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https://libwebsockets.org/repo/libwebsockets
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51 lines
1.7 KiB
Markdown
51 lines
1.7 KiB
Markdown
## Using UDP in lws
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UDP is supported in lws... the quickest way is to use the api
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`lws_create_adopt_udp()` which returns a wsi bound to the provided
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vhost, protocol, `lws_retry` struct, dns address and port.
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The wsi can be treated normally and `lws_write()` used to write on
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it.
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## Implementing UDP retries
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Retries are important in udp but there's no standardized ack method
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unlike tcp. Lws allows you to bind an `lws_retry` struct describing
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the policy to the udp wsi, but since one UDP socket may have many
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transactions in flight, the `lws_sul` and `uint16_t` to count the
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retries must live in the user's transaction object like this
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```
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...
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lws_sorted_usec_list_t sul;
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uint16_t retry;
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...
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```
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in the `LWS_CALLBACK_RAW_WRITEABLE` callback, before doing the write,
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set up the retry like this
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```
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if (lws_dll2_is_detached(&transaction->sul_write.list) &&
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lws_retry_sul_schedule_retry_wsi(wsi, &transaction->sul_write,
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transaction_retry_write_cb,
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&transaction->retry_count_write)) {
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/* we have reached the end of our concealed retries */
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lwsl_warn("%s: concealed retries done, failing\n", __func__);
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goto retry_conn;
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}
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```
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This manages the retry counter in the transaction object, guards against it wrapping,
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selects the timeout using the policy bound to the wsi, and sets the `lws_sul` in the
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transaction object to call the given callback if the sul time expires.
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In the callback, it should simply call `lws_callback_on_writable()` for the udp wsi.
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## Simulating packetloss
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You can simulate udp packetloss at tx and rx by using the Fault Injection apis
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with the well-known fault names "udp_tx_loss" and "udp_rx_loss", typically
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with the probabilistic setting, in commandline format something like
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`--fault-injection "wsi/udp_tx_loss(10%)"`
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