[ippm] https://datatracker.ietf.org/doc/html/draft-ravi-ippm-csig-01

[Sebastian Moeller <[e-mail]>]

https://datatracker.ietf.org/doc/html/draft-ravi-ippm-csig-01

4.1.1. Compact Format
CSIG-tag compact format is as shown, with 2B allocated for the CSIG Tag Protocol ID (TPID) and 2B allocated for the data fields. 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TPID | T |R| S | LM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

|0-15| TPID : IEEE allocated Tag Protocol ID for 4 Byte CSIG tag
|16-18| T : Signal Type (0:min(ABW), 1: min(ABW/C), 2:max(PD))
|19| R : Reserved
|20-24| S : Signal Value: Bucketed (32 configurable buckets)
|25-31| LM : Locator Metadata of bottleneck device / port

Figure 1: CSIG-tag Compact version
4.1.2. Expanded Format
CSIG-tag expanded format is as shown, with 2B allocated for the Tag Protocol ID (TPID) and 6B allocated for the data fields 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TPID | LM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T | S | R |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

|0-15| TPID : IEEE allocated Tag Protocol ID for 8 Byte CSIG tag
|16-31| LM : Locator Metadata of bottleneck device / port
|0-3| T : Signal Type (0:min(ABW), 1: min(ABW/C), 2:max(PD))
|4-23| S : Signal Value: Uniformly quantized
|24-31| R : Reserved for future use

[...]

5.1. Minimum Available Bandwidth - min(ABW)
min(ABW) captures the minimum absolute available bandwidth (in bps) across all the ports in the packet path. Available bandwidth is defined per egress port on each device.

[...]

5.3.3. Bucketing / Quantization Requirements
The computed ABW or ABW/C values MUST be compressed to fit in the available Signal value bits on the CSIG-tag. The device MUST support 32 fully configurable ABW buckets and ABW/C buckets for compact CSIG, and configurable quanta for uniform quantization in expanded CSIG. All devices along the packet path MUST be configured with the same buckets / quanta per signal type in order to correctly compute min(ABW) or min(ABW/C) along the path. Appendix A provides examples of these configurations.
Each transit device performs a compare-and-replace, i.e., updates the signal value on the CSIG tag if the incoming ABW or ABW/C signal value on the packet is higher than the device's locally computed ABW or ABW/C value for the packet's egress port, post bucketization / quantization. E.g.,
// Update the signal value on packet if current hop is the bottleneck
pkt->csig_tag->abw = min(pkt->csig_tag->abw, egr_port->abw)

[...]


So we have either 4 or 20 bits, to carry a signal that is supposed to be in units of bsp (so nominally 0-32 or 0 to 1048576 bps), but that then is quantised in a way that needs to be specified and configured out of band. After the quantisation the signal is not in units of bsp anymore, hence at the very least the text in section 5.1 needs to be adjusted...

However, that IMHO is not how a protocol should be specified, the quantisation level would need to be part of the actual record, as this becomes completely infeasible over the internet if "All devices along the packet path MUST be configured with the same buckets / quanta per signal type" is kept as requirement.

If that is not changes I would argue that this draft should be moved to the informational track, as it clearly is not fit for internet deployment (this is in addition to using non end to end L2 records), congestion control is inherently and end to end problem, so proposals should come with a clear path for internet wide deployment, and making all L2 domains across arbitrary internet paths honor and use L2 CSIG records does not appear feasible. Incremental deployment IMHO is not helped with a design that strips end to end relevant congestion ibnformation, just because a packet crosses an L2 domain boundary.

Regards
	Sebastian