File content as of revision 0:faa09bd4e6bf:

#include "mbed.h"
#include "net.h"
#include "gps.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(time_t seconds, int ms)
{
    uint64_t timestampMs = ms;
    timestampMs <<= 32;
    timestampMs /= 1000;
    
    uint64_t timestampS = seconds;
    timestampS += 2208988800ULL; //Add 1970 - 1900 to align the epochs
    timestampS <<= 32;           //put the seconds into the upper 32 bits
    
    return NetToHost64(timestampS + timestampMs);
}
int NtpHandleRequest(int size, void * pPacket)
{
    if (size < HEADER_SIZE) return 0;
    
    struct header* pHeader = (struct header*)pPacket;
    
    switch (pHeader->Mode)
    {
        case CLIENT:            
            pHeader->Mode       = SERVER;
            pHeader->LI         = 0;
            if (GpsStartTime) pHeader->Stratum = 1;
            else              pHeader->Stratum = 0;
            pHeader->Poll       = 0;
            pHeader->Precision  = 0;
            pHeader->RootDelay  = 0;
            pHeader->Dispersion = 0;
            if (GpsStartTime)
            {
                pHeader->RefIdentifier[0] = 'G'; //For stratum 1 (reference clock), this is a four-octet, left-justified, zero-padded ASCII string.
                pHeader->RefIdentifier[1] = 'P';
                pHeader->RefIdentifier[2] = 'S';
                pHeader->RefIdentifier[3] =  0 ;
            }
            else
            {
                pHeader->RefIdentifier[0] = 'I'; //For stratum 1 (reference clock), this is a four-octet, left-justified, zero-padded ASCII string.
                pHeader->RefIdentifier[1] = 'N';
                pHeader->RefIdentifier[2] = 'I';
                pHeader->RefIdentifier[3] = 'T' ;
            }
            
            pHeader->RefTimeStamp = makeNtpTimeStamp(GpsStartTime, GpsStartMs);
            pHeader->OriTimeStamp = pHeader->TraTimeStamp;
            pHeader->RecTimeStamp = makeNtpTimeStamp(GpsTime, GpsMs);
            pHeader->TraTimeStamp = makeNtpTimeStamp(GpsTime, GpsMs);
            return HEADER_SIZE;
        default:
            printf("\r\nNTP packet unknown\r\n");
            printf("Mode           %d\r\n", pHeader->Mode);
            printf("Version        %d\r\n", pHeader->VN);
            printf("Stratum        %d\r\n", pHeader->Stratum);
            printf("Reference      %4s\r\n", pHeader->RefIdentifier);
    }
    return 0;
}