GTP echo implementation

I am currently adding GTP echo handling to osmo-upf, which uses the GTP kernel module to handle GTP G-PDUs. What I need is a GTPv1-U ECHO response when an GTPv1-U ECHO request comes in on the user land GTP socket. We already have GTP implementations in - libgtp (osmo-ggsn.git/gtp/) - gtp_echo_responder.c (osmo-ggsn.git/utils/) - osmo-hnodeb/gtp.c I first used libgtp to do GTP ECHO handling in osmo-upf, but that basically includes all of the GTP-C GSN code, in a rather inflexible way. It seems to work, but we don't want to include this clunky dependency. 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. 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. So I need to dig deeper to understand the GTP Echo landscape... IIUC, there are GTPv0, GTPv1-C, GTPv1-U, GTPv2-C. 3GPP TS 29.281 is GTPv1-U. Is 29.060 GTPv1-C? Any others? 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? 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. I think I am ok solving these questions, but am writing this specifically because it feels like I or at least the next person shouldn't have to re-invent this wheel yet again. 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? 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. 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. ...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. Any thoughts? ~N