On Mon, Oct 31, 2016 at 02:07:01PM +0100, Delio Brignoli wrote: > > @Delio, I see you allow announce without the TLV: > > > > > > https://github.com/audioscience/linuxptp/commit/0b6d8ca73332391af9bddc25177df254b066d669 > > > > Can you tell us why? > > This is tricky because in some places 802.1AS-2011 seems to imply a > TLV is mandatory but when you dig down into the implementation > details you find that: in 10.3.13.2.1 (f) txAnnounce() appends the > TLV *only if* it doesn’t exceed the transport’s MTU and in > 10.3.10.2.1 (c) and (d) qualifyAnnounce() accepts an announce > message without a TLV. So a TLV is present *if* it fits in the > transport’s MTU but a compliant implementation has to accept an > announce message without path trace TLV.
Those passages permit dropping the TLV when it would cause the message to exceed the frame size. Let's do a little math. The Announce message takes 78 bytes, the TLV takes 4 + 8N bytes, and an Ethernet frame can be as large as 1500 bytes. So the gPTP network would have to have 177 hops before dropping the TLV is allowed. Was the switch you had operating in such a large network? If the TLV is missing, chances are that the sender is out of compliance, don't you think? > Additionally the path trace TLV could be one of many TLVs appended > to the announce message and it doesn’t have to be the first. Even > with my patch applied linuxptp expects the path trace TLV to be the > first one to appear. I would like to fix it but we considered it a > non-urgent issue and had to shift focus to something else so it > never got done. You are right, in the presence of multiple TLVs, the Path Trace TLV could come appear at another position, and you are right that it is a non-issue. When we come to the place where 177 switch AVB networks are common and many more TLVs are defined, then we'll have to fix these points. Thanks, Richard ------------------------------------------------------------------------------ The Command Line: Reinvented for Modern Developers Did the resurgence of CLI tooling catch you by surprise? Reconnect with the command line and become more productive. Learn the new .NET and ASP.NET CLI. Get your free copy! http://sdm.link/telerik _______________________________________________ Linuxptp-users mailing list Linuxptp-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linuxptp-users
