Lecture 18

Internet Routing: The goal is to find any route that is loop free-Global optimization is a distant dream depending on economic and political drivers

Homework: 4.1-35, 37-45

Distance-vector routing: A’s routing table just after link A-E failure

D C NH

B B

C C

D C

E -

F 1 F

G F

Dest Cost NH

B B

C C

D C

E -

F 1 F

G F

Dest. Cost NHop

A A

B A

C A

D G

E 2 A

G G

A

F

Dest. Cost NHop

A A

B A

C A

D G

E -

G G

Dest. Cost NHop

A F

B F or C

C D

D D

E 3 F

F F

G

Dest. Cost NHop

A F

B F

C D

D D

E 3 F

F F

Dest. Cost NHop

A F

B F

C D

D D

E -

F F

What happens when G advertises its table to D? How does D get the message that A-E has failed?

Loops -A

• A advertises [E, ] to B and B advertises [E, 2] to A (they cross intransit)

• A advertises [E,3] and B advertises [E, ]

• etc.

• B resets [E, , -] (since A is next hop) A updates [E, 3, B] since 3<

• B updates [E, 4,A] and A resets [E, , -] since B is next hop

• etc.

Split horizon: B should not advertise a route

[E, 2] it got from A (in the first step)

Loops -B

• A advertises [E, ] to B & C

Advertisement to C is delayed

• B may advertise [E, ] to C

• C advertises [E, 2] to B

• A’s advertisement [E, ] arrives at C

• B advertises [E,3] to A

• C advertises [E, ] to B

• A advertises [E,4] to C

• B advertises [E, ] to A

• C advertises [E,5] to B

• B resets [E, , -] since A is next hop

• C will not update (why)

• B updates [E, , C] since 3<• C updates [E, , -] since A is next hop

• A updates [E,4, B] since 4<• B updates [E, , -] since C is next hop

• C updates [E, 5, A] since 5<

• A updates [E, , -] since B is next hop

• B updates [E, 6, A] since 6<

Split horizon will not solve this problem

Link State Routing

A B C D

A

B

C

D

You have a global view: routing table is spanning tree as seen from root node

The Internet circa 1990: A hierarchical collection of autonomous systems (AS)

Today’s multiple backbone

Stub AS

Multi-homed AS

Transit AS

Inefficiency of IP address classes

• If you have 257 end users, you need class B and then you have 16K addresses. We need finer distinctions.

• Two issues:– How do you give different network addresses to

physical networks within 1 class A, B or C network---subnetting

– How do you aggregate networks within an domain to simplifier routing outside the domain--supernetting

Forwarding Table of R1

Subnet Number Subnet Mask NextHop

128.96.34.0 255.255.255.128 Interface0

128.96.34.128 255.255.255.128 Interface 1

128.96.33.0 255.255.255.0 R2

More Subnetting

• You can break the same physical network into subnets—forcing hosts to speak through a router

• Each host has it’s own subnet in the new engineering network

• Subnet mask=IP address---non-contiguous 1’s and 0’s

Classless Interdomain Routing (CIDR)

• We can get better utilization if we hand out Class C addresses

• This would increase the size of forwarding tables

• We aggregate contiguous class C blocks

Aggregation of 16 Class C network addresses into a single 20 bit CIDER address

Class C network numbers

192.4.00010000.XYZ

192.4.00010001.XYZ

192.4.00010010.XYZ CIDR 20 bit network address

192.4.00010011.XYZ 192.4.0001 =194.4.16/20

192.4.00010100.XYZ

192.4.00010101.XYZ

192.4.00010110.XYZ

192.4.00010111.XYZ

192.4.00011000.XYZ

192.4.00011001.XYZ

192.4.00011010.XYZ

192.4.00011011.XYZ

192.4.00011100.XYZ

192.4.00011101.XYZ

192.4.00011110.XYZ

192.4.00011111.XYZ

Intradomain Routing

Border Gateway Routers—Default router for outbound traffic

Intradomain routing issues

• Scale—100,000 network addresses

• Autonomy

• Trust

• Flexibility—hot potato routing

BGP example

1 BGP speaker/AS: Advertise routes-prevents looping

Withdrawn broken routes

Routing Areas in OSPF