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Network Working Group                                  D. Crocker
Internet Draft                                        Brandenburg
     draft-crocker-mast-analysis-00.doc           InternetWorking
Expires: <2-04>                                September 16, 2003



     This document is an Internet-Draft and is in full conformance
     with all provisions of Section 10 of RFC2026. Internet-Drafts are
     working documents of the Internet Engineering Task Force (IETF),
     its areas, and its working groups.  Note that other groups may
     also distribute working documents as Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other
     documents at any time.  It is inappropriate to use Internet-
     Drafts as reference material or to cite them other than as "work
     in progress."

     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at


     Copyright (C) The Internet Society (2003).  All Rights Reserved.


     Classic Internet transport protocols use a single source IP
     address and a single destination IP address, as part of the
     identification for an individual data flow.  TCP includes these
     in its definition of a connection and its calculation of the
     header checksum.  Hence the transport service is tied to a
     particular IP address pair. This is problematic for multihomed
     hosts and for mobile hosts. They cannot use more than one, for
     any single transport association (context).  In recent years,
     there have been efforts to overcome many of these limitations,
     through different approaches at different places in the Internet
     architecture. This paper reviews the requirements for support of
     multiaddressing (mobility and multihoming), and the efforts to
     support them. Barriers to adoption, administrative overhead, and
     operational efficiency are of particular concern.


     1.2. SCENARIOS

     2.1. MOBILITY
     2.3. SECURITY
     2.8. SIGNALING

     3.5. IP ENDPOINT



     Classic Internet transport protocols use a single source IP
     address and a single destination IP address, as part of the
     identification for an individual transport data flow.  For
     example, TCP includes these in its definition of a connection and
     its calculation of the header checksum.  Hence a classic
     transport association is tied to a particular IP address pair.
     This is problematic for multihomed hosts and for mobile hosts.
     Both have access to multiple IP addresses, but they are prevented
     from using more than one within an existing transport exchange.
     For a host to use a different IP address pair, participants must
     initiate a new exchange.  In the case of TCP, this means a new

     In recent years, there have been efforts to overcome many of
     these limitations, through different approaches at different
     places in the Internet architecture. Some modify the IP
     infrastructure, with embedded redirection services.  Some define
     transport enhancements to support a set of addresses directly,
     and some define a layer between classic IP and classic transport.
     Each of the existing proposals has notable limitations in
     functionality, implementation, deployment or use.

     This paper reviews the requirements for support of
     multiaddressing (mobility and multihoming), and the efforts to
     support them. Barriers to adoption, administrative overhead, and
     operational efficiency are of particular concern.

1.1. Terminology

     This paper discusses requirements and methods for enabling an
     endpoint (host) to use multiple addresses during single
     application associations (sessions).

     "Agent" refers to a forwarding service that represents an
     endpoint for multiaddressing. For mobility, the agent resides on
     the "home" network and relays datagrams to the endpoints actual
     location on the Internet.  The endpoints are modified to support
     this forwarding technique. For multihoming, an agent hides the
     presence of multiple addresses from the endpoint located on the
     local network.

     "Address" refers to a string that indicates a location, usually
     in terms of network topology. IP addresses specify a topological
     network access point. They usually are considered to specify an
     endpoint interface.  However discussions about mobility are
     enhanced by viewing the value as belonging to the network
     (interface) rather than to the endpoint.

     "Association" refers to a transport-level exchange context
     between endpoints, such as a TCP connection.

     "Endpoint" refers to an end-system that participates in an
     association. Endpoints are distinguished from intermediate,
     infrastructure nodes and hosts.

     "Identifier" refers to a unique label for an endpoint. The label
     is used simply for distinguishing one endpoint from another. If
     the location information in an address is ignored, it can serve
     as an identifier. However an address will usually suffer
     administrative and referential limitations as a global identifier
     for mobile endpoints.

     "Initiator" refers to an endpoint that initiates contact with a
     target endpoint. In client/server architecture it is the client.

     "Mobility" refers to the availability of different addresses at
     the same endpoint, over time. This may even include
     discontinuities, at times having no available addresses. It also
     may include overlapping availability of addresses. Interestingly,
     this looks the same as multihoming.

     "Multiaddressing" refers to the availability of different
     addresses at the same endpoint. It encompasses both multihoming
     and mobility.

     "Multihoming" refers to the availability of multiple addresses at
     the same endpoint, simultaneously. It is typically used to refer
     to multiple network attachments for a host, but works equally
     well for multiple upstream network attachments by the local
     network, when the different upstream addresses are visible to the
     host. Interestingly, multihomed environments often must support
     dynamic changes, such as when adding a new upstream provider.
     Therefore, multihoming can include mobility features and mobility
     can include multihoming features.

     "Path discovery" provides a sender with the means for learning
     about the addresses from which they can send.

     "Path selection" is required when more than one address is
     available to the sender. Although the sender is limited to
     specifying an address, rather than a path, it appears that
     thinking of it as path selection aids consideration of solutions.
     In effect, it formulates the selection task as being similar to
     the job of routers. Route formulation is mature technology, so
     that this aspect of multiaddress processing will be tractable, if
     not straightforward.

     "Rendezvous" permits a host that is initiating an association to
     find the target of the association, such as a client finding a
     server. "Finding" means obtaining a valid address for the target.
     A public process is required for rendezvous. The primary Internet
     mechanism for rendezvous has been the Domain Name Service (DNS).
     The DNS uses long, variable-length strings (names) and is
     tailored for large-scale rendezvous with names and addresses
     (mappings) that change infrequently.

     "Target" refers to an endpoint that receives contact from an
     Initiator endpoint. In a client/server architecture, this is the

1.2. Scenarios

     What are the situations and concerns that affect design and use
     of a mechanism for the support of multiaddressing?

          Section 3 of [MOBHOM], has an excellent discussion of these
          It is included here by reference without section 3.2.
          Section 3.2 covers an interesting topic that appears to be
          independent of multiaddressing.
          The included text comprises the following sub-

          3.     Usage scenarios
          3.1    End-host mobility
          3.2    Location privacy à
          3.3    End-host multi-homing
          3.4    Site multi-homing
          3.5    Combined mobility and multi-homing
          3.6    Network renumbering
          3.7    Combined all

1.3. IETF Background

     Historically, IETF focus on mobility has split between initial
     attachment configurations, into an otherwise static environment
     such as by using DHCP, versus forwarding mechanisms, such as by
     modifying the IP infrastructure with Mobile IP.  Multihoming has
     largely been ignored, except in routing protocol work. Recent
     efforts are pursuing direct enhancements to transport or
     insertion of a mapping layer between IP and transport. There has
     also been adjunct activity, relevant to this topic.

     The following summary of IETF activities relies on text from the
     Abstracts of documents for those activities.  Analysis of the
     different architectural and protocol efforts is in Section 3,
     "Internet Stack Placement".

          The Name Space Research Group [NSRG] considered
          modifications to the Internet architecture, including
          whether an additional level of naming above layer 3, but
          below the application layer, is needed. Purpose-Built Keys
          [PBK] specifies a template for the use of specially
          generated public/private key pairs, to provide assurance
          that successive messages in the communication come from the
          same source. This is accomplished without the use of
          external certification authorities.
          Stream Control Transmission Protocol [SCTP] is a reliable
          transport protocol for multiplexed data streams.  It
          includes modern mechanisms for safe initiation of a
          connection, as well as the necessary tools for reliability
          and congestion control.  It also has a mechanism for
          communication access to multiple IP addresses between the
          participation host pair.  [TCP-MH] uses TCP options to
          support multihoming. Datagram Congestion Control Protocol
          [DCCP] is a proposal for a network-friendly, unreliable
          transport-level datagram delivery service.
          Mobile IP [MIP] provides an agent service to allow
          transparent routing of IP datagrams to mobile nodes in the
          Internet. Host Identity Protocol [HIP] is used to establish
          a rapid authentication between two hosts and to provide
          continuity of communications between those hosts independent
          of the networking layer. The [LIN6] protocol defines a layer
          that supports multiple addresses, between IPv6 and
          transport. Multiple Address Service for Transport [MAST]
          supports association of multiple IP addresses during the
          life of any transport instantiation, by defining a layer
          between IP and transport. It operates only in the endpoints
          and works with IPv4 and IPv6.

1.4. Discussion Venue

     Discussion and commentary are encouraged about the topics
     presented in this document. The preferred forum is the
     <mailto:multi6@ops.ietf.org> mailing list, for which archives and
     subscription information are available at

     NOTE:     The early drafts of a review document, like
               this, are certain to have significant errors.
               The author strongly requests guidance for
               clarifying and correcting any problematic text.

1.5. Document History

     -00       Derived from draft-crocker-mast-proposal-00.
               Extended discussions about alternative proposals
               and architectural issues, separated from the -
               proposal- draft.

     NOTE:     The author has put forward the MAST proposal.
               Clearly that colors the perspective in this
               discussion paper.


2.1. Mobility

     Mobility is time-varying access to multiple addresses for the
     same endpoint. Key parameters to mobility are scope of change,
     rate of change and source(s) of the change. Over what portion of
     the Internet topology might a change take place; how often will
     changes occur; and which of the participants will change their

     It is generally accepted that rapid, local changes should be
     handled by a layer below IP and therefore should be invisible to
     IP.  For initiator endpoints that are subject to occasional
     detachment, with eventual reconnection, the current set of
     technologies is probably sufficient.

     What is missing is support for initiator and target systems that
     move over the course of minutes or hours and need to maintain
     existing transport associations or need to maintain their
     availability for new associations. There are no IP-related
     standards for maintaining associations during mobility. For
     maintaining target availability, DNS dynamic update [DNSDYN] is
     plausible; however it is not widely deployed and the typical DNS
     record lifetime settings and client caching behaviors suggest
     that existing DNS use is better tailored for changes over days,
     rather than shorter times. Separately the core role of DNS for
     Internet infrastructure operations suggests avoiding major
     changes to its operational model.  Supporting potentially high
     volumes of rapid changes probably require very different software
     and administration than are used for the current DNS.

     The difference between mobility prior to initial contact and
     mobility during an association is significant.  In the latter
     case, the mobile host can use the association state when needing
     to inform the other endpoint about the change.  Prior to an
     association -- or when both endpoints are mutually mobile -- an
     independent rendezvous venue is required.

     The difference between initiator mobility and target mobility is
     also significant, with respect to initial contact.  In particular
     the initiator needs to be able to find the target. Again, this
     requires a rendezvous mechanism, such as having the routing
     system map from identifiers to routes, rather than addresses to
     routes.  Either it must be provided implicitly within the network
     or there must be an external "rendezvous" mechanism.  For static
     servers, the DNS already provides this rendezvous quite well.
     However current DNS use does not support frequent address changes
     over short periods. Hence enhancements are needed to support
     rendezvous with a mobile target.

2.2. Multihoming

     The Internet already supports a number of types of "indirect"
     multihoming. The core of dynamic packet-switched routing is
     exploitation of alternative routes, so that the path between
     endpoints might vary considerable over the course of an
     association. For networks with multiple attachments to a
     backbone, external routing technology already permits propagation
     of alternate routing information.  Further a domain name may have
     multiple address records that point to the same network. (However
     there is no indication whether the same records are, instead,
     pointing to different, redundant systems; on the other hand the
     importance of this ambiguity is not clear.)

     What is notably missing is a means for an existing association to
     directly use multiple paths, in particular when the paths
     terminate at one of the endpoints. Here, the fact that classic
     Internet transport services rely on single, specific IP addresses
     is the barrier.

     Support of multihoming can be useful for robustness and
     throughput.  The former makes loss of a path transparent to the
     association.  The latter increases the effective bandwidth for an
     association.  In general, the former goal is dominating current
     work. At the least, using multiple paths for increased bandwidth
     ensures a high degree of out-of-order arrivals. This usually
     reduces target endpoint performance, rather than increasing it.

2.3. Security

     The level of security built into IP is minimal.  Some would say
     it is non-existent. However classic transport services rely on
     having a significant degree of correlation between the IP address
     in the source field of an IP datagram and the likelihood that the
     IP datagram came from that address.  The context of repeated
     exchanges between source and destination addresses is taken as a
     validation of this correlation. Permitting the IP address of a
     source to vary during an association is an invitation to
     connection hijacking.  Hence, any support for multiple addresses
     must contain a strong anti-hijacking mechanism.

     All other security concerns are independent of multiaddressing;
     and they are probably best handled by additional mechanisms, such
     as IPSec and TLS.  There is no indication that any of these other
     mechanisms need to be changed, so support multiaddressing.

     Once there is an effort to design protection against hijacking,
     it is easy to consider adding more protections, such as privacy
     or, perhaps, other kinds of authentication. Although such
     mechanisms obviously would be useful, they are not essential to
     the basic requirements of multiaddressing.  Further, they might
     be redundant with mechanisms provided elsewhere in the

     Any effort related to multiaddress support, which goes beyond
     preventing hijacking, needs to have explicit discussion about its
     relationship to other security mechanisms and the need for
     attaching these additional capabilities to multiaddress support.
     As with any opportunity for adding features to a design effort,
     there should be concern about causing unnecessary design
     complexity, delays to the specification effort, and difficulty in

2.4. Implementation

     The software that supports IP and classic transport services is
     mature. Usually it is highly tuned and highly robust. Often it is
     also complex. Hence it can be risky to introduce modifications to
     one or more of these modules.  On the other hand, attempting to
     introduce multiaddress support through additional modules runs
     the risk of being awkward and inefficient.

2.5. Deployment and Use

     However difficult it is to have vendors make major modifications
     to mature software, it is far more difficult to deploy the
     changes to a global installed base of hundreds of millions of
     platforms. Changes to support multiaddressing need to consider
     barriers to adoption by users and operators, both ISPs and
     enterprises. What is the effort needed to deploy the changes?
     What is the effort needed to use it?  How broad must the adoption
     be before users can obtain benefit? What dependencies do the
     changes have on existing or new services?

     Making one new service depend upon the reliable performance of
     another new service greatly increases the riskiness of the
     effort. Making a change require modification to the Internet's
     infrastructure typically creates a long delay before it is
     useful.  In particular, early adopters gain no immediate benefit
     from their efforts; this acts as a disincentive for adoption.
     Everyone waits for others to take the first step.

2.6. Matters of State

     Support for multiple addresses requires adding a conceptual layer
     of referential indirection.  Beyond simple use of the DNS,
     endpoints currently use individual endpoint addresses within an
     association.  In order to use multiple addresses, to refer to the
     same endpoint, some type of aggregation and mapping mechanism
     must be added.  The mechanism defines a relationship between the
     referenced endpoint and a set of addresses. Where should this
     state information be placed in the Internet architecture?

     If the major lesson of the Internet is scaling, the major
     embodiment of that lesson is to place complexity in the edges,
     rather than the infrastructure. Generally, this does not mean
     that there is a balanced debate between the choices.  Rather,
     there is an assumption that a change should be made to the edges
     rather than the infrastructure.  It is made in the infrastructure
     only when there is a clear agreement that doing otherwise will
     seriously reduce the utility of the change.

     This methodology can even be applied to some infrastructure
     changes. A change that will clearly have an infrastructure impact
     might be introduced incrementally, via endpoint modifications.
     Two major examples of this are DNS and MIME.  Both were added to
     operational, infrastructure services (the IP internet and the
     Internet mail service, respectively) but were added in a fashion
     that made no immediate changes to the existing services.  Rather,
     edge systems independently chose to adopt the changes.  Any two
     endpoints wishing to exploit the change, for interacting with
     each other, immediately benefited from the adoption.  Over time,
     adoption became sufficiently broad-based to make the change
     effectively part of the infrastructure service.  Although the IP
     network works well without the DNS, end-user utility of the
     Internet, without the DNS, would be nil.  Similarly the ability
     to use attachments has become a fundamental part of the Internet
     mail experience.

     Addition of support for multiaddressing faces a similar type of
     choice.  Should the change be made above the transport layer, in
     the transport layer, in the IP layer, or perhaps between IP and
     transport?  How is the aggregation established and how is it
     maintained?  Do IP (or TCP, or...) packets contain the mappings
     or are they maintained in the endpoints or, perhaps, in the IP

     The answers to these questions need to be determined by their
     effect on barriers to adoption, operational overhead, and
     administrative convenience.

2.7. Endpoint Identifiers

     Historically, IP addresses have served the dual role of network
     interface locator and endpoint identifier (EID).  Adding support
     for multiaddressing serves to highlight the need for splitting
     these two roles.  IP addresses work quite well as network
     interface locators. However their topological dependence makes
     them work poorly as identifiers, in the richer world of

     Does an EID need to be assigned by a registry or can it be
     dynamically computed? Does it need to be publicly visible across
     the Internet or can it be kept private to individual
     associations? Does it need to be used frequently, such as in
     every datagram, or is it needed only for specific transactions,
     such as initiating or recovering an association?

     It is appealing to define an EID to be publicly registered and
     carried in every datagram. This provides the maximum amount of
     decoupling from addressing and appears to offer an especially
     clean modification to the transport layer interface. Transport
     header calculation merely needs to switch to use of the EID,
     rather than the address. With sufficiently strong protection
     against hijacking, this approach can almost make the address
     irrelevant to the transport layer.

     However there still must be a mapping between EID and addresses,
     so the IP service knows where to send the datagram.  Hence, the
     state information of an EID/addresses "routing" table must reside
     somewhere.  Unless the IP infrastructure is modified to directly
     support EIDs, this state information is most probably in the

     Having a public EID means that a new, global registration service
     must be developed and operated.  Some believe network operators
     will not mind this additional work; others disagree.

     Having an EID in every datagram means that the string must be as
     short as possible.  Even then it will add significant overhead to
     datagram header size.  However given the need to process
     multiaddressing, having the EID in every datagram probably will
     not alter datagram processing overhead, in the endpoints, from
     any other approach to using EIDs.

     If an EID is used only occasionally, one candidate is a domain
     name. Domain names already have an administrative structure, and
     they are well engrained into Internet use. Their length is not a
     problem, when they are need only periodically. One objection to
     using domain names is that they are already used in a number of
     ways that do not suit the role of EID.  It is unclear how the
     fact that domain names serve multiple roles prevents their
     serving the role of EID.

2.8. Signaling

     How does an endpoint learn its addresses?  The notable challenge
     is when a NAT modifies the address an endpoint uses directly, to
     a different address that is visible to the rest of the network.

     How does an endpoint communicate its available set of addresses
     to another endpoint?

     DNS is currently useful for registering essentially static sets.
     More dynamic or tailored communication requires a signaling
     exchange between endpoints.  This can be done through a distinct
     signaling protocol, such as is done with MAST, or inline -- that
     is, as a sub-exchange -- within an existing protocol, such as is
     done with TCP-MH.

2.9. Operation Through NATs

     A Network Address Translation (NAT) device maps between one set
     of addresses, and another.  In typical cases, addresses from the
     interior of a network are mapped to different ports on a single,
     public address on the outside of the network.

     This mapping task must be performed with knowledge of transport
     protocol details because it must adjust transport headers, as
     well as IP-level addresses.

     Stateless NATs are likely to work with most multihoming solutions
     and some mobility solutions. The NAT will simply do its usual
     task of replacing IP addresses and adjusting dependent transport
     headers accordingly.  However, there is the basic question of
     whether a multiaddressed initiator correctly knows its own
     addresses.  Typically it will not.  Given the prevalence of NATs,
     a solution to multiaddressing needs to deal with this scenario.

     Some solutions require that NATs be upgraded to support the
     solution. This is another example of an infrastructure


     From a purely technical standpoint, multiaddressing can be
     supported through a number of different mechanisms. This section
     discusses the possible venues within the Internet stack, and
     existing efforts that are pursuing these choices.

     The current architecture for transport use of IP addresses makes
     a direct linkage to a specific IP address pair:

             (IP.a, Port.l, IP.y, Port.r)
                  | Port.l          | Port.r
               +-----+           +-----+
               | TCP |           | TCP |
               +-----+           +-----+
                  | IP.a            | IP.y
               +-----+           +-----+
               | IP  |           | IP  |
               +-----+           +-----+
                |   |             |   |
             IP.a   IP.f       IP.q   IP.y

     This example shows each host being multihomed.  However a given
     association must choose a single IP address, at each end, and
     bind the connection to it.

3.1. IP Infrastructure

     In the classic Internet infrastructure model, a datagram contains
     topological references to the source and destination network
     interfaces.  The network knows nothing about higher-level issues,
     such as whether two interfaces are attached to the same endpoint.
     This design derives from the explicit desire to keep the Internet
     infrastructure as simple as possible, by putting as much
     functionality as possible into the endpoints rather than in the
     Internet's switching devices.

     The Mobile IP [MIP] effort provides an encapsulation-based
     forwarding service. An agent intercepts datagrams using an
     original destination IP address, and then forwards the datagram
     to the destination's new IP address. An optimization may (later)
     permit direct transmission to the new venue. This is achieved by
     use of datagram encapsulation -- tunneling the original IP
     datagram inside a new one -- and by having datagrams carry both
     an address and an end-point identifier.  [HOWIE] provides an
     interesting discussion of MIPv6 adoption and use issues.

                (IP.f, Port.l, IP.q, Port.r)
                | Port.l                   | Port.r
            +-------+                  +-------+
            |  TCP  |                  |  TCP  |
            +-------+                  +-------+
                | IP.f                     |
            +-------+                      |
            | IP-es |                      |
            +-------+                      |
            IP.a|                          |
            +-------+    +-------+     +-------+
            | IP-is |    | Agent |     | IP-is |
            +-------+    +-------+     +-------+
                |            |             |
             IP.a           IP.f           IP.q

     Conceptually, the biggest problem with this approach is that it
     attempts to take topology-related information -- the IP address -
     - and use it as the basis for contacting an endpoint non-

     Operationally, the biggest problems with this approach are that
     forwarding services are inefficient, multi-layer encapsulation
     adds complexity, and the service requires infrastructure change.

     Therefore, this approach changes the infrastructure and changes
     the IP datagram. Hence it changes several different aspects of
     the Internet architecture, with each change constituting a
     significant barrier to adoption or efficiency.

3.2. Transport-Level

     Recent transport protocols, such as [SCTP], [TCPMH] and the
     proposal for [DCCP], use multiple IP addresses directly in the
     transport association. These efforts have primarily focused on
     multihoming, with the time-varying nature of mobility being
     ignored or retrofitted. TCP-MH notably uses TCP options for
     inline signaling of multihoming information between the
     endpoints; its current specification appears to have weak
     protection against hijacking but this can be remedied.

     A transport-level approach has the benefit of placing the
     necessary functionality only in end-systems and avoiding possible
     address translation problems.

                (IP.?, Port.l, IP.?, Port.r)
                | Port.l                   | Port.r
            +-------+                  +-------+
            |  TCP  |                  |  TCP  |
            +-------+                  +-------+
          IP.a|   |IP.f              IP.q|   |IP.y
            +-------+                  +-------+
            |  IP   |                  |   IP  |
            +-------+                  +-------+
              |   |                      |   |
              |   |                      |   |
           IP.a   IP.f                IP.q   IP.y

     NOTE:     Given that multiaddressing is directly visible
               to the transport level, it is not clear how to
               formally define a connection. Are "virtual"
               addresses used?  Is one of the addresses used?

     It also has the considerable benefit of leaving the IP
     infrastructure unchanged.  Given the complexity and robustness of
     that infrastructure, as well as the considerable time and effort
     that was needed to achieve its stability, any design that avoids
     changing the infrastructure is to be commended.

     The fact that the functionality is applicable across all
     transport services suggests that there might be benefit in having
     IP multiaddressing functionality reside in a single architectural
     module, separate from any specific transport service. In any case
     these new transport protocol efforts cannot affect the
     considerable installed base of services using older transport
     protocols, such as TCP and UDP.

3.3. Session-Level

     The session layer provides functionality above transport and
     below the application. In effect it is a way of
     institutionalizing application-level support.  The merit of
     placing multiaddressing support at the session layer is that it
     can use multiple transport services.

           +---------+                +---------+
           |   App   |                |   App   |
           +---------+                +---------+
                |                          |
           +---------+                +---------+
           | Session |                | Session |
           +---------+                +---------+
          IP.a|   | IP.f             IP.q|   |IP.y
           +---------+                +---------+
           |   TCP   |                |   TCP   |
           +---------+                +---------+
          IP.a|   | IP.f             IP.q|   |IP.y
           +---------+                +---------+
           |   IP    |                |   IP    |
           +---------+                +---------+
              |   |                      |   |
              |   |                      |   |
           IP.a   IP.f                IP.q   IP.y

     The problem with this approach is that a full session layer
     typically replicates substantial portions of the transport
     service, in order to ensure reliability and in-order data
     sequencing.  This makes the session-level approach notably
     complicated and inefficient.

3.4. Application-Level

     Applications often provide themselves with enhanced
     infrastructure support services, to compensate for limitations in
     the lower protocol, or to optimize functionality and performance
     according to the peculiarities of the specific application.  A
     typical example is with reliable data transfer, when using an
     unreliable datagram service.  The obvious difficulty with this
     approach is that it burdens each new application with re-creating
     these (common) underlying services.

             +-------+                  +-------+
             |  App  |                  |  App  |
             +-------+                  +-------+
          TCP.1|   |TCP.2            TCP.1|   |TCP.2
             +-------+                  +-------+
             |  TCP  |                  |  TCP  |
             +-------+                  +-------+
           IP.a|   | IP.f             IP.q|   |IP.y
             +-------+                  +-------+
             |  IP   |                  |   IP  |
             +-------+                  +-------+
               |   |                      |   |
               |   |                      |   |
            IP.a   IP.f                IP.q   IP.y

     There well might be some benefit in permitting applications to
     discover details about multiple-address capabilities, and
     possibly even to specify some controls over their use, through an
     enhanced API.  However the prevalence of multiaddressing dictate
     their support in lower layers.

3.5. IP Endpoint

     A recent approach to multiaddressing defines a new "convergence"
     layer that exists only in the endpoint systems (hosts) and
     operates between classic IP and the transport layer. Hence these
     capabilities are invisible to the IP relaying infrastructure and
     can be invisible to the transport layer. However they may specify
     new or modified adjunct infrastructure services, especially to
     obtain full rendezvous capabilities.

     This type of approach can be viewed as using a "shim" or "wedge"
     partial-layer, between IP and transport, or it can be viewed as
     partitioning IP, between a lower, relaying module that is common
     to all IP nodes, versus an upper module that performs IP-related
     functions specific to endpoints.

     The remainder of this sub-section considers these architectural
     views and then discusses the three IP Endpoint proposals.

     3.5.1.    Choosing an IP Endpoint Model  Shim Model

     For the Shim, or wedge, approach, a portion of functionality is
     "intercepted" and modified by the shim module:

               (IP.a, Port.l, IP.y, Port.r)
               | Port.l                  | Port.r
            +------+                  +------+
            | TCP  |                  | TCP  |
            +------+                  +------+
          IP.a |                         | IP.y
             +----+                    +----+
            < shim >                  < shim >
             +----+                    +----+
              |  |                      |  |
          IP.a|  |IP.f              IP.q|  |IP.y
            +------+                  +------+
            |  IP  |                       |  IP  |
            +------+                  +------+
              |  |                      |  |
           IP.a  IP.f                IP.q  IP.y  IP/Transport Convergence Layer Model

     Rather than viewing this type of service as being ad hoc, it can
     be seen as an example of  IP-level services that reside only in
     the end-systems.  That is, there is a distinction between the
     relaying activities in every "intermediate" system (IP-is),
     versus IP functions that are needed only in the end-systems at
     the endpoints (IP-es).  For multiaddressing, the architectural
     impact is embodied by using an "endpoint identifier" (EID) in the
     interface between IP-es and the transport layer, rather than
     using an endpoint address.  Significantly, the EID might be
     private to the endpoint(s), rather than needing to be globally

     IPSec is another example of and IP-es service. Note that this
     architectural change also must affect the upper-layer access to
     DNS, since DNS address records must be converted to EIDs.

           (IP.eid1, Port.l, IP.eid2, Port.r)
               | Port.l                  | Port.r
            +-------+                  +-------+
            |  TCP  |                  |  TCP  |
            +-------+                  +-------+
                | IP.eid1          IP.eid2 |
            +-------+                  +-------+
            | IP-es |                  | IP-es |
            +-------+                  +-------+
          IP.a|   | IP.f             IP.q|   |IP.y
            +-------+    +-------+     +-------+
            | IP-is |    | IP-is |     | IP-is |
            +-------+    +-------+     +-------+
              |   |        |   |         |   |
              |   +--------+   +---------+   |
           IP.a   IP.f               IP.q   IP.y

     3.5.2.    Host Identity Protocol (HIP)

     HIP works with IPv4 and IPv6.  Also, it:

          *    Creates a new, globally unique name space
          *    Uses strong, cryptographically based protocol details,
               overloading some HIP functionality with security
          *    Is tied significantly to [IPSEC]
          *    Creates a new DNS RR entry
          *    Requires a Rendezvous server for mobility support
          *    Requires that NATs be aware of HIP

     Many of the HIP features are appealing, such as the cleanliness
     of the architectural model depicted in Section 4 of the HIP
     architecture document.  Were the Internet stack being created
     now, HIP well might be an excellent approach.  However
     retrofitting this approach into the existing, deployed Internet
     entails serious barriers to adoption, such as its dependence on

     In general, addition of a DNS SRV record can be useful for
     achieving efficient rendezvous, with or without mobility.  It
     permits participants to know whether a service is supported by
     its partner, without requiring a probe packet.  While beneficial,
     this enhancement to DNS data structures is not required for
     multihoming or client (initiator) mobility.

     3.5.3.    LIN6

     LIN6 defines a new, globally unique 64-bit end-point identifier
     that is used by upper layers, within an IPv6 address format.
     This is then mapped to one or more IPv6 IP-layer addresses.

     The LIN6 specification also provides for the rendezvous function,
     using DNS for basic name resolution and a separate, dynamically
     updated service to provide accurate information about rapidly
     changing addresses.

     3.5.4.    MAST

     MAST is a control protocol for the exchange of IP address
     notification and authorization, to use additional IP addresses in
     a given host-pair context.

     The primary MAST exchange transmits:

          *    A list of current IP addresses supported by the sender

     Support exchanges:

          *    Establish a host-pair context
          *    Establish relevant authentication between the pair

     MAST takes a more modest approach than HIP or LIN6. It does not
     define a new identifier space, has a simpler specification,
     permits easier implementation and adoption, and works equally
     with IPv4 and IPv6.

     Rendezvous with a mobile target is provided as an adjunct
     function and relies on domain names and an existing presence

     MAST differs from the list of HIP requirements in that it:

          *    Uses a name space that is transient and local to the
          *    Uses existing security mechanisms, limited to the sole
               requirement to prevent association hijacking
          *    Treats rendezvous as an adjunct requirement and has no
               special requirements on DNS, in the base service
          *    Is transparent to NATs


     This is a discussion paper and specifies no actions. Hence it has
     no security impact, except in terms of generally discussing
     security issues for the IP architecture.


A.   Acknowledgements

     Funding for the RFC Editor function is currently provided by the
     Internet Society.

     Commenters on this text include: Marcelo Bagnulo, Iljitsch van
     Beijnum, Vint Cerf, Spencer Dawkins, Robert Honore, James Kempf ,
     Eugene Kim, Eliot Lear, Pekka Nikander, Erik Nordmark, Tim
     Shepard, Randall R. Stewart, and Fumio Teraoka

B.   References

     B.1. Non-Normative

     [DCCP]    Kohler, E., M. Handley, S. Floyd, J. Padhye,
               "Datagram Congestion Control Protocol (DCCP)",
               draft-ietf-dccp-spec-04.txt, 30 June 2003

     [DNSDYN]  Vixie, P., Thomson, S., Rekhter, Y., Bound, J.,
               Dynamic Updates in the Domain Name System (DNS
               UPDATE)", RFC2136, April 1997

               Wellington , B., "Secure Domain Name System
               (DNS) Dynamic Update", RFC 3007, November 2000

     [EID]     Chiappa, J.N.,   "Endpoints and Endpoint Names:
               A Proposed Enhancement to the Internet

     [ETCP]    Zhang, B., Zhang, B.,  Wu,  I., "Extended
               Transmission Control Protocol (ETCP) Project--
               Extension to TCP for Mobile IP Support",

     [HIP]     Moskowitz, R., "Host Identity Protocol
               Architecture", < http://www.ietf.org/internet-
               drafts/draft-moskowitz-hip-arch-03.txt >

               Moskowitz, R., "Host Identity Protocol", <ietf-
               id: draft-moskowitz-hip-07>

               Nikander, P., "End-Host Mobility and Multi-
               Homing with Host Identity Protocol", <

     [HOWIE]   Howie, D., "Consequences of using MIPv6 to
               Achieve Mobile Ubiquitous Multimedia",

     [IPSEC]   Kent, S. and R. Atkinson, "Security Architecture
               for the Internet Protocol", RFC 2401, November

     [LIN6]    Teraoka, F.,  Ishiyama, M.,  Kunishi, M., "LIN6:
               A Solution to Mobility and Multi-Homing in
               IPv6", draft-teraoka-ipng-lin6-02.txt, 24 June

     [NAT]     Egevang, K., and P. Francis, "The IP Network
               Address Translator (NAT)", RFC1631, May 1994

     [NSRG]    Lear, E., Droms, R., "What's In A Name: Thoughts
               from the NSRG", draft-irtf-nsrg-report-09.txt,
               March 2003

     [MAST]    Crocker, D., "Multiple Address Service for
               Transport (MAST):
               An Extended Proposal", draft-crocker-mast-
               proposal-00.txt, September 13,2003

     [MIP]     Perkins, C., "IP Mobility Support", RFC 2002,
               October 1996

               Johnson, D., Perkins, C., Arkko, J., "Mobility
               Support in IPv6", draft-ietf-mobileip-ipv6-
               24.txt, June 30, 2003

               Bagnulo, M., Garcia-Martinez, A., Soto, I.,
               "Application of the MIPv6 protocol to the multi-
               homing problem", draft-bagnulo-multi6-mnm-00,
               February 25, 2003

     [PBK]     Bradner, S., Mankin,  AS., Schiller, J.,  "A
               Framework for Purpose-Built Keys (PBK)",  draft-
               bradner-pbk-frame-06.txt, June 2003

     [SCTP]    L. Ong, and J. Yoakum "An Introduction to the
               Stream Control Transmission Protocol (SCTP)",
               May 2002

               R. Stewart, et al, "Stream Control Transmission
               Protocol (SCTP) Dynamic Address
               Reconfiguration", draft-ietf-tsvwg-addip-sctp-
               07.txt, February 26, 2003

     [TCPMH]   Matsumoto, A. Kozuka, M., Fujikawa, K., Okabe,
               Y., "TCP Multi-Home Options", draft-arifumi-tcp-
               mh-00.txt, 10 Sep 2003

     [TLS]     Dierks, T., C. Allen , "The TLS Protocol Version
               1.0", RFC 2246, January 1999.

C.   Author's Adress

     Dave Crocker
     Brandenburg InternetWorking
     675 Spruce Drive
     Sunnyvale, CA  94086  USA

     tel: +1.408.246.8253

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