netdev - Re: [RFC PATCH 15/16] gtp: add ability to send GTP controls headers

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Date:   Tue, 2 Feb 2021 06:24:49 +0100
From:   Jonas Bonn <jonas@...rbonn.se>
To:     Jakub Kicinski <kuba@...nel.org>,
        Pravin Shelar <pravin.ovn@...il.com>
Cc:     Harald Welte <laforge@...monks.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Pravin B Shelar <pbshelar@...com>,
        Pablo Neira Ayuso <pablo@...filter.org>
Subject: Re: [RFC PATCH 15/16] gtp: add ability to send GTP controls headers

Hi Jakub,

On 01/02/2021 21:44, Jakub Kicinski wrote:
> On Sat, 30 Jan 2021 12:05:40 -0800 Pravin Shelar wrote:
>> On Sat, Jan 30, 2021 at 10:44 AM Jakub Kicinski <kuba@...nel.org> wrote:
>>> On Fri, 29 Jan 2021 22:59:06 -0800 Pravin Shelar wrote:
>>>> On Fri, Jan 29, 2021 at 6:08 AM Jonas Bonn <jonas@...rbonn.se> wrote:
>>>> Following are the reasons for extracting the header and populating metadata.
>>>> 1. That is the design used by other tunneling protocols
>>>> implementations for handling optional headers. We need to have a
>>>> consistent model across all tunnel devices for upper layers.
>>>
>>> Could you clarify with some examples? This does not match intuition,
>>> I must be missing something.
>>
>> You can look at geneve_rx() or vxlan_rcv() that extracts optional
>> headers in ip_tunnel_info opts.
> 
> Okay, I got confused what Jonas was inquiring about. I thought that the
> extension headers were not pulled, rather than not parsed. Copying them
> as-is to info->opts is right, thanks!
> 

No, you're not confused.  The extension headers are not being pulled in 
the current patchset.

Incoming packet:

---------------------------------------------------------------------
| flags | type | len | TEID | N-PDU | SEQ | Ext | EXT.Hdr | IP | ...
---------------------------------------------------------------------
<--------- GTP header ------<<Optional GTP elements>>-----><- Pkt --->

The "collect metadata" path of the patchset copies 'flags' and 'type' to 
info->opts, but leaves the following:

-----------------------------------------
| N-PDU | SEQ | Ext | EXT.Hdr | IP | ...
-----------------------------------------
<--------- GTP header -------><- Pkt --->

So it's leaving _half_ the header and making it a requirement that there 
be further intelligence down the line that can handle this.  This is far 
from intuitive.

/Jonas