NR-SACKs for SCTP (Non-Renegable SACKs)

draft-natarajan-tsvwg-sctp-nrsack-01

Preethi NatarajanPaul Amer

Ertugrul Yilmaz

University of Delaware

Randall Stewart

The Research Group

Janardhan Iyengar

Franklin and Marshall College

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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 on gap-acked data

– due to buffer overbooking– sender does not discard gap-acked data

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Unordered Data Transfer

With SACKs

sacked, but still in RtxQ

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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)

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Unordered Data Transfer

With SACKs With NR-SACKs

sacked, but still in RtxQ

nr-sacked data removed;

more data flowing

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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.

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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

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RtxQ_Util_L: Utilization during Loss Recovery

• SCTP with SACKs wastes ~50% of RtxQ during loss recovery• NR-SACKs always utilize RtxQ most efficiently

SACKs and NR-SACKs differ in RtxQ_Util only when data received out-of-order ─ during loss

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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

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Questions…

• Questions• Is this acceptable as a working group

document (just say yes ) ?

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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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×= TtTrktUtilRtxQ ii

ii ;_