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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

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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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