ECE544: Communication Networks-II, Spring 2007

D. Raychaudhuri

Lecture 4,5

Includes teaching materials from L. Peterson

Today’s Lecture

• IP basics• Routing principles

– distance vector (RIP)– link state (OSPF)

IP Basics

Best Effort Service ModelGlobal Addressing SchemeARP & DHCP

IP Internet

• Concatenation of Networks

• Protocol Stack

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8

R1

ETH FDDI

IPIP

ETH

TCP R2

FDDI PPP

IP

R3

PPP ETH

IP

H1

IP

ETH

TCP

H8

Service Model• Connectionless (datagram-based)• Best-effort delivery (unreliable service)

– packets are lost– packets are delivered out of order– duplicate copies of a packet are delivered– packets can be delayed for a long time

• Datagram format Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

Fragmentation and Reassembly

• Each network has some MTU• Strategy

– fragment when necessary (MTU < Datagram)– try to avoid fragmentation at source host– re-fragmentation is possible – fragments are self-contained datagrams– use CS-PDU (not cells) for ATM– delay reassembly until destination host– do not recover from lost fragments

Example

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

Ident = x Offset = 0

Start of header

0

Rest of header

1400 data bytes

Ident = x Offset = 0

Start of header

1

Rest of header

512 data bytes

Ident = x Offset = 512

Start of header

1

Rest of header

512 data bytes

Ident = x Offset = 1024

Start of header

0

Rest of header

376 data bytes

Global Addresses• Properties

– globally unique– hierarchical: network + host

• Dot Notation– 10.3.2.4– 128.96.33.81– 192.12.69.77

Network Host

7 24

0A:

Network Host

14 16

1 0B:

Network Host

21 8

1 1 0C:

Datagram Forwarding • Strategy

– every datagram contains destination’s address– if directly connected to destination network, then forward

to host– if not directly connected to destination network, then

forward to some router– forwarding table maps network number into next hop– each host has a default router– each router maintains a forwarding table

• Example (R2) Network Number Next Hop

1 R3 2 R1 3 interface 1 4 interface 0

Address Translation • Map IP addresses into physical addresses

– destination host– next hop router

• Techniques– encode physical address in host part of IP address– table-based

• ARP– table of IP to physical address bindings– broadcast request if IP address not in table– target machine responds with its physical address– table entries are discarded if not refreshed

ARP Details

• Request Format– HardwareType: type of physical network (e.g., Ethernet)– ProtocolType: type of higher layer protocol (e.g., IP)– HLEN & PLEN: length of physical and protocol addresses– Operation: request or response – Source/Target-Physical/Protocol addresses

• Notes– table entries timeout in about 10 minutes– update table with source when you are the target – update table if already have an entry– do not refresh table entries upon reference

ARP Packet Format

TargetHardwareAddr (bytes 2 – 5)

TargetProtocolAddr (bytes 0 – 3)

SourceProtocolAddr (bytes 2 – 3)

Hardware type = 1 ProtocolType = 0x0800

SourceHardwareAddr (bytes 4 – 5)

TargetHardwareAddr (bytes 0 – 1)

SourceProtocolAddr (bytes 0 – 1)

HLen = 48 PLen = 32 Operation

SourceHardwareAddr (bytes 0 – 3)

0 8 16 31

ATM ARP

H2

R

H1

LIS 10

LIS 12

ATM network

10.0.0.2

10.0.0.112.0.0.3

12.0.0.5

• ATM ARP for mapping IP<->ATM addr– medium is not a broadcast type unlike Ethernet– requires servers which maintain ARP tables– concept of multiple “logical IP subnets” (LIS)

Dynamic Host Control Protocol (DHCP)

• DHCP server per network for IP address assignment

• Static list of IP<->physical addr or dynamic binding from common pool

• Host boot-up via well-known address 255.255.255.255

• DHCP “relay agent” can be used to avoid one server per network

Dynamic Host Control Protocol (DHCP)

• DHCP packet format (runs over UDP)

Operation HType HLen Hops

Xid

Secs Flag

ciaddr

yiaddr

siaddr

giaddr

chaddr (16B)

....

Internet Control Message Protocol (ICMP)

• Echo (ping)• Redirect (from router to source host)• Destination unreachable (protocol, port, or

host)• TTL exceeded (so datagrams don’t cycle

forever)• Checksum failed • Reassembly failed• Cannot fragment

Routing Basics

Routing Problem• Network as a Graph

Problem: Find lowest cost path between two nodes

• Factors– static: topology– dynamic: load

4

3

6

21

9

1

1D

A

FE

B

C

Two main approaches• DV: Distance-vector protocols• LS: Link state protocols• Variations of above methods applied

to:– Intra-domain routing (small/med networks)

• RIP, OSPF

– Inter-domain routing (large/global networks)• BGP-4

Distance Vector Protocols

• Employed in the early Arpanet• Distributed next hop computation

– adaptive

• Unit of information exchange – vector of distances to destinations

• Distributed Bellman-Ford Algorithm

Distance Vector• Each node maintains a set of triples

– (Destination, Cost, NextHop)

• Exchange updates directly connected neighbors– periodically (on the order of several seconds)– whenever table changes (called triggered update)

• Each update is a list of pairs:– (Destination, Cost)

• Update local table if receive a “better” route– smaller cost– came from next-hop

• Refresh existing routes; delete if they time out

Distributed Bellman-FordStart Conditions:

Each router starts with a vector of (zero) distancesto all directly attached networks

Send step:Each router advertises its current vector to allneighboring routers.

Receive step:Upon receiving vectors from each of its neighbors,router computes its own distance to each neighbor.Then, for every network X, router finds that neighborwho is closer to X than to any other neighbor.Router updates its cost to X. After doing thisfor all X, router goes to send step.

Example - initial distances

A

B

E

C

D

Info atnode

A

B

C

D

A B C

0 7 ~

7 0 1~ 1 0

~ ~ 2

7

1

1

2

28

Distance to node

D

~

~2

0

E 1 8 ~ 2

1

8~

2

0

E

E receives D’s routes

A

B

E

C

D

Info atnode

A

B

C

D

A B C

0 7 ~

7 0 1~ 1 0

~ ~ 2

7

1

1

2

28

Distance to node

D

~

~2

0

E 1 8 ~ 2

1

8~

2

0

E

E updates cost to C

A

B

E

C

D

Info atnode

A

B

C

D

A B C

0 7 ~

7 0 1~ 1 0

~ ~ 2

7

1

1

2

28

Distance to node

D

~

~2

0

E 1 8 4 2

1

8~

2

0

E

A receives B’s routes

A

B

E

C

D

Info atnode

A

B

C

D

A B C

0 7 ~

7 0 1~ 1 0

~ ~ 2

7

1

1

2

28

Distance to node

D

~

~2

0

E 1 8 4 2

1

8~

2

0

E

A updates cost to C

A

B

E

C

D

Info atnode

A

B

C

D

A B C

0 7 8

7 0 1~ 1 0

~ ~ 2

7

1

1

2

28

Distance to node

D

~

~2

0

E 1 8 4 2

1

8~

2

0

E

A receives E’s routes

A

B

E

C

D

Info atnode

A

B

C

D

A B C

0 7 8

7 0 1~ 1 0

~ ~ 2

7

1

1

2

28

Distance to node

D

~

~2

0

E 1 8 4 2

1

8~

2

0

E

A updates cost to C and D

A

B

E

C

D

Info atnode

A

B

C

D

A B C

0 7 5

7 0 1~ 1 0

~ ~ 2

7

1

1

2

28

Distance to node

D

3

~2

0

E 1 8 4 2

1

8~

2

0

E

Final distances

A

B C

D

Info atnode

A

B

C

D

A B C

0 6 5

6 0 15 1 0

3 3 2

7

1

1

2

28

Distance to node

D

3

32

0

E 1 5 4 2

1

54

2

0

E

E

Final distances after link failure

A

B C

D

Info atnode

A

B

C

D

A B C

0 7 8

7 0 1

8 1 0

10 3 2

7

1

1

2

28

Distance to node

D

10

3

2

0

E 1 8 9 11

1

8

9

11

0

E

E

View from a node

A

B

E

C

D

dest

A

B

C

D

A B D

1 14 5

7 8 56 9 4

4 11 2

7

1

1

2

28

Next hop

E’s routing table

The bouncing effect

A

25

1

1

B

C

B

C 21

dest costA

C 11

dest cost

A

B 12

dest cost

C sends routes to B

A

25 1

B

C

B

C 21

dest costA

C 1~

dest cost

A

B 12

dest cost

B updates distance to A

A

25 1

B

C

B

C 21

dest costA

C 13

dest cost

A

B 12

dest cost

B sends routes to C

A

25 1

B

C

B

C 21

dest costA

C 13

dest cost

A

B 14

dest cost

C sends routes to B

A

25 1

B

C

B

C 21

dest costA

C 15

dest cost

A

B 14

dest cost

How are these loops caused?

• Observation 1:– B’s metric increases

• Observation 2:– C picks B as next hop to A– But, the implicit path from C to A

includes itself!

Avoiding the Bouncing Effect

• Select loop-free paths• One way of doing this:

– each route advertisement carries entire path

– if a router sees itself in path, it rejects the route

• BGP does it this way– Space proportional to diameter

Cheng, Riley et al

Computing Implicit Paths

• To reduce the space requirements– propagate for each destination not only the

cost but also its predecessor– can recursively compute the path– space requirements independent of diameter

x y z

w u

v v u

u w

w z

z y

y z

Distance Vector in Practice

• RIP and RIP2– uses split-horizon/poison reverse

• BGP/IDRP– propagates entire path– path also used for effecting policies

Link State Routing

• Each node assumed to know state of links to its neighbors

• Step 1: Each node broadcasts its state to all other nodes

• Step 2: Each node locally computes shortest paths to all other nodes from global state

Link State Routing: Building blocks

• Reliable broadcast mechanism– flooding– sequence number issues

• Shortest path tree (SPT) algorithm– Dijkstra’s SPT algorithm

Link state packets (LSPs)

• Periodically, each node creates a Link state packet containing:– Node ID– List of neighbors and link cost– Sequence number– Time to live (TTL)

• Node outputs LSP on all its links

Reliable flooding

• When node i receives LSP from node j:– If LSP is the most recent LSP from j

that i has seen so far, i saves it in database and forwards a copy on all links except link LSP was received on.

– Otherwise, discard LSP.

• Problem: sequence number may wrap around

• Solution: treat space as circular, continue after wrap around:– A is less than B if

• A<B and B-A < N/2, or• A>B and A-B > N/2

Sequence number space issues

0 N

B A

Wrap around

Problem: Router Failure

• A failed router and comes up but does not remember the last sequence number it used before it crashed

• New LSPs may be ignored if they have lower sequence number

One solution: LSP Aging

• Nodes periodically decrement age (TTL) of stored LSPs

• LSPs expire when TTL reaches 0– LSP is re-flooded once TTL = 0

• Rebooted router waits until all LSPs have expired

• Trade-off between frequency of LSPs and router wait after reboot

OSPF Sequencing and Aging

• 32-bit sequence number field, does not wrap• LSP’s compared on basis of sequence

number• LSP’s purged after about an hour• Synchronized expiration of LSPs

– expired LSP reflooded with age zero

• On startup, router need not wait– can start with lowest sequence number– will be informed if its own LSP is in network

SPT algorithm (Dijkstra)

• SPT = {a}• for all nodes v

– if v adjacent to a then D(v) = cost (a, v)– else D(v) = infinity

• Loop– find w not in SPT, where D(w) is min– add w in SPT– for all v adjacent to w and not in SPT

• D(v) = min (D(v), D(w) + C(w, v))

• until all nodes are in SPT

Link State Algorithm

Flooding:1) Periodically distribute link-state advertisement (LSA) to neighbors

- LSA contains delays to eachneighbor

2) Install received LSA in LS database3) Re-distribute LSA to all neighbors

Path Computation1) Use Dijkstra’s shortest path algorithmto compute distances to all destinations2) Install <destination, nexthop> pair inforwarding table

Example

A F

B

D E

C2

2

2

3

1

1

1

3

5

step SPT D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f)0 A 2, A 5, A 1, A ~ ~

5

B C D E F

Example

A F

B

D E

C2

2

2

3

1

1

1

3

5

step SPT D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f)0 A 2, A 5, A 1, A ~ ~1 AD 2, A 4, D 2, D ~

5

B C D E F

Example

A F

B

D E

C2

2

2

3

1

1

1

3

5

step SPT D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f)0 A 2, A 5, A 1, A ~ ~1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E

5

B C D E F

Example

A F

B

D E

C2

2

2

3

1

1

1

3

5

step SPT D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f)0 A 2, A 5, A 1, A ~ ~1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E3 ADEB 3, E 4, E

5

B C D E F

Example

A F

B

D E

C2

2

2

3

1

1

1

3

5

step SPT D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f)0 A 2, A 5, A 1, A ~ ~1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E3 ADEB 3, E 4, E4 ADEBC 4, E

5

B C D E F

Example

A F

B

D E

C2

2

2

3

1

1

1

3

5

step SPT D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f)0 A 2, A 5, A 1, A ~ ~1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E3 ADEB 3, E 4, E4 ADEBC 4, E5 ADEBCF

5

B C D E F

Link State Characteristics

• With consistent LSDBs, all nodes compute consistent loop-free paths

• Limited by Dijkstra computation overhead, space requirements

• Can still have transient loops

A

B

C

D

13

5 2

1

Packet from C->Amay loop around BDC

Link State in Practice

• OSPF (Open Shortest Path First Protocol)– most commonly used routing protocol

in the Internet– support for authentication, addl

hierarchy, load balancing

OSPF PacketsLS Age Options Type=1

0 Flags 0 Number of links

Link type Num_TOS Metric

Link state ID

Advertising router

LS sequence number

Link ID

Link data

Optional TOS information

More links

LS checksum Length

61

Today’s Homework• Peterson & Davie, Chap 4

-4.12-4.13-4.16-4.21Download and browse RIP and OSPF RFC’s

Due on Fri (2/23)