diff --git a/keyserver/src/socket/tunnelbroker.js b/keyserver/src/socket/tunnelbroker.js
--- a/keyserver/src/socket/tunnelbroker.js
+++ b/keyserver/src/socket/tunnelbroker.js
@@ -2,16 +2,19 @@
 
 import WebSocket from 'ws';
 
+import { type TBKeyserverConnectionInitializationMessage } from 'lib/types/tunnelbroker-messages.js';
+
 function createTunnelbrokerWebsocket() {
   try {
     const tunnelbrokerSocket = new WebSocket('ws://localhost:51001');
     tunnelbrokerSocket.on('open', () => {
       // TODO: Replace keyserver details with actual details
-      const message = {
+      const message: TBKeyserverConnectionInitializationMessage = {
         type: 'sessionRequest',
         accessToken: 'foobar',
-        deviceId: 'foo',
+        deviceID: 'foo',
         deviceType: 'keyserver',
+        userID: 'alice',
       };
       console.log(
         'Sending message to tunnelbroker: ' + JSON.stringify(message),
diff --git a/lib/types/tunnelbroker-messages.js b/lib/types/tunnelbroker-messages.js
new file mode 100644
--- /dev/null
+++ b/lib/types/tunnelbroker-messages.js
@@ -0,0 +1,30 @@
+// @flow
+
+type TBSharedConnectionInitializationMessage = {
+  +type: 'sessionRequest',
+  +deviceID: string,
+  +accessToken: string,
+  +deviceAppVersion?: string,
+  +userID: string,
+};
+
+export type TBKeyserverConnectionInitializationMessage = {
+  ...TBSharedConnectionInitializationMessage,
+  +deviceType: 'keyserver',
+};
+
+export type TBClientConnectionInitializationMessage = {
+  ...TBSharedConnectionInitializationMessage,
+  +deviceType: 'web' | 'mobile',
+};
+
+export type TBNotifyClientConnectionInitializationMessage = {
+  ...TBClientConnectionInitializationMessage,
+  +notifyToken: string,
+  +notifyPlatform: 'apns' | 'fcm' | 'web' | 'wns',
+};
+
+export type TBConnectionInitializationMessage =
+  | TBKeyserverConnectionInitializationMessage
+  | TBClientConnectionInitializationMessage
+  | TBNotifyClientConnectionInitializationMessage;
diff --git a/services/commtest/tests/tunnelbroker_integration_test.rs b/services/commtest/tests/tunnelbroker_integration_test.rs
--- a/services/commtest/tests/tunnelbroker_integration_test.rs
+++ b/services/commtest/tests/tunnelbroker_integration_test.rs
@@ -20,6 +20,7 @@
       "type": "sessionRequest",
       "accessToken": "xkdeifjsld",
       "deviceId": "foo",
+      "userId": "alice",
       "deviceType": "keyserver"
     }"#;
 
diff --git a/services/tunnelbroker/src/websockets/session.rs b/services/tunnelbroker/src/websockets/session.rs
--- a/services/tunnelbroker/src/websockets/session.rs
+++ b/services/tunnelbroker/src/websockets/session.rs
@@ -19,7 +19,7 @@
     message: &str,
   ) -> Result<(), serde_json::Error> {
     match serde_json::from_str::<Messages>(message)? {
-      Messages::SessionRequest(session_info) => {
+      Messages::ConnectionInitializationMessage(session_info) => {
         ACTIVE_CONNECTIONS.insert(session_info.device_id, self.tx.clone());
       }
       _ => {
diff --git a/shared/tunnelbroker_messages/src/messages/mod.rs b/shared/tunnelbroker_messages/src/messages/mod.rs
--- a/shared/tunnelbroker_messages/src/messages/mod.rs
+++ b/shared/tunnelbroker_messages/src/messages/mod.rs
@@ -11,5 +11,5 @@
 #[serde(untagged)]
 pub enum Messages {
   RefreshKeysRequest(RefreshKeyRequest),
-  SessionRequest(SessionRequest),
+  ConnectionInitializationMessage(ConnectionInitializationMessage),
 }
diff --git a/shared/tunnelbroker_messages/src/messages/session.rs b/shared/tunnelbroker_messages/src/messages/session.rs
--- a/shared/tunnelbroker_messages/src/messages/session.rs
+++ b/shared/tunnelbroker_messages/src/messages/session.rs
@@ -3,7 +3,7 @@
 use serde::{Deserialize, Serialize};
 
 /// The workflow when estabilishing a tunnelbroker connection:
-///   - Client sends SessionRequest
+///   - Client sends ConnectionInitializationMessage
 ///   - Tunnelbroker validates access_token with identity service
 ///   - Tunnelbroker emits an AMQP message declaring that it has opened a new
 ///     connection with a given device, so that the respective tunnelbroker
@@ -31,8 +31,9 @@
 /// service before continuing with the request.
 #[derive(Serialize, Deserialize)]
 #[serde(tag = "type", rename_all = "camelCase")]
-pub struct SessionRequest {
+pub struct ConnectionInitializationMessage {
   pub device_id: String,
+  pub user_id: String,
   pub access_token: String,
   pub notify_token: Option<String>,
   pub device_type: DeviceTypes,
@@ -41,7 +42,7 @@
 }
 
 #[derive(Serialize, Deserialize)]
-pub struct SessionResponse {
+pub struct ConnectionInitializationResponse {
   pub session_id: String,
 }
 
@@ -59,7 +60,8 @@
     }"#;
 
     let request =
-      serde_json::from_str::<SessionRequest>(example_payload).unwrap();
+      serde_json::from_str::<ConnectionInitializationMessage>(example_payload)
+        .unwrap();
     assert_eq!(request.device_id, "foo");
     assert_eq!(request.access_token, "xkdeifjsld");
     assert_eq!(request.device_os, None);