在每个片段上发送带有UDP头的片段数据报

Question

I am working with an embedded box that must be able to communicate with traditional computers using UDP. When the box sends large UDP messages (that need to be fragmented), a UDP header is included for each fragment. Thus if I want to a send a large datagram, it will be fragmented like this:

[eth hdr][ip hdr][udp hdr][    data 1    ] /* first fragment */
[eth hdr][ip hdr][udp hdr][    data 2    ] /* second fragment */
[eth hdr][ip hdr][udp hdr][  data 3  ]     /* last fragment */

I understand that this is not customary, as usually the udp header would only be included in only the first ip packet of the fragmented message. However, this works perfectly for communicating with the other machines I need to talk to (ex. using recvfrom), so I have no reason to dig in and try to change it.

My issue, however, is in reading messages. The box seems to expect fragmented udp datagrams to be sent to it in the same manner. By this I mean that it expects every ipv4 fragment to have a udp header. Before trying to change this (it's a rather specialized and complicated platform) I would like to know if there is any way to configure sendto() or any other such function for sending udp messages in this format. I see when monitoring the traffic that those udp headers aren't present.

Thank you very much for the help.

Answer 1

No. Socket's don't work this way. Just write your own sendto wrapper to manually fragment the frames across multiple UDP packets on whatever buffer size boundary you choose. This will achieve the desired effect that you want.

Sample code as follows:

ssize_t fragmented_sendto(int sockfd, const void *buf, size_t len, int flags,
           const struct sockaddr *dest_addr, socklen_t addrlen, size_t MAX_PACKET_SIZE)
{
    unsigned char* ptr = (unsigned char*) buf;

    size_t total = 0;

    while (total <= len)
    {
       size_t newsize = len - total;
       if (newsize > MAX_PACKET_SIZE)
       {
           newsize = MAX_PACKET_SIZE;
       }
       ssize_t result = sendto(sockfd, ptr, newsize, flags, dest_addr, addrlen);
       if (result < 0)
       {
          // handle error
          return -1;
       }
       else
       {
          total += result;
          ptr += result;
       }
    }
    return (ssize_t)total;
}