Cmpe 150 winter 2009
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CMPE 150 – Winter 2009. Lecture 17 March 5, 2009 P.E. Mantey. CMPE 150 -- Introduction to Computer Networks. Instructor: Patrick Mantey [email protected] http://www.soe.ucsc.edu/~mantey/ Office: Engr. 2 Room 595J Office hours: Tues 3-5 PM, Mon 5-6 PM*

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CMPE 150 – Winter 2009

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Cmpe 150 winter 2009

CMPE 150 – Winter 2009

  • Lecture 17

  • March 5, 2009

  • P.E. Mantey


Cmpe 150 introduction to computer networks

CMPE 150 -- Introduction to Computer Networks

  • Instructor: Patrick Mantey [email protected] http://www.soe.ucsc.edu/~mantey/

  • Office: Engr. 2 Room 595J

  • Office hours: Tues 3-5 PM, Mon 5-6 PM*

  • TA: Anselm Kia [email protected]

  • Web site: http://www.soe.ucsc.edu/classes/cmpe150/Winter09/

  • Text: Tannenbaum: Computer Networks

    (4th edition – available in bookstore, etc. )


Syllabus

Syllabus


Text readings

Text Readings

  • Today:

    • Chapter 6, Section 6.6 (TCP Performance)

  • Tuesday March 10

    • Chapter 7.1 (DNS), 7.2-3

    • Chapter 7.4 (Multimedia)


Internet layering

Internet Layering

  • Level 5 -- Application Layer

  • (rlogin, ftp, SMTP, POP3, IMAP, HTTP..)

  • Level 4-- Transport Layer(a.k.a Host-to-Host)

  • (TCP, UDP)

  • Level 3-- Network Layer (a.k.a. Internet)

  • (IP, ICMP, ARP)

  • Level 2-- (Data)Link Layer / MAC sub-layer

  • (a.k.a. Network Interface or Network Access Layer)

  • Level 1-- Physical Layer


Transport layer connection oriented service

Transport Layer – Connection Oriented Service

  • Addressing / Access Points

  • Connection Request / Establishment

    • Delayed Duplicate Packet Problem

    • Timer management

    • Connection points

  • Connection Release


Today s agenda

Today’s Agenda

  • Transport Layer

    • TCP: Timers / RTT estimates

    • Exponential backoff

    • Wireless vs. wired

    • Transactional TCP

    • Performance


Port detective

Port Detective

http://www.softpedia.com/get/Network-Tools/Network-Testing/Port-Detective.shtml


Cmpe 150 winter 2009

3389 Remote desktop (Windows XP) Reference:

http://www.microsoft.com/windowsxp/using/networking/expert/northrup_03may16.mspx

==========================================================See also:http://www.chebucto.ns.ca/~rakerman/port-table.html


Tcp congestion control

TCP Congestion Control

  • (a) A fast network feeding a low capacity receiver.

  • (b) A slow network feeding a high-capacity receiver.


Jacobson s algorithm 1

Jacobson’s Algorithm 1

  • Determining the round-trip time:

    • TCP keeps RTT variable.

    • When segment sent, TCP measures how long it takes to get ACK back (M).

    • RTT = alpha*RTT + (1-alpha)M.

    • alpha: smoothing factor; determines weight given to previous estimate.

    • Typically, alpha=7/8.


Jacobson s algorithm 2

Jacobson’s Algorithm 2

  • Determining timeout value:

    • Measure RTT variation, or |RTT-M|.

    • Keeps smoothed value of cumulative variation D=alpha*D+(1-alpha)|RTT-M|.

    • Alpha may or may not be the same as value used to smooth RTT.

    • Timeout = RTT+4*D.


Karn s algorithm

Karn’s Algorithm

  • How to account for ACKs of retransmitted segments?

    • Count it for first or second transmission?

    • Karn proposed not to update RTT on any retransmitted segment.

    • Instead RTT is doubled on each failure until segments get through.


Tcp congestion control example

TCP Congestion Control: Example

cwin

timeout

threshold

threshold

time


Persistence timer

Persistence Timer

  • Prevents deadlock if an window update packet is lost and advertised window = 0.

  • When persistence timer goes off, sender probes receiver; receiver replies with its current advertised window.

  • If = 0, persistence timer is set again.


Keepalive timer

Keepalive Timer

  • Goes off when a connection is idle for a long time.

  • Causes one side to check whether the other side is still alive.

  • If no answer, connection terminated.


Tcp timer management

TCP Timer Management

  • (a) Probability density of ACK arrival times in the data link layer.

  • (b) Probability density of ACK arrival times for TCP.


Tcp retransmission timer

TCP Retransmission Timer

  • When segment sent, retransmission timer starts.

    • If segment ACKed, timer stops.

    • If time out, segment retransmitted and timer starts again.


How to set timer

How to set timer?

  • Based on round-trip time: time between a segment is sent and ACK comes back.

  • If timer is too short, unnecessary retransmissions.

  • If timer is too long, long retransmission delay.


Jacobson s algorithm 11

Jacobson’s Algorithm 1

  • Determining the round-trip time:

    • TCP keeps RTT variable.

    • When segment sent, TCP measures how long it takes to get ACK back (M).

    • RTT = alpha*RTT + (1-alpha)M.

    • alpha: smoothing factor; determines weight given to previous estimate.

    • Typically, alpha=7/8.


Jacobson s algorithm 21

Jacobson’s Algorithm 2

  • Determining timeout value:

    • Measure RTT variation, or |RTT-M|.

    • Keeps smoothed value of cumulative variation D=alpha*D+(1-alpha)|RTT-M|.

    • Alpha may or may not be the same as value used to smooth RTT.

    • Timeout = RTT+4*D.


Karn s algorithm1

Karn’s Algorithm

  • How to account for ACKs of retransmitted segments?

    • Count it for first or second transmission?

    • Karn proposed not to update RTT on any retransmitted segment.

    • Instead RTT is doubled on each failure until segments get through.


Persistence timer1

Persistence Timer

  • Prevents deadlock if an window update packet is lost and advertised window = 0.

  • When persistence timer goes off, sender probes receiver; receiver replies with its current advertised window.

  • If = 0, persistence timer is set again.


Keepalive timer1

Keepalive Timer

  • Goes off when a connection is idle for a long time.

  • Causes one side to check whether the other side is still alive.

  • If no answer, connection terminated.


Performance issues

Performance Issues

  • Performance Problems in Computer Networks

  • Network Performance Measurement

  • System Design for Better Performance

  • Fast TPDU Processing

  • Protocols for Gigabit Networks


System design for better performance

System Design for Better Performance

  • Rules:

  • CPU speed is more important than network speed.

  • Reduce packet count to reduce software overhead.

  • Minimize context switches.

  • Minimize copying.

  • You can buy more bandwidth but not lower delay.

  • Avoiding congestion is better than recovering from it.

  • Avoid timeouts.


Wireless tcp and udp

Wireless TCP and UDP

  • Splitting a TCP connection into two connections.


Wireless tcp 1

Wireless TCP 1

  • According to layered system design principles, transport protocol should be independent of underlying technology.

  • However, wireless networks invalidate this principle.

    • Ignoring properties of wireless medium can lead to poor TCP performance.

    • Problem: TCP’s congestion control.


Wireless tcp 2

Wireless TCP 2

  • Problem: packet loss as congestion indicator.

    • When retransmission timer times out, sender slows down.

  • Wireless links are lossy!

    • Dealing with losses in this case should be re-sending lost segments asap.


Indirect tcp i tcp

Indirect TCP (I-TCP)

  • [Bakne and Badrinath, 1995].

  • Split TCP connection in 2: one from sender to base station and the other from base station to receiver.

    • Base station serves as “repeater”: copies segments between connections in both directions.

    • Connections are homogeneous; timeouts on 1st. connection, slow down sender.

    • Problem: violates TCP’s e2e’ness.

      • Example: ACKs to sender mean base station received segments, not necessarily receiver.


Snoop tcp

Snoop TCP

  • [Balakrishnan et al., 1995].

  • Does not break connection.

  • Modifications to base station’s network layer code.

    • Snooping agent on base station observes and caches TCP segments sent to mobile host and ACKs coming back.

    • If it doesn’t see an ACK for a segment or sees duplicate ACKs, it times out and retransmits.

    • But source may time out anyway.


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