Packet Switching: Packet Switching COM1337/3501 Textbook: Computer Networks: A Systems Approach,
L. Peterson, B. Davie, Morgan Kaufmann
Chapter 3. Outline: Outline Packet switching paradigms
Bridges and extended LANs
Scalable Networks : Scalable Networks Switch
forwards packets from input port to output port
port selected based on address in packet header
cover large geographic area (tolerate latency)
support large numbers of hosts (scalable bandwidth) Packet Switching Paradigms: Packet Switching Paradigms Virtual circuit switching (routing)
Datagram switching (routing)
Source Routing: Source Routing The information to route the packet is provided by the source host and included in the packet
Example of implementing source routing:
Assign a number to each switch output port
Include the list of output ports that the packet has to go through
The list is rotated by the intermediate switches before forwarding
Packet initiators need to have a sufficient information about the network topology
The header has a variable length Source Routing: Source Routing Virtual Circuit (VC) Switching: Virtual Circuit (VC) Switching Explicit connection setup (and tear-down) phase
Subsequent packets follow same circuit (path)
Sometimes called connection-oriented model Analogy: phone call
Each switch maintains a VC table Virtual Circuit Switching: Virtual Circuit Switching Connection Setup approaches:
Permanent Virtual Circuits (PVC): manually setup/removed by network administrators
Switched Virtual Circuits (SVC): dynamically setup through signaling over some control channels
Connection state => VC table
incoming interface, VC Identifier (VCI), outgoing interface, outgoing VCI
The setup message is forwarded over the network
New entries are created in the VC table and destination switches choose incoming VCI
When the setup message reaches the destination, connection acknowledgements and chosen VCI are communicated back to the source
Virtual Circuits: Virtual Circuits Examples of Virtual Circuit Technology:
Frame Relay, X.25, Asynchronous Transfer Mode (ATM)
Frame Relay was popular for creating virtual private networks (VPNs) using PVC.
ATM is a more complex technology that provides mechanisms for supporting quality of service Datagram Switching: Datagram Switching No connection setup phase
Each packet forwarded independently
Sometimes called connectionless model 0 1 3 2 0 1 3 2 0 1 3 2 Switch 3 Switch 2 Host A Switch 1 Host C Host D Host E Host G Host H Analogy: postal system
Each switch maintains a forwarding (routing) table Switch 4 Virtual Circuit Model: Virtual Circuit Model Setup: Typically wait full RTT for connection setup before sending first data packet.
Header: While the connection request contains the full destination address, each data packet contains only a small identifier, making the per-packet header overhead small.
Quality of Service (QoS):
Connection setup allows resource reservation
If a switch or a link in a connection fails, the connection is broken and a new one needs to be established.
Datagram Model: Datagram Model Setup: There is no round trip time delay waiting for connection setup; a host can send data as soon as it is ready.
Header: Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.
Quality of Service (QoS):
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.
Successive packets may follow different paths and be received out of order.
Outline: Outline Packet switching paradigms
Bridges and extended LANs
Bridges and Extended LANs: Bridges and Extended LANs LANs have physical limitations (e.g., 2500m)
Connect two or more LANs with a bridge
accept and forward strategy
level 2 connection (does not add packet header)
Ethernet Switch is a LAN Switch = Bridge Learning Bridges : Learning Bridges Do not forward when unnecessary
Maintain forwarding table
Learn table entries based on source address
Table is an optimization; need not be complete
Always forward broadcast frames Spanning Tree Algorithm : Spanning Tree Algorithm Problem: loops
Bridges run a distributed spanning tree algorithm
select which bridges actively forward
developed by Radia Perlman
now IEEE 802.1 specification Algorithm Overview : 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 designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge Algorithm Details: Algorithm Details 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 Algorithm Detail (cont): Algorithm Detail (cont) When learn not root, stop generating config messages
in steady state, only root generates configuration messages
When learn not designated bridge, stop forwarding config messages
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 Broadcast and Multicast: Broadcast and Multicast Forward all broadcast/multicast frames
Learn when no group members downstream
Accomplished by having each member of group G send a frame to bridge multicast address with G in source field Limitations of Bridges: Limitations of Bridges Do not scale
spanning tree algorithm does not scale
broadcast does not scale
Do not accommodate heterogeneity
Caution: beware of transparency
Bridged LANs do not always behave as single shared medium LAN: they drop packets when congested, higher latency Virtual LANs (VLAN): Virtual LANs (VLAN) VLANs are used to:
increase scalability: reduce broadcast messages
provide some basic security by separating LANs
VLANs have an ID (color).
Bridges insert the VLAN ID between the ethernet header and its payload
Packets (unicast and multicast) are only forwarded to VLAN with the same ID as the source VLAN