On 9/6/23 12:01, Saku Ytti wrote:
Fun fact about the real world, devices do not internally guarantee order. That is, even if you have identical latency links, 0 congestion, order is not guaranteed between packet1 coming from interfaceI1 and packet2 coming from interfaceI2, which packet first goes to interfaceE1 is unspecified. This is because packets inside lookup engine can be sprayed to multiple lookup engines, and order is lost even for packets coming from interface1 exclusively, however after the lookup the order is restored for _flow_, it is not restored between flows, so packets coming from interface1 with random ports won't be same order going out from interface2. So order is only restored inside a single lookup complex (interfaces are not guaranteed to be in the same complex) and only for actual flows.
Yes, this has been my understanding of, specifically, Juniper's forwarding complex.
Packets are chopped into near-same-size cells, sprayed across all available fabric links by the PFE logic, given a sequence number, and protocol engines ensure oversubscription is managed by a request-grant mechanism between PFE's.
I'm not sure what mechanisms other vendors implement, but certainly OoO cells in the Juniper forwarding complex is not a concern within the same internal system itself.
Mark.
