diff --git a/API-Protocol.md b/API-Protocol.md
index 9361489..d2b6801 100644
--- a/API-Protocol.md
+++ b/API-Protocol.md
@@ -36,6 +36,7 @@ The client submits the verification code it received via voice or SMS to the ser
 1. `basic_auth` are the authorization credentials the client would like to create.  These are in the form of `Base64({number}:{password})`, where `number` is the client's verified PSTN number and `password` is a randomly generated 16 byte string.
 1. `signalingKey` is a randomly generated 32 byte AES key and a 20 byte HMAC-SHA1 MAC key, concatenated together and Base64 encoded.
 1. `supportsSms` indicates whether a client supports SMS as a transport.
+1. `registrationId` is a 14 bit integer that's randomly generated at client install time. This will be used for clients to detect whether an app has reinstalled and lost their session state.
 
 **Returns**: