Considerations of Considerations of SCTP Retransmission Delays SCTP Retransmission Delays
for Thin Streamsfor Thin Streams
Jon Pedersen1, Carsten Griwodz1,2 & Pål Halvorsen1,2
1Department of Informatics, University of Oslo, Norway 2Simula Research Laboratory, Norway
{jonped, griff, paalh}@ifi.uio.no
LCN 2006: 31st IEEE Conference on Local Computer Networks, Tampa, FL, USA, November 2006
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Overview
Latency problems for thin streams
SCTP as an alternative to TCP
Experiments
New experiments
Conclusions
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Thins Streams Transport protocols being developed for throughput-bound
applications BUT, there exist several low-rate, time-dependent applications
Anarchy Online MMORPG Case Study
average delay: ~250 ms max delay: 67 seconds (6 retransmissions) packets per second: < 4 (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]
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Stream Control Transmission Protocol
Network
sender receiver
(re)t
ransm
issi
on q
ueue
SA
CK
SCTP should support signaling acknowledged error-free transfers data fragmentation according to MTU packet boundary maintenance sequenced delivery within multiple
streams bundling partial reliability … suppose to address low latencies
“require response between 500 – 1200 ms” … or “initiation of error procedures” [rfc 2719]
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Test Set Up Linux 2.6.15 with lksctp 100 bytes packets 4 packets per second 3.2 Kbps
SCTP
Network emulated using netem• dropp• delays (RTTs: 0, 100, 200, 400 ms)
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Results: lksctp for Thins Streams
Even worse than TCP!!!
Why these high delays?
Two ways of triggering retransmissions of a lost chunk…
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Retransmission by Time-Out
Network
sender receiver
(re)t
ransm
issi
on q
ueue
retransmission of packet with green chunks due to timeout
Timeout is dependent on
minRTO = 1000 ms
estimated RTT based on SACKs BUT SACKs are delayed
o one ACK for two packets oro 200 ms timer
influences estimated RTT, especially for thin streams
RTO value grows
SA
CK
SA
CK
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Retransmission by Fast Retransmit
Network
sender receiver
SA
CK
no
SA
CK
no
SA
CK
no
SA
CK
no
4 SACKs needed for fast retransmit
+ thin streams
= “all” retransmissions due to timeouts
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Enhancement: Removal of Exponential Backoff
Network
sender receiver
(re)t
ransm
issi
on q
ueue
retransmission of green packet due to timeout
ENHANCEMENT: remove exponential backoff
retransmission number
time in RTTS
2
4
6
8
1 2 3 4
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Enhancement: Fast Retransmit Bundling
Network
sender receiver
retr
ansm
issi
on q
ueue
retransmission of green packet (chunks) due to dupACKs
blue packet is NOT piggybacked when dupACKs(but would be if due to timeout)
ENHANCEMENT: piggyback all chunks in retransmission queue
SA
CK
no
SA
CK
no
SA
CK
no
SA
CK
no
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Enhancements Modified retransmission timer
removal of exponential backoff minRTO = 200 ms (as in TCP)
Modified retransmission bundling always allow aggressive bundling for fast retransmit
Modified fast retransmit tested fast retransmit after 1 SACK
Thin stream detection fewer packets in flight to trigger a fast retransmit added tracking of outstanding packets less than 4 in flight = thin stream
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Enhancement: Results (200 ms)
Considerable reduction in average and maximum latencies
Increase in number of fast retransmissions compared to timeouts
Increase in number of retransmissions
original SCTP
reduced minRTO & fast retransmit
modified timer restart
no SACK delay
Timeout 266 197 331 633
Fast retransmit 35 284 288 1
Total 301 481 619 634
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
New lksctp versions & New Test Set Up
New lksctp versions has been developed
lksctp in 2.6.16 (2.5.72-0.7.1) only one retransmission due to fast
retransmit, next timeout only 3 SACKs required for fast
retransmits
lksctp in 2.6.17 has no major changesfor our scenario
New 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
Many web-connections generating cross traffic (and thus losses)
SCTP
WEBWEB
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Results: New lksctp
Still high average and worst case latencies
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Results: Fast Retransmission Modification
Reduction in maximum and average latency As expected a large increase in fast retransmit
An increase in spurious retransmissions
Fast retransmit modification – 1 SACK
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Results: Removed Exponential Backoff
Reduction in maximum and average latency
An increase in spurious retransmissions
Retransmission aggressiveness does not really pose a congestion threat since the amount of data waiting to be sent is always less than the minimum transmission window
Removed exponential backoff
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Results: Reduced Minimum Time-out
Faster timeouts Reduction in maximum and average latency
An increase in spurious retransmissions
Reduced minRTO
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Results: All Modifications Combined
A further reduction in maximum and average latency As expected an increase in fast retransmit
An increase in spurious retransmissions
All thin stream modifications
2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen
LCN 2006, Tampa, FL, USA, November 2006
Conclusions Based on SCTP description we expected (hoped for)
reduced latencies compared to TCP
Enhancements like reduced minRTO removal of exponential backoff removal of delayed SACKs …
reduce latencies for thin streams
The enhancements increase the number of spurious retransmissions, but maybe not important for thin streams!!??
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