1. Introduction

In IPv4, the Identification (ID) field is a 16-bit value that is unique for every datagram for a given source address, destination address, and protocol, such that it does not repeat within the maximum datagram lifetime (MDL) [RFC791] [RFC1122]. As currently specified, all datagrams between a source and destination of a given protocol must have unique IPv4 ID values over a period of this MDL, which is typically interpreted as two minutes and is related to the recommended reassembly timeout [RFC1122]. This uniqueness is currently specified as for all datagrams, regardless of fragmentation settings.

Uniqueness of the IPv4 ID is commonly violated by high-speed devices; if strictly enforced, it would limit the speed of a single protocol between two IP endpoints to 6.4 Mbps for typical MTUs of 1500 bytes (assuming a 2-minute MDL, using the analysis presented in [RFC4963]). It is common for a single connection to operate far in excess of these rates, which strongly indicates that the uniqueness of the IPv4 ID as specified is already moot. Further, some sources have been generating non-varying IPv4 IDs for many years (e.g., cellphones), which resulted in support for such in RObust Header Compression (ROHC) [RFC5225].

This document updates the specification of the IPv4 ID field to more closely reflect current practice and to include considerations taken into account during the specification of the similar field in IPv6.