IPv4 – Understanding the Identification Field in IPv4 Header

ipv4

Identification field is of 16 bits in Ipv4 header. This means we can have $2^16$ unique datagrams from a single host at a time.
Also each datagram can have a total length of $2^16$ bytes.

So can the sender send only $2^32$ or 4GB of data at a time ? If he sends more data, will the identification number wrap around ?

Best Answer

the rfc6864 - Updated Specification of the IPv4 ID Field precisely address this issue.

Extract from this rfc:

  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].

and

4.1. IPv4 ID Used Only for Fragmentation

Although RFC 1122 suggests that the IPv4 ID field has other uses,
including datagram de-duplication, such uses are already not
interoperable with known implementations of sources that do not vary
their ID. This document thus defines this field's value only for
fragmentation and reassembly:

>> The IPv4 ID field MUST NOT be used for purposes other than fragmentation and reassembly.

Datagram de-duplication can still be accomplished using hash-based duplicate detection for cases where the ID field is absent (IPv6 unfragmented datagrams), which can also be applied to IPv4 atomic datagrams without utilizing the ID field [RFC6621].

(emphasis mine)

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