Internet Core Protocols: The Definitive Guide

Name

Echo Request and Echo Reply

Synopsis

Echo Request and Echo Reply query messages provide the ability for one system to see if another system is on the network and functional. Any Echo Request message received by a system must be responded to with an Echo Reply query message.

ICMP Message Type

The Message Type for Echo Request is 8. The Message Type for Echo Reply is 0.

ICMP Message Codes

Neither the Echo Request nor Echo Reply query messages uses any Codes, and this field must be zero. However, some vendors (such as Cisco) use some message codes for specific services. If you see a non-zero value in this field, check the documentation for the sending system to determine what purpose it serves.

Defined In

RFC 792.

Additional Fields

Both the Echo Request and Echo Reply query messages use three additional fields. Table 5.12 lists these fields, their size (in bytes), and their usage.

Table 5.12. The Additional Fields used in Echo Request and Echo Reply Query Messages

Field	Bytes	Description
Identifier	2	Provides a “session identification number” for the request and reply messages
Sequence Number	2	Provides a “counter” for the unique Echo Request and Echo Reply messages, allowing multiple messages to be sent using a single Identifier
Optional Data	varies	Used to store any test data desired

These fields are explained below, but if you are familiar with the ping program, then you’re probably already aware of the functions they provide.

• The Identifier field provides a kind of “session identification number” ...