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588 Section 6. Neil Spring May 11, 1999. Schedule. Notes (1 slide) Multicast review (3slides) RLM (the paper you didn’t read) (3 slides) ALF & SRM (8 slides). Reminders & Notes. Programming Assignment 2 due May 24 Homework 3 will be due June 1 Project 3 will be due June 7

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588 section 6

588 Section 6

Neil Spring

May 11, 1999

schedule
Schedule
  • Notes
    • (1 slide)
  • Multicast review
    • (3slides)
  • RLM (the paper you didn’t read)
    • (3 slides)
  • ALF & SRM
    • (8 slides)
reminders notes
Reminders & Notes
  • Programming Assignment 2 due May 24
  • Homework 3 will be due June 1
  • Project 3 will be due June 7
  • Final Project too!
  • Project 1 seems to have gone well
  • Thanks for your help with the images
multicast summary review
Multicast Summary (Review)
  • What?
    • Queries to anyone who is listening
    • Updates to shared state
  • Why?
    • Economic use of resources
    • Scale
  • Scope control
  • Plenty of ways to generate distribution trees
multicast trees review
Multicast Trees (Review)
  • Reverse Path Flooding
    • find a good tree
  • Reverse Path Broadcasting
    • avoid duplicates on a wire by electing a parent
  • Truncated RPB
    • prune uninterested leaves
  • Reverse Path Multicasting
    • prune on demand
multicast challenges review
Multicast Challenges (Review)
  • Differences in receiver bandwidth
  • Reliability
    • redundant transmission
    • retransmission
  • Ordering/consistency of multiple senders
multicast challenge heterogeneity
Multicast Challenge: Heterogeneity
  • We all want to watch the rolling stones on our computers.
  • Our world includes links speeds that differ by 3 orders of magnitude (at least!)
    • modems, isdn
    • lans, cable modems
    • wireless links
  • One broadcast does not fit all!
response simulcast
Response: Simulcast
  • 28.8 modems get this stream.
  • Direct network links get another.
  • Problems?
response rlm
Response: RLM
  • Receiver-driven Layered Multicast
  • Send a bunch of streams of increasing detail
    • base layer: includes the most important stuff, small
    • additional layers: add detail, may be large
  • Receivers dynamically decide how many layers to subscribe to.
  • Loss implies congestion implies over-subscription.
rlm how many layers
RLM: How many layers?
  • Startup:
    • get the first one, wait a few seconds
    • ask for the next one, wait a few seconds,
    • repeat until drop (skipped sequence number)
    • go back to the previous layer
  • With exponentially increasing timer:
    • Try the next layer
      • maybe there’s new bandwidth or less congestion
      • maybe the drop wasn’t your fault.
multicast challenge reliability
Multicast Challenge: Reliability
  • The SRM paper is one approach (little later)
  • Redundant transmission is another
    • messages may be small so redundancy is cheap
    • explicit: here are the previous 6 commands again
    • implicit: redundancy in video or audio streams
      • real audio has some (doesn’t rely on it exclusively)
  • Or you just tolerate missing a frame
why is reliability hard
Why is reliability hard?
  • Can’t keep state about all receivers and still scale
  • Can’t reason about RTT/cwnd
  • No fate sharing
what is alf
What is ALF
  • Application Level Framing
  • Someone explained it to me as ‘it’s just UDP’.
  • Application defines an atomic unit
    • roughly like a packet
    • approaches a record, block, file, frame, etc
  • Application deals with ordering
    • may accept out of order packets (real audio)
response srm
Response: SRM
  • All interaction is multicast
  • Receivers learn they’re missing something when:
    • hole in sequence space
    • receive a report from someone else about a newer packet
  • Receivers missing data ask everyone for it
  • Imagine a student asking for a copy of the handout
  • Anyone can reply
srm retransmit requests
SRM Retransmit Requests
  • Avoid too many retransmit requests:
    • deterministic suppression: nodes farther away see our request and don’t make one of their own
    • probabilistic suppression: nodes equally far have a random timer, not all will fire before they see our request.
  • Is there a better solution for containing retransmissions and retransmission requests?
srm retransmit containment
SRM Retransmit Containment
  • Administrative Scoping (I don’t know)
  • Separate Groups
    • for local receivers that could help out
    • possibly separate channels for each missed packet (can a host subscribe that fast?)
  • TTL Scope Control
    • how to reach those who also want a retransmit?
srm ttl based scope hacks
SRM TTL-based Scope Hacks
  • Reply with TTL*2
    • ?
  • Request from the requestor with TTL
    • ?
srm requirements
SRM Requirements
  • How many packets do you hold on to?
  • How do you order updates?
    • Eg. Whose writing goes on top?
    • Thomas’ write rule?
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