[PATCH] gre: copy ToS/DiffServ bits to outer IP header

[Andreas Jaggi <aj@open.ch>]

When tunneling IP traffic with GRE this patch makes it possible to export the
ToS/DiffServ information to the outer IP header. This is particularly useful in
a scenario with ESP/AH where the inner IP header is encrypted but the packet
priority/DiffServ information should still be respected by the transporting
routers (for example in an MPLS backbone network).

The feature is disabled by default and can be enabled on per-interface basis.
The flag is stored in an unused bit of ip_tunnel_parm.o_flags, and can be
modified through the rntl_link interface.

Also does this bring Linux back in the game, as JunOS/IOS provide this for
quite some time:
http://www.cisco.com/en/US/docs/ios/11_3/feature/guide/greqos.html
http://www.juniper.net/techpubs/software/junos/junos94/swconfig-services/configuring-a-gre-tunnel-to-copy-tos-bits-to-the-outer-ip-header.html

Signed-off-by: Andreas Jaggi <aj@open.ch>

diff -urN vanilla-linux-2.6.29.4/include/linux/if_tunnel.h dev-gre/include/linux/if_tunnel.h
--- vanilla-linux-2.6.29.4/include/linux/if_tunnel.h	2009-05-19 01:52:34.000000000 +0200
+++ dev-gre/include/linux/if_tunnel.h	2009-07-13 15:15:26.000000000 +0200
@@ -24,6 +24,7 @@
 #define GRE_REC		__constant_htons(0x0700)
 #define GRE_FLAGS	__constant_htons(0x00F8)
 #define GRE_VERSION	__constant_htons(0x0007)
+#define GRE_COPY_TOS	__constant_htons(0x0008)
 
 struct ip_tunnel_parm
 {
diff -urN vanilla-linux-2.6.29.4/net/ipv4/ip_gre.c dev-gre/net/ipv4/ip_gre.c
--- vanilla-linux-2.6.29.4/net/ipv4/ip_gre.c	2009-05-19 01:52:34.000000000 +0200
+++ dev-gre/net/ipv4/ip_gre.c	2009-07-01 15:30:44.000000000 +0200
@@ -677,7 +677,7 @@
 	}
 
 	tos = tiph->tos;
-	if (tos&1) {
+	if (tunnel->parms.o_flags&GRE_COPY_TOS || tos&1) {
 		if (skb->protocol == htons(ETH_P_IP))
 			tos = old_iph->tos;
 		tos &= ~1;
@@ -804,7 +804,7 @@
 			iph->ttl = dst_metric(&rt->u.dst, RTAX_HOPLIMIT);
 	}
 
-	((__be16 *)(iph + 1))[0] = tunnel->parms.o_flags;
+	((__be16 *)(iph + 1))[0] = tunnel->parms.o_flags&~GRE_FLAGS;
 	((__be16 *)(iph + 1))[1] = (dev->type == ARPHRD_ETHER) ?
 				   htons(ETH_P_TEB) : skb->protocol;
 
@@ -1080,7 +1080,7 @@
 	__be16 *p = (__be16*)(iph+1);
 
 	memcpy(iph, &t->parms.iph, sizeof(struct iphdr));
-	p[0]		= t->parms.o_flags;
+	p[0]		= t->parms.o_flags&~GRE_FLAGS;
 	p[1]		= htons(type);
 
 	/*
@@ -1503,6 +1503,7 @@
 	t->parms.iph.ttl = p.iph.ttl;
 	t->parms.iph.tos = p.iph.tos;
 	t->parms.iph.frag_off = p.iph.frag_off;
+	t->parms.o_flags |= p.o_flags&GRE_COPY_TOS;
 
 	if (t->parms.link != p.link) {
 		t->parms.link = p.link;