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

one 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

0 0

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

• Tim

eout

• Packet

• ACK

• Tim

eout

• Packet

• Tim

eout

• Packet

• ACK

• Tim

eout

• Packet

• ACK

• Tim

eout

• Packet

• ACK

• Tim

eout

• 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