From ca6a6f93867a9763bdf8685c788e2e558d10975f Mon Sep 17 00:00:00 2001 From: Jakub Sitnicki Date: Mon, 9 Dec 2024 20:38:04 +0100 Subject: [PATCH] tcp: Add sysctl to configure TIME-WAIT reuse delay Today we have a hardcoded delay of 1 sec before a TIME-WAIT socket can be reused by reopening a connection. This is a safe choice based on an assumption that the other TCP timestamp clock frequency, which is unknown to us, may be as low as 1 Hz (RFC 7323, section 5.4). However, this means that in the presence of short lived connections with an RTT of couple of milliseconds, the time during which a 4-tuple is blocked from reuse can be orders of magnitude longer that the connection lifetime. Combined with a reduced pool of ephemeral ports, when using IP_LOCAL_PORT_RANGE to share an egress IP address between hosts [1], the long TIME-WAIT reuse delay can lead to port exhaustion, where all available 4-tuples are tied up in TIME-WAIT state. Turn the reuse delay into a per-netns setting so that sysadmins can make more aggressive assumptions about remote TCP timestamp clock frequency and shorten the delay in order to allow connections to reincarnate faster. Note that applications can completely bypass the TIME-WAIT delay protection already today by locking the local port with bind() before connecting. Such immediate connection reuse may result in PAWS failing to detect old duplicate segments, leaving us with just the sequence number check as a safety net. This new configurable offers a trade off where the sysadmin can balance between the risk of PAWS detection failing to act versus exhausting ports by having sockets tied up in TIME-WAIT state for too long. [1] https://lpc.events/event/16/contributions/1349/ Signed-off-by: Jakub Sitnicki Reviewed-by: Eric Dumazet Reviewed-by: Jason Xing Link: https://patch.msgid.link/20241209-jakub-krn-909-poc-msec-tw-tstamp-v2-2-66aca0eed03e@cloudflare.com Signed-off-by: Jakub Kicinski --- Documentation/networking/ip-sysctl.rst | 14 ++++++++++++++ .../net_cachelines/netns_ipv4_sysctl.rst | 1 + include/net/netns/ipv4.h | 1 + net/ipv4/sysctl_net_ipv4.c | 10 ++++++++++ net/ipv4/tcp_ipv4.c | 4 +++- 5 files changed, 29 insertions(+), 1 deletion(-) diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index eacf8983e230..2f2b00295836 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -1000,6 +1000,20 @@ tcp_tw_reuse - INTEGER Default: 2 +tcp_tw_reuse_delay - UNSIGNED INTEGER + The delay in milliseconds before a TIME-WAIT socket can be reused by a + new connection, if TIME-WAIT socket reuse is enabled. The actual reuse + threshold is within [N, N+1] range, where N is the requested delay in + milliseconds, to ensure the delay interval is never shorter than the + configured value. + + This setting contains an assumption about the other TCP timestamp clock + tick interval. It should not be set to a value lower than the peer's + clock tick for PAWS (Protection Against Wrapped Sequence numbers) + mechanism work correctly for the reused connection. + + Default: 1000 (milliseconds) + tcp_window_scaling - BOOLEAN Enable window scaling as defined in RFC1323. diff --git a/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst b/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst index 629da6dc6d74..de0263302f16 100644 --- a/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst +++ b/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst @@ -79,6 +79,7 @@ u8 sysctl_tcp_retries1 u8 sysctl_tcp_retries2 u8 sysctl_tcp_orphan_retries u8 sysctl_tcp_tw_reuse timewait_sock_ops +unsigned_int sysctl_tcp_tw_reuse_delay timewait_sock_ops int sysctl_tcp_fin_timeout TCP_LAST_ACK/tcp_rcv_state_process unsigned_int sysctl_tcp_notsent_lowat read_mostly tcp_notsent_lowat/tcp_stream_memory_free u8 sysctl_tcp_sack tcp_syn_options diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h index 3c014170e001..46452da35206 100644 --- a/include/net/netns/ipv4.h +++ b/include/net/netns/ipv4.h @@ -175,6 +175,7 @@ struct netns_ipv4 { u8 sysctl_tcp_retries2; u8 sysctl_tcp_orphan_retries; u8 sysctl_tcp_tw_reuse; + unsigned int sysctl_tcp_tw_reuse_delay; int sysctl_tcp_fin_timeout; u8 sysctl_tcp_sack; u8 sysctl_tcp_window_scaling; diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index a79b2a52ce01..42cb5dc9cb24 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -45,6 +45,7 @@ static unsigned int tcp_child_ehash_entries_max = 16 * 1024 * 1024; static unsigned int udp_child_hash_entries_max = UDP_HTABLE_SIZE_MAX; static int tcp_plb_max_rounds = 31; static int tcp_plb_max_cong_thresh = 256; +static unsigned int tcp_tw_reuse_delay_max = TCP_PAWS_MSL * MSEC_PER_SEC; /* obsolete */ static int sysctl_tcp_low_latency __read_mostly; @@ -1065,6 +1066,15 @@ static struct ctl_table ipv4_net_table[] = { .extra1 = SYSCTL_ZERO, .extra2 = SYSCTL_TWO, }, + { + .procname = "tcp_tw_reuse_delay", + .data = &init_net.ipv4.sysctl_tcp_tw_reuse_delay, + .maxlen = sizeof(unsigned int), + .mode = 0644, + .proc_handler = proc_douintvec_minmax, + .extra1 = SYSCTL_ONE, + .extra2 = &tcp_tw_reuse_delay_max, + }, { .procname = "tcp_max_syn_backlog", .data = &init_net.ipv4.sysctl_max_syn_backlog, diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 3b6ba1d16921..e45222d5fc2e 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -163,7 +163,8 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp) and use initial timestamp retrieved from peer table. */ ts_recent_stamp = READ_ONCE(tcptw->tw_ts_recent_stamp); - reuse_thresh = READ_ONCE(tw->tw_entry_stamp) + MSEC_PER_SEC; + reuse_thresh = READ_ONCE(tw->tw_entry_stamp) + + READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tw_reuse_delay); if (ts_recent_stamp && (!twp || (reuse && time_after32(tcp_clock_ms(), reuse_thresh)))) { /* inet_twsk_hashdance_schedule() sets sk_refcnt after putting twsk @@ -3458,6 +3459,7 @@ static int __net_init tcp_sk_init(struct net *net) net->ipv4.sysctl_tcp_fin_timeout = TCP_FIN_TIMEOUT; net->ipv4.sysctl_tcp_notsent_lowat = UINT_MAX; net->ipv4.sysctl_tcp_tw_reuse = 2; + net->ipv4.sysctl_tcp_tw_reuse_delay = 1 * MSEC_PER_SEC; net->ipv4.sysctl_tcp_no_ssthresh_metrics_save = 1; refcount_set(&net->ipv4.tcp_death_row.tw_refcount, 1);