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Wide Area Networks and InternetCT1403

Lecture-8: Internet Network Layer (Part-3)

By : Najla Al-Nabhan

Lecture goals:

understand principles behind:

Address Resolution Protocol (ARP) in the Internet

Network layer service models (Connection & Connectionless)

forwarding versus routing how a router works routing (path selection) Internet broadcast, multicast

Network layer: Recall! transport segment

from sending to receiving host

on sending side encapsulates segments into datagrams

on receiving side, delivers segments to transport layer

network layer protocols in every host, router

router examines header fields in all IP datagrams passing through it

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

networkdata linkphysical network

data linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysicalnetwork

data linkphysical

Network Layer4-33

The Internet network layer

forwarding

table

host, router network layer functions:

routing protocols• path

selection• RIP, OSPF,

BGP

IP protocol• addressing

conventions• datagram format• packet handling

conventionsICMP protocol• error reporting• router

“signaling”

transport layer: TCP, UDP

link layer

physical layer

network

layer

IP Routing Process

When packet arrives, look up dest addr

local network? send immediately to destination

distant network? forward to next router on the interface given

in routing table not in the routing table?

forward to default gateway

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP)

Because there are both network -layer addresses (IP address) and link-layer addresses (that is MAC address), there is a need to translate between them

For Internet, this translation is the job of the Address Resolution Protocol (ARB)

MAC address allocation administered by IEEE. Manufacturer buys portion of MAC address space (to ensure uniqueness)

Analogy:MAC address: like Social Security NumberIP address: like postal address

ARP: address resolution protocol

ARP table: each IP node (host, router) on LAN has table

IP/MAC address mappings for some LAN nodes:

< IP address; MAC address; TTL>

TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

Question: how to determineinterface’s MAC address, knowing its IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137.196.7.23

137.196.7.78

137.196.7.14

137.196.7.88

ARP protocol in the Internet: same LAN

1. A wants to send datagram to B

B’s MAC address not in A’s ARP table.

2. A broadcasts ARP query packet, containing B's IP address

dest MAC address = FF-FF-FF-FF-FF-FF

all nodes on LAN receive ARP query

3. B receives ARP packet, replies to A with its (B's) MAC address

frame sent to A’s MAC address (unicast)

4. A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

soft state: information that times out (goes away) unless refreshed

5. ARP is “plug-and-play”:

nodes create their ARP tables without intervention from net administrator

walkthrough: send datagram from A to B via R focus on addressing – at IP (datagram) and MAC layer (frame)

assume A knows B’s IP address assume A knows IP address of first hop router, R (how?)

assume A knows R’s MAC address (how?)

Addressing: routing to another LAN

R

1A-23-F9-CD-06-9B

222.222.222.220

111.111.111.110

E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

74-29-9C-E8-FF-55

A

222.222.222.222

49-BD-D2-C7-56-2A

222.222.222.221

88-B2-2F-54-1A-0F

B

R

1A-23-F9-CD-06-9B

222.222.222.220

111.111.111.110

E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

74-29-9C-E8-FF-55

A

222.222.222.222

49-BD-D2-C7-56-2A

222.222.222.221

88-B2-2F-54-1A-0F

B

Addressing: routing to another LAN

IPEthPhy

IP src: 111.111.111.111 IP dest: 222.222.222.222

A creates IP datagram with IP source A, destination B

A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram

MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B

R

1A-23-F9-CD-06-9B

222.222.222.220

111.111.111.110

E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

74-29-9C-E8-FF-55

A

222.222.222.222

49-BD-D2-C7-56-2A

222.222.222.221

88-B2-2F-54-1A-0F

B

Addressing: routing to another LAN

IP src: 111.111.111.111 IP dest: 222.222.222.222

R forwards datagram with IP source A, destination B

R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram

MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A

IPEthPhy

IPEthPhy

R

1A-23-F9-CD-06-9B

222.222.222.220

111.111.111.110

E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

74-29-9C-E8-FF-55

A

222.222.222.222

49-BD-D2-C7-56-2A

222.222.222.221

88-B2-2F-54-1A-0F

B

Addressing: routing to another LAN R forwards datagram with IP source A, destination B

R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram

IP src: 111.111.111.111 IP dest: 222.222.222.222

MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A

IPEthPhy

IPEthPhy

R

1A-23-F9-CD-06-9B

222.222.222.220

111.111.111.110

E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

74-29-9C-E8-FF-55

A

222.222.222.222

49-BD-D2-C7-56-2A

222.222.222.221

88-B2-2F-54-1A-0F

B

Addressing: routing to another LAN R forwards datagram with IP source A, destination B

R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram

IP src: 111.111.111.111 IP dest: 222.222.222.222

MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A

IPEthPhy

Two key network-layer functions

network layer has three major functions:

1. forwarding: move packets from router’s input to appropriate router output

2. routing: determine route taken by packets from source to dest.

routing algorithms

analogy:routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader

valueoutput

link0100

0101

0111

1001

3221

Interplay between routing and forwarding

routing algorithm determinesend-end-path through networkforwarding table determineslocal forwarding at this router

Connection setup Connection setup is the 3rd important function

in some network architectures; (such as ATM, frame relay, X.25)

Occurs before data transfer: 3-way handshake in TCP Connection All routers in the Virtual Circuit (VC) need to

handshake with each other in order to establish virtual connection

routers get involved network vs transport layer connection

service: network: between two hosts (may also involve

intervening routers in case of VCs) transport: between two processes

Network service modelQ: What service model for “channel” transporting datagrams from sender to receiver?example services for an individual datagram:

guaranteed delivery guaranteed delivery

with less than 40 msec delay

example services for a flow of datagrams:

in-order datagram delivery

guaranteed minimum bandwidth to flow

restrictions on changes in inter-packet spacing

Network layer service models:

NetworkArchitectu

re

Internet

ATM

ATM

ServiceModel

best effort

CBR

ABR

Bandwidth

none

constantrateguaranteed minimum

No loss

no

yes

no

Order

no

yes

yes

Timing

no

yes

no

Congestionfeedback

no (inferredvia loss)nocongestionyes (indicated)

Guarantees ?

virtual circuit and datagram networks

Connection, connection-less service Transport layer provides connection &

connection-less services between two processes Network layer provides connection OR

connection-less services between two hosts datagram network provides network-layer

connectionless service virtual-circuit network provides network-layer

connection service analogous to TCP/UDP connection-oriented /

connectionless transport-layer services, but: service: host-to-host no choice: network provides one or the other

(not both) implementation: in network core

Virtual circuits

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host

address) every router on source-dest path maintains “state” for

each passing connection link, router resources (bandwidth, buffers) may be

allocated to VC (dedicated resources = predictable service)

“source-to-dest path behaves much like telephone circuit”

performance-wise network actions along source-to-dest path

VC implementation

a VC consists of:1. path from source to destination2. VC numbers, one number for each link

along path3. entries in forwarding tables in routers

along path packet belonging to VC carries VC

number (rather than dest address) VC number can be changed on each

link. new VC number comes from forwarding

table

VC forwarding table12

22

32

1 23

VC numberinterfac

enumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

forwarding table innorthwest router:

VC routers maintain connection state information!

applicationtransportnetworkdata linkphysical

Virtual circuits: signaling protocols

used to setup, maintain and terminate VC

used in ATM, frame-relay, X.25 not used in today’s Internet

1. initiate call

2. incoming call

3. accept call

4. call connected

5. data flow begins

6. receive data

applicationtransportnetworkdata linkphysical

Datagram networks no call setup at network layer routers: no state about end-to-end

connections no network-level concept of “connection”

packets forwarded using destination host address

1. send datagrams

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

2. receive datagrams

1

23

Datagram forwarding table

IP destination address in arriving packet’s header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1

address-range 2

address-range 3

address-range 4

3221

4 billion IP addresses, so rather than list individual destination addresslist range of addresses(aggregate table entries)

Destination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q: but what happens if ranges don’t divide up so nicely?

Datagram forwarding table

Longest prefix matching

Destination Address Range

11001000 00010111 00010*** *********

11001000 00010111 00011000 *********

11001000 00010111 00011*** *********

otherwise

DA: 11001000 00010111 00011000 10101010

examples: DA: 11001000 00010111 00010110

10100001 which interface?which interface?

when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.

longest prefix matching

Link

interface

0

1

2

3

Datagram or VC network: why?Internet (datagram) data exchange among

computers “elastic” service, no

strict timing req. many link types

different characteristics uniform service difficult

“smart” end systems (computers) can adapt, perform

control, error recovery simple inside

network, complexity at “edge”

ATM (VC) evolved from

telephony human conversation:

strict timing, reliability requirements

need for guaranteed service

“dumb” end systems telephones complexity inside

network

what’s inside a router

Router architecture overviewtwo key router functions:

run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link

high-seed

switching

fabric

routing process

or

router input ports

router output ports

forwarding data plane (hardware)

routing, managementcontrol plane

(software)

forwarding tables computed,pushed to input ports

lineterminati

on

link layer

protocol(receive

)

lookup,forwardi

ng

queueing

Input port functions

decentralized switching: given datagram dest., lookup output

port using forwarding table in input port memory (“match plus action”)

goal: complete input port processing at ‘line speed’

queuing: if datagrams arrive faster than forwarding rate into switch fabric

physical layer:bit-level

receptiondata link layer:e.g.,

Ethernetsee chapter

5

switchfabric

Switching fabrics transfer packet from input buffer to

appropriate output buffer switching rate: rate at which packets

can be transfer from inputs to outputsoften measured as multiple of input/output line rateN inputs: switching rate N times line rate desirable

three types of switching fabrics

memory

memory

bus

crossbar

Switching via memoryfirst generation routers: traditional computers with switching under direct control of CPU packet copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)

inputport(e.g.,

Ethernet)

memory

outputport(e.g.,

Ethernet)

system bus

Switching via a bus

datagram from input port memory

to output port memory via a shared bus

bus contention: switching speed limited by bus bandwidth

32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers

bus

Switching via interconnection network

overcome bus bandwidth limitations

banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor

advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.

Cisco 12000: switches 60 Gbps through the interconnection network

crossbar

Output ports

buffering required when datagrams arrive from fabric faster than the transmission rate

scheduling discipline chooses among queued datagrams for transmission

lineterminati

on

link layer

protocol(send)

switchfabric

datagram

buffer

queueing

Output port queueing

buffering when arrival rate via switch exceeds output line speed

queueing (delay) and loss due to output port buffer overflow!

at t, packets morefrom input to output

one packet time later

switchfabric

switchfabric

Input port queuing

fabric slower than input ports combined -> queueing may occur at input queues

queueing delay and loss due to input buffer overflow!

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

output port contention:only one red datagram can

be transferred.lower red packet is blocked

switchfabric

one packet time later: green

packet experiences HOL

blocking

switchfabric

Midterm Revision:

Your Questions: Please Ask!Difficult to Understand Topics?