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Published byRalph Cole Modified over 10 years ago
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Optimizing Buffer Management for Reliable Multicast Zhen Xiao AT&T Labs – Research Joint work with Ken Birman and Robbert van Renesse
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Why important? Many applications desire reliable or semi-reliable delivery. IP multicast is best-effort. Buffering is necessary for retransmission. Buffer space is limited! How to utilize the amount of buffer space most efficiently?
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Previous Work RMTP: Buffer all messages on repair servers. –Impractical for long-lived sessions. SRM: Regenerate messages at the application. –Buffer management at the application level remains a challenge. Stability Detection: Buffer messages until they are stable (i.e. received by all members in the group). –It takes a long time to achieve stability in a large multicast group. Bimodal Multicast: Buffer messages for a fixed amount of time. –Optimization: buffer messages on a sub-group of members.
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Talk Overview RRMP: Randomized Reliable Multicast Protocol Error recovery algorithm in RRMP: Infocom 2001 Buffering algorithms in RRMP: DSN 2002 –Feedback based short-term buffering –Randomized long-term buffering Simulation results Summary
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RRMP: Randomized Reliable Multicast Protocol Key idea: combine previous work on randomized error recovery with the Bimodal Multicast protocol and hierarchical error recovery similar to that employed by tree-based protocols. Group receivers into a hierarchy. Do not use any repair server. parent region: the least upstream region of a receiver in the hierarchy. Each receiver maintains group membership information about receivers in its region and receivers in its parent region.
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Two-phase Error Recovery Assume a receiver p detects a message loss. local loss: the loss affects a fraction of receivers in p’s region regional loss: the loss affects all receivers in p’s region Local recovery: a receiver tries to recover the loss from randomly selected neighbors. Remote recovery: some receivers in the region request retransmissions from the parent region.
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s s routers receivers sender
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s p q s routers receivers sender
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s p q s routers receivers sender
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Overview of Buffering Scheme Local recovery Remote recovery Error Recovery Long-term buffering Short-term buffering Buffering Short-term buffering: when a message is first introduced into the system. Long-term buffering: when almost all receivers in a region have received the message.
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Not All Messages Are Created Equal!
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idle message: no request for this message has been received for a time interval T. (T is the idle threshold.) Short-term buffering: buffer a received message until it becomes idle. Result: messages most needed in the system stay in the buffer longer. No extra traffic overhead! n: the size of a region p: the percentage of members in this region missing a message The probability that a member will not receive any request: As, this probability can be approximated by idea: a member uses the retransmission requests it received as feedback to estimate how many members in the region still miss the message. Feedback-based Short-term Buffering
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Simulation Results Short-term buffering in a local region. –100 members in the region, fully connected. –RTT between any two members: 10ms. –idle threshold: 40ms. Outcome of IP multicast: select a random subset of members to hold a message initially. –Measure how long these members buffer the message.
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96 %
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Why Long-term Buffering ???
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s s routers receivers sender
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s s routers receivers sender idle Sorry, you are out of luck! p q
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Randomized Long-term Buffering idea: provide long-term buffering for an idle message at a small subset of receivers in each region. Load balancing: spread the load of buffering across all receivers in a region. Randomized algorithm: each member independently tosses a coin to decide whether to become a long-term bufferer. C: the expected number of long-term bufferers. Saving in buffer space: n / C Network dynamics: message transfer
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The probability that k members buffer an idle message for different values of C, the expected number of long-term bufferers.
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The probability that no member buffers an idle message decreases exponentially with C
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How to find a long-term bufferer ???
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s p q s routers receivers sender
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help! Do you have the msg? have the msg?
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The search is over!
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Search Overhead Evaluate penalty in recovery time due to search for a bufferer in a region with 100 members. –RTT between any two members: 10ms. –Assume a remote request arrives at a random member. –Simulation repeated 100 times with different random seeds. Question I: how does the search time change with the number of bufferers? Question II: how does the search time changes with the region size?
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Search time as the number of bufferers increases.
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Search time as the size of the region increases
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Summary Efficient buffer management is essential for reliable multicast in a large group. Two phase buffering to address variances in delivery latency in a large group. Retransmission requests can be used as feedback to allocate buffer space adaptively. Spread the load of buffering among all members in a group through randomization.
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