DCCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-tsvwg-multipath-dccp-24
| Document | Type | Active Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | Markus Amend , Anna Brunstrom , Andreas Kassler , Veselin Rakocevic , Stephen Johnson | ||
| Last updated | 2025-04-30 (Latest revision 2025-04-29) | ||
| Replaces | draft-amend-tsvwg-multipath-dccp | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
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draft-ietf-tsvwg-multipath-dccp-24
Transport Area Working Group M. Amend, Ed.
Internet-Draft DT
Intended status: Standards Track A. Brunstrom
Expires: 31 October 2025 A. Kassler
Karlstad University
V. Rakocevic
City, University of London
S. Johnson
BT
29 April 2025
DCCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-tsvwg-multipath-dccp-24
Abstract
Datagram Congestion Control Protocol (DCCP) communications, as
defined in RFC 4340, are inherently restricted to a single path per
connection, despite the availability of multiple network paths
between peers. The ability to utilize multiple paths simultaneously
for a DCCP session can enhance network resource utilization, improve
throughput, and increase resilience to network failures, ultimately
enhancing the user experience.
Use cases for Multipath DCCP (MP-DCCP) include mobile devices (e.g.,
handsets, vehicles) and residential home gateways that maintain
simultaneous connections to distinct network types, such as cellular
and Wireless Local Area Networks (WLANs) or cellular and fixed access
networks. Compared to existing multipath transport protocols, such
as Multipath TCP (MPTCP), MP-DCCP is particularly suited for latency-
sensitive applications with varying requirements for reliability and
in-order delivery.
This document specifies a set of protocol extensions to DCCP that
enable multipath operations. These extensions maintain the same
service model as DCCP while introducing mechanisms to establish and
utilize multiple concurrent DCCP flows across different network
paths.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 31 October 2025.
Copyright Notice
Copyright (c) 2025 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 (https://trustee.ietf.org/
license-info) 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
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Multipath DCCP in the Networking Stack . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Operation Overview . . . . . . . . . . . . . . . . . . . . . 6
2.1. MP-DCCP Concept . . . . . . . . . . . . . . . . . . . . . 7
3. MP-DCCP Protocol . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Multipath Capable Feature . . . . . . . . . . . . . . . . 10
3.2. Multipath Option . . . . . . . . . . . . . . . . . . . . 12
3.2.1. MP_CONFIRM . . . . . . . . . . . . . . . . . . . . . 14
3.2.2. MP_JOIN . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.3. MP_FAST_CLOSE . . . . . . . . . . . . . . . . . . . . 18
3.2.4. MP_KEY . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.5. MP_SEQ . . . . . . . . . . . . . . . . . . . . . . . 20
3.2.6. MP_HMAC . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.7. MP_RTT . . . . . . . . . . . . . . . . . . . . . . . 23
3.2.8. MP_ADDADDR . . . . . . . . . . . . . . . . . . . . . 25
3.2.9. MP_REMOVEADDR . . . . . . . . . . . . . . . . . . . . 28
3.2.10. MP_PRIO . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.11. MP_CLOSE . . . . . . . . . . . . . . . . . . . . . . 31
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3.2.12. Experimental Multipath option MP_EXP for private
use . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3. MP-DCCP Handshaking Procedure . . . . . . . . . . . . . . 32
3.4. Address knowledge exchange . . . . . . . . . . . . . . . 34
3.4.1. Advertising a new path (MP_ADDADDR) . . . . . . . . . 34
3.4.2. Removing a path (MP_REMOVEADDR) . . . . . . . . . . . 36
3.5. Closing an MP-DCCP connection . . . . . . . . . . . . . . 37
3.6. Fallback . . . . . . . . . . . . . . . . . . . . . . . . 38
3.7. State Diagram . . . . . . . . . . . . . . . . . . . . . . 39
3.8. Congestion Control Considerations . . . . . . . . . . . . 40
3.9. Maximum Packet Size Considerations . . . . . . . . . . . 41
3.10. Maximum number of Subflows Considerations . . . . . . . . 41
3.11. Path usage strategies . . . . . . . . . . . . . . . . . . 42
3.11.1. Path mobility . . . . . . . . . . . . . . . . . . . 42
3.11.2. Concurrent path usage . . . . . . . . . . . . . . . 42
4. Security Considerations . . . . . . . . . . . . . . . . . . . 43
5. Interactions with Middleboxes . . . . . . . . . . . . . . . . 45
6. Implementation . . . . . . . . . . . . . . . . . . . . . . . 45
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
8.1. New Multipath Capable DCCP feature . . . . . . . . . . . 46
8.2. New MP-DCCP version registry . . . . . . . . . . . . . . 46
8.3. New Multipath option and registry . . . . . . . . . . . . 47
8.4. New DCCP Reset Code . . . . . . . . . . . . . . . . . . . 48
8.5. New Multipath Key Type registry . . . . . . . . . . . . . 48
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 49
9.1. Normative References . . . . . . . . . . . . . . . . . . 49
9.2. Informative References . . . . . . . . . . . . . . . . . 50
Appendix A. Differences from Multipath TCP . . . . . . . . . . . 52
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 55
1. Introduction
Datagram Congestion Control Protocol (DCCP) [RFC4340] is a transport
protocol that provides bidirectional unicast connections of
congestion-controlled unreliable datagrams. DCCP communications are
restricted to one single path. Other fundamentals of the DCCP
protocol are summarized in section 1 of [RFC4340], such as the
reliable handshake process in section 4.7 and the reliable
negotiation of features in section 4.5. These are an important basis
for this document. This also applies to the DCCP sequencing scheme,
which is packet-based (section 4.2), and the principles for loss and
retransmission of features as described in more detail in section
6.6.3. 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.
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Multipath DCCP (MP-DCCP) enables a DCCP connection to simultaneously
establish a 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.
In addition to the integration into DCCP services, implementers or
future specification could choose MP-DCCP for other use cases like
3GPP 5G multi-access solutions (e.g., Access Traffic Steering,
Switching, and Splitting (ATSSS) specified in [TS23.501]) or hybrid
access networks that either combine a 3GPP and a non-3GPP access or a
fixed and cellular access between user-equipment/residential gateway
and operator network. MP-DCCP can be used in these scenarios for
load balancing, seamless session handover and bandwidth aggregation
when non-DCCP traffic like IP, UDP or TCP is encapsulated into MP-
DCCP. More details on potential use cases for MP-DCCP are provided
in [multipath-dccp.org], [IETF105.Slides], and [MP-DCCP.Paper]. All
these use cases profit from an Open Source Linux reference
implementation provided under [multipath-dccp.org].
The encapsulation of non-DCCP traffic (e.g., UDP or IP) in MP-DCCP to
enable the above-mentioned use cases is not considered in this
specification. Also out of scope is the encapsulation of DCCP
traffic in UDP to pass middleboxes (e.g., NATs, firewalls, proxies,
intrusion detection systems (IDSs), etc) that do not support DCCP. A
possible method is defined in [RFC6773] or is considered in
[I-D.amend-tsvwg-dccp-udp-header-conversion] to achieve the same with
less overhead.
MP-DCCP is based exclusively on the lean concept of DCCP. For
traffic that is already encrypted or does not need encryption, MP-
DCCP is an efficient choice as it does not apply its own encryption
mechanisms. Also, the procedures defined by MP-DCCP, which allow
subsequent reordering of traffic and efficient traffic scheduling,
improve performance, as shown in [MP-DCCP.Paper], and take into
account the interaction of the protocol with the further elements
required for multi-path transport.
1.1. Multipath DCCP in the Networking Stack
MP-DCCP provides a set of features to DCCP; Figure 1 illustrates this
layering. MP-DCCP is designed to be used by applications in the same
way as DCCP with no changes to the application itself.
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+-------------------------------+
| Application |
+---------------+ +-------------------------------+
| Application | | MP-DCCP |
+---------------+ + - - - - - - - + - - - - - - - +
| DCCP | |Subflow (DCCP) |Subflow (DCCP) |
+---------------+ +-------------------------------+
| IP | | IP | IP |
+---------------+ +-------------------------------+
Figure 1: Comparison of standard DCCP and MP-DCCP protocol stacks
A CLI at the endpoint (or another method) could be used to configure
and manage the DCCP Connections. This could be extended to also
support MP-DCCP, but this specification does not define this.
1.2. Terminology
This document uses terms that are either specific for multipath
transport as defined in [RFC8684] or are defined in the context of
MP-DCCP, 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 and the
source and destination ports. This definition follows [RFC8684] and
is illustrated in the following two examples for IPv6 and IPv4, which
each show a pair of sender IP-address:port and a pair of receiver IP-
address:port, which together form the 4-tuple:
* IPv6: [2001:db8:3333:4444:5555:6666:7777:8888]:1234,
[2001:db8:3333:4444:cccc:dddd:eeee:ffff]:4321
* IPv4: 203.0.113.1:1234, 203.0.113.2:4321
Subflow: A subflow refers to a DCCP flow 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.
(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.
Connection Identifier (CI): A unique identifier that is assigned to a
multipath connection by the host to distinguish several multipath
connections locally. The CIs must therefore be locally unique per
host and do not have to be the same across the peers.
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Host: An end host operating an MP-DCCP implementation, and either
initiating or accepting an MP-DCCP connection.
'+': The plus symbol means concatenation of values.
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 Section 3.
1.3. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Operation Overview
DCCP transmits congestion-controlled unreliable datagrams over a
single path. 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).
However, DCCP does not provide built-in support for managing multiple
subflows within one DCCP connection. The extension of DCCP for
Multipath-DCCP (MP-DCCP) is described in detail in Section 3.
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 paths, for example using paths
via different links. 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.
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A Multipath DCCP connection provides a bidirectional connection of
datagrams between two hosts exchanging data using DCCP. It does not
require any change to the applications. Multipath DCCP enables the
hosts to use multiple paths with different 4-tuples 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 Section 3.1. Once negotiated, all
subsequent MP-DCCP operations for that connection are signalled with
a variable length multipath-related option, as described in
Section 3. All MP-DCCP operations are signaled by Multipath options
described in Section 3.2. Options that require confirmation from the
remote peer are retransmitted by the sender until confirmed or until
confirmation is no longer considered relevant.
The following sections define MP-DCCP behavior in detail.
2.1. MP-DCCP Concept
Figure 2 provides a general overview of the MP-DCCP working mode,
whose main characteristics are summarized in this section.
Host A Host B
------------------------ ------------------------
Address A1 Address A2 Address B1 Address B2
---------- ---------- ---------- ----------
| | | |
| (DCCP subflow setup) | |
|----------------------------------->| |
|<-----------------------------------| |
| | | |
| | (DCCP subflow setup)| |
| |--------------------->| |
| |<---------------------| |
| merge individual DCCP subflows to one MP-DCCP connection
| | | |
Figure 2: Example MP-DCCP usage scenario
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* An MP-DCCP connection begins with a 4-way handshake, between two
hosts. In Figure 2, an MP-DCCP connection is established between
addresses A1 and B1 on Hosts A and B. In the handshake, a
Multipath Capable feature is used to negotiate multipath support
for the connection. Host specific keys are also exchanged between
Host A and Host B during the handshake. The details of the MP-
DCCP handshaking procedure is described in Section 3.3. 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 can be created on these paths
and attached to the existing MP-DCCP connection. An MP_JOIN
option is used to connect a new DCCP subflow to an existing MP-
DCCP connection. It contains a Connection Identifier during the
setup of the initial subflow and is exchanged in the 4-way
handshake for the subflow together with the Multipath Capable
feature. The example in Figure 2 illustrates creation of an
additional DCCP subflow between Address A2 on Host A and Address
B1 on Host B. The two subflows continue to provide a single
connection to the applications at both endpoints.
* 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 the additional subflow in the example is shown
as being initiated from A2, an additional subflow 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
the procedures that enable a host to initiate new subflows or to
signal available IP addresses between peers. However, the
definition of a path management method, in which sequence and when
subflows are created, is outside the scope of this document. This
method is subject to a corresponding policy and the specifics of
the implementation. If an MP-DCCP peer host wishes to limit the
maximum number of paths that can be maintained (e.g. similar to
that discussed in section 3.4 of [RFC8041]), the creation of new
subflows from that peer host is omitted when the threshold of
maximum paths is exceeded and incoming subflow requests MUST be
rejected.
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* Through the use of multipath options, MP-DCCP adds connection-
level sequence numbers and exchange of Round-Trip Time (RTT)
information to enable optional reordering features. As a hint for
scheduling decisions, a multipath option that allows a peer to
indicate its priorities for what path to use is also defined.
* Subflows are terminated in the same way as regular DCCP
connections, as described in ([RFC4340], Section 8.3). MP-DCCP
connections are closed by including an MP_CLOSE option in subflow
DCCP-CloseReq or DCCP-Close messages. An MP-DCCP connection may
also be reset through the use of an MP_FAST_CLOSE option. Key
data from the initial handshake is included in the MP_CLOSE and
MP_FAST_CLOSE to protect from unauthorized shutdown of MP-DCCP
connections.
3. MP-DCCP Protocol
The DCCP protocol feature list (Section 6.4 of [RFC4340]) is extended
in this document by adding a new Multipath feature with Feature
number 10, as shown in Table 1.
+========+===================+============+===============+=======+
| Number | Meaning | Rec'n Rule | Initial Value | Req'd |
+========+===================+============+===============+=======+
| 10 | Multipath Capable | SP | 0 | N |
+--------+-------------------+------------+---------------+-------+
Table 1: Multipath feature
Rec'n Rule: The reconciliation rule used for the feature. SP
indicates the server-priority as defined in section 6.3 of
[RFC4340].
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
([RFC4340], Section 5.8) providing a new Multipath related variable-
length option with option type 46, as shown in Table 2.
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+======+===============+===========+============+
| Type | Option Length | Meaning | DCCP-Data? |
+======+===============+===========+============+
| 46 | variable | Multipath | Y |
+------+---------------+-----------+------------+
Table 2: Multipath option set
Note to the RFC Editor: The Feature Number and Option Type reflect
the temporary assignment by IANA and must be verified once again.
3.1. 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 10 and
follows the structure for features given in [RFC4340] Section 6.
Beside the negotiation of the feature itself, also one or several
values can be exchanged. The value field specified here for the
Multipath Capable feature has a length of one-byte and can be
repeated several times within the DCCP option for feature
negotiation. This can be for example required to announce support of
different versions of the protocol. For that, the leftmost four bits
in Figure 3 specify the compatible version of the MP-DCCP
implementation and MUST be set to 0 following this specification.
The four bits following the Version field are unassigned in version 0
and MUST be set to zero by the sender and MUST be ignored by the
receiver.
0 1 2 3 4 5 6 7
+-----------+------------+
| Version | Unassigned |
+-----------+------------+
Figure 3: Format of the Multipath Capable feature value field
The setting of the MP_CAPABLE feature MUST follow the server-priority
reconciliation rule described in ([RFC4340], 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 Section 3.3. No subsequent re-negotiation of the
MP_CAPABLE feature is allowed for the same MP-DCCP connection.
Clients MUST include a Change R ([RFC4340], Section 6) option during
the initial handshake request to supply a list of supported MP-DCCP
protocol versions, ordered by preference.
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Servers MUST include a Confirm L ([RFC4340], Section 6) 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 10 and no values.
An example of successful version negotiation is shown hereafter and
follows the negotiation example shown in [RFC4340] Section 6.5. For
better understanding, this example uses the unspecified MP-DCCP
versions 1 and 2 in addition to the MP-DCCP version 0 specified in
this document:
Client Server
------ ------
DCCP-Req + Change R(MP_CAPABLE, 1 0)
----------------------------------->
DCCP-Resp + Confirm L(MP_CAPABLE, 1, 2 1 0)
<-----------------------------------
* agreement on version = 1 *
Figure 4: Example of MP-DCCP support negotiation using MP_CAPABLE
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 indicated by the first
value, and supplies its own preference list with the following
values.
3. MP-DCCP is then enabled between the Client and Server with
version 1.
Unlike the example in Figure 4, this document only allows the
negotiation of MP-DCCP version 0. Therefore, successful negotiation
of MP-DCCP as defined in this document, the client and the server
MUST both support MP-DCCP version 0.
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 MUST either fall back to
regular DCCP or MUST close the connection. Further details are
specified in Section 3.6
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3.2. Multipath Option
MP-DCCP uses one single option to signal various multipath-related
operations. The format of this multipath option is shown in
Figure 5.
1 2 3
01234567 89012345 67890123 45678901 23456789
+--------+--------+--------+--------+--------+
|00101110| Length | MP_OPT | Value(s) ...
+--------+--------+--------+--------+--------+
Type=46
Figure 5: Multipath option format
The fields used by the multipath option are described in Table 3.
MP_OPT refers to a Multipath option.
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+======+========+================+================================+
| Type | Option | MP_OPT | Meaning |
| | Length | | |
+======+========+================+================================+
| 46 | var | 0 =MP_CONFIRM | Confirm reception and |
| | | | processing of an MP_OPT option |
+------+--------+----------------+--------------------------------+
| 46 | 12 | 1 =MP_JOIN | Join subflow to an existing |
| | | | MP-DCCP connection |
+------+--------+----------------+--------------------------------+
| 46 | var | 2 | Close an MP-DCCP connection |
| | | =MP_FAST_CLOSE | 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 | Hash-based message auth. code |
| | | | for MP-DCCP |
+------+--------+----------------+--------------------------------+
| 46 | 12 | 6 =MP_RTT | Transmit RTT values and |
| | | | calculation parameters |
+------+--------+----------------+--------------------------------+
| 46 | var | 7 =MP_ADDADDR | Advertise additional |
| | | | address(es)/port(s) |
+------+--------+----------------+--------------------------------+
| 46 | 8 | 8 | Remove address(es)/ port(s) |
| | | =MP_REMOVEADDR | |
+------+--------+----------------+--------------------------------+
| 46 | 4 | 9 =MP_PRIO | Change subflow priority |
+------+--------+----------------+--------------------------------+
| 46 | var | 10 =MP_CLOSE | Close an MP-DCCP connection |
+------+--------+----------------+--------------------------------+
| 46 | var | 11 =MP_EXP | Experimental option for |
| | | | private use |
+------+--------+----------------+--------------------------------+
| 46 | TBD | >11 | Reserved for future Multipath |
| | | | options. |
+------+--------+----------------+--------------------------------+
Table 3: MP_OPT option types
Future MP options could be defined in a later version or extension to
this specification.
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These operations are largely inspired by the signals defined in
[RFC8684]. The procedures for handling faulty or unknown MP options
are described in Section 3.6.
3.2.1. MP_CONFIRM
1 2 3 4 5
01234567 89012345 67890123 45678901 23456789 01234567 89012345
+--------+--------+--------+--------+--------+--------+--------+
|00101110| var |00000000| List of confirmations ...
+--------+--------+--------+--------+--------+--------+--------+
Type=46 Length MP_OPT=0
Figure 6: Format of the MP_CONFIRM option
Some multipath options require confirmation from the remote peer (see
Table 4). Such options 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.
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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 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_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.
+======+===============+==================+=========================+
| Type | Option Length | MP_OPT | MP_CONFIRM Sending path |
+======+===============+==================+=========================+
| 46 | var | 7 =MP_ADDADDR | Any available |
+------+---------------+------------------+-------------------------+
| 46 | 4 | 8 | Any available |
| | | =MP_REMOVEADDR | |
+------+---------------+------------------+-------------------------+
| 46 | 4 | 9 =MP_PRIO | Any available |
+------+---------------+------------------+-------------------------+
Table 4: Multipath options requiring confirmation
An example to illustrate the MP-DCCP confirm procedure for the
MP_PRIO option is shown in Figure 7. 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 (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).
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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) --------|
| | | |
Figure 7: Example MP-DCCP CONFIRM procedure
A second example to illustrate 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 confirming receipt of the MP_PRIO(1) with sequence
number 2.
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) --------|
| | | |
Figure 8: Example MP-DCCP CONFIRM procedure with outdated suboption
3.2.2. MP_JOIN
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1 2 3
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
|00101110|00001100|00000001| Addr ID|
+--------+--------+--------+--------+
| Connection Identifier |
+--------+--------+--------+--------+
| Nonce |
+--------+--------+--------+--------+
Type=46 Length=12 MP_OPT=1
Figure 9: 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 Connection Identifier (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, 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 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.
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The Nonce is a 32-bit random value locally generated for every
MP_JOIN option. Together with the derived key from the both hosts
Key Data described in Section 3.2.4, the Nonce value builds the basis
to calculate the HMAC used in the handshaking 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
Figure 10: Format of the MP_FAST_CLOSE suboption
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 unauthorized
shutdown of an MP-DCCP connection, the selected Key Data of the peer
host during the handshaking 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 multipath DCCP connection immediately
terminates.
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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
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
Figure 11: Format of the MP_KEY suboption
The MP_KEY suboption is used to exchange a Connection Identifier (CI)
and key material between hosts (host A, 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 2^32. 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.
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+===============+====================+==================+
| Key Type | Key Length (bytes) | Meaning |
+===============+====================+==================+
| 0 =Plain Text | 8 | Plain Text Key |
+---------------+--------------------+------------------+
| 1-254 | | Reserved for |
| | | future Key Types |
+---------------+--------------------+------------------+
| 255 | 64 | For private use |
| =Experimental | | only |
+---------------+--------------------+------------------+
Table 5: MP_KEY key types
Plain Text
Key Data is exchanged in plain text between hosts (Host A, Host
B), and the respective key parts (KeyA, 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 to use 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 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
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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
Figure 12: Format of the MP_SEQ suboption
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 in [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 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 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 considering the current globally
routable maximum packet size of 1500 bytes, which corresponds to
roughly 375 PiB of data within the sequence number space.
3.2.6. MP_HMAC
1 2 3 4
01234567 89012345 67890123 45678901 23456789 01234567
+--------+--------+--------+--------+--------+--------+
|00101110|00010111|00000101| HMAC-SHA256 (20 bytes) ...
+--------+--------+--------+--------+--------+--------+
Type=46 Length=23 MP_OPT=5
Figure 13: Format of the MP_HMAC suboption
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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 SHA256 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, and
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 associated IP
address and if defined port, otherwise two bytes of value 0. 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)
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MP_JOIN, MP_ADDADDR and MP_REMOVEADDR can co-exist 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. Therefore,
an MP_HMAC MUST directly follow its associated multipath option. In
the likely case of sending a 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 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 Section 3.2.8 and Section 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 (penultimate arrow in Figure 21).
3.2.7. MP_RTT
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
Figure 14: 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.
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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 -
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 procedure 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 which
corresponds to the absolute difference of both values. In the case
that 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 |----------------| |
+--------+ +-------------+
Figure 15: Exemplary flow of MP_RTT exchange and usage
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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 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. 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 d-KeyA, and in the case of Host B, d-KeyB. 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. 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 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.
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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
Figure 16: 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.
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.
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A host MAY advertise private addresses, e.g., because there is a NAT
on the path. It is desirable to allow this, since 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 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 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 (Section 3.2.1). This ensures reliable exchange of
address information.
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 to save resources. If a sender, however, 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.
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).
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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 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 Section 3.2.6. 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 d-KeyA, and in the case of Host B,
d-KeyB. 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 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 reliable exchange of
address information. To avoid inconsistent states, the sender
releases the address ID only after MP_REMOVEADDR has been confirmed.
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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 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 MP_HMAC option
Figure 17: Format of the MP_REMOVEADDR suboption
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 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
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
Figure 18: Format of the MP_PRIO suboption
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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 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 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 Wi-Fi path to Primary and Cellular 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 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 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
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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 depend 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 be the appropriate one that has the highest
available priority value including also 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
1 2 3
01234567 89012345 67890123 45678901 23456789
+--------+--------+--------+--------+--------+
|00101110| var |00001010| Key Data ...
+--------+--------+--------+--------+--------+
Type=46 Length MP_OPT=10
Figure 19: 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 [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 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.
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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 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 CLOSE 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 optimization of 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 TBD |
+---------------+---------------+---------------+---------------+
| ...
+---------------------------------------------------------------+
Type=46 Length MP_OPT=11
Figure 20: 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.
3.3. MP-DCCP Handshaking Procedure
An example to illustrate the MP-DCCP handshake procedure is shown in
Figure 21.
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Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| DCCP-Request + Change R (MP_CAPABLE,...) |
|----- MP_KEY(CI-A + KeyA(1), KeyA(2),...) ---------->|
|<------------------- MP_KEY(CI-B + KeyB) ------------|
| DCCP-Response + Confirm L (MP_CAPABLE, ...) |
| | |
| DCCP-Ack | |
|---------------------------------------------------->|
|<----------------------------------------------------|
| DCCP-Ack | |
| | |
| |DCCP-Request + Change R(MP_CAPABLE,...)|
| |--- MP_JOIN(CI-B,RA) ----------------->|
| |<------MP_JOIN(CI-A,RB) + MP_HMAC(B)---|
| |DCCP-Response+Confirm L(MP_CAPABLE,...)|
| | |
| |DCCP-Ack |
| |-------- MP_HMAC(A) ------------------>|
| |<--------------------------------------|
| |DCCP-ACK |
Figure 21: Example MP-DCCP handshake
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 a Host-specific CI-A and a KeyA
for each of the supported key types as described in Section 3.2.4.
CI-A is a unique identifier during the lifetime of an MP-DCCP
connection.
* Host B sends a DCCP-Response with Confirm feature for MP-Capable
and the MP_Key option with a unique Host-specific CI-B and a
single Host-specific KeyB. The type of the key is chosen from the
list of supported types from the previous request.
* Host A sends a DCCP-Ack to confirm the proper key exchange.
* Host B sends a DCCP-Ack to complete the handshake and set both
connection ends to the OPEN state.
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It should be noted that DCCP is protected against corruption of DCCP
header data (section 9 of [RFC4340]), so no additional mechanisms
beyond the general confirmation are required to ensure that the
header data has been properly received.
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 CI-B, obtained during
the initial handshake. 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 CI-A and a random nonce RB together with
the computed MP_HMAC. As specified in Section 3.2.6, 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
Section 3.2.4 as key:
MP_HMAC(B) = HMAC-SHA256(Key=d-keyB, Msg=RB+RA)
* Host A sends a DCCP-Ack with the HMAC computed for the DCCP-
Response. As specified in Section 3.2.6, 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
Section 3.2.4 as key:
MP_HMAC(A) = HMAC-SHA256(Key=d-keyA, Msg=RA+RB)
* Host B sends a DCCP-Ack to confirm the HMAC and to conclude the
handshaking.
3.4. Address knowledge exchange
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:
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* 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
handshaking procedure described in Section 3.3 and Section 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
handshaking 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) -------------|
Figure 22: Example MP-DCCP ADDADDR procedure
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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
which 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
handshaking procedure described in Section 3.3 and Section 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
handshaking 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) ----------|
Figure 23: Example MP-DCCP REMOVEADDR procedure
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3.5. Closing an 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 [RFC4340], 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
[DCCP.Parameter] 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 MUST 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 [RFC4340] on each subflow
with the first packet on each subflow carrying MP_CLOSE (see
Section 3.2.11).
Host A Host B
------ ------
<- Optional DCCP-CloseReq +
MP_CLOSE [A's key]
[on all subflows]
DCCP-Close + MP_CLOSE ->
[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 Section 3.2.3, where a DCCP-Reset
is sent on all subflows, each carrying the MP_FAST_CLOSE option.
Host A Host B
------ ------
DCCP-Reset + MP_FAST_CLOSE ->
[B's key] [on all subflows]
<- DCCP-Reset
[on all subflows]
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3.6. Fallback
When a subflow fails to operate following MP-DCCP intended behavior,
it is necessary to proceed with a fall back. This may be either
falling back to regular DCCP [RFC4340] 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
Multipath options, 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 fall back to regular DCCP. If this is not
possible the connection MUST be closed.
If the endpoints fail to agree on the protocol version to use during
the Multipath Capable feature negotiation, the connection MUST either
be closed or fall back to regular DCCP. This is described in
Section 3.1. 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 fall back to regular DCCP.
The fall back procedure to regular DCCP MUST be also applied if the
MP_KEY Section 3.2.4 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 (Section 3.2.6) or an invalid CI used in MP_JOIN
(Section 3.2.2) 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 KeyA/KeyB 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 [RFC4340]. 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
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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. The same procedure applies if the MP
option is unknown.
3.7. State Diagram
The MP-DCCP per subflow state transitions to a large extent follow
the state transitions defined for DCCP in [RFC4340], with some
modifications due to the MP-DCCP four-way handshake and fast close
procedures. The state diagram below illustrates the most common
state transitions. The diagram is illustrative. For example, there
are arcs (not shown) from several additional states to TIMEWAIT,
contingent on the receipt of a valid DCCP-Reset.
The states transitioned when moving from the CLOSED to OPEN state
during the four-way handshake remain the same as for DCCP, but it is
no longer possible to transmit application data while in the REQUEST
state. The fast close procedure can be triggered by either the
client or the server and results in the transmission of a Reset
packet. The fast close procedure moves the state of the client and
server directly to TIMEWAIT and CLOSED, respectively.
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+----------------------------+ +------------------------------+
| v v |
| +----------+ |
| +-------------+ CLOSED +-------------+ |
| | passive +----------+ active | |
| | open open | |
| | snd Request | |
| v v |
| +-----------+ +----------+ |
| | LISTEN | | REQUEST | |
| +-----+-----+ +----+-----+ |
| | rcv Request rcv Response | |
| | snd Response snd Ack | |
| v v |
| +-----------+ +----------+ |
| | RESPOND | | PARTOPEN | |
| +-----+-----+ +----+-----+ |
| | rcv Ack rcv Ack/DataAck | |
| | snd Ack | |
| | +-----------+ | |
| +------------>| OPEN |<-----------+ |
| +--+-+-+-+--+ |
| server active close | | | | active close |
| snd CloseReq | | | | or rcv CloseReq |
| | | | | snd Close |
| | | | | |
| +-----------+ | | | | +----------+ |
| | CLOSEREQ |<---------+ | | +----------->| CLOSING | |
| +-----+-----+ | | +----+-----+ |
| | rcv Close | | rcv Reset | |
| | snd Reset | | | |
| | | | active FastClose | |
|<----------+ rcv Close | | or rcv FastClose v |
| or server active FastClose | | snd Reset +----+-----+ |
| or server rcv FastClose | +------------->| TIMEWAIT | |
| snd Reset | +----+-----+ |
+------------------------------+ | |
+-----------+
2MSL timer expires
Figure 24: Most common state transitions of an MP-DCCP subflow
3.8. 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.
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3.9. 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 [RFC4340], 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.
3.10. Maximum number of Subflows Considerations
MP-DCCP does not support any explicit procedure to negotiate the
maximum number of subflows between endpoints. In practical
scenarios, however, there will be resource limitations on the host or
use cases that do not benefit from additional subflows.
It is RECOMMENDED to limit the number of subflows in implementations
and to reject incoming subflow requests with a DCCP-Reset using the
Reset Code "too busy" according to [RFC4340] if the resource limit is
exceeded or it is known that the multipath connection will not
benefit from further subflows. Likewise, the host that wants to
create the subflows is RECOMMENDED to consider the aspect of
available resources and the possible gains.
To avoid further inefficiencies with subflows due to short-lived
connections, it MAY be useful to delay the start of additional
subflows. The decision on the initial number of subflows can be
based on the occupancy of the socket buffer and/or the timing.
While in the socket buffer based approach the number of initial
subflows can be derived by opening new subflows until their initial
windows cover the amount of buffered application data, the timing
based approach delays the start of additional subflows based on a
certain time period, load or knowledge of traffic and path
properties. The delay based approach also provides resilience for
low-bandwidth but long-lived applications. All this could also be
supported by advanced APIs that signal application traffic requests
to the MP-DCCP.
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3.11. 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 an 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.
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, 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 the MP_PRIO
suboption 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 [multipath-dccp.org].
3.11.2. 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
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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 [RFC6356]
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 [OLIA].
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.
4. 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
[RFC5238] or other end-to-end security is recommended; Secure Real-
time Transport Protocol (SRTP) [RFC3711] is one candidate protocol
for authentication. Together with Encryption of Header Extensions in
SRTP, as provided by [RFC6904], also integrity would be provided.
DCCP [RFC4340] 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. The security
objectives for MP-DCCP are:
* Provide assurance that the parties involved in an MP-DCCP
handshake procedure are identical to those in the original DCCP
connection.
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* Before a path is used, verify that the new advertised path is
valid for receiving traffic.
* Provide replay protection, i.e., ensure that a request to add/
remove a subflow is 'fresh'.
* Allow a party to limit the number of subflows that it allows.
To achieve these goals, MP-DCCP includes a hash-based handshake
algorithm documented in Sections Section 3.2.4, Section 3.2.6 and
Section 3.3. 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. Depending on the security
requirements, different Key Types can be negotiated in the handshake
procedure or must follow the fallback scenario described in
Section 4. If there are security requirements that go beyond the
capabilities of Key Type 0, then it is RECOMMENDED that Key Type 0 is
not enabled to avoid downgrade attacks that result in the key being
exchanged as plain text. To ease demultiplexing while not revealing
cryptographic material, subsequent subflows use the initially
exchanged CI information. The keys exchanged once at the beginning
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 of subsequent subflows are
the same as in the original connection setup. This 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) for
both parties -- 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 [RFC4086]. 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 Section 3.2.8, a host may advertise its private
addresses, but these might point to different hosts in the receiver's
network. The MP_JOIN handshake (Section 3.2.2) 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.
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As described in Section 3.9, a Maximum Packet Size (MPS) is
maintained for an MP-DCCP connection. If MP-DCCP exposes a minimum
MPS across all paths, any change to one path impacts the sender for
all paths. To mitigate attacks that seek to force a low MPS, MP-DCCP
could detect an attempt to reduce the MPS less than a minimum MPS,
and then stop using these paths.
5. 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 [RFC4043],
Section 16). When both hosts are located behind a NAT or firewall
entity, specific measures have to be applied such as the [RFC5596]
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 [RFC5597].
[RFC6773] specifies UDP Encapsulation for NAT Traversal of DCCP
sessions, similar to other UDP encapsulations such as for SCTP
[RFC6951]. Future specifications by the IETF could specify other
methods for DCCP encapsulation.
The security impact of MP-DCCP aware middleboxes is discussed in
Section 4.
6. 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 [multipath-dccp.org].
7. Acknowledgments
[RFC8684] defines 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, Olivier Bonaventure, Gorry
Fairhurst and Behcet Sarikaya.
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8. 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 the RFC Required
policy of [RFC8126], Section 4.7. 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.
8.1. New Multipath Capable DCCP feature
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 Section 3. The following entry in
Table 6 should be added to the Feature Numbers registry in the DCCP
registry group according to [RFC4340], Section 19.4. under the "DCCP
Protocol" heading.
+==============+===================+================+
| Value | Feature Name | Specification |
+==============+===================+================+
| 10 suggested | Multipath Capable | [ThisDocument] |
+--------------+-------------------+----------------+
Table 6: Addition to DCCP Feature Numbers registry
Note to RFC Editor: Please replace [ThisDocument] with a reference
to the final RFC
8.2. New MP-DCCP version registry
Section 3.1 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:
+=========+================+================+
| Version | Value | Specification |
+=========+================+================+
| 0 | 0000 suggested | [ThisDocument] |
+---------+----------------+----------------+
| 1-15 | unassigned | |
+---------+----------------+----------------+
Table 7: MP-DCCP Versions Registry
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Note to RFC Editor: Please replace [ThisDocument] with a reference
to the final RFC
Future MP-DCCP versions 1 to 15 are assigned from this registry using
the RFC Required policy (Section 4.7 of [RFC8126]).
8.3. New Multipath option and registry
This document requests IANA to assign value 46 in the DCCP "Option
Types" registry to "Multipath Options", as described in Section 3.2.
IANA is requested to create a new 'Multipath Options' registry within
the DCCP registry group. The following entries in Table 8 should be
added to the new 'Multipath Options' registry. The registry in
Table 8 has an upper boundary of 255 in the numeric value field.
+===========+===============+=====================+===========+
| Multipath | Name | Description | Reference |
| Option | | | |
+===========+===============+=====================+===========+
| MP_OPT=0 | MP_CONFIRM | Confirm reception/ | Section |
| | | processing of an | 3.2.1 |
| | | MP_OPT option | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=1 | MP_JOIN | Join subflow to an | Section |
| | | existing MP-DCCP | 3.2.2 |
| | | connection | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=2 | MP_FAST_CLOSE | Close an MP-DCCP | Section |
| | | connection | 3.2.3 |
| | | unconditionally | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=3 | MP_KEY | Exchange key | Section |
| | | material for | 3.2.4 |
| | | MP_HMAC | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=4 | MP_SEQ | Multipath sequence | Section |
| | | number | 3.2.5 |
+-----------+---------------+---------------------+-----------+
| MP_OPT=5 | MP_HMAC | Hash-based message | Section |
| | | auth. code for MP- | 3.2.6 |
| | | DCCP | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=6 | MP_RTT | Transmit RTT values | Section |
| | | and calculation | 3.2.7 |
| | | parameters | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=7 | MP_ADDADDR | Advertise | Section |
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| | | additional | 3.2.8 |
| | | address(es)/port(s) | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=8 | MP_REMOVEADDR | Remove address(es)/ | Section |
| | | port(s) | 3.2.9 |
+-----------+---------------+---------------------+-----------+
| MP_OPT=9 | MP_PRIO | Change subflow | Section |
| | | priority | 3.2.10 |
+-----------+---------------+---------------------+-----------+
| MP_OPT=10 | MP_CLOSE | Close an MP-DCCP | Section |
| | | connection | 3.2.11 |
+-----------+---------------+---------------------+-----------+
| MP_OPT=11 | MP_EXP | Experimental option | Section |
| | | for private use | 3.2.12 |
+-----------+---------------+---------------------+-----------+
| MP_OPT>11 | Unassigned | Reserved for future | |
| | | Multipath options | |
+-----------+---------------+---------------------+-----------+
Table 8: Multipath Options registry
Future Multipath options with MP_OPT>11 are assigned from this
registry using the RFC Required policy (Section 4.7 of [RFC8126]).
8.4. New DCCP Reset Code
IANA is requested to assign a new DCCP-Reset Code value 13 suggested
in the DCCP-Reset Codes Registry, with the short description "Abrupt
MP termination". Use of this reset code is defined in section
Section 3.2.3.
8.5. New Multipath Key Type registry
IANA is requested to assign for this version of the MP-DCCP protocol
a new 'Multipath Key Type' registry containing two different
suboptions to the MP_KEY option to identify the MP_KEY Key types in
terms of 8-bit values as specified in Section 3.2.4 according to the
entries in Table 9 below. Values in range 3-254 (decimal) inclusive
remain unassigned in this here specified version 0 of the protocol
and are assigned via RFC Required [RFC8126] in potential future
versions of the MP-DCCP protocol.
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+=======+==============+=========================+===============+
| Type | Name | Meaning | Reference |
+=======+==============+=========================+===============+
| 0 | Plain Text | Plain text key | Section 3.2.4 |
+-------+--------------+-------------------------+---------------+
| 1-254 | Unassigned | Reserved for future use | Section 3.2.4 |
+-------+--------------+-------------------------+---------------+
| 255 | Experimental | For private use only | Section 3.2.4 |
+-------+--------------+-------------------------+---------------+
Table 9: Multipath Key Type registry with the MP_KEY Key Types
for key data exchange on different paths
9. References
9.1. Normative References
[DCCP.Parameter]
"IANA Datagram Congestion Control Protocol (DCCP)
Parameters", n.d., <https://www.iana.org/assignments/dccp-
parameters/dccp-parameters.xhtml>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/rfc/rfc4340>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/rfc/rfc6234>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
9.2. Informative References
[I-D.amend-iccrg-multipath-reordering]
Amend, M. and D. Von Hugo, "Multipath sequence
maintenance", Work in Progress, Internet-Draft, draft-
amend-iccrg-multipath-reordering-03, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-amend-iccrg-
multipath-reordering-03>.
[I-D.amend-tsvwg-dccp-udp-header-conversion]
Amend, M., Brunstrom, A., Kassler, A., and V. Rakocevic,
"Lossless and overhead free DCCP - UDP header conversion
(U-DCCP)", Work in Progress, Internet-Draft, draft-amend-
tsvwg-dccp-udp-header-conversion-01, 8 July 2019,
<https://datatracker.ietf.org/doc/html/draft-amend-tsvwg-
dccp-udp-header-conversion-01>.
[IETF105.Slides]
Amend, M., "MP-DCCP for enabling transfer of UDP/IP
traffic over multiple data paths in multi-connectivity
networks", IETF105 , n.d.,
<https://datatracker.ietf.org/meeting/105/materials/
slides-105-tsvwg-sessa-62-dccp-extensions-for-multipath-
operation-00>.
[MP-DCCP.Paper]
Amend, M., Bogenfeld, E., Cvjetkovic, M., Rakocevic, V.,
Pieska, M., Kassler, A., and A. Brunstrom, "A Framework
for Multiaccess Support for Unreliable Internet Traffic
using Multipath DCCP", DOI 10.1109/LCN44214.2019.8990746,
October 2019,
<https://doi.org/10.1109/LCN44214.2019.8990746>.
[multipath-dccp.org]
"Multipath extension for DCCP", n.d.,
<https://multipath-dccp.org/>.
[OLIA] Khalili, R., Gast, N., Popovic, M., Upadhyay, U., and J.
Le Boudec, "MPTCP is not pareto-optimal: performance
issues and a possible solution", Proceedings of the 8th
international conference on Emerging networking
experiments and technologies, ACM , 2012.
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[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/rfc/rfc2104>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/rfc/rfc3711>.
[RFC4043] Pinkas, D. and T. Gindin, "Internet X.509 Public Key
Infrastructure Permanent Identifier", RFC 4043,
DOI 10.17487/RFC4043, May 2005,
<https://www.rfc-editor.org/rfc/rfc4043>.
[RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, DOI 10.17487/RFC5238, May 2008,
<https://www.rfc-editor.org/rfc/rfc5238>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
[RFC5596] Fairhurst, G., "Datagram Congestion Control Protocol
(DCCP) Simultaneous-Open Technique to Facilitate NAT/
Middlebox Traversal", RFC 5596, DOI 10.17487/RFC5596,
September 2009, <https://www.rfc-editor.org/rfc/rfc5596>.
[RFC5597] Denis-Courmont, R., "Network Address Translation (NAT)
Behavioral Requirements for the Datagram Congestion
Control Protocol", BCP 150, RFC 5597,
DOI 10.17487/RFC5597, September 2009,
<https://www.rfc-editor.org/rfc/rfc5597>.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<https://www.rfc-editor.org/rfc/rfc6356>.
[RFC6773] Phelan, T., Fairhurst, G., and C. Perkins, "DCCP-UDP: A
Datagram Congestion Control Protocol UDP Encapsulation for
NAT Traversal", RFC 6773, DOI 10.17487/RFC6773, November
2012, <https://www.rfc-editor.org/rfc/rfc6773>.
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[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<https://www.rfc-editor.org/rfc/rfc6904>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/rfc/rfc6951>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/rfc/rfc7323>.
[RFC8041] Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and
Operational Experience with Multipath TCP", RFC 8041,
DOI 10.17487/RFC8041, January 2017,
<https://www.rfc-editor.org/rfc/rfc8041>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/rfc/rfc8684>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/rfc/rfc9293>.
[TS23.501] 3GPP, "System architecture for the 5G System; Stage 2;
Release 16", December 2020,
<https://www.3gpp.org/ftp//Specs/
archive/23_series/23.501/23501-g70.zip>.
Appendix A. Differences from Multipath TCP
This appendix is Informative.
Multipath DCCP is similar to Multipath TCP [RFC8684], in that it
extends the related basic DCCP transport protocol [RFC4340] with
multipath capabilities in the same way as Multipath TCP extends TCP
[RFC9293]. However, because of the differences between the
underlying TCP and DCCP protocols, the transport characteristics of
MPTCP and MP-DCCP are different.
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Table 10 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 differently
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.
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+=======================+=================+======================+
| 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 | no | yes |
| boundaries | | |
+-----------------------+-----------------+----------------------+
| Path MTU discovery | yes | yes |
+-----------------------+-----------------+----------------------+
| Fragmentation | yes | no |
+-----------------------+-----------------+----------------------+
| SYN flood protection | yes | no |
+-----------------------+-----------------+----------------------+
| Half-open connections | yes | no |
+-----------------------+-----------------+----------------------+
Table 10: TCP and DCCP protocol comparison
Consequently, the multipath features, shown in Table 11, 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.
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+==================+=======================+==========+
| 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 |
+------------------+-----------------------+----------+
Table 11: MPTCP and MP-DCCP protocol comparison
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 Section 3.2.5) facilitates adding optional mechanisms
for data stream packet reordering at the receiver. Information from
the MP_RTT multipath option (Section 3.2.7), DCCP path sequencing and
the DCCP Timestamp Option provide further means for advanced
reordering approaches, e.g., as proposed in
[I-D.amend-iccrg-multipath-reordering]. 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
Markus Amend (editor)
Deutsche Telekom
Deutsche-Telekom-Allee 9
64295 Darmstadt
Germany
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Email: Markus.Amend@telekom.de
Anna Brunstrom
Karlstad University
Universitetsgatan 2
SE-651 88 Karlstad
Sweden
Email: anna.brunstrom@kau.se
Andreas Kassler
Karlstad University
Universitetsgatan 2
SE-651 88 Karlstad
Sweden
Email: andreas.kassler@kau.se
Veselin Rakocevic
City, University of London
Northampton Square
London
United Kingdom
Email: veselin.rakocevic.1@city.ac.uk
Stephen Johnson
BT
Adastral Park
Martlesham Heath
IP5 3RE
United Kingdom
Email: stephen.h.johnson@bt.com
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