1. Introduction

1. Introduction

When one IP host has a large amount of data to send to another host, the data is transmitted as a series of IP datagrams. It is usually preferable that these datagrams be of the largest size that does not require fragmentation anywhere along the path from the source to the destination. (For the case against fragmentation, see [5].) This datagram size is referred to as the Path MTU (PMTU), and it is equal to the minimum of the MTUs of each hop in the path. A shortcoming of the current Internet protocol suite is the lack of a standard mechanism for a host to discover the PMTU of an arbitrary path.

Note: The Path MTU is what in [1] is called the "Effective MTU for sending" (EMTU_S). A PMTU is associated with a path, which is a particular combination of IP source and destination address and perhaps a Type-of-service (TOS).

The current practice [1] is to use the lesser of 576 and the first-hop MTU as the PMTU for any destination that is not connected to the same network or subnet as the source. In many cases, this results in the use of smaller datagrams than necessary, because many paths have a PMTU greater than 576. A host sending datagrams much smaller than the Path MTU allows is wasting Internet resources and probably getting suboptimal throughput. Furthermore, current practice does not prevent fragmentation in all cases, since there are some paths whose PMTU is less than 576.

It is expected that future routing protocols will be able to provide accurate PMTU information within a routing area, although perhaps not across multi-level routing hierarchies. It is not clear how soon that will be ubiquitously available, so for the next several years the Internet needs a simple mechanism that discovers PMTUs without wasting resources and that works before all hosts and routers are modified.