TCP socket timestamping option for Linux

Question

Print the form of this online kernel document

• SO_TIMESTAMPING Creates timestamps on reception, transmission, or both. supports multiple timestamp sources, including hardware. Supports generation of timestamps for streams .

Linux supports TCP time binding, and I tried to write demo code to get a timestamp for a TCP packet.

Server code as below:

//Bind
if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
    perror("bind failed. Error");
    return 1;
}
puts("bind done");

//Listen
listen(socket_desc , 3);

//Accept and incoming connection
puts("Waiting for incoming connections...");
int c = sizeof(struct sockaddr_in);

client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
if (client_sock < 0)
{
    perror("accept failed");
    return 1;
}

// Note: I am trying to get software timestamp only here..
int oval = SOF_TIMESTAMPING_RX_SOFTWARE | SOF_TIMESTAMPING_SOFTWARE;
int olen = sizeof( oval );
if ( setsockopt( client_sock, SOL_SOCKET, SO_TIMESTAMPING, &oval, olen ) < 0 )
    { perror( "setsockopt TIMESTAMP"); exit(1); }

puts("Connection accepted");

char    buf[] = "----------------------------------------";
int len = strlen( buf );

struct iovec    myiov[1] = { {buf, len } };

unsigned char   cbuf[ 40 ] = { 0 };
int     clen = sizeof( cbuf ); 

struct msghdr   mymsghdr = { 0 };
mymsghdr.msg_name   = NULL;
mymsghdr.msg_namelen    = 0;
mymsghdr.msg_iov    = myiov;
mymsghdr.msg_iovlen = 1;
mymsghdr.msg_control    = cbuf;
mymsghdr.msg_controllen = clen;
mymsghdr.msg_flags  = 0;

int read_size = recvmsg( client_sock, &mymsghdr, 0);

if(read_size == 0)
{
  puts("Client disconnected");
  fflush(stdout);
}
else if(read_size == -1)
{
  perror("recv failed");
}
else
{
  struct msghdr *msgp = &mymsghdr;
  printf("msg received: %s \n",(char*)msgp->msg_iov[0].iov_base);// This line is successfully hit.
  // Additional info: print msgp->msg_controllen inside gdb is 0.
  struct cmsghdr    *cmsg;
  for ( cmsg = CMSG_FIRSTHDR( msgp );
      cmsg != NULL;
      cmsg = CMSG_NXTHDR( msgp, cmsg ) )
  {
    printf("Time GOT!\n"); // <-- This line is not hit.
    if (( cmsg->cmsg_level == SOL_SOCKET )
        &&( cmsg->cmsg_type == SO_TIMESTAMPING ))
      printf("TIME GOT2\n");// <-- of course , this line is not hit
  } 

}

Any ideas why there is no timestamp? thank

Solution I can get the software’s timestamp along with the hardware’s timestamp using onload with the solar panel's network card. Still don't know how to get only the timestamp of the software.

Answer 1

, , :

I've discovered why it doesn't work. SIOCGSTAMP only works for UDP
packets or RAW sockets, but does not work for TCP. – Gio Mar 17 '16 at 9:331    

it doesn't make sense to ask for timestamps for TCP, because there's
no direct correlation between arriving packets and data becoming 
available. If you really want timestamps for TCP you'll have to use 
RAW sockets and implement your own TCP stack (or use a userspace TCP
library). – ecatmur Jul 4 '16 at 10:39