Subject: [iccrg] Follow-up: PIE performance in cable modem environments

From: Preethi Natarajan <[email-masked]>
Date: Tue, 23 Apr 2013 16:18:43 -0700

Hello,

This is a follow-up to Greg White's (from Cable Labs) talk at the recent
ICCRG meeting on PIE's performance in cable modem environments.

Post the meeting, Greg was kind to share his ns-2 DOCSIS model with us. We
investigated PIE's performance using this model. The key items from this
investigation: 
1. Bug in PIE code: The previous PIE release (that Greg used for
evaluations) was missing a line of code. This missing line brings down drop
probability under certain conditions and turns out to be critical for the
cable modem scenario. Without this line of code, the drop probability
remains high and takes longer to come down even when the queue delay has
remained lower than the reference. The updated ns-2 PIE code can be found
here ‹ ftp://ftpeng.cisco.com/pie/.
2. Bug in ns-2 TCP/Linux: Greg's cable modem simulations used the TCP Cubic
variant. We discovered a serious bug in ns-2 TCP/Linux Agent (confirmed by
Dr. Injong Rhee's team) that makes TCP/Cubic senders very aggressive and
unresponsive to packet drops/notifications, pretty much like UDP traffic.
Please find more details about the bug here --
http://sourceforge.net/tracker/?func=detail&aid=3608750&group_id=149743&atid
=775392.
We are working with Cable Labs to verify the cable modem results, they'll
soon be available on our FTP site along with the PIE code.

A technical paper about PIE was recently accepted at the IEEE Conference on
High Performance Switching and Routing 2013. A copy of the paper is attached
here.

The Linux PIE implementation is expected to be ready by next week and we'll
follow-up on that as well.

Many thanks,
Preethi on behalf of PIE team.

---

From: "Eggert, Lars" <[email-masked]>
Date: Wed, 24 Apr 2013 14:17:38 +0000

Hi,

for those of you using ns2 for TCP experimentation, the following bug has been mentioned on the aqm list.

Lars

Begin forwarded message:

Post the meeting, Greg was kind to share his ns-2 DOCSIS model with us. We investigated PIE's performance using this model. The key items from this investigation:

  1.  Bug in PIE code: The previous PIE release (that Greg used for evaluations) was missing a line of code. This missing line brings down drop probability under certain conditions and turns out to be critical for the cable modem scenario. Without this line of code, the drop probability remains high and takes longer to come down even when the queue delay has remained lower than the reference. The updated ns-2 PIE code can be found here — ftp://ftpeng.cisco.com/pie/.
  2.  Bug in ns-2 TCP/Linux: Greg's cable modem simulations used the TCP Cubic variant. We discovered a serious bug in ns-2 TCP/Linux Agent (confirmed by Dr. Injong Rhee's team) that makes TCP/Cubic senders very aggressive and unresponsive to packet drops/notifications, pretty much like UDP traffic. Please find more details about the bug here -- http://sourceforge.net/tracker/?func=detail&aid=3608750&group_id=149743&atid=775392.