L116: C++-Core: Loosen behavior of GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA

L116-core-loosen-max-pings-without-data.md

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+## L116: C++-Core: Loosen behavior of `GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA`
+
+*   Author: Yash Tibrewal (@yashykt)
+*   Approver: Mark Roth (@markdroth), Craig Tiller (@ctiller)
+*   Status: Draft
+*   Implemented in:
+*   Last updated: 2024-04-25
+*   Discussion at: https://groups.google.com/g/grpc-io/c/X_iApPGvwmo
+
+## Abstract
+
+Modify the behavior of `GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA` to throttle pings
+to a frequency of 1 minute instead of completely blocking pings when too many
+pings have been sent without data/header frames being sent.
+
+## Background
+
+[A8: Client-side Keepalive](A8-client-side-keepalive.md) and
+[A9: Server-side Connection Management](A9-server-side-conn-mgt.md) designed the
+use of HTTP2 pings as a keepalive mechanism in gRPC. In gRPC C++-Core, the
+following channel arguments implement keepalives. (Refer
+[Keepalive User Guide for gRPC Core](https://github.com/grpc/grpc/blob/master/doc/keepalive.md)
+for details.) -
+
+*   `GRPC_ARG_KEEPALIVE_TIME_MS` - Period after which a keepalive ping is sent.
+*   `GRPC_ARG_KEEPALIVE_TIMEOUT_MS` - Period after which sender of keepalive
+    pings closes transport if it does not receive an acknowledgement of the
+    ping.
+*   `GRPC_ARG_KEEPALIVE_PERMIT_WITHOUT_CALLS` - Allows keepalive pings to be
+    sent even if there are no calls in flight.
+*   `GRPC_ARG_HTTP2_MIN_RECV_PING_INTERVAL_WITHOUT_DATA_MS` - Minimum allowed
+    time between a server receiving successive ping frames without sending
+    data/header frames.
+*   `GRPC_ARG_HTTP2_MAX_PING_STRIKES` - Number of bad pings server will tolerate
+    before closing the connection.
+
+In addition to these channel arguments, the following channel arguments were
+introduced in gRPC C++-Core to play nicely with proxies that break connections
+that send too many pings.
+
+*   `GRPC_ARG_HTTP2_MIN_SENT_PING_INTERVAL_WITHOUT_DATA_MS` - Minimum time
+    sender of a ping would wait between consecutive ping frames without
+    receiving a data/header frame.
+*   `GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA` - Maximum number of pings that can
+    be sent without a data/header frame.
+
+These two additional channel arguments have historically caused a lot of pain
+and confusion among users of gRPC C++-Core and dependent languages when
+configuring keepalives. For example, long-lived streams with sparse
+communication get affected by these channel arguments and pings are blocked
+completely, defeating the purpose of keepalives. To help with this,
+[grpc/grpc#24063](https://github.com/grpc/grpc/pull/24063) deprecated
+`GRPC_ARG_HTTP2_MIN_SENT_PING_INTERVAL_WITHOUT_DATA_MS`.
+
+### Related Proposals:
+
+*   [A8: Client-side Keepalive](A8-client-side-keepalive.md)
+*   [A9: Server-side Connection Management](A9-server-side-conn-mgt.md)
+
+## Proposal
+
+Modify the behavior of `GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA` to throttle pings
+to a frequency of 1 minute instead of completely blocking pings when too many
+pings have been sent without data/header frames being sent.
+
+The default setting of `GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA` is 2, and as
+stated earlier, long-lived streams with sparse communication get blocked by this
+channel arg if not explicitly disabled by setting it to 0. By loosening the
+behavior to throttle the pings instead, these use-cases would be less impacted.
+
+### Temporary experiment protection
+
+The C++-Core experiment "max_pings_wo_data_throttle" will be used to guard this
+change in behavior.
+
+## Rationale
+
+Ideally, we would be able to deprecate `GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA`
+similar to `GRPC_ARG_HTTP2_MIN_SENT_PING_INTERVAL_WITHOUT_DATA_MS` and remove
+the pain completely. Unfortunately, this would break current users that go
+through proxies that limit how often a ping can be sent.
+
+If there's user interest and if the behavior of
+`GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA` is still considered useful, we could
+consider adding a channel argument that controls how long pings are throttled.
+
+## Implementation
+
+Implemented in [grpc/grpc#36374](https://github.com/grpc/grpc/pull/36374).