3.2.1. MP_CONFIRM

Some Multipath Options require confirmation from the remote peer (see Table 4) for which MP_CONFIRM is specified.¶

            1          2          3           4          5
 01234567 89012345 67890123 45678901 23456789 01234567 89012345
+--------+--------+--------+--------+--------+--------+--------+
|00101110|  var   |00000000| List of confirmations ...
+--------+--------+--------+--------+--------+--------+--------+
 Type=46   Length  MP_OPT=0

Multipath Options that require confirmation will be retransmitted by the sender until an MP_CONFIRM is received or the confirmation of options is considered irrelevant because the data contained in the options has already been replaced by newer information.¶

This can happen, for example, with an MP_PRIO option if the path prioritization is changed while the previous prioritization has not yet been confirmed. The further processing of the Multipath Options in the receiving host is not the subject of MP_CONFIRM.¶

Multipath Options could arrive out of order; therefore, Multipath Options defined in Table 4 MUST be sent in a DCCP datagram with MP_SEQ (see Section 3.2.5). This allows a receiver to identify whether Multipath Options are associated with obsolete datasets (information carried in the option header) that would otherwise conflict with newer datasets. In the case of MP_ADDADDR or MP_REMOVEADDR, the same dataset is identified based on AddressID, whereas the same dataset for MP_PRIO is identified by the subflow in use. An outdated multipath Option is detected at the receiver if a previous Multipath Option referring to the same dataset contained a higher sequence number in the MP_SEQ. An MP_CONFIRM MAY be generated for Multipath Options that are identified as outdated.¶

Similarly, an MP_CONFIRM could arrive out of order. The associated MP_SEQ received MUST be echoed to ensure that the most recent Multipath Option is confirmed. This protects from inconsistencies that could occur, e.g., if three MP_PRIO options are sent one after the other on one path in order to first set the path priority to 0, then to 1, and finally to 0 again. Without an associated MP_SEQ, a loss of the third MP_PRIO option and a loss of the MP_CONFIRM of the second update and the third update would cause the sender to incorrectly interpret that the priority value was set to 0 without recognizing that the receiver has applied priority value 1.¶

The length of the MP_CONFIRM option and the path over which the option is sent depend on the confirmed Multipath Options and the received MP_SEQ, which are both copied verbatim and appended as a list of confirmations. The list is structured by first listing the received MP_SEQ followed by the related Multipath Option or options to confirm. The same rules apply when Multipath Options with different MP_SEQs are confirmed at once. This could happen if the following are received in short succession: a datagram with MP_PRIO and a first MP_SEQ_1 and another datagram with MP_ADDADDR and a second MP_SEQ_2. In this case, the structure described above is concatenated resulting in MP_SEQ_2 + MP_ADDADDR + MP_SEQ_1 + MP_PRIO. The order of the confirmed Multipath Options in the list of confirmations MUST reflect the incoming order at the host who sends the MP_CONFIRM, with the most recent suboption received listed first. This could allow the host receiving the MP_CONFIRM to verify that the options were applied in the correct order and to take countermeasures if they were not, e.g., if an MP_REMOVEADDR overtakes an MP_ADDADDR that refers to the same dataset.¶

An example to illustrate the MP-DCCP confirm procedure for the MP_PRIO option is shown in Figure 7. Host A sends a DCCP-Request on path A2-B2 with an MP_PRIO option with value 1 and an associated sequence number of 1. Host B replies on the same path in this instance (any path can be used) with a DCCP-Response containing the MP_CONFIRM option and a list containing the original sequence number (1) together with the associated option (MP_PRIO).¶

          Host A                                     Host B
------------------------                     ------------------------
Address A1    Address A2                     Address B1    Address B2
----------    ----------                     ----------    ----------
     |             |                                   |       |
     |             | DCCP-Request(seqno 1) + MP_PRIO(1)|       |
     |             |------------------------------------------>|
     |             |                                   |       |
     |             | DCCP-Response +                   |       |
     |             |<---- MP_CONFIRM(seqno 1, MP_PRIO) --------|
     |             |                                   |       |

A second example that illustrates the same MP-DCCP confirm procedure but where an out-of-date option is also delivered is shown in Figure 8. Here, the first DCCP-Data is sent from Host A to Host B with option MP_PRIO set to 4. Host A subsequently sends the second DCCP-Data with option MP_PRIO set to 1. In this case, the delivery of the first MP_PRIO is delayed in the network between Host A and Host B and arrives after the second MP_PRIO. Host B ignores this second MP_PRIO as the associated sequence number is earlier than the first. Host B sends a DCCP-Ack with sequence number 2 to confirm the receipt of the MP_PRIO(1).¶

          Host A                                     Host B
------------------------                     ------------------------
Address A1    Address A2                     Address B1    Address B2
----------    ----------                     ----------    ----------
     |             |                                   |       |
     |             | DCCP-Data(seqno 1) +  MP_PRIO(4)  |       |
     |             |------------                       |       |
     |             |            \                      |       |
     |             | DCCP-Data(seqno 2) +  MP_PRIO(1)  |       |
     |             |--------------\--------------------------->|
     |             |               \                   |       |
     |             |                -------------------------->|
     |             |                                   |       |
     |             | DCCP-Ack +                        |       |
     |             |<---- MP_CONFIRM(seqno 2, MP_PRIO) --------|
     |             |                                   |       |

3.2.2. MP_JOIN

The MP_JOIN option is used to add a new subflow to an existing MP-DCCP connection, and a successful establishment of the first subflow using MP_KEY is REQUIRED.¶

            1          2          3
 01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
|00101110|00001100|00000001| Addr ID|
+--------+--------+--------+--------+
| Connection Identifier             |
+--------+--------+--------+--------+
| Nonce                             |
+--------+--------+--------+--------+
 Type=46  Length=12 MP_OPT=1

The CI is the one from the peer host, which was previously exchanged with the MP_KEY option. MP_HMAC MUST be set when using MP_JOIN within a DCCP-Response packet; see Section 3.2.6 for details. Similar to the setup of the first subflow, MP_JOIN also exchanges the Multipath Capable Feature MP_CAPABLE as described in Section 3.1. This procedure includes the DCCP Confirm principle and thus ensures a reliable exchange of the MP_JOIN in accordance with Section 6.6.4 of [RFC4340].¶

The MP_JOIN option includes an "Addr ID" (Address ID) generated by the sender of the option, which is used to identify the source address of this packet, even if the IP header was changed in transit by a middlebox. The 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 3.2.9) without the need to know the source address at the receiver, thus allowing address removal through NATs. The Address ID also allows correlation between new subflow setup attempts and address signaling (Section 3.2.8), to prevent setting up duplicate subflows on the same path, if an MP_JOIN and MP_ADDADDR are sent at the same time.¶

The Address IDs of the subflow used in the initial DCCP Request/Response 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 for both 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. An Address ID MUST always be unique over the lifetime of a subflow and can only be reassigned if sender and receiver no longer have them in use.¶

The Nonce is a 32-bit random value locally generated for every MP_JOIN option. Together with the derived key from both hosts' Key Data (as described in Section 3.2.4), the Nonce value builds the basis to calculate the Hash-based Message Authentication Code (HMAC) used in the handshake process (as described in Section 3.3) to avoid replay attacks.¶

If the CI cannot be verified by the receiving host during a handshake negotiation, the new subflow MUST be closed, as specified in Section 3.6.¶

3.2.3. MP_FAST_CLOSE

DCCP can send a Close or Reset signal to abruptly close a connection. Using MP-DCCP, a regular Close or Reset only has the scope of the subflow over which a signal was received. As such, it will only close the subflow and does not affect other remaining subflows or the MP-DCCP connection (unless it is the last subflow). This permits break-before-make handover between subflows.¶

In order to provide an MP-DCCP-level "reset" and thus allow the abrupt closure of the MP-DCCP connection, the MP_FAST_CLOSE suboption can be used.¶

            1          2          3
 01234567 89012345 67890123 45678901 23456789
+--------+--------+--------+--------+--------+
|00101110|  var   |00000010| Key Data ...
+--------+--------+--------+--------+--------+
 Type=46   Length  MP_OPT=2

When Host A wants to abruptly close an MP-DCCP connection with Host B, it will send out the MP_FAST_CLOSE. The MP_FAST_CLOSE suboption MUST be sent from Host A on all subflows using a DCCP-Reset packet with Reset Code 13. The requirement to send the MP_FAST_CLOSE on all subflows increases the probability that Host B will receive the MP_FAST_CLOSE to take the same action. To protect from an unauthorized shutdown of an MP-DCCP connection, the selected Key Data of the peer host during the handshake procedure is carried by the MP_FAST_CLOSE option.¶

After sending the MP_FAST_CLOSE on all subflows, Host A MUST tear down all subflows, and the MP-DCCP connection immediately terminates.¶

Upon reception of the first MP_FAST_CLOSE with successfully validated Key Data, Host B will send a DCCP-Reset packet response on all subflows to Host A with Reset Code 13 to clean potential middlebox states. Host B MUST then tear down all subflows and terminate the MP-DCCP connection.¶

3.2.4. MP_KEY

MP-DCCP protects against some on-path attacker as further outlined in Section 4. The basis of this protection is laid by an initial exchange of keys during the MP-DCCP connection setup, for which MP_KEY is introduced. The basis of this protection is laid by an initial exchange of keys during the MP-DCCP connection setup, for which MP_KEY is introduced.¶

                     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
+---------------+---------------+---------------+---------------+
|0 0 1 0 1 1 1 0|      var      |0 0 0 0 0 0 1 1|     resvd     |
+---------------+---------------+---------------+---------------+
|                     Connection Identifier                     |
+---------------+---------------+---------------+---------------+
|  Key Type (1) |  Key Data (1) |  Key Type (2) |  Key Data (2) |
+---------------+---------------+---------------+---------------+
|  Key Type (3) | ...
+---------------+---------------+
    Type=46          Length         MP_OPT=3

The MP_KEY suboption is used to exchange a CI and key material between hosts (Host A and Host B) for a given connection. The CI is a unique number in the host for each multipath connection and is generated for inclusion in the first exchange of a connection with MP_KEY. With the CI, it is possible to connect other DCCP subflows to an MP-DCCP connection with MP_JOIN (Section 3.2.2). Its size of 32 bits also defines the maximum number of simultaneous MP-DCCP connections in a host to 2³². According to the Key-related elements of the MP_KEY suboption, the Length varies between 17 and 73 bytes for a single-key message and up to 82 bytes when all specified Key Types 0 and 255 are provided. The Key Type field specifies the type of the following Key Data. The set of Key Types are shown in Table 5.¶

Plain Text:

Key Data is exchanged in plain text between hosts (Host A and Host B), and the respective key parts (KeyA and KeyB) are used by each host to generate the derived key (d-key) by concatenating the two parts with the local key in front. That is,¶

Host A:

d-keyA=(KeyA+KeyB)¶

Host B:

d-keyB=(KeyB+KeyA)¶

Experimental:

This Key Type allows the use of other Key Data and can be used to validate other key exchange mechanisms for a possible future specification.¶

Multiple keys are only permitted in the DCCP-Request message of the handshake procedure for the first subflow. This allows the hosts to agree on a single Key Type to be used, as described in Section 3.3¶

It is possible that not all hosts will support all Key Types, and this specification does not recommend or enforce the announcement of any particular Key Type within the MP_KEY option as this could have security implications. However, at least Key Type 0 (Plain Text) MUST be supported for interoperability tests in implementations of MP-DCCP. If the Key Type cannot be agreed in the handshake procedure, the MP-DCCP connection MUST fall back to not using MP-DCCP, as indicated in Section 3.6.¶

3.2.5. MP_SEQ

DCCP [RFC4340] defines a packet sequencing scheme that continues to apply to the individual DCCP subflows within an MP-DCCP connection. However, for the operation of MP-DCPP, the order of packets within an MP-DCCP connection MUST be known before assigning packets to subflows to apply the received Multipath Options in the correct order or to recognize whether delayed Multipath Options are obsolete. Therefore, MP_SEQ is introduced and can also be used to reorder data packets on the receiver side.¶

            1          2          3           4          5
 01234567 89012345 67890123 45678901 23456789 01234567 89012345
+--------+--------+--------+--------+--------+--------+--------+
|00101110|00001001|00000100| Multipath Sequence Number
+--------+--------+--------+--------+--------+--------+--------+
                  |
+--------+--------+
 Type=46  Length=9 MP_OPT=4

The MP_SEQ suboption is used for end-to-end 48-bit datagram-based sequence numbers of an MP-DCCP connection. The initial data sequence number (IDSN) SHOULD be set randomly [RFC4086]. As with the standard DCCP sequence number, the data sequence number should not start at zero but at a random value to make blind session hijacking more difficult; see also Section 7.2 of [RFC4340].¶

The MP_SEQ number space is independent of the path individual sequence number space and MUST be sent with all DCCP-Data and DCCP-DataACK packets.¶

When the sequence number space is exhausted, the sequence number MUST be wrapped. [RFC7323] provides guidance on selecting an appropriately sized sequence number space according to the Maximum Segment Lifetime (MSL) of TCP. 64 bits is the recommended size for TCP to avoid the sequence number space going through within the segment lifetime. For DCCP, the MSL is the same as that of TCP as specified in Section 3.4 of [RFC4340]. Compared to TCP, the sequence number for DCCP is incremented per packet rather than per byte transmitted. For this reason, the 48 bits chosen in MP_SEQ are considered sufficiently large per the current globally routable maximum packet size (MPS) of 1500 bytes, which corresponds to roughly 375 pebibytes (PiBs) of data within the sequence number space.¶

3.2.6. MP_HMAC

MP-DCCP protects against some on-path attacker as further outlined in Section 4. Once an MP-DCCP connection has been established, the MP_HMAC option introduced here provides further protection based on the key material exchanged with MP_KEY when the connection is established.¶

            1          2          3           4
 01234567 89012345 67890123 45678901 23456789 01234567
+--------+--------+--------+--------+--------+--------+
|00101110|00010111|00000101| HMAC-SHA256 (20 bytes) ...
+--------+--------+--------+--------+--------+--------+
 Type=46  Length=23 MP_OPT=5

The MP_HMAC suboption is used to provide authentication for the MP_ADDADDR and MP_REMOVEADDR suboptions. In addition, it provides authentication for subflows joining an existing MP_DCCP connection, as described in the second and third step of the handshake of a subsequent subflow in Section 3.3. For this specification of MP-DCCP, the HMAC code is generated according to [RFC2104] in combination with the SHA-256 hash algorithm described in [RFC6234], with the output in big-endian format truncated to the leftmost 160 bits (20 bytes). It is possible that other versions of MP-DCCP will define other hash algorithms in the future.¶

The "Key" used for the HMAC computation is the derived key (d-keyA for Host A or d-KeyB for Host B) described in Section 3.2.4, while the HMAC "Message" for MP_JOIN, MP_ADDADDR, and MP_REMOVEADDR must be calculated in both hosts in order to protect the Multipath Option when sending and to validate the Multipath Option when receiving; it is a concatenation of:¶

• For MP_JOIN: The Nonces of the MP_JOIN messages for which authentication shall be performed. Depending on whether Host A or Host B performs the HMAC-SHA256 calculation, it is carried out as follows:¶

  • MP_HMAC(A) = HMAC-SHA256(Key=d-keyA, Msg=RA+RB)¶

  • MP_HMAC(B) = HMAC-SHA256(Key=d-keyB, Msg=RB+RA)¶

A usage example is shown in Figure 21.¶

• For MP_ADDADDR: The Address ID and Nonce with an associated IP address and a port, if defined; otherwise, 2 bytes of value 0. The IP address and port MUST be used in network byte order (NBO). Depending on whether Host A or Host B performs the HMAC-SHA256 calculation, it is carried out as follows:¶

  • MP_HMAC(A) = HMAC-SHA256(Key=d-keyA, Msg=Address ID+Nonce+NBO(IP)+NBO(Port))¶

  • MP_HMAC(B) = HMAC-SHA256(Key=d-keyB, Msg=Address ID+Nonce+NBO(IP)+NBO(Port))¶

• For MP_REMOVEADDR: Solely the Address ID. Depending on whether Host A or Host B performs the HMAC-SHA256 calculation, it is carried out as follows:¶

  • MP_HMAC(A) = HMAC-SHA256(Key=d-keyA, Msg=Address ID+Nonce)¶

  • MP_HMAC(B) = HMAC-SHA256(Key=d-keyB, Msg=Address ID+Nonce)¶

MP_JOIN, MP_ADDADDR, and MP_REMOVEADDR can coexist or be used multiple times within a single DCCP packet. All these Multipath Options require an individual MP_HMAC option. This ensures that the MP_HMAC is correctly associated. Otherwise, the receiver cannot validate multiple MP_JOIN, MP_ADDADDR, or MP_REMOVEADDR options. Therefore, an MP_HMAC MUST directly follow its associated Multipath Option. In the likely case of sending an MP_JOIN together with an MP_ADDADDR, this results in concatenating MP_JOIN + MP_HMAC_1 + MP_ADDADDR + MP_HMAC_2, whereas the first MP_HMAC_1 is associated with the MP_JOIN and the second MP_HMAC_2 is associated with the MP_ADDADDR suboption.¶

On the receiver side, the HMAC validation of the suboptions MUST be carried out according to the sending sequence in which the associated MP_HMAC follows a suboption. If the suboption cannot be validated by a receiving host because the HMAC validation fails (HMAC is wrong or missing), the subsequent handling depends on which suboption was being verified. If the suboption to be authenticated was either MP_ADDADDR or MP_REMOVEADDR, the receiving host MUST silently ignore it (see Sections 3.2.8 and 3.2.9). If the suboption to be authenticated was MP_JOIN, the subflow MUST be closed (see Section 3.6).¶

In the event that an MP_HMAC cannot be associated with a suboption, this MP_HMAC MUST be ignored, unless it is a single MP_HMAC that was sent in a DCCP-Ack corresponding to a DCCP response packet with MP_JOIN (see the penultimate arrow in Figure 21).¶

3.2.7. MP_RTT

The MP_RTT suboption is used to transmit RTT values and Age (represented in milliseconds) that belong to the path over which this information is transmitted. This information is useful for the receiving host to calculate the RTT difference between the subflows and to estimate whether missing data has been lost.¶

            1          2          3           4          5
 01234567 89012345 67890123 45678901 23456789 01234567 89012345
+--------+--------+--------+--------+--------+--------+--------+
|00101110|00001100|00000110|RTT Type| RTT
+--------+--------+--------+--------+--------+--------+--------+
         | Age                               |
+--------+--------+--------+--------+--------+
 Type=46  Length=12 MP_OPT=6

The RTT and Age information is a 32-bit integer. This covers a period of approximately 1193 hours.¶

The Field RTT type indicates the type of RTT estimation, according to the following description:¶

Raw RTT (=0)

Raw RTT value of the last Datagram round trip.¶

Min RTT (=1)

Min RTT value over a given period.¶

Max RTT (=2)

Max RTT value over a given period.¶

Smooth RTT (=3)

Averaged RTT value over a given period.¶

Each CCID specifies the algorithms and period applied for their corresponding RTT estimations. The availability of the above-described types, to be used in the MP_RTT option, depends on the CCID implementation in place.¶

Age:

The Age parameter defines the time difference between now -- the creation of the MP_RTT option -- and the conducted RTT measurement in milliseconds. If no previous measurement exists, e.g., when initialized, the value is 0.¶

An example of a flow showing the exchange of path individual RTT information is provided in Figure 15. RTT1 refers to the first path and RTT2 to the second path. The RTT values could be extracted from the sender's congestion control algorithm and are conveyed to the receiving host using the MP_RTT suboption. With the reception of RTT1 and RTT2, the receiver is able to calculate the path_delta that corresponds to the absolute difference of both values. In the case where the path individual RTTs are symmetric in the down-link and up-link directions and there is no jitter, packets with missing sequence number MP_SEQ, e.g., in a reordering process, can be assumed lost after path_delta/2.¶

MP-DCCP                   MP-DCCP
Sender                    Receiver
+--------+  MP_RTT(RTT1)  +-------------+
|   RTT1 |----------------|             |
|        |                | path_delta= |
|        |  MP_RTT(RTT2)  | |RTT1-RTT2| |
|   RTT2 |----------------|             |
+--------+                +-------------+

3.2.8. MP_ADDADDR

The MP_ADDADDR suboption announces additional addresses (and, optionally, port numbers) by which a host can be reached. This can be sent at any time during an existing MP-DCCP connection, when the sender wishes to enable multiple paths and/or when additional paths become available. Multiple instances of this suboption within a packet can simultaneously advertise new addresses.¶

The Length is variable depending on the address family (IPv4 or IPv6) and whether a port number is used. This field is in the range between 12 and 26 bytes.¶

The Nonce is a 32-bit random value that is generated locally for each MP_ADDADDR option and is used in the HMAC calculation process to prevent replay attacks.¶

The final 2 bytes optionally specify the DCCP port number to use, and their presence can be inferred from the length of the option. Although it is expected that the majority of use cases will use the same port pairs as used for the initial subflow (e.g., port 80 remains port 80 on all subflows, as does the ephemeral port at the client), there could be cases (such as port-based load balancing) where the explicit specification of a different port is required. If no port is specified, the receiving host MUST assume that any attempt to connect to the specified address uses the port already used by the subflow on which the MP_ADDADDR signal was sent.¶

Along with the MP_ADDADDR option, an MP_HMAC option MUST be sent for authentication. The truncated HMAC parameter present in this MP_HMAC option is the leftmost 20 bytes of an HMAC, negotiated and calculated as described in Section 3.2.6. Similar to MP_JOIN, the key for the HMAC algorithm will be d-KeyA when the message is transmitted by Host A and d-KeyB when transmitted by Host B. These are the keys that were exchanged and selected in the original MP_KEY handshake. The message for the HMAC is the Address ID, Nonce, IP address, and port number that precede the HMAC in the MP_ADDADDR option. If the port number is not present in the MP_ADDADDR option, the HMAC message will include 2 bytes of value zero. The rationale for the HMAC is to prevent unauthorized entities from injecting MP_ADDADDR signals in an attempt to hijack a connection. Additionally, note that the presence of this HMAC prevents the address from being changed in flight unless the key is known by an intermediary. If a host receives an MP_ADDADDR option for which it cannot validate the HMAC, it MUST silently ignore the option.¶

The presence of an MP_SEQ (Section 3.2.5) MUST be ensured in a DCCP datagram in which MP_ADDADDR is sent, as described in Section 3.2.1.¶

                     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
+---------------+---------------+-------+-------+---------------+
|0 0 1 0 1 1 1 0|      var      |0 0 0 0 0 1 1 1|  Address ID   |
+---------------+---------------+-------+-------+---------------+
|                             Nonce                             |
+-------------------------------+-------------------------------+
|          Address (IPv4 - 4 bytes / IPv6 - 16 bytes)           |
+-------------------------------+-------------------------------+
|   Port (2 bytes, optional)    | + MP_HMAC option
+-------------------------------+
     Type=46         Length         MP_OPT=7

Each address has an Address ID that could be used for uniquely identifying the address within a connection for address removal. Each host maintains a list of unique Address IDs, and it manages these as it wishes. The Address ID is also used to identify MP_JOIN options (see Section 3.2.2) relating to the same address, even when address translators are in use. The Address ID MUST uniquely identify the address for the sender of the option (within the scope of the connection); the mechanism for allocating such IDs is implementation specific.¶

All Address IDs learned via either MP_JOIN or MP_ADDADDR can be stored by the receiver in a data structure that gathers all the Address-ID-to-address mappings for a connection (identified by a CI pair). In this way, there is a stored mapping between the Address ID, the observed source address, and the CI pair for future processing of control information for a connection. Note that an implementation MAY discard incoming address advertisements. Reasons for this are, for example:¶

• to avoid the required mapping state, or¶

• because advertised addresses are of no use to it.¶

Possible scenarios in which this applies are the lack of resources to store a mapping or when IPv6 addresses are advertised even though the host only supports IPv4. Therefore, a host MUST treat address announcements as soft state. However, a sender MAY choose to update the announcements periodically to overcome temporary limitations.¶

A host MAY advertise private addresses, e.g., because there is a NAT on the path. It is desirable to allow this as there could be cases where both hosts have additional interfaces on the same private network. The advertisement of broadcast or multicast IP addresses MUST be ignored by the recipient of this option, as it is not permitted according to the unicast principle of the basic DCCP.¶

The MP_JOIN handshake used to create a new subflow (Section 3.2.2) provides mechanisms to minimize security risks. The MP_JOIN message contains a 32-bit CI that uniquely identifies a connection to the receiving host. If the CI is unknown, the host MUST send a DCCP-Reset.¶

Further security considerations around the issue of MP_ADDADDR messages that accidentally misdirect, or maliciously direct, new MP_JOIN attempts are discussed in Section 4. If a sending host of an MP_ADDADDR knows that no incoming subflows can be established at a particular address, an MP_ADDADDR MUST NOT announce that address unless the sending host has new knowledge about the possibility to do so. This information can be obtained from local firewall or routing settings, knowledge about availability of an external NAT or a firewall, or connectivity checks performed by the host/application.¶

The reception of an MP_ADDADDR message is acknowledged using MP_CONFIRM (Section 3.2.1). This ensures a reliable exchange of address information.¶

A host that receives an MP_ADDADDR but finds that the IP address and port number is unsuccessful at connection setup SHOULD NOT perform further connection attempts to this address/port combination for this connection to save resources. However, if a sender wishes to trigger a new incoming connection attempt on a previously advertised address/port combination, they can refresh the MP_ADDADDR information by sending the option again.¶

A host MAY send an MP_ADDADDR message with an already-assigned Address ID using the IP address previously assigned to this Address ID. The new MP_ADDADDR could have the same port number or a different port number. The receiver MUST silently ignore the MP_ADDADDR if the IP address is not the same as that previously assigned to this Address ID. A host wishing to replace an existing Address ID MUST first remove the existing one (Section 3.2.9).¶

3.2.9. MP_REMOVEADDR

If, during the lifetime of an MP-DCCP connection, a previously announced address becomes invalid (e.g., if an interface disappears), the affected host SHOULD announce this. The peer can remove a previously added address with an Address ID from a connection using the Remove Address (MP_REMOVEADDR) suboption. This will terminate any subflows currently using that address.¶

MP_REMOVEADDR is only used to close already-established subflows that have an invalid address. Functional flows with a valid address MUST be closed with a DCCP Close exchange (as with regular DCCP) instead of using MP_REMOVEADDR. For more information see Section 3.5.¶

The Nonce is a 32-bit random value that is generated locally for each MP_REMOVEADDR option and is used in the HMAC calculation process to prevent replay attacks.¶

Along with the MP_REMOVEADDR suboption, an MP_HMAC option MUST be sent for authentication. The truncated HMAC parameter present in this MP_HMAC option is the leftmost 20 bytes of an HMAC, negotiated and calculated as described in Section 3.2.6. Similar to MP_JOIN, the key for the HMAC algorithm will be d-KeyA when the message is transmitted by Host A and d-KeyB when transmitted by Host B. These are the keys that were exchanged and selected in the original MP_KEY handshake. The message for the HMAC is the Address ID.¶

The rationale for using an HMAC is to prevent unauthorized entities from injecting MP_REMOVEADDR signals in an attempt to hijack a connection. Additionally, note that the presence of this HMAC prevents the address from being modified in flight unless the key is known by an intermediary. If a host receives an MP_REMOVEADDR option for which it cannot validate the HMAC, it MUST silently ignore the option.¶

A receiver MUST include an MP_SEQ (Section 3.2.5) in a DCCP datagram that sends an MP_REMOVEADDR. Further details are given in Section 3.2.1.¶

The reception of an MP_REMOVEADDR message is acknowledged using MP_CONFIRM (Section 3.2.1). This ensures a reliable exchange of address information. To avoid inconsistent states, the sender releases the Address ID only after MP_REMOVEADDR has been confirmed.¶

The sending and receiving of this message SHOULD trigger the closing procedure described in [RFC4340] between the client and the server on the affected subflow(s), if possible. This helps remove middlebox state before removing any local state.¶

Address removal is done by the Address ID to allow the use of NATs and other middleboxes that rewrite source addresses. If there is no address at the requested Address ID, the receiver will silently ignore the request.¶

                     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
+---------------+---------------+---------------+---------------+
|0 0 1 0 1 1 1 0|0 0 0 0 0 1 0 0|0 0 0 0 1 0 0 0|   Address ID  |
+---------------+---------------+---------------+---------------+
|                             Nonce                             |
+-------------------------------+-------------------------------+
     Type=46        Length=8         MP_OPT=8

-> followed by the MP_HMAC option

3.2.10. MP_PRIO

The path priority signaled with the MP_PRIO option provides hints for the packet scheduler when making decisions about which path to use for payload traffic. When a single specific path from the set of available paths is treated with higher priority compared to the others when making scheduling decisions for payload traffic, a host can signal such change in priority to the peer. This could be used when there are different costs for using different paths (e.g., Wi-Fi is free while cellular has a limit on volume, and 5G has higher energy consumption). The priority of a path could also change, for example, when a mobile host runs out of battery, and the usage of only a single path may be the preferred choice of the user.¶

The MP_PRIO suboption, shown below, can be used to set a priority value for the subflow over which the suboption is received.¶

                     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
+---------------+---------------+---------------+---------------+
|0 0 1 0 1 1 1 0|0 0 0 0 0 1 0 0|0 0 0 0 1 0 0 1|(resvd)| prio  |
+---------------+---------------+---------------+---------------+
    Type=46         Length=4        MP_OPT=9

The following values are available for the Prio field:¶

• 0: Do not use. The path is not available.¶

• 1: Standby: Do not use this path for traffic scheduling if another path (secondary or primary) is available. The path will only be used if other secondary or primary paths are not established.¶

• 2: Secondary: Do not use this path for traffic scheduling if the other paths are good enough. The path will be used occasionally for increasing the available capacity temporarily, e.g., when primary paths are congested or are not available. This is the recommended setting for paths that have costs or data caps as these paths will be used less frequently then primary paths.¶

• 3 - 15: Primary: The path can be used for packet scheduling decisions. The priority number indicates the relative priority of one path over the other for primary paths. Higher numbers indicate higher priority. The peer should consider sending traffic first over higher priority paths. This is the recommended setting for paths that do not have a cost or data caps associated with them as these paths will be frequently used.¶

Example use cases include:¶

1. Setting the Wi-Fi path to Primary and Cellular path to Secondary. In this case, Wi-Fi will be used and Cellular will be used only if the Wi-Fi path is congested or not available. Such setting results in using the Cellular path only temporally, if more capacity is needed than the Wi-Fi path can provide, indicating a clear priority of the Wi-Fi path over the Cellular due to, e.g., cost reasons.¶

2. Setting the Wi-Fi path to Primary and Cellular path to Standby. In this case, Wi-Fi will be used and Cellular will be used only if the Wi-Fi path is not available.¶

3. Setting the Wi-Fi path to Primary and Cellular path to Primary. In this case, both paths can be used when making packet scheduling decisions.¶

If not specified, the default behavior is to always use a path for packet scheduling decisions (MP_PRIO=3), when the path has been established and added to an existing MP-DCCP connection. At least one path ought to have an MP_PRIO value greater than or equal to one for it to be allowed to send on the connection. It is RECOMMENDED to update at least one path to a non-zero MP_PRIO value when an MP-DCCP connection enters a state where all paths remain with an MP_PRIO value of zero. This helps an MP-DCCP connection to schedule when the multipath scheduler strictly respects MP_PRIO value 0. To ensure reliable transmission, the MP_PRIO suboption MUST be acknowledged via an MP_CONFIRM (see Table 4).¶

The relative ratio of the primary path values 3-15 depends on the path usage strategy, which is described in more detail in Section 3.11. In the case of path mobility (Section 3.11.1), only one path can be used at a time and MUST have the highest available priority value. That also includes the prio numbers 1 and 2. In the other case of concurrent path usage (Section 3.11.2), the definition is up to the multipath scheduler logic.¶

An MP_SEQ (Section 3.2.5) MUST be present in a DCCP datagram in which the MP_PRIO suboption is sent. Further details are given in Section 3.2.1.¶

3.2.11. MP_CLOSE

The mechanism available in DCCP [RFC4340] for closing a connection cannot give an indication for closing an MP-DCCP connection, which typically contains several DCCP subflows; therefore, one cannot conclude from the closing of a subflow to the closing of an MP-DCCP connection. This is solved by introducing MP_CLOSE.¶

            1          2          3
 01234567 89012345 67890123 45678901 23456789
+--------+--------+--------+--------+--------+
|00101110|  var   |00001010| Key Data ...
+--------+--------+--------+--------+--------+
 Type=46   Length  MP_OPT=10

An MP-DCCP connection can be gracefully closed by sending an MP_CLOSE to the peer host. On all subflows, the regular termination procedure described in [RFC4340] MUST be initiated using MP_CLOSE in the initial packet (either a DCCP-CloseReq or a DCCP-Close). When a DCCP-CloseReq is used, the following DCCP-Close MUST also carry the MP_CLOSE to avoid keeping a state in the sender of the DCCP-CloseReq. At the initiator of the DCCP-CloseReq, all sockets, including the MP-DCCP connection socket, transition to CLOSEREQ state. To protect from unauthorized shutdown of a multipath connection, the selected Key Data of the peer host MUST be included in the MP_CLOSE option during the handshake procedure and MUST be validated by the peer host. Please note that the Key Data sent in DCCP-CloseReq will not be the same as the Key Data sent in DCCP-Close as these originate from different ends of the connection.¶

On reception of the first DCCP-CloseReq carrying an MP_CLOSE with valid Key Data, or due to a local decision, all subflows transition to the CLOSING state before transmitting a DCCP-Close carrying MP_CLOSE. The MP-DCCP connection socket on the host sending the DCCP-Close reflects the state of the initial subflow during the handshake with MP_KEY option. If the initial subflow no longer exists, the state moves immediately to CLOSED.¶

Upon reception of the first DCCP-Close carrying an MP_CLOSE with valid Key Data at the peer host, all subflows, as well as the MP-DCCP connection socket, move to the CLOSED state. After this, a DCCP-Reset with Reset Code 1 MUST be sent on any subflow in response to a received DCCP-Close containing a valid MP_CLOSE option.¶

When the MP-DCCP connection socket is in CLOSEREQ or CLOSED state, new subflow requests using MP_JOIN MUST be ignored.¶

Contrary to an MP_FAST_CLOSE (Section 3.2.3), no single-sided abrupt termination is applied.¶

3.2.12. Experimental Multipath Option MP_EXP for Private Use

This section reserves a Multipath Option to define and specify any experimental additional feature for improving and optimizing the MP-DCCP protocol. This option could be applicable to specific environments or scenarios according to potential new requirements and is meant for private use only. MP_OPT Feature Number 11 is specified with an exemplary description as below:¶

                     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
+---------------+---------------+---------------+---------------+
|0 0 1 0 1 1 1 0|      var      |0 0 0 0 1 0 1 1|     Data      |
+---------------+---------------+---------------+---------------+
|   ...
+---------------------------------------------------------------+
     Type=46         Length         MP_OPT=11

The Data field can carry any data according to the foreseen use by the experimenters with a maximum Length of 252 bytes.¶

3.4.1. Advertising a New Path (MP_ADDADDR)

When a host (Host A) wants to advertise the availability of a new path, it should use the MP_ADDADDR option (Section 3.2.8) as shown in the example in Figure 22. The MP_ADDADDR option passed in the DCCP-Data contains the following parameters:¶

• an identifier (id 2) for the new IP address, which is used as a reference in subsequent control exchanges¶

• a Nonce value to prevent replay attacks¶

• the IP address of the new path (A2_IP)¶

• a pair of bytes specifying the port number associated with this IP address. The value of 00 here indicates that the port number is the same as that used for the initial subflow address A1_IP.¶

According to Section 3.2.8, the following options are required in a packet carrying MP_ADDADDR:¶

• the leftmost 20 bytes of the HMAC(A) generated during the initial handshake procedure described in Sections 3.3 and 3.2.6¶

• the MP_SEQ option with the sequence number (seqno 12) for this message, according to Section 3.2.5¶

Host B acknowledges receipt of the MP_ADDADDR message with a DCCP-Ack containing the MP_CONFIRM option. The parameters supplied in this response are as follows:¶

• an MP_CONFIRM containing the MP_SEQ number (seqno 12) of the packet carrying the option that we are confirming together with the MP_ADDADDR option¶

• the leftmost 20 bytes of the HMAC(B) generated during the initial handshake procedure (Section 3.3)¶

          Host A                                         Host B
------------------------                              -----------
Address A1    Address A2                               Address B1
----------    ----------                              -----------
     |             |                                       |
     |   DCCP-Data +  MP_ADDADDR(id 2, Nonce, A2_IP, 00) + |
     |------- MP_HMAC(A) + MP_SEQ(seqno 12) -------------->|
     |             |                                       |
     |   DCCP-Ack + MP_HMAC(B) +                           |
     |<----- MP_CONFIRM(seqno 12, MP_ADDADDR) -------------|

3.4.2. Removing a Path (MP_REMOVEADDR)

When a host (Host A) wants to indicate that a path is no longer available, it should use the MP_REMOVEADDR option (Section 3.2.9) as shown in the example in Figure 23. The MP_REMOVEADDR option passed in the DCCP-Data contains the following parameters:¶

• an identifier (id 2) for the IP address to remove (A2_IP) and that was specified in a previous MP_ADDADDR message¶

• a Nonce value to prevent replay attacks¶

According to Section 3.2.9, the following options are required in a packet carrying MP_REMOVEADDR:¶

• the leftmost 20 bytes of the HMAC(A) generated during the initial handshake procedure described in Sections 3.3 and 3.2.6¶

• the MP_SEQ option with the sequence number (seqno 33) for this message, according to Section 3.2.5¶

Host B acknowledges receipt of the MP_REMOVEADDR message with a DCCP-Ack containing the MP_CONFIRM option. The parameters supplied in this response are as follows:¶

• an MP_CONFIRM containing the MP_SEQ number (seqno 33) of the packet carrying the option that we are confirming, together with the MP_REMOVEADDR option¶

• the leftmost 20 bytes of the HMAC(B) generated during the initial handshake procedure (Section 3.3)¶

          Host A                                         Host B
------------------------                              -----------
Address A1    Address A2                               Address B1
----------    ----------                              -----------
     |             |                                       |
     |   DCCP-Data +  MP_REMOVEADDR(id 2, Nonce) +         |
     |------- MP_HMAC(A) + MP_SEQ(seqno 33) -------------->|
     |             |                                       |
     |   DCCP-Ack + MP_HMAC(B) +                           |
     |<----- MP_CONFIRM(seqno 33, MP_REMOVEADDR) ----------|

3.11.1. Path Mobility

The path mobility strategy provides the use of a single path with a seamless handover function to continue the connection when the currently used path is deemed unsuitable for service delivery. Some of the DCCP subflows of an MP-DCCP connection might become inactive due to either the occurrence of certain error conditions (e.g., DCCP timeout, packet loss threshold, RTT threshold, and closed/removed) or adjustments from the MP-DCCP user. When there is outbound data to send and the primary path becomes inactive (e.g., due to failures) or deprioritized, the MP-DCCP endpoint SHOULD try to send the data through an alternate path with a different source or destination address (depending on the point of failure), if one exists. This process SHOULD respect the path priority configured by the MP_PRIO suboption; otherwise, if the path priority is not available, pick the most divergent source-destination pair from the originally used source-destination pair.¶

3.11.2. Concurrent Path Usage

Different from a path mobility strategy, the selection between MP-DCCP subflows is a per-packet decision that is a part of the multipath scheduling process. This method would allow multiple subflows to be simultaneously used to aggregate the path resources to obtain higher connection throughput.¶

In this scenario, the selection of congestion control, per-packet scheduling, and a potential reordering method determines a concurrent path utilization strategy and result in a particular transport characteristic. A concurrent path usage method uses a scheduling design that could seek to maximize reliability, maximize throughput, minimize latency, etc.¶

Concurrent path usage over the Internet can have implications. When an MP-DCCP connection uses two or more paths, there is no guarantee that these paths are fully disjoint. When two (or more) subflows share the same bottleneck, using a standard congestion control algorithm could result in an unfair distribution of the capacity with the multipath connection using more capacity than competing single-path connections.¶

Multipath TCP uses the coupled congestion control Linked Increases Algorithm (LIA) specified in an experimental specification [RFC6356] to solve this problem. This scheme could also be specified for MP-DCCP. The same applies to other coupled congestion control algorithms that have been proposed for Multipath TCP such as the Opportunistic Linked Increases Algorithm [OLIA].¶

The specification of scheduling for concurrent multipath and related congestion control algorithms and reordering methods for use in the general Internet are outside the scope of this document. If, and when, the IETF specifies a method for concurrent usage of multiple paths for the general Internet, the framework specified in this document could be used to provide an IETF-recommended method for MP-DCCP.¶