Date:	Wed, 22 Oct 2014 17:36:36 +0200
From:	Kristian Evensen <kristian.evensen@...il.com>
To:	netdev@...r.kernel.org
Cc:	Kristian Evensen <kristian.evensen@...il.com>
Subject: [PATCH net-next] tcp: Add TCP_FREEZE socket option

From: Kristian Evensen <kristian.evensen@...il.com>

This patch introduces support for Freeze-TCP [1].

Devices that are mobile frequently experience temporary disconnects, for example
due to signal fading or a technology change. These changes can last for a
substantial amount of time (>10 seconds), potentially causing multiple RTOs to
expire and the sender to enter slow start. Even though a device has
reconnected, it can take a long time for the TCP connection to recover.

Operators of mobile broadband networks mitigate this issue by placing TCP
splitters at the edge of their networks. However, the splitters typically only
operate on some ports (mostly only port 80) and violate the end-to-end
principle. The operator's TCP splitter receives a notification when a temporary
disconnect occurs and starts sending Zero Window Announcements (ZWA) to the
remote part of the connection. When a devices regains connectivity, the window
is reopened.

Freeze-TCP is a client-side only approach for enabling application developers to
trigger sending ZWAs. It is implemented as a socket option and accepts three
different values. If the value is set to one, the connection is frozen. A ZWA is
sent and the window size set to 0 in any reply to additional packets arriving
from remote party. If the value is set to two, the connection is unfrozen and a
window update announcement is sent. If the value is set to three, two additional
window update announcements are sent. This is referred to as TR-ACK in the paper
and is used to increase probability that a window update announcement will be
received.

When to trigger Freeze-TCP depends on the application requirements and
underlaying network, is not the responsibility of the kernel. One approach is to
have the application, or a daemon, analyze the meta data exported from a mobile
broadband modem. A temporary disconnect can often be detected in advance by
looking at different statistics.

[1] - T. Goff, J. Moronski, D. S. Phatak, and V. Gupta, "Freeze-TCP: a True
End-to-end TCP Enhancement Mechanism for Mobile Environments," In Proceedings of
IEEE INFOCOM 2000. URL: http://www.csee.umbc.edu/~phatak/publications/ftcp.pdf

Signed-off-by: Kristian Evensen <kristian.evensen@...il.com>
---
 include/linux/tcp.h      |  3 ++-
 include/uapi/linux/tcp.h |  1 +
 net/ipv4/tcp.c           | 33 +++++++++++++++++++++++++++++++++
 net/ipv4/tcp_output.c    |  8 +++++++-
 4 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index c2dee7d..7ed26c1 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -187,7 +187,8 @@ struct tcp_sock {
 		syn_data:1,	/* SYN includes data */
 		syn_fastopen:1,	/* SYN includes Fast Open option */
 		syn_data_acked:1,/* data in SYN is acked by SYN-ACK */
-		is_cwnd_limited:1;/* forward progress limited by snd_cwnd? */
+		is_cwnd_limited:1,/* forward progress limited by snd_cwnd? */
+		frozen:1;	/* Artifically deflate announced window to 0 */
 	u32	tlp_high_seq;	/* snd_nxt at the time of TLP retransmit. */
 
 /* RTT measurement */
diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 3b97183..bc0684d 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -112,6 +112,7 @@ enum {
 #define TCP_FASTOPEN		23	/* Enable FastOpen on listeners */
 #define TCP_TIMESTAMP		24
 #define TCP_NOTSENT_LOWAT	25	/* limit number of unsent bytes in write queue */
+#define TCP_FREEZE		26	/* Freeze TCP connection by sending ZWA */
 
 struct tcp_repair_opt {
 	__u32	opt_code;
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 1bec4e7..5bf30d0 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2339,6 +2339,7 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	int val;
 	int err = 0;
+	u8 itr = 0;
 
 	/* These are data/string values, all the others are ints */
 	switch (optname) {
@@ -2600,6 +2601,35 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 		tp->notsent_lowat = val;
 		sk->sk_write_space(sk);
 		break;
+	case TCP_FREEZE:
+		if (val < 1 || val > 3 ||
+		    !((1 << sk->sk_state) & TCPF_ESTABLISHED)) {
+			err = -EINVAL;
+			break;
+		}
+
+		if (val == 1) {
+			tp->frozen = 1;
+			tcp_send_ack(sk);
+			break;
+		} else if (!tp->frozen) {
+			err = -EINVAL;
+			break;
+		}
+
+		tp->frozen = 0;
+		tcp_send_ack(sk);
+
+		if (val == 2)
+			break;
+
+		/* If val is three, send two additional reconnection ACKs to
+		 * increase chance of a non-zero windows announcement arriving.
+		 */
+		for (itr = 0; itr < 2; itr++)
+			tcp_send_ack(sk);
+
+		break;
 	default:
 		err = -ENOPROTOOPT;
 		break;
@@ -2832,6 +2862,9 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 	case TCP_NOTSENT_LOWAT:
 		val = tp->notsent_lowat;
 		break;
+	case TCP_FREEZE:
+		val = tp->frozen;
+		break;
 	default:
 		return -ENOPROTOOPT;
 	}
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 3af2129..9c1429b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -958,7 +958,13 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 		 */
 		th->window	= htons(min(tp->rcv_wnd, 65535U));
 	} else {
-		th->window	= htons(tcp_select_window(sk));
+		/* Because window is only artifically deflated to zero, we
+		 * postpone updating tcp state until connection is unfrozen
+		 */
+		if (unlikely(tp->frozen))
+			th->window = 0;
+		else
+			th->window = htons(tcp_select_window(sk));
 	}
 	th->check		= 0;
 	th->urg_ptr		= 0;
-- 
1.8.3.2

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