On Fri, Aug 15, 2025 at 1:40 AM chia-yu.chang@nokia-bell-labs.com wrote:
From: Ilpo Järvinen ij@kernel.org
The Accurate ECN allows echoing back the sum of bytes for each IP ECN field value in the received packets using AccECN option. This change implements AccECN option tx & rx side processing without option send control related features that are added by a later change.
Based on specification: https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt (Some features of the spec will be added in the later changes rather than in this one).
A full-length AccECN option is always attempted but if it does not fit, the minimum length is selected based on the counters that have changed since the last update. The AccECN option (with 24-bit fields) often ends in odd sizes so the option write code tries to take advantage of some nop used to pad the other TCP options.
The delivered_ecn_bytes pairs with received_ecn_bytes similar to how delivered_ce pairs with received_ce. In contrast to ACE field, however, the option is not always available to update delivered_ecn_bytes. For ACK w/o AccECN option, the delivered bytes calculated based on the cumulative ACK+SACK information are assigned to one of the counters using an estimation heuristic to select the most likely ECN byte counter. Any estimation error is corrected when the next AccECN option arrives. It may occur that the heuristic gets too confused when there are enough different byte counter deltas between ACKs with the AccECN option in which case the heuristic just gives up on updating the counters for a while.
tcp_ecn_option sysctl can be used to select option sending mode for AccECN: TCP_ECN_OPTION_DISABLED, TCP_ECN_OPTION_MINIMUM, and TCP_ECN_OPTION_FULL.
This patch increases the size of tcp_info struct, as there is no existing holes for new u32 variables. Below are the pahole outcomes before and after this patch:
[BEFORE THIS PATCH] struct tcp_info { [...] __u32 tcpi_total_rto_time; /* 244 4 */
/* size: 248, cachelines: 4, members: 61 */}
[AFTER THIS PATCH] struct tcp_info { [...] __u32 tcpi_total_rto_time; /* 244 4 */ __u32 tcpi_received_ce; /* 248 4 */ __u32 tcpi_delivered_e1_bytes; /* 252 4 */ __u32 tcpi_delivered_e0_bytes; /* 256 4 */ __u32 tcpi_delivered_ce_bytes; /* 260 4 */ __u32 tcpi_received_e1_bytes; /* 264 4 */ __u32 tcpi_received_e0_bytes; /* 268 4 */ __u32 tcpi_received_ce_bytes; /* 272 4 */
/* size: 280, cachelines: 5, members: 68 */}
This patch uses the existing 1-byte holes in the tcp_sock_write_txrx group for new u8 members, but adds a 4-byte hole in tcp_sock_write_rx group after the new u32 delivered_ecn_bytes[3] member. Therefore, the group size of tcp_sock_write_rx is increased from 96 to 112. Below are the pahole outcomes before and after this patch:
[BEFORE THIS PATCH] struct tcp_sock { [...] u8 received_ce_pending:4; /* 2522: 0 1 */ u8 unused2:4; /* 2522: 4 1 */ /* XXX 1 byte hole, try to pack */
[...] u32 rcv_rtt_last_tsecr; /* 2668 4 */ [...] __cacheline_group_end__tcp_sock_write_rx[0]; /* 2728 0 */ [...] /* size: 3200, cachelines: 50, members: 167 */}
[AFTER THIS PATCH] struct tcp_sock { [...] u8 received_ce_pending:4;/* 2522: 0 1 */ u8 unused2:4; /* 2522: 4 1 */ u8 accecn_minlen:2; /* 2523: 0 1 */ u8 est_ecnfield:2; /* 2523: 2 1 */ u8 unused3:4; /* 2523: 4 1 */
[...] u32 rcv_rtt_last_tsecr; /* 2668 4 */ u32 delivered_ecn_bytes[3];/* 2672 12 */ /* XXX 4 bytes hole, try to pack */ [...] __cacheline_group_end__tcp_sock_write_rx[0]; /* 2744 0 */ [...] /* size: 3200, cachelines: 50, members: 171 */}
Signed-off-by: Ilpo Järvinen ij@kernel.org Signed-off-by: Neal Cardwell ncardwell@google.com Co-developed-by: Chia-Yu Chang chia-yu.chang@nokia-bell-labs.com Signed-off-by: Chia-Yu Chang chia-yu.chang@nokia-bell-labs.com
v13
- Move TCP_ACCECN_E1B_INIT_OFFSET, TCP_ACCECN_E0B_INIT_OFFSET, and TCP_ACCECN_CEB_INIT_OFFSET to this patch
- Use static array lookup in tcp_accecn_optfield_to_ecnfield()
- Return false when WARN_ON_ONCE() is true in tcp_accecn_process_option()
- Make synack_ecn_bytes as static const array and use const u32 pointer in tcp_options_write()
- Use ALIGN() and ALIGN_DOWN() in tcp_options_fit_accecn() to pad TCP AccECN option to dword
v10
- Add documentation of tcp_ecn_option in ip-sysctl.rst to this patch
- Remove the global variable u32 synack_ecn_bytes[3]
- Add READ_ONCE() over every reads of sysctl
v9:
- Restruct the code in the for loop of tcp_accecn_process_option()
- Remove ecn_bytes and add use_synack_ecn_bytes flag in tcp_out_options struct to identify whether syn_ack_bytes or received_ecn_bytes is used
- Replace leftover_bytes and leftover_size with leftover_highbyte and leftover_lowbyte and add comments in tcp_options_write()
v8:
- Reset leftover_size to 2 once leftover_bytes is used
Documentation/networking/ip-sysctl.rst | 19 ++ .../networking/net_cachelines/tcp_sock.rst | 3 + include/linux/tcp.h | 9 +- include/net/netns/ipv4.h | 1 + include/net/tcp.h | 13 ++ include/net/tcp_ecn.h | 89 +++++++++- include/uapi/linux/tcp.h | 7 + net/ipv4/sysctl_net_ipv4.c | 9 + net/ipv4/tcp.c | 15 +- net/ipv4/tcp_input.c | 94 +++++++++- net/ipv4/tcp_ipv4.c | 1 + net/ipv4/tcp_output.c | 165 +++++++++++++++++- 12 files changed, 412 insertions(+), 13 deletions(-)
minlen);
} }}
@@ -263,6 +347,9 @@ static inline void tcp_accecn_init_counters(struct tcp_sock *tp) tp->received_ce = 0; tp->received_ce_pending = 0; __tcp_accecn_init_bytes_counters(tp->received_ecn_bytes);
__tcp_accecn_init_bytes_counters(tp->delivered_ecn_bytes);tp->accecn_minlen = 0;tp->est_ecnfield = 0;}
/* Used for make_synack to form the ACE flags */ diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h index bdac8c42fa82..53e0e85b52be 100644 --- a/include/uapi/linux/tcp.h +++ b/include/uapi/linux/tcp.h @@ -316,6 +316,13 @@ struct tcp_info { * in milliseconds, including any * unfinished recovery. */
__u32 tcpi_received_ce; /* # of CE marks received */__u32 tcpi_delivered_e1_bytes; /* Accurate ECN byte counters */__u32 tcpi_delivered_e0_bytes;__u32 tcpi_delivered_ce_bytes;__u32 tcpi_received_e1_bytes;__u32 tcpi_received_e0_bytes;__u32 tcpi_received_ce_bytes;};
We do not add more fields to tcp_info, unless added fields are a multiple of 64 bits.
Otherwise a hole is added and can not be recovered.