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

['Toerless Eckert' <[e-mail]>]

On Fri, May 19, 2023 at 01:37:05PM -0400, [e-mail] wrote:
> P4 supports variable length parsing.

Not via the parser itself, but through explicit P4 programming, which is
a significant limitation. I can find our older papers on variable length
packet parsing issues. Deparsing is then also equally more complex.

P4 is based on a well known type of parsing hardware, which is employed
in Tofino. But there are more intelligent parsing engines.

> Regarding memory optimizations on cpu, vpp has optimized memory for both instructions and data.

For processing that needs to operate on more than just integers,
such as monitoring, accounting and inference, you definitely want
a specification language that supports the desired data types natively.
Such as real or fixed-point numbers.  Even allow to define new types.
But also beyond that.  For example, in one of my drafts, i need to do
have bit-accurate pointers for calculation into complex bitstring
structures. Same is likely true if we where to pseudocode
specify some of the IoT header compression mechanisms, or other
compressed structures.

Production VPP code is certainly not useful as specification reference code.
I know how much of that code was written by developers (Ed et. al.)
who knew exactly what x86 instructions where generated from C code and
their cycle count. Which of course was (and i guess still is) then a good
amount of re-optimization need for ARM (or other CPU). Not to talk about SIMD.

> All these optimizations are available in my company's P4toVPP compiler.

Sure, but a better specification language than P4 would be even more opportunity for
a future versions of a compiler like yours.  For example, fixedpoint math
looks ugly in P4. Good enoough for validation code, but not for a spec language.
It could look very nice if the language had such data
types natively. And your compiler could translate into a fitting P4
fixedpoint math library. Or extern libraries, such as for actual floating
point calculation. Or even skip mapping into P4 completely and creating
better code directly mapping to the underlying platform features.

> TM is still an open issue to work on.

There is good work happening for high-speed PIFO type FPGA forwarding code.
I think some form of extended PIFO functionality could become the basis for
programmable TM in the future. Still has to be seen how well it would possible
to efficiently PoC this on lower end CPU forwarders. If this direction pans
out, then the forwarding plane impact is just the compute of the TM
parameters such as rank/timestamps - and processing on dequeueing.

Cheers
    Toerless

> 
> Hemant
> 
> -----Original Message-----
> From: 'Toerless Eckert' <[e-mail]> 
> Sent: Friday, May 19, 2023 12:33 PM
> To: Haoyu Song <[e-mail]>
> Cc: [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
> 
> Right.
> 
> For high speed router/switch (>= 100gbps/interface) validation:
>   P4 implementations on Tofino are great as proof of concept - if it can be done.
>   If P4 implementation on Tofino does not work, other proof points are needed,
> 
> For medium speed "software forwarding" validation:
>   P4 creates too many limitations. All CPU have float, and/or SMDI, and/or
>   more flexible memory access options and likely several other aspects not
>   easily utilized by P4.
> 
> For generic specification:
>   IMHO above limitations are at the core of why P4 is insufficient.
>   I very much like several structural aspects of P4, they would be great
>   starting points for general purpose spec. Such as header parsing specs
>   (need to be extended for variable length though).
>   Aka: Anything we can use from P4 which is more/better declarative than
>   C/C++ is IMHO useful. Just lets not introduce constraints: Validation on
>   different type of forwarding platforms is different from a single
>   specification.
>   And we are also missing good DSL spec options for TM i fear...
> 
> Cheers
>     Toerless
>   
> On Fri, May 19, 2023 at 04:02:57PM +0000, Haoyu Song wrote:
> > I think the discussion is off the point. We are discussing whether or not P4 is suitable for a standard specification language for general dataplane processing and forwarding applications. To fulfill that purpose,  it needs to be target/architecture independent; it needs to be concise and expressive to cover various dataplane functions which may not be conceivable today (e.g., in-network computing applications).  
> > Unfortunately, I don't think P4 can meet these requirements now 
> > (imagine all the pseudo code in RFCs are translated to P4). Of course, 
> > it can be limited to just support certain functions, e.g., to document 
> > the header formats and parsing process (but here I don't see the 
> > advantage of P4 either)
> > 
> > Haoyu
> > 
> > -----Original Message-----
> > From: [e-mail] <[e-mail]>
> > Sent: Thursday, May 18, 2023 6:19 PM
> > To: 'Toerless Eckert' <[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
> > 
> > Toerless,
> > 
> > Only externs defined in core.p4 are in a library. Externs defined in an architecture include file, e.g., v1model.p4 are implemented in target-specific back end and are extensions. An example of a target extern is vpp accelerator in Octeon 10. Neither P4-16 spec nor PNA support floating point and neither does my compiler. Floating point in used be AI/ML. The float point numbers would have to be converted to integer. Google's Tensor chip also converts float to integer for ML.
> > 
> > If you want to work with the linux kernel, see the ebpf architecture include file, https://github.com/p4lang/p4c/blob/main/p4include/ebpf_model.p4. Doesn't the file have externs which have totally abstracted linux kernel data structs and functions?
> > 
> > Hemant
> > 
> > -----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. 
> > > > 
> > > 
> > > 
> > > 
> > > --
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> > 
> > 
> > 
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