© oxford university press 2011 computer networks bhushan trivedi, director, mca programme, at the...

© Oxford University Press 2011

Computer NetworksComputer Networks

Bhushan Trivedi, Director, MCA Programme, at the GLS Institute of

Computer Technology, Ahmadabad


Chapter 7Chapter 7

The Network LayerThe Network Layer


Routing and Forwarding

• Routing is – to know about other routers – where each of them is located– which networks they are connected to

• Forwarding is – finding out about one’s own neighbouring routers – forward a specified packet to its nearest

neighbour to make it reach to a given destination


Routing and forwarding

• Routing table contains information about nearest router for given destination

• Routing algorithms decide placement of routers

• Virtual circuit is mechanism used for connection oriented forwarding

• Datagrams are units of data sent in connectionless forwarding


Network layer duties

• Handling accounting for usage of network resources

• Devise and implement mechanisms of identifying each machine uniquely

• Implement connectionless or connection-oriented forwarding

• Multiplexing and demultiplexing the transport layer and the data link layer jobs


Two different types of routing

• Collection of networks organized by a single party is known as an autonomous system (AS)

• Exterior routing is across AS– BGP (Border Gateway Protocol)

• Interior routing is within AS– Distance Vector– Link state– AODV


Other duties

• Accounting

• Receive Service from the Data Link Layer

• Provide Service to the Transport Layer


Provide Service to DLL


Provide service to transport layer


Global Machine-Level Addressing

• Classful addressing

• The dotted decimal notation

• Classless addressing

• Aggregating multiple routing entries

• Unique address requirement

• Network Address Translation

• The real solution, IPv6


Aggregating multiple route entries


Routing table without aggregation

Network ID Next hop address

Interface … (other fields)

11.1.0.0/16 R1 111.2.0.0/16 R1 111.3.0.0/16 R1 1

292.12.10.0/24 R3 2292.12.12.0/24 R3 2292.12.13.0/24

R3 2


Routing table after aggregation

Network ID

Next hop address

Interface … (other fields)

11.*/16 R1 1292.12.*/24 R3 2


Unique address requirement


Multiplexing and Demultiplexing

• Multiplexing and demultiplexing multiple transport layer connections

• Multiplexing and De-multiplexing the data link layer connections


Forwarding

• Connection-oriented forwarding using virtual Circuits

• Connectionless forwarding using datagrams

• Connection-oriented vs. Connectionless Forwarding

• forwarding examples


Steps involved in CO forwarding

• Find out a complete path to the other end

• Informs the routers along the path and gets their confirmation

• The service may be degraded or another path is tried if some router is not willing


Steps involved in CO forwardingIntermediaries are to

• (a) reserve resources

• (b) set the priorities

• (c) take a call on other services needed

• (d) reserve an amount of bandwidth


Steps involved in CO forwarding

• Connection ID is provided• Independent of other routers • No provision for a central authority

– The central authority needs to be consulted for every connection establishment process

– If the central authority fails, no connection is possile

– Assigning the connection number locally is simple and effective

• similar confirmation from the receiver is sought


Forwarding

• Virtual circuit switching

• Multi protocol Label Switching

• Flow label in IPV6


Forwarding

• Datagram based connectionless forwarding

• Comparison between Connection-oriented and Connectionless Forwarding

•


Forwarding: Datagram based connectionless

forwarding

• Not wasteful

• Autonomy and recovery

• Better fault tolerance


Normal Path when no fault


A router out of order


A communication line out of order


CO vs CL forwarding

• Connection establishment VC requires this phase, Datagram-based solutions do not

• Routing Datagrams are checked and processed at every router while not in VC

• Speed of operation more time to route datagrams

• Congestion control Admission control vs routing around the congested path


Avoiding congested path while routing


CO vs CL forwarding

• Addressing Datagrams are designed to re-route

• Robustness Datagrams are more robust when congestions are expected

• Quality of service Connection-oriented service is better to provide QOS


D is working


D is not working


B’s routing table before and after D stopped functioning

Destination Next node InterfaceA A 1K A 1I D 2

…… ….. …..

Destination Next node InterfaceA A 1K A 1I C 3

…… ….. …..


Requirements of a good Routing Algorithms

• Simple enough to be implemented

• Should not oscillate

• Fault tolerance

• Speed

• Dynamism and flexibility

• Performance


Issues in router’s performance

• A router must scale up to the size of the network attached to it

• Quality of service– Delay– delay tolerance– bandwidth, particularly for real-time

applications– Fault tolerance– High availability


Router architecture


Functions of a router: Lookup

Address prefix Next router Interface …..11* R1 1

1010* R1 110* R2 2

…

Why addresses should be truncated by prefixes

• Router should divide the addresses in as many groups as the number of interfaces available. • Easy management of the entries


Address prefixes help in routing


Issues in router’s performance

• Switching

• Queuing

• Robustness


Robustness of a routing algorithm


Routing

• Internal and External routing algorithms

• Distance Vector Routing

• Problem with count-to-infinity

• The split horizon hack

• Link State Routing

• Routing in MANet


The subnet for routing and J sender


Partial Routing table for J

Network Next router

Interface

E G 1A I 2B I 2X I 2V G 1W G 1Q C 3

….. ….. …..


Facts related to Routing

• In a large network, there may be more than one path to a given destination

• Based on the parameter considered, a path can score better than others

• When we consider a different parameter, the best path may change


Constructing routing tablesEstimated delay from I

Network Next router

Delay

A D 2B D 2K K 1O O 1X P 2U P 2V P 3

….. ….. …..


Constructing routing tablesEstimated delay from G

Network Next router

Delay

E E 1A J 4B E 4X L 4F F 1H M 2M M 1

….. ….. …..


Constructing routing tablesEstimated delay from C

Network Next router

Delay

E E 1A J 4B E 4X L 4F F 1H M 2M M 1

….. ….. …..


Constructing routing tablesAnother Example


Constructing routing tablesAnother Example

Estimated delay from B Estimated delay from DNetwor

kNext

routerDelay Networ

kNext

routerDelay

A A 3 A A 5D D 6 B B 6K A 10 K A 12H D 8 H H 2J D 12 J H 6G D 11 G G 5


Constructing routing tables

E’s Routing tableNetwork Next

routerDelay

A B 8B B 5D D 4K B 15H D 6G D 9J D 10

constructing routing table from two of the neighbor’s inputs


Count to infinity problemE’s routing table when D goes

downNetwork Next

routerDelay

A B 8B B 5D B 9K B 15H B 13G B 16J B 17


Count to Infinity ProblemAnother example


Count to infinity: C’s and B’s routing tables before D is down

Network Next router Delay A B 4B B 2D D 2

Network Next router DelayA A 2C C 2D C 4


A’s routing table and C’s modified routing table when D is down

Network Next router DelayC B 4B B 2D B 6

Network Next router Delay A B 4B B 2D B 6


How C’s routing table is further modified

Network Next router Delay A B 4B B 2D A 8

Network Next router Delay A B 4B B 2D infinity


Solution by lying: the split horizon hack

B’s routing tableNetwork Next router Delay

A A 2C C 2D C infinity


Link state routing steps

• All routers have information about their neighbours

• The router that collects the data from other routers constructs the AS topology.

• Parallel to the fist two steps, data about neighbours collected and Link state packets are generated

• They are sent to every other router


Link state algorithm


LS, step by step

• Collecting Information from other routers and estimating the AS topology

• Finding neighbours and delay

• Sequence number and Age fields

• Routing table

construction

HeaderNode Delay

E 5D 6A 3


Refinements

• All LS packets are acknowledged

• No LS packet or acknowledgement competes with normal packets.

• The sender waits for a newer packet to arrive for some time.

• No packet is forwarded to destination from where the LS packet is received, only acked


Link state packets from all neighbours

From B From A From KNode Dela

yNode Dela

yNode Dela

yE 5 B 3 A 7D 6 D 5 H 8A 3 K 7From D From G From H From J

Node Delay

Node Delay

Node Delay

Node Delay

B 6 D 5 K 8 G 3A 5 J 3 D 2 H 4H 2 J 4G 5


The graph constructed from all the packets


E’s routing table

Network Next router DelayA B 8B B 5D D 4K B 15H D 6G D 9J D 10


The issue when LAN is a part


Solution: depict LAN as a node


Routing in MANet

• No special-purpose routers

• No fixed topology

• Best path assumptions

• Power requirements

• Security issues


Route Discovery in AODV


AODV: Format of a RouteRequest packet

Source ID

Request ID

Destination

ID

Source sequence no.

Destination

sequence no.

Hop count

Time-to‑Liv

e


Route Maintenance: D’s Current routing table

Destination Next hop Distance Active neighbours

…

B B 1 G, H …E E 1 H …A A 1 G …K A 2 G, E …H H 1 A …G G 1 B …J H 2 E …


Exterior Gateway Protocol

• Border Gateway Protocol

• BGP and DV

• Path Vector Routing

• No count to infinity

• Complexity

• Impact of business relationship


The remaining AS are of second and third type.

BGP network. C and F are the first type of network, i.e. they are customers


Autonomous systems and AS

Path


Factors considered in BGP

• Local preference

• Avoiding false alarms

• Check willingness to be intermediary

• Hot potato

• Interior Routing


Hot Potato


Interior Routing


other issues

• Hierarchical routing

• Broadcast routing

• Individual delivery

• Multicast routing

• Congestion

• Congestion Control and Algorithms

• Network Layer switching


Broadcast routing

• Individual delivery

• Flooding

• Reverse path forwarding

• Spanning tree


Individual delivery


Flooding-1


Flooding-2


Flooding-3


Spanning tree


Multicast routing

• There may be multiple groups present.

• intermediaries and members

• Member belongs to multiple groups

• A sender sending to multiple groups.

• Router having information – about its downstream members– members of all the groups that pass through

it.


Congestion and control

• Bursty traffic and Traffic Jam

• Congestion control– Explicit control

• Admission control• Prevention of congested routes

– Implicit control• RED• Jitter Contro


Ignoring congested areas


The rest

• Multiprotocol Label Switching

• Internetworking issues

• Heterogeneity in network

• Fragmentation

• Tunneling

• Security issues at the Network layer and IPSec

• Overview of IPSec


Network Layer Switching

• Routing process is time consuming

• Tagging to improve

• Switching is forwarding based on tags


Advantage of labeling

• IP header does not need to change.• Simpler and faster to look up• Tags relates to required services. • independent of the network layer protocol• multiple labels pasted on a packet• Forwarding process becomes faster• Possible to monitor SLA• Layer 4 and 5 are also used to label


MPLS


MPLS and VC differences

• The labels are independent of the senders’ and receivers’ addresses

• MPLS can provide the same label to different connections

• Faster processing

• No connection establishment and termination

• No router entries in MPLS


MPLS frame format


MPLS frame format

• The MPLS header is attached above IP

• The router-router links use MPLS

• When used with Ethernet, Type is set to 8847

• The label field has local significance

• The QOS field indicates the quality of service

• TTL stands for time to live

• The S field is either 1 or 0


Heterogeneity in Networks

• Maximum transferrable unit• QOS field• Address differences• CO vs CL networks • Error, congestion and security• Fragmentation• Tunneling

– VPN– Mobile IP


The Dual Stack solution


Fragmentation


Fragmentation in Internet


Tunneling


Mobile IP

© oxford university press 2011 computer networks bhushan trivedi, director, mca programme, at the...

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