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

[[e-mail]]

-----Original Message-----
From: Coin <[e-mail]> On Behalf Of 'Toerless Eckert'
Sent: Thursday, May 18, 2023 8:23 PM
To: [e-mail]
Cc: 'Haoyu Song' <[e-mail]>; 'Hesham ElBakoury' <[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

Hemant,

Externs and functions are not an extension to the language. 
They are really just libraries.

Syntactic changes/extensions such as for parsing/deparsing, new types, such as float/double - those would be language extensions.

This makes the use of P4 cumbersome on any platform that can do more than the worst-case-chip against which the P4 language is designed (aka: Tofino).

For example on your OCTEON P4 compiler: The ARM A72 does support floating point numbers. If you implemented support for them via the mechanisms supported by P4, the way i understood it, you could still not write simple P4 programs for OCTEON then like:

    float a, b, c
    a = b * c / 3.2;

You may have a set of externs/functions through which you could do this, but it would look a lot more ugly.

Right ? If not, pls. explain - i would be happy!

And Haoyu's argument was that when we want to suggest to use P4 as the reference specification or base forwarding programming in the linux kernel, then this need to hide stuff outside of P4 written externs and functions does limit P4's useful as a complete specification and reference code language.

Cheers
    Toerless

On Thu, May 18, 2023 at 04:56:31PM -0400, [e-mail] wrote:
> Why do you care for internal behavior of checksum when the P4 api is able to provide data to compute checksum.
> 
> https://github.com/p4lang/p4c/blob/main/p4include/v1model.p4#L505
> 
> Hemant
> 
> -----Original Message-----
> From: Coin <[e-mail]> On Behalf Of Haoyu Song
> Sent: Thursday, May 18, 2023 3:30 PM
> To: 'Toerless Eckert' <[e-mail]>; [e-mail]
> Cc: 'Hesham ElBakoury' <[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
> 
> Toerless,
> 
> This is exactly the problem: you can't use the core language to describe an arbitrary dataplane function. 
> "Extern" can be a blackbox but what need is the spec for what's in the blackbox.
> 
> For P4 spec: " Extern objects are architecture-specific constructs that can be manipulated by P4 programs through well-defined APIs, but whose internal behavior is hard-wired (e.g., checksum units) and hence not programmable using P4"
> 
> Haoyu
> 
> -----Original Message-----
> From: 'Toerless Eckert' <[e-mail]>
> Sent: Thursday, May 18, 2023 12:15 PM
> To: [e-mail]
> Cc: Haoyu Song <[e-mail]>; 'Hesham ElBakoury' 
> <[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
> 
> On Thu, May 18, 2023 at 02:19:12PM -0400, [e-mail] wrote:
> > 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.
> 
> So far, my limited experience has been, that the language itself has rather evolved over its series of spec to further reduce the base language functionality and move any improvemenets into implementation extensions ("extern") calls, so that no possible hardware would not be able to not support any base language feature. 
> 
> This is a good strategy to gain more acceptance in the industry, but 
> not to get to the easiest to use forwarding plane behavioral 
> definition language
> 
> For exmple, i would need axtual syntactical language extensions to define a good way to parse andd deparsing variable length header fields. This can not just be done with extern calls.
> 
> Has P4 adopted the approach to have optional language feature extensions ?
> Sorry. Did not have time to follow the spec evolution in detail.
> 
> Cheers
>     Toerless
> > 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. 
> > 
> 
> 
> 
> --
> ---
> [e-mail]
> --
> Coin mailing list
> [e-mail]
> https://www.irtf.org/mailman/listinfo/coin



--
---
[e-mail]

--
Coin mailing list
[e-mail]
https://www.irtf.org/mailman/listinfo/coin