https://lwn.net/Articles/617824/
tcp: Add TCP_FREEZE socket option
| From: | Kristian Evensen <kristian.evensen@gmail.com> | |
| To: | netdev@vger.kernel.org | |
| Subject: | [PATCH net-next] tcp: Add TCP_FREEZE socket option | |
| Date: | Wed, 22 Oct 2014 17:36:36 +0200 | |
| Message-ID: | <1413992196-4891-1-git-send-email-kristian.evensen@gmail.com> | |
| Cc: | Kristian Evensen <kristian.evensen@gmail.com> | |
| Archive-link: | Article, Thread |
From: Kristian Evensen <kristian.evensen@gmail.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@gmail.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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