TCPOLEN_SIGNATURE
#define TCPOLEN_SIGLEN (TCPOLEN_SIGNATURE+2) /* padding */
if (optlen != TCPOLEN_SIGNATURE)
if (!OPT_FITS(TCPOLEN_SIGNATURE))
*optp++ = TCPOLEN_SIGNATURE;
optlen += TCPOLEN_SIGNATURE;
*optp++ = TCPOLEN_SIGNATURE;
optlen += TCPOLEN_SIGNATURE;