Network Working Group T. Bates
Request for Comments: 1786 MCI Telecommunications Corporation
Category: Informational E. Gerich
Merit, Inc.
L. Joncheray
Merit, Inc.
J-M. Jouanigot
CERN
D. Karrenberg
RIPE NCC
M. Terpstra
Bay Networks, Inc.
J. Yu
Merit, Inc.
March 1995
Representation of IP Routing Policies
in a Routing Registry
(ripe-81++)
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This document was originally published as a RIPE document known as
ripe-181 but is also being published as an Informational RFC to reach
a larger audience than its original scope. It has received community
wide interest and acknowledgment throughout the Internet service
provider community and will be used as the basic starting point for
future work on Internet Routing Registries and routing policy
representation. It can also be referred to as ripe-81++. This
document is an update to the original `ripe-81'[1] proposal for
representing and storing routing polices within the RIPE database. It
incorporates several extensions proposed by Merit Inc.[2] and gives
details of a generalized IP routing policy representation to be used
by all Internet routing registries. It acts as both tutorial and
provides details of database objects and attributes that use and make
up a routing registry.
Bates, et al. [Page 1]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Table of Contents
1. Introduction ................................................ 3
2. Organization of this Document ............................... 3
3. General Representation of Policy Information ............... 5
4. The Routing Registry and the RIPE Database .................. 11
5. The Route Object ............................................ 16
6. The Autonomous System Object ................................ 26
7. AS Macros ................................................... 36
8. The Community Object ........................................ 38
9. Representation of Routing Policies .......................... 41
10. Future Extensions .......................................... 50
11. References ................................................. 51
12. Security Considerations .................................... 52
13. Authors' Addresses ......................................... 53
Appendix A - Syntax for the "aut-num" object ................... 55
Appendix B - Syntax for the "community" object ................. 68
Appendix C - Syntax for the "as-macro" object .................. 72
Appendix D - Syntax for the "route" object ..................... 76
Appendix E - List of reserved words ............................ 80
Appendix F - Motivations for RIPE-81++ ......................... 81
Appendix G - Transition strategy ............................... 83
Bates, et al. [Page 2]
RFC 1786 Representing IP Routing Policies in a RR March 1995
1. Introduction
This document is a much revised version of the RIPE routing registry
document known as ripe-81 [1]. Since its inception in February, 1993
and the establishment of the RIPE routing registry, several additions
and clarifications have come to light which can be better presented
in a single updated document rather than separate addenda.
Some of the text remains the same the as the original ripe-81
document keeping its tutorial style mixed with details of the RIPE
database objects relating to routing policy representation. However
this document does not repeat the background and historical remarks
in ripe-81. For these please refer to the original document. It
should be noted that whilst this document specifically references the
RIPE database and the RIPE routing registry one can easily read
"Regional routing registry" in place of RIPE as this representation
is certainly general and flexible enough to be used outside of the
RIPE community incorporating many ideas and features from other
routing registries in this update.
This document was originally published as a RIPE document known as
ripe-181 but is also being published as an Informational RFC to reach
a larger audience than its original scope. It has received large
interest and acknowledgment within the Internet service provider
community and will be used as the basic starting point for future
work on Internet Routing Registries and routing policy
representation. It but can also be referred to as ripe-81++.
We would like to acknowledge many people for help with this document.
Specifically, Peter Lothberg who was a co-author of the original
ripe-81 document for his many ideas as well as Gilles Farrache,
Harvard Eidnes, Dale Johnson, Kannan Varadhan and Cengiz Alaettinoglu
who all provided valuable input. We would also like to thank the
RIPE routing working group for their review and comment. Finally, we
like to thank Merit Inc. for many constructive comments and ideas and
making the routing registry a worldwide Internet service. We would
also like to acknowledge the funding provided by the PRIDE project
run in conjunction with the RARE Technical Program, RIPE and the RIPE
NCC without which this paper would not have been possible.
2. Organization of this Document
This document acts as both a basic tutorial for understanding routing
policy and provides details of objects and attributes used within an
Internet routing registry to store routing policies. Section 3
describes general issues about IP routing policies and their
representation in routing registries. Experienced readers may wish to
skip this section. Section 4 provides an overview of the RIPE
Bates, et al. [Page 3]
RFC 1786 Representing IP Routing Policies in a RR March 1995
database, its basic concepts, schema and objects which make up the
database itself. It highlights the way in which the RIPE database
splits routing information from allocation information. Sections 5,
6, 7 and 8 detail all the objects associated with routing policy
representation. Section 9 gives a fairly extensive "walk through" of
how these objects are used for expressing routing policy and the
general principles behind their use. Section 10 provides a list of
references used throughout this document. Appendix A, B, C and D
document the formal syntax for the database objects and attributes.
Appendix F details the main changes from ripe-81 and motivations for
these changes. Appendix G tackles the issues of transition from
ripe-81 to ripe-81++.
Bates, et al. [Page 4]
RFC 1786 Representing IP Routing Policies in a RR March 1995
3. 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 classless IP network number: Layer 3 entities. We do not mean
organizations.
We call the organizations operating networks "network operators".
For the sake of the examples we divide network operators into two
categories: "service providers" and "customers". A "service provider"
is a network operator who operates a network to provide Internet
services to different organizations, its "customers". The
distinction between service providers and customers is not clear cut.
A national research networking organization frequently acts as a
service provider to Universities and other academic organizations,
but in most cases it buys international connectivity from another
service provider. A University networking department is a customer of
the research networking organization 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 (IGPs). In most cases interior routing
decisions are based on metrics derived from technical parameters like
topology, link speeds and load. 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 directly with the database
objects and routing tools. By using only one term we hope to reduce
the number of concepts and to avoid confusion. The academically
inclined reader may forgive us.
ASes exchange routing information with other ASes using Exterior
Routing Protocols (EGPs). Exterior routing decisions are frequently
based on policy based rules rather than purely on technical
parameters. Tools are needed 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 [8] and
BGP-4 [9] 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
Bates, et al. [Page 5]
RFC 1786 Representing IP Routing Policies in a RR March 1995
document describes a "Routing Registry" providing this functionality.
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 considers another route more appropriate to reach NET1.
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:
Bates, et al. [Page 6]
RFC 1786 Representing IP Routing Policies in a RR March 1995
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 acceptance
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.
Usually policies are not configured for each network separately but
for groups of networks. In practise these groups are almost always
defined by the networks forming one or more ASes.
Routing Policy limitations
It is important to realize that with current destination based
forwarding technology routing policies must eventually be expressed
in these terms. It is relatively easy to formulate 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.
The generic example of a reasonable but un-implementable routing is a
split of already joined packet streams based on something other than
destination address. 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 destination (disregarding
special cases like "type of service" routing, load sharing and
Bates, et al. [Page 7]
RFC 1786 Representing IP Routing Policies in a RR March 1995
routing instabilities).
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 concerns
traffic towards AS Z. Consider the following topology:
B AS ---) "W"
W AS ---) --->
B AS ---)>> AS A ---> ... AS Z .... NET 3
B AS ---) --->
W AS ---) "B"
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 depending on the
routing policies of AS A and all ASes between it and AS Z.
Bates, et al. [Page 8]
RFC 1786 Representing IP Routing Policies in a RR March 1995
The important thing to note is that unless routing and forwarding
decisions can be made based on both source and destination addresses,
policies like the "black and white" example cannot be implemented in
general because "once joined means joined forever".
Access Policies
Access policies contrary to routing policies are not necessarily
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 inappropriate 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 particular
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 sometimes hard to diagnose. Therefore
they should also be documented in the routing registry according to
similar requirements as outlined above.
Routing vs. Allocation information
The RIPE database contains both routing registry and address space
allocation registry information. In the past the database schema
combined this information. Because RIPE was tasked with running both
an allocation and routing registry it seemed natural to initially
combine these functions. However, experience has shown that a clear
separation of routing information from allocation is desirable. Often
the maintainer of the routing information is not the same as the
maintainer of the allocation information. Moreover, in other parts
of the world there are different registries for each kind of
information.
Whilst the actual routing policy objects will be introduced in the
next section it is worthy of note that a transition from the current
objects will be required. Appendix G details the basic steps of such
a transition.
This split in information represents a significant change in the
representational model of the RIPE database. Appendix F expands on
the reasons for this a little more.
Bates, et al. [Page 9]
RFC 1786 Representing IP Routing Policies in a RR March 1995
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 PRIDE tools [3] from the PRIDE project started in
September 1993 as well as others produced by Merit Inc [4] and CERN
[5].
These tools will enable them to use the routing policy stored in the
RIPE routing registry 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.
Work continues on producing more useful tools to service the Internet
community.
Bates, et al. [Page 10]
RFC 1786 Representing IP Routing Policies in a RR March 1995
4. The Routing Registry and the RIPE Database
One of the activities of RIPE is to maintain a database of European
IP networks, DNS domains and their contact persons along with various
other kinds of network management information. The database content
is public and can be queried using the whois protocol as well as
retrieved as a whole. This supports NICs/NOCs all over Europe and
beyond to perform their respective tasks.
The RIPE database combines both allocation registry and routing
registry functions. The RIPE allocation registry contains data about
address space allocated to specific enterprises and/or delegated to
local registries as well as data about the domain name space. The
allocation registry is described in separate documents [6,7] and
outside the scope of this document.
Database Objects
Each object in the database describes a single entity in the real
world. This basic principle means that information about that
entity should only be represented in the corresponding
database object and not be repeated in other objects. The whois
service can automatically display referenced objects where
appropriate.
The types of objects stored in the RIPE database are summarized in
the table below:
R Object Describes References
____________________________________________________________________
B person contact persons
A inetnum IP address space person
A domain DNS domain person
R aut-num autonomous system person
(aut-num,community)
R as-macro a group of autonomous systems person, aut-num
R community community person
R route a route being announced aut-num, community
R clns CLNS address space and routing person
The first column indicates whether the object is part of the
allocation registry (A), the routing registry (R) or both (B). The
Bates, et al. [Page 11]
RFC 1786 Representing IP Routing Policies in a RR March 1995
last column indicates the types of objects referenced by the
particular type of object. It can be seen that almost all objects
reference contact persons.
Objects are described by attributes value pairs, one per line.
Objects are separated by empty lines. An attribute that consists of
multiple lines should have the attribute name repeated on
consecutive lines. The information stored about network 192.87.45.0
consists of three objects, one inetnum object and two person
objects and looks like this:
Bates, et al. [Page 12]
RFC 1786 Representing IP Routing Policies in a RR March 1995
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
rev-srv: ns.ripe.net
rev-srv: ns.eu.net
notify: ops@ripe.net
changed: tony@ripe.net 940110
source: RIPE
person: Daniel Karrenberg
address: RIPE Network Coordination Centre (NCC)
address: Kruislaan 409
address: NL-1098 SJ Amsterdam
address: Netherlands
phone: +31 20 592 5065
fax-no: +31 20 592 5090
e-mail: dfk@ripe.net
nic-hdl: DK58
changed: ripe-dbm@ripe.net 920826
source: RIPE
person: Marten Terpstra
address: RIPE Network Coordination Centre (NCC)
address: PRIDE Project
address: Kruislaan 409
address: NL-1098 SJ Amsterdam
address: Netherlands
phone: +31 20 592 5064
fax-no: +31 20 592 5090
e-mail: Marten.Terpstra@ripe.net
nic-hdl: MT2
notify: marten@ripe.net
changed: marten@ripe.net 931230
source: RIPE
Objects are stored and retrieved in this tag/value format. The RIPE
NCC does not provide differently formatted reports because any
desired format can easily be produced from this generic one.
Bates, et al. [Page 13]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Routing Registry Objects
The main objects comprising the routing registry are "aut-num" and
"route", describing an autonomous system and a route respectively. It
should be noted that routes not described in the routing registry
should never be routed in the Internet itself.
The autonomous system (aut-num) object provides contact information
for the AS and describes the routing policy of that AS. The routing
policy is described by enumerating all neighboring ASes with which
routing information is exchanged. For each neighbor the routing
policy is described in terms of exactly what is being sent
(announced) and allowed in (accepted). It is important to note that
this is exactly the part of the global policy over which an AS has
direct control. Thus each aut-num object describes what can indeed be
implemented and enforced locally by the AS concerned. Combined
together all the aut-num objects provide the global routing graph and
permit to deduce the exact routing policy between any two ASes.
While the aut-num objects describe how routing information is
propagated, the route object describes a single route injected into
the external routing mesh. The route object references the AS
injecting (originating) the route and thereby indirectly provides
contact information for the originating AS. This reference also
provides the primary way of grouping routes into larger collections.
This is necessary because describing routing policy on the level of
single routes would be awkward to impractical given the number of
routes in the Internet which is about 20,000 at the time of this
writing. Thus routing policy is most often defined for groups of
routes by originating AS. This method of grouping is well supported
by current exterior routing protocols. The route object also
references community objects described below to provide another
method of grouping routes. Modification of aut-num object itself and
the referencing by route objects is strictly protected to provide
network operators control over the routing policy description and the
routes originated by their ASes.
Sometimes even keeping track of groups of routes at the AS level is
cumbersome. Consider the case of policies described at the transit
provider level which apply transitively to all customers of the
transit provider. Therefore another level of grouping is provided by
the as-macro object which provides groups of ASes which can be
referenced in routing policies just like single ASes. Membership of
as-macro groups is also strictly controlled.
Sometimes there is a need to group routes on different criteria than
ASes for purposes like statistics or local access policies. This is
provided by the community object. A community object is much like an
Bates, et al. [Page 14]
RFC 1786 Representing IP Routing Policies in a RR March 1995
AS but without a routing policy. It just describes a group of
routes. This is not supported at all by exterior routing protocols
and depending on aggregation of routes may not be generally usable to
define routing policies. It is suitable for local policies and non-
routing related purposes.
These routing related objects will be described in detail in the
sections below.
Bates, et al. [Page 15]
RFC 1786 Representing IP Routing Policies in a RR March 1995
5. The Route Object
As stated in the previous chapter routing and address space
allocation information are now clearly separated. This is performed
with the introduction of the route object. The route object will
contain all the information regarding a routing announcement.
All routing related attributes are removed from the inetnum object.
Some old attributes are obsoleted: connect, routpr-l, bdryg-l, nsf-
in, nsf-out, gateway). The currently useful routing attributes are
moved to the route object: aut-sys becomes origin, ias-int will be
encoded as part of the inet-rtr [15] object and comm-list simply
moves. See [6] for detail of the "inetnum" object definition.
The information in the old inetnum object
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 WCW
aut-sys: AS3333
comm-list: SURFNET
ias-int: 192.87.45.80 AS1104
ias-int: 192.87.45.6 AS2122
ias-int: 192.87.45.254 AS2600
rev-srv: ns.ripe.net
rev-srv: ns.eu.net
notify: ops@ripe.net
changed: tony@ripe.net 940110
source: RIPE
will be distributed over two objects:
Bates, et al. [Page 16]
RFC 1786 Representing IP Routing Policies in a RR March 1995
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
rev-srv: ns.ripe.net
rev-srv: ns.eu.net
notify: ops@ripe.net
changed: tony@ripe.net 940110
source: RIPE
route: 192.87.45.0/24
descr: RIPE Network Coordination Centre
origin: AS3333
comm-list: SURFNET
changed: dfk@ripe.net 940427
source: RIPE
The route object is used to represent a single route originated into
the Internet routing mesh. The actual syntax is given in Appendix D.
However, there are several important aspects of the attributes worthy
of note.
The value of the route attribute will be a classless address. It
represents the exact route being injected into the routing mesh. The
representation of classless addresses is described in [10].
The value of the origin attribute will be an AS reference of the form
AS1234 referring to an aut-num object. It represents the AS
injecting this route into the routing mesh. The "aut-num" object
(see below) thus referenced provides all the contact information for
this route.
Special cases: There can only be a single originating AS in each
route object. However in todays Internet sometimes a route is
injected by more than one AS. This situation is potentially dangerous
as it can create conflicting routing policies for that route and
requires coordination between the originating ASes. In the routing
registry this is represented by multiple route objects.
Bates, et al. [Page 17]
RFC 1786 Representing IP Routing Policies in a RR March 1995
This is a departure from the one route (net), one AS principle of the
ripe-81 routing registry. The consequences for the different tools
based in the routing registry will need to be evaluated and possibly
additional consistency checking of the database is needed.
The examples below will illustrate the usage of the route object
further. Suppose three chunks of address space of 2 different
enterprises represented by the following inetnum objects:
Examples
inetnum: 193.0.1.0
netname: ENT-1
descr: Enterprise 1
...
inetnum: 193.0.8.0
netname: ENT-2
descr: Enterprise 2
...
inetnum: 193.0.9.0
netname: ENT-2-SPEC
descr: Enterprise 2
...
Supposing that the Enterprises have their own AS numbers straight
application of routing without aggregation would yield:
route: 193.0.1.0/24
descr: Enterprise 1
origin: AS1
...
route: 193.0.8.0/24
descr: Enterprise 2
origin: AS2
...
route: 193.0.9.0/24
descr: Enterprise 2
origin: AS2
...
Bates, et al. [Page 18]
RFC 1786 Representing IP Routing Policies in a RR March 1995
NB: This representation can be achieved by straight translation from
the ripe-81 representation. See Appendix G for more details.
Homogeneous Aggregation
The two chunks of address space of Enterprise 2 can be represented by
one aggregate route turning two route objects into one and
potentially saving routing table space for one route.
route: 193.0.8.0/23
descr: Enterprise 2
origin: AS2
...
Note that AS2 can also decide to originate all routes mentioned so
far, two 24-bit prefixes and one 23-bit prefix. This case would be
represented by storing all three route objects in the database. In
this particular example the additional routes will not add any
functionality however and only increase the amount of routes
announced unnecessarily.
Heterogeneous Aggregation
Consider the following case however:
route: 193.0.8.0/24
descr: Enterprise 2
origin: AS2
...
route: 193.0.9.0/24
descr: Enterprise 2 / Special
origin: AS2
comm-list: SPECIAL
...
Now the prefix 193.0.9.0/24 belongs to community SPECIAL (this
community may well not be relevant to routing) and the other prefix
originated by AS2 does not. If AS2 aggregates these prefixes into the
193.0.8.0/23 prefix, routing policies based on the community value
SPECIAL cannot be implemented in general, because there is no way to
distinguish between the special and the not-so-special parts of AS2.
Bates, et al. [Page 19]
RFC 1786 Representing IP Routing Policies in a RR March 1995
If another AS has the policy to accept only routes to members of
community SPECIAL it cannot implement it, because accepting the route
to 193.0.8.0/23 would also route to 193.0.8.0/24 and not accepting
this route would lose connectivity to the special part 193.0.9.0/24.
We call aggregate routes consisting of components belonging to
different communities or even different ASes "heterogeneous
aggregates".
The major problem introduced with heterogeneous aggregates is that
once the homogeneous more specific routes are withdrawn one cannot
tell if a more specific part of the heterogeneous route has a
different policy. However, it can be counter argued that knowing this
policy is of little use since a routing policy based on the less
specific heterogeneous aggregate only cannot be implemented. In fact,
this displays a facet of CIDR itself in that one may actually trade
off implementing slight policy variations over announcing a larger
(albeit heterogeneous in terms of policy) aggregate to save routing
table space.
However, it is still useful to be able to document these variations
in policy especially when this homogeneous more specific route is
just being withdrawn. For this one can use the "withdrawn" attribute.
The withdrawn attribute can serve to both indicate that a less
specific aggregate is in fact heterogeneous and also allow the
general documenting of route withdrawal.
So there has to be a way for AS2 to document this even if it does not
originate the route to 193.0.9.0/24 any more. This can be done with
the "withdrawn" attribute of the route object. The aggregate route
to 193.0.8.0/23 is now be registered as:
route: 193.0.8.0/23
descr: Enterprise 2
origin: AS2
...
With the two homogeneous routes marked as withdrawn from the Internet
routing mesh but still preserving their original routing information.
Bates, et al. [Page 20]
RFC 1786 Representing IP Routing Policies in a RR March 1995
route: 193.0.8.0/24
descr: Enterprise 2
origin: AS2
withdrawn: 940701
...
route: 193.0.9.0/24
descr: Enterprise 2 / Special
origin: AS2
comm-list: SPECIAL
withdrawn: 940701
...
It should be noted that the date value used in the withdrawn
attribute can only be in the past.
Proxy Aggregation
The next step of aggregation are aggregates consisting of more than
one AS. This generally means one AS is aggregating on behalf of
another. It is called proxy aggregation. Proxy aggregation should be
done with great care and always be coordinated with other providers
announcing the same route.
Consider the following:
route: 193.0.0.0/20
descr: All routes known by AS1 in a single package
origin: AS1
...
route: 193.0.1.0/24
descr: Foo
origin: AS1
withdrawn: 940310
...
Bates, et al. [Page 21]
RFC 1786 Representing IP Routing Policies in a RR March 1995
route: 193.0.8.0/24
descr: Bar
origin: AS2
withdrawn: 940310
...
route: 193.0.9.0/24
descr: Bar-2
origin: AS2
withdrawn: 940310
comm-list: SPECIAL
...
If AS1 announced no other routes to a single homed neighboring AS,
that neighbor can in general either take that route or leave it but
not differentiate between AS1 and AS2.
Note: If the neighbor was previously configured to accept routes
originating in AS2 but not in AS1 they lose connectivity to AS2 as
well. This means that proxy aggregation has to be done carefully and
in a well coordinated fashion. The information in the withdrawn route
object can help to achieve that.
Aggregates with Holes
If we assume that the world of our example still consists of only
three chunks of address space the aggregate above contains what are
called holes, parts of an aggregate that are not reachable via the
originator of the route. From the routing information itself one
cannot tell whether these are holes and what part of the route falls
inside one. The only way to tell is to send a packet there and see
whether it gets to the destination, or an ICMP message is received
back, or there is silence. On the other hand announcing aggregates
with holes is quite legitimate. Consider a 16-bit aggregate with
only one 24-bit prefix unreachable. The savings in routing table
size by far outweigh the hole problem.
For operational reasons however it is very useful to register these
holes in the routing registry. Consider the case where a remote
network operator experiences connectivity problems to addresses
Bates, et al. [Page 22]
RFC 1786 Representing IP Routing Policies in a RR March 1995
inside an aggregate route. If the packets are getting to the AS
announcing the aggregate and there are no more specific routes, the
normal cause of action is to get in touch with the originating AS of
the aggregate route and ask them to fix the problem. If the address
falls into a hole this is futile. Therefore problem diagnosis can be
sped up and unnecessary calls prevented by registering the holes in
the routing registry. We do this by using the "hole" attribute. In
our example the representation would be:
route: 193.0.0.0/20
descr: All routes known by AS1
origin: AS1
hole: 193.0.0.0/24
hole: 193.0.2.0/23
hole: 193.0.4.0/22
hole: 193.0.10.0/23
hole: 193.0.12.0/22
...
Note: there would also be two routes with the withdrawn attribute as
displayed above (i.e. 193.0.8.0/24 and 193.0.9.0/24). It is not
mandatory to document all holes. It is recommended all holes routed
by another service provider are documented.
Multiple Proxy Aggregation
Finally suppose that AS2 decides to announce the same aggregate, as
in the previous example, they would add the following route object to
the registry:
route: 193.0.0.0/20
descr: All routes known by AS2
origin: AS2
hole: 193.0.0.0/24
hole: 193.0.2.0/23
hole: 193.0.4.0/22
hole: 193.0.10.0/23
hole: 193.0.12.0/22
...
Both AS1 and AS2 will be notified that there already is a route to
the same prefix in the registry.
Bates, et al. [Page 23]
RFC 1786 Representing IP Routing Policies in a RR March 1995
This multiple proxy aggregation is very dangerous to do if the sub-
aggregates of the route are not the same. It is still dangerous when
the sub-aggregates are consistent but connectivity to the sub-
aggregates varies widely between the originators.
Route object update procedures
Adding a route object will have to be authorised by the maintainer of
the originating AS. The actual implementation of this is outside the
scope of this document. This guarantees that an AS guardian has full
control over the registration of the routes it announces [11].
What is an Inter-AS network ?
An inter-AS network (Inter-AS IP networks are those networks are
currently called FIXes, IXFs, DMZs, NAPs, GIX and many other
acronyms) exists for the purpose of passing traffic and routing
information between different autonomous systems. The most simple
example of an inter-AS network is a point-to-point link, connecting
exactly two ASes. Each end of such a link is connected to an
interface of router belonging to each of the autonomous systems.
More complex examples are broadcast type networks with multiple
interfaces connecting multiple ASes with the possibility of more than
one connection per AS. Consider the following example of three
routers 1, 2 and 3 with interfaces a through f connected by two
inter-AS networks X and Y:
X Y
a1b --- c2d --- e3f
Suppose that network X is registered in the routing registry as part
of AS1 and net Y as part of AS3. If traffic passes from left to right
prtraceroute will report the following sequence of interfaces and
ASes:
a in AS1
c in AS1
e in AS3
The traceroute algorithm enumerates only the receiving interfaces on
the way to the destination. In the example this leads to the passage
Bates, et al. [Page 24]
RFC 1786 Representing IP Routing Policies in a RR March 1995
of AS2 going unnoticed. This is confusing to the user and will also
generate exceptions when the path found is checked against the
routing registry.
For operational monitoring tools such as prtraceroute it is necessary
to know which interface on an inter-AS network belongs to which AS.
If AS information is not known about interfaces on an inter-AS
network, tools like prtraceroute cannot determine correctly which
ASes are being traversed.
All interfaces on inter-AS networks will are described in a separate
object know as the `inet-rtr' object [15].
Bates, et al. [Page 25]
RFC 1786 Representing IP Routing Policies in a RR March 1995
6. The Autonomous System Object
Autonomous Systems
An Autonomous System (AS) is a group of IP networks operated by one
or more network operators which has a single and clearly defined
external routing policy.
An AS has a unique number associated with it which is used both in
exchange of exterior routing information and as an identifier 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 interior gateway
protocols (IGPs) in conjunction with some sort of common agreed
metrics when exchanging network information within its own AS.
The term AS is often confused or even misused as a convenient 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
external 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 routing
announcement. It should be noted that there is a limited number of
AS numbers available. Also creating an AS may well increase the
number of AS paths modern EGPs will have to keep track of. This
aggravates what is known as "the routing table growth problem". This
may mean that by applying the general rules for the creation and
allocation 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:
+ Creation of an AS is only required when exchanging routing
information with other ASes. Some router implementations make
use of an AS number as a form of tagging 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.
Bates, et al. [Page 26]
RFC 1786 Representing IP Routing Policies in a RR March 1995
+ For a simple case of customer networks connected to a single
service provider, the IP network should normally be a member of
the service providers AS. In terms of routing 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.
+ 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 networks connected to
two service providers there are at least two different routing
policies to a given customer 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 different service
providers. This is the ONLY case where a network operator
should create its own AS number.
+ As a general rule one should always try to populate the AS with
as many routes as possible, providing all routes conform to the
same routing policy.
Each AS is represented in the RIPE database by both an aut-num object
and the route objects representing the routes originated by the AS.
The aut-num object stores descriptive, administrative and contact
information about the AS as well as the routing policies of the AS in
relation to all neighboring ASes.
The origin attributes of the route objects define the set of routes
originated by the AS. Each route object can have exactly one origin
attribute. Route objects can only be created and updated by the
maintainer of the AS and not by those immediately responsible for the
particular routes referenced therein. This ensures that operators,
especially service providers, remain in control of AS routing
announcements.
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.
Bates, et al. [Page 27]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example:
aut-num: AS1104
descr: NIKHEF-H Autonomous system
as-in: from AS1213 100 accept AS1213
as-in: from AS1913 100 accept AS1913
as-in: from AS1755 150 accept ANY
as-out: to AS1213 announce ANY
as-out: to AS1913 announce ANY
as-out: to AS1755 announce AS1104 AS1913 AS1213
tech-c: Rob Blokzijl
admin-c: Eric Wassenaar
guardian: as-guardian@nikhef.nl
changed: ripe-dbm@ripe.net 920910
source: RIPE
See Appendix A for a complete syntax definition of the "aut-num"
object.
It should be noted that this representation provides two things:
+ a set of routes.
+ a description of administrative details and routing policies.
The set of routes 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.
Bates, et al. [Page 28]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Description of routing policies between ASs with multiple connections
- "interas-in/interas-out"
The following section is only relevant for ASes which use different
policies on multiple links to the same neighboring AS. Readers not
doing this may want to skip this section.
Description of multiple connections between ASs defines how two ASs
have chosen to set different policies for the use of each or some of
the connections between the ASs. This description is necessary only
if the ASs are connected in more than one way and the routing policy
and differs at these two connections.
Example:
LINK1
193.0.1.1 +----------+ 193.0.1.2
| |
AS1------AS2== ==AS3-----AS4
| |
193.0.1.5 +----------+ 193.0.1.6
LINK2
Note: LINK here denotes the peer connection points between
ASs. It is not necessarily just a serial link. It could
be ethernet or any other type of connection as well. It
can also be a peer session where the address is the same at
one end and different at the other end.
It may be that AS2 wants to use LINK2 only for traffic towards AS4.
LINK1 is used for traffic to AS3 and as backup to AS4, should LINK2
fail. To implement this policy, one would use the attribute
"interas-in" and "interas-out." This attribute permits ASs to
describe their local decisions based on its preference such as
multi-exit-discriminators (MEDs) as used in some inter-domain routing
protocols (BGP4, IDRP) and to communicate those routing decisions.
This information would be useful in resolving problems when some
traffic paths changed from traversing AS3's gateway in Timbuktu
rather than the gateway in Mogadishu. The exact syntax is given in
Appendix A. However, if we follow this example through in terms of
AS2 we would represent this policy as follows:
Bates, et al. [Page 29]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example:
aut-num: AS2
as-in: from AS3 10 accept AS3 AS4
as-out: to AS3 announce AS1 AS2
interas-in:from AS3 193.0.1.1/32 193.0.1.2/32 (pref=5) accept AS3
interas-in:from AS3 193.0.1.1/32 193.0.1.2/32 (pref=9) accept AS4
interas-in:from AS3 193.0.1.5/32 193.0.1.6/32 (pref=7) accept AS4
...
Here we see additional policy information between two ASs in terms of
the IP addresses of the connection. The parentheses and keyword are
syntactic placeholders to add the readability of the attributes. If
pref=MED is specified the preference indicated by the remote AS via
the multi-exit- discriminator metric such as BGP is used. Of course
this type on inter-AS policy should always be bilaterally agreed upon
to avoid asymmetry and in practice there may need to be
corresponding interas-out attributes in the policy representation of
AS3.
The interas-out attribute is similar to interas-in as as-out is to
as-in. The one major difference being that interas-out allows you to
associate an outgoing metric with each route. It is important to note
that this metric is just passed to the peer AS and it is at the peer
AS's discretion to use or ignore it. A special value of IGP
specifies that the metric passed to the receiving AS will be derived
from the IGP of the sending AS. In this way the peer AS can choose
the optimal link for its traffic as determined by the sending AS.
If we look at the corresponding interas-out for AS3 we would see the
following:
Example:
aut-num: AS3
as-in: from AS2 10 accept AS1 A2
as-out: to AS2 announce AS3 AS4
interas-out:to AS2 193.0.1.2/32 193.0.1.1/32 (metric-out=5) announce AS3
interas-out:to AS2 193.0.1.2/32 193.0.1.1/32 (metric-out=9) announce AS4
interas-out:to AS2 193.0.1.6/32 193.0.1.5/32 (metric-out=7) announce AS4
...
Bates, et al. [Page 30]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Descriptions of interas policies do not replace the global
policy described in as-in, as-out and other policy attributes which
should be specified too. If the global policy mentions more routes
than the combined local policies then local preferences for these
routes are assumed to be equal for all links.
Any route specified in interas-in/out and not specified in as-in/out
is assumed not accepted/announced between the ASes concerned.
Diagnostic tools should flag this inconsistency as an error. It
should be noted that if an interas-in or interas-out policy is
specified then it is mandatory to specify the corresponding global
policy in the as-in or as-out line. Please note there is no relevance
in the cost associated with as-in and the preferences used in
interas-in.
The interaction of interas-in/interas-out with as-in/as-out
Although formally defined above, the rules associated with policy
described in terms of interas-in and interas-out with respect to as-
in and as-out are worthy of clarification for implementation.
When using interas-in or interas-out policy descriptions, one must
always make sure the set of policies described between two ASes is
always equal to or a sub-set of the policy described in the global
as-in or as-out policy. When a sub-set is described remember the
remaining routes are implicitly shared across all connections. It is
an error for the interas policies to describe a superset of the
global policies, i.e. to announce or accept more routes than the
global policies.
When defining complex interas based policies it is advisable to
ensure that any possible ambiguities are not present by explicitly
defining your policy with respect to the global as-in and as-out
policy.
If we look at a simple example, taking just in-bound announcements to
simplify things. If we have the following global policy:
aut-num: AS1
as-in: from AS2 10 accept AS100 OR {10.0.0.0/8}
Suppose there are three peerings between AS1 and AS2, known as L1-R1,
L2-R2 and L3-R3 respectively. The actual policy of these connections
is to accept AS100 equally on these three links and just route
10.0.0.0/8 on L3-R3. The simple way to mention this exception is to
just specify an interas policy for L3-R3:
Bates, et al. [Page 31]
RFC 1786 Representing IP Routing Policies in a RR March 1995
interas-in: from AS2 L3 R3 (pref=100) accept {10.0.0.0/8}
The implicit rule that all routes not mentioned in interas policies
are accepted on all links with equal preference ensures the desired
result.
The same policy can be written explicitly as:
interas-in: from AS2 L1 R1 (pref=100) accept AS100
interas-in: from AS2 L2 R2 (pref=100) accept AS100
interas-in: from AS2 L3 R3 (pref=100) accept AS100 OR {10.0.0.0/8}
Whilst this may at first sight seem obvious, the problem arises when
not all connections are mentioned. For example, if we specified only
an interas-in line for L3-R3 as below:
aut-num: AS1
as-in: from AS2 10 accept AS100 OR {10.0.0.0/8}
interas-in: from AS2 L3 R3 (pref=100) accept AS100 OR {10.0.0.0/8}
then the policy for the other links according to the rules above
would mean they were equal to the global policy minus the sum of the
local policies (i.e. ((AS100 OR {10.0.0.0/0}) / (AS100 OR
{10.0.0.0/0})) = empty) which in this case would mean nothing is
accepted on connections L1-R1 and L2-R2 which is incorrect.
Another example: If we only registered the policy for link L2-
R2:
interas-in: from AS2 L2 R2 (pref=100) accept AS100
The implicit policy for both L1-R1 and L3-R3 would be as follows:
interas-in: from AS2 L1 R1 (pref=100) accept {10.0.0.0/8}
interas-in: from AS2 L3 R3 (pref=100) accept {10.0.0.0/8}
This is derived as the set of global policies minus the set of
interas-in policies (in this case just accept AS100 as it was the
L2-R2 interas-in policy we registered) with equal cost for the
remaining connection. This again is clearly not what was intended.
Bates, et al. [Page 32]
RFC 1786 Representing IP Routing Policies in a RR March 1995
We strongly recommend that you always mention all policies for all
interas connections explicitly, to avoid these possible errors. One
should always ensure the set of the interas policies is equal to the
global policy. Clearly if interas policies differ in complex ways it
is worth considering splitting the AS in question into separate ASes.
However, this is beyond the direct scope of this document.
It should also be noted there is no direct relationship between the
cost used in as-in and the preference used in interas-in.
Bates, et al. [Page 33]
RFC 1786 Representing IP Routing Policies in a RR March 1995
How to describe the exclusion policy of a certain AS - "as-exclude"
Some ASes have a routing policy based on the exclusion of certain
routes if for whatever reason a certain AS is used as transit.
Whilst, this is in general not good practice as it makes implicit
assumptions on topology with asymmetry a possible outcome if not
coordinated, this case needs to be accommodated within the routing
policy representation.
The way this is achieved is by making use of the "as-exclude"
attribute. The precise syntax of this attribute can be found in
Appendix A along with the rest of the defined syntax for the "aut-
num" object. However, some explanation of the use of this attribute
is useful. If we have the following example topology.
Example:
AS4--------AS3
| | |
| | |
AS1--------AS2--------AS5
With a simple corresponding policy like so:
Example:
aut-num: AS1
as-in: from AS2 100 accept ANY
as-out: to AS2 announce AS1
as-exclude: exclude AS4 to ANY
....
We see an interesting policy. What this says in simple terms is AS1
doesn't want to reach anything if it transits AS4. This can be a
perfectly valid policy. However, it should be realized that if for
whatever reason AS2 decides to route to AS3 via AS4 then immediately
AS1 has no connectivity to AS3 or if AS1 is running default to AS2
packets from AS1 will still flow via AS4. The important point about
this is that whilst AS1 can advise its neighbors of its policy it has
no direct control on how it can enforce this policy to neighbors
upstream.
Bates, et al. [Page 34]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Another interesting scenario to highlight the unexpected result of
using such an "as-exclude" policy. If we assume in the above example
AS2 preferred AS4 to reach AS3 and AS1 did not use default routing
then as stated AS1 would have no connectivity to AS3. Now lets
suppose that for example the link between AS2 and AS4 went down for
some reason. Like so:
Example:
AS4--------AS3
|
|
AS1--------AS2--------AS5
Suddenly AS1 now has connectivity to AS3. This unexpected behavior
should be considered when created policies based on the "as-exclude"
attribute.
The second problem with this type of policy is the potential of
asymmetry. In the original example we saw the correct policy from
AS1's point of view but if ASes with connectivity through AS4 do not
use a similar policy you have asymmetric traffic and policy. If an
AS uses such a policy they must be aware of the consequences of its
use. Namely that the specified routes which transit the AS (i.e.
routing announcements with this AS in the AS path information) in
question will be excluded. If not coordinated this can easily cause
asymmetry or even worse loss of connectivity to unknown ASes behind
(or in front for that matter) the transit AS in question. With this
in mind this attribute can only be viewed as a form of advisory to
other service providers. However, this does not preclude its use with
policy based tools if the attribute exists.
By having the ability to specify a route keyword based on any of the
four notations given in the syntax it allows the receiving AS to
specify what routes it wishes to exclude through a given transit AS
to a network granularity.
Bates, et al. [Page 35]
RFC 1786 Representing IP Routing Policies in a RR March 1995
7. AS Macros
It may be difficult to keep track of each and every new AS that is
represented in the routing registry. A convenient way around this is
to define an `AS Macro' which essentially is a convenient way to
group ASes. This is done so that each and every AS guardian does not
have to add a new AS to it's routing policy as described by the as-in
and as-out attributes of it's AS object.
However, it should be noted that this creates an implicit trust on
the guardian of the AS-Macro.
An AS-Macro can be used in <routing policy expressions> for the "as-
in" and "as-out" attributes in the aut-num object. The AS-Macro
object is then used to derive the list or group of ASes.
A simple example would be something like:
Example:
aut-num: AS786
as-in: from AS1755 100 accept AS-EBONE AND NOT AS1104
as-out to AS1755 announce AS786
.....
Where the as-macro object for AS-EBONE is as follows:
as-macro: AS-EBONE
descr: ASes routed by EBONE
as-list: AS2121 AS1104 AS2600 AS2122
as-list: AS1103 AS1755 AS2043
guardian: guardian@ebone.net
......
So the policy would be evaluated to:
aut-num: AS786
as-in: from AS1755 100 accept (AS2121 OR AS1104 OR AS2600 OR AS2122
as-in: from AS1755 100 accept AS1103 OR AS1755 OR
as-in: from AS1755 100 accept AS2043) AND NOT AS1104
......
Bates, et al. [Page 36]
RFC 1786 Representing IP Routing Policies in a RR March 1995
It should be noted that the above examples incorporates the rule for
line wrapping as defined in Appendix A for policy lines. See
Appendix C for a definition on the AS-Macro syntax.
Bates, et al. [Page 37]
RFC 1786 Representing IP Routing Policies in a RR March 1995
8. The Community Object
A community is a group of routes that cannot be represented by an AS
or a group of ASes. It is in some circumstances useful to define a
group of routes that have something in common. This could be a
special access policy to a supercomputer centre, a group of routes
used for a specific mission, or a disciplinary group that is
scattered among several autonomous systems. Also these communities
could be useful to group routes for the purpose of network
statistics.
Communities do not exchange routing information, since they do not
represent an autonomous system. More specifically, communities do
not define routing policies, but access or usage policies. However,
they can be used as in conjunction with an ASes routing policy to
define a set of routes the AS sets routing policy for.
Communities should be defined in a strict manner, to avoid creating
as many communities as there are routes, or even worse. Communities
should be defined following the two rules below;
+ Communities must have a global meaning. Communities that have
no global meaning, are used only in a local environment and
should be avoided.
+ Communities must not be defined to express non-local policies.
It should be avoided that a community is created because some
other organization forces a policy upon your organization.
Communities must only be defined to express a policy defined by
your organization.
Community examples
There are some clear examples of communities:
BACKBONE -
all customers of a given backbone 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.
Bates, et al. [Page 38]
RFC 1786 Representing IP Routing Policies in a RR March 1995
HEPNET -
the High Energy Physics community partly shares infrastructure
with other organizations, and the institutes it consists of are
scattered all over Europe, often being part of a non HEPNET
autonomous system. To allow statistics, access or part of a
routing policy , a community HEPNET, consisting of all routes
that are part of HEPNET, conveniently groups all these routes.
NSFNET -
the National Science Foundation Network imposes an acceptable
use policy on routes that wish to make use of it. A community
NSFNET could imply the set of routes that comply with this
policy.
MULTI -
a large multinational corporation that does not have its own
internal infrastructure, but connects to the various parts of
its organizations by using local service 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 community tags on the
route objects representing the routes belonging to the community.
The community object stores descriptive, administrative and contact
information about the community.
The community tags on the route objects define the set of routes
belonging to a community. A route can have multiple 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 community 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 route 192.16.199.0/24 has a single routing policy (i.e. that of
AS 1104), but is part of several different communities of interest.
We use the tag "comm-list" to represent the list of communities
associated with this route. NIKHEF-H uses the service provider
SURFNET (a service provider with customers with more than one routing
Bates, et al. [Page 39]
RFC 1786 Representing IP Routing Policies in a RR March 1995
policy), is also part of the High Energy Physics community as well as
having the ability to access the Supercomputer at CERN (the community
`CERN-SUPER', is somewhat national, but is intended as an example of
a possible use of an access policy constraint).
Example:
route: 192.16.199.0/24
descr: Local Ethernet
descr: NIKHEF section H
origin: AS1104
comm-list: HEPNET CERN-SUPER SURFNET
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.
Bates, et al. [Page 40]
RFC 1786 Representing IP Routing Policies in a RR March 1995
9. 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 information 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 routing information.
Example 1:
AS1------AS2
This specifies a simple routing exchange of two presumably isolated
ASes. Even if either of them has routing information about routes in
ASes other than AS1 and AS2, none of that will be announced to the
other.
aut-num: AS1
as-out: to AS2 announce AS1
as-in: from AS2 100 accept AS2
aut-num: AS2
as-out: to AS1 announce AS2
as-in: from AS1 100 accept AS1
The number 100 in the in-bound 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:
Bates, et al. [Page 41]
RFC 1786 Representing IP Routing Policies in a RR March 1995
AS1------AS2------AS3
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 routing registry 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: to AS2 announce AS1
as-in: from AS2 100 accept AS2
aut-num: AS2
as-out: to AS1 announce AS2
as-in: from AS1 100 accept AS1
as-out: to AS3 announce AS2
as-in: from AS3 100 accept AS3
aut-num: AS3
as-out: to AS2 announce AS3
as-in: from AS2 100 accept AS2
Note that in this example, AS2 keeps full control over its resources.
Even if AS3 and AS1 were to allow each others routes in from AS2, the
routing information would not flow because AS2 is not announcing it.
Of course AS1 and AS3 could just send traffic to each other to AS2
even without AS2 announcing the routes, hoping that AS2 will forward
it correctly. Such questionable practices however are beyond the
scope of this document.
Bates, et al. [Page 42]
RFC 1786 Representing IP Routing Policies in a RR March 1995
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:
aut-num: AS1
as-out: to AS2 announce AS1
as-in: from AS2 100 accept AS2 AS3
aut-num: AS2
as-out: to AS1 announce AS2 AS3
as-in: from AS1 100 accept AS1
as-out: to AS3 announce AS2 AS1
as-in: from AS3 100 accept AS3
aut-num: AS3
as-out: to AS2 announce AS3
as-in: from AS2 100 accept AS1 AS2
Note that the amount of routing information exchanged with a neighbor
AS is defined in terms of routes belonging to ASes. In BGP terms
this is the AS where the routing information originates and the
originating AS information carried in BGP could be used to implement
the desired policy. However, using BGP or the BGP AS-path
information is not required to implement the policies thus specified.
Configurations based on route 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 expression:
ANY - means any routing information known. For output this means that
all routes an AS knows about are announced. For input it means
that anything is accepted from the neighbor AS.
Bates, et al. [Page 43]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example 3:
AS4 is a stub customer AS, which only talks to service provider
AS123.
|
|
-----AS123------AS4
|
|
aut-num: AS4
as-out: to AS123 announce AS4
as-in: from AS123 100 accept ANY
aut-num: AS123
as-in: from AS4 100 accept AS4
as-out: to AS4 announce ANY
<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 routing registry 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
as-out: to AS123 announce AS4
default: AS123 100
aut-num: AS123
as-in: from AS4 100 accept AS4
<further neighbors>
Bates, et al. [Page 44]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example 4:
AS4 now connects to a different operator, AS5. AS5 uses AS123 for
outside connectivity but has itself no direct connection 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
as-out: to AS123 announce AS4 AS5
as-in: from AS123 100 accept ANY
as-out: to AS5 announce ANY
as-in: from AS5 50 accept AS5
aut-num: AS5
as-in: from AS4 100 accept ANY
as-out: to AS4 announce AS5
aut-num: AS123
as-in: from AS4 100 accept AS4 AS5
as-out: to AS4 announce ANY
<further neighbors>
Now AS4 has two sources of external routing information. AS5 which
provides only information about its own routes 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 routing towards
AS123:
Bates, et al. [Page 45]
RFC 1786 Representing IP Routing Policies in a RR March 1995
aut-num: AS4
as-out: to AS123 announce AS4 AS5
default: AS123 11
as-in: from AS5 50 accept AS5
Note no announcements to AS5, they default to us.
aut-num: AS5
as-out: to AS4 announce AS5
default: AS4 100
aut-num: AS123
as-in: from AS4 100 announce AS4 AS5
<further neighbors>
Note that the relative cost associated with default routing is
totally separate from the relative cost associated with in-bound
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 behavior 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
certain datagram applications which do not require acknowledgments
however, significant amounts of traffic may be uselessly directed at
AS123. Similarly default routing should not be used if there are
stringent security policies which prescribe any traffic intended for
AS5 to ever touch AS123.
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. Therefore, generally it can be
said that default routing should only be used in very simple
topologies.
Bates, et al. [Page 46]
RFC 1786 Representing IP Routing Policies in a RR March 1995
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 destined 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.
Bates, et al. [Page 47]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example 5a:
aut-num: AS4
as-in: from AS123 100 accept NOT AS6
as-out: to AS123 announce AS4
as-in: from AS6 50 accept AS6
as-out: to AS6 announce AS4
aut-num: AS123
as-in: from AS4 100 accept AS4
as-out: to AS4 announce ANY
as-in: from AS6 100 accept AS6
as-out: to AS6 announce ANY
<further neighbors>
aut-num: AS6
as-in: from AS123 100 accept NOT AS4
as-out: to AS123 announce AS6
as-in: from AS4 50 accept AS4
as-out: to AS4 announce 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 checking tools might flag these cases however.
Bates, et al. [Page 48]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example 5b:
aut-num: AS4
as-in: from AS123 100 accept ANY
as-out: to AS123 announce AS4
as-in: from AS6 50 accept AS6
as-out: AS6 AS4
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: from AS123 100 accept ANY
as-out: to AS123 announce AS6
as-in: from AS4 50 accept AS4
as-out: to AS4 announce 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.
Bates, et al. [Page 49]
RFC 1786 Representing IP Routing Policies in a RR March 1995
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: from AS123 100 accept ANY
as-out: to AS123 announce AS4
as-in: from AS6 50 accept AS6
as-in: from AS6 110 accept ANY
as-out: to AS6 AS4
aut-num: AS123
as-in: from AS4 1 accept AS4
as-out: to AS4 announce ANY
as-in: from AS6 1 accept AS6
as-in: from AS6 2 accept AS4
as-out: to AS6 announce ANY
<further neighbors>
aut-num: AS6
as-in: from AS123 100 accept ANY
as-out: to AS123 AS6 announce AS4
as-in: from AS4 50 accept AS4
as-out: to AS4 announce ANY
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.
10. Future Extensions
We envision that over time the requirements for describing routing
policy will evolve. The routing protocols will evolve to support the
requirements and the routing policy description syntax will need to
evolve as well. For that purpose, a separate document will describe
experimental syntax definitions for policy description. This
document [14] will be updated when new objects or attributes are
proposed or modified.
Bates, et al. [Page 50]
RFC 1786 Representing IP Routing Policies in a RR March 1995
11. References
[1] Bates, T., Jouanigot, J-M., Karrenberg, D., Lothberg, P.,
Terpstra, M., "Representation of IP Routing Policies in the RIPE
Database", RIPE-81, February 1993.
[2] Merit Network Inc.,"Representation of Complex Routing Policies
of an Autonomous System", Work in Progress, March 1994.
[3] PRIDE Tools Release 1.
See ftp.ripe.net:pride/tools/pride-tools-1.tar.Z.
[4] Merit Inc. RRDB Tools.
See rrdb.merit.edu:pub/meritrr/*
[5] The Network List Compiler.
See dxcoms.cern.ch:pub/ripe-routing-wg/nlc-2.2d.tar
[6] Lord, A., Terpstra, M., "RIPE Database Template for Networks and
Persons", RIPE-119, October 1994.
[7] Karrenberg, D., "RIPE Database Template for Domains", RIPE-49,
April 1992.
[8] Lougheed, K., Rekhter, Y., "A Border Gateway Protocol 3 (BGP-
3)", RFC1267, October 1991.
[9] Rekhter, Y., Li, T., "A Border Gateway Protocol 4 (BGP-4)",
RFC-1654, May 1994.
[10] Bates, T., Karrenberg, D., Terpstra, M., "Support for Classless
Internet Addresses in the RIPE Database", RIPE-121, October
1994.
[11] Karrenberg, D., "Authorisation and Notification of Changes in
the RIPE Database", RIPE-120, October 1994.
[12] Bates, T., "Support of Guarded fields within the RIPE Database",
ripe-117, July 1994.
[13] Estrin, D., Li, T., Rekhter, Y., Varadhan, K., Zappala, D.,
"Source Demand Routing: Packet Format and Forwarding
Specification (Version 1)", Work in Progress, March 1994.
[14] Joncheray, L., "Experimental Objects and attributes for the
Routing Registry", RIPE-182, October1994.
[15] Bates, T., "Specifying an `Internet Router' in the Routing
Bates, et al. [Page 51]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Registry", RIPE-122, October 1994.
[16] Bates, T., Karrenberg, D., Terpstra, M., "RIPE Database
Transition Plan", RIPE-123, October 1994.
12. Security Considerations
Security issues are beyond the scope of this memo.
Bates, et al. [Page 52]
RFC 1786 Representing IP Routing Policies in a RR March 1995
13. Authors' Addresses
Tony Bates
MCI Telecommunications Corporation
2100 Reston Parkway
Reston, VA 22094
USA
+1 703 715 7521
Tony.Bates@mci.net
Elise Gerich
The University of Michigan
Merit Computer Network
1075 Beal Avenue
Ann Arbor, MI 48109
USA
+1 313 936 2120
epg@merit.edu
Laurent Joncheray
The University of Michigan
Merit Computer Network
1075 Beal Avenue
Ann Arbor, MI 48109
USA
+1 313 936 2065
lpj@merit.edu
Jean-Michel Jouanigot
CERN, European Laboratory for Particle Physics
CH-1211 Geneva 23
Switzerland
+41 22 767 4417
Jean-Michel.Jouanigot@cern.ch
Daniel Karrenberg
RIPE Network Coordination Centre
Kruislaan 409
NL-1098 SJ Amsterdam
The Netherlands
+31 20 592 5065
D.Karrenberg@ripe.net
Bates, et al. [Page 53]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Marten Terpstra
Bay Networks, Inc.
2 Federal St
Billerica, MA 01821
USA
+1 508 436 8036
marten@BayNetworks.com
Jessica Yu
The University of Michigan
Merit Computer Network
1075 Beal Avenue
Ann Arbor, MI 48109
USA
+1 313 936 2655
jyy@merit.edu
Bates, et al. [Page 54]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Appendix A - Syntax for the aut-num object.
Here is a summary of the tags associated with aut-num object itself
and their status. The first column specifies the attribute, the
second column whether this attribute is mandatory in the aut-num
object, and the third column whether this specific attribute can
occur only once per object [single], or more than once [multiple].
When specifying multiple lines per attribute, the attribute name must
be repeated. See [6] the example for the descr: attribute.
aut-num: [mandatory] [single]
as-name: [optional] [single]
descr: [mandatory] [multiple]
as-in: [optional] [multiple]
as-out: [optional] [multiple]
interas-in: [optional] [multiple]
interas-out: [optional] [multiple]
as-exclude: [optional] [multiple]
default: [optional] [multiple]
tech-c: [mandatory] [multiple]
admin-c: [mandatory] [multiple]
guardian: [mandatory] [single]
remarks: [optional] [multiple]
notify: [optional] [multiple]
mnt-by: [optional] [multiple]
changed: [mandatory] [multiple]
source: [mandatory] [single]
Each attribute has the following syntax:
aut-num:
The 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, only one line allowed
Bates, et al. [Page 55]
RFC 1786 Representing IP Routing Policies in a RR March 1995
as-name:
The name associated with this AS. This should as short but as
informative as possible.
Format:
Text consisting of capitals, dashes ("-") and digits, but must
start with a capital.
Example:
as-name: NIKHEF-H
Status: single, only one line allowed
descr:
A short description of the Autonomous System.
Format:
free text
Example:
descr: NIKHEF section H
descr: Science Park Watergraafsmeer
descr: Amsterdam
Status: mandatory, multiple lines allowed
as-in:
A description of accepted routing information between AS peers.
Format:
from <aut-num> <cost> accept <routing policy expression>
The keywords from and accept are optional and can be omitted.
<aut-num> refers to your AS neighbor.
<cost> is a positive integer used to express a relative 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, AS Macros, Communities or
Route Lists.
A Route List is a list of routes in prefix length format,
Bates, et al. [Page 56]
RFC 1786 Representing IP Routing Policies in a RR March 1995
separated by commas, and surrounded by curly brackets
(braces, i.e. `{' and '}').
Examples:
as-in: from AS1103 100 accept AS1103
as-in: from AS786 105 accept AS1103
as-in: from AS786 10 accept AS786 HEPNET
as-in: from AS1755 110 accept AS1103 AS786
as-in: from AS3333 100 accept {192.87.45.0/16}
2. A set of KEYWORDS. The following KEYWORD is currently
defined:
ANY this means anything the neighbor AS knows.
3. A logical expression of either 1 or 2 above The current
logical operators are defined as:
AND
OR
NOT
This operators are defined as true BOOLEAN operators even
if the operands themselves do not appear to be BOOLEAN.
Their operations are defined as follows:
Operator Operation Example
OR UNION AS1 OR AS2
|
+-> all routes in AS1
or AS2.
AND INTERSECTION AS1 AND HEPNET
|
+-> a route in AS1 and
belonging to
community HEPNET.
NOT COMPLEMENT NOT AS3
|
+-> any route except
AS3 routes.
Bates, et al. [Page 57]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Rules are grouped together using parenthesis i.e "(" and
")".
The ordering of evaluation of operators and there
association is as follows:
Operator Associativity
() left to right
NOT right to left
AND left to right
OR left to right
NOTE: if no logical operator is given between ASes, AS-
macros, Communities, Route Lists and KEYWORDS it is
implicitly evaluated as an `OR' operation. The OR can be
left out for conciseness. However, please note the
operators are still evaluated as below so make sure you
include parentheses whenever needed. To highlight this
here is a simple example. If we denoted a policy of for
example; from AS1755 I accept all routes except routes
from AS1, A2 and AS3 and you enter the following as-in
line.
as-in: from AS1755 100 accept NOT AS1 AS2 AS3
This will be evaluated as:
as-in: from AS1755 100 accept NOT AS1 OR AS2 OR AS3
Which in turn would be evaluated like this:
(NOT AS1) OR AS2 OR AS3
-> ((ANY except AS1) union AS2) union AS3)
--> (ANY except AS1)
This is clearly incorrect and not the desired result. The
correct syntax should be:
as-in: from AS1755 100 accept NOT (AS1 AS2 AS3)
Bates, et al. [Page 58]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Producing the following evaluation:
NOT (AS1 OR AS2 OR AS3)
-> (ANY) except (union of AS1, AS2, AS3)
Which depicts the desired routing policy.
Note that can also be written as below which is perhaps
somewhat clearer:
as-in: from AS1755 100 accept ANY AND NOT
as-in: from AS1755 100 accept (AS1 OR AS2 OR AS3)
Examples:
as-in: from AS1755 100 accept ANY AND NOT (AS1234 OR AS513)
as-in: from AS1755 150 accept AS1234 OR {35.0.0.0/8}
A rule can be wrapped over lines providing the associated
<aut-num>, <cost> values and from and accept keywords are
repeated and occur on consecutive lines.
Example:
as-in: from AS1755 100 accept ANY AND NOT (AS1234 AS513)
and
as-in: from AS1755 100 accept ANY AND NOT (
as-in: from AS1755 100 accept AS1234 AS513)
are evaluated to the same result. Please note that the
ordering of these continuing lines is significant.
Status: optional, multiple lines allowed
Bates, et al. [Page 59]
RFC 1786 Representing IP Routing Policies in a RR March 1995
as-out:
A description of generated routing information sent to other AS
peers.
Format:
to <aut-num> announce <routing policy expression
The to and announce keywords are optional and can be omitted.
<aut-num> refers to your AS neighbor.
<routing policy expression> is explained in the as-in
attribute definition above.
Example:
as-out: to AS1104 announce AS978
as-out: to AS1755 announce ANY
as-out: to AS786 announce ANY AND NOT (AS978)
Status: optional, multiple lines allowed
interas-in:
Describes incoming local preferences on an inter AS connection.
Format:
from <aut-num> <local-rid> <neighbor-rid> <preference> accept
<routing policy expression>
The keywords from and accept are optional and can be omitted.
<aut-num> is an autonomous system as defined in as-in.
<local-rid> contains the IP address of the border router in
the AS describing the policy. IP address must be in prefix
length format.
<neighbor-rid> contains the IP address of neighbor AS's border
router from which this AS accept routes defined in the
<routing policy expression>. IP addresses must be in prefix
length format.
<preference> is defined as follows:
(<pref-type>=<value>)
It should be noted the parenthesis "(" and ")" and the
"<pref-type>" keyword must be present for this preference to
Bates, et al. [Page 60]
RFC 1786 Representing IP Routing Policies in a RR March 1995
be valid.
<pref-type> currently only supports "pref". It could be
expanded to other type of preference such as TOS/QOS as
routing technology matures.
<value> can take one of the following values:
<cost>
<cost> is a positive integer used to express a relative
cost of routes learned. The lower the cost the more
preferred the route. This <cost> value is only comparable
to other interas-in attributes, not to as-in attributes.
MED
This indicates the AS will use the
MUTLI_EXIT_DISCRIMINATOR (MED) metric, as implemented in
BGP4 and IDRP, sent from its neighbor AS.
NOTE: Combinations of MED and <cost> should be avoided
for the same destinations.
CAVEAT: The pref-type values may well be enhanced in the
future as more inter-ASs routing protocols introduce
other metrics.
Any route specified in interas-in and not specified in
as-in is assumed not accepted between the ASes concerned.
Diagnostic tools should flag this inconsistency as an
error. It should be noted that if an interas-in policy
is specified then it is mandatory to specify the
corresponding global policy in the as-in line. Please
note there is no relevance in the cost associated with
as-in and the preferences used in interas-in.
<routing policy expression> is an expression as defined in
as-in above.
Examples:
NB: This line is wrapped for readability.
interas-in: from AS1104 192.(pref=10)/accept.AS786.AS987
interas-in: from AS1104 192.87.45.(pref=20)2accept.AS987
interas-in: from AS1103 192.87.45.2(pref=MED)8accept2ANY
Status: optional, multiple lines allowed
Bates, et al. [Page 61]
RFC 1786 Representing IP Routing Policies in a RR March 1995
interas-out:
Format:
to <aut-num> <local-rid> <neighbor-rid> [<metric>] announce
<routing policy expression>
The keywords to and announce are optional and can be omitted.
The definitions of <aut-num>, <local-rid> <neighbor-rid>, and
<routing policy expression> are identical to those defined in
interas-in.
<metric> is optional and is defined as follows:
(<metric-type>=<value>)
It should be noted the parenthesis "(" and ")" and the
keywords of "<metric-type>" must be present for this metric to
be valid.
<metric-type> currently only supports "metric-out". It could
be expanded to other type of preference such as TOS/QOS as
routing technology matures.
<value> can take one of the following values:
<num-metric>
<num-metric> is a pre-configured metric for out-bound
routes. The lower the cost the more preferred the route.
This <num-metric> value is literally passed by the
routing protocol to the neighbor. It is expected that it
is used there which is indicated by pref=MED on the
corresponding interas-in attribute. It should be noted
that whether to accept the outgoing metric or not is
totally within the discretion of the neighbor AS.
IGP
This indicates that the metric reflects the ASs internal
topology cost. The topology is reflected here by using
MED which is derived from the AS's IGP metric.
NOTE: Combinations of IGP and <num-metric> should be
avoided for the same destinations.
CAVEAT: The metric-out values may well be enhanced in the
future as more interas protocols make use of metrics.
Any route specified in interas-out and not specified in
as-out is assumed not announced between the ASes
Bates, et al. [Page 62]
RFC 1786 Representing IP Routing Policies in a RR March 1995
concerned. Diagnostic tools should flag this
inconsistency as an error. It should be noted that if an
interas-out policy is specified then it is mandatory to
specify the corresponding global policy in the as-out
line.
Examples:
interas-out:ntoiAS1104p192.87.45.254/32t192.87.45.80/32
interas-out: to AS1104m192.87.45.254/32n192.87.45.80/32
interas-out: to AS1103 192.87.45.254/325192.87.45.80/32
(metric-out=IGP) announce ANY
Status: optional, multiple lines allowed
as-exclude:
A list of transit ASes to ignore all routes from.
Format:
exclude <aut-num> to <exclude-route-keyword>
Keywords exclude and to are optional and can again be omitted.
<aut-num> refers to the transit AS in question.
an <exclude-route-keyword> can be ONE of the following.
1. <aut-num>
2. AS macro
3. Community
4. ANY
Examples:
as-exclude: exclude AS690 to HEPNET
This means exclude any HEPNET routes which have a route via
AS690.
as-exclude: exclude AS1800 to AS-EUNET
This means exclude any AS-EUNET routes which have a route via
AS1800.
as-exclude: exclude AS1755 to AS1104
Bates, et al. [Page 63]
RFC 1786 Representing IP Routing Policies in a RR March 1995
This means exclude any AS1104 route which have a route via
AS1755.
as-exclude: exclude AS1104 to ANY
This means exclude all routes which have a route via AS1104.
Status: optional, multiple lines allowed
default:
An indication of how default routing is done.
Format:
<aut-num> <relative cost> <default-expression>
where <aut-num> is the AS peer you will default route to,
and <relative cost> is the relative cost is a positive integer
used to express a preference for default. There is no
relationship to the cost used in the as-in tag. The AS peer
with the lowest cost is used for default over ones with higher
costs.
<default-expression> is optional and provides information on
how a default route is selected. It can take the following
formats:
1. static. This indicates that a default is statically
configured to this AS peer.
2. A route list with the syntax as described in the as-in
attribute. This indicates that this list of routes is
used to generate a default route. A special but valid
value in this is the special route used by some routing
protocols to indicate default: 0.0.0.0/0
3. default. This is the same as {0.0.0.0/0}. This means that
the routing protocol between these two peers generates a
true default.
Examples:
default: AS1755 10
default: AS786 5 {140.222.0.0/16, 192.87.45.0/24}
default: AS2043 15 default
Status: optional, multiple lines allowed
Bates, et al. [Page 64]
RFC 1786 Representing IP Routing Policies in a RR March 1995
tech-c:
Full name or uniquely assigned NIC-handle of a technical contact
person. This is someone to be contacted for technical problems such
as misconfiguration.
Format:
<firstname> <initials> <lastname> or <nic-handle>
Example:
tech-c: John E Doe
tech-c: JED31
Status: mandatory, multiple lines allowed
admin-c:
Full name or uniquely assigned NIC-handle of an administrative
contact person. In many cases this would be the name of the
guardian.
Format:
<firstname> <initials> <lastname> or <nic-handle>
Example:
admin-c: Joe T Bloggs
admin-c: JTB1
Status: mandatory, multiple lines allowed
guardian:
Mailbox of the guardian of the Autonomous system.
Format:
<email-address>
The <email-address> should be in RFC822 domain format wherever
possible.
Example:
guardian: as1104-guardian@nikhef.nl
Status: mandatory, only one line and e-mail address allowed
Bates, et al. [Page 65]
RFC 1786 Representing IP Routing Policies in a RR March 1995
remarks:
Remarks/comments, to be used only for clarification.
Format:
free text
Example:
remarks: Multihomed AS talking to AS1755 and AS786
remarks: Will soon connect to AS1104 also.
Status: optional, multiple lines allowed
notify:
The notify attribute contains an email address to which
notifications of changes to this object should be sent. See also
[11].
Format:
<email-address>
The <email-address> should be in RFC822 domain syntax wherever
possible.
Example:
notify: Marten.Terpstra@ripe.net
Status: optional, multiple lines allowed
mnt-by:
The mnt-by attribute contains a registered maintainer name. See
also [11].
Format:
<registered maintainer name>
Example:
mnt-by: RIPE-DBM
Status: optional, multiple lines allowed
changed:
Who changed this object last, and when was this change made.
Format:
<email-address> YYMMDD
Bates, et al. [Page 66]
RFC 1786 Representing IP Routing Policies in a RR March 1995
<email-address> should be the address of the person who made
the last change. YYMMDD denotes the date this change was made.
Example:
changed: johndoe@terabit-labs.nn 900401
Status: mandatory, multiple lines allowed
source:
Source of the information.
This is used to separate information from different sources kept by
the same database software. For RIPE database entries the value is
fixed to RIPE.
Format:
RIPE
Status: mandatory, only one line allowed
Bates, et al. [Page 67]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Appendix B - Syntax details for the community object.
Here is a summary of the tags associated with community object itself
and their status. The first column specifies the attribute, the
second column whether this attribute is mandatory in the community
object, and the third column whether this specific attribute can
occur only once per object [single], or more than once [multiple].
When specifying multiple lines per attribute, the attribute name must
be repeated. See [6] the example for the descr: attribute.
community: [mandatory] [single]
descr: [mandatory] [multiple]
authority: [mandatory] [single]
guardian: [mandatory] [single]
tech-c: [mandatory] [multiple]
admin-c: [mandatory] [multiple]
remarks: [optional] [multiple]
notify: [optional] [multiple]
mnt-by: [optional] [multiple]
changed: [mandatory] [multiple]
source: [mandatory] [single]
Each attribute has the following syntax:
community:
Name of the community. The name of the community should be
descriptive of the community it describes.
Format:
Upper case text string which cannot start with "AS" or any
of the <routing policy expression> KEYWORDS. See Appendix
A.
Example:
community: WCW
Status: mandatory, only one line allowed
Bates, et al. [Page 68]
RFC 1786 Representing IP Routing Policies in a RR March 1995
descr:
A short description of the community represented.
Format:
free text
Example:
descr: Science Park Watergraafsmeer
descr: Amsterdam
Status: mandatory, multiple lines allowed
authority:
The formal authority for this community. This could be an
organisation, institute, committee, etc.
Format:
free text
Example:
authority: WCW LAN Committee
Status: mandatory, only one line allowed
guardian:
Mailbox of the guardian of the community.
Format:
<email-address>
The <email-address> should be in RFC822 domain format
wherever possible.
Example:
guardian: wcw-guardian@nikhef.nl
Status: mandatory, only one line and email address allowed
tech-c:
Full name or uniquely assigned NIC-handle of an technical
contact person for this community.
Bates, et al. [Page 69]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Format:
<firstname> <initials> <lastname> or <nic-handle>
Example:
tech-c: John E Doe
tech-c: JED31
Status: mandatory, multiple lines allowed
admin-c:
Full name or uniquely assigned NIC-handle of an administrative
contact person. In many cases this would be the name of the
guardian.
Format:
<firstname> <initials> <lastname> or <nic-handle>
Example:
admin-c: Joe T Bloggs
admin-c: JTB1
Status: mandatory, multiple lines allowed
remarks:
Remarks/comments, to be used only for clarification.
Format:
free text
Example:
remarks: Temporary community
remarks: Will be removed after split into ASes
Status: optional, multiple lines allowed
notify:
The notify attribute contains an email address to which
notifications of changes to this object should be send. See also
[11].
Format:
<email-address>
The <email-address> should be in RFC822 domain syntax
wherever possible.
Bates, et al. [Page 70]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example:
notify: Marten.Terpstra@ripe.net
Status: optional, multiple lines allowed
mnt-by:
The mnt-by attribute contains a registered maintainer name. See
also [11].
Format:
<registered maintainer name>
Example:
mnt-by: RIPE-DBM
Status: optional, multiple lines allowed
changed:
Who changed this object last, and when was this change made.
Format:
<email-address> YYMMDD
<email-address> should be the address of the person who
made the last change. YYMMDD denotes the date this change
was made.
Example:
changed: johndoe@terabit-labs.nn 900401
Status: mandatory, multiple lines allowed
source:
Source of the information.
This is used to separate information from different sources kept
by the same database software. For RIPE database entries the
value is fixed to RIPE.
Format:
RIPE
Status: mandatory, only one line allowed
Bates, et al. [Page 71]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Appendix C - AS Macros syntax definition.
Here is a summary of the tags associated with as-macro object itself
and their status. The first column specifies the attribute, the
second column whether this attribute is mandatory in the as-macro
object, and the third column whether this specific attribute can
occur only once per object [single], or more than once [multiple].
When specifying multiple lines per attribute, the attribute name must
be repeated. See [6] the example for the descr: attribute.
as-macro: [mandatory] [single]
descr: [mandatory] [multiple]
as-list: [mandatory] [multiple]
guardian: [mandatory] [single]
tech-c: [mandatory] [multiple]
admin-c: [mandatory] [multiple]
remarks: [optional] [multiple]
notify: [optional] [multiple]
mnt-by: [optional] [multiple]
changed: [mandatory] [multiple]
source: [mandatory] [single]
Each attribute has the following syntax:
as-macro:
The name of a macro containing at least two Autonomous Systems
grouped together for ease of administration.
Format:
AS-<string>
The <string> should be in upper case and not contain any
special characters.
Example:
as-macro: AS-EBONE
Status: mandatory, only one line allowed
descr:
A short description of the Autonomous System Macro.
Format:
free text
Bates, et al. [Page 72]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example:
descr: Macro for EBONE connected ASes
Status: mandatory, multiple lines allowed
as-list:
The list of ASes or other AS macros that make up this macro. It
should be noted that recursive use of AS macros is to be
encouraged.
Format:
<aut-num> <as-macro> ...
See Appendix A for <aut-num> definition.
Example:
as-list: AS786 AS513 AS1104
as-list: AS99 AS-NORDUNET
Status: mandatory, multiple lines allowed
guardian:
Mailbox of the guardian of this AS macro.
Format:
<email-address>
The <email-address> should be in RFC822 domain format
wherever possible.
Example:
guardian: as-ebone-guardian@ebone.net
Status: mandatory, only one line and e-mail address allowed
tech-c:
Full name or uniquely assigned NIC-handle of a technical contact
person for this macro. This is someone to be contacted for
technical problems such as misconfiguration.
Format:
<firstname> <initials> <lastname> or <nic-handle>
Bates, et al. [Page 73]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Examples:
tech-c: John E Doe
tech-c: JED31
Status: mandatory, multiple lines allowed
admin-c:
Full name or uniquely assigned NIC-handle of an administrative
contact person. In many cases this would be the name of the
guardian.
Format:
<firstname> <initials> <lastname> or <nic-handle>
Examples:
admin-c: Joe T Bloggs
admin-c: JTB1
Status: mandatory, multiple lines allowed
remarks:
Remarks/comments, to be used only for clarification.
Format:
free text
Example:
remarks: AS321 will be removed from this Macro shortly
Status: optional, multiple lines allowed
notify:
The notify attribute contains an email address to which
notifications of changes to this object should be send. See also
[11].
Format:
<email-address>
The <email-address> should be in RFC822 domain syntax
wherever possible.
Example:
notify: Marten.Terpstra@ripe.net
Bates, et al. [Page 74]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Status: optional, multiple lines allowed
mnt-by:
The mnt-by attribute contains a registered maintainer name. See
also [11].
Format:
<registered maintainer name>
Example:
mnt-by: RIPE-DBM
Status: optional, multiple lines allowed
changed:
Who changed this object last, and when was this change made.
Format:
<email-address> YYMMDD
<email-address> should be the address of the person who
made the last change. YYMMDD denotes the date this change
was made.
Example:
changed: johndoe@terabit-labs.nn 900401
Status: mandatory, multiple lines allowed
source:
Source of the information.
This is used to separate information from different sources kept
by the same database software. For RIPE database entries the
value is fixed to RIPE.
Format:
RIPE
Status: mandatory, only one line allowed
Bates, et al. [Page 75]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Appendix D - Syntax for the "route" object.
There is a summary of the tags associated with route object itself
and their status. The first column specifies the attribute, the
second column whether this attribute is mandatory in the community
object, and the third column whether this specific attribute can
occur only once per object [single], or more than once [multiple].
When specifying multiple lines per attribute, the attribute name must
be repeated. See [6] the example for the descr: attribute.
route: [mandatory] [single]
descr: [mandatory] [multiple]
origin: [mandatory] [single]
hole: [optional] [multiple]
withdrawn: [optional] [single]
comm-list: [optional] [multiple]
remarks: [optional] [multiple]
notify: [optional] [multiple]
mnt-by: [optional] [multiple]
changed: [mandatory] [multiple]
source: [mandatory] [single]
Each attribute has the following syntax:
route:
Route being announced.
Format:
Classless representation of a route with the RIPE database
known as the "prefix length" representation. See [10] for
more details on classless representations.
Examples:
route: 192.87.45.0/24
This represents addressable bits 192.87.45.0 to
192.87.45.255.
route: 192.1.128.0/17
This represents addressable bits 192.1.128.0 to
192.1.255.255.
Status: mandatory, only one line allowed
Bates, et al. [Page 76]
RFC 1786 Representing IP Routing Policies in a RR March 1995
origin:
The autonomous system announcing this route.
Format:
<aut-num>
See Appendix A for <aut-num> syntax.
Example:
origin: AS1104
Status: mandatory, only one line allowed
hole:
Denote the parts of the address space covered this route object
to which the originator does not provide connectivity. These
holes may include routes that are being currently routed by
another provider (e.g., a customer using that space has moved to
a different service provider). They may also include space that
has not yet been assigned to any customer.
Format:
Classless representation of a route with the RIPE database
known as the "prefix length" representation. See [10] for
more details on classless representations. It should be
noted that this sub-aggregate must be a component of that
registered in the route object.
Example:
hole: 193.0.4.0/24
Status: optional, multiple lines allowed
withdrawn:
Used to denote the day this route has been withdrawn from the
Internet routing mesh. This will be usually be used when a less
specific aggregate route is now routed the more specific (i.e.
this route) is not need anymore.
Format:
YYMMDD
YYMMDD denotes the date this route was withdrawn.
Bates, et al. [Page 77]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Example:
withdrawn: 940711
Status: optional, one line allowed.
comm-list:
List of one or more communities this route is part of.
Format:
<community> <community> ...
See Appendix B for <community> definition.
Example:
comm-list: HEP LEP
Status: optional, multiple lines allowed
remarks:
Remarks/comments, to be used only for clarification.
Format:
free text
Example:
remarks: Multihomed AS talking to AS1755 and AS786
remarks: Will soon connect to AS1104 also.
Status: optional, multiple lines allowed
notify:
The notify attribute contains an email address to which
notifications of changes to this object should be send. See also
[11].
Format:
<email-address>
The <email-address> should be in RFC822 domain syntax
wherever possible.
Example:
notify: Marten.Terpstra@ripe.net
Bates, et al. [Page 78]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Status: optional, multiple lines allowed
mnt-by:
The mnt-by attribute contains a registered maintainer name. See
also [11].
Format:
<registered maintainer name>
Example:
mnt-by: RIPE-DBM
Status: optional, multiple lines allowed
changed:
Who changed this object last, and when was this change made.
Format:
<email-address> YYMMDD
<email-address> should be the address of the person who
made the last change. YYMMDD denotes the date this change
was made.
Example:
changed: johndoe@terabit-labs.nn 900401
Status: mandatory, multiple lines allowed
source:
Source of the information.
This is used to separate information from different sources kept
by the same database software. For RIPE database entries the
value is fixed to RIPE.
Format:
RIPE
Status: mandatory, only one line allowed
Bates, et al. [Page 79]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Appendix E - List of reserved words
The following list of words are reserved for use within the
attributes of the AS object. The use of these words is solely for the
purpose of clarity. All keywords must be lower case.
accept
announce
exclude
from
to
transit
Examples of the usage of the reserved words are:
as-in: from <neighborAS> accept <route>
as-out: to <neighborAS> announce <route>
as-exclude: exclude <ASpath> to <destination>
as-transit: transit <ASpath> to <destination>
default: from <neighborAS> accept <route>
default: to <neighborAS> announce <route>
Note: that as-transit is an experimental attribute. See section 10.
Bates, et al. [Page 80]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Appendix F - Motivations for RIPE-81++
This appendix gives motivations for the major changes in this
proposal from ripe-81.
The main goals of the routing registry rework are:
SPLIT
Separate the allocation and routing registry functions into
different database objects. This will facilitate data management
if the Internet registry and routing registry functions are
separated (like in other parts of the world). It will also make
more clear what is part of the routing registry and who has
authority to change allocation vs. routing data.
CIDR
Add the possibility to specify classless routes in the routing
registry. Classless routes are being used in Internet
production now. Aggregation information in the routing registry
is necessary for network layer troubleshooting. It is also
necessary because aggregation influences routing policies
directly.
CALLOC
Add the possibility to allocate address space on classless
boundaries in the allocation registry. This is a way to preserve
address space.
CLEAN
To clean up some of the obsolete and unused parts of the routing
registry.
The major changes are now discussed in turn:
Introduce Classless Addresses
CIDR, CALLOC
Introduce route object.
SPLIT, CIDR and CALLOC.
Bates, et al. [Page 81]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Delete obsolete attributes from inetnum.
CLEAN.
Delete RIPE-DB and LOCAL from routing policy expressions.
CLEAN
Allow multiple ASes to originate the same route
Because it is being done. CIDR. Made possible by SPLIT.
Bates, et al. [Page 82]
RFC 1786 Representing IP Routing Policies in a RR March 1995
Appendix G - Transition strategy from RIPE-81 to RIPE-81++
Transition from the routing registry described by ripe-81 to the routing
registry described in this document is a straightforward process once
the new registry functions have been implemented in the database
software and are understood by the most commonly used registry tools.
The routing related attributes in the classful inetnum objects of ripe-
81 can be directly translated into new routing objects. Then these
attributes can be deleted from the inetnum object making that object if
conform to the new schema.
Proposed transition steps:
1) Implement classless addresses and new object definition in the
database software.
2) Make common tools understand the new schema and prefer it if both
old and new are present.
3) Invite everyone to convert their data to the new format. This can
be encouraged by doing conversions automatically and proposing them
to maintainers.
4) At a flag day remove all remaining routing information from the
inetnum objects. Before the flag day all usage of obsoleted
inetnum attributes has to cease and all other routing registry
functions have to be taken over by the new objects and attributes.
Bates, et al. [Page 83]
|
