Charles M. Kozierok The TCP-IP Guide

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In our consultant analogy, this type of care-of address is like forwarding from the London

post office to the Tokyo post office. The London personnel would take a letter for John

Smith sent to his London address, and repackage it for delivery to “John Smith, care of the

Tokyo post office”. The Tokyo post office (or John Smith himself) would need to worry about

the last leg of the delivery.

Co-Located Care-Of Address

This is a care-of address assigned directly to the mobile node using some means external

to Mobile IP. For example, it may be assigned on the foreign network manually, or automat-

ically using DHCP. In this situation, the care-of address is used to forward traffic from the

home agent directly to the mobile node. This was the type of address shown in Figure 128.

In our consultant analogy, this is like John Smith obtaining a temporary address for his use

while in Tokyo. The London post office would forward directly to his Tokyo address. They

would not specifically send it to the Tokyo post office (though of course that PO would

handle the mail at some point).

Key Concept: In Mobile IP, each mobile device uses a temporary, care-of address

while on a foreign network. A co-located care-off address is one that is assigned

directly to the mobile node, and enables direct delivery of datagrams to the node.

The alternative is to use a foreign agent care-of address. In this situation the mobile node

actually uses the IP address of the foreign agent; datagrams are sent to the foreign agent,

which delivers them to the mobile node.

Advantages and Disadvantages of the Care-Of Address Types

The foreign agent care-of address is considered the type used in “classical” Mobile IP,

where there is both a home agent and a foreign agent. While it seems less efficient than the

co-located address method, it offers some important advantages. A key one is that the

same foreign agent care-of address can be used for all mobile nodes visiting that network.

Datagrams for all mobile nodes on that network are sent to the foreign agent, which

completes the delivery to the individual nodes. Since the mobile nodes use the foreign

agent's address, no extra addresses or extra work are required for each mobile node.

The co-located care-of address has the advantage that traffic can be forwarded directly

from the home agent to the mobile node. In this type of arrangement, it is possible for a

Mobile IP device to travel to a foreign network where there is no Mobile-IP-aware router to

act as a foreign agent. This does mean, however, that the Mobile IP implementation must

include all the functions of communicating with the home agent that the foreign agent

normally performs.

When co-located care-of addresses are used, an issue is how the temporary address is

obtained. In many foreign networks automatic assignment of an IP address using

something like DHCP may be possible, but if not, a temporary IP address would need to be

assigned. Either way, some of the foreign network's limited IP address space would need to

be set aside for mobile nodes, each of which would use an address while present on the

network. In some cases this could lead to an address depletion issue.

Foreign agent care-of addressing is usually preferred due to its more automatic nature,

when a foreign agent is present on the visited network. Considering that all datagrams will

need to go through some router on the foreign network to reach the mobile node anyway,

we might as well save the extra IP addresses. Co-located care-of addresses would be used

when there is no foreign agent, or might be practical for long term connections even when a

foreign agent is present.

Figure 129: Mobile IP Operation With A Foreign Agent “Care-Of” Address

This diagram is similar to Figure 128, except that instead of the mobile node having a co-located (distinct) IP

address as in that example, here the mobile node is using a foreign agent care-of address. This means that

the node’s care-of address is actually that of the foreign agent itself. Step #1 is the same as in Figure 128, but

in step #2 the home agent forwards not to the mobile node directly, but to the foreign agent (since that router

is the one whose IP address the mobile is using). In step #3 the foreign agent strips off the home agent’s

packaging and delivers the original datagram to the mobile node. This is typically done using whatever layer

two (LAN or wireless LAN) technology connects the mobile node and foreign agent together.

Foreign Network (Tokyo)

210.4.79.0/24

Home Network (London)

71.13.204.0/24

Mobile Node

Home Location

Home Address 71.13.204.20

Home

Age nt

Foreign

Age nt

210.4.79.2

To: 71.13.204.20

Mobile Node

Foreign Location

Care-Of Address 210.4.79.2

Tunnel To: 210.4.79.2

L2 Dest: Mobile Node

Remote

Client

Addess 56.1.78.22

To: 71.13.204.20

Remember that the care-of address represents only the destination to which mobile node

datagrams are forwarded. Foreign agents provide services other than forwarding, so it is

possible for a mobile node to use a co-located care-of address even when a foreign agent

is present, while continuing to take advantage of the other foreign agent services.

Key Concept: In Mobile IP, co-located care-of addresses have the advantage of

flexibility, but require each device to have a unique IP address on the remote

network. Foreign agent care-of addresses have the chief advantage of allowing

many mobile devices on a foreign network without each requiring a distinct IP address.

For more information on how datagrams are forwarded between the home agent and the

mobile node's care-of address, see the topic on Mobile IP encapsulation and tunneling.

Mobile IP Agent Discovery, and Agent Advertisement and Solicitation

Messages

When a mobile node is first turned on, it cannot assume that it is still “at home” the way

normal IP devices do. It must first determine where it is, and if it is not at home, begin the

process of setting up datagram forwarding from its home network. This process is accom-

plished by communicating with a local router serving as an agent, through the process

called agent discovery.

Agent Discovery Process

Agent discovery encompasses the first three steps in the simplified five-step Mobile IP

operational summary I gave in the topic on general operation. The main goals of agent

discovery include the following:

☯ Agent/Node Communication: Agent discovery is the method by which a mobile node

first establishes contact with an agent on the local network to which it is attached.

Messages are sent from the agent to the node containing important information about

the agent; a message can also be sent from the node to the agent asking for this infor-

mation to be sent.

☯ Orientation: The node uses the agent discovery process to determine where it is.

Specifically, it learns whether it is on its home network or a foreign network by identi-

fying the agent that sends it messages.

☯ Care-Of Address Assignment: The agent discovery process is the method used to

tell a mobile node the care-of address it should use, when foreign agent care-of

addressing is used.

Mobile IP agents are routers that have been given additional programming to make them

“Mobile IP aware”. The communication between a mobile node and the agent on its local

network is basically the same as the normal communication required between a device on

an IP network and its local router, except more information needs to be sent when the

router is an agent.

Agent Advertisement and Agent Solicitation Messages

Provision already exists for exchanges of data between a router and a node, in the form of

ICMP messages that are used for the regular IP router discovery process. Two messages

are used for this purpose: Router Advertisement messages that let routers tell local nodes

that they exist and describe their capabilities, and Router Solicitation messages that let a

node prompt a router to send an advertisement. These are described in their own topic in

the ICMP section.

Given the similarity to normal router discovery, it made sense to implement agent discovery

as a modification to the existing process rather than set up a whole new system. The

messages used in the agent discovery process are:

☯ Agent Advertisement: This is a message transmitted regularly by a router acting as a

Mobile IP agent. It consists of a regular Router Advertisement message that has one

or more extensions added that contain Mobile-IP-specific information for mobile

nodes.

☯ Agent Solicitation: This message can be sent by a mobile IP device to nudge a local

agent to send an Agent Advertisement.

The use of these messages is described in the Mobile IP standard in detail, and unsurpris-

ingly, is very similar to how regular Router Advertisement and Router Solicitation messages

are employed. Agents are normally configured to send out Agent Advertisements on a

regular basis, with the rate set to ensure reasonably fast contact with mobile nodes without

consuming excessive network bandwidth. They are required to respond to any Agent Solic-

itation messages they receive by sending an Advertisement. It is possible that some agents

may be configured to send Advertisements only upon receipt of a Solicitation.

Mobile nodes are required to accept and process Agent Advertisements. They distinguish

these from regular Router Advertisements by looking at the size of the message. They then

parse the extension(s) to learn the capabilities of the local agent. They determine whether

they are on their home network or a foreign network, and in the case of a foreign agent, how

the agent should be used. Mobile nodes are required to use Agent Advertisements to detect

when they have moved, using one of two algorithms defined in the standard, and also to

detect when they have returned back to their home network after they have been traveling.

They are also required to be able to send Agent Solicitation messages if they don't receive

an Agent Advertisement after a certain period of time. They are restricted to sending these

only infrequently, however, again to keep traffic manageable.

Now let's look at the formats of the two message types.

Agent Solicitation Message Format

Let’s start with the Agent Solicitation message because it is much simpler. In fact, there is

no new message format defined for this at all; it is identical to the format of a Router Solici-

tation message.

The reason no new message type is required here is that a solicitation is an extremely

simple message: “hey, if there are any routers out there, please tell me who you are and

what you can do”. No extra Mobile IP information needs to be sent. When a regular IP

router receives a Router Solicitation it will send a Router Advertisement, but a Mobile IP

router automatically sends the longer Agent Advertisement instead when prompted by any

solicitation, whether it comes from a Mobile IP node or a regular IP device.

Agent Advertisement Message Format

The Agent Advertisement begins with the normal fields of an ICMP Router Advertisement

message. The destination of the message is either the “all devices” multicast address

(224.0.0.1) if multicast is supported on the local network, or the broadcast address

(255.255.255.255) otherwise. The Router Address fields are filled in with the address(es) of

the agent.

Note: It is possible that a device may wish to advertise its ability to handle Mobile

IP messages, but not act as a regular router. In this case it changes the normal

Code field in the header of the Router Advertisement message from 0 to 16.

Following the regular fields, one or more extensions are added to the message format.

There are three extensions defined: the Mobility Agent Advertisement Extension, Prefix-

Lengths Extension, and One-Byte Padding Extension.

Mobility Agent Advertisement Extension

This is the main extension used to convey Mobile IP capabilities of the agent to mobile

nodes on the local network. It contains numerous fields used as part of the various

processes described in this overall section on Mobile IP.

The structure of the Mobility Agent Advertisement Extension is described in Table 82 and

shown graphically in Figure 130.

Table 82: Mobile IP Mobility Agent Advertisement Extension Format

Field Name

Size

(bytes)

Description

Type 1

Extension Type: Identifies the Agent Advertisement extension type. For

the Mobility Agent Advertisement Extension, it is set to 16.

Length 1

Length: Length of the extension in bytes, excluding the Type and Length

fields. Thus, it is equal to 6 plus 4 for each care-of address in the message.

Sequence

Number

Sequence Number: A sequential counter set to zero when the router

initializes and then incremented for each advertisement sent out.

Registration

Lifetime

Registration Lifetime: The maximum length of time, in seconds, the agent

is willing to accept for registration requests. A value of 65,535 (all ones)

means “infinite”. Note that this field is for registration only and has no

relation to the regular Lifetime field in the regular Router Advertisement

part of the message.

Flags 1

Reserved 1 Reserved: Sent as zero and ignored by recipient.

Care-Of

Addresses

Variable

(4 per

address)

Care-Of Addresses: Zero or more addresses provided by a foreign agent

for a mobile node to use as a foreign agent care-of address. A foreign

agent must always provide at least one address in its advertisement; a

router that cannot act as a foreign agent will typically omit this field.

ags:

one-

con

ng severa

orma

ona

ags

convey

specific information about the agent's capabilities and status. There are

seven one-bit flags, which when set to one convey the meanings given in

the table below:

Subfield

Name

Size

(bytes)

Description

1/8

(1 bit)

Registration Required: The mobile node must

using a co-located care-of address.

1/8

(1 bit)

Busy: The agent is currently too busy to accept

further registrations from mobile nodes.

1/8

(1 bit)

Home Agent: The agent is willing to function as a

home agent on this link (it will forward datagrams,

etc.) Note that a device can offer services as both

a home agent and a foreign agent.

1/8

(1 bit)

Foreign Agent: The agent is willing to function as

a foreign agent. Again, a device can act as both a

home agent and a foreign agent simultaneously.

1/8

(1 bit)

Minimal Encapsulation: The agent can receive

tunneled datagrams using minimal encapsulation.

1/8

(1 bit)

GRE Encapsulation: The agent can receive

tunneled datagrams using GRE encapsulation.

1/8

(1 bit)

Reserved: Not used; sent as zero.

1/8

(1 bit)

Reverse Tunneling: The agent supports reverse

tunneling.

Related Information: See the topic on Mobile IP encapsulation for more on

minimal and GRE encapsulation and reverse tunneling.

Prefix-Lengths Extension

This is an optional extension that tells a mobile node the prefix length(s) of the router

address(es) contained in the regular portion of the Agent Advertisement message; that is,

the Router Address field in the regular Router Advertisement part of the message. The

prefix length is another term for the number of bits in an IP address is the network ID, so

this tells the node the network ID in each of the router's addresses

Figure 130: Mobile IP Mobility Agent Advertisement Extension Format

This extension appears after the normal fields of a Router Advertisement message, as shown in Figure 148.

Extension Type = 16 Length Sequence Number

Registration Lifetime Flags Reserved

4 8 12 16 20 24 28 320

...

Regis-

tration

Required

(R)

Busy (B)

Home

Agent (H)

Foreign

Agent (F)

Minimal

Encap-

sulation

(M)

GRE

Encap-

sulation

(G)

Res er v ed

(r)

Rev er s e

Tunneling

(T)

Care-Of Address 1

Care-Of Address 2

Care-Of Address #N

The format of the Prefix-Lengths Extension is shown in Table 83 and also in Figure 131.

One-Byte Padding Extension

Some implementations require ICMP messages to be an even number of bytes, so a byte

of padding is needed. (The One-Byte Padding Extension is just a single byte of all zeroes.

No tables or figures needed for that!)

Key Concept: Mobile IP agent discovery is the process by which a mobile node

determines where it is located and establishes contact with a home or foreign agent.

Routers that can function as agents regularly send Agent Advertisement messages

to indicate their capabilities, which are modified versions of regular Router Advertisements.

A mobile node can also send an Agent Solicitation to request the sending of an Adver-

tisement, which is the same as regular Router Solicitation.

Note: Mobile IP does not include any provisions for authentication of Agent Adver-

tisement and Agent Solicitation messages. They may be authenticated using

IPSec, of course, if that has been implemented.

Table 83: Mobile IP Prefix-Lengths Extension Format

Field Name

Size

(bytes)

Description

Type 1

Extension Type: Identifies the Agent Advertisement extension type. For

the Prefix-Lengths Extension, it is set to 19.

Length 1

Length: Length of the extension in bytes, excluding the Type and Length

fields. Thus, it is equal to the number of prefix lengths (since each takes 1

byte).

Prefix Lengths

Variable

(1 per

length)

Prefix Lengths: One prefix length number for each router address in the

regular, Router Advertisement portion of the Agent Advertisement.

Figure 131: Mobile IP Prefix-Lengths Extension Format

This extension appears after the normal fields of a Router Advertisement message, as shown in Figure 148.

Extension Type = 19 Length Prefix Length #1 Prefix Length #2

Prefix Length #3 Prefix Length #4

...

Prefix Length #N

4 8 12 16 20 24 28 320

Mobile IP Home Agent Registration and Registration Messages

Once a mobile node has completed agent discovery, it knows whether it is on its home

network or a foreign network. If on its home network it communicates as a regular IP

device, but if on a foreign network it must activate Mobile IP. This requires that it commu-

nicate with its home agent so information and instructions can be exchanged between the

two. This process is called home agent registration, or more simply, just registration.

The main purpose of registration is to actually start Mobile IP working. The mobile node

must contact the home agent and tell it that it is on a foreign network and request that

datagram forwarding be turned on. It also must let the home agent know its care-of address

so the home agent knows where to send the forwarded datagrams. The home agent in turn

needs to communicate various types of information back to the mobile node when regis-

tration is performed. Note that the foreign agent is not really involved in registration, except

perhaps to relay messages, as we will see.

Mobile Node Registration Events

Successful registration establishes what is called in the standard a mobility binding between

a home agent and a mobile node. For the duration of the registration, the mobile node's

regular home address is tied to its current care-of address and the home agent will encap-

sulate and forward datagrams addressed to the home address over to the care-of address.

The mobile node is supposed to manage its registration and handle various events using

several actions:

☯ Registration: The mobile node initiates a registration when it first detects it has

moved from its home network to a foreign network.

☯ Deregistration: When the mobile node returns home, it should tell the home agent to

cancel forwarding, a process called deregistration.

☯ Reregistration: If the mobile node moves from one foreign network to another, or if its

care-of address changes, it must update its registration with the home agent. It also

must do so if its current registration is about to expire, even if it remains stationary on

one foreign network.

Each registration is established only for a specific length of time, which is why regular rereg-

istration is required whether the device moves or not. Registrations are time-limited to

ensure that they do not become stale. If, for example, a node forgets to de-register when it

returns home, the datagram forwarding will eventually stop when the registration expires.

New Registration Request and Registration Reply Messages

To perform registration, two new message types have been defined in Mobile IP: the Regis-

tration Request and the Registration Reply. Each of these does what you would expect from

its name. Interestingly, these are not ICMP messages like the ones used in agent discovery;

they are User Datagram Protocol (UDP) messages. Thus, technically speaking, registration

is performed at a higher layer than the rest of Mobile IP communication. Agents listen for

Registration Requests on well-known UDP port #434, and respond back to mobile nodes

using whatever ephemeral port the node used to send the message.

The formats of these two messages are described later in this topic.

Registration Procedures

There are two different procedures defined for registration, depending on the type of care-of

address used by the mobile node and other specifics we will get into shortly. The first is the

direct registration method, which has just two steps:

1. Mobile node sends Registration Request to home agent.

2. Home agent sends Registration Reply back to mobile node.

In some cases, however, a slightly more complex process is required, where the foreign

agent conveys messages between the home agent and the mobile node. In this situation,

the process has four steps:

1. Mobile node sends Registration Request to foreign agent.

2. Foreign agent processes Registration Request and forwards to home agent.

3. Home agent sends Registration Reply to foreign agent.

4. Foreign agent processes Registration Reply and sends back to mobile node.

The first, simpler method is normally used when a mobile node is using a co-located care-of

address. In that situation, the node can easily communicate directly with the home agent,

and the mobile node is also set up to directly receive information and datagrams from the

home agent. When there is no foreign agent, this is obviously the method that must be

used. It is also obviously the method used when a mobile node is de-registering with its

home agent after it arrives back on the home network.

The second method is required when a mobile node is using a foreign care-of address.

Recall that in this situation, the mobile node doesn't have its own unique IP address at all; it

is using a shared address given it by the foreign agent, which precludes direct communi-

cation between the node and the home agent. Also, if a mobile node receives an Agent

Advertisement with the “R” flag set, it also should go through the foreign agent, even if it has

a co-located care-of address.

Note that the foreign agent really is just a “middleman”; the exchange is still really between

the home agent and the mobile node. However, the foreign agent can deny registration if

the request violates whatever rules are in place for using the foreign network. It is for this

reason that some foreign agents may require that they be the conduit for registrations even

if the mobile node has a co-located care-of address. Of course, if the foreign agent can't

contact the home agent the registration will not be able to proceed.

The description above is really a highly simplified explanation of the basics of registration.

The Mobile IP standard specifies many more details on exactly how agents and nodes

perform registration, including particulars on when requests and replies are sent, how to

handle various special conditions such as invalid requests, rules for how home agents

maintain a table of mobility bindings, and much more. The standard covers the definition of

extensions to the regular registration messages to support authentication, which is required