4a1

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Miscellaneous

• Last Modified
• 11/14/2009 95419 AM

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Grades

• Resubmit graded HW2 so that we can note who
  graded each assignment
• Will compute means for each grader and adjust
  accordingly
• If you are not satisfied with that can submit
  written request for regrade
• Midterm one person graded all instances of a
  given question so should be fair
• If you have a question, may submit written
  request for regrade

3
Programming Assignment 2 Resources

• Standard Assignment
• Linux home directories (coming today)
• All keiko, splash, stella, namu, nova
  (.csuglab.cornell.edu)
• Meng Additional cfs01 - cfs20 (.cs.cornell.edu)
• 20 GB for everyone (40 MB per team if run out
  of disk space let Dora Abdullah know)
• Optional Assignment
• 7 machines in 330 Upson Linux Installation
  (marked with red dots)
• 7 machines in 315 Upson Linux Installation
  (marked with red dots)
• Get more information (including passwords) from
  Ranveer
• You must officially sign up!! ASAP please

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Note on homeworks

• Details in homework High level information in
  class why?
• Any particular gory detail is not essential a
  feel for reality is
• Homeworks are the place for gory details in this
  class (too many details to cover in lectures)
• Confront inconsistencies of reality when have
  all references available to you
• Test what you are learning against the real world
• Would people like to see us cover homework
  solutions in class?

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Clarifications

• Path MTU Discovery
• Count to Infinity

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Path MTU DiscoveryTCP

• If doing Path MTU Discovery, start with minimum
  of receivers specified MSS or local MTU and set
  the Dont Fragment Bit
• If ICMP message received indicating that
  fragmentation was required, then segment size
  will be reduced
• Periodically (every 10 min or so), TCP will try a
  higher segment size up to the receivers MSS to
  see if new route is being used that would allow
  larger segments
• Not all implementations support this

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Path MTU Discovery UDP

• Not like TCP where sender sends stream in chunks
  as they see fit and receiver reads in chunks as
  they see fit
• With UDP, the size of the UDP packet is much more
  visible to the application
• May send with DF bit off
• May send with DF bit on and if get ICMP messages
  then IP on host may fragment before sent but not
  exposed to application layer to encourage smaller
  amounts of data sent
• Again not all implementations support

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Path MTU Discovery

• Look in Ethereal at TCP segments, will see Do Not
  Fragment Bit is set
• If on Ethernet, dont usually see adjustment
• Ethernet has one of the smaller MTUs so never
  get an ICMP error saying needs to be smaller
• If sent from local network with larger MTU might
  see this activity

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Distance Vector link cost changes

• Link cost changes
• node detects local link cost change
• updates distance table (line 15)
• if cost change in least cost path, notify
  neighbors (lines 23,24)

algorithm terminates
good news travels fast
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Distance Vector link cost changes

• Link cost changes
• good news travels fast
• bad news travels slow - count to infinity
  problem!

algorithm continues on!
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Distance Vector poisoned reverse

• If Z routes through Y to get to X
• Z tells Y its (Zs) distance to X is infinite (so
  Y wont route to X via Z)
• will this completely solve count to infinity
  problem?

algorithm terminates
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Bigger Loops and Poison Reverse
Loop back through E! Poison reverse will fix this
Loop back through E! Poison reverse will not fix
this E will try to send through B Bs route is
through C so no poison reverse
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Count to Infinity Example with Bigger Loop
B will learn bad news C will have told B
infinity because its route is through B, so B
wont reroute through C E however will have told
B about a good route through D (cost 6) B will
choose that route instead and advertise it as the
new best to C (cost 68 14) it will be worse
than the old one it advertised to C (old cost
1) C will propagate this updated best route to
D (cost 15) D will propagate this new best
route to E (cost 17) E will update the best
route to B (cost 19) Last time it advertised
cost 6 to B It will loop around adding 13 each
time (cost of loop) Will continue until cost E
advertises to B is bigger than 500