netdev - [RFC] gre: conform to RFC6040 ECN progogation

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Date:	Mon, 24 Sep 2012 14:44:57 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Chris Wright <chrisw@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: [RFC] gre: conform to RFC6040 ECN progogation

Linux GRE was likely written before this RFC and therefore does not
conform to one of the rules in Section 4.2.  Default Tunnel Egress Behaviour.

The new code addresses:
 o If the inner ECN field is Not-ECT, the decapsulator MUST NOT
      propagate any other ECN codepoint onwards.  This is because the
      inner Not-ECT marking is set by transports that rely on dropped
      packets as an indication of congestion and would not understand or
      respond to any other ECN codepoint [RFC4774].  Specifically:

      *  If the inner ECN field is Not-ECT and the outer ECN field is
         CE, the decapsulator MUST drop the packet.

      *  If the inner ECN field is Not-ECT and the outer ECN field is
         Not-ECT, ECT(0), or ECT(1), the decapsulator MUST forward the
         outgoing packet with the ECN field cleared to Not-ECT.

This was caught by Chris Wright while reviewing VXLAN.
This code has not been tested with real ECN through tunnel.

Signed-off-by: Stephen Hemminger <shemminger@...tta.com>

--- a/net/ipv4/ip_gre.c	2012-09-21 08:45:55.948772761 -0700
+++ b/net/ipv4/ip_gre.c	2012-09-24 14:35:54.666185603 -0700
@@ -567,15 +567,16 @@ out:
 	rcu_read_unlock();
 }
 
-static inline void ipgre_ecn_decapsulate(const struct iphdr *iph, struct sk_buff *skb)
+static int ipgre_ecn_decapsulate(const struct iphdr *iph, struct sk_buff *skb)
 {
 	if (INET_ECN_is_ce(iph->tos)) {
 		if (skb->protocol == htons(ETH_P_IP)) {
-			IP_ECN_set_ce(ip_hdr(skb));
+			return IP_ECN_set_ce(ip_hdr(skb));
 		} else if (skb->protocol == htons(ETH_P_IPV6)) {
-			IP6_ECN_set_ce(ipv6_hdr(skb));
+			return IP6_ECN_set_ce(ipv6_hdr(skb));
 		}
 	}
+	return 1;
 }
 
 static inline u8
@@ -703,17 +704,18 @@ static int ipgre_rcv(struct sk_buff *skb
 			skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);
 		}
 
+		__skb_tunnel_rx(skb, tunnel->dev);
+
+		skb_reset_network_header(skb);
+		if (!ipgre_ecn_decapsulate(iph, skb))
+			goto drop;
+
 		tstats = this_cpu_ptr(tunnel->dev->tstats);
 		u64_stats_update_begin(&tstats->syncp);
 		tstats->rx_packets++;
 		tstats->rx_bytes += skb->len;
 		u64_stats_update_end(&tstats->syncp);
 
-		__skb_tunnel_rx(skb, tunnel->dev);
-
-		skb_reset_network_header(skb);
-		ipgre_ecn_decapsulate(iph, skb);
-
 		netif_rx(skb);
 
 		rcu_read_unlock();
--
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