Download Data Link Layer Switching

Switching and Routing
Foreleser: Carsten Griwodz
Email: [email protected]
8.+15. Mar. 2006
1
INF-3190: Switching and Routing
Motivation

It is desirable to connect networks (instead of using a single
large one):





Limits in physical size and number of nodes per network
Limits in amount of concurrent traffic per network
Different kinds of network for different needs
Separate networks for increased availability
Administrative boundaries
one frame at a time,
Min 512 bits
Host
802.5
LAN
wireless
network
WAN-WAN
SNA-WAN
G
G
company
802.11
LAN
802.3
LAN
G
LAN-LAN
8.+15. Mar. 2006
ISP
Max 500m
802.3 LAN
ATM WAN
G
G
LAN-WAN
LAN-WAN-LAN
2
INF-3190: Switching and Routing
Connecting Networks by Relays
End system
Intermediate
system
End system
5
4
Gateway
5
Application layer
4
Transport layer
3
Router
3
Network layer
2
Bridge
2
Data link layer
1
Repeater
1
Physical layer
8.+15. Mar. 2006
3
INF-3190: Switching and Routing
Connecting Networks by Relays

Layer 1: Repeater / Hub






copies all bits between
cable segments
works solely as a
repeater
does not influence the
traffic between networks
Layer 3: Router (or
Layer 3 Gateway)



Layer 2: Bridge / Switch


relays frames between
LANs (MAC level)
minor frame
modifications, increases
the number of stations

relays packets between
different networks
modifies the packets
converts different
addressing concepts
Layer 4 - 5: Gateway
(or Protocol Converter)


converts one protocol
into another
usually no 1-to-1
mapping of functions
Note:


names (in products) are often confused
e. g. bridge and switch
8.+15. Mar. 2006
4
INF-3190: Switching and Routing
Layer 1: Physical Layer
8.+15. Mar. 2006
5
INF-3190: Switching and Routing
Repeater and Hub
A
Repeater ex.:
IEEE 802.3
“Thinwire”

C
Hub ex.:
IEEE 802.3
Twisted Pair
Repeater
E
D
Hub
F
G
H
Function



B
To amplify the electrical signals
To increase the range
Limitation


Extends the broadcast medium: every bit is copied
One collision domain
8.+15. Mar. 2006
6
INF-3190: Switching and Routing
Layer 2: Data Link Layer
8.+15. Mar. 2006
7
INF-3190: Switching and Routing
Bridges, Switches

Bridges




Connects two or more LANs (potentially
different types)
Each line is its own collision domain
Traditionally: store-and-forward and CPUbased
Switches



Typically connects two or more
computers
Each port / line is its own collision
domain (no collisions)
Typically cut-through switching devices




A
B
C
Bridge
LAN
Host
E
F
G
H
A
B
C
D
Switch
begin forwarding as soon as possible
when destination header has been
detected, before rest of frame arrived
Hardware-based
D
E
F
G
H
Bridges vs. Switches

Sometimes difference seems to be more a marketing issue
than technical one
8.+15. Mar. 2006
8
INF-3190: Switching and Routing
Bridge

Tasks






Coupling of different LANs
Scalability of networks
To increase capacity
To increase reliability
To improve security
To cover large distances
B
BackboneLAN
B
LAN
B
2,5km
802.3 LAN
To offer independence from
protocols



B
File
Server
2,5km

B
IP  OSI layer 2 protocols
Ethernet versions
Important goal: to achieve transparency


Change attachment point without changes to HW, SW, configuration
Hide different types of LAN to communicating machines
8.+15. Mar. 2006
9
INF-3190: Switching and Routing
Bridge Connecting 2 Different Networks
Host A
Example:
Bridge between
IEEE 802.3 (CSMA/CD)
and
IEEE 802.4 (Token Bus)
Network
LLC
Host B
Pkt
Pkt
Pkt
Pkt
802.3 Pkt
802.3 Pkt 802.4 Pkt
802.4 Pkt
Physical 802.3 Pkt
802.3 Pkt 802.4 Pkt
802.4 Pkt
MAC
802.3 Pkt

Approach



Pkt
Bridge
802.4 Pkt
CSMA/CD LAN
Token bus LAN
LLC as common layer
Frames are routed to the respective MAC
Bridge contains


Its own implementation for each MAC
For each to it belonging physical layer the corresponding
implementation
8.+15. Mar. 2006
10
INF-3190: Switching and Routing
802.x  802.y Bridging

Some different 802.x frame formats:


There are even more different frame formats ...
Some fields are technically necessary in one case but useless in
another

e.g. duration of 802.11
Start
P ream ble delimite r
A ccess
Frame Destination S ource
control
control
address address
End
Length
Data
Pad
Checksum delimiter
Frame
status
802.3
802.4
802.5
802.11
Frame
con trol
Du ration
8.+15. Mar. 2006
Addr 1
A ddr 2 Add r 3 Seq
Addr 4
11
Data
Che cksum
INF-3190: Switching and Routing
802.x  802.y Bridging

Different transmission rates (4/10/11/16/100/1000/... Mbps)



Bridge between fast LAN and slow LAN (or several LANs to one)
Buffering frames which cannot be transmitted immediately
Potentially many frames must be buffered within bridge


Different frame lengths


802.3: 1518 bytes, 802.4: 8191 bytes, 802.5: unlimited, 802.11:
2346 bytes
802.x protocols do not support reassembly





If bridge is out of memory, frames are dropped
Bridge must not segment frames that are too large
Frames that are too long are dropped
Implies a loss of transparency
Special case 802.6 – DQDB transmits each frame in several cells
Different features

Priorities


Supported (in various forms) from both 802.4 and 802.5
Not supported by 802.3
8.+15. Mar. 2006
12
INF-3190: Switching and Routing
802.x  802.y Bridging

Different checksum calculations


Security



802.11 provides some data link layer encryption
802.3 does not
Quality of Service / Priorities




Means conversion, delay, buffering
Supported (in various forms) by both 802.4 and 802.5
Not supported by 802.3
’Kind of’ in 802.11 (PCF / DCF)
Acknowledgements



Supported by 802.4 (temporary token handoff)
Supported by 802.5 (C+A bits)
Not supported by 802.3
8.+15. Mar. 2006
13
INF-3190: Switching and Routing
Self–Learning Bridges


Also called Transparent Bridges
Transparency


Transparent bridge



Bridges not visible for the other components of the network
 simplifies other components
Bridge works in
promiscuous mode
(receives every frame of
each connected LAN)
Bridge manages table:
station  LAN
(output line)
LAN 1
LAN 2
Bridge 1
LAN 4
Bridge 2
A
B
address
LAN
A
1
B
2
C
2
D
2
D
LAN 3
C
Decision procedure



Destination unknown: flooding
Source and destination LANs identical: frame dropped
Source and destination LANs differ: frame rerouted to destination
LAN
8.+15. Mar. 2006
14
INF-3190: Switching and Routing
Self–Learning Bridges

Learning procedure

Bridge table initially empty


Use flooding for unknown destination
Backward learning

Bridge works in promiscuous mode:


Bridge receives frames from source address Q on LAN L



Receives any frame on any of its LANs
Q can be reached over L
Create table entry accordingly
Adaptation to changes in topology



Entry associated with timestamp (last frame arrival time)
Timestamp of an entry (Z, LAN, TS) is updated when frame
received from Z
Entries that are not updated are purged
8.+15. Mar. 2006
15
INF-3190: Switching and Routing
Self–Learning Bridges: Spanning Tree

Increase reliability


Connect LANs via various bridges in parallel
Problem


This creates a loop in the topology
Frames with unknown destination are flooded

Frame is copied again and again
Frame copied
by B1
F1
LAN 2
Frame copied
by B2
F2
Bridge
B1
B2
LAN 1
F

Solution


Initial frame
Communication among bridges
Overlay actual topology by spanning tree reaching every LAN

Exactly one path from any LAN to every other LAN
8.+15. Mar. 2006
16
INF-3190: Switching and Routing
Self–Learning Bridges: Spanning Tree

Algorithm

Choose a bridge as root of tree


Generation of spanning tree



All bridges broadcast their serial number, lowest wins
Configured with bridges representing the edges within the tree
Thereby avoiding loops
Adaptation if configuration is changed
A
B
1
2
C
3
1
D
E
G
5
6
I
8
D
7
Bridge that 5
is part of the
spanning tree H
J
3
C
4
E
F
8
6
7
J
9
Bridge that is
not part of the
spanning tree
9
A Spanning Tree
Bridges between LANs

B
F
Bridge
H
2
4
LAN
LAN
A
Drawback

Ignores some potential connections between LANs
i.e., not all bridges are necessarily present in the tree
8.+15. Mar. 2006
17
INF-3190: Switching and Routing
Source Routing Bridges

Alternative to self-learning bridges

Principle



The frame’s sender defines path
Bridge routes the frame
Prerequisite



LAN has a unique address
Bridge at the respective LAN is also unique
Then



Sender flags the frame (top bit of its own address = 1),
if destination address is not reachable in LAN
Bridge routes only frames that have been flagged in such a way
Determining Path




Sender sends discovery frames as broadcast
Each bridge forwards on all attached LANs
Each bridge on the path adds own address to return packet
Problem: high traffic
8.+15. Mar. 2006
18
INF-3190: Switching and Routing
Connecting Equal Networks:
Encapsulation
Bridge
Example:
remote bridge
Point-to-point line
LAN 1
LAN 2
LAN 3

Principle
1.
2.
3.

Incoming data unit is packaged as payload,
Transmitted and
Then fed into the destination network
Properties

Certain protocol on connecting route



e.g. PPP
i.e. MAC frames encapsulated in PPP
Only bridge at the destination network can be reached
Simple
8.+15. Mar. 2006
19
INF-3190: Switching and Routing
Layer 3: Network Layer
8.+15. Mar. 2006
20
INF-3190: Switching and Routing
Network Layer

Goal

End systems
Enable data transfer from
end system to end system






Subnetworks
Standardized for end systems
Independent from network technology
Independent from number, type and topology of the subnetworks
Subnetworks (ISO definition):


Several hops, (heterogeneous)
subnetworks
Compensate for differences
between end systems during
transmission
The provided services are

Intermediate systems
A multiple of one or several intermediary systems that provide
switching functionalities and through which open end systems can
establish network connections
Routers are such Intermediate Systems
8.+15. Mar. 2006
21
INF-3190: Switching and Routing
Network Layer

Primary task from a layer model perspective

To provide service to the transport layer






Connectionless or connection-oriented service
Uniform addressing
Internetworking: provide transitions between networks
Routing
Congestion control
Quality of Service (QoS)
8.+15. Mar. 2006
22
INF-3190: Switching and Routing
Inside: Types of Switching

Circuit switching

Switching a physical connection
Physical copper
connection set up
when call is made
circuit switching

Packet switching

Store-and-forward, but transmissions packets limited in size
Packet queued up
for subsequent
transmission
packet switching

Message switching

Message is stored and passed one by one hop
8.+15. Mar. 2006
23
INF-3190: Switching and Routing
Circuit Switching

Connection exists physically for the duration of the
conversation

Refers to



Switching centers
Connections between switching centers
(frequency spectrum, dedicated ports)
Implementation examples



Historically: on switching boards
Mechanical positioning of the dialers
Setting coupling points in circuits
8.+15. Mar. 2006
24
INF-3190: Switching and Routing
Packet Switching

Data entity
Transport
…
Network
Packet
Data link
Frame
Physical
Bit/byte (bit stream)
Datagrams


Layer
Every packet chooses its path
Virtual circuits

Packets (or cells) over a pre-defined path
8.+15. Mar. 2006
25
INF-3190: Switching and Routing
Packet Switching



Packets of limited size
Dynamic route search (no connect phase)
No dedicated path from source to destination
Data to
Multiple
Users
Destination
Node F
Node D
Node B
Node E
Node A
Source
Node G
Data
From
Multiple
Users
Node C
: Packets
8.+15. Mar. 2006
26
INF-3190: Switching and Routing
Message Switching

All data to be sent are treated as a "message"

“Store and forward" network
1.
2.
3.
4.
Accept
Treat of possible errors
Store
Forward
8.+15. Mar. 2006
27
INF-3190: Switching and Routing
Comparison: Temporal Performance
Call request signal
Msg
Pkt 1
Propagation
delay
Pkt 2
Queueing
delay
Time
Msg
Time spent
hunting for
an outgoing
trunk
Pkt 3
Pkt 1
Pkt 2
Pkt 3
Pkt 1
Pkt 2
Pkt 3
Msg
Call accept
signal
Data
A
B
C
D
Circuit switching
8.+15. Mar. 2006
A
B
C
D
Message switching
28
A
B
C
D
Packet switching
Virtual circuit
INF-3190: Switching and Routing
Comparison: Properties

Circuit switching





Packet switching




Connection has to occur before transmission
Establishing a connection takes time
Resource allocation too rigid (possibly waste of resources)
Once connection is established it cannot be blocked anymore
Possibly only reservation of average bandwidth (static reservation)
Possibility of congestion
High utilization of resources
Message switching


High memory requirements at the node (switching centers)
Node may be used to its full capacity over a longer period of time
by one message
8.+15. Mar. 2006
29
INF-3190: Switching and Routing
Comparison: Circuit and Packet Switching

Circuit switching


Connection establishment
can take a long time
Bandwidth is reserved






No danger of congestion
Possibly poor bandwidth
utilization (burst traffic)
Continuous transmission
time

Packet switching



30
Danger of congestion
Optimized bandwidth
utilization
Varying transmission time

all data is transmitted
over the same path
8.+15. Mar. 2006
Connect phase not
absolutely necessary
Dynamic allocation of
bandwidth
packets between same
end systems may use
different paths
INF-3190: Switching and Routing
Virtual Circuits and Datagrams
8.+15. Mar. 2006
31
INF-3190: Switching and Routing
Virtual Circuits

Connection set-up phase




Select a path
Intermediate systems store path information
Network reserves all resources required for the connection
Data transfer phase


All packets follow the selected path
Packet contains VC’s number



Identification of connection, no address information
IS uses the stored path information to determine the
successor
Disconnect phase


Network forgets the path
Releases reserved resources
8.+15. Mar. 2006
32
INF-3190: Switching and Routing
Implementation Virtual Circuit
A
End systems ES allocate VCnumbers independently
Problem: the same VCidentifiers may be allocated
to different paths
1
2
C
1
IS
?
IS
1
B

1
1
1
D
Solution: allocate VC-numbers for virtual circuit segments

IS differentiates between incoming and outgoing VC-number



IS receives incoming VC-number when connect request arrives
IS creates outgoing VC-number
(unique between IS and successor(IS))
IS sends outgoing VC-number in connect request
8.+15. Mar. 2006
33
INF-3190: Switching and Routing
Implementation Virtual Circuit
H
B
H
B
A
IN OUT
H
H
B
H
H
H
0
1
0
2
3
4
B
E
E
B
E
E
A
H
H
A
H
F
0
0
1
1
2
0
C
C
A
F
F
H
0
1
0
0
1
0
0
1
0
1
D
D
H
D
0
1
0
2
Host
0
0
1
1
2
3
C
C
F
F
C
A
A
A
A
0
1
0
1
2
H
H
H
H
F
0
1
2
3
0
F
H
C
C
0
0
0
1
E
B
B
D
0
0
1
0
D
D
H
B
E
F
IMP
H
8 Simplex virtual circuits
Originating
Originating
at A
at B
F
0
1
2
3
H
D
H
D
E
C
A
C
B
B
E
E
H
0
1
0
0
0 - ABCD
0 - BCD
1 - AEFD
1 - BAE
2 - ABFD
2 - BF
3 - AEC
4 - AECDFB
8.+15. Mar. 2006
34
INF-3190: Switching and Routing
Implementation Datagram

Datagram passes through the network as an isolated
unit




Has complete source and destination addresses
Individual route selection for each datagram
Generally no resource reservation
Correct sequence not guaranteed
8.+15. Mar. 2006
35
INF-3190: Switching and Routing
Datagram vs. Virtual Circuit

Datagram: IS routing table
specifies possible path(s)




but




No connection setup delay
Less sensible to IS and link
failures
Route selection for each
datagram: quick reaction to
failures
Virtual Circuit: destination
address defined by connection




Each packet contains the full
destination and source address
Route selection for each
datagram: overhead
QoS guarantees hardly possible

but




8.+15. Mar. 2006
36
Packets contain short VCnumber only
Low overhead during transfer
phase
“Perfect" channel throughout
the net
Resource reservation: "Quality
of Service" guarantees possible
Overhead for connection setup
Memory for VC tables and state
information needed in every IS
Sensible to IS and link failures
Resource reservation:
potentially poor utilization
INF-3190: Switching and Routing
Services of the Network Layer
8.+15. Mar. 2006
37
INF-3190: Switching and Routing
Services of the Network Layer

Concepts


Connection oriented






Connection oriented vs. connectionless communication
Error free communication channel
Usually error control, flow control, ...
Usually duplex communication
More favorable for real-time communications
Favored by telephone and telecommunication companies
Connectionless





Unreliable communication
Hardly any error control: left to layer 4 or higher
Simplex communication
More favorable for simple data communication:
Favored by Internet community
8.+15. Mar. 2006
38
INF-3190: Switching and Routing
Connection Oriented Communication

Connection Oriented Communication

3-phase interaction








Connect
Data transfer
Disconnect
(allows for) Quality of Service Negotiation
(typically) Reliable Communication in both directions
Flow Control
Relatively complex protocols
Connection-Oriented Service

Service provider offers



Queues in both directions
Ordered transmission of objects
Delivery of objects at most once
8.+15. Mar. 2006
39
INF-3190: Switching and Routing
Connectless communication

Connectionless Communication


Network transmits packets as isolated Units (datagram)
Unreliable Communication




loss, duplication, modification, sequence errors possible
No flow control
Comparatively simple protocols
Connectionless Service

Service provider can



Delete objects in a queue
Duplicate objects in a queue
Change the object sequence within a queue
8.+15. Mar. 2006
40
INF-3190: Switching and Routing
Comparison of Concepts

Arguments pro a connection
oriented service





Simple, powerful paradigm
Simplification of the higher
layers
Relieves end systems
For some applications efficiency
in time is more important than
error-free transmission


Arguments pro a
connectionless service



e. g. real-time applications,
digital voice transmission)
suitable for a wide range of
applications

High flexibility and low
complexity
Costs for connects and
disconnects are high for
transaction oriented
applications
Easier to optimize the network
load
Compatibility and costs


‘End-to-End Arguments’

8.+15. Mar. 2006
41
IP common, can’t change now
secure communication
requires error control within
the application but error
control in one layer can
replace the error control in the
layer underneath it
INF-3190: Switching and Routing
Routing
8.+15. Mar. 2006
42
INF-3190: Switching and Routing
Routing: Foundations

Task

To define the route of packets through the network



Routing algorithm


From the source
To the destination system
Defines on which outgoing line an incoming packet will be
transmitted
Route determination

Datagram


Routing algorithm makes individual decision for each packet
Virtual circuit

Routing algorithm runs only during connect (session routing)
8.+15. Mar. 2006
43
INF-3190: Switching and Routing
Routing: Routing and Forwarding

Distinction can be made


Routing: makes decision which route to use
Forwarding: what happens when a packet arrives
Topology, link utilization, etc.
information
Router
Routing
Process
destination
A
Fills & Updates
Uses & Looks up
link
0
B
3
C
1
D
4
Routing
table
Data packets
Incoming
lines
8.+15. Mar. 2006
Forwarding
Process
44
Outgoing
lines
INF-3190: Switching and Routing
Good Properties for Routing Algorithms


Correctness
Simplicity


Robustness




Consistent results
No volatile adaptations to new conditions
Fairness


Compensation for IS and link failures
Handling of topology and traffic changes
Stability


Minimize load of routers
Among different sources compared to each other
Optimality
8.+15. Mar. 2006
45
INF-3190: Switching and Routing
Routing Algorithms: Conflicting Properties


Often conflicting: fairness and optimization
Some different
A
optimization criteria



Average packet delay
Total throughput
Individual delay




C
X’
X
Example:

A’
B’
C’
Communication among A  A’, B  B’, C  C’ uses full capacity of
horizontal line
Optimized throughput, but
No fairness for X and X’


Conflict
B
Tradeoff between fairness and optimization
Therefore often

Hop minimization per packet


It tends to reduce delays and decreases required bandwidth
Also tends to increase throughput
8.+15. Mar. 2006
46
INF-3190: Switching and Routing
Classes of Routing Algorithms

Class Non-adaptive Algorithms

Current network state not taken into consideration




With knowledge of the overall topology



Spanning tree
Flow-based routing
Without knowledge of the overall topology


Assume average values
All routes are defined off-line before the network is put into operation
No change during operation (static routing)
Flooding
Class Adaptive Algorithms

Decisions are based on current network state


Measurements / estimates of the topology and the traffic volume
Further sub-classification into



Centralized algorithms
Isolated algorithms
Distributed algorithms
8.+15. Mar. 2006
47
INF-3190: Switching and Routing
Optimality Principle and Sink Tree

General statement about optimal routes



Example:




If router J is on optimal path from router I to router K
Then the optimal path from router J to router K uses the same
route
r1: route from I to J
r1
r2: route from J to K
I
If better route r2’ from J to K
would exist
 Then
 Concatenation of r1 and r2’
would improve route from I to K
r1
J
K
r2’
Set of optimal routes
From all sources

To a given destination
form a tree rooted at the destination: Sink Tree

8.+15. Mar. 2006
48
INF-3190: Switching and Routing
Sink Tree
B
C
A
Subnet
D
E
G
F
J
I
K
H
L
Comments

Tree: no loops


Other trees with same path lengths may exist
Goal of all routing algorithms


Each optimal route is finite with bounded number of hops
Not necessarily unique


N
O
M

Sink Tree for
Destination B
Discover and use the sink trees for all routers
Not realistic to use Sink Trees as real-life routing algorithm


Need complete information about topology
Sink Tree is only a benchmark for routing algorithms
8.+15. Mar. 2006
49
INF-3190: Switching and Routing
Methodology & Metrics

Networks represented as graphs:


Node represents a router
Edge represents a communication line (link)

Compute the shortest path between a given pair of routers

Different metrics for path lengths can be used



Can lead to different results
Sometime even combined (but this leads to computational problems)
Metrics for the "ideal" route, e.g., a "short" route







Number of hops
Geographical distance
Bandwidth
Average data volume
Cost of communication
Delay in queues
...
8.+15. Mar. 2006
50
INF-3190: Switching and Routing
Non-Adaptive Routing
Shortest Path Routing
8.+15. Mar. 2006
51
INF-3190: Switching and Routing
Non-Adaptive Shortest Path Routing

Static Procedure


Network operator generates tables
Tables



Are loaded when IS operation is initiated and
Will not be changed any more
Characteristics



Simple
Good results with relatively consistent topology and
traffic
But

Poor performance if traffic volume or topologies change
over time
8.+15. Mar. 2006
52
INF-3190: Switching and Routing
Non-Adaptive Shortest Path Routing

Spanning Tree and Optimized Route

Information about the entire network has to be available


i. e. application actually as a benchmark comparison
Example



Link is labeled with distance / weight
Node is labeled with distance from source node along best
known path (in parentheses)
Find the shortest path from A to D
B (●,-)
2
2
A
E (●,-)
1
6
G (●,-)
C (●,-)
7
3
3
D (●,-)
2 F (●,-)
2
4
2
H (●,-)
2
8.+15. Mar. 2006
53
INF-3190: Switching and Routing
Non-Adaptive Shortest Path Routing

Procedure: e. g. according to Dijkstra

Find the shortest path from A to D


Labels may be permanent or tentative
Initially, no paths are known



1.
2.
3.
4.
All nodes are labeled with infinity (tentative)
Discover the labels that represent shortest possible path from
source to any node
Make those labels permanent
Node A labeled as permanent (filled-in circle)
Relabel all directly adjacent nodes with the distance to A
(path length, nodes adjacent to source)
Examine all tentatively labeled nodes, make the node with the
smallest label permanent
This node will be the new working node for the iterative
procedure
(i.e., continue with step 2.)
8.+15. Mar. 2006
54
INF-3190: Switching and Routing
Non-Adaptive Shortest Path Routing
B (2,A)
(●,-)
2
2
A
E (●,-)
1
6
G (6,A)
(●,-)


C (●,-)
7
3
3
D (●,-)
2 F (●,-)
2
4
2
H (●,-)
Procedure: e. g. according to Dijkstra
Find the shortest path from A to D:
1.
2.
A flagged as permanent (filled-in circle)
Relabel all directly adjacent nodes with the distance to A

8.+15. Mar. 2006
(path length, IS adjacent to the source):
55
INF-3190: Switching and Routing
Non-Adaptive Shortest Path Routing
B (2,A)
2
2
A
E (●,-)
1
6
G (6,A)


C (●,-)
7
3
3
D (●,-)
2 F (●,-)
2
4
2
H (●,-)
Procedure: e. g. according Dijkstra
Find the shortest path from A to D:

3.
...
Compare all recent, not firmly flagged IS;

4.
flag the one with the lowest number as fixed
This IS is the origin of the iterative procedure

8.+15. Mar. 2006
(i. e. continue with item 2.)
56
INF-3190: Switching and Routing
Non-Adaptive Shortest Path Routing
B (2,A)
2
2
A
E (●,-)
(4,B)
1
6
G (6,A)


C (9,B)
(●,-)
7
3
3
D (●,-)
2 F (●,-)
2
4
2
H (●,-)
Procedure: e.g., according to Dijkstra
Find the shortest path from A to D:
1.
2.
Node B has been labeled as permanent (filled-in circle)
relabel all directly adjacent nodes with the distance to B
(path length, nodes adjacent to source):

8.+15. Mar. 2006
A (does not apply, because it is the origin),
57
INF-3190: Switching and Routing
Non-Adaptive Shortest Path Routing
B (2,A)
2
2
A
E (4,B)
1
6
G (5,E)
(6,A)


C (9,B)
7
3
3
D (10,H)
(●,-)
2 F (●,-)
(6,E)
2
4
2
H (8,F)
(●,-)
(9,G)
Procedure: e.g., according to Dijkstra
find the shortest path from A to D:
1.
2.
3.
…
…
examine all tentatively labeled nodes;

4.
make the node with the smallest label permanent
this node will be the new working node for the iterative
procedure ...
8.+15. Mar. 2006
58
INF-3190: Switching and Routing
Non-Adaptive Routing
Flooding
8.+15. Mar. 2006
59
INF-3190: Switching and Routing
Flooding

Principle

IS transmits the received packet to all adjacent IS


But generates "an infinite amount" of packets
Flood limitations

Hop counter in the packet header



Initialize to destination’s distance, or subnet diameter is
unknown
Decrement per hop, discard packet at 0
Keeping history of transferred packets in ISs,
delete copies



Source router inserts sequence number into each packet
ISs keeps per-router history of sequence numbers
Old packets are dropped
8.+15. Mar. 2006
60
INF-3190: Switching and Routing
Selective Flooding

Approach





Do not send out on every line
IS transmits received packet to adjacent stations,
located in the Direction of the Destination
With ’regular’ topologies this makes sense and is
an optimization
But some topologies do not fit well to this
approach
Comment

Geographically-oriented routing got recent interest
for mobile scenarios
8.+15. Mar. 2006
61
INF-3190: Switching and Routing
Flooding

Evaluation and use





In most scenarios impractical because of
overhead
Extremely robust
Reaches all ISs
Does not need topology information, no
bootstrap
Always finds the shortest path
8.+15. Mar. 2006
62
INF-3190: Switching and Routing
Adaptive Routing
8.+15. Mar. 2006
63
INF-3190: Switching and Routing
Adaptive Routing

Class Adaptive Algorithms

Decisions are based on current network
state


Measurements / estimates of the topology and
the traffic volume
Further sub-classification into



Centralized algorithms
Isolated algorithms
Distributed algorithms
8.+15. Mar. 2006
64
INF-3190: Switching and Routing
Adaptive Routing
Centralized Routing
8.+15. Mar. 2006
65
INF-3190: Switching and Routing
Adaptive Centralized Routing

Principle


One routing control center (RCC) exists in the
network
Each IS sends periodically status updates to the
RCC





Available neighbors
Current queue length
Line utilization
…
RCC




Collects information
Computes the optimal path each IS pair
Forms routing tables
Distributes tables to ISs
8.+15. Mar. 2006
66
INF-3190: Switching and Routing
Adaptive Centralized Routing

Characteristics



RCC has complete information
IS is free of routing calculations
But





Re-calculations quite often
necessary (approx. once/min or
more often)
Low robustness
No correct decisions if network
is partitioned
ISs receive tables at different
times
Traffic concentration in RCC
proximity
8.+15. Mar. 2006
67
RCC
INF-3190: Switching and Routing
Adaptive Routing
Isolated Routing
8.+15. Mar. 2006
68
INF-3190: Switching and Routing
Adaptive Isolated Routing
through Backward Learning

Isolated routing

Every IS makes decision based on locally
gathered information only



No exchange of routing information among
nodes
Only limited adaptation possibility to changed
traffic or topology
IS ‘learns’ from received packets


Source IS
Distance estimate by hop count
8.+15. Mar. 2006
69
INF-3190: Switching and Routing
Adaptive Isolated Routing
through Backward Learning

Packet




Routing table in IS



From source S
received on line L
after C hops
 S is reachable on L within C hops
L - table (destination - IS, outgoing line, Cmin)
Update of the routing table
IS receives packet ( ..., S, C, ... ) on L
if not (S in L-Table)
then Add(S,L,C)
else if C < Cmin
then Update(S,L,C)
8.+15. Mar. 2006
70
INF-3190: Switching and Routing
Adaptive Isolated Routing
through Backward Learning

Example: D learns about A

packet ( ..., source - IS, section counter,
...)


P1 ( ..., A, 4, ... )  Add ( A, l1, 4 )
P2 ( ..., A, 3, ... )  Update ( A, l2, 3 )
?
?
I1
A
8.+15. Mar. 2006
?
?
71
I2
D
INF-3190: Switching and Routing
Adaptive Isolated Routing
through Backward Learning

Problem


Packets use a different route, e. g. because of
failures, high load
Algorithm retains only the old value (because it
was "better"),


i. e. algorithm does not react to deteriorations
Solution


Periodic deletion of routing tables
(new learning period)
Table deletion


Too often: mainly during the learning phase
Not often enough: reaction to deteriorations too slow
8.+15. Mar. 2006
72
INF-3190: Switching and Routing
Adaptive Routing
Distributed Routing
Distance Vector
8.+15. Mar. 2006
73
INF-3190: Switching and Routing
Distance Vector Routing

Distance-Vector Routing


Group of Distance Vector Routing Algorithms
Also known as


Use



Distributed Bellman-Ford algorithm, Ford-Fulkerson algorithm
Was the original ARPANET routing algorithm
Has been used in the Internet as RIP (Routing Information
Protocol)
Basic principle

IS maintains table (i.e., vector) stating



Best known distance to destinations
And line to be used
ISs update tables

By exchanging routing information with their neighbors
8.+15. Mar. 2006
74
INF-3190: Switching and Routing
Distance Vector Routing

Each IS


maintains routing table with one entry per router in the subnet
is assumed to know the ‘distances’ to each neighbor

IS sends list with estimated distances to each destination periodically to
its neighbors

X receives list E(Z) from neighbor Y




Distance X to Y:
e
Distance Y to Z:
E(Z)
Distance X to Z via Y: E(Z)+e
IS computes new routing table from the received lists
containing



Destination IS
Preferred outgoing path
Distance
8.+15. Mar. 2006
75
INF-3190: Switching and Routing
Distance Vector Routing
A
B
C
D
G
E
H
F
I
J
K
L
A
B
C
D
E
F
G
H
I
J
K
L
delay

A
0
12
25
40
14
23
18
17
21
9
24
29
I
24
36
18
27
7
20
31
20
0
11
22
33
H
20
31
19
8
30
19
6
0
14
7
22
9
K
21
28
36
24
22
40
31
19
22
10
0
9
JA
8
JI
10
JH
12
JK
6
8
20
28
20
17
30
18
12
10
0
6
15
line
A
A
I
H
I
I
H
H
I
K
K
Previous routing table will not be taken into account

Reaction to deteriorations
8.+15. Mar. 2006
76
INF-3190: Switching and Routing
Distance Vector Routing



Fast route improvement
Fast distribution of information about new short
paths (with few hops)
Example




initially A unknown
later: A connected with distance
1 to B, this will be announced
Distribution proportional to
topological spread
Synchronous (stepwise) update is
a simplification
8.+15. Mar. 2006
77
A
B
C
D
E
∞
∞
∞
∞
1
∞
∞
∞
1
2
∞
∞
1
2
3
∞
1
2
3
4
INF-3190: Switching and Routing
Distance Vector Routing



Example: deterioration




Slow distribution of information about new long paths
(with many hops)
“Count to Infinity” problem of DVR
Here: connection destroyed
A previously known, but now
detached
The values are derived from
(incorrect) connections of distant IS
Comment

Limit "infinite" to a finite value,
depending on the metrics, e.g.

‘infinite’ = maximum path length+1
8.+15. Mar. 2006
78
A
B
C
D
E
1
2
3
4
3
2
3
4
3
4
3
4
5
4
5
6
5
6
5
6
7
6
7
6
7
8
7
8
∞
∞
∞
∞
INF-3190: Switching and Routing
Distance Vector Routing



Variant: ‘Split Horizon Algorithm’
Objective: improve the "count to infinity" problem
Principle


In general, to publicize the "distance" to each neighbour
If neighbor Y exists on the reported route, X reports the response
"false" to Y


Example: deterioration (connection
destroyed)


distance X (via Y) according to arbitrary i:  A
B to C: A =  (real),
C to B: A =  (because A is on path), ...
But: still poor, depending on topology,
example



Connection CD is removed
A receives "false information" via B
B receives "false information" via A

B
C
D
E
1
2
3
4
∞
2
3
4
∞
∞
3
4
∞
∞
∞
4
∞
∞
∞
∞
A
Slow distribution (just as before)
C
D
B
8.+15. Mar. 2006
79
INF-3190: Switching and Routing
Adaptive Routing
Distributed Routing
Link State Routing
8.+15. Mar. 2006
80
INF-3190: Switching and Routing
Link State Routing

Basic principle




Procedure
1.
2.
3.
4.
5.

IS measures the "distance" to the directly adjacent IS
Distributes information
Calculates the ideal route
Use


Determine the address of adjacent IS
Measure the "distance" (delay, ...) to neighbouring IS
Organize the local link state information in a packet
Distribute the information to all IS
Calculate the route based on the information of all IS
Introduced into the ARPANET in 1979, nowadays most prevalent
IS-IS (Intermediate System-Intermediate System)




developed by DECNET
also used as ISO CLNP in NSFNET
Novell Netware developed its own variation from this (NLSP)
OSPF (Open Shortest Path First)

8.+15. Mar. 2006
since 1990 Internet RFC 1247
81
INF-3190: Switching and Routing
Link State Routing
1. Phase: gather information about the adjacent
intermediate systems
H
H
D
E
B
A
C
G
D
B
I
A
F
G
I
C
F
LAN
8.+15. Mar. 2006
E
82
INF-3190: Switching and Routing
Link State Routing
1. Phase: gather information about the adjacent
intermediate systems
H
H
D
E
B
A
C
G
D
E
B
I
A
F
LAN
G
I
F
C
N
Initialization procedure

New IS


Sends a HELLO message over each L2 channel
Adjacent IS

Responds with its own address, unique within the network
8.+15. Mar. 2006
83
INF-3190: Switching and Routing
Link State Routing
2. Phase: measure the "distance"

Definition of distance needed


Usually delay
Where to measure?
Topology, link utilization, etc.
information
Router
Routing
Process
Fills & Updates
Uses & Looks up
destination
A
lines
8.+15. Mar. 2006
0
B
3
C
1
D
4
Data packets
ECHO
Incoming
link
Routing
table
Queues
Forwarding
Process
When to start timer?
HELLO
Outgoing
lines
84
INF-3190: Switching and Routing
Link State Routing
2. Phase: measure the "distance“

Queuing delay


Measuring without does not take load into account
Measuring with does  usually better
West
B
East
C
G
F
H
A
E
I
D

But


J
Possibility for oscillations (route flapping)
Once per routing table update
8.+15. Mar. 2006
85
INF-3190: Switching and Routing
Link State Routing
3. Phase: organizing the information as link state packet

Including own address, sequence number, age, "distance"

Timing problems: validity and time of sending


B
Periodically
In case of major changes
2
Link State Packets:
C
4
3
A
1
5
E
D
6
8
7
A
Seq.
Age
B 4
E 5
F
8.+15. Mar. 2006
86
B
Seq.
Age
A 4
C 2
F 6
C
Seq.
Age
B 2
D 3
E 1
D
Seq.
Age
C 3
F 7
E
Seq.
Age
A 5
C 1
F 8
F
Seq.
Age
B 6
D 7
E 8
INF-3190: Switching and Routing
Link State Routing
4. Distributing the local information to all IS

By applying the flooding procedure (very robust)


Therefore sequence number in packets
Problem: inconsistency


Varying states simultaneously available in the network
Indicate and limit the age of packet,
i. e. IS removes packets that are too old
5. Computing new routes
 Each IS for itself
 Possibly larger amount of data available
8.+15. Mar. 2006
87
INF-3190: Switching and Routing
Adaptive Routing
Distributed Routing
Distance Vector
8.+15. Mar. 2006
88
INF-3190: Switching and Routing
Distance Vector Routing




Maintains own routing table
knows distances to every neighbor
sends its estimated distances to the
neighbors periodically
A
B
C
D
E
F
G
H
I
J
K
L
Add estimated distance to Y and Y’s
estimated distance to Y
Node computes new routing table
from the received lists
containing



Destination node
Preferred outgoing path
Distance
delay
8.+15. Mar. 2006
89
C
D
G
H
F
I
Node X estimates distance to node
Z through neighbor Y

B
E
Each node


A
J
A
0
12
25
40
14
23
18
17
21
9
24
29
I
24
36
18
27
7
20
31
20
0
11
22
33
H
20
31
19
8
30
19
6
0
14
7
22
9
K
21
28
36
24
22
40
31
19
22
10
0
9
JA
8
JI
10
JH
12
JK
6
K
8
20
28
20
17
30
18
12
10
0
6
15
L
line
A
A
I
H
I
I
H
H
I
K
K
INF-3190: Switching and Routing
Distance Vector Routing



Fast route improvement
Fast distribution of information about new short paths (with
few hops)
Example




initially A unknown
later: A connected with distance
1 to B, this will be announced
Distribution proportional to
topological spread
Synchronous (stepwise) update is
a simplification
8.+15. Mar. 2006
90
A
B
C
D
E
∞
∞
∞
∞
1
∞
∞
∞
1
2
∞
∞
1
2
3
∞
1
2
3
4
INF-3190: Switching and Routing
Distance Vector Routing



Slow distribution of information about new long paths
(with many hops)
“Count to Infinity” problem of DVR
Deterioration




A
Connection destroyed
A previously known
New estimates derived from old
estimates of distant nodes
Comment

Limit "infinite" to a finite value
8.+15. Mar. 2006
91
B
C
D
E
1
2
3
4
3
2
3
4
3
4
3
4
5
4
5
6
5
6
5
6
7
6
7
6
7
8
7
8
∞
∞
∞
∞
INF-3190: Switching and Routing
Distance Vector Routing

Split Horizon Algorithm




Deterioration


Objective to improve the "count to infinity" problem
In general, as before
Report  to best route neighbor (“poisoned reverse”)
A
B to C: A =  (real),
C to B: A =  (because A is on
path), ...
Still poor, depending on topology



Connection CD is removed
A receives "false information" via B
B receives "false information" via A
92
C
D
E
1
2
3
4
∞
2
3
4
∞
∞
3
4
∞
∞
∞
4
∞
∞
∞
∞
A
C
8.+15. Mar. 2006
B
D
B
INF-3190: Switching and Routing
Adaptive Routing
Distributed Routing
Link State Routing
8.+15. Mar. 2006
93
INF-3190: Switching and Routing
Link State Routing

Node measures the "distance" to the directly
adjacent node
Distributes information
Calculates the ideal route

Procedure


1.
2.
3.
4.
5.
Determine the address of adjacent node
Measure the "distance" (delay, ...) to neighboring node
Organize the local link state information in a packet
Distribute the information to all nodes
Calculate the route based on the information of all nods
8.+15. Mar. 2006
94
INF-3190: Switching and Routing
Link State Routing
1.
Gather information about the adjacent nodes

New node


Adjacent node

2.
Sends a HELLO message over each L2 channel
Responds with its own address, unique within the network
Measure the distance

Definition of distance needed


8.+15. Mar. 2006
Often delay
Two approaches to measuring the delay
95
INF-3190: Switching and Routing
Link State Routing
2. Phase: measure the "distance"
Topology, link utilization, etc.
information
Router
Routing
Process
Fills & Updates
Uses & Looks up
destination
A
0
B
3
C
1
D
4
Data packets
Routing
table
Queues
ECHO
Incoming
Forwarding
Process
lines

link
When to start timer?
HELLO
Outgoing
lines
Queuing delay


Measuring without does not take load into account
Measuring with does


often better performance
possibility for oscillations (route flapping)
once per routing table update
8.+15. Mar. 2006
96
INF-3190: Switching and Routing
Link State Routing
3. Organizing the information as a link state packet



Including own address, sequence number, age, "distance“
Send periodically and in case of major changes
Timing problems: validity and time of sending
4. Distributing the local information to all nodes


By applying the flooding procedure
Problem: inconsistency


Varying states simultaneously available in the network
Indicate and limit the age of packet,
i. e. node removes packets that are too old
5. Computing new routes


Each node for itself
Possibly larger amount of data available
8.+15. Mar. 2006
97
INF-3190: Switching and Routing
Other routing approaches
Snapshot

DVR and LSR are basic routing algorithms




Unicast communication
No node mobility
Flat structure
Why is that insufficient?

Increasing number of applications
have use for





Multi-homing
Multicast
Anycast
Courses that go deeper into routing
• INF5050
• INF5070
• INF5090
More and more mobile nodes
Too many nodes
• UNIK4200
• UNIK4290
Continuation

Multicast routing

Routing in mobile and ad hoc networks

The Internet protocols (IPv4 and IPv6) and routing in the Internet
8.+15. Mar. 2006
98
INF-3190: Switching and Routing
Broadcast Routing
Controlled Flooding
8.+15. Mar. 2006
99
INF-3190: Switching and Routing
Reverse Path Forwarding

When a multicast packet arrives at a node



from origin S
on an interface I
Test whether it would send unicast packets to S via I

Yes



No



Deliver multicast packet to all connected end systems
Forward multicast packet on all interfaces to other routers
except I
It does not arrive on the shortest path from S
Assume it’s a duplicate
Drop packet
8.+15. Mar. 2006
100
INF-3190: Switching and Routing
Multicast Routing
Shared Tree
Core-Based Tree
8.+15. Mar. 2006
101
INF-3190: Switching and Routing
Core-Based Tree
Core node
Non-Core node


Also known as "Trees with Rendezvous Points“
Principle





Select a core node (a node which is central to the group)
Determine spanning tree of the core node
Sender transmits a packet to core node
Core node transmits packet via the spanning tree
Properties
+
+
-
Simple central calculation
One tree common to all n senders (instead of n trees)
Route to the core node may not be optimized
8.+15. Mar. 2006
102
INF-3190: Switching and Routing
Multicast Routing
Source-based Tree
8.+15. Mar. 2006
103
INF-3190: Switching and Routing
Spanning Tree
1
2
Multicast source node
1
2
1
2
2
1
Spanning tree for group 1
2 Spanning tree for group 2
1
Spanning tree for source node

Principle



Every node maintains a spanning tree to all other nodes in the
network: their spanning tree
A subset of all nodes is a multicast group
Node does initially not know about groups of receivers


Distribution of this information depends on the underlying routing
protocol
Prune the spanning tree to include only group nodes
8.+15. Mar. 2006
104
INF-3190: Switching and Routing
Multicast Routing
Spanning Tree
with Link State Routing
8.+15. Mar. 2006
105
INF-3190: Switching and Routing
Spanning Tree with LSR

All nodes send link state packets periodically




Distance to neighbors

End-systems connected to a node and belonging to a group
Broadcast to all other nodes
Each node calculates a multicast tree



Containing information
From the current locally available and complete state
information
Decide locally whether neighboring node needs a packet
Based on the information about the multicast tree

Node determines the outgoing lines
on which packets have to be transmitted
8.+15. Mar. 2006
106
INF-3190: Switching and Routing
Multicast Routing
Reverse Path Forwarding
with Pruning
8.+15. Mar. 2006
107
INF-3190: Switching and Routing
Reverse Path Forwarding with Pruning



Used together with DVR as unicast routing protocol
Use DVR routing tables to determine whether packet
arrives on shortest path from direct neighbor
Principle



Sender sends first multicast packet to everybody
Use Reverse Path Forwarding
Then prune the tree
8.+15. Mar. 2006
108
INF-3190: Switching and Routing
Reverse Path Forwarding with Pruning

Pruning


When a multicast packet arrives from S on interface I
If



Then



To find end-systems that have joined
Benefit



Send a Non-Membership-Report to the previous node that forwarded
the packet
Do not forward messages for the group any more
Flooding and pruning must be repeated after some time


No directly connected end system is registered, or
Non-Membership-Reports are received from all nodes reachable via
interfaces other than I
Pruning only trees that are actually used
Unused trees are cut coarsely
Optimized for many receivers
8.+15. Mar. 2006
109
INF-3190: Switching and Routing
Summary: Multicast Routing


Save network resources: send same data to many
hosts with fewer packets
Pay with router resources: Routers need to know
group members

All algorithms aim at maintaining a spanning tree
from the sending node
Can rely on unicast routing or on broadcast

A bit more multicast in conjunction with IP multicast

8.+15. Mar. 2006
110
INF-3190: Switching and Routing
Ad hoc Routing
8.+15. Mar. 2006
111
INF-3190: Switching and Routing
Ad hoc Routing Protocols

Outside the Internet world
Infrastructure-less networks, often with mobile nodes

Reactive protocols




Determine route if and when needed
Source initiates route discovery
Proactive protocols




Traditional distributed shortest-path protocols
Maintain routes between every host pair at all times
Based on periodic updates
High routing overhead
8.+15. Mar. 2006
112
INF-3190: Switching and Routing
Ad hoc Routing
Reactive Routing
8.+15. Mar. 2006
113
INF-3190: Switching and Routing
Ad Hoc On-Demand Distance Vector
Routing (AODV)

AODV assumes symmetric (bi-directional) links
Every node maintains a routing table

When source S wants to send to D, but route to D is not known




When a node re-broadcasts a RREQ



Adds the reverse path to its routing table
Local routing table grows
When D receives RREQ


S initiates a route discovery
S floods Route Request (RREQ)
D replies with Route Reply (RREP)
RREP travels along the reverse path

Using routing table
8.+15. Mar. 2006
114
INF-3190: Switching and Routing
Route Requests in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents a node that has received RREQ for D from S
8.+15. Mar. 2006
115
INF-3190: Switching and Routing
Route Requests in AODV
Y
Broadcast transmission
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents transmission of RREQ
8.+15. Mar. 2006
116
INF-3190: Switching and Routing
Route Requests in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
Represents links on Reverse Path
8.+15. Mar. 2006
117
INF-3190: Switching and Routing
Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
8.+15. Mar. 2006
118
INF-3190: Switching and Routing
Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
J
A
L
G
H
K
I
8.+15. Mar. 2006
M
D
N
119
INF-3190: Switching and Routing
Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
• Node D does not forward RREQ, because node D
is the intended target of the RREQ
8.+15. Mar. 2006
120
INF-3190: Switching and Routing
Forward Path Setup in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
Forward links are setup when RREP travels along
the reverse path
8.+15. Mar. 2006
Represents a link on the forward path
121
INF-3190: Switching and Routing
AODV



Only last node must be included in RREQ packets
Nodes maintain routing tables containing entries only for routes
that are in active use
At most one next-hop per destination maintained at each node

Sequence numbers are used to avoid old/broken routes
Sequence numbers prevent formation of routing loops

Unused routes expire even if topology does not change

8.+15. Mar. 2006
122
INF-3190: Switching and Routing
Ad hoc Routing
Proactive Routing
8.+15. Mar. 2006
123
INF-3190: Switching and Routing
Destination-Sequenced Distance
Vector Routing (DSDV)

Each node maintains a routing table which stores




Each node periodically forwards routing table to neighbors



Overwrites the old table when the sequence number is higher
When a node decides that a route is broken



Increases sequence number each time
Includes it in routing table update
A node that receives a routing table for a destination


Next hop for each destination
Cost metric towards each destination
A sequence number that is created by the destination itself
It increments the sequence number of the route
Advertises it with infinite metric
Destination advertises new sequence number
8.+15. Mar. 2006
124
INF-3190: Switching and Routing
Reactive vs. Proactive Routing

Reactive protocols





Proactive protocols





Lower overhead since routes are determined on demand
Significant delay in route determination
Employ flooding (global search)
Control traffic may be bursty
Always maintain routes
Little or no delay for route determination
Consume bandwidth to keep routes up-to-date
Maintain routes which may never be used
Which approach achieves a better trade-off depends
on the traffic and mobility patterns
8.+15. Mar. 2006
125
INF-3190: Switching and Routing