Linklayer

data link layer

ethernet, wifi, fiber, telephony

transfer frames: ethernet, PPP, MAC address (add size, error correction codes)

• Inserts framing info to denote frame boundaries
• Inserts control, addressing and error correction info in header
• Detects transmission errors on link. May retransmit frames
• Activation, maintenance, & deactivation of link connection

• mission: transfer frames from one node, over a link
• scope: Service received from physical layer is the ability to move a bit across the link
  • A packet is transferred by different link protocols over different links
  • Each data link protocol provides different services
• addressing mechanism:MAC
• data types : frame
• responsibilities/services

  - Framing:  encapsulate datagram into frame, adding header, trailer; identify source, destination with addresses • Different from IP 
  

  addresses!!

  - Link access: use medium access control (MAC) protocol
  - Error Detection
  - Error Correction
  - Reliable Delivery: critical for wireless links ➙ high error rates
  

CSMA/CD

• Carrier Sense: Listen before talking Multiple Access: Broadcast Medium
• Collision Detection: Listen as you talk. If you hear someone else, be quiet
• Capturing a Channel: If no other station initiates transmission during this period, sender has captured the channel. t = tprop
• Ethernet is the most famous example
• Before transmitting, listen
• If channel is sensed idle, send the frame
• Else, defer transmission a random time
• If collision is detected, abort transmission
• reduces channel wastage
• Min transmission time must be long enough for collisions to propagate
• Length of the packet >= 2 Tp Bandwidth of the link worst case

ARP

• mission: Translate a network-layer address (IP) to a link-layer address (MAC), Similar to DNS
• frame fields:
  • Sender Ethernet and IP address
  • Target Ethernet and IP address
• transmission mechanism
  • Sent in an Ethernet frame to a broadcast address
  • ARP Reply takes the same form
  • broadcast request, if some one know, reply
  • if in another subnet, sent datagram to router
• caching
  • Each Ethernet adapter uses a table to keep track of known mappings between IP addresses and Ethernet MAC Addrs
  • When given IP address, ARP looks in ARP table and returns corresponding MAC address
    • If IP not in table, queue frame and broadcast ARP request
• security
  • ARP frames are not authenticateds
  • May be spoofed by a malicious entity
  • May be proxied by design
• gratuitous use
  • ARP can be an “announcement” protocol
  • Can send an ARP frame just to update other node’s ARP tables
  • Set target address and sender address to the same value

csma/ca

• CSMA / Collision Avoidance
• 1. Listen for a specified time (DIFS) If medium is not free:
  • 1. Exponential Backoff (下次请求的时间)
• If medium is free:
  • Transmit entire frame
  • Await ACK frame （AP send ACK to sender）
  • If no ACK, then Exponential Backoff
• ACK
  • protects against bit errors
  • Receiver only sends ACK if frame passes CRC
• Exponential Backoff
  • Each node chooses a random number
  • Max size increases from round to round
  • Can be modeled as a counter (wait time)
    • Decremented during any idle time
    • Put on hold if another node transmits and for a short time afterwards (SIFS)
    • When zero, the counting station may transmit

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Backlinks

• Network (my_note)