Direct Link Networks Reliable Transmission

  Computer Network Direct Link Networks Reliable Transmission

  

Outline

• Direct link networks (Ch. 2)
• – Encoding – Framing – Error detection
• – Reliable delivery
• – Media access control
• – Network Adapter

  

Hardware Building Blocks

• Nodes
• – Hosts: General-purpose computers
• – Switches: Typically special-purpose H/W
• – Routers
• Links
• – Cooper with electronic signaling
• – Glass fibre with optical signaling
• – Wireless with electromagnetic (radio, Infra Red,

  Microwave) signaling

  

Link

• Cooper Based Media
• – Cat 5 Twisted Pair 10 – 100 Mbps 100m
• – Thin Net Coax 10 – 100 Mbps 200m
• – Thick Net Coax 10 –100 Mbps 500m

  

Link

• Optical Media
• – Multimode Fibre 100 Mbps

  2Km

• – Singlemode Fibre 100-2400 Mbps 40Km

  

Link

• Comparison Of Octical Media
• – Single mode
• Lower attenuation (longer distances)
• Lower dispersion (higher data rates)
• – Multi mode
>Cheap (IF vs Laser)
• Easier to terminate

  

Link

• Advantages of optical communication
• – higher bandwidths
• – superior attenuation properties
• – immune from electromagnetic interference
• – no crosstalk between fibers
• – thin, lightweight, and cheap (the fiber, not the optical-electrical interfaces)

  

Link

• Wireless transmission
• – 2.4 GHz radio 11 Mbps 7Km
• – Infrared (IR) link 1 Mbps 9 m
• – Geosynchronous satellite 600-1000 Mbps continent

  Services available from carriers

  

Encoding (line coding)

‰problems with signal transmission

• – attenuation: signal power absorbed by medium

• – dispersion: a discrete signal spreads in space
• – noise: random background “signals&rd
• – Problem is how are 0’s and 1’s detected at receiving node in a robust fashion

  

Encoding Taxonomy

• Digital data, digital signal
• – codes which represent bits
• – our focus
• – many strategies!
• Analog data, digital signal
• – sampling to represent voltages
• Digital data, analog signal
• – modulation to represent bits
• Analog data, analog signal
• – modulation to represent voltages

Non-Return to Zero (NRZ)

• 0 Æ low signal; 1 Æ high signal
• Poor clock synchronization when there is a long string of 1’s or 0’s

  

NRZ-Inverted (NRZI)

• • 1 Æ make transition; 0 Æ stay at the current

  level
• Addresses clock synchronization problem for long strings of 1’s
• – still out of luck on consecutive strings of 0’s

  

Manchester Encoding

• Each bit contains a transition
• – 0 Æ high-to-low transition
• – 1 Æ low-to-high transition
>

• Enables effective clock recovery at receiver

• Used by 802.3 - 10 Mbps Ethernet

  

Manchester Encoding contd.

• • Disadvantage: needs a clock that is twice as

  fast as the transmission rate

  1

  1

  1

  1

  1 Manchester Clock

  

4B/5B Encoding

• Tries to address inefficiencies in Manchester • Idea is to insert extra bits in bit stream to break up long strings of 0’s or 1’s
• Every 4 bits of data are encoded in 5 bit code (see text for details)
• – at most one leading 0
• – at most two trailing 0’s
>therefore never more than three consecutive 0&rsq
• Uses NRZI to put bits on wire
• – reason why code focused on zeros

Framing

• Blocks of data (not just sequences of bits) are exchanged across links --- frames
• Problem: how to tell where a frame begins and where it ends

Character-Based Framing

• Special characters (tags) are used to indicate idle fill (ASCII SYN), start of text (STX) and end of text (ETX)
• Used by PPP (point-to-point protocol)
• Problem: what if tag characters occur in packet data

  

Character-Based Framing contd.

• Solution: Insert extra escape characters when a tag appears in data field

  

Bit-Oriented Framing

• Each frame begins with a start and end bit sequence (flag)

  6

• – 01111110 = 01 0 is the usual flag
• Example: HDLC (high-level data link protocol)
• Problem: what if flag appears in body

  

Bit Stuffing

• Sender: inserts a 0 after five consecutive 1s
• Receiver: when it sees five 1s makes decision based on next two bits
• – if next bit 0 (this is a stuffed bit), remove it
• – if next bit 1, look at next bit
• if 0 this is end of frame (receiver has seen

  01111110)

• if 1 this is an error, discard frame (receiver has seen

  01111111)

  Bit Stuffing Example

Framing with Length Fields

• Detecting the end of the frame done by sending length information (e.g. number of bytes in the frame) in the header
• Used by DECNET’s DDCMP
• Q: Is a flag still needed at the start of the frame ?

  Digital transmission over

analog

• modulation to represent bits
• – amplitude (AM)
• – frequency (FM)
• – phase/phase shift
• – combinations of these

  

Error Detection

• Bit errors can be introduced in packets
• This problem has been studied for a long time
• – Error detection (and correction) codes
• – Cyclic redundancy check (CRC) is a common error detection method
• Basic idea of any scheme is to add redundant data
• – A primary goal is to send minimal amount of redundant data
• – Another goal is to make generation of code fast

  

Error Detection

• We use error-detecting codes to help detect errors
• Example code: Consider the function that duplicates each bit in the message
• – E.g. the message 1011001 would be mapped to the code 11001111000011, and then transmitted to the sender. The receiver knows that bits come in pairs. If the two bits in a pair are different, it declares that there was a bit error.

  

Parity

  Parity added to make @ 1s even/odd

  0 1 1 1 0 1 0 1 1 If parity is wrong ERROR If parity is right NO ERROR

  (or an even number of errors has occurred) Two-Dimensional Parity

  Q: If an error is detected in this scheme, is it also correctable?

  Two-Dimensional Parity 1 0 0 1 0 0

  1 0 0 1 0 0

Example

  0 0 0 0 0 1 0 0 0 0 0 1 Two

  One 1 0 0 1 0 0 1 0 0 1 0 0 errors error

  1 1 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 1 1 0 0 1 1 1

  Arrows indicate failed check bits 1 0 0 1 0 0

  1 0 0 1 0 0 0 0 0 1 0 1 0 0 0 1 0 1 Four

  1 0 0 1 0 0 Three 1 0 0 1 0 0 errors errors 1 0 0 1 1 0

  1 0 0 0 1 0 1 0 0 1 1 1 1 0 0 1 1 1

  

Internet Checksum

• Used by IP, TCP, UDP, ...

• Method
• – Break message as a sequence of 16-bit integers
• – Add these integers using 16-bit one’s complement arithmetic
• – Take the one’s complement of the result
• – Resulting 16-bit number is the checksum
• It’s simple to implement
• – Reason why it’s used despite relatively weak detection capability

  

Reliable Transmission

• Big Q: How do we send a packet reliably when it can be lost and/or corrupted in the network?
• Mechanisms
• – Acknowledgements – Timeouts
• Simplest reliable protocol: Stop and Wait
• – send a packet
• – stop and wait until an ACK arrives from the receiver
• – retransmit if timeout occurs before ACK arrives

  Stop and Wait Recovering from Errors

  

Reliable Transmission

• How does the receiver recognize a duplicate transmission (e.g. in the case of a lost ACK)?
• – Solution: Put sequence number in packet
• Performance
• – No pipeline effect
• – Example
>For a l.5 Mbps link with 45 ms RTT
• Bandwidth-delay product = 67.5 Kb (8KB)
• 1KB packets imply 1/8th link utilization
• Solution: Sliding Window Protocols

  

How do we keep the pipe full?

• Send multiple packets without waiting for the first to be ACKed

• • Upper bound on un-ACKed packets, called

  Sender Receiver window ime T

  

Sliding Windows: Sender

• State
• – Packets sent and ACKed (LAR = last ACK received)
• – Packets sent but not ACKed (buffer for SWS packets)
• – Packets not yet sent (LFS = last packet sent)
• Keep LFS - LAR SWS

  ≤

  ≤ ^SWS

  LAR LFS …

  …

• Advance LAR when ACK arrives

  

Sliding Windows: Receiver

• State
• – Packets received and ACKed (NFE = next frame expected, LFR = last packet received)
• – Packets received out of order (buffer for RWS packets)
• – Packets not yet received (LFA = last frame acceptable)
• Keep LFA - LFR RWS

  ≤ _RWS ≤

  … _{LFR LFA} …

  

What if we lose a packet?

• Selective repeat/ack (SACK)
• – Receiver sends ACK for each packet in window
• – On timeout, sender resends only the missing packet
• – Proposed for TCP
• Go back N with buffering out-of-order packets

  (TCP)

• – Receiver ACKs “got up through packet k”
• If multiple packets are received, only one ACK is needed
>– On timeout, sender restarts from k + 1
• – Different from traditional Go back N

Traditional Go back N window size 4 Go back N with buffering window size 4

  

Sender Algorithm

• Send full window, set timeout
• On ACK
• – If it increases LAR (packets sent and ACKed)
>send remaining packets in wi
• On timeout
• – Resend all outstanding packets starting from

  LAR + 1 (first packet not yet ACKed)

  

Receiver Algorithm

• On packet arrival
• – If packet is the NFE (next frame expected)
• Send ACK
• Increase NFE
• Deliver packet(s) to the upper layer (could fill hole in buffer)
• – Else
>Send ACK
• Discard if < NFE or > LFA

  

How to determine timeouts?

• Problem:
• – If timeout too small, useless retransmits
• – If timeout too large, low utilization (pipe not full)
• Solution:

  ≈

• – On direct link, timeout 2 x PROP
• – Trickier to estimate at transport layer • RTT varies with congestion, route changes, etc.

  

Sequence Number Space

• Recall seq # (SeqNum) cannot grow arbitrarily
• SWS MaxSeqNum - 1 is not sufficient

  ≤

• – Suppose 3-bit SeqNum field (0…7)
• – SWS = RWS = 7
• – sender transmits packets 0..6
• – arrive successfully, but ACKs lost
• – sender retransmits 0..6
• – receiver expecting 7, 0..5, but receives the original incarnation of 0..5
• SWS (MaxSeqNum + 1)/2 is correct rule

  ≤

  

Sliding Window Summary

• Sliding window is best known algorithm in networking
• First role is to enable reliable delivery of packets
• – Timeouts and acknowledgements
• Second role is to enable in order delivery of packets
• – Receiver doesn’t pass data up to app until it has packets in order
• Third role is to enable flow control
• – Prevents sender from overflowing receiver’s buffer