draft-ietf-mobileip-ipv6-00.txt   draft-ietf-mobileip-ipv6-01.txt 
Mobile IP Working Group David B. Johnson
IPv6 Working Group Charles Perkins INTERNET-DRAFT Carnegie Mellon University
INTERNET DRAFT IBM Corporation Charles Perkins
David B. Johnson IBM Corporation
Carnegie Mellon University 13 June 1996
26 January 1996
Mobility Support in IPv6 Mobility Support in IPv6
<draft-ietf-mobileip-ipv6-00.txt> <draft-ietf-mobileip-ipv6-01.txt>
Abstract Abstract
This document specifies mobility messages that allow transparent This document specifies the operation of mobile computers using IPv6.
routing of IP datagrams to mobile nodes in the Internet. Each Each mobile node is always identified by its home address, regardless
mobile node is always identified by its home address, regardless of of its current point of attachment to the Internet. While situated
its current point of attachment to the Internet. While situated away from its home, a mobile node is also associated with a care-of
away from its home, a mobile node is also associated with a address, which provides information about the mobile node's current
care-of address, which provides information about its current location. IPv6 packets addressed to a mobile node's home address are
point of attachment to the Internet. The protocol provides for transparently routed to its care-of address. The protocol enables
notifying the mobile node's home agent, and any other interested IPv6 IPv6 nodes to cache the binding of a mobile node's home address with
addressable entities, about the care-of address of the mobile node. its care-of address, and to then send packets destined for the mobile
When necessary, the home agent sends packets destined for the mobile node directly to it at this care-of address.
node through a tunnel to the care-of address. After arriving at the
end of the tunnel, the packets are then delivered to the mobile node.
Status of This Memo Status of This Memo
This document is a submission by the Mobile IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the Working Group mailing list at "mobile-ip@SmallWorks.COM".
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
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Contents
Abstract i
Status of This Memo i
1. Introduction 1
1.1. Design Requirements . . . . . . . . . . . . . . . . . . . 2
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 2
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 2
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.6. Specification Language . . . . . . . . . . . . . . . . . 5
2. Overview of Mobile IPv6 Operation 7
3. Message and Option Formats 9
3.1. Binding Update Option . . . . . . . . . . . . . . . . . . 9
3.2. ICMP Binding Acknowledgement Message . . . . . . . . . . 13
4. Requirements for IPv6 Nodes 15
5. Binding Cache Management 17
5.1. Receiving Binding Updates . . . . . . . . . . . . . . . . 17
5.2. Requests to Cache a Binding . . . . . . . . . . . . . . . 17
5.3. Requests to Delete a Binding . . . . . . . . . . . . . . 18
5.4. Sending Binding Acknowledgements . . . . . . . . . . . . 18
5.5. Cache Replacement Policy . . . . . . . . . . . . . . . . 19
5.6. Receiving ICMP Error Messages . . . . . . . . . . . . . . 19
6. Mobile Node Considerations 21
6.1. Movement Detection . . . . . . . . . . . . . . . . . . . 21
6.2. Forming New Care-of Addresses . . . . . . . . . . . . . . 23
6.3. Sending Binding Updates to the Home Agent . . . . . . . . 24
6.4. Sending Binding Updates to Correspondent Nodes . . . . . 25
6.5. Sending Binding Updates to the Previous Default Router . 25
6.6. Rate Limiting for Sending Binding Updates . . . . . . . . 26
6.7. Receiving Binding Acknowledgements . . . . . . . . . . . 26
6.8. Using Multiple Care-of Addresses . . . . . . . . . . . . 27
6.9. Returning Home . . . . . . . . . . . . . . . . . . . . . 28
7. Home Agent Considerations 29
7.1. Home Agent Care-of Address Registration . . . . . . . . . 29
7.2. Home Agent Care-of Address De-registration . . . . . . . 31
7.3. Delivering Packets to a Mobile Node . . . . . . . . . . . 32
7.4. Renumbering the Home Network . . . . . . . . . . . . . . 32
8. Correspondent Node Considerations 34
8.1. Delivering Packets to a Mobile Node . . . . . . . . . . . 34
9. Authentication and Replay Protection 36
10. Routing Multicast Packets 37
11. Constants 38
Acknowledgements 38
References 39
A. Open Issues 40
A.1. Session Keys with Local Routers . . . . . . . . . . . . . 40
A.2. Source Address Filtering by Firewalls . . . . . . . . . . 40
Chair's Address 42
Authors' Addresses 42
1. Introduction 1. Introduction
A new version of the Internet Protocol, IPv6, is being developed with This document specifies the operation of mobile computers using
128-bit addresses, which remedies perceived flaws with the existing Internet Protocol Version 6 (IPv6) [6]. Mobile computers are likely
version (that is, IPv4). This document specifies messages and a to account for a majority or at least a substantial fraction of
simple protocol for the operation of mobile computers for IPv6. the population of the Internet during the lifetime of IPv6. The
Mobile computers are likely to account for a substantial fraction of protocol, known as Mobile IPv6, allows transparent routing of IPv6
the population of the Internet during the lifetime of IPv6. packets to mobile nodes using the mobile node's home IPv6 address,
regardless of the mobile node's current point of attachment to the
Internet.
The development of IPv6 presents a rare opportunity, in that there The most important function needed to support such routing to mobile
is no existing installed base of IPv6 hosts or routers with which nodes is the reliable and timely notification of a mobile node's
compatibility must be maintained, and all IPv6 nodes may be assumed current location to those other nodes that need it. Correspondent
to perform the few operations needed to support Internet-wide nodes communicating with a mobile node need this location information
mobility. The most important function needed to support mobility in order to correctly deliver their own packets to a mobile node;
is the reliable and timely notification of a mobile node's current Mobile IPv6 allows correspondent nodes to learn and cache a mobile
location those other nodes that need it. The home agent needs this node's location, and to use this cached information to route their
location information in order to forward intercepted packets from the own packets directly to a mobile node at its current location. The
home network to the mobile node, and correspondent nodes need this mobile node's "home agent", a router on the mobile node's home
information in order to send their own packets directly to the mobile network, also needs this location information in order to forward
node. intercepted packets from the home network to the mobile node, for
correspondent nodes that have not yet learned the mobile node's
location, and indeed, for correspondent nodes that do not even yet
know that the mobile node is currently away from home.
In this document, we specify the way that the mobile node can notify A mobile node's current location is represented as a "care-of
other nodes about its current whereabouts, using a Destination option address", an IPv6 address assigned to the mobile node (in addition
which fits naturally in IPv6. We describe the mechanism by which a to its home IPv6 address) within the foreign network currently being
routing header can be used to deliver packets to the mobile node at visited by the mobile node. The association between a mobile node's
its current whereabouts. All IPv6 nodes and routers are assumed to home address and its care-of address, along with the remaining
perform the few operations required for mobility, since doing so adds lifetime of that association, is known as a "binding", and the mobile
little additional overhead. This leads to dramatic simplifications node notifies other nodes about its current binding using a new
in the required protocols, compared to the methods required for IPv4. destination option called a Binding Update. IPv6 correspondent nodes
then use a Routing header to deliver subsequent packets to the mobile
node's care-of address. All IPv6 nodes and routers MUST be able to
cache mobile node bindings received in Binding Updates; this leads to
dramatic simplifications in the required protocols, compared to the
methods required for IPv4.
2. Basic Operation In this document, "movement" is considered to be a change in a mobile
node's point of attachment to the Internet such that it is no longer
link-level connected to the same IPv6 subnet (network prefix) as
it was previously. If a mobile node is not currently link-level
connected to its home IPv6 network, the mobile node is said to be
"away from home".
From the model of operation developed for enabling mobile networking 1.1. Design Requirements
for IPv4, we borrow the concepts of home network, home address,
home agent, care-of address, and binding. Mobile computers will
have assigned to their interface(s) (at least) two IPv6 addresses
whenever they are roaming away from their home network. One (the
home address) is permanent; the other (the IPv6 link-local address)
is used temporarily. In addition, the mobile node will typically
autoconfigure a globally-routable address at each new point of
attachment [12]. Every IPv6 router supports encapsulation, so every
router is capable of serving as a home agent on the network(s) to
which it is attached.
In brief, using the IPv4 language, we have a basic model of operation A mobile node must continue to be able to be addressed by its home
in which a mobile node can always be reached by sending packets IPv6 address, and to be able to communicate with other IPv6 nodes
to its home (permanent) address. Assuming the mobile node is not using its home address, after changing its link-level point of
present on its home network, packets arriving for it there will be attachment from one IPv6 subnet to another.
intercepted by the home agent, and tunneled to a care-of address.
Care-of addresses can be constructed by the mobile node using All messages used to update another node as to the location of a
the methods of automatic address configuration [12]. If the mobile node must be authenticated in order to protect against remote
mobile node receives router advertisments, it MUST use automatic redirection attacks.
address configuration to construct a globally unique, routable
address. This routable address can be used by the mobile node as its
care-of address. After determining its care-of address, a mobile
node must send a binding update containing that care-of address
to the home agent (and any other correspondent nodes that may
have out-of-date bindings in their binding cache). By default,
correspondent nodes send packets to mobile nodes by using routing
headers instead of encapsulation. As detailed in the next section,
correspondent nodes are usually expected to deliver packets directly
to the mobile node's care-of address, so that the home agent is
rarely involved with packet transmission to the mobile node.
It is essential for scalability and minimizing network load that 1.2. Goals
correspondent nodes be able to learn the care-of address of a mobile
node, and to be able to cache this information for use in sending
future packets to the mobile node's care-of address. By caching the
care-of address of a mobile node, optimal routing of packets can be
achieved between the correspondent node and the mobile node. Routing
packets directly to the mobile node's care-of address also eliminates
congestion at the home agent and thus contributes significantly to
the overall health of the Internet. Moreover, many communications
between the mobile nodes and its correspondent nodes can be carried
out with no assistance from the home agent. Thus, the impact
of failure at the home agent can be drastically reduced; this is
important because many administrative domains will have a single home
agent to serve a particular home network, and thus a single point of
failure for communications to nodes using that home agent. Besides
that, communications between the home agent and a mobile node may
depend on a number of intervening networks; thus, there are many more
ways that packets can fail to reach a mobile node when the home agent
is required as an intermediate node. This would be particularly
relevant on, say, trans-oceanic links between home agent and mobile
node. Caching the binding of a mobile node at the correspondent node
enables communication with the mobile nodes even if the home agent
fails or is difficult to contact over the Internet.
In the typical case when a mobile node has configured its The number of administrative messages sent over the link by which
care-of address at one of its own interfaces, transferring data to a mobile node is directly attached to the Internet should be
the mobile node means no more work for routers on link at its current minimized, and the size of these messages should be kept as small
point of attachment, than transferring data to any other node on that as is reasonably possible. This link may often be a wireless link,
link. This affords another substantial performance improvement in having a substantially lower bandwidth and higher error rate than
the typical case. traditional wired networks, and many mobile nodes are likely to
operate on limited battery power. By reducing the number and size
of administrative messages required for mobility support, network
resources and mobile node battery resources are conserved.
3. Terminology 1.3. Assumptions
Mobile IPv6 defines these terms: This protocol places no additional constraints on the assignment of
IPv6 addresses. That is, a mobile node may acquire its addresses
using stateless address autoconfiguration [12], or alternatively
using a stateful address configuration protocol such as DHCPv6 [3] or
PPPv6 [7].
This protocol assumes that any mobile node will generally not change
its link-level point of attachment from one IPv6 subnet to another
more frequently than once per second.
This protocol assumes that IPv6 unicast packets are routed based on
the Destination Address in the packet's IPv6 header (and not, for
example, by source address).
1.4. Applicability
Mobile IPv6 is intended to enable nodes to move from one IPv6 subnet
to another. It is just as suitable for mobility across homogeneous
media as it is for mobility across heterogeneous media. That is,
Mobile IPv6 facilitates node movement from one Ethernet segment to
another as well as it accommodates node movement from an Ethernet
segment to a wireless LAN, as long as the mobile node's IPv6 address
remains the same after such a movement.
One can think of Mobile IPv6 as solving the "macro" mobility
management problem. It is less well suited for more "micro" mobility
management applications -- for example, handoff amongst wireless
transceivers, each of which covers only a very small geographic
area. As long as node movement does not occur between link-level
points of attachment on different IPv6 subnets, link-layer mechanisms
for mobility management (i.e., link-layer handoff) may offer faster
convergence and far less overhead than Mobile IPv6.
1.5. Terminology
This document uses the following special terms:
Binding Binding
The association of a home address with a care-of address, along The association of the home address of a mobile node with a
with the remaining lifetime of that association. care-of address for that mobile node, along with the remaining
lifetime of that association.
Binding Cache
A cache, maintained by each IPv6 node, of bindings for other
nodes. An entry in a node's binding cache for which the node
is serving as a home agent is marked as a "home registration"
entry and SHOULD NOT be deleted by the node until the
expiration of its binding lifetime, whereas other Binding Cache
entries MAY be replaced at any time by any reasonable local
cache replacement policy. The Binding Cache is a conceptual
data structure used in this document, which may be implemented
in any manner consistent with the external behavior described
here, for example by being combined with the node's Destination
Cache as maintained through Neighbor Discovery [9].
Binding Update List
A list, maintained by each IPv6 mobile node, of the IPv6
address of each other node to which this node has sent a
Binding Update giving its binding, such that the lifetime of
the binding sent to that node has not yet expired. This is a
conceptual data structure used in this document, which may be
implemented in any manner consistent with the external behavior
described here.
Care-of Address Care-of Address
The care-of address is the termination point of a tunnel toward An IPv6 address associated with a mobile node while visiting a
a mobile node that is away from its home network. foreign network, which uses the network prefix of that foreign
network. Among the multiple care-of addresses that a mobile
node may have at a time (with different network prefixes), the
one registered with its home agent is called its "primary"
care-of address.
Correspondent Correspondent Node
A peer with which a mobile node is communicating. The A peer with which a mobile node is communicating. The
correspondent may be either mobile or stationary. correspondent node may be either mobile or stationary.
Foreign Network Foreign Network
Any network other than the mobile node's Home Network. Any network other than the mobile node's home network.
Home Address Home Address
An IPv6 address that is assigned for an extended period of time An IPv6 address that is assigned for an extended period of
to a mobile node. It remains unchanged regardless of where the time to a mobile node. It remains unchanged regardless of the
node is attached to the Internet. node's current link-level point of attachment to the Internet.
Home Agent Home Agent
A router on a mobile node's home network which tunnels packets A router on a mobile node's home network that, while the mobile
for delivery to the mobile node when it is away from home, and node is away from home, intercepts packets on the home network
maintains current location information for the mobile node. destined to the mobile node's home address, encapsulates them,
and tunnels them to the mobile node's current care-of address.
The home agent maintains a registry of the current binding for
mobile nodes whose home address is on the home network routed
by the home agent.
Home Network Home Network
A network, possibly virtual, having a network prefix matching A network, which may possibly be a virtual network, having a
that of a mobile node's home address. Note that standard IP network prefix matching that of a mobile node's home address.
routing mechanisms will deliver packets destined to a mobile Standard IPv6 routing mechanisms will deliver packets destined
node's Home Address to the mobile node's Home Network. for a mobile node's home address to the mobile node's home
network.
Link Link
A facility or medium over which nodes can communicate at the A facility or medium over which nodes can communicate at the
link layer. A link underlies the network layer. link layer. A link underlies the network layer.
Mobile Node Mobile Node
A host or router that changes its point of attachment from one A node that can change its link-level point of attachment from
network or subnetwork to another. A mobile node may change its one IPv6 subnet to another, while still being addressable via
location without losing connectivity and without changing its its IPv6 home address.
IPv6 address.
Node Node
A host or a router. A host or a router.
Tunnel Tunnel
The path followed by a packet while it is encapsulated. The The path followed by a packet while it is encapsulated. The
model is that, while it is encapsulated, a packet is routed model is that, while it is encapsulated, a packet is routed
to a knowledgeable decapsulating agent, which decapsulates to a knowledgeable decapsulating agent, which decapsulates
the packet and then correctly delivers it to its ultimate the packet and then correctly delivers it to its ultimate
destination. destination.
4. Binding Updates Virtual Network
In IPv6, all IPv6 nodes must be capable of caching the care-of A network with no physical instantiation beyond a home agent
address of mobile nodes with which they want to communicate. This (with a physical network interface on another network). The
cached address information can be integrated with the node's home agent generally advertises reachability to the network
Destination Cache [9]. Binding updates should be considered a prefix of the virtual network using conventional routing
form of routing updates; thus, handled incorrectly, they could protocols.
be a source of security problems and routing loops. Therefore,
packets which include binding updates MUST also include an IPv6
authentication header [1]; replay protection is then achieved by use
of the Identification field in the binding update.
4.1. Binding Update Option Format 1.6. Specification Language
The Binding Update Option is an option within the Destination In this document, several words are used to signify the requirements
Header [5]. of the specification. These words are often capitalized.
A mobile node uses the Binding Update destination option to notify MUST
another node (e.g., correspondent node or home agent) of its current
care-of address. The binding update should be placed in the IPv6 This word, or the adjective "required", means that the
packet after any routing header, since the binding update should definition is an absolute requirement of the specification.
only be processed by the destination node rather than by each hop
along the path. The binding update is encoded as destination option MUST NOT
type 16 (TBD). By encoding the binding update in this way, it can
be included in any normal data packet or can be sent in a separate This phrase means that the definition is an absolute
packet containing no data. The binding update contains the mobile prohibition of the specification.
node's care-of address, an identification for the update (to protect
against attempts to replay it), and a lifetime for the binding. SHOULD
This option format is adapted from that used in the IPv4 Route
Optimization [7]. Note that the home address MUST be the source This word, or the adjective "recommended", means that, in some
address of the IPv6 packet containing the binding update, and thus is circumstances, valid reasons may exist to ignore this item, but
not required to be located within the data of the destination option. the full implications must be understood and carefully weighed
before choosing a different course. Unexpected results may
result otherwise.
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.
2. Overview of Mobile IPv6 Operation
In addition to its (permanent) IPv6 home address, a mobile node
while away from home will have assigned to its network interface(s)
a "primary care-of address" and possibly other "care-of addresses".
A care-of address is an IPv6 address assigned to a mobile node only
while visiting a particular foreign network, typically acquired
through stateless [12] or stateful (e.g., DHCPv6 [3]) address
autoconfiguration. The decision about which manner of address
autoconfiguration to use is made according to the methods of IPv6
Neighbor Discovery [9].
Each time a mobile node moves its link-level point of attachment from
one IPv6 subnet to another, it will configure its primary care-of
address at its new point of attachment, and will send a Binding
Update containing that care-of address to its home agent. The
care-of address for a mobile node registered with its home agent is
known as the mobile node's "primary" care-of address, and the mobile
node may also have additional care-of addresses, one for each of the
network prefixes that it currently considers to be on-link. Each
time it changes its primary care-of address, a mobile node also sends
a Binding Update to each other (correspondent) node that may have an
out-of-date care-of address for the mobile node in its Binding Cache.
A mobile node attached to the Internet can always be reached by
sending packets to its home IPv6 address. If the mobile node is not
present on its home network, any packet arriving there for it will be
intercepted there by its home agent, which will tunnel the packet to
the mobile node's current primary care-of address. The home agent
uses IPv6 encapsulation [5] to tunnel the packet.
A correspondent node sending a packet checks its Binding Cache for
an entry for the Destination Address of the packet, and uses a
Routing header (instead of encapsulation) to route the packet to the
destination mobile node's care-of address if a cached binding is
found. Otherwise, the correspondent node sends the packet normally
(with no Routing header), and the packet is then intercepted and
tunneled by the mobile node's home agent as described above. When
the tunneled packet reaches the mobile node, the mobile node returns
a Binding Update to the correspondent node, allowing it to cache the
mobile node's binding for future packets.
Since correspondent nodes cache bindings, it is expected that
correspondent nodes usually will route packets directly to the mobile
node's care-of address, so that the home agent is rarely involved
with packet transmission to the mobile node. This is essential for
scalability and reliability, and for minimizing overall network load.
By caching the care-of address of a mobile node, optimal routing of
packets can be achieved between the correspondent node and the mobile
node. Routing packets directly to the mobile node's care-of address
also eliminates congestion at the mobile node's home agent and home
network. In addition, the impact of of any possible failure of the
home agent, the home network, or intervening networks leading to the
home network is drastically reduced, since these components are not
involved in the delivery of most packets to the mobile node.
3. Message and Option Formats
3.1. Binding Update Option
A Binding Update is a new IPv6 destination option, used by a mobile
node to notify a correspondent node or its home agent of its current
care-of address. As a destination option, it can appear in a
Destination Options header in any IPv6 packet [6], and thus can be
included in any normal data packet or can be sent in a separate
packet containing no data. The Binding Update contains the mobile
node's care-of address, an identification for the Update (to sequence
Updates and to protect against attempts to replay it), and a lifetime
for the binding. The mobile node's IPv6 home address MUST be the
source address of the packet containing the Binding Update, since
the option does not contain space to separately represent the mobile
node's home address.
Binding Updates should be considered a form of routing updates;
handled incorrectly, they could be a source of security problems and
routing loops. Therefore, packets which include Binding Updates MUST
also include an IPv6 Authentication header [1]; sequencing and replay
protection is then achieved by use of the Identification field in the
Binding Update.
The Binding Update option is encoded in type-length-value (TLV)
format as follows:
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | | Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|C|I|E|B| Reserved | Lifetime | |A|H|L| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| Care-of Address | + Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Care-of Address +
| | | |
+ +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification | | |
+ +
| |
+ Home Link-Local Address +
| (only present if L bit set) |
+ +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type Option Type
8-bit identifier of the type of option. The first three bits 16
of the option are 000, indicating first that a node receiving
the option may discard the option and still process the rest
of the packet, and second that the option may not be modified
enroute.
Option Length Option Length
8-bit unsigned integer. Length of the Option Data field of 8-bit unsigned integer. Length of the option, in octets,
this option, in octets. excluding the Option Type and Option Length fields. For the
current definition of the Binding Update option, this field
must be set to 28.
Acknowledge (A) Acknowledge (A)
The Acknowledge (A) bit is set by a node if it wants a a The Acknowledge (A) bit is set by the sending node to request a
Binding Acknowledge message to be returned upon receipt of the Binding Acknowledgement message be returned upon receipt of the
Binding Update Option. Binding Update option.
Co-location (C)
The mobile node is itself the agent receiving datagrams at the
care-of address.
Identification Present (I)
The (I) bit is set by the node sending the Binding Update
option to indicate whether or not the Identification field is
present.
Encapsulation (E) Home Registration (H)
The (E) bit is set by the mobile node to request that the The Home Registration (H) bit is set by the sending node to
receiving node use IPv6-within-IPv6 encapsulation when sending request the receiving node to act as this node's home agent.
any future packets to the mobile node's care-of address, The Destination Address in the IPv6 header of the packet
instead of packets containing the care-of address in a routing carrying this option MUST be that of a router sharing the same
header. See subsection 7. network prefix as the mobile node's home IPv6 address.
"All-Nodes Multicast" (B) Home Link-Local Address Present (L)
The (B) bit is set by the mobile node to request that the home The Home Link-Local Address Present (L) bit indicates the
agent encapsulate and send "all-nodes multicast" packets to the presence of the Home Link-Local Address field in the Binding
mobile node at its care-of address. The (B) bit must only be Update. This bit is set by the sending mobile node to request
used when sending binding updates to the home agent. Note that the receiving node to act as a proxy (for participating in
the home agent may be manually configured to send only a subset the Neighbor Discovery Protocol) for the node while it is
of such packets to the mobile node -- for instance, the home away from home. This bit MUST NOT be set unless the Home
agent may inspect the port number of UDP packets, or the ICMP Registration (H) bit is also set in the Binding Update.
packet type, to determine whether or not the packet should be
forwarded to the mobile node.
Reserved Reserved
Sent as 0; ignored on reception. Sent as 0; ignored on reception.
Lifetime Lifetime
The number of seconds remaining before the binding must be 16-bit unsigned integer. The number of seconds remaining
considered expired. A value of all ones indicates infinity. before the binding must be considered expired. A value of all
A value of zero indicates that the indicated binding (or ones (0xffff) indicates infinity. A value of zero indicates
route table entry, in the case of a mobile node's previous that the Binding Cache entry for the mobile node should be
router) for the mobile node should be deleted. The lifetime is deleted.
typically equal to the remaining lifetime of the mobile node's
binding with its care-of address. Identification
a 64-bit number used to sequence Binding Updates and to match
a returned Binding Acknowledgement message with this Binding
Update. The Identification field also serves to protect
against replay attacks for Binding Updates.
Care-of Address Care-of Address
The current care-of address of the mobile node. When set equal The current care-of address of the mobile node. When set equal
to the home address of the mobile node, the Binding Update to the home address of the mobile node, the Binding Update
option instead indicates that any existing binding for the option instead indicates that any existing binding for the
mobile node should be deleted; no binding for the mobile node mobile node should be deleted; no binding for the mobile node
should be created. should be created.
Identification Home Link-Local Address
If present, a 64-bit number used to protect against replay
attacks.
The receiver of this message must be able to determine that the
mobile node is truly the agent which has generated the binding
update, by verifying a subsequent IPv6 authentication header [1]
within the packet.
Extensions to the Binding Update Options format may be included after
the fixed portion of the Binding Update option. The presence of such
extensions will be indicated by the option length field. When the
option length is greater than the size of the fixed fields of the
Binding Update Option, the remainder is interpreted as extensions.
Currently no extensions have been defined.
5. Sending Binding Updates
After moving away from its home network to a new location (see
subsection 5.1), the mobile node registers its new binding with
its home agent by sending a packet containing a binding update to
its home agent. This binding update MUST have the (A) bit set,
instructing the home agent to send an acknowledgement. If not
already doing so, the home agent must send out onto the Home Network
a proxy Neighbor Advertisment on behalf of the mobile node, with the
Override flag set [9]. This will ensure that other nodes on the home
network are able to send packets to the mobile node by using the
services of the home agent.
In the case when the mobile node is returning to its home network,
the binding update sent to its home agent MUST contain the mobile
node's home address in place of any care-of address. The mobile node
MUST also send out the appropriate Neighbor Advertisment packets with
the Override flag set, so that its neighbors on its home network will
update the relevant information for the mobile node in their Neighbor
Caches. This Neighbor Advertisement packet can be repeated a small
number of times to guard against occasional loss of packets on the
home network.
A binding update may also be included, whenever necessary, in
a normal data packet sent to a correspondent node. For each
correspondent node, information is kept by the mobile node to
determine whether or not the correspondent node has been sent a
fresh binding update since the last time any movement by the mobile
node to a new care-of address has occurred. When a packet is to be
sent to a correspondent node which hasn't been sent a fresh binding
update, the mobile node SHOULD include the update within the packet,
and indicate that the update has been sent. Thus, correspondent
nodes are generally kept updated and can send almost all data packets
directly to the mobile node. Such binding updates are not generally
required to be acknowledged. However, if the mobile node wants to be
sure, an acknowledgment can be requested.
The binding update can also be sent in an otherwise empty packet
whenever the mobile node wishes to update its correspondents. This
is normally done only if the mobile suspects that its home agent is
not operational, too far away, a correspondent node is not sending
the traffic to the proper care-of address, or there is an immediate
need for the correspondent node to obtain the binding. The mobile
node must not send binding updates more often than MAX_UPDATE_RATE to
any correspondent host, since it is not allowed to change its point
of attachment more often than MAX_UPDATE_RATE. A mobile node can
detect that a correspondent node is not sending packets to the proper
care-of address because in that case the packets arrive at the mobile
node's care-of address by encapsulation instead by inclusion in a
routing header within the packet.
The mobile node may choose to keep its location private from certain
correspondent nodes. The mobile node need not send binding updates
to those correspondents. No other IPv6 nodes are authorized to send
binding updates on behalf of the mobile node.
When sending binding updates, a mobile node uses the Identification The link-local address of the mobile node used by the mobile
field of the destination option, in conjunction with the IPv6 node when it was last attached to its home network. This field
Authentication Header, to protect against replays. One style in the Binding Update is optional and is only present when the
of replay protection involves the use of a timestamp as the Home Link-Local Address (L) bit is set.
Identification data. The result would be that the mobile node and
the target of its binding update would have to roughly agree on
the current time, and that stale binding updates would have to be
rejected. The exact mechanisms by which the Identification field is
created and interpreted by the sending and receiving parties depends
on the Security Association existing between them. This subject is
discussed thoroughly in the mobile-IPv4 specification [6].
5.1. Detecting movement As with all IPv6 options, the highest-order three bits of the Option
Type Field (16) of the Binding Update option specify the following
properties of the option:
A mobile node may detect that it has changed its point of attachment - The highest-order two bits are 00: Any node receiving this
to the Internet by several means. The usual method involves option that does not recognize the Option Type MUST skip over
reception of router advertisements from previously undetected this option and continue processing the header.
routers. This may also be augmented by a determination that a
previously accessible router is no longer accessible (using Neighbor
Unreachability Detection (NUD) as specified in [9]).
It is also possible that indications about changes of point of - The third-highest-order bit is 0: The Option Data does not
attachment can be obtained from lower-level protocol or device driver change en-route, and thus, when an Authentication header is
software. present in the packet, the entire Binding Update option MUST be
included when computing or verifying the packet's authenticating
value.
6. Binding Acknowledgement Message Extensions to the Binding Update option format may be included after
the fixed portion of the Binding Update option specified above.
The presence of such extensions will be indicated by the Option
Length field. When the Option Length is greater than 28 octets,
the remaining octets are interpreted as extensions. Currently no
extensions have been defined.
A Binding Acknowledge message is used to acknowledge acceptance 3.2. ICMP Binding Acknowledgement Message
of a Binding Update (section 4.1) option, if that option has the
Acknowledge (A) bit set. Binding Acknowledgement messages should be
sent addressed to the mobile node originating the Binding Update,
using if necessary a routing header containing the care-of address
given in the Binding Update.
Since the Binding Acknowledgement is mostly used by home agents A Binding Acknowledgement message is an informational ICMP message
and is not associated with any transmission of data packets, it is used to acknowledge acceptance of a Binding Update (Section 3.1)
specified here as an informational ICMP message to the mobile node. option, if that Binding Update has the Acknowledge (A) bit set.
However, all of the error conditions specified in the Registration
Reply message of the IPv4 mobile-IP protocol may apply, so the
general format and codes of that message are adapted here to fit the
ICMP packet layout for IPv6 [4].
The acknowledgement message contains the necessary codes to inform Upon receipt of a Binding Update requesting an acknowledgement, the
the mobile node about the status of its binding. Additionally, the receiving node returns a Binding Acknowledgement message addressed to
home agent MAY shorten the lifetime to be smaller than indicated the care-of address in the Binding Update.
in the original binding update. When the lifetime of the reply is
greater than what was contained in the binding update, the excess
time MUST be ignored. When the lifetime of the reply is smaller than
the original request, another binding update SHOULD be sent before
the lifetime expires.
If a mobile node fails to receive an acceptable Binding If a mobile node fails to receive an acceptable Binding
Acknowledgement within INITIAL_BINDACK_TIMEOUT seconds after Acknowledgement message within INITIAL_BINDACK_TIMEOUT seconds
transmitting the binding update, it must retransmit the binding after transmitting the Binding Update, it SHOULD retransmit the
update with the same identification, and begin an exponential Binding Update until a Binding Acknowledgement is received. Such a
back-off process of retransmission. The time-out period is doubled retransmitted Binding Update MUST use he same Identification value as
upon each retransmission until the target of the binding update the original transmission. The retransmissions by the mobile node
sends an acknowledgement, or the time-out period reaches the value MUST use an exponential back-off process, in which timeout period
MAX_BINDACK_TIMEOUT. is doubled upon each retransmission until either the node receives
a Binding Acknowledgement message or the timeout period reaches the
value MAX_BINDACK_TIMEOUT.
The ICMP Binding Acknowledgment packet has the following format: The ICMP Binding Acknowledgement message has the following format:
0 1 2 3 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 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 | Checksum | | Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Lifetime | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + Identification +
| Identification |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 192 (TBD) Type
Code One of the following codes: 133
0 service will be provided Code
128 service denied: reason unspecified
129 service denied: administratively prohibited
130 service denied: insufficient resources
133 service denied: identification mismatch
134 service denied: poorly formed request
136 service denied: unknown home agent address
Lifetime The seconds remaining before the binding is
considered expired. A value of zero indicates
removal of a binding. A value of all ones
indicates infinity.
Identification The acknowledgment identification is derived 8-bit unsigned integer indicating the disposition of the
from the binding update message, for use by the Binding Update. Values of the Code field less than 128
mobile node in matching the acknowledgment with indicate that the Binding Update was accepted by the receiving
an outstanding Binding Update. node. The following such values are currently defined:
0 Binding Update accepted
Values of the Code field greater than or equal to 128 indicate
that the Binding Update was rejected by the receiving node.
The following such values are currently defined:
128 Reason unspecified
129 Poorly formed Binding Update
130 Administratively prohibited
131 Insufficient resources
132 Home registration not supported
133 Not home network
134 Identification field mismatch
135 Unknown home agent address
Checksum
The checksum of the message calculated as specified for ICMP
for IPv6 [4].
Identification
The acknowledgement Identification is derived from the Binding
Update option, for use by the mobile node in matching the
acknowledgement with an outstanding Binding Update.
Up-to-date values of the Code field are to be specified in the most Up-to-date values of the Code field are to be specified in the most
recent "Assigned Numbers" [10]. recent "Assigned Numbers" [10].
7. Delivering Packets to a Mobile Node Extensions to the Binding Acknowledgement message format may be
included after the fixed portion of the Binding Acknowledgement
message specified above. The presence of such extensions will be
indicated by the ICMP message length, derived from the IPv6 Payload
Length field. When the Option Length is greater than 16 octets,
the remaining octets are interpreted as extensions. Currently no
extensions have been defined.
If a routing header is not present, the routing infrastructure will 4. Requirements for IPv6 Nodes
route packets addressed to a mobile node to its home network. Since
the mobile node's location is known on the home network (namely, by
the home agent), packets can be addressed to the mobile node and
intercepted by the home agent without the sender knowing that the
node is mobile.
Correspondent nodes that have accepted a binding update for a Mobile IPv6 places some special requirements on the functions
mobile node, can send packets directly to that mobile node's current provided by different IPv6 nodes. This section itemizes those
care-of address by including a routing header in them. To use requirements, identifying the functionality each requirement is
the routing header for delivery of packets to a mobile node, a intended to support. Further details on this functionality is
correspondent host just specifies the care-of address as the (last) provided in the following sections.
intermediate routing point and the mobile node as the destination.
When the packet arrives at the care-of address, normal processing of
the routing header will ensure delivery to the mobile node. IPv6
routing headers do not carry the semantics which require reversal of
source routes.
Home agents cannot use routing headers to deliver packets to the Since any IPv6 node may at any time be a correspondent of a mobile
mobile node, because they can't modify the packet and add to it in node, all IPv6 nodes MUST support the following requirements:
flight. They must always use encapsulation [3] for this purpose
(section 8).
If a packet to the mobile node is encapsulated, it uses the care-of - Every IPv6 node MUST be able to process a received Binding Update
address as the destination address in the outer IPv6 header. Then, option, and to return a Binding Acknowledgement message if
when the the encapsulated packet arrives at the care-of address, requested.
the encapsulation is stripped away and the packet delivered (if
possible) to the mobile node. Of course, if the mobile node is
itself receiving packets at the care-of address, the delivery path is
trivial.
If a correspondent node receives ICMP Host Unreachable or Network - Every IPv6 node MUST be able to maintain a Binding Cache of the
Unreachable after sending a packet to a mobile node using its cached bindings received in accepted Binding Updates.
care-of address, it SHOULD delete the cache entry until information
about the mobile node's current care-of address becomes available
(via a binding update).
7.1. Smooth Handoffs - Every IPv6 node MUST be able to maintain Security Associations
for use in IPv6 Authentication Headers [2, 1, 6]. An IPv6
node receiving a packet containing a Binding Update option
MUST verify, using the Authentication Header in the packet,
the authenticity of the sender (the mobile node for which this
binding applies) before modifying its Binding Cache in response
to that Binding Update option.
If a mobile node obtains a new care-of address from an stateful Since any IPv6 router may at any time have a Binding Cache entry
address allocation authority (e.g, [2]), it should soon explicitly for a mobile node, all IPv6 router MUST support the following
deallocate the previous care-of address. For smooth handoffs, a requirement:
mobile client may still accept packets at both addresses for a short
time after configuring its newly allocated IPv6 address. If the
previous address is allocated by such a stateful address server, then
such a mobile client may not wish to release the address immediately
upon acquisition of a new care-of address. The stateful address
server will allow mobile clients to acquire new addresses while still
using previously allocated addresses.
Routers must (just as any IPv6 node) be able to accept authenticated - Every IPv6 router MUST be able to use its Binding Cache in
binding updates for the mobile node and, subsequently, act on the forwarding packets; if the router has a Binding Cache entry for
cached binding by encapsulating packets for intermediate delivery the Destination Address of a packet it is forwarding, then the
to the care-of address specified in the binding. In cases where router SHOULD encapsulate the packet and tunnel it to the care-of
a mobile node moves from one care-of address to another with no address in the Binding Cache entry.
delay, but without being able to maintain simultaneous connectivity
at both care-of addresses, it SHOULD send a binding update to the
router servicing the previous care-of address, so that packets
for the mobile node can be delivered to the new care-of address
immediately. For example, a mobile node may move from one radio link
to another on a different channel, and be unable to monitor packets
delivered over two channels at once. In this example, the mobile
node should send a binding update to the entity delivering packets
over the previous radio channel so that those packets will instead be
delivered via a new care-of address. This binding update associates
the mobile node's previous care-of address to the mobile node's new
care-of address, and is authenticated using the IPv6 Authentication
Header with whatever security association the previous router had
with the mobile node's previous care-of address.
For this purpose, the mobile node must have some security association In order for a mobile node to correctly operate while away from
with the entity serving the previous care-of address. In the typical home, at least one IPv6 router in its home network must support
case specified within this document, a mobile node has obtained a functioning as a home agent for the mobile node. All IPv6 routers
care-of address via autoconfiguration and is receiving tunneled capable of serving as a home agent MUST support the following special
packets at that care-of address. When the mobile node moves, routers requirements:
serving the link at its previous point of attachment may find that
the mobile node's previous care-of address has become inaccessible.
Note that the previous router does not necessarily know anything - Every home agent MUST be able to maintain a registry of mobile
about the mobile node's home address as part of this sequence of node bindings for those mobile nodes for which it is serving as
events; the routers may only know about things associated with the the home agent.
(e.g., autoconfigured) care-of addresses used by the mobile node at
the previous and current points of attachment.
Since only one binding update is expected to be sent to the previous - Every home agent MUST be able to intercept packets (e.g., using
router, the mobile node MAY elect to omit the Identification field. Neighbor Advertisements) on the local network addressed to
If the mobile node omits the Identification field from the binding a mobile node for which it holds a binding in its registry
update, there is no replay protection and the security association indicating that the mobile node is currently away from home.
with the previous router can only be used one time. In this case,
the router should only accept the binding update if the mobile node's
care-of address is still present in its Neighbor Cache. In this
situation, the mobile node SHOULD request an acknowledgment for the
binding update. Thus, the previous router should keep the security
association around for the mobile node's previous care-of address in
case the mobile node loses the acknowledgment and retransmits the
binding update (with the same new care-of address).
The previous router then operates the same way as when the mobile - Every home agent MUST be able to encapsulate such intercepted
node's home agent receives a binding update from the mobile node. packets in order to tunnel them to the care-of address for the
That is, the previous router must inspect packets, and redirect the mobile node indicated in its binding.
packets destined for the care-of address indicated in the binding
update. Packets which need to be redirected to the mobile node's new
care-of address are encapsulated and sent to the new care-of address.
In fact, the previous router is temporarily acting as a home agent
for the mobile node's previous care-of address. In particular,
the previous router does not use any routing header to effect the
redirected delivery. Moreover, the previous router should issue
Neighbor Advertisement packets for the previous care-of address, so
that on-link neighbors will send packets destined to the mobile node
to the previous router for encapsulation and further delivery to the
new care-of address.
Once the mobile node receives the encapsulated packet, it can then - Every home agent MUST be able to issue Binding Acknowledgement
typically follow the routing header contained in the decapsulated messages in response to Binding Updates received from a mobile
packet and deliver the final payload to internal protocol handling node.
using its IPv6 home address. The mobile node must ensure that a
binding update is sent to each source of such packets so that the
previous router is relieved of its duties at the earliest possible
moment.
8. Home Agent Considerations - Every home agent MUST be able to maintain Security Associations
for the mobile nodes from which it will accept Binding Updates.
When the home agent, or any other routing agent, receives a packet Finally, all IPv6 nodes capable of functioning as mobile nodes MUST
destined to a mobile node for which it has a binding cached, support the following requirements:
it encapsulates the packet for delivery to the mobile node's
care-of address. The agent cannot insert a routing header, or
modify the destination address of the mobile node, because of IPv6
authentication mechanisms [1]. Moreover, the home agent is expected
to be involved only rarely with the transmission of data to the
mobile node, because the mobile node will send binding updates as
soon as possible to its correspondent hosts.
It is useful to be able to send a packet to a mobile node's home - Every IPv6 mobile node MUST be able to perform IPv6
agent without explicitly knowing the home agent's address. For decapsulation [5].
example, a mobile node must communicate with its home agent to
send it a binding update; but since the mobile node was last at - Every IPv6 mobile node MUST support sending Binding Updates,
home, it may have become necessary to replace the node serving as as specified in Sections 6.3, 6.4, and 6.5; and MUST be able
its home agent due to the failure of the original node or due to to receive and process Binding Acknowledgement messages, as
reconfiguration of the home network. It thus may not always be specified in Section 6.7.
possible or convenient for a mobile node to know the exact address of
its own home agent. - Every IPv6 mobile node MUST maintain a Binding Update List in
which it keeps track of which other IPv6 nodes it has sent a
Binding Update to, for which the Lifetime sent in that binding
has not yet expired.
5. Binding Cache Management
The Binding Cache is the central data structure in Mobile IPv6.
All IPv6 nodes MUST support maintenance of a Binding Cache, and
MUST support processing of received Binding Updates. This section
describes the management aspects of a Binding Cache common to all
nodes.
5.1. Receiving Binding Updates
Upon receiving a Binding Update option in some packet, the receiving
node MUST validate the packet according to the following tests:
- The packet contains an IP Authentication header and the
authentication is valid [1]. The Authentication header is
assumed to provide both authentication and integrity protection.
- The length of the option specified in the Option Length field is
greater than or equal to 28 octets.
- The Identification field is valid.
Any Binding Update not satisfying all of these tests MUST be silently
ignored, although the remainder of the packet (i.e., other options,
extension headers, or payload) SHOULD be processed normally according
to any procedure defined for that part of the packet.
If the Binding Update is valid according to the tests above, then the
Binding Update is processed further as follows:
- If the Lifetime specified in the Binding Update is nonzero and
the specified Care-of Address differs from the Home Address,
this is a request to cache a binding for the mobile node.
Processing for this type of received Binding Update is described
in Section 5.2.
- If the Lifetime specified in the Binding Update is zero or the
specified Care-of Address matches the Home Address, then this is
a request to delete the mobile node's binding. Processing for
this type of received Binding Update is described in Section 5.3.
5.2. Requests to Cache a Binding
If a node receives a valid Binding Update requesting it to cache a
binding for a mobile node, as specified in Section 5.1, then the node
MUST examine the Home Registration (H) bit in the Binding Update
to determine how to further process the Binding Update. If the
Home Registration (H) bit is set, the Binding Update is processed
according to the procedure specified in Section 7.1.
If the Home Registration (H) bit is not set, then the receiving
node SHOULD create a new entry in its Binding Cache for this mobile
node's Home Address (or update its existing Binding Cache Entry for
this Home Address) to record the Care-of Address as specified in the
Binding Update, and begin a timer to delete this Binding Cache entry
after the expiration of the Lifetime period specified in the Binding
Update.
5.3. Requests to Delete a Binding
If a node receives a valid Binding Update requesting it to delete
a binding for a mobile node, as specified in Section 5.1, then the
node MUST examine the Home Registration (H) bit in the Binding Update
to determine how to further process the Binding Update. If the
Home Registration (H) bit is set, the Binding Update is processed
according to the procedure specified in Section 7.2.
If the Home Registration (H) bit is not set, and if a node receives a
valid Binding Update requesting it to delete a binding for a mobile
node, as specified in Section 5.1, then it MUST delete any existing
entry in its Binding Cache for this mobile node's Home Address.
5.4. Sending Binding Acknowledgements
When any node receives a packet containing a Binding Update option,
it SHOULD return a Binding Acknowledgement message acknowledging
receipt of the Binding Update. If the node accepts the Binding
Update and adds the binding contained in it to its Binding Cache, the
Code field in the Binding Acknowledgement MUST be set to a value less
than 128; if the node rejects the Binding Update and does not add
the binding contained in it to its Binding Cache, the Code field in
the Binding Acknowledgement MUST be set to a value greater than or
equal to 128. Specific values for the Code field are described in
Section 3.2 and in the most recent "Assigned Numbers" [10].
The Destination Address in the IPv6 header for the Binding
Acknowledgement MUST be set to the Care-of Address copied from the
Binding Update option. This ensures that the Binding Acknowledgement
will be routed to the current location of the node sending the
Binding Update, whether the Binding Update was accepted or rejected.
5.5. Cache Replacement Policy
Any entry in a node's Binding Cache MUST be deleted after the
expiration the Lifetime specified in the Binding Update from which
the entry was created. Conceptually, a node MUST maintain a separate
timer for each entry in its Binding Cache. When creating or updating
a Binding Cache entry in response to a received Binding Update, the
node sets the timer for this entry to the specified Lifetime period.
When a Binding Cache entry's timer expires, the node MUST delete the
entry.
Each node's Binding Cache will, by necessity, have a finite size.
A node MAY use any reasonable local policy for managing the space
within its Binding Cache, except that any entry marked as a "home
registration" (Section 7.1) SHOULD NOT be deleted from the cache
until the expiration of its lifetime period. When attempting to
add a new "home registration" entry in response to Binding Update
with the Home Registration (H) bit set, if insufficient space exists
(or can be reclaimed) in the node's Binding Cache, the node MUST
reject the Binding Update and SHOULD return a Binding Acknowledgement
message to the sending mobile node, in which the Code field is set to
131 (Insufficient resources). When otherwise attempting to add a new
entry to its Binding Cache, a node MAY if needed choose to drop any
entry already in the Binding Cache other than a "home registration"
entry, in order to make space for the new entry. For example, a
"least-recently used" (LRU) strategy for cache entry replacement is
likely to work well.
If a packet is sent by a node to a destination for which it has
dropped the cache entry from its Binding Cache, the packet will be
routed normally, leading to the mobile node's home network, where it
will be intercepted by the mobile node's home agent and tunneled to
the mobile node's current primary care-of address. As when a Binding
Cache entry is initially created, this indirect routing to the mobile
node will result in the mobile node sending a Binding Update to this
sending node, allowing it to add this entry again to its Binding
Cache.
5.6. Receiving ICMP Error Messages
When a correspondent node sends a packet to a mobile node, if the
correspondent node has a Binding Cache entry for the destination
mobile node's address (its home address), then the correspondent node
uses a Routing header to deliver the packet to the mobile node's
care-of address, and then to the mobile node's home address. Any
ICMP error message caused by the packet on its way to the mobile node
will be returned normally to the correspondent node.
On the other hand, if the correspondent node has no Binding Cache
entry for the mobile node, the packet will be routed to the mobile
node's home network, where it will be intercepted by the mobile
node's home agent, encapsulated, and tunneled to the mobile node's
care-of address. Similarly, if a packet for a mobile node arrives
at the mobile node's previous default router (e.g., the mobile node
moved after the packet was sent), the router will encapsulate and
tunnel the packet to the mobile node's new care-of address (if it has
a Binding Cache entry for the mobile node). Any ICMP error message
caused by the packet on its way to the mobile node while in the
tunnel, will be returned to the node that encapsulated the packet
(the home agent or the previous default router, respectively). By
the definition of IPv6 encapsulation [5], however, this encapsulating
node MUST relay certain ICMP error messages back to the original
sender of the packet (the correspondent node).
Thus, whether the correspondent node has a Binding Cache entry
for the destination mobile node or not, the correspondent node
will receive any meaningful ICMP error message that is caused by
its packet on its way to the mobile node. If the correspondent
node receives an ICMP Host Unreachable or Network Unreachable
error message after sending a packet to a mobile node using its
cached care-of address, the correspondent node SHOULD delete its
Binding Cache entry for this mobile node. If the correspondent node
subsequently transmits another packet to the mobile node, the packet
will be routed to the mobile node's home network, intercepted by the
mobile node's home agent, and tunneled to the mobile node's care-of
address using IPv6 encapsulation. The mobile node will then return a
Binding Update to the correspondent node, allowing it to recreate a
(correct) Binding Cache entry for the mobile node.
6. Mobile Node Considerations
6.1. Movement Detection
A mobile node MAY use any combination of mechanisms available to
it to detect when its link-level point of attachment has moved
from one IPv6 subnet to another. The primary movement detection
mechanism for Mobile IPv6 defined here uses the facilities of
IPv6 Neighbor Discovery, including Router Discovery and Neighbor
Unreachability Detection. The description here is based on the
conceptual model of the organization and data structures defined by
Neighbor Discovery [9].
Mobile nodes SHOULD use Router Discovery to discover new routers and
on-link network prefixes; a mobile node MAY send Router Solicitation
messages, or MAY wait for unsolicited (periodic) Router Advertisement
messages, as specified for Router Discovery [9]. Based on received
Router Advertisement messages, a mobile node (in the same way as any
other node) maintains an entry in its Default Router List for each
router, and an entry in its Prefix List for each network prefix,
that it currently considers to be on-link. Each entry in these
lists has an associated invalidation timer value (extracted from the
Advertisement) used to expire the entry when it becomes invalid.
While away from home, a mobile node SHOULD select one router from its
Default Router List to use as its default router, and one network
prefix advertised by that router from its Prefix List to use as
the network prefix in its primary care-of address. A mobile node
MAY also have associated additional care-of addresses, using other
network prefixes from its Prefix List. The method by which a mobile
node selects and forms a care-of address from the available network
prefixes is described in Section 6.2. The mobile node registers
its primary care-of address with its home agent, as described in
Section 6.3.
While away from home and using some router as its default router,
it is important for a mobile node to be able to quickly detect when
that router becomes unreachable, so that it can switch to a new
default router and to a new primary care-of address. Since some
links (notably wireless) do not necessarily work equally well in
both directions, it is likewise important for the mobile node to
detect when it becomes unreachable to its default router, so that any
correspondent nodes attempting to communicate with the mobile node
can still reach it.
To detect when its default router becomes unreachable, a mobile
node SHOULD use Neighbor Unreachability Detection. As specified
in Neighbor Discovery [9], while the mobile node is actively
sending packets to (or through) its default router, the mobile node
can detect that the router has become unreachable either through
indications from upper layer protocols on the mobile node that a
connection is not making "forward progress" (e.g., TCP timing out
waiting for an acknowledgement after a number of retransmissions),
or through the failure to receive a Neighbor Advertisement messages
form its default router in response to retransmitted explicit
Neighbor Solicitation messages to it. No exceptions to Neighbor
Unreachability Detection are necessary for this aspect of movement
detection in Mobile IPv6.
For a mobile node to detect when it has become unreachable to its
default router, however, the mobile node cannot efficiently rely on
Neighbor Unreachability Detection alone, since the network overhead
would be prohibitively high in many cases for a mobile node to
continually probe its default router with Neighbor Solicitation
messages even when it is not otherwise actively sending packets to
it. Instead, a mobile node SHOULD consider receipt of any IPv6
packets from its current default router as an indication that it is
still reachable from the router. Both packets from the router's IPv6
address and (IPv6) packets from its link-layer address (e.g., those
forwarded but not originated by the router) SHOULD be considered.
Since the router SHOULD be sending periodic multicast Router
Advertisement messages, the mobile node will have frequent
opportunity to check if it is still reachable to its default router,
even in the absence of other packets to it from the router. On some
types of network interfaces, the mobile node MAY also supplement
this by setting its network interface into "promiscuous" receive
mode, so that is able to receive all packets on the link, including
those not link-level addressed to it. The mobile node will then
be able to detect any packets sent by the router, in order to to
detect reachability from the router. This may be useful on very low
bandwidth (e.g., wireless) links, but its use MUST be configurable on
the mobile node.
If the above means do not provide indication that the mobile node
is still reachable from its current default router (i.e., the
mobile node receives no packets form the router for a period of
time), then the mobile node SHOULD actively probe the router with
Neighbor Solicitation messages, even if it is not otherwise actively
sending packets to the router. If it receives a solicited Neighbor
Advertisement message in response from the router, then the mobile
node can deduce that it is still reachable. It is expected that the
mobile node will in most cases be able to determine its reachability
from the router by listening for packets from the router as described
above, and thus, such extra Neighbor Unreachability Detection probes
should rarely be necessary.
With some types of networks, it is possible that additional
indications about link-layer mobility can be obtained from
lower-layer protocol or device driver software within the mobile
node. However, a mobile node MUST NOT assume that all link-layer
mobility indications from lower layers indicate a movement of the
mobile node's link-layer connection to a new IPv6 subnet, such that
the mobile node would need to switch to a new default router and
primary care-of address. Upon lower-layer indication of link-layer
mobility, the mobile node SHOULD send Router Solicitation messages
to determine if new routers (and new on-link network prefixes) are
present on its new link.
Such lower-layer information might also be useful to a mobile node in
deciding to switch its primary care-of address to one of the other
care-of addresses it has formed from the on-link network prefixes
currently available through different default routers from which the
mobile node is reachable. For example, a mobile node MAY use signal
strength or signal quality information (with suitable hysteresis)
for its link with the available default routers to decide when to
switch to a new primary care-of address using that default router
rather than its current default router (and current primary care-of
address). Even though the mobile node's current default router may
still be reachable in terms of Neighbor Unreachability Detection, the
mobile node MAY use such lower-layer information to determine that
switching to a new default router would provide a better connection.
6.2. Forming New Care-of Addresses
After detecting that its link-layer point of attachment has moved
from one IPv6 subnet to another (i.e., its current default router
has become unreachable and it has discovered a new default router),
a mobile node SHOULD form a new primary care-of address using one of
the on-link network prefixes advertised by the new router. A mobile
node MAY form a new primary care-of address at any time, except
that it MUST NOT do so too frequently (more often than once per
MAX_UPDATE_RATE seconds).
In addition, after discovering a new on-link network prefix, a
mobile node MAY form a new (non-primary) care-of address using that
network prefix, even when it has not switched to a new default
router. A mobile node can have only one primary care-of address
at a time (registered with its home agent), but it MAY have an
additional care-of address for each network prefix on its current
link. Furthermore, since a wireless network interface may actually
allow a mobile node to be reachable on more than one link at a time
(i.e., within wireless transmitter range of routers on more than one
separate link), a mobile node MAY have care-of addresses on more than
one link at a time. For more information on using more than one
care-of address at a time, see Section 6.8.
As described in Section 2, in order to form a new care-of address,
a mobile node MAY use either stateless [12] or stateful (e.g.,
DHCPv6 [3]) address autoconfiguration. If a mobile node needs to
send packets as part of the method of address autoconfiguration, it
MUST use an IPv6 link-local address rather than its own IPv6 home
address as the Source Address.
In some cases, a mobile node may already know a (constant) IPv6
address that has been assigned to it for its use while visiting this
network. For example, it may be statically configured with an IPv6
address assigned by the system administrator of the new network. If
so, rather than using address autoconfiguration to form a new care-of
address using this network prefix, the mobile node SHOULD use its own
pre-assigned address as its care-of address on this network.
6.3. Sending Binding Updates to the Home Agent
After changing its primary care-of address as described in
Sections 6.1 and 6.2, a mobile node SHOULD register its new primary
care-of address with its home agent. To do so, the mobile node sends
a packet to its home agent containing a Binding Update option with
the Acknowledge (A) bit set, requesting the home agent to return a
Binding Acknowledgement message in response to this Binding Update.
As described in Section 3.2, the mobile node SHOULD retransmit this
Binding Update to its home agent until it receives a matching Binding
Acknowledgement message. Once reaching a retransmission timeout
period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD continue
to periodically retransmit the Binding Update at this rate until
acknowledged.
It is useful for a mobile node to be able to send a Binding Update
its home agent without explicitly knowing the home agent's address.
For example, since the mobile node was last at home, it may have
become necessary to replace the node serving as its home agent due
to the failure of the original node or due to reconfiguration of the
home network. It thus may not always be possible or convenient for a
mobile node to know the exact address of its own home agent.
Mobile nodes can dynamically discover the address of a home agent Mobile nodes can dynamically discover the address of a home agent
by sending a binding update to the anycast address on their home by sending a Binding Update to the anycast address on their home
network. Each router on the home network which receives this binding network. Each router on the home network which receives this Binding
update MUST reject the binding update and include its address in the Update MUST reject the Binding Update and include its address in the
Binding Acknowledgement packet indicating the rejection. The mobile Binding Acknowledgement message indicating the rejection. The mobile
node is assumed to know a proper anycast address on its home network node is assumed to know a proper anycast address on its home network
before making use of this method for determining a particular home before making use of this method for determining a particular home
agent's address. agent's address.
Other routers on the home network must be instructed to forward 6.4. Sending Binding Updates to Correspondent Nodes
packets to the current router which is serving as the mobile node's
home agent. This can be done using the same proxy mechanisms
already made available in Neighbor Discovery. The current home agent
multicasts the equivalent of a Proxy ARP onto the home network, and
subsequently the other routers on the home network will forward
packets destined to the mobile node to the particular router which is
serving as the home agent for that mobile node.
8.1. Renumbering the Home Network A mobile node MAY also include a Binding Update in any normal data
packet sent to a correspondent node. For each correspondent node
to which it has sent a Binding Update, the mobile node MUST keep
information to determine whether or not the correspondent node has
been sent a fresh Binding Update since the last time the mobile node
switched to a new primary care-of address. When a packet is to be
sent to a correspondent node that has not been sent a fresh Binding
Update, the mobile node SHOULD include the Binding Update within the
packet. Thus, correspondent nodes are generally kept updated and
can send almost all data packets directly to the mobile node using
the mobile node's current binding. Such Binding Updates are not
generally required to be acknowledged; however, if the mobile node
wants to be sure, an acknowledgement can be requested, although in
this case, the mobile node SHOULD NOT continue to retransmit the
Binding Update once the retransmission timeout period has reached
MAX_BINDACK_TIMEOUT.
A mobile node MAY also send a Binding Update in any otherwise empty
packet, whenever the mobile node wishes to update a correspondent
node as to its current binding. This is normally done only if
the mobile suspects that its home agent is not operational or is
too far away, a correspondent node is not sending the traffic to
the proper care-of address, or there is an immediate need for the
correspondent node to obtain the binding. A mobile node can detect
that a correspondent node is not sending packets to the proper
care-of address because in that case the packets arrive at the mobile
node's care-of address by encapsulation instead by inclusion in a
routing header within the packet.
A mobile node MAY choose to keep its location private from certain
correspondent nodes, and thus need not send new Binding Updates to
those correspondents. A mobile node MAY also send a Binding Update
to such a correspondent node to instruct it to delete any existing
binding for the mobile node from its Binding Cache, as described in
Section 3.1. No other IPv6 nodes are authorized to send Binding
Updates on behalf of a mobile node.
6.5. Sending Binding Updates to the Previous Default Router
After switching to a new default router (and thus also changing
its primary care-of address), a mobile node SHOULD send a Binding
Update message to its previous default router, giving its new care-of
address. If it sends such a Binding Update, the mobile node MUST set
the Home Address field to its old primary care-of address (that it
used while using this default router), and set the Care-of Address
field to its new primary care-of address. Note that the previous
router does not necessarily know the mobile node's home address as
part of this sequence of events.
The mobile node's previous default router then, in effect,
temporarily act as a home agent for the mobile node's old primary
care-of address. If any subsequent packets arrive at this previous
router for forwarding to the mobile node's old primary care-of
address, the router SHOULD encapsulate each and tunnel it to the
mobile node at its new primary care-of address. Moreover, the
previous router should issue Neighbor Advertisement packets for the
previous care-of address, so that on-link neighbors will send packets
destined to the mobile node's old primary care-of address to the
previous router for encapsulation and tunneling to its new care-of
address.
6.6. Rate Limiting for Sending Binding Updates
A mobile node MUST NOT send Binding Update messages more often than
once per MAX_UPDATE_RATE seconds to any correspondent node. After
sending 5 consecutive Binding Updates to a particular correspondent
node with the same care-of address, the mobile node SHOULD reduce its
rate of sending Binding Updates to that correspondent node, to the
rate of SLOW_UPDATE_RATE per second. The mobile node MAY continue
to send Binding Updates at the slower rate indefinitely, in hopes
that the correspondent node will finally be able to process a Binding
Update and begin to route its packets directly to the mobile node at
its current primary care-of address.
6.7. Receiving Binding Acknowledgements
Upon receiving a packet carrying a Binding Acknowledgement message,
a mobile node MUST validate the packet according to the following
tests:
- The packet contains an IP Authentication header and the
authentication is valid [1]. The Authentication header is
assumed to provide both authentication and integrity protection.
- The ICMP Checksum is valid.
- The length of the ICMP message (derived from the IPv6 Payload
Length field) is greater than or equal to 16 octets.
- The Identification field is valid.
Any Binding Acknowledgement not satisfying all of these tests MUST be
silently discarded.
If the Binding Acknowledgement is valid, the mobile node MUST examine
the Code field as follows:
- If the Code field indicates that the Binding Update was accepted
(the Code field is less than 128), then the mobile node MUST
update the corresponding entry in its Binding Update List to
indicate that the Binding Update has been acknowledged. The
mobile node SHOULD thus stop retransmitting the Binding Update.
- If the Code field indicates that the Binding Update was not
accepted (the Code field is greater than or equal to 128), then
the mobile node MUST delete the corresponding Binding Update List
entry. Optionally, the mobile node MAY take steps to correct the
cause of the error and retransmit the Binding Update, subject to
the rate limiting restriction specified in Section 6.6.
6.8. Using Multiple Care-of Addresses
As described in Section 6.2, a mobile node MAY have more than
one care-of address at a time. Particularly in the case of many
wireless networks, a mobile node effectively may be reachable through
multiple link-level points of attachment at the same time (e.g.,
with overlapping wireless cells), on which different on-link network
prefixes may exist. A mobile node SHOULD select a primary care-of
address from among those care-of addresses it has formed using any
of these network prefixes, based on the movement detection mechanism
in use (Section 6.1). When the mobile node selects a new primary
care-of address, it MUST register it with its home agent through a
Binding Update message with the Acknowledge (A) bit set, as described
in Section 6.3.
To assist in smooth handoffs, a mobile node SHOULD retain its
previous primary care-of address as a care-of address, and SHOULD
still accept packets at this address, even after registering its new
primary care-of address with its home agent. This is reasonable,
since the mobile node could only receive packets at its previous
primary care-of address if it were indeed still connected to that
link. If the previous primary care-of address was allocated using
stateful address autoconfiguration [3], the mobile node may not wish
to release the address immediately upon switching to a new primary
care-of address. The stateful address autoconfiguration server
will allow mobile nodes to acquire new addresses while still using
previously allocated addresses.
6.9. Returning Home
A mobile node detects that it has returned to its home network
through the movement detection algorithm in use (Section 6.1),
when the mobile node detects that its home network prefix is again
on-link. The mobile node SHOULD then send a Binding Update to its
home agent, to instruct its home agent to no longer intercept or
tunnel packets for it. In this Binding Update, the mobile node MUST
set the Care-of Address field to its own IPv6 home address. As with
other Binding Updates sent to register with its home agent, the
mobile node MUST set the Acknowledge (A) and Home Registration (H)
bits and SHOULD retransmit the Binding Update until a matching
Binding Acknowledgement message is received.
The mobile node MUST also send out the appropriate Neighbor
Advertisement packets with the Override flag set, so that its
neighbors on its home network will update the relevant information
for the mobile node in their Neighbor Caches. The mobile node
MUST do this for both its link-local address and its home address.
The Neighbor Advertisement packets can be repeated a small number
of times to guard against occasional loss of packets on the home
network.
7. Home Agent Considerations
7.1. Home Agent Care-of Address Registration
General processing of a received Binding Update that requests a
binding to be cached, is described in Section 5.2. However, if the
Home Registration (H) bit is set in the Binding Update, then the
receiving node MUST process the Binding Update as specified in this
section, rather than following the generall procedure specified in
Section 5.2.
To begin processing the Binding Update, the home agent MUST perform
the following sequence of tests:
- If the node is not a router that implements home agent
functionality, then the node MUST reject the Binding Update and
SHOULD return a Binding Acknowledgement message to the mobile
node, in which the Code field is set to 132 (Home registration
not supported).
- Else, if the Home Address field in the Binding Update is not an
on-link IPv6 address with respect to the home agent's current
Prefix List, then the home agent MUST reject the Binding Update
and SHOULD return a Binding Acknowledgement message to the mobile
node, in which the Code field is set to 133 (Not home network).
- Else, if the home agent chooses to reject the Binding Update for
any other reason (e.g., insufficient resources to serve another
mobile node as a home agent), then the home agent SHOULD return a
Binding Acknowledgement message to the mobile node, in which the
Code field is set to an appropriate value to indicate the reason
for the rejection.
If the home agent does not reject the Binding Update as described
above, then it becomes the home agent for the mobile node. The
new home agent (the receiving node) MUST then create a new entry
(or update the existing entry) in its Binding Cache for this
mobile node's Home Address, as described in Section 5.2. In
addition, the home agent MUST mark this Binding Cache entry as a
"home registration" to indicate that the node is serving as a home
agent for this binding. Binding Cache entries marked as a "home
registration" SHOULD be excluded from the normal cache replacement
policy used for the Binding Cache (Section 5.5) and SHOULD NOT be
removed from the Binding Cache until the expiration of the Lifetime
period.
If the home agent was not already serving as a home agent for the
Home Address specified in the Binding Update (the home agent did
not already have a Binding Cache entry for this address marked as
a "home registration"), then the home agent MUST multicast onto
the home network (to the all-nodes multicast address), a Neighbor
Advertisement message on behalf of the mobile node, with the fields
in the Neighbor Advertisement set as follows:
Router Flag (R)
1 -- the sending node (the home agent) is a router.
Solicited Flag (S)
0 -- the Neighbor Advertisement message is unsolicited.
Override Flag (O)
1 -- the advertisement SHOULD override any existing Neighbor
Cache entry at the receiver, updating the receiver's cached
link-layer address for this Target Address.
Target Address
The mobile node's home address, copied from the Home Address
field of the Binding Update.
Options
The home agent MUST include at least a Target Link-layer
Address option in the Neighbor Advertisement message, in which
the Link-Layer Address gives the link-layer address of the home
agent itself.
Any node on the home network receiving this Neighbor Advertisement
message will thus update its Neighbor Cache to associate the mobile
node's home address with the home agent's link layer address, causing
it to transmit future packets for the mobile node instead to the
mobile node's home agent. Since multicasts on the local link (such
as Ethernet) are typically not guaranteed to be reliable, the home
agent MAY retransmit this Neighbor Advertisement message a small
number of times to increase its reliability. It is still possible
that some nodes on the home network will not receive any of these
Neighbor Advertisements, but these nodes will eventually be able
to detect the link-layer address change for the mobile node's home
address, through use of Neighbor Unreachability Detection [9].
In addition, while this node is serving as a home agent to any mobile
node (it has at least one entry marked as a "home registration" in
its Binding Cache), it SHOULD act as a proxy for each such mobile
node to reply to any received Neighbor Solicitation messages for
it. When a home agent receives a Neighbor Solicitation message, it
MUST check if the Target Address specified in the message matches
the Home Address of any mobile node for which it has a Binding Cache
entry marked as a "home registration". If such an entry exists
in its Binding Cache, the home agent MUST reply to the Neighbor
Solicitation message with a Neighbor Advertisement message, giving
the home agent's own link-layer address as the link-layer address for
the specified Target Address.
7.2. Home Agent Care-of Address De-registration
General processing of a received Binding Update that requests a
binding to be deleted, is described in Section 5.3. However, if the
Home Registration (H) bit is set in the Binding Update, then the
receiving node MUST process the Binding Update as specified in this
section, rather than following the generall procedure specified in
Section 5.3.
To begin processing the Binding Update, the home agent MUST perform
the following sequence of tests:
- If the node is not a router that implements home agent
functionality, then the node MUST reject the Binding Update and
SHOULD return a Binding Acknowledgement message to the mobile
node, in which the Code field is set to 132 (Home registration
not supported).
- Else, if the Home Address field in the Binding Update is not an
on-link IPv6 address with respect to the home agent's current
Prefix List, then it MUST reject the Binding Update and SHOULD
return a Binding Acknowledgement message to the mobile node, in
which the Code field is set to 133 (Not home network).
If the home agent does not reject the Binding Update as described
above, then it MUST delete any existing entry in its Binding Cache
for this mobile node's Home Address, as specified in the Binding
Update.
In addition, the home agent SHOULD multicast a Neighbor Advertisement
message (to the all-nodes multicast address), giving the mobile
node's home address as the Target Address, and specifying the mobile
node's link-layer address in a Target Link-layer Address option in
the Neighbor Advertisement message. The home agent MAY retransmit
this Neighbor Advertisement message a small number of times to
increase its reliability, and any nodes on the home network that miss
all of these Neighbor Advertisements can also eventually detect the
link-layer address change for the mobile node's home address, through
use of Neighbor Unreachability Detection [9].
7.3. Delivering Packets to a Mobile Node
Home agents cannot use Routing headers to deliver packets to the
mobile node, because they can't modify the packet and add to it
in flight. They must always use IPv6 encapsulation [5] for this
purpose.
When a home agent encapsulates a packet for delivery to the mobile
node, the home agent uses the care-of address as the destination
address in the outer IPv6 header. Since the mobile node is presumed
to be receiving packets at the care-of address, the delivery path
from the care-of address to the mobile node's home address is then
trivial.
Note that the home agent cannot insert a routing header, or
modify the destination address of the mobile node, because of IPv6
authentication mechanisms [1]. The home agent is expected to be
involved only rarely with the transmission of data to the mobile
node, because the mobile node will send Binding Updates as soon as
possible to its correspondent nodes.
7.4. Renumbering the Home Network
Neighbor Discovery [9] specifies a mechanism by which all nodes on a Neighbor Discovery [9] specifies a mechanism by which all nodes on a
network can gracefully autoconfigure new addresses, say by combining network can gracefully autoconfigure new addresses, say by combining
a new routing prefix with their existing MAC address. As currently a new routing prefix with their existing MAC address. As currently
specified, this mechanism works when the nodes are on the same link specified, this mechanism works when the nodes are on the same link
as the router issuing the necessary multicast packets to advertise as the router issuing the necessary multicast packets to advertise
the new routing prefix(es) appropriate for the link. the new routing prefix(es) appropriate for the link.
However, for mobile nodes not currently attached to the same link However, for mobile nodes away from home, special care must be taken
as their home agent, special care must be taken to allow the mobile to allow the mobile nodes to renumber gracefully. The most direct
nodes to renumber gracefully. The most direct method of insuring method of insuring this is for the home agent to encapsulated and
this is for the home agent to tunnel the multicast packets to the tunnel the multicast packets to the care-of address of the mobile
care-of address of the mobile node as necessary. The rules for this node as necessary. The rules for this are as follows:
are as follows:
- A mobile node assumes that its routing prefix has not changes - A mobile node assumes that its routing prefix has not changes
unless it receives authenticated router advertisement messages unless it receives authenticated router advertisement messages
from its home agent that the prefix has changed. from its home agent that the prefix has changed.
- When the mobile node is at home, the home agent does not tunnel - When the mobile node is at home, the home agent does not tunnel
router advertisements to it. router advertisements to it.
- When a home network prefix changes, the home agent tunnels router - When a home network prefix changes, the home agent tunnels router
advertisement packets to each mobile node which is currently advertisement packets to each mobile node which is currently
skipping to change at page 15, line 41 skipping to change at page 34, line 5
Neighbor Discovery specification [9] Neighbor Discovery specification [9]
Note that a mobile node is guaranteed that its home address is unique Note that a mobile node is guaranteed that its home address is unique
and used by no other mobile node. However, in some circumstances it and used by no other mobile node. However, in some circumstances it
may nevertheless be true that other nodes on its home network form may nevertheless be true that other nodes on its home network form
the same link-local address as the mobile node during the time when the same link-local address as the mobile node during the time when
the mobile node is away from its home network. Thus, there is the the mobile node is away from its home network. Thus, there is the
requirement above that the mobile node perform Duplicate Address requirement above that the mobile node perform Duplicate Address
Detection when it returns again to its home network. Detection when it returns again to its home network.
9. Multicast Packet Routing 8. Correspondent Node Considerations
8.1. Delivering Packets to a Mobile Node
The routing infrastructure of the Internet will normally route a
packet destined to a mobile node to the mobile node's home network,
if the Destination Address in the packet's IPv6 header is the mobile
node's home address. Once the packet reaches the home network, it
will be intercepted by the mobile node's home agent if the mobile
node is away from home, and will then be encapsulated using IPv6
encapsulation and tunneled to the mobile node's current primary
care-of address. Using this delivery mechanism, the sender need not
know that the node is mobile.
Correspondent nodes that have received and cached a Binding Update
for a mobile node, MAY instead route packets directly to that mobile
node's care-of address. To do so, the correspondent node includes
a Routing header in each packet to the mobile node, to cause the
packet to be routed to the mobile node's care-of address as the last
intermediate routing point before reaching the final destination
of the mobile node's home address. When the packet arrives at the
care-of address (which the mobile node has associated with its
network interface), normal processing of the Routing header by the
mobile node will result in delivery of the packet to the mobile node
as the final destination of the packet.
For example, assuming no other use of the Routing header in the
packet, the sender initializes the Destination Address in the IPv6
header to the mobile node's care-of address, and includes a Type 0
Routing header [6] in the packet initialized as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type=0|Segments Left=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Strict/Loose Bit Map |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the Routing header.
Hdr Ext Len
8-bit unsigned integer. Length of the Routing header in
8-octet units, not including the first 8 octets. For this use
of the Type 0 Routing header, Hdr Ext Len is equal to 2.
Routing Type
0
Segments Left
8-bit unsigned integer. Number of route segments remaining
before reaching the final destination. For this use of the
Type 0 Routing header, Segments Left is initialized to 1 by the
sender.
Reserved
8-bit reserved field. Initialized to zero for transmission;
ignored on reception.
Strict/Loose Bit Map
24-bit bit-map, numbered 0 to 23, left-to-right. For this use
of the Type 0 Routing header, bit 0 of the Strict/Loose Bit Map
is set to 1, indicating strict routing from the care-of
address to the mobile node's home address (both addresses are
associated with the mobile node itself).
Home Address
The home address of the destination mobile node.
If a correspondent node receives an ICMP Host Unreachable or Network
Unreachable message after sending a packet to a mobile node using
its cached care-of address, it SHOULD delete the cache entry from
its Binding Cache until information about the mobile node's current
care-of address becomes available (via a Binding Update).
9. Authentication and Replay Protection
When sending Binding Updates, a mobile node uses the Identification
field in the option, in conjunction with the IPv6 Authentication
Header, to protect against replays of the Binding Update. The style
of replay protection specified for the IPv6 Binding Update involves
the use of a timestamp as the Identification data. Accordingly the
mobile node and the target of its Binding Update have to roughly
agree on the current time. Stale Binding Updates MUST be rejected.
10. Routing Multicast Packets
A mobile node that is connected to its home network functions just A mobile node that is connected to its home network functions just
like any other (stationary) host or router. Thus, when it is at like any other (stationary) node. Thus, when it is at home, a mobile
home, a mobile node functions identically to other multicast senders node functions identically to other multicast senders and receivers.
and receivers. This section therefore describes the behavior of a This section therefore describes the behavior of a mobile node that
mobile node that is not on its home network. is not on its home network.
In order receive multicasts, a mobile node must join the multicast In order receive multicasts, a mobile node must join the multicast
group. Mobile nodes MAY join multicast groups in order to receive group. Mobile nodes MAY join multicast groups in order to receive
transmissions in one of two ways. First, they MAY join the group transmissions in one of two ways. First, they MAY join the group
via a (local) multicast router on the visited subnet. This option via a (local) multicast router on the visited subnet. This option
assumes that there is a multicast router present on the visited assumes that there is a multicast router present on the visited
subnet. The mobile node SHOULD use its dynamically acquired care-of subnet. The mobile node SHOULD use its dynamically acquired care-of
address (if it has acquired one) as the source IP address of its address (if it has acquired one) as the source IPv6 address of its
multicast group membership control message packets. Otherwise, it multicast group membership control message packets. Otherwise, it
MAY use its home address. MAY use its home address.
Alternatively, a mobile node which wishes to receive multicasts can Alternatively, a mobile node which wishes to receive multicasts can
join groups via a bi-directional tunnel to its home agent, assuming join groups via a bi-directional tunnel to its home agent, assuming
that its home agent is a multicast router. The mobile node tunnels that its home agent is a multicast router. The mobile node tunnels
the appropriate multicast group membership control packets to its the appropriate multicast group membership control packets to its
home agent and the home agent forwards multicast packets down the home agent and the home agent forwards multicast packets down the
tunnel to the mobile node. The home agent must tunnel the packet tunnel to the mobile node. The home agent must tunnel the packet
directly to the mobile node's dynamically acquired care-of address, directly to the mobile node's dynamically acquired care-of address,
or, the packet must be tunneled first to the mobile node's home or, the packet must be tunneled first to the mobile node's home
address and then recursively tunneled to the mobile node's care-of address and then recursively tunneled to the mobile node's care-of
address. address.
A mobile node which wishes to send packets to a multicast group A mobile node which wishes to send packets to a multicast group also
also has two options: (1) send directly on the visited network; or has two options: (1) send directly on the visited network; or (2)
(2) send via a tunnel to its home agent. Because multicast routing send via a tunnel to its home agent. Because multicast routing in
in general depends upon the IP source address, a mobile node which general depends upon the IPv6 source address, a mobile node which
sends multicast packets directly on the visited network MUST use sends multicast packets directly on the visited network MUST use a
a dynamically acquired care-of address as the IP source address. dynamically acquired care-of address as the IPv6 source address.
Similarly, a mobile node which tunnels a multicast packet to its home Similarly, a mobile node which tunnels a multicast packet to its home
agent MUST use its home address as the IP source address of both the agent MUST use its home address as the IPv6 source address of both
(inner) multicast packet and the (outer) encapsulating packet. This the (inner) multicast packet and the (outer) encapsulating packet.
second option assumes that the home agent is a multicast router. This second option assumes that the home agent is a multicast router.
10. Compatibility with ICMP 11. Constants
When sending a packet to a mobile node, it is important to correctly INITIAL_BINDACK_TIMEOUT 1 second
return to the original sender any ICMP error messages generated by
this packet. Since in most cases such packets use a routing header
containing the care-of address, this is usually not a problem.
However, when a packet encapsulated at the home agent encounters such MAX_BINDACK_TIMEOUT 256 seconds
an error condition, ICMP error messages are returned to the sender as
specified in [3]. ICMP for IP version 6 has been specified to return
as much of the original packet as will fit in the ICMP error message
without the ICMP packet exceeding 576 octets [4]. This size should
be sufficient for correctly returning ICMP error messages backwards
along the tunnel.
11. Protocol Requirements MAX_UPDATE_RATE 1 per second
This section summarizes the requirements introduced by the above SLOW_UPDATE_RATE once per 10 seconds
protocol operations for IPv6 nodes and for routers.
11.1. Requirements for IPv6 Nodes Acknowledgements
Every IPv6 node must be able to interpret Binding Update packets. We would like to thank Thomas Narten for contributing valuable
Every IPv6 node must be able to maintain Security Associations for discussion and reviewing this draft, and for helping to shape some of
use in IPv6 Authentication Headers [1] which are used to authenticate the recent changes relevant to the operation of Neighbor Discovery.
Binding Updates and protect against replay attacks. Every IPv6
node must be able to associate care-of addresses with IPv6 target
addresses, and use routing headers to deliver packets to IPv6 target
addresses (e.g., mobile node addresses) using the care-of address as
an intermediate router address.
11.2. Requirements for IPv6 Mobile Nodes References
Every IPv6 mobile node must be able to perform IPv6 decapsulation. [1] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.
Every mobile node must be able to send Binding Updates as outlined
above, and receive Binding Acknowledgements from routers. Every IPv6
mobile node must keep track of which other IPv6 nodes may need to
receive Binding Updates as a result of recent movement by the mobile
node. In particular, every IPv6 mobile node must be able to send
Binding Updates when a packet is received that does not use a routing
header to specify its care-of address.
11.3. Requirements for IPv6 Routers [2] R. Atkinson. Security Architecture for the Internet Protocol.
RFC 1825, August 1995.
Every IPv6 router must perform the mobility-related functions listed [3] J. Bound and C. Perkins. Dynamic Host Configuration Protocol
in the previous subsection (11.1) for IPv6 nodes, but not necessarily for IPv6. draft-ietf-dhc-dhcpv6-05.txt -- work in progress,
the functions for mobile nodes. June 1996.
Every IPv6 router must be able to issue Binding Acknowledgements in [4] A. Conta and S. Deering. Internet Control Message Protocol
response to Binding Updates received and accepted from a mobile node. (ICMPv6) for the Internet Protocol Version 6 (IPv6). RFC 1885,
Every IPv6 router must be able to encapsulate packets in order to December 1995.
tunnel them to a care-of address known for a mobile node from which
it has received a binding update. Every IPv6 router must be able to
maintain security associations for the mobile nodes from which it
will accept binding updates.
A. Constants [5] A. Conta and S. Deering. Generic Packet Tunneling in IPv6.
draft-ietf-ipngwg-ipv6-tunnel-01.txt - work in progress,
February 1996.
INITIAL_BINDACK_TIMEOUT == 1 second [6] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883, December 1995.
MAX_BINDACK_TIMEOUT == 256 seconds [7] D. Haskin and E. Allen. IP Version 6 over PPP.
draft-ietf-ipngwg-pppext-ipv6cp-03.txt - work in progress, June
1996.
MAX_UPDATE_RATE == 1 per second [8] David B. Johnson and Charles E. Perkins. Route Optimization
in Mobile-IP. draft-ietf-mobileip-optim-04.txt -- work in
progress, February 1996.
B. Open issues [9] T. Narten, E. Nordmark, and W. Simpson. IPv6 Neighbor
Discovery. draft-ietf-ipngwg-discovery-03.txt -- work in
progress, November 1995.
B.1. Using Encapsulation Protocols [10] Joyce K. Reynolds and Jon Postel. Assigned Numbers. RFC 1700,
October 1994.
Should alternative encapsulation techniques be defined for use with [11] Fumio Teraoka. draft-teraoka-ipv6-mobility-sup-02.txt.
these protocols? Should a minimal encapsulation be defined and Internet Draft -- work in progress, January 1996.
specified as the default?
There is only one possible advantage afforded by the use of [12] S. Thomson and T. Narten. IPv6 Stateless Address
encapsulation, compared to the use of the existing routing header Autoconfiguration. draft-ietf-addrconf-ipv6-auto-06.txt
defined for IPv6, and it only occurs when a mobile node uses a - work in progress, November 1995.
care-of address associated with a router attached to the same link as
the mobile node's point of attachment as in B.3. If a mobile node
has a link to a router over a low speed wireless link, and the router
receives encapsulated packets for the mobile node, the encapsulation
is stripped away before final delivery is made to the mobile node.
In that case, fewer bytes are transmitted over the low-speed link,
than would be the case for a normally processed routing header
specifying the care-of address. Perhaps this would be better taken
care of by defining something like TCP header compression over the
link from the router to the mobile node. Such a compression scheme
would eliminate the need to include the routing header information in
every packet delivered over a slow-speed connection between a router
and a mobile node.
Another alternative would be to provide another type of routing A. Open Issues
header (routing type == 2, say) which would allow an intermediate
node to delete itself from the list instead of just rearranging the
list. This would completely eliminate the need for encapsulation for
normal datagrams from correspondent host to mobile node. However,
having routers remove addresses to shrink the packet size may be a
slow operation (relatively speaking).
B.2. Session keys with local routers A.1. Session Keys with Local Routers
In the IPv4 route optimization proposal, a mechanism is outlined In the IPv4 route optimization proposal [8], a mechanism is outlined
whereby a session key can be established between foreign agents whereby a session key can be established between foreign agents
and mobile clients, without requiring any pre-established security and mobile nodes, without requiring any pre-established security
relationship between them. A similar mechanism should be defined for relationship between them. A similar mechanism could be defined for
IPv6, to avoid the need for a possibly time-consuming negotiation IPv6, to avoid the need for a possibly time-consuming negotiation
between routers and mobile nodes for the purpose of obtaining the between routers and mobile nodes for the purpose of obtaining the
session key, which under many circumstances would only be used once. session key, which under many circumstances would only be used once.
This mechanism, if needed, can be specified completely outside the This mechanism, if needed, can be specified completely outside
mobile-IPv6 protocol and would amount to a way of creating a dynamic the Mobile IPv6 protocol and would amount to a way of creating a
SPI between two nodes which do not share a trust relationship, but dynamic security association between two nodes which do not share an
which need to agree on a key for some particular purpose (here, existing trust relationship, but which need to agree on a key for
allowing the future authentication of a binding update). Hopefully, some particular purpose (here, allowing the future authentication of
Photuris [8] will allow this function to be performed appropriately a Binding Update). Hopefully, the work of the IP Security Working
for mobile nodes, say by a Diffie-Hellman key exchange. Group will allow this function to be performed appropriately for
mobile nodes, say by a Diffie-Hellman key exchange.
B.3. Local Router Considerations
In previous versions of this specification, routers local to the
current point of attachment of the mobile node ("local routers")
were expected to offer services to mobile nodes. That is still
quite feasible, and requires only that the routers support the
decapsulation procedure required to extract the packet for final
delivery to the mobile node. If every router supports decapsulation
(in addition to the operations required from every IPv6 router and
IPv6 node), then addresses formed using any prefix advertised by
the router could be used as a care-of address except the router's
link-local address. Enabling this style of care-of address
acquisition will likely require some straightforward enhancements
to the IPv6 Neighbor Discovery packet formats. In particular, a
Router Advertisement should probably define another per-prefix bit
to specify whether the prefix is available to the mobile nodes for
constructing a care-of address. For stateful address configuration,
an option could be defined to allow the stateful server to notify a
mobile node of a legitimate care-of address appropriate for use at
the new point of attachment.
Many other operations, related to registration of the mobile node in
a new service area, are likely to become important as mobile nodes
become more prevalent. For instance, it may be required to:
- authenticate the identity of mobile clients
- charge for connection services
- produce or share a session key for use by new mobile clients
(say, for encryption)
- negotiate a compression algorithm
- manage the resources of router's communications devices
B.4. Source Address Filtering by Firewalls A.2. Source Address Filtering by Firewalls
The current specification does nothing to permit mobile nodes to The current specification does nothing to permit mobile nodes to
send their packets through firewalls which filter out packets with send their packets through firewalls which filter out packets with
the "wrong" source IPv6 addresses in the IPv6 packet header. The the "wrong" source IPv6 addresses in the IPv6 packet header. The
mobile node's home address may be unlikely to fall within the ranges mobile node's home address may be unlikely to fall within the ranges
required to satisfy the firewall's criteria for further delivery. required to satisfy the firewall's criteria for further delivery.
This subject needs serious discussion soon. As indicated by As indicated by recent discussion, firewalls are unlikely to
recent discussion, such firewalls are unlikely to disappear. Any disappear. Any standardized solution [11] to the firewall problem
standardized solution [11] to the firewall problem based on hiding based on hiding the non-local source address outside the source
the non-local source address outside the source addres field address field of the IPv6 header is likely to fail. Any vendor or
of the IPv6 header is likely to fail. Any vendor or facilities facilities administrator wanting to filter based on the address in
administrator wanting to filter based on the address in the IPv6 the IPv6 source address field would also quickly begin filtering on
source address field would also quickly begin filtering on hidden hidden source addresses.
source addresses.
C. Acknowledgments
Thanks to Thomas Narten for contributing valuable discussion and
reviewing this draft, as well as helping to shape some recent changes
relevant to the operation of Neighbor Discovery.
References
[1] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.
[2] J. Bound. Dynamic Host Configuration Protocol for IPv6.
draft-ietf-dhc-dhcpv6-03.txt -- work in progress, November 1995.
[3] A. Conta and S. Deering. Generic Packet Tunneling in IPv6.
draft-ietf-ipngwg-ipv6-tunnel-00.txt - work in progress,
November 1995.
[4] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6). RFC 1885,
December 1995.
[5] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883, December 1995.
[6] IETF Mobile-IP Working Group. IPv4 Mobility Support.
ietf-draft-mobileip-protocol-12.txt - work in progress,
September 1995.
[7] David B. Johnson and Charles E. Perkins. Route Optimization
in Mobile-IP. draft-ietf-mobileip-optim-03.txt -- work in
progress, November 1995.
[8] P. Karn and B. Simpson. draft-ietf-ipsec-photuris-08.txt. Assume, for the moment, that a mobile node is able to establish a
Internet Draft -- work in progress, November 1995. secure tunnel through a firewall protecting the domain in which
a correspondent node is located. The mobile node could then
encapsulate its packet so that the outer IPv6 header was addressed
to the firewall and used the mobile node's care-of address as the
source address. When the firewall decapsulates, it would be able to
authenticate the inner packet based (correctly) on the mobile node's
home address. After the authentication is performed, the firewall
could forward the packet to the correspondent node as desired. This
simple procedure has the feature that it requires the minimal amount
of encapsulation, no assistance by routers or other agents, and that
the firewall can establish a security relationship with the mobile
node based on its home (i.e., permanent) address.
[9] T. Narten, E. Nordmark, and W. Simpson. IPv6 Neighbor Chair's Address
Discovery. draft-ietf-ipngwg-discovery-03.txt -- work in
progress, November 1995.
[10] J. Reynolds and J. Postel. Assigned Numbers. RFC 1700, October The Working Group can be contacted via its current chair:
1994.
[11] Fumio Teraoka. draft-teraoka-ipv6-mobility-sup-02.txt. Jim Solomon
Internet Draft -- work in progress, January 1996. Motorola, Inc.
1301 E. Algonquin Rd.
Schaumburg, IL 60196
[12] S. Thomson and T. Narten. IPv6 Stateless Address Work: +1-847-576-2753
Autoconfiguration. draft-ietf-addrconf-ipv6-auto-06.txt E-mail: solomon@comm.mot.com
- work in progress, November 1995.
Authors' Addresses Authors' Addresses
Charles Perkins Questions about this document can also be directed to the authors:
Room J1-A25
T. J. Watson Research Center
IBM Corporation
30 Saw Mill River Rd.
Hawthorne, NY 10532
Work: +1 914 789-7350
Fax: +1 914 784-7007
E-mail: perk@watson.ibm.com
David B. Johnson David B. Johnson
Computer Science Department Computer Science Department
Carnegie Mellon University Carnegie Mellon University
5000 Forbes Avenue 5000 Forbes Avenue
Pittsburgh, PA 15213-3891 Pittsburgh, PA 15213-3891
Work: +1 412 268-7399 Work: +1 412 268-7399
Fax: +1 412 268-5576 Fax: +1 412 268-5576
E-mail: dbj@cs.cmu.edu E-mail: dbj@cs.cmu.edu
Charles Perkins
Room H3-D34
T. J. Watson Research Center
IBM Corporation
30 Saw Mill River Rd.
Hawthorne, NY 10532
Work: +1 914 789-7350
Fax: +1 914 784-6205
E-mail: perk@watson.ibm.com
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