tcp: Allow full IP tos/IPv6 tclass to be reflected in L3 header (861602b5) · Commits · 戴 / test

An issue was recently found where DCTCP SYN/ACK packets did not have the ECT bit set in the L3 header. A bit of code review found that the recent change referenced below had gone though and added a mask that prevented the ECN bits from being populated in the L3 header. This patch addresses that by rolling back the mask so that it is only applied to the flags coming from the incoming TCP request instead of applying it to the socket tos/tclass field. Doing this the ECT bits were restored in the SYN/ACK packets in my testing. One thing that is not addressed by this patch set is the fact that tcp_reflect_tos appears to be incompatible with ECN based congestion avoidance algorithms. At a minimum the feature should likely be documented which it currently isn't. Fixes: ("tcp: reflect tos value received in SYN to the socket") Signed-off-by:

Alexander Duyck <alexanderduyck@fb.com> Acked-by:

Wei Wang <weiwan@google.com> Signed-off-by:

Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_ipv4.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -981,7 +981,8 @@ static int tcp_v4_send_synack(const struct sock sk, struct dst_entry dst,
		skb = tcp_make_synack(sk, dst, req, foc, synack_type, syn_skb);

		tos = sock_net(sk)->ipv4.sysctl_tcp_reflect_tos ?
		tcp_rsk(req)->syn_tos : inet_sk(sk)->tos;
		tcp_rsk(req)->syn_tos & ~INET_ECN_MASK :
		inet_sk(sk)->tos;

		if (skb) {
		__tcp_v4_send_check(skb, ireq->ir_loc_addr, ireq->ir_rmt_addr);
		@@ -990,7 +991,7 @@ static int tcp_v4_send_synack(const struct sock sk, struct dst_entry dst,
		err = ip_build_and_send_pkt(skb, sk, ireq->ir_loc_addr,
		ireq->ir_rmt_addr,
		rcu_dereference(ireq->ireq_opt),
		tos & ~INET_ECN_MASK);
		tos);
		rcu_read_unlock();
		err = net_xmit_eval(err);
		}

net/ipv6/tcp_ipv6.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -530,12 +530,12 @@ static int tcp_v6_send_synack(const struct sock sk, struct dst_entry dst,
		rcu_read_lock();
		opt = ireq->ipv6_opt;
		tclass = sock_net(sk)->ipv4.sysctl_tcp_reflect_tos ?
		tcp_rsk(req)->syn_tos : np->tclass;
		tcp_rsk(req)->syn_tos & ~INET_ECN_MASK :
		np->tclass;
		if (!opt)
		opt = rcu_dereference(np->opt);
		err = ip6_xmit(sk, skb, fl6, sk->sk_mark, opt,
		tclass & ~INET_ECN_MASK,
		sk->sk_priority);
		tclass, sk->sk_priority);
		rcu_read_unlock();
		err = net_xmit_eval(err);
		}

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