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tcp: Add sysctl to configure TIME-WAIT reuse delay

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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 <jakub@cloudflare.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20241209-jakub-krn-909-poc-msec-tw-tstamp-v2-2-66aca0eed03e@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Sitnicki 2024-12-09 20:38:04 +01:00 committed by Jakub Kicinski
parent 19ce8cd304
commit ca6a6f9386
5 changed files with 29 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
Documentation/networking/ip-sysctl.rst
View File

@ -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.

1
Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst
View File

@ -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

1
include/net/netns/ipv4.h
View File

@ -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;

10
net/ipv4/sysctl_net_ipv4.c
View File

@ -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,

4
net/ipv4/tcp_ipv4.c
View File

@ -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);