INF-3190: Switching and Routing

1
Switching and Routing

• Foreleser Carsten Griwodz
• Email griff_at_ifi.uio.no

2
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
wireless network
Max 500m
3
Connecting Networks by Relays
End system
End system
Intermediate system
5
5
Application layer
Gateway
4
4
Transport layer
3
3
Network layer
Router
2
2
Data link layer
Bridge
1
1
Physical layer
Repeater
4
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 2 Bridge / Switch
• relays frames between LANs (MAC level)
• minor frame modifications, increases the number
  of stations

• Layer 3 Router (or Layer 3 Gateway)
• 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

5
Layer 1 Physical Layer
6
Repeater and Hub
Hub ex. IEEE 802.3 Twisted Pair
Repeater ex. IEEE 802.3 Thinwire

• Function
• To amplify the electrical signals
• To increase the range
• Limitation
• Extends the broadcast medium every bit is copied
• One collision domain

7
Layer 2 Data Link Layer
8
Bridges, Switches

• Bridges
• Connects two or more LANs (potentially different
  types)
• Each line is its own collision domain
• Traditionally store-and-forward and CPU-based
• Switches
• Typically connects two or more computers
• Each port / line is its own collision domain (no
  collisions)
• Typically cut-through switching devices
• begin forwarding as soon as possible
• when destination header has been detected, before
  rest of frame arrived
• Hardware-based

• Bridges vs. Switches
• Sometimes difference seems to be more a marketing
  issue than technical one

9
Bridge

• Tasks
• Coupling of different LANs
• Scalability of networks
• To increase capacity
• To increase reliability
• To improve security
• To cover large distances
• To offer independence from protocols
• 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

10
Bridge Connecting 2 Different Networks
Example Bridge between IEEE 802.3 (CSMA/CD)
and IEEE 802.4 (Token Bus)

• Approach
• 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

11
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

12
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
• If bridge is out of memory, frames are dropped
• 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
• 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

13
802.x ? 802.y Bridging

• Different checksum calculations
• Means conversion, delay, buffering
• Security
• 802.11 provides some data link layer encryption
• 802.3 does not
• Quality of Service / Priorities
• 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 (CA bits)
• Not supported by 802.3

14
SelfLearning Bridges

• Also called Transparent Bridges
• Transparency
• Bridges not visible for the other components of
  the network? simplifies other components

• Transparent bridge
• Bridge works in promiscuous mode(receives every
  frame of each connected LAN)
• Bridge manages table station ? LAN(output line)

• Decision procedure
• Destination unknown flooding
• Source and destination LANs identical frame
  dropped
• Source and destination LANs differ frame
  rerouted to destination LAN

15
SelfLearning Bridges

• Learning procedure
• Bridge table initially empty
• Use flooding for unknown destination
• Backward learning
• Bridge works in promiscuous mode
• Receives any frame on any of its LANs
• Bridge receives frames from source address Q on
  LAN L
• 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

16
SelfLearning 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

• Solution
• Communication among bridges
• Overlay actual topology by spanning tree reaching
  every LAN
• Exactly one path from any LAN to every other LAN

17
SelfLearning Bridges Spanning Tree

• Algorithm
• Choose a bridge as root of tree
• All bridges broadcast their serial number, lowest
  wins
• Generation of spanning tree
• Configured with bridges representing the edges
  within the tree
• Thereby avoiding loops
• Adaptation if configuration is changed

• Drawback
• Ignores some potential connections between
  LANsi.e., not all bridges are necessarily
  present in the tree

18
Source Routing Bridges

• Alternative to self-learning bridges
• Principle
• The frames 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

19
Connecting Equal Networks Encapsulation
Example remote bridge

• Principle
• Incoming data unit is packaged as payload,
• Transmitted and
• Then fed into the destination network
• Properties
• Certain protocol on connecting route
• e.g. PPPi.e. MAC frames encapsulated in PPP
• Only bridge at the destination network can be
  reached
• Simple

20
Layer 3 Network Layer
21
Network Layer

• Goal
• Enable data transfer fromend system to end
  system
• Several hops, (heterogeneous) subnetworks
• Compensate for differences between end systems
  during transmission
• The provided services are
• Standardized for end systems

• Independent from network technology
• Independent from number, type and topology of the
  subnetworks
• Subnetworks (ISO definition)
• 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

22
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)

23
Inside Types of Switching

• Circuit switching
• Switching a physical connection

• Packet switching
• Store-and-forward, but transmissions packets
  limited in size

• Message switching
• Message is stored and passed one by one hop

24
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

25
Packet Switching
Layer Data entity
Transport
Network Packet
Data link Frame
Physical Bit/byte (bit stream)

• Datagrams
• Every packet chooses its path
• Virtual circuits
• Packets (or cells) over a pre-defined path

26
Packet Switching

• Packets of limited size
• Dynamic route search (no connect phase)
• No dedicated path from source to destination

27
Message Switching

• All data to be sent are treated as a "message"
• Store and forward" network
• Accept
• Treat of possible errors
• Store
• Forward

28
Comparison Temporal Performance
Circuit switching
Message switching
Packet switching Virtual circuit
29
Comparison Properties

• Circuit 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
• Packet switching
• 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

30
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
• all data is transmitted over the same path

• Packet switching
• Connect phase not absolutely necessary
• Dynamic allocation of bandwidth
• Danger of congestion
• Optimized bandwidth utilization
• Varying transmission time
• packets between same end systems may use
  different paths

31
Virtual Circuits and Datagrams
32
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 VCs 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

33
Implementation Virtual Circuit
End systems ES allocate VC-numbers
independently Problem the same VC-identifiers
may be allocated to different paths

• 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

34
Implementation Virtual Circuit
B
C
A
IN
OUT
D
8 Simplex virtual circuits
Originating at A Originating at B





E
F
0 - ABCD
0 - BCD
1 - AEFD
1 - BAE
2 - ABFD
2 - BF
3 - AEC
4 - AECDFB
35
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

36
Datagram vs. Virtual Circuit

• Datagram IS routing table specifies possible
  path(s)
• No connection setup delay
• Less sensible to IS and link failures
• Route selection for each datagram quick reaction
  to failures
• but
• Each packet contains the full destination and
  source address
• Route selection for each datagram overhead
• QoS guarantees hardly possible

• Virtual Circuit destination address defined by
  connection
• Packets contain short VC-number only
• Low overhead during transfer phase
• Perfect" channel throughout the net
• Resource reservation "Quality of Service"
  guarantees possible
• but
• 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

37
Services of the Network Layer
38
Services of the Network Layer

• Concepts
• Connection oriented vs. connectionless
  communication
• Connection oriented
• 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

39
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

40
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

41
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
• e. g. real-time applications, digital voice
  transmission)
• suitable for a wide range of applications

• Arguments pro a connectionless service
• 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
• IP common, cant change now
• End-to-End Arguments
• secure communication requires error control
  within the application but error control in one
  layer can replace the error control in the layer
  underneath it

42
Routing
43
Routing Foundations

• Task
• To define the route of packets through the
  network
• From the source
• To the destination system
• Routing algorithm
• 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)

44
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
Routing table
Fills Updates
Uses Looks up
Forwarding Process
45
Good Properties for Routing Algorithms

• Correctness
• Simplicity
• Minimize load of routers
• Robustness
• Compensation for IS and link failures
• Handling of topology and traffic changes
• Stability
• Consistent results
• No volatile adaptations to new conditions
• Fairness
• Among different sources compared to each other
• Optimality

46
Routing Algorithms Conflicting Properties

• Often conflicting fairness and optimization

• Some different optimization criteria
• Average packet delay
• Total throughput
• Individual delay
• Conflict

• Example
• Communication among A ? A, B ? B, C ? C uses
  full capacity of horizontal line
• Optimized throughput, but
• No fairness for X and X
• 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

47
Classes of Routing Algorithms

• Class Non-adaptive Algorithms
• Current network state not taken into
  consideration
• Assume average values
• All routes are defined off-line before the
  network is put into operation
• No change during operation (static routing)
• With knowledge of the overall topology
• Spanning tree
• Flow-based routing
• Without knowledge of the overall topology
• 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

48
Optimality Principle and Sink Tree

• General statement about optimal routes
• 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
• Example
• r1 route from I to J
• r2 route from J to K
• If better route r2 from J to Kwould exist
• Then
• Concatenation of r1 and r2 would improve route
  from I to K
• Set of optimal routes
• From all sources
• To a given destination
• form a tree rooted at the destination Sink Tree

49
Sink Tree
Sink Tree for Destination B
Subnet

• Comments
• Tree no loops
• Each optimal route is finite with bounded number
  of hops
• Not necessarily unique
• Other trees with same path lengths may exist
• Goal of all routing algorithms
• 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

50
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
• ...

51
Non-Adaptive Routing

• Shortest Path Routing

52
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

53
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 (?,-)
C (?,-)
E (?,-)
A
D (?,-)
F (?,-)
G (?,-)
H (?,-)
2
54
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
• All nodes are labeled with infinity (tentative)
• Discover the labels that represent shortest
  possible path from source to any node
• Make those labels permanent
• 1. Node A labeled as permanent (filled-in circle)
• 2. Relabel all directly adjacent nodes with the
  distance to A(path length, nodes adjacent to
  source)
• 3. Examine all tentatively labeled nodes, make
  the node with the smallest label permanent
• 4. This node will be the new working node for the
  iterative procedure(i.e., continue with step 2.)

55
Non-Adaptive Shortest Path Routing
B (?,-)
C (?,-)
B (2,A)
E (?,-)
A
D (?,-)
F (?,-)
G (?,-)
H (?,-)
G (6,A)

• Procedure e. g. according to Dijkstra
• Find the shortest path from A to D
• A flagged as permanent (filled-in circle)
• Relabel all directly adjacent nodes with the
  distance to A
• (path length, IS adjacent to the source)

56
Non-Adaptive Shortest Path Routing
C (?,-)
B (2,A)
E (?,-)
A
D (?,-)
F (?,-)
H (?,-)
G (6,A)

• Procedure e. g. according Dijkstra
• Find the shortest path from A to D
• ...
• Compare all recent, not firmly flagged IS
• flag the one with the lowest number as fixed
• This IS is the origin of the iterative procedure
• (i. e. continue with item 2.)

57
Non-Adaptive Shortest Path Routing
C (?,-)
B (2,A)
C (9,B)
E (4,B)
E (?,-)
A
D (?,-)
F (?,-)
H (?,-)
G (6,A)

• Procedure e.g., according to Dijkstra
• Find the shortest path from A to D
• 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)
• A (does not apply, because it is the origin),

58
Non-Adaptive Shortest Path Routing
B (2,A)
C (9,B)
E (4,B)
A
D (?,-)
D (10,H)
F (?,-)
F (6,E)
H (?,-)
G (6,A)
H (9,G)
G (5,E)
H (8,F)

• Procedure e.g., according to Dijkstra
• find the shortest path from A to D
• 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 ...

59
Non-Adaptive Routing

• Flooding

60
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 destinations 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

61
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

62
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

63
Adaptive Routing
64
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

65
Adaptive Routing

• Centralized Routing

66
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

67
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

68
Adaptive Routing

• Isolated Routing

69
Adaptive Isolated Routingthrough 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

70
Adaptive Isolated Routingthrough Backward
Learning

• Packet
• From source S
• received on line L
• after C hops
• ? S is reachable on L within C hops
• Routing table in IS
• 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 lt Cmin
• then Update(S,L,C)

71
Adaptive Isolated Routingthrough 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
D
I2
72
Adaptive Isolated Routingthrough 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

73
Adaptive Routing

• Distributed Routing
• Distance Vector

74
Distance Vector Routing

• Distance-Vector Routing
• Group of Distance Vector Routing Algorithms
• Also known as
• Distributed Bellman-Ford algorithm,
  Ford-Fulkerson algorithm
• Use
• 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

75
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
  listscontaining
• Destination IS
• Preferred outgoing path
• Distance

76
Distance Vector Routing
line
A
B
C
D
G
28
I
E
H
F
20
H
17
I
30
I
I
J
K
L
18
H
0
-
15
K
JA 8
JI 10
JH 12
JK 6
delay

• Previous routing table will not be taken into
  account
• Reaction to deteriorations

77
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

78
Distance Vector Routing

• Slow distribution of information about new long
  paths (with many hops)
• Count to Infinity problem of DVR

• Example deterioration
• 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 length1

79
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
• distance X (via Y) according to arbitrary i ?

• Example deterioration (connection destroyed)
• 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
• Slow distribution (just as before)

80
Adaptive Routing

• Distributed Routing
• Link State Routing

81
Link State Routing

• Basic principle
• IS measures the "distance" to the directly
  adjacent IS
• Distributes information
• Calculates the ideal route
• Procedure
• 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
• Use
• 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)
• since 1990 Internet RFC 1247

82
Link State Routing

• 1. Phase gather information about the adjacent
  intermediate systems

83
Link State Routing

• 1. Phase gather information about the adjacent
  intermediate systems

• Initialization procedure
• New IS
• Sends a HELLO message over each L2 channel
• Adjacent IS
• Responds with its own address, unique within the
  network

84
Link State Routing

• 2. Phase measure the "distance"
• Definition of distance needed
• Usually delay
• Where to measure?

Router
When to start timer?
Uses Looks up
Data packets
Queues
Forwarding Process
Incoming lines
Outgoing lines
85
Link State Routing

• 2. Phase measure the "distance
• Queuing delay
• Measuring without does not take load into account
• Measuring with does ? usually better

• But
• Possibility for oscillations (route flapping)
• Once per routing table update

86
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
• Periodically
• In case of major changes

87
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

88
Adaptive Routing

• Distributed Routing
• Distance Vector

89
Distance Vector Routing
A
B
C
D
G
E
H

• Each node
• Maintains own routing table
• knows distances to every neighbor
• sends its estimated distances to the neighbors
  periodically
• Node X estimates distance to node Z through
  neighbor Y
• Add estimated distance to Y and Ys estimated
  distance to Y
• Node computes new routing table from the received
  listscontaining
• Destination node
• Preferred outgoing path
• Distance

F
I
J
K
L
line
28
I
20
H
17
I
30
I
18
H
0
-
15
K
JA 8
JI 10
JH 12
JK 6
delay
90
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

91
Distance Vector Routing

• Slow distribution of information about new long
  paths (with many hops)
• Count to Infinity problem of DVR

• Deterioration
• Connection destroyed
• A previously known
• New estimates derived from old estimates of
  distant nodes
• Comment
• Limit "infinite" to a finite value

92
Distance Vector Routing

• Split Horizon Algorithm
• Objective to improve the "count to infinity"
  problem
• In general, as before
• Report ? to best route neighbor (poisoned
  reverse)

• Deterioration
• 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

93
Adaptive Routing

• Distributed Routing
• Link State Routing

94
Link State Routing

• Node measures the "distance" to the directly
  adjacent node
• Distributes information
• Calculates the ideal route
• Procedure
• 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

95
Link State Routing

• Gather information about the adjacent nodes
• New node
• Sends a HELLO message over each L2 channel
• Adjacent node
• Responds with its own address, unique within the
  network
• Measure the distance
• Definition of distance needed
• Often delay
• Two approaches to measuring the delay

96
Link State Routing

• 2. Phase measure the "distance"

Router
When to start timer?
Uses Looks up
Data packets
Queues
Forwarding Process
Incoming lines
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

97
Link State Routing

• 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
• 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
• Computing new routes
• Each node for itself
• Possibly larger amount of data available

98
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 applicationshave use for
• Multi-homing
• Multicast
• Anycast
• More and more mobile nodes
• Too many nodes
• Continuation
• Multicast routing
• Routing in mobile and ad hoc networks
• The Internet protocols (IPv4 and IPv6) and
  routing in the Internet

• Courses that go deeper into routing
• INF5050
• INF5070
• INF5090
• UNIK4200
• UNIK4290

99
Broadcast Routing

• Controlled Flooding

100
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
• Deliver multicast packet to all connected end
  systems
• Forward multicast packet on all interfaces to
  other routers except I
• No
• It does not arrive on the shortest path from S
• Assume its a duplicate
• Drop packet

101
Multicast Routing

• Shared Tree
• Core-Based Tree

102
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

103
Multicast Routing

• Source-based Tree

104
Spanning Tree
Multicast source node
Spanning tree for group 1
Spanning tree for group 2
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

105
Multicast Routing

• Spanning Tree
• with Link State Routing

106
Spanning Tree with LSR

• All nodes send link state packets periodically
• Containing information
• 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
• 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 lineson which
  packets have to be transmitted

107
Multicast Routing

• Reverse Path Forwarding
• with Pruning

108
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

109
Reverse Path Forwarding with Pruning

• Pruning
• When a multicast packet arrives from S on
  interface I
• If
• No directly connected end system is registered,
  or
• Non-Membership-Reports are received from all
  nodes reachable via interfaces other than I
• Then
• 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
• To find end-systems that have joined
• Benefit
• Pruning only trees that are actually used
• Unused trees are cut coarsely
• Optimized for many receivers

110
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

111
Ad hoc Routing
112
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

113
Ad hoc Routing

• Reactive Routing

114
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
• S initiates a route discovery
• S floods Route Request (RREQ)
• When a node re-broadcasts a RREQ
• Adds the reverse path to its routing table
• Local routing table grows
• When D receives RREQ
• D replies with Route Reply (RREP)
• RREP travels along the reverse path
• Using routing table

115
Route Requests in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents a node that has received RREQ for D
from S
116
Route Requests in AODV
Y
Broadcast transmission
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents transmission of RREQ
117
Route Requests in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents links on Reverse Path
118
Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N

• Node C receives RREQ from G and H, but does not
  forward
• it again, because node C has already forwarded
  RREQ once

119
Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
120
Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N

• Node D does not forward RREQ, because node D
• is the intended target of the RREQ

121
Forward Path Setup in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Forward links are setup when RREP travels
along the reverse path Represents a link on the
forward path
122
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

123
Ad hoc Routing

• Proactive Routing

124
Destination-Sequenced Distance Vector Routing
(DSDV)

• Each node maintains a routing table which stores
• Next hop for each destination
• Cost metric towards each destination
• A sequence number that is created by the
  destination itself
• Each node periodically forwards routing table to
  neighbors
• Increases sequence number each time
• Includes it in routing table update
• A node that receives a routing table for a
  destination
• Overwrites the old table when the sequence number
  is higher
• When a node decides that a route is broken
• It increments the sequence number of the route
• Advertises it with infinite metric
• Destination advertises new sequence number

125
Reactive vs. Proactive Routing

• Reactive protocols
• Lower overhead since routes are determined on
  demand
• Significant delay in route determination
• Employ flooding (global search)
• Control traffic may be bursty
• Proactive protocols
• 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