Network Working Group D. Meyer Request for Comments: 2650 Cisco Systems Category: Informational J. Schmitz America On-Line C. Orange RIPE NCC M. Prior Connect C. Alaettinoglu USC/ISI August 1999
Using RPSL in Practice
Status of this Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This document is a tutorial on using the Routing Policy Specification Language (RPSL) to describe routing policies in the Internet Routing Registry (IRR). We explain how to specify various routing policies and configurations using RPSL, how to register these policies in the IRR, and how to analyze them using the routing policy analysis tools, for example to generate vendor specific router configurations.
This document is a tutorial on RPSL and is targeted towards an Internet/Network Service Provider (ISP/NSP) engineer who understands Internet routing, but is new to RPSL and to the IRR. Readers are referred to the RPSL reference document (RFC 2622) [1] for completeness. It is also good to have that document at hand while working through this tutorial.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
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The IRR is a repository of routing policies. Currently, the IRR repository is a set of five repositories maintained at the following sites: the CA*Net registry in Canada, the ANS, CW and RADB registries in the United States of America, and the RIPE registry in Europe. The five repositories are run independently. However, each site exchanges its data with the others regularly (at least once a day and as often as every ten minutes). CW, CA*Net and ANS are private registries which contain the routing policies of the networks and the customer networks of CW, CA*Net, and ANS respectively. RADB and RIPE are both public registries, and any ISP can publish their policies in these registries.
The registries all maintain up-to-date copies of one another's data. At any of the sites, the five registries can be inspected as a set. One should refrain from registering his/her data in more than one of the registries, as this practice leads almost invariably to inconsistencies in the data. The user trying to interpret the data is left in a confusing (at best) situation. CW, ANS and CA*Net customers are generally required to register their policies in their provider's registry. Others may register policies either at the RIPE or RADB registry, as preferred.
RPSL is based on RIPE-181 [2, 3], a language used to register routing policies and configurations in the IRR. Operational use of RIPE-181 has shown that it is sometimes difficult (or impossible) to express a routing policy which is used in practice. RPSL has been developed to address these shortcomings and to provide a language which can be further extended as the need arises. RPSL obsoletes RIPE-181.
RPSL constructs are expressed in one or more database "objects" which are registered in one of the registries described above. Each database object contains some routing policy information and some necessary administrative data. For example, an address prefix routed in the inter-domain mesh is specified in a route object, and the peering policies of an AS are specified in an aut-num object. The database objects are related to each other by reference. For example, a route object must refer to the aut-num object for the AS in which it is originated. Implicitly, these relationships define sets of objects, which can be used to specify policies effecting all members. For example, we can specify a policy for all routes of an ISP, by referring to the AS number in which the routes are registered to be originated.
When objects are registered in the IRR, they become available for others to query using a whois service. Figure 1 illustrates the use of the whois command to obtain the route object for 128.223.0.0/16. The output of the whois command is the ASCII representation of the route object. The syntax and semantics of the route object are
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described in Appendix A.3. Registered policies can also be compared with others for consistency and they can be used to diagnose operational routing problems in the Internet.
% whois -h whois.ra.net 128.223.0.0/16 route: 128.223.0.0/16 descr: UONet descr: University of Oregon descr: Computing Center descr: Eugene, OR 97403-1212 descr: USA origin: AS3582 mnt-by: MAINT-AS3582 changed: meyer@ns.uoregon.edu 19960222 source: RADB
Figure 1: whois command and a route object.
The RAToolSet [6] is a suite of tools which can be used to analyze the routing registry data. It includes tools to configure routers (RtConfig), tools to analyze paths on the Internet (prpath and prtraceroute), and tools to compare, validate and register RPSL objects (roe, aoe and prcheck).
In the following section, we will describe how common routing policies can be expressed in RPSL. The objects themselves are described in Appendix A. Authoritative information on the IRR objects, however, should be sought in RFC-2622, and authoritative information on general database objects (person, role, and maintainers) and on querying and updating the registry databases, should be sought in RIPE-157 [4]. Section 3.2 describes the use of RtConfig to generate vendor specific router configurations.
The key purpose of RPSL is to allow you to specify your routing configuration in the public Internet Routing Registry (IRR), so that you and others can check your policies and announcements for consistency. Moreover, in the process of setting policies and configuring routers, you take the policies and configurations of others into account.
In this section, we begin by showing how some simple peering policies can be expressed in RPSL. We will build on that to introduce various database objects that will be needed in order to register policies in the IRR, and to show how more complex policies can be expressed.
The peering policies of an AS are registered in an aut-num object which looks something like that in Figure 2. We will focus on the semantics of the import and export attributes in which peering policies are expressed. We will also describe some of the other key attributes in the aut-num object, but the reader should refer to RFC-2622 or to RIPE-157 for the definitive descriptions.
aut-num: AS2 as-name: CAT-NET descr: Catatonic State University import: from AS1 accept ANY import: from AS3 accept <^AS3+$> export: to AS3 announce ANY export: to AS1 announce AS2 AS3 admin-c: AO36-RIPE tech-c: CO19-RIPE mnt-by: OPS4-RIPE changed: orange@ripe.net source: RIPE
Figure 2: Autonomous System Object
Now consider Figure 3 (AS4 and AS5 in the figure will be discussed later). The peering policies expressed in the AS2 aut-num object in Figure 2 are typical for a small service provider providing connectivity for a customer AS3 and using AS1 for transit. That is, AS2 only accepts announcements from AS3 which:
o are originated in AS3; and
o have paths composed of only AS3's (^ in <^AS3+$> means that AS3 is the first member of the path, + means that AS3 occurs one or more times in the path, and $ means that no other AS can be present in the path after AS3) (1).
To AS1, AS2 announces only those routes which originate in their AS or in their customer's AS.
AS1--------AS2--------AS3 | | | | AS4--------AS5
Figure 3: Some Neighboring ASes.
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In the example above, "accept ANY" in the import attribute indicates that AS2 will accept any announcements that AS1 sends, and "announce ANY" in the export attribute indicates that any route that AS2 has in its routing table will be passed on to AS3. Assuming that AS1 announces "ANY" to AS2, AS2 is taking full routing from AS1.
Note that with this peering arrangement, if AS1 adds or deletes route objects, there is no need to update any of the aut-num objects to continue the full routing policy. Added (or deleted) route objects will implicitly update AS1's and AS2's policies.
While the peering policy specified in Figure 2 for AS2 is common, in practice many peering agreements are more complex. Before we consider more examples, however, let's first consider the aut-num object itself. Note that it is just a set of attribute labels and values which can be submitted to one of the registry databases. This particular object is specified as being in (or headed for) the RIPE registry (see the last line in Figure 2). The source should be specified as one of ANS, CANET, CW, RADB, or RIPE depending on the registry in which the object is maintained. The source attribute must be specified in every database object.
It is also worth noting that this object is "maintained by" OPS4-RIPE (the value of the mnt-by attribute), which references a "mntner" object. Because the aut-num object may be used for router configuration and other operational purposes, the readers need to be able to count on the validity of its contents. It is therefore required that a mntner be specified in the aut-num and in most other database objects, which means you must create a mntner object before you can register your peering policies. For brief information on the "mntner" object and object writeability, see Appendix A of this document. For more extensive information on how to set up and use a mntner to protect your database objects, see Section 2.3 of RIPE-157.
It is not uncommon for an ISP to acquire connectivity from a transit provider which announces all routes to it, which it in turn passes on to its customers to allow them to access hosts on the global Internet. Meanwhile, the ISP will generally announce the routes of its customers networks to the transit ISP, making them accessible on the global Internet. This is the service that is specified in Figure 2 for AS3.
Consider again Figure 3. Suppose now that AS2 wants to provide the same service to AS4. Clearly, it would be easy to modify the import and export lines in the aut-num object for AS2 (Figure 2) to those shown in Figure 4.
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import: from AS1 accept ANY import: from AS3 accept <^AS3+$> import: from AS4 accept <^AS4+$> export: to AS3 announce ANY export: to AS4 announce ANY export: to AS1 announce AS2 AS3 AS4
Figure 4: Policy for AS3 and AS4 in the AS2 as-num object
These changes are trivial to make of course, but clearly as the number of AS2 customers grows, it becomes more difficult to keep track of, and to prevent errors. Note also that if AS1 is selective about only accepting routes from the customers of AS2 from AS2, the aut-num object for AS1 would have to be adjusted to accommodate AS2's new customer.
By using the RPSL "as-set" object, we can simplify this significantly. In Figure 5, we describe the customers of AS2. Having this set to work with, we can now rewrite the policies in Figure 2 as shown in Figure 6.
import: from AS1 accept ANY import: from AS2:AS-CUSTOMERS accept <^AS2:AS-CUSTOMERS+$> export: to AS2:AS-CUSTOMERS announce ANY export: to AS1 announce AS2 AS2:AS-CUSTOMERS
Figure 6: Policy in the AS2 aut-num object for all AS2 customers
Note that if the aut-num object for AS1 contains the line:
import: from AS2 accept <^AS2+ AS2:AS-CUSTOMERS*$>
then no changes will need to be made to the aut-num objects for AS1 or AS2 as the AS2 customer base grows. The AS numbers for new customers can simply be added to the as-set AS2:AS-CUSTOMERS, and everything will work as for the existing customers. Clearly in terms of readability, scalability and maintainability, this is a far better mechanism when compared to adding policy for the customer AS's to the aut-num objects directly. The policy in this particular example states that AS1 will accept route announcements from AS2 in which the first element of the path is AS2, followed by more occurrences of
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AS2, and then 0 or more occurrences of any AS2 customer (e.g. any member of the as-set AS2:AS-CUSTOMERS).
Alternatively, one may wish to limit the routes one accepts from a peer, especially if the peer is a customer. This is recommended for several reasons, such as preventing the improper use of unassigned address space, and of course malicious use of another organization's address space.
Such filtering can be expressed in various ways in RPSL. Suppose the address space 7.7.0.0/16 has been allocated to the ISP managing AS3 for assignment to its customers. AS3 may want to announce part or all of this block on the global Internet. Suppose AS2 wants to be certain that it only accepts announcements from AS3 for address space that has been properly allocated to AS3. AS2 might then modify the AS3 import line in Figure 2 to read:
import: from AS3 accept { 7.7.0.0/16^16-19 }
which states that route announcements for this address block will be accepted from AS3 if they are of length upto /19. This of course will have to be modified if and when AS3 gets more address space. Moreover, it is again clear that for an ISP with a growing or changing customer base, this mechanism will not scale well.
Luckily RPSL supports the notion of a "route-set" which, as shown in Figure 7, can be used to specify the set of routes that will be accepted from a given customer. Given this set (and a similar one for AS4), the manager of AS2 can now filter on the address space that will be accepted from their customers by modifying the import lines in the AS2 aut-num object as shown in Figure 8.
import: from AS1 accept ANY import: from AS3 accept AS2:RS-ROUTES:AS3 import: from AS4 accept AS2:RS-ROUTES:AS4 export: to AS2:AS-CUSTOMERS announce ANY export: to AS1 announce AS2 AS2:AS-CUSTOMERS
Figure 8: Policy in the AS2 aut-num object for address based filtering on AS2 customers
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Note that this is now only slightly more complex than the example in Figure 6. Furthermore, nothing need be changed in the AS2 aut-num object due to address space changes for a customer, and this filtering can be supported without any changes to the AS1 aut-num object. The additional complexity is due to the two route set names being different, otherwise we could have combined the two import statements into one. Please note that the set names are constructed hierarchically. The first AS number denotes whose sets these are, and the last AS number parameterize these sets for each peer. RPSL allows the peer's AS number to be replaced by the keyword PeerAS.
Hence,
import: from AS3 accept AS2:RS-ROUTES:PeerAS import: from AS4 accept AS2:RS-ROUTES:PeerAS
has the same meaning as the corresponding import statements in Figure 6. This lets us combine the two import statements into one as shown in Figure 9.
import: from AS1 accept ANY import: from AS2:AS-CUSTOMERS accept AS2:RS-ROUTES:PeerAS export: to AS2:AS-CUSTOMERS announce ANY export: to AS1 announce AS2 AS2:AS-CUSTOMERS
Figure 9: Policy in the AS2 aut-num object using PeerAS
In the above examples peerings were only given among ASes. However, the peerings may be described in much more detail by RPSL, where peerings can be specified between physical routers using IP addresses in the import and export attributes. Figure 10 shows a simple example in which AS1 and AS2 are connected to an exchange point IX. While AS1 has only one connection to the exchange point via a router interface with IP address 7.7.7.1, AS2 has two different connections with IP address 7.7.7.2 and 7.7.7.3. The first AS may then define its routing policy in more detail by specifying its boundary router.
Figure 10: Including interfaces in peerings definitions
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aut-num: AS1 import: from AS2 at 7.7.7.1 accept <^AS2+$>
Because AS1 has only one connection to the exchange point in this example, this specification does not differ from that in which no boundary router is specified. However, AS1 might want to choose to accept only those announcements from AS2 which come from the router with IP address 7.7.7.2 and disregard those announcements from router 7.7.7.3. AS1 can specify this routing policy as follows:
aut-num: AS1 import: from AS2 7.7.7.2 at 7.7.7.1 accept <^AS2+$>
By selecting certain pairs of routers in a peering specification, others can be denied. If no routers are included in a policy clause then it is assumed that the policy applies to all peerings among the ASes involved.
The specification of peerings among ASes is not limited to one router for each AS. In figure 10 one of the two connections of AS2 to the exchange point IX might be used as backup in case the other connection fails. Let us assume that AS1 wants to use the connection to router 7.7.7.2 of AS2 during regular operations, and router 7.7.7.3 as backup. In a router configuration this may be done by setting a local preference. The equivalent in RPSL is a corresponding action definition in the peering description. The action definitions are inserted directly before the accept keyword.
aut-num: AS1 import: from AS2 7.7.7.2 at 7.7.7.1 action pref=10; from AS2 7.7.7.3 at 7.7.7.1 action pref=20; accept <^AS2+$>
pref is opposite to local-pref in that the smaller values are preferred over larger values. Actions may also be defined without specifying IP addresses of routers. If no routers are included in the policy clause then it is assumed that the actions are carried out for all peerings among the ASes involved.
In the previous example AS1 controls where it sends its traffic and which connection is used as backup. However, AS2 may also define a backup connection in an export clause:
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aut-num: AS2 export: to AS1 7.7.7.1 at 7.7.7.2 action med=10; to AS1 7.7.7.1 at 7.7.7.3 action med=20; announce <^AS2+$>
The definition given here for AS2 is the symmetric counterpart to the routing policy of AS1. The selection of routing information is done by setting the multi exit discriminator metric med. Actually, med metrics will not be used in practice like this; they are more suitable for load balancing including backups. For more details on med metrics refer to the BGP-4 RFC [7]. To use the med to achieve load balancing, one often sets it to the "IGP metric". This is specified in RPSL as:
aut-num: AS2 export: to AS1 action med=igp_cost; announce <^AS2+$>
Hence, both routers will set the med to the IGP metric at that router, causing some routes to be preferred at one of the routers and other routes at the other router.
2.5 Multi-Home Routing Policies using the community Attribute
RFC 1998 [9] describes the use of the BGP community attribute to provide support for load balancing and backup connections of multi- homed autonomous systems. In this section, we use stepwise refinement of an example to illustrate how those policies might be specified using RPSL.
The basic premise of RFC 1998 is to use the BGP community attribute to allow a customer to configure the BGP "LOCAL_PREF" on a provider's routers. This will allow the customer to influence the provider's route selection, normally by lowering the BGP "LOCAL_PREF" to indicate backup arrangements.
In this example, we illustrate how AS1 (the provider) might specify their policy so that a customer (AS4) connected to two of AS1's direct customers (AS2 and AS3) might signal to AS1 which connection is to be preferred.
AS1's base policy is to only accept routes from customers that are originated by the customer, or by the customer's customers. This leads to a policy such as:
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aut-num: AS1 import: from AS2 accept (AS2 OR AS4) AND <^AS2+ AS4*$> import: from AS3 accept (AS3 OR AS4) AND <^AS3+ AS4*$> import: from AS5 accept AS5 AND <^AS5+$>
Note that AS4 is a customer of AS2 and AS3, and AS5 does not have its own customers.
Now suppose we want to add some policy to describe that if a customer tags a route with community 1:1 then AS1 will act on this to reduce the BGP "LOCAL_PREF" by 10.
aut-num: AS1 import: from AS2 action pref=10; accept (AS2 OR AS4) AND <^AS2+ AS4*$> AND community.contains(1:1) import: from AS2 action pref=0; accept (AS2 OR AS4) AND <^AS2+ AS4*$> import: from AS3 action pref=10; accept (AS3 OR AS4) AND <^AS3+ AS4*$> AND community.contains(1:1) import: from AS3 action pref=0; accept (AS3 OR AS4) AND <^AS3+ AS4*$> import: from AS5 action pref=10; accept AS5 AND <^AS5+$> AND community.contains(1:1) import: from AS5 action pref=0; accept AS5 AND <^AS5+$>
We can see here that basically we are adding identical statements for each peering to the policy. This is the ideal candidate for RPSL's refine statement. This will make the policy more concise and avoid some of the potential for errors as more peering statements are added in the future:
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aut-num: AS1 import: { from AS-ANY action pref=10; accept community.contains(1:1); from AS-ANY action pref=0; accept ANY; } refine { from AS2 accept (AS2 OR AS4) AND <^AS2+ AS4*$>; from AS3 accept (AS3 OR AS4) AND <^AS3+ AS4*$>; from AS5 accept AS5 AND <^AS5+$>; }
Now, we can clearly see that any route that has been accepted from a customer that contains the community 1:1 will have it's local preference value reduced by 10.
The refinement has cleaned up some of the policy but we still have a large number of individual policies representing the same basic provider policy "from the customer, accept customer routes". These can be simplified by using AS sets.
First, we will collect together all of AS1's customers into a single AS set, AS1:AS-CUSTOMERS. We use a hierarchical set name that start with AS1 to avoid possible set name clashes in IRR with other ASes:
as-set: AS1:AS-CUSTOMERS members: AS2, AS3, AS5
We also define one set for each customer which lists the AS numbers of any of their customers.
as-set: AS1:AS-CUSTOMERS:AS2 members: AS4
as-set: AS1:AS-CUSTOMERS:AS3 members: AS4
as-set: AS1:AS-CUSTOMERS:AS5 members: # AS5 has no customers yet, so keep blank for now
We can now use the keyword PeerAS with these AS sets to simplify the policy further:
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aut-num: AS1 import: { from AS-ANY action pref=10; accept community.contains(1:1); from AS-ANY action pref=0; accept ANY; } refine { from AS1:AS-CUSTOMERS accept (PeerAS OR AS1:AS-CUSTOMER:PeerAS) AND <^PeerAS+ AS1:AS-CUSTOMER:PeerAS*$> }
The use of PeerAS with AS1:AS-CUSTOMERS is basically equivalent to looping over the members of AS1:AS-CUSTOMERS, expanding the policy by replacing PeerAS with a member from the set AS1:AS-CUSTOMERS.
To illustrate how this policy might be utilised by AS4, we present the following policy fragment:
aut-num: AS4 export: to AS2 action community.append(1:1); announce AS1 export: to AS3 announce AS1
Here, AS4 is signalling AS1 to prefer the routes from AS3.
In this section, we briefly introduce a number of tools which can be used to inspect data in the database, to determine optimal routing policies, and enter new data.
All the examples shown in the previous sections may well be edited by hand. They may be extracted one by one from the IRR using the whois program, edited, and then handed back to the registry robots. However, again the RAToolSet [6] provides a very nice tool which makes working with aut-num objects much easier: the aut-num object editor aoe.
The aut-num object editor has a graphical user interface to view and manipulate aut-num objects registered at any IRR. New aut-num objects may be generated using templates and submitted to the registries.
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Moreover, the routing policy from the databases may be compared to real life peerings. Therefore, aoe is highly recommended as an interface to the IRR for aut-num objects. Further information on aoe is available together with the RAToolSet [6].
RtConfig is a tool developed by the Routing Arbiter project [8] to generate vendor specific router configurations from the policy data held in the various IRRs. RtConfig currently supports Cisco, gated and RSd configuration formats. It has been publicly available since late 1994, and is currently being used by many sites for router configuration. The next section describes a methodology for generating vendor specific router configurations using RtConfig (2).
The general paradigm for using RtConfig involves registering policy in an IRR, building a RtConfig source file, then running running RtConfig against the source file and the IRR database to create vendor specific router configuration for the specified policy. The source file will contain vendor specific commands as well as RtConfig commands. To make a source file, pick up one of your router configuration files and replace the vendor specific policy configuration commands with the RtConfig commands.
Commands beginning with @RtConfig instruct RtConfig to perform special operations. An example source file is shown in Figure 11. In this example, commands such as "@RtConfig import AS3582 198.32.162.1 AS3701 198.32.162.2" instruct RtConfig to generate vendor specific import policies where the router 198.32.162.1 in AS3582 is importing routes from router 198.32.162.2 in AS3701. The other @RtConfig commands instruct the RtConfig to use certain names and numbers in the output that it generates (please refer to RtConfig manual [8] for additional information).
Once a source file is created, the file is processed by RtConfig (the default IRR is the RADB, and the default vendor is Cisco; however, command line options can be used to override these values). The result of running RtConfig on the source file in Figure 11 is shown in Figure 19 in Appendix B.
In this appendix, we introduce the RPSL objects required to implement many typical Internet routing policies. RFC-2622 and RIPE-157 provide the authoritative description for these objects and for the RPSL syntax, but this appendix will often be sufficient in practice.
The frequently needed objects are:
o maintainer objects (mntner)
o autonomous system number objects (aut-num)
o route objects (route)
o set objects (as-set, route-set)
and they are described in the following sections. To make your routing policies and configuration public, these objects should be registered in exactly one of the IRR registries.
In general, you can register your information by sending the appropriate objects to an email address for the registry you use. The email should consist of the objects you want to have registered or modified, separated by empty lines, and preceded by some kind of authentication details (see below). The registry robot processes your mail and enters new objects into the database, deletes old ones (upon request), or makes the requested modifications.
You will receive a response indicating the status of your submission. As the emails are handled automatically, the response is generally very fast. However, it should be remembered that a significant number of updates are also sometimes submitted to the database (by other robots), so the response time cannot be guaranteed. The email addresses for submitting objects to the existing registries are listed in Figure 12.
ANS auto-dbm@ans.net CANET auto-dbm@canet.net CW auto-rr@cw.net RADB auto-dbm@ra.net RIPE auto-dbm@ripe.net
Figure 12: Email addresses to register policy objects in IRR.
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Because it is required that a maintainer be specified in many of the database objects, a mntner is usually the first to be created. To have it properly authenticated, a mntner object is added manually by registry staff. Thereafter, all database submissions, deletions and modifications should be done through the registry robot.
Each of the registries can provide additional information and support for users. For the public registries this support is available from the email addresses listed in Figure 13.
RADB db-admin@ra.net RIPE ripe-dbm@ripe.net
Figure 13: Support email addresses.
If you are using one of the private registries, your service provider should be able to address your questions.
The maintainer object is used to introduce some kind of authorization for registrations. It lists various contact persons and describes security mechanisms that will be applied when updating objects in the IRR. Registering a mntner object is the first step in creating policies for an AS. An example is shown in Figure 14. The maintainer is called MAINT-AS3701. The contact person here is the same for administrative (admin-c) and technical (tech-c) issues and is referenced by the NIC-handle DMM65. NIC-handles are unique identifiers for persons in registries. Refer to registry documentation for further details on person objects and usage of NIC-handles.
The example shows two authentication mechanisms: CRYPT-PW and MAIL- FROM. CRYPT-PW takes as its argument a password that is encrypted with Unix crypt (3) routine. When sending updates, the maintainer adds the field password: <cleartext password> to the beginning of any requests that are to be authenticated. MAIL-FROM takes an argument that is a regular expression which covers a set of mail addresses. Only users with any of these mail addresses are authorized to work with objects secured by the corresponding maintainer (3).
The security mechanisms of the mntner object will only be applied on those objects referencing a specific mntner object. The reference is done by adding the attribute mnt-by to an object using the name of the mntner object as its value. In Figure 14, the maintainer MAINT- AS3701 is maintained by itself.
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mntner: MAINT-AS3701 descr: Network for Research and Engineering in Oregon remark: Internal Backbone admin-c: DMM65 tech-c: DMM65 upd-to: noc@nero.net auth: CRYPT-PW 949WK1mirBy6c auth: MAIL-FROM .*@nero.net notify: noc@nero.net mnt-by: MAINT-AS3701 changed: meyer@antc.uoregon.edu 970318 source: RADB
The autonomous system object describes the import and export policies of an AS. Each organization registers an autonomous system object (aut-num) in the IRR for its AS. Figure 15 shows the aut-num for AS3582 (UONET).
The autonomous system object lists contacts (admin-c, tech-c) and is maintained by (mnt-by) MAINT-AS3701 which is the maintainer displayed in Figure 14.
The most important attributes of the aut-num object are import and export. The import clause of an aut-num specifies import policies, while the export clause specifies export policies. The corresponding clauses allow a very detailed description of the routing policy of the AS specified. The details are given in section 2.
With these clauses, an aut-num object shows its relationship to other autonomous systems by describing its peerings. In addition, it also defines a routing entity comprising a group of IP networks which are handled according to the rules defined in the aut-num object. Therefore, it is closely linked to route objects.
In this example, AS3582 imports all routes from AS3701 by using the keyword ANY. AS3582 imports only internal routes from AS4222, AS5650, and AS1798. The import policy for for AS2914 is slightly more complex. Since AS2914 provides transit to various other ASes, AS3582 accepts routes with ASPATHs that begin with AS2194 followed by members of AS-WNA, which is an as set (see section A.4.1 below) describing those customers that transit AS2914.
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Since AS3582 is a multi-homed stub AS (i.e., it does not provide transit), its export policy consists simply of "announce AS3582" clauses; that is, announce internal routes of AS3582. These routes are those in route objects where the origin attribute is AS3582.
aut-num: AS3582 as-name: UONET descr: University of Oregon, Eugene OR import: from AS3701 accept ANY import: from AS4222 accept <^AS4222+$> import: from AS5650 accept <^AS5650+$> import: from AS2914 accept <^AS2914+ (AS-WNA)*$> import: from AS1798 accept <^AS1798+$> export: to AS3701 announce AS3582 export: to AS4222 announce AS3582 export: to AS5650 announce AS3582 export: to AS2914 announce AS3582 export: to AS1798 announce AS3582 admin-c: DMM65 tech-c: DMM65 notify: nethelp@ns.uoregon.edu mnt-by: MAINT-AS3582 changed: meyer@antc.uoregon.edu 970316 source: RADB
Figure 15: Autonomous System Object
The aut-num object forms the basis of a scalable and maintainable router
route: 128.223.0.0/16 origin: AS3582 descr: UONet descr: University of Oregon descr: Computing Center descr: Eugene, OR 97403-1212 descr: USA mnt-by: MAINT-AS3582 changed: meyer@ns.uoregon.edu 960222 source: RADB
Figure 16: Example of a route object
configuration system. For example, if AS3582 originates a new route, it need only create a route object for that route with origin AS3582. AS3582 can now build configuration using this route object without changing its aut-num object.
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Similarly, if for example, AS3701 originates a new route, it need only create a route object for that route with origin AS3701. Both AS3701 and AS3582 can now build configuration using this route object without modifying its aut-num object.
In contrast to aut-num objects which describe propagation of routing information for an autonomous system as a whole, route objects define single routes from an AS. An example is given in Figure 16.
This route object is maintained by MAINT-AS3582 and references AS3582 by the origin attribute. By this reference it is grouped together with other routes of the same origin AS, becoming member of the routing entity denoted by AS3582. The routing policies can then be defined in the aut-num objects for this group of routes.
Consequently, the route objects give the routes from this AS which are distributed to peer ASes according to the rules of the routing policy. Therefore, for any route in the routing tables of the backbone routers a route object must exist in one of the registries in IRR. route objects must be registered in the IRR only for the routes seen outside your AS. Normally, this set of external routes is different from the routes internally visible within your AS. One of the major reasons is that external peers need no information at all about your internal routing specifics. Therefore, external routes are in general aggregated combinations of internal routes, having shorter IP prefixes where applicable according to the CIDR rules. Please see the CIDR FAQ [5] for a tutorial introduction to CIDR. It is strongly recommended that you aggregate your routes as much as possible, thereby minimizing the number of routes you inject into the global routing table and at the same time reducing the corresponding number of route objects in the IRR.
While you may easily query single route objects using the whois program, and submit objects via mail to the registry robots, this becomes kind of awkward for larger sets. The RAToolSet [6] offers several tools to make handling of route objects easier. If you want to read policy data from the IRR and process it by other programs, you might be interested in using peval which is a low level policy evaluation tool. As an example, the command
peval -h whois.ra.net AS3582
will give you all route objects from AS3582 registered with RADB.
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A much more sophisticated tool from the RAToolSet to handle route objects interactively is the route object editor roe. It has a graphical user interface to view and manipulate route objects registered at any IRR. New route objects may be generated from templates and submitted to the registries. Moreover, the route objects from the databases may be compared to real life routes. Therefore, roe is highly recommended as an interface to the IRR for route objects. Further information on peval and roe is available together with the RAToolSet [6].
With routing policies it is often necessary to reference groups of autonomous systems or routes which have identical properties regarding a specific policy. To make working with such groups easier RPSL allows to combine them in set objects. There are two basic types of predefined set objects, as-set, and route-set. The RPSL set objects are described below.
Autonomous system set objects (as-set) are used to group autonomous system objects into named sets. An as-set has an RPSL name that starts with "AS-". In the example in Figure 17, an as-set called AS-NERO-PARTNERS and containing AS3701, AS4201, AS3582, AS4222, AS1798 is defined. The as-set is the RPSL replacement for the RIPE- 181 as-macro. It has been extended to include ASes in the set indirectly by referencing as set names in the aut-num objects.
AS-SETs are particularly useful when specifying policies for groups such as customers, providers, or for transit. You are encouraged to register sets for these groups because it is most likely that you will treat them alike, i.e. you will have a very similar routing policy for all your customers which have an autonomous system of their own. You may as well discover that this is also true for the providers you are peering with, and it is most convenient to have the ASes combined in one as-set for which you offer transit. For example, if a transit provider specifies its import policy using its customer's as-set (i.e., its import clause for the customer contains the customer's as-set), then that customer can modify the set of ASes that its transit provider accepts from it. Again, this can be accomplished without requiring the customer or the transit provider to modify its aut-num object.
The ASes of the set are simply compiled in a comma delimited list following the members attribute of the as-set. This list may also contain other AS-SET names.
A route-set is a way to name a group of routes. The syntax is similar to the as-set. A route-set has an RPSL name that starts with "RS-". The members attribute lists the members of the set. The value of a members attribute is a list of address prefixes, or route-set names. The members of the route-set are the address prefixes or the names of other route sets specified.
Figure 18 presents some example route-set objects. The set rs-uo contains two address prefixes, namely 128.223.0.0/16 and 198.32.162.0/24. The set rs-bar contains the members of the set rs- uo and the address prefix 128.7.0.0/16. The set rs-martians illustrate the use of range operators. 0.0.0.0/0^32 are the length 32 more specifics of 0.0.0.0/0, i.e. the host routes; 224.0.0.0/3^+ are the more specifics of 224.0.0.0/3, i.e. the routes falling into the multicast address space. For more complete list of range operators please refer to RFC-2622.
no access-list 100 access-list 100 permit ip 128.223.0.0 0.0.0.0 255.255.0.0 0.0.0.0 access-list 100 deny ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 ! no route-map AS3701-EXPORT route-map AS3701-EXPORT permit 1 match ip address 100 ! router bgp 3582 neighbor 198.32.162.2 route-map AS3701-EXPORT out ! no route-map AS3701-IMPORT route-map AS3701-IMPORT permit 1 set local-preference 1000 ! router bgp 3582 neighbor 198.32.162.2 route-map AS3701-IMPORT in ! ! WNA/VERIO neighbor 198.32.162.6 remote-as 2914 ! no route-map AS2914-EXPORT route-map AS2914-EXPORT permit 1 match ip address 100 ! router bgp 3582 neighbor 198.32.162.6 route-map AS2914-EXPORT out no ip as-path access-list 100 ip as-path access-list 100 permit ^_2914(((_[0-9]+))*_ \ (13|22|97|132|175|668|1914|2905|2914|3361|3381|3791|3937| \ 4178|4354|4571|4674|4683|5091|5303|5798|5855|5856|5881|6083 \ |6188|6971|7790|7951|8028))?$ ! no route-map AS2914-IMPORT route-map AS2914-IMPORT permit 1 match as-path 100 set local-preference 998
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! router bgp 3582 neighbor 198.32.162.6 route-map AS2914-IMPORT in
Figure 19: Output of RtConfig
Security Considerations
This document is a tutorial to RPSL, it does not define protocols or standards that need to be secured.
Endnotes
(1) AS-PATH regular expressions are POSIX compliant regular expressions.
(2) Discussion of RtConfig internals is beyond the scope of this document.
(3) Clearly, neither of these mechanisms is sufficient to provide strong authentication or authorization. Other public key (e.g., PGP) authentication mechanisms are available from some of the IRRs.
References
[1] Alaettinoglu, C., Villamizar, C., Gerich, E., Kessens, D., Meyer, D., Bates, T., Karrenberg, D. and M. Terpstra, "Routing Policy Specification Language (RPSL)", RFC 2622, June 1999.
[2] Bates, T., Jouanigot, J-M., Karrenberg, D., Lothberg, P. and M. Terpstra, "Representation of IP Routing Policies in the RIPE database", Technical Report ripe-81, RIPE, RIPE NCC, Amsterdam, Netherlands, February 1993.
[3] T. Bates, E. Gerich, J. Joncharay, J-M. Jouanigot, D. Karrenberg, M. Terpstra, and J. Yu. Representation of IP Routing Policies in a Routing Registry, Technical Report ripe-181, RIPE, RIPE NCC, Amsterdam, Netherlands, October 1994.
[4] A. M. R. Magee. RIPE NCC Database Documentation. Technical Report RIPE-157, RIPE NCC, Amsterdam, Netherlands, May 1997.
[9] Chen, E. and T. Bates, "An Application of the BGP Community Attribute in Multi-Home Routing", RFC 1998, August 1996.
Authors' Addresses
David Meyer Cisco Systems
EMail: dmm@cisco.com
Joachim Schmitz America On-Line
EMail: SchmitzJo@aol.com
Carol Orange RIPE NCC
EMail: orange@spiritone.com
Mark Prior connect.com.au pty ltd
EMail: mrp@connect.com.au
Cengiz Alaettinoglu USC/Information Sciences Institute
EMail: cengiz@isi.edu
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