Internet Engineering Task Force C. Perkins, editor
INTERNET DRAFT IBM
21 October 1994
IP Mobility Support
draft-ietf-mobileip-protocol-07.txt
Abstract
This document specifies protocol enhancements that allow transparent
routing of IP datagrams to mobile node 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.
Status of This Memo
This document is a submission to the Mobile-IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the mobile-ip@sunroof.eng.sun.com mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
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or ``work in progress.''
To learn the current status of any Internet-Draft, please check
the ``1id-abstracts.txt'' listing contained in the internet-drafts
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(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
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Contents
Abstract i
Status of This Memo i
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 . . . . . . . . . . . . . . . . . . . . . 6
2.2. Agent Solicitation . . . . . . . . . . . . . . . . . . . 6
2.3. Agent Advertisement . . . . . . . . . . . . . . . . . . . 7
3. Registration 9
3.1. Authentication . . . . . . . . . . . . . . . . . . . . . 10
3.2. Registration Request . . . . . . . . . . . . . . . . . . 10
3.3. Registration Reply . . . . . . . . . . . . . . . . . . . 12
4. Mobility Message Extensions 14
4.1. Mobility Extension . . . . . . . . . . . . . . . . . . . 15
4.2. Home Address Extension . . . . . . . . . . . . . . . . . 16
4.3. Key Identifier . . . . . . . . . . . . . . . . . . . . . 17
4.4. Mobile-Home Authentication Extension . . . . . . . . . . 17
4.5. Minimal Encapsulation Extension . . . . . . . . . . . . . 18
5. Forwarding Datagrams to the Mobile Node 19
5.1. IP in IP Encapsulation . . . . . . . . . . . . . . . . . 19
5.2. Minimal Encapsulation . . . . . . . . . . . . . . . . . . 19
6. Mobile Node Considerations 22
6.1. Configuration and Registration Tables . . . . . . . . . . 22
6.2. Registration When Away From Home . . . . . . . . . . . . 23
6.3. Registration without a foreign agent . . . . . . . . . . 23
6.4. De-registration When At Home . . . . . . . . . . . . . . 24
6.5. Registration Replies . . . . . . . . . . . . . . . . . . 25
6.6. Registration Retransmission . . . . . . . . . . . . . . . 25
6.7. Simultaneous Registrations . . . . . . . . . . . . . . . 26
6.8. Mobile Routers . . . . . . . . . . . . . . . . . . . . . 26
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7. Foreign Agent Considerations 27
7.1. Configuration and Registration Tables . . . . . . . . . . 28
7.2. Receiving Registration Requests . . . . . . . . . . . . . 28
7.3. Receiving Registration Replies . . . . . . . . . . . . . 28
7.4. Decapsulation . . . . . . . . . . . . . . . . . . . . . . 29
7.5. Mobility . . . . . . . . . . . . . . . . . . . . . . . . 29
8. Home Agent Considerations 30
8.1. Configuration and Registration Tables . . . . . . . . . . 30
8.2. Receiving Registration Requests . . . . . . . . . . . . . 30
8.3. Simultaneous Registrations . . . . . . . . . . . . . . . 31
8.4. Registration Expiration . . . . . . . . . . . . . . . . . 31
8.5. Encapsulation . . . . . . . . . . . . . . . . . . . . . . 32
8.6. Mobility . . . . . . . . . . . . . . . . . . . . . . . . 32
9. Security Considerations 33
9.1. Message Authentication Codes . . . . . . . . . . . . . . 33
9.2. Tunneling to Care-of Addresses . . . . . . . . . . . . . 33
9.3. Key management . . . . . . . . . . . . . . . . . . . . . 34
9.4. Picking good keys . . . . . . . . . . . . . . . . . . . . 34
9.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.6. Replay Protection for Registration Requests . . . . . . . 34
10. Acknowledgements 35
A. Gratuitous and Proxy ARP 36
B. Link-Layer considerations 37
B.1. Point-to-Point Link-Layers . . . . . . . . . . . . . . . 37
B.2. Multi-Point Link-Layers . . . . . . . . . . . . . . . . . 38
C. TCP Considerations 38
C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . . 38
C.2. TCP Congestion Management . . . . . . . . . . . . . . . . 38
D. Tunnel Management 39
Chair's Address 42
Editor's Address 42
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1. Introduction
Current versions of the Internet Protocol make an implicit assumption
that a node's point of attachment remains fixed. Datagrams are sent
to a node based on the location information contained in the node's
IP address.
If a node moves while keeping its IP address unchanged, its network
number will not reflect its new point of attachment. The routing
protocols will not be able to route datagrams to it correctly.
This document defines new functions that allow a node to roam on the
Internet, without changing its IP address.
The following entities are defined:
Mobile Node
A host or router that changes its point of attachment from one
network or subnetwork to another.
Home Agent
A router that maintains a registry of the current mobility
bindings for 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 that is
away from its home network.
Care-of Address
The care-of address terminates the end of a tunnel toward 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.
The following support services are defined:
Agent Discovery
Home agents and foreign agents advertise their availability
on each link for which they provide service. A newly arrived
mobile node can send a solicitation on the link to learn if any
prospective agents are present.
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Registration
When the mobile node is away from home, it registers a
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.
Encapsulation
Once a mobile node has registered a care-of address with its
home agent, that home agent intercepts datagrams destined for
the mobile node, builds another datagram with the intercepted
datagram enclosed within, and forwards the resulting datagram
to the entity at the care-of address.
Decapsulation
At the care-of address, the enclosed datagram is extracted.
When the mobile node receives packets sent to 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. Only a few administrative messages should be sent between a
mobile node and an agent. The size of these messages should be kept
as short as possible.
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As few messages as possible which duplicate functionality are sent
on mobile links. This is particularly important on wireless and
congested links.
1.3. Assumptions
The protocols defined in this document place no additional
requirements on assignment of IP addresses. That is, a mobile node
will be assigned an IP address by the organization that owns the
machine, and will be able to use that IP address regardless of the
current point of attachment.
It is assumed that mobile nodes are able to change their point of
attachment to the Internet no more frequently than once per second.
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.
It is assumed that IP datagrams are routed to a destination without
regard to the source of the datagram.
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.
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
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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:
Agent Advertisement
A periodic advertisement constructed by attaching a special
extension to a Router Advertisement [5] message.
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)(see [16], [12]).
Correspondent
A peer with which a mobile node is communicating. The
correspondent may be either mobile or stationary.
Home Address
A long-term IP 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; a link underlies the network
layer.
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Mobile Agent
Either a home agent or a foreign agent.
Mobility Binding
The association of a home address with a care-of address, and
the remaining lifetime of the association.
Mobility Security Association
The security relationship between two nodes that is used with
Mobile IP protocol messages. This relationship includes
the authentication type (including algorithm and algorithm
mode) and the secret (such as a shared key, or appropriate
public/private key pair).
Routing Prefix
The high-order bits in an address, which are used by routers to
locate a link for delivery of a datagram.
Source Address
An IP address belonging to the interface on which this message
is sent.
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2. Agent Discovery
To communicate with a foreign or home agent, a mobile node must
learn either the IP 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. After
establishing a link-layer connection, the mobile node learns whether
there are any agents available. If the address of any agent matches
the mobile node's stored address for its home agent, the mobile node
is at home.
An agent which is not identified by a link-layer protocol MUST
implement ICMP Router Discovery [5]. The Router Advertisements
indicate whether the router is also a home agent or a foreign 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
(see subsection 4.1) is envisioned for service providers with packet
filtering fire-walls, or visiting policies (such as accounting) which
require exchanges of authorization.
2.1. Authentication
No authentication is required for the advertisement and solicitation
process. These messages MAY be authenticated using the IP
Authentication Header [1], which is external to the messages
described here. Further work on authentication of advertisement and
solicitation is outside of the scope of this document.
Whenever an externally authenticated message fails authentication,
the message is silently discarded.
2.2. Agent Solicitation
Every mobile node MUST implement ICMP Router Solicitation if it needs
to obtain a care-of address in an agent advertisement. However, the
solicitation is only sent when no care-of address has been determined
through a link-layer protocol or prior Router Advertisement. Any
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foreign agent or home agent which is not identified by a link- layer
protocol MUST respond to ICMP Router Solicitation.
The same procedures, defaults, and constants are used as described
in "ICMP Router Discovery Messages" [5], except that the mobile
node may solicit more often than once every three seconds and
MAX_SOLICITATIONS does not apply for mobile nodes that are currently
unconnected to any foreign agent. A mobile node MAY send a
solicitation once each MOBILE_SOLICITATION_INTERVAL (1 second?) until
the solicitation is answered by a mobile agent, and the mobile node
can finally issue a registration request.
2.3. Agent Advertisement
Every mobile node MUST correctly process ICMP Router Advertisements.
Any foreign agent or home agent which is not identified by a link-
layer protocol MUST send ICMP Router Advertisements. An agent which
is identified by a link-layer protocol SHOULD also implement Router
Advertisements. However, the advertisements need not be sent, except
when the site policy requires registration with the agent, or as a
response to a specific solicitation.
The same procedures, defaults, and constants are used as described
in "ICMP Router Discovery Messages" [5], except as specified herein;
a foreign agent MUST NOT send Router Advertisements more often than
once per second.
The Router Advertisements are extended by examining the number of
advertised addresses. When the IP total length indicates that the
ICMP message is longer than needed for the number of addresses
present, the remainder is interpreted as extensions. The extensions
are described in section 4.
The Mobility Extension (subsection 4.1) is required, and indicates
that the router is an mobile agent. Other extensions indicate
optionally supported features (see, e.g., subsection 5.2).
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The Code field of the ICMP Router Advertisement is interpreted as
follows:
0 If the Mobility Extension is present, the router supports
mobility registration. The router also handles common
traffic -- that is, IP data packets not necessarily related
to mobile nodes.
16 A home or foreign agent which supports registration, but is
not routing common traffic.
A foreign agent includes the care-of address as a router address.
Upon receipt of an agent advertisement, a mobile node compares the
route address to that of the home agent(s) in its list. If there
is an exact match, the mobile node is at home. Otherwise, the
care-of address may be chosen from among advertising agents in the
same fashion as the mobile node would choose a first hop router. The
highest preference router address which falls within a subnet that
the mobile node has configured on its mobile interface(s) is used for
the care-of address.
It is very likely that no advertised routing prefix matches when the
mobile node is not at home. In this case, the highest preference
non-matching router address is 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.
DISCUSSION: What is this value?
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3. Registration
The registration function exchanges information between mobile
nodes and home agents. Registration creates a mobility binding,
associating the mobile node's home address with a care-of address
which can be used to reach the mobile node.
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 (see the description of the "F" bit, in subsection 4.1).
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 to the home agent, asking
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
mobile node to inform it of the disposition of its request.
The registration messages defined in this section(3.2, 3.3) use the
User Datagram Protocol header [18]. The UDP checksum is required.
Any mobility message with an incorrect or zero UDP checksum is
silently discarded.
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3.1. Authentication
Each mobile node, foreign agent, and home agent MUST support
the maintenance of an internal table holding a list of security
associations for mobile entities, indexed by their IP address.
Mobile node to home agent registration messages are required to be
authenticated with the Mobile-Home Authentication Extension (see
subsection 4.4). See section 9.1 for support requirements for
authentication algorithms.
Mobile-Foreign and Foreign-Home Authentications use the IP
Authentication Header [1].
Only one mobility security association at a time is in effect between
any given pair of participating nodes. Whenever a mobility security
association exists between a pair of nodes, all registration messages
between these nodes MUST be authenticated.
3.2. Registration Request
The registration request message is sent by a mobile node to its
home agent, so that the home agent can create a new mobility binding
for the mobile node (with a new lifetime). The registration may be
relayed to the home agent by the foreign agent from which the mobile
node is accepting service, or it may be sent directly in case the
mobile node has received a temporary care-of address by some other
means (e.g, DHCP [7]).
IP fields:
Source An IP address belonging to the interface on which
this message is sent.
A mobile node MUST use the transient care-of address
when assigned; otherwise, the home address is used.
Destination The IP address of the agent, when known.
When the IP address is unknown (the agent was
discovered via a link-layer protocol), the "All
Mobile Agents" multicast address (224.0.0.11). The
link-layer unicast address is used to deliver the
datagram to the correct agent.
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UDP fields:
Source Port variable
Destination Port 434
The UDP Header is followed by the Mobile-IP fields shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type 1
Code Optional capabilities:
0 remove prior registrations
1 retain prior registrations
Lifetime The number of seconds remaining before the
registration is considered expired. A value of
zero indicates a request for deregistration. A
value of all ones indicates infinity.
Home Agent The IP address of a home agent.
Care-of Address The IP address for the decapsulation end of a
tunnel.
Identification A 64-bit sequence number, assigned by the mobile
node, used to assist in matching requests with
replies, and in protecting against replay
attacks (see section 9.6).
The Home Address Extension (subsection 4.2) is required. The
Mobile-Home Authentication Extension (subsection 4.4) is required,
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and immediately follows all non-authentication extensions, except
those foreign agent specific extensions which may be added to the
packet after the mobile node computes the authentication.
3.3. Registration Reply
The registration reply message is returned by a home agent to a
mobile node which has sent a registration request (subsection 3.2)
message. If the mobile node is accepting service from a foreign
agent, that foreign agent will receive the reply from the home agent
and subsequently relay it to the mobile node. The reply message
contains the necessary codes to inform the mobile node about the
status of its request, along with the lifetime granted by the home
agent, which MAY be smaller than the original request. 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, another registration SHOULD occur
before the lifetime expires.
IP fields:
The source and destination IP addresses of the request message
are swapped for the reply message.
UDP fields:
The source port and destination port of the request message are
swapped for the reply message.
Note that the source IP address and the source UDP port of the
original registration request must be saved in order for the foreign
agent to return the reply to the correct mobile node UDP port.
DISCUSSION: I think this means that well-known port 435
will go unused.
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The UDP Header is followed by the Mobile-IP fields shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type 3
Code One of the following codes:
0 service will be provided.
Service denied by the foreign agent:
16 reason unspecified.
17 administratively prohibited.
18 insufficient resources.
19 mobile node failed authentication.
20 home agent failed authentication.
21 requested lifetime too long.
Service denied by the home agent:
32 reason unspecified.
33 administratively prohibited.
34 insufficient resources.
35 mobile node failed authentication.
36 foreign agent failed authentication.
Up-to-date values of the Code field are specified
in the most recent "Assigned Numbers" [20].
Lifetime The seconds remaining before the registration is
considered expired. A value of zero confirms a
request for de-registration. A value of all ones
indicates infinity.
Identification The registration identification is copied from
the request message.
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4. Mobility Message Extensions
Each message begins with a short fixed part, followed by one or more
mobility message extensions in type-length-value format. These
extensions may apply to agent advertisement messages (subsection 2.3)
and registration messages (section 3).
The Home Address Extension (subsection 4.2) is required. The
Mobile-Home Authentication Extension (subsection 4.4) is required,
and immediately follows all non-authentication extensions, except
those foreign agent specific extensions which may be added to the
packet when it is being relayed through a foreign agent. When
forwarded by a foreign agent, extensions which are specific to
the foreign agent are removed. All others are copied without
modification.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Extension | Length | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Extension
Current values are assigned as follows:
16 Mobility
17 Home Address
18 Key Identifier
32 Mobile-Home Authentication
64 Minimal Encapsulation
65 GRE Encapsulation (see [10])
Up-to-date values are specified in the most recent "Assigned
Numbers" [20].
Length
Indicates the length (in bytes) of the data field. The length
does not include the Extension and Length bytes.
Data
This field is zero or more bytes in length and contains the
value(s) for this extension. The format and length of the data
field is determined by the extension and length fields.
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Extensions allow variable amounts of information to be carried within
each datagram. The end of the list of extensions is indicated by the
total length of the IP datagram.
When an extension is encountered which is not recognized, it is
ignored. The length field of the extension is used to skip the data
field in searching for the next extension.
4.1. Mobility Extension
The Mobility Extension is used to indicate that a Router
Advertisement message is actually an agent advertisement being sent
by a home agent or foreign agent (see subsection 2.3). All agents
which send agent advertisements are presumed to be foreign agents.
When agents cannot accept new requests for service from mobile
clients, they will set the Busy bit; if the Busy bit is turned off,
the agent may attract new mobile clients. An agent which wishes to
serve only as a home agent, MUST set the Busy bit in addition to the
"H" bit in the mobility extension.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension | Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|B|H|F|reservd|
+-+-+-+-+-+-+-+-+
Extension 16
Length 3
Sequence Number Contains the number of advertisement messages
sent since the node was initialized.
F Foreign agent registration required bit. When
this bit is set to 1, the mobile node SHOULD
register through the foreign agent, even
when the mobile node has acquired a transient
care-of address.
B Busy bit. The agent is not willing to
accept any more registrations, even though it
continues to send advertisements with a positive
preference.
H Agent is offering service as a home agent.
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reservd Sent as zero; ignored on reception.
The sequence number MUST count the current advertisement; that
is, it begins with one (1) and wraps to zero (0). When this
value decreases, or the value one (1) follows any other value
than zero (0), the mobile node MUST assume that any current
registration has been lost. This field cannot roll over in less
than MIN_ADVERTISEMENT_INTERVAL*(2**16) seconds (more than 18 hours),
and rollover is unambiguously indicated by the value zero (0) and
followed by the value one (1).
4.2. Home Address Extension
The home address extension is be found in registration requests
(see subsection 3.2). This extension requests that packets for
entire subnets be tunneled and delivered to the mobile node, not just
packets for the mobile node's particular IP address. More than one
home address extension MAY be present. If any have nonzero prefix
size, a prefix is inferred by retaining the most significant bits
(specified by the prefix size) of the home address. Then the home
agent will intercept packets for any destinations that match that
prefix and tunnel them to the mobile node. After decapsulation, the
foreign agent will then deliver the packets to that mobile node in
its visitor list, which has been associated with the inferred prefix
matching the tunneled packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension | Length | reserved | Prefix Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Extension 17
Length 6
reserved Sent as zero; ignored on reception.
Prefix Size The size of the left-justified bit-mask that is
applied to the home address to determine the IP
routing prefix. Ranges from 0 to 30. Set to zero
by mobile nodes which are not routers.
Home Address The IP address of the mobile node.
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4.3. Key Identifier
The key identifier extension is be found in registration requests
(see subsection 3.2). This extension informs the home agent that
authentication is performed using a cryptographic key or algorithm
different than the home agent would use by default. If a home
agent receives a registration request which does not contain
this extension, the home agent assumes that the mobile node used
the default Message Authentication Code (see subsection 9.1) to
authenticate the registration.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension | Length | Key Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Extension 18
Length 4
reserved Sent as zero; ignored on reception.
Key Identifier
The key identifier may be chosen from a list which is privately
configured between the home agent and the mobile node. In this case,
the identifier is completely opaque; no information about which
cryptographic algorithm to be used is obvious from the specific value
of the key identifier.
However, it is also possible for the home agent and mobile node
to agree that the value of the key identifier corresponds to the
identifier of the cryptographic algorithm used, according to a table
to be established and kept by IANA, the Internet Assigned Numbers
Authority. So, if there have not been any specific values for key
identifier configured between the home agent and the the mobile
host, the home agent will understand the key identifier to select a
particular cryptographic algorithm and mode. See subsection 9.1 for
a primitive list of algorithms and modes.
4.4. Mobile-Home Authentication Extension
This extension is found in all registration requests and replies,
and is intended to eliminate problems which are well known to result
from the uncontrolled propagation of remote redirects in the Internet
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(section 9). See subsection 9.1 for information about support
requirements for message authentication codes, etc.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension | Length | Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Extension 32
Length The number of data bytes in the extension.
Authenticator (variable length) A hash value taken over a stream
of bytes including the shared secret, all prior
extensions in their entirety, and the type and
length of this extension, but not including the
Authenticator field itself.
4.5. Minimal Encapsulation Extension
The Minimal Encapsulation Extension is found in agent advertisements
(subsection 2.3) and registration requests (subsection 5.2).
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Extension 64
Length 0
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5. Forwarding Datagrams to the Mobile Node
5.1. IP in IP Encapsulation
Support for IP in IP encapsulated datagrams is required.
An outer, full-sized IP fragmentation header is inserted before the
datagram's IP header:
+---------------------------+
| Outer IP Header |
+---------------------------+ +---------------------------+
| IP Header | | IP Header |
+---------------------------+ ====> +---------------------------+
| | | |
| IP Payload | | IP Payload |
| | | |
+---------------------------+ +---------------------------+
The format of the IP header is described in [19]. The outer IP
header source and destination addresses identify the "endpoints" of
the tunnel. The inner IP header source and destination addresses
identify the sender and recipient of the datagram.
The protocol field in the outer IP header is set to protocol number
4 for the encapsulation protocol. The destination field in the
outer IP header set to the care-of address of the mobile node. The
source field in the outer IP header is set to the IP address of the
encapsulating agent.
When the datagram is encapsulated, the Time To Live (TTL) field in
the outer IP header is set to be the same as the original datagram.
When decapsulating, the outer IP TTL minus one is inserted into the
inner IP TTL. Thus, IP hops are counted, but the actual routers
interior to the tunnel are not identified.
5.2. Minimal Encapsulation
A minimal forwarding header is defined for datagrams which are not
fragmented prior to encapsulating. When a datagram is already
fragmented prior to encapsulating, IP in IP is used.
Use of this encapsulating method is optional.
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The minimal header is inserted between the datagram's IP header and
the rest of the datagram:
+---------------------------+ +---------------------------+
| IP Header | | Modified IP Header |
+---------------------------+ ====> +---------------------------+
| | | Forwarding Header |
| IP Payload | +---------------------------+
| | | |
+---------------------------+ | IP Payload |
| |
+---------------------------+
A foreign agent which is capable of decapsulating the minimal header
will include the Minimal Encapsulation Extension (subsection 4.5) in
its Router Advertisements.
A mobile node indicates the capability of decapsulating the minimal
header at the care-of address by the inclusion of the Minimal
Encapsulation Extension in its registration request.
The Minimal Encapsulation Extension is not included in the
registration reply. The use of the minimal header is entirely at the
discretion of the home agent.
The format of the minimal forwarding header is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol |S| reserved | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Correspondent Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Protocol
Copied from the protocol field in the original IP header.
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S
Source field present bit, which indicates that the
Correspondent Source Address field is present.
0 not present.
1 present.
reserved
Sent as zero; ignored on reception.
Header Checksum
The 16-bit one's complement of the one's complement sum of the
encapsulation header. For computing the checksum, the checksum
field is set to 0.
Home Address
Copied from the destination field in the original IP header.
Correspondent Source Address
Copied from the source field in the original IP header.
Present only if the S-bit is set.
The protocol field in the IP header is replaced by protocol number 55
for the minimal encapsulation protocol. The destination field in the
IP header is replaced by the care-of address of the mobile node. If
the encapsulating agent is not the original source of the datagram,
the source field in the IP header is replaced by the IP address of
the encapsulating agent. The Don't Fragment bit is set in the IP
header.
When decapsulating a datagram, the fields in the forwarding header
are restored to the IP header, and the forwarding header is removed
from the datagram.
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6. 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.
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 create a mobility binding by registering with its home agent
(see subsections 3.2, 2.3). If the mobile node is returning to its
home network, it MUST deregister with its home agent. A mobile node
will operate without the support of mobility functions when it is at
home.
See appendix B for some considerations regarding the interaction of
this mobility specification with features commonly found in link
layer implementations for media which may be used with mobile nodes.
Receipt of an ICMP Redirect from a registered agent MUST NOT affect
the choice of agent for re-registrations. ICMP Redirect only affects
the choice of preferred router for forwarding decisions.
If desired, the Mobile Node can create a tunnel to its Home Agent.
The definition of such a mechanism is outside the scope of this
document.
DISCUSSION: Why are we keeping this here?
Any node can do encapsulation!
Does anyone remember the motivation for it?
6.1. Configuration and Registration Tables
Each mobile node will need:
- home address
- prefix size
- one or more home agents
- mobility security association for each home agent
For each pending registration:
- MAC address of agent
- care-of address
- registration identification
- lifetime
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6.2. Registration When Away From Home
In the absence of link-layer indications of changes in point of
attachment, agent advertisements from new agents do not necessarily
affect a current registration. In the absence of link-layer
indications, a mobile node must not attempt to register more often
than once per second. A mobile node may register with a different
agent when:
- transport-layer protocols indicate excessive retransmissions.
- its 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.
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 MUST register again. Such a reduction
does not include the wrap of the Sequence Number to zero.
A mobile node SHOULD NOT request a lifetime for its registration that
exceeds 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)
may be used. The lifetime MAY be modified by the home agent in its
reply.
A mobile node SHOULD register again before the lifetime of its
registration expires. The mobile node MAY register again 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. See also subsection 8.2.
6.3. Registration without a foreign agent
In cases where a mobile node away from home is able to dynamically
acquire a transient IP address (e.g, DHCP [7]), the mobile node
can serve without a foreign agent, using the transient address as
the care-of address. Then all communication between the mobile
node and its home agent can proceed without the intervention of
foreign agents. This eliminates the need to deploy foreign agents as
separate entities.
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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 can initiate
acquisition of a new transient address each time such a change
occurs.
The lifetime of such a registration is chosen by the mobile node.
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
(see subsection 2.3), the mobile node SHOULD register through an
appropriate foreign agent, even when it might otherwise be able to
register directly with a home agent.
6.4. De-registration When At Home
At times, a mobile node might become attached to its home link, and
will no longer need any forwarding service from its home agent. A
deregistration procedure MUST be used between the mobile node and its
home agent.
The deregistration 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.
The mobile node is not required to register with its home agent.
It MAY deregister each foreign agent, or it MAY allow its mobility
bindings to simply expire.
DISCUSSION: This appears to contradict the combination
of the previous MUST, and the specification of the
deregistration procedure.
It is not necessary to register again 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.
procedure MUST be used between the mobile node and its home agent.
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6.5. Registration Replies
In order for a mobile node to accept a registration reply, the
reply must have the same registration identification of its most
recent registration request to the sender; otherwise, 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
supersedes 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.
DISCUSSION: Is this a reference to the need for resynchronization?
Otherwise, when a reply is received with an invalid Authenticator,
the message is silently discarded.
The mobile node is not required to issue any message in response to a
registration reply.
6.6. Registration Retransmission
When no reply has been received within a reasonable time, the
registration request is retransmitted. A new registration
identification is chosen for each retransmission.
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.
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.
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The maximum retransmission time SHOULD be no greater than the
lifetime of the registration request.
6.7. Simultaneous Registrations
Multiple simultaneous registrations are useful in several situations,
for example when a mobile node is on a border between multiple
cellular systems.
IP explicitly allows duplication of datagrams. When the home agent
allows simultaneous registrations, it 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.
In order to request this optional capability, 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. When the need for multiple
registrations has passed, the mobile node SHOULD register again with
the Code set to 0, to remove the other registrations.
6.8. 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 router registers through a foreign agent. This
allows a foreign agent to recognize all addresses attached to the
mobile node when they are decapsulated at the care-of address (see
subsection 4.2).
When a transient IP 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 deregisters with the home
agent, it participates directly in routing with other routers in its
home routing domain.
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7. Foreign Agent Considerations
The role of the foreign agent is passive and minimal, relaying
registration requests between the home agent and the mobile node, and
decapsulating datagrams for delivery to the mobile node.
When no mobility security association exists, this also reduces the
risks resulting from absence of authentication from foreign agent
messages.
DISCUSSION: Does anyone know why this is here?
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.
The foreign agent SHOULD NOT advertise the presence of the mobile
node which is a router to other routers in its routing domain.
The 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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7.1. 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 containing:
- Media address of mobile node
- home address
- prefix size
- home agent
- registration identification
- 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.
7.2. 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 forwards 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.
7.3. 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
accordingly.
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7.4. Decapsulation
Every foreign agent MUST examine all arriving encapsulated traffic
and compare the destination address to those entries in its visitor
list, considering both the home address and routing prefix for the
entries (see subsection 4.2), in order to forward to the correct
mobile node.
When the destination does not match any node currently in the visitor
list, the datagram SHOULD be silently discarded. The datagram MUST
NOT be further forwarded without modifications to the original IP
header, because otherwise a routing loop is likely to result. ICMP
Destination Unreachable MUST NOT be sent when a foreign agent is
unable to forward a datagram.
7.5. 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.
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8. 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, if the home network is not merely a
virtual network.
The home agent SHOULD advertise the presence of the mobile node which
is a router to other routers in its routing domain.
8.1. Configuration and Registration Tables
Each home agent will need:
- an IP address
- prefix size for the home network, if any
DISCUSSION: If the home agent controls a virtual home network,
the home agent does NOT need an IP address on the
virtual home network.
For each authorized mobile node, the home agent will need:
- home address
- mobility security association
- prefix size(s) for the mobile network(s), if any
For each registered mobile node, the home agent will need a
forwarding list entry containing:
- care-of address
- registration identification
- lifetime
8.2. Receiving Registration Requests
Upon receipt of a registration request (subsection 3.2), the
home agent grants or denies the service requested by sending a
registration reply (subsection 3.2) to the sender of the request,
with the appropriate code set. If service permission is granted, the
home agent will update its forwarding list with the care-of address
of the tunnel.
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The request is validated by checking that the registration
identification is not the same as a preceding request, and the
Mobile-Home Authentication Extension (subsection 4.4) is correct.
Other Authentication Extensions are also validated when present.
When a registration request is invalid, a registration reply is
sent with the appropriate error code. This reply will be used by a
foreign agent to clear its pending request list, if a foreign agent
was involved in relaying the registration request.
The home agent MAY shorten the lifetime of the request.
A mobile node requests termination of service by indicating a
lifetime of zero. If the Code field set to 1, the home agent removes
the mobility binding for that care-of address from its forwarding
list. Otherwise, if the Code field is 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 visitor lists are allowed to expire naturally.
8.3. 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 (subsection 3.3) 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.
DISCUSSION: Does anyone know why no status
indication can be returned?
8.4. Registration Expiration
If the lifetime for a given mobile node expires before the home agent
has received a another 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 visitor lists will expire naturally, and probably at
the same time.
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8.5. 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 that 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. Other previously encapsulated datagrams, which are
not associated with the routing prefix, are recursively encapsulated.
8.6. 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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9. 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.
9.1. Message Authentication Codes
Home agents and mobile nodes MUST be able to perform keyed MD5 [21],
with a key size of 128 bits, for authentication, with the key
prefixed and suffixed to the data to be hashed. 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, key
distribution methods, and key sizes MAY also be supported.
Here is a primitive list of algorithms and modes which may be used by
home agents and mobile nodes.
0 128+ Keyed-MD5 with both suffix and prefix
1 128+ Keyed-MD5 with suffix
3 160 Keyed-SHA with both suffix and prefix
4 160 Keyed-SHA with suffix
6 160 Keyed-SHA1 with both suffix and prefix
7 160 Keyed-SHA1 with suffix
16 56 DES (Block cipher in MAC mode)
17 168 Triple-DES (Block cipher in MAC mode)
24 ? RC2 Variable Key Size symmetric
block cipher in MAC mode
DISCUSSION: If anyone knows of a better list described in
an existing RFC, please let me know
9.2. Tunneling to Care-of Addresses
The registration protocol described in this document will result
in a mobile node's traffic being tunneled to its care-of address.
This tunneling feature could be a significant vulnerability if the
registration were not authentic. Such remote redirection, for
instance as performed by the mobile registration protocol, is widely
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understood to be a security problem in the current Internet [2].
Moreover, the Address Resolution Protocol (ARP) is not authenticated,
and can potentially be used to steal another host's traffic. The use
of "Gratuitous ARP"(see Appendix A) brings with it all of the risks
associated with the use of ARP.
9.3. Key management
This specification requires 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.
9.4. Picking good keys
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 authorized parties) and
pseudo-random. Eastlake, et.al. [8] provides more information on
generating pseudo-random numbers.
9.5. Privacy
Users who have sensitive data that they do not wish others to see
should use mechanisms outside the scope of this document (such as
encryption) to provide appropriate protection. Users concerned about
traffic analysis should consider appropriate use of link encryption.
9.6. Replay Protection for Registration Requests
A Network Time Protocol [15] formatted value is preferred. The
low-order 32 bits of the NTP format represent fractional seconds,
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and those bits which are not available from a time source SHOULD be
generated from a good source of randomness.
A battery-backed clock is the usual source of this value. In more
robust implementations, Global Positioning System or authenticated
NTP values MAY be used. The elapsed time since system startup or
another such monotonically increasing counter MAY be used, but is
considered less secure, as it could repeat each time the machine
is restarted, or when a poor source of randomness is used for the
low-order bits. See Eastlake, et.al. [8] for a discussion of sources
of randomness.
The value MUST NOT be the same as an immediately preceding request,
and SHOULD NOT repeat during the lifetime of the mobility security
association between the mobile node and the home agent.
It is possible to use an entirely random "nonce" in this field, or to
generate nonces from previous registration exchanges.
DISCUSSION: The use of nonces for replay
protection may depend partially on the resolution
of a patent issue. Moreover, there is a problem
with interoperability; a mobile node and its home
agent must agree on the use of nonces, because if
a home agent expects only a nonce, it is unlikely
that the mobile node's time value will be accepted.
10. Acknowledgements
Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
and John Ioannidis (Columbia), for forming the working group,
chairing it, and putting so much effort into its early development.
Thanks also to Greg Minshall for his contributions to the group while
performing the duties of chairperson.
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Thanks to the active members of the Working Group, particularly those
who contributed text, including (in alphabetical order)
- Ran Atkinson (Naval Research Lab),
- Dave Johnson (Carnegie Mellon University),
- Andrew Myles (Macquarie University),
- John Penners (US West),
- Al Quirt (Bell Northern Research),
- Yakov Rekhter (IBM), and
- Fumio Teraoka (Sony).
Thanks to Charlie Kunzinger, the editor who produced the first drafts
for the Working Group, and to Bill Simpson, who has produced most of
the text of this draft, reflecting the discussions of the Working
Group.
Thanks to Greg Minshall (Novell) and Phil Karn (Qualcomm) for their
generous support in hosting interim Working Group meetings.
A. Gratuitous and Proxy ARP
Many people will use their computers for extended periods of time
on a single link, whether or not it is at their home network. When
doing so, they will expect the same level of service from their
infrastructure as they receive today on the home network.
A separate "virtual" IP address block is not required for mobile
nodes. This would require a small network to have an extra router
between the mobile and non-mobile nodes, which is an unacceptable
expense.
This section details the special care to be taken when using ARP [17]
with nodes on the same link as a mobile node.
A problem can arise if a mobile node which has previously answered an
ARP Request moves away from the link, leaving behind a stale entry in
another node's ARP cache. For example, if a router which forwards
datagrams into the home network has a stale ARP cache entry for the
mobile node, any datagrams arriving through that router for the
mobile node will be lost. Thus, it is important that ARP caches of
nodes populating the link be updated as soon as possible.
A gratuitous ARP is an ARP Reply that is broadcast to all nodes on
a link, which is not in response to any ARP Request. When an ARP
Reply is broadcast, all hosts are required to update their local
ARP caches, whether or not the ARP Reply was in response to an ARP
Request they had issued.
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When there is a physical link which corresponds to the home network,
a gratuitous proxy ARP is issued by the home agent on behalf of a
mobile node whenever the home agent receives a valid registration.
The gratuitous proxy ARP will indicate that all remaining nodes
should associate the home address of the mobile node with the
link-layer address of the home agent which is now serving the mobile
node.
In the gratuitous ARP, the source IP address is the home address, the
MAC address is the source link-layer address for the interface used,
the target IP address would be the all-systems multicast address, and
the target link-layer address would be the general broadcast address.
While the mobile node is away from its home network, the home agent
performs proxy ARP Replies for the mobile node. When a mobile node
returns to its home network, it SHOULD issue a gratuitous ARP on its
own behalf, immediately before sending the de- registration request
to the home agent.
Although the gratuitous ARP can be lost, this is not different from
the usual ARP Reply problems, which are outside the scope of this
document. A home agent may repeat the gratuitous ARP a small number
of times to allow for the possibility of the first packet getting
lost.
B. Link-Layer considerations
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 [13], and changes in cell or
administration. The mechanisms will be specific to the particular
link-layer technology, and are outside the scope of this document.
B.1. Point-to-Point Link-Layers
The Point-to-Point-Protocol (PPP) [22] and its Internet Protocol
Control Protocol (IPCP) [14], negotiates the use of IP addresses.
The mobile node SHOULD first attempt to specify its home address.
This allows an unrouted link to function correctly.
When the home address is not accepted by the peer, but a transient
IP address is dynamically assigned, that address MAY be used as the
care-of address for registration.
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When the peer specifies its own IP address, that address MUST NOT
be assumed to be the care-of address of a foreign agent or the IP
address of a home agent.
DISCUSSION: Can anyone explain better what's going on here?
When Router Advertisements are received which contain the Mobility
Extension, registration with the agent SHOULD take place as usual.
If the link is bandwidth limited, this method is preferred over use
of the transient care-of address, The encapsulation will be removed
by the peer, allowing header compression techniques to function
correctly [11].
B.2. Multi-Point Link-Layers
Another link establishment protocol, IEEE 802.11 [6], might yield the
link address of an agent. This link-layer address SHOULD be used to
attempt registration.
The receipt of a Router Advertisement supersedes the link-layer
address, and a new registration MUST occur.
C. TCP Considerations
C.1. 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 medium 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.
C.2. TCP Congestion Management
Mobility nodes are likely to use media which have low bandwidth and
are more likely to introduce errors, effectively causing more packets
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to be dropped. This introduces a conflict with the mechanisms for
congestion management found in modern versions of TCP. Now, when
a packet is dropped, the correspondent's TCP implementation is
likely to react as if there were a source of network congestion, and
initiate the slow-start mechanisms [4] designed for controlling that
problem. However, those mechanisms are inappropriate for overcoming
errors introduced by the links themselves, and have the effect of
magnifying the discontinuity introduced by the dropped packet. This
problem has been analyzed by Caceres, et. al. ( [3]); there is
no easy solution available, and certainly no solution likely to
be installed soon on all correspondents. While this problem has
nothing to do with any of the specifications in this document, it
does illustrate that providing performance transparency to mobile
nodes involves understanding mechanisms outside the network layer.
It also indicates the need to avoid designs which systematically drop
packets; such designs might otherwise be considered favorably when
making engineering tradeoffs.
D. 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 IP 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, ICMP only requires IP routers to return 8 bytes (64
bits) of the datagram beyond the IP 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
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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.
DISCUSSION: Would anyone like to provide more explanation?
Or, should we just delete most of it
and be satisfied with a reference in the
section about Home Agent Considerations?
Tunnel soft state was originally developed for the IP address
encapsulation (IPAE) specification [9].
References
[1] R. Atkinson. SIPP Authentication Header. Internet Draft --
work in progress, April 1994.
[2] S.M. Bellovin. Security Problems in the TCP/IP Protocol Suite.
ACM Computer Communications Review, 19(2), March 1989.
[3] Ramon Caceres and Liviu Iftode. The Effects of Mobility on
Reliable Transport Protocols. In Proceedings of the 14th
International Conference on Distributed Computing Systems, June
1994.
[4] Douglas E. Comer. Internetworking with TCP/IP, volume 1.
Prentice Hall, 1991.
[5] S. Deering. Router Discovery. RFC 1256, September 1991.
[6] Wim Diepstraten, Greg Ennis, and Phil Belanger. DFWMAC -
Distributed Foundation Wireless Medium Access Control. IEEE
Document P802.11-93/190, Nov 1993.
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[7] R. Droms. Dynamic Host Configuration Protocol. RFC 1541,
October 1993.
[8] D.E. Eastlake, S.D. Crocker, and J.I. Schiller. Randomness
Requirements for Security. Internet Draft -- work in progress,
October 1994.
[9] R. Gilligan, E. Nordmark, and B. Hinden. IPAE: The SIPP
Interoperability and Transition Mechanism. Internet Draft --
work in progress, March 1994.
[10] S. Hanks, T. Li, D. Farinacci, and P. Traina. Generic routing
encapsulation (gre). draft-hanks-gre-00.txt -- work in
progress, October 1994.
[11] V. Jacobson. Compressing TCP/IP Headers for Low-Speed Serial
Links. RFC 1144, February 1990.
[12] J. Kohl and C. Newman. The Kerberos Network Authentication
Service (V5). RFC 1510, September 1993.
[13] K. McCloghrie and F. Kastenholz. Evolution of the Interfaces
Group MIP-II. RFC 1573, January 1994.
[14] G. McGregor. The PPP Internet Procotol Control Protocol (IPCP).
RFC 1332, May 1992.
[15] D. Mills. Network Time Protocol (Version 3). RFC 1305, March
1992.
[16] National Bureau of Standards. Data Encryption Standard.
Federal Information Processing Standards, 1977.
[17] D. Plummer. An Ethernet Address Resolution Protocol. RFC 826,
November 1982.
[18] J. Postel. User Datagram Protocol. RFC 768, August 1980.
[19] J. Postel. Internet Protocol. RFC 791, September 1981.
[20] J. Reynolds and J. Postel. Assigned Numbers. RFC 1700, October
1994.
[21] R. Rivest. The MD5 Message-Digest Algorithm. RFC 1321, April
1992.
[22] W. Simpson (Editor). The Point-to-Point Protocol (PPP). RFC
1661, July 1994.
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Chair's Addresses
The working group can be contacted via the current chairs:
Kannan Alagappan Tony Li
170 W. Tasman Dr.
San Jose CA 95134
Work: +1 222 3334444 Work: +1 408 5268186
E-mail: kannan@emc.com E-mail: tli@cisco.com
Editor's Address
Questions about this memo can also be directed to:
Charles Perkins
Room J1-A25
T. J. Watson Research Center
IBM Corporation
P. O. Box 218
Yorktown Heights, NY 10598
Work: +1 914 7847350
Fax: +1 914 7847007
E-mail: perk@watson.ibm.com
Perkins, editor Expires 21 April 1994 [Page 42]
