linux.git/include/linux/tcp.h, branch v7.1-rc2

tcp: implement RFC 7323 window retraction receiver requirements

2026-03-14T15:01:49+00:00

By default, the Linux TCP implementation does not shrink the
advertised window (RFC 7323 calls this "window retraction") with the
following exceptions:

- When an incoming segment cannot be added due to the receive buffer
running out of memory. Since commit 8c670bdfa58e ("tcp: correct
handling of extreme memory squeeze") a zero window will be
advertised in this case. It turns out that reaching the required
memory pressure is easy when window scaling is in use. In the
simplest case, sending a sufficient number of segments smaller than
the scale factor to a receiver that does not read data is enough.

- Commit b650d953cd39 ("tcp: enforce receive buffer memory limits by
allowing the tcp window to shrink") addressed the "eating memory"
problem by introducing a sysctl knob that allows shrinking the
window before running out of memory.

However, RFC 7323 does not only state that shrinking the window is
necessary in some cases, it also formulates requirements for TCP
implementations when doing so (Section 2.4).

This commit addresses the receiver-side requirements: After retracting
the window, the peer may have a snd_nxt that lies within a previously
advertised window but is now beyond the retracted window. This means
that all incoming segments (including pure ACKs) will be rejected
until the application happens to read enough data to let the peer's
snd_nxt be in window again (which may be never).

To comply with RFC 7323, the receiver MUST honor any segment that
would have been in window for any ACK sent by the receiver and, when
window scaling is in effect, SHOULD track the maximum window sequence
number it has advertised. This patch tracks that maximum window
sequence number rcv_mwnd_seq throughout the connection and uses it in
tcp_sequence() when deciding whether a segment is acceptable.

rcv_mwnd_seq is updated together with rcv_wup and rcv_wnd in
tcp_select_window(). If we count tcp_sequence() as fast path, it is
read in the fast path. Therefore, rcv_mwnd_seq is put into rcv_wnd's
cacheline group.

The logic for handling received data in tcp_data_queue() is already
sufficient and does not need to be updated.

Signed-off-by: Simon Baatz
Reviewed-by: Eric Dumazet
Link: https://patch.msgid.link/20260309-tcp_rfc7323_retract_wnd_rfc-v3-1-4c7f96b1ec69@gmail.com
Signed-off-by: Jakub Kicinski

tcp: add tcp_release_cb_cond() helper

2026-03-12T12:22:03+00:00

Majority of tcp_release_cb() calls do nothing at all.

Provide tcp_release_cb_cond() helper so that release_sock()
can avoid these calls.

Also hint the compiler that __release_sock() and wake_up()
are rarely called.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-77 (-77)
Function                                     old     new   delta
release_sock                                 258     181     -77
Total: Before=25235790, After=25235713, chg -0.00%

Signed-off-by: Eric Dumazet 
Reviewed-by: Kuniyuki Iwashima 
Link: https://patch.msgid.link/20260310124451.2280968-1-edumazet@google.com
Signed-off-by: Paolo Abeni

tcp: move tp->chrono_type next tp->chrono_stat[]

2026-03-10T02:33:43+00:00

chrono_type is currently in tcp_sock_read_txrx group, which
is supposed to hold read-mostly fields.

But chrono_type is mostly written in tx path, it should
be moved to tcp_sock_write_tx group, close to other
chrono fields (chrono_stat[], chrono_start).

Note this adds holes, but data locality is far more important.

Use a full u8 for the time being, compiler can generate
more efficient code.

Signed-off-by: Eric Dumazet 
Reviewed-by: Neal Cardwell 
Link: https://patch.msgid.link/20260308122302.2895067-1-edumazet@google.com
Signed-off-by: Jakub Kicinski

tcp: accecn: detect loss ACK w/ AccECN option and add TCP_ACCECN_OPTION_PERSIST

2026-02-03T14:13:25+00:00

Detect spurious retransmission of a previously sent ACK carrying the
AccECN option after the second retransmission. Since this might be caused
by the middlebox dropping ACK with options it does not recognize, disable
the sending of the AccECN option in all subsequent ACKs. This patch
follows Section 3.2.3.2.2 of AccECN spec (RFC9768), and a new field
(accecn_opt_sent_w_dsack) is added to indicate that an AccECN option was
sent with duplicate SACK info.

Also, a new AccECN option sending mode is added to tcp_ecn_option sysctl:
(TCP_ECN_OPTION_PERSIST), which ignores the AccECN fallback policy and
persistently sends AccECN option once it fits into TCP option space.

Signed-off-by: Chia-Yu Chang 
Acked-by: Paolo Abeni 
Reviewed-by: Eric Dumazet 
Link: https://patch.msgid.link/20260131222515.8485-13-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni

tcp: try to avoid safer when ACKs are thinned

2026-02-03T14:13:24+00:00

Add newly acked pkts EWMA. When ACK thinning occurs, select
between safer and unsafe cep delta in AccECN processing based
on it. If the packets ACKed per ACK tends to be large, don't
conservatively assume ACE field overflow.

This patch uses the existing 2-byte holes in the rx group for new
u16 variables withtout creating more holes. Below are the pahole
outcomes before and after this patch:

[BEFORE THIS PATCH]
struct tcp_sock {
    [...]
    u32                        delivered_ecn_bytes[3]; /*  2744    12 */
    /* XXX 4 bytes hole, try to pack */

    [...]
    __cacheline_group_end__tcp_sock_write_rx[0];       /*  2816     0 */

    [...]
    /* size: 3264, cachelines: 51, members: 177 */
}

[AFTER THIS PATCH]
struct tcp_sock {
    [...]
    u32                        delivered_ecn_bytes[3]; /*  2744    12 */
    u16                        pkts_acked_ewma;        /*  2756     2 */
    /* XXX 2 bytes hole, try to pack */

    [...]
    __cacheline_group_end__tcp_sock_write_rx[0];       /*  2816     0 */

    [...]
    /* size: 3264, cachelines: 51, members: 178 */
}

Signed-off-by: Ilpo Järvinen 
Co-developed-by: Chia-Yu Chang 
Signed-off-by: Chia-Yu Chang 
Acked-by: Paolo Abeni 
Reviewed-by: Eric Dumazet 
Link: https://patch.msgid.link/20260131222515.8485-2-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni

tcp: preserve const qualifier in tcp_rsk() and inet_rsk()

2026-01-22T03:20:04+00:00

We can change tcp_rsk() and inet_rsk() to propagate their argument
const qualifier thanks to container_of_const().

Signed-off-by: Eric Dumazet 
Reviewed-by: Kuniyuki Iwashima 
Link: https://patch.msgid.link/20260120125353.1470456-1-edumazet@google.com
Signed-off-by: Jakub Kicinski

tcp: move mtu_info to remove two 32bit holes

2025-09-23T00:55:25+00:00

This removes 8bytes waste on 64bit builds.

Signed-off-by: Eric Dumazet 
Reviewed-by: Kuniyuki Iwashima 
Link: https://patch.msgid.link/20250919204856.2977245-8-edumazet@google.com
Signed-off-by: Jakub Kicinski

tcp: move tcp_clean_acked to tcp_sock_read_tx group

2025-09-23T00:55:25+00:00

tp->tcp_clean_acked is fetched in tx path when snd_una is updated.

This field thus belongs to tcp_sock_read_tx group.

Signed-off-by: Eric Dumazet 
Reviewed-by: Kuniyuki Iwashima 
Link: https://patch.msgid.link/20250919204856.2977245-7-edumazet@google.com
Signed-off-by: Jakub Kicinski

tcp: move recvmsg_inq to tcp_sock_read_txrx

2025-09-23T00:55:25+00:00

Fill a hole in tcp_sock_read_txrx, instead of possibly wasting
a cache line.

Note that tcp_recvmsg_locked() is also reading tp->repair,
so this removes one cache line miss in tcp recvmsg().

Signed-off-by: Eric Dumazet 
Reviewed-by: Kuniyuki Iwashima 
Link: https://patch.msgid.link/20250919204856.2977245-6-edumazet@google.com
Signed-off-by: Jakub Kicinski

tcp: move tcp->rcv_tstamp to tcp_sock_write_txrx group

2025-09-23T00:55:24+00:00

tcp_ack() writes this field, it belongs to tcp_sock_write_txrx.

Signed-off-by: Eric Dumazet 
Reviewed-by: Kuniyuki Iwashima 
Link: https://patch.msgid.link/20250919204856.2977245-5-edumazet@google.com
Signed-off-by: Jakub Kicinski