tcp: use dctcp if enabled on the route to the initiator

Currently, the following case doesn't use DCTCP, even if it should:
A responder has f.e. Cubic as system wide default, but for a specific
route to the initiating host, DCTCP is being set in RTAX_CC_ALGO. The
initiating host then uses DCTCP as congestion control, but since the
initiator sets ECT(0), tcp_ecn_create_request() doesn't set ecn_ok,
and we have to fall back to Reno after 3WHS completes.

We were thinking on how to solve this in a minimal, non-intrusive
way without bloating tcp_ecn_create_request() needlessly: lets cache
the CA ecn option flag in RTAX_FEATURES. In other words, when ECT(0)
is set on the SYN packet, set ecn_ok=1 iff route RTAX_FEATURES
contains the unexposed (internal-only) DST_FEATURE_ECN_CA. This allows
to only do a single metric feature lookup inside tcp_ecn_create_request().

Joint work with Florian Westphal.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 2 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 8771530df45e..f45cac6f8356 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1698,6 +1698,7 @@ out:
 static int ip6_convert_metrics(struct mx6_config *mxc,
                                const struct fib6_config *cfg)
 {
+        bool ecn_ca = false;
         struct nlattr *nla;
         int remaining;
         u32 *mp;
@@ -1722,7 +1723,7 @@ static int ip6_convert_metrics(struct mx6_config *mxc,
                         char tmp[TCP_CA_NAME_MAX];
 
                         nla_strlcpy(tmp, nla, sizeof(tmp));
-                        val = tcp_ca_get_key_by_name(tmp);
+                        val = tcp_ca_get_key_by_name(tmp, &ecn_ca);
                         if (val == TCP_CA_UNSPEC)
                                 goto err;
                 } else {
@@ -1735,8 +1736,12 @@ static int ip6_convert_metrics(struct mx6_config *mxc,
                 __set_bit(type - 1, mxc->mx_valid);
         }
 
-        mxc->mx = mp;
+        if (ecn_ca) {
+                __set_bit(RTAX_FEATURES - 1, mxc->mx_valid);
+                mp[RTAX_FEATURES - 1] |= DST_FEATURE_ECN_CA;
+        }
 
+        mxc->mx = mp;
         return 0;
  err:
         kfree(mp);