27 Mar
2022
27 Mar
'22
12:15 p.m.
Hi Neels,
On Thu, Mar 24, 2022 at 11:29:20PM +0100, Neels Hofmeyr wrote:
Looking at gtp_echo_responder.c, I see that the code
uses none of the Osmocom
structures (osmo_fd, logging, osmo_select, osmocom/core/endian.h), and it
implements GTPv1-C and GTPv2-C, but I need GTPv1-U.
yes, look at the license. It was specifically created to be used under permissive
license (MIT) and hence cannot use the existing libosmo* infrastructure.
This is also outlined in the commit message:
https://gerrit.osmocom.org/c/osmo-ggsn/+/25652
osmo-hnodeb/gtp.c does use Osmocom structures. It has
its own struct gtp1u_hdr.
But apparently osmo-hnodeb doesn't do any GTP ECHO handling at all.
The latter part is likely a bug, and should be addressed.
Though, to be honest, I'm not 100% sure if the use of GTP-U on any other interface
than that between GSNs (SGSN/SGW and GGSN/PGW) mandates GTP ECHO support...
I am now only concerned with GTPv1-U, so TS 29.281
should be all I need. Still
interesting to know, do the echos differ between the protocol versions and
planes? Can I use the GTPv1-C code from gtp_echo_responder.c for GTPv1-U?
GTPv0 is not really relevant anymore in 99.9% of all situations.
GTPv1C and GTPv2C are completely differnt protocols.
GTP-U v1 is used by GTPv1C and GTPv2C as well as on many other interfaces that don't
use GTP-C for the control plane.
The fact that the GTPv1-U header contains a TEID
confused me at first, then I
found in 29.281 that the TEID shall be all zeros in the ECHO req + resp
messages. So, yes, ECHO is done between GSNs as a whole, not on each tunnel.
ACK.
Will we spawn all-new GTP implementations in every
osmocom repository that
touches GTP, or should I rather implement a re-usable GTP echo response now?
to be fair, responding to a GTP-U echo request is hardly a very complex task.
One proper (TM) way seems to be to rearrange libgtp in
such a way that a caller
can just use the msg coding part for specific messages, and can use UDP sockets
without having to set up a complete struct gsn_t. That's some work.
I think libgtp is fundamentlaly un-osmocom-like due to its history and it doesn't
make sense to spend time on it.
Another way that comes to mind is opening a
libosmo-gtp section in libosmocore,
absorb protocol definitions across the various GTP versions there, and use them
in the places where we do GTP coding now. Seems a lot of work.
I would say for the struct definitions it would make sense to put it in
include/osmocom/gsm/protocol/ or even better include/osmocom/gprs/protocol/
osmo-hnodeb and osmo-upf could then use that code. But that's about it.
...or I go the apparently quickest, easiest way, do a
copy/paste/reimplement
from scratch of GTP echo coding, so that we have yet another partial GTP
implementation in osmo-upf.git. That's what I'm doing now, but it feels wrong.
In general, I hear you, but in this case "partial" means 0.01% of what GTP
is...
--
- Harald Welte <laforge(a)osmocom.org> http://laforge.gnumonks.org/
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