers in the Internet in any finite time. A Datagram Too Big message from an unmodified router can be recognized by the presence of a zero in the (newly-defined) Next-Hop MTU field. (This is required by the ICMP specification [7], which says that "unused" fields must be zero.) In section 5, we discuss possible strategies for a host to follow in response to an old-style Datagram Too Big message (one sent by an unmodified router).
A host MUST never reduce its estimate of the Path MTU below 68 octets.
A host MUST not increase its estimate of the Path MTU in response to the contents of a Datagram Too Big message. A message purporting to announce an increase in the Path MTU might be a stale datagram that has been floating around in the Internet, a false packet injected as part of a denial-of-service attack, or the result of having multiple paths to the destination.