Internet Core Protocols: The Definitive Guide

Name

Timestamp

Synopsis

Identifies the time at which a router processed the IP datagram.

Class and Code

Class 2, Code 4

Size

Varies as needed.

Copy to all fragments?

No (first fragment only).

Defined In

RFC 791.

Status

Standard.

Notes

The Timestamp option is conceptually similar to the Record Route option, with the critical exception being that the router will also place a timestamp into the Option-Data field (actually, the source device can choose the specific information that it wants to have recorded).

In order to facilitate this process, the Timestamp option uses an 8-bit pointer field similar to the pointer found in the Source Route and Record Route options, as well as a four-bit overflow field, and a four-bit set of flags.

The overflow field provides a counter for the routers that could not register their timestamps. This allows an administrator to see how much of the network they could not record, due to lack of space. The flags are used to define the behavior that an administrator wishes the routers to adhere to. These behaviors are listed in Table 2.14.

Table 2.14. Flags Used with the Timestamp Option

Flag Value	Description
0	Timestamps only (do not record router addresses)
1	Record router addresses, followed by timestamps
2	Match timestamps with preexisting router addresses

Timestamps are recorded as 32-bit integers that represent the number of milliseconds since midnight, Universal Time.

As the datagram is passed around the Internet, the routers use the pointer to indicate the byte position where ...