1. Packet Switching Outline Store-and-Forward Switches
Bridges and Extended LANs
Cell Switching
Segmentation and Reassembly
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2. Packet Switching Problem: “Not all networks are Directly Connected”
Directly connected networks have two limitations:
- number of hosts that can be accommodated
- geographic limitation
Goal – networks that are global in scale
Like Telephone using (circuit) switches,
we use packet switches(store-and-forward) that take packets that arrive on an input and forward (switch) them to the right output. Two ways to do it:
- connection
- connectionless
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3. Packet Switching Key problems that a switch must deal with:
Contention – packet arrival exceeds packet dispatch (in this chapter)
Congestion – packets discards (due to running out of buffer space) too frequently (in future chapter)
Key issues covered
- forwarding
- contention
Two switching technologies
- LAN switching – Popular in LAN, evolved from
Ethernet bridging
- Asynchronous Transfer Mode(ATM) – popular in WAN
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4. Star Topology Large numbers of switches can be connected
Connecting switches/hosts using Point-to-Point links
Scaling doesn’t always mean performance  (switches designed with enough aggregate capacity)
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5. Scalable Networks Switch Advantages
forwards packets from input port to output port, this is referred to as either switching or forwarding. In terms of the OSI architecture, it is the main function of the network layer.
port selected based on address in packet header
Advantages
cover large geographic area (tolerate latency)
support large numbers of hosts (scalable bandwidth)
Input
ports T3
STS-1
Switch
Output
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6. Switches Q. How does the switch determine output port?
By looking at an identifier in the packet header
Three approaches:
Virtual Circuit (connection-oriented)
Datagram (connectionless)
Source Routing
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7. Virtual Circuits Explicit connection setup (and tear-down) phase
Switch 1 3 1 2
Switch 2 2 3 1 5 11
Explicit connection setup (and tear-down) phase
Subsequence packets follow same circuit
Sometimes called connection-oriented model
Host A
Two types:
- PVC Permanent Virtual Circuit
Need network administrator configure the state
- SVC “Signalled” Virtual Circuit
Send a message into the network(signalling)
Analogy: phone call
Each switch maintains a VC table 7
Switch 3 1 3 4
Host B 2
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8. Virtual Circuits Tables
VC Table
for Switch 1
Incoming Incoming Outgoing Outgoing
Interface VCI Interface VCI
. . .
Switch 1 3 1 2 2 3 1 5 11
VC Table for Switch 2
Switch 2
Incoming Incoming Outgoing Outgoing
Interface VCI Interface VCI
Host A
VCI Virtual Circuit Identifier (0, 1, 2, …)
combined with incoming/outgoing interface (e.g. 0, 1, 2, 3)can uniquely identify the virtual connection (VC)
assigned whenever a new connection is created
not a globally significant identifier for the VC; rather, only on a given link 7
Switch 3 1 3 4
Host B 2
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9. Example: Give Virtual Circuit Tables for all switches
Host C
Host E
Switch 1
Host D
Switch 3 2
Host F 3 1 3 1 1
Switch 2
Host B 2 2
Host G 3
Host A
Host H
Incoming
Interface#
VCI#
Outgoing 1 2
Connections
Established
in the order of
(a) to (d)
Switch
Incoming
Interface# VCI#
Outgoing
(a) Host A connects to Host E 1 2 3
(b) Host C connects to Host G
(c) Host G connects to Host A
(d) Host D
connects to Host E
VC Table
for Switch 1
Incoming
Interface#
VCI#
Outgoing 1 2 3
VC Table
for Switch 2
Incoming
Interface#
VCI#
Outgoing 3 1 2
VC Table
for Switch 3
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10. Virtual Circuit Model
Typically wait full RTT for connection setup before sending first data packet.
While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small.
If a switch or a link in a connection fails, the connection is broken and a new one needs to be established.
Connection setup provides an opportunity to reserve resources(QoS reservations).
ATM utilizes virtual circuits
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11. Datagrams
Idea – provide just enough information for the switch to forward the packet
Host D
Switch 1
Host E
Host F 3 1 2
Switch 2
Host C 2 3 1
No setup time
Independent forwarding packets
Analogy – postal system
Each switch maintains a forwarding (routing) table.
More routing in the next chapter
Host A
destination
Port A 3 B C D E 2 F 1 G H
Forwarding table for Switch 2
Host G
Switch 3
Host B 1 3 2
Host H
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12. Workstation Used As a Switch
Main problem: all packets must pass through a single point of contention. (I/O bus, read to/write from the main memory)
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13. Forwarding Table for NodesC E
Give the datagram forwarding table for each node:
Node A Node B Node C Node D Node E
Destination
Next Hop B C D E
Destination
Next Hop A C D E
Destination
Next Hop A B D E
Destination
Next Hop A C B E
Destination
Next Hop A C B D
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14. Datagram Model
There is no round trip time delay for connection setup; a host can send data as soon as it is ready.
Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up.
Since packets are treated independently, it is possible to route around link and node failures.
Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.
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15. Source Routing
Does not need to use either VCs or Datagrams although can be used in combination with
IP for instance uses datagrams but has a source routing option
Can be used for VC setup
Source host contains all
information
- rotates data
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16. Bridges and LAN Switches
Q. What can be used to share data between two shared-media LAN’s?
LAN switch (or bridge) – like a host in promiscuous mode
LANs connected with >= 1 bridge are called Extended LANs
Problem – what about when node A wants to send node B a message, what happens? A B C
Port 1
Bridge
Port 2 X Z Y
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17. Learning Bridges Do not forward when unnecessary Maintain forwarding
Host Port A B C X Y A B C
Port 1
Bridge
Port 2 X Y Z
Learn table entries based on source address
Table is an optimization; need not be complete
Always forward broadcast frames
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18. Spanning Tree Algorithm
Problem: loops
Bridges run a distributed
spanning tree algorithm
- select which bridges actively
forward
- developed by Radia Perlman
- now IEEE specification
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19. What is a Spanning Tree? The problem:
Tree - is connected graph with no cycles.
A Spanning Tree of G is a tree which contains all vertices
in G.
Example: Given a graph G
Is G a Spanning Tree?
Yes No
Note: Connected graph with n vertices and exactly n – 1 edges is Spanning Tree.
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20. Spanning Tree Example
Example: G: 1 2 3 7 6 4 5 8
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21. Centralized Spanning Tree Algorithm1 #1 #7 7 2 3 #2 #6 5 4 6 #4 #3 #5 8
        DFS (Depth First Search)
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22. Centralized Spanning Tree Algorithm1 #1 #2 2 3 #3 #4 #5 #6 4 5 6 7 #7 8
BFS (Breadth First Search)
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23. Distributed Spanning Tree Algorithm - Overview
Each bridge has unique id(e.g., B1, B2 , B3 )
Select bridge with smallest id as root
Select bridge on each LAN closest to root as the designated bridge(use id to break ties)
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24. Distributed Spanning Tree Algorithm - Detail
Bridges exchange configuration messages
id for bridge sending the message
id for what the sending bridge believes to be root bridge
distance(hops) from sending bridge to root bridge
Each bridge records current “best” configuration message for each port
Initially, each bridge believes it is the root and sends messages out on all its ports(distance to root = 0)
When learn not root, stop generating configuration messages
in steady state, only root generates configuration messages
When learn not designated bridge, stop forwarding config messages(disconnected)
in steady state, only designated bridges forward config messages
Root continues to periodically send config messages
If any bridge does not receive config message after a period of time, it starts generating config messages claiming to be the root
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25. Distributed Spanning Tree Algorithm - Overview
Ports which are not selected (disconnected) by the Distributed Spanning Tree Algorithm × × × × ×
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26. Asynchronous Transfer Mode(ATM)
Connection-oriented packet-switched network (virtual circuits)
Used in both WAN and LAN settings
Signaling(connection setup) Protocol: Q.2931
Packets are called cells
5-byte header + 48-byte payload(fixed length)
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