Charles M. Kozierok The TCP-IP Guide

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ICMPv6 Echo Request and Echo Reply Messages

IP is a relatively simple protocol that does not itself include any method for performing tests

between devices to help in diagnosing internetwork problems. This means that this job, like

other support tasks, falls to ICMP. The simplest test performed when there is a problem

using TCP/IP is usually a check that a pair of devices is able to send datagrams to each

other. This is most often done by an initiating device sending a test message to a second

device, which receives it and replies back to tell the first device it received the message.

Like ICMPv4, ICMPv6 includes a pair of messages specifically for connection testing. To

use them, Device A begins the test process by sending an ICMPv4 Echo Request message

to Device B. Device B responds back to Device A with an Echo Reply message. When

Device A receives this message, it knows that it is able to communicate (both send and

receive) successfully with Device B.

Note: As a side note, in ICMPv4, the first message type was named just Echo but

was often called Echo Request instead. In ICMPv6 they finally just added the

“Request” to the message name and were done with it. A modest but useful

improvement from a clarity standpoint.

ICMPv6 Echo and Echo Reply Message Format

The format for ICMPv6 Echo Request and Echo Reply messages is very similar to that of

the ICMPv4 version, as shown in Table 109 and Figure 157.

Table 109: ICMPv6 Echo Request and Echo Reply Message Format

Field Name

Size

(bytes)

Description

Type 1

Type: Identifies the ICMPv6 message type; for Echo Request messages

the value is 128, and for Echo Reply messages 129. (Recall that in

ICMPv6, informational messages always have a Type value of 128 or

higher.)

Code 1 Code: Not used; set to 0.

Checksum 2

Checksum: 16-bit checksum field for the ICMP header, as described in the

topic on the ICMP common message format.

Identifier 2

Identifier: An optional identification field that can be used to help in

matching Echo Request and Echo Reply messages.

Sequence

Number

Sequence Number: A sequence number to help in matching Echo

Request and Echo Reply messages.

Optional Data Variable

Optional Data: Additional optional data to be sent along with the message.

If this is sent in the Echo Request it is copied into the Echo Reply to be

sent back to the source.

As with ICMPv4, it is often necessary to match an Echo Reply message with the Echo

Request message that led to it being generated. Two special fields are used within the

format of these messages to allow Echo Request and Echo Reply messages to be matched

together, and to allow a sequence of messages to be exchanged. The Identifier field is

provided so a particular test session can be identified, and the Sequence Number field

allows a series of tests in a session to be numbered. The use of both fields is optional.

Application of Echo and Echo Reply Messages

ICMPv6 Echo Request and Echo Reply messages are used via the IPv6 version of the IP

ping utility, commonly called ping6. Like its IPv4 predecessor this utility allows an adminis-

trator to configure a number of test options to perform either a simple or rigorous test of the

connection between a pair of devices. See the topic describing ping for a full explanation.

Key Concept: ICMPv6 Echo Request and Echo Reply messages are used to facil-

itate network reachability testing. A device tests its ability to communicate with

another by sending it an Echo Request message and waiting for an Echo Reply in

response. The ping utility, a widely-used diagnostic tool in TCP/IP internetworks, makes use

of these messages.

ICMPv6 Router Advertisement and Router Solicitation Messages

At the highest level, we can separate IP devices into two groups: hosts and routers. Both

participate in the use of the internetwork, but they have different roles. An important IP

principle related to this division is that routers take care of routing—moving data between

networks—while hosts generally don't need to worry about this job. Hosts rely on the

routers on their local networks to facilitate communication to all other hosts except those on

the local network.

Figure 157: ICMPv6 Echo Request and Echo Reply Message Format

Type = 128 or 129 Code = 0 Checksum

Identifier Sequence Number

Optional Data

4 8 12 16 20 24 28 320

The implications of this are clear: a host cannot really use an internetwork until it knows the

identity of at least one local router, and the method by which that router is to be used. In

IPv4, a technique known as router discovery was invented, which provides a means by

which a host can locate a router and learn important parameters related to the operation of

the local network.

Router discovery in IPv6 works in a very similar manner, by having routers send Router

Advertisement messages both on a regular basis and in response to hosts prompting for

them using Router Solicitation messages. The router discovery function has been incorpo-

rated into the Neighbor Discovery (ND) protocol, where it is part of a larger class of tools

that I call host-router discovery functions.

ICMPv6 Router Advertisement Message Format

The formats of the ICMPv6 Router Advertisement and Router Solicitation messages are

fairly similar to their counterparts in ICMPv4. The main differences are in the parameters

that are communicated. Since routers are responsible for a few more functions in IPv6 than

they are in IPv4, the Router Advertisement message in ICMPv6 has a few more fields than

the older version.

The format of an ICMPv6 Router Advertisement message is described in Table 110 and

shown in Figure 158.

Table 110: ICMPv6 Router Advertisement Message Format (Page 1 of 2)

Field Name

Size

(bytes)

Description

Type 1

Type: Identifies the ICMPv6 message type; for Router Advertisement

messages the value is 134.

Code 1 Code: Not used; set to 0.

Checksum 2

Checksum: 16-bit checksum field for the ICMP header, as described in the

topic on the ICMP common message format.

Cur Hop Limit 1

Current Hop Limit: This is a default value the router is telling hosts on the

local network they should put in the Hop Limit field of datagrams they send.

If 0, the router is not recommending a Hop Limit value in this Router

Advertisement.

Autoconfig Flags 1

Router Lifetime 2

Router Lifetime: Tells the host receiving this message how long, in

seconds, this router should be used as a default router. If 0, tells the host

this router should not be used as a default router.

Note that this is an expiration interval only for the status of the router as a

default, not for other information in the Router Advertisement message.

Reachable Time 4

Reachable Time: Tells hosts how long, in milliseconds, they should

consider a neighbor to be reachable after they have received reachability

confirmation. See the Neighbor Discovery topic discussing neighbor

unreachability detection for more.

Retrans Timer 4

Retransmission Timer: The amount of time, in milliseconds, that a host

should wait before retransmitting Neighbor Solicitation messages.

Options Variable

Options: Router Advertisement messages may contain three possible

options (see the topic on ICMPv6 options for details):

☯ Source Link-Layer Address: Included when the router sending the

Advertisement knows its link-layer (layer two) address.

☯ MTU: Used to tell local hosts the MTU of the local network when this

information may not be known by hosts on the network.

☯ Prefix Information: Tells local hosts what prefix or prefixes to use

for the local network. (Recall that the “prefix” indicates which bits of

an IPv6 address are the network identifier compared to the host

identifier; it is thus analogous to an IPv4 subnet mask).

Table 110: ICMPv6 Router Advertisement Message Format (Page 2 of 2)

Field Name

Size

(bytes)

Description

ocon

gura

ags:

ags

e rou

autoconfiguration is performed on the local network. See the topic on IPv6

autoconfiguration for more details:

Subfield

Name

Size

(bytes)

Description

1/8

(1 bit)

Managed Address Configuration Flag: When

set, this flag tells hosts to use an administered or

“stateful” method for address autoconfiguration,

such as DHCP.

1/8

(1 bit)

Other Stateful Configuration Flag: When set,

tells hosts to use an administered or “stateful”

autoconfiguration method for information other

than addresses.

Reserved

6/8

(6 bits)

Reserved: Reserved for future use; sent as

zeroes.

ICMPv6 Router Solicitation Message Format

The format of an ICMPv6 Router Solicitation message is as shown Tab le 111 and Figure

159.

Figure 158: ICMPv6 Router Advertisement Message Format

Table 111: ICMPv6 Router Solicitation Message Format

Field Name

Size

(bytes)

Description

Type 1

Type: Identifies the ICMPv6 message type; for Router Solicitation

messages the value is 133.

Code 1 Code: Not used; set to 0.

Checksum 2

Checksum: 16-bit checksum field for the ICMP header, as described in the

topic on the ICMP common message format.

Reserved 4 Reserved: 4 reserved bytes set to 0.

Options Variable

Options: If the device sending the Router Solicitation knows its layer two

address, it should be included in a Source Link-Layer Address option.

Option formats are described in a separate topic.

Type = 134 Code = 0 Checksum

Current Hop Limit

Autoconfig Flags

Router Lifetime

ICMPv6 Options

4 8 12 16 20 24 28 320

240

Managed

Address

Config

Flag (M)

Other

Stateful

Config

Flag (O)

Reserved

Retransmission Timer

Reachable Time

Addressing of Router Advertisement and Router Solicitation Messages

Router Solicitation messages are normally sent to the IPv6 “all routers” multicast address;

this is the most efficient method since routers are required to subscribe to this multicast

address while hosts will ignore it. A routine (unsolicited) Router Advertisement is sent to all

devices using the “all nodes” multicast address for the local network. A Router Adver-

tisement that is sent in response to a Router Solicitation goes unicast back to the device

that sent the solicitation.

Key Concept: ICMPv6 Router Advertisement messages are sent regularly by IPv6

routers to inform hosts of their presence and characteristics, and to provide hosts

with parameters they need to function properly on the local network. A host that

wants to find out immediately what routers are present may send a Router Solicitation,

which will prompt listening routers to send out Router Advertisements.

ICMPv6 Neighbor Advertisement and Neighbor Solicitation Messages

The previous topic described the Router Advertisement and Router Solicitation messages,

which are used to facilitate host-router discovery functions as part of the IPv6 Neighbor

Discovery (ND) protocol. The other main group of tasks for which ND is responsible relates

to the exchange of information between neighboring hosts on the same network. I call these

host-host communication or host-host discovery functions.

Arguably, the most important additions to the Neighbor Discovery protocol are the functions

that formalize the exchange of parameters, and the methods for determining the existence

of neighboring hosts. These tasks include the new method of address resolution in IPv6, as

well the processes of next-hop determination and neighbor unreachability detection. They

require the use of two ICMPv6 messages: the Neighbor Solicitation message and the

Neighbor Advertisement message.

Figure 159: ICMPv6 Router Solicitation Message Format

Type = 133 Code = 0 Checksum

Reserved

ICMPv6 Options

4 8 12 16 20 24 28 320

The Neighbor Solicitation message allows a device to check that a neighbor exists and is

reachable, and to initiate address resolution. The Neighbor Advertisement message

confirms the existence of a host or router, and also provides layer-two address information

when needed. As you can see, these two messages are comparable to the Router Adver-

tisement and Router Solicitation messages, but are used differently and include different

parameters.

ICMPv6 Neighbor Advertisement Message Format

The format for the Neighbor Advertisement message is described in Table 112 and Figure

160.

Table 112: ICMPv6 Neighbor Advertisement Message Format (Page 1 of 2)

Field Name

Size

(bytes)

Description

Type 1

Type: Identifies the ICMPv6 message type; for Neighbor Advertisement

messages the value is 136.

Code 1 Code: Not used; set to 0.

Checksum 2

Checksum: 16-bit checksum field for the ICMP header, as described in the

topic on the ICMP common message format.

Flags 4

ags:

ree

ags

convey

orma

on a

e message

(

s o

emp

space for future use):

Subfield

Name

Size

(bytes)

Description

1/8

(1 bit)

Router Flag: Set when a router sends a Neighbor

Advertisement, and cleared when a host sends one.

This identifies the type of device that sent the

datagram, and is also used as part of Neighbor

Unreachability Detection to detect when a device

changes from acting as a router to functioning as a

regular host.

1/8

(1 bit)

Solicited Flag: When set, indicates that this message

was sent in response to a Neighbor Solicitation

message. Cleared for unsolicited Neighbor

Advertisements.

1/8

(1 bit)

Override Flag: When set, tells the recipient that the

information in this message should override any

existing cached entry for the link-layer address of this

device. This bit is normally set in unsolicited Neighbor

Advertisements since these are sent when a host

needs to force a change of information in the caches of

its neighbors.

Reserved

3 5/8

(29 bits)

Reserved: A big whomping set of reserved bits. ☺

Target Address 16

Target Address: If the Neighbor Advertisement is being sent in response

to a Neighbor Solicitation, this is the same value as in the Target Address

field of the Solicitation. This field will commonly contain the IPv6 address of

the device sending the Neighbor Advertisement, but not in all cases. For

example, if a device responds as a proxy for the target of the Neighbor

Solicitation, the Target Address field contains the address of the target, not

the device sending the response. See the topic on address resolution

proxying for details.

If the Neighbor Advertisement is being sent unsolicited, then this is the

IPv6 address of the device sending it.

Options Variable

Options: When sent in response to a multicast Neighbor Solicitation, a

Neighbor Advertisement message must contain a Target Link-Layer

Address option, which carries the link-layer address of the device sending

the message. This is a good example of an “option” that's not really

“optional”. ☺

When the Neighbor Advertisement is sent in response to a unicast

Neighbor Solicitation, this option is technically not required (since the

sender of the Solicitation must already have the target's link-layer address

to have sent it unicast.) Despite this, it is still normally included, to ensure

that the link-layer address of the target is refreshed in the cache of the

device that sent the Neighbor Solicitation.

Table 112: ICMPv6 Neighbor Advertisement Message Format (Page 2 of 2)

Field Name

Size

(bytes)

Description

ICMPv6 Neighbor Solicitation Message Format

The Neighbor Solicitation message format is much simpler, as depicted in Table 113 and

Figure 161.

Figure 160: ICMPv6 Neighbor Advertisement Message Format

Table 113: ICMPv6 Neighbor Solicitation Message Format (Page 1 of 2)

Field Name

Size

(bytes)

Description

Type 1

Type: Identifies the ICMPv6 message type; for Neighbor Solicitation

messages the value is 135.

Code 1 Code: Not used; set to 0.

Checksum 2

Checksum: 16-bit checksum field for the ICMP header, as described in the

topic on the ICMP common message format.

Reserved 4 Reserved: 4 reserved bytes set to 0.

Type = 136 Code = 0 Checksum

Flags Reserved

Target Address

4 8 12 16 20 24 28 320

240

Router

Flag

(R)

Solicited

Flag

(S)

Override

Flag

(O)

Reserved

ICMPv6 Options

Key Concept: ICMPv6 Neighbor Advertisement and Neighbor Solicitation messages

are similar in many ways to the Router Advertisement and Router Solicitation

messages. However, rather than being used to communicate parameters from

routers to hosts, they are used for various types of communication between hosts on a

physical network, such as address resolution, next-hop determination and neighbor

unreachability detection.

Target Address 16

Target Address: The IPv6 address of the target of the solicitation. For

IPv6 address resolution, this is the actual unicast IP address of the device

whose layer two (link-layer) address we are trying to resolve.

Options Variable

Options: If the device sending the Neighbor Solicitation knows both its

own IP address and layer two address, it should include the layer two

address in a Source Link-Layer Address option. The inclusion of this option

will allow the destination of the Neighbor Solicitation to enter the layer two

and layer three addresses of the source of this message into its own

address cache. See the discussion of IPv6 address resolution.

Figure 161: ICMPv6 Neighbor Solicitation Message Format

Table 113: ICMPv6 Neighbor Solicitation Message Format (Page 2 of 2)

Field Name

Size

(bytes)

Description

Type = 135 Code = 0 Checksum

Reserved

Target Address

ICMPv6 Options

4 8 12 16 20 24 28 320