Hi Peter, Yes, I carefully read your draft and understand its purported benefits. I think, more importantly, all three drafts attempt to tackle the same problem space, i.e. how to respond to local congestion, and unfortunately none of the proposals are perfect.
WRT DCM: * If you're creating a congestion free topology, why not just always use the congestion free topology? A colleague reminded me of https://www.cs.princeton.edu/~jrex/teaching/spring2005/reading/khanna89.pdf It would seem to me that you risk recreating the oscillation problems described there. * What happens if/when an endpoint in the OFA topology becomes unreachable due to multiple simultaneous congestion events? I presume packets get dropped? * Terminating offloading for topological events associated with the OFA path to an endpoint, on any reasonably size network, would suggest this has very limited applicability. * For SRv6 networks, a packet arriving at an endpoint with 2 SRv6 headers would be quite problematic for many forwarding engines As far as your criticisms of TTE go: * 1st, I’d argue that none of the proposals really address strategic resource management in an SR network. Said another way, all the proposals need the congestion to be abated via some means. i.e. global optimization, human intervention, magic controllers, etc. * In that regard, all three proposals are incomplete from an over resource management perspective * Wrt #2, loops are easily preventable in SR networks through the use of SSFP SIDs when a flow is forwarded along the TI-LFA path. Maybe I should update the TTE draft so this is specifically addressed and not left to the reader to infer. * TTE does support multiple congestion events, however, the same flow will not be “TTE’d” more than once. The flow selection techniques described in the TTE draft easily facilitate this * For all practical purposes, introducing 1 or more additional flex-algo’s results in non-trivial scaling considerations on any reasonably sized network. * Further, if flex-algo is already in use, do you also need per flex-algo OFA topologies so as to maintain the optimization objective for the flex-algo topology you’re off-loading from? * In contrast, TTE will work with flex-algo to maintain the optimization objective along the backup path. Cheers, —Colby From: Peter Psenak <[email protected]> Date: Friday, July 17, 2026 at 3:14 AM To: Colby Barth <[email protected]>; lsr <[email protected]>; [email protected] <[email protected]> Subject: Re: [Lsr] Comments on draft-psenak-lsr-igp-dcm-00 Colby, if you read the DCM draft carefully, you will find out that what you put below is maybe funny, but not more than that. Here are the significant differences between the two: 1. TTE is based on a backup paths, DCM calculates its own offload paths 2. LFA/TI-LFA does not take congestion into account, DCM offload path is calculated to be congestion free 2. TTE does not support multiple congested links in the network at the same time. Using multiple LFA/TI-LFA paths simultaneously on different nodes can easily lead to loops. 3. TTE is proposed to be used for a short duration of the time and then the TE is expected to recompute. DCM is meant to be used for long time and does not need any TE action. 4. As a result of (3), DCM offload path is fully protected by LFA/TI-LFA, TTE paths are note protected. Contrary, the other draft that I pointed out in the RTGWG list, is way more closer to the DCM. regards, Peter On 16/07/2026 19:42, Colby Barth wrote: All, I would like to point authors of draft-psenak-lsr-igp-dcm-00 to an existing draft: https://datatracker.ietf.org/doc/draft-li-rtgwg-tte/02/ There is a significant overlap between the two drafts, which makes me think that either: a) the authors of the draft-psenak-lsr-igp-dcm-00 were aware of the original TTE work, but decided to publish their own draft anyway. If that was the case, they should have at minimum referenced the TTE draft and described the differences between the two, including the motivations behind these differences. or b) the authors of the draft-psenak-lsr-igp-dcm-00 were not aware of the original TTE work, in which case I would advise them to read the TTE draft before they take any further action with their draft. Thanks, —Colby _______________________________________________ Lsr mailing list -- [email protected]<mailto:[email protected]> To unsubscribe send an email to [email protected]<mailto:[email protected]>
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