Home Agent assisted Route Optimization between Mobile IPv4 Networks

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Abstract

This document describes a Home Agent assisted route optimization extension to IPv4 Network Mobility Protocol.

1. Introduction and motivations
2. Terms and definitions
3. Mobile IPv4 route optimization between mobile networks
    3.1. Maintaining route optimization information
        3.1.1. Advertising route-optimizable prefixes
        3.1.2. Route Optimization cache
        3.1.3. Correspondent Router Mobility Bindings
    3.2. Return routability procedure
        3.2.1. Router keys
        3.2.2. Nonces
    3.3. Mobile-Correspondent Router operations
        3.3.1. Triggering Route Optimization
        3.3.2. Mobile Router routing tables
        3.3.3. Inter-Mobile Router registration
        3.3.4. Inter-Mobile Router tunnels
        3.3.5. Constructing route-optimized packets
        3.3.6. Handovers and Mobile Routers leaving network
    3.4. Convergence and synchronization issues
4. Data compression schemes
    4.1. Prefix compression
    4.2. Realm compression
        4.2.1. Encoding of compressed realms
        4.2.2. Searching algorithm
        4.2.3. Encoding example
5. New Mobile IPv4 messages and extensions
    5.1. Route optimization prefix advertisement
    5.2. Mobile router Route optimization capability
    5.3. Home-Test Init message
    5.4. Care-of-Test Init message
    5.5. Home Test message
    5.6. Care-of test message
    5.7. Mobile-Correspondent authentication extension
    5.8. Care-of address Extension
6. Special Considerations
    6.1. NATs and stateful firewalls
    6.2. Foreign Agents
    6.3. Multiple Home Agents
    6.4. Mutualness of Route Optimization
    6.5. Extensibility
    6.6. Load Balancing
7. Scalability
8. Example signaling scenarios
    8.1. Registration request
    8.2. Route optimization with return routability
    8.3. Handovers
9. IANA Considerations
10. Security Considerations
    10.1. Trust relationships
11. Acknowledgements
12. References
    12.1. Normative References
    12.2. Informative References
§ Authors' Addresses

1. Introduction and motivations

From service provider perspective a common topology for enterprise customer network consists of one to several sites (typically headquarters and various branch offices). These sites are typically connected via various Layer 2 technologies (ATM or Frame relay PVCs), MPLS VPNs or Layer 3 site-to-site VPNs.

Recently, a trend has emerged where customers prefer to maintain connectivity via multiple service providers. Reasons include redundancy, reliability and availability issues. These kinds of multi-homing scenarios have traditionally been solved by using such technologies as multihoming BGP. However, a more lightweight solution is desirable.

Mobile IP, especially mobile routers, can accommodate for this. The customer becomes a client of a virtual service provider, which does not take part in the actual access technology. The service provider has a backend system and an IP pool that it distributes to customers. The drawback of this solution is that it creates a star topology; All Mobile IP tunnels end up at the service provider hosted home agent, causing heavy load at the backend. Route optimization between mobile networks addresses this issue, by taking network load off the home agent and the backend.

        +----------------------------+
        |  Virtual Operator Backend  |
        +------------+         +-----+
        | Home Agent |         | AAA |
        +------------+---------+-----+
                     |
                   .--.
                 _(.   `)
               _(   ISP `)_
              (   Peering  `)
             ( `  . Point )  )
              `--(_______)--'
        ____ /     |         \
       /           |          \
    .--.         .--.         .--.
  _(    `.     _(    `.     _(    `.
 (  ISP A )   (  ISP B )   (  ISP C )
( `  .  )  ) ( `  .  )  ) ( `  .  )  )
 `--(___.-'   `--(___.-'   `--(___.-'
     |     ______/    \       /
     |    /            \     /
     |   /              \   /
   +----+               +----+
   |MR A|               |MR B|
   +----+               +----+
     |                    |
    .--.                 .--.
  _(    `.             _(    `.
 ( Site A )           ( Site B )
( `  .  )  )         ( `  .  )  )
 `--(___.-'           `--(___.-'

In this case, organization network consists of two sites, that are connected via 2 ISPs for redundancy reasons. Mobile IP allows fast handovers without problematics of multi-homing and BGP peering between each individual ISP and the organization. The traffic however takes a non-optimal route through the virtual operator backend.

Route optimization would address this issue, allowing traffic between Sites A and B to flow through ISP B's network, or in case of a link failure, via the ISP peering point (such as MAE-WEST). The backend will not suffer from heavy loads.

The primary design goal is to limit the load to the backend to minimum. Additional design goals include extensibility to a more generalized scope, beyond the need of a single, coordinating Home Agent.

2. Terms and definitions

3. Mobile IPv4 route optimization between mobile networks

The Home Agent-assisted route optimization is primarily intended for helping to optimize traffic patterns between multiple sites in an single organization or administrative domain; However, extranets can also be reached with optimized routes, as long as all Mobile Routers connect to the same Home Agent. The procedure aim to maintain backwards compatibility; With legacy nodes or routers full connectivity is always preserved even though optimal routing cannot be guaranteed.

The schema requires a Mobile Router to be able to receive messages from Home Agent and other Mobile Routers unsolicited - that is, without first initiating a request. This behavior is similar to the registration revocation procedure (Glass, S. and M. Chandra, “Registration Revocation in Mobile IPv4,” August 2003.) [RFC3543]. Many of the mechanisms are same - including the fact that advertising route optimization support upon registration implies capability to receive registration requests and return routability messages from other Mobile Routers.

Compared to IPv6, where Mobile Node <-> Correspondent node bindings are maintained via Mobility Routing header and Home Address options, Mobile IPv4 always requires the use of tunnels. Therefore, inter-mobile-router tunnel establishment has to be conducted.

3.1. Maintaining route optimization information

During registration, a joining Mobile Router MAY request information on route-optimizable networks and let the Home Agent allow re-distribution of information of its own networks. This is indicated with Mobile Router route optimization capability extension, see Section 5.2 (Mobile router Route optimization capability).

Note that the redistribution of networks from the Home Agent happens only during the registration phase. There are no "routing updates" from Home Agent except in the form of re-registration. Besides timeouts, the re-registration may occur if a Correspondent Router receives a registration request from a unknown Mobile Router (see Section 3.3.3 (Inter-Mobile Router registration)).

3.1.1. Advertising route-optimizable prefixes

A NEMO-supporting Home Agent already maintains information on which network(s) are reachable behind certain Mobile Routers. Only change to this functionality is that this information can now be distributed to other nodes upon request. This request is defined in Section 5.2 (Mobile router Route optimization capability).

When a Home Agent receives a registration request, it conducts the normal authentication and authorization procedures.

If registration is successful and the route optimization request was present in the registration request, the reply message MAY include one route optimization prefix advertisement extensions which inform the Mobile Router of all existing mobile networks and the Mobile Routers that manage them. The networks SHOULD be included in order of priority, with the networks most desired to conduct optimization listed first. The extension is constructed as shown in Section 5.1 (Route optimization prefix advertisement). The extension consists of a list where each Mobile Router is listed with according prefix(es) and their respective realm(s).

Each network prefix can be associated to a realm, usually of form '@organization.example.com'. Besides the routers in customer's own organization, the prefix list may also include other Mobile Routers, e.g. default prefix (0.0.0.0/0) pointing towards Internet gateway for Internet connectivity, and possible extranets. The realm information can be used to make policy decisions on the Mobile Router, such as preferring optimization within specific realm only.

In common scenarios, where prefixes are allocated to Mobile Routers connecting to a single Home Agent, the prefixes are usually either continuous or at least very close to each other. Due to these qualities, a prefix compression mechanism is provided. A similar schema is in use for realm information, where realms often share same higher-level domains. These compression mechanisms are further examined in Section 4 (Data compression schemes).

Upon receiving registration reply with the route optimization prefix advertisement extension, the Mobile Router SHALL insert the Mobile Router HoA as a host-prefix to the local Route Optimization Cache if it does not already exist. If they are included, any additional prefixes information SHALL also be inserted to the Route Optimization Cache.

The Mobile Router MAY discard entries from a desired starting point onwards, due to memory or other policy related constraints. The intention of listing the prefixes in order of priority is to provide implicit guidance for this decision. If the capacity of the device allows, the Mobile Router SHOULD use information on all advertised prefixes.

3.1.2. Route Optimization cache

Mobile routers supporting route optimization will maintain a Route Optimization Cache.

The Route Optimization Cache contains mappings between Correspondent Router HoA's, network(s) associated with each HoA and return routability procedure status.

The Route Optimization Cache contains the following information for all Correspondent Routers:

3.1.3. Correspondent Router Mobility Bindings

Each Correspondent Router will maintain Mobility Bindings, in a similar way to the Home Agent.

3.2. Return routability procedure

The return routability procedure for Mobile IPv6 is described in [RFC3775] (Johnson, D., Perkins, C., and J. Arkko, “Mobility Support in IPv6,” June 2004.). Same principles apply to the Mobile IPv4 version: Two messages are sent to Correspondent Router's Home Address, one via Home Agent and the other directly from the Mobile Router CoA, with two responses coming through same routes. Registration management key is derived from token information carried on these messages. This registration management key (KRm) can then be used to authenticate registration requests (comparable to Binding Updates in Mobile IPv6).

The Return Routability procedure is a method provided by Mobile IP protocol to establish the KRm in a relatively lightweight fashion. If desired, the KRm's can be configured to Mobile Routers statically, or using an appropriate secure key provisioning mechanism. If KRm's are known to the Mobile Routers via some other mechanism, Return Routability procedure can be skipped. Such provisioning mechanisms are out of scope for this document.

Assumption on traffic patterns is that the Mobile Router that initiates the RR procedure can always send outbound messages, even when behind NAT or firewall. This basic assumption made for NAT Traversal in [RFC3519] (Levkowetz, H. and S. Vaarala, “Mobile IP Traversal of Network Address Translation (NAT) Devices,” April 2003.) is also applicable here. In case the Correspondent Router is behind such obstacles, it receives these messages via the reverse tunnel to CR's Home Address, thus any problem regarding the CR's connectivity is addressed during the registration phase.

3.2.1. Router keys

Each Correspondent Router maintains a router key, Kcr, which is not shared with anyone else. This is analogous to node key, Kcn, in Mobile IPv6. Correspondent Router uses router key to verify that the keygen tokens sent by Mobile Router in registration request are its own. The router key MUST be a random number, 96 bits in length.

3.2.2. Nonces

Each Correspondent Router also maintains one or more indexed nonces. Nonces should be generated periodically with a good random number generator. The Correspondent Router may use same nonces with all Mobile Routers.

Correspondent Routers keep both the current nonce and small set of valid previous nonces whose lifetime have not expired yet.

Return Routability procedure may be initiated only when the Route Optimization Cache's RRSTATE field for the Correspondent Router is INACTIVE. When Return Routability procedure is initiated, the state MUST be set to IN PROGRESS.

The Return Routability procedure consists of four new Mobile IP messages: Home Test Init, Care-of Test Init, Home Test and Care-of Test. They are constructed as shown in Section 5.3 (Home-Test Init message) through Section 5.6 (Care-of test message). If the Mobile Router has included the Mobile Router optimization capability extension in its Registration Request, it MUST be able to accept Return Routability messages. The messages are delivered as standard Mobile IP packets. The addresses are set to Correspondent Router's HoA and Mobile Router's CoA.

The return routability procedure begins with the Mobile Router sending HoTI and CoTI messages, each containing a cookie.

Upon receiving the HoTI or CoTI message the Correspondent Router MUST have a secret Kcr. If the Kcr does not exist, it must be produced before continuing with the return routability procedure.

Correspondent Router responds to HoTI and CoTI messages by constructing HoT and CoT messages, respectively, as replies. HoT message contains current home nonce index and CoT message contains current care-of nonce index.

Upon completion of Return Routability procedure, the Routing Optimization Cache's RRSTATE field is set to ACTIVE. The Mobile Router will establish a registration management key KRm:

Like in Mobile IPv6, the Correspondent Router does not maintain any state for the Mobile Router until after receiving a registration request.

3.3. Mobile-Correspondent Router operations

This section deals with the operation of Mobile and Correspondent Routers performing route optimization. Note that in a typical case all routers work as both Mobile Router and Correspondent Router.

Especially compared to Mobile IPv6 route optimization there are two issues that are different regarding IPv4. First of all, since Mobile IPv4 always uses tunnels, there must be a tunnel established between MR and CR's Care-of addresses. This is accomplished with a new extension, "Care-of Address", in registration reply. Second issue is rising from security standpoint: In a registration request, the Mobile Router claims to represent an arbitrary IPv4 network. If the CR has not yet received this information (HoA <-> Network prefix), it SHOULD perform a re-registration to Home Agent to verify the claim.

3.3.1. Triggering Route Optimization

Since each Mobile Router knows all the route-optimizable networks, the route optimization between all Correspondent Routers can be established at any time; However a better general practice is to conduct Route Optimization activation on-demand only. Route optimization SHOULD only be started when receiving a packet where destination address is local (and the subnet is registered as route optimizable) and source address exists in the Route Optimization Cache.

3.3.2. Mobile Router routing tables

Each Mobile Router maintains a routing table. In a typical situation, it contains interface(s) to the local networks, interface to wide-area network (such as ISP), and a tunnel interface to the Home Agent. Additional entries consist of route-optimized tunnel interfaces and networks associated with each tunnel.

3.3.3. Inter-Mobile Router registration

If route optimization between Mobile Router and Correspondent Router is desired, either Return Routability procedure must have been performed ( See Section 3.2 (Return routability procedure)), or key KRm must be pre-shared between the Mobile and Correspondent Router. If a known KRm exists, a Mobile Router MAY send a registration request to the Correspondent Router's HoA.

The encapsulation can be set as desired, except in the case where the Correspondent Router's Route Optimization Cache Entry has NAT set or the Mobile Router itself is behind NAT or firewall. If either of the conditions apply, registration request MUST specify UDP encapsulation.

The Correspondent Router first checks the registration request's authentication against Kcr and nonce indexes negotiated during Return Routability procedure. This ensures that the registration request is coming from a correct Mobile Router. If the check passes, the Correspondent Router MUST check whether the Mobile Router already exists in it's Route Optimization Cache and is linked with the prefixes included in the request.

If the prefixes don't match, the Correspondent Router SHOULD send a re-registration request to Home Agent with the 'S' (solicitation) bit set, thus obtaining the latest information on mobile prefixes managed by incoming Mobile Router. If, even after this update, the prefixes still don't match, the Correspondent Router MUST reject the registration request. This verification is done to protect against Mobile Router claiming to represent arbitrary networks; However, since Home Agent provides trusted information, it can authorize Mobile Router's claim. If the network is considered trusted, the Correspondent Router can, as a policy, accept registrations from without this check; however, this is NOT RECOMMENDED as a general practice.

If the prefixes match, the Correspondent Router MAY accept the registration. Upon receiving the registration reply, the Mobile Router MUST check if a tunnel to the Mobile Router already exists. Otherwise a tunnel MUST be established and Mobility Binding updated. This is covered in Section 3.3.4 (Inter-Mobile Router tunnels).

The Correspondent Router's routing table MUST be updated to include the Mobile Router's networks are reachable via the tunnel to the Mobile Router.

After the tunnel is established, the Mobile Router MAY update it's routing tables to reach all Correspondent Router's Prefixes via the tunnel. This is primarily a policy decision depending on the network environment. See section Section 6.4 (Mutualness of Route Optimization).

3.3.4. Inter-Mobile Router tunnels

Inter-Mobile Router tunnel establishment follows establishing standard reverse tunnels to the Home Agent. The registration request to Correspondent Router includes information on the desired encapsulation. In the case of GRE, IP over IP or minimal encapsulation no special considerations regarding the reachability are necessary; The tunnel has no stateful information; The packets are simply encapsulated within the GRE, IP, or minimal header.

The tunnel origination point for the Correspondent Router is its Care-of Address, not the address where the registration requests were sent. This is different from creation of the Reverse Tunnel to Home Agent.

Special considerations rise from using UDP encapsulation, especially in cases where one of the Mobile Routers is located behind NAT or firewall. If the Route Optimization Cache has NAT bit set for a specific network, the UDP tunnel establishment MUST be initiated from the Correspondent Router. However, the procedure otherwise follows [RFC3519] (Levkowetz, H. and S. Vaarala, “Mobile IP Traversal of Network Address Translation (NAT) Devices,” April 2003.). Once the first UDP keepalive is sent, the tunnel can be considered active.

If both the Mobile Router and the Correspondent Router are behind NAT, route optimization cannot be performed between them. Possibilities to set up mutual tunneling when both routers are behind NAT, are outside the scope of this draft. However, some of these issues are addressed in Section 6.1 (NATs and stateful firewalls ).

Due to the fact that the route optimization procedures may occur concurrently at two Mobile Routers, each working as each other's Correspondent Router, there may be a situation where two routers are attempting to establish separate tunnels between them at the same time. If a router with a smaller Home Address receives a registration request (in CR role) while its own registration request (sent in MR role) has not been answered yet, the reply must be held until the tunnel initiated by its registration request is up. This avoids the problem of maintaining two separate tunnels forming concurrently between two Mobile Routers.

Since typical routers act as both MR's and CR's, tunnels can be only torn down if there is no Mobility Bindings on the tunnel (in Correspondent Router role) AND no Mobility Sessions are bound to the tunnel (Mobile Router role). Both Mobility Sessions and Mobility Bindings will go down when their lifetime expires.

3.3.5. Constructing route-optimized packets

All packets received by the Mobile Router are forwarded using normal routing rules according to the routing table. There are no special considerations when constructing the packets, the interface procedure will encapsulate any packet automatically.

If prefixes overlap each other, e.g. 192.0.2.128/25 and 192.0.2.128/29, the standard longest match rule for routing is in effect. However, overlapping private address SHOULD be considered an error situation. Any aggregation for routes in private address space SHOULD be conducted only at HA.

3.3.6. Handovers and Mobile Routers leaving network

Handovers and connection breakdowns can be categorized as either ungraceful or graceful, or break-before-make and make-before-break situations.

In a typical situation, router act as both Mobile Router and Correspondent Router for other routers' prefixes.

When a Mobile Router wishes to leave network, it SHOULD send a re-registration request to all Correspondent Routers with the lifetime set to zero. If the Correspondent Router is also acting as a Mobile Router with active Route Optimization with the leaving router, it SHOULD send a similar re-registration request with the lifetime set to zero. This causes both the Mobility Bindings and active Mobility Sessions to go down, allowing for teardown of the tunnel.

If the Mobile Router was unable to send the re-registration request before handover, it MUST send it immediately after handover is completed and binding with the Home Agent is up. In a similar fashion, Correspondent Router acting as a Mobile Router MUST realize that the old Mobility Session is no longer valid and establish a new one. Thus, route optimization can resume.

3.4. Convergence and synchronization issues

Home Agent state and Mobile Routers' Route Optimization Caches do not need to be explicitly synchronized. The assumption is that at least some of the traffic between mobile networks is always bidirectional. This will cause Mobile Routers to perform re-registrations and thus they will receive updates on-demand only.

Consider a situation with three mobile networks, A, B, C handled by three Mobile Routers, MR A, MR B and MR C. If they register to a Home Agent in this order, the situation goes as follows:

MR A registers; Receives no information on other networks from HA, as no other MR has registered yet.

MR B registers; Receives information on mobile network A being reachable via MR A.

If a node in mobile network C receives traffic from mobile network A, the route optimization is straightforward; MR C already has network A in its Route Optimization Cache. When it registers to MR A (after Return Routability procedure is completed), MR A does not have information on mobile network C; Thus it will perform a re-registration to the Home Agent on-demand. This allows MR A to verify that MR C is indeed managing network C.

If a node in mobile network B receives to traffic from mobile network C, MR B has no information on network C. No route optimization is triggered. However, when network B's reply reaches MR C, route optimization happens as above. Further examples of signaling are in Section 8 (Example signaling scenarios).

Even in the very rare case of completely unidirectional traffic from an entire network, the re-registrations caused by timeouts will eventually cause convergence. However, this should be treated as a special case.

Note that all Mobile Routers are connected to same Home Agent. For possibilities concerning multiple Home Agents, see Section 6.3 (Multiple Home Agents)

4. Data compression schemes

This section defines the two compression formats used in Route Optimization Prefix Advertisement extensions.

4.1. Prefix compression

The prefix-compression is based on the idea that prefixes usually share common properties. The scheme is simple delta-compression. In the prefix information advertisement, Section 5.1 (Route optimization prefix advertisement), the D bit indicates whether receiving a "master" or a "delta" prefix. This, combined with the Prefix Length information, allows for compression and decompression of prefix information.

If D=0, what follows in the "Prefix" field are bits 1..n of the a new master prefix, where n is PLen. This is rounded up to nearest full octet. Thus, prefix lengths of /4 and /8 take 1 octet, /12 and /16 take 2 octets, /20 and /24 three, and larger than that full 4 octets.

If D=1, what follows in the "Prefix" field are bits m..PLen of the prefix, where m is the first changed bit of previous master prefix, with padding from master prefix filling the field to full octet. Maximum value of Plen-m is 8 (that is, delta MUST fit into one octet). If this is not possible, a new master prefix has to be declared.

Determining the order of prefix transmission should be based on saving maximum space during transmission.

Example of compression and transmitted data, where network prefixes 192.0.2.0/28, 192.0.2.64/26 and 192.0.2.128/25 are transmitted are illustrated. Because of the padding to full octets, redundant information is also sent. The bit-patterns being transmitted are:

 =+= shows the prefix mask
 --- shows the master prefix for delta coded prefixes
 192.0.2.0/28, D=0

 0                   1                     2                     3
 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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|0|0|0|0|0|0|0|0|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+
 ^                                                                   ^
 +---------------------------- encoded ------------------------------+
                                                               ^     ^
                                                               +-pad-+
 192.0.2.64/26, D=1

 0                   1                     2                     3
 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 2
+-------------------------------------------------------------+-+-+-+-+
|1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|0|1|0|0|0|0|0|0|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+-+-+
                                         ^               ^
                                         +--- encoded ---+
                                         ^             ^
                                         +-- padding --+
 192.0.2.128/25, D=1

 0                   1                     2                     3
 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 2
+-------------------------------------------------------------+-+-+-+-+
|1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|1|0|0|0|0|0|0|0|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+-+-+-+
                                       ^               ^
                                       +--- encoded ---+
                                       ^           ^
                                       +- padding -+

First prefix, 192.0.2.0/28, is considered a master prefix and is transmitted in full. The PLen of 28 bits determines that all four octets must be transmitted. If the prefix would have been eg. 192.0.2.0/24, three octets would have sufficed since 24 bits fit into 3 octets.

For the following prefixes, the D=1. Thus, they are deltas of the previous prefix where D was zero.

192.0.2.64/26 includes bits 19-26 (full octet). Bits 19-25 are copied from master prefix, but bit 26 is changed to 1. The final notation in binary is "1001", or 0x09.

192.0.2.128/25 includes bits 18-25 (full octet). Bits 18-24 are copied from master prefix, but bit 25 is changed to 1. The final notation in binary is "101", or 0x05.

+----------------+--------+-+---------------------+
|     Prefix     |  Plen  |D| Transmitted Prefix  |
+----------------+--------+-+---------------------+
| 192.0.2.0/28   |  28    |0| 0xc0 0x00 0x02 0x00 |
| 192.0.2.64/26  |  26    |1| 0x09                |
| 192.0.2.128/25 |  25    |1| 0x05                |
+----------------+--------+-+---------------------+

It should be noted that in this case the order of prefix transmission would not affect compression efficiency. If prefix 192.0.2.128/25 would have been considered the master prefix and the others as deltas instead, the resulting encoding still fits into one octet for the subsequent prefixes. There would be no need to declare a new master prefix.

4.2. Realm compression

4.2.1. Encoding of compressed realms

In order to reduce the size of messages, the system introduces a realm compression scheme, which reduces the size of realms in a message. The compression scheme is a simple dynamically updated dictionary based algorithm, which is designed to compress arbitrary length text strings. In this scheme, an entire realm, a single label or a list of labels may be replaced with an index to a previous occurance of the same string stored in the dictionary. The realm compression defined in this specification was inspired by the RFC 1035 (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) [RFC1035] DNS domain name label compression. Our algorithm is, however, improved to gain more compression.

When compressing realms, the dictionary is first resetted and does not contain a single string. The realms are processed one by one so the algorithm does not expect to see them all or the whole message at once. The state of the compressor is the current content of the dictionary. The realms are compressed label by label or as a list of labels. The dictionary can hold maximum 128 strings. Thus, when adding the 129th string into the dictionary, the dictionary MUST first be resetted to the initial state (i.e. emptied) and the index of the string will become 0.

The encoding of an index to the dictionary or an uncompressed run of octets representing a single label has purposely been made simple and the whole encoding works on an octet granularity. The encoding of an uncompressed label takes the form of a one octet:

This encoding allows label lengths from 1 to 127 octets. A label length of zero (0) is not allowed. The "label length" tag octet is then followed by up to 127 octets of the actual encoded label string.

The index to the dictionary (the "label index" tag octet) takes the form of a one octet:

The above encodings do not allow generating an output octet value of zero (0). The encapsulating Mobile IPv4 extension makes use of this property and uses the value of zero (0) to mark the end of compressed realm or to indicate an empty realm. It is also possible to encode the complete realm using only "label length" tags. In this case no compression takes place. This allows the sender to skip compression, for example to reduce computation requirements when generating messages. However, the receiver MUST always be prepared to receive compressed realms.

4.2.2. Searching algorithm

When compressing the input realm, the dictionary is searched for a matching string. If no match could be found, the last label is removed from the right-hand side of the used input realm. The search is repeated until the whole input realm has been processed. If no match was found at all, then the first label of the original input realm is encoded using the "label length" tag and the label is inserted into the dictionary. The previously described search is repeated with the remaining part of the input realm, if any. If nothing remains, the realm encoding is complete

When a matching string is found in the dictionary the matching part of the input realm is encoded using the "label index" tag. The matching part of the input realm is removed and the search is repeated with the remaining part of the input realm, if any. If nothing remains, the octet value of zero (0) is inserted to mark the end of encoded realm.

The search algorithm also maintains the "longest non-matching string" for each input realm. Each time the search in dictionary fails and a new label gets encoded using the "label length" tag and inserted into the dictionary, the "longest non-matching string" is concatenated by this label. When a match is found in the dictionary the "longest non-matching string" is resetted (i.e. emptied). Once the whole input realm has been processed and encoded, all possible suffixes longer than one label are taken from the string and inserted into the dictionary.

4.2.3. Encoding example

This section shows an example how to encode a set of realms using the specified realm compression algorithm. For example, a message might need to compress the realms "foo.example.com", "bar.foo.example.com", "buz.foo.example.org", "example.com" and "bar.example.com.org". The following example shows the processing of input realms on the left side and the contents of the dictionary on the right hand side. The example uses hexadecimal representation of numbers.

COMPRESSOR:                                 DICTIONARY:
1) Input "foo.example.com"
Search("foo.example.com")
Search("foo.example")
Search("foo")
Encode(0x03,'f','o','o')                    0x00 "foo"
  +-> "longest non-matching string" = "foo"
Search("example.com")
Search("example")
Encode(0x07,'e','x','a','m','p','l','e')    0x01 "example"
  +-> "longest non-matching string" = "foo.example"
Search("com")
Encode(0x03,'c','o','m')                    0x02 "com"
  +-> "longest non-matching string" = "foo.example.com"
                                            0x03 "foo.example.com"
                                            0x04 "example.com"
Encode(0x00)
2) Input "bar.foo.example.com"
Search("bar.foo.example.com")
Search("bar.foo.example")
Search("bar.foo"
Search("bar")
Encode(0x03,'b','a','r')                    0x05 "bar"
  +-> "longest non-matching string" = "bar"
Search("foo.example.com") -> match to 0x03
Encode(0x83)
  +-> "longest non-matching string" = NUL
Encode(0x00)
3) Input "buz.foo.example.org"
Search("buz.foo.example.org")
Search("buz.foo.example")
Search("buz.foo")
Search("buz")
Encode(0x03,'b','u','z')                    0x06 "buz"
  +-> "longest non-matching string" = "buz"
Search("foo.example.org")
Search("foo.example")
Search("foo") -> match to 0x00
Encode(0x80)
  +-> "longest non-matching string" = NUL
Search("example.org")
Search("example") -> match to 0x01
Encode(0x81)
  +-> "longest non-matching string" = NUL
Search("org")
Encode(0x03,'o','r','g')                    0x07 "org"
  +-> "longest non-matching string" = "org"
Encode(0x00)
4) Input "example.com"
Search("example.com") -> match to 0x04
Encode(0x84)
Encode(0x00)
5) Input "bar.example.com.org"
Search("bar.example.com.org")
Search("bar.example.com")
Search("bar.example")
Search("bar") -> match to 0x05
Encode(0x85)
Search("example.com.org")
Search("example.com") -> match to 0x04
Encode(0x84)
Search("org") -> match to 0x07
Encode(0x87)
Encode(0x00)

As can be seen from the example, due the greedy approach of encoding matches, the search algorithm and the dictionary update function is not the most optimal one. However, we do not claim the algorithm would be the most efficient. It functions efficiently enough for most inputs. In this example, the original input realm data was 79 octets and the compressed output excluding the end mark is 35 octets.

5. New Mobile IPv4 messages and extensions

5.1. Route optimization prefix advertisement

This non-skippable extension MAY be sent by a Home Agent to a Mobile Router in the registration reply message. The extension is only included when explicitly requested by the Mobile Router in the registration request message. Implicit prioritization of prefixes is caused by the order of extensions.

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Sub-type   |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  1-n times the following information structure
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |O|D|M| Plen    |  Optional Mobile Router HoA, 4 octets         ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ~               |  Optional Prefix, 1,2,3 or 4 octets           ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ~                 Realm  (1-n characters)                       ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2. Mobile router Route optimization capability

This skippable extension MAY be sent by a Mobile Router to a Home Agent in the registration request message.

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |   Sub-Type    |A|R|S| Reserved|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ~                 Optional Mobile Router HoA                    ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.3. Home-Test Init message

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |           Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                       Home Init Cookie                        +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.4. Care-of-Test Init message

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |           Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                    Care-of Init Cookie                        +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.5. Home Test message

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |       Home Nonce Index        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                       Home Init Cookie                        +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                       Home Keygen Token                       +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.6. Care-of test message

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |     Care-of Nonce Index       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                     Care-of Init Cookie                       +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                     Care-of Keygen Token                      +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.7. Mobile-Correspondent authentication extension

Nonce indices extension is used in registration requests sent from Mobile Router to Correspondent Router

Mobile-Correspondent authentication extension is included in registration requests sent from Mobile Router to Correspondent Router. It may also be used with Foreign Agents. The format is similar to the other Authentication Extensions defined in [RFC3344] (Perkins, C., “IP Mobility Support for IPv4,” August 2002.), with SPI's replaced by Nonce Indexes.

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |      Home Nonce Index         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Care-of Nonce Index       |       Authenticator...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Home Nonce Index field tells the Correspondent Router which nonce value to use when producing the home keygen token. The Care-of Nonce Index field is ignored in requests to remove a binding. Otherwise, it tells the Correspondent Router which nonce value to use when producing the Care-of Keygen Token.

The protected data, just like on other cases where Authenticator is used, consists of

5.8. Care-of address Extension

  0               1               2               3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |           Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Care-of Address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Care-of Address extension is added to a registration reply sent by the Correspondent Router to inform the Mobile Router of the upcoming tunnel endpoint.

6. Special Considerations

6.1. NATs and stateful firewalls

If Mobile Router and Correspondent Router are behind same NAT from HA's point of view, it is possible to establish tunnel between them. This may also be the situation in the case of nested NATs. This calls for development of some sort of "Route optimization discovery" protocol (see Section 6.5 (Extensibility)) , or more information in the Route Optimization capability advertisements.

If both the Mobile Router and the Correspondent Router are behind two separate NATs, some sort of proxy or hole-punching technique may be needed. This is out of scope of this draft.

6.2. Foreign Agents

Since Foreign Agents have been dropped from Network Mobility for Mobile IPv4 work, they are not considered here.

6.3. Multiple Home Agents

In fact, Mobile Routers can negotiate and perform route optimization without the assistance of Home Agent - if they can discover each others existence. This draft only addresses a single Home Agent that distributes network prefix information to the Mobile Routers. Problems arise from possible trust relationships; In this draft the Home Agent serves as a way to provide verification that a specific network is managed by a specific router. For host-routes (route optimization between single nodes), the requirements may not be so strict.

Several possibilities exist for achieving Route Optimization between Mobile Routers attached to separate Home Agents, such as a discovery/probing protocol, routing protocol between Home Agents or DNS SRV records, or a common AAA architecture. There already is a framework for HA to retrieve information from AAA so it can be considered as the most viable possibility. See Section 6.5 (Extensibility) for information on possibility to generalize the method.

6.4. Mutualness of Route Optimization

The procedure as specified is asymmetric; That is, if bidirectional route optimization is desired while maintaining consistency, the route optimization (RR check and registration) has to be performed in both directions, but this is not strictly necessary. This is primarily a policy decision depending on how often the mobile prefixes are reconfigured.

Consider the case where two networks, A and B, are handled by Mobile Routers A and B respectively. If the route optimization is triggered by receiving packet from a network in Route Optimization Cache, the following occurs if a node in network A starts pinging a node in network B.

MR B sees the incoming message. It sees that the destination is in network B, and furthermore, source is in network A.

MR B completes RR procedure and registration with MR A, which is now acting as Correspondent Router. A tunnel is created between the routers. MR A updates its routing table so that network B is reachable via MR A <-> MR B tunnel.

The traffic pattern is now that packets from network B to network A are sent over the direct tunnel, but the packets from A to B are transmitted via the Home Agent and reverse tunnels. MR A now performs its own route optimization towards MR B. Upon completion, MR A notices that a tunnel to MR B already exists, but updates its routing table so that network B is now reachable via the MR A <-> MR B tunnel. From this point onward, traffic is bidirectional.

In this scenario, if MR A does NOT perform a separate route optimization (RR check and registration), but instead simply updates its routing table to reach network B via the tunnel, problems may arise if MR B has started to manage another network B' before the information has propagated to MR A. The end result is that MR B starts to receive packets for network B' via the Home Agent and for network B via direct tunnel. If RPF checking or similar mechanism is in use on MR B, packets from network A could be blackholed.

6.5. Extensibility

The design considerations include several mechanisms which might not be strictly necessary if Route Optimization would only be desired between individual customer sites in a managed network. The registration procedure (with the optional Return Routability part), which allows for Correspondent Routers to learn Mobile Router's Care-of Addresses is not strictly necessary; The CoA's could have been provided by HA directly.

However, this approach allows the method to be extended to a more generic route optimization. The primary driver for having Home Agent to work as a centralized information distributer is to provide Mobile Routers with the knowledge of not only the other routers, but to provide information on which networks are managed by which routers.

The Home Agent provides the information on all feasible nodes with which it is possible to establish Route Optimization. If representing a whole Mobile Network is not necessary, in effect the typical Mobile Node <-> Correspondent Node situation, the mechanisms in this draft work just as well - only problem is discovering if the target Correspondent Node can provide Route Optimization capability. This can be performed by not including any prefixes in the information extension, just the HoA address of Mobile Router.

Correspondent node/router discovery protocols (whether they are based on probing or a centralized directory beyond the single Home Agent) are outside the scope of this draft.

6.6. Load Balancing

7. Scalability

Home Agent assisted Route Optimization scalability issues stem from the general Mobile IPv4 architecture which is based on tunnels. Creating, maintaining and destroying tunnel interfaces can cause load on the Mobile Routers. However, the MRs can always fall back to normal, reverse tunnelled routing if resource constraints are apparent.

If there is a large number of optimization-capable prefixes, maintaining state for all of these may be an issue also, due to limits on routing table sizes.

Registration responses from Home Agent to Mobile Router may provide information on large number of network prefixes. If thousands of networks are involved, the registration reply messages are bound to grow very large. The prefix- and realm compression mechanisms defined in Section 4 (Data compression schemes) mitigates this problem to an extent. There will, however, be some practical upper limit after which point some other delivery mechanism for the prefix information will be needed.

8. Example signaling scenarios

8.1. Registration request

The following example signaling assumes that there are three Mobile Routers, MR A, B, C, each managing network prefixes A, B, and C. At the beginning, no networks are registered to the Home Agent. Any AAA processing at the Home Agent is omitted from the diagram.

+--------+ +--------+ +--------+ +--------------+
| [MR A] | | [MR B] | | [MR C] | | [Home Agent] |
+--------+ +--------+ +--------+ +--------------+
   |          |          |          |
   x------------------------------->|  Registration request,
   |          |          |          |  includes Mobile Router
   |          |          |          |  route optimization
   |          |          |          |  capability extension
   |          |          |          |
   |<-------------------------------x  Registration response,
   |          |          |          |  no known networks from HA
   |          |          |          |  in response
   |          |          |          |
   |          x-------------------->|  Registration request, similar
   |          |          |          |  to the one sent by MR A
   |          |          |          |
   |          |<--------------------x  Registration reply, includes
   |          |          |          |  network A in route optimization
   |          |          |          |  prefix advertisement extension
   |          |          |          |
   |          |          x--------->|  Registration request, similar
   |          |          |          |  the one sent by MR A
   |          |          |          |
   |          |          |<---------x  Registration reply, includes
   |          |          |          |  networks A and B in route
   |          |          |          |  optimization prefix
   |          |          |          |  advertisement extension.
   |          |          |          |  Network B is sent in
   |          |          |          |  compressed form.
   |          |          |          |

8.2. Route optimization with return routability

The following example signaling has same network setup as in Section 8.1 (Registration request) - Three mobile routers, each corresponding to their respective network. Node A is in network A and Node C is in network C.

At the beginning, no mobile routers know KRm's of each other. If the KRm's would be pre-shared or provisioned with some other method, the Return Routability messages can be omitted. Signaling in Section 8.1 (Registration request) has occured, thus MR A is not aware of the other networks, and MR C is aware of networks A and B.

  ======= Traffic inside Mobile IP tunnel to/from HA
  =-=-=-= Traffic inside Mobile IP tunnel between MRs
  ------- Traffic outside Mobile IP tunnel

+----------+ +--------+ +------+ +--------+ +----------+
| [Node A] | | [MR A] | | [HA] | | [MR C] | | [Node C] |
+----------+ +--------+ +------+ +--------+ +----------+
   |            |          |         |       |
   |<-----------O==========O=========O-------x  Mobile Router A is
   |            |          |         |       |  unaware of network C,
   |            |          |         |       |  thus, nothing happens
   |            |          |         |       |
   x------------O==========O=========O------>|  Mobile Router C
   |            |          |         |       |  notices packet from
   |            |          |         |       |  Net A - inits RO
   |            |          |         |       |
   |            |          |         |       |  Return Routability
   |            |          |         |       |  (If no preshared KRms)
   |            |          |         |       |
   |            |<=========O---------x       |  CoTI
   |            |<=========O=========x       |  HoTI
   |            |          |         |       |
   |            x==========O-------->|       |  CoT
   |            x==========O========>|       |  HoT
   |            |          |         |       |
   |            |          |         |       |  KRm between MR A <-> C
   |            |          |         |       |  established
   |            |          |         |       |
   |            |<=========O---------x       |  Registration request
   |            |          |         |       |
   |            x--------->|         |       |  Registration request
   |            |          |         |       |  to HA due to MR A
   |            |          |         |       |  being unaware of
   |            |          |         |       |  network C.
   |            |          |         |       |  Solicit bit set.
   |            |          |         |       |
   |            |<---------x         |       |  Registration reply,
   |            |          |         |       |  contains info on Net A
   |            |          |         |       |
   |            x==========O-------->|       |  Registration reply,
   |            |          |         |       |  includes MR A's CoA in
   |            |          |         |       |  Care-of-Address
   |            |          |         |       |  extension
   |            |          |         |       |
   |<-----------O=-=-=-==-=-=-=-==-=-O-------x  Packet from Node C -> A
   |            |          |         |       |  routed to direct tunnel
   |            |          |         |       |  at MR C, based on
   |            |          |         |       |  MR C now knowing MR A's
   |            |          |         |       |  CoA and tunnel being up
   |            |          |         |       |
   x------------O=-=-=-==-=-=-=-==-=-O------>|  Packet from Node A -> C
   |            |          |         |       |  routed to direct tunnel
   |            |          |         |       |  at MR A, based on MR A
   |            |          |         |       |  now knowing MR C's CoA
   |            |          |         |       |  and tunnel being up

8.3. Handovers

In these example signalings, MR C changes care-of-address while Route Optimization between MR A is operating and data is being transferred. Both cases where the handover is graceful ("make before break") and ungraceful ("break before make") occur in similar fashion, except in the graceful version no packets get lost.

  ======= Traffic inside Mobile IP tunnel to/from HA
  =-=-=-= Traffic inside Mobile IP tunnel between MRs
  ------- Traffic outside Mobile IP tunnel

+----------+ +--------+ +------+ +--------+ +----------+
| [Node A] | | [MR A] | | [HA] | | [MR C] | | [Node C] |
+----------+ +--------+ +------+ +--------+ +----------+
   |            |          |         |       |
   x------------O=-=-=-==-=-=-=-==-=-O------>| Nodes A and C
   |<-----------O=-=-=-==-=-=-=-==-=-O-------x exchanging traffic
   |            |          |         |       |
   |            |          xxxxxxxxxxx       | Break occurs: MR C
   |            |          |         |       | loses connectivity to
   |            |          |         |       | current attachment point
   |            |          |         |       |
   x------------O=-=-=-==-=-=-=->    |       | Traffic from A -> C
   |            |          |         |       | lost and
   |            |          |   x<=-=-O-------x vice versa
   |            |          |         |       |
   |            |          |<--------x       | MR C finds a new
   |            |          |         |       | point of attachment,
   |            |          |         |       | registers to HA, clears
   |            |          |         |       | routing tables
   |            |          |         |       |
   |            |          x-------->|       | Registration reply
   |            |          |         |       |
   x------------O=-=-=-==-=-=-=->    |       | Traffic from A -> C
   |            |          |         |       | lost
   |<-----------O==========O=========O-------| Traffic from C -> A
   |            |          |         |       | sent via HA
   |            |          |         |       |
   |            |<=========O---------x       | Registration request,
   |            |          |         |       | reusing still active KRm
   |            |          |         |       |
   |            x==========O-------->|       | Registration reply
   |            |          |         |       |
   x------------O=-=-=-==-=-=-=-==-=-O------>| Traffic from A -> C
   |            |          |         |       | forwarded again
   |<-----------O=-=-=-==-=-=-=-==-=-O-------x Traffic from C -> A
   |            |          |         |       | switches back to direct
   |            |          |         |       | tunnel
   |            |          |         |       |

9. IANA Considerations

10. Security Considerations

10.1. Trust relationships

The network of trust relationships in Home Agent assisted Route Optimization solve the issues where arbitrary Correspondent Router can trust an arbitrary Mobile Router that it is indeed the proper route to reach an arbitrary mobile network.

It is assumed that all Mobile Routers have a trust relationship with the Home Agent. Thus, they trust information provided by Home Agent.

The Home Agent provides information matching Home Addresses and network prefixes. Each Mobile Router trusts this information.

Mobile Routers may perform Return Routability procedure between each other. This creates a trusted association between Mobile Router Home Address and Care-of Address. The Mobile Router also claims to represent a specific network. This information is not trustworthy as such.

The claim can be verified by checking the Home Address <-> network prefix information received, either earlier, or due to on-demand request, from the Home Agent. If they match, the Mobile Router's claim is authentic. If the network is considered trusted, a policy decision can be made to skip this check. Exact definitions on situations where such decision can be made are out of scope of this draft. The RECOMMENDED general practice is to perform the check.

[RFC2119]	Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC3344]	Perkins, C., “IP Mobility Support for IPv4,” RFC 3344, August 2002 (TXT).
[RFC3519]	Levkowetz, H. and S. Vaarala, “Mobile IP Traversal of Network Address Translation (NAT) Devices,” RFC 3519, April 2003 (TXT).
[RFC5177]	Leung, K., Dommety, G., Narayanan, V., and A. Petrescu, “Network Mobility (NEMO) Extensions for Mobile IPv4,” RFC 5177, April 2008 (TXT).

[I-D.gundavelli-mip4-multiple-tunnel-support]	Gundavelli, S. and K. Leung, “Multiple Tunnel Support for Mobile IPv4,” draft-gundavelli-mip4-multiple-tunnel-support-02 (work in progress), February 2010 (TXT).
[I-D.ietf-mobileip-optim]	Perkins, C. and D. Johnson, “Route Optimization in Mobile IP,” September 2001.
[RFC1035]	Mockapetris, P., “Domain names - implementation and specification,” STD 13, RFC 1035, November 1987 (TXT).
[RFC3543]	Glass, S. and M. Chandra, “Registration Revocation in Mobile IPv4,” RFC 3543, August 2003 (TXT).
[RFC3775]	Johnson, D., Perkins, C., and J. Arkko, “Mobility Support in IPv6,” RFC 3775, June 2004 (TXT).

	Antti Makela
	Helsinki University of Technology
	P.O. BOX 3000
	FIN-02105 TKK
	FINLAND
Phone:	+358 9 451 5590
Email:	antti.makela@tkk.fi

	Jouni Korhonen
	Nokia Siemens Networks
	Linnoitustie 6
	FI-02600 Espoo
	FINLAND
Email:	jouni.nospam@gmail.com

Home Agent assisted Route Optimization between Mobile IPv4 Networksdraft-makela-mip4-nemo-haaro-04

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