Network Working Group W A Simpson
Internet Draft Daydreamer
expires in six months November 1994
IPv6 Mobility Support
draft-simpson-ipv6-mobility-00.txt
Status of this Memo
This document is a submission to the IPv6 Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the ipng@sunroof.eng.sun.com mailing list.
Distribution of this memo is unlimited.
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Abstract
This document specifies protocol enhancements that allow transparent
routing of IPv6 datagrams to Mobile Nodes in the Internet. The
Mobile Node is always identified by its Home-Address, regardless of
its current point of attachment to the Internet. While situated away
from its home, a Mobile Node is also associated with a Care-Of-
Address, which provides information about its current point of
attachment to the Internet. The protocol provides for registering
the Care-Of-Address with a Home Agent. The Home Agent sends traffic
destined for the Mobile Node through a tunnel to the Care-Of-Address.
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1. Introduction
The following actors participate in roaming on the Internet:
Mobile Node
A host or router that changes its point of attachment from one
link to another.
Home Agent
A router that advertises reachability for a Mobile Node, maintains
a registry of the current point of attachment of that Mobile Node,
and encapsulates datagrams for delivery to the Mobile Node while
it is away from home.
Foreign Agent
A router that assists a locally reachable Mobile Node while it is
away from home.
The following support services are needed:
Agent Discovery
All Agents advertise their availability on each link for which
they provide service. Since all agents are routers, this is
provided by the Router Advertisement.
A Mobile Node which changes its point of attachment can send a
Router Solicitation to learn if any routers are present.
Care-Of-Address Assignment
The Care-Of-Address identifies the point of attachment of a Mobile
Node. Depending on the foreign network configuration, the Care-
Of-Address may be either dynamically assigned to the Mobile Node
or associated with a Foreign Agent.
Registration
When the Mobile Node is away from home, it registers the Care-Of-
Address with a Home Agent.
Depending on its method of attachment, the Mobile Node will
register either directly with a Home Agent, or through a Foreign
Agent which forwards the registration to the Home Agent.
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Encapsulation
Once a Mobile Node has registered a Care-Of-Address with a Home
Agent, the Home Agent intercepts datagrams destined for the Mobile
Node, and forwards the resulting datagram to the Care-Of-Address.
Decapsulation
At the Care-Of-Address, the enclosed datagram is extracted.
When the Mobile Node has its own Care-Of-Address, it decapsulates
its own datagrams.
When the Care-Of-Address is associated with a Foreign Agent, the
Foreign Agent decapsulates the datagrams. If the datagram is
addressed to a Mobile Node which the Foreign Agent is currently
serving, it will deliver the datagram to the Mobile Node.
1.1. Requirements
A Mobile Node using its Home-Address shall be able to communicate
with other nodes after having been disconnected from the Internet,
and then reconnected at a different point of attachment.
A Mobile Node shall continue to be capable of communicating directly
with existing nodes which do not implement the mobility functions
described in this document.
A Mobile Node shall provide authentication in its registration
messages.
1.2. Goals
The Mobile Node's directly attached link is likely to be bandwidth
limited. Few administrative messages are sent between a Mobile Node
and an Agent. The size of these messages are kept as short as
possible.
As few messages as possible which duplicate functionality are sent on
mobile links. This is particularly important on low bandwidth and
congested links.
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1.3. Assumptions
The protocols defined in this document place no additional
requirements on assignment of Internet Addresses. That is, a Mobile
Node will be assigned an Internet Address by the organization that
owns the machine, and will be able to use that Internet Address
regardless of the current point of attachment.
Mobile Nodes are able to change their point of attachment to the
Internet no more frequently than once per 4 seconds, which is also
the default frequency of advertisements [D-Send].
Changes in topology which occur more frequently must be handled at
the link layer transparently to the internetwork layer. It is
further noted that engineering margins may require the link layer to
handle all changes at a frequency in the neighborhood of 10 seconds.
No protocol enhancements are required in hosts or routers that are
not serving any of the mobility functions. Similarly, no additional
protocols are needed by a router (that is not acting as a Home Agent
or a Foreign Agent) to route datagrams to or from a Mobile Node.
The operation of this specification assumes that Internet datagrams
are routed to a Destination without regard to the Source of the
datagram.
If desired, the Mobile Node can create tunnel(s) to its Home Agent.
Such mechanisms are beyond the scope of this document.
1.4. Specification Language
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.
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MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
silently discard
The implementation discards the packet without further
processing, and without indicating an error to the sender.
The implementation SHOULD provide the capability of logging
the error, including the contents of the discarded packet,
and SHOULD record the event in a statistics counter.
1.5. Terminology
This document frequently uses the following terms:
Authentication Type
This includes the algorithm and algorithm mode. Note that
a single algorithm (such as DES) might have several modes
(for example, CBC and ECB).
Correspondent
A peer with which a Mobile Node is communicating. The
Correspondent may be either mobile or stationary.
Home-Address
A long-term Internet Address that is assigned to a Mobile
Node. It remains unchanged regardless of where the node is
attached to the Internet. Datagrams addressed to the
Home-Address are intercepted by the Home Agent while the
Mobile Node is registered with that Home Agent.
Link A communication facility or medium over which nodes can
communicate at the link layer; underlying the network
layer.
Mobility Binding
The association of a Home-Address with a Care-Of-Address,
and the remaining LifeTime of the association.
Routing Prefix
The high-order bits in an address, which are used by
routers to locate a link for delivery of a datagram.
Mobility Security Association
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The security relationship between two nodes that is used
with Mobile Registration messages. This relationship
includes the authentication type (including algorithm and
algorithm mode), the secret (such as a shared key, or
appropriate public/private key pair), and possibly other
information such as labelling.
Triangle Routing
A path followed by a datagram destined for a Mobile Node,
when that datagram arrives first at the Home Agent, and
then is encapsulated and forwarded by the Home Agent.
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2. Agent Discovery
To communicate with a Foreign or Home Agent, a Mobile Node must learn
either the Internet Address or the link address of that Agent.
It is assumed that a link-layer connection has been established
between the Agent and the Mobile Node. The method used to establish
such a link-layer connection is not specified in this document.
Recommended link-layer facilities are described in the Appendices.
After establishing a link-layer connection that supports the
attachment of Mobile Nodes, the node learns whether there are any
Agents available. If the Home Agent is recognized, the Mobile Node
is at home.
All Agents MUST implement Router Advertisements [D-Send]. The Router
Advertisements indicate whether the router is also an Agent.
When multiple methods of Agent identification are in use, the Mobile
Node SHOULD first attempt registration with routers sending Router
Advertisements in preference to those sending link-layer
advertisements. This ordering maximizes the likelihood that the
registration will be recognized, thereby minimizing the number of
registration attempts.
An Administrative Domain MAY require registration with a Foreign
Agent even when another registration method is in use. This facility
is envisioned for service providers with packet filtering fire-walls,
or visiting policies (such as accounting) which require exchanges of
authorization.
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2.1. Authentication
No authentication is required for the advertisement and solicitation
process.
These messages MAY be authenticated using the IPv6 Authentication
Header [IPv6-AH].
Whenever an externally authenticated message fails authentication,
the message is silently discarded.
There is the potential for a key management problem:
- if the Mobile Node doesn't know the authentication type and key
used by the advertiser.
- if the Foreign Agent doesn't know the authentication type and key
used by the Mobile Host.
This key management issue is simplified when asymmetric
authentication algorithms are used, because each node's public
authentication key can published without enabling masquerading
attacks. However, asymmetric algorithms are often more
computationally intensive than symmetric algorithms.
2.2. Agent Solicitation
Every Mobile Node is required to implement IPv6 Router Solicitation
[D-Send].
However, the Router Solicitation is only sent when no Care-Of-Address
has been determined through a link-layer protocol or prior Router
Advertisement.
All Foreign Agents and Home Agents MUST respond to Router
Solicitations.
The same procedures, defaults, and constants are used as described in
[D-Send].
2.3. Agent Advertisement
Every Mobile Node is required to correctly process IPv6 Router
Advertisements.
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All Foreign Agents and Home Agents MUST implement IPv6 Router
Advertisements.
When an Agent is identified by a link-layer protocol, the Router
Advertisements need not be sent, except when the site policy requires
registration with the Agent, or as a response to a specific Router
Solicitation.
The same procedures, defaults, and constants are used as described in
[D-Send], except as specified herein.
The Mobility Extension is required, and indicates that the router is
an Agent. Other extensions indicate optionally supported features.
The Mobile Node examines the Router Advertisement. If any Routing-
Information extension exactly matches a Home Agent in its list, the
Mobile Node is at home.
Otherwise, the Care-Of-Address is chosen from among advertising
Agents in the same fashion as the Mobile Node would choose a first
hop router.
If a Cluster-prefix exactly matches the Home-Address prefix extracted
by the same Prefix-Size, then that router is one of the preferred
routers for that Home-Address. The Mobile Node selects the highest
preference such IPv6 Cluster for the Care-Of-Address.
It is very likely that no Cluster-prefix matches when the Mobile Node
is not at home. In this case, the highest preference non-matching
Router Identifying-Address and Prefix-Size is used to calculate the
IPv6 Cluster-Address to be used for the Care-Of-Address.
A Home Agent which does not provide Foreign Agent services will have
preference values less than the highest Foreign Agent preference.
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3. Registration
The registration function exchanges information between Mobile Nodes
and Home Agents. This function creates a Mobility Binding, linking
the Home-Address with a Care-Of-Address to be used to reach the
Mobile Node.
3.1. Direct
When assigned a transient Care-Of-Address, a Mobile Node can act
without a Foreign Agent, and register or de-register directly with a
Home Agent. This registration process involves the exchange of only
2 messages:
a) The Mobile Node sends a Registration Request to a Home Agent,
to ask that Home Agent to provide the requested service.
b) The Home Agent sends a Registration Reply to the Mobile Node to
grant or deny service.
An Administrative Domain MAY require registration through a Foreign
Agent, as indicated in Agent Advertisements.
This method may also be less desirable when the link is low
bandwidth. The encapsulation will not be removed on the final hop.
3.2. Relayed
When the Care-Of-Address is associated with a Foreign Agent, the
Foreign Agent acts as a relay between the Mobile Node and Home Agent.
This extended registration process involves the exchange of 4
messages:
a) The Mobile Node sends a Registration Request to the prospective
Foreign Agent to begin the registration process.
b) The Foreign Agent relays the request by sending a Registration
Request to the Home Agent, to ask that Home Agent to provide
the requested service.
c) The Home Agent sends a Registration Reply to the Foreign Agent
to grant or deny service.
d) The Foreign Agent sends a copy of the Registration Reply to the
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Mobile Node to inform it of the disposition of its request.
3.3. Authentication
Each Mobile Node, Foreign Agent, and Home Agent MUST support an
internal table holding a list of Internet Addresses, and the Mobility
Security Association for each address.
Mobile Node to Home Agent registration messages are required to be
authenticated with the Mobile-Home Authentication Extension. The
Mobile Node and Home Agent MUST support authentication using keyed
MD5 and key sizes of 128 bits or greater, with manual key
distribution. Additional authentication algorithms, algorithm modes,
and key distribution methods MAY also be supported.
In addition, the Foreign Agent SHOULD support authentication using
keyed MD5 and key sizes of 128 bits or greater, with manual key
distribution. Additional authentication algorithms, algorithm modes,
and key distribution methods MAY also be supported.
Mobile-Foreign and Foreign-Home Authentication use the IPv6
Authentication Header [IPv6-AP].
Only one Mobility Security Association exists between any given pair
of participating nodes at any given time.
Whenever a Mobility Security Association exists between a pair of
nodes, all registration messages between these nodes MUST be
authenticated.
3.4. ICMP Message Formats
The Packet format and basic facilities are already defined for ICMP
as modified for IPv6 [IPv6-ICMP].
The Mobility Registration and Reply message formats are documented in
[D-Form].
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4. Forwarding Datagrams to the Mobile Node
Support for IPv6 in IPv6 encapsulated datagrams is required.
+---------------------------+
| Tunnel IPv6 Header |
+---------------------------+ +---------------------------+
| IPv6 Header | | IPv6 Header |
+---------------------------+ ====> +---------------------------+
| | | |
| IPv6 Payload | | IPv6 Payload |
| | | |
+---------------------------+ +---------------------------+
The format of the IPv6 Header is described in [IPv6-Base]. The outer
IPv6 Source and Destination identify the "endpoints" of the tunnel.
The inner IPv6 Source and Destination identify the sender and
recipient of the datagram.
The Protocol field in the outer IPv6 header is set to Payload number
41 for the direct IPv6 encapsulation.
The outer IPv6 Header Destination is set to the Care-Of-Address of
the Mobile Node.
The outer IPv6 Header Source is set to the Internet Address of the
encapsulating agent.
When the datagram is encapsulated, the outer IPv6 Header Hop Count
field is set to be the same as the original datagram.
When decapsulating, the outer Hop Count minus one is inserted into
the inner Hop Count.
4.1. Tunnel Management
It is possible that one of the routers along the tunnel interior
might encounter an error while processing the datagram, causing it to
return an ICMP error message to the source end of the tunnel. The
three types of ICMP errors that can occur in this circumstance are:
- Datagram too big.
- Time Exceeded.
- Destination Unreachable.
Unfortunately, IPv6 ICMP only requires routers to return 8 bytes (64
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bits) of the datagram beyond the IPv6 header. This is not enough to
include the encapsulated header, so it is not generally possible for
the Home Agent to immediately reflect the ICMP message from the
interior of a tunnel back to the source host.
However, by carefully maintaining "soft state" about its tunnels, the
encapsulating router can return accurate ICMP messages in most cases.
The router SHOULD maintain at least the following soft state
information about each tunnel:
- MTU of the tunnel.
- TTL (path length) of the tunnel
- Reachability of the end of the tunnel.
The router uses the ICMP messages it receives from the interior of a
tunnel to update the soft state information for that tunnel. When
subsequent datagrams arrive that would transit the tunnel, the router
checks the soft state for the tunnel. If the datagram would violate
the state of the tunnel (such as, the TTL is less than the tunnel
TTL) the router sends an ICMP error message back to the source, but
also forwards the datagram into the tunnel.
Using this technique, the ICMP error messages sent by encapsulating
routers will not always match up one-to-one with errors encountered
within the tunnel, but they will accurately reflect the state of the
network.
The Don't Fragment bit is always set within the tunnel. This enables
the proper MTU of the tunnel to be determined.
Fragmentation which occurs because of the size of the encapsulation
header is done before encapsulation, preventing more than one layer
of fragmentation in a single datagram.
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5. Mobile Node Considerations
A Mobile Node listens for Agent Advertisements at all times that it
has a link connection. In this manner, it can learn that its Foreign
Agent has changed, or that it has arrived home. The determination
that the point of attachment is at home or away from home is entirely
at the discretion of the Mobile Node, based on the information
obtained from Agent Advertisements.
Whenever a Mobile Node detects a change in its point of attachment,
it MUST initiate the registration process. If it is away from home,
it MUST either register through a Foreign Agent where required, or
directly with a Home Agent. If it is returning home, it MUST de-
register with its Home Agent.
A Mobile Node will operate without the support of mobility functions
when it is at home.
The Mobile Node primarily uses link-layer mechanisms to decide that
its point of attachment has changed. Such indications include the
Down/Testing/Up interface status [RFC-1573], and changes in cell or
administration. The mechanisms will be specific to the particular
link-layer technology, and are beyond the scope of this document.
In the absence of link-layer indications of changes in point of
attachment, Agent Advertisements from new Agents do not affect a
current registration. A Mobile Node which has already registered
MUST NOT register with a different Agent until:
a) transport-layer protocols indicate excessive re-transmissions.
b) the current Registration LifeTime has expired.
The Mobile Node MUST NOT register with a new Agent simply because a
higher preference Agent has appeared, or the preference values change
for the Agent with which it is currently registered. The preference
value is used only for initial selection of an Agent.
Receipt of a Local Redirect from a registered Agent MUST NOT affect
the choice of Agent for re-registrations. Local Redirect only
affects the choice of preferred router for forwarding decisions.
5.1. Configuration and Registration Tables
Each Mobile Node will need:
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- Home-Address
- Prefix-Size
- one or more Home Agents
For each pending registration:
- Media Address of Agent
- Care-Of-Address
- Identification used
- LifeTime
For each Mobility Security Association:
- Authentication Type
- Authentication Key
5.2. Registration When Away From Home
If a Mobile Node detects a reduction in the Sequence Number of an
Agent Advertisement from a Foreign Agent through which it has
registered, the Mobile Node SHOULD re-register. Such a reduction
does not include the wrap of the Sequence Number to zero.
The LifeTime of the registration SHOULD NOT be set to greater than
the LifeTime learned in an Agent Advertisement. When the method by
which the Care-Of-Address is learned does not include a LifeTime, the
default Router Advertisement LifeTime (1800 seconds) is used.
The LifeTime MAY be modified by the Home Agent in its reply.
A Mobile Node SHOULD re-register before the LifeTime of its
registration expires. The Mobile Node MAY re-register at any time.
A Mobile Node MAY ask a Home Agent to terminate forwarding service to
a particular Care-Of-Address, by sending a registration with a
LifeTime of zero.
5.3. Registration without a Foreign Agent
In cases where a Mobile Node away from home is able to dynamically
acquire a transient Internet Address, the Mobile Node can serve
without a Foreign Agent, using the transient address as the Care-Of-
Address.
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This feature MUST NOT be used unless the Mobile Node has mechanisms
to detect changes in its link-layer connectivity, and to initiate
acquisition of a new transient address each time such a change
occurs.
In these cases, all communication between the Mobile Node and its
Home Agent is direct. This eliminates the need to deploy separate
entities as Foreign Agents.
The LifeTime of such a registration is chosen by the Mobile Node. By
default, the Router Advertisement LifeTime (1800 seconds) is used.
The LifeTime MAY be modified by the Home Agent in its reply.
However, on those links where the Mobile Node detects an Agent
Advertisement that has the "F" bit set in the Mobility Extension, the
Mobile Node SHOULD register through an appropriate Foreign Agent,
even when it could otherwise register directly with a Home Agent.
5.4. De-registration When At Home
At times, a Mobile Node might attach itself to its home link. Since
a Mobile Node that is at home needs no forwarding, a de-registration
procedure MUST be used between the Mobile Node and its Home Agent.
The de-registration process involves the exchange of only two
messages:
a) The Mobile Node sends a Registration Request directly to its
Home Agent, with the LifeTime set to zero, and the Code field
set to 0, to indicate that the Home Agent remove all related
entries.
b) The Home Agent sends a Registration Reply to the Mobile Node to
grant or deny service.
In this special case, the Care-Of-Address is set to the Home-Address.
This procedure is specified for the sake of convenience. The Mobile
Node is not required to register with its Home Agent. It MAY de-
register each Foreign Agent, or it MAY allow its Mobility Bindings to
simply expire.
It is not necessary to re-register with a Home Agent when a change of
Sequence Number occurs, or the Advertisement LifeTime expires, since
the Mobile Node is not seeking encapsulating service.
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5.5. Registration Replies
When a Mobile Node receives a Registration Reply which has an
Identification which is not the same as the Identification of its
most recent Registration Request to the putative sender, the message
is silently discarded.
When a Reply is received which has a Code indicating information from
the Foreign Agent, the Mobile-Home Authenticator will be missing or
invalid. However, if no other reply has as yet been received, the
reason for denial SHOULD be accepted, and result in an appropriate
action. If a later authenticated reply is received, that reply
supercedes the unauthenticated reply.
When a Reply is received which has a Code indicating that
authentication failed with the Home Agent, the reason for denial
SHOULD result in an appropriate action.
Otherwise, when a Reply is received with an invalid Authenticator,
the message is silently discarded.
When the LifeTime of the reply is greater than the original request,
the excess time SHOULD be ignored. When the LifeTime of the reply is
smaller than the original request, re-registration SHOULD occur
before the LifeTime expires.
The Mobile Node is not required to issue any message in reply to a
Registration Reply.
5.6. Registration Retransmission
When no Reply has been received within a reasonable time, the
Registration Request is re-transmitted. A new Identification is
chosen for each retransmission.
The preferred technique is to re-register each time a new Agent
Advertisement is received. By default, the advertisements occur at
1/3 the LifeTime. This gives sufficient protection from missed
advertisements, or lost registration requests and replies.
The minimum retransmission time SHOULD be related to the speed of the
link. The minimum value SHOULD be large enough to account for the
size of the packets, twice the round trip time for transmission at
the link speed, and at least an additional 100 milliseconds to allow
for processing the packets before responding. Some circuits add
another 200 milliseconds of satellite delay.
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The initial time MUST NOT be less than 1 second. At 9,600 bps or
less, the recommended initial time is 3 seconds. At 1,200 bps or
less, the recommended initial time is 5 seconds.
Each successive value less than the maximum value SHOULD be at least
twice the previous value.
The maximum retransmission time SHOULD be no greater than the
LifeTime of the Registration Request.
5.7. Simultaneous Registrations
Under normal circumstances, sending a new Registration Request
removes other unexpired registrations for a Mobile Node from the Home
Agent.
An optional capability is to allow multiple simultaneous
registrations. For example, this is particularly useful when a
Mobile Node is on a border between multiple cellular systems.
In order to request simultaneous registrations, the Mobile Node sends
the Registration Request with the Code set to 1.
The return Code in the Registration Reply is the same. No error
occurs if the Home Agent is unable to fulfill the request.
IPv6 explicitly allows duplication of datagrams. When the Home Agent
is able to fulfill the request, the Home Agent will encapsulate a
separate copy of each arriving datagram to each Care-Of-Address, and
the Mobile Node will receive multiple copies of its datagrams.
When the need for multiple registrations has passed, the Mobile Node
SHOULD re-register with the Code set to 0, to remove the other
registrations.
5.8. Mobile Routers
A Mobile Node can be a router, which is responsible for the mobility
of an entire network moving together, such as on an airplane, a ship,
a train, an automobile, a bicycle, or a kayak.
Provision for a Routing-Prefix in registration messages is needed
when a Mobile Node registers through a Foreign Agent. This allows a
Foreign Agent to recognize all addresses attached to the Mobile Node
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when they are decapsulated at the Care-Of-Address.
When a transient Internet Address has been assigned, the Mobile Node
can register directly with the Home Agent, as described previously.
Such a Mobile Node MAY advertise to other routers in the foreign
routing domain.
The Mobile Node MAY register multiple times with different Home-
Addresses and Routing-Prefixes. This permits multiple prefixes to be
routed through the Mobile Node.
When the Mobile Node returns home, and de-registers with the Home
Agent, it participates directly in routing with other routers in its
home routing domain.
6. Foreign Agent Considerations
It is the intent that Foreign Agent involvement be as minimal as
possible. The role of the Foreign Agent is passive, passing
registration requests to the Home Agent, and decapsulating datagrams
to pass to the Mobile Node.
When no Mobility Security Association exists, this also reduces the
risks resulting from absence of authentication from Foreign Agent
messages.
The Foreign Agent MUST NOT originate a Request or Reply that has not
been prompted by the Mobile Node. No Request or Reply is generated
to indicate that the service LifeTime has expired.
A Foreign Agent MUST NOT originate a message which revokes the
registration of a different Foreign Agent. A Foreign Agent SHOULD
forward such revocations without modification when such revocation
messages originated from an appropriate Mobile Node or Home Agent.
The Foreign Agent SHOULD NOT advertise the presence of the Mobile
Node which is a router to other routers in its routing domain.
The Agent Advertisement preference is used to regulate the number of
Mobile Nodes which register with the Foreign Agent. When the Foreign
Agent would otherwise need to reject new registrations because of
insufficient resources, the Foreign Agent SHOULD reduce its
preference values until resources become available.
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6.1. Configuration and Registration Tables
Each Foreign Agent will need:
- Care-Of-Address
For each pending or current registration, the Foreign Agent will need
a Visitor List entry:
- Media Address of Mobile
- Home-Address
- Prefix-Size
- Home Agent
- Identification used
- LifeTime
A Foreign Agent that has implemented and is using authentication will
also need to have the Mobility Security Association information for
each pending or current authenticated registration. Even if a
Foreign Agent implements authentication, it might not use
authentication with each registration, because of the key management
difficulties.
6.2. Receiving Registration Requests
Upon receipt of a Registration Request, if the Foreign Agent is
unable to satisfy the request for some reason, then the Foreign Agent
sends a Registration Reply to the Mobile Node with an appropriate
Code, and does not forward the Request to the Home Agent. Otherwise,
the Foreign Agent will forward the Request to the Home Agent.
The Foreign Agent must maintain a list of pending Requests, which
includes the IP Source Address and UDP Source Port, in order that a
correctly addressed Reply can be returned to the Mobile Node.
6.3. Receiving Registration Replies
The fields of the Registration Reply MUST be examined for validity.
A Registration Reply which does not relate to a pending Registration
Request, or to a currently registered Mobile Node, is silently
discarded.
If the Registration Reply granted permission to provide service to
the Mobile Node, then the Foreign Agent updates its Visitor List
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accordingly.
6.4. Decapsulation
Every Foreign Agent MUST examine all arriving encapsulated traffic
for both the Home-Address and Routing-Prefix in order to forward to
the correct Mobile Node.
When the Destination does not match any node currently in the Visitor
List, the datagram MUST be silently discarded (rather than being
further forwarded). IPv6 Destination Unreachable MUST NOT be sent
when a Foreign Agent is unable to forward a datagram.
6.5. Mobility
The Foreign Agent can be mobile, if the link identified by the Care-
Of-Address is mobile. The Foreign Agent could be either a node on a
mobile network, or another Mobile Node itself.
7. Home Agent Considerations
It is the intent that the Home Agent have primary responsibility for
processing and coordinating mobility services.
The Home Agent for a given Mobile Node SHOULD be located on the link
identified by the Home-Address. This link MAY be virtual.
The Home Agent SHOULD advertise the presence of the Mobile Node which
is a router to other routers in its routing domain.
7.1. Configuration and Registration Tables
Each Home Agent will need:
- an IPv6 Address
- Prefix-Size for the Home Network, if any
For each authorized Mobile Node, the Home Agent will need:
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- Home-Address
- Prefix-Size for the Mobile Network, if any
For each registered Mobile Node, the Home Agent will need a
Forwarding List entry:
- Care-Of-Address
- Identification used
- LifeTime
For each Mobility Security Association:
- Authentication Type
- Authentication Key
7.2. Receiving Registration Requests
Upon receipt of a Registration Request, the Home Agent grants or
denies the service requested by sending a Registration Reply to the
sender of the request, with the appropriate Code set.
The Request is validated by checking that the Identification is not
the same as a preceeding Request, and the Mobile-Home Authentation
Extension is correct. Other Authentication Extensions are also
validated when present.
The Home Agent MAY shorten the LifeTime of the request.
If service permission is granted, the Home Agent will update its
Forwarding List with the Care-Of-Address of the tunnel.
If the Request asks for termination of service by indicating a
LifeTime of zero, and the Code field set to 1, the Home Agent removes
the Mobility Binding for that Care-Of-Address from its Forwarding
List.
If the Request asks for termination of service by indicating a
LifeTime of zero, and the Code field set to 0, the Home Agent removes
the Mobility Bindings for all Foreign Agents associated with that
Mobile Node from its Forwarding List.
On termination, no special Reply is sent to additional associated
Foreign Agents. The entries in their Visiting Lists are allowed to
expire naturally.
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7.3. Receiving Requests through a Foreign Agent
When a Registration Request is invalid, a Reply is sent to the
Foreign Agent, in order that the Foreign Agent can clear its pending
request list.
7.4. Simultaneous Registrations
When a Home Agent supports the optional capability of multiple
simultaneous registrations, any datagrams forwarded are simply
duplicated, and a copy is sent to each Care-Of-Address.
The return Code in the Registration Reply is the same. No error
occurs if the Home Agent is unable to fulfill the request, and
earlier entries in the Forwarding List are removed.
7.5. Registration Expiration
If the LifeTime for a given Mobile Node expires before the Home Agent
has received a re-registration request, then the associated Mobility
Binding is erased from the Forwarding List.
No special Registration Reply is sent to the Foreign Agents. The
entries in the Visiting Lists will expire naturally, and probably at
the same time.
7.6. Encapsulation
Every Home Agent MUST examine all arriving traffic for both the
Home-Address and Routing-Prefix in order to forward to the correct
Mobile Node.
When previously encapsulated datagrams arrive which are associated
with the Routing-Prefix of the Mobile Node, the Home Agent simply
alters the Destination to the Care-Of-Address. This avoids recursive
encapsulation.
Previously encapsulated datagrams which are not associated with the
Routing-Prefix are recursively encapsulated.
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7.7. Mobility
The Home Agent can be mobile, if the link identified by the Home-
Address it serves is mobile. The Home Agent could be either a node
on a mobile network, or another Mobile Node itself.
A datagram would be encapsulated on its way to the mobile network,
decapsulated for delivery to the Mobile Node, intercepted by the Home
Agent, and re-encapsulated to the Mobile Node.
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A. Point-to-Point Link-Layers
The Point-to-Point-Protocol (PPP) [RFC-1548] Internet Protocol
Control Protocol (IPCP) [RFC-1332], does not yet negotiate the use of
IPv6 addresses.
Instead, IPv6 Neighbor Discovery [D-Send] is used to exchange
identities with the peer. IPv6 Router Advertisements indicate
whether the router is also an Agent.
When a transient IPv6 Unicast Address is dynamically assigned, that
address MAY be used as the Care-Of-Address in registration.
B. Multi-Point Link-Layers
Another link establishment protocol, IEEE 802.11, might yield the
link address of an Agent. This link-layer address SHOULD be used to
attempt registration.
The receipt of a Router Advertisement supercedes the link-layer
address, and a new registration MUST occur.
C. TCP Timers
Most hosts and routers which implement TCP/IP do not permit easy
configuration of the TCP Timer values. When high-delay (e.g. SATCOM)
or low-bandwidth (e.g. High-Frequency Radio) links are in use, the
default TCP Timer values in many systems will cause retransmissions
or timeouts when the link and network is actually operating properly,
though with greater than usual delays because of the media in use.
This can cause an inability to create or maintain connections over
such links, and can also cause unneeded retransmissions which consume
already scarce bandwidth. Vendors are encouraged to make TCP Timers
more configurable. Vendors of systems designed for the mobile
computing markets should pick default timer values more suited to
low-bandwidth, high-delay links. Users of Mobile Nodes should be
sensitive to the possibility of timer-related difficulties.
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Security Considerations
The mobile computing environment is potentially very different from
the ordinary computing environment. In many cases, mobile computers
will be connected to the network via wireless links. Such links are
particularly vulnerable to passive eavesdropping, active replay
attacks, and other active attacks.
The registration protocol described here will result in a host's
traffic being source routed to its mobile location. Such traffic
redirection could be a significant vulnerability when the
registration were not authentic. Also, source routing is widely
understood to be a security problem in the current Internet.
[Bellovin89] The Address Resolution Protocol (ARP) is not
authenticated, and can potentially be used to steal another host's
traffic.
This specification includes a strong authentication mechanism (keyed
MD5) which precludes many potential attacks based on the Mobile IP
registration protocol. However, because key distribution is
difficult in the absence of a network key management protocol, not
all messages with the Foreign Agent are authenticated.
Vulnerabilities remain in the registration protocol whenever a
registration message is not authenticated. For example, in a
commercial environment it might be important to authenticate all
messages between the Foreign Agent and the Home Agent, so that
billing is possible, and service providers don't provide service to
users that are not legitimate customers of that service provider.
The strength of any authentication mechanism is dependent on several
factors, including the innate strength of the authentication
algorithm, the secrecy of the key used, the strength of the key used,
and the quality of the particular implementation. This specification
requires implementation of keyed MD5 for authentication, but does not
preclude the use of other authentication algorithms and modes. For
keyed MD5 authentication to be useful, the 128-bit key must be both
secret (that is, known only to authorised parties) and pseudo-random.
[Eastlake] provides more information on generating pseudo-random
numbers.
Users who have sensitive data that they do not wish others to see
should use mechanisms (such as encryption) to provide appropriate
protection. Users concerned about traffic analysis should consider
appropriate use of link encryption.
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References
[Atkinson] Atkinson, R., "Authentication Header", work in
progress.
[Bellovin89] Bellovin, S.M., "Security Problems in the TCP/IP
Protocol Suite", ACM Computer Communications Review,
Vol. 19, No. 2, March 1989.
[Eastlake] Eastlake, D.E.3, S.D. Crocker, J.I. Schiller,
"Randomness Requirements for Security", work in
progress.
[Voydock83] Voydock, V.L., S.T. Kent, "Security Mechanisms in
High-level Networks", ACM Computing Surveys, Vol.
15, No. 2, June 1983.
[RFC-768]
[RFC-791]
[RFC-826]
[RFC-1122]
[RFC-1144]
[RFC-1256]
[RFC-1310]
[RFC-1305]
[RFC-1321]
[RFC-1332]
[RFC-1573]
[RFC-1661]
Acknowledgements
Special thanks to John Ioannidis (Columbia University), for his
inspiration and experimentation which began this most recent round of
IP mobility development.
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Special thanks also to Steve Deering (Xerox PARC), for his early
support as Chair of the Simple-IP and Mobile-IP Working Groups.
Charlie Perkins (IBM) coalesced the terms of Home and Foreign Agents.
Security details are primarily the work of Randall Atkinson (Naval
Research Laboratory).
Tunnel soft state was originally developed for the "IP Address
Encapsulation (IPAE)" specification, by Robert E. Gilligan, Erik
Nordmark, and Bob Hinden (all of Sun Microsystems).
Much of the text of this specification is derived from earlier drafts
by Charlie Kunzinger (IBM), and the verbose members of the Mobile-IP
Working Group who contributed text, including Dave Johnson (Carnegie
Mellon University), Tony Li (Cisco Systems), Andrew Myles (Macquarie
University), John Penners (US West), Fumio Taraoka (Sony), and John
Zao (Harvard).
Finally, the Editor wishes to thank Phil Karn (Qualcomm), whose
decade of IP mobility experimentation in the amateur radio community,
and widespread freeware dissemination of his KA9Q software, provided
the impetus and availability for many thousands throughout the world
to join the Internet community.
Author's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
Bill.Simpson@um.cc.umich.edu
bsimpson@MorningStar.com
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Table of Contents
1. Introduction .......................................... 1
1.1 Requirements .................................... 2
1.2 Goals ........................................... 2
1.3 Assumptions ..................................... 3
1.4 Specification Language .......................... 3
1.5 Terminology ..................................... 4
2. Agent Discovery ....................................... 6
2.1 Authentication .................................. 7
2.2 Agent Solicitation .............................. 7
2.3 Agent Advertisement ............................. 7
3. Registration .......................................... 9
3.1 Direct .......................................... 9
3.2 Relayed ......................................... 9
3.3 Authentication .................................. 10
3.4 ICMP Message Formats ............................ 10
4. Forwarding Datagrams to the Mobile Node ............... 11
4.1 Tunnel Management ............................... 11
5. Mobile Node Considerations ............................ 13
5.1 Configuration and Registration Tables ........... 13
5.2 Registration When Away From Home ................ 14
5.3 Registration without a Foreign Agent ............ 14
5.4 De-registration When At Home .................... 15
5.5 Registration Replies ............................ 16
5.6 Registration Retransmission ..................... 16
5.7 Simultaneous Registrations ...................... 17
5.8 Mobile Routers .................................. 17
6. Foreign Agent Considerations .......................... 18
6.1 Configuration and Registration Tables ........... 19
6.2 Receiving Registration Requests ................. 19
6.3 Receiving Registration Replies .................. 19
6.4 Decapsulation ................................... 20
6.5 Mobility ........................................ 20
7. Home Agent Considerations ............................. 20
7.1 Configuration and Registration Tables ........... 20
7.2 Receiving Registration Requests ................. 21
7.3 Receiving Requests through a Foreign Agent ...... 22
7.4 Simultaneous Registrations ...................... 22
7.5 Registration Expiration ......................... 22
7.6 Encapsulation ................................... 22
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7.7 Mobility ........................................ 23
APPENDICES ................................................... 24
A. Point-to-Point Link-Layers ............................ 24
B. Multi-Point Link-Layers ............................... 24
C. TCP Timers ............................................ 24
SECURITY CONSIDERATIONS ...................................... 25
REFERENCES ................................................... 25
ACKNOWLEDGEMENTS ............................................. 26
AUTHOR'S ADDRESS ............................................. 27