Re: [Coin] Fwd: The Future of P4, Revisited

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A specific P4 architecture, e.g., PNA, may override the base P4 spec. PNA supports writing table entry by data plane. Using registers in P4 is a bit clunky but state can be maintained. Anyone is welcome to present a better solution to the P4 forums.

 

Hemant

 

From: Haoyu Song <[e-mail]> 
Sent: Thursday, May 18, 2023 1:56 PM
To: Hesham ElBakoury <[e-mail]>
Cc: [e-mail]; Toerless Eckert <[e-mail]>; hemant=[e-mail] <[e-mail]>; Bernier, Daniel <[e-mail]>; Marie-Jose Montpetit <[e-mail]>; coin <[e-mail]>; coinrg-chairs <[e-mail]>
Subject: RE: [Coin] Fwd: The Future of P4, Revisited

 

Hi Hesham,

 

Please see P4 spec section 6.5.2 for the current “stateful” support. So far the “table” element, which is the most important construct for a P4 program,  is not stateful (i.e., dataplane writable)

I think it’s a direction to extend P4 for better stateful support (part of my recent research). At least now it’s unnatural and difficult to describe many stateful functions.  

 

Haoyu

 

From: Coin <[e-mail] <mailto:[e-mail]> > On Behalf Of Hesham ElBakoury
Sent: Thursday, May 18, 2023 10:42 AM
To: Haoyu Song <[e-mail] <mailto:[e-mail]> >
Cc: [e-mail] <mailto:[e-mail]> ; Toerless Eckert <[e-mail] <mailto:[e-mail]> >; hemant=[e-mail] <mailto:hemant=[e-mail]> ; Bernier, Daniel <[e-mail] <mailto:[e-mail]> >; Marie-Jose Montpetit <[e-mail] <mailto:[e-mail]> >; coin <[e-mail] <mailto:[e-mail]> >; coinrg-chairs <[e-mail] <mailto:[e-mail]> >
Subject: Re: [Coin] Fwd: The Future of P4, Revisited

 

Hi Haoyu,

Actually, P4 supports dataplane-modifiable tables -- see PNA.

 

More generally, the *language* is fully extensible. You can have whatever architecture and state externs you want. So one needs to be careful to separate language (non) limitations from target limitations.

 

Hesham

 

On Thu, May 18, 2023, 10:15 AM Haoyu Song <[e-mail] <mailto:[e-mail]> > wrote:

Hi Hesham,

 

I said “it’s very limited”. P4 table is only readable (i.e., not writable) by dataplane, so it basically eliminates any dataplane stateful function that need to use tables. The stateful function can only use register arrays but unfortunately registers are local to a pipeline stage so the state update logic must be very simple and can be finish in a single stage. What you said “event” must be something very simple. Use case such as stateful load balancer can’t be implemented by P4.

 

There are some works addressing the issue. FlowBlaze is the most recent one, which needs a new chip architecture but it is still limited to simple stateful functions which can be realized in a single pipeline stage. 

 

Haoyu

 

From: Hesham ElBakoury <[e-mail] <mailto:[e-mail]> > 
Sent: Thursday, May 18, 2023 10:00 AM
To: Haoyu Song <[e-mail] <mailto:[e-mail]> >
Cc: [e-mail] <mailto:[e-mail]> ; Toerless Eckert <[e-mail] <mailto:[e-mail]> >; hemant=[e-mail] <mailto:[e-mail]> ; Bernier, Daniel <[e-mail] <mailto:[e-mail]> >; Marie-Jose Montpetit <[e-mail] <mailto:[e-mail]> >; coin <[e-mail] <mailto:[e-mail]> >; coinrg-chairs <[e-mail] <mailto:[e-mail]> >
Subject: Re: [Coin] Fwd: The Future of P4, Revisited

 

Hi Haoyu,

You say: "P4 has very limited support for stateful processing"

 

I am not sure I can agree with your characterization of P4. Perhaps can you elaborate on this.

 

Thanks

Hesham

 

On Thu, May 18, 2023, 9:19 AM Haoyu Song <[e-mail] <mailto:[e-mail]> > wrote:

Interesting discussion. See my comments below  [HS]

Haoyu

> For example, in the multicast drafts i write, we use C-pseudocode
> to specify behavior, but we do attempt to implemnt on Tofino in P4. Should we really
> use P4 code for the RFC specs... ? (much longer than C Pseudocode). Aka: quite selfish
> (but IETF relevant ;-) reason to highlight this point.

[EH]: This is an area I'm very interested in. Having a standardized and formal language to describe protocols and behavior can bring a lot of functionality and benefits to the IETF. 
My initial thinking is that having such a blueprint, the IETF could generate tools to create a reference implementation that can be used for interoperability purposes therefore decreasing time to test and implement protocols and therefore RFC publications.

[HS] P4 can only describe dataplane behaviors, so any control plane stuff is out of scope. For dataplane, if it's used to describe header format, it's not better than the "struct" in C. The language uses the match-action table abstraction with an implication of pipeline implementation which may make it cumbersome or even impossible to describe the  behavior (e.g.,  P4 has very limited support for stateful processing). In general, I don't think P4 at its current form can undertake the role for formal protocol specification.