Re: UDP path MTU discovery

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mardi 30 mars 2010 à 07:20 +0200, Andi Kleen a écrit :
> On Mon, Mar 29, 2010 at 04:38:49PM -0700, Templin, Fred L wrote:
> > > 1) 4096 bytes UDP messages... well...
> > > 2) Using regular TCP for DNS servers... well...
> > > 
> > > I believe some guys were pushing TCPCT (Cookie Transactions) for this
> > > case ( http://tools.ietf.org/html/draft-simpson-tcpct-00.html )
> > > 
> > > (That is, using an enhanced TCP for long DNS queries... but not only for
> > > DNS...)
> > 
> > IPv4 gets by this by setting DF=0 in the IP header, and
> > lets the network fragment the packet if necessary. IPv6 can
> > similarly get by this by having the sending host fragment
> > the large UDP packet into IPv6 fragments no longer than
> > 1280 bytes each.
> 
> That's true -- in theory the UDP app unwilling/unable to do proper ptmudisc 
> could set the path mtu to 1280 + header and still keep path mtu discovery off 
> and then just fragment. 
> 
> Drawback would be of course suboptimal network use with too small MTUs
> in the common case.
> 
> Right now there is no right socket option to set the path mtu. We
> have a IP_MTU option, but it only works for getting the MTU.
> That's because the PMTU is in the routing cache entry and shared
> by multiple sockets. Presumably one could add a special case
> with an MTU in the socket overriding the one in the destination entry.

We have IP_MTU_DISCOVER option with four existing values



/* IP_MTU_DISCOVER values */
#define IP_PMTUDISC_DONT                0       /* Never send DF frames */
#define IP_PMTUDISC_WANT                1       /* Use per route hints  */
#define IP_PMTUDISC_DO                  2       /* Always DF            */
#define IP_PMTUDISC_PROBE               3       /* Ignore dst pmtu      */

We might add a fifth value (or open full range) and change 

static inline int ip_skb_dst_mtu(struct sk_buff *skb)
{
        struct inet_sock *inet = skb->sk ? inet_sk(skb->sk) : NULL;

        return (inet && inet->pmtudisc == IP_PMTUDISC_PROBE) ?
               skb_dst(skb)->dev->mtu : dst_mtu(skb_dst(skb));
}

->

static inline int ip_skb_dst_mtu(struct sk_buff *skb)
{
	if (skb->sk) {
		struct inet_sock *inet = inet_sk(skb->sk);

		if (inet->pmtudisc > IP_PMTUDISC_PROBE)
			return inet->pmtudisc;
		if (inet->pmtudisc == IP_PMTUDISC_PROBE)
			return skb_dst(skb)->dev->mtu;
	}
	return dst_mtu(skb_dst(skb));
}