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Message-ID: <1413993272.9031.3.camel@edumazet-glaptop2.roam.corp.google.com>
Date: Wed, 22 Oct 2014 08:54:32 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Kristian Evensen <kristian.evensen@...il.com>
Cc: netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: Add TCP_FREEZE socket option
On Wed, 2014-10-22 at 17:36 +0200, Kristian Evensen wrote:
> 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;
> }
This asymmetry looks strange
Following sequence should be allowed :
getsockopt(... TCP_FREEZE, &val, ...)
setsockopt(... TCP_FREEZE, &val, ...)
So setsockopt() should accept val = 0
--
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