Subject: ping6 sets IPV6_PMTUDISC_DO option on multicast packets

From: Tushar Gohad <[email-masked]>
Date: Fri, 16 Nov 2007 14:29:09 -0700

Yoshifuji-san,

I noticed that ping6 forces the IPV6_PMTUDISC_DO option on packets sent 
to multicast destinations. Is this a requirement?

Due to the following check recently introduced in the ip6_fragment() 
code, multicast packets that are over PMTU in size are never sent out, 
since ping6 code always sets the PMTUDISC_DO option and ip6_fragment() 
always fails.

=== net/ipv6/ip6_output.c ===

  573 static int ip6_fragment(struct sk_buff *skb, int (*output)
                              (struct sk_buff *))
  574 {
  ......
  592         /* We must not fragment if the socket is set to force MTU
                 discovery
  ......

  596         if (!np || np->pmtudisc >= IPV6_PMTUDISC_DO) {
  597                 skb->dev = skb->dst->dev;
  598                 icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu,
                                    skb->dev);
  599                 IP6_INC_STATS(ip6_dst_idev(skb->dst),
                                    IPSTATS_MIB_FRAGFAILS);
  600                 kfree_skb(skb);
  601                 return -EMSGSIZE;
  602         }
  ......
--

This check in the kernel code makes perfect sense, since the option is 
implies "do not fragment."

Should the ping6 code be fixed in that case?

Thanks,
- Tushar