--- 1/draft-ietf-grow-bgp-med-considerations-03.txt 2006-02-04 23:23:50.000000000 +0100 +++ 2/draft-ietf-grow-bgp-med-considerations-04.txt 2006-02-04 23:23:50.000000000 +0100 @@ -1,50 +1,42 @@ - INTERNET-DRAFT Danny McPherson Arbor Networks, Inc. Vijay Gill AOL Category Informational -Expires: September 2005 March 2005 +Expires: December 2005 June 2005 BGP MED Considerations - - -Status of this Memo + Status of this Memo - This document is an Internet-Draft and is subject to all provisions - of Section 3 of RFC 3667. By submitting this Internet-Draft, each - author represents that any applicable patent or other IPR claims of - which he or she is aware have been or will be disclosed, and any of - which he or she become aware will be disclosed, in accordance with - RFC 3668. + By submitting this Internet-Draft, each author represents that any + applicable patent or other IPR claims of which he or she is aware have + been or will be disclosed, and any of which he or she becomes aware + will be disclosed, in accordance with Section 6 of 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. + Task Force (IETF), its areas, and its working groups. 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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 @@ -52,53 +44,56 @@ This document discusses implementation and deployment considerations regarding BGP MEDs and provides information which implementors and network operators should be familiar with. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. About the MULTI_EXIT_DISC (MED) Attribute . . . . . . . . . 4 1.2. MEDs and Potatos. . . . . . . . . . . . . . . . . . . . . . 5 - 2. Implementation and Protocol Considerations . . . . . . . . . . 6 + 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 Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4. MEDs, Route Reflection and AS Confederations for BGP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.5. Route Flap Damping and MED Churn. . . . . . . . . . . . . . 9 - 2.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 9 + 2.6. Effects of MEDs on Update Packing Efficiency. . . . . . . . 10 2.7. Temporal Route Selection. . . . . . . . . . . . . . . . . . 10 - 3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 10 + 3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 11 3.1. Comparing MEDs Between Different Autonomous Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 - 3.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 11 - 4. Security Considerations. . . . . . . . . . . . . . . . . . . . 12 - 4.1. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12 - 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 5.1. Normative References. . . . . . . . . . . . . . . . . . . . 14 - 5.2. Informative References. . . . . . . . . . . . . . . . . . . 15 - 6. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 + 3.2. Effects of Aggregation on MEDs` . . . . . . . . . . . . . . 12 + 4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 12 + 5. Security Considerations. . . . . . . . . . . . . . . . . . . . 12 + 5.1. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12 + 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 + 6.1. Normative References. . . . . . . . . . . . . . . . . . . . 14 + 6.2. Informative References. . . . . . . . . . . . . . . . . . . 15 + 7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 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 - network operators should be familiar with. + While reading this document it's important to keep in mind that the + goal is to discuss both implementation and deployment considerations + regarding BGP MEDs and provide and guidance which both implementors + and network operators should be familiar with. In some instances + implementation advice varies from deployment advice. 1.1. About the MULTI_EXIT_DISC (MED) Attribute The BGP MULTI_EXIT_DISC (MED) attribute, formerly known as the INTER_AS_METRIC, is currently defined in section 5.1.4 of [BGP4], as follows: The MULTI_EXIT_DISC is an optional non-transitive attribute which is intended to be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS. @@ -180,21 +175,25 @@ get rid of it quickly. Hot potato routing is accomplished by not passing the EGBP learned MED into IBGP. This minimizes transit traffic for the provider routing the traffic. Far less common is "cold potato routing" (or best-exit) where the transit provider uses their own transit capacity to get the traffic to the point that adjacent transit provider advertised as being closest to the destination. Cold potato routing is accomplished by passing the EBGP learned MED into IBGP. If one transit provider uses hot potato routing and another uses cold - potato, traffic between the two tends to be more symmetric. + potato, traffic between the two tends to be more symmetric. However, + if both providers employ cold potato routing, or both providers + employ hot potato routing between their networks, it's likely that a + larger amount of asymmetry would exist. + Depending on the business relationships, if one provider has more capacity or a significantly less congested backbone network, then that provider may use cold potato routing. An example of widespread use of cold potato routing was the NSF funded NSFNET backbone and NSF funded regional networks in the mid 1990s. In some cases a provider may use hot potato routing for some destinations for a given peer AS and cold potato routing for others. An example of this is the different treatment of commercial and research traffic in the NSFNET in the mid 1990s. Today many @@ -244,26 +243,34 @@ 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 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. + variances, many network operators today explicitly reset (i.e., set + to zero or some other 'fixed' value) 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. + + Because implementations don't normally provide a mechanism to disable + MED comparisons in the decision algorithm, "not using MEDs" usually + entails explicitly setting all MEDs to some fixed value upon ingress + to the routing domain. By assigning a fixed MED value consistently + to all routes across the network, MEDs are a effectively a non-issue + in the decision algorithm. 2.3. Comparing MEDs Between Different Autonomous Systems The MED was intended to be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS. However, a large number of MED applications now employ MEDs for the purpose of determining route preference between like routes received from different autonomous systems. A large number of implementations provide the capability to enable @@ -276,31 +283,32 @@ 2.4. MEDs, Route Reflection and AS Confederations for BGP In particular configurations, the BGP scaling mechanisms defined in "BGP Route Reflection - An Alternative to Full Mesh IBGP" [RFC 2796] and "Autonomous System Confederations for BGP" [RFC 3065] will 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 based on incomparable MEDs. Of course, IGP metric constraints may be unreasonably onerous for some applications. - Comparing MEDs between differing adjacent autonomous systems (which - is discussed in other sections), or not utilizing MEDs at all, + Comparing MEDs between differing adjacent autonomous systems + discussed in section 2.3), 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 session (e.g., standard IBGP, AS confederations EIBGP, route reflection, etc..) is NOT recommended. 2.5. Route Flap Damping and MED Churn @@ -360,21 +368,21 @@ decrease in update packing efficiency. 2.7. Temporal Route Selection Some implementations have had bugs which lead to temporal behavior in 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 + 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. 3. Deployment Considerations It has been discussed 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. @@ -420,68 +428,72 @@ 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. -4. Security Considerations +4. IANA Considerations + + This document introduces no new IANA considerations. + +5. Security Considerations The MED was purposely designed to be a "weak" metric that would only be used late in the best-path decision process. The BGP working group was concerned that any metric specified by a remote operator would only affect routing in a local AS IF no other preference was specified. A paramount goal of the design of the MED was to ensure that peers could not "shed" or "absorb" traffic for networks that they advertise. As such, accepting MEDs from peers may in some sense increase a network's susceptibility to exploitation by peers. -4.1. Acknowledgments +5.1. Acknowledgments Thanks to John Scudder for applying his usual keen eye and constructive insight. Also, thanks to Curtis Villamizar, JR Mitchell and Pekka Savola for their valuable feedback. -5. References +6. References -5.1. Normative References +6.1. Normative 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 2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection - An Alternative to Full Mesh IBGP", RFC 2796, April 2000. [RFC 3065] Traina, P., McPherson, D., Scudder, J.. "Autonomous System Confederations for BGP", RFC 3065, February 2001. [BGP4] Rekhter, Y., T. Li., and Hares. S, Editors, "A Border Gateway Protocol 4 (BGP-4)", BGP Draft, Work in Progress. -5.2. Informative References +6.2. Informative References [RFC 2439] Villamizar, C. and Chandra, R., "BGP Route Flap Damping", RFC 2439, November 1998. [RFC 3345] McPherson, D., Gill, V., Walton, D., and Retana, A, "BGP Persistent Route Oscillation Condition", RFC 3345, August 2002. -6. Authors' Addresses +7. Authors' Addresses Danny McPherson Arbor Networks Email: danny@arbor.net Vijay Gill AOL Email: VijayGill9@aol.com Intellectual Property Statement