The Accumulated IGP Metric Attribute for BGP
Author(s): James Uttaro, Rex Fernando, Pradosh Mohapatra, Eric Rosen
Routing protocols that have been designed to run within a single administrative domain ('IGPs') generally do so by assigning a metric to each link, and then choosing as the installed path between two nodes the path for which...
Network Working Group Pradosh Mohapatra Internet Draft Rex Fernando Intended Status: Proposed Standard Eric C. Rosen Expires: May 27, 2013 Cisco Systems, Inc. James Uttaro ATT November 27, 2012 The Accumulated IGP Metric Attribute for BGP draft-ietf-idr-aigp-09.txt Abstract Routing protocols that have been designed to run within a single administrative domain ("IGPs") generally do so by assigning a metric to each link, and then choosing as the installed path between two nodes the path for which the total distance (sum of the metric of each link along the path) is minimized. BGP, designed to provide routing over a large number of independent administrative domains ("autonomous systems"), does not make its path selection decisions through the use of a metric. It is generally recognized that any attempt to do so would incur significant scalability problems, as well as inter-administration coordination problems. However, there are deployments in which a single administration runs several contiguous BGP networks. In such cases, it can be desirable, within that single administrative domain, for BGP to select paths based on a metric, just as an IGP would do. The purpose of this document is to provide a specification for doing so. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference Mohapatra, et al. [Page 1] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright and License Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Mohapatra, et al. [Page 2] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 Table of Contents 1 Specification of requirements ......................... 3 2 Introduction .......................................... 3 3 AIGP Attribute ........................................ 5 3.1 Applicability Restrictions and Cautions ............... 6 3.2 Restrictions on Sending/Receiving ..................... 6 3.3 Creating and Modifying the AIGP Attribute ............. 7 3.3.1 Originating the AIGP Attribute ........................ 7 3.3.2 Modifications by the Originator ....................... 8 3.3.3 Modifications by a Non-Originator ..................... 8 4 Decision Process ...................................... 10 4.1 When a Route has an AIGP Attribute .................... 10 4.2 When the Route to the Next Hop has an AIGP attribute .. 11 5 Deployment Considerations ............................. 12 6 IANA Considerations ................................... 12 7 Security Considerations ............................... 12 8 Acknowledgments ....................................... 12 9 Authors' Addresses .................................... 13 10 Normative References .................................. 13 11 Informative References ................................ 14 1. Specification of requirements 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 [RFC2119]. 2. Introduction There are many routing protocols that have been designed to run within a single administrative domain. These are known collectively as "Interior Gateway Protocols" (IGPs). Typically, each link is assigned a particular "metric" value. The path between two nodes can then be assigned a "distance", which is the sum of the metrics of all the links that belong to that path. An IGP selects the "shortest" (minimal distance) path between any two nodes, perhaps subject to the constraint that if the IGP provides multiple "areas", it may prefer the shortest path within an area to a path that traverses more than one area. Typically the administration of the network has some Mohapatra, et al. [Page 3] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 routing policy which can be approximated by selecting shortest paths in this way. BGP, as distinguished from the IGPs, was designed to run over an arbitrarily large number of administrative domains ("autonomous systems", or "ASes") with limited coordination among the various administrations. BGP does not make its path selection decisions based on a metric; there is no such thing as an "inter-AS metric". There are two fundamental reasons for this: - The distance between two nodes in a common administrative domain may change at any time due to events occurring in that domain. These changes are not propagated around the Internet unless they actually cause the border routers of the domain to select routes with different BGP attributes for some set of address prefixes. This accords with a fundamental principle of scaling, viz., that changes with only local significance must not have global effects. If local changes in distance were always propagated around the Internet, this principle would be violated. - A basic principle of inter-domain routing is that the different administrative domains may have their own policies, which do not have to be revealed to other domains, and which certainly do not have to be agreed to by other domains. Yet the use of inter-AS metric in the Internet would have exactly these effects. There are, however, deployments in which a single administration runs a network which has been sub-divided into multiple, contiguous ASes, each running BGP. There are several reasons why a single administrative domain may be broken into several ASes (which, in this case, are not really "autonomous".) It may be that the existing IGPs do not scale well in the particular environment; it may be that a more generalized topology is desired than could be obtained by use of a single IGP domain; it may be that a more finely grained routing policy is desired than can be supported by an IGP. In such deployments, it can be useful to allow BGP to make its routing decisions based on the IGP metric, so that BGP chooses the "shortest" path between two nodes, even if the nodes are in two different ASes within that same administrative domain. We will refer to the set of ASes in a common administrative domain as an "AIGP Administrative Domain". There are in fact some implementations that already do something like this, using BGP's MULTI_EXIT_DISC (MED) attribute to carry a value based on IGP metrics. However, that doesn't really provide IGP-like "shortest path" routing, as the BGP decision process gives priority to other factors, such as the AS_PATH length. Also, the standard procedures for use of the MED do not ensure that the IGP metric is Mohapatra, et al. [Page 4] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 properly accumulated so that it covers all the links along the path. In this document, we define a new optional, non-transitive BGP attribute, called the "Accumulated IGP Metric Attribute", or "AIGP attribute", and specify the procedures for using it. The specified procedures prevent the AIGP attribute from "leaking out" past an AIGP administrative domain boundary into the Internet. The specified procedures also ensure that the value in the AIGP attribute has been accumulated all along the path from the destination, i.e., that the AIGP attribute does not appear when there are "gaps" along the path where the IGP metric is unknown. 3. AIGP Attribute The AIGP Attribute is an optional non-transitive BGP Path Attribute. The attribute type code for the AIGP Attribute is 26. The value field of the AIGP Attribute is defined here to be a set of elements encoded as "Type/Length/Value" (i.e., a set of "TLVs"). Each such TLV is encoded as shown in Figure 1. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ ~ | Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.......................... AIGP TLV Figure 1 - Type: A single octet encoding the TLV Type. Only type 1, "AIGP TLV", is defined in this document. - Length: Two octets encoding the length in octets of the TLV, including the type and length fields. The length is encoded as an unsigned binary integer. (Note that the minimum length is 3, indicating that no value field is present.) Mohapatra, et al. [Page 5] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 - A value field containing zero or more octets. This document defines only a single such TLV, the "AIGP TLV". The AIGP TLV is encoded as follows: - Type: 1 - Length: 11 - Accumulated IGP Metric. The value field of the AIGP TLV is always 8 bytes long. IGP metrics are frequently expressed as 4-octet values, and this ensures that the AIGP attribute can be used to hold the sum of an arbitrary number of 4-octet values. 3.1. Applicability Restrictions and Cautions This document only considers the use of the AIGP attribute in networks where each router uses tunneling of some sort to deliver a packet to its BGP next hop. Use of the AIGP attribute in networks that do not use tunneling is outside the scope of this document. If a Route Reflector supports the AIGP attribute, but some of its clients do not, then the routing choices that result may not all reflect the intended routing policy. 3.2. Restrictions on Sending/Receiving An implementation that supports the AIGP attribute MUST support a per-session configuration item, AIGP_SESSION, that indicates whether the attribute is enabled or disabled for use on that session. - The default value of AIGP_SESSION, for EBGP sessions, MUST be "disabled". - The default value of AIGP_SESSION, for IBGP and confederation- EBGP sessions, MUST be "enabled." The AIGP attribute MUST NOT be sent on any BGP session for which AIGP_SESSION is disabled. If an AIGP attribute is received on a BGP session for which AIGP_SESSION is disabled, the attribute MUST be treated exactly as if it were an unrecognized non-transitive attribute. That is, "it MUST be quietly ignored and not passed along to other BGP peers" (see Mohapatra, et al. [Page 6] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 [BGP], section 5). 3.3. Creating and Modifying the AIGP Attribute 3.3.1. Originating the AIGP Attribute An implementation that supports the AIGP attribute MUST support a configuration item, AIGP_ORIGINATE, that enables or disables its creation and attachment to routes. The default value of AIGP_ORIGINATE MUST be "disabled". A BGP speaker MUST NOT add the AIGP attribute to any route whose path leads outside the "AIGP administrative domain" to which the BGP speaker belongs. It may be added only to routes that satisfy one of the following conditions: - The route is a static route that is being redistributed into BGP - The route is an IGP route that is being redistributed into BGP - The route is an IBGP-learned route whose AS_PATH attribute is empty. - The route is an EBGP-learned route whose AS_PATH contains only ASes that are in the same AIGP Administrative Domain as the BGP speaker. A BGP speaker R MUST NOT add the AIGP attribute to any route for which R does not set itself as the next hop. It SHOULD be possible to set AIGP_ORIGINATE to "enabled for the routes of a particular IGP that are redistributed into BGP" (where "a particular IGP" might be "OSPF" or "ISIS"). Other policies determining when and whether to originate an AIGP attribute are also possible, depending on the needs of a particular deployment scenario. When originating an AIGP attribute for a BGP route to address prefix P, the value of the attribute is set according to policy. There are a number of useful policies, some of which are in the following list: - When a BGP speaker R is redistributing into BGP an IGP route to address prefix P, the IGP will have computed a "distance" from R to P. This distance MAY be assigned as the value of AIGP attribute. Mohapatra, et al. [Page 7] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 - A BGP speaker R may be redistributing into BGP a static route to address prefix P, for which a "distance" from R to P has been configured. This distance MAY be assigned as the value of AIGP attribute. - A BGP speaker R may have received and installed a BGP-learned route to prefix P, with next hop N. Or it may be redistributing a static route to P, with next hop N. Then: * If R has an IGP route to N, the IGP-computed distance from R to N MAY be used as the AIGP attribute value of the route to P. * If R has a BGP route to N, and an AIGP attribute value has been computed for that route (see section 3.3.3), that value MAY be used as the AIGP attribute value of the route to P. 3.3.2. Modifications by the Originator If BGP speaker R is the originator of the AIGP attribute of prefix P, and at some point the "distance" from R to P changes, R SHOULD issue a new BGP update containing the new value of the AIGP attribute. (Here we use the term "distance" to refer to whatever value the originator assigns to the AIGP attribute, however it is computed; see section 3.3.1.) However, if the difference between the new distance and the distance advertised in the AIGP attribute is less than a configurable threshold, the update MAY be suppressed. 3.3.3. Modifications by a Non-Originator Suppose a BGP speaker R1 receives a route with an AIGP attribute whose value is A, and a Next Hop whose value is R2. Suppose also that R1 is about to redistribute that route on a BGP session that is enabled for sending/receiving the attribute. If R1 does not change the Next Hop of the route, then R1 MUST NOT change the AIGP attribute value of the route. If R1 changes the Next Hop of the route from R2 to R1, and if R1's route to R2 is an IGP-learned route, or a static route that does not require recursive next hop resolution, then R1 must increase the value of the AIGP attribute by adding to A the distance from R1 to R2. This distance is either the IGP-computed distance from R1 to R2, or some value determined by policy. However, A MUST be increased by a non-zero amount. Mohapatra, et al. [Page 8] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 Note that if R1 and R2 above are EBGP neighbors, and there is a direct link between them on which no IGP is running, then when R1 changes the next hop of a route from R2 to R1, the AIGP metric value MUST be increased by a non-zero amount. The amount of the increase SHOULD be such that it is properly comparable to the IGP metrics. E.g., if the IGP metric is a function of latency, then the amount of the increase should be a function of the latency from R1 to R2. If R1 changes the Next Hop of the route from R2 to R1, and if R1's route to R2 is a BGP-learned route, or a static route that requires recursive next hop resolution, then the AIGP attribute value needs to be increased in several steps, according to the following procedure. (Note that this procedure is ONLY used when recursive next hop resolution is needed.) 1. Let Xattr be the new AIGP attribute value. 2. Initialize Xattr to A. 3. Set the XNH to R2. 4. Find the route to XNH. 5. If the route to XNH does not require recursive next hop resolution, get the distance D from R1 to XNH. (Note that this condition cannot be satisfied the first time through this procedure.) If D is above a configurable threshold, set the AIGP attribute value to Xattr+D. If D is below a configurable threshold, set the AIGP attribute value to Xattr. In either case, exit this procedure. 6. If the route to XNH is a BGP-learned route, and the route does NOT have an AIGP attribute, then exit this procedure and do not pass on any AIGP attribute. 7. If the route to XNH is a BGP-learned route, and the route has an AIGP attribute value of Y, then set Xattr=Xattr+Y, and set XNH to the next hop of this route. (The intention here is that Y is the AIGP value of the route as it was received by R1, without having been modified by R1.) 8. Go to step 4. The AIGP value of a given route depends on (a) the AIGP values of all the next hops that are recursively resolved during this procedure, and (b) the IGP distance to any next hop that is not recursively resolved. Any change due to (a) in any of these values MUST trigger a new AIGP computation for that route. Whether a change due to (b) Mohapatra, et al. [Page 9] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 triggers a new AIGP computation depends upon whether the change in IGP distance exceeds a configurable threshold. If the AIGP attribute is carried across several ASes, each with its own IGP domain, it is clear that these procedures are unlikely to give a sensible result if the IGPs are different (e.g., some OSPF and some IS-IS), or if the meaning of the metrics is different in the different IGPs (e.g., if the metric represents bandwidth in some IGP domains but represents latency in others). These procedures also are unlikely to give a sensible result if the metric assigned to inter-AS BGP links (on which no IGP is running) or to static routes is not comparable to the IGP metrics. All such cases are outside the scope of the current document. 4. Decision Process Support for the AIGP attribute involves several modifications to the tie breaking procedures of the BGP "phase 2" decision described in [BGP], section 188.8.131.52. These modifications are described below in sections 4.1 and 4.2. In some cases, the BGP decision process may install a route without executing any tie breaking procedures. This may happen, e.g., if only one route to a given prefix has the highest degree of preference (as defined in [BGP] section 9.1.1). In this case, the AIGP attribute is not considered. In other cases, some routes may be eliminated before the tie breaking procedures are invoked, e.g., routes with AS-PATH attributes indicating a loop, or routes with unresolvable next hops. In these cases, the AIGP attributes of the eliminated routes are not considered. 4.1. When a Route has an AIGP Attribute Assuming that the BGP decision process invokes the tie breaking procedures, the procedures in this section MUST be executed BEFORE any of the tie breaking procedures described in [BGP] section 184.108.40.206 are executed. If any routes have an AIGP attribute, remove from consideration all routes that do not have an AIGP attribute. If router R is considering route T, where T has an AIGP attribute, Mohapatra, et al. [Page 10] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 - then R must compute the value A, defined as follows: set A to the sum of (a) T's AIGP attribute value and (b) the IGP distance from R to T's next hop. - remove from consideration all routes that are not tied for the lowest value of A. 4.2. When the Route to the Next Hop has an AIGP attribute Suppose that a given router R1 is comparing two BGP-learned routes, such that either: - the two routes have equal AIGP attribute values, or else - neither of the two routes has an AIGP attribute. The BGP decision process as specified in [BGP] makes use, in its tie breaker procedures, of "interior cost", defined as follows: "interior cost of a route is determined by calculating the metric to the NEXT_HOP for the route using the Routing Table." Suppose route T has a next hop of N. We modify the notion of the "interior cost" from node R1 to node N as follows: - Let R2 be the BGP next hop of the route to N, after all recursive resolution of the next hop is done. Let m be the IGP distance (or in the case of a static route, the configured distance) from R1 to R2. - If the installed route to N has an AIGP attribute, set A to the AIGP value of the route to N, computing the AIGP value of the route according to the procedure of section 3.3.3. - If the installed route to N does not have an AIGP value, set A to 0. - The "interior cost" of route T is the quantity A+m. Mohapatra, et al. [Page 11] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 5. Deployment Considerations Using the AIGP attribute to achieve a desired routing policy will be more effective if each BGP speaker can use it to choose from among multiple routes. Thus is it highly recommended that the procedures of [BESTEXT] and [ADDPATH] be used in conjunction with the AIGP Attribute. If a Route Reflector does not pass all paths to its clients, then it will tend to pass the paths for which the IGP distance from the Route Reflector itself to the next hop is smallest. This may result in a non-optimal choice by the clients. 6. IANA Considerations IANA has assigned the codepoint 26 in the "BGP Path Attributes" registry to the AIGP attribute. IANA shall create a registry for "BGP AIGP Attribute Types". The type field consists of a single octet, with possible values from 0 to 255. The allocation policy for this field is to be "Standards Action with Early Allocation". Type 1 should be defined as "AIGP", and should refer to this document. 7. Security Considerations The spurious introduction, though error or malfeasance, of an AIGP attribute, could result in the selection of paths other than those desired. Improper configuration on both ends of an EBGP connection could result in an AIGP attribute being passed from one service provider to another. This would likely result in an unsound selection of paths. 8. Acknowledgments The authors would like to thank Waqas Alam, Rajiv Asati, Clarence Filsfils, Robert Raszuk, Yakov Rekhter, Eric Rosenberg, Samir Saad, John Scudder, and Shyam Sethuram for their input. Mohapatra, et al. [Page 12] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 9. Authors' Addresses Rex Fernando Cisco Systems, Inc. 170 Tasman Drive San Jose, CA 95134 Email: email@example.com Pradosh Mohapatra Cisco Systems, Inc. 170 Tasman Drive San Jose, CA 95134 Email: firstname.lastname@example.org Eric C. Rosen Cisco Systems, Inc. 1414 Massachusetts Avenue Boxborough, MA, 01719 Email: email@example.com James Uttaro AT&T 200 S. Laurel Avenue Middletown, NJ 07748 Email: firstname.lastname@example.org 10. Normative References [BGP], "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter, T. Li, S. Hares, RFC 4271, January 2006. Mohapatra, et al. [Page 13] Internet Draft draft-ietf-idr-aigp-09.txt November 2012 11. Informative References [ADDPATH] "Fast Connectivity Restoration Using BGP Add-Path", P. Mohapatra, R. Fernando, C. Filsfils, R. Raszuk, draft-pmohapat-idr- fast-conn-restore-02.txt, October 2011. [BESTEXT], "Advertisement of the Best External Route in BGP", P. Marques, R. Fernando, E. Chen, P. Mohapatra, H. Gredler, draft-ietf- idr-best-external-05.txt, January 2012. [RFC2119] "Key words for use in RFCs to Indicate Requirement Levels.", S. Bradner, March 1997. Mohapatra, et al. [Page 14]