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NR-SACKs for SCTP ( N on- R enegable SACKs )

NR-SACKs for SCTP ( N on- R enegable SACKs ). draft-natarajan-tsvwg-sctp-nrsack-01. Reneging and SCTP. Retransmission queue (RtxQ) – Portion of send buffer containing copies of transmitted data Receiver cannot reneg on cum-acked / delivered data sender discards cum-acked data from rtxq

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NR-SACKs for SCTP ( N on- R enegable SACKs )

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  1. NR-SACKs for SCTP(Non-Renegable SACKs) draft-natarajan-tsvwg-sctp-nrsack-01

  2. Reneging and SCTP • Retransmission queue (RtxQ) – Portion of send buffer containing copies of transmitted data • Receiver cannot reneg on cum-acked / delivered data • sender discards cum-acked data from rtxq • Receiver may reneg ongap-acked data • due to buffer overbooking • sender does not discard gap-acked data

  3. sacked, but still in RtxQ Unordered Data Transfer With SACKs

  4. Proposal: NR-SACKs for SCTP • Peers negotiate NR-SACK capability at INIT time • Replace SACKs with NR-SACKs during transfer • Sender is expected to perform congestion and flow control as it would with SACKs • NR-SACKs contain non-renegable gap-ack blocks; sender can use NR-SACK info to free buffer • (see draft for more on structure and use cases)

  5. sacked, but still in RtxQ nr-sacked data removed; more data flowing Unordered Data Transfer With SACKs With NR-SACKs

  6. Simulation Setup • Dumb-bell topology [1] • Access link topology • Application-level cross-traffic generation • More details in [2] • Unordered SCTP transfer using SACKs vs. NR-SACKs [1] L. Andrew et. al., ”Towards a Common TCP Evaluation Suite,” In PFLDnet 2008, March 2008 [2] P. Natarajan et. al., “Non-Renegable Selective Acknowledgments (NR-SACKs) for SCTP,” TR, CIS Dept., Univ of Delaware, April 2008.

  7. Retransmission Queue Utilization • Two types of data in retransmission queue (RtxQ) • Necessary: Necessary to maintain these data copies: data in-flight or received but renegable • Unnecessary: Data received and non-renegable (ex: TSNs 12-18 in slide 2). • Rtxq is most efficiently utilized when all data in rtxq are necessary • RtxQ_Util • Fraction of RtxQ utilized efficiently

  8. RtxQ_Util_L: Utilization during Loss Recovery SACKs and NR-SACKs differ in RtxQ_Util only when data received out-of-order ─ during loss • SCTP with SACKs wastes ~50% of RtxQ during loss recovery • NR-SACKs always utilize RtxQ most efficiently

  9. RtxQ_Util during Complete Transfer • As frequency of loss events and loss recovery durations increase, SCTP with SACKs wastes more of RtxQ • The amount of wasted memory increases as the number of transport connections increase

  10. Questions… • Questions • Is this acceptable as a working group document (just say yes ) ?

  11. Retransmission Queue (Rtxq) Utilization • Efficient utilization at time t = k/r • k = # necessary (in flight or renegable) TPDUs in the rtxq • r = size of the rtxq • Efficient utilization for the entire file transfer • ki/ri = efficient rtxq utilization during ti • T = File Transfer time

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