Friday, November 23, 2012

Notes - Data Link Layer

The following are notes from Computer Networks written by Tanenbaum 5th edition.
  • VLANs(Virtual LANs)
  • connections called bridges to join lans together
Uses of Bridges
  • Reasons why an organization may have multiple LANs
    • Many university and corporate departments have their own LANs
    • Organization geographically spread wants to be joined
    • Split a single LAN into separate LANs for logical organization
  • bridges let LANs be joined while keeping their own capacity
  • bridges increases reliability
    • can reroute nodes that are malfunctioning giving garbage data
  • transparent bridges
    • create with backward learning algorithm to stop traffic from being sent to unneeded areas and spanning tree algorithm to break loops
Learning Bridges
  • example topology
  • bridges developed when classic ethernets were in use
  • bridge operates in promiscuous mode accepts every frame to its ports
    • chooses to forward or discard
  • simple way to implement this scheme is to have a big hash table
    • initially empty
    • backward learning every frame sent tells the bridge about the topology
    • arrival time of frames noted in entry, tells the bridge the last time a frame is seen
    • bridge scans and purges entries more than a few minutes old
  • routing procedure for incoming frame
    • if the port for destination is same as source discard frame
    • if destination port is different from source forward to destination
    • if destination unknown send frame on all ports except the source
  • as each frame arrives algorithm must be applied
  • cut through switching/wormhole routing
    • bridges only look at MAC addresses to reduce latency
Spanning Tree Bridges
  • to increase reliability redundant links used between bridges
    • this introduces loops in the topology
    • so in order to do this, overlay the topology with a spanning tree to reach every bridge, ignore some links, example as follows
  • spanning tree is built with distributed algorithm, bridge periodically broadcasts a configuration message on all ports
    • tree of shortest paths from root to every bridge constructed, and spanning tree used between bridges
  • Radia Perlman poem
Repeaters, Hubs, Bridges, Switches, Routers and Gateways
  • These devices operate in different layers which is why even though they perform similar roles, they are all necessary

    • repeaters amplify signals
    • hubs don't amplify, but allow for multiple lines
    • bridges connects two or more LANs has multiple ports
      • isolated in its own collision domain, CSMA/CD not needed
      • better performance than hubs
      • can join different types of LANs such as Ethernet and Token Ring
        • joins different max frame lengths
        • join different security protocols
      • switches are modern bridges by another name
        • utilizes twisted pair cables, often used for ethernet
    • routers totally different
      • frame header and trailer gets stripped off and the packet located in payload field is passed to routing software
      • chooses outgoing line with address
      • does not see fram address or where packet comes in on
    • transport gateways connect two computers using different connection oriented protocols such as
      • TCP/IP
      • SCTP
      • understands formats such as email into sms messages
Virtual LANs
  • early days of LAN setup was designed in this way

  • issues
    • difficult to replace, geographically new workers have to sit at open workstations instead of with their coworkers
    • load issue not even
    • broadcast traffic to reach correct persons
    • vulnerable to broadcast storm when LAN malfunctions and sends random messages
  • solution is to set up VLAN(Virtual LAN)
    • based on VLAN aware switches
    • network admin decides how many VLANs are possible
    • association as configuration tables are built into bridges
  • The IEEE 802.1Q Standard
    • To implement this scheme bridges need to  know which VLAN an incoming 
    • issues
      • do we need to throw out existing ethernet cards to accomodate this standard?
      • who generates fields?
      • what happens to max size frames
    • VLAN fields are only used by bridges and switches and not by user machines
    • bridge is also allowed to use higher layer protocol to select colors for VLAN creation

  • for 802.1Q Ethernet frames, the tag composed of
    • 3 bit priority field
    • CFI(Canonical Format Indicator)
      • indicate order of bits in MAC adress
    • VLAN Identifier specifies color of VLAN
    • when this comes to VLAN aware switch the identifier is matched to a table to figure out which ports it should send to

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