diff --git a/design/protocol.md b/design/protocol.md
index ca37998..48e4c9b 100644
--- a/design/protocol.md
+++ b/design/protocol.md
@@ -129,8 +129,9 @@ on the same machine. Both protocols are supported through METADAPT.
 ## Message and Transaction Demarcation Protocol (METADAPT)
 The Message and Transaction Demarcation Protocol is used to break one or more
 reliable data streams into transactions, which are broken down further into
-messages. A message, as well as its associated metadata (length, transaction,
-method, etc.) together is referred to as METADAPT Message Block (MMB).
+messages. The representation of a message (or a part thereof) on the protocol,
+including its associated metadata (length, transaction, method, etc.) is
+referred to as METADAPT Message Block (MMB).
 
 For transports that offer multiple multiplexed data streams that can be created
 and destroyed on-demand (such as QUIC) each stream is used as a transaction. If
@@ -145,8 +146,12 @@ METADAPT-A requires a transport which offers a single full-duplex data stream
 that persists for the duration of the connection. All transactions are
 multiplexed onto this single stream. Each MMB contains a 12-octet long header,
 with the transaction ID, then the method, and then the payload size (in octets).
-The transaction ID is encoded as an I64, and the method and payload size are
-both encoded as U16s. The remainder of the message is the payload. Since each
+The transaction ID is encoded as an I64, the method is encoded as a U16 and the
+and payload size is encoded as a U64. Only the 63 least significant bits of the
+payload size describe the actual size, the most significant bit controlling
+chunking. See the section on chunking for more information.
+
+The remainder of the message is the payload. Since each
 MMB is self-describing, they are sent sequentially with no gaps in-between them.
 
 Transactions "open" when the first message with a given transaction ID is sent.
@@ -162,13 +167,25 @@ used up, the connection must fail. Don't worry about this though, because the
 sun will have expanded to swallow earth by then. Your connection will not last
 that long.
 
+#### Message Chunking
+
+The most significant bit of the payload size field of an MMB is called the Chunk
+Control Bit (CCB). If the CCB of a given MMB is zero, the represented message is
+interpreted as being self-contained and the data is processed immediately. If
+the CCB is one, the message is interpreted as being chunked, with the data of
+the current MMB being the first chunk. The data of further MMBs sent along the
+transaction will be appended to the message until an MMB is read with a zero
+CCB, in which case the MMB will be the last chunk and any more MMBs will be
+interpreted as normal.
+
 ### METADAPT-B
 METADAPT-B requires a transport which offers multiple multiplexed full-duplex
 data streams per connection that can be created and destroyed on-demand. Each
 data stream is used as an individual transaction. Each MMB contains a 4-octet
-long header with the method and then the payload size (in octets) both encoded
-as U16s. The remainder of the message is the payload. Since each MMB is
-self-describing, they are sent sequentially with no gaps in-between them.
+long header with the method and then the payload size (in octets) encoded as a
+U16 and U64 respectively. The remainder of the message is the payload. Since
+each MMB is self-describing, they are sent sequentially with no gaps in-between
+them.
 
 The ID of any transaction will reflect the ID of its corresponding stream. The
 lifetime of the transaction is tied to the lifetime of the stream, that is to