Network Layer: IP Addressing and Forwarding - CS 242, Wellesley College, Fall 2007, Study Guides, Projects, Research of Computer Systems Networking and Telecommunications

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Network Layer:Addressing andforwarding

CS 242: Computer NetworksWeek 09: Monday, 11/05/07Daniel BilarWellesley CollegeFall 2007

Goals today

Progress report, project proposals

How routers forward packets



Longest Prefix matching with Patricia tree

IP is the “lingua franca” of the Internet



How hosts obtain information needed toparticipate: the DHCP protocol

Some slides adapted and pictures gratefully

acknowledged from Kurose, Schul,Rexford

Review: IP layer responsibilities

forwarding

table

Routing protocols •path selection•RIP, OSPF, BGP

IP protocol •addressing conventions•datagram format•packet handling conventions

ICMP protocol •error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Network

layer

The Network ‘IP’ layer’s job is to permit hosts to inject packetsinto

any

network and have them travel independently to the

destination (potentially on different types of networks)

Review: Grouping Related Hosts

The Internet is an “inter-network”



Used to connect

networks

together, not

hosts



Needs a way to address a network (i.e., groupof hosts)

host

host

host

LAN 1

...

host

host

host

LAN 2

...

router

router

router

WAN

WAN

LAN = Local Area NetworkWAN = Wide Area Network

Review: Classless Inter-Domain Routing IP Address : 12.4.22.

IP Mask: 255.254.0.

Address

Mask

for hosts

Network Prefix

Use two 32-bit numbers to represent a network.

Network number = IP address + Mask

Written as 12.4.0.0/

AND

Key Functions of IP Layer

Routing:

Determine

route taken by packetsfrom source to dest.



Process of planning tripfrom source to destination



We’ll see more of this with routing algorithms

Forwarding:

Move

packets from router’sinput to appropriaterouter output



Process of getting throughsingle interchange



See it today

☺

1 2

3

0111

value in arrivingpacket’s header

routing algorithm

local forwarding table

header value output link

0100010101111001

3 2 2 1

What prefixes and

subnet masks areused for ..

Possibility: Entries Per Address

If a router had a forwarding entry per IP address and notsubnet



Match

destination address

of incoming packet



… to the

forwarding-table entry



… to determine the

outgoing interface

host

host

host

LAN 1

...

host

host

host

LAN 2

...

router

router

router

WAN

WAN

1.2.3.

5.6.7.

2.4.6.

1.2.3.

5.6.7.

2.4.6.

1.2.3.4^ 1.2.3. forwarding table

Possibility: Entry per class

If the router had an entry per classful prefix



Mixture of Class A, B, and C addresses



Depends on the first couple of bits of the destination

Identify the mask automatically from theaddress



First bit of 0: class A address (/8)



First two bits of 10: class B address (/16)



First three bits of 110: class C address (/24)

Then, look in the forwarding table for the match



E.g., 1.2.3.4 maps to 1.2.3.0/



Then, look up the entry for 1.2.3.0/



… to identify the outgoing interface

CIDR and forwarding

Tradeoff



CIDR allows efficient use of the limited address space but CIDRmakes packet forwarding much harder

Forwarding table

may have many matches



E.g., table entries for 201.10.0.0/21 and 201.10.6.0/



The IP address 201.10.6.17 would match

both

!

201.10.0.0/

201.10.0.0/

201.10.4.0/

201.10.5.0/

201.10.6.0/

Provider 1

Provider 2

Simplest Algorithm is Too Slow

Scan the forwarding table one entry at a time



See if the destination matches the entry



If so, check the size of the mask for the prefix



Keep track of the entry with longest-matching prefix

Overhead is linear in size of the forwarding table



Today, that means 150,000-200,000 entries!



And, the router may have just a few nanoseconds



… before the next packet is arriving

Need greater efficiency to keep up with line rate



Better algorithms



Hardware implementations

A “Patricia” Tree (1988)

Practical Algorithm to Retrieve Information Coded in Alphanumeric

Store the prefixes as a tree



One bit for each level of the tree



Some nodes correspond to valid prefixes



... which have next-hop interfaces in a table

When a packet arrives



Traverse the tree based on the destination address



Stop upon reaching the longest matching prefix

0

1

00

10

11

100

101

Genesis of Forwarding Tables?

Where do the table entries come from? Recall:Routers map prefix to outgoing link(s)

Entries can be statically configured



E.g., “map 12.34.158.0/24 to Serial0/0.1”

But this doesn’t adapt or scale to failures, newequipment, load balancing!

That is where DHCP, ARP



And routing protocols (BGP, OSPF) we’ll see soon

DHCP: How host learns its address

Q:

How does host get IP

address (subnet +host part)?

Hard-coded by systemadmin in a file



Windows: control-panel->network->configuration->tcp/ip->properties

DHCP: D

ynamic

H

ost

C

onfiguration

P

rotocol

ARP: Address Resolutionprotocol (we’ll see thismore in coming lecture)