Network Working Group D. Crocker
Internet Draft Brandenburg
draft-crocker-mast-analysis-00.doc InternetWorking
Expires: <2-04> September 16, 2003
CHOICES FOR SUPPORT OF MULTIADDRESSING
STATUS OF THIS MEMO
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),
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COPYRIGHT NOTICE
Copyright (C) The Internet Society (2003). All Rights Reserved.
ABSTRACT
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.
CONTENTS
1. INTRODUCTION
1.1. TERMINOLOGY
1.2. SCENARIOS
1.3. IETF BACKGROUND
1.4. DISCUSSION VENUE
1.5. DOCUMENT HISTORY
2. REQUIREMENTS AND CONSTRAINTS
2.1. MOBILITY
2.2. MULTIHOMING
2.3. SECURITY
2.4. IMPLEMENTATION
2.5. DEPLOYMENT AND USE
2.6. MATTERS OF STATE
2.7. ENDPOINT IDENTIFIERS
2.8. SIGNALING
2.9. OPERATION THROUGH NATS
3. INTERNET STACK PLACEMENT
3.1. IP INFRASTRUCTURE
3.2. TRANSPORT-LEVEL
3.3. SESSION-LEVEL
3.4. APPLICATION-LEVEL
3.5. IP ENDPOINT
4. SECURITY CONSIDERATIONS
APPENDIX
A. ACKNOWLEDGEMENTS
B. REFERENCES
C. AUTHOR'S ADRESS
D. FULL COPYRIGHT STATEMENT
INTRODUCTION
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
connection.
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
server.
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
issues.
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-
sections:
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
<http://ietf.org/html.charters/multi6-charter.html>.
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. REQUIREMENTS AND CONSTRAINTS
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
addresses?
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
architecture.
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
implementation.
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
infrastructure?
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
multiaddressing.
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
endpoints.
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
dependency.
3. INTERNET STACK PLACEMENT
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:
Connection
(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.
Connection
(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-
topologically.
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.
Connection
(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
3.5.1.1. Shim Model
For the Shim, or wedge, approach, a portion of functionality is
"intercepted" and modified by the shim module:
Connection
(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
3.5.1.2. 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
registered.
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.
Connection
(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
functionality
* 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
IPSec.
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
service.
MAST differs from the list of HIP requirements in that it:
* Uses a name space that is transient and local to the
host-pair
* 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
4. SECURITY CONSIDERATIONS
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.
APPENDIX
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
Architecture",
<http://users.exis.net/~jnc/tech/endpoints.txt>,
1999
[ETCP] Zhang, B., Zhang, B., Wu, I., "Extended
Transmission Control Protocol (ETCP) Project--
Extension to TCP for Mobile IP Support",
<http://www.cs.ucla.edu/~bzhang/etcp/report.html
>
[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", <
http://www.ietf.org/internet-drafts/draft-
nikander-hip-mm-00.txt>
[HOWIE] Howie, D., "Consequences of using MIPv6 to
Achieve Mobile Ubiquitous Multimedia",
http://www.mediateam.oulu.fi/publications/pdf/38
4.pdf
[IPSEC] Kent, S. and R. Atkinson, "Security Architecture
for the Internet Protocol", RFC 2401, November
1998
[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
2003
[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)",
<http://ietf.org/rfc/rfc3286.txt?number=3286>,
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
dcrocker@brandenburg.com
D. Full Copyright Statement
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