1. Introduction

This document describes a method for Packetization Layer Path MTU Discovery (PLPMTUD), which is an extension to existing Path MTU Discovery methods described in [RFC1191] and [RFC1981]. In the absence of ICMP messages, the proper MTU is determined by starting with small packets and probing with successively larger packets. The bulk of the algorithm is implemented above IP, in the transport layer (e.g., TCP) or other "Packetization Protocol" that is responsible for determining packet boundaries.

This document does not update RFC 1191 or RFC 1981; however, since it supports correct operation without ICMP, it implicitly relaxes some of the requirements for the algorithms specified in those documents.

The methods described in this document rely on features of existing protocols. They apply to many transport protocols over IPv4 and IPv6. They do not require cooperation from the lower layers (except that they are consistent about which packet sizes are acceptable) or from peers. As the methods apply only to senders, variants in implementations will not cause interoperability problems.

For sake of clarity, we uniformly prefer TCP and IPv6 terminology. In the terminology section, we also present the analogous IPv4 terms and concepts for the IPv6 terminology. In a few situations, we describe specific details that are different between IPv4 and IPv6.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

This document is a product of the Path MTU Discovery (PMTUD) working group of the IETF and draws heavily on RFC 1191 and RFC 1981 for terminology, ideas, and some of the text.