Representation of IP Routing Policies
in the RIPE Database
Tony Bates
Jean-Michel Jouanigot
Daniel Karrenberg
Peter Lothberg
Marten Terpstra
ripe-81
February, 1993
Table of Contents
1 Organisation of this Document ....................... 2
2 Background .......................................... 2
3 Requirements ........................................ 4
4 General Representation of Policy Information ........ 5
5 Proposed Database Objects ........................... 9
6 Representation of Routing Policies .................. 16
7 RIPE Database Update Procedure ...................... 26
8 Summary ............................................. 27
9 Glossary ............................................ 27
10 Author's Addresses ................................. 29
Appendix A - Syntax for the "aut-num" object .......... 30
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Appendix B - Syntax for the "community" object ........ 34
Appendix C - The idea of Autonomous System Macros ..... 37
Appendix D - Transitioning an AS ...................... 40
Appendix E - Routing Policy limitations ............... 43
1. Organisation of this Paper
This paper acts as both a basic tutorial for understanding
routing policy and a proposal for additional objects within
the RIPE database to store routing policies. Section 2
details some of the history and motivations behind this
paper. Section 3 details the general requirements sought for
such a method of representation of IP routing policies.
Section 4 examines the main concepts, how routing policies
are viewed and introduces the terms used throughout this
document. Section 5 details the proposed database objects
themselves. Section 6 gives a fairly extensive "walk
through" of how these objects are used for expressing rout-
ing policy and the general principles behind their use. Sec-
tion 7 explains how these objects should be managed and
updated in terms of the RIPE database itself. A short sum-
mary is given in section 8. A glossary is also included for
clarification and reference. Appendix A and B document the
formal syntax for the database objects proposed. The remain-
ing appendices are essentially "for further study" items
which may be included or are felt useful to the reader to
understand how the representation concept will work in prac-
tice.
2. Background
RIPE is a collaborative body of European Regional IP network
operators and service providers. European IP networking
through its natural evolvement has grown into an extremely
complex infrastructure and topology which in essence has
even today no real single `coordinated core'. Consequently,
there are many differing routing policies used within
Europe. The RIPE Routing Working Group was formed to tackle
the issues involved in large scale routing collaboration as
is needed within Europe. To achieve the needed global con-
nectivity, the network operators and service providers need
a way to easily and clearly express the various routing pol-
icies and provide a method for problem diagnosis and trouble
shooting when problems occur.
A RIPE recommendation paper known as `ripe-60', titled "Pol-
icy based routing within RIPE", was the outcome of a large
May 3, 1993
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amount of effort and discussion within the RIPE Routing
Working Group. This tackled many of the issues relevant to
the representation of IP routing policies and policy based
routing within the RIPE community. It cites many of the per-
tinent documents addressing the issues of policy based rout-
ing and proposes a method for representing routing policy
within the RIPE database. At the time of writing of `ripe-
60', it was very unclear as to what direction the European
IP infrastructure and routing implementations would take and
was very much focused in terms of generality.
It has become apparent that some of the problems with
`ripe-60' lie in its generalised approach and perhaps some-
what abstract notation that in practice are not easy for
network operators to understand. Consequently, the authors
of this document feel some clarification is needed and also
some enhancement in terms of the current update in routing
technology. A conscious effort has been made to be somewhat
less general than `ripe-60' in order to improve applicabil-
ity and clarity.
We would like to express our thanks to Antonio Blasco Boni-
tio, Piet Beertema, Andrew Partan and Brian Schifflet for
their valuable input and comments.
May 3, 1993
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3. Requirements
This section describes the requirements of a scheme for
representing routing information in the RIPE database.
Clarity
The representation must be easily explainable to network
operators. If it is not, then it will not be used or worse,
will be used incorrectly. This also includes that it must
either map well to the way network operators think about
routing policy or provide them with a convenient way to
start thinking about it.
Translatability
The representation must be translatable to and from router
configuration files. The resulting configurations must be
efficient in the sense that they are implementable with
current router resources even in complex environments. If
this requirement is not met there will be too many incon-
sistencies between the published and the implemented poli-
cies which will in turn cause hard to detect routing prob-
lems.
Checkability
It must be possible to check the consistency of the pub-
lished policies. Thus some level of redundancy is welcome if
it adds value by making it possible to reveal inconsisten-
cies.
Applicability
It must be possible to easily build diagnostic tools which
help network operators to diagnose problems by making use of
the policy information. These tools should answer questions
like:
o "Which paths between network A and B are allowed by
published policies ?"
o "Which path will traffic between network A and network
B take when a certain routing path cannot be used for
transit ?"
o "Does network A somehow have connectivity to all other
May 3, 1993
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networks within RIPE ?"
Generality
The "Clarity" and "Translatability" requirements may push
the design into the direction of large aggregations of net-
works by routing and administrative domains. While this
serves those requirements well the representation must make
it possible to specify particular policies down to network
granularity. Specifically, it should be possible to specify
forwarding restrictions.
4. General Representation of Policy Information
Networks, Network Operators and Autonomous Systems
Throughout this document an effort is made to be consistent
with terms so as not to confuse the reader.
When we talk about "networks" we mean physical networks
which have a unique IP network number: Layer 3 entities. We
do not mean organisations.
We call the organisations operating networks "network opera-
tors". For the sake of the examples we divide network
operators into two categories: "service providers" and "cus-
tomers". A "service provider" is a network operator who
operates a network to provide Internet services to different
organisations, its "customers". The distinction between
service providers and customers is not clear cut. A national
research networking organisation frequently acts as a ser-
vice provider to Universities and other academic organisa-
tions, but in most cases it buys international connectivity
from another service provider. A University networking
department is a customer of the research networking organi-
sation but in turn may regard University departments as its
customers.
An Autonomous System (AS) is a group of IP networks having a
single clearly defined routing policy which is run by one or
more network operators. Inside ASes IP packets are routed
using one or more Interior Routing Protocols (IGP). In most
cases interior routing decisions are based on metrics
derived from technical parameters like topology, link speeds
and load (1).
_________________________
(1) The entity we refer to as an AS is frequently and
more generally called a routing domain with the AS just
being an implementation vehicle. We have decided to use
the term AS exclusively because it relates more direct-
ly with the database objects and routing tools. By us-
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ASes exchange routing information with other ASes using
Exterior Routing Protocols (EGP). Exterior routing decisions
are frequently based on policy based rules rather than
purely on technical parameters. Until now, there are no
tools to configure complex policies and to communicate those
policies between ASes while still ensuring proper operation
of the Internet as a whole. Some EGPs like BGP-3 and BGP-4
provide tools to filter routing information according to
policy rules and more. None of them provides a mechanism to
publish or communicate the policies themselves. Yet this is
critical for operational coordination and fault isolation
among network operators and thus for the operation of the
global Internet as a whole.
This paper proposes to use the RIPE database as a repository
for publishing the policies of European ASes. There are two
types of policy information which need to be represented in
the database. These are "routing policies" and "access poli-
cies".
Routing Policies
The exchange of routing information between ASes is subject
to routing policies. Consider the case of two ASes, X and Y
exchanging routing information:
NET1 ...... ASX <---> ASY ....... NET2
ASX knows how to reach a network called NET1. It does not
matter whether NET1 is belonging to ASX or some other AS
which exchanges routing information with ASX either directly
or indirectly; we just assume that ASX knows how to direct
packets towards NET1. Likewise ASY knows how to reach NET2.
In order for traffic from NET2 to NET1 to flow between ASX
and ASY, ASX has to announce NET1 to ASY using an external
routing protocol. This states that ASX is willing to accept
traffic directed to NET1 from ASY. Policy thus comes into
play first in the decision of ASX to announce NET1 to ASY.
In addition ASY has to accept this routing information and
use it. It is ASY's privilege to either use or disregard
the information that ASX is willing to accept traffic for
NET1. ASY might decide not to use this information if it
does not want to send traffic to NET1 at all or if it con-
siders another route more appropriate to reach NET1.
_________________________
ing only one term we hope to reduce the number of con-
cepts and to avoid confusion. The academically inclined
reader may forgive us.
May 3, 1993
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So in order for traffic in the direction of NET1 to flow
between ASX and ASY, ASX must announce it to ASY and ASY
must accept it from ASX:
resulting packet flow towards NET1
<<===================================
|
|
announce NET1 | accept NET1
--------------> + ------------->
|
AS X | AS Y
|
<------------- + <--------------
accept NET2 | announce NET2
|
|
resulting packet flow towards NET2
===================================>>
Ideally, and seldom practically, the announcement and accep-
tance policies of ASX and ASY are identical.
In order for traffic towards NET2 to flow, announcement and
acceptance of NET2 must be in place the other way round. For
almost all applications connectivity in just one direction
is not useful at all.
It is important to realise that with current destination
based forwarding technology routing policies must eventually
be expressed in these terms. It is relatively easy to formu-
late reasonable policies in very general terms which CANNOT
be expressed in terms of announcing and accepting networks.
With current technology such policies are almost always
impossible to implement. See appendix E for further explana-
tion of routing policy limitations.
Usually separate policies are not configured for each net-
work separately but for groups of networks. In practise
these groups are almost always defined by the networks form-
ing one or more ASes.
By expressing routing policy in this way it should be noted
that this information is only of use when you have full
information for all networks within the global Internet.
Equally, the current detail is only limited to the RIPE
database and for an AS exchanging routing reachability
information for an AS whose information is not in the data-
base we obviously cannot give a full representation of the
ASes routing policy.
May 3, 1993
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Access Policies
Access policies contrary to routing policies are not neces-
sarily defined in terms of ASes. The very simplest type of
access policy is to block packets from a specific network S
from being forwarded to another network D. A common example
is when some unappropriate use of resources on network D has
been made from network S and the problem has not been
resolved yet. Other examples of access policies might be
resources only accessible to networks belonging to a partic-
ular disciplinary group or community of interest. While
most of these policies are better implemented at the host or
application level, network level access policies do exist
and are a source of connectivity problems which are some-
times hard to diagnose. Therefore they should also be docu-
mented in the database according to similar requirements as
outlined above.
Tools
The network operators will need a series of tools for policy
routing. Some tools are already available to perform some of
the tasks. Most notably, the "Network List Compiler", NLC
produced as part of earlier work done by the RIPE Routing
Working Group. These tools will enable them to use the
routing policy stored in the RIPE database to perform such
tasks as check actual routing against policies defined,
ensure consistency of policies set by different operators,
and simulate the effects of policy changes.
The tools are expected to include:
"prcheck"
A tool to check the consistency of routing policies
stored in the database. This tool will flag if two
neighboring network operators specify conflicting or
inconsistent routing information exchanges with each
other and also detect global inconsistencies where pos-
sible.
"prpath"
Extract all AS-paths between two networks which are
allowed by routing policy from the routing policy data-
base.
"prconn"
May 3, 1993
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Display the connectivity a given network has according
to current policies. This will of course also be able
to find the set of networks a given network can not
reach.
"prtraceroute"
A version of the existing traceroute tool which will be
able to display whether a route in use is allowed by
policy and where deviations from policy occur.
5. Proposed Database Objects
Autonomous System (AS)
An Autonomous System (AS) is a group of IP networks run by
one or more network operators which has a single and clearly
defined routing policy.
An AS has a unique number associated with it which is used
in both exchange of exterior routing information (i.e. net-
work reachability information between ASes) and as an iden-
tifier of the AS itself. Exterior routing protocols such as
BGP and EGP are used to exchange routing information between
ASes.
In routing terms an AS will normally use one or more inte-
rior gateway protocols in conjunction with some sort of com-
mon agreed metrics when exchanging network information
within its own AS.
The term AS is often confused or even misused as a con-
venient way of grouping together a set of networks which
belong under the same administrative umbrella even if within
that group of networks there are various different routing
policies. We provide the "community" concept for such use.
ASes can strictly have only one single routing policy.
The creation of an AS should be done in a conscious and well
coordinated manner to avoid creating ASes for the sake of
it, perhaps resulting in the worst case scenario of one AS
per IP network number. It should be noted that there is a
limited number of AS numbers available. This may mean that
by applying the general rules for the creation and alloca-
tion of an AS below, some re-engineering may well be needed.
However, this may be the only way to actually implement the
desired routing policy anyway. The creation and allocation
of an AS should be done with the following recommendations
in mind:
May 3, 1993
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o Creation of an AS is only required when exchanging
routing information with other ASes. Some router imple-
mentations make use of an AS number as a form of tag-
ging to identify the routing process. However, it
should be noted that this tag does not need to be
unique unless routing information is indeed exchanged
with other ASes.
o An IP network number can and must only belong to one
AS. This is a direct consequence of the fact that at
each point in the Internet there can only be exactly
one routing policy for traffic destined to each net-
work. In the case of the IP network which is used in
neighbor peering between two ASes, say at the border
between two ASes, a conscious decision must be made as
to which AS this IP network number actually resides in.
o For a simple case of customer networks connected to a
single service provider, the IP network should be a
member of the service provider's AS. In terms of rout-
ing policy the IP network has exactly the same policy
as the service provider and there is no need to make
any distinction in routing information. This idea may
at first seem slightly alien to some, but it highlights
the clear distinction in the use of the AS number as a
representation of routing policy as opposed to some
form of administrative use.
o If a network operator connects to more than one AS with
different routing policies then they need to create
their own AS. In the case of multi-homed customer net-
works connected to two service providers there are at
least two different routing policies to a given custo-
mer network. At this point the customer networks will
be part of a single AS and this AS would be distinct
from either of the service providers ASes. This allows
the customer the ability of having a different
representation of policy and preference to the dif-
ferent service providers. This is the ONLY case where a
network operator should create its own AS number.
o As a general rule one should always try to populate the
AS with as many IP networks as possible, providing all
IP networks conform to the same routing policy.
With the above rules in mind, it follows that when a network
operator changes service provider, it will move from one AS
to another. Once the database is being used as hoped for
routing then a clear procedure is needed for making sure any
May 3, 1993
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AS transition takes place in a smooth and transparent manner
without any loss of connectivity. See Appendix D for detail
of how this should be done.
Each AS is represented in the RIPE database by both an AS
object and AS tags on the network objects representing the
networks belonging to the AS. The AS object stores descrip-
tive, administrative and contact information about the AS as
well as the routing policies of the AS in relation to all
neighboring ASes.
The AS tags on the network objects define the set of net-
works belonging to an AS. Each network can have exactly one
AS tag. The AS tags can only be created and updated by the
"guardian" of the AS and not by those immediately responsi-
ble for the particular network. This ensures that operators,
especially service providers, remain in control of AS
membership. The example below shows how this would be
represented in terms of the network object. The tag "aut-
sys" is used. Refer to `ripe-50', for a complete description
of the network object in the RIPE database.
Example:
inetnum: 192.87.45.0
netname: RIPE-NCC
descr: RIPE Network Coordination Centre
descr: Amsterdam, Netherlands
country: NL
admin-c: Daniel Karrenberg
tech-c: Marten Terpstra
connect: RIPE NSF
aut-sys: 1104
rev-srv: ns.ripe.net
changed: marten@ripe.net 930121
source: RIPE
The AS object itself is used to represent a description of
administrative details and the routing policies of the AS
itself. The AS object definition is depicted as follows.
May 3, 1993
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Example:
aut-num: AS1104
descr: NIKHEF-H Autonomous system
as-in: AS1213 100 AS1213
as-in: AS1913 100 AS1913
as-in: AS1755 150 ANY
as-out: AS1213 ANY
as-out: AS1913 ANY
as-out: AS1755 AS1104 AS1913 AS1213
tech-c: Rob Blokzijl
admin-c: Eric Wassenaar
guardian: as-guardian@nikhef.nl
source: RIPE
changed: k13@nikhef.nl 920713
changed: ripe-dbm@ripe.net 920910
See Appendix A for a complete syntax definition of the
"aut-num" object.
It should be noted that this representation provides two
things:
o a set of networks.
o a description of administrative details and routing
policies.
The set of networks can be used to generate network list
based configuration information as well as configuration
information for exterior routing protocols knowing about
ASes. This means an AS can be defined and is useful even if
it does not use routing protocols which know about the AS
concept!
Community
A community is a group of networks that cannot be
represented by an AS or a group of ASes. It is in some cir-
cumstances useful to define a group of networks that have
something in common. This could be a special access policy
to a supercomputer centre, a group of networks used for a
specific mission, or a disciplinary group that is scattered
among several autonomous systems. Also these communities
could be useful to group networks for the purpose of network
statistics.
Communities do not exchange routing information, since they
do not represent an autonomous system. More specifically,
May 3, 1993
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communities do not define routing policies, but access or
usage policies. Contrary to an AS, networks can belong to
more than one community.
Communities should be defined in a strict manner, to avoid
creating as many communities as there are networks, or even
worse. Communities should be defined following the two
rules below;
o Communities must have a global meaning. Communities
that have no global meaning, are used only in a local
environment and must be avoided.
o Communities must not be defined to express non-local
policies. It should be avoided that a community is
created because some other organisation forces a policy
upon your organisation. Communities must only be
defined to express a policy defined by your organisa-
tion.
Community examples
There are some clear examples of communities:
PROVIDER -
all customers of a given service provider even though
they can have various different routing policies and
hence belong to different ASes. This would be extremely
useful for statistics collection.
HEPnet -
the High Energy Physics community partly shares infras-
tructure with other organisations, and the institutes
it consists of are scattered all over Europe, often
being part of a non HEPnet autonomous system. To allow
statistics or access policies, a community HEPnet, con-
sisting of all networks that are part of HEPnet, con-
veniently groups all these networks.
NSFNET -
the National Science Foundation Network imposes an
acceptable use policy on networks that wish to make use
of it. A community NSFNET could imply the set of net-
works that comply to this policy.
MULTI -
a large multinational corporation that does not have
May 3, 1993
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its own internal infrastructure, but connects to the
various parts of its organisations by using local ser-
vice providers that connect them all together, may
decide to define a community to restrict access to
their networks, only by networks that are part of this
community. This way a corporate network could be
defined on shared infrastructure. Also, this community
could be used by any of the service providers to do
statistics for the whole of the corporation, for
instance to do topology or bandwidth planning.
Similar to Autonomous systems, each community is represented
in the RIPE database by both a community object and commun-
ity tags on the network objects representing the networks
belonging to the community. The community object stores
descriptive, administrative and contact information about
the community.
The community tags on the network objects define the set of
networks belonging to a community. A network can have multi-
ple community tags. The community tags can only be created
and updated by the "guardian" of the community and not by
those directly responsible for the particular network. This
ensures that guardians remain in control of community
membership.
Here's an example of how this might be represented in terms
of the community tags within the network object. We have an
example where the network 192.16.199.0 has a single routing
policy (i.e. that of AS 1104), but is part of several dif-
ferent communities of interest. We use the tag "comm-list"
to represent the list of communities associated with this
network. NIKHEF-H uses the service provider SURFnet (a ser-
vice provider with customers with more than one routing pol-
icy), is also part of the High Energy Physics community as
well as having the ability to access the Supercomputer at
CERN (2).
_________________________
(2) The community `CERN-SUPER', is somewhat notional,
but is intended as an example of a possible use of an
access policy constraint.
May 3, 1993
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Example:
inetnum: 192.16.199.0
netname: HEFNET
descr: Local Ethernet
descr: NIKHEF section H
descr: Science Park Watergraafsmeer
descr: Amsterdam
descr: The Netherlands
country: NL
admin-c: Eric Wassenaar
tech-c: Rob Blokzijl
aut-sys: 1104
comm-list: HEPNET CERN-SUPER SURFNET
gateway: nih
changed: ripe-dbm@ripe.net 920604
source: RIPE
In the above examples some communities have been defined.
The community object itself will take the following format:
Example:
community: SURFnet
descr: Dutch academic research network
authority: SURFnet B.V.
guardian: comm-guardian@surfnet.nl
admin-c: Erik-Jan Bos
tech-c: Erik-Jan Bos
changed: ripe-dbm@ripe.net 920604
source: RIPE
For a complete explanation of the syntax please refer to
Appendix B.
May 3, 1993
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6. Representation of Routing Policies
Routing policies of an AS are represented in the autonomous
system object. Initially we show some examples, so the
reader is familiar with the concept of how routing informa-
tion is represented, used and derived. Refer to Appendix A,
for the full syntax of the "aut-num" object.
The topology of routing exchanges is represented by listing
how routing information is exchanged with each neighboring
AS. This is done separately for both incoming and outgoing
routing information. In order to provide backup and back
door paths a relative cost is associated with incoming rout-
ing information.
Example 1:
AS1------AS2
This specifies a simple routing exchange of two presumably
isolated ASes. Even if either of them has routing informa-
tion about networks in ASes other than AS1 and AS2, none of
that will be announced to the other.
aut-num: AS1
as-out: AS2 AS1 # announce all networks in AS1 to AS2
as-in: AS2 100 AS2 # accept all AS2 networks from AS2
aut-num: AS2
as-out: AS1 AS2 # announce all networks in AS2 to AS1
as-in: AS1 100 AS1 # accept all AS1 networks from AS1
The number 100 in the inbound specifications is a relative
cost, which is used for backup and back door routes. The
absolute value is of no significance. The relation between
different values within the same AS object is. A lower
value means a lower cost. This is consciously similar to the
cost based preference scheme used with DNS MX RRs.
Example 2:
Now suppose that AS2 is connected to one more AS, besides
AS1, and let's call that AS3:
AS1------AS2------AS3
May 3, 1993
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In this case there are two reasonable routing policies:
a) AS2 just wants to exchange traffic with both AS1 and
AS3 itself without passing traffic between AS1 and AS3.
b) AS2 is willing to pass traffic between AS3 and AS1,
thus acting as a transit AS
Example 2a:
In the first case AS1's representation in the database will
remain unchanged as will be the part of AS2's representation
describing the routing exchange with AS1. A description of
the additional routing exchange with AS3 will be added to
AS2's representation:
aut-num: AS1
as-out: AS2 AS1 # announce all networks in AS1 to AS2
as-in: AS2 100 AS2 # accept all AS2 networks from AS2
aut-num: AS2
as-out: AS1 AS2 # announce only networks in AS2 to AS1
as-in: AS1 100 AS1 # accept all AS1 networks from AS1
as-out: AS3 AS2 # announce only networks in AS2 to AS3
as-in: AS3 100 AS3 # accept all AS3 networks from AS3
aut-num: AS3
as-out: AS2 AS3 # announce all networks in AS3 to AS2
as-in: AS2 100 AS2 # accept all AS2 networks from AS2
Note that in this example, AS2 keeps full control over its
resources. Even if AS3 and AS1 were to allow each other's
routes in from AS2, the routing information would not flow
because AS2 is not announcing it (3).
Example 2b:
If contrary to the previous case, AS1 and AS3 are supposed
to have connectivity to each other via AS2, all AS objects
have to change:
_________________________
(3) Of course AS1 and AS3 could just send traffic to
each other to AS2 even without AS2 announcing the net-
works, hoping that AS2 will forward it correctly. Such
questionable practises however are beyond the scope of
this document.
May 3, 1993
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aut-num: AS1
as-out: AS2 AS1 # announce all networks in AS1 to AS2
as-in: AS2 100 AS2 AS3 # accept AS2 and AS3 networks from AS2
aut-num: AS2
as-out: AS1 AS2 AS3 # announce all networks in AS2 and AS3 to AS1
as-in: AS1 100 AS1 # accept all AS1 networks from AS1
as-out: AS3 AS2 AS1
as-in: AS3 100 AS3
aut-num: AS3
as-out: AS2 AS3 # announce all networks in AS3 to AS2
as-in: AS2 100 AS1 AS2 # accept AS2 and AS1 networks from AS2
Note that the amount of routing information exchanged with a
neighbor AS is defined in terms of networks belonging to
ASes. In BGP terms this is the AS where the routing infor-
mation originates and the originating AS information carried
in BGP could be used to implement the desired policy. How-
ever, using BGP or the BGP AS-path information is not
required to implement the policies thus specified. Confi-
gurations based on network lists can easily be generated
from the database. The AS path information, provided by BGP
can then be used as an additional checking tool as desired.
The specification understands one special expression and
this can be expressed as a boolean expressions:
ANY - means any routing information known. For output this
means that all networks an AS knows about are
announced. For input it means that anything is accepted
from the neighbor AS.
May 3, 1993
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Example 3:
AS4 is a stub customer AS, which only talks to service pro-
vider AS123.
|
|
-----AS123------AS4
|
|
aut-num: AS4 # representation of AS4 in the database
as-out: AS123 AS4 # announce only networks in AS4 to AS123
as-in: AS123 100 ANY # accept any announcement from AS123
aut-num: AS123 # representation of the service provider AS123
as-in: AS4 100 AS4 # accept routes for customer AS4's networks
as-out: AS4 ANY # provide all routes to customer AS4
<further neighbors>
Since AS4 has no other way to reach the outside world than
AS123 it is not strictly necessary for AS123 to send routing
information to AS4. AS4 can simply send all traffic for
which it has no explicit routing information to AS123 by
default. This strategy is called default routing. It is
expressed in the database by adding one or more default tags
to the autonomous system which uses this strategy. In the
example above this would look like:
aut-num: AS4 # representation of AS4 in the database
as-out: AS123 AS4 # announce only networks in AS4 to AS123
default: AS123 100 # send default traffic to AS123
aut-num: AS123 # representation of the service provider AS123
as-in: AS4 100 AS4 # accept routes for customer AS4's networks
# no announcements to AS4, AS4 defaults to us
<further neighbors>
May 3, 1993
- 20 -
Example 4:
AS4 now connects to a different operator, AS5. AS5 uses
AS123 for outside connectivity but has itself no direct con-
nection to AS123. AS5 traffic to and from AS123 thus has to
pass AS4. AS4 agrees to act as a transit AS for this
traffic.
|
|
-----AS123------AS4-------AS5
|
|
aut-num: AS4 # representation of AS4 in the database
as-out: AS123 AS4 AS5 # own and AS5 networks to AS123
as-in: AS123 100 ANY # accept any announcement from AS123
as-out: AS5 ANY # send all announcements through to AS5
as-in: AS5 50 AS5 # accept only AS5 announcements from them
aut-num: AS5
as-in: AS4 100 ANY
as-out: AS4 AS5
aut-num: AS123 # representation of the service provider AS123
as-in: AS4 100 AS4 AS5 # accept AS4's own and AS5 from AS4
as-out: AS4 ANY # all to customer AS4 and via it to AS5
<further neighbors>
Now AS4 has two sources of external routing information. AS5
which provides only information about its own networks and
AS123 which provides information about the external world.
Note that AS4 accepts information about AS5 from both AS123
and AS5 although AS5 information cannot come from AS123
since AS5 is connected only via AS4 itself. The lower cost
of 50 for the announcement from AS5 itself compared to 100
from AS123 ensures that AS5 is still believed even in case
AS123 will unexpectedly announce AS5.
In this example too, default routing can be used by AS5 much
like in the previous example. AS4 can also use default rout-
ing towards AS123:
May 3, 1993
- 21 -
aut-num: AS4 # representation of AS4 in the database
as-out: AS123 AS4 AS5 # own and AS5 networks to AS123
default: AS123 11 # default to AS123
as-in: AS5 50 AS5 # accept only AS5 announcements from them
# no announcements to AS5, they default to us
aut-num: AS5
as-out: AS4 AS5
default: AS4 100
aut-num: AS123 # representation of the service provider AS123
as-in: AS4 100 AS4 AS5 # accept AS4's own and AS5 from AS4
# no announcements to AS4, they default to us
<further neighbors>
Note that the relative cost associated with default routing
is totally separate from the relative cost associated with
inbound announcements. The default route will never be taken
if an explicit route is known to the destination. Thus an
explicit route can never have a higher cost than the default
route. The relative cost associated with the default route
is only useful in those cases where one wants to configure
multiple default routes for redundancy.
Note also that in this example the configuration using
default routes has a subtly different behaviour than the one
with explicit routes: In case the AS4-AS5 link fails AS4
will send traffic to AS5 to AS123 when using the default
configuration. Normally this makes not much difference as
there will be no answer and thus little traffic. With cer-
tain datagram applications which do not require acknowledge-
ments however, significant amounts of traffic may be use-
lessly directed at AS123. Similarly default routing should
not be used if there are stringent security policies which
proscribe any traffic intended for AS5 to ever touch AS123.
Generally it can be said that default routing should only be
used in very simple topologies. Once the situation gets more
complex using default routes can lead to unexpected results
or even defeat the routing policies established when links
fail. As an example consider how Example 5a) below could be
implemented using default routing.
May 3, 1993
- 22 -
Example 5:
In a different example AS4 has a private connection to AS6
which in turn is connected to the service provider AS123:
|
|
-----AS123------AS4
| |
| |
| |
AS6 ---------+
There are a number of policies worth examining in this case:
a) AS4 and AS6 wish to exchange traffic between themselves
exclusively via the private link between themselves;
such traffic should never pass through the backbone
(AS123). The link should never be used for transit
traffic, i.e. traffic not both originating in and des-
tined for AS4 and AS6.
b) AS4 and AS6 wish to exchange traffic between themselves
via the private link between themselves. Should the
link fail, traffic between AS4 and AS6 should be routed
via AS123. The link should never be used for transit
traffic.
c) AS4 and AS6 wish to exchange traffic between themselves
via the private link between themselves. Should the
link fail, traffic between AS4 and AS6 should be routed
via AS123. Should the connection between AS4 and AS123
fail, traffic from AS4 to destinations behind AS123 can
pass through the private link and AS6's connection to
AS123.
d) AS4 and AS6 wish to exchange traffic between themselves
via the private link between themselves. Should the
link fail, traffic between AS4 and AS6 should be routed
via AS123. Should the backbone connection of either
AS4 or AS6 fail, the traffic of the disconnected AS
should flow via the other AS's backbone connection.
May 3, 1993
- 23 -
Example 5a:
aut-num: AS4
as-in: AS123 100 NOT AS6 # accept anything besides AS6
as-out: AS123 AS4 # announce only ourselves to AS123
as-in: AS6 50 AS6 # accept only AS6 from AS6
as-out: AS6 AS4 # announce only own nets to AS6
aut-num: AS123
as-in: AS4 100 AS4
as-out: AS4 ANY
as-in: AS6 100 AS6
as-out: AS6 ANY
<further neighbors>
aut-num: AS6
as-in: AS123 100 NOT AS4
as-out: AS123 AS6
as-in: AS4 50 AS4
as-out: AS4 AS6
Note that here the configuration is slightly inconsistent.
AS123 will announce AS6 to AS4 and AS4 to AS6. These
announcements will be filtered out on the receiving end.
This will implement the desired policy. Consistency check-
ing tools might flag these cases however.
May 3, 1993
- 24 -
Example 5b:
aut-num: AS4
as-in: AS123 100 ANY # accept anything from AS123
as-out: AS123 AS4 # announce only ourselves to AS123
as-in: AS6 50 AS6 # accept AS6 only from AS6
as-out: AS6 AS4 # announce only own nets to AS6
aut-num: AS123
as-in: AS4 100 AS4
as-out: AS4 ANY
as-in: AS6 100 AS6
as-out: AS6 ANY
<further neighbors>
aut-num: AS6
as-in: AS123 100 ANY
as-out: AS123 AS6
as-in: AS4 50 AS4
as-out: AS4 AS6
The thing to note here is that in the ideal operational
case, `all links working' AS4 will receive announcements for
AS6 from both AS123 and AS6 itself. In this case the
announcement from AS6 will be preferred because of its lower
cost and thus the private link will be used as desired. AS6
is configured as a mirror image.
May 3, 1993
- 25 -
Example 5c:
The new feature here is that should the connection between
AS4 and AS123 fail, traffic from AS4 to destinations behind
AS123 can pass through the private link and AS6's connection
to AS123.
aut-num: AS4
as-in: AS123 100 ANY # expect anything from AS123
as-out: AS123 AS4 # announce only ourselves to AS123
as-in: AS6 50 AS6 # accept AS6 only from AS6
as-in: AS6 110 ANY # accept anything from AS6 (worse than AS123)
as-out: AS6 AS4 # announce only own nets to AS6
aut-num: AS123
as-in: AS4 1 AS4
as-out: AS4 ANY
as-in: AS6 1 AS6
as-in: AS6 2 AS4 # now also accept AS4 from AS6 in case backup
as-out: AS6 ANY
<further neighbors>
aut-num: AS6
as-in: AS123 100 ANY
as-out: AS123 AS6 AS4 # for backup also pass AS4 to AS123
as-in: AS4 50 AS4
as-out: AS4 ANY # for backup export all we know to AS4
Note that it is important to make sure to propagate routing
information for both directions in backup situations like
this. Connectivity in just one direction is not useful at
all for almost all applications.
Note also that in case the AS6-AS123 connection breaks, AS6
will only be able to talk to AS4. The symmetrical case (5d)
is left as an exercise to the reader.
May 3, 1993
- 26 -
7. RIPE Database Update Procedure
This section, describes a procedure for updating the routing
attributes in the RIPE database as mentioned in sections 5
and 6. These routing tags are essential for the routing of
networks that are known in the RIPE database. Therefore
updates of these attributes must be:
o Properly authorised
o Guarded against typing errors
o Efficient for both maintainers of the attributes and
the maintainers of the whole database
The procedure
For each of the AS and Community tags, a list of all net-
works having this attribute is kept separately from the gen-
eral database itself. These lists will be the `only' source
of AS and community membership information used in the data-
base. Normal database updates `never' change these attri-
butes. If an update includes such an attribute and a
discrepancy between the values in the update and those in
the database is found, a diagnostic message will be sent to
the originator of the update. The attributes as defined in
the files are incorporated in the database at each normal
update run. To ensure proper control and authorisation, we
propose to maintain the lists on the central NCC database
server, where each of the guardians of a tag keeps track of
his or her own list. The lists are maintained through indi-
vidual logins for each of the guardians. The guardians can
themselves decide in what manner they want to update their
list. The NCC will offer interactive logins, ftp logins or
any other means that might be deemed useful.
File Format
We propose to keep the file format as simple as possible.
The name of the file should indicate the name of the attri-
butes. This means that there cannot be two attributes with
the same name. The format within the file we propose as the
"inetnum:" entry for each of the networks, separated by an
empty line. If a guardian feels that he would want more
fields to identify a network that will get his attribute, he
is free to do so. An attribute will only be added to a net-
work if all fields defined in the list match a network in
May 3, 1993
- 27 -
the database.
Advantages
The update procedure as proposed above has the following
advantages:
o Authorisation of adding/deleting is guaranteed.
o No need for mailing back and forth of authorisation
messages.
o Simple procedure for both database maintainers and
guardians.
o Guardians keep full control of their attribute.
o Could be implemented at the NCC without too much delay.
8. Summary
In summary this paper basically introduces two new RIPE
database objects, the autonomous system - "aut-num" and the
community - "community" object. It attempts to clarify terms
in such a way as to make them easier and more usable for
network operators. It provides a mechanism to easily and
clearly express routing policy and provide a mechanism to
create, maintain and administer network based filter lists
for routing policy. Through its combination of tutorial and
outlined proposal style we have hopefully managed to satisfy
the requirements as set out in Section 3. The paper also
keeps in mind the original need for generalisation whilst at
the same time catering somewhat for the growing deployment
of newer more advanced routing protocols and their use.
9. Glossary
It is felt useful to include a basic glossary of some of the
terms used within this paper to act as both a reference and
as possible clarification tool.
- Announce
In terms of this paper, to announce means to convey
reachability information for a given IP network. This
can be done using a variety of routing techniques. For
example, by use of redistribution of routes into a
dynamic routing protocol. Equally this can be done by
May 3, 1993
- 28 -
installing a static route which effectively announces
the route to ones own router in the routing context.
This then can also re-distributed if so desired.
- Autonomous System (AS)
A group of IP networks run by one or more network
operators having a single clearly defined routing pol-
icy.
- BGP-3/BGP-4
BGP is the Border Gateway Protocol, BGP-3 is the
current Internet standard with BGP-4 as the proposed
standard. This is an exterior based IP routing protocol
with some interior based routing properties. It has the
ability to carry AS-Path information along with routing
reachability information. For further information,
refer to the following papers;
BGP-3 o "A Border Gateway Protocol 3 (BGP-3)", RFC
1276. K. Lougheed, Y. Rekhter, October 1991.
BGP-4 o "A Border Gateway Protocol 4 (BGP-4)", Work in
Progress. Y. Rekhter, T.Li, December 1992.
- Network
A layer 3 entity, not any form of organisation. For all
intent and purposes, the physical network represented
by a unique IP network number.
- Network Operator
An organisation which operates networks. A network
operator can be a customer of another service provider
as well as being a service provider to customers
itself.
- RIPE database
A European based database containing information on IP
networks, persons, namespace administration data and
routing policies.
- Service Provider
A network operator who provides Internet services to
different organisations.
May 3, 1993
- 29 -
10. Author's Addresses
Tony Bates
RIPE Network Coordination Centre
Kruislaan 409
NL-1098 SJ Amsterdam
The Netherlands
+31 20 592 5065
T.Bates@ripe.net
Jean-Michel Jouanigot
CERN, European Laboratory for Particle Physics
CN division
CH-1211 Geneva 23
Switzerland
+41 22 767 4417
jimi@dxcoms.cern.ch
Daniel Karrenberg
RIPE Network Coordination Centre
Kruislaan 409
NL-1098 SJ Amsterdam
The Netherlands
+31 20 592 5065
D.Karrenberg@ripe.net
Peter Lothberg
STUPI
Box 9129
102 72 Stockholm
Sweden
+46 8 6699720
roll@stupi.se
Marten Terpstra
RIPE Network Coordination Centre
Kruislaan 409
NL-1098 SJ Amsterdam
The Netherlands
+31 20 592 5065
M.Terpstra@ripe.net
May 3, 1993
- 30 -
Appendix A - Syntax for the "aut-num" object.
Here is a summary of the tags associated with aut-num object
itself:
aut-num: [mandatory]
descr: [mandatory] [multiple]
as-in: [optional] [multiple]
as-out: [optional] [multiple]
default: [optional] [multiple]
tech-c: [mandatory] [multiple]
admin-c: [mandatory] [multiple]
guardian: [mandatory]
remarks: [optional] [multiple]
changed: [mandatory] [multiple]
source: [mandatory] [RIPE]
Each item has the following syntax:
aut-num:
Autonomous system number.
This must be a uniquely allocated autonomous system number
from an AS registry (i.e. the RIPE NCC, the Inter-NIC, etc).
Format: AS<positive integer between 1 and 65535>
Example:
aut-num: AS1104
Status: mandatory
descr:
A short description of the Autonomous system.
Format: free text, one line per entry, multiple lines in sequence
Example:
descr: NIKHEF section H
descr: Science Park Watergraafsmeer
descr: Amsterdam
Stautus: mandatory
as-in:
A description of accepted routing information between AS peers.
Format: <aut-num> <cost> <routing policy expression>,
on multiple lines.
<aut-num> refers to your AS neighbor.
<cost> is a positive integer used to express a relative
May 3, 1993
- 31 -
cost of routes learned. The lower the cost the more
preferred the route.
<routing policy expression> can take the following formats.
1. A list of one or more ASes.
Example:
as-in: AS1103 100 AS1103
as-in: AS786 105 AS1103
as-in: AS786 10 AS786
as-in: AS1755 110 AS1103 AS786
2. A set of KEYWORDS.
The following keywords are currently defined.
ANY - this means anything the neighbor AS knows.
RIPE-DB - any network currently in the RIPE database.
LOCAL - any network in the RIPE database which is part of the
community LOCAL (i.e. no connectivity outside its own
organisation).
3. A logical expression of either 1 or 2 above
The current logical operators are defined as:
AND
OR
NOT
Rules are grouped together using parenthesis i.e "(" and ")".
Example:
as-in: AS1755 100 RIPE-DB AND NOT (LOCAL AS1234 AS513)
as-in: AS1755 150 DEFAULT
Stautus: optional
as-out:
A description of generated routing information sent to other AS peers.
Format: <aut-num> <routing policy expression>, multiple lines.
<aut-num> refers to your AS neighbor.
<routing policy expression> is explained in the as-in
definition above.
Example:
as-out: AS1104 AS978
as-out: AS1755 ANY
as-out: AS786 RIPE-DB AND NOT (AS978 LOCAL)
Stautus: optional
May 3, 1993
- 32 -
default:
An indication of how default routing is done.
Format: <aut-num> <relative cost>, multiple lines.
<aut-num> The AS peer you will default route to.
<relative cost> The relative cost is a positive integer use to
express a preference for default. There is no direct relationship
to to the cost used in the "as-in" tag. The lower the cost
indicates which AS peer is chosen for default.
Example:
default: AS1755 10
default: AS786 5
Stautus: optional
tech-c:
Name or NIC-handle of technical contact person.
This is someone to be contacted for technical problems
such as misconfiguration, etc.
Format: <firstname> <initials> <lastname>, multiple lines.
Example:
tech-c: John E. Doe
Status: mandatory
admin-c:
Name or NIC-handle of administrative contact person. This is probably
the name of the guardian but it is not mandatory to be.
Format: <firstname> <initials> <lastname>, multiple lines.
Example:
admin-c: Joe T. Bloggs
Status: mandatory
guardian:
Malibox of the guardian of the Autonomous system.
Format: <email-address>, single line.
<email-address> This should be in RFC822 format wherever possible.
Example:
guardian: as1104-guardian@nikhef.nl
Status: mandatory
source: RIPE
Source of the information.
May 3, 1993
- 33 -
This is used to separate information from different sources
kept by the same database software.
Format: RIPE
Status: mandatory
changed:
Who and when changed this last.
Format: <email-address> YYMMDD
Example:
changed: johndoe@terabit-labs.nn 900401
Status: mandatory
remark:
Remarks/comments
Format: text string, multiple lines
Example:
remark: Multihomed AS talking to AS 1755 and AS786
remark: Will soon connect to AS 1104 also.
Status: optional
May 3, 1993
- 34 -
Appendix B - Syntax details for the "community" object.
Here is a summary of the tags associated with "community"
object itself:
community: [mandatory]
descr: [mandatory] [multiple]
authority: [mandatory]
guardian: [mandatory]
tech-c: [mandatory] [multiple]
admin-c: [mandatory] [multiple]
remarks: [optional] [multiple]
changed: [mandatory] [multiple]
source: [mandatory] [RIPE]
Each item has the following syntax:
community:
Name of community. Should be as representative of the community as
possible rather than being something obscure.
Format: Text string which cannot start with "AS" or any of the <routing
policy expression> KEYWORDS. See Appendix A.
Example:
community: WCW
Stautus: mandatory
descr:
A short description of the community represented.
Format: free text, one line per entry, multiple lines in sequence
Example:
descr: Science Park Watergraafsmeer
descr: Amsterdam
Stautus: mandatory
authority:
The formal authority for this community. This could be an
organisation, institute, etc.
Format: text
Example:
authority: WCW LAN committee
Stautus: mandatory
May 3, 1993
- 35 -
guardian:
Malibox of the guardian of the community.
Format: <email-address>, single line.
Example:
guardian: wcw-guardian@nikhef.nl
Status: mandatory
tech-c:
Name or NIC-handle of technical contact person.
Format: <firstname> <initials> <lastname>, multiple lines.
Example:
tech-c: John E. Doe
Status: mandatory
admin-c:
Name or NIC-handle of administrative contact person.
Format: <firstname> <initials> <lastname>, multiple lines.
Example:
admin-c: Joe T. Bloggs
Status: mandatory
source: RIPE
Source of the information.
This is used to separate information from different sources
kept by the same database software.
Format: RIPE
Status: mandatory
changed:
Who and when changed this last.
Format: <email-address> YYMMDD
Example:
changed: johndoe@terabit-labs.nn 900401
Status: mandatory
remark:
Remarks/comments
Format: text string, multiple lines
Example:
remark: This is a group of institutes at WCW.
May 3, 1993
- 36 -
Status: optional
May 3, 1993
- 37 -
Appendix C - The idea of Autonomous System Macros.
AS Macros
For an AS that doesn't use default routing it may be diffi-
cult to keep track of each and every new AS that is
represented in the database. A convenient way around this is
to define an `AS Macro' which essentially is a convenient
way to group or aggregate ASes. This would be done so that
each and every AS guardian does not have to add a new AS to
it's as-in or as-out tags for it's AS object.
However, it should be noted that this creates an implicit
trust on the guardian of the AS-Macro.
The proposed idea would be to allow an AS-Macro as part of
the <routing policy expression> for the "as-in" and "as-out"
tags. The object could then be used as a list or group of
ASes. So for example we could create an AS object as fol-
lows:
Example:
aut-num: AS786
as-in: AS1755 100 AS-EBONE AND NOT AS1104
as-out AS1755 AS786
Here is a summary of the proposed tags associated with the
as-macro object itself:
as-macro: [mandatory]
descr: [mandatory]
as-list: [mandatory] [multiple]
guardian: [mandatory]
tech-c: [mandatory] [multiple]
admin-c: [mandatory] [multiple]
remarks: [optional] [multiple]
changed: [mandatory] [multiple]
source: [mandatory] [RIPE]
Each item has the following syntax:
as-macro:
A macro containing at least two Autonomous Systems grouped together for
ease of administration.
May 3, 1993
- 38 -
Format: AS-<String>, The string should be in upper case and not contain any
special characters.
Example:
as-macro: AS-EBONE
Status: mandatory
as-list:
A list of ASes depicted by the macro.
Format: <aut-num> <aut-num> ...., mulitple. See Appendix A for <aut-num>
definition.
Example:
as-list: AS786 AS513 AS1104
Status: mandatory
descr:
A short description of what the AS-macro represents.
Format: free text, one line per entry, multiple lines in sequence
Example:
descr: Ebone AS macro
descr: Details all ASes that make use of Ebone as a transit carrier
Stautus: mandatory
guardian:
Malibox of the guardian of the Autonomous system macro.
Format: <email-address>, single line.
Example:
guardian: as-ebone-guardian@ebone.net
Status: mandatory
tech-c:
Name of technical contact person.
Format: <firstname> <initials> <lastname>, multiple lines.
Example:
tech-c: John E. Doe
Status: mandatory
admin-c:
Name of administrative contact person.
Format: <firstname> <initials> <lastname>, multiple lines.
Example:
admin-c: Joe T. Bloggs
Status: mandatory
May 3, 1993
- 39 -
source: RIPE
Source of the information.
This is used to separate information from different sources
kept by the same database software.
Format: RIPE
Status: mandatory
changed:
Who and when changed this last.
Format: <email-address> YYMMDD
Example:
changed: johndoe@terabit-labs.nn 900401
Status: mandatory
remark:
Remarks/comments
Format: text string, multiple lines
Example:
remark: Pan-European backbone connecting several
remark: Regional Boundary Systems within Europe.
Status: optional
May 3, 1993
- 40 -
Appendix D - Transitioning from one AS to another.
As mentioned in Section 5, when changing service provider
there will be a need for the network operator to move from
one AS to another. Assuming that network operators use the
RIPE database as proposed to maintain the needed routing
filter lists, then there may be a synchronisation problem
with the database image in use by the provider. The current
frequency for database updates is once a day so if we look
at this in terms of a transition, a clearly defined pro-
cedure is needed to make sure there is no loss of connec-
tivity.
If we have the following scenario:
|
__|___ ______
+-------+ ( ) ( )
| NET X |-------------( AS A )----------( AS C )----
+-------+ (______) (______)
|
|
______ ___|__
( ) ( )
( AS B )----------( AS D )----
(______) (______)
|
A network or even a group of networks, represented by NET X
are connected to a single service provider, here shown as AS
A. NET X has the same routing policy as AS A and as such has
its network as-tag in the database as AS A. AS C, AS D and
AS B for that matter all apply their own routing policies.
Exactly what those polices are is not relevant for this dis-
cussion. However, when routing announcements from AS A
regarding NET X are sent to AS C and beyond it is clear by
using the database and the tools provided that NET X is part
of AS A. The ASes will expect to see NET X as part of AS A's
list of registered networks.
For whatever reason, the network operator of NET X decides
to change its service provider from AS A to AS B. So the
following transition takes place;
May 3, 1993
- 41 -
|
__|___ ______
+-------+ ( ) ( )
| NET X |- - - - - - -( AS A )----------( AS C )----
+-------+ (______) (______)
| |
| |
| ______ ___|__
| ( ) ( )
+-----------------( AS B )----------( AS D )----
(______) (______)
|
NET X now connects to AS B. As soon as this is done this
now means that NET X is announced as part of AS B as it now
has the same routing policy as AS B. This is fine for AS B
as it will presumably update the routing filter list because
NET X is now a customer network. The problem is however, if
AS D does not also update it routing filter list at the same
time then it will not expect NET X announcements to ori-
ginate from AS B. It will still think NET X will be part of
AS A lists of networks and hence reject this routing infor-
mation. This will be so until it has synchronised with the
latest RIPE database. Even though the database update pro-
cedure is done on a scheduled basis this is obviously unac-
ceptable. However, in reality this problem is not at bad as
it may first seem. There is a simple solution to this and it
is one that in practice would happen anyway. In the diagram
above, the connection from NET X to AS A is shown as a dot-
ted line. This is done intentionally to denote that most
network operators would always factor in a small amount of
transition time when changing service providers. With this
assumption in mind (and the authors feel this is a fair
one), then all that needs to be done whilst in transition is
that AS A must continue to announce NET X to it's neighbors
whilst AS B is also announcing NET X derived from the new
connection. As the various ASes re-align with the RIPE
database there would be a smooth transition in both routing
and traffic from provider AS A to provider AS B. Obviously
for this to take place, a small amount of coordination is
needed between providers AS A and AS B but this is not an
unreasonable expectation of any of todays service providers.
As the database is updated at least once every 24 hours then
it should not take more than two days for most network
operators to have aligned and all traffic to flow via the
new connection. At this point AS A can stop its announce-
ments of NET X and drop the connection as desired. So we
finish up with this as a final configuration with of course
no loss in connectivity for NET X;
May 3, 1993
- 42 -
|
__|___ ______
+-------+ ( ) ( )
| NET X | ( AS A )----------( AS C )----
+-------+ (______) (______)
| |
| |
| ______ ___|__
| ( ) ( )
+-----------------( AS B )----------( AS D )----
(______) (______)
|
This simple procedure is what could be called a "Basic Pro-
vider Transition" and really should be followed by any con-
scientious service provider.
May 3, 1993
- 43 -
Appendix E - Routing Policy limitations
The generic example of a reasonable but un-implementable
routing policy might be called "once joined means joined
forever". Once traffic for the same destination network
passes the same router, or the same AS at our level of
abstraction, it will take exactly the same route to the des-
tination (4).
In a concrete example AS Z might be connected to the outside
world by two links. AS Z wishes to reserve these links for
different kinds of traffic, let's call them black and white
traffic. For this purpose the management of AS Z keeps two
lists of ASes, the black and the white list. Together these
lists comprise all ASes in the world reachable from AS Z.
"W"
<--->
... AS Z .... NET 3
<--->
"B"
It is quite possible to implement the policy for traffic
originating in AS Z: AS Z will only accept announcements for
networks in white ASes on the white link and will only
accept announcements for networks in black ASes on the black
link. This causes traffic from networks within AS Z towards
white ASes to use the white link and likewise traffic for
black ASes to use the black link.
Note that this way of implementing things makes it necessary
to decide on the colour of each new AS which appears before
traffic can be sent to it from AS Z. A way around this would
be to accept only white announcements via the white link and
to accept all but white announcements on the black link.
That way traffic from new ASes would automatically be sent
down the black link and AS Z management would only need to
keep the list of white ASes rather than two lists.
Now for the unimplementable part of the policy. This con-
cerns traffic towards AS Z. Consider the following topol-
ogy:
B AS ---) "W"
W AS ---) --->
B AS ---)>> AS A ---> ... AS Z .... NET 3
B AS ---) --->
W AS ---) "B"
_________________________
(4) Disregarding special cases like "type of service"
routing, load sharing and routing instabilities.
May 3, 1993
- 44 -
As seen from AS Z there are both black and white ASes
"behind" AS A. Since ASes can make routing decisions based
on destination only, AS A and all ASes between AS A and the
two links connecting AS Z can only make the same decision
for traffic directed at a network in AS Z, say NET 3. This
means that traffic from both black and white ASes towards
NET 3 will follow the same route once it passes through AS
A. This will either be the black or the white route depend-
ing on the routing policies of AS A and all ASes between it
and AS Z.
The important thing to note is that unless routing and for-
warding decisions can be made based on both source and des-
tination addresses, policies like the "black and white"
example cannot be implemented in general because "once
joined means joined forever".
May 3, 1993