CS 457 Lecture 4 Link Layer & Reliable Delivery Part 1 Fall - - PowerPoint PPT Presentation

CS 457 – Lecture 4 Link Layer & Reliable Delivery Part 1

Fall 2011

Message, Segment, Packet, & Frame

• HTTP
• TCP
• IP
• Ethernet
• interface
• HTTP
• TCP
• IP
• Ethernet
• interface
• IP
• IP
• Ethernet
• interface
• Ethernet
• interface
• SONET
• interface
• SONET
• interface
• host
• host
• router
• router
• HTTP message
• TCP segment
• IP packet
• IP packet
• IP packet
• Ethernet frame
• Ethernet frame
• SONET frame

WE ARE HERE IN THE LAYERS

Point to Point Data Links

• One sender, One receiver, One link:

– no Media Access Control – no need for explicit MAC addressing – e.g., dialup link, ISDN line

• Popular Point-to-Point DLC protocols:

– PPP (point-to-point protocol) – HDLC: High level data link control

Encoding Bits On The Wire

• Simple idea of “high” for 1 and low for 0

– Say 1 usec per bit. Sender sends 4 bits with 4 usec “high” – Receiver sees 4.6 usec of “high”…. Was that 4 or 5 bits?

• Allows clocks in sending and receiving nodes to

synchronize to each other

– no need for a centralized, global clock among nodes!

• Physical-layer stuff for Electrical Engineers!

Variety of Encoding Approaches

Data Frame

• Link Layer Protocols Define Data Frames

– Header information, error checking, – Data to transmit (Body)

Ethernet Frame Structure

• Sending adapter encapsulates packet in

frame

• Preamble: synchronization

– Seven bytes with pattern 10101010, followed by

• ne byte with pattern 10101011

– Used to synchronize receiver, sender clock rates

Ethernet Frame Structure (Cont.)

• Addresses: source and destination MAC addresses

– Adaptor passes frame to network-level protocol

• If destination address matches the adaptor
• Or the destination address is the broadcast address

– Otherwise, adapter discards frame

• Type: indicates the higher layer protocol

– Usually IP – But also Novell IPX, AppleTalk, …

• CRC: cyclic redundancy check

– Checked at receiver – If error is detected, the frame is simply dropped

Error Detection

• EDC= Error Detection and Correction bits (redundancy)
• D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction

Parity Checking

• Single Bit Parity:
• Detect single bit errors
• Two Dimensional Bit Parity:
• Detect and correct single bit errors

Internet Checksum

Sender:

• treat segment contents as

sequence of 16-bit integers

• checksum: addition (1’s

complement sum) of segment contents

• sender puts checksum

value into UDP checksum field Receiver:

• compute checksum of received

segment

• check if computed checksum

equals checksum field value: – NO - error detected – YES - no error detected. But maybe errors nonetheless? More later ….

• Goal: detect “errors” (e.g., flipped bits) in transmitted

segment

Checksumming: Cyclic Redundancy Check

• view data bits D as a polynomial
• choose r+1 polynomial C
• goal: choose r CRC bits, R, such that

– <D,R> exactly divisible by C (modulo 2) – receiver knows C, divides <D,R> by C. If non-zero remainder: error detected! – can detect all burst errors less than r+1 bits

• widely used in practice (ATM, HDCL)

CRC Example

Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R R = remainder[ ] D.2r G

Reliable Transfer

Fundamental Networking Topic

Most important topic thus far in this course

• Frames may be lost or corrupted

– Encoding failures where clocks get out of sync – Error detection reports packet is corrupted – And a vast number of other reasons….

• Can we build link so it appears reliable?

– Build link so errors never occur??

• not feasible

– Add error correction to frames?

• Requires lots of additional overhead bytes for each frame
• Doesn’t help if entire frame lost (e.g. clock sync error)

Stop and Wait (first attempt… has an error)

• Sender writes frame onto wire

and sets a timeout to wait for an ACK

– Sender now “stops” and “waits” for the ACK

• Upon receiving a packet,

receiver sends an ACK

• Sender Will Either

1) receive an ACK and can send next frame 2) times out and receives the “lost” frame

Reasons for Retransmission

• Packet
• Timeout
• Packet
• ACK
• Timeout
• Packet
• Timeout
• Packet
• ACK
• Timeout
• Packet
• ACK
• Timeout
• Packet
• ACK
• Timeout
• ACK lost
• DUPLICATE

PACKET

• Packet lost
• Early timeout
• DUPLICATE

PACKETS

Stop and Wait (second attempt)

• Sender writes frame onto wire with Seq #0

and sets a timeout to wait for an ACK

– Sender now “stops” and “waits” for the ACK

• Upon receiving a packet with Seq #0

receiver sends an ACK for Seq #0

• Sender Will Either

1) receive an ACK with Seq #0 and send next frame with Seq #1 2) times out and resends the frame with Seq #0

Stop and Wait in Action

Stop and Wait In Action (2)

What’s Next

• Read Chapter 1, 2.1 - 2.5
• Next Lecture Topics from Chapter 2.5

– Reliable Transmission

• Critical Topic In Networking
• Homework

– Due Thursday

• Project 1

– Due 9/16 by 11:45