Add explainer for congestion control

This rescues the proposal for congestion control that was removed
from the PR that became the keyframe request indication interface.

Partial fix for #90

explainer-congestion-control-api.md

@@ -0,0 +1,153 @@
+# Congestion Control API
+
+This document describes how the congestion control API is expected to work.
+
+The CC API can be applied multiple places in the WebRTC model; the immediate
+application is in the Encoded Transform API.
+
+
+The role of a congestion control API is to inform upstream entities about
+reasonable restrictions on how much data they can send downstream. The
+expectation from the client should be that as long as they stay within the
+indicated restriction, the chance of data loss from buffer overflows should be
+reasonably low. The given limits are not a guarantee - either that no packets
+will be lost, or that packets will be lost if more than the indicated amount is
+sent.
+
+## Representing congestion control status
+
+The following structure represents a congestion control status:
+```
+interface BandwidthInfo {
+    readonly attribute double allocatedBitrate;  // bits per second
+    readonly attribute double availableBitrate;
+    readonly attribute boolean writable;
+    readonly attribute long maxMessageSize; // bytes
+};
+```
+The meaning of the fields is as follows:
+
+* allocatedBitrate: The recommended amount of data for the upstream entity to
+  send. By staying below this bitrate, it is likely that packet loss will be
+  low, and that the bandwidth is fairly shared with other users.
+* availableBitrate: The total available bitrate estimate for the
+  downstream link. Exceeding this bitrate will have a high probability of packet
+  loss.
+* writable: Whether or not a single write of a single message of up to
+  maxMessageSize will succeed. If it is false, downstream buffers are known to
+  be too full to guarantee success at the time that the information is given.
+* maxMessageSize: The size (in bytes) of the largest message that can safely be
+  written to the downstream interface.
+
+For the bitrates, an unconstrained status is indicated by a value of positive
+infinity.
+
+
+## Receiving information about downstream congestion control
+
+A downstream interface is expected to expose the following mixin:
+```
+interface mixin CongestionControlledSink {
+  readonly attribute BandwidthInfo bandwidthInfo;
+}
+```
+It is reasonable to read this attribute whenever a send operation has completed,
+or just before a send operation is attempted.
+
+If the bandwidthInfo.writable is false, the write operation SHOULD be aborted.
+
+## Sending information upstream about congestion control state
+An upstream interface is expected to expose the following mixin:
+```
+interface mixin CongestionControlledSource {
+  undefined sendBandwidthInfo(BandwidthInfo bandwidthInfo);
+}
+```
+This can be called whenever the consumer of the upstream interface deems that
+there has been significant change to the congestion control state it wishes the
+upstream source to conform to.
+
+Examples of significant changes are:
+* The value of "writable" has changed
+* The value of "allocatedBitrate" or "availableBitrate" has decreased
+* The value of "allocatedBitrate" or "availableBitrate" has increased
+  significantly
+
+"Significantly" varies by application.
+
+## Code examples
+
+These examples all assume that RTCRtpScriptTransformer is extended with both
+mixins above.
+
+### Transform that doubles message sizes if there's room enough
+This might, for example, be a RED type redundancy data appender.
+
+```
+function relayBandwidth(transformer) {
+  const newBandwidthInfo = transformer.bandwidthInfo;
+  newBandwidthInfo.allocatedBitrate =
+        newBandwidthInfo.allocatedBitrate / 2;
+  transformer.sendBandwidthinfo(newBandwidthInfo);
+}
+
+onrtctransform = function(transformerEvent) {
+  transformer = transformerEvent.transformer;
+  const transform = new TransformStream({
+     async transform(encodedFrame, controller) {
+        if (encodedFrame.data.byteLength < 
+            transformer.bandwidthInfo.maxMessageSize / 2) {
+           doDataSizeDoubling(encodedFrame.data);
+        }
+        if (transformer.bandwidthInfo.writable) {
+          controller.enqueue(encodedFrame);
+        }
+        // Tell upstream about the new half bitrate
+        relayBandwidth(transformer);
+     }
+  });
+  relayBandwidth(transformer);
+  transformer.sendBandwidthInfo(newbandwidthInfo);
+  transformer.readable.pipeThrough(transform)
+    .pipeTo(transformer.writable);
+}
+```
+
+### Transform that allocates half the available bandwith to "important"
+
+```
+function relayBandwidth(transformer, important) {
+  if (important) {
+    const newBandwidthInfo = transformer.bandwidthInfo;
+    newBandwidthInfo.allocatedBitrate = newBandwidthInfo.availableBitrate / 2;
+    transformer.sendBandwidthinfo(newBandwidthInfo);
+  } else {
+    // Do a fair division of everyone else's bandwidth
+  }
+}
+
+function isImportant(transformerEvent) {
+  // decide if it's important; if so return true.
+  return false;
+}
+
+onrtctransform = function(transformerEvent) {
+  transformer = transformerEvent.transformer;
+  const transform = new TransformStream({
+     important: isImportant(transformerEvent),
+     async transform(encodedFrame, controller) {
+        if (transformer.bandwidthInfo.writable) {
+          controller.enqueue(encodedFrame);
+        }
+        // Tell upstream about the new half bitrate
+        relayBandwidth(transformer, important);
+     }
+  });
+  relayBandwidth(transformer, transform.important);
+  transformer.sendBandwidthInfo(newbandwidthInfo);
+  transformer.readable.pipeThrough(transform)
+    .pipeTo(transformer.writable);
+}
+```
+
+