[PATCH v1 net] TCP_USER_TIMEOUT and tcp_keepalive should conform to RFC5482

Change from version 0: Rationale behind the change:

The man page for tcp(7) states

when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will
override keepalive to  determine  when to close a connection due to keepalive
failure.

This is ambigious at best. user expectation is most likely that the connection
will be reset after TCP_USER_TIMEOUT milliseconds of inactivity.

The code however waits for the keepalive to kick-in (default 2hrs) and than
after one failure resets the conenction. 

What is the rationale for that ? The same effect can be obtained by simply
changing the value of tcp_keep_alive_probes.

Since the TCP_USER_TIMEOUT option was added based on RFC 5482 we need to follow 
the RFC. Which states

4.2 TCP keep-Alives:
   Some TCP implementations, such as those in BSD systems, use a
   different abort policy for TCP keep-alives than for user data.  Thus,
   the TCP keep-alive mechanism might abort a connection that would
   otherwise have survived the transient period without connectivity.
   Therefore, if a connection that enables keep-alives is also using the
   TCP User Timeout Option, then the keep-alive timer MUST be set to a
   value larger than that of the adopted USER TIMEOUT.

This patch enforces the MUST and also dis-associates user timeout from keep
alive.  A man page patch will be submitted separately.

Signed-off-by: Rao Shoaib <redacted>
---
 net/ipv4/tcp.c       | 10 ++++++++--
 net/ipv4/tcp_timer.c |  9 +--------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 71ce33d..f2af44d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -2628,7 +2628,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 		break;
 
 	case TCP_KEEPIDLE:
-		if (val < 1 || val > MAX_TCP_KEEPIDLE)
+		/* Per RFC5482 keepalive_time must be > user_timeout */
+		if (val < 1 || val > MAX_TCP_KEEPIDLE ||
+		    ((val * HZ) <= icsk->icsk_user_timeout))
 			err = -EINVAL;
 		else {
 			tp->keepalive_time = val * HZ;

@@ -2724,8 +2726,12 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 	case TCP_USER_TIMEOUT:
 		/* Cap the max time in ms TCP will retry or probe the window
 		 * before giving up and aborting (ETIMEDOUT) a connection.
+		 * Per RFC5482 TCP user timeout must be < keepalive_time.
+		 * If the default value changes later -- all bets are off.
 		 */
-		if (val < 0)
+		if (val < 0 || (tp->keepalive_time &&
+				tp->keepalive_time <= msecs_to_jiffies(val)) ||
+		   net->ipv4.sysctl_tcp_keepalive_time <= msecs_to_jiffies(val))
 			err = -EINVAL;
 		else
 			icsk->icsk_user_timeout = msecs_to_jiffies(val);

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index c0feeee..d39fe60 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c

@@ -664,14 +664,7 @@ static void tcp_keepalive_timer (unsigned long data)
 	elapsed = keepalive_time_elapsed(tp);
 
 	if (elapsed >= keepalive_time_when(tp)) {
-		/* If the TCP_USER_TIMEOUT option is enabled, use that
-		 * to determine when to timeout instead.
-		 */
-		if ((icsk->icsk_user_timeout != 0 &&
-		    elapsed >= icsk->icsk_user_timeout &&
-		    icsk->icsk_probes_out > 0) ||
-		    (icsk->icsk_user_timeout == 0 &&
-		    icsk->icsk_probes_out >= keepalive_probes(tp))) {
+		if (icsk->icsk_probes_out >= keepalive_probes(tp)) {
 			tcp_send_active_reset(sk, GFP_ATOMIC);
 			tcp_write_err(sk);
 			goto out;

-- 
2.7.4

On Mon, Aug 7, 2017 at 11:16 AM, Rao Shoaib [off-list ref] wrote:

Change from version 0: Rationale behind the change:

The man page for tcp(7) states

when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will
override keepalive to  determine  when to close a connection due to keepalive
failure.

This is ambigious at best. user expectation is most likely that the connection
will be reset after TCP_USER_TIMEOUT milliseconds of inactivity.

ccing the original author Jerry Chu who can tell more.

[try to recover from long lost memory]

On Tue, Aug 8, 2017 at 10:25 AM, Yuchung Cheng [off-list ref] wrote:

On Mon, Aug 7, 2017 at 11:16 AM, Rao Shoaib [off-list ref] wrote:

quoted

Change from version 0: Rationale behind the change:

The man page for tcp(7) states

when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will
override keepalive to  determine  when to close a connection due to keepalive
failure.

This is ambigious at best. user expectation is most likely that the connection
will be reset after TCP_USER_TIMEOUT milliseconds of inactivity.

ccing the original author Jerry Chu who can tell more.

There was a reason for the above otherwise I wouldn't have explicitly
spelled it out in
my commit msg. But unfortunately it was seven years ago and I can't
remember why.
It could range from micro-optimization (saving a syscall() because
this facility was
used by servers handling millions of Android clients) to something more critical
but I can't remember.

quoted

The code however waits for the keepalive to kick-in (default 2hrs) and than
after one failure resets the conenction.

What is the rationale for that ? The same effect can be obtained by simply
changing the value of tcp_keep_alive_probes.

Since the TCP_USER_TIMEOUT option was added based on RFC 5482 we need to follow
the RFC. Which states

Well the patch has little to do with RFC5482 other than borrowing the name, and
also conveniently providing a mechanism for RFC5482 apps to program the local
timeout value. As far as I knew back when I worked on the patch, RFC5482 was
under little use (told directly by Lars).

Your proposed change may not be unreasonable but my fear is it may
cause breakage
on apps that depend on "TCP_USER_TIMEOUT will overtake keepalive to determine
when to close a connection due to keepalive failure". What is your
case for "RFC5482
compliance" after all? I know the TCP_USER_TIMEOUT option has been very popular
among apps since its inception.

quoted

4.2 TCP keep-Alives:
   Some TCP implementations, such as those in BSD systems, use a
   different abort policy for TCP keep-alives than for user data.  Thus,
   the TCP keep-alive mechanism might abort a connection that would
   otherwise have survived the transient period without connectivity.
   Therefore, if a connection that enables keep-alives is also using the
   TCP User Timeout Option, then the keep-alive timer MUST be set to a
   value larger than that of the adopted USER TIMEOUT.

This patch enforces the MUST and also dis-associates user timeout from keep
alive.  A man page patch will be submitted separately.

Signed-off-by: Rao Shoaib <redacted>
---
 net/ipv4/tcp.c       | 10 ++++++++--
 net/ipv4/tcp_timer.c |  9 +--------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 71ce33d..f2af44d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -2628,7 +2628,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
                break;

        case TCP_KEEPIDLE:
-               if (val < 1 || val > MAX_TCP_KEEPIDLE)
+               /* Per RFC5482 keepalive_time must be > user_timeout */
+               if (val < 1 || val > MAX_TCP_KEEPIDLE ||
+                   ((val * HZ) <= icsk->icsk_user_timeout))
                        err = -EINVAL;
                else {
                        tp->keepalive_time = val * HZ;

@@ -2724,8 +2726,12 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
        case TCP_USER_TIMEOUT:
                /* Cap the max time in ms TCP will retry or probe the window
                 * before giving up and aborting (ETIMEDOUT) a connection.
+                * Per RFC5482 TCP user timeout must be < keepalive_time.
+                * If the default value changes later -- all bets are off.
                 */
-               if (val < 0)
+               if (val < 0 || (tp->keepalive_time &&
+                               tp->keepalive_time <= msecs_to_jiffies(val)) ||
+                  net->ipv4.sysctl_tcp_keepalive_time <= msecs_to_jiffies(val))
                        err = -EINVAL;
                else
                        icsk->icsk_user_timeout = msecs_to_jiffies(val);

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index c0feeee..d39fe60 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c

@@ -664,14 +664,7 @@ static void tcp_keepalive_timer (unsigned long data)
        elapsed = keepalive_time_elapsed(tp);

        if (elapsed >= keepalive_time_when(tp)) {
-               /* If the TCP_USER_TIMEOUT option is enabled, use that
-                * to determine when to timeout instead.
-                */
-               if ((icsk->icsk_user_timeout != 0 &&
-                   elapsed >= icsk->icsk_user_timeout &&
-                   icsk->icsk_probes_out > 0) ||
-                   (icsk->icsk_user_timeout == 0 &&
-                   icsk->icsk_probes_out >= keepalive_probes(tp))) {
+               if (icsk->icsk_probes_out >= keepalive_probes(tp)) {
                        tcp_send_active_reset(sk, GFP_ATOMIC);
                        tcp_write_err(sk);
                        goto out;
--

2.7.4

On Wed, Aug 9, 2017 at 4:52 PM, Jerry Chu [off-list ref] wrote:

[try to recover from long lost memory]

On Tue, Aug 8, 2017 at 10:25 AM, Yuchung Cheng [off-list ref] wrote:

quoted

On Mon, Aug 7, 2017 at 11:16 AM, Rao Shoaib [off-list ref] wrote:

quoted

Change from version 0: Rationale behind the change:

The man page for tcp(7) states

when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will
override keepalive to  determine  when to close a connection due to keepalive
failure.

This is ambigious at best. user expectation is most likely that the connection
will be reset after TCP_USER_TIMEOUT milliseconds of inactivity.

ccing the original author Jerry Chu who can tell more.

There was a reason for the above otherwise I wouldn't have explicitly
spelled it out in
my commit msg. But unfortunately it was seven years ago and I can't
remember why.
It could range from micro-optimization (saving a syscall() because
this facility was
used by servers handling millions of Android clients) to something more critical
but I can't remember.

The issue is that the man page is ambiguous and does not conform to
any standard.
Whether  RFC 5482 is in little use or not that was cited as the basis
of this change and
I want to change the behavior to conform to it as users are confused.

I doubt that saving a syscall is of any benefit when the connection
has been idle for 2hrs. If anything the user expects the keep alive
probes to start after TCP_USER_TIMEOUT of inactivity. In which case
keep alive should be adjusted.

quoted

quoted

The code however waits for the keepalive to kick-in (default 2hrs) and than
after one failure resets the conenction.

What is the rationale for that ? The same effect can be obtained by simply
changing the value of tcp_keep_alive_probes.

Since the TCP_USER_TIMEOUT option was added based on RFC 5482 we need to follow
the RFC. Which states

Well the patch has little to do with RFC5482 other than borrowing the name, and
also conveniently providing a mechanism for RFC5482 apps to program the local
timeout value. As far as I knew back when I worked on the patch, RFC5482 was
under little use (told directly by Lars).

Your proposed change may not be unreasonable but my fear is it may
cause breakage
on apps that depend on "TCP_USER_TIMEOUT will overtake keepalive to determine
when to close a connection due to keepalive failure". What is your
case for "RFC5482
compliance" after all? I know the TCP_USER_TIMEOUT option has been very popular
among apps since its inception.

The only use of TCP_USER_TIMEOUT has been for flushing unacknowledged
data (evident from all the fixes). That behavior is not being touched.

Making Linux conform to standards and behavior that is logical seems
like a good enough reason. Mixing keep alive and TCP_USER_TIMEOUT does
not make any sense. I doubt very much if this change will break
anything but if it does than we need to see why that is needed and
implement a proper fix and document it.

Shoaib

quoted

quoted

4.2 TCP keep-Alives:
   Some TCP implementations, such as those in BSD systems, use a
   different abort policy for TCP keep-alives than for user data.  Thus,
   the TCP keep-alive mechanism might abort a connection that would
   otherwise have survived the transient period without connectivity.
   Therefore, if a connection that enables keep-alives is also using the
   TCP User Timeout Option, then the keep-alive timer MUST be set to a
   value larger than that of the adopted USER TIMEOUT.

This patch enforces the MUST and also dis-associates user timeout from keep
alive.  A man page patch will be submitted separately.

Signed-off-by: Rao Shoaib <redacted>
---
 net/ipv4/tcp.c       | 10 ++++++++--
 net/ipv4/tcp_timer.c |  9 +--------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 71ce33d..f2af44d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -2628,7 +2628,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
                break;

        case TCP_KEEPIDLE:
-               if (val < 1 || val > MAX_TCP_KEEPIDLE)
+               /* Per RFC5482 keepalive_time must be > user_timeout */
+               if (val < 1 || val > MAX_TCP_KEEPIDLE ||
+                   ((val * HZ) <= icsk->icsk_user_timeout))
                        err = -EINVAL;
                else {
                        tp->keepalive_time = val * HZ;

@@ -2724,8 +2726,12 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
        case TCP_USER_TIMEOUT:
                /* Cap the max time in ms TCP will retry or probe the window
                 * before giving up and aborting (ETIMEDOUT) a connection.
+                * Per RFC5482 TCP user timeout must be < keepalive_time.
+                * If the default value changes later -- all bets are off.
                 */
-               if (val < 0)
+               if (val < 0 || (tp->keepalive_time &&
+                               tp->keepalive_time <= msecs_to_jiffies(val)) ||
+                  net->ipv4.sysctl_tcp_keepalive_time <= msecs_to_jiffies(val))
                        err = -EINVAL;
                else
                        icsk->icsk_user_timeout = msecs_to_jiffies(val);

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index c0feeee..d39fe60 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c

@@ -664,14 +664,7 @@ static void tcp_keepalive_timer (unsigned long data)
        elapsed = keepalive_time_elapsed(tp);

        if (elapsed >= keepalive_time_when(tp)) {
-               /* If the TCP_USER_TIMEOUT option is enabled, use that
-                * to determine when to timeout instead.
-                */
-               if ((icsk->icsk_user_timeout != 0 &&
-                   elapsed >= icsk->icsk_user_timeout &&
-                   icsk->icsk_probes_out > 0) ||
-                   (icsk->icsk_user_timeout == 0 &&
-                   icsk->icsk_probes_out >= keepalive_probes(tp))) {
+               if (icsk->icsk_probes_out >= keepalive_probes(tp)) {
                        tcp_send_active_reset(sk, GFP_ATOMIC);
                        tcp_write_err(sk);
                        goto out;
--

2.7.4

-- 
JS

From: Joe Smith <redacted>
Date: Wed, 9 Aug 2017 17:20:32 -0700

Making Linux conform to standards and behavior that is logical seems
like a good enough reason.

That's an awesome attitude to have when we're implementing something
new and don't have the facility already.

But when we have something already the only important consideration is
not breaking existing apps which rely on that behavior.

That is much, much, more important than standards compliance.

If users are confused, just fix the documentation.

On 08/09/2017 05:20 PM, Joe Smith wrote:

On Wed, Aug 9, 2017 at 4:52 PM, Jerry Chu [off-list ref] wrote:

quoted

[try to recover from long lost memory]

On Tue, Aug 8, 2017 at 10:25 AM, Yuchung Cheng [off-list ref] wrote:

quoted

On Mon, Aug 7, 2017 at 11:16 AM, Rao Shoaib [off-list ref] wrote:

quoted

Change from version 0: Rationale behind the change:

The man page for tcp(7) states

when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will
override keepalive to  determine  when to close a connection due to keepalive
failure.

This is ambigious at best. user expectation is most likely that the connection
will be reset after TCP_USER_TIMEOUT milliseconds of inactivity.

ccing the original author Jerry Chu who can tell more.

There was a reason for the above otherwise I wouldn't have explicitly
spelled it out in
my commit msg. But unfortunately it was seven years ago and I can't
remember why.
It could range from micro-optimization (saving a syscall() because
this facility was
used by servers handling millions of Android clients) to something more critical
but I can't remember.

The issue is that the man page is ambiguous and does not conform to
any standard.
Whether  RFC 5482 is in little use or not that was cited as the basis
of this change and
I want to change the behavior to conform to it as users are confused.

I doubt that saving a syscall is of any benefit when the connection
has been idle for 2hrs. If anything the user expects the keep alive
probes to start after TCP_USER_TIMEOUT of inactivity. In which case
keep alive should be adjusted.

quoted

quoted

quoted

The code however waits for the keepalive to kick-in (default 2hrs) and than
after one failure resets the conenction.

What is the rationale for that ? The same effect can be obtained by simply
changing the value of tcp_keep_alive_probes.

Since the TCP_USER_TIMEOUT option was added based on RFC 5482 we need to follow
the RFC. Which states

Well the patch has little to do with RFC5482 other than borrowing the name, and
also conveniently providing a mechanism for RFC5482 apps to program the local
timeout value. As far as I knew back when I worked on the patch, RFC5482 was
under little use (told directly by Lars).

Your proposed change may not be unreasonable but my fear is it may
cause breakage
on apps that depend on "TCP_USER_TIMEOUT will overtake keepalive to determine
when to close a connection due to keepalive failure". What is your
case for "RFC5482
compliance" after all? I know the TCP_USER_TIMEOUT option has been very popular
among apps since its inception.

The only use of TCP_USER_TIMEOUT has been for flushing unacknowledged
data (evident from all the fixes). That behavior is not being touched.

Making Linux conform to standards and behavior that is logical seems
like a good enough reason. Mixing keep alive and TCP_USER_TIMEOUT does
not make any sense. I doubt very much if this change will break
anything but if it does than we need to see why that is needed and
implement a proper fix and document it.

The behavior for the main use case has been previously changed as part of bug fixes. This is a very low risk change and makes the code logical and clean.

Shoaib

On 08/09/2017 05:30 PM, David Miller wrote:

From: Joe Smith <redacted>
Date: Wed, 9 Aug 2017 17:20:32 -0700

quoted

Making Linux conform to standards and behavior that is logical seems
like a good enough reason.

That's an awesome attitude to have when we're implementing something
new and don't have the facility already.

But when we have something already the only important consideration is
not breaking existing apps which rely on that behavior.

That is much, much, more important than standards compliance.

If users are confused, just fix the documentation.

David,

If it was just confusion than sure fixing the documentation is fine. What if the logic is incorrect, does not conform to the standard that is says it is implementing and easy to fix with little or no risk of breakage.

The proposed patch changes a feature that no one uses. It also imposes the relation ship between keepalive and timeout values that is required by the RFC and make sense.

You are the final authority, if you say we should just fix the documentation than that is fine.

Shoaib

From: Rao Shoaib <redacted>
Date: Wed, 9 Aug 2017 17:47:57 -0700

On 08/09/2017 05:30 PM, David Miller wrote:

quoted

From: Joe Smith <redacted>
Date: Wed, 9 Aug 2017 17:20:32 -0700

quoted

Making Linux conform to standards and behavior that is logical seems
like a good enough reason.

That's an awesome attitude to have when we're implementing something
new and don't have the facility already.

But when we have something already the only important consideration is
not breaking existing apps which rely on that behavior.

That is much, much, more important than standards compliance.

If users are confused, just fix the documentation.

David,

If it was just confusion than sure fixing the documentation is
fine. What if the logic is incorrect, does not conform to the standard
that is says it is implementing and easy to fix with little or no risk
of breakage.

The proposed patch changes a feature that no one uses. It also imposes
the relation ship between keepalive and timeout values that is
required by the RFC and make sense.

You are the final authority, if you say we should just fix the
documentation than that is fine.

I want to hear more about what hkchu and ycheng have to say about
this.

On Wed, Aug 9, 2017 at 5:47 PM, Rao Shoaib [off-list ref] wrote:

On 08/09/2017 05:30 PM, David Miller wrote:

quoted

From: Joe Smith <redacted>
Date: Wed, 9 Aug 2017 17:20:32 -0700

quoted

Making Linux conform to standards and behavior that is logical seems
like a good enough reason.

That's an awesome attitude to have when we're implementing something
new and don't have the facility already.

But when we have something already the only important consideration is
not breaking existing apps which rely on that behavior.

That is much, much, more important than standards compliance.

If users are confused, just fix the documentation.

David,

If it was just confusion than sure fixing the documentation is fine. What if
the logic is incorrect, does not conform to the standard that is says it is

Not sure what part of logic is "incorrect" when it was a homegrown Linux API
with no need to conform with any "standard"? Note that the new API was invented
7 years ago not out of need for RFC5482. In fact I initially call the option
TCP_FAILFAST and did not even know the existence of RFC5482 until someone
around the same time proposed a UTO option specifically for RFC5482 and I
thought the two can be combined. (This is roughly the memory I can
recollect so far.)

So you see my focus back then was to devise a "failfast" option whereas RFC5482
was meant for a "failslow" case. I think that explains why I let the
option override
keepalive so a TCP connection can "fail fast" while RFC5482 4.2 tries to prevent
keepalive failure ahead of UTO, favoring "fail slow".

If we start from a clean slate then perhaps one can argue the semantic
either way
but we do not have a clean slate. For that I still slightly favor not
changing the code
because the risk of breakage is definitely non-zero and the issue you're having
seem to be only related to documentation.

Jerry

implementing and easy to fix with little or no risk of breakage.

The proposed patch changes a feature that no one uses. It also imposes the
relation ship between keepalive and timeout values that is required by the
RFC and make sense.

You are the final authority, if you say we should just fix the documentation
than that is fine.

Shoaib

On Wed, Aug 9, 2017 at 8:32 PM, Jerry Chu [off-list ref] wrote:

On Wed, Aug 9, 2017 at 5:47 PM, Rao Shoaib [off-list ref] wrote:

quoted

On 08/09/2017 05:30 PM, David Miller wrote:

quoted

From: Joe Smith <redacted>
Date: Wed, 9 Aug 2017 17:20:32 -0700

quoted

Making Linux conform to standards and behavior that is logical seems
like a good enough reason.

That's an awesome attitude to have when we're implementing something
new and don't have the facility already.

But when we have something already the only important consideration is
not breaking existing apps which rely on that behavior.

That is much, much, more important than standards compliance.

If users are confused, just fix the documentation.

David,

If it was just confusion than sure fixing the documentation is fine. What if
the logic is incorrect, does not conform to the standard that is says it is

Not sure what part of logic is "incorrect" when it was a homegrown Linux API
with no need to conform with any "standard"? Note that the new API was invented
7 years ago not out of need for RFC5482. In fact I initially call the option
TCP_FAILFAST and did not even know the existence of RFC5482 until someone
around the same time proposed a UTO option specifically for RFC5482 and I
thought the two can be combined. (This is roughly the memory I can
recollect so far.)

So you see my focus back then was to devise a "failfast" option whereas RFC5482
was meant for a "failslow" case. I think that explains why I let the
option override
keepalive so a TCP connection can "fail fast" while RFC5482 4.2 tries to prevent
keepalive failure ahead of UTO, favoring "fail slow".

If we start from a clean slate then perhaps one can argue the semantic
either way
but we do not have a clean slate. For that I still slightly favor not
changing the code
because the risk of breakage is definitely non-zero and the issue you're having
seem to be only related to documentation.

One more thing - the proposed patch compares TCP_KEEPIDLE against
TCP_USER_TIMEOUT. But I don't think TCP_KEEPIDLE is what the
"keep-alive
timer" referred to in RFC5482. Linux keepalive implementation seems to use #
of retries (TCP_KEEPCNT) rather than time duration (keep-alive time) to
determine when to quit. If that is the case then your proposed change is not
fully "compliant" either and the best is probably just don't change.

Jerry

quoted

implementing and easy to fix with little or no risk of breakage.

The proposed patch changes a feature that no one uses. It also imposes the
relation ship between keepalive and timeout values that is required by the
RFC and make sense.

You are the final authority, if you say we should just fix the documentation
than that is fine.

Shoaib

On 08/09/2017 09:59 PM, Jerry Chu wrote:

On Wed, Aug 9, 2017 at 8:32 PM, Jerry Chu [off-list ref] wrote:

quoted

On Wed, Aug 9, 2017 at 5:47 PM, Rao Shoaib [off-list ref] wrote:

quoted

On 08/09/2017 05:30 PM, David Miller wrote:

quoted

From: Joe Smith <redacted>
Date: Wed, 9 Aug 2017 17:20:32 -0700

quoted

Making Linux conform to standards and behavior that is logical seems
like a good enough reason.

That's an awesome attitude to have when we're implementing something
new and don't have the facility already.

But when we have something already the only important consideration is
not breaking existing apps which rely on that behavior.

That is much, much, more important than standards compliance.

If users are confused, just fix the documentation.

David,

If it was just confusion than sure fixing the documentation is fine. What if
the logic is incorrect, does not conform to the standard that is says it is

Not sure what part of logic is "incorrect" when it was a homegrown Linux API
with no need to conform with any "standard"? Note that the new API was invented
7 years ago not out of need for RFC5482. In fact I initially call the option
TCP_FAILFAST and did not even know the existence of RFC5482 until someone
around the same time proposed a UTO option specifically for RFC5482 and I
thought the two can be combined. (This is roughly the memory I can
recollect so far.)

So you see my focus back then was to devise a "failfast" option whereas RFC5482
was meant for a "failslow" case. I think that explains why I let the
option override
keepalive so a TCP connection can "fail fast" while RFC5482 4.2 tries to prevent
keepalive failure ahead of UTO, favoring "fail slow".

If we start from a clean slate then perhaps one can argue the semantic
either way
but we do not have a clean slate. For that I still slightly favor not
changing the code
because the risk of breakage is definitely non-zero and the issue you're having
seem to be only related to documentation.

We all make mistakes and over look things, that seems to be the case here. If this was so important than I am sure there was a use case. None has been presented. Without a use case I do not understand why we have to live with broken logic when we have a chance to fix it and make the code better.

If this change does break something (very very unlikely) we will understand the use case and provide an appropriate solution.

One more thing - the proposed patch compares TCP_KEEPIDLE against
TCP_USER_TIMEOUT. But I don't think TCP_KEEPIDLE is what the
"keep-alive
timer" referred to in RFC5482. Linux keepalive implementation seems to use #
of retries (TCP_KEEPCNT) rather than time duration (keep-alive time) to
determine when to quit. If that is the case then your proposed change is not
fully "compliant" either and the best is probably just don't change.

Did you look at the patch and what it changes ?

Take a look at the TCP_KEEPIDLE socket option and see what it does or just look at the man page of tcp(7)

TCP_KEEPIDLE (since Linux 2.4) The time (in seconds) the connection needs to remain idle before TCP starts sending keepalive probes, if the socket option SO_KEEPALIVE has been set on this socket. This option should not be used in code intended to be portable.

Shoaib.

quoted

Jerry

quoted

implementing and easy to fix with little or no risk of breakage.

The proposed patch changes a feature that no one uses. It also imposes the
relation ship between keepalive and timeout values that is required by the
RFC and make sense.

You are the final authority, if you say we should just fix the documentation
than that is fine.

Shoaib

On Thu, Aug 10, 2017 at 2:05 PM, Rao Shoaib [off-list ref] wrote:

On 08/09/2017 09:59 PM, Jerry Chu wrote:

quoted

On Wed, Aug 9, 2017 at 8:32 PM, Jerry Chu [off-list ref] wrote:

quoted

On Wed, Aug 9, 2017 at 5:47 PM, Rao Shoaib [off-list ref] wrote:

quoted

On 08/09/2017 05:30 PM, David Miller wrote:

quoted

From: Joe Smith <redacted>
Date: Wed, 9 Aug 2017 17:20:32 -0700

quoted

Making Linux conform to standards and behavior that is logical seems
like a good enough reason.

That's an awesome attitude to have when we're implementing something
new and don't have the facility already.

But when we have something already the only important consideration is
not breaking existing apps which rely on that behavior.

That is much, much, more important than standards compliance.

If users are confused, just fix the documentation.

David,

If it was just confusion than sure fixing the documentation is fine.
What if
the logic is incorrect, does not conform to the standard that is says it
is

Not sure what part of logic is "incorrect" when it was a homegrown Linux
API
with no need to conform with any "standard"? Note that the new API was
invented
7 years ago not out of need for RFC5482. In fact I initially call the
option
TCP_FAILFAST and did not even know the existence of RFC5482 until someone
around the same time proposed a UTO option specifically for RFC5482 and I
thought the two can be combined. (This is roughly the memory I can
recollect so far.)

So you see my focus back then was to devise a "failfast" option whereas
RFC5482
was meant for a "failslow" case. I think that explains why I let the
option override
keepalive so a TCP connection can "fail fast" while RFC5482 4.2 tries to
prevent
keepalive failure ahead of UTO, favoring "fail slow".

If we start from a clean slate then perhaps one can argue the semantic
either way
but we do not have a clean slate. For that I still slightly favor not
changing the code
because the risk of breakage is definitely non-zero and the issue you're
having
seem to be only related to documentation.

We all make mistakes and over look things, that seems to be the case here.

First of all, I do not understand what "mistake" has been committed? Like I
said, the API was not invented for RFC5482 specifically. It was really meant for
a "fail fast" case that was requested of me fir our internal use case
for a product
that back then had only 100 million users (but has mostly like grown to many
billions now).

If this was so important than I am sure there was a use case. None has been
presented. Without a use case I do not understand why we have to live with

I don't understand this one either. If you want a use case, you can
ask politely,
no need to flame or fume. Also why would I waste my energy on something
no one would use, then even more energy arguing now for maintaining its
semantic?

This API was requested of me by our Android group and has since been
deployed on all our Android servers. I initially pushed back and asked them to
just program a short KEEPALIVE time for all the Android connections but they
can't use it because it would wake up clients too often, unnecessarily
waste a lot
of battery power.

Its uses inside Google have proliferated since. Most recently (a few months ago)
I got an email from our Nest division saying they hit a bug related to
TCP_USER_TIMEOUT and even pointed me to
https://bugzilla.redhat.com/show_bug.cgi?id=1189241 (but i haven't got the time
to look into it and in the meantime they found a workaround)

So what's your use case then that really requires changing the implementation?

broken logic when we have a chance to fix it and make the code better.

If this change does break something (very very unlikely) we will understand
the use case and provide an appropriate solution.

quoted

One more thing - the proposed patch compares TCP_KEEPIDLE against
TCP_USER_TIMEOUT. But I don't think TCP_KEEPIDLE is what the
"keep-alive
timer" referred to in RFC5482. Linux keepalive implementation seems to use
#
of retries (TCP_KEEPCNT) rather than time duration (keep-alive time) to
determine when to quit. If that is the case then your proposed change is
not
fully "compliant" either and the best is probably just don't change.

Did you look at the patch and what it changes ?

Take a look at the TCP_KEEPIDLE socket option and see what it does or just
look at the man page of tcp(7)

TCP_KEEPIDLE (since Linux 2.4) The time (in seconds) the connection needs to
remain idle before TCP starts sending keepalive probes, if the socket option
SO_KEEPALIVE has been set on this socket. This option should not be used in
code intended to be portable.

Yes precisely my point above - the intent of section 4.2 in RFC5482, if i guess
correctly, is to avoid the case in some implementations where keep-alive code
uses a different timer than the regular retrans timer where TCP_USER_TIMEOUT
logic is allied, hence not subject to TCP_USER_TIMEOUT. E.g., if the
app programs
TCP_USER_TIMEOUT to be one hour, but a keepalive timeout to be 10 minutes,
RFC5482 does not want a connection to be aborted after 10 minutes due to
keepalive probe failure. This can be readily translated in some OSes if the
keepalive API is time/duration based. E.g., one can use the same
TCP_USER_TIMEOUT value to program the keepalive API.

But Linux's keepalive API has a TCP_KEEPIDLE period plus TCP_KEEPCNT of
retries plus TCP_KEEPINTVL. So depending on TCP_KEEPCNT and
TCP_KEEPINTVL, the total duration for keepalive may be >> TCP_KEEPIDLE
hence my comment above.

Again my main concern is backward compatibility. Our existing use cases most
likely have TCP_USER_TIMEOUT value < TCP_KEEPIDLE value so the risk
is mainly in removing the icsk_user_timeout check inside tcp_keepalive_timer().

Jerry

Shoaib.

quoted

quoted

Jerry

quoted

implementing and easy to fix with little or no risk of breakage.

The proposed patch changes a feature that no one uses. It also imposes
the
relation ship between keepalive and timeout values that is required by
the
RFC and make sense.

You are the final authority, if you say we should just fix the
documentation
than that is fine.

Shoaib

On Mon, 2017-08-07 at 11:16 -0700, Rao Shoaib wrote:

When TCP_USER_TIMEOUT socket option is attempted, maybe keepalive option
was not used.

Yet your new tests assume it is engaged.

It might break some usages.