]> UCLouvain & NGI Zero Core

matttbe@kernel.org

Web and Internet Transport TCP Maintenance and Minor Extensions mptcp This document proposes an extension to improve Multipath TCP based on operational experience by allowing Multipath TCP to use DSS mappings that are longer than 64 KBytes. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the TCP Maintenance and Minor Extensions Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction From a performance viewpoint, TCP stacks are optimised to leverage large segments and use TCP Segment Offload / Generic Receive Offload (TSO/GRO). The DSS option defined in Multipath TCP allows to map a series of bytes from the bytestream on a specific subflow. Unfortunately, the length of this mapping is encoded in a 16-bit field. Since each Multipath TCP segment must include a DSS mapping before being sent to the network interface, this restricts the size of the segments that Multipath TCP can use. In particular in IPv6, it is impossible for Multipath TCP to leverage IPv6 jumbograms in contrast to regular TCP. This document proposes a modification of the DSS option to support longer mappings.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Extending the DSS option The extension proposed in this document is pretty simple. Given that the DSS Checksum is rarely used in practice, we propose to reuse the space reserved for this checksum in the DSS option to support 32-bit data-level mappings. This enables Multipath TCP servers to send segments that are longer than 64 KBytes. This extension is negotiated using the TBD flag in the MP_CAPABLE option during the handshake.

Negotiation of the DSS option with longer mappings [flags (TBD is set)] <- MP_CAPABLE [B's key, flags (TBD is set)] ACK + MP_CAPABLE (+ data) -> [A's key, B's key, flags, (data-level details)] ]]>

The DSS option defined in reserves 16 bits for the Checksum. However, operational experience indicates that this checksum is almost never used by Multipath TCP deployments. It was designed to detect middlebox interference caused notably by Application Level Gateways that modify TCP payloads . Given the widespread adoption of TLS, such ALGs are rarely used by applications using Multipath TCP .

The DSS option in RFC8684

This document proposes to use a 32-bit Data-Level Length to support large TCP segments. The new DSS option is shown in . The other fields of this option and the procedures defined in are unchanged.

The new DSS option

defines the MP_CAPABLE option as shown in . This option contains several flags, A-H. Flags A, B, C, and H are specified in . This document uses Flag TBD to indicate in a SYN that the initiator of a connection requests the utilization of 32-bit Data-Level Length. If this Flag is set in a SYN, Flag A must also obviously be set to 0 to indicate that the Checksum is not required on this connection. If both Flags A and TBD are set in a SYN, the receiver MUST not continue the MPTCP connection, and SHOULD fallback to TCP. A server that receives a SYN with the TBD Flag set can reply with:

• a SYN+ACK with the TBD Flag set to 1 to confirm that it accepts to use 32-bit Data-Level Length
• a SYN+ACK with the TBD Flag set to 0 to indicate that it prefers to use 16-bit Data-Level Length

Even when the TBD Flag is set to 1, the MP_CAPABLE options continue to use a 16-bit Data-Level Length like before, to allow fallback if the receiver doesn't support a 32-bit Data-Level Length.

The MP_CAPABLE option 4) | | | +---------------------------------------------------------------+ | Option Receiver's Key (64 bits) | | (if option Length > 12) | | | +-------------------------------+-------------------------------+ | Data-Level Length (16 bits) | Checksum (16 bits, optional) | +-------------------------------+-------------------------------+ ]]>

Security Considerations This document does not change the security considerations defined in .

IANA Considerations This document requests the IANA to reserve flag TBD of the MP_CAPABLE option as defined in this document. It also proposes to change the format of the DSS option. This document suggests using the D flag of the MP_CAPABLE option.

References Normative References TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and thus improve user experience through higher throughput and improved resilience to network failure. Multipath TCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional TCP to support multipath operation. The protocol offers the same type of service to applications as TCP (i.e., a reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths. This document specifies v1 of Multipath TCP, obsoleting v0 as specified in RFC 6824, through clarifications and modifications primarily driven by deployment experience. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document describes the IPv6 Jumbo Payload option, which provides the means of specifying such large payload lengths. It also describes the changes needed to TCP and UDP to make use of jumbograms. [STANDARDS-TRACK] This document discusses both use cases and operational experience with Multipath TCP (MPTCP) in real networks. It lists several prominent use cases where Multipath TCP has been considered and is being used. It also gives insight to some heuristics and decisions that have helped to realize these use cases and suggests possible improvements. arXiv

Acknowledgments This project is funded through NGI Zero Core, a fund established by NLnet with financial support from the European Commission's Next Generation Internet program.