Considerations of SCTP Retransmission Delays for Thin Streams Jon Pedersen 1, Carsten Griwodz 1,2 & Pål Halvorsen 1,2 1 Department of Informatics, University of Oslo, Norway 2 Simula Research Laboratory, Norway {jonped, griff, 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 senderreceiver (re)transmission queue SACK 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 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 senderreceiver (re)transmission queue 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 or o 200 ms timer  influences estimated RTT, especially for thin streams  RTO value grows SACK

2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen LCN 2006, Tampa, FL, USA, November 2006 Retransmission by Fast Retransmit Network senderreceiver SACK no SACK no SACK no SACK 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 senderreceiver (re)transmission queue retransmission of green packet due to timeout ENHANCEMENT: remove exponential backoff retransmission number time in RTTS

2006 Jon Pedersen, Carsten Griwodz & Pål Halvorsen LCN 2006, Tampa, FL, USA, November 2006 Enhancement: Fast Retransmit Bundling Network senderreceiver retransmission queue 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 SACK no SACK no SACK no SACK 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 SCTPreduced minRTO & fast retransmit modified timer restartno SACK delay Timeout Fast retransmit Total

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 ( ) 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 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 WEB

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!!??