Outline. Internet Routing. Alleviating the Problem. DV Algorithm. Routing Information Protocol (RIP) Link State Routing. Routing algorithms

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Spring 2001 Venkat Padmanabhan 1

Internet Routing

Venkat Padmanabhan

Microsoft Research

9 April 2001

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Outline

• Routing algorithms

– distance-vector (DV) – link-state (LS)

• Internet Routing

– border gateway protocol (BGP)

• BGP convergence paper

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DV Algorithm

• Each router maintains a vector of costs to all destinations as well as routing table

– Initialize neighbors with known cost, others with infinity

• Periodically send copy of distance vector to neighbors

– On reception of a vector, determine if path via the neighbor better and if so update routing table

• If no changes, will converge to shortest paths

– but changes can create loops (count-to-infinity)

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Alleviating the Problem

• Split horizon

– Router never advertises the cost of a destination back to its next hop – that’s where it learned it from! – Solves trivial count-to-infinity problem

• Poison reverse

– go even further – advertise back infinity – why is this useful?

• Triggered updates

– count to infinity faster!

• However, DV protocols are still subject to the same problem with more complicated topologies

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Routing Information Protocol

(RIP)

• DV protocol with hop count as metric

– Infinity defined to be 16 hops! Limits network size – Includes split horizon with poison reverse

• Routers send vectors every 30 seconds

– With triggered updates for link failures – Time-out in 180 seconds to detect failures

• RIPv1 (RFC1058), RIPv2 (RFC1388)

– v2 includes subnet mask, authentication

• Main advantage: simplicity

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Link State Routing

• Same assumptions/goals, but different idea:

– Tell all routers the topology and have each compute best paths

– Two phases:

• Topology dissemination (flooding)

• Shortest-path calculation (Dijkstra’s algorithm)

• Why?

– In DV, routers hide their computation, making it difficult to make good decisions upon change

– With LS, faster convergence and hopefully better stability

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• Each router maintains link state database

and periodically sends link state packets

(LSPs) to its neighbors

– Contain [router, neighbors, costs]

• Each router forwards LSPs not already in

its database on all ports except where

received

– Each LSP will travel over the same link at most once in each direction

Flooding

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Example

• LSP generated by X at T=0

• Nodes become yellow as they receive it

X A C B D X A C B D X A C B D X A C B D T=0 T=1 T=2 T=3 CSE561

Link-State Routing Issues

• Distinguishing between old and new LSPs

– LSP carry sequence numbers

– Why is this not an issue for DV?

• Scalability

– overhead of flooding, SPF computation – use hierarchy (OSPF areas, IGP/EGP split)

• Metrics

– LSP can contain multiple metrics

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Open Shortest Path First

(OSPF)

• Most widely-used Link State protocol

today

• Basic link state algorithms plus many

features:

– Authentication of routing messages – Extra hierarchy: partition into routing areas – Load balancing: multiple equal cost routes

Routing Metrics

• Protocols such as OSPF don’t specify this • ARPANET history:

– Original metric: instantaneous queue length – D-SPF (late 70s): delay metric

• okay under light load (delay dominated by static quantities)

• oscillations under heavy load

– HN-SPF (late 80s): normalized “hops” metric

• delay used to estimate link utilization

• link utilization is normalized using a linear transform • cost of heavily-loaded link ≤3*cost of idle link

Internet Routing

• Main concern: scalability

– size of routing tables – volume of routing tables – amount of routing computation

• Tools for scaling

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Address Allocation and

Aggregation

• IP address indicates topological location

– unlike “flat” Ethernet addresses

• Hosts in a network share a common prefix

– prefix obtained from IANA or ISP – e.g., 128.32.X.Y for Berkeley

• Address aggregation

– only advertise routes to aggregates – subnetting

– supernetting (CIDR)

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Network Host 7 24 0 Network Host 14 16 1 0 Network Host 21 8 1 1 0

IPv4 Address Formats

Class A

Class B

Class C

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Network number Host number Class B address

Subnet mask (255.255.255.0)

Subnetted address 111111111111111111111111 00000000

Network number Subnet ID Host ID

Subnetting

• Split up one network number into multiple physical networks • Internal structure isn’t propagated • Helps allocation efficiency CSE561

Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 128.96.34.15 128.96.34.1 H1 R1 128.96.34.130 Subnet mask: 255.255.255.128 Subnet number: 128.96.34.128 128.96.34.129 128.96.34.139 R2 H2 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 H3

Subnet Example

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CIDR (Supernetting)

• CIDR: Classless Inter-Domain Routing • Aggregate advertised network routes

– e.g., ISP has class C addresses 192.4.16 through 192.4.31

– Really like one larger 20 bit address class … – Advertise as such (network number, prefix length) – Reduces size of routing tables

• But IP forwarding is more involved

– Based on Longest Matching Prefix operation

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Border gateway (advertises path to 128.32.2/23) Regional network Corporation X (128.32.2/24) Corporation Y (128.32.3/24)

CIDR Example

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Hierarchical Routing

• Several levels of hierarchy

• Intra-domain versus inter-domain

routing

– break problem down into more manageable pieces

– IGP: RIP, OSPF – EGP: EGP, IDRP, BGP

• Are RIP and OSPF suitable for

inter-domain routing?

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Backbone service provider Peering point Peering point Large corporation Large corporation Small corporation “Consumer ” ISP “Consumer”ISP “ Consumer” ISP You at home You at work

Structure of the Internet

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Inter-Domain Routing

• Network comprised of many Autonomous Systems (ASs)

– each AS is assigned a number

• Kinds of ASs

– stub AS – multi-homed AS – transit AS

• Does the AS number have to be unique? 12 44 7 321 23 1123 CSE561

Inter-Domain Routing

• Border routers summarize and advertise internal routes to external neighbors and vice-versa • Border routers apply policy • Internal routers can use

notion of default routes • Core is “default-free” R1 Autonomous system 1 R2 R3 Autonomous system 2 R4 R5 R6 AS1 AS2 Border router Border router NSFNET backbone Stanford BARRNET regional Berkeley PARC NCAR UNM Westnet regional UNL KU ISU MidNet regional …

Exterior Gateway Protocol

(EGP)

• First major interdomain routing protocol

• Constrained Internet to tree structure

Border Gateway Protocol (BGP-4)

• EGP used in the Internet backbone

today

• Features:

– path vector routing

– incremental updates (except initially) – application of policy

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Path Vectors

• Similar to distance vector, except send entire paths

– e.g. 321 hears [7,12,44] – stronger avoidance of loops – multiple BGP speakers per AS

• Shorter paths preferred (modulo policy)

• Reachability only

– announcements & withdrawals – explicit/implicit withdrawals – hard to ensure “optimal” routing

12 44 7 321 23 1123 CSE561

BGP Policies

• Impact of policies

– which routes to accept and preference – which routes to advertise

• Policies are generally local to an AS

– business considerations

– cost – robustness

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BGP Policies: Example

C2 C3 ISP1 ISP2 ISP3 C1

– ISP2may not provide transit service for ISP1and

ISP3

– ISP2may not blindly announce any route it hears

from C2

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Impact of Policies – Example #1

• Early Exit / Hot Potato

– “if it’s not for you, bail”

• Combination of best local policies not globally best • Side-effect: asymmetry • Inter-domain connectivity

cannot be modeled as a simple directed graph!

B A

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Impact of Policies: Example #2

• Persistent oscillations

• Example:

– (Varadhan et al. 1996) – AS1 prefers R2 – AS2 prefers R3 – AS3 prefers R1

• Solution?

AS1 AS3 AS2 R1 R2 R3 CSE561

Operation over TCP

• Most routing protocols operate over UDP/IP • BGP uses TCP

– TCP handles error control; reacts to congestion – Allows for incremental updates

• Issue: Data vs. Control plane

– Should routing messages receive a higher priority than data?

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When should we use BGP?

• Main benefit of BGP is greater control

– makes sense for multi-homed site, transit network

• How about a stub network?

– default/static route will suffice

– several costs to running BGP and advertising a separate prefix

• need BGP router

• additional routing entry in every BGP router • instability due to transient faults

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BGP Convergence

• Paper by Labovitz, Ahuja, Bose,

Jahanian

• Fast fail-over of Internet routes is a

myth

– can take several minutes

• BGP maintains an alternate path per

neighbor

– protocol doesn’t indicate cause of failure – blindly explores all paths upon failure

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Experimental Observations

• Tup & Tshort converge faster than

Tdown & Tlong

• No correlation between convergence

latency and geographic distance

– topology is the key (# of alternate paths)

• No correlation between convergence

latency and congestion

– previous study on routing instability had demonstrated correlation

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BGP Convergence Model

• Complete graph: O((n-1)!) time

• Reason:

– monotonically increasing rather than strictly increasing path lengths

• Basic problem:

– nodes advertise new paths as soon as they receive updates

Doing better

• Synchronizing updates

– at most one announce per destination during a MinRouteAdver interval

– ensures that each round only considers paths longer than that in previous rounds – O(max length path)

• Loop detection

– receiver-side as well as sender-side

Doing still better

• BGP-CT

– “cause tag” indicates the reason that a route was withdrawn

– can tell if an alternate route is also affected by a failure