From 19ce8cd3046587efbd2c6253947be7c22dfccc18 Mon Sep 17 00:00:00 2001 From: Jakub Sitnicki Date: Mon, 9 Dec 2024 20:38:03 +0100 Subject: [PATCH 1/2] tcp: Measure TIME-WAIT reuse delay with millisecond precision Prepare ground for TIME-WAIT socket reuse with subsecond delay. Today the last TS.Recent update timestamp, recorded in seconds and stored tp->ts_recent_stamp and tw->tw_ts_recent_stamp fields, has two purposes. Firstly, it is used to track the age of the last recorded TS.Recent value to detect when that value becomes outdated due to potential wrap-around of the other TCP timestamp clock (RFC 7323, section 5.5). For this purpose a second-based timestamp is completely sufficient as even in the worst case scenario of a peer using a high resolution microsecond timestamp, the wrap-around interval is ~36 minutes long. Secondly, it serves as a threshold value for allowing TIME-WAIT socket reuse. A TIME-WAIT socket can be reused only once the virtual 1 Hz clock, ktime_get_seconds, is past the TS.Recent update timestamp. The purpose behind delaying the TIME-WAIT socket reuse is to wait for the other TCP timestamp clock to tick at least once before reusing the connection. It is only then that the PAWS mechanism for the reopened connection can detect old duplicate segments from the previous connection incarnation (RFC 7323, appendix B.2). In this case using a timestamp with second resolution not only blocks the way toward allowing faster TIME-WAIT reuse after shorter subsecond delay, but also makes it impossible to reliably delay TW reuse by one second. As Eric Dumazet has pointed out [1], due to timestamp rounding, the TW reuse delay will actually be between (0, 1] seconds, and 0.5 seconds on average. We delay TW reuse for one full second only when last TS.Recent update coincides with our virtual 1 Hz clock tick. Considering the above, introduce a dedicated field to store a millisecond timestamp of transition into the TIME-WAIT state. Place it in an existing 4-byte hole inside inet_timewait_sock structure to avoid an additional memory cost. Use the new timestamp to (i) reliably delay TIME-WAIT reuse by one second, and (ii) prepare for configurable subsecond reuse delay in the subsequent change. We assume here that a full one second delay was the original intention in [2] because it accounts for the worst case scenario of the other TCP using the slowest recommended 1 Hz timestamp clock. A more involved alternative would be to change the resolution of the last TS.Recent update timestamp, tw->tw_ts_recent_stamp, to milliseconds. [1] https://lore.kernel.org/netdev/CANn89iKB4GFd8sVzCbRttqw_96o3i2wDhX-3DraQtsceNGYwug@mail.gmail.com/ [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b8439924316d5bcb266d165b93d632a4b4b859af 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-v2-1-66aca0eed03e@cloudflare.com Signed-off-by: Jakub Kicinski --- include/net/inet_timewait_sock.h | 4 ++++ net/ipv4/tcp_ipv4.c | 5 +++-- net/ipv4/tcp_minisocks.c | 7 ++++++- 3 files changed, 13 insertions(+), 3 deletions(-) diff --git a/include/net/inet_timewait_sock.h b/include/net/inet_timewait_sock.h index 62c0a7e65d6b..67a313575780 100644 --- a/include/net/inet_timewait_sock.h +++ b/include/net/inet_timewait_sock.h @@ -74,6 +74,10 @@ struct inet_timewait_sock { tw_tos : 8; u32 tw_txhash; u32 tw_priority; + /** + * @tw_reuse_stamp: Time of entry into %TCP_TIME_WAIT state in msec. + */ + u32 tw_entry_stamp; struct timer_list tw_timer; struct inet_bind_bucket *tw_tb; struct inet_bind2_bucket *tw_tb2; diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index a38c8b1f44db..3b6ba1d16921 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -120,6 +120,7 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp) const struct tcp_timewait_sock *tcptw = tcp_twsk(sktw); struct tcp_sock *tp = tcp_sk(sk); int ts_recent_stamp; + u32 reuse_thresh; if (READ_ONCE(tw->tw_substate) == TCP_FIN_WAIT2) reuse = 0; @@ -162,9 +163,9 @@ 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; if (ts_recent_stamp && - (!twp || (reuse && time_after32(ktime_get_seconds(), - ts_recent_stamp)))) { + (!twp || (reuse && time_after32(tcp_clock_ms(), reuse_thresh)))) { /* inet_twsk_hashdance_schedule() sets sk_refcnt after putting twsk * and releasing the bucket lock. */ diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index 7121d8573928..b089b08e9617 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -157,8 +157,11 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, rcv_nxt); if (tmp_opt.saw_tstamp) { + u64 ts = tcp_clock_ms(); + + WRITE_ONCE(tw->tw_entry_stamp, ts); WRITE_ONCE(tcptw->tw_ts_recent_stamp, - ktime_get_seconds()); + div_u64(ts, MSEC_PER_SEC)); WRITE_ONCE(tcptw->tw_ts_recent, tmp_opt.rcv_tsval); } @@ -316,6 +319,8 @@ void tcp_time_wait(struct sock *sk, int state, int timeo) tw->tw_mark = sk->sk_mark; tw->tw_priority = READ_ONCE(sk->sk_priority); tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; + /* refreshed when we enter true TIME-WAIT state */ + tw->tw_entry_stamp = tcp_time_stamp_ms(tp); tcptw->tw_rcv_nxt = tp->rcv_nxt; tcptw->tw_snd_nxt = tp->snd_nxt; tcptw->tw_rcv_wnd = tcp_receive_window(tp); From ca6a6f93867a9763bdf8685c788e2e558d10975f Mon Sep 17 00:00:00 2001 From: Jakub Sitnicki Date: Mon, 9 Dec 2024 20:38:04 +0100 Subject: [PATCH 2/2] 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);