Packet Switching
9 important questions on Packet Switching
What is main problem in this chapter?
Directly connected networks have limitations
- number of hosts
- geographical area
Requirements
- interconnect links
- interconnect networks
- scalability
Switching and forwarding
Bridges and LAN switches (layer 2)
Internetworking (layer 3)
Routing (layer 3)
Switch & Router Design
What is Datagram Switching?
Each packet forwarded independently
- analogy: postal system
Each switch maintains forwarding table
- the hard problem is constructing this table
Forwarding table
- list with addresses (A....Z), take path X to get there. Keep going through the path and tables till destination
What is Virtual Circuit Switching?
- explicit connection setup (and tear-down) phase
Subsequent packets follow same circuit
- analogy: phone call
Each switch maintains VC table
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What is Source Routing?
- no forwarding table in switches
- packet gives rotating list of ports for switches to use. Switch uses port and places it back of the list
Variable length path info
- where? In header? Variable length
Scaling problems!
Examples
- IP (option), Token Ring (bridges), SDN (controller)
What are Bridges and LAN switches
Connect two or more LANs with a bridge
- bridge operates in promiscuous mode (accept all frames)
- accept and forward strategy
- using only frame header (does not use/change layer 3 packet header)
Ethernet Switch = Bridge on Steroids
Extended Lan: a collection of interconnected LANs
Collissions only happen in 1 ethernet. Instead of the full extended Ethernet.
What are learning Bridges
Learn table entries based on source ddress
- initially table is empty
Table is an optimization; need not be complete
- timout to protect against invalid information
Always forward broadcast frames
Problem: Loops
- loops are causing "broadcast storms"
- loops disturb learning process
Explain the Spanning Tree Algorithm
Bridges run a distributed Spanning Tree Protocol (STP)
- select which bridges actively forward
- Developed by Radia Perlman
- now part of IEEE 802.1 specification (802.1D)
Overview
- Each bridge has unique ID
- Select bridge with smaller ID as root
- Select bridge on each LAN closest to root as designated bridge
- Each bridge forwards frames to/from each LAN for which it is the designated bridge
How Broadcast & Multicast for bridges
- forward frame on each active port
Multicast:
- broadcast all multicast frames
Better: learn when no group members downstream
- accomplished by having each member of group G send a frame with multicast address G in source address field
- (not used in practice)
What are the limitations of bridges?
- use of hardware addresses does not scale
- spanning tree algorithm does not scale
- broadcast does not scale
- solution: VLAN (infra)
How to accommodate hetergeneity?
- different frame formats
- different address formats
- different transmission speeds and throughputs
Transparency is an advantage, but can be dangerous
- frames can be delayed, dropped, reordered
- result: an extended LAN does not behave as an single Ethernet
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