diff --git a/deps.ts b/deps.ts
index e8c9551..774464a 100644
--- a/deps.ts
+++ b/deps.ts
@@ -1,5 +1,6 @@
 export * from "https://deno.land/x/willow_utils@0.2.1/mod.ts";
 
+export { FIFO } from "https://deno.land/x/fifo@v0.2.2/mod.ts";
 export { deferred } from "https://deno.land/std@0.202.0/async/deferred.ts";
 export { concat } from "https://deno.land/std@0.202.0/bytes/concat.ts";
 export { equals as equalsBytes } from "https://deno.land/std@0.202.0/bytes/equals.ts";
diff --git a/src/wgps/ready_transport.test.ts b/src/wgps/ready_transport.test.ts
new file mode 100644
index 0000000..9bea678
--- /dev/null
+++ b/src/wgps/ready_transport.test.ts
@@ -0,0 +1,132 @@
+import { assertEquals } from "https://deno.land/std@0.202.0/assert/mod.ts";
+import { delay } from "https://deno.land/std@0.202.0/async/mod.ts";
+import { ReadyTransport } from "./ready_transport.ts";
+import { transportPairInMemory } from "./transports/in_memory.ts";
+import { concat } from "../../deps.ts";
+
+Deno.test("Ready transport receives max payload ", async () => {
+  // Happy path
+  {
+    const [alfie, betty] = transportPairInMemory();
+
+    const readyTransport = new ReadyTransport({
+      transport: alfie,
+      challengeLength: 4,
+    });
+
+    let received = new Uint8Array();
+
+    (async () => {
+      for await (const bytes of readyTransport) {
+        received = concat(received, bytes);
+      }
+    })();
+
+    await betty.send(new Uint8Array([8]));
+    await betty.send(new Uint8Array([1, 2, 3, 4]));
+    await betty.send(new Uint8Array([7, 7, 7, 7]));
+
+    const maxPayloadSize = await readyTransport.maximumPayloadSize;
+    const receivedCommitment = await readyTransport.receivedCommitment;
+
+    assertEquals(maxPayloadSize, BigInt(256));
+    assertEquals(receivedCommitment, new Uint8Array([1, 2, 3, 4]));
+
+    await delay(0);
+
+    assertEquals(received, new Uint8Array([7, 7, 7, 7]));
+  }
+
+  // All at once.
+  {
+    const [alfie, betty] = transportPairInMemory();
+
+    const readyTransport = new ReadyTransport({
+      transport: alfie,
+      challengeLength: 4,
+    });
+
+    let received = new Uint8Array();
+
+    (async () => {
+      for await (const bytes of readyTransport) {
+        received = concat(received, bytes);
+      }
+    })();
+
+    await betty.send(new Uint8Array([8, 1, 2, 3, 4, 7, 7, 7, 7]));
+
+    const maxPayloadSize = await readyTransport.maximumPayloadSize;
+    const receivedCommitment = await readyTransport.receivedCommitment;
+
+    assertEquals(maxPayloadSize, BigInt(256));
+    assertEquals(receivedCommitment, new Uint8Array([1, 2, 3, 4]));
+
+    await delay(0);
+
+    assertEquals(received, new Uint8Array([7, 7, 7, 7]));
+  }
+
+  // Partial commitment.
+  {
+    const [alfie, betty] = transportPairInMemory();
+
+    const readyTransport = new ReadyTransport({
+      transport: alfie,
+      challengeLength: 4,
+    });
+
+    let received = new Uint8Array();
+
+    (async () => {
+      for await (const bytes of readyTransport) {
+        received = concat(received, bytes);
+      }
+    })();
+
+    await betty.send(new Uint8Array([8, 1, 2]));
+    await betty.send(new Uint8Array([3, 4, 7, 7, 7, 7]));
+
+    const maxPayloadSize = await readyTransport.maximumPayloadSize;
+    const receivedCommitment = await readyTransport.receivedCommitment;
+
+    assertEquals(maxPayloadSize, BigInt(256));
+    assertEquals(receivedCommitment, new Uint8Array([1, 2, 3, 4]));
+
+    await delay(0);
+
+    assertEquals(received, new Uint8Array([7, 7, 7, 7]));
+  }
+
+  // Even more partial commitment.
+  {
+    const [alfie, betty] = transportPairInMemory();
+
+    const readyTransport = new ReadyTransport({
+      transport: alfie,
+      challengeLength: 4,
+    });
+
+    let received = new Uint8Array();
+
+    (async () => {
+      for await (const bytes of readyTransport) {
+        received = concat(received, bytes);
+      }
+    })();
+
+    await betty.send(new Uint8Array([8, 1, 2]));
+    await betty.send(new Uint8Array([3]));
+    await betty.send(new Uint8Array([4, 7, 7, 7, 7]));
+
+    const maxPayloadSize = await readyTransport.maximumPayloadSize;
+    const receivedCommitment = await readyTransport.receivedCommitment;
+
+    assertEquals(maxPayloadSize, BigInt(256));
+    assertEquals(receivedCommitment, new Uint8Array([1, 2, 3, 4]));
+
+    await delay(0);
+
+    assertEquals(received, new Uint8Array([7, 7, 7, 7]));
+  }
+});
diff --git a/src/wgps/ready_transport.ts b/src/wgps/ready_transport.ts
new file mode 100644
index 0000000..971c4a7
--- /dev/null
+++ b/src/wgps/ready_transport.ts
@@ -0,0 +1,81 @@
+import { concat, deferred } from "../../deps.ts";
+import { SyncRole, Transport } from "./types.ts";
+
+/** A transport which only emits encoded messages, following the initial max payload size and commitment.
+ *
+ * _Doesn't_ send our own max payload size and commitment.
+ */
+export class ReadyTransport implements Transport {
+  private transport: Transport;
+  private challengeLength: number;
+
+  role: SyncRole;
+
+  /** The maximum payload size derived from the first byte sent over the transport. */
+  maximumPayloadSize = deferred<bigint>();
+  /** The received commitment sent after the first byte over the transport */
+  receivedCommitment = deferred<Uint8Array>();
+
+  constructor(opts: {
+    transport: Transport;
+    challengeLength: 1 | 2 | 4 | 8;
+  }) {
+    this.role = opts.transport.role;
+    this.transport = opts.transport;
+    this.challengeLength = opts.challengeLength;
+  }
+
+  send(bytes: Uint8Array): Promise<void> {
+    return this.transport.send(bytes);
+  }
+
+  private commitmentAcc: Uint8Array = new Uint8Array();
+
+  async *[Symbol.asyncIterator]() {
+    for await (const bytes of this.transport) {
+      if (
+        this.maximumPayloadSize.state === "fulfilled" &&
+        this.receivedCommitment.state === "fulfilled"
+      ) {
+        yield bytes;
+      }
+
+      if (this.maximumPayloadSize.state === "pending") {
+        const view = new DataView(bytes.buffer);
+
+        const power = view.getUint8(0);
+
+        this.maximumPayloadSize.resolve(BigInt(2) ** BigInt(power));
+
+        const rest = bytes.slice(1);
+
+        if (rest.byteLength < this.challengeLength) {
+          this.commitmentAcc = rest;
+        } else if (rest.byteLength === this.challengeLength) {
+          this.receivedCommitment.resolve(rest);
+        } else {
+          this.receivedCommitment.resolve(rest.slice(0, this.challengeLength));
+          yield rest.slice(this.challengeLength);
+        }
+
+        continue;
+      }
+
+      if (this.receivedCommitment.state === "pending") {
+        const combined = concat(this.commitmentAcc, bytes);
+
+        if (combined.byteLength === this.challengeLength) {
+          this.receivedCommitment.resolve(combined);
+        } else if (combined.byteLength < this.challengeLength) {
+          this.commitmentAcc = combined;
+        } else {
+          this.receivedCommitment.resolve(
+            combined.slice(0, this.challengeLength),
+          );
+
+          yield combined.slice(this.challengeLength);
+        }
+      }
+    }
+  }
+}
diff --git a/src/wgps/transports/in_memory.ts b/src/wgps/transports/in_memory.ts
new file mode 100644
index 0000000..c693954
--- /dev/null
+++ b/src/wgps/transports/in_memory.ts
@@ -0,0 +1,44 @@
+import { FIFO } from "../../../deps.ts";
+import { IS_ALFIE, IS_BETTY, SyncRole, Transport } from "../types.ts";
+
+export class TransportInMemory implements Transport {
+  private incoming: FIFO<Uint8Array>;
+  private outgoing: FIFO<Uint8Array>;
+
+  role: SyncRole;
+
+  constructor(
+    syncRole: SyncRole,
+    incoming: FIFO<Uint8Array>,
+    outgoing: FIFO<Uint8Array>,
+  ) {
+    this.role = syncRole;
+    this.incoming = incoming;
+    this.outgoing = outgoing;
+  }
+
+  send(bytes: Uint8Array): Promise<void> {
+    this.outgoing.push(bytes);
+
+    return Promise.resolve();
+  }
+
+  async *[Symbol.asyncIterator]() {
+    for await (const bytes of this.incoming) {
+      yield bytes;
+    }
+  }
+}
+
+export function transportPairInMemory(): [
+  TransportInMemory,
+  TransportInMemory,
+] {
+  const alfie = new FIFO<Uint8Array>();
+  const betty = new FIFO<Uint8Array>();
+
+  const alfieTransport = new TransportInMemory(IS_ALFIE, alfie, betty);
+  const bettyTransport = new TransportInMemory(IS_BETTY, betty, alfie);
+
+  return [alfieTransport, bettyTransport];
+}
diff --git a/src/wgps/types.ts b/src/wgps/types.ts
new file mode 100644
index 0000000..d6c7552
--- /dev/null
+++ b/src/wgps/types.ts
@@ -0,0 +1,17 @@
+/** The peer which initiated the synchronisation session. */
+export const IS_ALFIE = Symbol("alfie");
+/** The peer which did not initiate the synchronisation session. */
+export const IS_BETTY = Symbol("betty");
+
+/** we refer to the peer that initiated the synchronisation session as Alfie, and the other peer as Betty. */
+export type SyncRole = typeof IS_ALFIE | typeof IS_BETTY;
+
+/** A transport for receiving and sending data to with another peer */
+export interface Transport {
+  /** Whether this transport comes from the initiating party (Alfie), or not (Betty). */
+  role: SyncRole;
+  /** Send bytes to the other peer using this transport. */
+  send(bytes: Uint8Array): Promise<void>;
+  /** An async iterator of bytes received from the other peer via this transport. */
+  [Symbol.asyncIterator](): AsyncIterator<Uint8Array>;
+}