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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
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. )

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

slide12
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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