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Lecture 3 - Ethernet networks
Alejandro Saucedo - Comp2008 Lecture 3 FlashCard Set
Computer Networking
Undergraduate 2

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What is the responsability of the Ethernet as the Link layer?
  • Receive/transmit datagrams from the network layer
  • Encapsulate these as the payload of an Ethernet frame
  • Detect transmission errors
  • Use acknowledgements in the case of 802.11
  • Implies IP layer can determine Ethernet addresses
    • Need a method to resolve an IP address to mac address
What are the elements of the Link Layer?
  • Logical Link Control
    • Provides the interface to layer 3 (IP layer)
    • Supports error detection, ARQ pipelining
    • May use ACKs
      • Not used in wired Ethernet
  • Medium Access Control (MAC) protocol
    • Controls access to the broadcast media
    • Handles multiple host access to the same collision domain
      • Shared Ethernet uses CSMA/CD
What is the Logical Link Control (LLC)?

Controls frame synchronization, flow control and error checking:

  • Provides interface to the IP Layer
  • Supports error detection , ARQ pipelining
  • May use ACKs (not in wired Ethernet)
What is the Medium Access Control (MAC) protocol?
  • Controls access to the broadcast media
  • Handles mutliple host access to the same collision domain
What are the characteristics of Ethernet Addresses?
  • Link Layer addresses
    • Often referred as MAC addresses
    • Currently defined to be 48 bits, extensible to 64 bits
  • Needs to be unique
    • 24 bits used for vendor allocations
    • Last 24 bits assigned by vendor
What are some of the components of the Ethernet frame?
  • Reminder - includes:
    • 48-bit source and destination addresses
    • 1 byte SFD to sync Tx and Rx for the start of frame
    • 802.1Q tag for optional VLAN ID and frame priority
  • Minimum data size 46 bytes
    • Maximum set by link Maximum Transmission Unit (MTU)
What are some types of ethernet transmission?
  • Generally uses block encoding, e.v. 8b/10b
  • 10/100Gig Ethernet uses 64b/66b
What are some principles of Ethernet reliability?
  • Wired Ethernet doesn't use acknowledgements
  • It may however detect errors
    • CRC checksum
  • On error, the 'bad' frame is simply dropped
    • Not passed up to receiving host's IP layer
    • No negative acknowledgement (NACK) sent
  • With no ACKs there is no inherent retransmission
    • Recovery needs to be handled by higher layer
      • By TCP which inherently provides retransmission
      • By the application, if UDP, which does not
  • Wired iternet is a reliable medium
What is address resolution protocol (ARP)?
  • Maps an IP address to a physical machine address (MAC) that is recognized in the local network.
  • A table is used to maintain a correlation between each MAC address and its corresponding IP address.
  • If the current entry does not exist in the table, a broadcast message is sent within the link layer of the network asking for this person - and this person will respond with the required information
How does the Address Resolution Protocol work?
  • Uses a link layer broadcast message
    • Ethernet broadvast address is ff:ff:ff:ff:ff:ff
    • Asks "Who has this IP address?"
  • This is seen by all hosts in the same layer 2 network
    • Host with the target IP address responds
What are some of the flaws of ARP?
  • Potentially open to spoofing
    • Middle-man attacks can respond as the IP address being requested
  • Sleeping nodes
    • Can use proxy ARP to answer for them
  • Change of IP or MAC addresss
  • ARP probe
    • To detect IP address clashes
What is ARP cache?
A table which contains entries of IP and MAC addresses which are checked before sending a broadcast message
What are the classes of data link networks?
  • Broadcast
    • Multiple hosts in one layer 2 broadcast domain
    • Hosts in the same broadcast domain fall into the same LAN
    • The layer 2 broadcast domain cotrols where ARP travels
  • Point to point
    • Typically router to router
      • Only two devices in the broadcast domain and corresponding subnet
What are some Ethernet topologies?
  • Bus (old)
    • Hosts 'tap' into a single medium (cable)
  • Star (current)
    • Each host connects to a port in concentrator device
What are hubs?
Connection point for devices in a network with multiple ports - when a packet arrives at one port it is copied to all the other ports.
What are switches?
  • Device that filters and forwards packets between LAN segments.
  • Operate at data link and network layer.
  • Learns ethernet addresses of host(s) seen on each switch port.
What are some characteristics of Ethernet switching?
  • Commodity switching new at 1Gbit/s
  • Internal switch fabric between switch ports
  • One switch port connects  each host
    • No need for CSMA/CD (Unitil a researcher plugs a hub in)
  • Can run Cat5 at 1Gbit/s to about 100m
  • May have a higher speed uplink port or module
What are the 3 key principles of Switched Ethernet networks?
  1. How switches learn which ports to forward received frames on to, to minimise traffic propagation
  2. How loops in switch topologies can be handled
  3. How virtual layer 2 networks can be deployed
What are the principles of MAC learning in Ethernet Switching?
  • Allows bridge or switch to only forward frames to ports which serve devices they are addressed to
  • Uses an Ethernet (MAC) table per port
  • Observes incoming source MAC addresses
    • Stores observed MAC source addresses in port table
  • Then forward frames to taht address to taht port, and that port only
    • Hosts only see traffic for them, or any broadcast/multicast
    • If MAC address is not in any table, switch must flood to all ports
  • Purge MAC tables periodically
    • Devices may be moved
How to avoid loops in switched networks?
  • Using Spanning Tree Algorithms
  • Using redundant paths
What does the Spanning Tree (802.1d) Algorithm for Switched networks consist of?

It uses BPDU (Bridge Protocol Data Unit)

>Switches use these to determine the root and the best path to root<

  • Lets you create physical loops without breaking network
  • Refers to'bridges' which in today's terms are switches
  • Algorithm decides(elects) a root bridge (lowest MAC)
  • Determines least cost paths to the root
  • Enables root (facing) ports on each switch
  • Other ports with a path to the root are blocked
  • Alternative paths may be enabled if topology changes
What is the 802.1d protocol?
Spanning Tree
What is the 802.1Q protocol?
Virtual LANs
What are the principles of Virtual LANs (802.1Q)?
  • Used to create 'virtual' switched networks
    • And therefore virtual broadcast domains
  • Ethernet frame may include a VLAN identifier
    • 12-bit value within 802.1Q tag, so 4,096 values
    • Can then place switch ports in specific VLANs
    • Can carry multiple VLANs over one uplink
  • Avoid needs to physically re-cable
    • Can place one switch port in a different VLAN without touching cabling
What protocol is the 802.1p?
Ethernet frame priority
What does the Ethernet frame priority (802.1p) protocol consist of?
  • A specification for giving Layer 2 switches the ability to prioratize traffic (and perform dynamic multicast filtering)
  • Supported by use of the 802.1Q tag
    • Three bits of the tag are used for 802.1p priority
      • 1 is low, 7 is high priority
      • 0 is just treated as regular traffic 
What is the 802.11 protocol?
Wireless LAN
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