fix: revert WebRTC message size increase

packages/transport-webrtc/package.json

@@ -72,6 +72,7 @@
     "protons-runtime": "^5.4.0",
     "race-signal": "^1.0.2",
     "react-native-webrtc": "^118.0.7",
+    "uint8-varint": "^2.0.4",
     "uint8arraylist": "^2.4.8",
     "uint8arrays": "^5.1.0"
   },

packages/transport-webrtc/src/stream.ts

@@ -6,6 +6,7 @@ import pDefer from 'p-defer'
 import { pEvent, TimeoutError } from 'p-event'
 import pTimeout from 'p-timeout'
 import { raceSignal } from 'race-signal'
+import { encodingLength } from 'uint8-varint'
 import { Uint8ArrayList } from 'uint8arraylist'
 import { Message } from './pb/message.js'
 import type { DataChannelOptions } from './index.js'
@@ -35,22 +36,40 @@ export const MAX_BUFFERED_AMOUNT = 2 * 1024 * 1024
 export const BUFFERED_AMOUNT_LOW_TIMEOUT = 30 * 1000
 
 /**
- * protobuf field definition overhead + length encoding prefix length
+ * Max message size that can be sent to the DataChannel. In browsers this is
+ * 256KiB but go-libp2p and rust-libp2p only support 16KiB at the time of
+ * writing.
+ *
+ * @see https://blog.mozilla.org/webrtc/large-data-channel-messages/
+ * @see https://issues.webrtc.org/issues/40644524
  */
-export const PROTOBUF_OVERHEAD = 7
+export const MAX_MESSAGE_SIZE = 16 * 1024
 
 /**
- * Length of varint, in bytes
+ * max protobuf overhead:
+ *
+ * ```
+ * [message-length][flag-field-id+type][flag-field-length][flag-field][message-field-id+type][message-field-length][message-field]
+ * ```
  */
-export const VARINT_LENGTH = 2
+function calculateProtobufOverhead (maxMessageSize = MAX_MESSAGE_SIZE): number {
+  // these have a fixed size
+  const messageLength = encodingLength(maxMessageSize - encodingLength(maxMessageSize))
+  const flagField = 1 + encodingLength(Object.keys(Message.Flag).length - 1) // id+type/value
+  const messageFieldIdType = 1 // id+type
+  const available = maxMessageSize - messageLength - flagField - messageFieldIdType
+
+  // let message-length/message-data fill the rest of the message
+  const messageFieldLengthLength = encodingLength(available)
+
+  return messageLength + flagField + messageFieldIdType + messageFieldLengthLength
+}
 
 /**
- * Max message size that can be sent to the DataChannel
- *
- * @see https://blog.mozilla.org/webrtc/large-data-channel-messages/
- * @see https://issues.webrtc.org/issues/40644524
+ * The protobuf message overhead includes the maximum amount of all bytes in the
+ * protobuf that aren't message field bytes
  */
-export const MAX_MESSAGE_SIZE = 256 * 1024
+export const PROTOBUF_OVERHEAD = calculateProtobufOverhead()
 
 /**
  * When closing streams we send a FIN then wait for the remote to
@@ -132,7 +151,7 @@ export class WebRTCStream extends AbstractStream {
     this.incomingData = pushable<Uint8Array>()
     this.bufferedAmountLowEventTimeout = init.bufferedAmountLowEventTimeout ?? BUFFERED_AMOUNT_LOW_TIMEOUT
     this.maxBufferedAmount = init.maxBufferedAmount ?? MAX_BUFFERED_AMOUNT
-    this.maxMessageSize = (init.maxMessageSize ?? MAX_MESSAGE_SIZE) - PROTOBUF_OVERHEAD - VARINT_LENGTH
+    this.maxMessageSize = (init.maxMessageSize ?? MAX_MESSAGE_SIZE) - PROTOBUF_OVERHEAD
     this.receiveFinAck = pDefer()
     this.finAckTimeout = init.closeTimeout ?? FIN_ACK_TIMEOUT
     this.openTimeout = init.openTimeout ?? OPEN_TIMEOUT

packages/transport-webrtc/test/stream.spec.ts

@@ -26,8 +26,12 @@ describe('Max message size', () => {
       }
     })
 
-    // Make sure that the data that ought to be sent will result in a message with exactly MAX_MESSAGE_SIZE
-    const messageLengthEncoded = lengthPrefixed.encode.single(Message.encode({ message: data }))
+    // Make sure that a message with all fields will be exactly MAX_MESSAGE_SIZE
+    const messageLengthEncoded = lengthPrefixed.encode.single(Message.encode({
+      flag: Message.Flag.STOP_SENDING,
+      message: data
+    }))
+
     expect(messageLengthEncoded.length).eq(MAX_MESSAGE_SIZE)
     const webrtcStream = createStream({
       channel,