Can the Border Gateway Protocol (BGP) be fixed? UCL Oct 15, 2003 Timothy G. Griffin Intel Research, Cambridge UK

How do you connect to the Internet? Physical connectivity is just the beginning of the story….

Architecture of Dynamic Routing AS 1 AS 2 EGP (= BGP) EGP = Exterior Gateway Protocol IGP = Interior Gateway Protocol Metric based: OSPF, IS-IS, RIP, EIGRP (cisco) Policy based: BGP The Routing Domain of BGP is the entire Internet IGP

BGP Table Growth Thanks to Geoff Huston. on May 30, 2003

How Many ASNs are there? Thanks to Geoff Huston. on May 30, 2003

Partial View of ( ) Neighborhood AS 786 ja.net (UKERNA) AS 1239 Sprint AS 4373 Online Computer Library Center Originates > 180 prefixes, Including /16 AS 3356 Level 3 AS 6461 AboveNet AS 1213 HEAnet (Irish academic and research) AS 7 UK Defense Research Agency AS 4637 REACH AS Hanse AS 3257 Tiscali AS 5089 NTL Group AS Versatel AS 5459 LINX AS 702 UUNET AS GEANT

Topology information is flooded within the routing domain Best end-to-end paths are computed locally at each router. Best end-to-end paths determine next-hops. Based on minimizing some notion of distance Works only if policy is shared and uniform Examples: OSPF, IS-IS Each router knows little about network topology Only best next-hops are chosen by each router for each destination network. Best end-to-end paths result from composition of all next-hop choices Does not require any notion of distance Does not require uniform policies at all routers Examples: RIP, BGP Link StateVectoring Technology of Distributed Routing

8 BGP Route Processing Best Route Selection Apply Import Policies Best Route Table Apply Export Policies Install forwarding Entries for best Routes. Receive BGP Updates Best Routes Transmit BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values IP Forwarding Table Apply Policy = filter routes & tweak attributes Open ended programming. Constrained only by vendor configuration language

9 Shedding Inbound Traffic with ASPATH Prepending Prepending will (usually) force inbound traffic from AS 1 to take primary link AS /24 ASPATH = customer AS 2 provider /24 backupprimary /24 ASPATH = 2 Yes, this is a Glorious Hack …

10 … But Padding Does Not Always Work AS /24 ASPATH = customer AS 2 provider /24 ASPATH = 2 AS 3 provider AS 3 will send traffic on “backup” link because it prefers customer routes and local preference is considered before ASPATH length! Padding in this way is often used as a form of load balancing backupprimary

11 COMMUNITY Attribute to the Rescue! AS 1 customer AS 2 provider /24 ASPATH = 2 AS 3 provider backupprimary /24 ASPATH = 2 COMMUNITY = 3:70 Customer import policy at AS 3: If 3:90 in COMMUNITY then set local preference to 90 If 3:80 in COMMUNITY then set local preference to 80 If 3:70 in COMMUNITY then set local preference to 70 AS 3: normal customer local pref is 100, peer local pref is 90

Don’t celebrate just yet… customer peering provider/customer Provider B (Tier 1) Provider A (Tier 1) Provider C (Tier 2) Now, customer wants a backup link to C…. provider/customer

Customer installs a “backup link” … customer Provider B (Tier 1) Provider A (Tier 1) Provider C (Tier 2) customer sends “lower my preference” Community value primary backup

Disaster Strikes! customer Provider B (Tier 1) Provider A (Tier 1) Provider C (Tier 2) primary backup customer is happy that backup was installed …

The primary link is repaired, and something odd occurs… customer Provider B (Tier 1) Provider A (Tier 1) Provider C (Tier 2) primary backup YIKES --- routing DOES NOT return to normal!!!

WAIT! It Gets Better… A P B B B C B D P = primaryB = backup

OOOOOPS! A P B B B C B D Suppose A, B, C all break ties in the same direction (clockwise or counter-clockwise) No solution = Protocol Divergence

What the heck is going on? There is no guarantee that a BGP configuration has a unique routing solution. –When multiple solutions exist, the (unpredictable) order of updates will determine which one is wins. There is no guarantee that a BGP configuration has any solution! –And checking configurations NP-Complete [GW1999] Complex policies (weights, communities setting preferences, and so on) increase chances of routing anomalies. –… yet this is the current trend!

Larry Speaks Is this any way to run an Internet?

What Problem is BGP Solving? Underlying problem Shortest Paths Distributed means of computing a solution. ???? RIP, OSPF, IS-IS BGP [GSW1998, GSW2002] Stable Paths

Separate dynamic and static semantics SPVP = Simple Path Vector Protocol, a distributed algorithm for solving SPP BGP SPVP Booo Hooo, Many, many complications... BGP Policies Stable Paths Problem (SPP) “static” semantics dynamic semantics Worst case, This is an exponential Time and space translation

1 An instance of the Stable Paths Problem (SPP) A graph of nodes and edges, Node 0, called the origin, For each non-zero node, a set or permitted paths to the origin. This set always contains the “null path”. 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 routers, and 0 represents a node originating some address block most preferred … least preferred

A Solution to a Stable Paths Problem 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, each node is assigned the highest ranked path among those consistent with the paths assigned to its neighbors. A Solution need not represent a shortest path tree, or a spanning tree. A solution is an assignment of permitted paths to each node such that

An SPP may have multiple solutions First solution Second solution DISAGREE

BAD GADGET : No Solution This is an SPP version of the example first presented in Persistent Route Oscillations in Inter-Domain Routing. Kannan Varadhan, Ramesh Govindan, and Deborah Estrin. Computer Networks, Jan. 2000

SURPRISE! Becomes a BAD GADGET if link (4, 0) goes down. BGP is not robust : it is not guaranteed to recover from network failures.

Can BGP be fixed? Joint work with Aaron Jaggard (UPenn Math) and Vijay Ramachandran (Yale CS) SIGCOMM 2003 BGP policy languages have evolved organically A policy language really should be designed! But how?

Design Dimensions Robustness (required!) Transparency (required!) Expressive Power Autonomy (“freedom of independent action”) Global Consitency Policy Opaqueness Tradeoffs abound

Robustness Partially Partially Ordered ( PP0 ): For all paths P and Q, (P < Q and Q < P) implies (P = Q or last(P) = last(Q)) Checking robustness is an NP-hard P < Q : transitive closure of (subpath relation on permitted paths union the path ranking relation at each node) This is a sufficient condition for robustness

Transparency, Autonomy Transparency: protocol will compose its transformation with transformation of policy writer. Autonomy: measure of “wiggle room” –Weak autonomy: neighbors can’t dictate relative ranking of routes –Stronger: policy writer can classify neighbors and rank routes based on class (“autonomy of neighbor ranking”).

Need Global Constraints Theorem: Any robust system supporting both transparency and autonomy must have a non-trivial global constraint Global constraints must be a part of design from the start

A Partial Ordered for the Design Space ( J, L ) < ( J, L ) 1122 if and only if for all S : SPP 1.J(S) implies J(S) 2.L(S) implies L(S) Local ConstraintGlobal Constraint

Robust Designs ( J, L ) is robust if and only if 2 (J and L ) implies PPO Examples: ( True, SP ) ( PPO, True )

Robust Subspace ( PPO, True ) ( True, SP ) Expressive Power Constraint Simplicity Not tractable Tractable

Hierarchical BGP (HBGP) HBGP HBGP +PEER + BU HBGP +PEERHBGP + BU [GR2000, GGR2001]

Next? Need techniques for constructing policy languages. Design of protocols to enforce global constraints. Is there a general formalism to capture autonomy?

References [VGE1996, VGE2000] Persistent Route Oscillations in Inter-Domain Routing. Kannan Varadhan, Ramesh Govindan, and Deborah Estrin. Computer Networks, Jan (Also USC Tech Report, Feb. 1996) [GW1999] An Analysis of BGP Convergence Properties. Timothy G. Griffin, Gordon Wilfong. SIGCOMM 1999 [GSW1999] Policy Disputes in Path Vector Protocols. Timothy G. Griffin, F. Bruce Shepherd, Gordon Wilfong. ICNP 1999 [GW2001] A Safe Path Vector Protocol. Timothy G. Griffin, Gordon Wilfong. INFOCOM 2001 [GR2000] Stable Internet Routing without Global Coordination. Lixin Gao, Jennifer Rexford. SIGMETRICS 2000 [GGR2001] Inherently safe backup routing with BGP. Lixin Gao, Timothy G. Griffin, Jennifer Rexford. INFOCOM 2001 –[GW2002a] On the Correctness of IBGP Configurations. Griffin and Wilfong.SIGCOMM –[GW2002b] An Analysis of the MED oscillation Problem. Griffin and Wilfong. ICNP 2002.