I'm new to TICTOC. I've worked on the AVB standards and am working on other high-performance media networking systems and standards that would benefit from improvements in clock distribution. I know a lot about Ethernet and IP but I'm not an MPLS expert. Here's my contribution on the topic of QoS:
RFC 4596 specifically recommends using EF for NTP messaging. EF is also routinely used for VoIP payloads. Ideally we'd want 1588 to take precedence over both of these but there's no standard service level higher than EF. Network administrators are free to define custom service levels above EF and if there's any chance of a warm reception for such a recommendation, I think it should be made. As allowed by the other DiffServ RFCs and implementation by many vendors, there's no magic to the DSCP numbers. Best practice is for edge network devices to ignore incoming DSCP values and re-mark traffic based on characteristics such as protocol identifier, destination address and port number. The DCSP numbers used internally are then enterprise specific. In this context you can appreciate that a 1588 QoS recommendation can be descriptive and may not need to conform strictly to RFC 4596 recommendations. 1588 prioritization under AVB is largely irrelevant as no PTP packets are ever bridged so there are never any queuing delays for QoS to address. Kevin Gross AVA Networks From: [email protected] [mailto:[email protected]] On Behalf Of Shahram Davari Sent: Monday, August 08, 2011 11:37 AM To: Ron Cohen; [email protected] Subject: Re: [TICTOC] DSCP for PTP 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]] On Behalf Of Ron Cohen Sent: Sunday, August 07, 2011 1:00 AM To: [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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