[IPSec]Psudo code for handling PMTU message for IPSec/IPv4

["Zhao, Forrest" <forrest.zhao@intel.com>]

Hi, Alexey

I'd like to give the psudo code to describe my proposed way to handle
the PMTU message for IPSec/IPv4. Welcome your comment!

1 add a new data field "u32 pmtu" to struct xfrm_state
  When received "ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED" message, store mtu
to specific xfrm_state.pmtu
2 add a new data field " struct dst_entry *dst_xfrm" to struct sk_buff
to track the dst associated with transformation
  When xfrm_lookup() return successfully, store dst to sk_buff.dst_xfrm 
3 Propagate "ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED" message to the
originating host, the following modified code is to finish this task
static inline int dst_output(struct sk_buff *skb)
{
        int err;
+       u32 mtu;
+       struct dst_entry *dst;


	for (;;) {
+           if (skb->dst->xfrm)
+                   mtu = skb->dst->xfrm->pmtu;

		err = skb->dst->output(skb);

		if (likely(err == 0))
			return err;
		if (unlikely(err != NET_XMIT_BYPASS))
			return err;

+           if (mtu != 0 && mtu < skb->len){
+                    if( (dst = __sk_dst_check(sdb->sk, 0)) == NULL){
+                        if( !skb->dst_xfrm )
+                             return err = ???;
+                        dst = skb->dst_xfrm;
                     }
+                    mtu = xfrm_get_mss(dst,mtu);                     
+                    send "ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED" mtu
message to originating host;

            }   
	}
}
4 remove the invocation to xfrm_get_mss() in function tcp_sync_mss();

Am I right? Welcome your advice!

Thanks,
Forrest
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