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

[Harald Welte <laforge@gnumonks.org>]

Hi Jonas,

thanks for your detailed analysis and review of the changes.  To me, they
once again show that the original patch was merged too quickly, without
a detailed review by people with strong GTP background.

On Sun, Jan 24, 2021 at 03:21:21PM +0100, Jonas Bonn wrote:
> struct gtpu_metadata {
>         __u8    ver;
>         __u8    flags;
>         __u8    type;
> };
> 
> Here ver is the version of the metadata structure itself, which is fine.
> 'flags' corresponds to the 3 flag bits of GTP header's first byte:  E, S,
> and PN.
> 'type' corresponds to the 'message type' field of the GTP header.

One more comment on the 'type': Of how much use is it?  After all, the
GTP-U kernel driver only handles a single message type at all (G-PDU /
255 - the only message type that encapsulates user IP data), while all
other message types are always processed in userland via the UDP socket.

Side-note: 3GPP TS 29.060 lists 5 other message types that can happen in
GTP-U:
* Echo Request
* Echo Response
* Error Indication
* Supported Extension Headers Notification
* End Marker

It would be interesting to understand how the new flow-based tunnel would
treat those, if those 

> The 'control header' (strange name) example below allows the flags to be
> set; however, setting these flags alone is insufficient because each one
> indicates the presence of additional fields in the header and there's
> nothing in the code to account for that.

Full ACK from my side here.  Setting arbitrary bits in the GTP flags without
then actually encoding the required additional bits that those flags require
will produce broken packets.  IMHO, the GTP driver should never do that.

-- 
- Harald Welte <laforge@gnumonks.org>           http://laforge.gnumonks.org/
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