1.1. Design Assumptions

In order to limit the potentially huge design space, the MPTCP Working Group imposed two key constraints on the Multipath TCP design presented in this document:¶

• It must be backward compatible with current, regular TCP, to increase its chances of deployment.¶
• It can be assumed that one or both hosts are multihomed and multiaddressed.¶

To simplify the design, we assume that the presence of multiple addresses at a host is sufficient to indicate the existence of multiple paths. These paths need not be entirely disjoint: they may share one or many routers between them. Even in such a situation, making use of multiple paths is beneficial, improving resource utilization and resilience to a subset of node failures. The congestion control algorithm defined in [RFC6356] ensures that the use of multiple paths does not act detrimentally. Furthermore, there may be some scenarios where different TCP ports on a single host can provide disjoint paths (such as through certain Equal-Cost Multipath (ECMP) implementations [RFC2992]), and so the MPTCP design also supports the use of ports in path identifiers.¶

There are three aspects to the backward compatibility listed above (discussed in more detail in [RFC6182]):¶

External Constraints:

The protocol must function through the vast majority of existing middleboxes such as NATs, firewalls, and proxies, and as such must resemble existing TCP as far as possible on the wire. Furthermore, the protocol must not assume that the segments it sends on the wire arrive unmodified at the destination: they may be split or coalesced; TCP options may be removed or duplicated.¶

Application Constraints:

The protocol must be usable with no change to existing applications that use the common TCP API (although it is reasonable that not all features would be available to such legacy applications). Furthermore, the protocol must provide the same service model as regular TCP to the application.¶

Fallback:

The protocol should be able to fall back to standard TCP with no interference from the user, to be able to communicate with legacy hosts.¶

The complementary application considerations document [RFC6897] discusses the necessary features of an API to provide backward compatibility, as well as API extensions to convey the behavior of MPTCP at a level of control and information equivalent to that available with regular, single-path TCP.¶

Further discussion of the design constraints and associated design decisions is given in the MPTCP architecture document [RFC6182] and in [howhard].¶

1.2. Multipath TCP in the Networking Stack

MPTCP operates at the transport layer and aims to be transparent to both higher and lower layers. It is a set of additional features on top of standard TCP; Figure 1 illustrates this layering. MPTCP is designed to be usable by legacy applications with no changes; detailed discussion of its interactions with applications is given in [RFC6897].¶

                                +-------------------------------+
                                |           Application         |
   +---------------+            +-------------------------------+
   |  Application  |            |             MPTCP             |
   +---------------+            + - - - - - - - + - - - - - - - +
   |      TCP      |            | Subflow (TCP) | Subflow (TCP) |
   +---------------+            +-------------------------------+
   |      IP       |            |       IP      |      IP       |
   +---------------+            +-------------------------------+

1.4. MPTCP Concept

This section provides a high-level summary of normal operation of MPTCP; this type of scenario is illustrated in Figure 2. A detailed description of how MPTCP operates is given in Section 3.¶

            Host A                               Host B
   ------------------------             ------------------------
   Address A1    Address A2             Address B1    Address B2
   ----------    ----------             ----------    ----------
       |             |                      |             |
       |     (initial connection setup)     |             |
       |----------------------------------->|             |
       |<-----------------------------------|             |
       |             |                      |             |
       |            (additional subflow setup)            |
       |             |--------------------->|             |
       |             |<---------------------|             |
       |             |                      |             |
       |             |                      |             |

• To a non-MPTCP-aware application, MPTCP will behave the same as normal TCP. Extended APIs could provide additional control to MPTCP-aware applications [RFC6897]. An application begins by opening a TCP socket in the normal way. MPTCP signaling and operation are handled by the MPTCP implementation.¶
• An MPTCP connection begins similarly to a regular TCP connection. This is illustrated in Figure 2, where an MPTCP connection is established between addresses A1 and B1 on Hosts A and B, respectively.¶
• If extra paths are available, additional TCP sessions (termed MPTCP "subflows") are created on these paths and are combined with the existing session, which continues to appear as a single connection to the applications at both ends. The creation of the additional TCP session is illustrated between Address A2 on Host A and Address B1 on Host B.¶
• MPTCP identifies multiple paths by the presence of multiple addresses at hosts. Combinations of these multiple addresses equate to the additional paths. In the example, other potential paths that could be set up are A1<->B2 and A2<->B2. Although this additional session is shown as being initiated from A2, it could equally have been initiated from B1 or B2.¶
• The discovery and setup of additional subflows will be achieved through a path management method; this document describes a mechanism by which a host can initiate new subflows by using its own additional addresses or by signaling its available addresses to the other host.¶
• MPTCP adds connection-level sequence numbers to allow the reassembly of segments arriving on multiple subflows with differing network delays.¶
• Subflows are terminated as regular TCP connections, with a four‑way FIN handshake. The MPTCP connection is terminated by a connection-level FIN.¶

3.1. Connection Initiation

Connection initiation begins with a SYN, SYN/ACK, ACK exchange on a single path. Each packet contains the Multipath Capable (MP_CAPABLE) MPTCP option (Figure 4). This option declares its sender capable of performing Multipath TCP and wishes to do so on this particular connection.¶

                       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) |
  +-------------------------------+-------------------------------+

The MP_CAPABLE exchange in this specification (v1) is different than that specified in v0. If a host supports multiple versions of MPTCP, the sender of the MP_CAPABLE option SHOULD signal the highest version number it supports. In return, in its MP_CAPABLE option, the receiver will signal the version number it wishes to use, which MUST be equal to or lower than the version number indicated in the initial MP_CAPABLE. There is a caveat, though, with respect to this version negotiation with old listeners that only support v0. A listener that supports v0 expects that the MP_CAPABLE option in the SYN segment will include the initiator's key. If, however, the initiator already upgraded to v1, it won't include the key in the SYN segment. Thus, the listener will ignore the MP_CAPABLE of this SYN segment and reply with a SYN/ACK that does not include an MP_CAPABLE. The initiator MAY choose to immediately fall back to TCP or MAY choose to attempt a connection using MPTCP v0 (if the initiator supports v0), in order to discover whether the listener supports the earlier version of MPTCP. In general, an MPTCP v0 connection will likely be preferred over a TCP connection; however, in a particular deployment scenario, it may be known that the listener is unlikely to support MPTCP v0 and so the initiator may prefer not to attempt a v0 connection. An initiator MAY cache information for a peer about what version of MPTCP it supports, if any, and use this information for future connection attempts.¶

The MP_CAPABLE option is of variable length, with different fields included, depending on which packet the option is used on. The full MP_CAPABLE option is shown in Figure 4.¶

The MP_CAPABLE option is carried on the SYN, SYN/ACK, and ACK packets that start the first subflow of an MPTCP connection, as well as the first packet that carries data, if the initiator wishes to send first. The data carried by each option is as follows, where A = initiator and B = listener.¶

• SYN (A->B): only the first 4 octets (Length = 4).¶
• SYN/ACK (B->A): B's key for this connection (Length = 12).¶
• ACK (no data) (A->B): A's key followed by B's key (Length = 20).¶
• ACK (with first data) (A->B): A's key followed by B's key followed by Data-Level Length, and optional Checksum (Length = 22 or 24).¶

The contents of the option are determined by the SYN and ACK flags of the packet, along with the option's Length field. In Figure 4, "Sender" and "Receiver" refer to the sender or receiver of the TCP packet (which can be either host).¶

The initial SYN, containing just the MP_CAPABLE header, is used to define the version of MPTCP being requested and also to exchange flags to negotiate connection features, as described later.¶

This option is used to declare the 64-bit keys that the end hosts have generated for this MPTCP connection. These keys are used to authenticate the addition of future subflows to this connection. This is the only time the key will be sent in the clear on the wire (unless "Fast Close" (Section 3.5) is used); all future subflows will identify the connection using a 32-bit "token". This token is a cryptographic hash of this key. The algorithm for this process is dependent on the authentication algorithm selected; the method of selection is defined later in this section.¶

Upon reception of the initial SYN segment, a stateful server generates a random key and replies with a SYN/ACK. The key's method of generation is implementation specific. The key MUST be hard to guess, and it MUST be unique for the sending host across all its current MPTCP connections. Recommendations for generating random numbers for use in keys are given in [RFC4086]. Connections will be indexed at each host by the token (a one-way hash of the key). Therefore, an implementation will require a mapping from each token to the corresponding connection, and in turn to the keys for the connection.¶

There is a risk that two different keys will hash to the same token. The risk of hash collisions is usually small, unless the host is handling many tens of thousands of connections. Therefore, an implementation SHOULD check its list of connection tokens to ensure that there is no collision before sending its key, and if there is, then it should generate a new key. This would, however, be costly for a server with thousands of connections. The subflow handshake mechanism (Section 3.2) will ensure that new subflows only join the correct connection, however, through the cryptographic handshake, as well as checking the connection tokens in both directions, and ensuring that sequence numbers are in-window. So, in the worst case, if there was a token collision, the new subflow would not succeed, but the MPTCP connection would continue to provide a regular TCP service.¶

Since key generation is implementation specific, there is no requirement that they simply be random numbers. An implementation is free to exchange cryptographic material out of band and generate these keys from this material, in order to provide additional mechanisms by which to verify the identity of the communicating entities. For example, an implementation could choose to link its MPTCP keys to those used in higher-layer TLS or SSH connections.¶

If the server behaves in a stateless manner, it has to generate its own key in a verifiable fashion. This verifiable way of generating the key can be done by using a hash of the 4-tuple, sequence number, and a local secret (similar to what is done for the TCP sequence number [RFC4987]). It will thus be able to verify whether it is indeed the originator of the key echoed back in the subsequent MP_CAPABLE option. As for a stateful server, the tokens SHOULD be checked for uniqueness; however, if uniqueness is not met and there is no way to generate an alternative verifiable key, then the connection MUST fall back to using regular TCP by not sending an MP_CAPABLE in the SYN⁠/ACK.¶

The ACK carries both A's key and B's key. This is the first time that A's key is seen on the wire, although it is expected that A will have generated a key locally before the initial SYN. The echoing of B's key allows B to operate statelessly, as described above. Therefore, A's key must be delivered reliably to B, and in order to do this, the transmission of this packet must be made reliable.¶

If B has data to send first, then the reliable delivery of the ACK + MP_CAPABLE is ensured by the receipt of this data with an MPTCP Data Sequence Signal (DSS) option (Section 3.3) containing a DATA_ACK for the MP_CAPABLE (which is the first octet of the data sequence space). If, however, A wishes to send data first, it has two options to ensure the reliable delivery of the ACK + MP_CAPABLE. If it immediately has data to send, then the first ACK (with data) would also contain an MP_CAPABLE option with additional data parameters (the Data-Level Length and optional Checksum as shown in Figure 4). If A does not immediately have data to send, it MUST include the MP_CAPABLE on the first ACK, but without the additional data parameters. When A does have data to send, it must repeat the sending of the MP_CAPABLE option from the first ACK, with additional data parameters. This MP_CAPABLE option is used in place of the DSS and simply specifies (1) the Data-Level Length of the payload and (2) the checksum (if the use of checksums is negotiated). This is the minimal data required to establish an MPTCP connection -- it allows validation of the payload, and given that it is the first data, the Initial Data Sequence Number (IDSN) is also known (as it is generated from the key, as described below). Conveying the keys on the first data packet allows the TCP reliability mechanisms to ensure that the packet is successfully delivered. The receiver will acknowledge this data at the connection level with a Data ACK, as if a DSS option has been received.¶

There could be situations where both A and B attempt to transmit initial data at the same time. For example, if A did not initially have data to send but then needed to transmit data before it had received anything from B, it would use an MP_CAPABLE option with data parameters (since it would not know if the MP_CAPABLE on the ACK was received). In such a situation, B may also have transmitted data with a DSS option, but it had not yet been received at A. Therefore, B has received data with an MP_CAPABLE mapping after it has sent data with a DSS option. To ensure that these situations can be handled, it follows that the data parameters in an MP_CAPABLE are semantically equivalent to those in a DSS option and can be used interchangeably. Similar situations could occur when the MP_CAPABLE with data is lost and retransmitted. Furthermore, in the case of TCP segmentation offloading, the MP_CAPABLE with data parameters may be duplicated across multiple packets, and implementations must also be able to cope with duplicate MP_CAPABLE mappings as well as duplicate DSS mappings.¶

Additionally, the MP_CAPABLE exchange allows the safe passage of MPTCP options on SYN packets to be determined. If any of these options are dropped, MPTCP will gracefully fall back to regular single-path TCP, as documented in Section 3.7. If at any point in the handshake either party thinks the MPTCP negotiation is compromised -- for example, by a middlebox corrupting the TCP options or by unexpected ACK numbers being present -- the host MUST stop using MPTCP and no longer include MPTCP options in future TCP packets. The other host will then also fall back to regular TCP using the fallback mechanism. Note that new subflows MUST NOT be established (using the process documented in Section 3.2) until a DSS option has been successfully received across the path (as documented in Section 3.3).¶

Like all MPTCP options, the MP_CAPABLE option starts with the Kind and Length to specify the TCP option's kind and length. This information is followed by the MP_CAPABLE option. The first 4 bits of the first octet in the MP_CAPABLE option (Figure 4) define the MPTCP Option Subtype (see Section 7; for MP_CAPABLE, this value is 0x0), and the remaining 4 bits of this octet specify the MPTCP version in use (for this specification, this value is 1).¶

The second octet is reserved for flags, allocated as follows:¶

A:

The leftmost bit, labeled "A", SHOULD be set to 1 to indicate "Checksum required", unless the system administrator has decided that checksums are not required (for example, if the environment is controlled and no middleboxes exist that might adjust the payload).¶

B:

The second bit, labeled "B", is an extensibility flag. It MUST be set to 0 for current implementations. This flag will be used for an extensibility mechanism in a future specification, and the impact of this flag will be defined at a later date. It is expected, but not mandated, that this flag would be used as part of an alternative security mechanism that does not require a full version upgrade of the protocol but does require redefining some elements of the handshake. If receiving a message with the "B" flag set to 1 and this is not understood, then the MP_CAPABLE in this SYN MUST be silently ignored, which triggers a fallback to regular TCP; the sender is expected to retry with a format compatible with this legacy specification. Note that the length of the MP_CAPABLE option, and the meanings of bits "D" through "H", may be altered by setting B=1.¶

C:

The third bit, labeled "C", is set to 1 to indicate that the sender of this option will not accept additional MPTCP subflows to the source address and port, and therefore the receiver MUST NOT try to open any additional subflows toward this address and port. This improves efficiency in situations where the sender knows a restriction is in place -- for example, if the sender is behind a strict NAT or operating behind a legacy Layer 4 load balancer.¶

D through H:

The remaining bits, labeled "D" through "H", are used for crypto algorithm negotiation. In this specification, only the rightmost bit, labeled "H", is assigned. Bit "H" indicates the use of HMAC-SHA256 (as defined in Section 3.2). An implementation that only supports this method MUST set bit "H" to 1 and bits "D" through "G" to 0.¶

A crypto algorithm MUST be specified. If flag bits "D" through "H" are all 0, the MP_CAPABLE option MUST be treated as invalid and ignored (that is, it must be treated as a regular TCP handshake).¶

The selection of the authentication algorithm also impacts the algorithm used to generate the token and the IDSN. In this specification, with only the SHA-256 algorithm (bit "H") specified and selected, the token MUST be a truncated (most significant 32 bits) SHA-256 hash [RFC6234] of the key. A different, 64-bit truncation (the least significant 64 bits) of the SHA-256 hash of the key MUST be used as the IDSN. Note that the key MUST be hashed in network byte order. Also note that the "least significant" bits MUST be the rightmost bits of the SHA-256 digest, as per [RFC6234]. Future specifications of the use of the crypto bits may choose to specify different algorithms for token and IDSN generation.¶

Both the crypto and checksum bits negotiate capabilities in similar ways. For the "Checksum required" bit (labeled "A"), if either host requires the use of checksums, checksums MUST be used. In other words, the only way for checksums not to be used is if both hosts in their SYNs set A=0. This decision is confirmed by the setting of the "A" bit in the third packet (the ACK) of the handshake. For example, if the initiator sets A=0 in the SYN but the responder sets A=1 in the SYN/ACK, checksums MUST be used in both directions, and the initiator will set A=1 in the ACK. The decision regarding whether to use checksums will be stored by an implementation in a per-connection binary state variable. If A=1 is received by a host that does not want to use checksums, it MUST fall back to regular TCP by ignoring the MP_CAPABLE option as if it was invalid.¶

For crypto negotiation, the responder has the choice. The initiator creates a proposal setting a bit for each algorithm it supports to 1 (in this version of the specification, there is only one proposal, so bit "H" will always be set to 1). The responder responds with only 1 bit set -- this is the chosen algorithm. The rationale for this behavior is that the responder will typically be a server with potentially many thousands of connections, so it may wish to choose an algorithm with minimal computational complexity, depending on the load. If a responder does not support (or does not want to support) any of the initiator's proposals, it MUST respond without an MP_CAPABLE option, thus forcing a fallback to regular TCP.¶

The MP_CAPABLE option is only used in the first subflow of a connection, in order to identify the connection; all subsequent subflows will use the MP_JOIN option (see Section 3.2) to join the existing connection.¶

If a SYN contains an MP_CAPABLE option but the SYN/ACK does not, it is assumed that the sender of the SYN/ACK is not multipath capable; thus, the MPTCP session MUST operate as a regular, single-path TCP session. If a SYN does not contain an MP_CAPABLE option, the SYN/ACK MUST NOT contain one in response. If the third packet (the ACK) does not contain the MP_CAPABLE option, then the session MUST fall back to operating as a regular, single-path TCP session. This is done to maintain compatibility with middleboxes on the path that drop some or all TCP options. Note that an implementation MAY choose to attempt sending MPTCP options more than one time before making this decision to operate as regular TCP (see Section 3.9).¶

If the SYN packets are unacknowledged, it is up to local policy to decide how to respond. It is expected that a sender will eventually fall back to single-path TCP (i.e., without the MP_CAPABLE option) in order to work around middleboxes that may drop packets with unknown options; however, the number of multipath-capable attempts that are made first will be up to local policy. It is possible that MPTCP and non-MPTCP SYNs could get reordered in the network. Therefore, the final state is inferred from the presence or absence of the MP_CAPABLE option in the third packet of the TCP handshake. If this option is not present, the connection SHOULD fall back to regular TCP, as documented in Section 3.7.¶

The IDSN on an MPTCP connection is generated from the key. The algorithm for IDSN generation is also determined from the negotiated authentication algorithm. In this specification, with only the SHA-256 algorithm specified and selected, the IDSN of a host MUST be the least significant 64 bits of the SHA-256 hash of its key, i.e., IDSN-A = Hash(Key-A) and IDSN-B = Hash(Key-B). This deterministic generation of the IDSN allows a receiver to ensure that there are no gaps in sequence space at the start of the connection. The SYN with MP_CAPABLE occupies the first octet of data sequence space, although this does not need to be acknowledged at the connection level until the first data is sent (see Section 3.3).¶

3.2. Starting a New Subflow

Once an MPTCP connection has begun with the MP_CAPABLE exchange, further subflows can be added to the connection. Hosts have knowledge of their own address(es) and can become aware of the other host's addresses through signaling exchanges as described in Section 3.4. Using this knowledge, a host can initiate a new subflow over a currently unused pair of addresses. It is permissible for either host in a connection to initiate the creation of a new subflow, but it is expected that this will normally be the original connection initiator (see Section 3.9 for heuristics).¶

A new subflow is started as a normal TCP SYN/ACK exchange. The Join Connection (MP_JOIN) MPTCP option is used to identify the connection to be joined by the new subflow. It uses keying material that was exchanged in the initial MP_CAPABLE handshake (Section 3.1), and that handshake also negotiates the crypto algorithm in use for the MP_JOIN handshake.¶

This section specifies the behavior of MP_JOIN using the HMAC-SHA256 algorithm. An MP_JOIN option is present in the SYN, SYN/ACK, and ACK of the three-way handshake, although in each case with a different format.¶

In the first MP_JOIN on the SYN packet, illustrated in Figure 5, the initiator sends a token, random number, and Address ID.¶

                       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 = 12  |Subtype|(rsv)|B|   Address ID  |
  +---------------+---------------+-------+-----+-+---------------+
  |                   Receiver's Token (32 bits)                  |
  +---------------------------------------------------------------+
  |                Sender's Random Number (32 bits)               |
  +---------------------------------------------------------------+

The token is used to identify the MPTCP connection and is a cryptographic hash of the receiver's key, as exchanged in the initial MP_CAPABLE handshake (Section 3.1). In this specification, the tokens presented in this option are generated by the SHA-256 algorithm [RFC6234], truncated to the most significant 32 bits. The token included in the MP_JOIN option is the token that the receiver of the packet uses to identify this connection; i.e., Host A will send Token-B (which is generated from Key-B). Note that the hash generation algorithm can be overridden by the choice of cryptographic handshake algorithm, as defined in Section 3.1.¶

The MP_JOIN SYN sends not only the token (which is static for a connection) but also random numbers (nonces) that are used to prevent replay attacks on the authentication method. Recommendations for the generation of random numbers for this purpose are given in [RFC4086].¶

The MP_JOIN option includes an "Address ID". This is an identifier generated by the sender of the option, used to identify the source address of this packet, even if the IP header has been changed in transit by a middlebox. The numeric value of this field is generated by the sender and must map uniquely to a source IP address for the sending host. The Address ID allows address removal (Section 3.4.2) without needing to know what the source address at the receiver is, thus allowing address removal through NATs. The Address ID also allows correlation between new subflow setup attempts and address signaling (Section 3.4.1), to prevent setting up duplicate subflows on the same path, if an MP_JOIN and ADD_ADDR are sent at the same time.¶

The Address IDs of the subflow used in the initial SYN exchange of the first subflow in the connection are implicit and have the value zero. A host MUST store the mappings between Address IDs and addresses both for itself and the remote host. An implementation will also need to know which local and remote Address IDs are associated with which established subflows, for when addresses are removed from a local or remote host.¶

The MP_JOIN option on packets with the SYN flag set also includes 4 bits of flags, 3 of which are currently reserved and MUST be set to 0 by the sender. The final bit, labeled "B", indicates whether the sender of this option (1) wishes this subflow to be used as a backup path (B=1) in the event of failure of other paths or (2) wants the subflow to be used as part of the connection immediately. By setting B=1, the sender of the option is requesting that the other host only send data on this subflow if there are no available subflows where B=0. Subflow policy is discussed in more detail in Section 3.3.8.¶

When receiving a SYN with an MP_JOIN option that contains a valid token for an existing MPTCP connection, the recipient SHOULD respond with a SYN/ACK also containing an MP_JOIN option containing a random number and a truncated (leftmost 64 bits) HMAC. This version of the option is shown in Figure 6. If the token is unknown or the host wants to refuse subflow establishment (for example, due to a limit on the number of subflows it will permit), the receiver will send back a reset (RST) signal, analogous to an unknown port in TCP, containing an MP_TCPRST option (Section 3.6) with an "MPTCP specific error" reason code. Although calculating an HMAC requires cryptographic operations, it is believed that the 32-bit token in the MP_JOIN SYN gives sufficient protection against blind state exhaustion attacks; therefore, there is no need to provide mechanisms to allow a responder to operate statelessly at the MP_JOIN stage.¶

                       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 = 16  |Subtype|(rsv)|B|   Address ID  |
  +---------------+---------------+-------+-----+-+---------------+
  |                                                               |
  |                Sender's Truncated HMAC (64 bits)              |
  |                                                               |
  +---------------------------------------------------------------+
  |                Sender's Random Number (32 bits)               |
  +---------------------------------------------------------------+

An HMAC is sent by both hosts -- by the initiator (Host A) in the third packet (the ACK) and by the responder (Host B) in the second packet (the SYN/ACK). Doing the HMAC exchange at this stage allows both hosts to have first exchanged random data (in the first two SYN packets) that is used as the "message". This specification defines that HMAC as defined in [RFC2104] is used, along with the SHA-256 hash algorithm [RFC6234], and that the output is truncated to the leftmost 160 bits (20 octets). Due to option space limitations, the HMAC included in the SYN/ACK is truncated to the leftmost 64 bits, but this is acceptable, since random numbers are used; thus, an attacker only has one chance to correctly guess the HMAC that matches the random number previously sent by the peer (if the HMAC is incorrect, the TCP connection is closed, so a new MP_JOIN negotiation with a new random number is required).¶

The initiator's authentication information is sent in its first ACK (the third packet of the handshake), as shown in Figure 7. This data needs to be sent reliably, since it is the only time this HMAC is sent; therefore, receipt of this packet MUST trigger a regular TCP ACK in response, and the packet MUST be retransmitted if this ACK is not received. In other words, sending the ACK/MP_JOIN packet places the subflow in the PRE_ESTABLISHED state, and it moves to the ESTABLISHED state only on receipt of an ACK from the receiver. It is not permissible to send data while in the PRE_ESTABLISHED state. The reserved bits in this option MUST be set to 0 by the sender.¶

                       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 = 24  |Subtype|      (reserved)       |
  +---------------+---------------+-------+-----------------------+
  |                                                               |
  |                                                               |
  |              Sender's Truncated HMAC (160 bits)               |
  |                                                               |
  |                                                               |
  +---------------------------------------------------------------+

The key for the HMAC algorithm, in the case of the message transmitted by Host A, will be Key-A followed by Key-B; and in the case of Host B, Key-B followed by Key-A. These are the keys that were exchanged in the original MP_CAPABLE handshake. The "message" for the HMAC algorithm in each case is the concatenations of random numbers for each host (denoted by R): for Host A, R-A followed by R-B; and for Host B, R-B followed by R-A.¶

These various MPTCP options fit together to enable authenticated subflow setup as illustrated in Figure 8.¶

                Host A                                  Host B
       ------------------------                       ----------
       Address A1    Address A2                       Address B1
       ----------    ----------                       ----------
           |             |                                |
           |             |  SYN + MP_CAPABLE              |
           |--------------------------------------------->|
           |<---------------------------------------------|
           |          SYN/ACK + MP_CAPABLE(Key-B)         |
           |             |                                |
           |        ACK + MP_CAPABLE(Key-A, Key-B)        |
           |--------------------------------------------->|
           |             |                                |
           |             |   SYN + MP_JOIN(Token-B, R-A)  |
           |             |------------------------------->|
           |             |<-------------------------------|
           |             | SYN/ACK + MP_JOIN(HMAC-B, R-B) |
           |             |                                |
           |             |     ACK + MP_JOIN(HMAC-A)      |
           |             |------------------------------->|
           |             |<-------------------------------|
           |             |             ACK                |

       HMAC-A = HMAC(Key=(Key-A + Key-B), Msg=(R-A + R-B))
       HMAC-B = HMAC(Key=(Key-B + Key-A), Msg=(R-B + R-A))

If the token received at Host B is unknown or local policy prohibits the acceptance of the new subflow, the recipient MUST respond with a TCP RST for the subflow. If appropriate, an MP_TCPRST option with an "Administratively prohibited" reason code (Section 3.6) should be included.¶

If the token is accepted at Host B but the HMAC returned to Host A does not match the one expected, Host A MUST close the subflow with a TCP RST. In this and all subsequent cases of sending a RST as described in this section, the sender SHOULD send an MP_TCPRST option (Section 3.6) on this RST packet with the reason code for an "MPTCP-specific error".¶

If Host B does not receive the expected HMAC or the MP_JOIN option is missing from the ACK, it MUST close the subflow with a TCP RST.¶

If the HMACs are verified as correct, then both hosts have verified each other as being the same peers as those that existed at the start of the connection, and they have agreed of which connection this subflow will become a part.¶

If the SYN/ACK as received at Host A does not have an MP_JOIN option, Host A MUST close the subflow with a TCP RST.¶

This covers all cases of the loss of an MP_JOIN. In more detail, if an MP_JOIN is stripped from the SYN on the path from A to B and Host B does not have a listener on the relevant port, it will respond with a RST in the normal way. If in response to a SYN with an MP_JOIN option a SYN/ACK is received without the MP_JOIN option (because it was either stripped on the return path, or stripped on the outgoing path leading to Host B responding as if it was a new regular TCP session), then the subflow is unusable and Host A MUST close it with a RST.¶

Note that additional subflows can be created between any pair of ports (but see Section 3.9 for heuristics); no explicit application-level accept calls or bind calls are required to open additional subflows. To associate a new subflow with an existing connection, the token supplied in the subflow's SYN exchange is used for demultiplexing. This then binds the 5-tuple of the TCP subflow to the local token of the connection. One consequence is that it is possible to allow any port pairs to be used for a connection.¶

Demultiplexing subflow SYNs MUST be done using the token; this is unlike traditional TCP, where the destination port is used for demultiplexing SYN packets. Once a subflow is set up, demultiplexing packets is done using the 5-tuple, as in traditional TCP. The 5-tuples will be mapped to the local connection identifier (token). Note that Host A will know its local token for the subflow even though it is not sent on the wire -- only the responder's token is sent.¶

3.3. MPTCP Operation and Data Transfer

This section discusses the operation of MPTCP for data transfer. At a high level, an MPTCP implementation will take one input data stream from an application and split it into one or more subflows, with sufficient control information to allow it to be reassembled and delivered reliably and in order to the recipient application. The following subsections define this behavior in detail.¶

The Data Sequence Mapping and the Data ACK are signaled in the DSS option (Figure 9). Either or both can be signaled in one DSS, depending on the flags set. The Data Sequence Mapping defines how the sequence space on the subflow maps to the connection level, and the Data ACK acknowledges receipt of data at the connection level. These functions are described in more detail in the following two subsections.¶

                       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)     |
  +-------------------------------+------------------------------+

The flags, when set, define the contents of this option, as follows:¶

• A = Data ACK present¶
• a = Data ACK is 8 octets (if not set, Data ACK is 4 octets)¶
• M = Data Sequence Number (DSN), Subflow Sequence Number (SSN), Data-Level Length, and Checksum (if negotiated) present¶
• m = Data Sequence Number is 8 octets (if not set, DSN is 4 octets)¶

The flags "a" and "m" only have meaning if the corresponding "A" or "M" flags are set; otherwise, they will be ignored. The maximum length of this option, with all flags set, is 28 octets.¶

The "F" flag indicates "Data FIN". If present, this means that this mapping covers the final data from the sender. This is the connection-level equivalent of the FIN flag in single-path TCP. A connection is not closed unless there has been a Data FIN exchange, an MP_FASTCLOSE (Section 3.5) message, or an implementation-specific connection-level send timeout. The purpose of the Data FIN and the interactions between this flag, the subflow-level FIN flag, and the Data Sequence Mapping are described in Section 3.3.3. The remaining reserved bits MUST be set to 0 by an implementation of this specification.¶

Note that the checksum is only present in this option if the use of MPTCP checksumming has been negotiated at the MP_CAPABLE handshake (see Section 3.1). The presence of the checksum can be inferred from the length of the option. If a checksum is present but its use had not been negotiated in the MP_CAPABLE handshake, the receiver MUST close the subflow with a RST, as it is not behaving as negotiated. If a checksum is not present when its use has been negotiated, the receiver MUST close the subflow with a RST, as it is considered broken. In both cases, this RST SHOULD be accompanied by an MP_TCPRST option (Section 3.6) with the reason code for an "MPTCP-specific error".¶

                       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
  +--------------------------------------------------------------+
  |                                                              |
  |                Data Sequence Number (8 octets)               |
  |                                                              |
  +--------------------------------------------------------------+
  |              Subflow Sequence Number (4 octets)              |
  +-------------------------------+------------------------------+
  |  Data-Level Length (2 octets) |        Zeros (2 octets)      |
  +-------------------------------+------------------------------+

                       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|(rsv)|B|
  +---------------+---------------+-------+-----+-+

3.4. Address Knowledge Exchange (Path Management)

We use the term "path management" to refer to the exchange of information about additional paths between hosts, which in this design is managed by multiple addresses at hosts. For more details regarding the architectural thinking behind this design, see the MPTCP architecture document [RFC6182].¶

This design makes use of two methods of sharing such information, and both can be used on a connection. The first is the direct setup of new subflows (described in Section 3.2), where the initiator has an additional address. The second method (described in the following subsections) signals addresses explicitly to the other host to allow it to initiate new subflows. The two mechanisms are complementary: the first is implicit and simple, while the second (explicit) is more complex but is more robust. Together, these mechanisms allow addresses to change in flight (and thus support operation through NATs, since the source address need not be known); they also allow the signaling of previously unknown addresses and of addresses belonging to other address families (e.g., both IPv4 and IPv6).¶

Here is an example of typical operation of the protocol:¶

• An MPTCP connection is initially set up between address⁠/port A1 of Host A and address⁠/port B1 of Host B. If Host A is multihomed and multiaddressed, it can start an additional subflow from its address A2 to B1, by sending a SYN with an MP_JOIN option from A2 to B1, using B's previously declared token for this connection. Alternatively, if B is multihomed, it can try to set up a new subflow from B2 to A1, using A's previously declared token. In either case, the SYN will be sent to the port already in use for the original subflow on the receiving host.¶
• Simultaneously (or after a timeout), an ADD_ADDR option (Section 3.4.1) is sent on an existing subflow, informing the receiver of the sender's alternative address(es). The recipient can use this information to open a new subflow to the sender's additional address(es). In our example, A will send the ADD_ADDR option informing B of address⁠/port A2. The mix of using the SYN‑based option and the ADD_ADDR option, including timeouts, is implementation specific and can be tailored to agree with local policy.¶
• If subflow A2-B1 is successfully set up, Host B can use the Address ID in the MP_JOIN option to correlate this source address with the ADD_ADDR option that will also arrive on an existing subflow; now B knows not to open A2-B1, ignoring the ADD_ADDR. Otherwise, if B has not received the A2-B1 MP_JOIN SYN but received the ADD_ADDR, it can try to initiate a new subflow from one or more of its addresses to address A2. This permits new sessions to be opened if one host is behind a NAT.¶

Other ways of using the two signaling mechanisms are possible; for instance, signaling addresses in other address families can only be done explicitly using the Add Address (ADD_ADDR) option.¶

                       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|(rsv)|E|  Address ID   |
  +---------------+---------------+-------+-------+---------------+
  |           Address (IPv4: 4 octets / IPv6: 16 octets)          |
  +-------------------------------+-------------------------------+
  |   Port (2 octets, optional)   |                               |
  +-------------------------------+                               |
  |                Truncated HMAC (8 octets, if E=0)              |
  |                               +-------------------------------+
  |                               |
  +-------------------------------+

                     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 = 3 + n |Subtype|(resvd)|   Address ID  | ...
+---------------+---------------+-------+-------+---------------+
                           (followed by n-1 Address IDs, if required)

3.5. Fast Close

Regular TCP has the means of sending a RST signal to abruptly close a connection. With MPTCP, a regular RST only has the scope of the subflow; it will only close the applicable subflow and will not affect the remaining subflows. MPTCP's connection will stay alive at the data level, in order to permit break-before-make handover between subflows. It is therefore necessary to provide an MPTCP-level "reset" to allow the abrupt closure of the whole MPTCP connection; this is done via the MP_FASTCLOSE option.¶

MP_FASTCLOSE is used to indicate to the peer that the connection will be abruptly closed and no data will be accepted anymore. The reasons for triggering an MP_FASTCLOSE are implementation specific. Regular TCP does not allow the sending of a RST while the connection is in a synchronized state [RFC0793]. Nevertheless, implementations allow the sending of a RST in this state if, for example, the operating system is running out of resources. In these cases, MPTCP should send the MP_FASTCLOSE. This option is illustrated in Figure 14.¶

                       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)       |
  +---------------+---------------+-------+-----------------------+
  |                      Option Receiver's Key                    |
  |                            (64 bits)                          |
  |                                                               |
  +---------------------------------------------------------------+

If Host A wants to force the closure of an MPTCP connection, it can do so via two options:¶

• Option A (ACK): Host A sends an ACK containing the MP_FASTCLOSE option on one subflow, containing the key of Host B as declared in the initial connection handshake. On all the other subflows, Host A sends a regular TCP RST to close these subflows and tears them down. Host A now enters FASTCLOSE_WAIT state.¶
• Option R (RST): Host A sends a RST containing the MP_FASTCLOSE option on all subflows, containing the key of Host B as declared in the initial connection handshake. Host A can tear down the subflows and the connection immediately.¶

If Host A decides to force the closure by using Option A and sending an ACK with the MP_FASTCLOSE option, the connection shall proceed as follows:¶

• Upon receipt of an ACK with MP_FASTCLOSE by Host B, containing the valid key, Host B answers on the same subflow with a TCP RST and tears down all subflows also through sending TCP RST signals. Host B can now close the whole MPTCP connection (it transitions directly to CLOSED state).¶
• As soon as Host A has received the TCP RST on the remaining subflow, it can close this subflow and tear down the whole connection (transition from FASTCLOSE_WAIT state to CLOSED state). If Host A receives an MP_FASTCLOSE instead of a TCP RST, both hosts attempted fast closure simultaneously. Host A should reply with a TCP RST and tear down the connection.¶
• If Host A does not receive a TCP RST in reply to its MP_FASTCLOSE after one retransmission timeout (RTO) (the RTO of the subflow where the MP_FASTCLOSE has been sent), it SHOULD retransmit the MP_FASTCLOSE. To keep this connection from being retained for a long time, the number of retransmissions SHOULD be limited; this limit is implementation specific. A RECOMMENDED number is 3. If no TCP RST is received in response, Host A SHOULD send a TCP RST with the MP_FASTCLOSE option itself when it releases state in order to clear any remaining state at middleboxes.¶

If, however, Host A decides to force the closure by using Option R and sending a RST with the MP_FASTCLOSE option, Host B will act as follows: upon receipt of a RST with MP_FASTCLOSE, containing the valid key, Host B tears down all subflows by sending a TCP RST. Host B can now close the whole MPTCP connection (it transitions directly to CLOSED state).¶

3.6. Subflow Reset

An implementation of MPTCP may also need to send a regular TCP RST to force the closure of a subflow. A host sends a TCP RST in order to close a subflow or reject an attempt to open a subflow (MP_JOIN). In order to let the receiving host know why a subflow is being closed or rejected, the TCP RST packet MAY include the MP_TCPRST option (Figure 15). The host MAY use this information to decide, for example, whether it tries to re-establish the subflow immediately, later, or never.¶

                       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|U|V|W|T|    Reason     |
  +---------------+---------------+-------+-----------------------+

The MP_TCPRST option contains a reason code that allows the sender of the option to provide more information about the reason for the termination of the subflow. Using 12 bits of option space, the first 4 bits are reserved for flags (only one of which is currently defined), and the remaining octet is used to express a reason code for this subflow termination, from which a receiver MAY infer information about the usability of this path.¶

The "T" flag is used by the sender to indicate whether the error condition that is reported is Transient ("T" bit set to 1) or Permanent ("T" bit set to 0). If the error condition is considered to be Transient by the sender of the RST segment, the recipient of this segment MAY try to re-establish a subflow for this connection over the failed path. The time at which a receiver may try to re‑establish this subflow is implementation specific but SHOULD take into account the properties of the failure as defined by the provided reason code. If the error condition is considered to be Permanent, the receiver of the RST segment SHOULD NOT try to re‑establish a subflow for this connection over this path. The "U", "V", and "W" flags are not defined by this specification and are reserved for future use. An implementation of this specification MUST set these flags to 0, and a receiver MUST ignore them.¶

"Reason" is an 8-bit field that indicates the reason code for the termination of the subflow. The following codes are defined in this document:¶

• Unspecified error (code 0x00). This is the default error; it implies that the subflow is no longer available. The presence of this option shows that the RST was generated by an MPTCP-aware device.¶
• MPTCP-specific error (code 0x01). An error has been detected in the processing of MPTCP options. This is the usual reason code to return in the cases where a RST is being sent to close a subflow because of an invalid response.¶
• Lack of resources (code 0x02). This code indicates that the sending host does not have enough resources to support the terminated subflow.¶
• Administratively prohibited (code 0x03). This code indicates that the requested subflow is prohibited by the policies of the sending host.¶
• Too much outstanding data (code 0x04). This code indicates that there is an excessive amount of data that needs to be transmitted over the terminated subflow while having already been acknowledged over one or more other subflows. This may occur if a path has been unavailable for a short period and it is more efficient to reset and start again than it is to retransmit the queued data.¶
• Unacceptable performance (code 0x05). This code indicates that the performance of this subflow was too low compared to the other subflows of this Multipath TCP connection.¶
• Middlebox interference (code 0x06). Middlebox interference has been detected over this subflow, making MPTCP signaling invalid. For example, this may be sent if the checksum does not validate.¶

3.7. Fallback

Sometimes, middleboxes will exist on a path that could prevent the operation of MPTCP. MPTCP has been designed to cope with many middlebox modifications (see Section 6), but there are still some cases where a subflow could fail to operate within the MPTCP requirements. Notably, these cases are the following: the loss of MPTCP options on a path and the modification of payload data. If such an event occurs, it is necessary to "fall back" to the previous, safe operation. This may be either falling back to regular TCP or removing a problematic subflow.¶

At the start of an MPTCP connection (i.e., the first subflow), it is important to ensure that the path is fully MPTCP capable and the necessary MPTCP options can reach each host. The handshake as described in Section 3.1 SHOULD fall back to regular TCP if either of the SYN messages does not have the MPTCP options: this is the same, and desired, behavior in the case where a host is not MPTCP capable or the path does not support the MPTCP options. When attempting to join an existing MPTCP connection (Section 3.2), if a path is not MPTCP capable and the MPTCP options do not get through on the SYNs, the subflow will be closed according to the MP_JOIN logic.¶

There is, however, another corner case that should be addressed: the case where MPTCP options get through on the SYN but not on regular packets. If the subflow is the first subflow and thus all data in flight is contiguous, this situation can be resolved by using the following rules:¶

• A sender MUST include a DSS option with Data Sequence Mapping in every segment until one of the sent segments has been acknowledged with a DSS option containing a Data ACK. Upon reception of the acknowledgment, the sender has the confirmation that the DSS option passes in both directions and may choose to send fewer DSS options than once per segment.¶
• If, however, an ACK is received for data (not just for the SYN) without a DSS option containing a Data ACK, the sender determines that the path is not MPTCP capable. In the case of this occurring on an additional subflow (i.e., one started with MP_JOIN), the host MUST close the subflow with a RST, which SHOULD contain an MP_TCPRST option (Section 3.6) with a "Middlebox interference" reason code.¶
• In the case of such an ACK being received on the first subflow (i.e., that started with MP_CAPABLE), before any additional subflows are added, the implementation MUST drop out of MPTCP mode and fall back to regular TCP. The sender will send one final Data Sequence Mapping, with the Data-Level Length value of 0 indicating an infinite mapping (to inform the other end in case the path drops options in one direction only), and then revert to sending data on the single subflow without any MPTCP options.¶
• If a subflow breaks during operation, e.g., if it is rerouted and MPTCP options are no longer permitted, then once this is detected (by the subflow-level receive buffer filling up, since there is no mapping available in order to DATA_ACK this data), the subflow SHOULD be treated as broken and closed with a RST, since no data can be delivered to the application layer and no fallback signal can be reliably sent. This RST SHOULD include the MP_TCPRST option (Section 3.6) with a "Middlebox interference" reason code.¶

These rules should cover all cases where such a failure could happen -- whether it's on the forward or reverse path and whether the server or the client first sends data.¶

So far, this section has discussed the loss of MPTCP options, either initially or during the course of the connection. As described in Section 3.3, each portion of data for which there is a mapping is protected by a checksum, if checksums have been negotiated. This mechanism is used to detect if middleboxes have made any adjustments to the payload (added, removed, or changed data). A checksum will fail if the data has been changed in any way. The use of a checksum will also detect whether the length of data on the subflow is increased or decreased, and this means the Data Sequence Mapping is no longer valid. The sender no longer knows what subflow-level sequence number the receiver is genuinely operating at (the middlebox will be faking ACKs in return), and it cannot signal any further mappings. Furthermore, in addition to the possibility of payload modifications that are valid at the application layer, it is possible that such modifications could be triggered across MPTCP segment boundaries, corrupting the data. Therefore, all data from the start of the segment that failed the checksum onward is not trustworthy.¶

Note that if checksum usage has not been negotiated, this fallback mechanism cannot be used unless there is some higher-layer or lower‑layer signal to inform the MPTCP implementation that the payload has been tampered with.¶

When multiple subflows are in use, the data in flight on a subflow will likely involve data that is not contiguously part of the connection-level stream, since segments will be spread across the multiple subflows. Due to the problems identified above, it is not possible to determine what adjustments have been done to the data (notably, any changes to the subflow sequence numbering). Therefore, it is not possible to recover the subflow, and the affected subflow must be immediately closed with a RST that includes an MP_FAIL option (Figure 16), which defines the data sequence number at the start of the segment (defined by the Data Sequence Mapping) that had the checksum failure. Note that the MP_FAIL option requires the use of the full 64-bit sequence number, even if 32-bit sequence numbers are normally in use in the DSS signals on the path.¶

                       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=12   |Subtype|      (reserved)      |
  +---------------+---------------+-------+----------------------+
  |                                                              |
  |                 Data Sequence Number (8 octets)              |
  |                                                              |
  +--------------------------------------------------------------+

The receiver of this option MUST discard all data following the data sequence number specified. Failed data MUST NOT be DATA_ACKed and so will be retransmitted on other subflows (Section 3.3.6).¶

A special case is when there is a single subflow and it fails with a checksum error. If it is known that all unacknowledged data in flight is contiguous (which will usually be the case with a single subflow), an infinite mapping can be applied to the subflow without the need to close it first, essentially turning off all further MPTCP signaling. In this case, if a receiver identifies a checksum failure when there is only one path, it will send back an MP_FAIL option on the subflow-level ACK, referring to the data-level sequence number of the start of the segment on which the checksum error was detected. The sender will receive this information and, if all unacknowledged data in flight is contiguous, will signal an infinite mapping. This infinite mapping will be a DSS option (Section 3.3) on the first new packet, containing a Data Sequence Mapping that acts retroactively, referring to the start of the subflow sequence number of the most recent segment that was known to be delivered intact (i.e., was successfully DATA_ACKed). From that point onward, data can be altered by a middlebox without affecting MPTCP, as the data stream is equivalent to a regular, legacy TCP session. While in theory paths may only be damaged in one direction -- and the MP_FAIL signal affects only one direction of traffic -- for simplicity of implementation, the receiver of an MP_FAIL MUST also respond with an MP_FAIL in the reverse direction and entirely revert to a regular TCP session.¶

In the rare case that the data is not contiguous (which could happen when there is only one subflow but it is retransmitting data from a subflow that has recently been uncleanly closed), the receiver MUST close the subflow with a RST with MP_FAIL. The receiver MUST discard all data that follows the data sequence number specified. The sender MAY attempt to create a new subflow belonging to the same connection and, if it chooses to do so, SHOULD immediately place the single subflow in single-path mode by setting an infinite Data Sequence Mapping. This mapping will begin from the data-level sequence number that was declared in the MP_FAIL.¶

After a sender signals an infinite mapping, it MUST only use subflow ACKs to clear its send buffer. This is because Data ACKs may become misaligned with the subflow ACKs when middleboxes insert or delete data. The receiver SHOULD stop generating Data ACKs after it receives an infinite mapping.¶

When a connection has fallen back with an infinite mapping, only one subflow can send data; otherwise, the receiver would not know how to reorder the data. In practice, this means that all MPTCP subflows will have to be terminated except one. Once MPTCP falls back to regular TCP, it MUST NOT revert to MPTCP later in the connection.¶

It should be emphasized that MPTCP is not attempting to prevent the use of middleboxes that want to adjust the payload. An MPTCP-aware middlebox could provide such functionality by also rewriting checksums.¶

3.8. Error Handling

In addition to the fallback mechanism described above, the standard classes of TCP errors may need to be handled in an MPTCP‑specific way. Note that changing semantics -- such as the relevance of a RST -- are covered in Section 4. Where possible, we do not want to deviate from regular TCP behavior.¶

The following list covers possible errors and the appropriate MPTCP behavior:¶

• Unknown token in MP_JOIN (or HMAC failure in MP_JOIN ACK, or missing MP_JOIN in SYN/ACK response): send RST (analogous to TCP's behavior on an unknown port)¶
• DSN out of window (during normal operation): drop the data; do not send Data ACKs¶
• Remove request for unknown Address ID: silently ignore¶

3.9. Heuristics

There are a number of heuristics that are needed for performance or deployment but that are not required for protocol correctness. In this section, we detail such heuristics. Note that discussions of buffering and certain sender and receiver window behaviors are presented in Sections 3.3.4 and 3.3.5, and retransmission is discussed in Section 3.3.6.¶

7.1. TCP Option Kind Numbers

IANA has updated the "TCP Option Kind Numbers" registry to point to this document for Multipath TCP, as shown in Table 1:¶

7.2. MPTCP Option Subtypes

The 4-bit MPTCP subtype in the "MPTCP Option Subtypes" subregistry under the "Transmission Control Protocol (TCP) Parameters" registry was defined in [RFC6824]. Since [RFC6824] is an Experimental RFC and not a Standards Track RFC, and since no further entries have occurred beyond those pointing to [RFC6824], IANA has replaced the existing registry with the contents of Table 2 and with the following explanatory note.¶

Note: This registry specifies the MPTCP Option Subtypes for MPTCP v1, which obsoletes the Experimental MPTCP v0. For the MPTCP v0 subtypes, please refer to [RFC6824].¶

Values 0x9 through 0xe are currently unassigned. Option 0xf is reserved for use by private experiments. Its use may be formalized in a future specification. Future assignments in this registry are to be defined by Standards Action as defined by [RFC8126]. Assignments consist of the MPTCP subtype's symbolic name, its associated value, and a reference to its specification.¶

7.3. MPTCP Handshake Algorithms

The "MPTCP Handshake Algorithms" subregistry under the "Transmission Control Protocol (TCP) Parameters" registry was defined in [RFC6824]. Since [RFC6824] is an Experimental RFC and not a Standards Track RFC, and since no further entries have occurred beyond those pointing to [RFC6824], IANA has replaced the existing registry with the contents of Table 3 and with the following explanatory note.¶

Note: This registry specifies the MPTCP Handshake Algorithms for MPTCP v1, which obsoletes the Experimental MPTCP v0. For the MPTCP v0 subtypes, please refer to [RFC6824].¶

Note that the meanings of bits "D" through "H" can be dependent upon bit "B", depending on how the Extensibility parameter is defined in future specifications; see Section 3.1 for more information.¶

Future assignments in this registry are also to be defined by Standards Action as defined by [RFC8126]. Assignments consist of the value of the flags, a symbolic name for the algorithm, and a reference to its specification.¶

7.4. MP_TCPRST Reason Codes

IANA has created a further subregistry, "MPTCP MP_TCPRST Reason Codes" under the "Transmission Control Protocol (TCP) Parameters" registry, based on the reason code in the MP_TCPRST (Section 3.6) message. Initial values for this registry are given in Table 4; future assignments are to be defined by Specification Required as defined by [RFC8126]. Assignments consist of the value of the code, a short description of its meaning, and a reference to its specification. The maximum value is 0xff.¶

As guidance to the designated expert [RFC8126], assignments should not normally be refused unless codepoint space is becoming scarce, provided that there is a clear distinction from other, already-existing codes and also provided that there is sufficient guidance for implementers both sending and receiving these codes.¶

B.1. TFO Cookie Request with MPTCP

When a TFO initiator first connects to a listener, it cannot immediately include data in the SYN for security reasons [RFC7413]. Instead, it requests a cookie that will be used in subsequent connections. This is done with the TCP cookie request/response options, of 2 bytes and 6-18 bytes, respectively (depending on the chosen cookie length).¶

TFO and MPTCP can be combined, provided that the total length of all the options does not exceed the maximum 40 bytes possible in TCP:¶

• In the SYN: MPTCP uses a 4-byte MP_CAPABLE option. The sum of the MPTCP and TFO options is 6 bytes. With typical TCP options using up to 19 bytes in the SYN (24 bytes if options are padded at a word boundary), there is enough space to combine the MP_CAPABLE with the TFO cookie request.¶
• In the SYN + ACK: MPTCP uses a 12-byte MP_CAPABLE option, but now the TFO option can be as long as 18 bytes. Since the maximum option length may be exceeded, it is up to the listener to avoid this problem by using a shorter cookie. As an example, if we consider that 19 bytes are used for classical TCP options, the maximum possible cookie length would be 7 bytes. Note that, for the SYN packet, the same limitation applies to subsequent connections (because the initiator then echoes the cookie back to the listener). Finally, if the security impact of reducing the cookie size is not deemed acceptable, the listener can reduce the amount of space used by other TCP options by omitting the TCP timestamps (as outlined in Appendix A).¶

B.2. Data Sequence Mapping under TFO

In the TCP establishment phase, MPTCP uses a key exchange that is used to generate the Initial Data Sequence Numbers (IDSNs). In particular, the SYN with MP_CAPABLE occupies the first octet of data sequence space. With TFO, one way to handle the data sent together with the SYN would be to consider an implicit DSS mapping that covers that SYN segment (since there is not enough space in the SYN to include a DSS option). The problem with that approach is that if a middlebox modifies the TFO data, this will not be noticed by MPTCP because of the absence of a DSS checksum. For example, a TCP‑aware (but not MPTCP-aware) middlebox could insert bytes at the beginning of the stream and adapt the TCP checksum and sequence numbers accordingly. With an implicit mapping, this information would give to the initiator and listener a different view of the DSS mapping; there would be no way to detect this inconsistency, because the DSS checksum is not present.¶

To solve this issue, the TFO data must not be considered part of the data sequence number space: the SYN with MP_CAPABLE still occupies the first octet of data sequence space, but then the first non-TFO data byte occupies the second octet. This guarantees that, if the use of the DSS checksum is negotiated, all data in the data sequence number space is checksummed. We also note that this does not entail a loss of functionality, because TFO data is always only sent on the initial subflow, before any attempt to create additional subflows.¶

B.3. Connection Establishment Examples

A few examples of possible "TFO + MPTCP" establishment scenarios are shown below.¶

Before an initiator can send data together with the SYN, it must request a cookie from the listener, as shown in Figure 18. (Note: The sequence number and length are annotated in Figure 18 as Seq(Length) (e.g., "S. 0(0)") and used as such in the subsequent figures (e.g., "S  0(20)" in Figure 19).) This is done by simply combining the TFO and MPTCP options.¶

initiator                                                    listener
    |                                                           |
    |   S Seq=0(Length=0) <MP_CAPABLE>, <TFO cookie request>    |
    | --------------------------------------------------------> |
    |                                                           |
    |   S. 0(0) ack 1 <MP_CAPABLE>, <TFO cookie>                |
    | <-------------------------------------------------------- |
    |                                                           |
    |   .  0(0) ack 1 <MP_CAPABLE>                              |
    | --------------------------------------------------------> |
    |                                                           |

Once this is done, the received cookie can be used for TFO, as shown in Figure 19. In this example, the initiator first sends 20 bytes in the SYN. The listener immediately replies with 100 bytes following the SYN-ACK, to which the initiator replies with 20 more bytes. Note that the last segment in the figure has a TCP sequence number of 21, while the DSS subflow sequence number is 1 (because the TFO data is not part of the data sequence number space, as explained in Appendix B.2.¶

initiator                                                    listener
    |                                                           |
    |    S  0(20) <MP_CAPABLE>, <TFO cookie>                    |
    | --------------------------------------------------------> |
    |                                                           |
    |    S. 0(0) ack 21 <MP_CAPABLE>                            |
    | <-------------------------------------------------------- |
    |                                                           |
    |    .  1(100) ack 21 <DSS ack=1 seq=1 ssn=1 dlen=100>      |
    | <-------------------------------------------------------- |
    |                                                           |
    |    .  21(0) ack 1 <MP_CAPABLE>                            |
    | --------------------------------------------------------> |
    |                                                           |
    |    .  21(20) ack 101 <DSS ack=101 seq=1 ssn=1 dlen=20>    |
    | --------------------------------------------------------> |
    |                                                           |

In Figure 20, the listener does not support TFO. The initiator detects that no state is created in the listener (as no data is ACKed) and now sends the MP_CAPABLE in the third packet, in order for the listener to build its MPTCP context at the end of the establishment. Now, the TFO data, when retransmitted, becomes part of the Data Sequence Mapping because it is effectively sent (in fact re‑sent) after the establishment.¶

initiator                                                    listener
    |                                                           |
    |    S  0(20) <MP_CAPABLE>, <TFO cookie>                    |
    | --------------------------------------------------------> |
    |                                                           |
    |    S. 0(0) ack 1 <MP_CAPABLE>                             |
    | <-------------------------------------------------------- |
    |                                                           |
    |    .  1(0) ack 1 <MP_CAPABLE>                             |
    | --------------------------------------------------------> |
    |                                                           |
    |    .  1(20) ack 1 <DSS ack=1 seq=1 ssn=1 dlen=20>         |
    | --------------------------------------------------------> |
    |                                                           |
    |    .  0(0) ack 21 <DSS ack=21 seq=1 ssn=1 dlen=0>         |
    | <-------------------------------------------------------- |
    |                                                           |

It is also possible that the listener acknowledges only part of the TFO data, as illustrated in Figure 21. The initiator will simply retransmit the missing data together with a DSS mapping.¶

initiator                                                    listener
    |                                                           |
    |    S  0(1000) <MP_CAPABLE>, <TFO cookie>                  |
    | --------------------------------------------------------> |
    |                                                           |
    |    S. 0(0) ack 501 <MP_CAPABLE>                           |
    | <-------------------------------------------------------- |
    |                                                           |
    |    .  501(0) ack 1 <MP_CAPABLE>                           |
    | --------------------------------------------------------> |
    |                                                           |
    |    .  501(500) ack 1 <DSS ack=1 seq=1 ssn=1 dlen=500>     |
    | --------------------------------------------------------> |
    |                                                           |