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

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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>
This commit is contained in:
Daniel Borkmann
2015-08-31 15:58:47 +02:00
committed by David S. Miller
parent b8d3e4163a
commit c3a8d94746
7 changed files with 36 additions and 9 deletions
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9
net/ipv4/tcp_cong.c
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@@ -114,16 +114,19 @@ void tcp_unregister_congestion_control(struct tcp_congestion_ops *ca)
}
EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);
u32 tcp_ca_get_key_by_name(const char *name)
u32 tcp_ca_get_key_by_name(const char *name, bool *ecn_ca)
{
const struct tcp_congestion_ops *ca;
u32 key;
u32 key = TCP_CA_UNSPEC;
might_sleep();
rcu_read_lock();
ca = __tcp_ca_find_autoload(name);
key = ca ? ca->key : TCP_CA_UNSPEC;
if (ca) {
key = ca->key;
*ecn_ca = ca->flags & TCP_CONG_NEEDS_ECN;
}
rcu_read_unlock();
return key;