I am confused about the packet queue management approach in P4. Do all packets go into the egress port and queued? Or do they just go into the traffic management process and there are many queues in it and the packets are saved?

@omega00 , Thank you. It was not clear that you were asking about Tofino and TNA. I’ll give you quick answers here, but the best places to ask questions about Tofino and TNA are Tofino-specific forums, maintained by Intel. If you represent an academic or research organization, please use Intel Conne…

P4 specification specifically says (in the section 1 “Scope”) that it does not define “Mechanisms by which data are received by one packet-processing system and delivered to another system”. Therefore, the answer to your question differs from system to system and it is defined by the system’s P4 Ar…

Hi, thanks for your reply! I still have some questions about that. First, when multiple ingress ports send traffic to the same egress port, are the excessive packets buffered at the TM (traffic manager)? and then send to the egress port? Second, I have seen the queue length field in the egress pr…

In most P4 architectures for switch ASICs that I know of (e.g. v1model, PSA, TNA), yes the traffic manager buffers packets that are destined to the same output port, if those packets arrive faster than that output port can send them. That is one of the primary reasons for the existence of a traffic…

I just wanted to re-emphasize a very important point @andyfingerhut has made. You wrote “I have seen the queue length field in the egress process of P4 program”. Instead, the truth is that you have seen it in a P4 program written for a specific architecture (be it v1model, PSA, TNA – you neglected …

Hi, sorry for missing the detailed description. :fearful: I have seen the fields enq_qdepth and deq_qdepth in the tofino.p4 file and I am running the P4 switch with tofino asic. I learned from your reply that the excessive packets are buffered at TM (queues in TM?) and then transmitted to the egre…

Thanks for your detailed reply. :grinning: I will turn to Intel Premier Support for more detailed answers.

Thank you very much for your detailed answer before. I would like to ask some similar questions about V1 model. Q1: Can we get the length of each queue from the ingress pipeline? I want to compare the lengths of two specified queues and do some scheduling operations. Q2: Is it possible to dequeue …

Q1: There is no direct way designed into the v1model architecture to read queue depths from ingress. You could implement a way with some P4 code written by you, and probably a bit of control plane code as well, that would periodically send packets through ingress, then egress, where packet X would …

Thanks for the ideas you provided, I know what to do next

What is the queue management approach in P4?

omega00 March 17, 2022, 1:49am 8

Thanks for your detailed reply.
I will turn to Intel Premier Support for more detailed answers.

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What is the queue management approach in P4?

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