NR-SACKs for SCTP (Non-Renegable SACKs)

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

3
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
Unordered Data Transfer
With SACKs
With NR-SACKs
6
Simulation Setup

• Unordered bulk data transfer
• Bernoulli losses loss rates 1-10
• Currently working on simulations with losses from
  realistic application workloads using Tmix

M. Weigle et al., Tmix a tool for generating
realistic TCP application workloads in ns-2, ACM
CCR, 36(3), 2006
7
Throughput at varying send buffer sizes
45ms 1-way delay

• NR-SACK gt SACK at
• shorter delays

10ms 1-way delay
NR-SACK SACK
8
Average RtxQ

• When there is loss, NR-SACKs always use less
  memory for RtxQ

9
Questions
10
Mean RTOs for 45ms and 10ms Delays
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

12
Retransmission queue utilization 45ms 1-way Delay

• SACKs most efficient during no losses
• SACKs inefficiency increases as loss rate
  increases
• Increasing send buffer size cannot improve SACKs
  utilization upper bound to how well SACKs can
  utilize rtxq