CS 268: Lecture 9 Inter-domain Routing Protocol

Transcription

1 Overview CS 68: Lecture 9 Inter-domain Routing Protocol An Introduction to BGP BGP and the Stable Paths problem Convergence of BGP in the real world The End-to-End Effects of Internet Path Selection Scott Shenker and Ion Stoica Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA (*slides from Timothy Griffin and Craig Labovitz) Internet Routing Example Internet organized as a two level hierarchy First level autonomous systems (AS s) Interior router BGP router - AS region of network under a single administrative domain AS s run an intra-domain routing protocols - Distance Vector, e.g., RIP - Link State, e.g., OSPF Between AS s runs inter-domain routing protocols, e.g., Border Gateway Routing (BGP) AS- AS- AS- - De facto standard today, BGP- Inter-domain Routing basics Inter-domain Routing Internet is composed of over 6 autonomous systems BGP = Border Gateway Protocol - Is a Policy-Based routing protocol - Is the de facto inter-domain routing protocol of today s global Internet Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes. Use TCP Border Gateway Protocol (BGP), based on Bellman-Ford path vector AS s exchange reachability information through their BGP routers, only when routes change BGP routing information a sequence of AS s indicating the path traversed by a route; next hop General operations of a BGP router: - Learns multiple paths - Picks best path according to its AS policies - Install best pick in IP forwarding tables 5 6

2 BGP Operations (Simplified) Customers and Providers Establish session on TCP port 79 AS '( )+*-,/. ( BGP session Exchange all active routes AS D&E FG!H I&J!E K&L!MON F!PJ E 5#6 758:9; < =>?@#A AB C Exchange incremental updates While connection is ALIVE exchange route UPDATE messages 7! #" $% & % 8 The Peering Relationship Peering Provides Shortcuts QRRS QS X/Z[\/RS >?@#A AB r a5g`] c QRRS TUV W XY RS >?@#A AB r a5g`] c = ]]?/^`_5?a5b5B c]>!?@ de^b > f5]e>g`]] d > h ]B??]5^5_] C#>!B b]:c5i5^ > a j`]5?o^ = ]]?/^`c5a:dae>_?aebb c] >k?@ de^b > f]e>!gl]] d`_ ]]5?O^ = ]]?/^`m a#a >!]d nc à d ä > ] oc h5@dp ]lq5q q 9 = ]]?B d p`@ r r r a5g^`ca dd5] C#> B bb >w:f] >!g:]5]d >kh]:cie^#> a5j:]? ^:a#ax!vb ]?zy {_?/a5bb c5]?/^5 QRRS QS XOZ[\/RS QRRS TUV W XY RS }~~- Peering Wars : - +ž }Ÿ~ ~- Architecture of Dynamic Routing Reduces upstream transit costs Can increase end-to-end performance May be the only way to connect your customers to some part of the Internet ( Tier ) You would rather have customers Peers are usually your competition Peering relationships may require periodic renegotiation }~~-e + ƒ 5e /~ƒ ˆ-k~ ŠŒ!ˆ: Ž+~ ƒ - e~- + O - ƒ ƒƒ- ~ƒ Ž+~ O } -e /š }~~-e + ˆ -k~ ~ ~ kƒ ˆ ~! e~- /š~- + Oˆ- /œ ãä+ å : }» «l»/¼ ±!²³& µ³«± ² z³!µ ±!µ¹ µº ªz» «læ ½:¾ À&Á ÂÃÄÅ ¾ÆÇ ÈÉ+ÊË ÌÍ ÉÎ Í É ÌÏ Í/ÊÌ ÐÍ ÑŸÏÊÓÒÂÁ Å ÂÔ Õ ª«l ª ±!²³! µ³«± ² z³µ ± µ ¹µº ÊÔÖ Á Â Ã Ä Å¾ÆÇØÑlÊ Ù-Ú ¾ ÏÔzÛ Á ÜÝÞÔß ÄÁ Ü Ô à ØÑŸÊáÁ Å Ú ¾ ¾zÜ âá À¾Í/Ü ¾À!Ü ¾

3 AS-Path Sequence of AS s a route traverses Used for loop detection and to apply policy AS-.../6 AS-.../6 Four Types of BGP Messages Open : Establish a peering session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawing previously announced routes. AS- AS- AS-5.../6.../6 AS- AS- AS-.../6 AS- AS-.../6 AS- AS-5!#"%$&! Two Types of BGP Neighbor Relationships ibgp Peers Must be Fully Meshed AS ebgp External Neighbor (ebgp) in a different Autonomous Systems Internal Neighbor (ibgp) in the same Autonomous System : } ƒ e~ š ƒ- + & + #~# O - ˆ e~ ˆŒ } # - '! Ó})(k ibgp AS 5 ibgp updates ebgp update * +-,/.* :<;=8<68?>A@668B6C 9 8B?>D=: 9 C=* E=;<E=*@F* +-,/.G> 8G8?> 569* +/,/. * :?H I : } ƒ +~ ~ š~ š e ˆJ /š 5 + / )K ƒ / 5Ž ˆ MLt I! N#~ 5 + ~PO- e~-e ˆ e~ƒ e / } š ~ƒ + ƒ ˆ /~ ˆ- +š ˆ ƒ ~ƒ : } K+ - / Œ + & # ˆ O - # Š+~ / +ƒ I : } š ~ƒ + K k J- Oš ~RQeƒ-Ž e~ƒ Kˆ ŽPSÓk Important BGP attributes Route Selection Summary T LocalPREF - Local preference policy to choose most preferred route T Multi-exit Discriminator (MED) - Which peering point to choose? T Import Rules - What route advertisements do I accept? T Export Rules - Which routes do I forward to whom? UWV XZY&[]\ ^)_ ` afbfcfdge?[gf=[fe?[zhgaf[ i)yg`&e?^[ \ ^jikd<jlpu _ `gmn[g\o^wpqzr V sdtgudwvw[ s tgud _ `gmn[g\o^wx udwaf`g\o^ ^ỳtgudw[<x&e?[g\ \ _ `gmn[g\o^we?`&zg^6[fe{xdr g -?ˆ]}? ]ƒ } ƒ/ ~ 6 ˆ/ Š]Œ] Š 6}?~- ƒ] g / ]ƒgƒ]}d g Ž Œ]}?ˆ/ / Œ]~g ] Š~g ] ]}?ƒ]~g k 6 ƒ Š 7 8

4 Implementing Customer/Provider and Peer/Peer relationships T Enforce transit relationships - Outbound route filtering T Enforce order of route preference - provider < peer < customer Import Routes / ' 56 78(5 /556 78(5 97:"8(;<56 78(5 "!# $&% ' (!# $% = >?@ 78(5 & )*+-,.% )*+-,.% 9 A6 Export Routes / ' 5D 78"5 /55D 7&8(5 97:"8";<56 78"5 B (!&# $%& (!# $% A6 = >?E 78"5 Overview T An Introduction to BGP N BGP and the Stable Paths problem T Convergence of BGP in the real world T The End-to-End Effects of Internet Path Selection B )*+C,.% B )*+-,.% F-G H I J KH LM What Problem is BGP solving? Underlying problem Shortest Paths X? Having an X can Distributed means of computing a solution. RIP, OSPF, IS-IS BGP O aid in the design of policy analysis algorithms and heuristics, O aid in the analysis and design of BGP and extensions, O help explain some BGP routing anomalies, O provide a fun way of thinking about the protocol This talk Q : How simple can X get? A: The Stable Paths Problem (SPP) An instance of the SPP : O A graph of nodes and edges, O Node, called the origin, P For each non-zero node, a set or permitted paths to the origin. This set always contains the null path. P A ranking of permitted paths at each node. Null path is always least preferred. (Not shown in diagram) When modeling BGP : nodes represent BGP speaking border routers, and represents a node originating some address block 5 5 most preferred least preferred (not null)

5 A Solution to a Stable Paths Problem Example: SHORTEST A solution is an assignment of permitted paths to each node such that P node u s assigned path is either the null path or is a path uwp, where wp is assigned to node w and {u,w} is an edge in the graph, P each node is assigned the highest ranked path among those consistent with the paths assigned to its neighbors. 5 5 A Solution need not represent a shortest path tree, or a spanning tree. 5 6 Example: SHORTEST (Solution) Example: SHORTEST 7 8 Example: SHORTEST (Solution) Example: GOOD GADGET 9 5

6 Example: GOOD GADGET (Solution) A Stable Paths Problem may have multiple solutions DISAGREE First solution Second solution Example: NAUGHTY GADGET Example: NAUGHTY GADGET (Solution ) Example: NAUGHTY GADGET (Solution ) SPP helps explain possibility of BGP divergence P BGP is not guaranteed to converge to a stable routing. Policy inconsistencies can lead to livelock protocol oscillations. P See Persistent Route Oscillations in Inter-domain Routing by K. Varadhan, R. Govindan, and D. Estrin. ISI report, 996 The SPP view : Solvable must converge Can Diverge must diverge 5 6 6

7 Example: NAUGHTY GADGET 7 8 chooses () chooses ( ) chooses ( ) chooses ( ) 9 chooses () chooses ( ) chooses ( ) 7

8 chooses () chooses ( ) LOOP! BAD GADGET : No Solution SURPRISE : Beware of Backup Policies With a BGP-like protocol, each node will do the best it can, so at least one node will always have the opportunity to improve its path. Result : persistent oscillation. BGP is not robust : it is not guaranteed to recover from network failures. Becomes BAD GADGET if link (, ) goes down. 5 6 PRECARIOUS Has a solution, but can get trapped This part has a solution only when node is assigned the direct path ( ). As with DISAGREE, this part has two distinct solutions 7 Static Approach Automated Analysis of Routing Policies (This is very hard). What is to be done? Inter-AS coordination Dynamic Approach Extend BGP with a dynamic means of detecting and suppressing policy-based oscillations? These approaches are complementary 8 8

9 Theoretical Results Overview The problem of determining whether an instance of stable paths problem is solvable is NPcomplete Shortest path route selection is provably safe An Introduction to BGP BGP and the Stable Paths problem Convergence of BGP in the real world The End-to-End Effects of Internet Path Selection 9 5 Convergence in the real-world? [Labovitz99] Experimental results from two year study which measured 5, BGP faults injected into peering sessions at several IXPs Found - Internet averages minutes to converge after failover - Some multihomed failovers (short to long ASPath) require 5 minutes AS AS BGP Convergence Example AS AS R 5 *B R via AS *B R via AS *B R via AS * B R via AS,AS * B R via AS,AS *B B R via AS,AS B R via AS,AS *B B R via AS,AS B R via AS,AS 5 AS AS AS Convergence Result Outline of the Today s class If we assume. unbounded delay on BGP processing and propagation. Full BGP mesh BGP peers. Constrained shortest path first selection algorithm There exists possible ordering of messages such that BGP will explore all possible ASPaths of all possible lengths BGP is O(N!), where N number of default-free BGP speakers An Introduction to BGP BGP and the Stable Paths problem Convergence of BGP in the real world The End-to-End Effects of Internet Path Selection 5 5 9

10 End-to-end effects of Path Selection Approximating the best path Goal of study: Quantify and understand the impact of path selection on end-to-end performance Basic metric - Let X = performance of default path - Let Y = performance of best path - Y-X = cost of using default path Technical issues - How to find the best path? - How to measure the best path? Key Idea - Use end-to-end measurements to extrapolate potential alternate paths Rough Approach - Measure paths between pairs of hosts - Generate synthetic topology full NxN mesh - Conservative approximation of best path Question: Given a selection of N hosts, how crude is this approximation? Methodology For each pair of end-hosts, calculate: - Average round-trip time - Average loss rate - Average bandwidth Generate synthetic alternate paths (based on long-term averages) For each pair of hosts,graph difference between default path and alternate path 57 58! "#! $! %! "&! $! % 59 6

11 Why Path Selection is imperfect?! "#! $! % Technical Reasons - Single path routing - Non-topological route aggregation - Coarse routing metrics (AS_PATH) - Local policy decisions Economic Reasons - Disincentive to offer transit - Minimal incentive to optimize transit traffic 6 6