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Gett Communications
Expiration Date: May 2000 October 1999

To Be Multihomed: Requirements & Definitions

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),
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 Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

2. Abstract

As organizations find their Internet connectivity increasingly critical
to their mission, they seek ways of making that connectivity more
robust. The term ''multi-homing'' often is used to describe means of
fault-tolerant connection. Unfortunately, this term covers a variety of
mechanisms, including naming/directory services, routing, and physical
connectivity. The "home" may be identified by a DNS name, an IP address, or
an IP address and transport-layer port identifier. Any of these identifiers
may be translated in the path between source and destination. This
memorandum presents a systematic way to define the requirement for
resilience, and a taxonomy for describing mechanisms to achieve it. Its
intended audience is primarily people concerned with planning and performing
multihoming deployments, rather than protocol developers. It examines both
requirements and applications, with the emphasis on the former.

3. Introduction

As the Internet becomes more ubiquitous, more and more enterprises
connect to it. Some of those enterprises, such as Web software vendors,
have no effective business if their connectivity fails. Other
enterprises do not have mission-critical Internet applications, but
become so dependent on routine email, news, web, and similar access that
a loss of connectivity becomes a crisis.

As this Internet dependence becomes more critical, prudent management
suggests there be no single point of failure that can break all Internet
connectivity. The term "multihoming" has come into vogue to describe
various means of enterprise-to-service provider connectivity that avoid
a single point of failure. Multihoming also can describe connectivity
between Internet Service Providers and "upstream" Network Service

Several terms have become overloaded to the point of confusion, including
multihoming, virtual private networks, and load balancing. This document
attempts to bring some order to the definition of multihoming. It partially
overlaps definitions of virtual private networks [Ferguson & Huston].

If we take the word "multihoming" in the broadest context, it implies there
are multiple ways to reach a "home" destination. This "home" may be
identified by a name, an IP address, or a combination of IP address and
TCP/UDP port. In this document, TCP/UDP ports will be referred to as TU

There are other motivations for complex connectivity from enterprises to
the Internet. Mergers and acquisitions, where the joined enterprises
each had their own Internet access, often mean complex connectivity, at
least for a transition period. Consolidation of separate divisional
networks also creates this situation. A frequent case arises when a
large enterprise decides that Internet access should be available
corporate-wide, but their research labs have had Internet access for
years -- and it works, as opposed to the new corporate connection that
at best is untried.

Many discussions of multihoming focus on the details of implementation,
using such techniques as the Border Gateway Protocol (BGP) [RFC number
of the Applicability Statement], multiple DNS entries for a server, etc.
This document suggests that it is wise to look systematically at the
requirements before selecting a means of resilient connectivity.

One implementation technique is not appropriate for all requirements.
There are special issues in implementing solutions in the general
Internet, because poor implementations can jeopardize the proper
function of global routing or DNS. An incorrect BGP route advertisement
injected into the global routing system is a problem whether it
originates in an ISP or in an enterprise.

4. Goals

Requirements tend to be driven by one or more of several major goals for
server availability and performance. Availability goals are realized
with resiliency mechanisms, to avoid user-perceived failures from single
failures in servers, routing systems, or media. Performance goals are
realized by mechanisms that distribute the workload among multiple
machines such that the load is distributed in an useful manner. Like
multi-homing, the terms load-balancing and load-sharing have many

In defining requirements, the servers themselves may either share or
balance the load, there may be load-sharing or load-balancing routing
paths to them, or the routed traffic may be carried over load-shared or
load-balanced media.

4.1 Analyzing Application Requirements

Several questions need to be answered in the process of refining goals:

-- the administrative model and administrative awareness of endpoints
-- availability requirements
-- the security model
-- addressing requirements
-- scope of multihoming

4.1.1 Administrative Model

A key question is: are endpoints predefined in the multihoming process, or
will either the client or server end be arbitrary?

The servers of interest may be inside the enterprise, or outside it. If they
are outside, their names or addresses may or may not be preconfigured into
multihoming mechanisms.

In this document, intranet clients and servers are inside the enterprise and
intendedprimarily for enterprise use. The existence of both can be
preconfigured. Intranet clients have access only to machines on the

Multinet clients servers are inside the
enterprise, but there is pre-authorized access by known external partners.

Internet servers are operated by the enterprise but intended to be
accessible to the general Internet. Internet clients have general Internet
access that may be mediated by a firewall. The client administrator will
know the prior identity of clients, but not of servers. The server
administrator will know the prior identity of servers, but not of clients.

4.1.2 Availability Requirements

There are major implications between defining a requirement for high
availability of initial access, and making the connection stay up once
access has been achieved. The latter tends to require transport layer

In the terminology of RFC1775, "To be 'on' the Internet," servers
described here have "full" or a subset of "client" access. Client
servers may not directly respond to specific IP packet from an arbitrary
host, but a system such as a firewall MUST respond for them unless a
security policy precludes that. Some valid security policies, for
example, suppress the response of ICMP Destination Administratively
Prohibited responses, because that would reveal there is an information
resource being protected.

RFC1775 defines full access as " a permanent (full-time) Internet
attachment running TCP/IP, primarily appropriate for allowing the
Internet community to access application servers, operated by Internet
service providers. Machines with Full access are directly visible to
others attached to the Internet, such as through the Internet Protocol's
ICMP Echo (ping) facility. The core of the Internet comprises those
machines with Full access." This definition is extended here to allow
full firewalls or screening routers always to be present.

If a proxy or address translation service exists between the real
machine and the Internet, if this service is available on a full-time
basis, and consistently responds to requests sent to a DNS name of the
server, it is considered to be full-time.

In this discussion, we generalize the definition beyond machines
primarily appropriate for the Internet community as a whole, to include
in-house and authorized partner machines that use the Internet for
connectivity. Both the cases described in 4.2.3 and 4.2.4 have been termed
"Virtual Private Networks."

4.1.3 Security Model

Security requirements can include various cryptographic schemes, as well as
mechanisms to hinder denial of service attacks. The requirements analyst
must determine whether cryptography is needed, and, if so, whether
cryptographic trust must be between end hosts or between end hosts and a
trusted gateway. Such gateways can be routers or multiported application

4.1.4. Addressing Refinements and Issues

It is arguable that addressing used to support multihoming is a routing
deployment issue, beyond the scope of this document. Rationales for
including it here is that addressing MAY affect application behavior. There
also may be administrative requirements for addressing, such as a service
provider that contracts to run a multinet may require addresses to be
registered, possibly from the provider's address space.

If the enterprise runs applications that embed network layer addresses
in higher-level data fields, solutions that employ address translation,
at the packet or virtual connection level, MAY NOT work. Use of such
applications inherently is a requirement for the eventual multihoming

Consideration also needs to be given to application caches in addition
to those of DNS. Firewall proxy servers are a good example where
multiple addresses associated with a given destination may not be

Network Address Translation (NAT) is a widespread technique undergoing
significant enhancement in the NAT Working Group. The traditional approached
either did a one-to-one translation from inside to outside address, or a
many-to-one mapping from a large number of addresses on one side to a much
smaller number of addresses (with a larger number ot TCP/UDP ports). The
traditional approaches, in practice, include:

RFC1918 internal, static network address translation (NAT) to outside
The outside space may be an extranet partner's private or registered
address space, or may be registered space allocated to an ISP.

RFC1918 internal, dynamic port address translation (PAT) to outside
The outside space may be an extranet partner's private or registered
address space, or may be registered space allocated to an ISP.

Registered internal, Provider Assigned (PA)

Registered internal, Provider Independent (PI)

More powerful translation technologies such as Load-Sharing NAT [RFC2391]or
Address Layer Gateways (ALG) [RFC2663] may be needed.

4.1.5 Scope of multihoming

Multihoming may be defined between an end host and a router or application
gateway, on an end-to-end basis possibly involving virtual servers, among
routers, or among elements in a transmission system. Different multihoming
scopes may support the same application requirement.

4.2. Application Goals

These goals need to be agreed to by the people or organization responsible
for the applications. Not to reach fairly formal agreement here can lead to
problems of inappropriate expectations.

At the application layer, there will be expectations of connectivity. Not
all applications will operate through classical NAT devices. Application
designers should proceed on two fronts: following NAT-friendly application
design principles [Senie 1999a] and being aware of potential application
protocol interactions with NAT technologies [Holdredge 1999a].

The term "service level agreement" often refers to expectations of
performance, such as throughput or response time. Ideas here extend the
performance-based model to include availability.

4.2.1 Specific server availability

The first goal involves well-defined applications that run on specific
servers visible to the Internet at large. This will be termed "endpoint
multihoming", emphasizing the need for resilience of connectivity to
well-defined endpoints. Solutions here often involve DNS mechanisms.

There are both availability and performance goals here. Availability
goals arise when there are multiple routing paths that can reach the
server, protecting it from single routing failures. Other availability
goals involve replicated servers, so that the client will reach a server
regardless of single server failures.

Performance goals include balancing client requests over multiple
servers, so that one or more servers do not become overloaded and
provide poor service. Requests can be distributed among servers in a
round-robin fashion, or more sophisticated distribution mechanisms can
be employed. Such mechanisms can consider actual real-time workload on
the server, routing metric from the client to the server, known server
capacity, etc.

>From an application standpoint, this is either a many-to-one topology, many
clients to one server, or a many-to-many topology when multiple servers are
involved. It can be worthwhile to consider a many-to-few case, when the few
are multiple instances of a server function, which may appear as a single
server to the general Internet. The idea of many-to-few topology allows for
a local optimization of inter-server communications, without affecting the
global many-to-one model.

Addresses on interfaces that connect to the general Internet need to be
unique in the global Internet routing system, although they may be
translated, at the network address or port level, from public to internal

4.2.2 General Internet connectivity from the enterprise

The second is high availability of general Internet connectivity for
arbitrary enterprise users to the outside. This will be called
"internetwork multihoming". Solutions here tend to involve routing

This can be viewed as a few-to-many application topology.

Addresses on interfaces that connect to the general Internet need to be
unique in the global Internet routing system, although they may be
translated, at the network address or port level, from internal private
address to public space.

4.2.3 Use of Internet services to interconnect "intranet" enterprise

The third involves the growing number of situations where Internet
services are used to interconnect parts of an enterprise. This is
"intranetwork multihoming". This will usually involve dedicated or
virtual circuits, or some sort of tunneling mechanisms.

This case may be termed a "virtual private network." The "multihoming"
of this case is associated with high availability to the connectivity
network that underlies the tunneling system.

In this case, addresses only need to be unique within the enterprise.

4.2.4 Use of Internet services to connect to "multinet" partners

A fourth category involves use of the Internet to connect with strategic
partners. True, this does deal with endpoints, but the emphasis is
different than the first case. In the first case, the emphasis is on
connectivity from arbitrary points outside the enterprise to points
within it. This second case deals with pairs of well-known endpoints.

These endpoints may be linked with dedicated or virtual circuits defined
at the physical or data link layer. Tunneling or other virtual private
networks may be relevant here as well. There will be coordination
issues that do not exist for the third case, where all resources are
under common control.

Addresses need to be unique in the different enterprises, but do not need to
be unique in the global Internet.

5. Planning and Budgeting

In each of these scenarios, organization managers need to assign some
economic cost to outages. Typically, there will be an incident cost and
an incremental cost based on the length or scope of the connectivity

Ideally, this cost is then weighted by the probability of outage.

A weighted exposure cost results when the outage cost is multiplied by
the probability of the outage.

Resiliency measures modify the probability, but increase the cost of

Operational costs obviously include the costs of redundant mechanisms
(i.e., the addititional multihomed paths), but also the incremental
costs of personnel to administer the more complex mechanisms -- their
training and salaries.

6. Issues

6.1 Performance vs. Robustness: the Cache Conundrum

Goals of many forms of "multi-homing" conflict with goals of improving
local performance. For example, DNS queries normally are cached in DNS
servers, and in the requesting host. From the performance standpoint,
this is a perfectly reasonable thing to do, reducing the need to send
out queries.

>From the multihoming standpoint, it is far less desirable, as
application-level multihoming may be based on rapid changes of the DNS
master files. The binding of a given IP address to a DNS name can
change rapidly.

6.2 Service Level Agreements

Enterprise networks, especially mainframe-based, are accustomed to building
and enforcing service level agreements for application performance. A key to
being able to do this is total control of the end-to-end communications

In the current Internet, the enterprise(s) at one or both ends control their
local environments, and have contractual control over connections to their
direct service providers.

If service level control is a requirement, and both ends of the path are not
under control (i.e., cases 1 and 2), the general Internet cannot now provide
service level guarantees. The need for control should be reexamined, and, if
it still exists, the underlying structure will need to be dedicated
resources at the network layer or below. A network service provider may be
able to engineer this so that some facilities are shared to reduce cost, but
the sharing is planned and controlled.

6.3 Symmetry

One of the reasons service level agreements are not enforceable in the
general Internet is the reality that global routing cannot be guaranteed to
be symmetrical. Symmetrical routing assumes the path to a destination is
simply reversed to return a response from that destination. Both legs of a
symmetrical path are assumed to have the same performance characteristics.

Global Internet routing is not necessarily optimized for best end-to-end
routing, but for efficient handling in the Autonomous Systems along the
path. Many service providers use "closest exit" routing, where they will
go to the closest exit point from their perspective to get to the next hop
AS. The return path, however, is not necessarily of a mirror image of the
path from the original source to the destination.

Closest exit routing is, in fact, a "feature" rather than a "bug" in some
multihoming schemes [Peterson] [Friedman].

Especially when the
enterprise network has multiple points of attachment to the Internet,
either to a single ISP AS or to multiple ISPs, it becomes likely that the
response to a given packet will not come back at the same entry point in
which it left the enterprise.

This is probably not avoidable, and troubleshooting procedures and traffic
engineering have to consider this characteristic of multi-exit routing.

6.4 Security

ISPs may be reluctant to let user routing advertisements or DNS zone
information flow directly into their routing or naming systems. Users
should understand that BGP is not intended to be a plug-and-play
mechanism; manual configuration often is considered an important part of
maintaining integrity. Supplemental mechanisms may be used for
additional control, such as registering policies in a registry [RPS, RA
documents] or egress/ingress filtering [Ferguson draft]

Challenges may arise when client security mechanisms interact with fault
tolerance mechanisms associated with servers. For example, if a server
address changes to that of a backup server, a stateful packet screening
firewall might not accept a valid return. Similarly, unless servers back
one another up in a full mirroring mode, if one end of a TCP-based
application connection fails, the user will need to reconnect. As long
as another server is ready to accept that connection, there may not be
major user impact, and the goal of high availability is realized. High
availability and user transparent high availability are not synonymous.

6.5 Load Balancing vs. Load Sharing

These terms are often interchanged, but they really mean different things.
Load balancing is deterministic, and at a finer level of control than load
sharing, which is statistical. Load balancing is generally not something
that can be realized in general Internet routing, other than in special and
local cases between adjacent AS. A degree of load sharing is achievable in
routing, but it may introduce significant resource demands and operational

Paul Ferguson defines load-balancing as "a true "50/50"
sharing of equal paths. This can be done by either (a) round robin per-
packet transmission, (b) binding pipes a the lower layers such that bits
are either 'bit-striped' across all parallel paths (like the
etherchannel stuff), or binding pipes so that SAR functions are done in
a method such as multilink PPP. These are fundamentally the same.

"Load-sharing is quite different. It simply implies that no link is
sitting idle -- that at least all links get utilized in some fashion.
Usually in closest exit routing. The equity of utilization may be
massively skewed. It may also resemble something along the lines of
60/40, which is reasonable."

6.6 Application Compatibility

Some deployment mechanisms involve network address, or network address and
TCP/UDP port, translation (NAT and NAPT). If the application protocols embed
IP addresses in their protocol fields, NAT or NAPT may cause protocol
failures. Translation mechanisms for such cases may require knowledge of the
application protocol, as typified by application proxies in firewalls, or in
application gateways with multiple interfaces.

7. Multihoming Deployment Technologies

A basic way to tell which technology(ies) is applicable is to ask oneself
whether the functional requirement is defined in terms of multihoming to
specific hosts, or to specific networks. If the former, some type of
application or transport technology is needed, because only these
technologies have awareness of specific hosts.

A given multihoming implementation may draw on several of these
technologies. For example, the Cisco Distributed Director does DNS
name-level redirection based in part on routing metrics.

7.1 Application/Name Based

Techologies in this category may involve referring a client request to
different instances of the endpoint represented by a single name. Another
aspect of application/name multihoming may work at the level of IP
addressing, but specifically is constrained to endpoint (i.e., server)
activities that redirect the client request to a different endpoint.

Application-level firewall proxy services can provide this functionality,
although their application protocol modification emphasizes security while a
multihoming application service emphasizes availability and quality of

7.2 Transport Based

Transport based technologies are based on maintaining tunnels through an
underlying network or transmission system. The tunnels may be true end to
end, connecting a client host with a server host, or may interconnect
between proxy servers or other gateways.

7.3 Network Based

Network based approaches to multihoming are router-based. They involve
having alternate next-hops, or complete routes, to alternate destinations.

7.4 Data Link Based

Data link layer methods may involve coordinated use of multiple physical
paths, as in multilink PPP or X.25. If the underlying WAN service has a
virtual circuit mechanism, as in frame relay or ATM, the service provider
may have multihomed paths provided as part of the service. Such functions
blur between data link and physical layers.

Other data link methods may manipulate MAC addresses to provide virtual
server functions.

7.5 Physically-based

Physical multihoming strategies can use diverse media, often of different
types such as a wire backed up with a wireless data link. They also involve
transmission media which have internal redundancy, such as SONET.

8. Application/Name Multihoming

While many people look at the multihoming problem as one of routing,
the goal is often multihoming to endpoints. Finding an endpoint usually
begins in DNS. Once an endpoint address is found, some application
protocols, notably HTTP and FTP, may redirect the request to a different

The basic
idea here is that arbitrary clients will first request access to a
resource by its DNS name, and certain DNS servers will resolve the same
name to different addresses based on conditions of which DNS is aware,
or using some statistical load-distribution mechanism.

There are some general DNS issues here. DNS was not really designed to
do this. A key issue is that of DNS cacheing. Cacheing and frequent
changes in name resolution are opposite goals. Traditional DNS schemes
emphasize performance over resiliency.

8.1 DNS Caching

DNS standards do provide the capability for short cache lifetimes, which in
principle support name based multihoming. "The meaning of the TTL field is a
time limit on how long an
RR can be kept in a cache. This limit does not apply to authoritative
data in zones; it is also timed out, but by the refreshing policies for
the zone. The TTL is assigned by the administrator for the zone where
the data originates. While short TTLs can be used to minimize caching,
and a zero TTL prohibits caching, the realities of Internet performance
suggest that these times should be on the order of days for the typical
host. If a change can be anticipated, the TTL can be reduced prior to
the change to minimize inconsistency during the change, and then
increased back to its former value following the change. [RFC1034] "

Several real-world factors limit the utility of simply shortening the cache
time. Widely used BIND, the most widely used DNS implementation, does not
accept cache lifetimes less than 5 minutes.

Dynamic DNS may be a long-term solution here. In the short term, setting
very short TTL values may be help in some cases, but is not likely to help
elow a granularity of 5 minutes. Remember that the name
normally is resolved when an application session first is established,
and the decisions are made over a longer time base than per-packet
routing decisions.

8.2 DNS Multiple Hosts and Round Robin Response

The DNS protocol allows it to return multiple host addresses in respondse to
a single query. At the first level of DNS-based multihoming, this can
provide additional reliability.

A DNS server knows three IP addresses for the server function identified by
server.example.com,,, and A simple response to a
query for server.example.com returns all three addresses. Assume the
response provides server addresses in the order,, and

Whether this will provide multihoming now depends on the DNS client. Not all
host client implementations will, if the first address returned (i.e., does not respond, try the additional addresses. In this example, might be operating perfectly,

A variant suggested by Kent England is to have the addresses returned in the
DNS response come from the CIDR blocks of different ISPs that provide
connectivity to the server function [England]. This approach combines
aspects of name and network multihoming.

Again, this will work when intelligent clients try every IP address returned
until a server responds.

8.3 Replication

One approach [Peterson] uses DNS as the main method, but makes assumptions
about the underlying routing. The two assumptions it makes, which appears
generally valid for the Internet, are that connectivity providers use
closest exit routing, and once a packet reaches a provider network, that
provider knows best how to reach the specific server inside that network.

One implementation of this approach is Cisco's DistributedDirector product.
It involves communication between DNS servers and routers in the multihomed
domain. When a DNS request is received by a DistributedDirector DNS server,
the DD server selects the IP address to be returned based on information on
routing cost from the client entry point to closest server, on
administrative weight, or other generally routing-associated factors.

8.4 Cache Servers and Application Multihoming

The cache server is the primary home for servicing the client request, but
the true server backs it up.

9. Transport Multihoming

Transport layer functions are conceptually end-to-end. There are two broad
classes of transport multihoming function, those maintained by the endpoints
and those that involve intermediate translation devices.

9.1 End-to-end Tunnel Maintenance

Basic point-to-point tunneling mechanisms include GRE, PPTP and L2TP. DVMRP
is a special case. Choices here will depend in part on the security policy
and the administrative model by which multihoming is provided. GRE, for
example, does not itself provide encryption, while PPTP and L2TP do.

"The differences between PPTP and L2TP are more of where one wishes the
PPP session to terminate, and one of control. It really depends on who
you are, and where you are, in the scheme of the control process. If
your desire is to control where the PPP session terminates (as an ISP
might wish to control), then L2TP might be the right approach. On the
other hand, however, if you are a subscriber, and you wish to control
where the PPP session terminates (to, say, a PPTP server somewhere
across the cloud), then PPTP might be the right approach -- and it
would be transparent to the service provider). It really depends on
what problem one is trying to solve, and if you are in the business
of trying to create "services." [Ferguson-1998-2]

9.2 Tunneling with Translation

Various proxy and address translation mechanisms can play a role in
multihoming. They can be divided into several levels of topological

-- all servers are colocated in a single address domain "behind" the

-- servers are in different parts of a single address domain. These
parts are connected by tunnels.
-- the servers are at arbitrary network addresses, but the translator
knows how to reach them.

Application-aware proxies can have even more knowledge of application load.

A variant of NAT, called load sharing NAT (LS-NAT), offers a load sharing
mechanism at the transport/network level rather than the DNS level
[Srishuresh]. When considering LSNAT-style load sharing, remember that it
emphasizes providing a pool of servers capable of servicing requests.

In its "local" form, it does not easily provide mechanisms for increasing
reliability by mapping the user request to geographically distributed
servers. More advanced variants can combine with DNS- and routing-aware
mechanisms to increase reliability as well as performance.

The LSNAT function is visible globally as a server address. It is actually a
virtual server. When a client request arrives at the LSNAT, the LSNAT
translates the destination address, transparently to the client, and passes
it to a server in the LSNAT's server pool.

LSNATs, in their basic form, do have topological restriction. It has been
suggested that all request/response traffic in a single session must go from
the real client, to one specific LSNAT, to the server. It is conceivable
that multiple routers could be used, but they would need to be tightly
LSNAT builds on NAPT, but adds intelligence to the port mapping function.
Also, general NAPT is oriented toward outgoing requests from the stub domain
to the outside, while LSNAT emphasizes incoming requests to a virtual server

As currently conceived, LSNATs operate at the TCP/UDP transport level, so
they could not easily change server hosts during a session. There are
potential workarounds to this, including using a multicast address as the
server pool destination, with coordination between the servers as to which
actually answers the request. For highly fault-tolerant applications, more
than one server conceivably could answer the NAT request, and the LSNAT
decide which to pass to the client. Typically, if servers are identical, it
would be the first response received by the server pool side of the LSNAT.

This general topology restriction suggests LSNAT functionality belongs on a
single NAT-capable border router, with the server pool inside the stub
domain. A LSNAT violates the Internet end-to-end model in the same way that
basic NAT does. There is no requirement that the addressing in the stub
domain be private, only that all access to the servers go through the NAT.

In basic LSNAT, an arbitrary external client attempts to establish a session
with what it believes to be a server. In reality, it is attempting to
establish a session with the virtual server address of the "outside"
interface of the LSNAT.

The LSNAT, based on internal criteria, redirects the external request to a
specific internal server in a server pool. Unique connections are
established from the LSNAT to the pool server for each request, translating
addresses and ports as needed.

9.2.2. Load Shared Network Address and Port Translation (LS-NAPT)

Adding the complexity of port as well as address translation gives
additional flexibility. In particular, adding port translation removes many
topological limitations between the real servers and the NAT.

In a LS-NAT router, inbound sessions identified by the tuple <real client
address, real client TU port, virtual server address, virtual server port)
are translated into the tuple:

o client address
o client TU port
o real server address
o real server TU port

Notice that the server needs to respond to the real client address in a
LS-NAT system. Assuming the servers do not participate in routing, the only
realistic way for the servers to send to an external address is to use a
default route.

It is not a given that servers do not participate in routing. Some cache
server schemes physically involve a cluster of servers with a dedicated

LS-NAPT involves defining a virtual server address as that of an external
interface of an LS-NAPT router. Incoming sessions translate into the tuple:

o virtual server address
o virtual server TU port
o real server address
o real server port

By replacing the real client address with the address of the virtual server
as the address source seen by the inside, the real server can use multiple
paths to return responses. Quite complex routing can provide multiple paths
to the LS-NAPT router. There remains the basic topological constraint that
there will be only one LS-NAPT router, but there can more easily be multiple
internal paths to it. This allows servers to be outside a stub domain. The
LS-NAPT router can direct traffic to an internal server inside the private
address space of a stub domain, or direct the traffic to a third-party
server using registered addresses and WAN connectivity.

The additional complexity of LS-NAPT does allow greater scalability, because
new links can be dropped into the routing system without problem. As long as
new client links can get to the virtual server address, the addition of
these links is transparent to both servers and clients.

10. Network/Routing Multihoming

A common concern of enterprise financial managers is that multihoming
strategies involve expensive links to ISPs, but, in some of these
scenarios, alternate links are used only as backups, idle much of the
time. Detailed analysis may reveal that the cost of forcing these links
to be used at all times, however, exceeds the potential savings.

The intention here is to focus on requirements rather than specifics of
the routing implementation, several approaches to which are discussed in
RFC1998 and draft-bates-multihoming-01.txt. Exterior routing policies can be
described with the Routing Policy Specification Language [RFC2280].

Operational as well as technical considerations apply here. While the
Border Gateway Protocol could convey certain information between user
and provider, many ISPs will be unwilling to risk the operational
integrity of their global routing by making the user network part of
their internal BGP routing systems.

ISPs may also be reluctant to accept BGP advertisements from
organizations that do not have frequent operational experience with this
complex protocol.

10.1 Single-homed (R1)

The enterprise generally does not have its own ASN; all its
advertisements are made through its ISP. The enterprise uses default
routes to the ISP. The customer is primarily concerned with protecting
against link or router failures, rather than failures in the ISP routing

10.1.1 Single-homed, single-link (R1.1)

There is a single active data link between the customer and provider.
Variations could include switched backup over analog or ISDN services.
Another alternative might be use of alternate frame relay or other PVCs
to an alternate ISP POP.

10.1.2 Single-homed, balanced link (R1.2)

In this configuration, multiple parallel data links exist from a single
customer router to an router. There is protection against link

The single customer router constraint allows this router to do round-
robin packet-level load balancing across the multiple links, for
resiliency and possibly additional bandwidth. The ability of a router
to do such load-balancing is implementation-specific, and may be a
significant drain on the router's processor.

10.1.3 Single-homed, multi-link (R1.3)

Here, we have separate paths from multiple customer routers to multiple
ISP routers at different POPs. Default routes generated at each of the
customer gateways are injected into the enterprise routing system, and
the combination internal and external metrics are considered by internal
routers in selecting the external gateway.

This often is attractive for enterprises that want resiliency but wish
to avoid the complexity of BGP.

10.1.4 Special Cases

While the customer in this configuration is still single-homed, an AS
upstream from the ISP has a routing policy that makes it necessary to
distinguish routes originating in the customer from those originating in
the ISP. In such cases, the enterprise may need to run BGP, or have the
ISP run it on its behalf, to generate advertisements of the needed
specificity. Since the same basic topologies discussed above apply, we
can qualify them as R1.1B, R1.2B, and R1.3B.

It MAY be possible for the customer to avoid using BGP, if its adjacent
ISP will set a BGP community attribute, understood by the upstream, on
the customer prefixes [RFC1998]. Doing so results in the cases R1.1C,
R1.2C, and R1.3C. This will involve more administrative coordination, but
offers the advantage of leaving complex BGP routing to professionals.

10.2 Multi-homed Routing

The enterprise connects to more than one ISP, and desires to protect
against problems in the ISP routing system. It will accept additional
complexity and router requirements to get this. The enterprise may also
have differing service agreements for Internet access for different

10.2.1 Multi-homed, primary/backup, single link (R2.1)

The enterprise connects to two or more ISPs from a single router, but
has a strict policy that only one ISP at a time will be used for
default. In an OSPF environment, this would be done by advertising
defaults to both ISPs, but with different Type 2 external metrics. The
primary ISP would have the lower metric. BGP is not necessary in this
case. This easily can be extended to multi-link.

10.2.2 Multi-homed, differing internal policies (R2.2)

In this example, assume OSPF interior routing, because OSPF can distinguish
between type 1 and type 2 external metrics. The main default for the
enterprise comes from one or more ASBRs in Area 0, all routing to the
same ISP. One or more organizations brought into the corporate network
have pre-existing Internet access agreements with an ISP other than the
corporate ISP, and wish to continue using this for their "divisional"
Internet access.

This is frequent when a corporation decides to have general Internet
access, but its research arm has long had its own Internet connectivity.
Mergers and acquisitions also produce this case.

In this situation, an additional ASBR(s) are placed in the OSPF areas
associated with the special-case, and this ASBR advertises default.
Filters at the Area Border Router block the divisional ASBR's default
from being advertised into Area 0, and the corporate default from being
advertised into the division. Note that these filters do not block OSPF
LSAs, but instead block the local propagation of selected default and
external routes into the Routing Information Base (i.e., main routing
table) of a specific router.

10.2.3 Multi-homed, "load shared" with primary/backup (R2.3)

While there still is a primary/backup policy, there is an
attempt to make active use of both the primary and backup providers.
The enterprise runs BGP, but does not take full Internet routing. It
takes partial routing from the backup provider, and prefers the backup
provider path for destinations in the backup provider's AS, and perhaps
directly connected to that AS. For all other destinations, the primary
provider is the preferred default. A less preferred default is defined
to the second ISP, but this default is advertised generally only if
connectivity is lost to the primary ISP.

10.2.4 Multi-homed, global routing aware (R2.4)

Multiple customer router receive a full routing table, and, using
appropriate filtering and aggregation, advertise different destinations
(i.e., not just default) internally. This requires BGP, and, unless
dealing with a limited number of special cases, requires significantly
more resources inside the organization.

`0.3 Transit.

While we usually think of this in terms of ISPs, some enterprises may
provide Internet connectivity to strategic partners. They do not offer
Internet connectivity on a general basis.

10.3.1 Full iBGP mesh (R3.1)

Connectivity and performance requirements are such that a full iBGP mesh
is practical.

10.3.2 Scalable IBGP required (R3.2)

The limits of iBGP full mesh have been reached, and confederations,
route reflectors, etc., are needed for growth.

11. Transmission Considerations in Multihoming

"Multihoming" is not logically complete until all single points of
failure are considered. With the current emphasis on routing and naming
solutions, the lowly physical layer often is ignored, until a physical
layer failure dooms a lovely and sophisticated routing system.

Physical layer diversity can involve significant cost and delay.
Nevertheless, it should be considered for mission-critical connectivity.

The principal transmission impairment, the backhoe, can be viewed at

11.1 Local Loop

>From a typical server room, analog and digital signals physically flow
to a wiring closet, where they join a riser cable. Depending on the specific
building, the closet and riser may be the responsibility of the enterprise
or ISP, the building management, or a telecommunications carrier.

The riser cable
joins with other riser cables in a cable vault, from which a cable
leaves the building and goes to the end switching office of the local
telecommunications provider. Most buildings have a single cable vault,
possibly with multiple cables following a single physical route back to
the end office. A single error by construction excavators can cut
multiple cables on a single path.

A failure in carrier systems can isolate a single end office. Highly
robust systems have physical connectivity to two or more POPs reached
through two or more end offices.

Alternatives here can become creative. On a campus, it can be feasible
to use some type of existing ductwork to run additional cables to
another building that has a physically diverse path to the end office.
Direct wire burial, fiber optic cables run in the air between buildings,
etc., are all possible.

In a non-campus environment, it is possible, in many urban areas, to
find alternate means of running physical media to other buildings with
alternate paths to end offices. Electrical power utilities may have
empty ducts which they will lease, and through which privately owned
fiber can be run.

11.2 Provider Core

As demonstrated by a rash of fiber cuts in early 1997, carriers lease
bandwidth from one another, so a cut to one carrier-owned facility may
affect connectivity in several carriers. This reality makes some
traditional diverse media strategies questionable.

Many organizations consciously obtain WAN connectivity from multiple
carriers, with the notion that a failure in carrier will not affect
another. This is not a valid assumption.

If the goal is to obtain diversity/resiliency among WAN circuits, it may
be best to deal with a single service provider. The contract with this
provider should require physical diversity among facilities, so the
provider's engineering staff will be aware of requirements not to put
multiple circuits into the same physical facility, owned by the carrier
or leased from other carriers.

12. Security Considerations

13. Acknowledgments

14. References

[RFC1775] D. Crocker. To Be "On" the Internet. March 1995.

[RFC1930] Hawkinson, J., and T. Bates. Guidelines for creation, selection,
and registration of an Autonomous System (AS). March 1996. (Also BCP0006)

[RFC1034] Mockapetris, P.V. Domain names - concepts and facilities.

[RFC1998] An Application of the BGP Community Attribute in Multi-home
Routing. E. Chen & T. Bates. August 1996

[RFC2071] Ferguson, P., and H. Berkowitz, "Network Renumbering
Overview: Why would I want it and what is it anyway?", RFC 2071,
January 1997.

[RFC2050] Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D., and J.
2050, November 1996.

[RFC1631] Egevang,, K., and P. Francis, "The IP Network Address
Translator (NAT)", RFC 1631, May 1994.

[Srishuresh] Srishuresh, P., and D. Gan, "Load Sharing using IP Network
Address Translation (LSNAT)", work in progress,

[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G-J.,
and E. Lear, "Address Allocation for Private Internets", RFC 1918,
February 1996.

[RFC1900] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work", RFC
1900, February 1996.

[RFC2280] Routing Policy Specification Language (RPSL). C. Alaettinoglu, T.
Bates, E. Gerich, D. Karrenberg, D. Meyer, M. Terpstra, C.
Villamizar. January 1998.

[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.

[Peterson] Peterson, A. "Dynamic Selection of Geographically Distributed
Servers," presentation at NANOG October 1997 meeting, notes at

[Freedman] Freedman, A. "Dynamic Selection of Geographically Distributed
Servers," presentation at NANOG October 1997 meeting, notes at

[Ferguson-1998-1] Ferguson, P. "Re: Comments on "What is a VPN?"" Message to
IETF VPN mailing list, 08 Mar 1998 19:52:29

2391 Load Sharing using IP Network Address Translation (LSNAT). P.
Srisuresh, D. Gan. August 1998. (Format: TXT=44884 bytes) (Status:

[RFC2663] IP Network Address Translator (NAT) Terminology and
Considerations. P. Srisuresh, M. Holdredge. August 1999.

[Senie1999a] NAT Friendly Application Design Guidelines. D. Senie. Work in
Progress, IETF NAT Working Group, September 1999. draft-nat-app-guide-02.txt

[Holdredge1999a] Protocol Complications with the IP Network Address
Translator (NAT). M. Holdredge, P. Srisuresh. Work in Progress, IETF NAT
Working Group, Junee 1999. draft-ietf-nat-protocol-complications-01.txt

15. Author's Address

Howard C. Berkowitz
PO Box 6897
Arlington VA 22206
Phone: +1 703 998 5819
EMail: hcb@clark.net

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