New IP Routing Algorithms

New IP Routing Algorithms

숙명여자대학교 : 최 종원이화여자대학교 : 이 미정

2000. 2. 14

목차

• 인터넷 라우팅• What needed in New IP routing

– Qos routing– routing table convergence

• 결론

Internet Routing

Goal: determine “good” path

(sequence of routers) thru network from source to

dest.

A

ED

CB

F

22

13

1

1

2

53

5

• “good” path:– typically means

minimum cost path

– other def’s possible

Routing Algorithm classificationGlobal or decentralized

information?Global:• all routers have complete

topology, link cost info• “link state” algorithms

Decentralized: • router knows physically-

connected neighbors, link costs to neighbors

• iterative process of computation, exchange of info with neighbors

• “distance vector” algorithms

Static or dynamic?Static:

• routes change slowly over time

Dynamic:

• routes change more quickly

– periodic update

– in response to link cost changes

Comparison of LS and DV algorithms

Speed of Convergence• LS:

– may have oscillations

• DV: – convergence time varies

– may be routing loops

– count-to-infinity problem

Robustness: what happens if router malfunctions?

LS: – node can advertise incorrect link cost

– each node computes only its own table

DV:– DV node can advertise

incorrect path cost– each node’s table used by

others • error propagate thru network

Routing in the Internet

• The Global Internet consists of Autonomous Systems (AS) interconnected with each other:– Stub AS: small corporation

– Multihomed AS: large corporation (no transit)

– Transit AS: provider

• Two-level routing: – Intra-AS: administrator is responsible for choice

– Inter-AS: unique standard

Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)• Most common IGPs:

– RIP: Routing Information Protocol

– OSPF: Open Shortest Path First

– IGRP: Interior Gateway Routing Protocol (Cisco propr.)

Inter-AS routing

• BGP (Border Gateway Protocol): the de facto standard

• Path Vector protocol: and extension of Distance Vector

• Each Border Gateway broadcast to neighbors (peers) the entire path (ie, sequence of ASs) to destination

Why different Intra- and Inter-AS routing ? • Policy: Inter is concerned with policies (which provider

we must select/avoid, etc). Intra is contained in a single organization, so, no policy decisions necessary

• Scale: Inter provides an extra level of routing table size and routing update traffic reduction above the Intra layer

• Performance: Intra is focused on performance metrics; needs to keep costs low. In Inter it is difficult to propagate performance metrics efficiently (latency, privacy etc). Besides, policy related information is more meaningful.

We need BOTH!

What needed in New IP Routing• QoS Routing

– E.Crawley, R.Nair, B.Rajagopalan and H.Sandick, “A Framework for QoS-based Routing in the Internet,” RFC 2386, Aug. 1998

– G.Apostolopoulos, R.Guerin, and S.Kamat, A.Orda, “QoS Routing Mechanisms and OSPF Extensions,” RFC 2676, Aug. 1999

– S.Chen and K.Nahrstedt, “An Overview of Quality of Service Routing for Next-Generation High-Speed Networks: Problems and Solutions,” IEEE Network Nov. 1998

• Convergence Time Consideration– C. Labovitz, “Delayed Internet Routing Convergence”, SIGCOMM,

Aug. 2000– T. Griffin, “An Analysis of BGP Convergence Properties”, SIGCOMM,

Aug. 1999

Objectives of QoS-based Routing

• Dynamic determination of feasible paths– Select a path that has a good chance of

accommodating the QoS of the given flow– May be subject to policy constraints such as path

cost, provider selection

• Optimization of resource usage– Network state dependent routing

• Graceful performance degradation– Compensate for transient inadequacies in network

engineering

Issues raised in QoS-based Routing

• How to determine the QoS capability of each outgoing link and reserve link resources?

• What is the granularity of routing decision?

• What routing metrics are used?

• How are QoS-accommodating paths computed?

• What are the admission control issues?• What factors affect the routing overheads?

• How is scalability achieved?

Difficulties of QoS Routing

• Diverse QoS constraints– Two or more independent additive constraints make

the problem NP-complete

• Carry both QoS and best-effort traffic– Hard to determine best operating point for both

traffic

• Network state changes dynamically– Performance of a QoS routing algorithm can be

seriously degraded if the state information used is outdated

Two Tasks of QoS Routing

• Collecting state information

• Computing a feasible path

Collecting state information

• Metrics are more dynamic– Requires more frequent exchange of state

information

– Induce more frequent path computation overhead

• Inaccurate– Non negligible propagation delay

– In order to reduce overhead of local state information broadcasting

Collecting state information (cont’d)

• Need to find an optimal compromise between the performance and overheads of the QoS routing – Triggers for network state updates– Triggers for path selection computations

• Overhead is proportional to the size of network and frequency of broadcasting local states– Hierarchical routing

Classification of QoS Routing Algorithms

• Routing classes– Unicast routing– Multicast routing

• Routing strategies– Source routing algorithms– Distributed routing algorithms– Hierarchical routing algorithms

When to Invoke a Routing Algorithm Computation?

• On-demand– For each new request– Computationally extensive– Use the most up to date network state information

• Pre-computation and caching– Amortizes the computational cost over multiple requests– Each computation instance is usually more expensive– Accuracy of the selected paths may be lower– Periodic pre-computation or after a given number of updates

Implementation Experiment• RFC 2676

– Apostolopoulos, Williams, Kamat, Guerin, Orda, Przygienda

– Proposed experimental protocol extending OSPF for QoS routing

– Implemented the proposed extensions and performed performance measurements

• BW-constrained cost-optimization

Implementation Experiment (cont’d)

• QoS routing extensions to OSPF demonstrated in RFC 2676 are fairly straightforward to implement

• By measuring the performance of the real system, it was demonstrated that the overheads associated with QoS routing are not excessive, and within the capabilities of modern processor

Future Research Should Focus On

• Efficient heuristic algorithms for the NP-complete routing problems

• State aggregation with multiple QoS metrics• Hierarchical routing with imprecise information• Multipath routing

– Search multiple paths for a feasible one– Select a set of paths instead of a single one for a

connection

Future Research Should Focus On(cont’d)

• Integration of QoS routing and best-effort routing

• Rerouting– To adapt to changing network state

• Efficient routing algorithms based on specific network models such as the rate-based scheduling network

Internet Routing Convergence Problem

Background

BGP exhibits poor convergence behavior: – Measured convergence times of up to 20 minutes

for BGP path changes/failures– Factorial (N!) theoretic upper bound on BGP

convergence complexity (explore all paths of all possible lengths)

BGP route table -- http://www.telstra.net/ops/bgp.html

Open question: In practice, what topological and policy factors impact convergence delay ?

Goal: Understand BGP convergence behavior under real topologies/policies

– Given a physical topology and ISP policies, can we estimate the time required for convergence?

– Do convergence behaviors of ISPs differ?– How does steady-state topology compare to paths

explored during failure?– Can we change policies/topology to improve BGP

convergence times?

Experiments

• Analyzed secondary paths between between 20 source/destination AS pairs– Inject and monitor BGP faults – Survey providers to determine policies behind paths

• To provide intuition, we will focus on faults injected into three ISPs at Mae-West– Observed faults via fourth ISP (in Japan)– Three ISPs roughly map onto tier1, tier2, tier3

providers– Results from these three ISPs representative of all data

Why we should care about convergence!

• Routing reliability/fault-tolerance on small time scale(minutes) not previously a priority

• Emerging transaction oriented and interactive applications(e.g. Internet Telephony) will require higher leverls of end2end network reliability

• How well does the Internet routing infrastructure tolerate faults?

Conventional Wisdom

• “Restoral is not an issue in the IP world”– Just reroute around in a few milliseconds or

whatever

• BGP convergence takes only a few minutes

• “Bad news travels fast”

• BGP has great convergence properties

• Enough bandwidth will solve anything

The Purpose

• Most of the conventional wisdom about routing convergence is not accurate

Instrument the Internet

• Inject routes into geographically and topologically diverse provider BGP peering sessions(Mae-West, Japan, Michigan, London)

• Periodically fail and change these routes(I.e. send withdraws or new attributes)

• Time envents using ICMP echos and NTP synchronized BGP “routeviess” monitoring mahines

• wait two years (and 250,000 faults)

Fault Secnarios

• Tup - A new route is advertised (route repair)

• Tdown - A route is withdrawn (route failure)

• Tshort - Advertised a shorter/better ASPath (route repair and fail over)

• Tlong - Advertised a longer/worse ASPath (route failure and failover)

Major Convergence Results• Routing convergence requires an order of

magnitude longer than expected (10s of minutes)• Routes converge more quickly following

Tup/Repair than Tdown/Failure events (“bad news travels more slowly”)

• Curiously, withdrawals (Tdown) generates several times the number of announcements than announcements (Tup)

Comparing ISP Convergence Latencies

• CDF of faults injected into three Mae-West providers and observed at Japanese ISP• Significant variations between providers• Not related to geography

Failures, Failovers and Repairs

• Bad news does not travel fast…

• Repairs (Tup) exhibit similar convergence properties as long-short ASPath failover

• Failures (Tdown) and short-long failovers also similar– slower than Tup – 60% take longer than two minutes

Delayed Convergencewhy does this happen?

• Well known that distance vector protocls exhibit poor convergence behaviors– counting to infinity, looping, bouncing problem

• RIP redefines infinity and adds split-horizon, poison reverse, etc.– Still, slow convergence and not scalable

• BGP avertises ASPaths instead of distance– Sloves counting to infinity and RIP looping

problem, but...

Towards Millisecond BGP Convergence

Three possible solutions

• Entirely new protocol

• Turn off MinRouteAdver timer

• “Tag” BGP updates– Provide hint so nodes can detect bogus state

information

결 론• QoS Routing Issues• An experimental implementation is done with OSPF

extensions for QoS• Tradeoffs between performance and overhead of

QoS routing are studied• convergence delays may grow exponentially in the

worst case• Need a fast convergence method

참고문헌• C. Labovitz, “Delayed Internet Routing Convergence”,

SIGCOMM, Aug. 2000, pp. 175-187• R.Guerin and A.Orda, ”QoS-based Routing in Networks with

Inaccurate Information: Theory and Algorithms,” IEEE INFOCOM’97, Japan, Apr. 1997

• Z.Wang and J.Crowcroft, ”QoS Routing for supporting Resource Reservation,” IEEE JSAC, Sept. 1996

• S.Chen and K.Nahrstedt, ”On Finding Multi-Constrained Paths,” IEEE ICC’98, June 1998

• Q.Ma and P.Steenkiste, “Quality-of-Service Routing with Performance Guarantees,” Proc. 4th Int’l. IFIP Wksp. QoS, may 1997

참고문헌 ( 계속 )• K.G.Shin and C.C.Chou, “ A distributed Route-

Selection Scheme for Establishing Real-Time Channel,” 6th IFIP Int’l. Conf. High Perf. Networking, Sept. 1995, pp. 319-29

• J.Behrens and J.J.Garcia-Luna-Aceves, “Hierarchical Routing Using Link Vectors,” IEEE INFOCOM’98, Mar. 1998

• D.H.Lorenz and A.Orda, “QoS Routing in Networks with Uncertain Parameters,” IEEE INFOCOM’98, Mar. 1998

• B.Awerbuch et al., “Throughput-Competitive On-line Routing,” 34th Annual Symp. Foundations of Comp. Sci., Palo alto, CA, Nov. 1993

참고문헌 ( 계속 )• J.-Y.Le Boudec and T.Przygienda, “A Route Pre-

Computation Algorithm for Integrated Services Networks,” J. Network and Sys. Mgmt., vol.3, no.4, 1995, pp. 427-49

• A.Iwata et al., “PNNI Routing Algorithms for Multimedia ATM Internet,” NEC R&D, vol.38, 1997

• M.Peyravian and A.D.Kshemkalyani, “Network Path Caching: Issues, Algorithms and A Simulation Study,” Comp. Commun. Rev., vol.20, 1997, pp. 605-14

• A.Shaikh, J.Rexford, and K.Shin, “Efficient precomputation of qulity-of-service routes,” Wksh. Network and Op. Sys. Support for Digital Audio and Video, July 1998

참고문헌 ( 계속 )• S.Chen and K.Nahrstedt, “Distributed QoS routing with

Imprecise State Information,” ICCCN’98, Oct. 1998• A.Banerjea, “Simulation Study of the Capacity Effects

of Dispersity Routing for Fault Tolerant Realtime Channels,” ACM SIGCOM’96, Aug. 1996

• N.S.V.Rao and S.G.Batsell, “QoS Routing Via Multiple Paths Using Bandwidth Reservation,” IEEE INFOCOM’98, San Francisco, CA, Mar. 1998

• C.Parris, H.Zhang, and D.Ferrari, “ A Mechanism for Dynamic Re-routing of Real-time Channels,” Tech. Rep. TR-92-053, Int’l. Comp. Sci. Inst., Berkeley, CA, Apr. 1992

참고문헌 ( 계속 )• G.Apostolopoulos and S.K.Tripathi, “On Reducing the

Processing Cost of On-Demand QoS Path Computation,” Proc. Of ICNP’98, Oct. 1998

• G.Apostolopoulos, R.Guerin, and S.Kamat, “Implementation and Performance Measurement of QoS Routing Extensions to OSPF,” Proc. of INFOCOM’99, Mar. 1999

• G.Apostolopoulos, R.Guerin, and S.Kamat, S.K.Tripathi, “QoS Routing: A Performance Perspective,” Proc. of ACM SIGCOMM’98

• R.Guerin, A.Orda, and D.Williams, “QoS Routing Mechanisms and OSPF Extensions,” Proc. of the 2nd IEEE Global Internet Mini-Conference, Nov. 1997

참고문헌 ( 계속 )• E.Crawley, R.Nair, B.Rajagopalan and H.Sandick, “A

Framework for QoS-based Routing in the Internet,” RFC 2386, Aug. 1998

• G.Apostolopoulos, R.Guerin, and S.Kamat, A.Orda, “QoS Routing Mechanisms and OSPF Extensions,” RFC 2676, Aug. 1999

• S.Chen and K.Nahrstedt, “An Overview of Quality of Service Routing for Next-Generation High-Speed Networks: Problems and Solutions,” IEEE Network Nov. 1998

• I. Cidon, R.Rom and Y.Shavitt, “Multi-Path Routing Combined with Resource Reservation,” IEEE INFOCOM’97, Japan, Apr.1997, pp. 92-100