| Internet-Draft | Multipath TCP with longer DSS mappings | July 2025 |
| Baerts | Expires 8 January 2026 | [Page] |
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.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://ipnetworkinglab.github.io/draft-mptcp-dss/draft-baerts-tcpm-mptcpdss.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-baerts-tcpm-mptcpdss/.¶
Discussion of this document takes place on the TCP Maintenance and Minor Extensions Working Group mailing list (mailto:tcpm@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tcpm/. Subscribe at https://www.ietf.org/mailman/listinfo/tcpm/.¶
Source for this draft and an issue tracker can be found at https://github.com/IPNetworkingLab/draft-mptcp-dss.¶
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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 [RFC2675] in contrast to regular TCP. This document proposes a modification of the DSS option to support longer mappings.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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.¶
Host A Host B
------ ------
MP_CAPABLE ->
[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 [RFC8684] 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 [RFC8041]. Given the widespread adoption of TLS, such ALGs are rarely used by applications using Multipath TCP [MPTCP-longitudinal].¶
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+----------------------+
| Kind | Length |Subtype| (reserved) |F|m|M|a|A|
+---------------+---------------+-------+----------------------+
| Data ACK (4 or 8 octets, depending on flags) |
+--------------------------------------------------------------+
| Data Sequence Number (4 or 8 octets, depending on flags) |
+--------------------------------------------------------------+
| Subflow Sequence Number (4 octets) |
+-------------------------------+------------------------------+
| Data-Level Length (2 octets) | Checksum (2 octets) |
+-------------------------------+------------------------------+
This document proposes to use a 32-bit Data-Level Length to support large TCP segments. The new DSS option is shown in Figure 3. The other fields of this option and the procedures defined in [RFC8684] are unchanged.¶
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+----------------------+
| Kind | Length |Subtype| (reserved) |F|m|M|a|A|
+---------------+---------------+-------+----------------------+
| Data ACK (4 or 8 octets, depending on flags) |
+--------------------------------------------------------------+
| Data Sequence Number (4 or 8 octets, depending on flags) |
+--------------------------------------------------------------+
| Subflow Sequence Number (4 octets) |
+-------------------------------+------------------------------+
| Data-Level Length (4 octets) |
+-------------------------------+------------------------------+
[RFC8684] defines the MP_CAPABLE option as shown in Figure 4. This option contains several flags, A-H. Flags A, B, C, and H are specified in [RFC8684]. 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.¶
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype|Version|A|B|C|D|E|F|G|H|
+---------------+---------------+-------+-------+---------------+
| Option Sender's Key (64 bits) |
| (if option Length > 4) |
| |
+---------------------------------------------------------------+
| Option Receiver's Key (64 bits) |
| (if option Length > 12) |
| |
+-------------------------------+-------------------------------+
| Data-Level Length (16 bits) | Checksum (16 bits, optional) |
+-------------------------------+-------------------------------+
This document does not change the security considerations defined in [RFC8684].¶
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.¶
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.¶