packet networks

8/18/2019 Packet Networks

1/75

Network Layer

Network Layer: the most complex layer Requires the coordinated actions of multiple,

geographically distributed network elements

(switches & routers !ust be able to deal with "ery large scales #illions of users (people & communicating de"ices

#iggest $hallenges

%ddressing: where should information be directed to Routing: what path should be used to get information

there


2/75

t 't

Network

Packet Switching

)ransfer of information as payload in data packets

*ackets undergo random delays & possible loss

+ifferent applications impose differing requirementson the transfer of information


3/75

Perspectives of Packet Networks

xternal -iew of the network .er"ices that the network pro"ides to the transport layer .er"ices are independent of the underlying network

/hether the network ser"ice requires setting up of connections

/hether data transfer requires any quality0of0ser"ice guarantees

1nternal 2peration of the network $onsiders physical topology of the network and its interconnection

%pproaches used to direct information 3 datagram, "irtual circuit %ddressing and routing procedures

+eal with congestion inside the network

)raffic management inside the network


4/75

ndsystem

4*hysical

layer

+ata linklayer

*hysical

layer

+ata linklayer nd

system5

Networklayer

Networklayer

*hysical

layer

+ata linklayer

Networklayer

*hysical

layer

+ata linklayer

Networklayer

)ransportlayer

)ransportlayer

!essages!essages

.egments

Networkser"ice

Networkser"ice

Network Service

Network layer can offer a "ariety of ser"ices to transport layer

$onnection0oriented ser"ice or connectionless ser"ice

#est0effort or delay6loss guarantees


5/75

Connectionless vs. Connection-oriented

$onnectionless : 2nly two basic interactions between transport and network layer

Request to network layer to send a packet

1ndication from network layer that a packet has arri"ed

7ser can request packet transmission at any time

No need to inform network layer ahead of time Responsibility for error control, sequencing and flow control on transport0layer

$onnection0oriented : $onnection0setup required

Network layer must be informed about the new flow to be sent to the network

Network layer maintains state information about the flows it is handling

%llows usage and quality0of0ser"ice negotiations Network resources may be allocated

$onnection0termination required

$omplex than connectionless ser"ice


6/75

Network Service vs. Operation

Network .er"ice $onnectionless (7+*

+atagram )ransfer

$onnection02riented ()$* Reliable and possibly

constant bit rate transfer

1nternal Network 2peration $onnectionless

+atagram operation

1*

$onnection02riented -irtual $ircuit operation

)elephone connection

%)!

-arious combinations are possible $onnection0oriented ser"ice o"er $onnectionless operation

$onnectionless ser"ice o"er $onnection02riented operation

$ontext & requirements determine what makes sense


7/75

(

8

8 9 9

9

89

9

9 (

9

!edium

% #

8 9 9 9

(

$

9 (

9 (

9 9 8

nd system5

nd system4

Network

9

*hysical layer entity

+ata link layer entity 8Network layer entity

8 Network layer entity

)ransport layer entity

Complexity at the Edge or in the Core

Need for the network to grow to "ery large scale 3

keep the core of the network simple (connectionless packet network

pro"ide necessary complexity at the edge


8/75

Network Layer !"nctions

ssential#o"ting: mechanisms for determining the

set of best paths for routing packets requires

the collaboration of network elements !orwarding: transfer of packets from N

inputs to outputsPriority $ Sched"ling: determining order of

packet transmission in each N2ptional: congestion control, segmentation &

reassembly, security


9/75

Packet-SwitchingNetworks

Datagrams and Virtual Circuits


10/75

%he Switching !"nction +ynamic interconnection of inputs to outputs

nables dynamic sharing of transmission resource

)wo fundamental approaches: $onnectionless

$onnection02riented: $all setup control, $onnection control

#ackbone Network

%ccess Network

.witch


11/75

*acketswitch

Network

)ransmissionline

7ser

Packet Switching Network

*acket switching network )ransfers packets

between users

)ransmission lines ;packet switches(routers

2rigin in messageswitching

)wo modes of operation: $onnectionless -irtual $ircuit


12/75

!essage switchingin"ented for telegraphy

ntire messagesmultiplexed onto sharedlines, stored & forwarded


13/75

t

t

t

t

+elay

.ource

+estination

T

τ

!inimum delay = 8τ ; 8T

.witch

.witch 9

&essage Switching 'elay

%dditional queueing delays possible at each link


14/75

Long &essages vs. Packets

%pproach : send !bitmessage

*robability messagearri"es correctly

2n a"erage it takes about8 transmissions6hop

)otal > bits transmitted ? @

!bits

!bitmessage source dest

#R=p='0@ #R='0@


15/75

Packet Switching - 'atagram

!essages broken into smallerunits (packets

.ource & destinationaddresses in packet header

$onnectionless, packets

routed independently(datagram *acket may arri"e out of

order *ipelining of packets across

network can reduce delay,increase throughput Lower delay that message

switching, suitable forinteracti"e traffic

*acket 9

*acket

*acket

*acket 9

*acket 9


16/75

t

t

t

t

+elay

8 9

8 9

89

!inimum +elay = 8τ ; B(T 68 (single path assumed

%dditional queueing delays possible at each link*acket pipelining enables message to arri"e sooner

Packet Switching 'elay

%ssume three packets corresponding to one messagetra"erse same path


17/75

t

t

t

t

8 9

8 9

89

8τ ; 9(T 68 first bit recei"ed

8τ ; 8(T 68 first bit released

8τ ; B (T 68 last bit released

Lτ + (L- )P first bit recei"ed

Lτ + LP first bit released

Lτ + LP + (k- )P last bit releasedwhere T = k P

( hops L hops

.ource

+estination

.witch

.witch 9

τ

'elay for k-Packet &essage over L

)ops


18/75

+estination

address

2utput

port

8B 9

9BC

D'DCB

@

9

B@@

#o"ting %a*les in 'atagram

Networks

Route determined by tablelookup

Routing decision in"ol"esfinding next hop in route to

gi"en destination Routing table has an entry

for each destinationspecifying output port that

leads to next hop .iEe of table becomes

impractical for "ery largenumber of destinations


19/75

Example+ ,nternet #o"ting

1nternet protocol uses datagram packetswitching across networks Networks are treated as data links


20/75

Packet Switching irt"al Circ"it

$all set0up phase sets up pointers in fixed path along network %ll packets for a connection follow the same path

%bbre"iated header identifies connection on each link *ackets queue for transmission -ariable bit rates possible, negotiated during call set0up +elays "ariable, cannot be less than circuit switching

-irtual circuit

*acket *acket

*acket

*acket


21/75

./

./9

./n

$onnectrequest

$onnectrequest

$onnectrequest

$onnectconfirm

$onnectconfirm

$onnectconfirm

/

Connection Set"p

.ignaling messages propagate as route is selected .ignaling messages identify connection and setup tables in

switches )ypically a connection is identified by a local tag, -irtual

$ircuit 1dentifier (-$1 ach switch only needs to know how to relate an incoming tag

in one input to an outgoing tag in the corresponding output 2nce tables are setup, packets can flow along path


22/75

t

t

t

t

8 9

8 9

89

Release

$onnectrequest

$R

$R$onnectconfirm

$$

$$

Connection Set"p 'elay

$onnection setup delay is incurred before any packetcan be transferred

+elay is acceptable for sustained transfer of largenumber of packets

)his delay may be unacceptably high if only a few

packets are being transferred


23/75

,np"t

C,

O"tp"t

port

O"tp"t

C,

01 01

12

0(

0(

3

43

04 55

4(

06

(5

irt"al Circ"it !orwarding %a*les

ach input port of packetswitch has a forwardingtable

Lookup entry for -$1 of

incoming packet +etermine output port (next

hop and insert -$1 for nextlink

-ery high speeds are

possible )able can also include

priority or other informationabout how packet should betreated


24/75

8

9

8 9

89

!inimum delay = 8τ ; T

t

t

t

t .ource

+estination

.witch

.witch 9

C"t-%hro"gh switching

.ome networks perform error checking on header only,so packet can be forwarded as soon as header isrecei"ed & processed

+elays reduced further with cut0through switching


25/75

&essage vs. Packet &inim"m

'elay

!essage:L τ + L T = L τ + (L 3 T + T

*acket

L τ + L P + (k – P = L τ + (L – P + T

$ut0)hrough *acket (1mmediate forwarding afterheader

= L τ + T

%bo"e neglect header processing delays


26/75

Example+ 7%& Networks

%ll information mapped into short fixed0lengthpackets called cells

$onnections set up across network

-irtual circuits established across networks )ables setup at %)! switches

.e"eral types of network ser"ices offered

$onstant bit rate connections -ariable bit rate connections


27/75


Routing in Packet Networks


28/75

9

8

B

@

Node(switch or router

#o"ting in Packet Networks

)hree possible (loopfree routes from to @:

080@, 00B0@, 090B0@/hich is FbestG !in delay !in hop !ax bandwidth !in cost

!ax reliability


29/75

Creating the #o"ting %a*les

Need information on state of links Link up6downH congestedH delay or other metrics

Need to distribute link state information using a

routing protocol /hat information is exchanged


30/75

#o"ting 7lgorithm #e8"irements

Responsi"eness to changes )opology or bandwidth changes, congestion

Rapid con"ergence of routers to consistent set of routes Ireedom from persistent loops

2ptimality Resource utiliEation, path length

Robustness $ontinues working under high load, congestion, faults, equipment failures, incorrect implementations

.implicity fficient software implementation, reasonable processing load


31/75

9

8

B

@ %

#

$

+

B

9

8

D

C

B

9

8

@

B

9

.witch or router


32/75

1ncoming 2utgoingNode -$1 Node -$1 % 8 9 % B 8 8 8 9 %

8 8 % B

1ncoming 2utgoingNode -$1 Node -$1 9 @ D 8 9 @ @ D 9 @ 9 8

1ncoming 2utgoingNode -$1 Node -$1 8 D # C 8 # B # B 8 # C 8 D

1ncoming 2utgoingNode -$1 Node -$1

$ @ 8 8 $ @

1ncoming 2utgoingNode -$1 Node -$1 9 8 8 9 8 B B 8 9 9 8

B B 8

1ncoming 2utgoingNode -$1 Node -$1 B + 9 + 9 B

Node

Node 9

Node 8

Node

Node @

Node B

#o"ting %a*les in C Packet

Networks

xample: -$1 from % to + Irom % & -$1 B J 8 & -$1 8 J & -$1

J B & -$1 B J + & -$1 9


33/75

9 9

8 8

B 9

@ 8

Node

Node 9

Node 8

Node

Node @

Node B

9

B @

@ @

8

9 B

8 8

8 B B

+estination Next node

8

B B

@ B

9 9

8

@ @

9 9

8 8

B B

@ 8






#o"ting %a*les in 'atagram

Packet Networks


34/75

''''''''

''''''''

'''''''

''''''''

R

9 B

8

'''' ' '' / /

''' ''' ' / /

R9

Non-)ierarchical 7ddresses and

#o"ting

No relationship between addresses & routingproximity

Routing tables require @ entries each


35/75

''''

''''''''

'''

'''''

'''

'

'''''

'''

R R9

9 B

8

''

' 8' 9 8

'' 8

' ' 8 B

)ierarchical 7ddresses and

#o"ting

*refix indicates network where host isattached

Routing tables require entries each


36/75

Speciali9ed #o"ting

Ilooding 7seful in starting up network

7seful in propagating information to all nodes

+eflection Routing Iixed, preset routing procedure

No route synthesis


37/75

!looding

.end a packet to all nodes in a networkNo routing tables a"ailable

Need to broadcast packet to all nodes (eAgA to

propagate link state information

%pproach

.end packet on all ports except one where it arri"ed

xponential growth in packet transmissions


38/75

0

9

8

B

@

Ilooding is initiated from Node :


39/75

4

8

B

@



40/75

9

(

B

@



41/75

Limited !looding

)ime0to0Li"e field in each packet limitsnumber of hops to certain diameter

ach switch adds its 1+ before floodingH

discards repeats.ource puts sequence number in each

packetH switches records source address and

sequence number and discards repeats


42/75

'eflection #o"ting

Network nodes forward packets to preferred port 1f preferred port busy, deflect packet to another port /orks well with regular topologies !anhattan street network

Rectangular array of nodes Nodes designated (i,K Rows alternate as one0way streets $olumns alternate as one0way a"enues

#ufferless operation is possible *roposed for optical packet networks %ll0optical buffering currently not "iable


43/75

',' ', ',9 ',8

,' , ,9 ,8

9,' 9, 9,9 9,8

8,' 8, 8,9 8,8

)unnel fromlast column tofirst column or

"ice "ersa


44/75

',' ', ',9 ',8

,' , ,9 ,8

9,' 9, 9,9 9,8

8,' 8, 8,9 8,8

busy

Example+ Node :;:0


45/75


Shortest Path Routing


46/75

Shortest Paths $ #o"ting

!any possible paths connect any gi"ensource and to any gi"en destination

Routing in"ol"es the selection of the path to

be used to accomplish a gi"en transfer )ypically it is possible to attach a cost or

distance to a link connecting two nodes

Routing can then be posed as a shortest pathproblem


47/75

#o"ting &etrics

!eans for measuring desirability of a path *ath Length = sum of costs or distances

*ossible metrics


48/75

Shortest Path 7pproaches

'istance ector Protocols

Neighbors exchange list of distances to destinations

#est next0hop determined for each destination

Iord0Iulkerson (distributed shortest path algorithmLink State Protocols

Link state information flooded to all routers

Routers ha"e complete topology information

.hortest path (& hence next hop calculated

+iKkstra (centraliEed shortest path algorithm

'istance ector


49/75

.an Mose 89

.an Mose B@

. a n M o s e 9 N :

. a n M o s e 9 B '

'istance ector

Do you know the way to San Jose?


50/75

'istance ector

Local Signpost +irection

+istance

Routing Table

Ior each destination list:

Next Node

+istance

Table Synthesis Neighbors exchange

table entries

+etermine current bestnext hop

1nform neighbors *eriodically

%fter changesdest next dist


51/75

Shortest Path to S?

i

j

Sa

n

Jose ij

! j

!i 1f !i is the shortest distance to .M from i

and if j is a neighbor on the shortest path,then !

i

= ij

+ ! j

Iocus on how nodes find their shortestpath to a gi"en destination node, iAeA .M

@"t we donAt know the shortest


52/75

i only has local infofrom neighbors

! j"

ij#

i

Sa

n

Jose

K ij ! j

!i j"

ij$

KO

! j$

*ick currentshortest path

@"t we donAt know the shortest

paths


53/75

Bhy 'istance ector Borks

Sa

n

Jose


54/75

@ellman-!ord 7lgorithm

onsi%er co&putations 'or one %estination % nitialiation

ach node table has row for destination % +istance of node % to itself is Eero: !% =* +istance of other node j to % is infinite: ! j =∝ for j ≠ % Next hop node n j = 0 to indicate not yet defined for j ≠ %

Sen% Step .end new distance "ector to immediate neighbors across local link

Recei,e Step

%t node j , find the next hop that gi"es the minimum distance to % ,in j . ij + ! j / Replace old (n j ! j (%)) by new (n j 0 ! j 0(%)) if new next node or distance

Po to send step


55/75

@ellman-!ord 7lgorithm

1ow consi%er parallel co&putations 'or all %estinations % nitialiation

ach node has row for each destination % +istance of node % to itself is Eero: !% (%)=* +istance of other node j to % is infinite: ! j (%)= ∝ for j ≠ %

Next node n j = 0 since not yet defined Sen% Step

.end new distance "ector to immediate neighbors across local link Recei,e Step

Ior each destination % , find the next hop that gi"es the minimum

distance to % , in j . ij + ! j (%) / Replace old (n j !i (%)) by new (n j 0 ! j 0(%)) if new next node or distance

found Po to send step


56/75

,teration Node 0 Node 4 Node ( Node 5 Node 1

1nitial (0, ∞ (0, ∞ (0, ∞ (0, ∞ (0, ∞

9

8

8

B

@

9

9

8

9

9

8

BSan

Jose

)able entry

Q node

for dest .M

)able entry

Q node 8

for dest .M


57/75


1nitial (0, ∞ (0, ∞ (0, ∞ (0, ∞ (0, ∞

(0, ∞ (0, ∞ (@, (0, ∞ (@,9

9

8

San

Jose

+@='

!2=!3 +4

n2=3

8

B

@

9

9

8

9

9

8

B

+@='!5 =!3 +6

n5 =3

;

4

0


58/75


1nitial (0, ∞ (0, ∞ (0, ∞ (0, ∞ (0, ∞

(0, ∞ (0, ∞ (@, (0, ∞ (@,9

9 (8,8 (B,@ (@, (8,8 (@,9

8

San

Jose

8

B

@

9

9

8

9

9

8

B;

0

4

(

(

6


59/75


1nitial (0, ∞ (0, ∞ (0, ∞ (0, ∞ (0, ∞

(0, ∞ (0, ∞ (@, (0, ∞ (@,9

9 (8,8 (B,@ (@, (8,8 (@,9

8 (8,8 (, (@, (8,8 (@,9

San

Jose

8

B

@

9

9

8

9

9

8

B;

0

46

(

(

5


60/75


1nitial (8,8 (, (@, (8,8 (@,9

(8,8 (, (, B (8,8 (@,9

9

8

San

Jose

8

B

@

9

9

8

9

9

8

B;

0

4

(

(

5

1etwork %isconnecte%7 Loop create% between no%es 2 an% 8

1


61/75


1nitial (8,8 (, (@, (8,8 (@,9

(8,8 (, (, B (8,8 (@,9

9 (8,D (, (, B (B,B (@,9

8

San

Jose

8

B

@

9

9

8

9

9

8

B;

4

1

(

(

5

3

1

Node could ha"e chosen 9 as next node because of tie

, i N d 0 N d 4 N d ( N d 5 N d


62/75


1nitial (8,8 (, (@, (8,8 (@,9

(8,8 (, (, B (8,8 (@,9

9 (8,D (, (, B (B,B (@,9

8 (8,D (,@ (, D (B,B (@,9

San

Jose

8

B

@

9

9

8

9

9

8

B ;

4

1

13

5

3

6

Node 9 could ha"e chosen B as next node because of tie


63/75

8

B

@

9

9

8

9

9

8

B


(8,8 (, (, B (8,8 (@,9

9 (8,D (, (, B (9,B (@,9

8 (8,D (,@ (, D (B,B (@,9

(9, (,@ (, D (B,B (@,9

San

Jose

;

33

1

6

4

Node could ha"e chose 8 as next node because of tie


64/75

8 9

8 9

(a

(b

Dpdate Node 0 Node 4 Node (

#efore break (9,8 (8,9 (,

%fter break (9,8 (8,9 (9,8

(9,8 (8, (9,8

9 (9,B (8, (9,B

8 (9,B (8,@ (9,B

(9,D (8,@ (9,D

B (9,D (8,C (9,D

/ / / /

Co"nting to ,nfinity Pro*lem

Nodes belie"e bestpath is through eachother

(+estination is node

P *l @ d N % l


65/75

Pro*lem+ @ad News %ravels

Slowly

Remedies .plit


66/75

8 9

8 9

(a

(b

Split )ori9on with Poison #everse

Nodes belie"e bestpath is througheach other

Dpdate Node 0 Node 4 Node (

#efore break (9, 8 (8, 9 (,

%fter break (9, 8 (8, 9 (0, ∞ Node 9 ad"ertiEes its route to tonode 8 as ha"ing distance infinityHnode 8 finds there is no route to

(9, 8 (0, ∞ (0, ∞ Node ad"ertiEes its route to tonode 9 as ha"ing distance infinityHnode 9 finds there is no route to

9 (0, ∞ (0, ∞ (0, ∞ Node finds there is no route to


67/75

Link-State 7lgorithm

#asic idea: two step procedure ach source node gets a map of all nodes and link metrics (link

state of the entire network

Iind the shortest path on the map from the source node to alldestination nodes

#roadcast of link0state information "ery node i in the network broadcasts to e"ery other node in the

network:

1+Ss of its neighbors: Ni=set of neighbors of

i+istances to its neighbors: T ij U j ∈1 i V

Ilooding is a popular method of broadcasting packets

'ikstra 7lgorithm+ !inding


68/75

'ikstra 7lgorithm+ !inding

shortest paths in order

s

w

w

"

w'

$losest node to s is hop away

w

"

x

x'

9nd closest node to s is hopaway from s or w”

x

z

z'

8rd closest node to s is hop

away from s, w”, or xw'

Iind shortest paths fromsource s to all otherdestinations


69/75

'ikstraAs algorithm

1 : set of nodes for which shortest path already found

1nitialiEation: (.tart with source no%e s) 1 = .s/ !s = ', Fs is distance Eero from itselfG

! j = sj for all j ≠ s distances of directly0connected neighbors

.tep %: (9in% ne:t closest no%e i Iind i ∉ 1 such that

!i = min !j for j ∉ 1

%dd i to 1

1f 1 contains all the nodes, stop

.tep #: (up%ate &ini&u& costs) Ior each node j ∉ 1

! j = min (! j !i + ij

Po to .tep %ini&u& %istance 'ro& s to

j through no%e i in 1


70/75

Exec"tion of 'ikstraAs algorithm

1teration N +9 +8 + +B +@

1nitial TV 8 9 B ∝ ∝

T,8V 8 4 ∝ 8

9 T,9,8V ( 4 D 8

8 T,9,8,@V ( 4 B (

T,9,8,,@V ( 4 5 B (

B T,9,8,,B,@V ( 4 5 1 (

1

2

4

5

6

1

1

2

3 2

35

2

4

3 1

2

4

5

6

1

1

2

3 2

35

2

4

331

2

4

5

6

1

1

2

3 2

35

2

4

3 1

2

4

5

6

1

1

2

3 2

35

2

4

331

2

4

5

6

1

1

2

3 2

35

2

4

33 1

2

4

5

6

1

1

2

3 2

35

2

4

331

2

4

5

6

1

1

2

3 2

35

2

4

33

Shortest Paths in 'ikstraAs


71/75

Shortest Paths in 'ikstra s

7lgorithm1

2

4

5

6

1

1

2

3 2

35

2

4

3 31

2

4

5

6

1

1

2

3 2

35

2

4

3

1

2

4

5

6

1

1

2

3 2

3

5

2

4

33 1

2

4

5

6

1

1

2

3 2

3

5

2

4

33

1

2

4

5

6

1

1

2

3 2

3

5

2

4

33 1

2

4

5

6

1

1

2

3 2

3

5

2

4

33


72/75

#eaction to !ail"re

1f a link fails, Router sets link distance to infinity & floods the

network with an update packet %ll routers immediately update their link database &

recalculate their shortest paths Reco"ery "ery quick

#ut watch out for old update messages %dd time stamp or sequence > to each update

message $heck whether each recei"ed update message is new 1f new, add it to database and broadcast 1f older, send update message on arri"ing link


73/75

Bhy is Link State @etter

Iast, loopless con"ergence.upport for precise metrics, and multiple

metrics if necessary (throughput, delay, cost,

reliability.upport for multiple paths to a destination

algorithm can be modified to find best two paths


74/75

So"rce #o"ting

.ource host selects path that is to be followed by apacket .trict: sequence of nodes in path inserted into header Loose: subsequence of nodes in path specified

1ntermediate switches read next0hop address andremo"e address

.ource host needs link state information or accessto a route ser"er

.ource routing allows the host to control the paths

that its information tra"erses in the network *otentially the means for customers to select what

ser"ice pro"iders they use


75/75

9

8

B

@

%

#

.ource host

+estination host

,8,@,#

8,@,# @,#

#

Example

packet networks

Documents