Ron, The interference effects of the plesiochronous traffic can be mitigated by randomizing the generation of the 1588 packets. Since the actual transmission time is recorded in the timestamp, there is no requirement for exact periodicity to the 1588 packet generation (just the 30% accuracy requirement of the standard 7.7.2.1).
I agree with your statement on there being a difference between an environment with full on path support (every node a port based Timestamping capable TC or BC) and an environment where this full support is not available. But I think there will be enough of the second class of deployments to warrant a recommendation on forwarding class. Peter R. ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Shahram Davari Sent: Monday, August 08, 2011 4:03 PM To: Ron Cohen Cc: [email protected] Subject: Re: [TICTOC] DSCP for PTP Ron, I have to look at this issue that you are raising. Do you have any reference for this effect? In I theory one could have 2x EF PHB, with different DSCP code points, where one has higher priority than the other one. Thx Shahram From: Ron Cohen [mailto:[email protected]] Sent: Monday, August 08, 2011 12:41 PM To: Shahram Davari Cc: [email protected]; [email protected] Subject: Re: [TICTOC] DSCP for PTP Hi Shahram, One possible drawback for using EF for PTP is that EF is also used by some video/voice and similar plesio-synchronous applications. The danger is that the noise introduced by such application might be harder to filter than 'normal' packet based traffic. Therefore, it might be better to use another PHB, with even higher forwarding priority if possible, or maybe simply a different PHB altogether. It would be great if someone did a study on this and can share the results with us. We should also differentiate between the PTP-aware (TC/BC) or PTP-unaware cases. In principle a TC doesn't need to send PTP messages with high forwarding priority as it compensates for the residence time anyhow. Best, Ron On Mon, Aug 8, 2011 at 8:37 PM, Shahram Davari <[email protected]<mailto:[email protected]>> wrote: Ron, I agree with you that all PTP messages must configured with highest priority (a.k.a EF PHB) and EF should be the default. In fact I would also think it is best to use L-LSP with EF PHB for 15880MPLS. If this is acceptable by the group then we could add this to the 1588oMPLS draft. Regards, Shahram From: [email protected]<mailto:[email protected]> [mailto:[email protected]<mailto:[email protected]>] On Behalf Of Ron Cohen Sent: Sunday, August 07, 2011 1:00 AM To: [email protected]<mailto:[email protected]> Subject: [TICTOC] DSCP for PTP Hi, Current standards do not recommend which DSCP values to set on PTP messages. As a result vendors have chosen different default values, and some vendors also differentiate between the DSCP set on PTP Event messages (Sync, Delay_Req, etc.) and PTP General messages (Announce, Follow_Up, Delay_Resp). I think TICTOC can help clarify the recommended DSCP usage, along the guidelines of RFC4595, 'Configuration Guidelines for DiffServ Service Classes'. In my opinion classifying PTP event and general messages into separate PHBs is problematic as discard of Follow_Up would render its associated Sync message useless. Forwarding of Sync messages in a higher priority queue compared to Follow_Up may lead to Sync message getting ahead of the previous Follow_Up message which again causes the slave to discard the previous Sync message. Hence the first order recommendation in my opinion should be to use the same PHB (and DSCP marking) to all PTP messages. Choosing one of the existing PHBs as the recommended one for PTP is somewhat harder. EF is a reasonable choice in my opinion, as per RFC4595 'The intent of Expedited Forwarding PHB [RFC3246] is to provide a building block for low-loss, low-delay, and low-jitter services.' TICTOC could in principle recommend defining a new PHB tailored for PTP and similar services, but I guess this would be a longer process. I would appreciate if others will share their insight on the recommended usage and whether this issue should or should not be addressed by TICTOC. Here is the relevant text from IEEE1588-2008 Annex D: "For PTP event messages, the value of the differentiated service (DS) field in the Type of Service (ToS) field should be set to the highest traffic class selector codepoint available." The PTP Telecom profile for frequency distribution, ITU-T G.8265.1, doesn't specify or recommend DSCP settings. Best, Ron
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