RE: small change in the connected mode draft
Michael Krause <[email protected]> Fri, 24 Mar 2006 09:47:46 -0800
| Newsgroups | gmane.ietf.ipoib |
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| Message-ID | <[email protected]> |
--===============2019877889== Content-Type: multipart/alternative; boundary="=====================_649358738==.ALT" --=====================_649358738==.ALT Content-Type: text/plain; charset="us-ascii"; format=flowed At 02:51 PM 3/23/2006, Dror Goldenberg wrote: > > >From: Michael Krause [mailto:[email protected]] >Sent: Thursday, March 23, 2006 10:58 PM > >At 04:04 PM 3/21/2006, H.K. Jerry Chu wrote: >>Hi folks, >> >>Allison Mankin, the transport area AD who brought up some >>concern during IESG review of the connected draft regarding >>simultaneous retransmissions at different layers, has suggested >>and Vivek agreed to the following change to section 7.1 >>"A Cautionary Note on IPoIB-RC". >> >>The revised section reads like this: >> >> The RC mode of InfiniBand guarantees in-order delivery of >> packets. Every message transmitted over the RC connection is >> broken into physical MTU sized packets by the RC connection. If >> any packet is lost, it is retransmitted until the complete >> message is exchanged. Therefore, there is a possibility of an >> upper transport layer experiencing a timeout, while the RC layer >> is still in the process of transferring the complete message. >> TCP will view the timeout as an indicator of congestion and >> enter slow-start thereby affecting throughput drastically >> [RFC2581]. Other upper layer protocols might insert >> retransmissions into the fabric adding to the already existing >> congestion. >> >> The applicability of Infiniband reliability is on a fabric >> with short latencies (not wide area). Therefore, the RC timer >> values should be short compared with the starting minimum >> time values used by the upper end-to-end transports. In >> addition, because the RC mode does not have measurement >> based reliable transmission, its use over fabrics with long >> latency or very dynamic latency may be a concern for congestion- >> aware traffic traversing those fabrics. >> >>If you have any comment/issue on the proposed change please post them >>to the list before COB this Friday (3/24/06). I'd like to move the >>draft past IESG review so we can wrap up the WG asap. >> >>BTW, please be informed that my email address will change after this >>Friday. My new address is [email protected]. > >What about RNR which can go into an infinite timer state? It should also >be noted that IB does not mandate timer ranges that necessarily correspond >to TCP timers. In fact, in the face of real IB congestion or a port / VL >arbitration policy that places such traffic in a best effort QoS slot, >then it is quite possible to see delays that would be interpreted by any >ULP such as TCP as congestion. Is this really an issue as the >applications continue to operate albeit at a slower rate? > >Mike > > >Mike, > >I am not sure what is the problem in question now. > >If people are afraid of retransmission of TCP while a retransmission of IB >RC is taking place, then: 1) packet drop is relatively rare in IB and 2) >usually retransmission timeouts are fast in IB due to being low latency >fabric. Given the above, I'd say that this event is rare anyway and if and >when IB retransmission happens, there is a good chance to recover it at >the IB level a while before TCP notices it. Packet drops in IB as measured are relatively rare - these measurements are largely within a small diameter fabric. IB over a WAN, which the protocol was never intended to operate, is another matter entirely. > >The case that you mentioned, RNR driven retransmission, is another case of >retransmission. It has nothing to do with congestion or packet drop in the >fabric. It is just being impacted by the ability of the receiver to post >receive buffers on the RQ/SRQ. Here the timeouts are application based and >as you wrote can be configured to infinity. I agree that infinit number of >retries would be a bad choice of RNR retry count. So, maybe we should >recommend on selecting also the RNR timeout and retry count to be low too. >I am also wondering what happens when there is a slow receiver, e.g. >posting too slow on the RQ/SRQ, in which case the RNR Nak will happen very >frequently and might cause the QP to get into error state because of RNR >timeout exahusted. The question raised was what impact will retransmission have on a ULP such as TCP which would treat timeouts as congestion events. I raised two cases which are not easy to control at the IB protocol level since they are admin managed: - RNR which can lead to very long delays in the connection transmission completion - Arbitration which can lead to very slow forward progress on a given QoS channel Both of these should be comprehended minimally as informative text to guide developers and management solutions to do the right thing. They also need to be aware that in the case of RNR, there is no single right answer and its usage may entail long delays while the OS does whatever it needs that triggered the RNR operation. >One approach would be to downgrade such peers to IPoIB-UD. Given either condition can occur at any time, you end up with split flows or require in essence a fail-over between the pairs to the UD paradigm. Neither is optimal to say the least. > Other option would be to indirectly detect those cases and provide hints > to the TCP stack to slow down. Need to treat all layer 2 technologies the same. >By indirectly detecting it, I am thinking along the lines of when the send >queue becomes full, we can start dropping packets at the transmitter (RED >algorithm or something similar). This packet drop can cause TCP to back >off and activate the congestion flow and reduce the transmission rate. >This has the right impact in my opinion. This is done for most layer 2 technologies. However, it does not solve the problem for RNR which can be very long in its delay. > >And, the last thing that you mentioned has to do with TCP retransmission >triggering when the IB is congested. IB packet delivery slows down because >of congestion in the IB fabric (slowing down can be because of real >congestion which backpressures the transmitters or because of IB >congestion management). Eventhough there is no packet drop at the IB level >and no retransmission at the IB level, still TCP times out. Luckily, TCP >is doing RTT evaluation, so hopefully this will be rare too. If it >happens, then the desired effect would be to slow down the TCP requester >(at the TCP level) which is OK. The side effect will be retransmission. >And the big question is whether this is so bad to have this >retransmission. BTW, I believe that also in this case, dropping packets at >the transmitter will ease the problem. IB can suffer congestion spreading even with its congestion management notification protocol enabled. Worse-case spreading can result in the fail-safe timers being triggered which is measured in seconds and which can yield packet loss. I agree that under most loads, there is a small probability of this occurring. However, it is a case where IB drops packets and is a case that can take a significant amount of time to realize what is going on and trigger recovery. There should be informative text perhaps in the spec to make sure developers understand exactly how IB operates when it comes to its own congestion or fail-safe modes. Mike --=====================_649358738==.ALT Content-Type: text/html; charset="us-ascii" <html> <body> <font size=3>At 02:51 PM 3/23/2006, Dror Goldenberg wrote:<br> <blockquote type=cite class=cite cite=""> <br> </font> <dl><font face="tahoma" size=2><br> <dd>From:</b> Michael Krause [<a href="mailto:[email protected]" eudora="autourl"> mailto:[email protected]</a>] <br> <dd>Sent:</b> Thursday, March 23, 2006 10:58 PM<br><br> </font> <dd><font size=3>At 04:04 PM 3/21/2006, H.K. Jerry Chu wrote:<br> <blockquote type=cite class=cite cite=""> <dd>Hi folks,<br><br> <dd>Allison Mankin, the transport area AD who brought up some<br> <dd>concern during IESG review of the connected draft regarding<br> <dd>simultaneous retransmissions at different layers, has suggested<br> <dd>and Vivek agreed to the following change to section 7.1<br> <dd>"A Cautionary Note on IPoIB-RC".<br><br> <dd>The revised section reads like this:<br><br> <dd><x-tab> </x-tab>The RC mode of InfiniBand guarantees in-order delivery of<br> <dd><x-tab> </x-tab> packets. Every message transmitted over the RC connection is<br> <dd><x-tab> </x-tab>broken into physical MTU sized packets by the RC connection. If<br> <dd><x-tab> </x-tab>any packet is lost, it is retransmitted until the complete<br> <dd><x-tab> </x-tab> message is exchanged. Therefore, there is a possibility of an<br> <dd><x-tab> </x-tab>upper transport layer experiencing a timeout, while the RC layer<br> <dd><x-tab> </x-tab>is still in the process of transferring the complete message.<br> <dd><x-tab> </x-tab>TCP will view the timeout as an indicator of congestion and<br> <dd><x-tab> </x-tab>enter slow-start thereby affecting throughput drastically<br> <dd><x-tab> </x-tab> [RFC2581]. Other upper layer protocols might insert<br> <dd><x-tab> </x-tab> retransmissions into the fabric adding to the already existing<br> <dd><x-tab> </x-tab> congestion.<br><br> <dd><x-tab> </x-tab>The applicability of Infiniband reliability is on a fabric<br> <dd><x-tab> </x-tab>with short latencies (not wide area). Therefore, the RC timer<br> <dd><x-tab> </x-tab>values should be short compared with the starting minimum<br> <dd><x-tab> </x-tab>time values used by the upper end-to-end transports. In<br> <dd><x-tab> </x-tab> addition, because the RC mode does not have measurement<br> <dd><x-tab> </x-tab>based reliable transmission, its use over fabrics with long<br> <dd><x-tab> </x-tab> latency or very dynamic latency may be a concern for congestion-<br> <dd><x-tab> </x-tab>aware traffic traversing those fabrics.<br><br> <dd>If you have any comment/issue on the proposed change please post them<br> <dd>to the list before COB this Friday (3/24/06). I'd like to move the<br> <dd>draft past IESG review so we can wrap up the WG asap.<br><br> <dd>BTW, please be informed that my email address will change after this<br> <dd>Friday. My new address is [email protected].</blockquote><br> <dd>What about RNR which can go into an infinite timer state? It should also be noted that IB does not mandate timer ranges that necessarily correspond to TCP timers. In fact, in the face of real IB congestion or a port / VL arbitration policy that places such traffic in a best effort QoS slot, then it is quite possible to see delays that would be interpreted by any ULP such as TCP as congestion. Is this really an issue as the applications continue to operate albeit at a slower rate?<br><br> <dd>Mike </font><font face="arial" size=2 color="#0000FF"> <br> </font> <dd><font size=3> <br><br> </font> </dl><font face="arial" size=2 color="#0000FF">Mike, <br> </font><font size=3> <br> </font><font face="arial" size=2 color="#0000FF">I am not sure what is the problem in question now.<br> </font><font size=3> <br> </font><font face="arial" size=2 color="#0000FF">If people are afraid of retransmission of TCP while a retransmission of IB RC is taking place, then: 1) packet drop is relatively rare in IB and 2) usually retransmission timeouts are fast in IB due to being low latency fabric. Given the above, I'd say that this event is rare anyway and if and when IB retransmission happens, there is a good chance to recover it at the IB level a while before TCP notices it.</font></blockquote><br> Packet drops in IB as measured are relatively rare - these measurements are largely within a small diameter fabric. IB over a WAN, which the protocol was never intended to operate, is another matter entirely.<br><br> <blockquote type=cite class=cite cite=""><font size=3> <br> </font><font face="arial" size=2 color="#0000FF">The case that you mentioned, RNR driven retransmission, is another case of retransmission. It has nothing to do with congestion or packet drop in the fabric. It is just being impacted by the ability of the receiver to post receive buffers on the RQ/SRQ. Here the timeouts are application based and as you wrote can be configured to infinity. I agree that infinit number of retries would be a bad choice of </font><font size=3> </font><font face="arial" size=2 color="#0000FF">RNR retry count. So, maybe we should recommend on selecting also the RNR timeout and retry count to be low too. I am also wondering what happens when there is a slow receiver, e.g. posting too slow on the RQ/SRQ, in which case the RNR Nak will happen very frequently and might cause the QP to get into error state because of RNR timeout exahusted.</font></blockquote><br> The question raised was what impact will retransmission have on a ULP such as TCP which would treat timeouts as congestion events. I raised two cases which are not easy to control at the IB protocol level since they are admin managed:<br> <x-tab> </x-tab>- RNR which can lead to very long delays in the connection transmission completion<br> <x-tab> </x-tab>- Arbitration which can lead to very slow forward progress on a given QoS channel<br><br> Both of these should be comprehended minimally as informative text to guide developers and management solutions to do the right thing. They also need to be aware that in the case of RNR, there is no single right answer and its usage may entail long delays while the OS does whatever it needs that triggered the RNR operation.<br><br> <blockquote type=cite class=cite cite=""> <font face="arial" size=2 color="#0000FF">One approach would be to downgrade such peers to IPoIB-UD.</font></blockquote><br> Given either condition can occur at any time, you end up with split flows or require in essence a fail-over between the pairs to the UD paradigm. Neither is optimal to say the least.<br><br> <blockquote type=cite class=cite cite=""> <font face="arial" size=2 color="#0000FF"> Other option would be to indirectly detect those cases and provide hints to the TCP stack to slow down. </font></blockquote><br> Need to treat all layer 2 technologies the same.<br><br> <blockquote type=cite class=cite cite=""> <font face="arial" size=2 color="#0000FF">By indirectly detecting it, I am thinking along the lines of when the send queue becomes full, we can start dropping packets at the transmitter (RED algorithm or something similar). This packet drop can cause TCP to back off and activate the congestion flow and reduce the transmission rate. This has the right impact in my opinion.</font></blockquote><br> This is done for most layer 2 technologies. However, it does not solve the problem for RNR which can be very long in its delay. <br><br> <blockquote type=cite class=cite cite=""><font size=3> <br> </font><font face="arial" size=2 color="#0000FF">And, the last thing that you mentioned has to do with TCP retransmission triggering when the IB is congested. IB packet delivery slows down because of congestion in the IB fabric (slowing down can be because of real congestion which backpressures the transmitters or because of IB congestion management). Eventhough there is no packet drop at the IB level and no retransmission at the IB level, still TCP times out. Luckily, TCP is doing RTT evaluation, so hopefully this will be rare too. If it happens, then the desired effect would be to slow down the TCP requester (at the TCP level) which is OK. The side effect will be retransmission. And the big question is whether this is so bad to have this retransmission. BTW, I believe that also in this case, dropping packets at the transmitter will ease the problem.</font></blockquote><br> IB can suffer congestion spreading even with its congestion management notification protocol enabled. Worse-case spreading can result in the fail-safe timers being triggered which is measured in seconds and which can yield packet loss. I agree that under most loads, there is a small probability of this occurring. However, it is a case where IB drops packets and is a case that can take a significant amount of time to realize what is going on and trigger recovery. There should be informative text perhaps in the spec to make sure developers understand exactly how IB operates when it comes to its own congestion or fail-safe modes.<br><br> Mike</body> </html> --=====================_649358738==.ALT-- --===============2019877889== Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline _______________________________________________ IPoverIB mailing list [email protected] https://www1.ietf.org/mailman/listinfo/ipoverib --===============2019877889==--