Internet DRAFT - draft-ietf-tsvwg-multipath-dccp
draft-ietf-tsvwg-multipath-dccp
Transport Area Working Group M. Amend, Ed.
Internet-Draft DT
Intended status: Experimental A. Brunstrom
Expires: 19 August 2023 A. Kassler
Karlstad University
V. Rakocevic
City University of London
S. Johnson
BT
15 February 2023
DCCP Extensions for Multipath Operation with Multiple Addresses
draft-ietf-tsvwg-multipath-dccp-07
Abstract
DCCP communications as defined in [RFC4340] are restricted to a
single path per connection, yet multiple paths often exist between
peers. The simultaneous use of available multiple paths for a DCCP
session could improve resource usage within the network and, thus,
improve user experience through higher throughput and improved
resilience to network failures. Use cases for a Multipath DCCP (MP-
DCCP) are mobile devices (e.g., handsets, vehicles) and residential
home gateways simultaneously connected to distinct networks as, e.g.,
a cellular and a Wireless Local Area (WLAN) networks or a cellular
and a fixed access networks. Compared to existing multipath
protocols, such as MPTCP, MP-DCCP provides specific support for non-
TCP user traffic (e.g., UDP or plain IP). More details on potential
use cases are provided in [website], [slide], and [paper]. All these
use cases profit from an Open Source Linux reference implementation
provided under [website].
This document specifies a set of extensions to DCCP to support
multipath operations. Multipath DCCP provides the ability to
simultaneously use multiple paths between peers. The protocol offers
the same type of service to applications as DCCP and it provides the
components necessary to establish and use multiple DCCP flows across
different paths simultaneously.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 19 August 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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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. MP-DCCP Concept . . . . . . . . . . . . . . . . . . . . . 5
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 7
2. Operation Overview . . . . . . . . . . . . . . . . . . . . . 7
3. MP-DCCP Protocol . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Multipath Capable Feature . . . . . . . . . . . . . . . . 9
3.2. Multipath Option . . . . . . . . . . . . . . . . . . . . 10
3.2.1. MP_CONFIRM . . . . . . . . . . . . . . . . . . . . . 11
3.2.2. MP_JOIN . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.3. MP_FAST_CLOSE . . . . . . . . . . . . . . . . . . . . 15
3.2.4. MP_KEY . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.5. MP_SEQ . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.6. MP_HMAC . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.7. MP_RTT . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.8. MP_ADDADDR . . . . . . . . . . . . . . . . . . . . . 20
3.2.9. MP_REMOVEADDR . . . . . . . . . . . . . . . . . . . . 23
3.2.10. MP_PRIO . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.11. MP_CLOSE . . . . . . . . . . . . . . . . . . . . . . 26
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3.2.12. Experimental MP-DCCP Sub-Option MP_EXP for private
use . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3. MP-DCCP Handshaking Procedure . . . . . . . . . . . . . . 28
3.4. Address knowledge exchange . . . . . . . . . . . . . . . 29
3.4.1. Removing a path (Section 3.2.9) . . . . . . . . . . . 30
3.5. Close a MP-DCCP connection . . . . . . . . . . . . . . . 31
3.6. Fallback . . . . . . . . . . . . . . . . . . . . . . . . 32
3.7. Congestion Control Considerations . . . . . . . . . . . . 33
3.8. Maximum Packet Size Considerations . . . . . . . . . . . 33
4. Security Considerations . . . . . . . . . . . . . . . . . . . 34
5. Interactions with Middleboxes . . . . . . . . . . . . . . . . 35
6. Implementation . . . . . . . . . . . . . . . . . . . . . . . 36
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
9. Informative References . . . . . . . . . . . . . . . . . . . 39
Appendix A. Differences from Multipath TCP . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
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. Multipath DCCP (MP-DCCP) is a set of
extensions to DCCP to enable DCCP flows to be established across
multiple paths simultaneously. Such extensions are beneficial to
applications that transfer
large amounts of data, due to the possibility to aggregate capacity
of the multiple paths. In addition, the multipath extensions enable
to tradeoff timeliness and reliability, which is important for low-
latency applications that do not require guaranteed delivery
services, such as Audio/Video streaming.
MP-DCCP has been first suggested in the context of the 3GPP work on
5G multi-access solutions
[I-D.amend-tsvwg-multipath-framework-mpdccp] and for hybrid access
networks [I-D.lhwxz-hybrid-access-network-architecture][I-D.muley-net
work-based-bonding-hybrid-access]. MP-DCCP can be applied for load-
balancing, seamless session handover, and bandwidth aggregation
purposes (referred to as Access Traffic Steering, Switching, and
Splitting (ATSSS) in the 3GPP terminology [TS23.501]).
This document presents the protocol changes required to add multipath
support to DCCP; specifically, those 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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DCCP, as stated in [RFC4340] does not provide reliable and ordered
delivery. Consequently, multiple application subflows may be
multiplexed over a single DCCP connection with no inherent
performance penalty for application subflows that do not require in-
ordered delivery. DCCP does not provide built-in support for those
multiple application subflows.
In the following, the term subflow refers to DCCP subflows
transmitted via different paths (4-tuple of source and destination
address/port pairs), not to be mixed up with the "application sub-
flows" mentioned in Section 17.2 of [RFC4340]. Application subflows
are differing content-wise by source and destination port per
application as, for example, enabled by Service Codes introduced to
DCCP in [RFC5595], and those application subflows can be multiplexed
over a single DCCP connection. For the sake of consistency we assume
that only a single application is served by a DCCP connection here as
shown in Figure 1 while use of that feature should not impact DCCP
operation on each single path as noted in (Section 2.4 of [RFC5595]).
Application subflows can co-exist with MP-DCCP operation as defined
in this document.
As pointed out in [I-D.amend-tsvwg-multipath-framework-mpdccp] the
proposed encapsulation in terms of lightweight DCCP flow headers is
more appropriate for unreliable IP traffic in terms of UDP and other
non-TCP packets in comparison to MPTCP. Such considerations are not
detailed in the present specification.
1.1. Multipath DCCP in the Networking Stack
MP-DCCP operates at the transport layer and aims to be transparent to
both higher and lower layers. It is a set of additional features on
top of 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.
+-------------------------------+
| Application |
+---------------+ +-------------------------------+
| Application | | MP-DCCP |
+---------------+ + - - - - - - - + - - - - - - - +
| DCCP | |Subflow (DCCP) |Subflow (DCCP) |
+---------------+ +-------------------------------+
| IP | | IP | IP |
+---------------+ +-------------------------------+
Figure 1: Comparison of Standard DCCP and MP-DCCP Protocol Stacks
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1.2. Terminology
Throughout this document we make use of terms that are either
specific for multipath transport or are defined in the context of MP-
DCCP, similar to [RFC8684], as follows:
Path: A sequence of links between a sender and a receiver, defined in
this context by a 4-tuple of source and destination address/ port
pairs.
Subflow: A flow of DCCP segments operating over an individual path,
which forms part of a larger MP-DCCP connection. A subflow is
started and terminated similar to a regular (single-path) DCCP
connection. The term subflow can also be used to refer to an MP-DCCP
connection with a single path.
(MP-DCCP) Connection: A set of one or more subflows, over which an
application can communicate between two hosts. The MP-DCCP
connection is exposed as single DCCP socket to the application.
Token: A locally unique identifier given to a multipath connection by
a host. May also be referred to as a "Connection ID".
Host: An end host operating an MP-DCCP implementation, and either
initiating or accepting an MP-DCCP connection.
In addition to these terms, within framework of MP-DCCP the
interpretation of, and effect on, regular single-path DCCP semantics
is discussed in Section 3.
1.3. MP-DCCP Concept
Figure 2 provides a general overview of the MP-DCCP working mode,
whose main characteristics are summarized in this section.
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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
* An MP-DCCP connection begins with a 4-way handshaking procedure,
between two hosts as described in Section 3.3. In Figure 2, an
MP-DCCP connection is established between addresses A1 and B1 on
Hosts A and B, respectively. It should be noted that MP-DCCP does
not require the presence of more than one address in both peers.
* In case extra paths and corresponding addresses/ports are
available, additional DCCP subflows are created on these paths and
are attached to the existing MP-DCCP session, which continues to
appear as a single connection to the applications at both ends.
The creation of an additional DCCP subflow is illustrated between
Address A2 on Host A and Address B1 on Host B.
* MP-DCCP identifies multiple paths by the presence of multiple
addresses/ports at hosts. Combinations of these multiple
addresses/ports equate to the additional paths. In the example,
other potential paths that could be set up are A1<->B2 and
A2<->B2. Although this additional subflow is shown as being
initiated from A2, it could equally have been initiated from B1 or
B2.
* The discovery and setup of additional subflows will be achieved
through a path management method including the logic and details
of the procedures for adding/removing subflows; this document
describes supportive measures by which a host can initiate new
subflows and signal available addresses between peers. The
definition of a path management method is, however, out of scope
of this document and subject to a corresponding policy and the
specifics of the implementation. In the same context, if any of
the MP-DCCP peer hosts has a limit on the maximum number of paths
that can be maintained (e.g., similar to what is discussed in
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Section 3.4 of [RFC8041], the creation of new subflows from that
peer host should be avoided and incoming subflow requests should
be terminated.
* MP-DCCP adds connection-level sequence numbers and exchange of
Round-Trip Time (RTT) information to enable optional reordering
features.
* Subflows are terminated as regular DCCP connections, as described
in ([RFC4340], Section 8.3). The MP-DCCP connection is terminated
by a connection-level DCCP-CloseReq or DCCP-Close message.
1.4. 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 (Section 17.2 of [RFC4340]) allows multiple application subflows
to be multiplexed over a single DCCP connection with potentially same
performance. However, DCCP does not provide built-in support for
multiple subflows and the Congestion Manager (CM) [RFC3124], as a
generic multiplexing facility, can not fully support multiple
congestion control mechanisms for multiple DCCP flows between same
source and destination addresses. Various congestion control
mechanisms have been specified to optimize DCCP performance for
specific traffic types in terms of profiles denoted by a Congestion
Control IDentifier (CCID).
The extension of DCCP towards Multipath-DCCP (MP-DCCP) is described
in detail in Section 3.
As a high level overview of the MP-DCCP operation, the data stream
from a DCCP application is split by MP-DCCP operation into one or
more subflows which can be transmitted via different also physically
isolated paths. The corresponding control information allows the
receiver to optionally re-assemble and deliver the received data in
the right order to the recipient application. The details of the
transmission scheduling mechanism and optional reordering mechanism
are up to the sender and receiver, respectively, and are outside the
scope of this document.
The following sections define MP-DCCP behavior in detail.
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A Multipath DCCP connection provides a bidirectional connection of
datagrams between two hosts exchanging data as in DCCP, thus, not
requiring any change to the applications. However, Multipath DCCP
enables the hosts to use different paths with different IP addresses
to transport the packets of an MP-DCCP connection. MP-DCCP manages
the request, set-up, authentication, prioritization, modification,
and removal of the DCCP subflows on different paths as well as the
exchange of performance parameters.
The Multipath Capability for MP-DCCP can be 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 with MP-DCCP
suboptions described in {#MP_OPT}.
The number of concurrent 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.
3. MP-DCCP Protocol
The DCCP protocol feature list ([RFC4340], Section 6.4) are enriched
with a new Multipath related feature with Feature number 10, as shown
in Table 1.
+========+===================+======+=============+===============+
| Number | Meaning | Rule | Rec'n Value | Initial Req'd |
+========+===================+======+=============+===============+
| 10 | Multipath Capable | SP | 0 | N |
+--------+-------------------+------+-------------+---------------+
Table 1: Multipath Feature
Rec'n Rule: The reconciliation rule used for the feature. SP means
server-priority, NN means non-negotiable.
Initial Value: The initial value for the feature. Every feature has
a known initial value.
Req'd: This column is "Y" if and only if every DCCP implementation
MUST understand the feature. If it is "N", then the feature
behaves like an extension, and it is safe to respond to Change
options for the feature with empty Confirm options.
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The DCCP protocol options as defined in ([RFC4340], Section 5.8) and
([RFC5634], Section 2.2.1) are enriched with a new Multipath related
variable-length option with option type 46, as shown in Table 2.
+======+===============+===========+============+
| Type | Option Length | Meaning | DCCP-Data? |
+======+===============+===========+============+
| 46 | variable | Multipath | Y |
+------+---------------+-----------+------------+
Table 2: Multipath Option Set
3.1. Multipath Capable Feature
DCCP endpoints uses the Multipath Capable Feature to decide whether
multipath extensions can be enabled for a DCCP connection.
Multipath Capable feature has feature number 10 and has length of
one-byte. The leftmost four bits are used to specify a compatible
version of the MP-DCCP implementation (0000 for this specification).
The following four bits are unassigned in version 0. The unassigned
bits 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 |
+-----------+------------+
The setting of Multipath Capable MUST follow the server-priority
reconciliation rule described in ([RFC4340], Section 6.3.1), which
allows multiple versions to be specified in order of priority.
The negotiation MUST be done as part of the initial handshake
procedure as described in Section 3.3, and no subsequent re-
negotiation of the Multipath Capable feature is allowed on the same
MP-DCCP connection.
Clients MUST include a Change R option during the initial handshake
request to supply a list of supported MP-DCCP protocol versions,
ordered by preference.
Servers MUST include a Confirm L option in the subsequent response to
agree on an MP-DCCP version to be used from the Client list, followed
by its own supported version(s) ordered by preference. Any subflow
addition to an existing MP-DCCP connection MUST use the same version
negotiated for the first subflow.
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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:
Client Server
------ ------
DCCP-Req + Change R(CAPABLE, 1 0)
----------------------------------->
DCCP-Resp + Confirm L(CAPABLE, 1, 2 1 0)
<-----------------------------------
* agreement on version = 1 *
1. The Client indicates support for both MP-DCCP versions 1 and 0,
with a preference for version 1.
2. Server agrees on using MP-DCCP version 1, and supplies its own
preference list.
3. MP-DCCP is then enabled between the Client and Server with
version 1.
If the version negotiation fails or the MP_CAPABLE feature is not
present in the DCCP-Request or DCCP-Response packets of the initial
handshake procedure, the MP-DCCP connection SHOULD fall back to
regular DCCP or MUST be closed. Further details are specified in
Section 3.6
3.2. Multipath Option
MP-DCCP uses one single option to signal various multipath-related
operations. The format of this option is shown in Figure 3.
1 2 3
01234567 89012345 67890123 45678901 23456789
+--------+--------+--------+--------+--------+
|00101110| Length | MP_OPT | Value(s) ...
+--------+--------+--------+--------+--------+
Type=46
Figure 3: Multipath Option Format
The description of the fields of the multipath option is shown in
Table 3. MP_OPT refers to an MP-DCCP suboption.
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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 path 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 | HMA Code for authentication |
+------+--------+----------------+--------------------------------+
| 46 | 12 | 6 =MP_RTT | Transmit RTT values |
+------+--------+----------------+--------------------------------+
| 46 | var | 7 =MP_ADDADDR | Advertise additional Address |
+------+--------+----------------+--------------------------------+
| 46 | 4 | 8 | Remove Address |
| | | =MP_REMOVEADDR | |
+------+--------+----------------+--------------------------------+
| 46 | 4 | 9 =MP_PRIO | Change Subflow Priority |
+------+--------+----------------+--------------------------------+
| 46 | var | 10 =MP_CLOSE | Close an MP-DCCP subflow |
+------+--------+----------------+--------------------------------+
| 46 | TBD | >10 | Reserved for future MP |
| | | | suboptions defined in Version |
| | | | > 0 or extension |
+------+--------+----------------+--------------------------------+
Table 3: MP_OPT Option Types
These operations are largely inspired by the signals defined in
[RFC8684].
3.2.1. MP_CONFIRM
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1 2 3 4 5
01234567 89012345 67890123 45678901 23456789 01234567 89012345
+--------+--------+--------+--------+--------+--------+--------+
|00101110| var |00000000| List of confirmations ...
+--------+--------+--------+--------+--------+--------+--------+
Type=46 Length MP_OPT=0
Some multipath options require confirmation from the remote peer (see
Table 4). Such options will be retransmitted by the sender until a
MP_CONFIRM is received or confirmation of options is identified
outdated. The further processing of the multipath options in the
receiving host is not the subject of MP_CONFIRM.
As the transmission of multipath suboptions is subject to out-of-
order arrival, suboptions defined in Table 4 SHALL be sent in a DCCP
datagram with MP_SEQ Section 3.2.5. This allows to identify outdated
suboptions which updates the same dataset. In case of MP_ADDADDR,
MP_REMOVEADDR the same dataset is identified based on AddressID,
whereas the same dataset for MP_PRIO is identified by the subflow in
use. An outdated suboption is detected at the receiver if a previous
suboption referring to the same dataset contained a higher sequence
number carried by MP_SEQ. Generating a MP_CONFIRM for suboptions
identified outdated is optional.
Similarly MP_CONFIRM is subject to out-of-order arrival. To ensure
that the most recent suboption is confirmed the associated MP_SEQ
received MUST be echoed. Otherwise inconsistency happens if between
updates of a dataset with the same value, another value is sent. If
the MP_CONFIRM of the second update and the third update itself gets
lost, the value of the second update is applied on receiver side
without being detected by the sender.
The length and sending path of the MP_CONFIRM are dependent on the
confirmed suboptions and the received MP_SEQ, which will be both
copied verbatim and appended as list of confirmations. The list
structures by first listing the received MP_SEQ followed by the
confirmed suboption or suboptions. The same rules apply when
suboptions with different MP_SEQs are confirmed at once. This might
happen if a datagram with MP_PRIO and a first MP_SEQ_1 and another
datagram with MP_ADDADDR and a second MP_SEQ_2 are received in short
succession. In this case, the structure described above is
concatenated resulting in MP_SEQ_1 + MP_PRIO + MP_SEQ_2 + MP_ADDADDR.
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+======+===============+==================+=========================+
| 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 4. The host A sends a DCCP-Request
on path A2-B2 with an MP_PRIO option with value 1 and associated
sequence number of 1. Host B replies on the same path in this
instance (but could be any path) with a DCCP-Response containing the
MP_CONFIRM option and a list containing the original sequence number
(1) together with the associated option (MP_PRIO)
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 4: 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 5.
Here, a first DCCP-Data is sent from Host A to Host B with option
MP_PRIO set to 4. Host A subsequently issues a second DCCP-Data with
option MP_PRIO set to 1. 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.
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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 5: Example MP-DCCP CONFIRM procedure with outdated suboption
3.2.2. MP_JOIN
1 2 3
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
|00101110|00001100|00000001| Addr ID|
+--------+--------+--------+--------+
| Path Token |
+--------+--------+--------+--------+
| Nonce |
+--------+--------+--------+--------+
Type=46 Length=12 MP_OPT=1
Figure 6: Format of the MP_JOIN Suboption
The MP_JOIN option is used by a host to add a new subflow to an
existing MP-DCCP connection. The Path Token is the SHA256 hash of
the derived key (d-key), which was previously exchanged with the
MP_KEY option. MP_HMAC MUST be set when using MP_JOIN to provide
authentication (See MP_HMAC for details). Also MP_KEY MUST be set to
provide key material for authentication purposes.
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 has been changed in transit by a
middlebox. The numeric value of this field is generated by the
sender and must map uniquely to a source IP address for the sending
host. The Address ID allows address removal (Section 3.2.9) without
needing to know what the source address at the receiver is, thus
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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 has to be unique over the lifetime of a subflow and
can only be re-assigned if sender and receiver no longer have them in
use.
The Nonce is a 32-bit random value locally generated for every
MP_JOIN option. Together with the Token, the Nonce value builds the
basis to calculate the HMAC used in the handshaking process as
described in Section 3.3.
If the path token can not be verified by the receiving host during a
handshake negotiation, the new subflow MUST be closed, as specified
in Section 3.6.
3.2.3. MP_FAST_CLOSE
Regular DCCP has the means of sending a Close or Reset signals to
abruptly close a connection. With MP-DCCP, a regular Close or Reset
only has the scope of the subflow over which the signal was received.
As such, it will only close the applicable subflow and will not
affect the remaining subflows concurrently in use on other paths. A
MP-DCCP connection will stay alive at the data level in order to
permit break-before-make handover between subflows. It is therefore
necessary to provide an MP-DCCP-level "Reset" to allow the abrupt
closure of the whole MP-DCCP connection; this is done via the
MP_FAST_CLOSE suboption.
1 2 3
01234567 89012345 67890123 45678901 23456789
+--------+--------+--------+--------+--------+
|00101110| var |00000010| Key Data ...
+--------+--------+--------+--------+--------+
Type=46 Length MP_OPT=2
Figure 7: Format of the MP_FAST_CLOSE Suboption
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For being effective, the MP_FAST_CLOSE suboption MUST be sent from an
initiating host on all subflows as part of a DCCP-Reset packet with
Reset Code 13. To protect unauthorized shutdown of a multipath DCCP
connection, the selected Key Data of the peer host during the
handshaking procedure is carried by the MP_FAST_CLOSE option.
With completion of this step, the initiating host can tear down the
subflows and the multipath DCCP connection immediately terminates.
Upon reception of the MP_FAST_CLOSE and successful validation of the
Key Data at the peer host, a DCCP Reset packet is replied on all
subflows to the initiating host with Reset Code 13. The peer host
can now close the whole MP-DCCP connection (i.e., it transitions the
connection state directly to CLOSED).
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| Key Type (1) |
+---------------+---------------+---------------+---------------+
| Key Data (1) | Key Type (2) | Key Data (2) | ....
+---------------+---------------+---------------+---------------+
Type=46 Length MP_OPT=3
Figure 8: Format of the MP_KEY Suboption
The MP_KEY suboption is used to exchange key material between hosts
for a given connection. The Length varies between 12 and 68 Bytes
for a single-key message, and up to 110 Bytes when all specified Key
Types 0-2 are provided. The Key Type field is used to specify the
type of the following key data. 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 =ECDHE-C25519-SHA256 | 32 | ECDHE with SHA256 |
| | | and Curve25519 |
+------------------------+--------------------+-------------------+
| 2 =ECDHE-C25519-SHA512 | 64 | ECDHE with SHA512 |
| | | and Curve25519 |
+------------------------+--------------------+-------------------+
| 3-255 | | Unassigned |
+------------------------+--------------------+-------------------+
Table 5: MP_KEY Key Types
Plain Text
Key Material is exchanged in plain text between hosts, and the key
parts (key-a, key-b) are used by each host to generate the derived
key (d-key) by concatenating the two parts with the local key in
front (e.g. hostA d-key(A)=(key-a+key-b), hostB d-key(B)=(key-
b+key-a)).
ECDHE-SHA256-C25519
Public Key Material is exchanged via ECDHE key exchange with
SHA256 and Curve 25519 to generate the derived key (d-key) from
the shared secret.
ECDHE-SHA512-C25519
Public Key Material is exchanged via ECDHE key exchange with
SHA512 and Curve 25519 to generate the derived key (d-key) from
the shared secret.
Providing multiple keys is only permitted in the DCCP-Request message
of the handshake procedure for the first subflow, and allows the
hosts to agree on a single key type to be used as described in
Section 3.3
If the key type can not be agreed when the MP_KEY option is sent as
part of the handshake procedure, the MP-DCCP connection should
fallback to regular 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 9: Format of the MP_SEQ Suboption
The MP_SEQ suboption is used for end-to-end datagram-based sequence
numbers of an MP-DCCP connection. The initial data sequence number
(IDSN) SHOULD be set randomly [RFC4086].
The MP_SEQ number space is different from the path individual
sequence number space and MUST be sent with any DCCP-Data and DCCP-
DataACK packet.
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 10: Format of the MP_HMAC Suboption
The MP_HMAC suboption is used to provide authentication for the
MP_JOIN, MP_ADDADDR, and MP_REMOVEADDR suboptions. The HMAC code is
generated according to [RFC2104] in combination with the SHA256 hash
algorithm described in [RFC6234], with the output truncated to the
leftmost 160 bits (20 bytes).
The "Key" used for the HMAC computation is the derived key (d-key)
described in Section 3.2.4, while the HMAC "Message" is a
concatenation of
* MP_JOIN: The token and nonce of the MP_JOIN for which
authentication shall be performed.
* MP_ADDADDR: The Address ID with associated IP address and if
defined port, otherwise two octets of value 0.
* MP_REMOVEADDR: Solely the Address ID.
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MP_JOIN, MP_ADDADDR and MP_REMOVEADDR can co-exist or be used
multiple times within a single DCCP packet. As all this multipath
options come along with an individual MP_HASH option, this requires
the MP_HASH to be correctly associated. Otherwise, the receiver
cannot validate multiple MP_JOIN, MP_ADDADDR or MP_REMOVEADDR.
Therefore, a MP_HASH MUST directly follow its associated multipath
option. In the likely case of sending a MP_JOIN together with a
MP_ADDADDR, this results in concatenating MP_JOIN + MP_HMAC_1 +
MP_ADDADDR + MP_HMAC_2, whereas the first MP_HMAC_1 is associated
with the MP_JOIN and the second MP_HMAC_2 with the MP_ADDADDR
suboption.
If the HMAC can not be validated by a receiving host, the subsequent
handling depends on which suboption was being authenticated. If the
suboption to be authenticated was either MP_ADDADDR or MP_REMOVEADDR,
the receiving host SHOULD silently ignore it (see Section 3.2.8 and
Section 3.2.9). If the suboption to be authenticated was MP_JOIN, it
MUST lead to a subflow closing (see Section 3.6)
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 11: Format of the MP_RTT Suboption
The MP_RTT suboption is used to transmit RTT values in milliseconds
and MUST belong to the path over which this information is
transmitted. Additionally, the age of the measurement is specified
in milliseconds. This information is in particular 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, which allows to
cover a period of approximately 1193 hours.
Raw RTT (=0)
Raw RTT value of the last Datagram Round-Trip preferably provided
by the CCID in use.
Min RTT (=1)
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Min RTT value over a given period preferably provided by the CCID
in use.
Max RTT (=2)
Max RTT value over a given period preferably provided by the CCID
in use.
Smooth RTT (=3)
Averaged RTT value over a given period preferably provided by the
CCID in use.
Age
The Age parameter defines the time difference between now -
creation of the MP_RTT option - and the conducted RTT measurement
in milliseconds. If no previous measurement exists, e.g., when
initialized, the value is 0.
In Figure 12 an exemplary flow shows the exchange of path individual
RTT information with RTT1 pointing to a first path and RTT2 to a
second path. Those RTT values might be extracted from the sender's
Congestion Control procedure and carried to the receiving host using
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 case the path individual RTTs are
symmetric in down- and uplink direction, 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 12: Exemplary flow of MP_RTT exchange and usage
3.2.8. MP_ADDADDR
The MP_ADDADDR suboption announces additional addresses (and,
optionally, port numbers) on which a host can be reached. This
option can be used at any time during an existing DCCP connection,
when the sender wishes to enable multiple paths and/or when
additional paths become available. Multiple instances of this
suboption within a packet advertise simultaneously new addresses.
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Length is variable depending on the address family (IPv4 or IPv6) and
whether a port number is used. This field is in range between 8 and
22 bytes.
The presence of the final 2 octets, specifying the DCCP port number
to use, are optional and 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 may 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 peer SHOULD assume that any attempt
to connect to the specified address has to be on the same port as is
already in use by the subflow on which the MP_ADDADDR signal was
sent.
Along with the MP_ADDADDR option a MP_HMAC option MUST be sent for
authentication. The truncated HMAC parameter present in this MP_HMAC
option is the leftmost 20 bytes of an HMAC, negotiated and calculated
as described in 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 Key-A followed by Key-B, and in the case of Host B,
Key-B followed by Key-A. These are the keys that were exchanged and
selected in the original MP_KEY handshake. The message for the HMAC
is the Address ID, IP address, and port that precede the HMAC in the
MP_ADDADDR option. If the port is not present in the MP_ADDADDR
option, the HMAC message will nevertheless include 2 octets of value
zero. The rationale for the HMAC is to prevent unauthorized entities
from injecting MP_ADDADDR signals in an attempt to hijack a
connection. Note that, additionally, the presence of this HMAC
prevents the address from being changed in flight unless the key is
known by an intermediary. If a host receives an MP_ADDADDR option
for which it cannot validate the HMAC, it SHOULD silently ignore the
option.
The presence of a MP_SEQ Section 3.2.5 MUST be ensured in a DCCP
datagram in which MP_ADDADDR is sent. Further details are given 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 |
+---------------+---------------+-------+-------+---------------+
| Address (IPv4 - 4 bytes / IPv6 - 16 bytes) |
+-------------------------------+-------------------------------+
| Port (2 bytes, optional) | + MP_HMAC option
+-------------------------------+
Type=46 Length MP_OPT=7
Figure 13: Format of the MP_ADDADDR Suboption
Every address has an Address ID that can be used for uniquely
identifying the address within a connection for address removal. The
Address ID is also used to identify MP_JOIN options (see
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 SHOULD be
stored by the receiver in a data structure that gathers all the
Address-ID-to-address mappings for a connection (identified by a
token pair). In this way, there is a stored mapping between the
Address ID, observed source address, and token pair for future
processing of control information for a connection. Note that an
implementation MAY discard incoming address advertisements at will,
for example, for avoiding the required mapping state, or because
advertised addresses are of no use to it (for example, IPv6 addresses
when it has IPv4 only). Therefore, a host MUST treat address
advertisements as soft state, and it MAY choose to refresh
advertisements periodically.
Due to the proliferation of NATs, it is reasonably likely that one
host may attempt to advertise private addresses. It is not desirable
to prohibit this, since there may be cases where both hosts have
additional interfaces on the same private network, and a host MAY
want to advertise such addresses. 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 token that uniquely
identifies the connection to the receiving host. If the token is
unknown, the host will return with a DCCP-Reset. In the unlikely
event that the token is known, subflow setup will continue, but the
HMAC exchange must occur for authentication. This will fail, and
will provide sufficient protection against two unconnected hosts
accidentally setting up a new subflow upon the signal of a private
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address. 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. In case a
sending host of a MP_ADDADDR knows about the inability to establish
incoming subflows on a particular address, a MP_ADDADDR SHOULD NOT
advertise this address unless sending host has new knowledge about
the ability. Such ability 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). Using this mechanism reliable exchange
of address information is ensured.
A host can send an MP_ADDADDR message with an already assigned
Address ID, but the Address MUST be the same as previously assigned
to this Address ID, and the Port MUST be different from one already
in use for this Address ID. If these conditions are not met, the
receiver SHOULD silently ignore the MP_ADDADDR. A host wishing to
replace an existing Address ID MUST first remove the existing one
(Section 3.2.9).
A host that receives an MP_ADDADDR but finds a connection set up to
that IP address and port number is unsuccessful SHOULD NOT perform
further connection attempts to this address/port combination for this
connection. However, a sender that wants to trigger a new incoming
connection attempt on a previously advertised address/port
combination can therefore refresh MP_ADDADDR information by sending
the option again.
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 so that the peer can remove
subflows related to this address.
This is achieved through the Remove Address (MP_REMOVEADDR) suboption
which will remove a previously added address with an Address ID from
a connection and terminate any subflows currently using that address.
Along with the MP_REMOVEADDR suboption a MP_HMAC option MUST be sent
for authentication. The truncated HMAC parameter present in this
MP_HMAC option is the leftmost 20 bytes of an HMAC, negotiated and
calculated as described in 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 Key-A followed by Key-B, and in the
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case of Host B, Key-B followed by Key-A. These are the keys that
were exchanged and selected in the original MP_KEY handshake. The
message for the HMAC is the Address ID. The rationale for the HMAC
is to prevent unauthorized entities from injecting MP_REMOVEADDR
signals in an attempt to hijack a connection. Note that,
additionally, the presence of this HMAC prevents the address from
being removed in flight unless the key is known by an intermediary.
If a host receives an MP_REMOVEADDR option for which it cannot
validate the HMAC, it SHOULD silently ignore the option.
The presence of a MP_SEQ Section 3.2.5 MUST be ensured in a DCCP
datagram in which MP_REMOVEADDR is sent. 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). Using this mechanism reliable exchange
of address information is ensured. To avoid inconsistent states, it
is recommended to release the sender address ID only after
MP_REMOVEADDR has been confirmed.
The sending and receipt of this message SHOULD trigger the sending of
DCCP-Close and DCCP-Reset by client and server, respectively on the
affected subflow(s) (if possible), as a courtesy to cleaning up
middlebox state, before cleaning up any local state.
Address removal is undertaken by ID, so as to permit the use of NATs
and other middleboxes that rewrite source addresses. If there is no
address at the requested 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 |
+---------------+---------------+---------------+---------------+
Type=46 Length=4 MP_OPT=8
-> followed by MP_HMAC option
Figure 14: Format of theMP_REMOVEADDR Suboption
A subflow that is still functioning MUST be closed with a DCCP-Close
or exchange as in regular DCCP, rather than using this option. For
more information, see Section Section 3.5.
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3.2.10. MP_PRIO
In the event that a single specific path out of the set of available
paths shall be treated with higher priority compared to the others
when making scheduling decisions for user plane traffic, a host may
wish to signal such change in priority to the peer. One reason for
such behavior is due to the different costs involved in using
different paths (e.g., WiFi is free while cellular has limit on
volume, 5G has higher energy consumption). Also, the priority of a
path may be subject to dynamic changes, for example when the mobile
runs out of battery, the usage of only a single path may be the
preferred choice of the user. Therefore, the path priority should be
considered as hints for the packet scheduler when making decisions
which path to use for user plane traffic.
The MP_PRIO suboption, shown below, can be used to set a priority
flag for the path 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 15: Format of the MP_PRIO Suboption
The following values are available for Prio field:
* 0: Do not use. The path is not available.
* 1: Standby: do not use this path for traffic scheduling, if
another path (secondary or primary) is available. The path will
only be used if other secondary or primary paths are not
established.
* 2: Secondary: do not use this path for traffic scheduling, if the
other paths are good enough. The path will be used occasionally
for increasing temporarily the available capacity, e.g. when
primary paths are congested or are not available. This is the
recommended setting for paths that have costs or data caps as
these paths will be used less frequently then primary paths.
* 3 - 15: Primary: can use the path in any way deemed reasonable by
peer. The path will always 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 more
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traffic over higher priority paths. This is the recommended
setting for paths that do not have a cost or data caps associated
with them as these paths will be frequently used.
Example use cases include: 1) Setting Wi-Fi path to Primary and
Cellular paths to Secondary. In this case Wi-Fi will be used and
Cellular only if the Wi-Fi path is congested or not available. Such
setting results in using the Cellular path only temporally, if more
capacity is needed than the WiFi path can provide, indicating a clear
priority of the Wi-Fi path over the Cellular due to e.g. cost
reasons. 2) Setting Wi-Fi path to Primary and Cellular to Standby.
In this case Wi-Fi will be used and Cellular only, if the Wi-Fi path
is not available. 3) Setting Wi-Fi path to Primary and Cellular path
to Primary. In this case, all packets can be scheduled over all
paths at all time.
If not specified, the default behavior is, that a path can always be
used for packet scheduling decisions (MP_PRIO=3), if the path has
been established and added to an existing MP-DCCP connection. At
least one path should have a MP_PRIO value greater or equal to one
for it to be allowed to send on the connection. MP_PRIO is assumed
to be exchanged reliably using the MP_CONFIRM mechanisms (see
Table 4).
The presence of a MP_SEQ Section 3.2.5 MUST be ensured in a DCCP
datagram in which MP_PRIO 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 16: Format of the MP_CLOSE Suboption
For a graceful shutdown of a MP-DCCP connection, MP_CLOSE is used to
communicate this to a 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). In the case where a DCCP-CloseReq is used, the
following DCCP-Close MUST carry the MP_CLOSE as well. At the
initiator of the DCCP-CloseReq all sockets, including the MP-DCCP
connection socket, transition to CLOSEREQ state. To protect
unauthorized shutdown of a multi-path connection, the selected Key
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Data of the peer host during the handshaking procedure MUST be
carried by the MP_CLOSE option and validated by the peer host. Note,
Key Data is different between MP_CLOSE option carried by DCCP-
CloseReq or DCCP-Close.
On reception of a first DCCP-CloseReq carrying a MP_CLOSE with valid
Key Data, or due to a local decision, all subflows transition to the
CLOSING state before transmitting a DCCP-Close carrying MP_CLOSE. In
this case, the MP-DCCP connection socket on the host sending the
DCCP-Close reflects the state of the initial subflow used during
handshake with MP_KEY option. If the initial subflow no longer
exists, the state moves immediately to CLOSED.
Upon reception of the first DCCP-Close carrying a MP_CLOSE with valid
Key Data at the peer host, all subflows, as well as the MP-DCCP
connection socket, move to the CLOSED state. After this, a DCCP-
Reset with Reset Code 1 MUST be sent on any subflow in response to a
received DCCP-Close containing a valid MP_CLOSE option.
When the MP-DCCP connection socket is in CLOSEREQ or CLOSE state, new
subflow requests using MP_JOIN MUST be ignored.
Contrary to a MP_FAST_CLOSE Section 3.2.3, no single-sided abrupt
termination is applied.
3.2.12. Experimental MP-DCCP Sub-Option MP_EXP for private use
This section reserves a MP-DCCP sub-option to define and specify any
experimental additional feature for improving and optimization of MP-
DCCP protocol. This option may be applicable to specific
environments or scenarios according to potential new requirements and
meant for private use only at this stage. MP_OPT feature number 11
is foreseen 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 17: Format of the MP_EXP Suboption
Details as length and type of data remain to be defined according to
the foreseen use by the experimenters.
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3.3. MP-DCCP Handshaking Procedure
An example to illustrate the MP-DCCP handshake procedure is shown in
Figure 18.
Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| DCCP-Request + MP_CAPABLE |
|------- MP_KEY(Key-A(1), Key-A(2),...) ------------->|
|<---------------------- MP_KEY(Key-B) ---------------|
| DCCP-Response + MP_CAPABLE |
| | |
| DCCP-Ack | |
|--------- MP_KEY(Key-A) + MP_KEY(Key-B) ------------>|
|<----------------------------------------------------|
| DCCP-Ack | |
| | |
| | DCCP-Request + MP_CAPABLE |
| |--- MP_JOIN(TB,RA) ------------------->|
| |<------MP_JOIN(TB,RB) + MP_HMAC(B)-----|
| |DCCP-Response + MP_CAPABLE |
| | |
| |DCCP-Ack |
| |-------- MP_HMAC(A) ------------------>|
| |<--------------------------------------|
| |DCCP-ACK |
Figure 18: 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 an Host-specific Key-A for each
of supported types as described in Section 3.2.4.
* Host B sends a DCCP-Response with Confirm feature for MP-Capable
and the MP_Key option with a single Host-specific Key-B. The type
of the key is chosen from the list of supported types from the
previous request.
* Host A sends a DCCP-Ack with both Keys echoed to Host B.
* Host B sends a DCCP-Ack to confirm both keys and conclude the
handshaking.
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Host A waits for the final DCCP-Ack from host B before starting any
establishment of additional subflow connections.
The handshake for subsequent subflows based on a successful initial
handshake is as follows:
* Host A sends a DCCP-Request with the MP-Capable feature Change
request and the MP_JOIN option with Host B's Token TB, generated
from the derived key by applying a SHA256 hash and truncating to
the first 32 bits. Additionally, an own random nonce RA is
transmitted with the MP_JOIN.
* Host B computes the HMAC of the DCCP-Request and sends a DCCP-
Response with Confirm feature option for MP-Capable and the
MP_JOIN option with the Token TB and a random nonce RB together
with the computed MP_HMAC. The HMAC is calculated by taking the
leftmost 20 bytes from the SHA256 hash of a HMAC code created by
using token and nonce received with MP_JOIN(A) as message and the
derived key described in Section 3.2.4 as key:
MP_HMAC(B) = HMAC-SHA256(Key=d-key(B), Msg=RB+RA)
* Host A sends a DCCP-Ack with the HMAC computed for the DCCP-
Response. The HMAC is calculated by taking the leftmost 20 bytes
from the SHA256 hash of a HMAC code created by using token and
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-key(A), Msg=RA+RB)
* Host B sends a DCCP-Ack to confirm the HMAC and to conclude the
handshaking.
3.4. Address knowledge exchange
### Advertising a new path (Section 3.2.8)
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 19. The MP_ADDADDR option passed in the DCCP-
Data contains the following parameters: * an identifier (id 2) for
the new IP address which is used as a reference in subsequent control
exchanges. * the IP address of the new path (A2_IP) * A pair of
octets specifying the port number associated with this IP address.
The value of 00 here, indicates that the port number is the same as
that used for the initial subflow address A1_IP
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The following options must be included in a packet carrying
MP_ADDADDR: * the leftmost 20 bytes of the HMAC(A) generated during
the initial handshaking procedure described in Section 3.3 and
Section 3.2.6 * the sequence number (seqno 12) for this message
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, A2_IP, 00) + |
|------- MP_HMAC(A) + MP_SEQ(seqno 12) -------------->|
| | |
| DCCP-Ack + MP_HMAC(B) + |
|<----- MP_CONFIRM(seqno 12, MP_ADDADDR) -------------|
Figure 19: Example MP-DCCP ADDADDR procedure
3.4.1. Removing a path (Section 3.2.9)
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 20. The MP_REMOVEADDR option passed
in the DCCP-Data contains the following parameters: * an identifier
(id 2) for the IP address to remove (A2_IP) and which was specified
in a previous MP_ADDADDR message.
The following options must be included in a packet carrying
MP_REMOVEADDR * the leftmost 20 bytes of the HMAC(A) generated during
the initial handshaking procedure described in Section 3.3 and
Section 3.2.6 * the sequence number (seqno 33) for this message
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
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Host A Host B
------------------------ -----------
Address A1 Address A2 Address B1
---------- ---------- -----------
| | |
| DCCP-Data + MP_REMOVEADDR(id 2) + |
|------- MP_HMAC(A) + MP_SEQ(seqno 33) -------------->|
| | |
| DCCP-Ack + MP_HMAC(B) + |
|<----- MP_CONFIRM(seqno 33, MP_REMOVEADDR) ----------|
Figure 20: Example MP-DCCP REMOVEADDR procedure
3.5. Close a MP-DCCP connection
When a host wants to close an existing subflow but not the whole MP-
DCCP connection, it initiates the regular DCCP connection termination
procedure as described in [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 is RECOMMENDED to use an appropriate DCCP
reset code as specified in Table 2 of [RFC4340]. This could be, for
example, sending reset code 5 (Option Error) when an MP-DCCP option
provides invalid data or reset code 9 (Too Busy) when the maximum
number of maintainable paths is reached. Note that receiving a reset
code 9 for secondary subflows SHOULD NOT impact already existing
active subflows. If necessary, these subflows are terminated in a
subsequent step using the procedures described in this section.
When a host wants to terminate an MP-DCCP connection, it is
RECOMMENDED that the host initiates the DCCP connection termination
as per [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.
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Host A Host B
------ ------
DCCP-Reset + MP_FAST_CLOSE ->
[B's key] [on all subflows]
<- DCCP-Reset
[on all subflows]
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 MP-DCCP
suboptions, faulty/non-supported MP-DCCP options or modification of
payload data.
At the start of an MP-DCCP connection, the handshake ensures exchange
of MP-DCCP feature and options and thus ensures that the path is
fully MP-DCCP capable. If during the handshake procedure it appears
that DCCP-Request or DCCP-Response messages don't carry the
MP_CAPABLE feature, the MP-DCCP connection will not be established
and the handshake SHOULD fall back to regular DCCP or MUST be closed.
The same fallback SHOULD take place if the endpoints fail to agree on
a protocol version to use during the Multipath Capable feature
negotiation, which 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.
Similar procedure MUST be applied if the MP_KEY Section 3.2.4 Key
Type cannot be negotiated, a final ACK carrying MP_KEY with wrong
Key-A/Key-B is received or MP_KEY option is malformed.
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. Also non-verifiable MP_HMAC
Section 3.2.6 or MP_JOIN Path Token Section 3.2.2 as part of the
subsequent flow establishment MUST lead to a subflow closing.
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
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changed in any way. All data from the start of the segment that
failed the checksum onwards cannot be considered trustworthy. DCCP
defines that if the checksum fails, the receiving endpoint MUST drop
the application data and report that data as dropped due to
corruption using a Data Dropped option (Drop Code 3, Corrupt). If
this happens in an MP-DCCP connection, the affected subflow can
either be closed or other action can be taken.
3.7. Congestion Control Considerations
Senders MUST manage per-path congestion status, and SHOULD avoid to
send more data on a given path than congestion control on that path
allows.
When a Multipath DCCP connection uses two or more paths, there is no
guarantee that these paths are fully disjoint. When two (or more
paths) share the same bottleneck, using a standard congestion control
scheme could result in an unfair distribution of the bandwidth with
the multipath connection getting more bandwidth than competing single
path connections. Multipath TCP uses the coupled congestion control
Linked Increases Algorithm (LIA) specified in [RFC6356] to solve this
problem. This scheme can be adapted also for Multipath DCCP. The
same applies to other coupled congestion control schemes, which have
been proposed for Multipath TCP such as Opportunistic Linked
Increases Algorithm [OLIA]. The details of the congestion control
algorithms are outside the scope of this document.
3.8. 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. As per this definition a DCCP application
interface SHOULD let the application discover the current MPS, this
is subject to ambiguity with potential different path MPS in a
multipath system.
For compatibility reasons, an MP-DCCP implementation SHOULD always
announce the minimum MPS across all paths.
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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 or some other kind of
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.
As described in [RFC4340], DCCP provides protection against hijacking
and limits the potential impact of some denial-of-service attacks,
but DCCP provides no inherent protection against attackers' snooping
on data packets. Regarding the security of MP-DCCP no additional
risks should be introduced compared to regular DCCP. Thereof derived
are the following key security requirements to be fulfilled by MP-
DCCP:
* Provide a mechanism to confirm that parties involved in a subflow
handshake are identical to those in the original connection setup.
* Provide verification that the new address to be included in a MP
connection is valid for a peer to receive traffic at before using
it.
* Provide replay protection, i.e., ensure that a request to add/
remove a subflow is 'fresh'.
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In order to achieve these goals, MP-DCCP includes a hash-based
handshake algorithm documented in Sections Section 3.2.4 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. To ease demultiplexing
while not giving away any cryptographic material, future subflows use
a truncated cryptographic hash of this key as the connection
identification "token". The keys are concatenated and used as keys
for creating Hash-based Message Authentication Codes (HMACs) used on
subflow setup, in order to verify that the parties in the handshake
are the same as in the original connection setup. It also provides
verification that the peer can receive traffic at this new address.
Replay attacks would still be possible when only keys are used;
therefore, the handshakes use single-use random numbers (nonces) at
both ends -- this ensures that the HMAC will never be the same on two
handshakes. Guidance on generating random numbers suitable for use
as keys is given in [RFC4086]. During normal operation, regular DCCP
protection mechanisms (such as header checksum to protect DCCP
headers against corruption) will 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) will ensure that this
does not succeed in setting 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.
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). In case, however, 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 NAT type
middleboxes are covered by [RFC5597].
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[RFC6773] specifies UDP Encapsulation for NAT Traversal of DCCP
sessions, similar to other UDP encapsulations such as for SCTP
[RFC6951]. The alternative U-DCCP approach proposed in
[I-D.amend-tsvwg-dccp-udp-header-conversion] would reduce tunneling
overhead. The handshaking procedure for DCCP-UDP header conversion
or use of a DCCP-UDP negotiation procedure to signal support for
DCCP-UDP header conversion would require encapsulation during the
handshakes and use of two additional port numbers out of the UDP port
number space, but would require zero overhead afterwards.
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 [website].
7. Acknowledgments
Due to the great spearheading work of the Multipath TCP authors in
[RFC6824]/[RFC8684], some text passages were copied almost unmodified
from these documents.
The authors gratefully acknowledge significant input into this
document from Dirk von Hugo, Nathalie Romo Moreno, Omar Nassef,
Mohamed Boucadair, and Behcet Sarikaya.
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 [RFC8126]. This
document defines one new value to be added to the DCCP Feature
Numbers registry and three new registries to be added to the DCCP
registry group.
This document requests IANA to assign a new DCCP feature parameter
for negotiating the support of multipath capability for DCCP sessions
between hosts as described in 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.
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+================+============================+================+
| Value | Feature Name | Specification |
+================+============================+================+
| 10 (suggested) | MP-DCCP capability feature | [ThisDocument] |
+----------------+----------------------------+----------------+
Table 6: Addition to DCCP Feature Numbers registry
As outlined in sect. Section 3.1 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 | Description | Specification |
+=============+=============+================+
| 0000 | Version 0 | [ThisDocument] |
+-------------+-------------+----------------+
| 0001 - 1111 | Unassigned | |
+-------------+-------------+----------------+
Table 7: MP-DCCP Versions Registry
Future MP-DCCP versions 1 to 15 are assigned from this registry using
the Specification Required policy (Section 4.6 of [RFC8126]).
This document requests IANA to assign a new DCCP protocol option of
type=46 as described in Section 3.2.
IANA is requested to create a new 'MP-DCCP Suboptions' registry
within the DCCP registry group. The following entries in Table 8
should be added to the new 'MP-DCCP Suboptions' registry. The
registry in Table 8 has an upper boundary of 255 in the numeric value
field.
+===========+===============+=====================+===========+
| Value | Symbol | Name | Reference |
+===========+===============+=====================+===========+
| Type=46 | MP_OPT | DCCP Multipath | Section |
| | | option | 3.2 |
+-----------+---------------+---------------------+-----------+
| 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 | Section |
| | | existing MP-DCCP | 3.2.2 |
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| | | connection | |
+-----------+---------------+---------------------+-----------+
| MP_OPT=2 | MP_FAST_CLOSE | Close MP-DCCP | Section |
| | | connection | 3.2.3 |
+-----------+---------------+---------------------+-----------+
| 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 |
| | | 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 MP-DCCP | Section |
| | | subflow | 3.2 |
+-----------+---------------+---------------------+-----------+
| MP_OPT=11 | MP_EXP | Experimental Sub- | Section |
| | | Option for private | 3.2.12 |
| | | use | |
+-----------+---------------+---------------------+-----------+
| MP_OPT>11 | Unassigned | Reserved for future | |
| | | MP-DCCP suboptions | |
+-----------+---------------+---------------------+-----------+
Table 8: MP-DCCP Suboptions registry
Future MP-DCCP sub-options with MP_OPT>11 can be assigned from this
registry using the Specification Required policy (Section 4.6 of
[RFC8126]).
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In addition IANA is requested to assign a new DCCP Reset Code value
13 (or TBD) 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.
In addition IANA is requested to assign for this version of the MP-
DCCP protocol a new 'MP_KEY' registry containing three different sub
options to the MP-KEY option to identify the MP_KEY Key types in
terms of 8-bit values as specified in Section 3.2.4 according to the
entries in Table 9 below. Values in range 3-255 (decimal) inclusive
remain unassigned in this version 0 of the protocol and are assigned
via Specification Required [RFC8126] in potential future versions of
the MP-DCCP protocol.
+=======+=====================+===================+===============+
| Value | Key Type | Name or Meaning | Reference |
+=======+=====================+===================+===============+
| 0 | Plain Text | Plain Text Key | Section 3.2.4 |
+-------+---------------------+-------------------+---------------+
| 1 | ECDHE-C25519-SHA256 | ECDHE with SHA256 | Section 3.2.4 |
| | | and Curve25519 | |
+-------+---------------------+-------------------+---------------+
| 2 | ECDHE-C25519-SHA512 | ECDHE with SHA512 | Section 3.2.4 |
| | | and Curve25519 | |
+-------+---------------------+-------------------+---------------+
Table 9: MP-DCCP MP_KEY registry with type sub-options for key
data exchange on different paths
9. 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>.
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[I-D.amend-tsvwg-multipath-framework-mpdccp]
Amend, M., Bogenfeld, E., Brunstrom, A., Kassler, A., and
V. Rakocevic, "A multipath framework for UDP traffic over
heterogeneous access networks", Work in Progress,
Internet-Draft, draft-amend-tsvwg-multipath-framework-
mpdccp-01, 8 July 2019,
<https://datatracker.ietf.org/doc/html/draft-amend-tsvwg-
multipath-framework-mpdccp-01>.
[I-D.lhwxz-hybrid-access-network-architecture]
Leymann, N., Heidemann, C., Cullen, M., Xue, L., and M.
Zhang, "Hybrid Access Network Architecture", Work in
Progress, Internet-Draft, draft-lhwxz-hybrid-access-
network-architecture-02, 13 January 2015,
<https://datatracker.ietf.org/doc/html/draft-lhwxz-hybrid-
access-network-architecture-02>.
[I-D.muley-network-based-bonding-hybrid-access]
Muley, P., Henderickx, W., Geng, L., Liu, H., Cardullo,
L., Newton, J., Seo, S., Draznin, S., and B. Patil,
"Network based Bonding solution for Hybrid Access", Work
in Progress, Internet-Draft, draft-muley-network-based-
bonding-hybrid-access-03, 22 October 2018,
<https://datatracker.ietf.org/doc/html/draft-muley-
network-based-bonding-hybrid-access-03>.
[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.
[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>.
[RFC0793] Postel, J., "Transmission Control Protocol", RFC 793,
DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/rfc/rfc793>.
[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>.
Amend, et al. Expires 19 August 2023 [Page 40]
Internet-Draft Multipath DCCP February 2023
[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>.
[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
RFC 3124, DOI 10.17487/RFC3124, June 2001,
<https://www.rfc-editor.org/rfc/rfc3124>.
[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>.
[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>.
[RFC5595] Fairhurst, G., "The Datagram Congestion Control Protocol
(DCCP) Service Codes", RFC 5595, DOI 10.17487/RFC5595,
September 2009, <https://www.rfc-editor.org/rfc/rfc5595>.
[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>.
[RFC5634] Fairhurst, G. and A. Sathiaseelan, "Quick-Start for the
Datagram Congestion Control Protocol (DCCP)", RFC 5634,
DOI 10.17487/RFC5634, August 2009,
<https://www.rfc-editor.org/rfc/rfc5634>.
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[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>.
[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>.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/rfc/rfc6824>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[slide] 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>.
[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>.
[website] "Multipath extension for DCCP", n.d.,
<https://multipath-dccp.org/>.
Appendix A. Differences from Multipath TCP
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
[RFC0793]. However, because of the differences between the
underlying TCP and DCCP protocols, the transport characteristics of
MPTCP and MP-DCCP are different.
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 in a different
way and does not guarantee to deliver any payload nor the order of
delivery. Since this is mainly affecting the receiving endpoint of a
TCP or DCCP communication, many similarities on the sender side can
be identified. Both transport protocols share the 3-way initiation
of a communication and both employ congestion control to adapt the
sending rate to the path characteristics.
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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
transport character of 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 described 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 deal with out-of-sequence data (e.g., through adaptive
audio and video buffers), and so additional reordering support may
not be necessary. The addition of optional reordering mechanisms are
most 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 might happen, since DCCP has no
mechanisms to prevent it.
The receiving process for MPTCP is on the other hand a rigid "just
wait" approach, since TCP guarantees reliable delivery.
Authors' Addresses
Markus Amend (editor)
Deutsche Telekom
Deutsche-Telekom-Allee 9
64295 Darmstadt
Germany
Email: Markus.Amend@telekom.de
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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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