• 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

• 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

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)

• Controls access to the broadcast media
• Handles mutliple host access to the same collision domain

• 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

• 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)

• Generally uses block encoding, e.v. 8b/10b
• 10/100Gig Ethernet uses 64b/66b

• 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

• 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

• 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

• 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

• 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

• Bus (old)
• Hosts 'tap' into a single medium (cable)
• Star (current)
• Each host connects to a port in concentrator device

• 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.

• 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

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

• 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

• Using Spanning Tree Algorithms
• Using redundant paths

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

• 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

• 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