Link Layer Switching

1
Link Layer Switching

• Connecting local networks
• Bridges
• Repeaters, Hubs, Bridges, Switches, Routers,
  Gateways
• Virtual LANs

2
Ethernet

• 50 ? thick 500 m
• 50 ? thinn 185 m
• max 4 repeaters
• traffic on one segment means traffic on all
  other segments

3
CSMA/CD (IEEE 802.3)
Logical Link Control
Link
A-MAC Phys. A
B-MAC Phys. B
C-MAC Phys. C
Physical
4
Bridges

• Connection on link layer
• forwarding based on MAC addresses
• self-learning bridges
• operation
• Advantages and limitations
• Spanning-tree bridges
• operation
• Advantages and limitations

5
Self-learning Bridge
Bridge
routing table
Forwarder
MAC_1 Phys_1
MAC_2 Phys_2
Network 1
Network 2
6
Self-learning Bridge
Routing table
Interface
MAC-adr
.
.
Learning
time

Mac-1
2
- -
routing
- - - - -
- -
- -
Mac-2
3
- -
Driver interface 1
.
Driver interfaec 3
.
Driver interface 2
.
 
LAN 2
LAN 1
LAN 3
7
Self-learning Bridge
Learning phase
Forwarding phase
Extract Sender and receiver
MAC-adresser
Start
Look up in Routing table
Look up in Routing table
Yes
No
Sender known?
New entry MAC-addr interface and timer
Broadcast frame, except on receiving interface
Update interface and timer
Put frame into correct outgoing queue
End
8
Link Layer Switching

• Multiple LANs connected by a backbone to handle a
  total load higher than the capacity of a single
  LAN.

9
Bridge from 802.x to 802.y

• IEEE 802 frame formats

10
Bridges from 802.x to 802.y

• Operation of a LAN bridge from 802.11 to 802.3.

11
Local Internetworking

• A configuration with four LANs and two bridges.

12
Problem with standard bridge

• Two parallel transparent bridges.

13
Spanning tree

• Goal each bridge should identify the interfaes
  for forwarding traffic
• Build a spanning tree
• From on root node
• Self-configuring
• To all nodes
• Only these interfaces in the spanning tree can
  forward traffic
• Provides the shortest path for all traffic

14
Spanning Tree Algorithm

• Configuration phase
• Each nodes sends out
• Its own identity (ID) (MAC-address)
• ID to the root-bridge
• Number of hops to root-bridge
• In this way, building up a spanning tree, bridge
  with lowest ID become root node
• Start forwarding frames

15
Spanning Tree Bridges

• (a) Interconnected LANs. (b) A spanning tree
  covering the LANs. The dotted lines are not part
  of the spanning tree.

16
Remote Bridges

• Bridges can be used to connection physically
  distant local networks

17
Repeaters, Hubs, Bridges, Switches, Routers and
Gateways

• (a) Which device is in which layer.
• (b) Frames, packets, and headers.

18
Hub (Nav)
lt 100 m
Transceiver
19
Repeaters, Hubs, Bridges, Switches, Routers and
Gateways

• (a) A hub. (b) A bridge. (c) a switch.

20
Switched Ethernet

• Switch
• Switches on MAC-addr
• Buffers frames, therefor no collision
• Competition only for switch capacity

10, 100, 1000 Mb/s
21
Gigabit Ethernet
Gigabit switch
1000 Mb/s 100 Mb/s
100/1000
100/1000
Switch
Switch
Working group 1
Working group 2
22
Virtual LANs

• A building with centralized wiring using hubs and
  a switch.

23
Virtual LANs (2)

• (a) Four physical LANs organized into two
  VLANs, gray and white, by two bridges. (b) The
  same 15 machines organized into two VLANs by
  switches.

24
The IEEE 802.1Q Standard

• Transition from legacy Ethernet to VLAN-aware
  Ethernet. The shaded symbols are VLAN aware.
  The empty ones are not.

25
The IEEE 802.1Q Standard (2)

• The 802.3 (legacy) and 802.1Q Ethernet frame
  formats.

26
Conclusion

• Bridges
• efficient connection alternative
• Limits/isolates collision domains
• Can be used for traffic isolation
• Do not consume IP addresses
• Switches
• High use degree, no danger of collisions
• Used for establishing virtual LANs

27
Routing and Packet Switching

• Goal
• Overview of how routing fits into the Internet
  architecture
• Principles for typical routing protocols
• Strengths and weaknesses
• Structure
• Primary tasks of the network layer
• Datagram and virtual line
• Some performance considerations
• Routing and forwarding

28
Network layer
Server
Client
Disk
Disk
link
29
Tasks of the Network Layer

• Responsible for end-to-end transport
• Addressing of machines
• Forwarding
• Connectionless
• datagram no fixed path through the network
• Connection-oriented (e.g. MPLS or ATM)
• Three phases connection establishment, data
  transmission, teardown
• Fixed path through the network
• Relatively reliable and ordered transmission
• Flow control

30
Forwarding
R
A
B
R
LAN-A
LAN-B
31
Routing and lookup

• Mail griff_at_ifi.uio.no
• Name to address conversion
• ifi.uio.no til IP address 129.240.64.2
• Find MAC-address to router and send packet(s)
• Forward through the network w.r.t. the network
  address
• Based on lookup in routing tables
• At the destination router
• Convert machines IP address to a MAC address
• Send packet to the receiving machine

32
Place of Routing in the architecture

• Structured
• Network dimensioning
• Where should lines be established?
• Capacity of lines
• Traffic directioning
• Mapping of connections down to paths through the
  net
• Routing to choose paths
• Routing of individual packets
• Best effort
• Routers choose the next hops separately for each
  packet

33
Routing

• Routing tables can be computed based on state
  information about the network
• Data exchanged between nodes
• Between neighbour nodes (distance vector routing
  RIP)
• Between all nodes in the network (link state
  routing OSPF, IS-IS)

34
Routing types

• Static vs. dynamic
• Dynamic with error handling, new links, changes
  of the load
• Centralized vs. distributed
• Distributed when routes are computed at all nodes
• Global vs. local topology knowledge
• Source routing vs. routing
• Kilde ruting vs. ruting
• In source routing the source chooses the routing
• In routing each router choose the next hop

35
Routing Parameters

• Performance parameters
• Number of hops
• Price
• Delay
• capacity

• Sources of routing information
• None
• Local to the node
• Neighbour nodes
• Nodes along the path
• All nodes in the network

• Routing decisions made
• In each node (distributed)
• In a central node (routing center)
• At the sender (source routing)

• Update interval
• Continously
• Periodic
• In case of large load variations
• In case of topology changes

36
Routing hierarchy

• In large networks
• Hierarchically structured
• Link state
• Open Shortest Path (OSPF)
• Intermediate System to Intermediate System
  (IS-IS)
• On campus or in companies
• Distance vector, RIP
• Static routing
• Ad-hoc networks, stationary or mobile wireless
  networks
• Many different protocols depending on scenarios

37
Router model
Routing prosess
Route
1
1
computation
Topology database
Routing table
2
2
Pre- process
3
3
Forwarding process
 
In
Out
e
Principle structure of a router with three
incoming and three outgoing connections
38
Routing alternatives

• Flooding
• Static routing
• Adaptive routing should handle
• Loss of a link (error, e.g. cable is broken)
• Loss of a node (error, e.g. power loss, OS crash)
• High traffic load (persistant of transient
  congestion, bottleneck)
• Disadvantages
• Complex, distributed, and not always correct
• Adaptivity must be balanced against additional
  overhead
• Can lead to oscillations (route flapping) if
  reactions are too fast
• Can be unattractive if reactions are too slow

39
Demands on a routing strategy

• Shall give correct routes
• Shall demand minimal load on nodes
• Shall be stable and converge quickly
• Fair towards different data streams
• Provide optimal routes
• Scale with the size of the network
• Size with the number of destinations

40
Plug-and-play capabilities

• Find neighbour nodes and routers
• Detect when neighbours go up and down
• Detect capacity of own links
• Send and receive topology information
• Send after timer or major changes to the network

41
Distance vector characteristics

• Nodes exchange a vector with their shortest
  distance to all destinations
• Periodic exchange
• Convergence is ensured
• Advantage
• Simple
• Disadvantages
• Vulnerable to errors
• Slow dissemination in case of problems

42
Distance Vector
5
3
C
5
B
2
9
A
2
1
F
2
1
D
E
Distance vector
Next node vector
Dest. delay Next node A 0 - B
2 B C 5 C D 1 D E
6 C F 8 C
Node A before change
43
Distance vector (2)
5
3
3
5
2
2
9
1
2
1
6
2
1

• 2 3 1
• 0 3 2
• 3 0 2
• 2 0
• 1 1
• 3 3
• D2 D3 D4

4
5
9
Dest. delay next node 1 0 - 2
2 2 3 3 4 4 1 4 5
2 4 6 4 4
Min dkj i ? A dij lki
Distance vector Send to node 1
A is the set of all neighbour nodes of k
Routing table in node 1 after
44
Router model
Routing prosess
Route
1
1
computation
Topology database
Routing table
2
2
Pre- process
3
3
Forwarding process
 
In
Out
e
Principle structure of a router with three
incoming and three outgoing connections
45
Link state

• Routing database
• Routing table
• Periodical and in case of changes
• Nodes flood their state onto the link to all
  other nodes
• At start, new nodes downlink the database from a
  neighbour
• Different kinds of link
• Point-to-point
• Point-to-multipoint
• Broadcast
• Each node calculates the best route to all other
  nodes
• Checkpoints
• Voting av entire database for link state at a
  sequence number

46
LS routing protocol architecture
change
route lookup
change
Protocol for handling of changes
Routing table
Routing algorithm
Link state database
47
Flooding of link state

• Statistically reliable
• Each node forwards on all interfaces
• All incoming link state packets
• If sequence number of large than earlier sequence
  numbers
• Will most probably reach all node in the network
• Content
• Sequence number
• Avoid broadcast storms
• Node ID of the source
• Topology
• Identify bi-directional links
• List of all direct neighbour nodes with a cost
  function
• Time-to-live

48
Link state, LSA
Routin database i D
A B C D E F A 2 5 1 B 2
2 C 5 D 1 2 9 E F
49
Link state problems/strengths

• Problems
• Selection of a node that reports for a shared
  medium
• Flooding does not scale for large networks
• Division into hierarchical networks to limit
  flooding
• Strengths
• All nodes have full topology knowledge
• Error have only local relevance

50
Link state problem
a
area 1
area 2
b

• We have two problems with the link state method
• Static cost factor
• Can be the source of congestion, all traffic is
  routing through a single link
• Oscillation effects in forwarding traffic
• At one point in time a is the preferred router
  between areas
• Then routing information is exchange
• New tables are computed and b becomes the
  preferred router

51
Router and Routecenter
Routers do not have to participate in a routing
protocol Routing center receives status reports
from routers Transfers forwarding table to routers
Network with router center
52
Connection-oriented forwarding

• Establish a channel a path through the network
• Examples ATM, MPLS, X.25
• Explicit signalling
• Data-driven signalling
• Signalling protocol
• Routing protocol to choose the nodes that should
  form the path
• In each node establishing a forwarding table
• Incoming interface, channel to outgoing
  interface, channel

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55
Virtual lines
C
B
1
2
1
2
b
b
1
1
2
1
A
3
D
4
a
a
c
2
c
2
node x
node y
V.C. tables
V.C. tables
 
c
b
a
c
a
b
1 a 3
1 b 1
1 a 1
1 c 3
1 b 2
1 c 2
2 a 4
2 b 2
2 a 2
2 c 4
2 b 1
2 c 1
3 c 1
3 a 1
4 c 2
4 a 2
56
Dynamic cost in route computation

• Adaptation of routes to load
• Move traffic to lines with lower load
• Main problem
• Delay between measurement and computation
• Delay between route computation and traffic
  arrival
• Fast variation in load
• Bad predictability
• Route flapping (oscillations)
• Overhead of exchanging the routing information

57
Performance of the network

• Performance of the networks means capacity,
  delay, delay variation (jitter), and reliable
• Has several elements
• Transmission delay
• Sending delay
• Signal propagiation time
• Node delay
• Processing time
• Queueing time

58
Measuring the link state
59
Topology example
60
Shortest path tree for nodes
 
61
Routing tables for shortest path trees
62
Packet size and delay
63
Modified load variation
 
64
Timing in line and packet switching