Re: [quicwg/base-drafts] An example ECN validation algorithm (#3320)

[Jana Iyengar <[e-mail]>]

janaiyengar commented on this pull request.

Thank you for working on this. 

> @@ -3488,6 +3489,54 @@ Even if validation fails, an endpoint MAY revalidate ECN on the same path at any
 later time in the connection.
 
 
+#### Exemplary ECN Validation Algorithm {#ecn-alg}
+
+Each time that an endpoint commences sending on a new network path, it

```suggestion
Each time an endpoint commences sending on a new network path, it
```

> @@ -3488,6 +3489,54 @@ Even if validation fails, an endpoint MAY revalidate ECN on the same path at any
 later time in the connection.
 
 
+#### Exemplary ECN Validation Algorithm {#ecn-alg}
+
+Each time that an endpoint commences sending on a new network path, it
+determines whether the path supports ECN.  If the path supports ECN, the goal is
+to use ECN.  Endpoints might also periodically reassess a path that was
+determined not support ECN.
+
+This section describes one method for testing new paths.  This algorithm is
+intended to show how a path might be tested for ECN support.  Endpoints can
+implement different methods.
+
+The path has an ECN state that is one of "testing", "unknown", "failed", or

Paths don't have ECN state. Suggestion: "For each path an endpoint sends on, it records known ECN support on that path as one of ..."

> @@ -3488,6 +3489,54 @@ Even if validation fails, an endpoint MAY revalidate ECN on the same path at any
 later time in the connection.
 
 
+#### Exemplary ECN Validation Algorithm {#ecn-alg}
+
+Each time that an endpoint commences sending on a new network path, it
+determines whether the path supports ECN.  If the path supports ECN, the goal is
+to use ECN.  Endpoints might also periodically reassess a path that was
+determined not support ECN.
+
+This section describes one method for testing new paths.  This algorithm is
+intended to show how a path might be tested for ECN support.  Endpoints can
+implement different methods.
+
+The path has an ECN state that is one of "testing", "unknown", "failed", or
+"capable".  In the "testing" and "capable" states the endpoint sends packets

```suggestion
"capable".  On paths that are recorded as "testing" or "capable", the endpoint sends packets
```

> +Each time that an endpoint commences sending on a new network path, it
+determines whether the path supports ECN.  If the path supports ECN, the goal is
+to use ECN.  Endpoints might also periodically reassess a path that was
+determined not support ECN.
+
+This section describes one method for testing new paths.  This algorithm is
+intended to show how a path might be tested for ECN support.  Endpoints can
+implement different methods.
+
+The path has an ECN state that is one of "testing", "unknown", "failed", or
+"capable".  In the "testing" and "capable" states the endpoint sends packets
+with an ECT(0) marking; otherwise, the endpoint sends unmarked packets.
+
+To start testing a path, the ECN state is set to "testing" and existing ECN
+counts are remembered as a baseline.  It should not be necessary to disregard
+the effect of packets sent prior to starting testing, though it is necessary for

It's not clear what "effect of packets" this is talking about.

> +
+This section describes one method for testing new paths.  This algorithm is
+intended to show how a path might be tested for ECN support.  Endpoints can
+implement different methods.
+
+The path has an ECN state that is one of "testing", "unknown", "failed", or
+"capable".  In the "testing" and "capable" states the endpoint sends packets
+with an ECT(0) marking; otherwise, the endpoint sends unmarked packets.
+
+To start testing a path, the ECN state is set to "testing" and existing ECN
+counts are remembered as a baseline.  It should not be necessary to disregard
+the effect of packets sent prior to starting testing, though it is necessary for
+a sender to remember what markings were used for every packet that is
+acknowledged; see {{ecn-ack}}.
+
+The testing period runs for a number of packets or round trip times as

```suggestion
The testing period runs for a number of packets or round-trip times, as
```

> +This section describes one method for testing new paths.  This algorithm is
+intended to show how a path might be tested for ECN support.  Endpoints can
+implement different methods.
+
+The path has an ECN state that is one of "testing", "unknown", "failed", or
+"capable".  In the "testing" and "capable" states the endpoint sends packets
+with an ECT(0) marking; otherwise, the endpoint sends unmarked packets.
+
+To start testing a path, the ECN state is set to "testing" and existing ECN
+counts are remembered as a baseline.  It should not be necessary to disregard
+the effect of packets sent prior to starting testing, though it is necessary for
+a sender to remember what markings were used for every packet that is
+acknowledged; see {{ecn-ack}}.
+
+The testing period runs for a number of packets or round trip times as
+determined by the endpoint.  During this time, packets sent are marked with

```suggestion
determined by the endpoint.
```

Redundant. You've already said this 2 paras above.

> +intended to show how a path might be tested for ECN support.  Endpoints can
+implement different methods.
+
+The path has an ECN state that is one of "testing", "unknown", "failed", or
+"capable".  In the "testing" and "capable" states the endpoint sends packets
+with an ECT(0) marking; otherwise, the endpoint sends unmarked packets.
+
+To start testing a path, the ECN state is set to "testing" and existing ECN
+counts are remembered as a baseline.  It should not be necessary to disregard
+the effect of packets sent prior to starting testing, though it is necessary for
+a sender to remember what markings were used for every packet that is
+acknowledged; see {{ecn-ack}}.
+
+The testing period runs for a number of packets or round trip times as
+determined by the endpoint.  During this time, packets sent are marked with
+ECT(0).  The goal is to limit the duration of the testing period, but to ensure

```suggestion
The goal is not to limit the duration of the testing period, but to ensure
```

> +implement different methods.
+
+The path has an ECN state that is one of "testing", "unknown", "failed", or
+"capable".  In the "testing" and "capable" states the endpoint sends packets
+with an ECT(0) marking; otherwise, the endpoint sends unmarked packets.
+
+To start testing a path, the ECN state is set to "testing" and existing ECN
+counts are remembered as a baseline.  It should not be necessary to disregard
+the effect of packets sent prior to starting testing, though it is necessary for
+a sender to remember what markings were used for every packet that is
+acknowledged; see {{ecn-ack}}.
+
+The testing period runs for a number of packets or round trip times as
+determined by the endpoint.  During this time, packets sent are marked with
+ECT(0).  The goal is to limit the duration of the testing period, but to ensure
+that enough marked packets are sent that it is likely that ECN counts will

```suggestion
that enough marked packets are sent for received ECN counts to
```

> +
+To start testing a path, the ECN state is set to "testing" and existing ECN
+counts are remembered as a baseline.  It should not be necessary to disregard
+the effect of packets sent prior to starting testing, though it is necessary for
+a sender to remember what markings were used for every packet that is
+acknowledged; see {{ecn-ack}}.
+
+The testing period runs for a number of packets or round trip times as
+determined by the endpoint.  During this time, packets sent are marked with
+ECT(0).  The goal is to limit the duration of the testing period, but to ensure
+that enough marked packets are sent that it is likely that ECN counts will
+provide a clear indication of how the path treats marked packets.
+
+<!-- Do we need a more concrete recommendation here?  For instance, I might say
+"Endpoints could test with packets that amount to between 1 to 2 times the
+initial congestion window over a period between 1 to 2 times the estimated RTT."

This is an example, and as such, it's useful for us to provide a specific value here. We should also expect that anyone implementing this is likely to use these exemplary values.

> +the effect of packets sent prior to starting testing, though it is necessary for
+a sender to remember what markings were used for every packet that is
+acknowledged; see {{ecn-ack}}.
+
+The testing period runs for a number of packets or round trip times as
+determined by the endpoint.  During this time, packets sent are marked with
+ECT(0).  The goal is to limit the duration of the testing period, but to ensure
+that enough marked packets are sent that it is likely that ECN counts will
+provide a clear indication of how the path treats marked packets.
+
+<!-- Do we need a more concrete recommendation here?  For instance, I might say
+"Endpoints could test with packets that amount to between 1 to 2 times the
+initial congestion window over a period between 1 to 2 times the estimated RTT."
+-->
+
+After the testing period ends, the ECN state for the path becomes "unknown".

Why does this have to go to "unknown" and not directly to "capable" or "failed"?

> +
+The testing period runs for a number of packets or round trip times as
+determined by the endpoint.  During this time, packets sent are marked with
+ECT(0).  The goal is to limit the duration of the testing period, but to ensure
+that enough marked packets are sent that it is likely that ECN counts will
+provide a clear indication of how the path treats marked packets.
+
+<!-- Do we need a more concrete recommendation here?  For instance, I might say
+"Endpoints could test with packets that amount to between 1 to 2 times the
+initial congestion window over a period between 1 to 2 times the estimated RTT."
+-->
+
+After the testing period ends, the ECN state for the path becomes "unknown".
+From the "unknown" state, successful validation of the ECN counts an ACK frame
+(see {{ecn-ack}}) causes the ECN state for the path to become "capable", unless
+no marked packet has been acknowledged.

You will be blocked, but only until the PTO timer fires. There is a question still of what to do afterwards. While it is rare, I suspect we need this fallback as a failsafe.

> +determined by the endpoint.  During this time, packets sent are marked with
+ECT(0).  The goal is to limit the duration of the testing period, but to ensure
+that enough marked packets are sent that it is likely that ECN counts will
+provide a clear indication of how the path treats marked packets.
+
+<!-- Do we need a more concrete recommendation here?  For instance, I might say
+"Endpoints could test with packets that amount to between 1 to 2 times the
+initial congestion window over a period between 1 to 2 times the estimated RTT."
+-->
+
+After the testing period ends, the ECN state for the path becomes "unknown".
+From the "unknown" state, successful validation of the ECN counts an ACK frame
+(see {{ecn-ack}}) causes the ECN state for the path to become "capable", unless
+no marked packet has been acknowledged.
+
+At any point, if validation of ECN counts fails, the ECN state becomes "failed".

```suggestion
If validation of ECN counts fails at any time, the endpoint records ECN state for the path as "failed".
```

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