Hi All,

 I need to configure the TCP MSS on a netbsd router. My requirement is that

 1. Either i want to configure the TCP MSS value from some ioctl and then
 use that integer value later
 2. or i want to configure that MSS is calculated form the interface mtu
and
 then use that further on.

Coudl anyone pls put some light over this ??

First i came to know that by setting tcp.mss_if_mtu to 1 will fufill my 2nd
requirement. But with that also i have some doubt

Acc to that method, there is no direct computation of the MSS on the basis
of tcp_if_mut. The MSS is computed from the interface mtu only and in the
last max value of the def MSS or mss from if_mtu is returned back.

By using the tcp_if_mtu, the check is made from for max value in the if_mtu
and the in_ifmtu value. Could you pls elaborate bit more.


I am attaching the code for that method below for your refrence.


u_long
tcp_mss_to_advertise(ifp, af)
      const struct ifnet *ifp;
      int af;
{
      extern u_long in_maxmtu;
      u_long mss = 0;
      u_long hdrsiz;

      /*
       * In order to avoid defeating path MTU discovery on the peer,
       * we advertise the max MTU of all attached networks as our MSS,
       * per RFC 1191, section 3.1.
       *
       * We provide the option to advertise just the MTU of
       * the interface on which we hope this connection will
       * be receiving.  If we are responding to a SYN, we
       * will have a pretty good idea about this, but when
       * initiating a connection there is a bit more doubt.
       *
       * We also need to ensure that loopback has a large enough
       * MSS, as the loopback MTU is never included in in_maxmtu.
       */

      if (ifp != NULL)
            switch (af) {
            case AF_INET:
                  mss = ifp->if_mtu;
                  break;
#ifdef INET6
            case AF_INET6:
                  mss = IN6_LINKMTU(ifp);
                  break;
#endif
            }

      if (tcp_mss_ifmtu == 0)
            switch (af) {
            case AF_INET:
                  mss = max(in_maxmtu, mss);
                  break;
#ifdef INET6
            case AF_INET6:
                  mss = max(in6_maxmtu, mss);
                  break;
#endif
            }

      switch (af) {
      case AF_INET:
            hdrsiz = sizeof(struct ip);
            break;
#ifdef INET6
      case AF_INET6:
            hdrsiz = sizeof(struct ip6_hdr);
            break;
#endif
      default:
            hdrsiz = 0;
            break;
      }
      hdrsiz += sizeof(struct tcphdr);
      if (mss > hdrsiz)
            mss -= hdrsiz;

      mss = max(tcp_mssdflt, mss);
      return (mss);
}


regards
- amit