Lightweight C library for HTML5 websockets
lws_retry_bo_t client connection management

This struct sets the policy for delays between retries, and for how long a connection may be 'idle' before it first tries to ping / pong on it to confirm it's up, or drops the connection if still idle.

Retry rate limiting

You can define a table of ms-resolution delays indexed by which connection attempt number is ongoing, this is pointed to by .retry_ms_table with .retry_ms_table_count containing the count of table entries.

.conceal_count is the number of retries that should be allowed before informing the parent that the connection has failed. If it's greater than the number of entries in the table, the last entry is reused for the additional attempts.

.jitter_percent controls how much additional random delay is added to the actual interval to be used... this stops a lot of devices all synchronizing when they try to connect after a single trigger event and DDoS-ing the server.

The struct and apis are provided for user implementations, lws does not offer reconnection itself.

Connection validity management

Lws has a sophisticated idea of connection validity and the need to reconfirm that a connection is still operable if proof of validity has not been seen for some time. It concerns itself only with network connections rather than streams, for example, it only checks h2 network connections rather than the individual streams inside (which is consistent with h2 PING frames only working at the network stream level itself).

Connections may fail in a variety of ways, these include that no traffic at all is passing, or, eg, incoming traffic may be received but no outbound traffic is making it to the network, and vice versa. In the case that tx is not failing at any point but just isn't getting sent, endpoints can potentially kid themselves that since "they are sending" and they are seeing RX, the combination means the connection is valid. This can potentially continue for a long time if the peer is not performing keepalives.

"Connection validity" is proven when one side sends something and later receives a response that can only have been generated by the peer receiving what was just sent. This can happen for some kinds of user transactions on any stream using the connection, or by sending PING / PONG protocol packets where the PONG is only returned for a received PING.

To ensure that the generated traffic is only sent when necessary, user code can report for any stream that it has observed a transaction amounting to a proof of connection validity using an api. This resets the timer for the associated network connection before the validity is considered expired.

.secs_since_valid_ping in the retry struct sets the number of seconds since the last validity after which lws will issue a protocol-specific PING of some kind on the connection. .secs_since_valid_hangup specifies how long lws will allow the connection to go without a confirmation of validity before simply hanging up on it.


The context defaults to having a 5m valid ping interval and 5m10s hangup interval, ie, it'll send a ping at 5m idle if the protocol supports it, and if no response validating the connection arrives in another 10s, hang up the connection.

User code can set this in the context creation info and can individually set the retry policy per vhost for server connections. Client connections can set it per connection in the client creation info .retry_and_idle_policy.

Checking for h2 and ws

Check using paired minimal examples with the -v flag on one or both sides to get a small validity check period set of 3s / 10s

Also give, eg, -d1039 to see info level debug logging


$ lws-minimal-http-server-h2-long-poll -v
$ lws-minimal-http-client -l -v


$ lws-minimal-ws-server-h2 -s -v
$ lws-minimal-ws-client-ping -n --server --port 7681 -v