[Detnet] On the sequence number in draft-varga-detnet-srv6-data-plane when applying PREOF

[LUIS MIGUEL CONTRERAS MURILLO <[e-mail]>]

Dear Balazs, all,

As continuation of my comment during DetNet session in Madrid, I would like to elaborate a bit more on the possible issues with the sequence number when applying PREOF.

We can naturally assume the applicability of draft-varga-detnet-srv6-data-plane in WAN scenarios where we can have nodes supporting high bit rate interfaces, such as 100 GE or 400 GE. Also in that WAN scenarios, it is common to have redundant paths supported over long-distance disjoint fiber paths. That is the case of metropolitan or national networks, or even international networks on top of submarine cables.

Then, in those situations, we have to consider with care the difference of the length of the fibers since it could create issues in terms of counter roll over when using shorter sequence number.

Putting some exemplary numbers, for instance, for flows with packet size of 512 bytes sent at 100 Gbps, a difference in the length of the paths of aprox. 540 km can imply that two packets with the same sequence number (one from the counter cycle N+1 through the shorter fiber path and another one from the cycle N through the longer one) are received at the same time. This means that if the shorter path fails, the PREOF function would recognize as the protection packet the one from the cycle N instead of the cycle N+1. In short, the PREOF functionality would not work properly. For the same example, for higher bit rate interfaces such 400 GE, the distance is reduced proportionally.

So, in summary, I think it would be convenient to add some text to the draft mentioning this possible situation so that operators can take this into account. Maybe adding some operational considerations for it. I suggest the following text, but for sure, feel free to ignore.

8< --
When deploying PREOF functionality in Wide Area Networks, operators SHOULD take into account the possibility of counter roll over when the same traffic is transmitted over multiple paths (e.g., for protection or redundancy). In particular, if the 16-bit Sequence Number field is used, wraparound can occur in millisecond (or even sub-millisecond) intervals at high transmission rates depending on the typical packet size of the flows in such links. If the differential delay between protecting paths exceeds the counter wraparound interval, it is possible for two distinct packets with the same sequence number to arrive at the receiver approximately at the same time, leading to ambiguity or mis-ordering at the time of applying PREOF. Therefore, to ensure correct identification and processing of packets, operators SHOULD ensure that the path delay difference is sufficiently small compared to the wraparound interval, or use the higher sequence number encodings (i.e., 28 bits) offering a sufficiently large space to avoid ambiguity.
8< --

Note that the text only refers to the delay due to the difference on the length of the fibers, but additional hops (introducing processing delay) could even shorten the fiber length difference in which problems can appear.

By the way, very interesting draft.

Best regards

Luis
___________________________________________
Luis M. Contreras

Transport & IP Networks
Systems and Network Global Direction
Telefónica Innovación Digital / CTIO unit / Telefónica

Distrito Telefónica, Edificio Sur 3, Planta 3
28050 Madrid
España / Spain

Mobile: [phone]
[e-mail]<mailto:[e-mail]>






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