Charles M. Kozierok The TCP-IP Guide

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Redirected Header Option Format

In a Redirect message, provides a copy of the original message (or a portion of it) that led

to the Redirect being generated. This option can be found in Table 119 and Figure 167.

MTU Option Format

The MTU option lets a router convey a recommended MTU value in Router Advertisement

messages. Its format is described in Table 120 and Figure 168.

Table 119: ICMPv6 Redirected Header Option Format

Field Name

Size

(bytes)

Description

Type 1

Type: Identifies the ICMPv6 option type. For the Redirected Header option,

the value is 4.

Length 1

Length: The length of the entire option (including the Type and Length

fields), expressed in units of 8 octets (64 bits).

Reserved 6 Reserved: 6 reserved bytes sent as zeroes.

IP Header + Data Variable

IP Header + Data: As much of the original IPv6 datagram as will fit without

causing the size of the ICMPv6 error message (including its own IP

header) to exceed the minimum IPv6 maximum transmission unit (MTU) of

1280 bytes.

Figure 167: ICMPv6 Redirected Header Option Format

Table 120: ICMPv6 MTU Option Format (Page 1 of 2)

Field Name

Size

(bytes)

Description

Type 1

Type: Identifies the ICMPv6 option type. For the MTU option, the value is

Length 1

Length: The length of the entire option (including the Type and Length

fields), expressed in units of 8 octets (64 bits). The MTU option is fixed in

length at 8 bytes so the value of this field is 1.

Type = 4 Length

Original IPv6 Datagram Portion

(As Much As Will Fit Without Exceeding The 1280-Byte IPv6 Minimum MTU)

4 8 12 16 20 24 28 320

Reserved

Reserved 2 Reserved: 2 reserved bytes sent as zeroes.

MTU 4

MTU: The maximum transmission unit (MTU) value, in bytes, that the

router is recommending for use on the local link.

Figure 168: ICMPv6 MTU Option Format

Table 120: ICMPv6 MTU Option Format (Page 2 of 2)

Field Name

Size

(bytes)

Description

Type = 5 Length Reserved

Maximum Transmission Unit (MTU)

4 8 12 16 20 24 28 320

TCP/IP IPv6 Neighbor Discovery Protocol (ND)

The new Internet Protocol version 6 (IPv6) represents an evolution of the venerable Internet

Protocol. It maintains the same basic operational principles of IPv4, but makes some

important modifications, particularly in the area of addressing. In fact, some of the more

significant changes in IPv6 are actually not in the IP protocol itself, but in the protocols that

support IP. One of the most interesting of these was the creation of an entirely new support

protocol for IPv6. It combines several tasks previously performed by other protocols in IPv4,

adds some new functions, and makes numerous improvements to the whole package. This

new standard is called the IPv6 Neighbor Discovery (ND) protocol.

In this section I describe the new Neighbor Discovery protocol used in IPv6. I begin with an

overview of the protocol, discussing its history, the motivation for its creation and the

standards that define it. I then describe its operation in general terms, listing the funda-

mental functions that ND performs, the three groups these functions fit into, and the

ICMPv6 message types used to carry them out. I describe the key differences between ND

and the way that its functions were carried out in IPv4. I then provide more information on

the three functional groups in ND: those that involve discovery of important internetwork

information from routers, those that are related to address resolution and neighbor commu-

nication between hosts, and finally, router redirection.

Background Information: This section assumes basic comprehension of IP

version 6, which in turn requires understanding IPv4. ND uses ICMP version 6

(ICMPv6) messages, so reference is made to the ICMP section. Finally, since

ICMP performs some functions done by the Address Resolution Protocol (ARP) in IPv4,

you may need to refer to the ARP discussion if unfamiliar with its operation.

IPv6 ND Overview, History, Motivation and Standards

The purpose of network layer protocols like the Internet Protocol is to provide a means of

connecting together individual local networks to create a much larger internetwork. To

higher protocol layers and to users, this internetwork behaves in most respects as if it were

a single large network, because the lower layers hide the details that “glue” together the

individual networks. Any device can send information to any other, regardless of where it is

located, and “like magic”, it will work. At least, most of the time.

The existence of an internetwork means that devices can treat all other devices as peers, at

least from the perspective of higher protocol layers and applications. From the standpoint of

lower layers, however, there is a very important difference between devices that are on a

host's local network and those that are elsewhere. In a general sense, most devices have a

more important relationship with the devices that are on its local network than those that are

far away. Some of the most obvious tasks that a device must perform specifically with other

devices on its local network include:

☯ Direct Datagram Delivery: Devices deliver data directly to other devices on their local

network, while data going to distant devices must be indirectly delivered (routed).

☯ Layer Two Addressing: To facilitate direct delivery, devices need to know the layer

two addresses of the other devices on the local network; they don't need to know them

for non-local devices.

☯ Router Identification: To deliver indirectly, a device needs to find a router on its local

network that it can talk to.

☯ Router Communication: The local router must communicate information to each of

the local hosts using it so they know how best to use it.

☯ Configuration: Hosts will usually look to information provided by local devices to let

them perform configuration tasks such as determining their own IP address.

To support these and other requirements, several special protocols and functions were

developed along with the original Internet Protocol (version 4). The IP addressing scheme

lets devices differentiate local addresses from distant ones. The Address Resolution

Protocol (ARP) lets devices determine layer two addresses from layer three addresses. The

Internet Control Message Protocol (ICMP) provides a messaging system to support various

communication requirements between local devices, including the ability of a host to find a

local router and the router to provide information to local hosts.

These features all work properly in IPv4, but they were developed sort of in an ad hoc

manner. They are defined not in a single place, but rather in a variety of different Internet

standards. There were also some limitations with the way these “local device” functions

were implemented.

Formalizing Local Network Functions: The Concept of Neighbors

IP version 6 represents the biggest change in decades to not just the Internet Protocol itself,

but the entire TCP/IP suite. It thus provided an ideal opportunity to formalize and integrate

the many disparate functions and tasks related to communication between local devices.

The result was the creation of a new protocol: Neighbor Discovery for IP Version 6, also

commonly called the IPv6 Neighbor Discovery protocol. Since this protocol is new in

version 6, there is no IPv4 version of it, so the name is usually just seen as the Neighbor

Discovery (ND) protocol with no further qualifications; its use with IPv6 is implied.

The term neighbor is one that has been used for years in various networking standards and

technologies to refer to devices that are local to each other. In the context of our current

discussion, two devices are neighbors if they are on the same local network, meaning that

they can send information to each other directly; the term can refer to either a regular host

or a router. I think this is a good analogy to the way humans refer to those who live or work

nearby. Just as most of us have a special relationship with people who are our neighbors

and communicate more with them than with those who are far away, so do IP devices.

Since a neighbor is a local device, the name of the Neighbor Discovery protocol would

seem to indicate that ND is all about how neighbors discover each other's existence. In the

context of this protocol, however, the term discovery has a much more generic meaning: it

refers to discovering not just who our neighbors are but also important information about

them. In addition to letting devices identify their neighbors, ND facilitates all the tasks in the

bullet list above, including such functions as address resolution, parameter communication,

autoconfiguration and much more as we will see in the next few topics.

Neighbor Discovery Standards

The Neighbor Discovery protocol was originally defined in RFC 1970, published in August

1996, and revised in the current defining standard, RFC 2461, published December 1998.

Most of the functions of the ND protocol are implemented using a set of five special ICMPv6

control messages. Thus, to some extent, the operation of ND is partially described by the

ICMPv6 standard, RFC 2463. Where ICMPv4 can be considered IPv4's “administrative

assistant”, IPv6 really has two such assistants working closely together: ICMPv6 and the

IPv6 ND protocol. I discuss more of the differences between the ways IPv4 and IPv6

implement ND's functions later in this section.

Key Concept: The new IPv6 Neighbor Discovery protocol formalizes for IPv6 a

number of functions related to communication between devices on a local network

that are performed in IPv4 by protocols such as ARP and ICMP. ND is considered

another “helper” protocol for IPv6, and is closely related to ICMPv6.

IPv6 ND General Operational Overview: ND Functions, Functional

Groups and Message Types

As I mentioned in the preceding overview, the name of the Neighbor Discovery (ND)

protocol really does not do it justice. The protocol facilitates not merely the discovery of

neighboring devices, but a substantial number of functions related to local network connec-

tivity, datagram routing and configuration. Both regular hosts and routers in an IPv6

environment count on the ND protocol to facilitate important exchanges of information that

are necessary for proper internetwork operation.

The Neighbor Discovery protocol has a number of similarities to the Internet Control

Message Protocol. An important one is that like ICMP, ND is a messaging protocol. It

doesn't implement a single specific function but rather a group of activities that are

performed through the exchange of messages. This means we can't explain the operation

of ND through a specific description of “what ND does”, but rather must define its operation

by means of a list of messages ND provides and specific ways that they are used.

Any local network on an internet will have both regular hosts and routers, and the term

neighbor can refer to either. Of course, hosts and routers play different roles on a network,

and as a result neighbor discovery is very different for each. The ND standard describes

nine specific functions performed by the protocol. To better understand these functions and

how they are related, we can divide them into three functional groups based on communi-

cation type and the kinds of devices involved (see Figure 169).

Host-Router Discovery Functions

One of the two main groups of functions in ND are those that facilitate the discovery of local

routers and the exchange of information between them and hosts. This includes four

specific functions:

☯ Router Discovery: This is the core function of this group: the method by which hosts

locate routers on their local network.

☯ Prefix Discovery: Closely related to the process of router discovery is prefix

discovery. Recall that the term “prefix” refers to the network portion of an IP address.

Hosts use this function to determine what network they are on, which in turn tells them

how to differentiate between local and distant destinations and whether to attempt

direct or indirect delivery of datagrams.

☯ Parameter Discovery: Also closely related to router discovery, this is the method by

which a host learns important parameters about the local network and/or routers, such

as the maximum transmission unit of the local link.

☯ Address Autoconfiguration: Hosts in IPv6 are designed to be able to automatically

configure themselves, but this requires information that is normally provided by a

router.

Host-Host Communication Functions

The other main group of functions is that associated with information determination and

communication directly between nodes, usually hosts. Some of these functions can be

performed between hosts and routers, but this group is not specifically related to router

discovery; it includes:

Figure 169: Neighbor Discovery Protocol (ND) Functional Groups and Functions

Neighbor Discovery Protocol

Host-Router

Discovery Functions

Router Discovery

Prefix Discovery

Address

Autoconfiguration

Parameter Discovery

Host-Host

Communication Functions

Address Resolution

Next-Hop Determination

Neighbor Unreachability

Detection

Duplicate Address

Detection

Redirect

Function

☯ Address Resolution: The process by which a device determines the layer two

address of another device on the local network from that device's layer three (IP)

address. This is the job performed by ARP in IP version 4.

☯ Next-Hop Determination: The method for looking at an IP datagram's destination

address and determining where it should next be sent.

☯ Neighbor Unreachability Detection: The process of determining whether or not a

neighbor device can be directly contacted.

☯ Duplicate Address Detection: Determining if an address that a device wishes to use

already exists on the network.

Redirect Function

The last functional group contains just one function: Redirect. The technique whereby a

router informs a host of a better next-hop node to use for a particular destination.

Key Concept: The Neighbor Discovery protocol encompasses nine individual

functions, many of which are related to each other. They are organized into three

functional groups: host-router discovery functions, host-host communication

functions, and the redirect function.

Relationships Between Functions

The division of ND's overall functionality into nine tasks in three groups is somewhat

arbitrary, but provides a good frame of reference for understanding what the protocol does.

Obviously some of the functions in different groups are related; next-hop determination

uses information obtained as part of parameter discovery. The redirect function is also a

form of router-host communication but is distinct from router discovery.

ICMPv6 Messages Used By the Neighbor Discovery Protocol

Just as ND is similar to ICMP in its operation, the two protocols are related in another way:

the method that messaging is done. ND actually implements its functions using ICMPv6

messages. A set of five message types is described in the ND standard:

☯ Router Advertisement Messages: Sent regularly by routers to tell hosts that they

exist and provide important prefix and parameter information to them.

☯ Router Solicitation Messages: Sent by hosts to request that any local routers send a

Router Advertisement message so they don't have to wait for the next regular adver-

tisement message.

☯ Neighbor Advertisement Messages: Sent by hosts to indicate the existence of the

host and provide information about it.

☯ Neighbor Solicitation Messages: Sent to verify the existence of another host and to

ask it to transmit a Neighbor Advertisement.

☯ Redirect Messages: Sent by a router to tell a host of a better method to route data to

a particular destination.

More details on how these message types are used can be found in the topics later in this

section that describe the three functional groups above in more detail. The structures of

each of the five ICMPv6 message types used by ND can be found in the section on ICMPv6

informational message types and formats.

IPv6 ND Functions Compared to Equivalent IPv4 Functions

The IPv6 Neighbor Discovery protocol has the distinction of being the only truly new

protocol created as part of the core of Internet Protocol version 6; there is no “NDv4” at all.

Of course, most of the services that ND provides to IPv6 were also required in version 4 of

the Internet Protocol. They were just provided in a rather diverse set of protocols and

standards that the ND protocol has formalized, integrated and improved.

What this means is that while ND is new, the jobs it does are equivalent to the tasks

performed by several other protocols in IPv4. Specifically, the bulk of ND functions corre-

spond to the following set of standards, features and message types in IPv4:

☯ ICMPv4 Router Discovery: Most of the functions associated with identifying and

obtaining information from routers in ND are based on the use of ICMPv4 Router

Advertisement and Router Solicitation messages as defined in RFC 1256.

☯ Address Resolution Protocol: ND provides enhanced address resolution capabilities

that are similar to the functions provided in IPv4 by ARP.

☯ ICMPv4 Redirect: ND's redirect function and Redirect messages are based on similar

functionality defined in IPv4 and ICMPv4.

Of course, there are other aspects of ND that only somewhat correlate to how things work

in IPv4. There are also improvements or new functionality compared to how these IPv4

functions work. Some of these are in fact due to differences in how IPv6 itself operates

compared to IPv4. For example, prefix discovery in ND is sort of related to the Address

Mask Request and Address Mask Reply messaging in ICMPv4.

Overall, ND represents a substantial improvement compared to the way its job was done in

IP version 4. Like IPv6 itself, ND is generally better suited to the needs of modern networks

than the older protocols. Some of the more important specific improvements made in ND

compared to how its job was done in IPv4 include the following:

☯ Formalizing Of Router Discovery: In IPv4 the process of router discovery and solici-

tation was arguably an “afterthought”; ND formalizes this process and makes it part of

the core of the TCP/IP protocol suite.

☯ Formalizing Of Address Resolution: In a similar manner, address resolution is

handled in a superior way in ND. ND functions at layer three and is tightly tied to IP just

like ICMP is. There is no more need for an “ambiguously-layered” protocol like ARP,

whose implementation is very dependent on the underlying physical and data link

layers.

☯ Ability To Perform Functions Securely: ND operates at the network layer, so it can

make use of the authentication and encryption capabilities of IPSec for tasks such as

address resolution or router discovery.

☯ Autoconfiguration: In combination with features built into IPv6, ND allows many

devices to automatically configure themselves even without the need for something

like a DHCP server (though DHCPv6 does also exist.)

☯ Dynamic Router Selection: Devices use ND to detect if neighbors are reachable or

not. If a device is using a router that stops being reachable it will detect this and

automatically switch to another one.

☯ Multicast-Based Address Resolution: Address resolution is performed using special

multicast addresses instead of broadcasts, reducing unnecessary disruption of

“innocent bystanders” when resolution messages must be sent.

☯ Better Redirection: Improvements have been made to the method by which redirects

are generated and used.

IPv6 ND Host-Router Discovery Functions: Router Discovery, Prefix

Discovery, Parameter Discovery and Address Autoconfiguration

Internetworks are created by connecting together individual networks. The devices that are

responsible for this connection of networks are routers, which send data from one network

to the next. A host must rely on a router to forward transmissions to all devices other than

those on the local network. For this reason, before a host can properly use an internetwork,

it needs to find a local router and learn important information about both the router and the

network itself. Enabling this information exchange is one of the most important jobs of the

IPv6 Neighbor Discovery protocol.

The general term used to describe most of the ND communication between hosts and

routers on a local network is discovery. As we discussed earlier in this section, the term

encompasses not merely discovery of the router but also communication of important

parameters. Most of this communication flows from the routers to the hosts, since routers

really control the way that each network is used. They provide information to hosts so the

hosts know how best to operate.

The various discovery features related to host-router communication are all facilitated by

the same exchange of two different ICMPv6 message types. Router Advertisement

messages are sent only by routers, and contain information about the router and also the

network on which it is located. Router Solicitation messages are optional, and sent by hosts

when they want to find a local router. The format of each of these messages is described in

the section on ICMPv6 informational messages.

Host-Router Discovery Functions Performed By Routers

The mechanisms for using these messages is not really that complicated. The best way to

see how the discovery process works overall is to look at the specific tasks performed both

by routers and hosts in ND. Let's start with those performed by routers:

☯ Routine Advertisement: The main job that routers do in ND is the regular trans-

mission of Router Advertisement messages. Each router maintains a timer that

controls how often an advertisement is sent out. Advertisements are also sent when

any sort of special situation arises. For example, a message will be sent if key infor-

mation about the router changes, such as its address on the local network.

Router Advertisement messages include key information about both the router and the

network. See the relevant topic in the ICMPv6 section for a full description of the

Router Advertisement message format.

☯ Parameter Maintenance: Routers are responsible for maintaining key parameters

about the local network so they can be sent in advertisements. These include the

default Hop Limit that should be used by hosts on the network, a default MTU value for

the network, and information such as network prefixes that are used for both first-hop

routing by hosts and autoconfiguration.

Again, some more details on these can be found in the topic that discusses Router

Advertisement fields.

☯ Solicitation Processing: Routers listen for Router Solicitation messages and when

one is received, will immediately send a Router Advertisement to the requesting host.

Host-Router Discovery Functions Performed By Hosts

For their part, hosts are responsible for three main functions.

☯ Advertisement Processing: Hosts listen for advertisements on their local network

and process them. They then set appropriate parameters based on the information in

these messages. This includes maintaining various data structures such as lists of

prefixes and routers, which are updated regularly as new advertisement information

comes in.

☯ Solicitation Generation: Under certain conditions a host will generate a Router Solic-

itation and send it out on the local network. This very simple message just requests

that any local routers that hear it immediately send a Router Advertisement message

back to the device that made the request. This is most often done when a host is first

turned on, so it doesn't have to sit “in limbo” waiting for the next routine advertisement.

☯ Autoconfiguration: When required, and if the network supports the function, the host

will use information from the local router to allow it to automatically configure itself with

an IP address and other parameters.

Key Concept: One of the two main functional groups of the Neighbor Discovery

protocol is the set of host-router discovery functions. They allow hosts on a local

network to discover the identity of a local router and learn important parameters

about how the network is to be used. Host-router discovery operations are performed using

ICMPv6 Router Advertisement and Router Solicitation messages.