T.J. Watson Research Center, IBM Corp.
Application of the Border Gateway Protocol in the Internet
Status of this Memo
This protocol is being developed by the Border Gateway Protocol Working Group (BGP) of the Internet Engineering Task Force (IETF). This RFC specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
This document, together with its companion document, "A Border Gateway Protocol (BGP-3)", define an inter-autonomous system routing protocol for the Internet. "A Border Gateway Protocol (BGP-3)" defines the BGP protocol specification, and this document describes the usage of the BGP in the Internet.
Information about the progress of BGP can be monitored and/or reported on the BGP mailing list (email@example.com).
Table of Contents
1. Introduction................................................... 2 2. BGP Topological Model.......................................... 3 3. BGP in the Internet............................................ 4 4. Policy Making with BGP......................................... 5 5. Path Selection with BGP........................................ 6 6. Required set of supported routing policies..................... 8 7. Conclusion..................................................... 9 Appendix A. The Interaction of BGP and an IGP..................... 9 References........................................................ 12 Security Considerations........................................... 12 Authors' Addresses................................................ 13
This document was original published as RFC 1164 in June 1990, jointly authored by Jeffrey C. Honig (Cornell University), Dave Katz (MERIT), Matt Mathis (PSC), Yakov Rekhter (IBM), and Jessica Yu (MERIT).
The following also made key contributions to RFC 1164 -- Guy Almes (ANS, then at Rice University), Kirk Lougheed (cisco Systems), Hans- Werner Braun (SDSC, then at MERIT), and Sue Hares (MERIT).
This updated version of the document is the product of the IETF BGP Working Group with Phillip Gross (ANS) and Yakov Rekhter (IBM) as editors. John Moy (Proteon) contributed Section 6 "Recommended set of supported routing policies".
We also like to explicitly thank Bob Braden (ISI) for the review of this document as well as his constructive and valuable comments.
This memo describes the use of the Border Gateway Protocol (BGP)  in the Internet environment. BGP is an inter-Autonomous System routing protocol. The network reachability information exchanged via BGP provides sufficient information to detect routing loops and enforce routing decisions based on performance preference and policy constraints as outlined in RFC 1104 . In particular, BGP exchanges routing information containing full AS paths and enforces routing policies based on configuration information.
All of the discussions in this paper are based on the assumption that the Internet is a collection of arbitrarily connected Autonomous Systems. That is, the Internet will be modeled as a general graph whose nodes are AS's and whose edges are connections between pairs of AS's.
The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS, and using an exterior gateway protocol to route packets to other AS's. Since this classic definition was developed, it has become common for a single AS to use several interior gateway protocols and sometimes several sets of metrics within an AS. The use of the term Autonomous System here stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other AS's to have a single coherent interior routing plan and presents a consistent picture of which networks are reachable through it. From the standpoint of exterior routing, an AS can be viewed as monolithic: networks within an AS must maintain connectivity via intra-AS paths.
AS's are assumed to be administered by a single administrative entity, at least for the purposes of representation of routing information to systems outside of the AS.
2. BGP Topological Model
When we say that a connection exists between two AS's, we mean two things:
Physical connection: There is a shared network between the two AS's, and on this shared network each AS has at least one border gateway belonging to that AS. Thus the border gateway of each AS can forward packets to the border gateway of the other AS without resort to Inter-AS or Intra-AS routing. BGP connection: There is a BGP session between BGP speakers on each of the AS's, and this session communicates to each connected AS those routes through the physically connected border gateways of the other AS that can be used for specific networks. Throughout this document we place an additional restriction on the BGP speakers that form the BGP connection: they must themselves share the same network that their border gateways share. Thus, a BGP session between the adjacent AS's requires no support from either Inter-AS or Intra-AS routing. Cases that do not conform to this restriction fall outside the scope of this document.
Thus, at each connection, each AS has one or more BGP speakers and one or more border gateways, and these BGP speakers and border gateways are all located on a shared network. Note that BGP speakers do not need to be a border gateway, and vice versa. Paths announced by a BGP speaker of one AS on a given connection are taken to be feasible for each of the border gateways of the other AS on the same connection, i.e. indirect neighbors are allowed.
Much of the traffic carried within an AS either originates or terminates at that AS (i.e., either the source IP address or the destination IP address of the IP packet identifies a host on a network directly connected to that AS). Traffic that fits this description is called "local traffic". Traffic that does not fit this description is called "transit traffic". A major goal of BGP usage is to control the flow of transit traffic.
Based on how a particular AS deals with transit traffic, the AS may now be placed into one of the following categories:
stub AS: an AS that has only a single connection to one other AS. Naturally, a stub AS only carries local traffic.
multihomed AS: an AS that has connections to more than one other AS, but refuses to carry transit traffic.
transit AS: an AS that has connections to more than one other AS, and is designed (under certain policy restrictions) to carry both transit and local traffic.
Since a full AS path provides an efficient and straightforward way of suppressing routing loops and eliminates the "count-to-infinity" problem associated with some distance vector algorithms, BGP imposes no topological restrictions on the interconnection of AS's.
3. BGP in the Internet
3.1 Topology Considerations
The overall Internet topology may be viewed as an arbitrary interconnection of transit, multihomed, and stub AS's. In order to minimize the impact on the current Internet infrastructure, stub and multihomed AS's need not use BGP. These AS's may run other protocols (e.g., EGP) to exchange reachability information with transit AS's. Transit AS's using BGP will tag this information as having been learned by some method other than BGP. The fact that BGP need not run on stub or multihomed AS's has no negative impact on the overall quality of inter-AS routing for traffic not local to the stub or multihomed AS's in question.
However, it is recommended that BGP may be used for stub and multihomed AS's as well, providing an advantage in bandwidth and performance over some of the currently used protocols (such as EGP). In addition, this would result in less need for the use of defaults and in better choices of Inter-AS routes for multihomed AS's.
3.2 Global Nature of BGP
At a global level, BGP is used to distribute routing information among multiple Autonomous Systems. The information flows can be represented as follows:
+-------+ +-------+ BGP | BGP | BGP | BGP | BGP ---------+ +---------+ +--------- | IGP | | IGP | +-------+ +-------+ <-AS A--> <--AS B->
This diagram points out that, while BGP alone carries information between AS's, a combination of BGP and an IGP carries information across an AS. Ensuring consistency of routing information between BGP and an IGP within an AS is a significant issue and is discussed at length later in Appendix A.
3.3 BGP Neighbor Relationships
The Internet is viewed as a set of arbitrarily connected AS's. BGP speakers in each AS communicate with each other to exchange network reachability information based on a set of policies established within each AS. Routers that communicate directly with each other via BGP are known as BGP neighbors. BGP neighbors can be located within the same AS or in different AS's. For the sake of discussion, BGP communications with neighbors in different AS's will be referred to as External BGP, and with neighbors in the same AS as Internal BGP.
There can be as many BGP speakers as deemed necessary within an AS. Usually, if an AS has multiple connections to other AS's, multiple BGP speakers are needed. All BGP speakers representing the same AS must give a consistent image of the AS to the outside. This requires that the BGP speakers have consistent routing information among them. These gateways can communicate with each other via BGP or by other means. The policy constraints applied to all BGP speakers within an AS must be consistent. Techniques such as using tagged IGP (see A.2.2) may be employed to detect possible inconsistencies.
In the case of External BGP, the BGP neighbors must belong to different AS's, but share a common network. This common network should be used to carry the BGP messages between them. The use of BGP across an intervening AS invalidates the AS path information. An Autonomous System number must be used with BGP to specify which Autonomous System the BGP speaker belongs to.
4. Policy Making with BGP
BGP provides the capability for enforcing policies based on various routing preferences and constraints. Policies are not directly encoded in the protocol. Rather, policies are provided to BGP in the form of configuration information.
BGP enforces policies by affecting the selection of paths from multiple alternatives, and by controlling the redistribution of routing information. Policies are determined by the AS administration.
Routing policies are related to political, security, or economic considerations. For example, if an AS is unwilling to carry traffic to another AS, it can enforce a policy prohibiting this. The following are examples of routing policies that can be enforced with the use of BGP:
- A multihomed AS can refuse to act as a transit AS for other AS's. (It does so by not advertising routes to networks other than those directly connected to it.)
- A multihomed AS can become a transit AS for a restricted set of adjacent AS's, i.e., some, but not all, AS's can use multihomed AS as a transit AS. (It does so by advertising its routing information to this set of AS's.)
- An AS can favor or disfavor the use of certain AS's for carrying transit traffic from itself.
A number of performance-related criteria can be controlled with the use of BGP:
- An AS can minimize the number of transit AS's. (Shorter AS paths can be preferred over longer ones.)
- The quality of transit AS's. If an AS determines that two or more AS paths can be used to reach a given destination, that AS can use a variety of means to decide which of the candidate AS paths it will use. The quality of an AS can be measured by such things as diameter, link speed, capacity, tendency to become congested, and quality of operation. Information about these qualities might be determined by means other than BGP.
- Preference of internal routes over external routes.
For consistency within an AS, equal cost paths, resulting from combinations of policies and/or normal route selection procedures, must be resolved in a consistent fashion.
Fundamental to BGP is the rule that an AS advertises to its neighboring AS's only those routes that it uses. This rule reflects the "hop-by-hop" routing paradigm generally used by the current Internet.
5. Path Selection with BGP
One of the major tasks of a BGP speaker is to evaluate different paths to a destination network from its border gateways at that connection, select the best one, apply applicable policy constraints, and then advertise it to all of its BGP neighbors at that same connection. The key issue is how different paths are evaluated and compared.
In traditional distance vector protocols (e.g., RIP) there is only one metric (e.g., hop count) associated with a path. As such, comparison of different paths is reduced to simply comparing two numbers. A complication in Inter-AS routing arises from the lack of a universally agreed-upon metric among AS's that can be used to evaluate external paths. Rather, each AS may have its own set of criteria for path evaluation.
A BGP speaker builds a routing database consisting of the set of all feasible paths and the list of networks reachable through each path. For purposes of precise discussion, it's useful to consider the set of feasible paths for a given destination network. In most cases, we would expect to find only one feasible path. However, when this is not the case, all feasible paths should be maintained, and their maintenance speeds adaptation to the loss of the primary path. Only the primary path at any given time will ever be advertised.
The path selection process can be formalized by defining a partial order over the set of all feasible paths to a given destination network. One way to define this partial order is to define a function that maps each full AS path to a non-negative integer that denotes the path's degree of preference. Path selection is then reduced to applying this function to all feasible paths and choosing the one with the highest degree of preference.
In actual BGP implementations, criteria for assigning degree of preferences to a path are specified in configuration information.
The process of assigning a degree of preference to a path can be based on several sources of information:
- Information explicitly present in the full AS path.
- A combination of information that can be derived from the full AS path and information outside the scope of BGP (e.g., policy routing constraints provided at configuration).
Possible criteria for assigning a degree of preference to a path are:
- AS count. Paths with a smaller AS count are generally better.
- Policy consideration. BGP supports policy-based routing based on the controlled distribution of routing information. A BGP speaker may be aware of some policy constraints (both within and outside of its own AS) and do appropriate path selection. Paths that do not comply with policy requirements are not considered further.
- Presence or absence of a certain AS or AS's in the path. By means of information outside the scope of BGP, an AS may know some performance characteristics (e.g., bandwidth, MTU, intra-AS diameter) of certain AS's and may try to avoid or prefer them.
- Path origin. A path learned entirely from BGP (i.e., whose endpoint is internal to the last AS on the path is generally better than one for which part of the path was learned via EGP or some other means.
- AS path subsets. An AS path that is a subset of a longer AS path to the same destination should be preferred over the longer path. Any problem in the shorter path (such as an outage) will also be a problem in the longer path.
- Link dynamics. Stable paths should be preferred over unstable ones. Note that this criterion must be used in a very careful way to avoid causing unnecessary route fluctuation. Generally, any criteria that depend on dynamic information might cause routing instability and should be treated very carefully.
6. Required set of supported routing policies.
Policies are provided to BGP in the form of configuration information. This information is not directly encoded in the protocol. Therefore, BGP can provides support for quite complex routing policies. However, it is not required for all BGP implementations to support such policies.
We are not attempting to standardize the routing policies that must be supported in every BGP implementation, we strongly encourage all implementors to support the following set of routing policies:
- BGP implementations should allow an AS to control announcements of BGP-learned routes to adjacent AS's. Implementations should also support such control with at least the granularity of a single network. Implementations should also support such control with the granularity of an autonomous system, where the autonomous system may be either the autonomous system that originated the route, or the autonomous system that advertised the route to the local system (adjacent autonomous system).
- BGP implementations should allow an AS to prefer a particular path to a destination (when more than one path is available). This function should be implemented by allowing system administrators to assign "weights" to AS's, and making route selection process to select a route with the lowest "weight" (where "weight" of a route is defined as a sum of "weights" of all AS's in the AS_PATH path attribute associated with that route).
- BGP implementations should allow an AS to ignore routes with certain AS's in the AS_PATH path attribute. Such function can be implemented by using technique outlined in (2), and by assigning "infinity" as "weights" for such AS's. The route selection process must ignore routes that have "weight" equal to "infinity".
The BGP protocol provides a high degree of control and flexibility for doing interdomain routing while enforcing policy and performance constraints and avoiding routing loops. The guidelines presented here will provide a starting point for using BGP to provide more sophisticated and manageable routing in the Internet as it grows.
Appendix A. The Interaction of BGP and an IGP
This section outlines methods by which BGP can exchange routing information with an IGP. The methods outlined here are not proposed as part of the standard BGP usage at this time. These methods are outlined for information purposes only. Implementors may want to consider these methods when importing IGP information.
This is general information that applies to any generic IGP. Interaction between BGP and any specific IGP is outside the scope of this section. Methods for specific IGP's should be proposed in separate documents. Methods for specific IGP's could be proposed for standard usage in the future.
By definition, all transit AS's must be able to carry traffic which originates from and/or is destined to locations outside of that AS. This requires a certain degree of interaction and coordination between BGP and the Interior Gateway Protocol (IGP) used by that particular AS. In general, traffic originating outside of a given AS is going to pass through both interior gateways (gateways that support the IGP only) and border gateways (gateways that support both the IGP and BGP). All interior gateways receive information about external routes from one or more of the border gateways of the AS via the IGP.
Depending on the mechanism used to propagate BGP information within a given AS, special care must be taken to ensure consistency between BGP and the IGP, since changes in state are likely to propagate at different rates across the AS. There may be a time window between the moment when some border gateway (A) receives new BGP routing information which was originated from another border gateway (B) within the same AS, and the moment the IGP within this AS is capable of routing transit traffic to that border gateway (B). During that time window, either incorrect routing or "black holes" can occur.
In order to minimize such routing problems, border gateway (A) should not advertise a route to some exterior network X via border gateway (B) to all of its BGP neighbors in other AS's until all the interior gateways within the AS are ready to route traffic destined to X via the correct exit border gateway (B). In other words, interior routing should converge on the proper exit gateway before/advertising routes via that exit gateway to other AS's.
A.2 Methods for Achieving Stable Interactions
The following discussion outlines several techniques capable of achieving stable interactions between BGP and the IGP within an Autonomous System.
A.2.1 Propagation of BGP Information via the IGP
While BGP can provide its own mechanism for carrying BGP information within an AS, one can also use an IGP to transport this information, as long as the IGP supports complete flooding of routing information (providing the mechanism to distribute the BGP information) and onepass convergence (making the mechanism effectively atomic). If an IGP is used to carry BGP information, then the period of desynchronization described earlier does not occur at all, since BGP information propagates within the AS synchronously with the IGP, and the IGP converges more or less simultaneously with the arrival of the new routing information. Note that the IGP only carries BGP information and should not interpret or process this information.
A.2.2 Tagged Interior Gateway Protocol
Certain IGPs can tag routes exterior to an AS with the identity of their exit points while propagating them within the AS. Each border gateway should use identical tags for announcing exterior routing information (received via BGP) both into the IGP and into Internal BGP when propagating this information to other border gateways within the same AS. Tags generated by a border gateway must uniquely identify that particular border gateway--different border gateways must use different tags.
All Border Gateways within a single AS must observe the following two rules:
- Information received via Internal BGP by a border gateway A declaring a network to be unreachable must immediately be propagated to all of the External BGP neighbors of A.
- Information received via Internal BGP by a border gateway A about a reachable network X cannot be propagated to any of the External BGP neighbors of A unless/until A has an IGP route to X and both the IGP and the BGP routing information have identical tags.
These rules guarantee that no routing information is announced externally unless the IGP is capable of correctly supporting it. It also avoids some causes of "black holes".
One possible method for tagging BGP and IGP routes within an AS is to use the IP address of the exit border gateway announcing the exterior route into the AS. In this case the "gateway" field in the BGP UPDATE message is used as the tag.
Encapsulation provides the simplest (in terms of the interaction between the IGP and BGP) mechanism for carrying transit traffic across the AS. In this approach, transit traffic is encapsulated within an IP datagram addressed to the exit gateway. The only requirement imposed on the IGP by this approach is that it should be capable of supporting routing between border gateways within the same AS.
The address of the exit gateway A for some exterior network X is specified in the BGP identifier field of the BGP OPEN message received from gateway A via Internal BGP by all other border gateways within the same AS. In order to route traffic to network X, each border gateway within the AS encapsulates it in datagrams addressed to gateway A. Gateway A then performs decapsulation and forwards the original packet to the proper gateway in another AS
Since encapsulation does not rely on the IGP to carry exterior routing information, no synchronization between BGP and the IGP is required.
Some means of identifying datagrams containing encapsulated IP, such as an IP protocol type code, must be defined if this method is to be used.
Note, that if a packet to be encapsulated has length that is very close to the MTU, that packet would be fragmented at the gateway that performs encapsulation.
A.2.4 Other Cases
There may be AS's with IGPs which can neither carry BGP information nor tag exterior routes (e.g., RIP). In addition, encapsulation may be either infeasible or undesirable. In such situations, the following two rules must be observed:
- Information received via Internal BGP by a border gateway A about a reachable network X cannot be propagated to any of the External BGP neighbors of A unless A has an IGP route to X and sufficient time (holddown) has passed for the IGP routes to have converged.
The above rules present necessary (but not sufficient) conditions for propagating BGP routing information to other AS's. In contrast to tagged IGPs, these rules cannot ensure that interior routes to the proper exit gateways are in place before propagating the routes other AS's.
If the convergence time of an IGP is less than some small value X, then the time window during which the IGP and BGP are unsynchronized is less than X as well, and the whole issue can be ignored at the cost of transient periods (of less than length X) of routing instability. A reasonable value for X is a matter for further study, but X should probably be less than one second.
If the convergence time of an IGP cannot be ignored, a different approach is needed. Mechanisms and techniques which might be appropriate in this situation are subjects for further study.
Security issues are not discussed in this memo.
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