Expanding on my comments in the meeting earlier today on: Credit-based Flow Control Based on RSVP for RDMA transmission in WAN https://datatracker.ietf.org/doc/draft-hu-rtgwg-cbfc-rsvp/
Credit-based flow control is complex and subtle. RSVP is already complex enough - this proposal adds not just credit-based flow control, but flow-based credit-based flow control, which significantly increases complexity. This draft's proposed mechanism depends on RSVP maintaining credit state. All RSVP state is "soft state" that is dropped if not refreshed - there will be situations (e.g., some error cases) in which RSVP drops state due to lack of refresh, which risks dropping credits. That's a specific example of an area of complexity and subtlety - the normal operation case of credit-based flow control is easy to specify ... but ... there are inevitably situations that lose credits ... and ... if such a situation repeats, enough credits are eventually lost to prevent transmission. A quick read of the draft suggests that no attempt is made to recover credits - any detected credit loss problem causes failover to the backup path. That response is likely to be too aggressive - an attempt should be made to recover credits before making that dramatic a change. There is some possibly-related IETF credit-based flow control work for the DLEP protocol in the manet WG - see draft-ietf-manet-dlep-credit-flow-control<https://datatracker.ietf.org/doc/draft-ietf-manet-dlep-credit-flow-control/>. Related drafts can be found from the manet WG's documents page<https://datatracker.ietf.org/wg/manet/documents/> . Thanks, --David David L. Black, Sr. Distinguished Engineer, Technology & Standards Infrastructure Solutions Group, Dell Technologies mobile +1 978-394-7754 [email protected]<mailto:[email protected]>
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