Re: [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.

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/* 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.

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+ 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.

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+{ + 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