computer networks - network layer1 the network layer n design issues n routing algorithms n...

Computer Networks - Network Layer 1

The Network Layer

Design Issues Routing Algorithms Congestion Control Internetworking Example Network Layer Protocols


Goals of the Network Layer

The network layer is concerned with getting packets from the source all the way to the destination

the network layer must know the topology of the communication subnet choose route to avoid overloading some of the commun

ication lines and routers while leaving others idle deal with problems when the source and destination are

in different networks


Services Provided to the Transport Layer Connectionless (unreliable) services

each packet must carry the full destination address no packet ordering and flow control should be done

Connection-oriented (reliable) services a network layer process on the sending site must set up a

connection to its peer on the receiving side when a connection is set up, two processes can enter a

negotiation about service parameters packets are delivered in sequence flow control is provided automatically


International Organization of the Network Layer virtual circuit

a route from the source to the destination is chosen as part of the connection setup

primarily for connection-oriented service datagrams

each packet sent is routed independently of its predecessors

for connection-oriented and connectionless services


Datagram Vs. Virtual Circuit

Issue Datagram Virtual Circuit (VC)Circuit Setup Not needed RequiredAddressing Each packet contains the full

source and destination addressEach packet contains a shortVC number

Stateinformation

Subnet does not hold stateinformation

Each VC requires subnet tablespace

Routing Each packet is routedindependently

Route chosen when a VC is setup; all packets follow this route

Effect of routerfailures

None, except for packets lostduring the crash

All VCs that passed through thefailed router are terminated

Congestioncontrol

Difficult Easy if enough buffers can beallocated in advance for eachVC


Combinations of Services and Subnet Structures

Connection-oriented Connectionless

DatagramVirtual Circuit

Services provided to Transport layer

Subnet Structures

Connection-oriented ConnectionlessServices provided by Transport layer

TCPoverIP

UDPoverIP

UDPover

ATM IP

ATM AAL1over ATM


Routing

Packets are often routed from the source to the destination hop by hop.

Two networks are connected by at least a router. The network is defined from the point of view of the network layer.


Types of Routing

Static Routing Routes to destinations are predetermined and are not

dependent on the current state (traffic, topology etc.) of the network.

Dynamic Routing (Adaptive Routing) Routes being learned via exchange of routing information

to reflect changes in the topology and traffic.

Default Routing: Traffic to destinations that are unknown to the router is

sent to a default “outlet”.


The Optimality Principle

If router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route.

the set of optimal routes from all sources to a destination form a tree, called a sink tree, rooted at the destination.

The goal of all routing algorithms is to discover and use the sink trees for all routers.

I J Kr1

r2


Routing Algorithms

Static Routing Algorithms Shortest Path Routing Flooding Flow-Based Routing

Dynamic Routing Algorithms Distance Vector Routing Link State Routing

Hierarchical Routing Routing for Mobile Hosts Broadcast Routing Multicast Routing


Shortest Path Routing

Find the shortest path between a given pair of routers.

Cost of a link may be a function of the distance, bandwidth, average traffic, communication cost, mean queue length, delay, etc.

The Dijkstra’s algorithm is used.


Flooding

Every incoming packet is sent out on every outgoing line except the one it arrived on.

Vast numbers of duplicate packets are generated.

For robustness Application:

Concurrent updates of databases Always choose the shortest path

I J

K L

M


Flow-Based Routing

For a given line, if the capacity and average flow are known in advance, it is possible to compute the mean packet delay on that line from queueing theory.

The routing problem then reduces to finding a routing algorithm that produces the minimum average delay for the subnet.


Distance Vector Routing

RIP, the distributed Bellman-Ford routing algorithm, the Ford-Fulkerson algorithm

Each router maintains a routing table giving the best known distance to each destination and which line to use to get there.

These tables are updated by exchanging information with the neighbors.


Update Routing Table

(a)

A B C D

E

I J K L

F GH

Ro uter

012254014231817219

2429

243618277

2031200

112233

2031198

301960

147

229

2128362422403119221009

8202820173018121006

15

AAI

HII

HHIKK

T o A I H K L ine

N ew estim ateddelay fro m J

ABCDEFGHIJKL

JA JI JH JKdelay delaydelaydelay

is is is is8 10 12 6

N ewro uting

tab lefo r J

V ecto rs received fro mJ's fo ur neighbo rs

(b)


Distance Vector Routing

Metric used to measure the “distance” number of hops time delay queue length

Drawback it reacts rapidly to good news, but leisurely to

bad news.


Count-to-Infinity Problem

1

1 2

1 2 3

1 2 3 4

A B C D E

Good News

down initially andcome up later

1 2 3 4

3 2 3 4

3 4 3 4

5 4 5 4

5 6 5 6

7 6 7 6

7 8 7 8

A B C D E

Bad News

Initially

After 1 exchange

After 2

exchanges

After 3

exchanges

After 4

exchanges

After 5

exchanges

After 6

exchanges

alive initially anddown later


Link State Routing

IS-IS, CLNP, NLSP, OSPF Each route must

1.Discover its neighbors and learn their network addresses.

2.Measure the delay or cost to each of its neighbors.

3.Construct a packet telling all it has just learned.

4.Send this packet to all other routers.

5.Compute the shortest path to every other router.


Learning About the Neighbors

Send a special HELLO packet on each point-to-point line

the router on the other end is expected to send back a reply telling who it is

I J

K

L

HELLO

I am J


Measuring Line Cost

Send an ECHO packet over a line the other side is required to send back ECH

O immediately the round-trip delay can be mesaured

if the load is taken into account results in better performance the routing table may be oscillate wildly


Building Link State Packets

B C

E F

A D61

2

8

5 7

4 3

(a)

A

Seq.

A ge

B C D E F

B 4

E 5

Seq.

A ge

A 4

C 2

Seq.

A ge

B 2

D 3

Seq.

A ge

C 3

F 7

Seq.

A ge

A 5

C 1

Seq.

A ge

B 6

D 7

F 6 E 1 F 8 E 8

L ink State P ackets

(b)


Distributing Link State Packets

Use flooding sequence number is used to determine among

new, duplicate, or obsolete packets three problems

sequence number may wrap around a crash router will lose track of its sequence number

when it restarts a corrupted bit in sequence number may cause

disaster


The Use of ‘Age’ Field

Include in each packet decremented once per second when the age hits zero, the information fro

m that router is discarded also decremented by each router during the

initial flooding process no packet can get lost and live for an indefinite

period of time


Reducing Flooding Costs

When a link state packet comes in it is put in a holding area to wait a short while when another packet from the source comes in

duplicate: discarded different: the older one is thrown out all link state packets are acknowledged

I J

K L

M


Computing the New Route

Once a router has accumulated a full set of link state packets it can construct the entire subnet graph by locall

y running Dijkstra’s algorithm for a subnet with n routers, each of which h

as k neighbors, the memory required to store the input data is proportional to kn.


Hierarchical Routing

When hierarchical routing is used, the routers are divided into regions each router knows all the details about how to

route packets to destinations within its own region

each router knows nothing about the internal structure of other regions.


Hierarchical RoutingRegion 1 Region 2

Region 5

A

BC

AB

CD

Region 3Region 4

A

BA

B

C

A

B CD

Full table for 1A

Dest. Line Hops1A ---- ---1B 1B 11C 1C 12A 1B 22B 1B 32C 1B 32D 1B 43A 1C 33B 1C 24A 1C 34B 1C 44C 1C 45A 1C 45B 1C 55C 1B 55D 1C 5

Dest. Line Hops1A ---- ---1B 1B 11C 1C 12 1B 23 1C 24 1C 35 1C 4

Hierarchicaltable for 1A


Routing for Mobile Hosts

Each mobile node is assumed to have a permanent home location that never changes.

Each area has one or more foreign agents, which keep track of all mobile nodes visiting the area.

In addition, each area has a home agent, which keep track of nodes whose home is in the area, but who are currently visiting another area.

When a new node enters an area, it must register itself with a foreign agent there. Then the mobile node will register either directly with its home agent, or through the foreign agent which forwards the registration to the home agent.



Home

WAN, M

AN, or LAN

Foreign

Foreign

moving

moving



Datagrams sent to the mobile node are intercepted by its home agent, then tunneled by the home agent towards the mobile

node, received at the tunnel foreign agent, and finally delivered to the mobile node.



1. Packet is sent to the mobile host’s home

2. Packet is tunneled to the foreign agent

3. Sender is given foreign agent’s address

4. Subsequent packets are tunneled to the foreign agent


Broadcast Routing

To simply send a distinct packet to each destination

Flooding Multidestination Routing Spanning Tree Routing Reverse Path Forwarding


Multidestination Routing

Each packet contains a list of desired destinations. When a packet arrives, the router checks all the

destinations to determine the set of output lines for forwarding the packet. An output line is selected if it is the best route to at least one of the destinations.

The router generates a new copy of the packet for selected output line, with a set of destinations that are to use the line.


Spanning Tree Routing

Assume each router has knowledge of a spanning tree (e.q. a sink tree) in the network.

Each router copies an incoming broadcast packet onto all the spanning tree lines except the one it arrives on.

Use minimum number of packets.


Reverse Path Forwarding

Without knowing any spanning tree

if a packet arrives at the line that is normally used for sending packets to the source of the broadcastthe router forwards copies of it onto all lines

except the one it arrived on.

otherwisethe packet is discarded


Reverse Path Forwarding: An Example

AB C

F

D

G

J

O

IE

H

K L

M

N

I

F NJH

A MGED OOK

E KDGC N

H LB

BLPreferred link


Multicasting

Send a message to all the other members of the group

group management create and destroy groups for processes to join and leave groups

routers knows which of their hosts belong to which group

routers tell their neighbors, so the information propagates through the subnet


Multicast Routing

Each router computes a spanning tree covering all other routers in the subnet.

When a multicast packet for a group arrives, the first router examines its spanning tree and prunes it, removing all lines that do not lead to hosts in the group.

Multicast packets are forwarded only along the pruned tree.

mn trees is needed with n groups, each with an average of m members.


Core-based Tree for Multicast Routing A single spanning tree,called core-based

tree, for a group is computed, with the root (core) near the middle of the group.

A host first sends a multicasting message to the core, which then does the multicasting along the spanning tree.

The tree is not optimal. However only n trees need to be stored.


Congestion

When too many packets are present in (a part of) a subnet, performance degrades. This situation is called congestion.

Packet

delivered

Packet sent

Maximun carryingcapacity of subnet

Perfect

Desirable

Congested


Congestion Control

goal make sure the subnet is able to carry the offered

traffic Congestion causes

bursty data insufficient memory slow processor low-bandwidth line


Flow Control vs. Congestion Control

1 Gbps

1000 GbpsPC

SuperComputer

100 Kbps

1 Mbps 1000

1000

FlowControl

CongestionControl


General Principles

Open Loop make sure congestion does not occur in the first

place Closed Loop

monitor the system to detect congestion (where and when)

pass this information to places where action can be taken

adjust system operation to correct the problem


Congestion Control Algorithm Taxonomy explicit feedback

Packets are sent back from the point of congestion to warn the source.

implicit feedback The source deduces the existence of congestion

by making local observations, such as the acknowledgement time.


Congestion Prevention Policies

Virtual circuit versus datagram Most algorithms work only with virtual circuit.

Packet queueing and service policy input queueing versus output queueing round robin, priority-based, random, ...

Packet discarded policy which packet is dropped when there is no space

Routing algorithm Load sharing

Packet lifetime management


Traffic Shaping/Policing

Congestion would be less common if traffic is less bursty

traffic shaping force packets to be transmitted at a more predic

table rate traffic policing

monitor a traffic flow


Leaky Bucket Algorithm

Water dripsout of thehole at aconstant rate.

Network

packets

regulatedflow

unregulatedflow

interfaceimplementing

the leakybucket

algorithm

The bucketholdspackets.


Token Bucket Algorithm

Before

tokens

packets

Thebucketholdstokens.

Thebucket

holdspackets.

tokensaddedperiodically

After

NetworkNetwork


Properties of Token Bucket

allows saving up permission to send large bursts later

throws away token when the bucket fills up but never discards packets

C: token bucket capacity; :token arrival rate; S: burst length; M: maximal output rate

C+ S=MS


Flow Specification

An agreement on the traffic pattern the source gives the flow specification to th

e subnet for approval

Maximal packet size (bytes)Token bucket rate (bytes/sec)Token bucket size (bytes)Maximal transmission rate(bytes/sec)

Characteristics of the input Service Desired

Loss sensitivity (bytes)Loss interval (sec)Burst loss sensitivity (packets)Min. delay noticed (sec)Max. delay variation (sec)Quality of guarantee


Congestion Control in Virtual Circuit Subnets Admission control

once congestion has been signaled, no more virtual circuits are set up until the problem has gone away

carefully route all new virtual circuits around problem areas

negotiate an agreement between the host and subnet when a virtual circuit is set up


Choke Packets

5

7

21 3

46

Heavyflow

Choke

Choke

ChokeReducedflow

Flow is reduced.


Hop-by-Hop Choke Packets

5

21 3

4

Heavyflow

Choke

Choke

Choke

Reduc

ed

flow

Flow is reduced.

To provide quickrelief at the pointof congestion atthe price of usingup more buffersupstream.


Load Shedding

when routers are being inundated by packets that they can not handle, they just throw them away.

Packet discarding policy Wine: Old is better than new. Milk: New is better than old. Priority Control


Jitter Control The jitter is the amount of variation in the end-to-end

packet transit time. The jitter can be bounded by computing the expected

transit time for each hop along the path. When a packet arrives at a router, the router checks to see how much the packet is behind or ahead of its schedule. This information is stored in the packet and updated at each hop. If the packet is ahead of schedule, it may be held just enough to get it back on schedule. If it is behind schedule, the router tries to get it out the door quickly.


Congestion Control for Multicasting Multicast flows from multiple sources to m

ultiple destinations (cable television) if it is the sender that reserves bandwidth

each sender should track membership changes regenerate the spanning tree at each change

RSVP (Resource reSerVation Protocol) it is the receiver that reserves bandwidth


RSVP (Resource reSerVation Protocol) allows multiple senders to transmit multiple

groups of receivers. permits individual receivers to switch

channels freely. optimizes bandwidth use while at the same

time eliminating congestion. uses multicast routing using spanning trees


Multicast Spanning Trees

Senders

Receivers

1 2

3 4 5

1 2

3 4 5

1 2

3 4 5

Multicast spanningtree for host 2

Multicast spanningtree for host 1


Bandwidth Reservation

Senders

Receivers

1 2

3 4 5

Senders

Receivers

1 2

3 4 5

Senders1 2

3 4 5

Bandwidthreservedfor source 1




RSVP (Resource reSerVation Protocol) When making a reservation, a receiver can

(optionally) specify one or more sources that it wants to receive from.

It can also specify whether these choices are fixed for the duration of the reservation, or whether the receiver wants to keep open the option of changing sources later.


RSVP (Resource reSerVation Protocol) Two receivers are only set up to share a

path if they both agree not to change sources later on.

Once a receiver has reserved bandwidth, it can switch to another source and keep that portion of the existing path that is valid for the new source.


X.25

Internetworking

B

802.4 LAN802.3 LAN

802.5 LAN

R

DECnet

R

SNA

R

R


Internetworking

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

7

6

5

4

3

2

1

Layer

APDU

PPDU

SPDU

TPDU

Packet

Frame

Bit

Application Protocol

Presentation Protocol

Session Protocol

Transport Protocol

Host A Host B

Network

Data Link

Physical

Network

Data Link

Physical

Router Router

Internal Subnet Protocol


Internetworking Devices

Layer 1: Repeater or Amplifier To amplify or regenerate signals

Layer 2: (Multi-protocol) Bridge Store-and-forward device Headers at layer 2 are translated if necessary.

Layer 3: (Multi-protocol) Router Headers at layer 3 (and layer 2) are translated if

necessary. Layer 4~7: Transport/Application Gateway


Devices on the Market

Repeater, Hub Bridge (Multi-protocol) Router, Layer 3 Switch Layer 4 Switch Gateway with interfaces of distinct layer

protocols Bridge/Router (or called B-Router)


How Networks Differ

Service offered Connection-oriented versus Connectionless

Protocol IP, IPX, CLNP, AppleTalk, DECnet, etc.

Addressing Flat (802) versus hierarchical (IP, PDN, PSTN, ISDN, e

tc.) Multicasting/Broadcasting

Present or absent


How Networks Differ (Cont.)

Packet size Every network has its own maximum

Quality of service Present or absent

Error handling Reliable, ordered, and unordered delivery

Flow control Sliding window, rate control, others, or none


How Networks Differ (Cont.)

Congestion control Leaky bucket, choke packets, etc.

Security Privacy rules, encryption, etc.

Parameters Different timeouts, flow specifications, etc.

Accounting By connection time, by packet, by byte, or not at all


Concatenated Virtual Circuit

1

M

M

M

M

2

SNA

OSI

X.25ATM

Router

Host

MultiprotocolRouter


Connectionless Interworking

1

M

M

M

M

2


Tunneling

EthernetEthernet

RR

WAN

IP

Ethernet header

Ethernet frame

IP

WAN packet header

WAN packet

IP

Ethernet header

Ethernet frame

Using encapsulation of IP packets

The same type of network


Internetwork Routing interior gateway protocol: within each network exterior gateway protocol: between networks

Network

1

2 3

4 5

A

B

C D

E

F

Gateway/Router

A

E

C

B

D

F


Autonomous System (AS)

An autonomous system is a set of routers having a single routing policy, running under a single technical administration.

Each AS is independent of all the others.


Fragmentation and Reassembly

Each network imposes some maximum size on its packets

When a large packet wants to travel through a network whose maximum packet size is too small allow gateway to break packets up into

fragments fragments are reassembled into packet latter


Transparent and Nontransparent Fragmentation

G 1 G 2 G 3 G 4

G 1 G 2 G 3 G 4

P acket

N etw o rk 1

G 1 fragm entsa large packet

G 2reassem bles

the fragm entsG 3 fragm ents

again

G 4reassem bles

again

N etw o rk 2

(a)

P acket

G 1 fragm entsa large packet

T he fragm ents are no t reassem bleduntil the final destinatio n (a ho st) is reached

(b)


Elementary FragmentationN um ber o f the first elem entary fragm ent in this packet

P acketnum ber

E nd o fpacket b it

27 0 1 A B C D E F G H I J

27 0 0 A B C D E F G H 27 8 1 I J

27 0 0 A B C D E 27 5 0 F G H 27 8 1 I J

H eader

1 byte

H eader H eader

H eader H eader H eader

(a)

(b)

(c)


Firewalls

Packet filter router is a router equipped with some extra functionality that allows every incoming or outgoing packet to be inspected.

Application gateway (e.g.a mail gateway) may examine headers and/or contents of messages.

ApplicationGateway

PacketFilteringRouter

PacketFilteringRouter

Inside Outsid

e


Internet Network Layer Protocol

The IP (Internal Protocol) Protocol IP Addressing Subnets Internet Control Protocols

The Internet Control Message Protocol (ICMP) The Address Resolution Protocol (ARP) The Reverse Address Resolution Protocol (RAR

P)


Internet Network Layer Protocol

The Interior Gateway Routing Protocol: Open Shortest Path First (OSPF)

The Exterior Gateway Routing Protocol: Border Gateway Protocol (BGP)

Internet Multicasting Mobile IP Classless InterDomain Routing (CIDR) IPv6


The IP Header

0 4 8 16 19 24 31 bit

Version IHL Type of service Total length

Identification Fragment offsetDF

MF

Time to live Protocol Header checksum

Source address

Destination address

Options (0 or more words)


The IP Protocol

Version: The current protocol version is 4. IP Header length (IHL): measured in 32-bit words

for example, without options, its value is 5.

Type of service Precedence (3 bits): 0 (normal precedence) ~ 7 (network

control) Delay (1 bit): low delay Throughput (1 bit): high throughput Reliability (1 bit): high reliability unused (2 bits)


The IP Protocol

Total length: measured in octets, including the length of the header and data

Identification: datagram identifier Flags

unused (1 bit) DF (1 bit): don’t fragment MF (1 bit): more fragment

Fragment offset: the offset of this fragment in the original datagram, measured in units of 8 octets


The IP Protocol Time to live (TTL): packet lifetime, measured in

seconds (hops, in practice) Protocol: protocol type (e.g., TCP, UDP, ...), RFC 170 Header checksum Source IP address Destination IP address Options Padding: to make the header extend to an exact

multiple of 32 bits, containing 0


IP Options Security

to specify how secret the datagram is

Strict source routing to give the complete path to be followed

Loose source routing to give a list of routers not to be missed

Record route to make each router append its IP address

Timestamp to make each router append its address and timestamp


IP Option Code

Copy (1 bit): 0: the option will only be copied into the first fragment and n

ot to all fragments 1: the option should be copied into all fragments

Class (2 bits) 0: datagram or network control 1: reserved 2: debugging and measurement 3: reserved

Number (5 bits)


IP Option Number

Class Number Length Description

0

0

0

0

0

0

0

2

1

0

2

3

7

8

9

4

1

1

11

var

var

4

var

var

end of option list

no operation

security and handling restriction

loose source routing

record route

stream identifier

strict source routing

internet timestamp


IP Addressing

32 bits long, represented in dotted decimal notation, like 192.41.6.20

Network number + Host number Network numbers are assigned by the NIC

(Network Information Center) to avoid conflicts.

All the hosts in a network must have the same network number.


IP Addresses

1 2 3 Range of hostClass 01234567890123456789012345678901 addresses

A 0Network Host1.0.0.0 to127.255.255.255

E 11110 Reserved for future use240.0.0.0 to247.255.255.255

D 1110 Multicast address224.0.0.0 to239.255.255.255

C 110 Network Host192.0.0.0 to223.255.255.255

B 10 Network Host128.0.0.0 to191.255.255.255


Special IP Addresses

00000000000000000000000000000000

11111111111111111111111111111111

127 (anything)

00 00 Host

Network 11 11

This host

A host on this network

Broadcast on thelocal network

Broadcast on adistant network

Loopback


Network Growth Problem

Problem Immense administrative overhead

Every time a new network is installed the system administrator has to contact NIC to get a new network number.

Then this number must be announced worldwide.

Large routing table Solution: To minimize network numbers by

sharing one network number among multiple physical networks


Subnet Addressing

G128.10.2.1

128.10.1.2

128.10.2.2

128.10.1.1

128.10.0.0

Network 128.10.2.0

Network 128.10.1.0

•ack like a single network to the outside world•split into several parts for internal use


Subnets

Network Subnet HostIP address

Subnet mask 11 11 11 11 00 00

The standard does not restrict subnet masks to select contiguous bits of the address.

Host PartA Class B Network


How Router Processes IP Packets

(network0, 0)

(network1, 0)

IP A

IP B

... ...

(this-net, host0) IP M

(this-net, host1) IP N... ...

(default router)

A

B

network0

network1

M N

(this-net, subnet, 0) IP X

(this-net, this-sub, host1) IP Y When subnetting is introduced


The Internet Control Message Protocol (ICMP)

Message type DescriptionDestination unreachable Packet could not be deliveredTime exceeded Time to live field hit 0Parameter problem Invalid header fieldSource quench Choke packetRedirect Teach a router about geographyEcho request Ask a machine if it is aliveEcho reply Yes, I am aliveTimestamp request Same as Echo request,

but with timestampTimestamp reply Same as Echo reply,

but with timestamp


The Internet Control Message Protocol (ICMP) RFC 792 Each ICMP message has its own format. Example: Echo request or reply

Type=8: request, Type=0: reply Code=0

IP headerType Code Checksum Identifier Sequence number

Optional data


The Address Resolution Protocol (ARP) RFC 826 To map an IP address onto data link layer address, such

as Ethernet. An IP host runs the ARP protocol to inquiry the unknow

n data link layer address of a destination IP address before a datagram is sent.

The ARP of a host may maintain a cache to record known IP address and DLL address pairs.

A machine may broadcast its own mapping when it boots by using ARP.


The Reverse Address Resolution Protocol (RARP) RFC 903 Allow a newly-booted (diskless) workstation

(with a DLL address) to discover its IP address Need a RARP server on each network Bootp, RFCs 951, 1048, and 1084

Use UDP messages which are forwarded over routers to find the file server that holds the mapping


The Interior Gateway Routing Protocol A Routing Protocol within an autonomous syst

em (AS). Internet interior gateway protocol

Original: a distance vector protocol, Routing Information Protocol (RIP), based on the Bellman-Ford algorithm

Successor (1979): a link state protocol Now (1990): Open Shortest Path First (OSPF), RF

C 1247 (ver. 1) and RFC 1583 (ver. 2).


Objectives of OSPF

published in the open literature to support a variety of distance metrics: distance,

delay, ... a dynamic algorithm to support routing based on type of service load balancing support for hierarchical systems security tunneling


Graph Representation of an AS

OSPF computes the shortest path from every router to every other router.

router

WAN

LAN

A B C

A B C Each arc is assigned a cost


Areas and Backbone Areas

An AS can be divided into numbered areas an area is a network or a set of continuous networks (a

generalization of a subnet) areas do not overlap Within an area, each router has the same link state data

bases and runs the same shortest path algorithm

backbone area (area 0) all areas within an AS are connected to the backbone, p

ossibly by tunnels


Classes of Routers

internal router

backbonerouter

backbonearea

area

area borderrouter

ASboundary

router

AS

BGP protocolconnects the ASes

router


Open Shortest Path First (OSPF)

Type of service routing: multiple graphs one labeled with the costs when delay is the metric one labeled with the costs when throughput is the

metric one labeled with the costs when reliability is the m

etric Three kinds of routes: intra-area, inter-area an

d inter-AS.


OSPF Messages

Message type Description

Hello Used to discover who the neighbors are

Link state update Provides the sender’s costs to its neighbors

Link state ack Acknowledges link state update

Database description Announces which updates the sender has

Link state request Requests information from the partner


OSPF Messages

When a router boots, it sends HELLO messages on all of its point-to-point lines and multicasts then on LANs to the group consisting of all the other routers.

During normal operation, each router periodically floods LINK STATE UPDATE messages to each of its adjacent routers. Routers also send these messages when a line goes up or down or its cost changes.


OSPF Messages

DATABASE DESCRIPTION messages give the sequence numbers of all the link state entries currently held by the sender. By comparing its own values with those of the sender, the receiver can determine who has the most recent values. These messages are used when a line a brought up.


The Exterior Gateway Routing Protocol A Routing Protocol between ASes

The main goal of an interior gateway protocol is to route efficiently, while the exterior gateway protocols have to worry about “politics”.

Border Gateway Protocol (BGP) described in RFC 1654 BGP1 in 1989, current version BGP4 in 1993. fundamentally a distance vector protocol.


Policies

Political, security, or economic considerations No transit traffic through certain ASes. Never put Iraq on a router starting at the Pentago

n. Only transit Albania if there is no alternative to th

e destination. Traffic starting or ending at IBM should not trans

it Microsoft.


Border Gateway Protocol (BGP)

AB C

D

E

FG

H

I J

Information F receivesfrom its neighbors about D

From B: “I use BCD”From G: “I use GCD”From I: “I use IFGCD”From E: “I use EFGCD”

Each BGP keeps track of the exact path used to each destination


Border Gateway Protocol (BGP)

After all the paths come in from the neighbors, the best can be determined.

Since each router keeps the exact routes used, the count-to-infinite problem can be easily solved.

BGPs uses TCP as its transport protocol (port 179) for reliable transmission.


How BGP Solves The Count-to-Infinity Problem

Information F receivesfrom its neighbors about D

From B: “I use BCD”Nothing from GFrom I: “I use IFGCD”From E: “I use EFGCD”

F chooses “FBCD” as its new route

AB C

D

E

FG

H

I J


Internet Multicasting

IP supports multicasting using class D addresses. Permanent addresses and temporary addresses.

Permanent addresses 224.0.0.1 all systems on a LAN 224.0.0.2 all routers on a LAN 224.0.0.5 all OSPF routers on a LAN 224.0.0.6 all designated routers on a LAN

A multicast router uses the IGMP (Internet Group Management Protocol) to group hosts the groups their processes currently belong to.

IGMP in RFC 1112. Multicast routing uses spanning trees.


IGMP (Internet Group Management Protocol)

Multicastrouter

Query

Multicastrouter

Response

1, 2 31, 3

1, 3 3

1, 2

Hardware Multicast


Mobile IP

Problems with host mobility in Internet IP address implies host location

Major goals of Mobile IP Mobile host can use its home IP address anywhere. Software changes to the fixed hosts were not permitted changes to the router software and tables were not

permitted most packets for mobile hosts should not make detours on

the way no overhead should be incurred when at home


How to Handle Mobility

Home agent Foreign agent Mobile host

register

care-of-addressHe is herePacket formobile host

(tunneling)

care-of-address Packet formobile host

Sender


Gratuitous ARP

Home agent Mobile Host

ARP Response

Router athome’sLAN

Packet formobile host

ARP Query

Packet formobile host

Packet formobile host gratuitous ARP

(about tomove)

(tunneling)


IP Addressing Problems

32-bit IP address space is not enough Organizing the address space by classes wast

es millions of them a class B address is far too large for most organiz

ations the routing table explosion

every router in the Internet would need a table with half a million entries, if that much class C networks are in use


Classless InterDomain Routing (CIDR) To solve the IP address depletion problem

and the routing table explosion problem RFC 1519 The basic idea behind CIDR is to allocate

the remaining class C networks in variable- sized blocks

192.0.0.0 223.255.255.255


Example

X University needs 2048 addresses and is assigned the addresses 194.24.0.0 through 194.24.7.255, along with mask 255.255.248.0.

Y University needs 4096 addresses and is assigned the addresses 194.24.16.0 through 194.24.31.255, along with mask 255.255.240.0.

Z University needs 1024 addresses and is assigned the addresses 194.24.8.0 through 194.24.11.255, along with mask 255.255.252.0.


Example

Routing table with entries base address: 194.24.0.0, mask: 255.255.248.0 base address: 194.24.16.0, mask:

255.255.240.0 base address: 194.24.8.0, mask: 255.255.252.0

Destination address with 194.24.17.4 matching the second entry, so the packet is sent to the Y router.


Classless InterDomain Routing (CIDR) The world was partitioned into zones, each given a portion

of the class C address space: Addresses 194.0.0.0 to 195.255.255.255 for Europe Addresses 196.0.0.0 to 197.255.255.255 for Others Addresses 198.0.0.0 to 199.255.255.255 for North America Addresses 200.0.0.0 to 201.255.255.255 for Central and South

America Addresses 202.0.0.0 to 203.255.255.255 for Asia and Pacific Addresses 204.0.0.0 to 207.255.255.255 for Others Addresses 208.0.0.0 to 223.255.255.255 reserved for future use


IP Address Allocation

Class A address allocation is restricted. Class B address are also restricted .They will be allocated onl

y if the need for them is justified. Class C addresses are allocated with a contiguous block of ad

dresses which consists of several contiguous class C addresses.Class C addresses are being distributed to ISPs so that the allocation could last at least two years.

If a subscriber has a requirement for more than 4096 IP address, a Class B network number may be allocated.

Organizations are encouraged to use Variable Length Subnet Mask for efficient use of address space.


IPv6

A modified combined version of Deering and Francis proposals. SIPP (Simple Internet Protocol Plus)

Longer addresses Simplification of headers Support for options Security and authentication Type of services


The IPv6 Header

Version Priority Flow labelPayload length Next header Hop limit

Source address (16 bytes)

Destination address (16 bytes)

0 4 8 16 24 31


The IPv6 Protocol

Version: 6 Priority

0~7: for transmission possibly slowed down in case of congestion

8~15: for real time traffic with constant sending rate suggestion

1: news 4: FTP 6: Telnet


The IPv6 Protocol

Flow label: for setting up a pseudo connection with particular properties and requirements

Payload length: information bytes following the 40 byte header

Next header: specify which of the (currently) six extension headers, if any, follows the header. If the header is the last IP header, the Next header specifies the transport protocol handler.

Hop limit: for limiting packet lifetime Source address and Destination address


IPv6 Address Expression

Written as eight groups of four hexadecimal digits with colons between groups8000:0000:0000:0000:0123:4567:89AB:CDEF

optimizations8000::123:4567:89AB:CDEF

IPv4 addresses::192.31.20.46

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