IRTF Routing Research Elwyn Davies, Avri Doria, Howard Berkowitz,
Internet Draft Dmitri Krioukov, Nortel Networks
Malin Carlzon, SUNET
Anders Bergsten, Olle Pers, Yong Jiang,
Telia Research
Lenka Carr Motyckova, Pierre Fransson, Olov Schelen
Lulea University of Technology
Tove Madsen, Utfors Bredband AB
Document: draft-davies-fdr-reqs-01.txt July, 2001
Future Domain Routing Requirements
<draft-davies-fdr-reqs-01.txt>
Status of this Memo
This document is an Internet Draft and is in full conformance with
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document will expire before September, 2001. Distribution of this
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document sets out a set of requirements which we believe are
desirable for the future routing architecture and routing protocols
of a successful Internet. This first version is, of necessity,
incomplete. It is hoped that this document will be useful as a
catalyst to the work that needs to be done in both the IRTF and the
IETF.
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CONTENTS
1. Introduction...............................................4
1.1 Background............................................5
1.2 Goals.................................................6
2. Historical Perspective.....................................9
2.1 The legacy of RFC1126.................................9
2.2 Nimrod Requirements..................................20
2.3 PNNI.................................................21
2.4 Recent Research Work.................................22
3. Existing problems of BGP and the current EGP/IGP Architecture
.....................................................24
3.1 BGP and Auto-aggregation.............................24
3.2 Convergence and Recovery Issues......................24
3.3 Non-locality of Effects of Instability and Misconfiguration
..................................................25
3.4 Multihoming Issues...................................25
3.5 AS-number exhaustion.................................26
3.6 Partitioned ASÆs.....................................26
3.7 Load Sharing.........................................27
3.8 Hold down issues.....................................27
3.9 Interaction between Inter domain routing and intra domain
routing...........................................27
3.10 Policy Issues.....................................28
3.11 Security Issues...................................29
3.12 Support of MPLS and VPNS..........................29
3.13 IPv4 / IPv6 Ships in the Night....................29
3.14 Existing Tools to Support Effective Deployment of Inter-
Domain Routing....................................30
4. Expected Users............................................32
4.1 Organisations........................................32
4.2 Human Users..........................................34
5. Mandated Constraints......................................34
5.1 The Federated Environment............................35
5.2 Working with different sorts of network..............35
5.3 Delivering Diversity.................................35
5.4 When will the new solution be required?..............36
6. Assumptions...............................................36
7. Functional Requirements...................................38
7.1 Topology.............................................38
7.2 Distribution.........................................39
7.3 Addressing...........................................40
7.4 Management Requirements..............................42
7.5 Mathematical Provability.............................42
7.6 Traffic Engineering..................................42
7.7 Support for NATs and other Mid-boxes.................43
7.8 Statistics support...................................43
8. Performance Requirements..................................44
9. Backwards compatibility (cutover) and Maintainability.....44
10. Security Requirements....................................44
11. Open Issues..............................................46
11.1 System Modeling...................................46
11.2 Advantages and Disadvantages of having the same protocols
for EGP and IGP...................................46
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11.3 Introduction of new control mechanisms............49
11.4 Robustness........................................49
11.5 VPN Support.......................................50
11.6 End to End Reliability............................50
12. Acknowledgements.........................................50
13. References...............................................51
14. Author's Addresses.......................................54
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1. Introduction
It is generally accepted that there are major shortcomings in the
inter-domain routing of the Internet today and that these may result
in meltdown within an unspecified period of time. Remedying these
shortcomings will require extensive research to tie down the exact
failure modes that lead to these shortcomings and identify the best
techniques to remedy the situation.
Various developments in the nature and quality of the services that
users want from the Internet are difficult to provide within the
current framework as they impose requirements never foreseen by the
original architects of the Internet routing system.
The potential advent of IPv6 and the application of IP mechanisms to
private commercial networks offering specific service guarantees as
an improvement over the best efforts services of the Public Internet
epitomize the kind of radical changes which have to be accommodated:
Major changes to the inter-domain routing system are inevitable if it
is to provide an efficient underpinning for the radically changed and
increasingly, commercially-based networks which rely on the IP
protocol suite.
Although inter-domain routing is seen as the major source of
problems, the interactions with intra-domain routing and the
constraints that confining changes to the inter-domain arena would
impose, means that we should consider the whole area of routing as an
integrated system. This is done for 2 reasons:
- Requirements should not presuppose the solution. A continued
commitment to the current definitions and split between inter-
domain and intra-domain routing would constitute such a
presupposition. Therefore the name Future Domain Routing(FDR) is
being used in this document,
- As an acknowledgement of how intertwined inter-domain and intra-
domain routing are within todayÆs routing architecture.
We are aware that using the term Domain Routing is already fraught
with danger because of possible misinterpretation due to prior usage:
The meaning of Domain Routing will be developed implicitly throughout
the document, but a little advance explicit definition of the word
ædomainÆ is required, as well as some expansion on the scope of
æroutingÆ.
This document uses domain in a very broad sense to mean any
collection of systems or domains which come under a common authority
that determines the attributes defining, and the policies controlling
that collection. The use of domain in this context is very similar to
the concept of Region that was put forth by John Wroclawski in his
Metanet model [10]. The idea includes the notion that within a domain
certain system attributes will characterize the behavior of the
systems in the domain and that there will be borders between domains.
The idea of domain presented does not inherently presuppose that the
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identifying behaviors between two domains are to be the same. Nor
does it presuppose anything about the hierarchical nature of domains.
Finally it does not place restrictions on the nature of the
attributes that might be used to determine membership in a domain.
Since todayÆs routing domains are a subset of all domains as
conceived of in this document, there has been no attempt to create a
new term.
Current practice stresses the need to separate the concerns of the
control plane in a router and the forwarding plane: This document
will follow this practice, but we still use the term æroutingÆ as a
global portmanteau to cover all aspects of the system.
This draft makes a start on the process of improving domain routing
in Section 2 by revisiting the requirements for a future routing
system which were last documented in RFC1126 - ôGoals and Functional
Requirements for Inter-Autonomous System Routingö [4] as a precursor
to the design of BGP in 1989. This section also looks at some other
work that has been carried out since RFC1126 was published in order
to flesh out the historical perspective. Some of the requirements
derive from the problems that are currently being experienced in the
Internet today. These will be discussed in Section 3. The
environment in which the future domain routing system will have to
work is covered in Sections 4 - 6. Specific requirements for a
future Domain routing system are discussed in Sections 7 - 10.
Inevitably this document is incomplete: Some known Open Issues are
discussed in Section 11.
1.1 Background
TodayÆs Internet uses an addressing and routing structure that has
developed in an ad hoc, more or less upwards-compatible fashion. It
has progressed from handling a single domain, non-commercial Internet
to a solution that is just about controlling todayÆs multi-domain,
federated, combination commercial and not-for-profit Internet. The
result is not ideal, particularly as regards inter-domain routing
mechanisms that have to implement a host of domain specific routing
policies for competing, communicating domains, but it does a pretty
fair job at its primary goal of providing any-to-any connectivity to
many millions of computers.
Based on a large body of anecdotal evidence, but also on experimental
evidence [11] and analytic work on the stability of BGP under certain
policy specifications [12], the main Internet inter-domain routing
protocol, BGP4, appears to have a number of problems that need to be
resolved. Additionally, the hierarchical nature of the inter-domain
routing problem appears to be changing as the connectivity between
domains becomes increasingly meshed [13] which alters some of the
scaling and structuring assumptions on which BGP4 is built. Patches
and fix-ups may relieve some of these problems but others may require
a new architecture and new protocols. The starting point of this work
is to step back from the current state, examine how the Internet
might develop in the future and derive a new set of requirements for
a routing architecture from this work.
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The development of the Internet is likely to be driven by a number of
changes that will affect the organization and the usage of the
network, including:
- Ongoing evolution of the commercial relationships between
(connectivity) service providers, leading to changes in the way in
which peering between providers is organised and the way in which
transit traffic is routed
- Requirements for traffic engineering within and between domains
including coping with multiple paths between domains
- Potential addition of a second IP addressing technique through
IPv6.
- Evolution of the end to end principle to deal with the expanded
role of the Internet as discussed in [32]. This paper discusses
the possibility that the range of new requirements, especially the
social and techno-political ones, that are being placed on the
future Internet may compromise the InternetÆs original design
principles. This might cause the Internet to lose some of its key
features, in particular its ability to support new and
unanticipated applications. The discussion is linked to the rise
of new stakeholders in the Internet, especially ISPs; new
government interests; the changing motivations of the ever growing
user base; and the tension between the demand for trustworthy
overall operation and the inability to trust the behaviour of
individual users.
- Incorporation of alternative forwarding techniques such as the
pipes supplied by the Generalised MPLS environment[25].
- Integrating additional constraints into route determination from
interactions with other layers (e.g. Shared Risk Link Groups [31])
- Support for alternative and multiple routing techniques that are
better suited to delivering types of content organised other than
into IP addressed packets.
Philosophically, the Internet has the mission of transferring
information from one place to another. Conceptually, this
information is rarely organised into conveniently sized, IP-addressed
packets and the FDR needs to consider how the information (content)
to be carried is identified, named and addressed. Routing techniques
can then be adapted to handle the expected types of content.
1.2 Goals
This section attempts to answer two questions:
- What are we trying to achieve in a new architecture?
- Why should the Internet community care?
There is a third question which needs to be answered as well, but
which, for the present, is mostly not explicitly discussed:
- How will we know when we have succeeded?
1.2.1 Providing a Routing System matched to Domain Organisation
Many of todayÆs routing problems are caused by a routing system which
is not well-matched to the organization and policies which it is
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trying to support. It is our goal to develop a routing architecture
where even domain organization which is not envisioned today can be
served by a routing architecture that matches its requirements.
We will know when this goal is achieved when the desired policies,
rules and organization can be mapped into the routing system in a
natural, consistent and simply understood way.
1.2.2 Supporting a range of different communication services
TodayÆs routing protocols only support a single data forwarding
service which is typically used to deliver a best efforts service in
the Public Internet. On the other hand, with, for example, DiffServ
it is possible to construct a number of different bit transport
services within the network. Using some of the PDBs which have been
discussed in the IETF, it should be possible to construct services
such as Virtual Wire [18] and Assured Rate [19].
Providers today offer rudimentary promises about how traffic will be
handled in the network, for example delay and long term packet loss
guarantees, and this will probably be even more relevant in the
future. Communicating the service characteristics of paths in routing
protocols will be necessary in the near future, and it will be
necessary to be able to route packets according to their service
requirements.
Thus, a goal of this architecture is to allow for adequate
information about path service characteristics passed between domains
and consequently allow the delivery of bit transport services other
than the best-effort datagram connectivity service which is the
current common denominator.
1.2.3 Scaleable well beyond current predictable needs
Any proposed new FDR system should scale beyond the size and
performance we can foresee for the next ten years. The previous IDR
proposal has, with some massaging, held up for somewhat over ten
years in which time the Internet has grown far beyond the predictions
that were made in the requirements that were placed on it originally.
Unfortunately, we will only know if we have succeeded in this goal if
the improved system survives beyond its design lifetime without
serious massaging: Failure will be much easier to spot!
1.2.4 Supporting alternative forwarding mechanisms
With the advent of circuit based technologies (e.g., MPLS [24] used
for RSVP-TE or CR-LDP for traffic engineering, G-MPLS [25]) managed
by IP routers there are forwarding mechanisms other than the datagram
service that need to be supported by the routing architecture.
An explicit goal of this architecture is to support more forwarding
mechanisms than just hop-by-hop datagram forwarding driven by
globally unique IP addresses.
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1.2.5 Supporting separation of topology map and connectivity service
It is envisioned that an organization can support multiple services
on top of a single network. These services can, for example, be of
different quality, of different type of connectivity, or different
protocols (e.g. IPv4 and IPv6). For all these services there may be
common domain topology, even though the policies controlling the
routing of information might differ from service to service.
Thus, a goal with this architecture is to support separation between
creation of a domain (or organization) topology map and service
creation.
1.2.6 Achieving full/appropriate separation of concerns between
routing and forwarding
The architecture of a router is composed of two main separable parts;
control and forwarding. These components, while inter-dependent,
perform functions that are largely independent of each other.
Control (routing, signaling, and management) is typically done in
software while forwarding typically is done with specialized ASICs or
network processors.
The nature of an IP based network today is that control and data
protocols share the same network and forwarding regime. This may not
always be the case in future networks and we should be careful to
avoid building this sharing in as an assumption in the FDR.
A goal of this architecture is to support full separation of control
and forwarding, and to consider what additional concerns might be
properly considered separately (e.g. adjacency management).
1.2.7 Providing means of using different routing paradigms seamlessly
in different areas of the same network
A number of different routing paradigms have been used or researched
in addition to conventional shortest path hop-by-hop paradigm that is
the current mainstay of the Internet. In particular, differences in
underlying transport networks may mean that other kinds of routing
are more relevant, and the perceived need for traffic engineering
will certainly alter the routing chosen in various domains.
Implicitly, one of these routing paradigms should be the current
routing paradigm, so that the new paradigms will inter-operate in a
backwards compatible way with todayÆs system. This will facilitate a
migration strategy which avoids flag days.
1.2.8 Preventing denial of service and other security attacks
Part of the problem here is that the Internet offers a global,
unmoderated connectivity service. Existence of a route to a
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destination effectively implies that anybody who can get a packet
onto the network is entitled to use that route. Whilst there are
limitations to this generalization, there is a clear invitation to
denial of service attacks. A goal of the FDR system should be to
allow traffic to be specifically linked to whole or partial routes so
that a destination or link resources can be protected from
unauthorized use.
1.2.9 Providing provable convergence with verifiable policy
interaction
It has been shown both analytically by Griffin et al (see [12]) and
practically (see [20]) that BGP will not converge stably or is only
meta-stable (i.e. will not reconverge in the face of a single
failure) when certain types of policy constraint are applied to
categories of network topology. The addition of policy to the basic
distance vector algorithm invalidates the mathematical proofs that
applied to RIP and could be applied to a policy free BGP
implementation.
A goal of the FDR should be to achieve mathematically provable
convergence of the protocols used which may involve constraining the
topologies used, vetting the polices imposed to ensure that they are
compatible across domain boundaries and result in a globally
consistent policy set.
1.2.10 Providing robustness despite errors and failures
From time to time in the history of the Internet there have been
occurrences where people misconfiguring routers have destroyed global
connectivity. This should never be possible.
A goal of the FDR is to be robust to configuration errors and
failures. This should probably involve ensuring that the effects of
misconfiguration and failure can be confined to some suitable
locality of the failure or misconfiguration: This is not currently
the case as both misconfigurations and problems in any AS can, under
the right circumstances, have global effects across the entire
Internet.
1.2.11 Simplicity in management
With the policy work ([26], [27] and [28]) that has been done at IETF
there exists an architecture that standardizes and simplifies
management of QoS. This kind of simplicity is needed in a future
domain routing architecture and its protocols.
A goal of this architecture is to make configuration and management
of inter-domain routing as simple as possible.
2. Historical Perspective
2.1 The legacy of RFC1126
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RFC1126 outlined a set of requirements that were to guide the
development of BGP. While the network is definitely different then it
was in 1989, many of the same requirements remain. As a first step
in setting requirements for the future, we need to understand the
requirements that were originally set for the current protocols. And
in charting a future architecture we must first be sure to do no
harm. Which means a future domain routing has to support as its base
requirement, the level of function that is available today.
The following sections each relate to a requirement, or non
requirement listed in RFC1126. In fact the section names are direct
quotes from the document. The discussion of these requirements
covers the following areas:
Optionally, interpretation for todayÆs audience of the intent of the
requirement
Relevance: Is the requirement of RFC1126 still relevant, and
to what degree? Should it be understood
differently in todayÆs environment?
Current practice: How well is the requirement met by current
protocols and practice?
2.1.1 ææGeneral RequirementsÆÆ
2.1.1.1 ææRoute to DestinationÆÆ
Timely routing to all reachable destinations, including multihoming
and multicast.
Relevance: Valid, but requirements for multihoming need further
discussion and elucidation. The requirement should include
multiple source multicast routing.
Current practice: Multihoming is not efficient and the proposed
inter-domain multicast protocol BGMP is an add-on to BGP
following many of the same strategies but not integrated
into the BGP framework [23].
2.1.1.2 ææRouting is AssuredÆÆ
This requires that a user be notified within a reasonable time period
of attempts, about inability to provide a service.
Relevance: Valid
Current practice: There are ICMP messages for this, but in many cases
they are not used, either because of fears about creating
message storms or uncertainty about whether the end system
can do anything useful with the resulting information.
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2.1.1.3 ææLarge SystemÆÆ
The architecture was designed to accommodate the growth of the
Internet.
Relevance: Valid. Properties of Internet topology might be an issue
for future scalability (topology varies from very sparse
to quite dense at present). Instead of setting growth in a
time-scale, indefinite growth should be accommodated. On
the other hand, such growth has to be accommodated without
making the protocols too expensive - trade-offs may be
necessary.
Current practice: Scalability of the protocols will not be sufficient
under the current rate of growth. There are problems with
BGP convergence for large dense topologies, problems with
routing information propagation between routers in transit
domain, limited support for hierarchy, etc.
2.1.1.4 ææAutonomous OperationÆÆ
Relevance: Valid. There may need to be additional requirements for
adjusting policy decisions to the global functionality and
to avoid contradictory policies would decrease a
possibility of unstable routing behavior.
There should also be a separate requirement for handling
various degrees of trust in autonomous operation, ranging
from no trust (e.g., between separate ISPs) to very high
trust where the domains have a common goal of optimizing
their mutual policies.
Policies for intra domain operations should in some cases
be revealed, using suitable abstractions, to a global
routing mechanism?
Current practice: Policy management is in the control of network
managers, as required, but there is little support for
handling policies at an abstract level for a domain.
Cooperating administrative entities decide about the
extent of cooperation independently. Lack of coordination
combined with global range of effects results in
occasional melt-down of Internet routing.
2.1.1.5 ææDistributed SystemÆÆ
The routing environment is a distributed system. The distributed
routing environment supports redundancy and diversity of nodes and
links. Both data and operations are distributed.
Relevance: Valid. RFC1126 is very clear that we should not be using
centralized solutions, but maybe we need a requirement on
trade-offs between common knowledge and distribution
(e.g., to allow for uniform policy routing) (e.g., GSM
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systems are in a sense centralized (but with hierarchies)
and they work) This requirement should not rule out
certain solutions that are needed to meet other
requirements in this document.
Current practice: Routing is very distributed, but lacking abilities
to consider optimization over several hops or domains.
2.1.1.6 ææProvide A Credible EnvironmentÆÆ
Routing mechanism information must be integral and secure (credible
data, reliable operation). Security from unwanted
modification and influence is required.
Relevance: Valid.
Current practice: BGP provides a mechanism for authentication and
security. There are however security problems with
current practice.
2.1.1.7 ææBe A Managed EntityÆÆ
Requires that a manager should get enough information on a state of
network so that (s)he could make informed decisions.
Relevance: The requirement is reasonable, but we might need to be
more specific on what information should be available,
e.g. to prevent routing oscillations.
Current practice: All policies are determined locally, where they may
appear reasonable but there is limited global coordination
through the routing policy databases operated by the
Internet registries (ARIN, RIPE, APNIC etc). Operators
are not required to register their policies; even when
policies are registered, it is difficult to check that the
actual policies in use match the declared policies and
therefore a manager cannot guarantee to make a globally
consistent decision.
2.1.1.8 ææMinimize Required ResourcesÆÆ
Relevance: Valid, however, the paragraph states that assumptions on
significant upgrades shouldnÆt be made. Although this is
reasonable, a new architecture should perhaps be prepared
to use upgrades when they occur.
Current practice: Most bandwidth is consumed by the exchange of the
NLRI. Usage of CPU depends on the stability of the
Internet. Both phenomena have a local nature, so there are
not scaling problems with bandwidth and CPU usage.
Instability of routing increases the consumption of
resources in any case. The number of networks in the
Internet dominates memory requirements - this is a scaling
problem.
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2.1.2 ææFunctional RequirementsÆÆ
2.1.2.1 ææRoute Synthesis RequirementsÆÆ
2.1.2.1.1 ææRoute around failures dynamicallyÆÆ
Relevance: Valid. Should perhaps be stronger. Only providing a best-
effort attempt may not be enough if real-time services are
to be provided for. Detections may need to be faster than
100ms to avoid being noticed by end-users.
Current practice: latency of fail-over is too high (minutes).
2.1.2.1.2 ææProvide loop free pathsÆÆ
Relevance: Valid. Loops should occur only with negligible probability
and duration.
Current practice: both link-state intra domain routing and BGP inter-
domain routing (if correctly configured) are forwarding-
loop free after having converged. However, convergence
time for BGP can be very long and poorly designed routing
policies may result in a number of BGP speakers engaging
in a cyclic pattern of advertisements and withdrawals
which never converges to a stable result [20].
2.1.2.1.3 ææKnow when a path or destination is unavailableÆÆ
Relevance: Valid to some extent, but there is a trade-off between
aggregation and immediate knowledge of reachability. It
requires that routing tables contain enough information to
determine that the destination is unknown or a path cannot
be constructed to reach it.
Current practice: Knowledge about lost reachability propagates slowly
through the networks due to slow convergence for route
withdrawals.
2.1.2.1.4 ææProvide paths sensitive to administrative policiesÆÆ
Relevance: Valid. Policy control of routing is of increasingly
importance as the Internet has turned into business.
Current practice: Supported to some extent. Policies are only
possible to apply locally in an AS and not globally. At
least there is very small possibilities to affect policies
in other ASÆs. Furthermore, only static policies are
supported; between static policies and `policies dependent
upon volatile events of great celerity` there should exist
events that routing should be aware of. Lastly, there is
no support for policies other than route-properties (such
as AS-origin, AS-path, destination prefix, MED-values
etc).
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2.1.2.1.5 ææProvide paths sensitive to user policiesÆÆ
Relevance: Valid to some extent, as they may conflict with the
policies of the network administrator. It is likely that
this requirement will be met by means of different bit
transport services offered by an operator, but at the cost
of adequate provisioning, authentication and policing when
utilizing the service.
Current practice: not supported in normal routing. Can be
accomplished to some extent with loose source routing,
resulting in inefficient forwarding in the routers. The
various attempts to introduce QoS (Integrated Services and
DiffServ) can also be seen as means to support this
requirement but they have met with limited success.
2.1.2.1.6 ææProvide paths which characterize user quality-of-service
requirementsÆÆ
Relevance: Valid to some extent, as they may conflict with the
policies of the operator. It is likely that this
requirement will be met by means of different bit
transport services offered by an operator, but at the cost
of adequate provisioning, authentication and policing when
utilizing the service. It has become clear that offering
to provide a particular QoS to any arbitrary destination
from a particular source is generally impossible: QoS
except in very æsoftÆ forms such as overall long term
average packet delay, is generally associated with
connection oriented routing.
Current practice: Creating routes with specified QoS is not generally
possible at present.
2.1.2.1.7 ææProvide autonomy between inter- and intra-autonomous system
route synthesisÆÆ
Relevance: Inter and intra domain routing should stay independent,
but one should notice that this to some extent contradicts
the previous three requirements. There is a trade-off
between abstraction and optimality.
Current practice: inter-domain routing is performed independently of
intra-domain routing. Intra-domain routing is, especially
in transit domains, very interrelated to inter-domain
routing.
2.1.2.2 ææForwarding RequirementsÆÆ
2.1.2.2.1 ææDecouple inter- and intra-autonomous system forwarding
decisionsÆÆ
Relevance: Valid.
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Current practice: As explained in 2.1.2.1.7, intra-domain forwarding
in transit domains is codependent with inter-domain
forwarding decisions.
2.1.2.2.2 ææDo not forward datagrams deemed administratively
inappropriateÆÆ
Relevance: Valid, and increasingly important in the context of
enforcing policies correctly expressed through routing
advertisements but flouted by rogue peers which send
traffic for which a route has not been advertised. On the
other hand, packets that have been misrouted due to
transient routing problems perhaps should be forwarded to
reach the destination, although along an unexpected path.
Current practice: at stub domains there is packet filtering, e.g., to
catch source address spoofing on outgoing traffic or to
filter out unwanted incoming traffic. Filtering can in
particular reject traffic (such as unauthorized transit
traffic) that has been sent to a domain even when it has
not advertised a route for such traffic on a given
interface. The growing class of æmid boxesÆ (e.g. NATs)
is quite likely to apply administrative rules that will
prevent forwarding of packets. Note that security
policies may deliberately hide administrative denials. In
the backbone, intentional packet dropping based on
policies is not common.
2.1.2.2.3 ææDo not forward datagrams to failed resourcesÆÆ
Relevance: Unclear, although it is clearly desirable to minimise
waste of forwarding resources by discarding datagrams
which cannot be delivered at the earliest opportunity.
There is a trade-off between scalability and keeping track
of unreachable resources. Equipment closest to a failed
node has the highest motivation to keep track of failures
so that waste can be minimised.
Current practice: routing protocols use both internal adjacency
management sub-protocols (e.g. Hello protocols) and
information from equipment and lower layer link watchdogs
to keep track of failures in routers and connecting links.
Failures will eventually result in the routing protocol
reconfiguring the routing to avoid (if possible) a failed
resource, but this is generally very slow (30s or more).
In the meantime datagrams may well be forwarded to failed
resources. In general terms, end hosts and some non-
router midboxes do not participate in these notifications
and failures of such boxes will not affect the routing
system.
2.1.2.2.4 ææForward datagram according to its characteristicsÆÆ
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Relevance: Valid. Is necessary in enabling differentiation in the
network, based on QoS, precedence, policy or security.
Current practice: ingress and egress filtering can be done on policy.
2.1.2.3 ææInformation RequirementsÆÆ
2.1.2.3.1 ææProvide a distributed and descriptive information baseÆÆ
Relevance: Valid, however hierarchical IBs might provide more
possibilities.
Current practice: IBs are distributed, not sure whether they support
all provided routing functionality.
2.1.2.3.2 ææDetermine resource availabilityÆÆ
Relevance: Valid. It should be possible for resource availability
and levels of resource availability to be determined.
This prevents needing to discover unavailability through
failure. Resource location and discovery is arguably a
separate concern which could be addressed outside the core
routing requirements.
Current practice: Resource availability is predominantly handled
outside of the routing system.
2.1.2.3.3 ææRestrain transmission utilizationÆÆ
Relevance: Valid. However certain requirements in the control plane,
such as fast detection of faults may be worth consumption
of more resources. Similarly, simplicity of
implementation may make it cheaper to æback haulÆ traffic
to central locations to minimise the cost of routing if
bandwidth is cheaper than processing.
Current practice: BGP messages probably do not ordinarily consume
excessive resources, but might during erroneous
conditions. In the data plane, the near universal
adoption of shortest path protocols could be considered to
result in minimization of transmission utilization.
2.1.2.3.4 ææAllow limited information exchangeÆÆ
Relevance: Valid. But perhaps routing could be improved if certain
information could be globally available.
Current practice: Policies are used to determine which reachability
information that is exported.
2.1.2.4 ææEnvironmental RequirementsÆÆ
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2.1.2.4.1 ææSupport a packet-switching environmentö
Relevance: Valid but should not be exclusive.
Current practice: supported.
2.1.2.4.2 ææAccommodate a connection-less oriented user transport
serviceÆÆ
Relevance: Valid, but should not be exclusive.
Current practice: accommodated.
2.1.2.4.3 ææAccommodate 10K autonomous systems and 100K networksÆÆ
Relevance: No longer valid. Needs to be increased potentially
indefinitely. It is extremely difficult to foresee the
future size expansion of the Internet so that the utopian
solution would be to achieve an Internet whose
architecture is scale invariant. Regrettably, this may
not be achievable without introducing undesirable
complexity and a suitable trade off between complexity and
scalability is likely to be necessary.
Current Practice: Yes but perhaps reaching the limit.
2.1.2.4.4 ææAllow for arbitrary interconnection of autonomous systemsÆÆ
Relevance: Valid. However perhaps not all interconnections should be
accessible globally.
Current practice: BGP-4 allows for arbitrary interconnections.
2.1.2.5 ææGeneral ObjectivesÆÆ
2.1.2.5.1 ææProvide routing services in a timely mannerÆÆ
Relevance: Valid, as stated before. The more complex a service is the
longer it should be allowed to take, but the
implementation of services requiring (say) NP-complete
calculation should be avoided.
Current practice: More or less, with the exception of convergence and
fault robustness.
2.1.2.5.2 ææMinimize constraints on systems with limited resourcesÆÆ
Relevance: Valid
Current practice: Systems with limited resources are typically stub
domains that advertise very little information.
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2.1.2.5.3 ææMinimize impact of dissimilarities between autonomous
systemsÆÆ
Relevance: Important. This requirement is critical to a future
architecture. In a domain routing environment where the
internal properties of domains may differ radically, it
will be important to be sure that these dissimilarities
are minimized at the borders.
Current: practice: for the most part this capability isnÆt required
in todayÆs networks since the intra-domain attribute are
nearly identical to start with.
2.1.2.5.4 ææAccommodate the addressing schemes and protocol mechanisms
of the autonomous systemsÆÆ
Relevance: Important, probably more so than when RFC1126 was
originally developed because of the potential deployment
of IPv6, wider usage of MPLS and the increasing usage of
VPNs.
Current practice: Largely only one global addressing scheme is
supported in most autonomous systems.
2.1.2.5.5 ææMust be implementable by network vendorsÆƳ
Requirement: Valid, but note that what can be implemented today is
different from what was possible when RFC1126 was written:
FDR should not be unreasonably constrained by past
limitations.
Current practice: BGP was implemented.
2.1.3 ææNon-Goalsö
RFC1126 also included a section discussing non goals. To what extent
are these still non goals? Does the fact that they were non-goals
adversely affect todayÆs IDR system?
2.1.3.1 ææUbiquityÆÆ
In a sense this ænon-goalÆ has effectively been achieved by the
Internet and IP protocols. This requirement reflects a different
world view where there was serious competition for network protocols
which is really no longer the case. Ubiquitous deployment of inter-
domain routing in particular has been achieved and must not be undone
by any proposed FDR. On the other hand:
- ubiquitous connectivity cannot be reached in a policy sensitive
environment and should not be an aim,
- it must not be required that the same routing mechanisms are
used throughout provided that they can interoperate
appropriately
- the information needed to control routing in a part of the
network should not necessarily be ubiquitously available and it
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must be possible for an operator to hide commercially sensitive
information that is not needed outside a domain.
Relevance: De facto essential for a FDR but what is required is
ubiquity of the routing system rather than ubiquity of
connectivity.
Current practice: de facto ubiquity achieved.
2.1.3.2 ææCongestion controlÆÆ
Relevance: It is not clear if this non-goal was to be applied to
routing or forwarding. It is definitely a non-goal to
adapt the choice of route at transient congestion.
However, to add support for congestion avoidance (e.g.,
ECN and ICMP messages) in the forwarding process would be
a useful addition. There is also extensive work going on
in traffic engineering which should result in congestion
avoidance through routing as well as in forwarding.
Current practice: Some ICMP messages (e.g. source quench) exist to
deal with congestion control but these are not generally
used as they either make the problem worse or there is no
mechanism to reflect the message into the application
which is providing the source.
2.1.3.3 ææLoad splittingÆÆ
Relevance: This should neither be a non-goal, nor an explicit goal.
It might be desirable in some cases.
Current practice: Can be implemented by exporting different prefixes
on different links, but this requires manual configuration
and does not consider actual load.
2.1.3.4 ææMaximizing the utilization of resourcesÆÆ
Relevance: Valid. Cost-efficiency should be strived for, maximizing
resource utilization does not always lead to greatest
cost-efficiency.
Current practice: To the extent possible.
2.1.3.5 ææSchedule to deadline serviceÆÆ
This non-goal was put in place to ensure that the IDR did not have to
meet real time deadline goals such as might apply to CBR services in
ATM.
Relevance: The hard form of deadline services is still a non-goal for
the FDR but overall delay bounds are much more of the
essence than was the case when RFC1126 was written.
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INTERNET DRAFT FDR Requirements 9 July, 2001
Current Practice: Service providers are now offering overall
probabilistic delay bounds on traffic contracts. To
implement these contracts there is a requirement for a
rather looser form of delay sensitive routing.
2.1.3.6 ææNon-interference policies of resource utilizationö
The requirement in RFC1126 is somewhat opaque, but appears to imply
that what we would today call QoS routing is a non-goal and that
routing would not seek to control the elastic characteristics of
Internet traffic whereby a TCP connection can seek to utilize all the
spare bandwidth on a route, possibly to the detriment of other
connections sharing the route or crossing it.
Relevance: Open Issue. It is not clear whether dynamic QoS routing
can or should be implemented. Such a system would seek to
control the admission and routing of traffic depending on
current or recent resource utilization. This would be
particularly problematic where traffic crosses an
ownership boundary because of the need for potentially
commercially sensitive information to be made available
outside the ownership boundary.
Current practice: Routing does not consider dynamic resource
availability. Forwarding can support service
differentiation.
2.2 Nimrod Requirements
Nimrod as expressed by Noel Chiappa in his early document, ææA New IP
Routing and Addressing ArchitectureÆÆ (1991) and later in the NIMROD
Working Group documents RFC 1753 and RFC1992 established a number of
requirements that need to be considered by any new routing
architecture. The Nimrod requirements took RFC1126 as a starting
point and went further.
The goals of Nimrod, quoted from RFC1992, were as follows:
1. To support a dynamic internetwork of *arbitrary size* (our
emphasis) by providing mechanisms to control the amount of
routing information that must be known throughout an
internetwork.
2. To provide service-specific routing in the presence of multiple
constraints imposed by service providers and users.
3. To admit incremental deployment throughout an internetwork.
It is certain that these goals remain as requirements for any new
domain routing architecture.
- As discussed in other sections of this document the amount of
information needed to maintain the routing system is growing at
a rate that does not scale. And yet, as the services and
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constraints upon those services grow there is a need for more
information to be maintained by the routing system.
One of the key terms in the first requirements is æcontrolÆ.
While increasing amounts of information need to be known and
maintained in the Internet, the amounts and kinds of information
that are distributed can be controlled.
This goal will be reflected in the requirements for the future
domain architecture.
- If anything, the demand for specific services in the Internet
has grown since 1996 when the Nimrod architecture was published.
Additionally the kinds of constraints that service providers
need to impose upon their networks and that services need to
impose upon the routing have also increased. Any changes made
to the network in the last half-decade have not significantly
improved this situation.
- The ability to incrementally deploy any new routing architecture
within the Internet is still a absolute necessity. It is
impossible to imagine that a new routing architecture could
supplant the current architecture on a flag day
At one point in time Nimrod, with its addressing and routing
architectures was seen as a candidate for IPng. History shows that
it was not accepted as the IPng, having been ruled out of the
selection process by the IESG in 1994 on the grounds that it was ætoo
much of a research effortÆ [35], although input for the requirements
of IPng was explicitly solicited from Chiappa [8]. IÆd still like to
know more about what those reasons wereà
Instead IPv6 has been put forth as the IPng. Without entering a
discussion of the relative merits of IPv6 versus Nimrod, it is
apparent that IPv6, while it may solve many problems, does not solve
the critical routing problems in the Internet today. In fact in some
sense it exacerbates them by adding a requirements for support of two
internet protocols and their respective addressing methods. In many
ways the addition of IPv6 to the mix of methods in todayÆs Internet
only points to the fact that the goals, as set forth by the Nimrod
team, remain as necessary goals.
There is another sense in which study of Nimrod and its architecture
may be important to deriving a FDR. Nimrod can be said to have two
derivatives:
- MPLS in that it took the notion of forwarding along well known
paths
- PNNI in that it took the notion of abstracting topological
information and using that information to create connections for
traffic.
It is important to note, that whilst MPLS and PNNI borrowed ideas
from Nimrod, neither of them can be said to be an implementation of
this architecture.
2.3 PNNI
PNNI was developed under the ATM ForumÆs auspices as a hierarchical
route determination protocol for ATM, a connection oriented
architecture. It is reputed to have developed several of it methods
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from a study of the Nimrod architecture. What can be gained from an
analysis of what did and did not succeed in PNNI?
The PNNI protocol includes the assumption that all peer groups are
willing to cooperate, and that the entire network is under the same
top administration. Are there limitations that stem from this æworld
nodeÆ presupposition? As we know [13], the Internet is no longer a
clean hierarchy and there is a lot of resistance to having any sort
of æultimate authorityÆ controlling or even brokering communication.
PNNI is the first deployed example of a routing protocol that uses
abstract map exchange (as opposed to distance vector or link state
mechanisms) for inter-domain routing information exchange. One
consequence of this is that domains need not all use the same
mechanism for map creation. What were the results of this
abstraction and source based route calculation mechanism?
Since the authors of this document do not have experience running a
PNNI network, the comments above are from a theoretical perspective.
Information on these issues, and any other relevant issues, is
solicited from those who do have such operational experience.
2.4 Recent Research Work
2.4.1 Developments in Internet Connectivity
The recent work commissioned from Geoff Huston by the Internet
Architecture Board [13] draws a number of conclusions from analysis
of BGP routing tables and routing registry databases:
- The connectivity between provider ASs is becoming more like a
dense mesh than the tree structure which was commonly assumed to
be commonplace a couple of years ago. This has been driven by the
increasing amounts charged for peering and transit traffic by
global service providers. Local direct peering and internet
exchanges are becoming steadily more common as the cost of local
fibre connections drops.
- End user sites are increasingly resorting to multi-homing onto two
or more service providers as a way of improving resiliency. This
has a knock-on effect of spectacularly fast depletion of the
available pool of AS numbers as end user sites require public AS
numbers to become multi-homed and corresponding increase in the
number of prefixes advertised in BGP.
- Multi-homed sites are using advertisement of longer prefixes in
BGP as a means of traffic engineering to load spread across their
multiple external connections with further impact on the size of
the BGP tables.
- Operational practices are not uniform, and in some cases lack of
knowledge or training is leading to instability and/or excessive
advertisement of routes by incorrectly configured BGP speakers.
- All these factors are quickly negating the advantages in limiting
the expansion of BGP routing tables that were gained by the
introduction of CIDR and consequent prefix aggregation in BGP. It
is also now impossible for IPv6 to realize the world view in which
the default free zone would be limited to perhaps 10,000 prefixes.
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- The typical æwidthÆ of the Internet in AS hops is now around five,
and much less in many cases.
These conclusions have a considerable impact on the requirements for
the FDR:
- Topological hierarchy (e.g. mandating a tree structured
connectivity) cannot be relied upon to deliver scalability of a
large Internet routing system
- Aggregation cannot be relied upon to constrain the size of routing
tables for an all-informed routing system
2.4.2 Defending the End To End Principle
DARPA is funding a project to think about a new architecture for
future generation Internet, called imaginatively NewArch
(http://www.isi.edu/newarch/). Work started in the first half of
2000 but the published results are limited to an introductory paper
and some slides.
The main development so far is to conclude that as the Internet
becomes mainstream infrastructure, fewer and fewer of the
requirements are truly global but may apply with different force or
not at all in certain parts of the network. This (it is claimed)
makes the compilation of a single, ordered list of requirements
deeply problematic. Instead we may have to produce multiple
requirement sets with support for differing requirement importance at
different times and in different places. This æmeta-requirementÆ
significantly impacts architectural design.
Potential new technical requirements identified so far include:
- Commercial environment concerns such as richer inter-provider
policy controls and support for a variety of payment models
- Trustworthiness
- Ubiquitous mobility
- Policy driven self-organisation (ædeep auto configurationÆ)
- Extreme short-time-scale resource variability
- Capacity allocation mechanisms
- Speed, propagation delay and Delay/BandWidth Product issues
Non-technical or political ærequirementsÆ include:
- Legal and Policy drivers such as
o Privacy and free/anonymous speech
o Intellectual property concerns
o Encryption export controls
o Law enforcement surveillance regulations
o Charging and taxation issues
- Reconciling national variations and consistent operation in a
world wide infrastructure
One of the participants in this work (Dave Clark) with one of his
associates has also just published a very interesting paper analyzing
the impact of some of these new requirements on the end to end
principle that has guided the development of the Internet to date
[32]. Their primary conclusion is that the loss of trust between the
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users at the ends of end to end has the most fundamental effect on
the Internet. This is clear in the context of the routing system,
where operators are unwilling to reveal the inner workings of their
networks for commercial reasons. Similarly, trusted third parties
and their avatars (mainly mid-boxes of one sort or another) have a
major impact on the end to end principles and the routing mechanisms
that went with them. Overall, the end to end principles should be
defended so far as is possible - some changes are already too deeply
embedded to make it possible to go back to full trust and openness -
at least partly as a means of staving off the day when the network
will ossify into an unchangeable form and function (much as the
telephone network has done). The hope is that by that time a new
Internet will appear to offer a context for unfettered innovation.
3. Existing problems of BGP and the current EGP/IGP Architecture
Although most of the people who have to work with BGP today believe
it to be a useful, working protocol, discussions have brought to
light a number of areas where BGP or the relationship between BGP and
the IGPs in use today could be improved. This section is, to a large
extent, a wish list for the FDR based on those areas where BGP is
seen to be lacking, rather than simply a list of problems with BGP.
The shortcomings of todayÆs inter-domain routing system have also
been extensively surveyed in æArchitectural Requirements for Inter-
Domain Routing in the InternetÆ [13], particularly with respect to
its stability and the problems produced by explosions in the size of
the Internet.
3.1 BGP and Auto-aggregation
The stability and later linear growth rates of the number of routing
objects (prefixes) that was achieved by the introduction of CIDR
around 1994, has now been once again been replaced by near-
exponential growth of number of routing objects. The granularity of
many of the objects advertised in the DFZ is very small (prefix
length of 22 or longer): This granularity appears to be a by-product
of attempts to perform precision traffic engineering related to
increasing levels of multi-homing. At present there is no mechanism
in BGP that would allow an AS to aggregate such prefixes without
advance knowledge of their existence, even if it was possible to
deduce automatically that they could be aggregated. Achieving
satisfactory auto-aggregation would also significantly reduce the
non-locality problems associated with instability in peripheral ASs.
On the other hand, it may be that alterations to the connectivity of
the net as described in [13] and Section 2.4.1 may limit the
usefulness of auto-aggregation
3.2 Convergence and Recovery Issues
BGP today is a stable protocol under most circumstances but this has
been achieved at the expense of making the convergence time of the
inter-domain routing system very slow under some conditions. This
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has a detrimental effect on the recovery of the network from
failures.
The timers that control the behavior of BGP are typically set to
values in the region of several tens of seconds to a few minutes,
which constrains the responsiveness of BGP to failure conditions.
In the early days of deployment of BGP, poor network stability and
router software problems lead to storms of withdrawals closely
followed by re-advertisements of many prefices. To control the load
on routing software imposed by these æroute flapsÆ, route flap
damping was introduced into BGP. Most operators have now implemented
a degree of route flap damping in their deployments of BGP. This
restricts the number of times that the routing tables will be rebuilt
even if a route is going up and down very frequently. Unfortunately,
the effect of route flap damping is exponential in its behavior which
can result in some parts of the Internet being inaccessible for hours
at a time.
There is evidence ([13] and our own measurements) that in todayÆs
network route flap is disproportionately associated with the fine
grain prefices (length 22 or longer) associated with traffic
engineering at the periphery of the network. Auto-aggregation as
previously discussed would tend to mask such instability and prevent
it being propagated across the whole network.
3.3 Non-locality of Effects of Instability and Misconfiguration
There have been a number of instances, some of which are well-
documented (e.g. The April 1997 incident when misconfiguration of BGP
at a small company in Virginia, USA, turned the company into a
traffic magnet for much of the traffic in the Internet resulting in
global problems until it was fixed) of a mistake in BGP configuration
in a single peripheral AS propagating across the whole Internet and
resulting in misrouting of most of the traffic in the Internet.
Similarly, route flap in a single peripheral AS can require route
table recalculation across the entire Internet.
This non-locality of effects is highly undesirable, and it would be a
considerable improvement if such effects were naturally limited to a
small area of the network around the problem.
3.4 Multihoming Issues
As discussed previously, the increasing use of multi-homing as a
robustness technique by peripheral ASs requires that multiple routes
have to be advertised for such domains. These routes must not be
aggregated close in to the multi-homed domain as this would defeat
the traffic engineering implied by multi-homing and currently cannot
be aggregated further away from the multi-homed domain due to the
lack of auto-aggregation capabilities. Consequentially the DFZ
routing table is growing exponentially again.
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The longest prefix match routing technique introduced by CIDR, and
implemented in BGP4, when combined with provider address allocation
is an obstacle to effective multi-homing if load sharing across the
multiple links is required: If an AS has been allocated its
addresses from an upstream provider, the upstream provider can
aggregate those addresses with those of other customers and need only
advertise a single prefix for a range of customers. But, if the
customer AS is also connected to another provider, the second
provider is not able to aggregate the customer addresses because they
are not taken from his allocation, and will therefore have to
announce a more specific route to the customer AS. The longest match
rule will then direct all traffic through the second provider, which
is not as required.
Example:
AS3 has received its addresses from AS1, which means AS1 can
Aggregate. But if AS3 want its traffic to be seen equally
both ways, AS1 is forced to announce both the aggregate and
the more specific route to AS3.
\ /
AS1 AS2
\ /
AS3
This problem has induced many ASs to apply for their own address
allocation even though they could have been allocated from an
upstream provider further exacerbating the DFZ route table size
explosion. This problem also interferes with the desire of many
providers in the DFZ to route only prefixes that are equal to or
shorter than 20 or 19 bits.
Note that some problems which are referred to as multihoming issues
are not and should not solvable through the routing system (e.g.
where a TCP load distributor is needed), and multihoming is not a
panacea for the general problem of robustness in a routing
system [38].
3.5 AS-number exhaustion
The domain identifier or AS-number is a 16-bit number. Allocation of
AS-numbers is currently increasing 51% p.a. [13] with the result that
exhaustion is likely around 2005. The IETF is currently studying
proposals to increase the available range of AS-numbers to 32 bits,
but this will present a deployment challenge during transition.
3.6 Partitioned ASÆs
Tricks with discontinuous ASs are used by operators, for example, to
implement anycast. Discontinuous ASs may also come into being by
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chance if a multi-homed domain becomes partitioned as a result of a
fault and part of the domain can access the Internet through each
connection. It may be desirable to make BGPÆs support for this kind
of situation more transparent than at present.
3.7 Load Sharing
Load splitting or sharing was not a goal of the original designers of
BGP and it is now a problem for todayÆs network designers and
managers. Trying to fool BGP into load sharing between several links
is a constantly recurring exercise for most operators today. Traffic
engineering extensions to the FDR which will facilitate load sharing
are essential.
3.8 Hold down issues
As with the interval between æhelloÆ messages in OSPF, the typical
size and defined granularity (seconds to tens of seconds) of the
ækeep-aliveÆ time negotiated at start-up for each BGP connection
constrains the responsiveness of BGP to link failures.
The recommended values and the available lower limit for this timer
were set to limit the overhead caused by keep-alive messages when
link bandwidths were typically much lower than today. Analysis and
experiment ([14], [15] & [33]) indicate that faster links could
sustain a much higher rate of keep-alive messages without
significantly impacting normal data traffic. This would improve
BGPÆs responsiveness to link and node failures but with a
corresponding increase in the risk of instability, if the error
characteristics of the link are not taken properly into account when
setting the keep-alive interval.
An additional problem with the hold-down mechanism in BGP is the
amount of information that has to be exchanged to re-establish the
database of route advertisements on each side of the link when it is
re-established after a failure. Currently any failure, however brief
forces a full exchange which could perhaps be constrained by
retaining some state across limited time failures and using revision
control, transaction and replication techniques to re-synchonise the
databases. Various techniques have been implemented to try to reduce
this problem but they have not yet been standardised.
3.9 Interaction between Inter domain routing and intra domain routing
Today, many operatorsÆ backbone routers run both I-BGP and an IGP
maintain the routes that reach between the borders of the domain.
Exporting routes from BGP into IGP and bringing them back up to BGP
is not recommended [29], but it is still necessary for all backbone
routers to run both protocols. BGP is used to find the egress point
and IGP to find the path (next hop router) to the egress point across
the domain. This is not only a management problem but may also create
other problems:
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INTERNET DRAFT FDR Requirements 9 July, 2001
- BGP is a distance vector protocol, as compared with most IGPs
which are link state protocols, and as such it is not optimised
for convergence speed although they generally require less
processing power. Incidentally, more efficient distance vector
algorithms are available such as [34].
- The metrics used in BGP and the IGP are rarely comparable or
combinable. Whilst there are arguments that the optimizations
inside a domain may be different from those for end-to-end paths,
there are occasions, such as calculating the ætopologically
nearestÆ server when computable or combinable metrics would be of
assistance.
- The policies that can be implemented using BGP are designed for
control of traffic exchange between operators, not for controlling
paths within a domain. Policies for BGP are most conveniently
expressed in RPSL and this could be extended if thought desirable
to include IGP policies.
- If the NEXT HOP destination for a set of BGP routes becomes
inaccessible because of IGP problems, the routes using the
vanished next hop have to be invalidated at the next available
UPDATE. Subsequently, if the next hop route reappears, this would
normally lead to the BGP speaker requesting a full table from its
neighbour(s). Current implementations may attempt to circumvent
the effects of IGP route flap by caching the invalid routes for a
period in case the next hop is restored.
- Synchronization between IGP and EGP is a problem as long as we use
different protocols for IGP and EGP, which will most probably be
the case even in the future because of the differing requirements
in the two situations. Some sort of synchronization between those
two protocols would be useful. The draft æOSPF Transient Blackhole
AvoidanceÆ [22], the IGP side of the story is covered.
- Synchronizing in BGP means waiting for the IGP to know about the
same networks as the EGP, which can take a significant period of
time and slows down the convergence of BGP by adding the IGP
convergence time into each cycle.
3.10 Policy Issues
There are several classes of issue with current BGP policy:
- Policy is installed in an ad-hoc manner in each autonomous
system. There isnÆt a method for ensuring that the policy
installed in one router is coherent with policies installed in
other routers.
- As described in Griffin [12] and in McPherson [20] it is
possible to create policies for ASs, and instantiate them in
routers, that will cause BGP to fail to converge in certain
types of topology
- There is no available network model for describing policy in a
coherent manner.
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Policy management is extremely complex and mostly done without the
aid of any automated procedures. The extreme complexity means that a
highly qualified specialist is required for policy management of
border routers. The training of these specialists is quite lengthy
and needs to involve long periods of hands-on experience. There is,
therefore, a shortage of qualified staff for installing and
maintaining the routing policies. Also many training courses cover
only the basic configuration aspects and do not cover policy issues.
3.11 Security Issues
While many of the issues with BPG security have been traced either to
implementation issues or to operational issues, BGP is vulnerable to
DDOS attacks. Additionally routers can be used as unwitting
forwarders in DDOS attacks on other systems.
Though DDOS attacks can be fought in a variety of ways, most
filtering methods, it is takes constant vigilance. There is nothing
in the current architecture or in the protocols that serves to
protect the forwarders from these attacks.
3.12 Support of MPLS and VPNS
Recently BGP has been modified to function as a signalling protocol
for MPLS and for VPNs [16]. Some people see this over-loading of
the BGP protocol as a boon whilst others see it as a problem. While
it was certainly convenient as a vehicle for vendors to deliver extra
functionality for to their products, it has exacerbated some of the
performance and complexity issues of BGP. Two important problems are,
the additional state that must be retained and refreshed to support
VPN tunnels and that BGP does not provide end-to-end notification
making it difficult to confirm that all necessary state has been
installed or updated.
In creating the future domain routing architecture, serious
consideration must be given to the argument that VPN signaling
protocols should remain separate from the route determination
protocols.
3.13 IPv4 / IPv6 Ships in the Night
The fact that service providers would need to maintain two completely
separate networks; one for IPv4 and one for IPv6 has been a real
hindrance to the introduction of IPv6. Even if IPv6 does get
deployed it will do so without causing the disappearance of IPv4.
This means that unless something is done, service providers would
need to maintain the two networks in perpetuity.
It is possible to use a single set of BGP speakers with multiprotocol
extensions [37] to exchange information about both IPv4 and IPv6
routes between domains, but the use of TCP as the transport protocol
for the information exchange results in an asymmetry when choosing to
use one of TCP over IPv4 or TCP over IPv6. Successful information
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exchange confirms one of IPv4 or IPv6 reachability between the
speakers but not the other, making it possible that reachability is
being advertised for a protocol for which it is not present.
Also, current implementations do not allow a route to be advertised
for both IPv4 and IPv6 in the same UPDATE message, because it is not
possible to explicitly link the reachability information for an
address family to the corresponding next hop information. This could
be improved, but currently results in independent UPDATEs being
exchanged for each address family.
The tools available to network operators
3.14 Existing Tools to Support Effective Deployment of Inter-Domain
Routing
The tools available to network operators to assist in configuring and
maintaining effective inter-domain routing in line with their defined
policies are limited, and almost entirely passive.
For example:
- there are no tools to facilitate the planning of the routing of a
domain (either intra- or inter-domain); there are a limited
number of display tools which will visualize the routing once it
has been configured
- there are no tools to assist in converting business policy
specifications into the RPSL language; there are limited tools to
convert the RPSL into BGP commands and to check, post-facto, that
the proposed policies are consistent with the policies in adjacent
domains (always provided that these have been revealed and
accurately documented).
- there are no tools to monitor BGP route changes in real time and
warn the operator about policy inconsistencies and/or
instabilities.
The following section summarises the tools that are available to
assist with the use of RPSL. Note they are all batch mode tools used
off-line from a real network. These tools will provide checks for
skilled inter-domain routing configurers but limited assistance for
the novice.
3.14.1 Routing Policy Specification Language RPSL (RFC 2622, 2650) and
RIPE NCC Database (RIPE 157)
Routing Policy Specification Language RPSL enables a network operator
to describe routes, routers and autonomous systems ASs that are
connected to the local AS.
Using the RPSL language a distributed database is created to describe
routing policies in the Internet as described by each AS
independently. The database can be used to check the consistency of
routing policies stored in the database.
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Tools exist (RIPE 81, 181, 103) that can be applied on the database
to answer requests of the form, e.g.
- flag when two neighboring network operators specify conflicting or
inconsistent routing information exchanges with each other and
also detect global inconsistencies where possible;
- extract all AS-paths between two networks which are allowed by
routing policy from the routing policy database; display the
connectivity a given network has according to current policies.
The database queries enable a partial static solution to the
convergence problem. They analyze routing policies of very limited
part of Internet and verify that they do not contain conflicts that
could lead to protocol divergence. The static analysis of convergence
of the entire system has exponential time complexity, so
approximation algorithms would have to be used.
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4. Expected Users
This section considers the requirements imposed by the target
audience of the FDR both in terms of organizations that might own
networks, which would use FDR, and the human users who will have to
interact with the FDR.
4.1 Organisations
The organizations that own networks connected to the Internet have
become much more diverse since RFC1126 [4] was published. In
particular a major part of the network is now owned by commercial
service provider organizations in the business of making profits from
carrying data traffic.
4.1.1 Commercial Service Providers
The routing system must take into account their desires for
commercial secrecy and security, as well as allowing them to organize
their business as flexibly as possible.
Service providers will often wish to conceal the details of the
network from other connected networks. So far as is possible, the
routing system should not require the service providers to expose
more details of the topology and capability of their networks than is
strictly necessary.
Many service providers will also offer contracts to their customers
in the form of Service Level Agreements (SLAs) and the routing system
must allow the providers to support these SLAs through traffic
engineering and load balancing as well as multihoming allowing them
to achieve the degree of resilience and robustness that they need.
Service providers can be categorized as
- Global Service Providers (GSPs) with networks which have a
global reach. Such providers may and usually will wish to
constrain traffic between their customers to run entirely on
their networks. Such providers will interchange traffic at
multiple peering points with other GSPs and need extensive
policy-based controls to control the interchange of traffic.
Peering may be through the use of dedicated private lines
between the partners or increasingly through Internet Exchange
Points.
- National Service Providers (NSPs)which are similar to GSPs but
typically cover one country. Such organizations may operate as
a federation which provides similar reach to a GSP and may wish
to be able to steer traffic preferentially to other federation
members to achieve global reach.
- Local Internet Service Providers (ISPs) operate regionally and
will typically purchase transit capacity from NSPs or GSPs to
provide global connectivity, but may also peer with neighbouring
ISPs.
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The routing system should be sufficiently flexible to accommodate the
continually changing business relationships of the providers, and the
various levels of trustworthiness that they apply to customers and
partners.
Service providers will need to be involved in accounting for Internet
usage, monitoring the traffic, and may be involved in government
action to tax the usage of the Internet, enforce social mores and
intellectual property rules or apply surveillance to the traffic to
detect or prevent crime.
4.1.2 Enterprises
The leaves of the network domain graph are in many cases networks
supporting a single enterprise. Such networks cover an enormous
range of complexity with some multi-national companies owning
networks that rival the complexity and reach of a GSP whereas many
fall into the Small Office-Home Office (SOHO) category. The routing
system should allow simple and robust configuration and operation for
the SOHO category, whilst effectively supporting the larger
enterprise.
Enterprises are particularly likely to lack the capability to
configure and manage a complex routing system and every effort should
be made to provide simple configuration and operation for such
networks.
Enterprises will also wish to be able to change their service
provider with ease. Whilst this is predominantly a naming and
addressing issue, the routing system must be able to support seamless
changeover, for example, by coping with a changeover period when both
sets of addresses are in use.
Enterprises will wish to be able to multihome to one or more
providers as one possible means of enhancing the resilience of their
network.
Enterprises will also frequently wish to control the trust that they
place both in workers and external connections through firewalls and
similar mid-boxes placed at their external connections.
4.1.3 Domestic Networks
Increasingly domestic networks are likely to be æalways onÆ and will
resemble SOHO enterprises networks with no special requirements of
the routing system.
In the meantime, the routing system must support dial-up users.
4.1.4 Internet Exchange Points
Peering of service providers, academic networks and larger
enterprises is increasingly happening at specific Internet Exchange
Points where many networks are linked together in a relatively small
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physical area. The resources of the exchange may be owned by a
trusted third party or jointly by the connecting networks. The
routing systems should support such exchange points without requiring
the exchange point to either operate as a superior entity with every
connected network logically inferior to it or requiring the exchange
point to be a member of one (or all) connected networks. The
connecting networks have to delegate a certain amount of trust to the
exchange point operator.
4.1.5 Content Providers
Content providers are at one level a special class of enterprise, but
the desire to deliver content efficiently means that a content
provider may provide multiple replicated origin servers or caches
across a network. These may also be provided by a separate content
delivery service. The routing system should facilitate delivering
content from the most efficient location.
4.2 Human Users
This section covers the most important human users of the FDR and
their expected interactions with the system.
4.2.1 Network Planners
The routing system should allow them to plan and implement a network
that can be proved to be stable and will meet their traffic
engineering requirements.
4.2.2 Network Operators
The routing system should, so far as is possible, be simple to
configure and operate, behave in a predictable, stable fashion and
deliver appropriate statistics and events to allow the network to be
managed and upgraded in an efficient and timely fashion.
4.2.3 Mobile End Users
The routing system must support mobile end users. The NewArch team
(see Section 2.4.2) considers that mobility will become æubiquitousÆ
5. Mandated Constraints
While many of the requirement to which the protocol must respond are
technical, some arenÆt. These mandated constraints are those that
are determined by conditions of the world around us. Understanding
these requirements requires and analysis of the world in which these
systems will be deployed,. The constraints include those that are
determined by:
- Environmental factors.
- Geography
- Political boundaries and considerations
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- Technological factors such as the prevalence of different
levels of technology in the developed world as opposed to in
the developing or undeveloped world.
5.1 The Federated Environment
The graph of the Internet network with routers and other control
boxes at the nodes and communication links along the edges is today
partitioned administratively into a large number of disjoint domains,
known as Autonomous Systems (ASs).
A common administration may have responsibility for one or more
domains that may or may not be adjacent in the graph.
Commercial and policy constraints affecting the routing system will
typically be exercised at the boundaries of these domains where
traffic is exchanged between domains.
The perceived need for commercial confidentiality will seek to
minimise the information transferred across these boundaries, leading
to requirements for aggregated information, abstracted maps of
connectivity exported from domains and mistrust of supplied
information.
The perceived desire for anonymity may require the use of zero-
knowledge security protocols to allow users to access resources
without exposing their identity.
One possible extension to the requirements would be to require the
protocols to provide the ability for groups of peering domains to be
treated as a (super-)domain. These domains would have a common
administrative policy.
5.2 Working with different sorts of networks
The diverse Layer 2 networks over which the layer 3 routing system is
implemented have typically been operated totally independently from
the layer 3 network. Consideration needs to be given to the degree
and nature of interchange of information between the layers that is
desirable. In particular, the desire for robustness through diverse
routing implies knowledge of the underlying networks to be able to
guarantee the robustness.
Mobile access networks may also impose extra requirements on Layer 3
routing.
5.3 Delivering Diversity
The routing system is operating at Layer 3 in the network. To
achieve robustness and resilience at this layer requires that where
multiple diverse routes are employed as part of delivering the
resilience, the routing system at Layer 3 needs to be assured that
the Layer 2 and lower routes are really diverse. The ædiamond
problemÆ is the simplest form of this problem - layer 3 provider
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attempting to provide diversity buys layer 2 services from two
separate providers who in turn buy wayleaves from the same provider:
Layer 3 service
/ \
/ \
Layer 2 Layer 2
Provider A Provider B
\ /
\ /
Trench provider
Now when the backhoe cuts the trench, the Layer 3 provider has no
resilience unless he had taken special steps to verify that the
trench wasnÆt common. The routing system should facilitate avoidance
of this kind of trap.
Some work is going on to understand the sort of problems that stemm
from this requirement, such as the work on Shared Risk Link Groups
[31]. Unfortunately, the full generality of the problem requires
diversity be maintained over time between an arbitrarily large set of
mutually distrustful providers. For some cases, it may be sufficient
for diversity to be checked at provisioning or route instantiation
time, but this remains a hard problem requiring research work.
5.4 When will the new solution be required?
There is a full range of opinion on this subject. An informal survey
indicates that the range varies from 2 years to 6 years. And while
there are those, possibly outliers, who think there is no need for a
new routing architecture as well as those who think a new
architecture was needed years ago, the median seems to lie at around
4 years. As in all projections of the future this is largely not
provable.
6. Assumptions
In projecting the requirements for the Future Routing Domain a number
of assumptions have been made. The requirements set out should be
consistent with these assumptions, but there are doubtless a number
of other assumptions which are not explicitly articulated here:
1. The number of hosts today is somewhere in the area of 100 Million.
With dial in and NATs this is likely to turn into up to 500
Million users (see [30]). In a number of years, with wireless
accesses and different ægizmosÆ attaching to the Internet, we are
likely to see a couple of Billion æusersÆ on the Internet. The
number of globally addressable hosts is very much dependent on how
common NATs will be in the future.
2. NATs and other mid-boxes exist and we cannot assume that they will
cease being a presence in the networks.
3. The number of operators in the Internet will probably not grow
very much, as there is a likelihood that operators will tend to
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merge. However, as Internet-connectivity expands to new countries,
new operators will emerge and then merge again.
4. Today, there are around 9,500 ASÆs with a growth rate of around
51% per annum [13]. With current use of ASÆs (for e.g., multi-
homing) the number of ASÆs grow to 70,000 within 3 - 5 years.
5. In contrast to the number of operators, the number of domains is
likely to grow significantly. Today, each operator has different
domains within an AS, but this also shows in SLAs and policies
internal to the operator. Making this globally visible would
create a number of domains 10-100 times the amount of ASs, i.e.,
between 100,000 and 1,000,000.
6. With more and more capacity at the edge of the network the IP
network will expand. Today there are operators with several
thousands of routers, but this is likely to be increased. A domain
will probably contain tens of thousands of routers.
7. The speed of connections in the (fixed) access will technically be
(almost) unconstrained. However, the cost for the links will not
be negligible so that the apparent speed will be effectively
bounded. Within a number of years some will have multi-Gigabit-
speed in the access.
8. At the same time, the bandwidth of wireless access still has a
strict upper-bound. Within the foreseeable future each user will
only have a tiny amount of resources available compared to fixed
accesses (10kbps to 2Mbps for UMTS with only a few achieving the
higher figure as the bandwidth is shared between the active users
in a cell and only small cells can actually reach this speed, but
11Mbps or more for wireless LAN connections).
9. Assumptions 7 and 8 taken together suggest a span of bandwidth
between 10 kbps to 10 Gbps.
10. The speed in the backbone has grown rapidly, and there is no
evidence that the growth will stop in the coming years. Terabit-
speed is likely to be the minimum backbone speed in a couple of
years. The range of bandwidths that might need to be represented
will require some thought to be given to how to represent the
values in the protocols.
11. There have been discussions as to whether Moore's law will
continue to hold for processor speed. If Moore's law does not
hold, then communication circuits might play a more important role
in the future. Also, optical routing is based on circuit
technology, which is the main reason for taking ³circuits³ into
account when designing an FDR.
12. However, the datagram model still remains the fundamental model
for the Internet.
13. The number of peering points in the network is likely to grow, as
multi-homing becomes important. Also traffic will become more
locally distributed, which will drive the demand for local
peering.
14. The FDR will achieve the same degree of ubiquity as the current
Internet and IP routing.
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7. Functional Requirements
This section includes a detailed discussion of new requirements for a
future domain routing architecture. As discussed in section 2.1 a
new architecture must build upon the requirements for past routing
architecture. For that reason, the requirements discussed in section
2.1 are not repeated here. In case where the requirement has changed
significantly, was omitted from the discussions in RFC1126 or was
treated as a non-goal in RFC1126 but may now be significant, it will
be discussed in further detail in this section.
7.1 Topology
7.1.1 Routers should be able to know and exploit the domain topology
Routers need to know the domain topology. BGP today operates with a
policy database, but does not provide a link state database for the
connectivity of each AS - the extent to which this is feasible or
desirable needs to be investigated.
The OSI Inter-Domain Routing Protocol (IDRP) [36] utilized a related
capability which allowed a collection of topologically related
domains to be replaced by a domain collection object in a similar way
to Nimrod domain hierarchies, allowing a route to be more compactly
represented by a single collection in place of a sequence of
individual domains. This abstraction and aggregation feature is
similar to but somewhat more powerful than the BGP community
capability.
7.1.2 The same topology information should support different path
selection ideas:
The same topology information needs to provide a more flexible
spectrum of path selection methods that we might expect to find in a
future Internet, including, amongst others, both distributed
techniques such as hop-by-hop, shortest path, local optimization
constraint-based, class of service, source address routing, and
destination address routing as well as the centralized, global
optimization constraint-based ætraffic engineeringÆ type (Open
constraints should be allowed). Allowing different path selection
techniques to be used will produce a much more predictable and
comprehensible result than the æclever tricksÆ that are currently
needed to achieve the same results. Traffic engineering functions
need to be combined.
7.1.3 Separation between the routing information topology from the
data transport topology.
The controlling network should be logically separate from the
controlled network. Physically, the two functional "planes" can
reside in the same nodes and share the same links, but this is not
the only possibility. Other options can also be feasible, and may
sometimes be necessary. An example is a pure circuit switch (that
cannot see individual IP packets), combined with an external
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controller. Another example may be where there are multiple links
between two routers, and all the links are used for data forwarding,
but only one is used for carrying the routing session.
7.2 Distribution
7.2.1 Distribution mechanisms
The important requirement is that every entity gets the information
it needs in a fast, reliable, and trusted way.
Possible distribution mechanisms for routing information exchange may
be for example full mesh, spanning tree, route reflections, flooding,
and multicast.
The current I-BGP seems to have unnecessary limitations in this
respect, where a router requires full mesh to all other I-BGP
speakers in the domain to obtain all available routes. Route
reflection avoids the need of full meshes but requires very careful
configuration to ensure that the best route available is still
selected as if all routers were connected in a full mesh.
7.2.2 Path advertisement
The inter-domain routing system must be able to advertise more kinds
of information than just connectivity and AS path. The FDR should
support the Service Level Specifications (SLSs) that are being
developed under the Differentiated Services imprimatur.
Careful attention should be paid to ensuring that the distribution of
additional information with path advertisements remains scalable as
domains and the Internet get larger.
Possible examples of such additional information that might be
carried include:
- QoS information
To allow an ISP to sell predictable end-to-end QoS service to any
destination, the routing system should have information about the
end-to-end QoS. This means that the routing system should be able to
support different paths for different services identified by DiffServ
PDBÆs or TOS-values. The routing system should also be able to carry
information about the expected (or actually, promised)
characteristics of the entire path and also the price for the
service. (If such information is exchanged at all between network
operators today, it is through bilateral management interfaces, and
not through the routing protocols.)
This would allow for the operator to optimise the choice of path
based on a price/performance trade-off.
It is possible that providing dynamic QoS information to control
routing is not scalable, and an alternative would be to use static
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class-of-service information such as is suggested in the
Differentiated Services work.
- security information
Security characteristics of other ASs (in the path or in the map) can
allow the routing entity to choose routing decision based on some
political reasons. The information itself is assumed to be so secure
that you can trust it.
- usage and cost information
This can be used for billing and traffic engineering purpose. In
order to support cost based routing policies for customers (ie peer
ISPs), information such as "traffic on this link or path costs XXX
USD per Gigabyte" needs to be advertised, so that the customer can
choose a cheap or an expensive route from an economic perspective.
- monitored performance
Some performance information such as delay and drop frequency can be
carried. (This is may only be suitable inside a domain because of
trust considerations). This should support at least the kind of
delay bound contractual terms that are currently being offered by
service providers. Note that these values refer to the outcome of
carrying bits on the path, whereas the QOS information refers to the
proposed behaviour which results in this outcome.
7.2.3 Stability of Routing Information
7.2.3.1 Avoiding Routing Oscillations
The FDR must minimize oscillations in route advertisements.
7.2.3.2 Providing Loop Free Routing and Forwarding
In line with the separation of concerns of routing and forwarding,
the distribution of routing information should be, so far as is
possible, loop-free, and the forwarding information created from this
routing information should also seek to minimize persistent loops in
the data forwarding paths. It is accepted that transient loops may
occur during convergence of the protocol and that there are trade-
offs between loop avoidance and global scalability.
7.3 Addressing
7.3.1 Support mix of IPv4, IPv6 and other types of addresses
The routing system must support a mix of different kinds of
addresses, including at least IPv4 and IPv6 addresses, and preferably
various types of non-IP addresses too. For instance networks like
SDH/SONET and WDM may prefer to use non-IP addresses. It may also be
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necessary to support multiple sets of æprivateÆ RFC1918 addresses
when dealing with multiple customer VPNs.
The routing system should support the capability to use a single
topology representation to generate routing and forwarding tables for
multiple address families on the same network. This capability would
minimise the protocol overhead when exchanging routes.
Note that both Integrated IS-IS and BGP with multi-protocol
extensions [37] can support different address families. Extended BGP
is used, for example, in RFC2547 [16] to carry the VPN-IPv4 address
family.
7.3.2 Support for domain renumbering/readdressing
The routing system must support readdressing (when a new prefix is
given to an old network, and the change is known in advance) by
maintaining routing during the changeover period [39], [40].
7.3.3 Multicast and Anycast
The routing system must support multicast addressing, both within a
domain and across multiple domains. It must also support anycast
addressing within a domain, and should support inter-domain anycast
addressing.
7.3.4 Address scoping
The routing system must support scoping of addresses, for each of the
unicast, multicast, and anycast types.
For unicast address scoping as of IPv6, there seems to be no special
problems with respect to routing. Inter-domain routing handles only
global addresses, while intra-domain routing also needs to be aware
of site-local addresses. Link-local addresses are never routed at
all.
For scoping in a more general sense, and for scoping of multicast and
anycast addresses, more study may be needed to identify the
requirements.
7.3.5 Mobility Support
The routing system shall support end system mobility (and movability,
and portability, whatever the differences may be).
We observe that the existing solutions based on re-numbering and/or
tunneling are designed to work with the current routing, so they do
not add any new requirements to future routing. But the requirement
is general, and future solutions may not be restricted to the ones we
have today.
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7.4 Management Requirements
7.4.1 Simple policy management
- Less manual configuration than today
- Operators/providers want easy handling, but cannot afford to lose
control.
- All the information should be available
- But should not be visible except for when desired.
- Advertise policy (not only the result of policy)
- Policy conflict Resolution
(e g one would like to have one default behavior, and possibilities
to choose other options. But much of this depends on implementation,
and not on the protocols)
7.5 Mathematical Provability
The protocol is required to be resistant to bad routing policy
decisions made by operators. Tools are needed to check compatibility
of routing policies. Routing policies are compatible if their global
interaction does not cause divergence (collection of ASes exchange
routing messages indefinitely never entering a stable state). Tools
must be provided to make routing system convergent. A routing system
is convergent if after an exchange of routing information, routing
tables reach a stable state that does not change until routing
policies change.
To achieve the above mentioned goals a mechanism is needed to publish
and communicate policies so that operational coordination and fault
isolation is possible. Tools are required that verify stable
properties routing system in specified parts of Internet. The tools
should be efficient (fast) and have a broad scope of operation (check
large portions of Internet).
Tools analyzing routing policies can be applied statically or
(preferably) dynamically. Dynamic solution requires tools that can be
used for run time checking for a source of oscillations that arise
from policy conflicts. Research is needed to prove that there is an
efficient solution to the dynamic checking of oscillations.
7.6 Traffic Engineering
7.6.1 Support for and Provision of Traffic Engineering Tools
At present there is an almost total lack of effective traffic
engineering tools, whether on-line at all times for network control
or off-line for network planning. The routing system should
encourage the provision of such tools and generate statistical and
accounting information in such a way that these tools can be used
both in real time and for off-line planning and management.
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7.6.2 Support of Multiple Parallel Paths
The routing system shall support the controlled distribution over
multiple links or paths, of traffic towards the same destination.
This applies to domains with two or more connections to the same
neighbor domain, and to domains with connections to more than one
neighbor domain. The paths need not have the same metric.
This capability should be provided to support both cases where the
offered traffic is known to exceed the available capacity of a single
link, and also cases where load is to be shared over paths for cost
or resiliency reasons.
Imposition of this requirement on the routing system requires that
the corresponding forwarding should avoid reordering of packets in
individual micro-flows, and should have mechanisms to allow the
traffic to be reallocated back on to a single path when the multiple
paths are not needed.
7.6.3 Peering support
The FDR must support peer-level connectivity as well as purely
hierarchical inter-domain connections. The network is becoming
increasingly complex with private peering arrangements set up between
providers at every level of the hierarchy of service providers and
even by certain large enterprises, in the form of dedicated
extranets.
The FDR must facilitate traffic engineering of these peer routes so
that traffic can be readily constrained to travel as the network
operators desire and they can consequentially make optimal use of the
available connectivity.
7.7 Support for NATs and other Mid-boxes
One of our assumptions is that NATs and other Mid-boxes such as
firewalls, web proxies and address family (e.g. IPv4 to IPv6)
translators are here to stay.
The FDR should seek to work with NATs to aid in bi-directional
connectivity through the NAT without compromising the additional
opacity and privacy which the NAT offers. This problem is closely
analogous to the abstraction problem, which is already under
discussion for the interchange of routing information between
domains.
7.8 Statistics support
Both the routing and forwarding parts of the FDR must maintain
statistical information about the performance of their functions.
This may be an extended version of the MIBs provided for IP
forwarding, BGP and the relevant IGP.
Davies, et al Expires: January 2002 43
INTERNET DRAFT FDR Requirements 9 July, 2001
8. Performance Requirements
Over the past several years, the perfomance of the routing system has
frequently been discussed. Some of the questions being asked
include:
- How fast does an AS converge (given that we understand what we
mean by convergence)? How fast must domains converge?
- How big are the Areas, the ASs? How big should domains be? How
many peers should a BGP Speaker be able to cope with? Can the
routing protocols manage domains of this size
- How much or how little data may be transferred in a routing
message?
- How much state can be stored and processed in route control
processors.
- Measures of network availability
- Measure of network reliability
- Global and Local measures of network Stability
- Capacity Measurement
In many cases there has been very little data or statistical evidence
for many of the performance claims being made. In recent years
several efforts have been initiated to gather data and do the
analyses required to make scientific assessments of the performance
issues and requirements. In order to complete this section of the
requirements analysis, the data and analyses from these studies needs
to be gathered and collated into this document. This work has been
started but has yet to be completed.
9. Backwards compatibility (cutover) and Maintainability
This area poses a dilemma. On one hand it is an absolute requirement
that introduction of FDR must not require any flag days. The network
currently in place has to keep running at least as well as it does
now while the new network is being brought in around it.
However, at the same time, it is also an absolute requirement that
the new architecture not be limited by the restrictions that plague
todayÆs network. Those restrictions cannot be allowed to become
permanent baggage on the new architecture. If they do, the effort to
create a new system will come to naught.
These two requirements have significance not only for the transition
strategy, but for the architecture itself implying that it must be
possible for an internet such as todayÆs BGP controlled network, or
one of its ASs, to exist as a domain within the new FDR.
10. Security Requirements
As previously discussed, one of the major changes to have overtaken
the Internet since its inception, is the erosion of trust between end
users making use of the net, between those users and the suppliers of
services, and between the multiplicity of providers. Hence security,
in all its aspects, will be much more important in the FDR.
Davies, et al Expires: January 2002 44
INTERNET DRAFT FDR Requirements 9 July, 2001
It must be possible to secure the routing communication: the
communicating entities shall be able to identify who sent and who
received the information (authentication), and verify that the
information has not been changed on the way (integrity).
Security is more important in inter-domain routing where the operator
has no control to the other domains, and less serious in intra-domain
routing since all the links and the nodes are under the
administration of the operator and can be expected to share a trust
relationship.
The routing communication mechanism shall be robust against denial-
of-service attacks.
Further considerations which may impose requirements include:
- Whether no one else but the intended recipient must be able to
access (privacy) or understand (confidentiality) the information.
- Whether it is possible to verify that all the information has been
received (non-repudiation).
- Whether there is a need to separate security of routing from
security of forwarding.
- Whether traffic flow security is needed (i.e. whether there is
value in concealing who can connect to whom, and what volumes of
data are exchanged).
Securing the BGP session, as done today, only secures the exchange of
messages from the peering AS, not the content of the information. In
other words, we can confirm that the information we got is what our
neighbor really sent us, but we do not know if this information (that
originated in some remote AS) is true or not.
A decision has to be made on whether to rely on chains of trust (we
trust our peers who trust their peers who..), or whether we also need
authentication and integrity of the information end-to-end. This
information includes both routes and addresses. There has been
interest in having digital signatures both on originated routes, but
also countersignatures by address authorities to confirm that the
originator has authority to advertise the prefix. Even understanding
who can confirm the authority is non-trivial as it might be the
provider who delegated the prefix (with a whole chain of authority
back to ICANN) or it may be straight to an address registry. Where a
prefix delegated by a provider is being advertised though another
provider as in multi-homing, both may have to be involved to confirm
that the prefix may be advertised through the provider who doesnÆt
have any interest in the prefix!
The FDR should seek to cooperate with the security policies of
firewalls and other third-party controlled mid-boxes whenever
possible. This is likely to involve further requirements for
abstraction of information, as, for example, the firewall is seeking
to minimize interchange of information that could lead to a security
breach. The effect of such changes on the end-to-end principle
should be carefully considered as discussed in [32].
Davies, et al Expires: January 2002 45
INTERNET DRAFT FDR Requirements 9 July, 2001
Provision may have to be made to override some of these requirements
when local laws mandate interception of communication capabilities.
11. Open Issues
This section covers issues that need to be considered and resolved in
deciding on a future domain routing architecture. While they canÆt
be described as requirements, they do affect the types of solution
that are acceptable. The discussions included below are very open-
ended.
11.1 System Modeling
The assumption that object modeling of a system is an essential first
step to creating a new system is still novel in this context.
Frequently the effort to object model becomes an end in itself and
does not lead to system creation. But there is a balance and a lot
that can be discovered in an ongoing effort to model a system such as
the future domain routing system.
It is recommended that this process be included in the requirements.
It should not, however be a gating event to all other work.
Some of the most important realizations will occur during the process
of determining the following:
- Object classification
- Relationships and containment
- Roles and Rules
11.2 Advantages and Disadvantages of having the same protocols for EGP
and IGP
Inter-domain and intra-domain routing have different targets and
business assumptions. An IGP figures out how each node in the network
gets to every other node in the network in the most optimal way. In
this context the word optimal refers to the cost of the path measured
by metrics associated with each link in the network. The area of
network infrastructure (primarily routers) over which an IGP runs is
typically under the same technical and administrative control, and it
defines the boundary of an AS (Autonomous System). The purpose of an
EGP is to allow two different ASs to exchange routing information so
that data traffic can be forwarded across the AS border. Because an
AS border router both separates and attaches two different areas of
technical and administrative control, the specifications and
implementations of EGPs include mechanisms for doing policy routing,
meaning that control can be exerted over which routing information
crosses the border between two ASs. EGPs contain features that are
like metrics in IGPs, but unlike IGPs, the function of an EGP is not
necessarily to optimize the path that data traffic takes through a
backbone. Having different protocols for EGP and IGP reflects this
difference.
However, there is increasing demand in IGP to do policy routing. The
shortest path may not be the best path in the light of the policies.
Davies, et al Expires: January 2002 46
INTERNET DRAFT FDR Requirements 9 July, 2001
Network operators need to have more flexibility in choosing routes
for reasons such as load balancing. This means both inter-domain
routing and intra-domain routing are for the same purpose of choosing
the best route according to operators' own policies. Having the same
protocol will emphasize the need to do policy control in IGP.
This comment touches on the fact that the level of manual control
(policy) is much larger in EGP. Why is this so?
EGP:
- Manifests business relations to peers, providers and customers.
- Borders to resources outside of our control. We don't trust others
to behave well when configuring routing. These resources are also
often be less stable (eg customer access).
- Network size extremely large. This gives many updates which means
we need to have a simple calculation of paths. It also gives an
extremely large amount of information (due to the network size)
which gives the need for aggregation. Also we need policy to
protect our network from receiving bad announcements causing our
egress traffic to take the "wrong" way and to avoid sending bad
announcements attracting the "wrong" traffic.
IGP:
- The network resources are under our control and we trust ourself
to behave well (in a sense defined by ourselves) when configuring
routing.
- The network resources of today are fairly stable in a backbone
network.
- The size of the network is limited. So, the domain is fairly
stable which gives a limited number of updates. Limited number of
updates gives the option of using processor intensive automation
(distributed link state routing). This gives us fast and easy to
manage dynamic routing. BUT stability and visibility issues still
constrain us from going further down the path of policy routing.
11.2.1 The necessity to clearly identify all identities related to
routing
As in all other fields, the words used to refer to concepts and to
describe operations about routing are important. Rather than describe
concepts using terms that are inaccurate or rarely used in the real
world of networking, it is necessary to make an effort to use the
correct words. Many networking terms are used casually, and the
result is a partial or incorrect understanding of the underlying
concept. Entities such as nodes, interfaces, sub-networks, tunnels,
and the grouping concepts such as ASs, domains, areas, and regions,
need to be clearly identified and defined to avoid mixing from each
other. And, even if they are all identified by IP numbers, the
routing entities should know what kind of entities they are.
There is also a need to separate identifiers (what or who) from
locators (where) from routes (how to reach). One of the problems with
the current BGP is if there is a topology change, the amount of
information circulated is a function of the number of IP prefixes
Davies, et al Expires: January 2002 47
INTERNET DRAFT FDR Requirements 9 July, 2001
being routed. This is a common problem for a distance vector
protocol. If the topology information is properly separated from
addressing information in a state map, then when a link between two
ASs goes down, this is the only information which needs to be
advertised, instead of advertising the inability to reach some
network prefixes. This example shows the need to separate end node
identifiers from routing information.
11.2.2 Map distribution and/or route Distribution
11.2.2.1 Class of protocol to use
BGP4 is an enhanced distance vector protocol, which is the oldest and
least sophisticated type of mechanism for distributing routes. It
would be possible to retain the basic route distribution mechanism
but use an improved convergence mechanism such as is described in
[34].
Alternatively, it would be possible to move to the more sophisticated
Map Distribution class of protocol such as PNNI. This has some
advantages in that it probably easier to isolate the routing
mechanisms on a per domain basis when exchanging information by maps
which are a more sophisticated data structure.
11.2.2.2 Map Abstraction
If every detail is advertised throughout the Internet, there will be
a lot of information. Scalable solutions require abstraction.
- If we summarise too much, some information will be lost on the
way.
- If we summarize too little, then more information then required is
available contributing to scaling limitations.
- One can allow more summarisation, if there also is a mechanism to
query for more details within policy limits.
- The basic requirement is not that the information shall be
advertised, but that the information shall be available to those
who need it. (We should not presuppose a solution where
advertising is the only possible mechanism.)
11.2.3 Robustness and redundancy:
The routing association between two domains should survive even if
some individual connection between two ASBR routers goes down.
The "session" should operate between logical "routing entities" on
each domain side, and not necessarily be bound to individual routers
or IP addresses. Such a logical entity can be physically distributed
over multiple network elements. Or it can reside in a single router,
which would default to the current situation.
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INTERNET DRAFT FDR Requirements 9 July, 2001
11.2.4 Hierarchy
A more flexible hierarchy with more levels and recursive groupings in
both upward and downward directions allows more structured routing.
The consequence is that no single level will get too big for routers
to handle.
On the other hand, it appears that the real world Internet is
becoming less hierarchical, so that it will be increasingly difficult
to use hierarchy for scaling control.
Note that groupings can look different depending on which aspect we
use to define them. A DiffServ area, a MPLS domain, a trusted domain,
a QoS area, a multicast domain, etc, do not always coincide. And
neither are they strict hierarchical subsets of each other. The basic
distinction at each level is "this grouping versus everything
outside".
Each AS is still independent, and forms the basis for policy
decisions. However, is there a need for a higher level aggregation
which is above AS? If yes, who will be responsible for this level?
Can a network make policy decisions on such aggregated ASs without
seeing the individual ASs?
11.3 Introduction of new control mechanisms
Is it be possible to apply a control theory framework, and analyze
the stability of the control system of the whole network domain, for
e.g. convergence speed and the frequency response, and then use the
results from that analysis to set the timers and other protocol
parameters.
Control theory could also play a part is QoS Routing, by modifying
current link state protocols with link costs dependent on load.
Control theory is used to increase the stability of such systems.
At best, it might be possible to construct a new totally dynamical
routing protocol solely on a control theoretic basis as opposed to
the current protocols which are based in graph theory and static in
nature.
11.4 Robustness
Is solution to the Byzantine Generals problem a requirement? This is
problem of reaching a consensus among distributed units if some of
them give misleading answers. The original problem concerns generals
plotting a coup. Some generals lie about whether they will support a
particular plan and what other generals told them. What percentage of
liars can a decision-making algorithm tolerate and still correctly
determine a consensus? The current intra-domain routing system is,
at one level, totally intolerant of misleading information, but the
effect of different sorts of misleading or incorrect information has
Davies, et al Expires: January 2002 49
INTERNET DRAFT FDR Requirements 9 July, 2001
vastly varying results, from total collapse after the æsmall Virginia
ISPÆ incident through to purely local disconnection of a single AS.
This sort of behaviour is not very desirable.
What are some of the other network robustness issues that must be
resolved?
11.5 VPN Support
Today BGP is also used for VPN and other stuff for example as
described in RFC2547
Internet routing and VPN routing have different purposes, and most
often exchange different information between different devices. Most
Internet routers do not need to know any VPN specific information.
The concepts should be clearly separated.
But when it comes to the mechanisms, VPN routing can share the same
protocol as ordinary Internet routing, it can use a separate instance
of the same protocol, or it can use a different protocol. All
variants are possible and have their own merits.
For example, all the AS Border Routers within one AS participate in a
full-mesh I-BGP process for distributing external IP routes. At the
same time a separate "VPN-routing" protocol can be operating between
all the PE routers of some "VPN provider". These PE routers can be
located in different ASs, and some of them may also be ASBRs.
11.6 End to End Reliability
The existing Internet architecture neither requires or provides end-
to-end reliability of control information dissemination. For
example, in distributing VPN information there is, however, a
requirement for end to end reliability of control information, i.e.
the ends of the VPN established need to have a acknowledgement of the
success in setting up the VPN. While it is not necessarily the
function of a routing architecture to provide end-to-end reliability
for this kind of purpose, we must be clear that end-to-end
reliability becomes a requirement if the network has to support such
reliable control signalling. There may be other requirements that
derive from requiring the FDR to support reliable control signaling.
12. Acknowledgements
The authors would like to acknowledge the helpful comments and
suggestions of the following individuals: Loa Andersson, Tomas
Ahlstr÷m, Niklas Borg, Nigel Bragg, Thomas Chmara, Krister Edlund,
Owe Grafford, Torbj÷rn Lundberg, Jasminko Mulahusic, Florian-Daniel
Otel Bernhard Stockman, Henrik Villf÷r, Tom Worster, Roberto
Zamparo,.
In addition, the authors are indebted to the folks who wrote all the
references we have consulted in putting this paper together. This
Davies, et al Expires: January 2002 50
INTERNET DRAFT FDR Requirements 9 July, 2001
includes not only the reference explicitly listed below, but those
who contributed to the mailing lists we have been participating in
for years.
13. References
[1] Clark, D., "Policy Routing in Internet
Protocols", RFC 1102, May 1989.
[2] Estrin, D., "Requirements for Policy Based
Routing in the Research Internet", RFC 1125,
November 1989.
[3] Steenstrup, M,. ææAn Architecture for Inter-
Domain Policy RoutingÆÆ, RFC 1478, June 1993
[4] Little, M., "Goals and Functional Requirements
for Inter-Autonomous System Routing", RFC 1126,
July 1989.
[5] Perlman, R., ææInterconnections Second EditionÆÆ,
1999, Addison Wesley Longman, Inc.
[6] Perlman, R., "Network Layer Protocols with
Byzantine Robust-ness", Ph.D. Thesis, Department
of Electrical Engineering and Computer Science,
MIT, August 1988.
[7] Castineyra, I., Chiappa, N., Steenstrup, M.,
ææthe Nimrod Routing ArchitectureÆÆ, RFC1992, Aug
1996
[8] Chiappa, N., ææIPng Technical Requirements of the
Nimrod Routing and Addressing ArchitectureÆÆ, RFC
1753, Dec 1994
[9] Chiappa, N., ææA New IP Routing and Addressing
ArchitectureÆÆ
[10] Wroclowski, J., The Metanet White Paper -
Workshop on Research Directions for the Next
Generation Internet, 1995
[11] Labovitz, C., Ahuja, A., Farnam J., Bose, A.,
Experimental Measurement of Delayed Convergence,
NANOG
[12] Griffin, T.G., Wilfong, G., An Analysis of BGP
Convergence Properties, SIGCOMM 1999
[13] Huston, G., Architectural Requirements for Inter-
Domain Routing in the Internet, Internet Draft -
draft-iab-bgparch-00, Feb 2001, Work in Progress
Davies, et al Expires: January 2002 51
INTERNET DRAFT FDR Requirements 9 July, 2001
[14] Alaettinoglu, C., Jacobson, V. and Yu, H, ,
Towards Milli-Second IGP Convergence, Internet
Draft - draft-alaettinoglu-isis-convergence-00,
Nov 2000 Work in Progress
[15] Sandick, H., Squire, M., Cain, B., Duncan, I.,
Haberman, B., Fast LIveness Protocol (FLIP),
Internet Draft - draft-sandiick-flip-00,
Feb 2000, Work in Progress
[16] Rosen, E. and Rekhter, Y., BGP/MPLS VPNs,
RFC2547, March 1999
[17] Clark, D., Chapin, L., Cerf, V., Braden, R.,
Hobby, R., æætowards the Future Internet
ArchitectureÆÆ, RFC1287, December 1991
[18] Jacobson, V., Nichols, K. and Poduri, K., The
æVirtual WireÆ Behavior Aggregate, Internet Draft
- draft-ietf-diffserv-pdb-vw-00, July 2000, Work
in Progress
[19] Seddigh, N., Nandy, B., and Heinanen, J.,
An Assured Rate Per-Domain Behaviour for
Differentiated Services, Internet Draft -
draft-ietf-diffserv-pdb-ar-00, Feb 2001, Work in
Progress
[20] McPherson, D., Gill, V., Walton, D. and Retana,
A., ææBGP Persistent Route Oscillation
ConditionÆÆ,
Internet Draft - draft-mcpherson-bgp-route-
oscillation-00, Dec 2000, Work in Progress
[21] Hain, T, ææArchitectural Implications of NATÆÆ,
RFC 2993, November 2000
[22] McPherson, D. and Przygienda, T., OSPF Transient
Blackhole Avoidance, Internet Draft - draft-
mcpherson-ospf-transient-00, July 2000 Work In
Progress
[23] Thaler, D., Estrin, D. and Meyer, D. (editors),
Border Gateway Multicast Protocol (BGMP):
Protocol Specification, Internet Draft - draft-
ietf-bgmp-spec-02, Nov 2000 Work in progress
[24] Rosen, E. Et al., Multiprotocol Label Switching
Architecture, RFC 3031
[25] Ashwood-Smith, P. Et al., Generalized MPLS -
Signaling Functional Description, Internet Draft
- draft-ietf-mpls-generalized-signaling-01.txt,
Work in progress
Davies, et al Expires: January 2002 52
INTERNET DRAFT FDR Requirements 9 July, 2001
[26] IETF Resource Allocation Protocol working group,
http://www.ietf.org/html.charters/rap-
charter.html
[27] IETF Configuration management with SNMP working
group,
http://www.ietf.org/html.charters/snmpconf-
charter.html
[28] IETF Policy working group,
http://www.ietf.org/html.charters/policy-
charter.html
[29] Yu J., ææScalable Routing Design PrinciplesÆÆ,
RFC 2791
[30] Telcordia Technologies Netsizer web site
http://www.netsizer.com/
[31] Inference of Shared Risk Link Groups,
draft-many-inference-srlg-00.txt,
Work in progress
[32] Blumenthal, Marjory S. and Clark, David D.,
Rethinking the design of the Internet:
The end to end arguments vs. the brave new world,
May 2001,
http://ana-www.lcs.mit.edu/anaweb/papers.html
[33] Lang et al, Link Management Protocol,
draft-lang-mpls-lmp-02.txt,
Work in progress
[34] Xu, Z., Dai, S. and Garcia-Luna-Aceves, J.J.,
A More Efficient Distance Vector Routing
Algorithm, Proc. IEEE MILCOM 97, Monterey,
California, November 2-5, 1997,
http://www.cse.ucsc.edu/research/ccrg/
publications/zhengyu.milcom97.pdf
[35] Bradner, S. and Mankin, A., "The Recommendation
for the IP Next Generation Protocol", RFC 1752,
January 1995.
[36] ISO/IEC, "Protocol for Exchange of Inter-Domain
Routeing Information among Intermediate
Systems to support Forwarding of ISO 8473 PDUs",
International Standard 10747,
ISO/IEC JTC 1,Switzerland 1993
[37] Bates, T., Rekhter, Y., Chandra, R. and Katz, D,
ææMultiprotocol Extensions to BGP-4ö,
RFC2858, June 2000
Davies, et al Expires: January 2002 53
INTERNET DRAFT FDR Requirements 9 July, 2001
[38] Berkowitz, H. and Krioukov, D, ææTo Be
Multihomed: Requirements and DefinitionsÆÆ,
draft-berkowitz-multirqmt-02.txt,
Work in progress.
[39] Ferguson, P. and Berkowitz, H. ææNetwork
Renumbering Overview: Why would I want it and
what is it anyway?ÆÆ, RFC2071, January 1997
[40] Berkowitz, H., ææRouter Renumbering GuideÆÆ,
RFC2072, January 1997
14. Author's Addresses
Elwyn Davies
Nortel Networks
London Road
Harlow, Essex CM17 9NA, UK
Phone: +44-1279-405498
Email: elwynd@nortelnetworks.com
Avri Doria
Nortel Networks
600 Technology Park Drive
Billerica, MA, USA
Phone: +1 978 288 6627
Email: avri@nortelnetworks.com
Howard Berkowitz
Nortel Networks
5012 South 25th St
Arlington
VA 22206, USA
Phone: +1 703 998-5819
Email: hcb@clark.net/hberkowi@nortelnetworks.com
Dmitri Krioukov
Nortel Networks
1st Floor
205 van Buren Street
Herndon
VA 20170, USA
Phone: +1 703 709 8518
Email: dima@nortelnetworks.com
Davies, et al Expires: January 2002 54
INTERNET DRAFT FDR Requirements 9 July, 2001
Malin Carlzon
Royal Institute of Technology
Network Operating Centre
KTHNOC
SE-100 44
Stockholm, Sweden
Phone: +46 70 269 6519
Email: malin@sunet.se
Anders Bergsten
Telia Research AB
Aurorum 6
S-977 75 Lulea, SWEDEN
Phone: +46 920 754 50
Email: anders.p.bergsten@telia.se
Olle Pers
Telia Research AB
Stockholm, SWEDEN
Phone: +46 8 713 8182
Email: olle.k.pers@telia.se
Yong Jiang
Telia Research AB
123 86 Farsta SWEDEN
Phone: +46 8 713 8125
Email: yong.b.jiang@telia.se
Lenka Carr Motyckova
Div. of Computer
Lulea University of Technology
S-971 87
Lulea, SWEDEN
Phone: (+46) 920 91769
Email: lenka@sm.luth.se
Pierre Fransson
Div. of Computer
Lulea University of Technology
S-971 87
Lulea, SWEDEN
Phone: (+46) 70 646 0384
Email: pierre@cdt.luth.se
Olov Schelen
Div. of Computer
Lulea University of Technology
S-971 87
Lulea, SWEDEN
Phone: (+46) 70 536 2030
Email: Olov.Schelen@cdt.luth.se
Davies, et al Expires: January 2002 55
INTERNET DRAFT FDR Requirements 9 July, 2001
Tove Madsen
Utfors Bredband AB
R
sundavgen 12
P.O. Box 525
SE-169 29 Solna
Sweden
Phone: +46 (8) 5270 5040
Email: tove.madsen@utfors.se
Davies, et al Expires: January 2002 56