Deutsche Telekom

Deutsche-Telekom-Allee 9 Darmstadt 64295 Germany Markus.Amend@telekom.de

Karlstad University

Universitetsgatan 2 Karlstad 651 88 Sweden anna.brunstrom@kau.se

Karlstad University

Universitetsgatan 2 Karlstad 651 88 Sweden andreas.kassler@kau.se

City University of London

Northampton Square London United Kingdom veselin.rakocevic.1@city.ac.uk

BT

Adastral Park Martlesham Heath IP5 3RE United Kingdom stephen.h.johnson@bt.com

transport Transport Area Working Group Internet-Draft DCCP communications as defined in are restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of available multiple paths for a DCCP session could improve resource usage within the network and, thus, improve user experience through higher throughput and improved resilience to network failures. Use cases for a Multipath DCCP (MP-DCCP) are mobile devices (e.g., handsets, vehicles) and residential home gateways simultaneously connected to distinct networks as, e.g., a cellular and a Wireless Local Area (WLAN) networks or a cellular and a fixed access networks. Compared to existing multipath protocols, such as MPTCP, MP-DCCP provides specific support for non-TCP user traffic (e.g., UDP or plain IP). More details on potential use cases are provided in , , and . All these use cases profit from an Open Source Linux reference implementation provided under . This document specifies a set of extensions to DCCP to support multipath operations. Multipath DCCP provides the ability to simultaneously use multiple paths between peers. The protocol offers the same type of service to applications as DCCP and it provides the components necessary to establish and use multiple DCCP flows across different paths simultaneously.

Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at . Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 14 April 2024.

Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents () in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

Table of Contents

• .  Introduction
  • .  Multipath DCCP in the Networking Stack
  • .  Terminology
• .  Operation Overview
  • .  MP-DCCP Concept
• .  Requirements Language
• .  MP-DCCP Protocol
  • .  Multipath Capable Feature
  • .  Multipath Option
    • .  MP_CONFIRM
    • .  MP_JOIN
    • .  MP_FAST_CLOSE
    • .  MP_KEY
    • .  MP_SEQ
    • .  MP_HMAC
    • .  MP_RTT
    • .  MP_ADDADDR
    • .  MP_REMOVEADDR
    • . MP_PRIO
    • . MP_CLOSE
    • . Experimental MP-DCCP Sub-Option MP_EXP for private use
  • .  MP-DCCP Handshaking Procedure
  • .  Address knowledge exchange
    • .  Advertising a new path (MP_ADDADDR)
    • .  Removing a path (MP_REMOVEADDR)
  • .  Close a MP-DCCP connection
  • .  Fallback
  • .  Congestion Control Considerations
  • .  Maximum Packet Size Considerations
  • .  Path usage strategies
    • .  Path mobility
    • .  Concurrent path usage
• .  Security Considerations
• .  Interactions with Middleboxes
• .  Implementation
• .  Acknowledgments
• .  IANA Considerations
• . References
  • .  Normative References
  • .  Informative References
• .  Differences from Multipath TCP
• Authors' Addresses

Introduction Datagram Congestion Control Protocol (DCCP) is a transport protocol that provides bidirectional unicast connections of congestion-controlled unreliable datagrams. DCCP communications are restricted to one single path. Multipath DCCP (MP-DCCP) provides a set of extensions to DCCP to enable a DCCP connection to simultaneously establish flow across multiple paths. This can be beneficial to applications that transfer large amounts of data, by utilizing the capacity/connectivity offered by multiple paths. In addition, the multipath extensions enable to tradeoff timeliness and reliability, which is important for low-latency applications that do not require guaranteed delivery services, such as Audio/Video streaming. MP-DCCP was first suggested in the context of the 3GPP work on 5G multi-access solutions and for hybrid access networks , where MP-DCCP can be applied for load-balancing, seamless session handover, and bandwidth aggregation (referred to as Access Traffic Steering, Switching, and Splitting (ATSSS) in the 3GPP terminology ). This document specifies a set of protocol changes that add multipath support to DCCP; specifically, support for signaling and setting up multiple paths (a.k.a, "subflows"), managing these subflows, reordering of data, and termination of sessions. DCCP, as stated in does not provide reliable and ordered delivery. Consequently, multiple application subflows may be multiplexed over a single DCCP connection with no inherent performance penalty for application subflows that do not require in-ordered delivery. DCCP does not provide built-in support for those multiple application subflows. Encapsulation for DCCP in UDP is defined in . proposes a lightweight encapsulation for DCCP flow headers appropriate for unreliable IP traffic in terms of UDP and other non-TCP packets in comparison to MPTCP. This is not considered in the present specification.

Multipath DCCP in the Networking Stack MP-DCCP provides a set of features to DCCP; illustrates this layering. It operates at the transport layer and can be used as a transparent for both higher and lower layers. It is designed to be used by applications in the same way as DCCP with no changes to the application itself.

Comparison of Standard DCCP and MP-DCCP Protocol Stacks

Terminology This document uses terms that are either specific for multipath transport or are defined in the context of MP-DCCP, similar to , as follows: Path: A sequence of links between a sender and a receiver, defined in this context by a 4-tuple of source and destination address/ port pairs. (MP-DCCP) Connection: A set of one or more subflows, over which an application can communicate between two hosts. The MP-DCCP connection is exposed as single DCCP socket to the application. Token: A locally unique identifier given to a multipath connection by a host. May also be referred to as a "Connection ID". Host: An end host operating an MP-DCCP implementation, and either initiating or accepting an MP-DCCP connection. SubFlow: A subflow refers to a DCCP flows transmitted using a specific path (4-tuple of source and destination address/port pairs) that forms one of the multipath flows used by a single connection. In addition to these terms, within the framework of MP-DCCP the interpretation of, and effect on, regular single-path DCCP semantics is discussed in . The term SubFlow is not to be confused with an "application sub-flows" mentioned in Section 17.2 of . Application subflows are differentiated by source and destination port per application as, for example, enabled by Service Codes introduced to DCCP in , and those application subflows can be multiplexed over a single DCCP connection. For the sake of consistency, this specification assumes a single application is served by a DCCP connection, as shown in . This ought to not impact DCCP operation on each single path as noted in (Section 2.4 of ). Application subflows can co-exist with MP-DCCP operation as specified in this document.

Operation Overview DCCP (Section 17.2 of ) allows multiple application subflows to be multiplexed over a single DCCP connection with potentially same performance. However, DCCP does not provide built-in support for managing multiple subflows within one DCCP connection. Various congestion control mechanisms have been specified to optimize DCCP performance for specific traffic types in terms of profiles denoted by a Congestion Control IDentifier (CCID). The extension of DCCP for Multipath-DCCP (MP-DCCP) is described in detail in . At a high level of the MP-DCCP operation, the data stream from a DCCP application is split by MP-DCCP operation into one or more subflows which can be transmitted via different also physically isolated paths. The corresponding control information allows the receiver to optionally re-assemble and deliver the received data in the originally transmitted order to the recipient application. This may be necessary because DCCP does not guarantee in-order delivery. The details of the transmission scheduling mechanism and optional reordering mechanism are up to the sender and receiver, respectively, and are outside the scope of this document. The following sections define MP-DCCP behavior in detail. A Multipath DCCP connection provides a bidirectional connection of datagrams between two hosts exchanging data using in DCCP. It does not require any change to the applications. Multipath DCCP enables the hosts to use multiples paths with different IP addresses to transport the packets of an MP-DCCP connection. MP-DCCP manages the request, set-up, authentication, prioritization, modification, and removal of the DCCP subflows on different paths as well as the exchange of performance parameters.
The number of DCCP subflows can vary during the lifetime of a Multipath DCCP connection. The details of the path management decisions for when to add or remove subflows are outside the scope of this document. The Multipath Capability for MP-DCCP is negotiated with a new DCCP feature, as specified in . Once negotiated, all subsequent MP-DCCP operations for that connection are signalled with a variable length multipath-related option, as described in . All MP-DCCP operations are signaled by MP-DCCP suboptions described in {#MP_OPT}.

MP-DCCP Concept provides a general overview of the MP-DCCP working mode, whose main characteristics are summarized in this section.

Example MP-DCCP Usage Scenario | | |<-----------------------------------| | | | | | | | (DCCP subflow setup)| | | |--------------------->| | | |<---------------------| | | merge individual DCCP subflows to one MP-DCCP connection | | | | ]]>

• An MP-DCCP connection begins with a 4-way handshake, between two hosts as described in . In , an MP-DCCP connection is established between addresses A1 and B1 on Hosts A and B, respectively. MP-DCCP does not require both peers to have more than one address.
• When additional paths and corresponding addresses/ports are available, additional DCCP subflows are created on these paths and are attached to the existing MP-DCCP session, which continues to provide a single connection to the applications at both endpoints. The example illustrates creation of an additional DCCP subflow between Address A2 on Host A and Address B1 on Host B.
• MP-DCCP identifies multiple paths by the presence of multiple addresses/ports at hosts. Combinations of these multiple addresses/ports indicate the additional paths. In the example, other potential paths that could be set up are A1<->B2 and A2<->B2. Although this additional subflow is shown as being initiated from A2, it could alternatively have been initiated from B1 or B2.
• The discovery and setup of additional subflows is achieved through a path management method including the logic and details of the procedures for adding/removing subflows; this document describes measures to allow a host to initiate new subflows and signal available addresses between peers. The definition of a path management method is, however, out of scope of this document and subject to a corresponding policy and the specifics of the implementation. If a MP-DCCP peer host limits the maximum number of paths that can be maintained (e.g., similar to what is discussed in Section 3.4 of , the creation of new subflows from that peer host needs to be avoided and incoming subflow requests terminated.
• MP-DCCP adds connection-level sequence numbers and exchange of Round-Trip Time (RTT) information to enable optional reordering features.
• Subflows are terminated in the same way as regular DCCP connections, as described in (, Section 8.3). The MP-DCCP connection is terminated by a connection-level DCCP-CloseReq or DCCP-Close message.

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

MP-DCCP Protocol The DCCP protocol feature list (, Section 6.4) is updated by adding a new Multipath feature with Feature number THIS-FEATURE, as shown in . Multipath Feature

Number	Meaning	Rule	Rec'n Value	Initial Req'd
THIS-FEATURE	Multipath Capable	SP	0	N

Rec'n Rule:

The reconciliation rule used for the feature. SP indicates the server-priority, NN indicates this is non-negotiable.

Initial Value:

The initial value for the feature. Every feature has a known initial value.

Req'd:

This column is "Y" if and only if every DCCP implementation MUST understand the feature. If it is "N", then the feature behaves like an extension, and it is safe to respond to Change options for the feature with empty Confirm options.

This specification adds a DCCP protocol option as defined in (, Section 5.8) providing a new Multipath related variable-length option with option type 46, as shown in . Multipath Option Set

Type	Option Length	Meaning	DCCP-Data?
46	variable	Multipath	Y

Multipath Capable Feature A DCCP endpoint negotiates the Multipath Capable Feature to determine whether multipath extensions can be enabled for a DCCP connection. The Multipath Capable feature (MP_CAPABLE) has feature number THIS-FEATURE and has a length of one-byte. The leftmost four bits specify the compatible version of the MP-DCCP implementation (THIS-VER for this specification). The following four bits are unassigned in version, THIS-VER. The unassigned bits MUST be set to zero by the sender and MUST be ignored by the receiver.

Format of the Multipath Capable feature

The setting of the MP_CAPABLE feature MUST follow the server-priority reconciliation rule described in (, Section 6.3.1). This allows multiple versions to be specified in order of priority. The negotiation MUST be a part of the initial handshake procedure described in . No subsequent re-negotiation of the MP_CAPABLE feature is allowed for the same MP-DCCP connection. Clients MUST include a Change R option during the initial handshake request to supply a list of supported MP-DCCP protocol versions, ordered by preference. Servers MUST include a Confirm L option in the subsequent response to agree on an MP-DCCP version to be used from the Client list, followed by its own supported version(s), ordered by preference. Any subflow added to an existing MP-DCCP connection MUST use the version negotiated for the first subflow. If no agreement is found, the Server MUST reply with an empty Confirm L option with feature number THIS-FEATURE and no values. An example of successful version negotiation is shown hereafter:

Example of MP-DCCP support negotiation using MP_CAPABLE DCCP-Resp + Confirm L(MP_CAPABLE, 1, 2 1 0) <----------------------------------- * agreement on version = 1 * ]]>

1. The Client indicates support for both MP-DCCP versions 1 and 0, with a preference for version 1.
2. Server agrees on using MP-DCCP version 1, and supplies its own preference list.
3. MP-DCCP is then enabled between the Client and Server with version 1.

If the version negotiation fails or the MP_CAPABLE feature is not present in the DCCP-Request or DCCP-Response packets of the initial handshake procedure, the MP-DCCP connection SHOULD fallback to regular DCCP or MUST close the connection. Further details are specified in

Multipath Option MP-DCCP uses one single option to signal various multipath-related operations. The format of this option is shown in .

Multipath Option Format

The fields used by the the multipath option are described in . MP_OPT refers to an MP-DCCP suboption. MP_OPT Option Types

Type	Option Length	MP_OPT	Meaning
46	var	0 =MP_CONFIRM	Confirm reception and processing of an MP_OPT option
46	12	1 =MP_JOIN	Join path to an existing MP-DCCP connection
46	var	2 =MP_FAST_CLOSE	Close an MP-DCCP connection unconditionally
46	var	3 =MP_KEY	Exchange key material for MP_HMAC
46	9	4 =MP_SEQ	Multipath Sequence Number
46	23	5 =MP_HMAC	HMA Code for authentication
46	12	6 =MP_RTT	Transmit RTT values
46	var	7 =MP_ADDADDR	Advertise additional Address
46	4	8 =MP_REMOVEADDR	Remove Address
46	4	9 =MP_PRIO	Change Subflow Priority
46	var	10 =MP_CLOSE	Close an MP-DCCP subflow
46	TBD	>10	Reserved for future MP suboptions.

Future MP suboptions could be defined in a later version or extension to this specification. These operations are largely inspired by the signals defined in .

MP_CONFIRM

Format of the MP_CONFIRM suboption

Some multipath options require confirmation from the remote peer (see ). Such options will be retransmitted by the sender until a MP_CONFIRM is received or confirmation of options is identified outdated. The further processing of the multipath options in the receiving host is not the subject of MP_CONFIRM. Multipath suboptions could arrive out-of-order, therefore suboptions defined in MUST be sent in a DCCP datagram with MP_SEQ . This allows a receiver to identify whether suboptions are associated with obsolete datasets that would otherwise conflict with newer datasets. In 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 suboption is detected at the receiver if a previous suboption referring to the same dataset contained a higher sequence number in the MP_SEQ. AN MP_CONFIRM MAY be generated for suboptions 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 suboption 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 suboptions and the received MP_SEQ, which is both copied verbatim and appended as a list of confirmations. The list is structured by first listing the received MP_SEQ followed by the confirmed suboption or suboptions. The same rules apply when suboptions with different MP_SEQs are confirmed at once. This could happen if a datagram with MP_PRIO and a first MP_SEQ_1 and another datagram with MP_ADDADDR and a second MP_SEQ_2 are received in short succession. In this case, the structure described above is concatenated resulting in MP_SEQ_1 + MP_PRIO + MP_SEQ_2 + MP_ADDADDR. Multipath options requiring confirmation

Type	Option Length	MP_OPT	MP_CONFIRM Sending path
46	var	7 =MP_ADDADDR	Any available
46	4	8 =MP_REMOVEADDR	Any available
46	4	9 =MP_PRIO	Any available

An example to illustrate the MP-DCCP confirm procedure for the MP_PRIO option is shown in . The host A sends a DCCP-Request on path A2-B2 with an MP_PRIO option with value 1 and associated sequence number of 1. Host B replies on the same path in this instance (but could be any path) 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)

Example MP-DCCP CONFIRM procedure | | | | | | | DCCP-Response + | | | |<---- MP_CONFIRM(seqno 1, MP_PRIO) --------| | | | | ]]>

A second example to illustrate the same MP-DCCP confirm procedure but where an out of date option is also delivered is shown in (. Here, a first DCCP-Data is sent from Host A to Host B with option MP_PRIO set to 4. Host A subsequently sends a 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 confirming receipt of the MP_PRIO(1) with sequence number 2.

Example MP-DCCP CONFIRM procedure with outdated suboption | | | \ | | | | -------------------------->| | | | | | | DCCP-Ack + | | | |<---- MP_CONFIRM(seqno 2, MP_PRIO) --------| | | | | ]]>

MP_JOIN

Format of the MP_JOIN Suboption

The MP_JOIN option is used to add a new subflow to an existing MP-DCCP connection and REQUIRES a successful establishment of the first subflow using MP_KEY. The Path Token is the SHA256 hash of the derived key (d-key), which was previously exchanged with the MP_KEY option. MP_HMAC MUST be set when using MP_JOIN to provide authentication (See for details). The MP_JOIN option includes an "Addr ID" (Address ID) generated by the sender of the option, 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 () 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 (), 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 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. An Address ID always MUST be unique over the lifetime of a subflow and can only be re-assigned 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 Token, the Nonce value builds the basis to calculate the HMAC used in the handshaking process as described in . If the path token can not be verified by the receiving host during a handshake negotiation, the new subflow MUST be closed, as specified in .

MP_FAST_CLOSE DCCP can send a Close or Reset signals 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. An MP-DCCP-level "Reset" allows the abrupt closure of the MP-DCCP connection using the MP_FAST_CLOSE suboption.

Format of the MP_FAST_CLOSE Suboption

The MP_FAST_CLOSE suboption MUST be sent from an initiating host on all subflows using a DCCP-Reset packet with Reset Code THIS-RESET-CODE. To protect from unauthorized shutdown of a multipath DCCP connection, the selected Key Data of the peer host during the handshaking procedure is carried by the MP_FAST_CLOSE option. On completion of this step, the initiating host tears down all subflows and the multipath DCCP connection immediately terminates. Upon reception of the MP_FAST_CLOSE and successful validation of the Key Data, a DCCP Reset packet response is sent on all subflows to the initiating host with Reset Code THIS-RESET-CODE. The host then closes the MP-DCCP connection (i.e., it transitions the connection state to CLOSED).

MP_KEY

Format of the MP_KEY Suboption

The MP_KEY suboption is used to exchange key material between hosts for a given connection. The Length varies between 12 and 68 Bytes for a single-key message, and up to 110 Bytes when all specified Key Types 0-2 are provided. The Key Type field specifies the type of the following key data. The set of key types are shown in . MP_KEY Key Types

Key Type	Key Length (Bytes)	Meaning
0 =Plain Text	8	Plain Text Key
1 =ECDHE-C25519-SHA256	32	ECDHE with SHA256 and Curve25519
2 =ECDHE-C25519-SHA512	64	ECDHE with SHA512 and Curve25519
3-255		Unassigned

Plain Text

Key Material is exchanged in plain text between hosts, and the key parts (key-a, key-b) are used by each host to generate the derived key (d-key) by concatenating the two parts with the local key in front (e.g. hostA d-key(A)=(key-a+key-b), hostB d-key(B)=(key-b+key-a)).

ECDHE-SHA256-C25519

Public Key Material is exchanged via ECDHE key exchange with SHA256 and Curve 25519 to generate the derived key (d-key) from the shared secret.

ECDHE-SHA512-C25519

Public Key Material is exchanged via ECDHE key exchange with SHA512 and Curve 25519 to generate the derived key (d-key) from the shared secret.

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 If the key type can not be agreed in the handshake procedure, the MP-DCCP connection MUST fallback to not using MP-DCCP, as indicated in

MP_SEQ

Format of the MP_SEQ Suboption

The MP_SEQ suboption is used for end-to-end datagram-based sequence numbers of an MP-DCCP connection. The initial data sequence number (IDSN) SHOULD be set randomly . The MP_SEQ number space is independent from the path individual sequence number space and MUST be sent with all DCCP-Data and DCCP-DataACK packet. When the sequence number space is exhausted, the sequence number MUST be wrapped. provides guidance on selecting an appropriately sized sequence number space according to the maximum segment lifetime 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 Maximum Segment Lifetime is the same as that of TCP as specified in , Section 3.4. 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.

MP_HMAC

Format of the MP_HMAC Suboption

The MP_HMAC suboption is used to provide authentication for the MP_JOIN, MP_ADDADDR, and MP_REMOVEADDR suboptions. The HMAC code is generated according to in combination with the SHA256 hash algorithm described in , with the output truncated to the leftmost 160 bits (20 bytes). The "Key" used for the HMAC computation is the derived key (d-key) described in , while the HMAC "Message" is a concatenation of

• MP_JOIN: The token and nonce of the MP_JOIN for which authentication shall be performed.
• MP_ADDADDR: The Address ID with associated IP address and if defined port, otherwise two octets of value 0.
• MP_REMOVEADDR: Solely the Address ID.

MP_JOIN, MP_ADDADDR and MP_REMOVEADDR can co-exist or be used multiple times within a single DCCP packet. All these multipath options include an individual MP_HASH option. This ensures the MP_HASH is correctly associated. Otherwise, the receiver cannot validate multiple MP_JOIN, MP_ADDADDR or MP_REMOVEADDR. Therefore, a MP_HASH MUST directly follow its associated multipath option. In the likely case of sending a MP_JOIN together with a 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 with the MP_ADDADDR suboption. If the HMAC can not be validated by a receiving host, the subsequent handling depends on which suboption was being authenticated. If the suboption to be authenticated was either MP_ADDADDR or MP_REMOVEADDR, the receiving host MUST silently ignore it (see and ). If the suboption to be authenticated was MP_JOIN, the subflow MUST be closed (see )

MP_RTT

Format of the MP_RTT Suboption

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

Raw RTT (=0)

Raw RTT value of the last Datagram Round-Trip, preferably provided by the CCID in use.

Min RTT (=1)

Min RTT value over a given period, preferably provided by the CCID in use.

Max RTT (=2)

Max RTT value over a given period, preferably provided by the CCID in use.

Smooth RTT (=3)

Averaged RTT value over a given period, preferably provided by the CCID in use.

Age

The Age parameter defines the time difference between now - 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 . RTT1 refers to the first path and RTT2 to the second path. The RTT values could be extracted from the sender's Congestion Control procedure and at the receiving host using the MP_RTT suboption. With the reception of RTT1 and RTT2, the receiver is able to calculate the path_delta which corresponds to the absolute difference of both values. In the case that the path individual RTTs are symmetric in the down- and uplink directions, packets with missing sequence number MP_SEQ, e.g., in a reordering process, can be assumed lost after path_delta/2.

Exemplary flow of MP_RTT exchange and usage

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 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 range between 8 and 22 bytes. The final 2 octets, 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 a 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 . In the same way as for MP_JOIN, 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 and selected in the original MP_KEY handshake. The message for the HMAC is the Address ID, IP address, and port that precede the HMAC in the MP_ADDADDR option. If the port is not present in the MP_ADDADDR option, the HMAC message will include 2 octets 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. Note that, additionally, 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 SHOULD silently ignore the option. The presence of a MP_SEQ MUST be ensured in a DCCP datagram in which MP_ADDADDR is sent, as described in .

Format of the MP_ADDADDR Suboption

Each address has an Address ID that could be used for uniquely identifying the address within a connection for address removal. The Address ID is also used to identify MP_JOIN options (see ) 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 token pair). In this way, there is a stored mapping between the Address ID, observed source address, and token pair for future processing of control information for a connection. Note that an implementation MAY discard incoming address advertisements, for example, for avoiding the required mapping state, or because advertised addresses are of no use to it (for example, IPv6 addresses when it has IPv4 only). Therefore, a host MUST treat address advertisements as soft state, and the sender MAY choose to refresh advertisements periodically. A host MAY advertise a private addresses, e.g., because there is a NAT on the path. It is not desirable to prohibit this, since there could be cases where both hosts have additional interfaces on the same private network. The MP_JOIN handshake to create a new subflow () provides mechanisms to minimize security risks. The MP_JOIN message contains a 32-bit token that uniquely identifies a connection to the receiving host. If the token is unknown, the host MUST send a DCCP-Reset. In the unlikely event that the token is already known, the SubFlow setup will continue, but the HMAC exchange must provide authentication, which will fail. This provides sufficient protection against two unconnected hosts accidentally setting up a new subflow using a private address. Further security considerations around the issue of MP_ADDADDR messages that accidentally misdirect, or maliciously direct, new MP_JOIN attempts are discussed in . If a sending host of an MP_ADDADDR knows that no incoming subflows can be established at a particular address, an MP_ADDADDR SHOULD 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 external NAT or firewall, or from connectivity checks performed by the host/application. The reception of an MP_ADDADDR message is acknowledged using MP_CONFIRM (). This ensures reliable exchange of address information. A host MAY send an MP_ADDADDR message with an already assigned Address ID, but the Address MUST be the same as previously assigned to this Address ID, and the Port MUST be different from one already in use for this Address ID. If these conditions are not met, the receiver SHOULD silently ignore the MP_ADDADDR. A host wishing to replace an existing Address ID MUST first remove the existing one (). A host that receives an MP_ADDADDR, but finds at connection set up that the IP address and port number is unsuccessful, SHOULD NOT perform further connection attempts to this address/port combination for this connection. However, a sender that wishes to trigger a new incoming connection attempt on a previously advertised address/port combination can therefore refresh the MP_ADDADDR information by sending the option again.

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. Along with the MP_REMOVEADDR suboption a 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 . In the same way as for MP_JOIN, 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 and selected in the original MP_KEY handshake. The message for the HMAC is the Address ID. The rationale for using a HMAC is to prevent unauthorized entities from injecting MP_REMOVEADDR signals in an attempt to hijack a connection. Note that, additionally, the presence of this HMAC prevents the address from being removed 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 SHOULD silently ignore the option. A receiver MUST include a MP_SEQ in a DCCP datagram that sends an MP_REMOVEADDR. Further details are given in . The reception of an MP_REMOVEADDR message is acknowledged using MP_CONFIRM (). This ensures 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 receipt of this message SHOULD trigger the closing procedure described in between client and server, respectively on the affected subflow(s) (if possible). This helps remove middlebox state, before removing any local state. Address removal is done by 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.

Format of the MP_REMOVEADDR Suboption followed by MP_HMAC option ]]>

A subflow that is still functioning MUST be closed with a DCCP-Close exchange as in regular DCCP, rather than using this option. For more information, see .

MP_PRIO The path priority SHOULD be considered as hints for the packet scheduler when making decisions 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., WiFi is free while cellular has limit on volume, 5G has higher energy consumption). The priority of a path could also change, for example, when a mobile host runs out of battery, 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 flag for the SubFlow over which the suboption is received.

Format of the MP_PRIO Suboption

The following values are available for 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 temporarily the available capacity, 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 Wi-Fi path to Primary and Cellular paths to Secondary. In this case Wi-Fi will be used and Cellular 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 WiFi path can provide, indicating a clear priority of the Wi-Fi path over the Cellular due to e.g. cost reasons. 2) Setting Wi-Fi path to Primary and Cellular to Standby. In this case Wi-Fi will be used and Cellular only, if the Wi-Fi path is not available. 3) Setting 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 a MP_PRIO value greater 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. MP_PRIO is assumed to be exchanged reliably using the MP_CONFIRM mechanisms (see ). A MP_SEQ MUST be present in a DCCP datagram in which MP_PRIO is sent. Further details are given in .

MP_CLOSE

Format of the MP_CLOSE Suboption

An MP-DCCP connection can be gracefully closed by sending and MP_CLOSE to the peer host. On all subflows, the regular termination procedure as described in 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 multi-path connection, the selected Key Data of the peer host during the handshaking procedure MUST be included in by the MP_CLOSE option and must be validated by the peer host. Note, the Key Data is different between MP_CLOSE option carried by DCCP-CloseReq or DCCP-Close. On reception of the first DCCP-CloseReq carrying a 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 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 a 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 CLOSE state, new subflow requests using MP_JOIN MUST be ignored. Contrary to a MP_FAST_CLOSE , no single-sided abrupt termination is applied.

Experimental MP-DCCP Sub-Option MP_EXP for private use This section reserves a MP-DCCP sub-option to define and specify any experimental additional feature for improving and optimization of MP-DCCP protocol. This option could be applicable to specific environments or scenarios according to potential new requirements and meant for private use only. MP_OPT feature number 11 is specified with an exemplary description as below:

Format of the MP_EXP Suboption

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

MP-DCCP Handshaking Procedure An example to illustrate the MP-DCCP handshake procedure is shown in .

Example MP-DCCP Handshake | |<---------------------- MP_KEY(Key-B) ---------------| | DCCP-Response + MP_CAPABLE | | | | | DCCP-Ack | | |--------- MP_KEY(Key-A) + MP_KEY(Key-B) ------------>| |<----------------------------------------------------| | DCCP-Ack | | | | | | | DCCP-Request + MP_CAPABLE | | |--- MP_JOIN(TB,RA) ------------------->| | |<------MP_JOIN(TB,RB) + MP_HMAC(B)-----| | |DCCP-Response + MP_CAPABLE | | | | | |DCCP-Ack | | |-------- MP_HMAC(A) ------------------>| | |<--------------------------------------| | |DCCP-ACK | ]]>

The basic initial handshake for the first subflow is as follows:

• Host A sends a DCCP-Request with the MP-Capable feature Change request and the MP_KEY option with an Host-specific Key-A for each of supported types as described in .
• Host B sends a DCCP-Response with Confirm feature for MP-Capable and the MP_Key option with a single Host-specific Key-B. The type of the key is chosen from the list of supported types from the previous request.
• Host A sends a DCCP-Ack with both Keys echoed to Host B.
• Host B sends a DCCP-Ack to confirm both keys and conclude the handshaking.

Host A waits for the final DCCP-Ack from host B before starting any establishment of additional subflow connections. The handshake for subsequent subflows based on a successful initial handshake is as follows:

• Host A sends a DCCP-Request with the MP-Capable feature Change request and the MP_JOIN option with Host B's Token TB, generated from the derived key by applying a SHA256 hash and truncating to the first 32 bits. Additionally, an own random nonce RA is transmitted with the MP_JOIN.
• Host B computes the HMAC of the DCCP-Request and sends a DCCP-Response with Confirm feature option for MP-Capable and the MP_JOIN option with the Token TB and a random nonce RB together with the computed MP_HMAC. The HMAC is calculated by taking the leftmost 20 bytes from the SHA256 hash of a HMAC code created by using the nonce received with MP_JOIN(A) and the local nonce RB as message and the derived key described in as key: MP_HMAC(B) = HMAC-SHA256(Key=d-key(B), Msg=RB+RA)
• Host A sends a DCCP-Ack with the HMAC computed for the DCCP-Response. The HMAC is calculated by taking the leftmost 20 bytes from the SHA256 hash of a HMAC code created by using the local nonce RA and the nonce received with MP_JOIN(B) as message and the derived key described in as key: MP_HMAC(A) = HMAC-SHA256(Key=d-key(A), Msg=RA+RB)
• Host B sends a DCCP-Ack to confirm the HMAC and to conclude the handshaking.

Address knowledge exchange

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 () as shown in the example in . 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. * the IP address of the new path (A2_IP) * A pair of octets 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 The following options MUST be included in a packet carrying MP_ADDADDR: * the leftmost 20 bytes of the HMAC(A) generated during the initial handshaking procedure described in and * the MP_SEQ option with the sequence number (seqno 12) for this message according to . 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 handshaking procedure

Example MP-DCCP ADDADDR procedure | | | | | DCCP-Ack + MP_HMAC(B) + | |<----- MP_CONFIRM(seqno 12, MP_ADDADDR) -------------| ]]>

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 () as shown in the example in . 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 which was specified in a previous MP_ADDADDR message. The following options must be included in a packet carrying MP_REMOVEADDR * the leftmost 20 bytes of the HMAC(A) generated during the initial handshaking procedure described in and * the MP_SEQ option with the sequence number (seqno 33) for this message according to . 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 handshaking procedure

Example MP-DCCP REMOVEADDR procedure | | | | | DCCP-Ack + MP_HMAC(B) + | |<----- MP_CONFIRM(seqno 33, MP_REMOVEADDR) ----------| ]]>

Close a MP-DCCP connection When a host wants to close an existing subflow but not the whole MP-DCCP connection, it MUST initiate the regular DCCP connection termination procedure as described in Section 5.6 of , i.e., it sends a DCCP-Close/DCCP-Reset on the subflow. This may be preceded by a DCCP-CloseReq. In the event of an irregular termination of a subflow, e.g., during subflow establishment, it MUST use an appropriate DCCP reset code as specified in IANA for DCCP operations. This could be, for example, sending reset code 5 (Option Error) when an MP-DCCP option provides invalid data or reset code 9 (Too Busy) when the maximum number of maintainable paths is reached. Note that receiving a reset code 9 for secondary subflows SHOULD NOT impact already existing active subflows. If necessary, these subflows are terminated in a subsequent step using the procedures described in this section. A host terminates an MP-DCCP connection using the DCCP connection termination specified in section 5.5 of on each subflow with the first packet on each subflow carrying MP_CLOSE (see ). [B's key] [on all subflows] <- DCCP-Reset [on all subflows] ]]> Additionally, an MP-DCCP connection may be closed abruptly using the "Fast Close" procedure described in , where a DCCP-Reset is sent on all subflows, each carrying the MP_FAST_CLOSE option. [B's key] [on all subflows] <- DCCP-Reset [on all subflows] ]]>

Fallback When a subflow fails to operate following MP-DCCP intended behavior, it is necessary to proceed with a fallback. This may be either falling back to regular DCCP or removing a problematic subflow. The main reasons for subflow failing include: no MP support at peer host, failure to negotiate protocol version, loss of MP-DCCP suboptions, faulty/non-supported MP-DCCP options or modification of payload data. At the start of an MP-DCCP connection, the handshake ensures exchange of MP-DCCP feature and options and thus ensures that the path is fully MP-DCCP capable. If during the handshake procedure it appears that DCCP-Request or DCCP-Response messages do not carry the MP_CAPABLE feature, the MP-DCCP connection will not be established and the handshake SHOULD fallback to regular DCCP (if this is not possible it MUST be closed). A connection SHOULD fallback to regular DCCP if the endpoints fail to agree on a protocol version to use during the Multipath Capable feature negotiation. This is described in . The protocol version negotiation distinguishes between negotiation for the initial connection establishment, and addition of subsequent subflows. If protocol version negotiation is not successful during the initial connection establishment, MP-DCCP connection will fallback to regular DCCP. The fallback procedure to regular DCCP MUST be also applied if the MP_KEY Key Type cannot be negotiated. If a subflow attempts to join an existing MP-DCCP connection, but MP-DCCP options or MP_CAPABLE feature are not present or are faulty in the handshake procedure, that subflow MUST be closed. This is especially the case if a different MP_CAPABLE version than the originally negotiated version is used. Reception of a non-verifiable MP_HMAC () or an invalid MP_JOIN Path Token during flow establishment MUST cause the subflow to be closed. The subflow closing procedure MUST be also applied if a final ACK carrying MP_KEY with wrong Key-A/Key-B is received or MP_KEY option is malformed. Another relevant case is when payload data is modified by middleboxes. DCCP uses checksum to protect the data, as described in section 9 of . A checksum will fail if the data has been changed in any way. All data from the start of the segment that failed the checksum onwards cannot be considered trustworthy. DCCP defines that if the checksum fails, the receiving endpoint MUST drop the application data and report that data as dropped due to corruption using a Data Dropped option (Drop Code 3, Corrupt). If data is dropped due to corruption for an MP-DCCP connection, the affected subflow MAY be closed.

Congestion Control Considerations Senders MUST manage per-path congestion status, and avoid to sending more data on a given path than congestion control for each path allows.

Maximum Packet Size Considerations A DCCP implementation maintains the maximum packet size (MPS) during operation of a DCCP session. This procedure is specified for single-path DCCP in , Section 14. Without any restrictions, this is adopted for MP-DCCP operations, in particular the PMTU measurement and the Sender Behaviour. The DCCP application interface SHOULD allow the application to discover the current MPS. This reflects the current supported largest size for the data stream that can be used across the set of all active MP-DCCP subflows.

Path usage strategies MP-DCCP can be configured to realize one of several strategies for path usage, via selecting one DCCP subflow of the multiple DCCP subflows within a MP-DCCP connection for data transmission. This can be a dynamic process further facilitated by the means of DCCP and MP-DCCP defined options such as path preference using MP-PRIO, adding or removing DCCP subflows using MP_REMOVEADDR, MP_ADDADDR or DCCP-Close/DCCP-Reset and also path metrics such as packet-loss-rate, CWND or RTT provided by the Congestion Control Algorithm. Selecting an appropriate method can allow MP-DCCP to realize different path utilization strategies that make MP-DCCP suitable for end-to-end implementation over the Internet or in controlled environments such as Hybrid Access or 5G ATSSS.

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 a MP-DCCP connection might become inactive due to either the occurrence of certain error conditions (e.g., DCCP timeout, packet loss threshold, RTT threshold, 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 de-prioritized, 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 MP_PRIO or if not available pick the most divergent source-destination pair from the original used source-destination pair. Note: Rules for picking the most appropriate source-destination pair are an implementation decision and are not specified within this document. Path mobility is supported in the current Linux reference implementation .

Concurrent path usage Different to 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 potential re-ordering 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, throughput, minimizing latency, etc. Concurrent path usage over the Internet can have implications. When a Multipath 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 scheme 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 the experimental specification to solve this problem. This scheme could also be specified for Multipath DCCP. The same applies to other coupled congestion control schemes that have been proposed for Multipath TCP such as Opportunistic Linked Increases Algorithm . The specification of scheduling for concurrent multipath and related the congestion control algorithms and re-ordering 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.

Security Considerations Similar to DCCP, MP-DCCP does not provide cryptographic security guarantees inherently. Thus, if applications need cryptographic security (integrity, authentication, confidentiality, access control, and anti-replay protection) the use of IPsec, DTLS over DCCP or other end-to-end security is recommended; Secure Real-time Transport Protocol (SRTP) is one candidate protocol for authentication. Together with Encryption of Header Extensions in SRTP, as provided by , also integrity would be provided. DCCP provides protection against hijacking and limits the potential impact of some denial-of-service attacks, but DCCP provides no inherent protection against an on-path attacker snooping on data packets. Regarding the security of MP-DCCP no additional risks should be introduced compared to regular DCCP. Thereof derived are the following key security requirements to be fulfilled by MP-DCCP:

• Provide a mechanism to confirm that parties involved in a subflow handshake are identical to those in the original connection setup.
• Provide verification that the new address to be included in a MP connection is valid for a peer to receive traffic at before using it.
• Provide replay protection, i.e., ensure that a request to add/remove a subflow is 'fresh'.

To achieve these goals, MP-DCCP includes a hash-based handshake algorithm documented in Sections and . The security of the MP-DCCP connection depends on the use of keys that are shared once at the start of the first subflow and are never sent again over the network. To ease demultiplexing while not revealing cryptographic material, future subflows use a truncated cryptographic hash of this key as the connection identification "token". The keys are concatenated and used as keys for creating Hash-based Message Authentication Codes (HMACs) used on subflow setup, in order to verify that the parties in the handshake are the same as in the original connection setup. It also provides verification that the peer can receive traffic at this new address. Replay attacks would still be possible when only keys are used; therefore, the handshakes use single-use random numbers (nonces) at both ends -- this ensures that the HMAC will never be the same on two handshakes. Guidance on generating random numbers suitable for use as keys is given in . During normal operation, regular DCCP protection mechanisms (such as header checksum to protect DCCP headers against corruption) is designed to provide the same level of protection against attacks on individual DCCP subflows as exists for regular DCCP. As discussed in , a host may advertise its private addresses, but these might point to different hosts in the receiver's network. The MP_JOIN handshake () is designed to ensure that this does not set up a subflow to the incorrect host. However, it could still create unwanted DCCP handshake traffic. This feature of MP-DCCP could be a target for denial-of-service exploits, with malicious participants in MP-DCCP connections encouraging the recipient to target other hosts in the network. Therefore, implementations should consider heuristics at both the sender and receiver to reduce the impact of this.

Interactions with Middleboxes Issues from interaction with on-path middleboxes such as NATs, firewalls, proxies, intrusion detection systems (IDSs), and others have to be considered for all extensions to standard protocols since otherwise unexpected reactions of middleboxes may hinder its deployment. DCCP already provides means to mitigate the potential impact of middleboxes, also in comparison to TCP (see , Section 16). When both hosts are located behind a NAT or firewall entity, specific measures have to be applied such as the -specified simultaneous-open technique that update the (traditionally asymmetric) connection-establishment procedures for DCCP. Further standardized technologies addressing middleboxes operating as NATs are provided in . specifies UDP Encapsulation for NAT Traversal of DCCP sessions, similar to other UDP encapsulations such as for SCTP . Future specifications by the IETF could specify other methods for DCCP encapsulation.

Implementation The approach described above has been implemented in open source across different testbeds and a new scheduling algorithm has been extensively tested. Also demonstrations of a laboratory setup have been executed and have been published at .

Acknowledgments and defined Multipath TCP and provided important inputs for this specification. The authors gratefully acknowledge significant input into this document from Dirk von Hugo, Nathalie Romo Moreno, Omar Nassef, Mohamed Boucadair, Simone Ferlin, and Behcet Sarikaya.

IANA Considerations This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to the MP extension of the DCCP protocol in accordance with . This document defines one new value which is requested to be allocated in the IANA DCCP Feature Numbers registry and three new registries to be allocated in the DCCP registry group. This document requests IANA to assign a new DCCP feature parameter for negotiating the support of multipath capability for DCCP sessions between hosts as described in . The following entry in should be added to the Feature Numbers registry in the DCCP registry group according to , Section 19.4. under the "DCCP Protocol" heading. Addition to DCCP Feature Numbers registry

Value	Feature Name	Specification
THIS-FEATURE (10 suggested)	MP-DCCP capability feature	[ThisDocument]

Sect. specifies the new 1-Byte entry above includes a 4-bit part to specify the version of the used MP-DCCP implementation. This document requests IANA to create a new 'MP-DCCP Versions' registry within the DCCP registry group to track the MP-DCCP version. The initial content of this registry is as follows: MP-DCCP Versions Registry

Version	Value	Specification
THIS-VER	Suggested 0000	[ThisDocument]
Unassigned	0001 - 1111

Future MP-DCCP versions 1 to 15 are assigned from this registry using the Specification Required policy (Section 4.6 of ). This document requests IANA to assign a new DCCP protocol option THIS-DCCP-OPTION (suggested type=46) as described in . In this document, THIS-DCCP-OPTION is replaced by 46 for better readability. IANA is requested to create a new 'MP-DCCP Suboptions' registry within the DCCP registry group. The following entries in should be added to the new 'MP-DCCP Suboptions' registry. The registry in has an upper boundary of 255 in the numeric value field. MP-DCCP Suboptions registry

Value	Symbol	Name	Reference
Type=46	MP_OPT	DCCP Multipath option
MP_OPT=0	MP_CONFIRM	Confirm reception/processing of an MP_OPT option
MP_OPT=1	MP_JOIN	Join subflow to existing MP-DCCP connection
MP_OPT=2	MP_FAST_CLOSE	Close MP-DCCP connection
MP_OPT=3	MP_KEY	Exchange key material for MP_HMAC
MP_OPT=4	MP_SEQ	Multipath Sequence Number
MP_OPT=5	MP_HMAC	Hash-based Message Auth. Code for MP-DCCP
MP_OPT=6	MP_RTT	Transmit RTT values and calculation parameters
MP_OPT=7	MP_ADDADDR	Advertise additional Address(es)/Port(s)
MP_OPT=8	MP_REMOVEADDR	Remove Address(es)/ Port(s)
MP_OPT=9	MP_PRIO	Change Subflow Priority
MP_OPT=10	MP_CLOSE	Close MP-DCCP subflow
MP_OPT=11	MP_EXP	Experimental Sub-Option for private use
MP_OPT>11	Unassigned	Reserved for future MP-DCCP suboptions

Future MP-DCCP sub-options with MP_OPT>11 are assigned from this registry using the Specification Required policy (Section 4.6 of ). In addition IANA is requested to assign a new DCCP Reset Code value THIS-RESET-CODE (13 suggested) in the DCCP Reset Codes Registry, with the short description "Abrupt MP termination". Use of this reset code is defined in section . In addition IANA is requested to assign for this version of the MP-DCCP protocol a new 'MP_KEY' registry containing three different sub options to the MP-KEY option to identify the MP_KEY Key types in terms of 8-bit values as specified in according to the entries in below. Values in range 3-255 (decimal) inclusive remain unassigned in this THIS-VER of the protocol and are assigned via Specification Required in potential future versions of the MP-DCCP protocol. MP-DCCP MP_KEY registry with type sub-options for key data exchange on different paths

Value	Key Type	Name or Meaning	Reference
0	Plain Text	Plain Text Key
1	ECDHE-C25519-SHA256	ECDHE with SHA256 and Curve25519
2	ECDHE-C25519-SHA512	ECDHE with SHA512 and Curve25519

References Normative References n.d. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Datagram Congestion Control Protocol (DCCP) is a transport protocol that provides bidirectional unicast connections of congestion-controlled unreliable datagrams. DCCP is suitable for applications that transfer fairly large amounts of data and that can benefit from control over the tradeoff between timeliness and reliability. [STANDARDS-TRACK] Informative References Deutsche Telekom Deutsche Telekom This document discusses the issue of packet reordering which occurs as a specific problem in multi-path connections without reliable transport protocols such as TCP. The topic is relevant for devices connected via multiple accesses technologies towards the network as is foreseen, e.g., within Access Traffic Selection, Switching, and Splitting (ATSSS) service of 3rd Generation Partnership Project (3GPP) enabling fixed mobile converged (FMC) scenario. Work in Progress Deutsche Telekom Deutsche Telekom Karlstad University Karlstad University City University of London More and more of today's devices are multi-homing capable, in particular 3GPP user equipment like smartphones. In the current standardization of the next upcoming mobile network generation 5G Rel.16, this is especially targeted in the study group Access Traffic Steering Switching Splitting [TR23.793]. ATSSS describes the flexible selection or combination of 3GPP untrusted access like Wi-Fi and cellular access, overcoming the single-access limitation of today's devices and services. Another multi-connectivity scenario is the Hybrid Access [I-D.lhwxz-hybrid-access-network-architecture][I-D. muley-network-based-bonding-hybrid-access], providing multiple access for CPEs, which extends the traditional way of single access connectivity at home to dual-connectivity over 3GPP and fixed access. A missing piece in the ATSSS and Hybrid Access is the access and path measurement, which is required for efficient and beneficial traffic steering decisions. This becomes particularly important in heterogeneous access networks with a multitude of volatile access paths. While MP-TCP has been proposed to be used within ATSSS, there are drawbacks when being used to encapsulate unreliable traffic as it blindly retransmits each lost frame leading to excessive delay and potential head-of-line blocking. A decision for MP-TCP though leaves the increasing share of UDP in today's traffic mix (<https://arxiv.org/abs/1801.05168>) unconsidered. In this document, a multi-access framework is proposed leveraging the MP-DCCP network protocol, which enables flexible traffic steering, switching and splitting also for unreliable traffic. A benefit is the support for pluggable congestion control which enables our framework to be used either independent or complementary to MP-TCP. Work in Progress Deutsche Telekom AG Deutsche Telekom AG Painless Security Huawei Huawei Residential and Enterprise customers require continuously increasing bandwidth, however it may be difficult for operators to update or rebuild existing fixed access networks, especially when they are deployed in certain areas. This document discusses a general way to address this problem by bundling together multiple heterogeneous access networks according to certain management policies. Work in Progress Nokia Nokia China Mobile D-Link Corp Vodafone Vodafone Korea Telecom Verizon Wireless AT&T In order to address increasing bandwidth demands, operators are considering bundling of multiple heterogeneous access networks (Hybrid access) for residential and enterprise customers. This document describes a solution for Hybrid access and covers the use case scenarios. Work in Progress n.d. n.d. This document describes HMAC, a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, e.g., MD5, SHA-1, in combination with a secret shared key. The cryptographic strength of HMAC depends on the properties of the underlying hash function. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind This document describes the Secure Real-time Transport Protocol (SRTP), a profile of the Real-time Transport Protocol (RTP), which can provide confidentiality, message authentication, and replay protection to the RTP traffic and to the control traffic for RTP, the Real-time Transport Control Protocol (RTCP). [STANDARDS-TRACK] This document defines a new form of name, called permanent identifier, that may be included in the subjectAltName extension of a public key certificate issued to an entity. The permanent identifier is an optional feature that may be used by a CA to indicate that two or more certificates relate to the same entity, even if they contain different subject name (DNs) or different names in the subjectAltName extension, or if the name or the affiliation of that entity stored in the subject or another name form in the subjectAltName extension has changed. The subject name, carried in the subject field, is only unique for each subject entity certified by the one CA as defined by the issuer name field. However, the new name form can carry a name that is unique for each subject entity certified by a CA. [STANDARDS-TRACK] Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the use of Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP). DTLS provides communications privacy for applications that use datagram transport protocols and allows client/server applications to communicate in a way that is designed to prevent eavesdropping and detect tampering or message forgery. DCCP is a transport protocol that provides a congestion-controlled unreliable datagram service. [STANDARDS-TRACK] This document describes the usage of Service Codes by the Datagram Congestion Control Protocol, RFC 4340. It motivates the setting of a Service Code by applications. Service Codes provide a method to identify the intended service/application to process a DCCP connection request. This provides improved flexibility in the use and assignment of port numbers for connection multiplexing. The use of a DCCP Service Code can also enable more explicit coordination of services with middleboxes (e.g., network address translators and firewalls). This document updates the specification provided in RFC 4340. [STANDARDS-TRACK] This document specifies an update to the Datagram Congestion Control Protocol (DCCP), a connection-oriented and datagram-based transport protocol. The update adds support for the DCCP-Listen packet. This assists DCCP applications to communicate through middleboxes (e.g., a Network Address Port Translator or a DCCP server behind a firewall), where peering endpoints need to initiate communication in a near- simultaneous manner to establish necessary middlebox state. [STANDARDS-TRACK] This document defines a set of requirements for NATs handling the Datagram Congestion Control Protocol (DCCP). These requirements allow DCCP applications, such as streaming applications, to operate consistently, and they are very similar to the TCP requirements for NATs, which have already been published by the IETF. Ensuring that NATs meet this set of requirements will greatly increase the likelihood that applications using DCCP will function properly. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Federal Information Processing Standard, FIPS Often endpoints are connected by multiple paths, but communications are usually restricted to a single path per connection. Resource usage within the network would be more efficient were it possible for these multiple paths to be used concurrently. Multipath TCP is a proposal to achieve multipath transport in TCP. New congestion control algorithms are needed for multipath transport protocols such as Multipath TCP, as single path algorithms have a series of issues in the multipath context. One of the prominent problems is that running existing algorithms such as standard TCP independently on each path would give the multipath flow more than its fair share at a bottleneck link traversed by more than one of its subflows. Further, it is desirable that a source with multiple paths available will transfer more traffic using the least congested of the paths, achieving a property called "resource pooling" where a bundle of links effectively behaves like one shared link with bigger capacity. This would increase the overall efficiency of the network and also its robustness to failure. This document presents a congestion control algorithm that couples the congestion control algorithms running on different subflows by linking their increase functions, and dynamically controls the overall aggressiveness of the multipath flow. The result is a practical algorithm that is fair to TCP at bottlenecks while moving traffic away from congested links. This document defines an Experimental Protocol for the Internet community. This document specifies an alternative encapsulation of the Datagram Congestion Control Protocol (DCCP), referred to as DCCP-UDP. This encapsulation allows DCCP to be carried through the current generation of Network Address Translation (NAT) middleboxes without modification of those middleboxes. This document also updates the Session Description Protocol (SDP) information for DCCP defined in RFC 5762. [STANDARDS-TRACK] TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and, thus, improve user experience through higher throughput and improved resilience to network failure. Multipath TCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional TCP to support multipath operation. The protocol offers the same type of service to applications as TCP (i.e., reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths. This document defines an Experimental Protocol for the Internet community. The Secure Real-time Transport Protocol (SRTP) provides authentication, but not encryption, of the headers of Real-time Transport Protocol (RTP) packets. However, RTP header extensions may carry sensitive information for which participants in multimedia sessions want confidentiality. This document provides a mechanism, extending the mechanisms of SRTP, to selectively encrypt RTP header extensions in SRTP. This document updates RFC 3711, the Secure Real-time Transport Protocol specification, to require that all future SRTP encryption transforms specify how RTP header extensions are to be encrypted. This document describes a simple method of encapsulating Stream Control Transmission Protocol (SCTP) packets into UDP packets and its limitations. This allows the usage of SCTP in networks with legacy NATs that do not support SCTP. It can also be used to implement SCTP on hosts without directly accessing the IP layer, for example, implementing it as part of the application without requiring special privileges. Please note that this document only describes the functionality required within an SCTP stack to add on UDP encapsulation, providing only those mechanisms for two end-hosts to communicate with each other over UDP ports. In particular, it does not provide mechanisms to determine whether UDP encapsulation is being used by the peer, nor the mechanisms for determining which remote UDP port number can be used. These functions are out of scope for this document. This document covers only end-hosts and not tunneling (egress or ingress) endpoints. This document specifies a set of TCP extensions to improve performance over paths with a large bandwidth * delay product and to provide reliable operation over very high-speed paths. It defines the TCP Window Scale (WS) option and the TCP Timestamps (TS) option and their semantics. The Window Scale option is used to support larger receive windows, while the Timestamps option can be used for at least two distinct mechanisms, Protection Against Wrapped Sequences (PAWS) and Round-Trip Time Measurement (RTTM), that are also described herein. This document obsoletes RFC 1323 and describes changes from it. This document discusses both use cases and operational experience with Multipath TCP (MPTCP) in real networks. It lists several prominent use cases where Multipath TCP has been considered and is being used. It also gives insight to some heuristics and decisions that have helped to realize these use cases and suggests possible improvements. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and thus improve user experience through higher throughput and improved resilience to network failure. Multipath TCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional TCP to support multipath operation. The protocol offers the same type of service to applications as TCP (i.e., a reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths. This document specifies v1 of Multipath TCP, obsoleting v0 as specified in RFC 6824, through clarifications and modifications primarily driven by deployment experience. 3GPP

Differences from Multipath TCP This appendix is Informative. Multipath DCCP is similar to Multipath TCP , in that it extends the related basic DCCP transport protocol with multipath capabilities in the same way as Multipath TCP extends TCP . However, because of the differences between the underlying TCP and DCCP protocols, the transport characteristics of MPTCP and MP-DCCP are different. compares the protocol characteristics of TCP and DCCP, which are by nature inherited by their respective multipath extensions. A major difference lies in the delivery of payload, which is for TCP an exact copy of the generated byte-stream. DCCP behaves in a different way and does not guarantee to deliver any payload nor the order of delivery. Since this is mainly affecting the receiving endpoint of a TCP or DCCP communication, many similarities on the sender side can be identified. Both transport protocols share the 3-way initiation of a communication and both employ congestion control to adapt the sending rate to the path characteristics. TCP and DCCP protocol comparison

Feature	TCP	DCCP
Full-Duplex	yes	yes
Connection-Oriented	yes	yes
Header option space	40 bytes	< 1008 bytes or PMTU
Data transfer	reliable	unreliable
Packet-loss handling	re-transmission	report only
Ordered data delivery	yes	no
Sequence numbers	one per byte	one per PDU
Flow control	yes	no
Congestion control	yes	yes
ECN support	yes	yes
Selective ACK	yes	depends on congestion control
Fix message boundaries	no	yes
Path MTU discovery	yes	yes
Fragmentation	yes	no
SYN flood protection	yes	no
Half-open connections	yes	no

Consequently, the multipath features, shown in , are the same, supporting volatile paths having varying capacity and latency, session handover and path aggregation capabilities. All of them profit by the existence of congestion control. MPTCP and MP-DCCP protocol comparison

Feature	MPTCP	MP-DCCP
Volatile paths	yes	yes
Session handover	yes	yes
Path aggregation	yes	yes
Data reordering	yes	optional
Expandability	limited by TCP header	flexible

Therefore, the sender logic is not much different between MP-DCCP and MPTCP. The receiver side for MP-DCCP has to deal with the unreliable delivery provided by DCCP. The multipath sequence numbers included in MP-DCCP (see ) facilitates adding optional mechanisms for data stream packet reordering at the receiver. Information from the MP_RTT multipath option (), DCCP path sequencing and the DCCP Timestamp Option provide further means for advanced reordering approaches, e.g., as proposed in . Such mechanisms do, however, not affect interoperability and are not part of the MP-DCCP protocol. Many applications that use unreliable transport protocols can also inherently process out-of-sequence data (e.g., through adaptive audio and video buffers), and so additional reordering support might not be necessary. The addition of optional reordering mechanisms are likely to be needed when the different DCCP subflows are routed across paths with different latencies. In theory, applications using DCCP are aware that packet reordering could occur, because DCCP does not provide mechanisms to restore the original packet order. In contrast to TCP, the receiver processing for MPTCP adopted a rigid "just wait" approach, because TCP guarantees reliable in-order delivery.

Authors' Addresses Deutsche Telekom

Deutsche-Telekom-Allee 9 Darmstadt 64295 Germany Markus.Amend@telekom.de

Karlstad University

Universitetsgatan 2 Karlstad 651 88 Sweden anna.brunstrom@kau.se

Karlstad University

Universitetsgatan 2 Karlstad 651 88 Sweden andreas.kassler@kau.se

City University of London

Northampton Square London United Kingdom veselin.rakocevic.1@city.ac.uk

BT

Adastral Park Martlesham Heath IP5 3RE United Kingdom stephen.h.johnson@bt.com