How do we find the MAC address of a receiving host, given its IP address?
ARP Table
Contains IP addr, MAC addr, TTL.
TTL refers to this ARP table entry’s TTL.
This forms a list of neighbours/edges between this interface and others.
When a frame is received from another interface, it is added to this table.
ARP Query
A sender broadcasts a query packet, with dest MAC address FF-FF-FF-FF-FF-FF.
All other nodes will check if the IP address in the query packet matches its own.
On a match, the node will reply with a frame containing it’s MAC address.
The sender then caches the IP address and MAC address pair in its ARP table.
Sending Frame in Same Subnet
Suppose A wants to send to B (A and B are in the same subnet).
Case 1: B’s MAC address known
Create a frame with B’s MAC address and send
Only B will process it
Other nodes process but ignore this frame.
Case 2: B’s MAC not known
A sends an ARP Query packet
On receiving B’s MAC address, goto Case 1.
Sending Frame in Another Subnet
Q: How do you know you are sending to a different subnet?
A: If subnet doesn’t have the same IP prefix.
A should create link layer frame with
dest MAC: gateway (router) interface (of the subnet) MAC
Hence router will receive a match for this frame.
Router accept and passes the datagram to the network layer
Network layer checks IP address, use routing protocol to send it to B’s gateway router.
dest IP: B’s IP address
B’s router receives the datagram
Sends from source (MAC is that of B’s router) to destination (B’s MAC)
If B’s MAC not known, ARP query is performed.
Router interface then receives and passes on to the next interface via RIP or other routing protocol.
Once the destination IP is in the same subnet as the src IP, the link-layer frame with B’s MAC address
is made.
LAN technologies
LAN is any computer network interconnecting computers within a geographical area.
Some LAN technologies:
IBM Token Ring (802.5)
Ethernet (802.3)
WiFi (802.11)
etc.
Physical topologies:
bus: all nodes can collide with each other
star: switch in center, nodes don’t collide
Ethernet
Ethernet standards have:
Different speeds
Different physical layer media (twisted pair/fibre physical layer)
MAC protocol and frame format
Twisted pair copper:
RJ45 connector
CAT 5/6/7 cable
Max speed 10Gbps
Max length 100m
Optical Fibre:
Left SC/PC, right SC/PC connectors
Single mode fibre
Ethernet frame: NIC adapter encapsulates IP datagram
Payload : minsize 46 bytes, maxsize MT
dest addr + src addr + type + payload + crc = 64 byte window frame soze;
Cell synchronization
MAin usage of the ethernet preamble: square wave pattern
width of a single bit
clock rate of the sender
Src/Dest address: MAC addresses
If NIC receive matching destination address, pass payload to network layer protocol
Type: Network layer protocol
CRC: Cheksum. Corrupted frame will be dropped
Delivery service features:
Connectionless: no handshaking between send/receive NIC
Unreliable: receiving NIC doesn’t send ACK or NAK to sender NIC.
Recovery only when higher level protocol detects missing frame.
Bus topology
If A sends frame at time $t$, propagation time between A and D is $d$,
and D sends at $t + d - 1$ => COLLISION!
Ethernet CSMA/CD:
Create frame with Network layer datagram.
If NIC senses the channel is idle, begin transmit.
If NIC transmits entire frame without interrupt: done
If another transmission is detected: abort + send jam signal to alert of collision.
Post abortion of signal: Binary/Exponential back-off
