1. 27/02/2019
Improving retransmission delays for thin streams Andreas Petlund Simula Research Laboratory and University of Oslo
Andreas Petlund 2007

2. Agenda Background / Funcom traces SCTP and retransmission mechanisms
27/02/2019
Agenda
Background / Funcom traces
SCTP and retransmission mechanisms
Tests and results
Fairness considerations
Ongoing and future work
2/27/20194/5/2001
Andreas Petlund 2007

3. Thin Streams
Transport protocols being developed for throughput-bound applications
BUT, there exists several low-rate, time-dependent applications
Anarchy Online MMORPG Case Study
average delay: ~250 ms
max delay: seconds (6 retransmissions)
packets per second: < (less then one per RTT)
average packet size: ~120 bytes
average bandwidth requirement: ~4 Kbps
All TCP variations available in Linux (2.6.15) fail to properly support time-dependent “thin streams”  targeted for high rate streams only [nossdav 2006]
2/27/20194/5/2001

4. Times of first retransmission, RTT=100 ms
TCP 1st retransmission
Times of first retransmission, RTT=100 ms
1% loss
5% loss
0% jitter
10% jitter
BIC
SACK
FACK
DSACK
Vegas
New Reno
Westwood+
DSACK&FACK
2/27/20194/5/2001

5. Thin Streams - Continued
Identifying thin streams
Ability to make use of retransmission mechanisms.
Fast retransmit (3 or 4 Duplicate ACKs).
Retransmission timeouts.
Criteria for triggering modifications:
Packets In Flight: -for SCTP tests: PIF = 4
Dependency of RTT: - has to be considered
Adapting to severe loss: PIF * 1 / (1 – p / 100)
Apply modifications only when criteria is fulfilled.
2/27/20194/5/2001

6. Stream Control Transmission Protocol
SCTP should support signaling
acknowledged error-free transfers
data fragmentation according to MTU
packet boundary maintenance
sequenced delivery within multiple streams
bundling
partial reliability …
supposed to address low latencies “require response between 500 – 1200 ms” … or “initiation of error procedures” [rfc 2719]
sender
receiver
(re)transmission queue
Network
SACK
2/27/20194/5/2001

7. Retransmission by Time-Out
27/02/2019
Retransmission by Time-Out
sender
receiver
Timeout is dependent on
minRTO = 1000 ms
estimated RTT based on SACKs
BUT SACKs are delayed
one ACK for two packets or
200 ms timer
influences estimated RTT, especially for thin streams
RTO value grows
retransmission of packet with green chunks due to timeout
(re)transmission queue
Network
SACK
SACK
2/27/20194/5/2001
Andreas Petlund 2007

8. Retransmission by Fast Retransmit
sender
receiver
4 SACKs needed for fast retransmit
+ thin streams
= “all” retransmissions due to timeouts
Network
SACK no
SACK no
SACK no
SACK no
2/27/20194/5/2001

9. Enhancement: Removal of Exponential Backoff
sender
receiver
retransmission number
time in RTTS 2 4 6 8 1 3
retransmission of
orange packet
due to timeout
(re)transmission queue
ENHANCEMENT:
remove exponential backoff
Network
2/27/20194/5/2001

10. Enhancement: Fast Retransmit Bundling
sender
receiver
ENHANCEMENT:
piggyback all chunks in retransmission queue
retransmission of orange packet (chunks) due to dupACKs
grey packet is NOT piggybacked when dupACKs (but would be if due to timeout)
retransmission queue
Network
SACK no
SACK no
SACK no
SACK no
2/27/20194/5/2001

11. lksctp versions & test set up
There have been some improvements in lksctp since we started this work.
lksctp in ( )
only one retransmission due to fast retransmit, next timeout
only 3 SACKs required for fast retransmits
lksctp in has no major changes for our scenario.
Most recent tests
100 B packets
RTTs: 0, 50, 100, 150, 200, 250 ms
Packet inter-arrival times: 50, 100, 150, 200, 250 ms
Dynamic thin stream detection
Tagging of chunks to indicate retr. type
Many web-connections generating cross traffic (and thus losses)
WEB
WEB
SCTP
2/27/20194/5/2001

12. Lksctp performance
Lksctp: RTT100, INT250
2/27/20194/5/2001

13. Lksctp performance Large reduction in maximum and average latency
RTT100, INT250
lksctp
All modifications
Large reduction in maximum and average latency
An increase in spurious retransmissions
-Tolerable due to the low datarate.
2/27/20194/5/2001

14. Performance comparison
27/02/2019
Max values
99 percentiles
Average
Om Exponential backoff: Ikke stor minskning paa gjennomsnitt, men stor reduksjon I max-verdier
Min RTO og FR: Tar gjennomsnittet ned.
Kombinasjon gir gode resultater paa begge
2/27/20194/5/2001
Andreas Petlund 2007

15. Fairness considerations and tests
Modifications increases aggressiveness of stream
- Exponential back-off.
- Fast retransmit.
- Minimum retransmission time out.
We want to test whether fairness is in jeopardy.
2/27/20194/5/2001

16. TCP: Removal of Exponential Backoff
Standard and modified timeout calculations
100 ms delay, average retransmission time
Standard and modified timeout calculations
300 ms delay, average retransmission time
1st retrans
2nd retrans
3rd retrans
1st retrans
2nd retrans
3rd retrans
Removal of the RTO backoff
Consistent reduction of delay for multiple losses
Reduced variation of smoothed RTO
But: effect is less pronounced when the RTT is big
2/27/20194/5/2001

17. Ongoing and future work
Similar tests for TCP with modifications.
Test retransmission mechanisms in middleware (UDP/application layer) in comparison with modifications.
Incorporate modifications into existing flight simulator (SilentWings developed at Simula/Kalkulo).
Analyse Internet Exchange dumps to identify more applications with thin stream properties.
Thin streams TCP tests across the Atlantic.
Fairness-tests with many concurrent modified thin streams.
If TCP-tests are as promising as expected:
Lobbying for standard Linux kernel acceptance (as TCP option).
2/27/20194/5/2001

18. Questions? ?
2/27/20194/5/2001