This document describes Multipath TCP (MPTCP), an extension of the Transmission Control Protocol (TCP) that allows a single TCP connection to use multiple paths simultaneously. MPTCP improves network resource usage and enhances user experience by increasing throughput and resilience to network failures. The protocol provides the same reliable bytestream service as traditional TCP but enables the use of multiple TCP flows across potentially disjoint paths. The document outlines the protocol changes required to add multipath capability to TCP, including signaling and setting up multiple paths ("subflows"), managing these subflows, reassembly of data, and termination of sessions. It also discusses the design assumptions, operation overview, and protocol details, including connection initiation, subflow setup, data transfer, and error handling. The document is published as an Experimental Protocol for the Internet community and is subject to BCP 78 and the IETF Trust's Legal Provisions. It includes a detailed table of contents and references to related documents, such as the MPTCP Architecture, Congestion Control, and Application Considerations. The document also provides information on the use of TCP options, control blocks, and finite state machines.This document describes Multipath TCP (MPTCP), an extension of the Transmission Control Protocol (TCP) that allows a single TCP connection to use multiple paths simultaneously. MPTCP improves network resource usage and enhances user experience by increasing throughput and resilience to network failures. The protocol provides the same reliable bytestream service as traditional TCP but enables the use of multiple TCP flows across potentially disjoint paths. The document outlines the protocol changes required to add multipath capability to TCP, including signaling and setting up multiple paths ("subflows"), managing these subflows, reassembly of data, and termination of sessions. It also discusses the design assumptions, operation overview, and protocol details, including connection initiation, subflow setup, data transfer, and error handling. The document is published as an Experimental Protocol for the Internet community and is subject to BCP 78 and the IETF Trust's Legal Provisions. It includes a detailed table of contents and references to related documents, such as the MPTCP Architecture, Congestion Control, and Application Considerations. The document also provides information on the use of TCP options, control blocks, and finite state machines.