--- 1/draft-ietf-grow-bgp-med-considerations-00.txt 2006-02-04 23:23:47.000000000 +0100 +++ 2/draft-ietf-grow-bgp-med-considerations-01.txt 2006-02-04 23:23:47.000000000 +0100 @@ -1,20 +1,20 @@ INTERNET-DRAFT Danny McPherson Arbor Networks, Inc. Vijay Gill AOL Category Informational -Expires: June 2004 December 2003 +Expires: November 2004 May 2004 BGP MED Considerations - + Status of this Document This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. @@ -32,34 +32,36 @@ 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 [RFC 2119]. This document is a product of an individual. Comments are solicited and should be addressed to the author(s). Copyright Notice - Copyright (C) The Internet Society (2003). All Rights Reserved. + Copyright (C) The Internet Society (2004). All Rights Reserved. +=0C Abstract The BGP MED attribute provides a mechanism for BGP speakers to convey to an adjacent AS the optimal entry point into the local AS. While BGP MEDs function correctly in many scenarios, there are a number of issues which may arise when utilizing MEDs in dynamic or complex topologies. This document discusses implementation and deployment considerations regarding BGP MEDs and provides information which implementors and network operators should be familiar with. +=0C Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. About the MULTI_EXIT_DISC (MED) Attribute . . . . . . . . . 4 1.2. MEDs and Potatoes . . . . . . . . . . . . . . . . . . . . . 5 2. Implementation and Protocol Considerations . . . . . . . . . . 7 2.1. MULTI_EXIT_DISC is a Optional Non-Transitive Attribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. MED Values and Preferences. . . . . . . . . . . . . . . . . 7 2.3. Comparing MEDs Between Different Autonomous @@ -72,20 +74,22 @@ 3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 11 3.1. Comparing MEDs Between Different Autonomous Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 12 4. Security Considerations. . . . . . . . . . . . . . . . . . . . 12 4.1. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13 7. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13 +=0C + 1. Introduction The BGP MED attribute provides a mechanism for BGP speakers to convey to an adjacent AS the optimal entry point into the local AS. While BGP MEDs function correctly in many scenarios, there are a number of issues which may arise when utilizing MEDs in dynamic or complex topologies. This document discusses implementation and deployment considerations regarding BGP MEDs and provides information which implementors and @@ -116,32 +120,33 @@ An implementation MAY also (based on local configuration) alter the value of the MULTI_EXIT_DISC attribute received over EBGP. This MAY be done prior to determining the degree of preference of the route and performing route selection (decision process phases 1 and 2). Section 9.1.2.2 (c) of [BGP4] defines the following route selection criteria regarding MEDs: +=0C Remove from consideration routes with less-preferred MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable between routes learned from the same neighboring AS (the neighboring AS is determined from the AS_PATH attribute). Routes which do not have the MULTI_EXIT_DISC attribute are considered to have the lowest possible MULTI_EXIT_DISC value. This is also described in the following procedure: - for m = all routes still under consideration - for n = all routes still under consideration - if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m)) + for m =3D all routes still under consideration + for n =3D all routes still under consideration + if (neighborAS(m) =3D=3D neighborAS(n)) and (MED(n) < MED(m)) remove route m from consideration In the pseudo-code above, MED(n) is a function which returns the value of route n's MULTI_EXIT_DISC attribute. If route n has no MULTI_EXIT_DISC attribute, the function returns the lowest possible MULTI_EXIT_DISC value, i.e. 0. If a MULTI_EXIT_DISC attribute is removed before re- advertising a route into IBGP, then comparison based on the received EBGP MULTI_EXIT_DISC attribute MAY still be performed. If an @@ -161,20 +166,22 @@ to cause route loops. Routes that have different MULTI_EXIT_DISC attribute SHALL NOT be aggregated. 1.2. MEDs and Potatoes In a situation where traffic flows between a pair of hosts, each connected to different transit networks, which are themselves interconnected at two or more locations, each transit network has the + +=0C choice of either sending traffic to the closest peering to the adjacent transit network or passing traffic to the interconnection location which advertises the least cost path to the destination host. DIAGRAM*** The former method is called "hot potato routing" (or closest-exit) because like a hot potato held in bare hands, whoever has it tries to get rid of it quickly. Hot potato routing is accomplished by not @@ -203,20 +210,22 @@ commercial networks exchange MEDs with customers but not bilateral peers. However, commercial use of MEDs varies widely, from ubiquitous use of MEDs to no use of MEDs at all. In addition, many deployments of MEDs today are likely behaving differently (e.g., resulting is sub-optimal routing) than the network operator intended, thereby resulting not in hot or cold potatoes, but mashed potatoes! More information on unintended behavior resulting from MEDs is provided throughout this document. +=0C + 2. Implementation and Protocol Considerations There are a number of implementation and protocol peculiarities relating to MEDs that have been discovered that may affect network behavior. The following sections provide information on these issues. 2.1. MULTI_EXIT_DISC is a Optional Non-Transitive Attribute MULTI_EXIT_DISC is a non-transitive optional attribute whose @@ -244,20 +253,22 @@ of the BGP draft specification have been all over the map with interpretation of missing-MED. For example, earlier versions of the specification called for a missing MED to be assigned the highest possible MED value (i.e., 2^32-1). In addition, some implementations have been shown to internally employ a maximum possible MED value (2^32-1) as an "infinity" metric (i.e., the MED value is used to tag routes as unfeasible), and would upon on receiving an update with an MED value of 2^32-1 rewrite the value to 2^32-2. Subsequently, the new MED value would be propagated + +=0C and could result in routing inconsistencies or unintended path selections. As a result of implementation inconsistencies and protocol revision variances, many network operators today explicitly reset all MED values on ingress to conform to their internal routing policies (i.e., to include policy that requires that MED values of 0 and 2^32-1 NOT be used in configurations, whether the MEDs are directly computed or configured), so as to not have to rely on all their routers having the same missing-MED behavior. @@ -286,20 +297,22 @@ introduce persistent BGP route oscillation [RFC 3345]. The problem is inherent in the way BGP works: a conflict exists between information hiding/hierarchy and the non-hierarchical selection process imposed by lack of total ordering caused by the MED rules. Given current practices, we see the problem most frequently manifest itself in the context of MED + route reflectors or confederations. One potential way to avoid this is by configuring inter-Member-AS or inter-cluster IGP metrics higher than intra-Member-AS IGP metrics and/or using other tie breaking policies to avoid BGP route selection + +=0C based on incomparable MEDs. Of course, IGP metric constraints may be unreasonably onerous for some applications. Comparing MEDs between differing adjacent autonomous systems (which will be discussed in later sections), or not utilizing MEDs at all, significantly decreases the probability of introducing potential route oscillation conditions into the network. Although perhaps "legal" as far as current specifications are concerned, modifying MED attributes received on any type of IBGP @@ -327,20 +340,22 @@ dampening behavior because it many cause routes to be re- advertised solely to reflect an internal topology change. Many implementations don't have a practical problem with IGP flapping, they either latch their IGP metric upon first advertisement or they employ some internal suppression mechanism. Some implementations regard BGP attribute changes as less significant than route withdrawals and announcements to attempt to mitigate the impact of this type of event. +=0C + 2.6. Effects of MEDs on Update Packing Efficiency Multiple unfeasible routes can be advertised in a single BGP Update message. In addition, one or more feasible routes can be advertised in a single Update message so long as all prefixes share a common attribute set. The BGP4 protocol permits advertisement of multiple prefixes with a common set of path attributes to be advertised in a single update message, this is commonly referred to as "update packing". When @@ -372,20 +387,22 @@ MED-based best path selection. These usually involved methods used to store the oldest route along with ordering routes for MED in earlier implementations that cause non-deterministic behavior on whether the oldest route would truly be selected or not. The reasoning for this is that "older" paths are presumably more stable, and thus more preferable. However, temporal behavior in route selection results in non-deterministic behavior, and as such, is often undesirable. +=0C + 3. Deployment Considerations Empirical data [MFN/Ixia Monitoring Project] has shown that accepting MEDs from other autonomous systems have the potential to cause traffic flow churns in the network. Some implementations only ratchet down the MED and never move it back up to prevent excessive churn. However, if that session is reset, the MEDs being advertised have the potential of changing. If an network is relying on received MEDs to @@ -414,20 +431,22 @@ This allows MEDs to reasonably reflect IGP topologies when advertising routes to peers. While this is fine when comparing MEDs between multiple paths learned from a single AS, it can result in potentially "weighted" decisions when comparing MEDs between different autonomous systems. This is most typically the case when the autonomous systems use different mechanisms to derive IGP metrics, BGP MEDs, or perhaps even use different IGP protocols with vastly contrasting metric spaces (e.g., OSPF v. traditional metric space in IS-IS). +=0C + 3.2. Effects of Aggregation on MEDs` Another MED deployment consideration involves the impact that aggregation of BGP routing information has on MEDs. Aggregates are often generated from multiple locations in an AS in order to accommodate stability, redundancy and other network design goals. When MEDs are derived from IGP metrics associated with said aggregates the MED value advertised to peers can result in very suboptimal routing. @@ -445,20 +464,22 @@ increase a network's susceptibility to exploitation by peers. 4.1. Acknowledgments Thanks to John Scudder for applying his usual keen eye and constructive insight. Also, thanks to Curtis Villamizar and JR Mitchell. Others to be supplied. +=0C + 5. References [RFC 1519] Fuller, V., Li. T., Yu J., and K. Varadhan, "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC 1519, September 1993. [RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995. [RFC 2439] Villamizar, C. and Chandra, R., "BGP Route Flap Damping", @@ -485,22 +506,23 @@ Danny McPherson Arbor Networks Email: danny@arbor.net Vijay Gill AOL Email: VijayGill9@aol.com 7. Full Copyright Statement - Copyright (C) The Internet Society (2003). All Rights Reserved. + Copyright (C) The Internet Society (2004). All Rights Reserved. +=0C This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for