Introduction to Computer Networks CS640 Intra-domain Routing

Ming Liu [email protected]

Introduction to Computer Networks

CS640 https://pages.cs.wisc.edu/~mgliu/CS640/F21/

Intra-domain Routing

1

Today

Last lecture • Subnetting/Supernetting

• ARP/DHCP/ICMP

2

Today • Introduction to Routing

• Distance Vector Algorithm

Announcements • Lab2 due on 10/19/2021 at 11:59PM

What makes the network — router (L1)

Network = nodes + links + switch + router • router: a multi-port bridge connecting different switched network

End Point

Link

Switch

Router

Switched network 1

Switched network 2

Switched network 3Internetwork

3

The Goal of Routing

Build router forwarding tables in an internetwork using intra-domain routing protocols

4

The Goal of Routing


Domain: a single administrative entity or organization that owns one or more switched networks

4

Switched network 1

CS

Switched network 2

ECE

Switched network 3

Others (…)Internetwork

UW-Madison Campus

Domain Example

5

Switched network 1

CS

Switched network 2

ECE

Switched network 3

Others (…)Internetwork

UW-Madison Campus

Domain Example

5

The Goal of Routing


High level approach • Distributed execution

• Send message about network links • Each router uses this info to compute routes

6

Forwarding v.s. Routing

Forwarding • To select an output port based on destination address and routing table

Routing • Process by which routing table is built

7

Forwarding Table v.s. Routing Table

Forwarding table • Used when a packet is being forwarded and so must contain enough information to

accomplish the forwarding function

• A row in the forwarding table contains the mapping from a network number to an

outgoing interface and some MAC information, such as Ethernet address of the next hop (inferred from ARP, or manual configured)

Routing table • Built by the routing algorithm as a precursor to build the forwarding table

• Generally contains mapping from network numbers to next hops

8

Routing Overview

We are computing paths to subnets (i.e., switched networks)

9

Routing Overview


Represent Internetwork as a Graph • Assume nodes in the graph are routers • Assume links have cost • This cost could be latency of the link, or a

function of link bandwidth

• Could also have a dynamic model for costs9

Routing Overview


Represent Internetwork as a Graph • Assume nodes in the graph are routers • Assume links have cost • This cost could be latency of the link, or a

function of link bandwidth

• Could also have a dynamic model for costs

The basic problem of routing is to find the lowest-cost path between any two nodes •Where the cost of a path equals the sum of the costs of all the edges that make up the path

9

Challenges of Designing Routing Protocols

#1: It may be simple to calculate least cost path if graph is static but …

#2: Links and routers are added or go down

#3: Traffic can cause links to be overloaded

#4: How are costs calculated to reflect current load on links?

#5: Algorithms must be distributed in order to scale 10

Challenges of Designing Routing Protocols

#1: It may be simple to calculate least cost path if graph is static but …

#2: Links and routers are added or go down

#3: Traffic can cause links to be overloaded

#4: How are costs calculated to reflect current load on links?

#5: Algorithms must be distributed in order to scale

Rich area for research due to distributed, dynamic nature of the problem

• Routing on Multiple Optimality Criteria, Sigcomm’20

10

Such a static approach has several shortcomings • It does not deal with node or link failures • It does not consider the addition of new nodes or links • It implies that edge costs cannot change

Routing Mechanism

For a simple network, we can calculate all shortest paths and load them into some non-volatile storage on each node

11

Routing Mechanism (cont’d)

What is the solution? • Need a distributed and dynamic protocol • Two main classes of protocols • Distance Vector • Link State

12

Distance Vector Overview

Key idea: Each node constructs a one dimensional array (a vector) containing the “distance” (costs) to all other nodes and distributes that vector to its immediate neighbors

Assumption • Each node knows the cost of the link to each of its directly connected neighbors

13

Distance Vector Algorithm

A

F

E

G

BC

D

Distance is defined as the number of hops

14

Distance Vector Alg. — Initial Distance

A

F

E

G

BC

D

Distance to Reach Node (Global View)

A B C D E F GABCDEFG 15


A

F

E

G

BC

D


A B C D E F GA 0 1 1 ∞ 1 1 ∞BCDEFG 15


A

F

E

G

BC

D


A B C D E F GA 0 1 1 ∞ 1 1 ∞B 1 0 1 ∞ ∞ ∞ ∞CDEFG 15


A

F

E

G

BC

D


A B C D E F GA 0 1 1 ∞ 1 1 ∞B 1 0 1 ∞ ∞ ∞ ∞C 1 1 0 1 ∞ ∞ ∞DEFG 15


A

F

E

G

BC

D


A B C D E F GA 0 1 1 ∞ 1 1 ∞B 1 0 1 ∞ ∞ ∞ ∞C 1 1 0 1 ∞ ∞ ∞D ∞ ∞ 1 0 ∞ ∞ 1

E 1 ∞ ∞ ∞ 0 ∞ ∞F 1 ∞ ∞ ∞ ∞ 0 1

G ∞ ∞ ∞ ∞ ∞ 1 015

Distance Vector Alg. — Initial Routing Table

A

F

E

G

BC

D

Destination Cost NextHopB 1 BC 1 CD ∞ —

E 1 EF 1 FG ∞ —

16


A

F

E

G

BC

D

Destination Cost NextHopA 1 AC 1 CD ∞ —

E ∞ —

F ∞ —

G ∞ —16


A

F

E

G

BC

D

Destination Cost NextHopA 1 AB 1 BD 1 DE ∞ —

F ∞ —

G ∞ —16


A

F

E

G

BC

D

Destination Cost NextHopA 1 AB ∞ —

C ∞ —

D ∞ —

F ∞ —

G ∞ —16


A

F

E

G

BC

D

Destination Cost NextHopA 1 AB ∞ —

C ∞ —

D ∞ —

E ∞ —

G 1 G16

Distance Vector Alg. — Exchange Distances

A

F

E

G

BC

D

Dest. Cost NextHopB 1 BC 1 CD ∞ —

E 1 EF 1 FG ∞ —

Dest. Cost NextHopBCDEFG

A

+

B

=

ADest. Cost NextHopA 1 AC 1 CD ∞ —

E ∞ —

F ∞ —

G ∞ —

t0

17



E 1 EF 1 FG ∞ —

Dest. Cost NextHopB 1 BCDEFG

A

+

B

=


E ∞ —

F ∞ —

G ∞ —

A

F

E

G

BC

Dt0

17



E 1 EF 1 FG ∞ —


E 1 EF 1 FG ∞ —

A

+

B

=


E ∞ —

F ∞ —

G ∞ —

A

F

E

G

BC

Dt0

17



E 1 EF 1 FG ∞ —


A

+

C

=

ADest. Cost NextHopA 1 AB 1 BD 1 DE ∞ —

F ∞ —

G ∞ —

A

F

E

G

BC

Dt0 t1

17



E 1 EF 1 FG ∞ —

Dest. Cost NextHopB 1 BC 1 CD 2 CE 1 EF 1 FG ∞ —

A

+

C

=

ADest. Cost NextHopA 1 AB 1 BD 1 DE ∞ —

F ∞ —

G ∞ —

A

F

E

G

BC

Dt0 t1

17




A

+

E

=

ADest. Cost NextHopA 1 AB ∞ —

C ∞ —

D ∞ —

F ∞ —

G ∞ —

A

F

E

G

BC

Dt0 t1 t2

17




A

+

E

=


C ∞ —

D ∞ —

F ∞ —

G ∞ —

A

F

E

G

BC

Dt0 t1 t2

17




A

+

F

=


C ∞ —

D ∞ —

F 0 FG 1 G

A

F

E

G

BC

Dt0 t1 t2 t3

17



Dest. Cost NextHopB 1 BC 1 CD 2 CE 1 EF 1 FG 2 F

A

+

F

=


C ∞ —

D ∞ —

F 0 FG 1 G

A

F

E

G

BC

Dt0 t1 t2 t3

17



Dest. Cost NextHopB 1 BC 1 CD 2 CE 1 EF 1 FG 2 F

A

+

F

=


C ∞ —

D ∞ —

F 0 FG 1 G

A

F

E

G

BC

Dt0 t1 t2 t3

• New_Cost (node) = Cost(node) (from neighbor) + Cost (node-neighbor) • Cost (node-neighbor) = 1 in the above discussion

• Cost = Min (New_Cost, Old_Cost) • If New_Cost is chosen, update the next hop to the neighbor node

• Routing table is evolving

• Based on the event sequence

17


Dest. Cost NextHopA 1 AB ∞ —

C ∞ —

D ∞ —

F ∞ —

G ∞ —

Dest. Cost NextHopA 1 AB 2 AC 2 AD 3 AF 2 AG ∞ —

E

+

A (t1)

=

EDest. Cost NextHopB 1 BC 1 CD 2 CE 1 EF 1 FG ∞ —

A

F

E

G

BC

D

t0 t1 t2 t3A

t0 t1 t2 t3E

17


Dest. Cost NextHopA 1 AB ∞ —

C ∞ —

D ∞ —

F ∞ —

G ∞ —

Dest. Cost NextHopA 1 AB 2 AC 2 AD 3 AF 2 AG 2 A

E

+

A (t3)

=

EDest. Cost NextHopB 1 BC 1 CD 2 CE 1 EF 1 FG 2 E

A

F

E

G

BC

D

t0 t1 t2 t3A

t0 t1 t2 t3E

17

A

F

E

G

BC

D


A B C D E F GA 0 1 1 2 1 1 2

B 1 0 1 2 2 2 3

C 1 1 0 1 2 2 2

D 2 2 1 0 3 2 1

E 1 2 2 3 0 2 3

F 1 2 2 2 2 0 1

G 2 3 2 1 3 1 0

Distance Vector Alg. — Final Distances

18

Distance Vector Discussion

The distance vector routing is based on the Bellman-Ford algorithm

Every T seconds each router sends a list of distance to all the routers to its neighbor

Each router then updates its table based on the new information

19

Distance Vector Discussion

The distance vector routing is based on the Bellman-Ford algorithm

Every T seconds each router sends a list of distance to all the routers to its neighbor

Each router then updates its table based on the new information

• Good

• Fast response to good news

• Bad

• Slow response to bad news

19

When a node detects a link failure

A

F

E

G

BC

D

X

20


A

F

E

G

BC

D

X• F detects that link to G has failed

• F sets distances to G to infinity and sends updates to A

• A sets distance to G to infinity since it uses F to reach G

20


A

F

E

G

BC

D




• A receives periodic update from C with 2-hop path to G

• A sets distance to G to 3 and sends update to F

• F decides it can reach G in 4 hops via A

20


A

F

E

G

BC

D




• A receives periodic update from C with 2-hop path to G

• A sets distance to G to 3 and sends update to F

• F decides it can reach G in 4 hops via A G notifies D that its distance to F is infinity

20

Distance Vector Converges Slowly

Slightly different circumstances can prevent the network from stabilizing

21


A

F

E

G

BC

DX

• At t0, A detects the link failure and advertises a distance of infinity to E

• At t1, B and C receive the message, and update the routing table accordingly

t0 t1A

t0 t1B

t0 t1C

21


A

F

E

G

BC

DX


• At t1, B receives the message from A and updates the routing table as <E, Infinity>

• At t2, B receives the message from C (saying the distance to E is 2), and updates the

routing table as <E, 3>

t0 t1 t2 t3A

t0 t1 t2 t3B

t0 t1 t2 t3C

21


A

F

E

G

BC

DX


• At t1, B receives the message from A and updates the routing table as <E, Infinity>

• At t2, B receives the message from C (saying the distance to E is 2), and updates the


• At t3, C receives the message from A and updates the routing table as <E, Infinity>

t0 t1 t2 t3A

t0 t1 t2 t3B

t0 t1 t2 t3C

21


A

F

E

G

BC

DX

• At t4, C receives the message from B (saying the distance to E is 3), and updates the


• At t4, A receives the message from B (saying the distance to E is 3), and updates the


t0 t1 t2 t3A

t0 t1 t2 t3B

t0 t1 t2 t3C

t4

t4

t4

21


A

F

E

G

BC

DX





• A will advertise this new changes to C, then C advertises B, B advertises A, …

t0 t1 t2 t3A

t0 t1 t2 t3B

t0 t1 t2 t3C

t4

t4

t4

21


A

F

E

G

BC

DX





• A will advertise this new changes to C, then C advertises B, B advertises A, …

t0 t1 t2 t3A

t0 t1 t2 t3B

t0 t1 t2 t3C

t4

t4

t4

This cycle stops only when the distances reach some threshold that is large enough to be considered infinite

• This is called the Count-to-infinity problem

21

Count-to-infinity Problem: Fixes

Use some relatively small number as an approximation of infinity

For example, the maximum number of hops to get across a certain network is never going to be more than 16

22

Count-to-infinity Problem More

A B S

Dest. Cost NextHopB 1 BS 2 B

Dest. Cost NextHopA 1 AS 1 S

• Initial state

23


A B S



X


Dest. Cost NextHopA 1 AS 1 ∞=16

• B to S link goes down

• Initial state

23


A B S



• Initial state

X


Dest. Cost NextHopA 1 AS ∞=16 —

• B to S link goes down


Dest. Cost NextHopA 1 AS 3 A

• B receive A’s advertisement before

B can advertise to A

• B finds shorter router to S

23


A B SXDest. Cost NextHopB 1 BS 4 B


• B advertises updated table to A

• A registers change in path length to

S via B

• A accordingly updates distance to S

23






S via B




• A advertises updated table to B

• B registers change in path length to

S via A

• B accordingly updates distance to S

23






S via B




• A advertises updated table to B

• B registers change in path length to

S via A

• B accordingly updates distance to S

Such back-and-forth advertisements between A and B continues => path lengths to S keeps increase => count-to-infinity

23

Count-to-infinity Problem: Another Fixes

Split horizon • When a node sends a routing update to its neighbors, it does not send those routes it learned from each neighbor back to that neighbor • A never advertises <S, 2, B> to B

A B S



X• Initial state

24

Count-to-infinity Problem: Another Fixes

Split horizon • When a node sends a routing update to its neighbors, it does not send those routes it learned from each neighbor back to that neighbor • A never advertises <S, 2, B> to B

Split horizon with poison reverse • When a node sends a routing update to its neighbors, it puts negative information in the

route to ensure that the route is never being used

• A advertises <S, ∞, B> to B

24

Split Horizon with Poison Reverse Example

A B S

Dest. Cost NextHopB 1 BS 2 BR 1 R

Dest. Cost NextHopS 2 BR 1 R

• w/o split horizon with poison reverse

R

Dest. Cost NextHopA 1 AS 1 SR 2 A

Dest. Cost NextHopS 2 ∞=16

R 1 R

• w/ split horizon with poison reverse

• This prevents B from using the path

A -> B

25

Split Horizon (/w Poison Reverse) Pitfalls

A

F

E

G

BC

DXt0 t1 t2 t3A

t0 t1 t2 t3B

t0 t1 t2 t3C

t4

t4

t4

Only works for small routing loops that involve two nodes • Please work through the example we discussed before

26

Routing Information Protocol (RIP)

Earliest IP routing protocol • 1982 BSD of Unix • Current standard is version 2 (RFC 1723)

Features • Every link has cost 1 -> Hop count • “Infinity” = 16, limits to network where everything

reachable within 15 hops

Sending updates • Every router listens for updates on UDP port 520

• Frequency: 30 seconds • Triggered when an entry is changed

27

Summary

Today • Introduction to Routing

• Distance Vector Algorithm

Next lecture • Link State Routing

• Link Weights • Router/Switch Design

28

Introduction to Computer Networks CS640 Intra-domain Routing

Documents