CS 457 Lecture 5 Reliable Delivery Part 2 Fall 2011 Stop and Wait - - PowerPoint PPT Presentation

CS 457 – Lecture 5 Reliable Delivery Part 2

Fall 2011

Stop and Wait in Action

Stop and Wait In Action (2)

How Long Should Sender Wait?

• Sender sets a timeout to wait for an ACK

– Too short: wasted retransmissions – Too long: excessive delays when packet lost

• TCP sets timeout as a function of the RTT

– Expect ACK to arrive after an RTT – … plus a fudge factor to account for queuing

• But, how does the sender know the RTT?

– Can estimate the RTT by watching the ACKs – Smooth estimate: keep a running average of the RTT

• EstimatedRTT = a * EstimatedRTT + (1 –a ) *

SampleRTT

– Compute timeout: TimeOut = 2 * EstimatedRTT

Example RTT Estimation

Performance of Stop and Wait

Example: (good test question) 1 Gbps link (1 *10^9 bits/sec), 15 ms propagation delay 8000 bit packet How does stop and wait perform?

Performance of Stop and Wait

T Transmit = 8kb/pkt 10**9 b/s = 8 us

– Utilization = fraction of time sender is busy sending – 1KB packet every 30 ms ->

• nly 33KB/s throughput over 1 Gbps link

U

sender

= .008

30.008

= 0.00027

microsec

L / R RTT + L / R =

L (packet length in bits) R (transmission rate, bps) =

Delay = T Transmit prop queue

T T + +

• +

Delay = 8 us + 15 ms = 15.008 ms

• ignore

Stop-and-Wait Operation

• first bit transmitted, t = 0
• sender
• receiver
• RTT
• last bit transmitted, t = L / R
• first bit arrives
• last bit arrives, send ACK
• ACK arrives, send next
• packet, t = RTT + L / R

U

sender

= .008

30.008

= 0.00027

microsec

L / R RTT + L / R =

Motivation for Sliding Window

• Stop-and-wait is inefficient

– Only one frame is “in flight” at a time – Especially bad when delay-bandwidth product is high

• Another Numerical example

– 1.5 Mbps link with a 45 msec round-trip time (RTT)

• Delay-bandwidth product is 67.5 Kbits (or 8 KBytes)

– But, sender can send at most one packet per RTT

• Assuming a segment size of 1 KB (8 Kbits)
• … leads to 8 Kbits/segment / 45 msec/segment  182

Kbps

• That’s just one-eighth of the 1.5 Mbps link capacity

Sliding Window

• Allow a larger amount of data “in flight”

– Allow sender to get ahead of the receiver – … though not too far ahead

• Sending process
• Receiving process
• Last byte ACKed
• Last byte sent
• TCP
• TCP
• Next byte expected
• Last byte written
• Last byte read
• Last byte received

Pipelined Protocols

Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged packets

– range of sequence numbers must be increased – buffering at sender and/or receiver

• Two generic forms of pipelined protocols:

go-Back-N (GBN) or selective repeat (SR)

Pipelining: Increased Utilization

• first bit transmitted, t = 0
• sender
• receiver
• RTT
• last bit transmitted, t = L / R
• first bit arrives
• last bit arrives, send ACK
• ACK arrives, send next
• packet, t = RTT + L / R
• last bit of 2nd packet arrives, send ACK
• last bit of 3rd packet arrives, send ACK

U

sender

= .024

30.008

= 0.0008

microsecon

3 * L / R RTT + L / R =

• Increase utilization
• by a factor of 3!

Go-Back-N

Sender:

• k-bit seq # in packet header
• “window” of up to N, consecutive unACK’ed pkts allowed
• ACK(n): cumulative ACK

– ACKs all pkts up to and including seq # n

– may receive duplicate ACKs (see receiver)

• timer for each in-flight packet
• timeout(n): retransmit packet n

and all higher seq # packets in window

GBN in action

Selective Repeat

• Receiver individually acknowledges all

correctly received packets

– buffers packets, as needed, for eventual in-order delivery to upper layer

• Sender only resends packets for which ACK

not received

– sender timer for each unACKed packet

• Sender window

– N consecutive seq #’s – again limits seq #s of sent, unACKed packets

Selective Repeat: Sender, Receiver Windows

Selective Repeat

data from above :

• if next available seq # in

window, send packet

timeout(n):

• resend packet n, restart

timer

ACK(n) in [sendbase,sendbase+N]:

• mark packet n as received
• if n smallest unACKed pkt,

advance window base to next unACKed seq #

• sender
• packet n in [rcvbase, rcvbase

+N-1]

• send ACK(n)
• ut-of-order: buffer
• in-order: deliver (also deliver

buffered, in-order packets), advance window to next not- yet-received packet

• packet n in [rcvbase-

N,rcvbase-1]

• ACK(n)
• otherwise:
• ignore
• receiver

Selective Repeat in Action

Selective Repeat and Seq Numbers

Example:

• seq #’s: 0, 1, 2, 3
• window size=3
• receiver sees no

difference in two scenarios!

• incorrectly passes

duplicate data as new in (a) Q: what relationship between seq # size and window size?

What’s Next

• Read Chapter 1 and 2.1-2.7
• Next Lecture Topics from Chapter 2.6 and 2.7

– Ethernet

• A critical topic in networking
• Homework

– Homework Due Thurs

• Project 1

– Due 9/16 at 11:45pm.