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proposals/2388-edu-as-pdu--receipt.md

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+# MSC2388 - Read-Receipt PDU type
+
+### Background
+
+Receipts are a feature in Matrix intended for clients to acknowledge a message. The only type of
+receipt at present is the m.read read-receipt for when the user has seen a message. This feature
+is specified as an EDU such that the reliability of their transmission is not guaranteed and there
+is no record of them. They were designed this way because an order of magnitude more receipts are
+expected by this system than other PDU messages.
+
+### Problem
+
+In practice, implementations treat read-receipt EDU's no different than other PDU messages. Upon
+reception they must be persisted for effective client synchronization. Implementations also employ
+additional complexity to transmit prior EDU's to other servers long after they were received.
+These implementation choices were made to provide a consistent, expected user experience, where
+the same receipts render the same for everyone. That consistency is beneficial to the social
+atmosphere.
+
+### Solution
+
+We specify the Event type m.receipt. Within the content of this event, an m.relates_to with a
+rel_type of m.read provides and replaces the functionality of the existing receipt EDU.