Message Session Relay Protocol

Description

MSRP is an application-layer protocol designed for session-oriented text messaging and file transfer within IP multimedia subsystem sessions. Instead of treating each instant message as a separate signaling request, MSRP establishes a persistent session between two endpoints and exchanges messages as chunks over a reliable transport. In 3GPP, it is used where chat, content sharing, or file transfer must be tightly coupled to a media session, such as IMS messaging, rich communication services, and companion features for voice or video calls.

Architecturally, MSRP sits alongside SIP and SDP. SIP creates the session, and SDP negotiates MSRP parameters such as the media type, transport, port, connection address, direction, maximum message size, and MSRP paths. Once negotiated, the endpoints open a TCP connection, optionally secured by TLS or IPsec, and then use MSRP methods such as SEND, REPORT, and failure reporting to exchange MIME parts. The protocol assumes a reliable ordered transport and focuses on session semantics, message segmentation, delivery confirmation, and failure handling. In some deployments, WebSocket or other transport adaptations may be used, but the core MSRP semantics remain the same.

A typical MSRP operation begins with an SDP offer and answer that define the session endpoints and their capabilities. The sender then transmits a SEND request containing a message or a fragment of a message. Headers such as To-Path, From-Path, Message-ID, Byte-Range, Continuation, and Failure-Report support matching, segmentation, and status reporting. If a message is larger than the negotiated maximum size, it is divided into chunks with byte ranges, and the Continuation flag indicates that more chunks belong to the same logical message. The receiver responds with REPORT messages, which can confirm individual chunks or the whole message. This chunked model is important for file transfer because it allows large payloads to be sent without requiring a single enormous message.

Key components include the MSRP endpoint, transport connection, message headers, MIME payload, and optional relay or proxy functions. For file transfer, MSRP can carry MIME parts with file metadata and data chunks, allowing progress reporting and interruption handling. Failure reports are important because they allow the sender to learn whether the transport or peer session has failed, enabling recovery or user notification. Because MSRP uses a persistent transport connection, it can reduce overhead for many short messages compared with repeatedly creating separate request and response transactions. It also supports mid-session data exchange, which is useful when users want to continue a conversation while a voice or video call remains active.

In a 3GPP network, MSRP matters because it gives operators a standardized way to add real-time chat and content sharing to voice, video, and messaging services. It can be used during a call to share text, images, or files without leaving the session, and it can support rich communication services that require reliable mid-session data exchange. Its session-oriented design makes it more suitable than simple one-shot messaging protocols for continuous conversations, while its use of TCP provides ordered, reliable delivery. Security is handled through transport security and network-layer protection, and interoperability is achieved through profile requirements in 3GPP specifications. For engineers, MSRP is best understood as the messaging and file-transfer plane that complements SIP signaling and media transport in IMS-based services.

Purpose & Motivation

MSRP exists because SIP signaling is well suited for session establishment, but it is not the most efficient or natural way to carry continuous mid-session chat and file transfer. Earlier approaches either used separate one-shot messages, relied on proprietary protocols, or attempted to embed large payloads in signaling, which created scaling, fragmentation, and interoperability problems. Operators and service providers needed a standardized session-oriented protocol that could deliver text, MIME parts, and files reliably while remaining tied to an established communication session.

The protocol addresses the limitations of simple messaging by introducing persistent sessions, chunking, delivery reports, and failure reporting. These mechanisms make it suitable for rich communication services, where users expect chat, presence, and content sharing to work alongside voice or video. MSRP also reduces the overhead of repeatedly establishing transport connections for every message, which is important in mobile networks where latency, battery use, and network signaling load matter.

Historically, the motivation came from the need to build interoperable IMS services across carriers and devices. By defining a common application protocol and 3GPP profiles, MSRP allows endpoints from different vendors to exchange chat and file data within the same session, supporting carrier-grade messaging features while preserving security and session management requirements.