I am trying the configuration for the  GTP_U to SRv6  function T.M.GTP4.D.
 
Here is my setup
 
Trex container ---------> VPP container---->SrV6 end point containers
                       GTP-U pkt
 
The VPP container has two interfaces provided by K8s IPVLAN CNI. One is with
Ipv6 address and the other one is IPv4 which is in the same subnet as the Trex container interface.
 
GTP tunnel is created between the interfaces of Trex and IPv4 interface of VPP container.
 
command => vppctl create gtpu tunnel src  <Trex Ipv4 addr> dst <VPP container ipv4 addr> teid 13
 
"vppctl show int addr" shows all the interfaces including gtp tunnel interface is UP.
 
Now created the SR policy rule-
command_1 => vppctl sr policy add  bsid fcf0:12::109 next fcf0:12::100 next fcf0:12::101  next fcf0:12::104:0E01:2042:1 behavior t.m.gtp4.d fcf0:12::104/64 v6src_prefix fcf0:12::109/64 nhtype ipv4
 
command2 => vppctl sr steer l3 <local ipv4 IP of VPP>/32 via bsid fcf0:12::109
 
command3 => vppctl set sr encaps source addr fcf0:12::109
 
Here fcf0:12::109 is the local sid of VPP container.  fcf0:12::100/64 and fcf0:12::101/64 are the intermediate SIDs of the SRv6 end points.  fcf0:12::104:0E01:2042:1 is the final destination SID. 
As per https://docs.fd.io/vpp/20.05/d7/d3c/srv6_mobile_plugin_doc.html, its coded as  destination prefix fcf0:12::104/64 followed by destination ipv4 ip, Teid(13=0xD) and QFI.
 
From VPP container, "ip -6 route" is executed to the SID of the next endpoint fcf0:12::100 over the ipv6 subnet.
 
Issue is when GTP-U packet with outer IP address as VPP destination ipv4 address is sent through Trex container, it reaches VPP but VPP is not de-encapsulating GTP-U pkt received and sent over the SRv6 network. 
 
 
Details about VPP version can be found at -
root@upf-0:/vpp# vppctl show version verbose cmdline
Version: v21.06-release
Compiled by: root
Compile host: f861a9bfa1ad
Compile date: 2021-06-30T14:33:32
Compile location: /w/workspace/vpp-merge-2106-ubuntu1804-x86_64
Compiler: Clang/LLVM 9.0.0 (tags/RELEASE_900/final)
Current PID: 29837
Command line arguments:
/usr/bin/vpp