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8/18/2019 Packet Networks
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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
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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
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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
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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
8/18/2019 Packet Networks
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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
8/18/2019 Packet Networks
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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
8/18/2019 Packet Networks
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(
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
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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
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Packet-SwitchingNetworks
Datagrams and Virtual Circuits
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%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
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*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
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!essage switchingin"ented for telegraphy
ntire messagesmultiplexed onto sharedlines, stored & forwarded
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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
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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@
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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
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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
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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
8/18/2019 Packet Networks
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+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
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Example+ ,nternet #o"ting
1nternet protocol uses datagram packetswitching across networks Networks are treated as data links
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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
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./
./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
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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
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,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
8/18/2019 Packet Networks
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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
8/18/2019 Packet Networks
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&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
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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
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Packet-SwitchingNetworks
Routing in Packet Networks
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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
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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
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#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
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9
8
B
@ %
#
$
+
B
9
8
D
C
B
9
8
@
B
9
.witch or router
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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
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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
+estination Next node
+estination Next node
+estination Next node
+estination Next node
+estination Next node
#o"ting %a*les in 'atagram
Packet Networks
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''''''''
''''''''
'''''''
''''''''
R
9 B
8
'''' ' '' / /
''' ''' ' / /
R9
Non-)ierarchical 7ddresses and
#o"ting
No relationship between addresses & routingproximity
Routing tables require @ entries each
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''''
''''''''
'''
'''''
'''
'
'''''
'''
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
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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
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!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
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0
9
8
B
@
Ilooding is initiated from Node :
8/18/2019 Packet Networks
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4
8
B
@
Ilooding is initiated from Node :
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9
(
B
@
Ilooding is initiated from Node :
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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
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'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
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',' ', ',9 ',8
,' , ,9 ,8
9,' 9, 9,9 9,8
8,' 8, 8,9 8,8
)unnel fromlast column tofirst column or
"ice "ersa
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',' ', ',9 ',8
,' , ,9 ,8
9,' 9, 9,9 9,8
8,' 8, 8,9 8,8
busy
Example+ Node :;:0
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Packet-SwitchingNetworks
Shortest Path Routing
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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
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#o"ting &etrics
!eans for measuring desirability of a path *ath Length = sum of costs or distances
*ossible metrics
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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
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.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?
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'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
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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
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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
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Bhy 'istance ector Borks
Sa
n
Jose
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@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
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@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
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,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
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,teration Node 0 Node 4 Node ( Node 5 Node 1
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
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,teration Node 0 Node 4 Node ( Node 5 Node 1
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
8/18/2019 Packet Networks
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,teration Node 0 Node 4 Node ( Node 5 Node 1
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
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,teration Node 0 Node 4 Node ( Node 5 Node 1
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
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,teration Node 0 Node 4 Node ( Node 5 Node 1
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
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,teration Node 0 Node 4 Node ( Node 5 Node 1
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
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8
B
@
9
9
8
9
9
8
B
,teration Node 0 Node 4 Node ( Node 5 Node 1
(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
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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
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Pro*lem+ @ad News %ravels
Slowly
Remedies .plit
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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
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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
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'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
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'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
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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
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Shortest Paths in 'ikstraAs
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Shortest Paths in 'ikstra s
7lgorithm1
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#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
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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
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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
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9
8
B
@
%
#
.ource host
+estination host
,8,@,#
8,@,# @,#
#
Example