Re: [Detnet-dp-dt] quick notes from call 2/14/15

[Jiangyuanlong <[e-mail]>]

Hi Jouni,

Some further comments based on our recent discussions.

Thanks,
Yuanlong

From: Detnet-dp-dt [mailto:[e-mail]] On Behalf Of Jouni Korhonen
Sent: Thursday, February 16, 2017 3:37 AM
To: [e-mail]
Subject: [Detnet-dp-dt] quick notes from call 2/14/15

Present: Jouni, Norm, Loa, Balazs, Janos, Carlos, Yuanlong.

Agenda:
- Was meant to be about CoS and QoS. However, we ended up discussion all time about proper layering and encapsulation of PW stack.

Discussion:
- Sequence number. We made a >>decision<< to settle down to 16 bits. This is the most “compatible” approach. At high speeds it has been argued 16 bits is not enough. In practical implementations even 16 bits of sequence number won’t be maintained. It is typically a much smaller window of the entire sequence number space (think about numbers.. 1M PWs times the seqnum bit vector etc..)
[YJ] Good to be compatible with the generic CW. We need to be careful to avoid sequence number wrap up. 32Mbps detnet flow is enough to overflow the sequence in a second (if 64 bytes/packet is assumed). If the "eliminate first and then replicate" mechanism is used, detnet traffic from a S-PE may be looped back to itself, thus there is also a risk of sequence overrun. "replicate first and then eliminate" will be more safe IMO.

- PWs and the label stack. See the latest slides from Loa and specifically the slide 9 (attached). I did add one slide to this deck, the slide 14.
- T-labels are typically per hop. L-labels are between DetNet aware S/P-PEs and essentially form an overlay over the underlying network. d-pw labels are end to end (at the moment.. to be discussed) between the T-PEs or in general between the DetNet aware end point that understand the detnet data plane.
[YJ] LSP is actually hop by hop allocated a label, though each hop can be overlaid in a tunnel itself. Thus T-labels and L-labels can be regarded as a single LSP label without loss of generality. PHP can be enabled optionally for this LSP.

- currently all d-pw (detnet PW labels) experience FRER if that functionality is enabled. d-pw labels are tied to sequence numbers (the detnet CW).
[YJ] Yes, d-pw label always go together with detnet CW.

- L-labels seen beneficial allowing the autoconfiguration of FRER i.e., build the overlay over the network and do not care configuring the PWs. This mimics one 802.1CB feature (see .1CB sub-clause 7.11).
[YJ] we don't need the L-label if MS-PW approach is taken, since each segment of PW can mimic the 802.1CB member stream.

- L-labels also allow simple label swap in a detnet S-PE i.e., no FRER would be applied. 
[YJ] we don't need the L-label. PW swap is the only action if no FRER is applied, this is the traditional MS-PW mechanism.

- This setup seems plausible. There was concerns overloading L-labels with some of the PW decision making in the fast path and thus possibly causing a lookup that needs to be done over two labels (L & d-pw). Essentially it is the L-label that signals whether the FRER gets applied.
[YJ] Using an L-label to trigger an S-PE looking into the CW inside a PW will be less clean compared with using a PW label directly.

- General consensus towards the PW instance “facing egress ports” in a detnet S-PE would do the elimination.  This discussion needs to be completed. An interesting use case was described by Norm (see slide 14). An example: packet arrives from A towards B (flow X), it always gets replicated towards C, but towards D only if the same packet (from flow Y) has not earlier arrived from C. Whether this works ok with the current S-PE and PW instance doing elimination concepts needs to be verified.
- Discussion to be completed whether there are cases where L-labels can be left out i.e., only use T-labels and d-pw labels. 
- Balazs said to provide a matrix/table of different permutations for labels & replications & eliminations.

We’ll have a call next Tuesday 2/21/17.


--
Jouni Korhonen, Broadcom Ltd.
M: [phone]
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