Internet DRAFT - draft-defoy-mptcp-considerations-for-5g
draft-defoy-mptcp-considerations-for-5g
Network Working Group X. de Foy
Internet-Draft M. Perras
Intended status: Informational InterDigital Communications
Expires: December 24, 2018 U. Chunduri
Huawei USA
K. Nguyen
M. Kibria
K. Ishizu
F. Kojima
NICT
June 22, 2018
Considerations for MPTCP operation in 5G
draft-defoy-mptcp-considerations-for-5g-01
Abstract
This document describes scenarios where the behavior of the 5G
mobility management framework is different from earlier cellular
generations, and describes how it may benefit from some form of
adaptation of MPTCP implementations and protocol aspects in the 5G
system. This document also describes how MPTCP may be leveraged in
5G system specifications.
Status of This Memo
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Need for Adapting MPTCP to 5G Session and Service
Continuity . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Opportunity for Leveraging MPTCP in 5G Dual Connectivity
Feature . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Impact of 5G Session and Service Continuity on MPTCP . . . . 3
2.1. New Inputs for MPTCP stacks . . . . . . . . . . . . . . . 4
2.2. SSC mode 1 . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. SSC mode 2 . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. SSC mode 3 . . . . . . . . . . . . . . . . . . . . . . . 7
2.5. Behavior of Remote Peer . . . . . . . . . . . . . . . . . 9
3. MPTCP with 5G Dual Connectivity . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
7. Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
MPTCP [RFC6824] is being deployed and widely adopted in today's smart
devices, which typically have multiple network interfaces such as
Cellular and Wifi. It provides reliability, bandwidth aggregation
capability, and handover efficiency.
This document describes scenarios where the behavior of the 5G
mobility management framework is different from earlier cellular
generations, and describes how it may benefit from some form of
adaptation of MPTCP implementations and protocol aspects in the 5G
system (see Section 1.1). This document also describes how MPTCP may
be leveraged in 5G system specifications (see Section 1.2).
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1.1. Need for Adapting MPTCP to 5G Session and Service Continuity
With regards to 5G session and service continuity mechanisms, MPTCP
stack behavior (including path manager and scheduling) should be
updated to achieve optimal performance:
o Proposed 5G MPTCP behavior is described in Section 2.2,
Section 2.3 and Section 2.4.
o To enable this behavior, MPTCP will need additional information
about local IP addresses: (1) its session continuity service type
and (2) a notification that a new IP address has been provided for
service continuity. This is described in Section 2.1.
o Remote MPTCP peers may need access to the IP address SCS type
(through MPTCP signaling or otherwise). This is discussed in
Section 2.5.
1.2. Opportunity for Leveraging MPTCP in 5G Dual Connectivity Feature
With regards to dual connectivity, MPTCP can be closely integrated
with the 5G stack to avoid duplicating its feature in 5G, as
discussed in Section 3. As a summary:
o MPTCP should be aware of the presence of multiple DC radio links,
which may be exposed as a single or distinct network interfaces/IP
addresses.
o MPTCP should optimally partition traffic or duplicate a data flow
over DC links, depending on the application's need, network policy
and conditions.
2. Impact of 5G Session and Service Continuity on MPTCP
One of the goals of 5G systems, as outlined in [_3GPP.23.501], is to
enable low latency services and access to local data networks where
mobility anchors can be deployed close to devices, thereby satisfying
use cases with stringent transmission delay and high reliability.
Mobility in 4G networks, as described at the architecture level in
[_3GPP.23.401], was based on a central mobility solution that made it
difficult to relocate mobility anchors closer to the end user. In
contrast, 5G uses a distributed mobility solution based on multiple
anchors providing different IP addresses as the device moves from one
area to another.
The base scenario in this section is: a 5G device connected to a
single-homed server is in an area with no usable Wifi coverage. An
application using MPTCP sends traffic over a single subflow, over the
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cellular air interface. Then, as the device moves, the 5G device
reacts to mobility events. Additionally, we briefly discuss cases
where the device switches from a Wifi to a cellular backup
connection, and other cases where both MPTCP peers are 5G mobile
devices.
In 5G, every unit of a network connectivity service (PDU session) has
a type which can be IP (IPv4 or IPv6), Ethernet or unstructured.
While session continuity is supported for all types, we will focus on
PDU sessions of IP type primarily. Different PDU sessions will
typically correspond to distinct network interfaces on the device
(though this is not explicit in the standard, and some
implementations may possibly behave differently).
In 4G networks, session continuity is enabled by anchoring a PDN
Connection (as PDU Sessions are referred to in 4G networks) to a P-GW
which allocates an IP address to the mobile device: PDN connection
and IP address allocation are maintained as long as the device
remains attached to the network, even when the device moves around.
In 5G, different types of session continuity can be provided, and are
indicated by a "Session and Service Continuity" (SSC) mode value of
1, 2 or 3 (defined in [_3GPP.23.501] section 5.6.9). Every PDU
session is associated with a single SSC mode, which cannot be changed
on this PDU session. The following sub-sections will study how 5G
handles each SSC mode, and potential effects on MPTCP.
2.1. New Inputs for MPTCP stacks
IP Address Session Continuity Service Type
The "session continuity service type" (SCS type) of an IP address
allocated by the network can be used as input by MPTCP
implementations. It has been defined for on-demand mobility
management [I-D.ietf-dmm-ondemand-mobility], as:
FIXED IP address: valid for a very long time, for session
continuity and IP address reachability.
SESSION_LASTING IP address: valid for the lifetime of an IP
session, even if the mobility host moves.
NON_PERSISTENT IP address: which does not provide session
continuity nor IP address reachability.
GRACEFUL_REPLACEMENT IP address: similar to a non-persistent
address, but adding a limited graceful period for the
transition from one address to another.
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This information needs to be conveyed to the device by the network
that allocates the address: for example, as described in
[I-D.feng-dmm-ra-prefixtype], the SCS type of IP address may be
conveyed through router advertisements. The MPTCP stack may
obtain the SCS type of a local IP address using a programmatic
API. This information does not directly tell which SSC mode the
application is using, nevertheless it is sufficient for MPTCP to
appropriately and non-ambiguously decide how to handle new IP
addresses in the context of SSC in 5G.
Notification of a New IP Address from Session Continuity
As in non-5G cases, MPTCP starts handling the initial socket
created by the application. The initial connection attempt
triggers the creation of the first PDU session (or the reuse of an
existing one), associated with a network interface and an IP
address on this interface. Later on, the network decides to use a
new anchor, e.g., when a mobility event occurs or when a Local
Data Network becomes available. The network then triggers the
modification of the existing PDU session, or creation of a new PDU
session, and ultimately a new IP address is provided to the 5G
device, respectively on the same or a new network interface.
The 5G stack on the device is aware of the relationship between
the old and new IP addresses, and can therefore notify the MPTCP
stack. Such a notification may include both the old and new IP
addresses , which ensures that the MPTCP stack has all necessary
information (for example, the MPTCP stack may then obtain the SCS
type of the new IP address, and the validity lifetime associated
with the old IP address, if it is still up).
2.2. SSC mode 1
In SSC mode 1 the same network anchor is kept regardless of device
location. An application running on the device will therefore be
able to keep using the same IP address on the same interface.
Additionally, in SSC mode 1, the network may decide to add and
remove, dynamically, additional network anchors (and therefore IP
addresses) to the PDU session, while always keeping the initial one.
PROPOSED MPTCP BEHAVIOR SPECIFICATION:
These steps are applicable when the initial IP address returned by
the network stack is FIXED or SESSION_LASTING.
MPTCP should not close all subflows originated from this original
IP address at any point during the session, since this IP address
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is the only one that is guaranteed, under normal circumstances, to
be maintained over time for this application.
At any time during the session, a new IP address of SCS type
NON_PERSISTENT may become available. MPTCP may create new
subflows for the application, using this IP address (this IP
address is likely to provide shorter path subflows, but may
disappear at any time).
2.3. SSC mode 2
SSC mode 2 has a break-before-make behavior. When the device leaves
the service area of its first network anchor, the network stops using
it and starts using a new second network anchor closer to the device.
(Such service areas may have a highly variable size depending on
network deployments.) On the device, this can result in the
currently used network interface being brought down, and after a
short time a new network interface being brought up. The time
between these 2 events is not standardized and implementation
dependent.
Break-before-make within cellular technology
When MPTCP is active on cellular only, this break-before-make
behavior is similar to the existing break-before-make capability
usually used in cellular/Wifi offload (section 3.1.3 of [RFC6897]
and section 2.2 of [RFC8041]). A similar MPTCP behavior is
therefore needed: wait for a given time for a new IP address to be
configured. As per [RFC6897], to benefit from this MPTCP
resilience feature, the application should not request using a
specific network interface.
Cellular and Wifi
Additionally, when Wifi is active and cellular is used as backup,
MPTCP implementations should also support this break-before-make
behavior to maintain a usable backup IP address on cellular. In
rare cases where a switch-to-cellular backup would be needed
during a break-before-make transition on cellular, MPTCP's
existing break-before-make capability can ensure MPTCP waits for a
new cellular-facing IP address to be available.
PROPOSED MPTCP BEHAVIOR SPECIFICATION:
These steps are applicable when the initial IP address returned by
the network stack is NON_PERSISTENT.
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At any point in time, the current NON_PERSISTENT IP address may be
taken down by the network stack. The MPTCP stack should wait for
another NON_PERSISTENT IP address to be made available by the
network stack. If such an address is made available within a
given time limit, the MPTCP stack should create new subflows using
this new address (effectively following the existing break-before-
make behavior present in MPTCP).
Additionally, if an initial backup IP address is a NON_PERSISTENT
address, the MPTCP stack should consider any subsequent
NON_PERSISTENT IP address as a backup IP address in replacement of
the initial NON_PERSISTENT address.
2.4. SSC mode 3
SSC mode 3 has a make-before-break behavior. When the device leaves
the service area of its first network anchor, the network selects a
second network anchor closer to the device, and either creates a new
PDU session (i.e. new IP address on new network interface) or share
the existing PDU session (i.e. new IP address on same network
interface). The first network anchor keeps being used for a given
time period, which is communicated to the device by the network using
the "valid lifetime" field of a prefix information option in a router
advertisement ([RFC4861], [RFC4862]). 5G does not mandate a specific
range for this valid lifetime. The first/older IP address should not
be used to create any new traffic ([RFC4862] section 5.5.4). In some
implementations, the network (SMF) may decide to release the first
network anchor as soon as it stops carrying traffic.
There is no limit set by the 5G standard for the number of
concurrently used network anchors. We expect that in usual cases the
first network anchor will be released before a third network anchor
starts being used. Nevertheless, to our knowledge nothing prevents a
5G system deployment to allow a third network anchor to be selected
while the first one is still in use.
On the 5G device, when using SSC mode 3, mobility will therefore
result in a new IP address being configured, either on the same
network interface initially used, or on a different interface. In
general, an application will see a single cellular-facing IP address,
and during transient phase it will see 2 IP addresses (with a
possibility for more than 2 concurrent IP addresses on some 5G system
implementations). In cases where the server is single-homed and the
Wifi interface is down, and assuming a full-mesh path manager policy,
there will be in general one subflow, and from time to time,
temporarily 2 subflows (or more on some 5G systems). In cases where
two mobile 5G devices are communicating with each other over MPTCP
and with the same assumptions on Wifi and path manager policy, there
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will be in general one subflow, and from time to time, temporarily 2
or even more rarely 4 subflows (again, possibly more on some 5G
systems).
MPTCP must create new subflows when a new IP address on a same or a
new cellular-facing network interface becomes available to the
application. MPTCP may keep using the first subflow during a
transient phase. Here are some considerations related to this
transient phase:
o When compared with simply waiting for the first IP address to be
brought down, ramping down usage of the first subflow will not
incur inefficiencies from resending lost segments. This may
especially help low-latency applications by avoiding throughput
drop.
o Assuming a lowest-rtt-first scheduling policy is used, after the
initial TCP slow start, the shortest path subflow should typically
carry the most traffic. Ramping down should ideally start after
the initial slow start is over.
o To make sure the ramping down completes before the interface is
brought down by the network, the MPTCP stack should be aware of
how long will the first network anchor be kept in use, e.g.
through configuration or communication with the local 5G stack.
o Ramping down and closing flows on the first network anchor as soon
as possible will help recycling network resources more rapidly.
This is especially true in cases where more than 2 network anchors
may be used concurrently.
o There may be some level of contention between subflows during the
transient phase, since they share the same air interface, and
especially if they share the same PDU session and QoS marking.
The shortest path subflow may therefore not reach its full
capacity during the transient phase.
o Additionally, the shortest subflow must not be closed during the
transient phase (even if it is less efficient for some reason), to
avoid losing all connectivity at the end of the transient phase.
To avoid this issue, the MPTCP stack could for example follow a
policy not to close any subflow created using the latest IP
address, during the transient period (in SSC mode 3).
In cases where cellular is used for backup, there is a possibility
that the switch to using backup occurs during a transient phase. To
support this case, MPTCP should keep creating and releasing subflows
as described above, even when cellular subflows are used as backup,
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to ensure that the backup is always usable. When a backup event
occurs during a transient phase, MPTCP should use the subflows
associated with the most recent cellular-facing IP address, i.e.
corresponding to the latest/closest network anchor.
PROPOSED MPTCP BEHAVIOR SPECIFICATION:
These steps are applicable when the initial IP address returned by
the network stack is GRACEFUL_REPLACEMENT.
At any point in time, a new GRACEFUL_REPLACEMENT IP address may be
made available by the network stack. The MPTCP stack must create
new subflows using this new address, gracefully transfer traffic
to these new subflow(s), and close subflow(s) using the previous
GRACEFUL_REPLACEMENT IP address before its scheduled closing
(known by obtaining the valid lifetime of the IP address from the
operating system).
Additionally, if an initial backup IP address is a
GRACEFUL_REPLACEMENT address, the MPTCP stack should consider any
subsequent GRACEFUL_REPLACEMENT IP address as the new backup IP
address, in replacement of the first GRACEFUL_REPLACEMENT IP
address.
2.5. Behavior of Remote Peer
The SCS type of an IP address is only available locally, which limits
to only one peer the MPTCP behavior specified in sections
Section 2.2, Section 2.3 and Section 2.4. This can potentially cause
problems:
A remote may for example close subflows that should be maintained
e.g. closing all subflows to a SESSION_LASTING IP address or to
the latest GRACEFUL_REPLACEMENT address.
A remote may also not behave optimally in some cases, e.g. because
it cannot distinguish between a transition from
GRACEFUL_REPLACEMENT to GRACEFUL_REPLACEMENT, from a temporary
addition of a NON_PERSISTENT address to a SESSION_LASTING address.
There are two possible ways to handle this: either explicitly
transmit the SCS type over MPTCP signalling, or use heuristics on the
remote peer (e.g. never close a subflow created by a peer, mirror
behavior of peer, etc.). In the explicit solution, a MPTCP peer
provides the SCS type of IP addresses to its remote peer, using
existing or new options (mp_capable, add_addr, mp_join, or an
experimental option). When compared to using heuristics,
transmitting the SCS type can lead to a less complex and more
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explicit implementation, which can help better handling unusual or
error cases, and adapt to future evolution of 5G or other mobility
management systems.
3. MPTCP with 5G Dual Connectivity
One of the key features of 5G [_3GPP.23.501] is dual connectivity
(DC). With DC, a 5G device can be served by two different base
stations. DC may play an essential role in leveraging the benefit of
5G new radio, especially in the evolving architecture with the
coexistence of 4G and 5G radios.
On a 5G device with DC, an application is able to send data to the
destination (e.g., a single-home server) through multiple radio
links, over one or more PDU sessions. Some PDU sessions may be over
a single radio link, while others may have flows over each radio
link. Therefore, in a first case, DC can be made visible to
applications through different IP addresses, while in a second case,
DC can be used by different flows terminated at the same IP address
on the device.
In any of those cases, the issues of out of order delivery and
diverse latency values need to be supported in DC. However, such
reliable communication scenarios have not been addressed in the
current DC architecture. Based on the design history of DC in
earlier systems, the 5G system will need to incorporate features to
support robustness/reliability (e.g., backup and duplication), that
will likely result in added complexity. Meanwhile, similar issues
have been solved in MPTCP (e.g., two types of sequences at subflow
and connection levels for out of order delivery, etc.). MPTCP hence
could be leveraged in those scenarios. On the other hand, to satisfy
new QoS requirements of 5G applications as well as to benefit the
most from DC, 5G devices are expected to include advanced algorithms
that control data transmissions over each radio link optimally. For
example, those algorithms could aim to minimize overall latency or
satisfy latency requirements of applications. Hence, the 5G device
needs to dynamically select the most suitable path for a given radio
condition. Additionally, algorithms for shifting, based on
congestion, ongoing traffic between transmissions over radio links
are also necessary.
MPTCP, which includes path manager, scheduler, and congestion control
functions, shows a lot of potential to address the issues mentioned
above. MPTCP could, therefore, be integrated with DC and the 5G
protocol stack, as an alternative to developing 5G-specific
solutions. As part of this integration, the MPTCP stack should be
aware of the presence of multiple radio links, whether they are
exposed using multiple IP addresses or hidden under a single IP
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address. MPTCP's scheduler should optimally partition traffic or
duplicate a data flow over different links, depending on the
application's need, network policy, and conditions.
4. IANA Considerations
This document requests no IANA actions.
5. Security Considerations
No new security considerations are identified at this time.
6. Acknowledgements
Thanks to following people for contributing, reviewing or providing
feedback: Debashish Purkayastha, Akbar Rahman, Ulises Olvera-
Hernandez, Sri Gundavelli.
7. Informative References
[_3GPP.23.401]
3GPP, "General Packet Radio Service (GPRS) enhancements
for Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access", 3GPP TS 23.401 15.3.0, 3 2018,
<http://www.3gpp.org/ftp/Specs/html-info/23401.htm>.
[_3GPP.23.501]
3GPP, "System Architecture for the 5G System", 3GPP
TS 23.501 15.14.0, 3 2018,
<http://www.3gpp.org/ftp/Specs/html-info/23501.htm>.
[I-D.feng-dmm-ra-prefixtype]
Feng, W. and D. Moses, "Router Advertisement Extensions
for On-Demand Mobility", draft-feng-dmm-ra-prefixtype-02
(work in progress), March 2018.
[I-D.ietf-dmm-ondemand-mobility]
Yegin, A., Moses, D., Kweon, K., Lee, J., Park, J., and S.
Jeon, "On Demand Mobility Management", draft-ietf-dmm-
ondemand-mobility-14 (work in progress), March 2018.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
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[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[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/info/rfc6824>.
[RFC6897] Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
Interface Considerations", RFC 6897, DOI 10.17487/RFC6897,
March 2013, <https://www.rfc-editor.org/info/rfc6897>.
[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/info/rfc8041>.
Authors' Addresses
Xavier de Foy
InterDigital Communications, LLC
1000 Sherbrooke West
Montreal
Canada
Email: Xavier.Defoy@InterDigital.com
Michelle Perras
InterDigital Communications, LLC
Montreal
Canada
Email: Michelle.Perras@InterDigital.com
Uma Chunduri
Huawei USA
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: uma.chunduri@huawei.com
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Kien Nguyen
National Institute of Information and Communications Technology
YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
Kanagawa 239-0847
Japan
Email: kienng@nict.go.jp
Mirza Golam Kibria
National Institute of Information and Communications Technology
YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
Kanagawa 239-0847
Japan
Email: mirza.kibria@nict.go.jp
Kentaro Ishizu
National Institute of Information and Communications Technology
YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
Kanagawa 239-0847
Japan
Email: ishidu@nict.go.jp
Fumihide Kojima
National Institute of Information and Communications Technology
YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
Kanagawa 239-0847
Japan
Email: f-kojima@nict.go.jp
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