RE: [PATCH] ethdev: introduce generic flow item and action

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Dumitrescu, Cristian [mailto:cristian.dumitrescu@intel.com]
> Sent: Wednesday, 2 August 2023 16.06
> 
> > From: Jerin Jacob <jerinjacobk@gmail.com>
> > Sent: Wednesday, August 2, 2023 12:22 PM
> >
> > On Wed, Aug 2, 2023 at 4:31 PM Morten Brørup
> > <mb@smartsharesystems.com> wrote:
> > >
> > > > From: Morten Brørup [mailto:mb@smartsharesystems.com]
> > > > Sent: Wednesday, 2 August 2023 12.25
> > > >
> > > > > From: Qi Zhang [mailto:qi.z.zhang@intel.com]
> > > > > Sent: Wednesday, 2 August 2023 19.35
> > > > >
> > > > > From: Cristian Dumitrescu <cristian.dumitrescu@intel.com>
> > > > >
> > > > > For network devices that are programmable through languages such as
> > > > > the P4 language, there are no pre-defined flow items and actions.
> > > > >
> > > > > The format of the protocol header and metadata fields that are used to
> > > > > specify the flow items that make up the flow pattern, as well as the
> > > > > flow actions, are all defined by the program, with an infinity of
> > > > > possible combinations, as opposed to being selected from a finite
> > > > > pre-defined list.
> > > > >
> > > > > It is virtually impossible to pre-define all the flow items and the
> > > > > flow actions that programs might ever use, as these are only limited
> > > > > by the set of HW resources and the program developer's imagination.
> > > > >
> > > > > To support the programmable network devices, we are introducing:
> > > > >
> > > > > * A generic flow item: The flow item is expressed as an array of bytes
> > > > > of a given length, whose meaning is defined by the program loaded by
> > > > > the network device.
> > > >
> > > > The flow item is not "generic", it is "opaque": Only the application
> knows
> > > > what this flow item does.
> > > >
> > > > I hate the concept for two reasons:
> > > > 1. The inability for applications to detect which flow items the
> underlying
> > > > hardware supports.
> > > > 2. The risk that vendors will use this instead of introducing new flow
> item
> > > > types, available for anyone to implement.
> > >
> > > After further consideration, there might be a middle ground.
> > >
> > > Consider Vendor-Specific attributes for DHCP and RADIUS, or SNMP MIBs...
> > >
> > > Any vendor is free to add his own, proprietary special-purpose attributes,
> > without going through the standardization process. (This is the key
> challenge
> > this patch seems to be aiming at.)
> > >
> > > The vendor might publish these attributes, and other vendors may
> > implement them too.
> > >
> > > And in order to prevent collisions, the Vendor-Specific attributes contain
> a
> > globally unique vendor ID, such as the Private Enterprise Number [1]
> > managed by IANA.
> > >
> > > If similar principles can be worked into the patch, I can support it.
> >
> > +1
> >
> 
> Morten, Jerin,
> 
> I think there is a fundamental misunderstanding here: we are not trying to
> provide support for some non-standard vendor-specific features here. What
> we are trying to do is add generic multi-vendor support in RTE_FLOW for
> P4 programmable network devices, which requires supporting flow items
> and actions that are defined directly by the user through their P4 programs
> as opposed to being selected from a pre-defined list.
> 
> There are an infinity of flow items and actions that the users can define
> through
> their P4 programs, and they cannot be supported with a finite list of RTE_FLOW
> items and actions:
> 
> 1/ Some flow items map directly to the IETF defined protocols, while some
> others do not, and only the user writing the program knows the exact answer;
> 
> 2/ Some flow items are simply application-specific (not vendor specific)
> meta-data that (I hope we all accept) is outside of the standardization
> process.

Such items can use a special "reserved" vendor-id.

> 
> 3/ Some flow actions map directly to the existing RTE_FLOW actions (especially
> the more straightforward actions such as: packet drop, packet redirection to
> an
> output queue, some specific packet modifications, etc), while the vast
> majority
> of possible actions do not.
> 
> Are you saying that the P4 programmable network devices should NOT be
> supported by DPDK and RTE_FLOW?

No, I get the need for this. And I understand that since P4 is compiled to hardware-specific binary blobs, there is a need to put such blobs into DPDK as flow items and actions, instead of the "uncompiled" P4 program.

I am suggesting that instead of adding a completely opaque data type:

Struct item {
Int len;      // Length of value in bytes.
Char value[]; // Application specific meaning.
};

...add a semi-opaque data type:

Struct tlv {
Int type;     // Vendor specific type.
Int len;      // Length of value in bytes.
Char value[]; // (Vendor, Type) specific meaning.
};

Struct item {
Int vendor;          // Vendor ID.
Int len;             // Length of values in bytes.
Struct tlv values[]; // Array of TLVs.
};

Like RADIUS Vendor-Specific attributes:
https://datatracker.ietf.org/doc/html/rfc2138#section-5.26

Then some (Vendor, Type) fields can be documented (and thus generally understood by DPDK), and some undocumented.

E.g. like Microsoft documented some of theirs in RFC 2548:
https://datatracker.ietf.org/doc/html/rfc2548


Another benefit is that these new "VENDOR-SPECIFIC" flow types can be reused for other purposes than compiled P4 programs.

> 
> >
> > >
> > > Preferably, there should also be a means for applications to query if
> specific
> > Vendor-Specific flow items and actions are supported or not.
> > >
> > >
> > > [1]: https://www.iana.org/assignments/enterprise-numbers/
> > >
> 
> Regards,
> Cristian