l4

Lecture 4: Dynamic routing protocols

Today:1. Overview of router architecture2. RIP, OSPF, BGP3. Notes on Lab 44. Midterm review

Router Architectures

An overview of router architectures.

3

Two key router functions

Control plane: run routing protocols (RIP, OSPF, BGP)

Data plane: forwarding packets from incoming to outgoing link

routingtable

Routingfunctions

IPForwarding

routing tablelookup

routing tableupdates

incoming IPdatagrams

outgoing IPdatagrams

routingprotocol

routingprotocol

4

Routing and Forwarding

Routing functions include:– route calculation– maintenance of the routing table– execution of routing protocols

• On commercial routers handled by a single general purpose processor, called route processor

IP forwarding is per-packet processing• On high-end commercial routers, IP forwarding is distributed• Most work is done on the interface cards

5

Router Hardware Components

• Hardware components of a router:– Network interfaces– Switching fabrics– Processor with a memory

and CPU

Interface Card

Switching fabric

Interface Card Interface Card

Processor

CPUMemory

6

PC Router versus commercial router

• On a PC router: – Switching fabric is the (PCI)

bus – Interface cards are NICs (e.g.,

Ethernet cards)– All forwarding and routing is

done on central processor

• On Commercial routers:– Switching fabrics and

interface cards can be sophisticated

– Central processor is the route processor (only responsible for control functions)

Interface Card

Switching fabric

Interface Card Interface Card

Processor

CPUMemory

7

Basic Architectural ComponentsPer-packet processing

8

Evolution of Router Architectures

• Early routers were essentially general purpose computers • Today, high-performance routers resemble supercomputers

• Exploit parallelism• Special hardware components

• Until 1980s (1st generation): standard computer• Early 1990s (2nd generation): delegate to interfaces• Late 1990s (3rd generation): Distributed architecture• Today: Distributed over multiple racks

9

1st Generation Routers (switching via memory)

• This architecture is still used in low end routers

• Arriving packets are copied to main memory via direct memory access (DMA)

• Switching fabric is a backplane (shared bus)

• All IP forwarding functions are performed in the central processor.

• Routing cache at processor can accelerate the routing table lookup.

Memory

Shared Bus

DMA

MAC

DMA

MAC

InterfaceCard

DMA

MAC

Route Processor

InterfaceCard

InterfaceCard

CacheCPU

10

Drawbacks of 1st Generation Routers

• Forwarding Performance is limited by memory and CPU

• Capacity of shared bus limits the number of interface cards that can be connected

InputPort

OutputPort

Memory

System Bus

11

SharedBus

InterfaceCards

DMA

MAC

DMA

MAC

DMA

MAC

Route Cache

Memory

Route Cache

Memory

Route Cache

Memory

Route Processor

MemoryCacheCPU

2nd Generation Routers (switching via a shared bus)

• Keeps shared bus architecture, but offloads most IP forwarding to interface cards

• Interface cards have local route cache and processing elements

Fast path: If routing entry is found in local cache, forward packet directly to outgoing interface

Slow path: If routing table entry is not in cache, packet must be handled by central CPU

slow pathfast path

12

CPU

Cache

Memory

MAC MAC

Memory

Forwarding Bus(IP headers only)

InterfaceCards

Data Bus

Control Bus

Memory

MAC

Memory

ForwardingEngine

CPU

Cache

Memory

ForwardingEngine Route Processor

CPU

Memory

Another 2nd Generation Architecture

• IP forwarding is done by separate components (Forwarding Engines)

Forwarding operations:1. Packet received on interface:

Store the packet in local memory. Extracts IP header and sent to one forwarding engine

2. Forwarding engine does lookup, updates IP header, and sends it back to incoming interface

3. Packet is reconstructed and sent to outgoing interface.

13

Drawbacks of 2nd Generation Routers

SharedBus

InterfaceCards

DMA

MAC

DMA

MAC

DMA

MAC

Route Cache

Memory

Route Cache

Memory

Route Cache

Memory

Route Processor

MemoryCacheCPU

Bus contentionlimits throughput

14

3rd Generation Architecture

• Switching fabric is an interconnection network (e.g., a crossbar switch)

• Distributed architecture: – Interface cards operate

independent of each other – No centralized processing for

IP forwarding• These routers can be scaled to

many hundred interface cards and to aggregate capacity of > 1 Terabit per second

CPU

Memory

RouteProcessor

Memory

RouteProcessing

MAC

SwitchFabric

Interface

SwitchFabric

Memory

RouteProcessing

MAC

SwitchFabric

Interface

15

Slotted Chassis

• Large routers are built as a slotted chassis– Interface cards are inserted in the slots– Route processor is also inserted as a slot

• This simplifies repairs and upgrades of components

Dynamic Routing ProtocolsPart 1: RIP

Relates to Lab 4.

The first module on dynamic routing protocols. This module introduces RIP.

17

Routing

• Recall: There are two parts to routing IP packets:1. How to pass a packet from an input interface to the output interface of a router (packet forwarding) ? 2. How to find and setup a route ?

• We already discussed the packet forwarding part– Longest prefix match

• There are two approaches for calculating the routing tables:– Static Routing (Lab 3)– Dynamic Routing: Routes are calculated by a routing protocol

18

Routing protocols versus routing algorithms

• Routing protocols establish routing tables at routers.• A routing protocol specifies

– What messages are sent between routers– Under what conditions the messages are sent– How messages are processed to compute routing tables

• At the heart of any routing protocol is a routing algorithm that determines the path from a source to a destination

19

What routing algorithms common routing protocols use

Routing information protocol (RIP) Distance vectorInterior Gateway routing protocol (IGRP, cisco proprietary)

Distance vector

Open shortest path first (OSPF) Link stateIntermediate System-to-Intermediate System (IS-IS

Link state

Border gateway protocol (BGP) Path vector

Routing protocol Routing algorithm

20

Intra-domain routing versus inter-domain routing

• Recall Internet is a network of networks.• Administrative autonomy

– internet = network of networks– each network admin may want to control routing in its own

network• Scale: with 200 million destinations:

– can’t store all dest’s in routing tables!– routing table exchange would swamp links

21

Autonomous systems

• aggregate routers into regions, “autonomous systems” (AS) or domain• routers in the same AS run the same routing protocol

– “intra-AS” or intra-domain routing protocol– routers in different AS can run different intra-AS routing protocol

Ethernet

Router

Ethernet

Ethernet

RouterRouter

Ethernet

Ethernet

EthernetRouterRouter

Router

AutonomousSystem 2

AutonomousSystem 1

22

Autonomous Systems

• An autonomous system is a region of the Internet that is administered by a single entity.

• Examples of autonomous regions are:• UCI’s campus network• MCI’s backbone network• Regional Internet Service Provider

• Routing is done differently within an autonomous system (intradomain routing) and between autonomous system (interdomain routing).

• RIP, OSPF, IGRP, and IS-IS are intra-domain routing protocols.

• BGP is the only inter-domain routing protocol.

23

RIP and OSPF computes shortest paths

• Shortest path routing algorithms• Goal: Given a network where each link is assigned a

cost. Find the path with the least cost between two nodes.

a

b

c d3 1

6

2

24

Distance vector algorithm

• A decentralized algorithm– A router knows physically-connected neighbors and link

costs to neighbors– A router does not have a global view of the network

• Path computation is iterative and mutually dependent.– A router sends its known distances to each destination

(distance vector) to its neighbors.– A router updates the distance to a destination from all its

neighbors’ distance vectors– A router sends its updated distance vector to its neighbors.– The process repeats until all routers’ distance vectors do

not change (this condition is called convergence).

25

A router updates its distance vectors using bellman-ford equation

Bellman-Ford EquationDefinedx(y) := cost of the least-cost path from x to y

Then• dx(y) = minv{c(x,v) + dv(y) }, where min is taken over all

neighbors of node x

26

Distance vector algorithm: initialization

• Let Dx(y) be the estimate of least cost from x to y• Initialization:

– Each node x knows the cost to each neighbor: c(x,v). For each neighbor v of x, Dx(v) = c(x,v)

– Dx(y) to other nodes are initialized as infinity.• Each node x maintains a distance vector (DV):

– Dx = [Dx(y): y 2 N ]

27

Distance vector algorithm: updates

• Each node x sends its distance vector to its neighbors, either periodically, or triggered by a change in its DV.

• When a node x receives a new DV estimate from a neighbor v, it updates its own DV using B-F equation:– If c(x,v) + Dv(y) < Dx(y) then

• Dx(y) = c(x,v) + Dv(y)• Sets the next hop to reach the destination y to the

neighbor v• Notify neighbors of the change

• The estimate Dx(y) will converge to the actual least cost dx(y)

28

Distance vector algorithm: an example

• t = 0• a = ((a, 0), (b, 3), (c, 6))• b = ((a, 3), (b, 0), (c,1))• c = ((a, 6), (b, 1), (c, 0) (d, 2))• d = ((c, 2), (d, 0))

a

b

c d3 1

6

2

• t = 1• a = ((a, 0), (b, 3), (c, 4), (d, 8))• b = ((a, 3), (b, 0), (c,1), (d, 3))• c = ((a, 4), (b, 1), (c, 0), (d, 2))• d = ((a, 8), (b, 3), (c, 2), (d,0))

• t = 2• a = ((a, 0), (b, 3), (c, 4), (d, 6))• b = ((a, 3), (b, 0), (c,1), (d, 3))• c = ((a, 4), (b, 1), (c, 0), (d, 2))• d = ((a, 6), (b, 3), (c, 2), (d,0))

29

How to map the abstract graph to the physical network

• Nodes (e.g., v, w, n) are routers, identified by IP addresses, e.g. 10.0.0.1• Nodes are connected by either a directed link or a broadcast link (Ethernet)• Destinations are IP networks, represented by the network prefixes, e.g.,

10.0.0.0/16– Net(v,n) is the network directly connected to router v and n.

• Costs (e.g. c(v,n)) are associated with network interfaces.– Router1(config)# router rip– Router1(config-router)# offset-list 0 out 10 Ethernet0/0– Router1(config-router)# offset-list 0 out 10 Ethernet0/1

n

vw

Net

Net(v,w)

Net(v,n)

c(v,w)

c(v,n)

30

Distance vector routing protocol: Routing Table

Dest

n

vw

D (v,Net)n

costvia(next hop)

Net

RoutingTable of node v

Net

Net(v,w)c(v,w)

Net(v,n)c(v,n)

Net(v,w): Network address of the network between v and w

c(v,w): cost to transmit on the interface to network Net(v,w)

D(v,net) is v’s cost to Net

31

Distance vector routing protocol: Messages

Dest

D (v,Net)n

costvia(next hop)

Net

RoutingTable of node v

• Nodes send messages to their neighbors which contain distance vectors• A message has the format: [Net , D(v,Net)] means“My cost to go to Net is D (v,Net)”

v n[Net , D(v,Net)]

32

Distance vector routing algorithm: Sending Updates

Dest

D (v,Net 2)n

costvia(next hop)

Net 2

RoutingTable of node v

D (v,Net 1)mNet 1

D (v,Net N)wNet N

Periodically, each node v sends the content of its routing table to its neighbors:

n

v wm

[Net N,D(v,Net N)]

[Net 1,D(v,Net 1)]

[Net N,D(v,Net N)]

[Net 1,D(v,Net 1)]

[Net N,D(v,Net N)]

[Net 1,D(v,Net 1)]

33

Initiating Routing Table I

Destc (v,w)

Net(v,w)

0m

costvia(next hop)

Net(v,m)

RoutingTablec(v,m)

Net(v,m)

c(v,n)Net(v,n) 0wNet(v,w)

0nNet(v,n)n

v wm

• Suppose a new node v becomes active.• The cost to access directly connected networks is zero:

– D (v, Net(v,m)) = 0– D (v, Net(v,w)) = 0– D (v, Net(v,n)) = 0

34

Initiating Routing Table II

Dest

0m

costvia(next hop)

Net(v,m)

RoutingTable

0wNet(v,w)

0nNet(v,n)

• Node v sends the routing table entry to all its neighbors:

n

v wm[w,0][n,0 ] [n,0 ]

[m,0]

[m,0][w,0]

n

v wm[Net(v,w),0][Net(v,n),0] [Net(v,n),0]

[Net(v,m),0]

[Net(v,w),0][Net(v,m),0]

n

v wm[Net(v,w),0][Net(v,n),0] [Net(v,n),0]

[Net(v,m),0]

[Net(v,w),0][Net(v,m),0]

35

n

v wm

[Net N,D(n,Net N)]

[Net 1,D(n,Net 1)]

[Net N,D(m,Net N)]

[Net 1,D(m,Net 1)]

[Net N,D(w,Net N)]

[Net 1,D(w,Net 1)]

Initiating Routing Table III

• Node v receives the routing tables from other nodes and builds up its routing table

36

Updating Routing Tables I

c(v,m)Net(v,m)

n

v wmNet[Net,D(m,Net)]

• Suppose node v receives a message from node m: [Net,D(m,Net)]

if ( D(m,Net) + c (v,m) < D (v,Net) ) {Dnew (v,Net) := D (m,Net) + c (v,m);Update routing table;send message [Net, Dnew (v,Net)] to all neighbors

}

Node v updates its routing table and sends out further messages if the message reduces the cost of a route:

37

Updating Routing Tables II

c(v,m)Net(v,m)

n

v wmNet[Net,D(m,Net)]

• Before receiving the message:

Dest

D(v,Net)??

costvia(next hop)

Net

RoutingTable

c(v,m)Net(v,m)

n

v wmNet[Net,D new (v,Net)]

[Net,D new (v,Net)]

Dest

m

costvia(next hop)

Net

RoutingTable

D new (v,Net)

• Suppose D (m,Net) + c (v,m) < D (v,Net):

38

Example

Router A Router B Router C Router D

10.0.2.0/24 10.0.3.0/24 10.0.4.0/24 10.0.5.0/2410.0.1.0/24

.1.2.2.2.2 .1.1.1

Assume: - link cost is 1, i.e., c(v,w) = 1 - all updates, updates occur simultaneously - Initially, each router only knows the cost of connected interfaces

t=0:10.0.1.0 - 010.0.2.0 - 0

Net via cost

t=0:10.0.2.0 - 010.0.3.0 - 0

Net via cost

t=0:10.0.3.0 - 010.0.4.0 - 0

Net via cost

t=0:10.0.4.0 - 010.0.5.0 - 0

Net via cost

t=1:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 1

t=2:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 2

t=2:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2

t=1:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 1

t=2:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

t=1:10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

t=2:10.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

t=1:10.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

39

Example

Router A Router B Router C Router D

10.0.2.0/24 10.0.3.0/24 10.0.4.0/24 10.0.5.0/2410.0.1.0/24

.1.2.2.2.2 .1.1.1

t=3:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 210.0.5.0 10.0.2.2 3

Net via cost

t=3:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2

Net via cost

t=3:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

Net via cost

t=3:10.0.1.0 10.0.4.1 310.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

Net via cost

Now, routing tables have converged !

t=2:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 2

t=2:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2

t=2:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

t=2:10.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

40

Characteristics of Distance Vector Routing Protocols

• Periodic Updates: Updates to the routing tables are sent at the end of a certain time period. A typical value is 30 seconds.

• Triggered Updates: If a metric changes on a link, a router immediately sends out an update without waiting for the end of the update period.

• Full Routing Table Update: Most distance vector routing protocol send their neighbors the entire routing table (not only entries which change).

• Route invalidation timers: Routing table entries are invalid if they are not refreshed. A typical value is to invalidate an entry if no update is received after 3-6 update periods.

41

The Count-to-Infinity Problem

A B C1 1

A's Routing Table B's Routing Table

C

to costvia(next hop)

2B C

to costvia(next hop)

1C

now link B-C goes down

C 2 C

C 1-C 2B

C C 3

C 3AC -

C 4 C

C -C 4B

11

11

1

42

Count-to-Infinity

• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”

• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B

• How can the Count-to-Infinity problem be solved?

43

Count-to-Infinity

• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”

• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B

• How can the Count-to-Infinity problem be solved?• Solution 1: Always advertise the entire path in an update

message to avoid loops (Path vectors)– BGP uses this solution

44

Count-to-Infinity

• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”

• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B

• How can the Count-to-Infinity problem be solved?• Solution 2: Never advertise the cost to a neighbor if this

neighbor is the next hop on the current path (Split Horizon)– Example: A would not send the first routing update to B, since B

is the next hop on A’s current route to C– Split Horizon does not solve count-to-infinity in all cases!

» You can produce the count-to-infinity problem in Lab 4.

45

RIP - Routing Information Protocol

• A simple intradomain protocol• Straightforward implementation of Distance Vector Routing• Each router advertises its distance vector every 30 seconds

(or whenever its routing table changes) to all of its neighbors• RIP always uses 1 as link metric• Maximum hop count is 15, with “16” equal to “”• Routes are timeout (set to 16) after 3 minutes if they are not

updated

46

RIP - History

• Late 1960s : Distance Vector protocols were used in the ARPANET

• Mid-1970s: XNS (Xerox Network system) routing protocol is the ancestor of RIP in IP (and Novell’s IPX RIP and Apple’s routing protocol)

• 1982 Release of routed for BSD Unix• 1988 RIPv1 (RFC 1058)

- classful routing• 1993 RIPv2 (RFC 1388)

- adds subnet masks with each route entry - allows classless routing

• 1998 Current version of RIPv2 (RFC 2453)

47

RIPv1 Packet Format

IP header UDP header RIP Message

Command Version Set to 00...0

32-bit address

Unused (Set to 00...0)

address family Set to 00.00

Unused (Set to 00...0)

metric (1-16)

one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

One RIP message can have up to 25 route entries

1: request2: response

2: for IP

Address of destination

Cost (measured in hops)

1: RIPv1

48

RIPv2

• RIPv2 is an extends RIPv1:– Subnet masks are carried in the route information– Authentication of routing messages– Route information carries next-hop address– Uses IP multicasting

• Extensions of RIPv2 are carried in unused fields of RIPv1 messages

49

RIPv2 Packet Format

IP header UDP header RIP Message

Command Version Set to 00...0

32-bit address

Unused (Set to 00...0)

address family Set to 00.00

Unused (Set to 00...0)

metric (1-16) one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

One RIP message can have up to 25 route entries

1: request2: response

2: for IP

Address of destination

Cost (measured in hops)

2: RIPv2

50

RIPv2 Packet Format

IP header UDP header RIPv2 Message

Command Version Set to 00.00

IP address

Subnet Mask

address family route tag

Next-Hop IP address

metric (1-16) one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

Used to provide a method of separating "internal" RIP routes (routes for networks within the RIP routing domain) from "external" RIP routes

Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0)

2: RIPv2

Subnet mask for IP address

51

RIP Messages

• This is the operation of RIP in routed. Dedicated port for RIP is UDP port 520.

• Two types of messages: – Request messages

• used to ask neighboring nodes for an update– Response messages

• contains an update

52

Routing with RIP

• Initialization: Send a request packet (command = 1, address family=0..0) on all interfaces:

• RIPv1 uses broadcast if possible, • RIPv2 uses multicast address 224.0.0.9, if possible

requesting routing tables from neighboring routers • Request received: Routers that receive above request send their entire

routing table• Response received: Update the routing table

• Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor in an response message

• Triggered Updates: Whenever the metric for a route change, send entire routing table.

53

RIP Security

• Issue: Sending bogus routing updates to a router• RIPv1: No protection• RIPv2: Simple authentication scheme

IP header UDP header RIPv2 Message

Command Version Set to 00.00

Password (Bytes 0 - 3)

Password (Bytes 4 - 7)

0xffff Authentication Type

Password (Bytes 8- 11)

Password (Bytes 12 - 15) Auth

etic

atio

nUp to 24 more routes (each 20 bytes)

32 bits

2: plaintext password

54

RIP Problems

• RIP takes a long time to stabilize– Even for a small network, it takes several minutes until the

routing tables have settled after a change• RIP has all the problems of distance vector algorithms, e.g.,

count-to-Infinity » RIP uses split horizon to avoid count-to-infinity

• The maximum path in RIP is 15 hops

Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol.

Dynamic Routing Protocols IIOSPF

56

Distance Vector vs. Link State Routing

• With distance vector routing, each node has information only about the next hop:

• Node A: to reach F go to B• Node B: to reach F go to D• Node D: to reach F go to E• Node E: go directly to F

• Distance vector routing makespoor routing decisions if directions are not completelycorrect (e.g., because a node is down).

• If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged.

A B C

D E F

57

Distance Vector vs. Link State Routing

• In link state routing, each node has a complete map of the topology

• If a node fails, each node can calculate the new route

• Difficulty: All nodes need to have a consistent view of the network

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

58

Link State Routing: Properties

• Each node requires complete topology information• Link state information must be flooded to all nodes• Guaranteed to converge

59

Link State Routing: Basic principles

1. Each router establishes a relationship (“adjacency”) with its neighbors2. Each router generates link state advertisements (LSAs) which are

distributed to all routers LSA = (link id, state of the link, cost, neighbors of the link)

Each router sends its LSA to all routers in the network (using a method called reliable flooding)

3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges

4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network

60

Link state routing: graphical illustration

a

b

c d3 1

6

2

a

3

6

b

c

a

b

c

3 1

a

b

c d

1

6

c d2

a’s view

b’s view

c’s view

d’s view

Collecting all pieces yielda complete view of the network!

61

Operation of a Link State Routing protocol

ReceivedLSAs

IP Routing Table

Dijkstra’s

Algorithm

Link StateDatabase

LSAs are flooded to other interfaces

62

Dijkstra’s Shortest Path Algorithm for a Graph

Input: Graph (N,E) with N the set of nodes and E the set of edges

cvw link cost (cvw = 1 if (v,w) E, cvv = 0)s source node.

Output: Dn cost of the least-cost path from node s to node n

M = {s};for each n M

Dn = csn;while (M all nodes) do

Find w M for which Dw = min{Dj ; j M};Add w to M;for each neighbor n of w and n M

Dn = min[ Dn, Dw + cwn ];Update route;

enddo

63

OSPF

• OSPF = Open Shortest Path First• The OSPF routing protocol is the most important link state routing

protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system)

• The complexity of OSPF is significant– RIP (RFC 2453 ~ 40 pages)– OSPF (RFC 2328 ~ 250 pages)

• History:– 1989: RFC 1131 OSPF Version 1 – 1991: RFC1247 OSPF Version 2– 1994: RFC 1583 OSPF Version 2 (revised)– 1997: RFC 2178 OSPF Version 2 (revised)– 1998: RFC 2328 OSPF Version 2 (current version)

64

Features of OSPF

• Provides authentication of routing messages• Enables load balancing by allowing traffic to be split evenly

across routes with equal cost• Type-of-Service routing allows to setup different routes

dependent on the TOS field• Supports subnetting• Supports multicasting• Allows hierarchical routing

65

Hierarchical OSPF

66

Hierarchical OSPF

• Two-level hierarchy: local area, backbone.

– Link-state advertisements only in area – each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.• Area border routers: “summarize” distances to nets in own area,

advertise to other Area Border routers.• Backbone routers: run OSPF routing limited to backbone.

67

Example Network

Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address

3

4 2

5

1

1

32

•Link costs are called Metric

• Metric is in the range [0 , 216]

• Metric can be asymmetric

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10.1.1.2 10.1.4.4 10.1.7.6

10.1.2.3 10.1.5.5

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Link State Advertisement (LSA)

• The LSA of router 10.1.1.1 is as follows:

• Link State ID: 10.1.1.1 = Router ID

• Advertising Router: 10.1.1.1 = Router ID• Number of links: 3 = 2 links plus router itself

• Description of Link 1: Link ID = 10.1.1.2, Metric = 4• Description of Link 2: Link ID = 10.1.2.2, Metric = 3• Description of Link 3: Link ID = 10.1.1.1, Metric = 0

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Network and Link State Database

Each router has a database which contains the LSAs from all other routers

LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age

Router-LSA 10.1.1.1 10.1.1.1 0x9b47 0x80000006 0

Router-LSA 10.1.1.2 10.1.1.2 0x219e 0x80000007 1618

Router-LSA 10.1.2.3 10.1.2.3 0x6b53 0x80000003 1712

Router-LSA 10.1.4.4 10.1.4.4 0xe39a 0x8000003a 20

Router-LSA 10.1.5.5 10.1.5.5 0xd2a6 0x80000038 18

Router-LSA 10.1.7.6 10.1.7.6 0x05c3 0x80000005 1680

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Link State Database

• The collection of all LSAs is called the link-state database• Each router has an identical link-state database

– Useful for debugging: Each router has a complete description of the network

• If neighboring routers discover each other for the first time, they will exchange their link-state databases

• The link-state databases are synchronized using reliable flooding

71

OSPF Packet Format

OSPF MessageIP header

Body of OSPF MessageOSPF MessageHeader

Message TypeSpecific Data LSA LSALSA ...

LSAHeader

LSAData

...

Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

TTL: set to 1 (in most cases)

OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89

72

OSPF Packet Format

source router IP address

authentication

authentication32 bits

version type message length

Area ID

checksum authentication type

Body of OSPF MessageOSPF MessageHeader

2: current version is OSPF V2

Message types:1: Hello (tests reachability)2: Database description3: Link Status request4: Link state update5: Link state acknowledgement

ID of the Area from which the packet originated

Standard IP checksum taken over entire packet

0: no authentication1: Cleartext password2: MD5 checksum(added to end packet)

Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits)

KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)

Prevents replay attacks

73

OSPF LSA Format

Link State ID

link sequence number

advertising router

Link Age Link Type

checksum length

Link ID

Link Data

Link Type Metric#TOS metrics

LSA

LSAHeader

LSAData

Link ID

Link Data

Link Type Metric#TOS metrics

LSA Header

Link 1

Link 2

74

Discovery of Neighbors

• Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.

• If two routers share a link, they can become neighbors, and establish an adjacency

• After becoming a neighbor, routers exchange their link state databases

OSPF Hello

OSPF Hello: I heard 10.1.10.2

10.1.10.1 10.1.10.2

Scenario:Router 10.1.10.2 restarts

75

Neighbor discovery and database synchronization

OSPF Hello

OSPF Hello: I heard 10.1.10.2

Database Description: Sequence = X

10.1.10.1 10.1.10.2

Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005

Database Description: Sequence = X+1

Sends empty database description

Scenario:Router 10.1.10.2 restarts

Discovery of adjacency

Sends database description. (description only contains LSA headers)

Database description of 10.1.10.2Acknowledges

receipt of description

After neighbors are discovered the nodes exchange their databases

76

Regular LSA exchanges

10.1.10.2 explicitly requests each LSA from 10.1.10.1

10.1.10.1 sends requested LSAs

10.1.10.1 10.1.10.2

Link State Request packets, LSAs = Router-LSA, 10.1.10.1, Router-LSA, 10.1.10.2, Router-LSA, 10.1.10.3, Router-LSA, 10.1.10.4, Router-LSA, 10.1.10.5, Router-LSA, 10.1.10.6,

Link State Update Packet, LSAs = Router-LSA, 10.1.10.1,0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

77

Routing Data Distribution

• LSA-Updates are distributed to all other routers via Reliable Flooding

• Example: Flooding of LSA from 10.10.10.1

LSA

LSA

Updatedatabase

Updatedatabase

ACK

ACK

LSA

LSA

LSA

LSA A

CK

AC

K

ACK

ACK

LSA

LSA

LSA

LSA

Updatedatabase

Updatedatabase

ACK

AC

K

ACK

AC

K

Updatedatabase

10.1.1.1 10.1.2.2 10.1.3.4 10.1.7.6

10.1.1.2 10.1.4.5

78

Dissemination of LSA-Update

• A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet)

• Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes.

• Acknowledgements of LSA-updates:• explicit ACK, or• implicit via reception of an LSA-Update

• Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

Border Gateway protocol (BGP)

80

BGP

• BGP = Border Gateway Protocol . Currently in version 4, specified in RFC 1771. (~ 60 pages)

• Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems.

• Interdomain routing protocol for routing between autonomous systems

• Uses TCP to establish a BGP session and to send routing messages over the BGP session

• BGP is a path vector protocol. Routing messages in BGP contain complete routes.

• Network administrators can specify routing policies

81

BGP policy routing

• BGP’s goal is to find any path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible.

• Network administrator sets BGP’s policies to determine the best path to reach a destination network.

82

BGP basics

• A route is defined as a unit of information that pairs a destination with the attributes of a path to that destination.

• EBGP session is a BGP session between two routers in different ASes.

• IBGP session is a BGP session between internal routers of an AS.

83

EBGP and IBGP

• IBGP is organized into a full mesh topology, or IBGP sessions are relayed using a route reflector.

128.195.0.0/16 0 128.195.0.0/16 0

128.195.0.0/16 1 0AS 0

AS 1

AS 2

AS 3128.195.0.0/16 2 1 0

R1R2 R3

R4

R5R6

R7

R8

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Commonly BGP attributes

• Origin: whether it is an internal prefix or an prefix learned from BGP peers• AS path• Next hop• Multi_Exit_Disc (MED, multiple exit discriminator): used to distinguish

routes learned from different peers of the same neighboring AS• Local_pref• Community: group routes to communities

85

BGP route selection process

• Input/output engine may filter routes or manipulate their attributes

InputPolicyEngine

Decisionprocess

Bestroutes

OutPolicyEnigne

Routes recved from peers Routes sentto peers

86

Best path selection algorithm

1. If next hop is inaccessible, ignore routes2. Prefer the route with the largest local preference value.3. If local prefs are the same, prefer route with the shortest AS

path4. If AS_path is the same, prefer route with lowest origin (IGP

< EGP < incomplete)5. If origin is the same, prefer the route with lowest MED6. IF MEDs are the same, prefer EBGP paths to IBGP paths7. If all the above are the same, prefer the route that can be

reached via the closest IGP neighbor.8. If the IGP costs are the same, prefer the router with lowest

router id.

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Example of BGP route selection

InputPolicyEngine

Decisionprocess

Bestroutes

OutPolicyEnigne

AS1

AS2

AS 5

AS3

AS4

128.195.0.0/16

0/0

128.195.0.0/16

0/0

•Deny 0/0 from AS1•Give 128.195.0.0/16•From AS1 higher•Local_pref•Accept other routes

•Accept 0/0 from AS2•Use AS1 to reach 128.195.0.0/16

0/0 AS2128.195.0.0/16 AS1

•Do not propagate 0/0 .

128.195.0.0/16

128.195.0.0/16

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Summary

• Router architectures• Dynamic routing protocols: RIP, OSPF, BGP• RIP uses distance vector algorithm, and converges slow (the

count-to-infinity problem)• OSPF uses link state algorithm, and converges fast. But it is

more complicated than RIP.• Both RIP and OSPF finds lowest-cost path.• BGP uses path vector algorithm, and its path selection

algorithm is complicated, and is influenced by policies.

Lab 4: dynamic routing protocols

90

Exercise (4B): count-to-infinity is optional

• Time consuming to reproduce, but interesting.• Why does count-to-infinity still exist with split horizon?• Lab report due after midterm

Router2

Router4

Router3

Router1101

1

11

1

91

Why does count-to-infinity still exist with split horizon?

Router2Router4

Router3

Router1101

1

11X1

10.0.1.0/24

Router3’s routing table:10.0.1.0/24 ?? 1

Router2’s routing table:10.0.1.0/24 ?? 1

Router4’s routing table:10.0.1.0/24 Router3 3

Router2 is not Router4’s next hop.Router4 sends to router2 the routing update

Router2’s routing table:10.0.1.0/24 Router 4 4

This lie will be told to Router3 andCirculates in the system count-to-infinity

Suppose updates happen in the following sequence:1. The update from PC3 arrives at Router2. The update from Router 3 arrives at Router 23. The update from Router 4 arrives at Router 2

PC3

Midterm review

93

What you’ll be tested on

• Basic lab commands– E.g., ping, traceroute, tcpdump, ethereal, ifconfig, how to

copy a file, how to list a directory• Basic trouble shooting

– E.g., I cannot ping 128.195.1.150, why?• Basic networking concepts

– E.g., layering principle, multiplexing, and encapsulation• Protocols we’ve covered so far

– ARP– ICMP– IP

94

Address translation protocol

• What is it used for?• What is an ARP cache used for?

95

ICMP

• What is it used for?– E.g. error reporting, route redirect

• When will an ICMP message be triggered?

96

IP

• Network order versus host order• CIDR addressing• Route aggregation• Longest prefix match• Fragmentation