The Transport Layer: Review Smith College, CSC 249 February 22, - - PDF document

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The Transport Layer: Review Smith College, CSC 249 February 22, - - PDF document

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The Transport Layer: Review Smith College, CSC 249 February 22, 2018 1 TCP Congestion Control 1. How does a sender sense congestion? v Loss event timeout vs. 3 duplicate ACKs 2. How does a sender determine its sending rate? v The dynamic

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The Transport Layer: Review

Smith College, CSC 249 February 22, 2018

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TCP Congestion Control

  • 1. How does a sender sense congestion?

v Loss event – timeout vs. 3 duplicate ACKs

  • 2. How does a sender determine its

sending rate?

v The dynamic variable “CWND” (congestion window) v Variable ssthresh to determine how to grow cwnd

  • 3. What algorithm is used to change the

send-rate?

v Many phases and alternatives...

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Reaction to Loss Events

q Exponential increase switches to linear increase

when CongWin gets to the ‘threshold’ value (size)

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Identify everything on this graph

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TCP Congestion Control Algorithm

Increase Sending Rate Phase Options:

1.

When CongWin is below Threshold, sender in slow-start phase, window grows exponentially.

  • 2. When CongWin is above Threshold, sender is in

congestion-avoidance phase, window grows linearly. Decrease Sending Rate Phase Options:

1.

When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold.

  • 2. When timeout occurs, Threshold set to

CongWin/2 and CongWin is set to 1 MSS.

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TCP Congestion Control Algorithm

Three major phases / mechanisms:

1) Slow start – at 1 max segment size

  • But increase _________

2) Congestion Avoidance phase

  • AIMD = additive incr, multiplicative decr
  • Using cwnd and ssthresh

3) Fast Recovery

  • Increase of cwnd each round trip time
  • Slow start: ________
  • Congestion avoidance: ________

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Summary TCP reaction to loss

§ Loss indicated by timeout

  • cwnd set to 1 MSS
  • ssthresh set to cwnd/2
  • Window (cwnd ) grows exponentially (slow start)

to the threshold, then grows linearly § Loss indicated by 3 duplicate ACKs

  • Network capable of delivering some segments, so...
  • cwnd is cut in half (=ssthresh)
  • Window grows linearly
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Transport Layer Review

qThe transport layer services are:

v v v v v v v v

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Transport Layer Review

qThe transport layer does not provide:

v v v v v v v v

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Transport Layer Review

qCompare TCP and UDP (pros and cons?)

v v v v v v v v

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Transport Layer Review

qTCP Connection Management includes

v v v v v v v v

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Transport Layer Review

qElements of TCP reliability:

v v v v v v v v

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Transport Layer Review

qElements of congestion control algorithm

v v v v v v v v

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TCP Flow Control

source port # dest port #

32 bits

application data (variable length) sequence number acknowledgement number

receive window Urg data pointer checksum

F S R P A U

head len not used

  • ptions (variable length)

URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands)

# of bytes rcvr willing to accept

counting by bytes

  • f data

(not segments) Internet checksum (as in UDP)

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TCP flow control (quick & easy)

q Receiver “advertises” free buffer space by including

rwnd value in TCP header of receiver-to-sender segments

v RcvBuffer size set via socket options (typical default is

4096 bytes)

v Many operating systems auto-adjust RcvBuffer

q Sender limits amount of un-ACKed (“in-flight”) data

to receiver’s rwnd value

v Guarantees receive buffer will not overflow

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TCP flow control

application process

TCP socket receiver buffers

TCP code IP code

application OS

receiver protocol stack The application may remove data from the TCP socket buffers …. … slower than the TCP receiver is delivering it into the buffers (sender is sending)

from sender

receiver controls sender, so sender won’t overflow receiver’s buffer by transmitting too much, too fast

flow control to the rescue!

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Transport Layer Review

qOther questions?

v v v v v v v v

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A fun tangent... Finite State Machines

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Finite State Machines

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TCP sender events:

(1) data received from application:

  • 1. Create a segment and assign a SEQ number

v SEQ # is byte-stream number of first data byte in segment

  • 2. Start timer if it is not already running

v Timer is for the oldest un-acked segment v Expiration interval: TimeOutInterval

(2) timeout:

  • 1. Retransmit segment that caused the timeout
  • 2. Restart the timer

(3) ACK received:

q For previously unacked segments

  • 1. update what is known to be acked
  • 2. start timer if there are outstanding segments

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TCP Congestion Control

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TCP Congestion Control: FSM

slow start congestion avoidance fast recovery

loss: timeout loss: timeout new ACK loss: 3dupACK loss: 3dupACK loss: timeout cwnd > ssthresh

Transport Layer Summary

q TCP and UDP Services q Encapsulation (create and attach header) q Multiplexing and demultiplexing q Checksum q Connection management q Reliable transport service q Congestion control q Detect loss and retransmit

v Detect out-of-order and reorder

q Flow Control

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Transport services and protocols

q Provide logical

communication, a virtual connection …between application processes running on different hosts This is not a physical path

including routers

application transport network data link physical application transport network data link physical

Introduction to the Network Layer

qDesired network layer services...

ØActual network layer services

qImplemented in hosts and routers qTwo main network layer functions qThree main network layer protocols

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Network Layer Services of IP?

q Guaranteed delivery? q Guaranteed minimum delay? q In-order datagram delivery? q Guaranteed minimum bandwidth to

flow?

q Restrictions on changes in inter-

packet spacing?

q IP Provides? à “Best-effort service”

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Key Network-Layer Functions

  • 1. routing: determine route taken by

packets from source to destination

vNetwork-wide routing algorithms

  • 2. forwarding: move packets from

router’s input link to appropriate

  • utput link

vInternal to a single router

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1

2 3

0111

Destination address value in arriving packet’s header

routing algorithm local forwarding table header value output link

0100 0101 0111 1001 3 2 2 1

Network Layer: Routing and Forwarding

Ø Create versus use the forwarding table

Network Layer, Chapter 4

q Router ‘switching fabric’

v Hardware / electrical pathways within a router

q Forwarding – use the forwarding table to

transmit, or forward, each packet to the correct output link, based on the destination IP address

q Routing – Create the forwarding tables

v Decentralized vs. Centralized algorithms v Within an ISP vs. between ISPs

q Softward Defined Networks, SDN

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