[EXT] Re: l2/l3_len fields

[Stephen Hemminger]

On Tue, 25 Jan 2022 17:38:42 +0000
Liron Himi <lironh at marvell.com> wrote:

> -----Original Message-----
> From: Stephen Hemminger <stephen at networkplumber.org> 
> Sent: Monday, 24 January 2022 19:21
> To: Liron Himi <lironh at marvell.com>
> Cc: dev at dpdk.org
> Subject: [EXT] Re: l2/l3_len fields
> 
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> ----------------------------------------------------------------------
> On Mon, 24 Jan 2022 08:39:59 +0000
> Liron Himi <lironh at marvell.com> wrote:
> 
> > Hi,
> > 
> > 
> > 
> > Can you please share the motivation of not filling l2/l3-len fields on PMD's RX function?
> > 
> > PMD already filling the mbuf with parsing info, and the l2/l3-len are also probably part of the HW descriptor, so why no propagate them as well?
> > 
> > The current way for application to find the l2_len (for example) is to examine the ptype using multiple if statement.
> > 
> > However, this may not always work if there are unknown/user headers (e.g. DSA header).
> > in addition, some of the examples are not checking the ptype and assumes a specific packet structure.
> > e.g. l3-fwd, assumes only ethernet header exist and hardcoded jump by 14B to access the IP header.
> > but if VLAN exist it will fail.
> > 
> > checking the l2_len will work in any case.
> > 
> > 
> > Regards,
> > Liron Himi
> > 
> > [A picture containing clipart  Description automatically generated]<https://www.marvell.com/>
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> 
> L2 len would be relatively easy since almost all devices are Ethernet only.
> L3 len is the problem, many devices don't interpret L3 and below headers.
> Only those that have flow type functionality are likely to do that.
> 
> [L.H.] Not sure I understand what you mean.
> what will be easy? Easy for the application to calculate the l2_len?
> 

If the hardware doesn't support doing it directly, then if the application needs
these it should do the calculations itself. Many chips don't support reporting L2 and L3 lengths
in the receive result.