File content as of revision 9:91dae5300a4d:

#include  "mbed.h"
#include   "log.h"
#include   "net.h"
#include "clock.h"
#include  "sync.h"
#include   "udp.h"

#define HEADER_SIZE 48

#define ERA_BASE     0
#define ERA_PIVOT 2016

#define CLIENT 3
#define SERVER 4

__packed struct header {
    unsigned Mode : 3;
    unsigned VN   : 3;
    unsigned LI   : 2;
    uint8_t  Stratum; 
     int8_t  Poll;
     int8_t  Precision;
    uint32_t RootDelay;
    uint32_t Dispersion;
    char     RefIdentifier[4];
    
    uint64_t RefTimeStamp;
    uint64_t OriTimeStamp;
    uint64_t RecTimeStamp;
    uint64_t TraTimeStamp;
};
uint64_t makeNtpTimeStamp(int64_t ns)
{    
    uint64_t timestamp = ns << 2;
    timestamp /= 1000; timestamp <<= 10;
    timestamp /= 1000; timestamp <<= 10;
    timestamp /= 1000; timestamp <<= 10;
    
    timestamp += 2208988800ULL << 32;
    
    return NetToHost64(timestamp);
}
int NtpHandleRequest(int* pSize, void * pPacket)
{
    if (*pSize < HEADER_SIZE) return DO_NOTHING;
    
    struct header* pHeader = (struct header*)pPacket;
    
    switch (pHeader->Mode)
    {
        case CLIENT:            
            pHeader->Mode       = SERVER;
            pHeader->LI         = 0;
            if (ClockStartNs) pHeader->Stratum = 1;
            else              pHeader->Stratum = 0;
            pHeader->Poll       = 0;
            pHeader->Precision  = 0;
            pHeader->RootDelay  = 0;
            pHeader->Dispersion = 0;
            char* pRef;
            if (ClockStartNs)
            {
                if (SyncedRate && SyncedTime) pRef = "GPS";
                else                          pRef = "LOCL";
            }
            else
            {
                pRef = "INIT";
            }
            pHeader->RefIdentifier[0] = pRef[0]; //For stratum 1 (reference clock), this is a four-octet, left-justified, zero-padded ASCII string.
            pHeader->RefIdentifier[1] = pRef[1];
            pHeader->RefIdentifier[2] = pRef[2];
            pHeader->RefIdentifier[3] = pRef[3];
            
            pHeader->RefTimeStamp = makeNtpTimeStamp(ClockStartNs);
            pHeader->OriTimeStamp = pHeader->TraTimeStamp;
            pHeader->RecTimeStamp = makeNtpTimeStamp(ClockGetNs());
            pHeader->TraTimeStamp = makeNtpTimeStamp(ClockGetNs());
            *pSize = HEADER_SIZE;
            return UNICAST;
        default:
            LogTimeF("\r\nNTP packet unknown\r\n");
            LogTimeF("Mode           %d\r\n",  pHeader->Mode);
            LogTimeF("Version        %d\r\n",  pHeader->VN);
            LogTimeF("Stratum        %d\r\n",  pHeader->Stratum);
            LogTimeF("Reference      %4s\r\n", pHeader->RefIdentifier);
    }
    return DO_NOTHING;
}