[PATCH v2 0/3] Retry SYNs for inbound connections

Yumei Huang

10 Oct 2025 10 Oct '25

9:46 a.m.

When a client connects, SYN would be sent to guest only once. If the guest is not connected or ready at that time, the connection will be reset in 10s. These patches introduce the SYN retry mechanism using the similar backoff timeout as linux kernel. Also update the data retransmission timeout. Yumei Huang (3): tcp: Rename "retrans" to "retries" tcp: Resend SYN for inbound connections tcp: Update data retransmission timeout tcp.c | 132 +++++++++++++++++++++++++++++++++++++++++++++-------- tcp.h | 2 + tcp_conn.h | 12 ++--- 3 files changed, 120 insertions(+), 26 deletions(-) -- 2.47.0

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Yumei Huang

10 Oct 10 Oct

9:46 a.m.

New subject: [PATCH v2 1/3] tcp: Rename "retrans" to "retries"

Rename "retrans" to "retries" so it can be used for SYN retries. Signed-off-by: Yumei Huang --- tcp.c | 12 ++++++------ tcp_conn.h | 12 ++++++------ 2 files changed, 12 insertions(+), 12 deletions(-) diff --git a/tcp.c b/tcp.c index 0f9e9b3..2ec4b0c 100644 --- a/tcp.c +++ b/tcp.c @@ -186,7 +186,7 @@ * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the * socket and reset sequence to what was acknowledged. If this persists for - * more than TCP_MAX_RETRANS times in a row, reset the connection + * more than TCP_MAX_RETRIES times in a row, reset the connection * * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE * with TAP_FIN_SENT event), and no ACK is received within this time, reset @@ -1127,7 +1127,7 @@ static void tcp_update_seqack_from_tap(const struct ctx *c, if (SEQ_LT(seq, conn->seq_to_tap)) conn_flag(c, conn, ACK_FROM_TAP_DUE); - conn->retrans = 0; + conn->retries = 0; conn->seq_ack_from_tap = seq; } } @@ -2414,7 +2414,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { flow_dbg(conn, "FIN timeout"); tcp_rst(c, conn); - } else if (conn->retrans == TCP_MAX_RETRANS) { + } else if (conn->retries == TCP_MAX_RETRIES) { flow_dbg(conn, "retransmissions count exceeded"); tcp_rst(c, conn); } else { @@ -2423,7 +2423,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) if (!conn->wnd_from_tap) conn->wnd_from_tap = 1; /* Zero-window probe */ - conn->retrans++; + conn->retries++; if (tcp_rewind_seq(c, conn)) return; @@ -3382,7 +3382,7 @@ static int tcp_flow_repair_opt(const struct tcp_tap_conn *conn, int tcp_flow_migrate_source(int fd, struct tcp_tap_conn *conn) { struct tcp_tap_transfer t = { - .retrans = conn->retrans, + .retries = conn->retries, .ws_from_tap = conn->ws_from_tap, .ws_to_tap = conn->ws_to_tap, .events = conn->events, @@ -3662,7 +3662,7 @@ int tcp_flow_migrate_target(struct ctx *c, int fd) memcpy(&flow->f.side, &t.side, sizeof(flow->f.side)); conn = FLOW_SET_TYPE(flow, FLOW_TCP, tcp); - conn->retrans = t.retrans; + conn->retries = t.retries; conn->ws_from_tap = t.ws_from_tap; conn->ws_to_tap = t.ws_to_tap; conn->events = t.events; diff --git a/tcp_conn.h b/tcp_conn.h index 38b5c54..e5c8146 100644 --- a/tcp_conn.h +++ b/tcp_conn.h @@ -13,7 +13,7 @@ * struct tcp_tap_conn - Descriptor for a TCP connection (not spliced) * @f: Generic flow information * @in_epoll: Is the connection in the epoll set? - * @retrans: Number of retransmissions occurred due to ACK_TIMEOUT + * @retries: Number of retries occurred due to timeouts * @ws_from_tap: Window scaling factor advertised from tap/guest * @ws_to_tap: Window scaling factor advertised to tap/guest * @tap_mss: MSS advertised by tap/guest, rounded to 2 ^ TCP_MSS_BITS @@ -38,9 +38,9 @@ struct tcp_tap_conn { bool in_epoll :1; -#define TCP_RETRANS_BITS 3 - unsigned int retrans :TCP_RETRANS_BITS; -#define TCP_MAX_RETRANS MAX_FROM_BITS(TCP_RETRANS_BITS) +#define TCP_RETRIES_BITS 3 + unsigned int retries :TCP_RETRIES_BITS; +#define TCP_MAX_RETRIES MAX_FROM_BITS(TCP_RETRIES_BITS) #define TCP_WS_BITS 4 /* RFC 7323 */ #define TCP_WS_MAX 14 @@ -102,7 +102,7 @@ struct tcp_tap_conn { * struct tcp_tap_transfer - Migrated TCP data, flow table part, network order * @pif: Interfaces for each side of the flow * @side: Addresses and ports for each side of the flow - * @retrans: Number of retransmissions occurred due to ACK_TIMEOUT + * @retries: Number of retries occurred due to timeouts * @ws_from_tap: Window scaling factor advertised from tap/guest * @ws_to_tap: Window scaling factor advertised to tap/guest * @events: Connection events, implying connection states @@ -122,7 +122,7 @@ struct tcp_tap_transfer { uint8_t pif[SIDES]; struct flowside side[SIDES]; - uint8_t retrans; + uint8_t retries; uint8_t ws_from_tap; uint8_t ws_to_tap; uint8_t events; -- 2.47.0

Yumei Huang

9:46 a.m.

New subject: [PATCH v2 2/3] tcp: Resend SYN for inbound connections

If a client connects while guest is not connected or ready yet, resend SYN instead of just resetting connection after 10 seconds. Use the same backoff calculation for the timeout as linux kernel. Signed-off-by: Yumei Huang --- tcp.c | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----- tcp.h | 2 ++ 2 files changed, 89 insertions(+), 8 deletions(-) diff --git a/tcp.c b/tcp.c index 2ec4b0c..85bbdac 100644 --- a/tcp.c +++ b/tcp.c @@ -179,9 +179,11 @@ * * Timeouts are implemented by means of timerfd timers, set based on flags: * - * - SYN_TIMEOUT: if no ACK is received from tap/guest during handshake (flag - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, reset the - * connection + * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during handshake + * (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, resend + * SYN. It's the starting timeout for the first SYN retry. If this persists + * for more than TCP_MAX_RETRIES or (tcp_syn_retries + + * tcp_syn_linear_timeouts) times in a row, reset the connection * * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the @@ -309,6 +311,7 @@ #include "tcp_internal.h" #include "tcp_buf.h" #include "tcp_vu.h" +#include "lineread.h" /* * The size of TCP header (including options) is given by doff (Data Offset) @@ -340,7 +343,7 @@ enum { #define WINDOW_DEFAULT 14600 /* RFC 6928 */ #define ACK_INTERVAL 10 /* ms */ -#define SYN_TIMEOUT 10 /* s */ +#define SYN_TIMEOUT_INIT 1 /* s */ #define ACK_TIMEOUT 2 #define FIN_TIMEOUT 60 #define ACT_TIMEOUT 7200 @@ -365,6 +368,10 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUEUE_MAX]; #define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller when above this */ +#define TCP_SYN_RETRIES_SYSCTL "/proc/sys/net/ipv4/tcp_syn_retries" +#define TCP_SYN_LINEAR_TIMEOUTS_SYSCTL \ + "/proc/sys/net/ipv4/tcp_syn_linear_timeouts" + /* "Extended" data (not stored in the flow table) for TCP flow migration */ static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX]; @@ -581,8 +588,13 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn) if (conn->flags & ACK_TO_TAP_DUE) { it.it_value.tv_nsec = (long)ACK_INTERVAL * 1000 * 1000; } else if (conn->flags & ACK_FROM_TAP_DUE) { - if (!(conn->events & ESTABLISHED)) - it.it_value.tv_sec = SYN_TIMEOUT; + if (!(conn->events & ESTABLISHED)) { + if (conn->retries < c->tcp.syn_linear_timeouts) + it.it_value.tv_sec = SYN_TIMEOUT_INIT; + else + it.it_value.tv_sec = SYN_TIMEOUT_INIT << + (conn->retries - c->tcp.syn_linear_timeouts); + } else it.it_value.tv_sec = ACK_TIMEOUT; } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { @@ -1961,6 +1973,7 @@ static void tcp_conn_from_sock_finish(const struct ctx *c, } tcp_send_flag(c, conn, ACK); + conn->retries = 0; /* The client might have sent data already, which we didn't * dequeue waiting for SYN,ACK from tap -- check now. @@ -2409,8 +2422,16 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) tcp_timer_ctl(c, conn); } else if (conn->flags & ACK_FROM_TAP_DUE) { if (!(conn->events & ESTABLISHED)) { - flow_dbg(conn, "handshake timeout"); - tcp_rst(c, conn); + if (conn->retries == MIN(TCP_MAX_RETRIES, + (c->tcp.tcp_syn_retries + c->tcp.tcp_syn_retries))) { + flow_dbg(conn, "handshake timeout"); + tcp_rst(c, conn); + } else { + flow_dbg(conn, "SYN timeout, retry"); + tcp_send_flag(c, conn, SYN); + conn->retries++; + tcp_timer_ctl(c, conn); + } } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { flow_dbg(conn, "FIN timeout"); tcp_rst(c, conn); @@ -2766,6 +2787,62 @@ static socklen_t tcp_probe_tcp_info(void) return sl; } +/** + * get_tcp_syn_param() - Read SYN parameters from /proc/sys + * @path: Path to the sysctl file + * @fallback: Default value if file can't be read + * + * Return: Parameter value, fallback on failure +*/ +int get_tcp_syn_param(const char *path, int fallback) +{ + char *line, *end; + struct lineread lr; + long value; + ssize_t len; + int fd; + + fd = open(path, O_RDONLY | O_CLOEXEC); + if (fd < 0) { + debug("Unable to open %s", path); + return fallback; + } + + lineread_init(&lr, fd); + len = lineread_get(&lr, &line); + close(fd); + + if (len < 0) { + debug("Unable to read %s", path); + return fallback; + } + + errno = 0; + value = strtol(line, &end, 10); + if (*end && *end != '\n') { + debug("Invalid format in %s", path); + return fallback; + } + if (errno || value < 0 || value > INT_MAX) { + debug("Invalid value in %s: %ld", path, value); + return fallback; + } + return (int)value; +} + +/** + * tcp_syn_params_init() - Get initial syn params for inbound connection + * @c: Execution context +*/ +void tcp_syn_params_init(struct ctx *c) +{ + c->tcp.tcp_syn_retries = get_tcp_syn_param(TCP_SYN_RETRIES_SYSCTL, 8); + c->tcp.syn_linear_timeouts = + get_tcp_syn_param(TCP_SYN_LINEAR_TIMEOUTS_SYSCTL, 1); + debug("TCP SYN parameters: retries=%d, linear_timeouts=%d", + c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts); +} + /** * tcp_init() - Get initial sequence, hash secret, initialise per-socket data * @c: Execution context @@ -2776,6 +2853,8 @@ int tcp_init(struct ctx *c) { ASSERT(!c->no_tcp); + tcp_syn_params_init(c); + tcp_sock_iov_init(c); memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)); diff --git a/tcp.h b/tcp.h index 234a803..df699a4 100644 --- a/tcp.h +++ b/tcp.h @@ -65,6 +65,8 @@ struct tcp_ctx { struct fwd_ports fwd_out; struct timespec timer_run; size_t pipe_size; + uint8_t tcp_syn_retries; + uint8_t syn_linear_timeouts; }; #endif /* TCP_H */ -- 2.47.0

Yumei Huang

9:47 a.m.

New subject: [PATCH v2 3/3] tcp: Update data retransmission timeout

Use an exponential backoff timeout with the initial timeout 1s and total timeout 60s. Signed-off-by: Yumei Huang --- tcp.c | 25 +++++++++++++++++++------ 1 file changed, 19 insertions(+), 6 deletions(-) diff --git a/tcp.c b/tcp.c index 85bbdac..0db7f30 100644 --- a/tcp.c +++ b/tcp.c @@ -185,10 +185,15 @@ * for more than TCP_MAX_RETRIES or (tcp_syn_retries + * tcp_syn_linear_timeouts) times in a row, reset the connection * - * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending - * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the - * socket and reset sequence to what was acknowledged. If this persists for - * more than TCP_MAX_RETRIES times in a row, reset the connection + * - ACK_TIMEOUT_INIT: if no ACK segment was received from tap/guest, after + * sending data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data + * from the socket and reset sequence to what was acknowledged. It's the + * starting timeout for the first retransmission. If this persists for more + * than TCP_MAX_RETRIES times in a row, reset the connection + * + * - ACK_TIMEOUT_TOTAL: if no ACK segment was received from tap/guest after + * retransmitting data repeatedly from the socket within this time, reset + * the connection * * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE * with TAP_FIN_SENT event), and no ACK is received within this time, reset @@ -344,7 +349,8 @@ enum { #define ACK_INTERVAL 10 /* ms */ #define SYN_TIMEOUT_INIT 1 /* s */ -#define ACK_TIMEOUT 2 +#define ACK_TIMEOUT_INIT 1 +#define ACK_TIMEOUT_TOTAL 60 #define FIN_TIMEOUT 60 #define ACT_TIMEOUT 7200 @@ -358,6 +364,9 @@ enum { ((conn)->events & (SOCK_FIN_RCVD | TAP_FIN_RCVD))) #define CONN_HAS(conn, set) (((conn)->events & (set)) == (set)) +#define RETRY_ELAPSED(timeout_init, retries) \ + ((timeout_init) * ((1 << ((retries) + 1)) - 2)) + /* Buffers to migrate pending data from send and receive queues. No, they don't * use memory if we don't use them. And we're going away after this, so splurge. */ @@ -596,7 +605,7 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn) (conn->retries - c->tcp.syn_linear_timeouts); } else - it.it_value.tv_sec = ACK_TIMEOUT; + it.it_value.tv_sec = ACK_TIMEOUT_INIT << conn->retries; } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { it.it_value.tv_sec = FIN_TIMEOUT; } else { @@ -2438,6 +2447,10 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) } else if (conn->retries == TCP_MAX_RETRIES) { flow_dbg(conn, "retransmissions count exceeded"); tcp_rst(c, conn); + } else if(RETRY_ELAPSED(ACK_TIMEOUT_INIT, conn->retries) >= + ACK_TIMEOUT_TOTAL) { + flow_dbg(conn, "retransmissions timeout exceeded"); + tcp_rst(c, conn); } else { flow_dbg(conn, "ACK timeout, retry"); -- 2.47.0

David Gibson

13 Oct 13 Oct

2:18 a.m.

New subject: [PATCH v2 2/3] tcp: Resend SYN for inbound connections

On Fri, Oct 10, 2025 at 03:46:59PM +0800, Yumei Huang wrote:

...

If a client connects while guest is not connected or ready yet, resend SYN instead of just resetting connection after 10 seconds.

Use the same backoff calculation for the timeout as linux kernel.

Signed-off-by: Yumei Huang --- tcp.c | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----- tcp.h | 2 ++ 2 files changed, 89 insertions(+), 8 deletions(-)

diff --git a/tcp.c b/tcp.c index 2ec4b0c..85bbdac 100644 --- a/tcp.c +++ b/tcp.c @@ -179,9 +179,11 @@ * * Timeouts are implemented by means of timerfd timers, set based on flags: * - * - SYN_TIMEOUT: if no ACK is received from tap/guest during handshake (flag - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, reset the - * connection + * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during handshake + * (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, resend + * SYN. It's the starting timeout for the first SYN retry. If this persists + * for more than TCP_MAX_RETRIES or (tcp_syn_retries + + * tcp_syn_linear_timeouts) times in a row, reset the connection * * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the @@ -309,6 +311,7 @@ #include "tcp_internal.h" #include "tcp_buf.h" #include "tcp_vu.h" +#include "lineread.h"

/* * The size of TCP header (including options) is given by doff (Data Offset) @@ -340,7 +343,7 @@ enum { #define WINDOW_DEFAULT 14600 /* RFC 6928 */

#define ACK_INTERVAL 10 /* ms */ -#define SYN_TIMEOUT 10 /* s */ +#define SYN_TIMEOUT_INIT 1 /* s */ #define ACK_TIMEOUT 2

As we've discussed in some places, the RFCs largely treat SYN retries the same as data retransmits. ACK_TIMEOUT controls the latter. 2s is a little odd, IIRC the RFC suggests 3, I'm not sure what Linux does by default. In any case, I'm wondering if we should use a single define for the initial value of both timeouts.

...

#define FIN_TIMEOUT 60 #define ACT_TIMEOUT 7200 @@ -365,6 +368,10 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUEUE_MAX];

#define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller when above this */

+#define TCP_SYN_RETRIES_SYSCTL "/proc/sys/net/ipv4/tcp_syn_retries" +#define TCP_SYN_LINEAR_TIMEOUTS_SYSCTL \ + "/proc/sys/net/ipv4/tcp_syn_linear_timeouts" + /* "Extended" data (not stored in the flow table) for TCP flow migration */ static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX];

@@ -581,8 +588,13 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn) if (conn->flags & ACK_TO_TAP_DUE) { it.it_value.tv_nsec = (long)ACK_INTERVAL * 1000 * 1000; } else if (conn->flags & ACK_FROM_TAP_DUE) { - if (!(conn->events & ESTABLISHED)) - it.it_value.tv_sec = SYN_TIMEOUT; + if (!(conn->events & ESTABLISHED)) { + if (conn->retries < c->tcp.syn_linear_timeouts) + it.it_value.tv_sec = SYN_TIMEOUT_INIT; + else + it.it_value.tv_sec = SYN_TIMEOUT_INIT << + (conn->retries - c->tcp.syn_linear_timeouts); + } else it.it_value.tv_sec = ACK_TIMEOUT; } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { @@ -1961,6 +1973,7 @@ static void tcp_conn_from_sock_finish(const struct ctx *c, }

tcp_send_flag(c, conn, ACK); + conn->retries = 0;

I know Stefano said you needed to add this, but on a closer examination I don't think you do. tcp_tap_handler() calls tcp_update_seqack_from_tap() if !ESTABLISHED. That will clear retrans/retries if the ack number from the guest has advanced. That will occur when the guest SYN-ACKs our SYN, which is exactly the point at which we should clear the retries count.

...

/* The client might have sent data already, which we didn't * dequeue waiting for SYN,ACK from tap -- check now. @@ -2409,8 +2422,16 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) tcp_timer_ctl(c, conn); } else if (conn->flags & ACK_FROM_TAP_DUE) { if (!(conn->events & ESTABLISHED)) { - flow_dbg(conn, "handshake timeout"); - tcp_rst(c, conn); + if (conn->retries == MIN(TCP_MAX_RETRIES, + (c->tcp.tcp_syn_retries + c->tcp.tcp_syn_retries))) {

Should this be tcp_syn_retries + linear_timeouts? Also, probably safer to use a >= rather than == - if we somehow send an extra retry, we don't want to then retry forever.

...

+ flow_dbg(conn, "handshake timeout"); + tcp_rst(c, conn); + } else { + flow_dbg(conn, "SYN timeout, retry"); + tcp_send_flag(c, conn, SYN); + conn->retries++; + tcp_timer_ctl(c, conn); + } } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { flow_dbg(conn, "FIN timeout"); tcp_rst(c, conn); @@ -2766,6 +2787,62 @@ static socklen_t tcp_probe_tcp_info(void) return sl; }

+/** + * get_tcp_syn_param() - Read SYN parameters from /proc/sys + * @path: Path to the sysctl file + * @fallback: Default value if file can't be read + * + * Return: Parameter value, fallback on failure +*/ +int get_tcp_syn_param(const char *path, int fallback)

I wonder if it might be worth making a new function in util.c to read a file containing a single number.

...

+{ + char *line, *end; + struct lineread lr; + long value; + ssize_t len; + int fd; + + fd = open(path, O_RDONLY | O_CLOEXEC); + if (fd < 0) { + debug("Unable to open %s", path); + return fallback; + } + + lineread_init(&lr, fd); + len = lineread_get(&lr, &line); + close(fd); + + if (len < 0) { + debug("Unable to read %s", path); + return fallback; + } + + errno = 0; + value = strtol(line, &end, 10); + if (*end && *end != '\n') { + debug("Invalid format in %s", path); + return fallback; + } + if (errno || value < 0 || value > INT_MAX) { + debug("Invalid value in %s: %ld", path, value); + return fallback; + } + return (int)value;

You return an (int) here, but store the value into a uint8_t in the caller. That will cause unexpected results if the value you read is greater than 255. Somewhere you need to check for this and clamp the value. I'd suggest the function actually reading /proc return a long long (for reuseability), then clamp it in the caller.

...

+} + +/** + * tcp_syn_params_init() - Get initial syn params for inbound connection + * @c: Execution context +*/ +void tcp_syn_params_init(struct ctx *c) +{ + c->tcp.tcp_syn_retries = get_tcp_syn_param(TCP_SYN_RETRIES_SYSCTL, 8); + c->tcp.syn_linear_timeouts = + get_tcp_syn_param(TCP_SYN_LINEAR_TIMEOUTS_SYSCTL, 1); + debug("TCP SYN parameters: retries=%d, linear_timeouts=%d", + c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts); +} + /** * tcp_init() - Get initial sequence, hash secret, initialise per-socket data * @c: Execution context @@ -2776,6 +2853,8 @@ int tcp_init(struct ctx *c) { ASSERT(!c->no_tcp);

+ tcp_syn_params_init(c); + tcp_sock_iov_init(c);

memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)); diff --git a/tcp.h b/tcp.h index 234a803..df699a4 100644 --- a/tcp.h +++ b/tcp.h @@ -65,6 +65,8 @@ struct tcp_ctx { struct fwd_ports fwd_out; struct timespec timer_run; size_t pipe_size; + uint8_t tcp_syn_retries; + uint8_t syn_linear_timeouts; };

#endif /* TCP_H */ -- 2.47.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson

David Gibson

1:52 a.m.

New subject: [PATCH v2 1/3] tcp: Rename "retrans" to "retries"

On Fri, Oct 10, 2025 at 03:46:58PM +0800, Yumei Huang wrote:

...

Rename "retrans" to "retries" so it can be used for SYN retries.

Signed-off-by: Yumei Huang

Reviewed-by: David Gibson I think the new name is clearer, although AFAICT the RFCs mostly seem to refer to both data retransmits and SYN retries as "retransmits".

...

--- tcp.c | 12 ++++++------ tcp_conn.h | 12 ++++++------ 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/tcp.c b/tcp.c index 0f9e9b3..2ec4b0c 100644 --- a/tcp.c +++ b/tcp.c @@ -186,7 +186,7 @@ * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the * socket and reset sequence to what was acknowledged. If this persists for - * more than TCP_MAX_RETRANS times in a row, reset the connection + * more than TCP_MAX_RETRIES times in a row, reset the connection * * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE * with TAP_FIN_SENT event), and no ACK is received within this time, reset @@ -1127,7 +1127,7 @@ static void tcp_update_seqack_from_tap(const struct ctx *c, if (SEQ_LT(seq, conn->seq_to_tap)) conn_flag(c, conn, ACK_FROM_TAP_DUE);

- conn->retrans = 0; + conn->retries = 0; conn->seq_ack_from_tap = seq; } } @@ -2414,7 +2414,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { flow_dbg(conn, "FIN timeout"); tcp_rst(c, conn); - } else if (conn->retrans == TCP_MAX_RETRANS) { + } else if (conn->retries == TCP_MAX_RETRIES) { flow_dbg(conn, "retransmissions count exceeded"); tcp_rst(c, conn); } else { @@ -2423,7 +2423,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) if (!conn->wnd_from_tap) conn->wnd_from_tap = 1; /* Zero-window probe */

- conn->retrans++; + conn->retries++; if (tcp_rewind_seq(c, conn)) return;

@@ -3382,7 +3382,7 @@ static int tcp_flow_repair_opt(const struct tcp_tap_conn *conn, int tcp_flow_migrate_source(int fd, struct tcp_tap_conn *conn) { struct tcp_tap_transfer t = { - .retrans = conn->retrans, + .retries = conn->retries, .ws_from_tap = conn->ws_from_tap, .ws_to_tap = conn->ws_to_tap, .events = conn->events, @@ -3662,7 +3662,7 @@ int tcp_flow_migrate_target(struct ctx *c, int fd) memcpy(&flow->f.side, &t.side, sizeof(flow->f.side)); conn = FLOW_SET_TYPE(flow, FLOW_TCP, tcp);

- conn->retrans = t.retrans; + conn->retries = t.retries; conn->ws_from_tap = t.ws_from_tap; conn->ws_to_tap = t.ws_to_tap; conn->events = t.events; diff --git a/tcp_conn.h b/tcp_conn.h index 38b5c54..e5c8146 100644 --- a/tcp_conn.h +++ b/tcp_conn.h @@ -13,7 +13,7 @@ * struct tcp_tap_conn - Descriptor for a TCP connection (not spliced) * @f: Generic flow information * @in_epoll: Is the connection in the epoll set? - * @retrans: Number of retransmissions occurred due to ACK_TIMEOUT + * @retries: Number of retries occurred due to timeouts * @ws_from_tap: Window scaling factor advertised from tap/guest * @ws_to_tap: Window scaling factor advertised to tap/guest * @tap_mss: MSS advertised by tap/guest, rounded to 2 ^ TCP_MSS_BITS @@ -38,9 +38,9 @@ struct tcp_tap_conn {

bool in_epoll :1;

-#define TCP_RETRANS_BITS 3 - unsigned int retrans :TCP_RETRANS_BITS; -#define TCP_MAX_RETRANS MAX_FROM_BITS(TCP_RETRANS_BITS) +#define TCP_RETRIES_BITS 3 + unsigned int retries :TCP_RETRIES_BITS; +#define TCP_MAX_RETRIES MAX_FROM_BITS(TCP_RETRIES_BITS)

#define TCP_WS_BITS 4 /* RFC 7323 */ #define TCP_WS_MAX 14 @@ -102,7 +102,7 @@ struct tcp_tap_conn { * struct tcp_tap_transfer - Migrated TCP data, flow table part, network order * @pif: Interfaces for each side of the flow * @side: Addresses and ports for each side of the flow - * @retrans: Number of retransmissions occurred due to ACK_TIMEOUT + * @retries: Number of retries occurred due to timeouts * @ws_from_tap: Window scaling factor advertised from tap/guest * @ws_to_tap: Window scaling factor advertised to tap/guest * @events: Connection events, implying connection states @@ -122,7 +122,7 @@ struct tcp_tap_transfer { uint8_t pif[SIDES]; struct flowside side[SIDES];

- uint8_t retrans; + uint8_t retries; uint8_t ws_from_tap; uint8_t ws_to_tap; uint8_t events; -- 2.47.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson

David Gibson

2:29 a.m.

New subject: [PATCH v2 3/3] tcp: Update data retransmission timeout

On Fri, Oct 10, 2025 at 03:47:00PM +0800, Yumei Huang wrote:

...

Use an exponential backoff timeout with the initial timeout 1s and total timeout 60s.

The commit message needs more information on why making this change to behaviour is desirable (e.g. referencing RFCs).

...

Signed-off-by: Yumei Huang --- tcp.c | 25 +++++++++++++++++++------ 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/tcp.c b/tcp.c index 85bbdac..0db7f30 100644 --- a/tcp.c +++ b/tcp.c @@ -185,10 +185,15 @@ * for more than TCP_MAX_RETRIES or (tcp_syn_retries + * tcp_syn_linear_timeouts) times in a row, reset the connection * - * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending - * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the - * socket and reset sequence to what was acknowledged. If this persists for - * more than TCP_MAX_RETRIES times in a row, reset the connection + * - ACK_TIMEOUT_INIT: if no ACK segment was received from tap/guest, after + * sending data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data + * from the socket and reset sequence to what was acknowledged. It's the + * starting timeout for the first retransmission. If this persists for more + * than TCP_MAX_RETRIES times in a row, reset the connection + * + * - ACK_TIMEOUT_TOTAL: if no ACK segment was received from tap/guest after + * retransmitting data repeatedly from the socket within this time, reset + * the connection * * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE * with TAP_FIN_SENT event), and no ACK is received within this time, reset @@ -344,7 +349,8 @@ enum {

#define ACK_INTERVAL 10 /* ms */ #define SYN_TIMEOUT_INIT 1 /* s */ -#define ACK_TIMEOUT 2 +#define ACK_TIMEOUT_INIT 1 +#define ACK_TIMEOUT_TOTAL 60 #define FIN_TIMEOUT 60 #define ACT_TIMEOUT 7200

@@ -358,6 +364,9 @@ enum { ((conn)->events & (SOCK_FIN_RCVD | TAP_FIN_RCVD))) #define CONN_HAS(conn, set) (((conn)->events & (set)) == (set))

+#define RETRY_ELAPSED(timeout_init, retries) \ + ((timeout_init) * ((1 << ((retries) + 1)) - 2)) + /* Buffers to migrate pending data from send and receive queues. No, they don't * use memory if we don't use them. And we're going away after this, so splurge. */ @@ -596,7 +605,7 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn) (conn->retries - c->tcp.syn_linear_timeouts); } else - it.it_value.tv_sec = ACK_TIMEOUT; + it.it_value.tv_sec = ACK_TIMEOUT_INIT << conn->retries; } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { it.it_value.tv_sec = FIN_TIMEOUT; } else { @@ -2438,6 +2447,10 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) } else if (conn->retries == TCP_MAX_RETRIES) { flow_dbg(conn, "retransmissions count exceeded"); tcp_rst(c, conn); + } else if(RETRY_ELAPSED(ACK_TIMEOUT_INIT, conn->retries) >= + ACK_TIMEOUT_TOTAL) { + flow_dbg(conn, "retransmissions timeout exceeded"); + tcp_rst(c, conn);

Having a test both for number of retries and time elapsed seems redundant. RETRY_ELAPSED is pure function of the number of retries, so it should be possible to just have a threshold on the number of retries, which can be calculated from the target total timeout.

...

} else { flow_dbg(conn, "ACK timeout, retry");

-- 2.47.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson

Yumei Huang

12:22 p.m.

New subject: [PATCH v2 2/3] tcp: Resend SYN for inbound connections

On Mon, Oct 13, 2025 at 8:21 AM David Gibson wrote:

...

On Fri, Oct 10, 2025 at 03:46:59PM +0800, Yumei Huang wrote:

...
If a client connects while guest is not connected or ready yet, resend SYN instead of just resetting connection after 10 seconds.

Use the same backoff calculation for the timeout as linux kernel.

Signed-off-by: Yumei Huang --- tcp.c | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----- tcp.h | 2 ++ 2 files changed, 89 insertions(+), 8 deletions(-)

diff --git a/tcp.c b/tcp.c index 2ec4b0c..85bbdac 100644 --- a/tcp.c +++ b/tcp.c @@ -179,9 +179,11 @@ * * Timeouts are implemented by means of timerfd timers, set based on flags: * - * - SYN_TIMEOUT: if no ACK is received from tap/guest during handshake (flag - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, reset the - * connection + * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during handshake + * (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, resend + * SYN. It's the starting timeout for the first SYN retry. If this persists + * for more than TCP_MAX_RETRIES or (tcp_syn_retries + + * tcp_syn_linear_timeouts) times in a row, reset the connection * * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the @@ -309,6 +311,7 @@ #include "tcp_internal.h" #include "tcp_buf.h" #include "tcp_vu.h" +#include "lineread.h"

/* * The size of TCP header (including options) is given by doff (Data Offset) @@ -340,7 +343,7 @@ enum { #define WINDOW_DEFAULT 14600 /* RFC 6928 */

#define ACK_INTERVAL 10 /* ms */ -#define SYN_TIMEOUT 10 /* s */ +#define SYN_TIMEOUT_INIT 1 /* s */ #define ACK_TIMEOUT 2

As we've discussed in some places, the RFCs largely treat SYN retries the same as data retransmits. ACK_TIMEOUT controls the latter. 2s is a little odd, IIRC the RFC suggests 3, I'm not sure what Linux does by default. In any case, I'm wondering if we should use a single define for the initial value of both timeouts.

Well, as Stefano pointed out, according to Appendix A in RFC 6298, the initial value is suggested to be one second. BTW, as this is for data retransmits which is addressed by the third patch, I kept it not changed in this second patch. I agree we could use one single define for both timeouts. Just not sure how we are supposed to split the patches. Maybe we could remove the SYN_TIMEOUT_INIT in the third patch? What do you think?

...

...
#define FIN_TIMEOUT 60 #define ACT_TIMEOUT 7200 @@ -365,6 +368,10 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUEUE_MAX];

#define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller when above this */

+#define TCP_SYN_RETRIES_SYSCTL "/proc/sys/net/ipv4/tcp_syn_retries" +#define TCP_SYN_LINEAR_TIMEOUTS_SYSCTL \ + "/proc/sys/net/ipv4/tcp_syn_linear_timeouts" + /* "Extended" data (not stored in the flow table) for TCP flow migration */ static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX];

@@ -581,8 +588,13 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn) if (conn->flags & ACK_TO_TAP_DUE) { it.it_value.tv_nsec = (long)ACK_INTERVAL * 1000 * 1000; } else if (conn->flags & ACK_FROM_TAP_DUE) { - if (!(conn->events & ESTABLISHED)) - it.it_value.tv_sec = SYN_TIMEOUT; + if (!(conn->events & ESTABLISHED)) { + if (conn->retries < c->tcp.syn_linear_timeouts) + it.it_value.tv_sec = SYN_TIMEOUT_INIT; + else + it.it_value.tv_sec = SYN_TIMEOUT_INIT << + (conn->retries - c->tcp.syn_linear_timeouts); + } else it.it_value.tv_sec = ACK_TIMEOUT; } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { @@ -1961,6 +1973,7 @@ static void tcp_conn_from_sock_finish(const struct ctx *c, }

tcp_send_flag(c, conn, ACK); + conn->retries = 0;

I know Stefano said you needed to add this, but on a closer examination I don't think you do. tcp_tap_handler() calls tcp_update_seqack_from_tap() if !ESTABLISHED. That will clear retrans/retries if the ack number from the guest has advanced. That will occur when the guest SYN-ACKs our SYN, which is exactly the point at which we should clear the retries count.

I see, thanks. Will remove it in v3.

...

...
/* The client might have sent data already, which we didn't * dequeue waiting for SYN,ACK from tap -- check now. @@ -2409,8 +2422,16 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) tcp_timer_ctl(c, conn); } else if (conn->flags & ACK_FROM_TAP_DUE) { if (!(conn->events & ESTABLISHED)) { - flow_dbg(conn, "handshake timeout"); - tcp_rst(c, conn); + if (conn->retries == MIN(TCP_MAX_RETRIES, + (c->tcp.tcp_syn_retries + c->tcp.tcp_syn_retries))) {

Should this be tcp_syn_retries + linear_timeouts?

Yes, sorry, there must be a copy error. I should have double checked before sending out.

...

Also, probably safer to use a >= rather than == - if we somehow send an extra retry, we don't want to then retry forever.

...
+ flow_dbg(conn, "handshake timeout"); + tcp_rst(c, conn); + } else { + flow_dbg(conn, "SYN timeout, retry"); + tcp_send_flag(c, conn, SYN); + conn->retries++; + tcp_timer_ctl(c, conn); + } } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { flow_dbg(conn, "FIN timeout"); tcp_rst(c, conn); @@ -2766,6 +2787,62 @@ static socklen_t tcp_probe_tcp_info(void) return sl; }

+/** + * get_tcp_syn_param() - Read SYN parameters from /proc/sys + * @path: Path to the sysctl file + * @fallback: Default value if file can't be read + * + * Return: Parameter value, fallback on failure +*/ +int get_tcp_syn_param(const char *path, int fallback)

I wonder if it might be worth making a new function in util.c to read a file containing a single number.

I can do that. So it could be reused in the future.

...

...
+{ + char *line, *end; + struct lineread lr; + long value; + ssize_t len; + int fd; + + fd = open(path, O_RDONLY | O_CLOEXEC); + if (fd < 0) { + debug("Unable to open %s", path); + return fallback; + } + + lineread_init(&lr, fd); + len = lineread_get(&lr, &line); + close(fd); + + if (len < 0) { + debug("Unable to read %s", path); + return fallback; + } + + errno = 0; + value = strtol(line, &end, 10); + if (*end && *end != '\n') { + debug("Invalid format in %s", path); + return fallback; + } + if (errno || value < 0 || value > INT_MAX) { + debug("Invalid value in %s: %ld", path, value); + return fallback; + } + return (int)value;

You return an (int) here, but store the value into a uint8_t in the caller. That will cause unexpected results if the value you read is greater than 255. Somewhere you need to check for this and clamp the value. I'd suggest the function actually reading /proc return a long long (for reuseability), then clamp it in the caller.

Yeah, I noticed that. Seems tcp_syn_retries should not be higher than 127 according to Documentation/networking/ip-sysctl.rst, but tcp_syn_linear_timeouts is not limited. Anyway, I should check them. Will update in v3.

...

...
+} + +/** + * tcp_syn_params_init() - Get initial syn params for inbound connection + * @c: Execution context +*/ +void tcp_syn_params_init(struct ctx *c) +{ + c->tcp.tcp_syn_retries = get_tcp_syn_param(TCP_SYN_RETRIES_SYSCTL, 8); + c->tcp.syn_linear_timeouts = + get_tcp_syn_param(TCP_SYN_LINEAR_TIMEOUTS_SYSCTL, 1); + debug("TCP SYN parameters: retries=%d, linear_timeouts=%d", + c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts); +} + /** * tcp_init() - Get initial sequence, hash secret, initialise per-socket data * @c: Execution context @@ -2776,6 +2853,8 @@ int tcp_init(struct ctx *c) { ASSERT(!c->no_tcp);

+ tcp_syn_params_init(c); + tcp_sock_iov_init(c);

memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)); diff --git a/tcp.h b/tcp.h index 234a803..df699a4 100644 --- a/tcp.h +++ b/tcp.h @@ -65,6 +65,8 @@ struct tcp_ctx { struct fwd_ports fwd_out; struct timespec timer_run; size_t pipe_size; + uint8_t tcp_syn_retries; + uint8_t syn_linear_timeouts; };

#endif /* TCP_H */ -- 2.47.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson

-- Thanks, Yumei Huang

Stefano Brivio

12:31 p.m.

New subject: [PATCH v2 2/3] tcp: Resend SYN for inbound connections

On Mon, 13 Oct 2025 18:22:00 +0800 Yumei Huang wrote:

...

On Mon, Oct 13, 2025 at 8:21 AM David Gibson wrote:

...
On Fri, Oct 10, 2025 at 03:46:59PM +0800, Yumei Huang wrote:

...
+/** + * get_tcp_syn_param() - Read SYN parameters from /proc/sys + * @path: Path to the sysctl file + * @fallback: Default value if file can't be read + * + * Return: Parameter value, fallback on failure +*/ +int get_tcp_syn_param(const char *path, int fallback)

I wonder if it might be worth making a new function in util.c to read a file containing a single number.

I can do that. So it could be reused in the future.

If it helps: we already have write_file(), so it might be worth adding a read_file() symmetric to it. Maybe it's overkill though. -- Stefano

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[PATCH v2 0/3] Retry SYNs for inbound connections

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