Internet DRAFT - draft-defoy-mptcp-5g-session-continuity-support
draft-defoy-mptcp-5g-session-continuity-support
Network Working Group X. de Foy
Internet-Draft U. Olvera-Hernandez
Intended status: Informational InterDigital Communications
Expires: August 17, 2019 U. Chunduri
Huawei USA
Feb 13, 2019
5G Session Continuity Support in MPTCP
draft-defoy-mptcp-5g-session-continuity-support-00
Abstract
This document describes how 5G session continuity can affect MPTCP.
For now only potential performance issues are identified. This
document aims to document discussions that took place at the IETF on
this subject, to facilitate future deployment of MPTCP over 5G.
Status of This Memo
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Table of Contents
1. Introduction and Goal . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
2.1. 5G Will Not Hide Session Continuity from MPTCP Any Longer 2
2.2. Detailed Issues . . . . . . . . . . . . . . . . . . . . . 3
3. Potential Solutions . . . . . . . . . . . . . . . . . . . . . 4
3.1. Alternative #1: No Change to MPTCP Protocol . . . . . . . 4
3.2. Alternative #2: Explicit signaling of Session Continuity
Type . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Alternative #3: Client-Driven Handling . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. Informative References . . . . . . . . . . . . . . . . . . . 7
Appendix A. IP Address Session Continuity Service Type . . . . . 8
Appendix B. Overview of 5G Session and Service Continuity . . . 8
B.1. SSC mode 1 . . . . . . . . . . . . . . . . . . . . . . . 9
B.2. SSC mode 2 . . . . . . . . . . . . . . . . . . . . . . . 9
B.3. SSC mode 3 . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix C. Example of MPTCP Client Implementations Behavior
with 5G SSC . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction and Goal
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 how 5G session continuity can affect MPTCP.
For now only potential performance issues are identified. This
document aims to document discussions that took place at the IETF on
this subject, to facilitate future deployment of MPTCP over 5G.
2. Problem Statement
2.1. 5G Will Not Hide Session Continuity from MPTCP Any Longer
In 4G [_3GPP.23.401], a single long-term IP address was provided to
the end device. Session continuity was performed through a fixed
anchor, and effectively hidden from MPTCP.
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In 5G, session continuity won't always be hidden from MPTCP. 3
session continuity modes are defined in [_3GPP.23.501]: in some
cases, what used to be a single IP address will now be visible by
MPTCP as multiple successive and possibly concurrent IP addresses.
More details on session continuity in 5G are provided in Appendix B.
In particular, the 5G term for session continuity is Session and
Service Continuity (SSC), and the 3 SSC modes correspond to: fixed
anchor (mode 1), distributed anchor with break-before-make (mode 2),
and distributed anchor with make-before-break (mode 3).
While it could be possible to hide 5G session continuity to MPTCP by
limiting its usage to SSC mode 1, it would limit the range of
applications that can benefit from MPTCP, since SSC modes 2 and 3
enable low-latency mobility. In the rest of this document, we will
study how MPTCP can deal with any SSC mode.
An MPTCP proxy function will be integrated with the 5G network in
3GPP Release-16, enabling a 5G device to use MPTCP over multiple
access technologies (e.g. cellular and WiFi) even if the server does
not support MPTCP. The normative phase for this feature has recently
started and is based on the concluded study [_3GPP.23.793].
Potential issues raised in this document may apply to 5G devices
directly using MPTCP without a proxy. Assuming that session
continuity modes 2 and 3 are available when using the MPTCP proxy
(which is possible but not yet established in the standard), these
issues may apply to 5G devices making use of the proxy as well.
2.2. Detailed Issues
Overall we don't expect SSC modes 2 and 3 will cause MPTCP to break,
but we do expect inefficiencies in some scenarios. The following
potential inefficiencies have been identified:
*Supporting make-before-break (MBB) without wasting resources*:
the old IP address should be released shortly after a MBB handover
has been performed. Not too early (wait for traffic using the new
IP address to ramp up), and not too late (to release resources to
the mobile network). Today's MPTCP implementations are likely to
keep using the old IP address as long as it is available, which
will prevent the network from releasing the resources early.
*Supporting break-before-make (BBM) without temporarily switching
to backup*: if there is a backup IP address, MPTCP peers should
not switch to using this backup IP address immediately, and
instead should wait for a new replacement IP address to be used
after BBM handover. Today's MPTCP implementations are likely to
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switch back-and-forth between active and backup IP addresses,
which can lead to network and power consumption inefficiencies.
3. Potential Solutions
Locally on the mobile node itself, a MPTCP implementation will need
some information to support session continuity. For each local IP
address "IP_A", MPTCP should be aware of the following information:
Is IP_A provided by a mobile network, and, if applicable, what is
its session continuity type? Session continuity type overview and
references are listed in Appendix A.
The original local IP address of the session (if it is not IP_A).
The client can use the tuple (address type, original IP address) to
locally support session continuity. An example of implementation
behavior is given in Appendix C. For example, a local MPTCP client
implementation can use this tuple to appropriately mark new subflows
as "backup", when they replace original subflows marked as "backup".
With regard to the behavior of the remote MPTCP peer, three
alternatives are identified at this point:
In alternative #1, we do not implement any specific support in the
MPTCP protocol.
In alternative #2, the MPTCP client sends to its remote peer, over
MPTCP signaling, the tuple (address type, original IP address) for
each IP address.
In alternative #3, we use an hybrid solution, where the tuple
(address type, original IP address) is not sent to the remote
peer; instead, the local MPTCP client influences the remote peer
using modified MPTCP signaling.
Some enhancements to the MPTCP protocol are proposed in alternatives
#2 and #3. Further discussions and analysis are expected to
determine which alternative is best suited for MPTCP.
3.1. Alternative #1: No Change to MPTCP Protocol
This section evaluates the impact of not implementing any specific
support in the MPTCP protocol, for the issues mentioned earlier
(although the MPTCP client implementation on a 5G device should still
be updated to be session continuity-aware, as in all 3 alternatives,
to implement client-side behavior such as properly assigning the
"backup" property).
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*Supporting make-before-break (MBB) without wasting resources*:
MBB can impact unmodified MPTCP (1) in term of network resource
usage and (2) in term of performances.
Resource usage: unmodified MPTCP will keep using the old IP
address until the network physically reclaims the network
resources when the lifetime of the old IP address is over.
This lifetime is not specified and may be implementation
specific. In the worst case, the network operator chooses a
long lifetime, with the option to remove the old IP address
after sensing it stopped being used, which would not happen
with MPTCP.
Performances: when the old IP address is brought down by the
network, some in-flight segments will need to be re-sent on
other subflows. While the client will be aware of the IP
address lifetime, and may therefore stop sending segments on
the associated subflow before reaching the end of the address
lifetime, the server may continue using this subflow until the
address is removed. The impact may therefore vary depending on
throughput and the nature of the application.
*Supporting break-before-make (BBM) without temporarily switching
to backup*: even if this issue is not addressed in the MPTCP
protocol, some applications, e.g. applications generating bursts
of traffic, may not be strongly impacted when temporarily
switching back and forth between radios, especially if the
occurrence is rare enough.
3.2. Alternative #2: Explicit signaling of Session Continuity Type
In this case, options that implicitly or explicitly add a new IP
address (MP_CAPABLE, ADD_ADDR, MP_JOIN) are associated with
additional fields (address type, original IP address index). This
way, both MPTCP peers share the same information about the IP
address, with regards to session continuity.
*Supporting make-before-break (MBB) without wasting resources*:
after the local client creates a new subflow using the new IP
address, local client and remote peer both start using it. They
continue sending traffic on the old subflow (i.e. subflow using
the old IP address), until the traffic usage ramps up on the new
subflow. At this point, both peers stop sending new segments on
the the old subflow. Once in-flight segments are received and
acked, the local client resets the old subflow and then remove the
old IP address, which makes it possible for the network to
ultimately reclaim the network resources.
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*Supporting break-before-make (BBM) without temporarily switching
to backup*: remote peer and local client are both aware that a BBM
is a normal occurrence for IP addresses associated with a "non
persistent" type. Therefore, remote peer and local client should
both wait for a given time before using a backup subflow. This
"BBM timeout" parameter may for example be sent in a new field by
the local client to the remote peer, when adding the original "non
persistent" IP address.
3.3. Alternative #3: Client-Driven Handling
In this case, session continuity type is not sent over MPTCP
signaling. The local client uses non-session-continuity-specific
MPTCP signaling to control the behavior of the remote peer. Some
minor modifications of the MPTCP protocol may be needed.
*Supporting make-before-break (MBB) without wasting resources*:
the local client creates a new subflow using the new IP address.
After enough time passed for traffic to ramp up on the new
subflow, the local client instructs the remote peer to stop using
the old subflow (i.e. subflow using the old IP address), without
abruptly closing the subflow, to avoid re-sending segments on the
new subflow and affect performance. To do this, the local client
sets the priority of the old subflow to "backup", and then waits
until in-flight segments are received and acked. At this point,
the local client resets the old, now unused subflow. Once no more
subflows are using the old IP address, the local client removes it
using REMOVE_ADDR.
A new subflow reset reason code "path management decision" may
be defined to indicate that a peer took the decision to
permanently remove a subflow.
As a minor improvement, a new priority "inactive" may also be
defined. "Inactive" would be similar to backup, except that it
would never become active, even if no other active subflow
exist. This could avoid rare issues when losing active
subflows while removing an old subflow.
*Supporting break-before-make (BBM) without temporarily switching
to backup*: the local client associates a timer value to a backup
priority on a subflow, e.g. using a new field in the MP_PRIO
option. When all active subflows are lost, MPTCP peers must wait
for the specified time before using the backup subflow. To avoid
switching between backup and active subflows in BBM, the local
client should ensure that all backup priority timers are set to a
value that is higher that the maximum BBM transition time.
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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 through discussions or
reviews: Christoph Paasch, Michelle Perras, Debashish Purkayastha,
Akbar Rahman, Sri Gundavelli, Philip Eardley, Yoshifumi Nishida.
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>.
[_3GPP.23.793]
3GPP, "Study on Access Traffic Steering, Switch and
Splitting support in the 5G system architecture", 3GPP
TR 23.793 16.0.0, 3 2018,
<http://www.3gpp.org/ftp/Specs/html-info/23793.htm>.
[I-D.feng-dmm-ra-prefixtype]
Feng, W. and D. Moses, "Router Advertisement Prefix Option
Extension for On-Demand Mobility", draft-feng-dmm-ra-
prefixtype-03 (work in progress), September 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-16 (work in progress), February 2019.
[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>.
Appendix A. IP Address Session Continuity Service Type
The "session continuity service type" (SCS type) characterises the
session continuity properties an IP address allocated by a mobile
network. 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.
This information can 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 an IP address may be
conveyed through router advertisements.
Although the session continuity service types are not 3GPP-specific,
they are planned to be used in the 5G specification from 3GPP, as
properties of the IP addresses allocated by the network to mobile
devices. There is a 1:1 relationship between the session continuity
service type of the initial IP address of a session and the SSC mode
of this session (SSC mode is a 3GPP concept discussed in the
following section).
Appendix B. Overview of 5G Session and Service Continuity
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
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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.
In 5G, every unit of a network connectivity service (PDU session) has
a type which can be IP (IPv4 or IPv6), Ethernet or unstructured.
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.
B.1. 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.
This would result in a second IP address being allocated on the
network interface with which the long-term IP address is associated.
This second IP address may be brought down at any time.
B.2. 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 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
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between these 2 events is not standardized and implementation
dependent.
B.3. 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 specifications 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 network
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).
Appendix C. Example of MPTCP Client Implementations Behavior with 5G
SSC
The following describes at high level how a MPTCP implementation
could be modified to on the client to support 5G session continuity.
If the solution alternative #2 is used, this behavior can be extended
to the MPTCP server as well.
For simplicity, we consider a case where MPTCP is used in a client
with 2 IP addresses, one of them being provided by a mobile network.
The behaviors described here depend on the session continuity type of
the initial mobile network-provided IP address, which has a 1:1
mapping to the 5G SCC mode used.
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When the initial IP address session continuity type is FIXED or
SESSION_LASTING (i.e. in SCC mode 1):
MPTCP should not close all subflows originated from this original
IP address at any point during the session, since this IP address
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).
When the initial IP address session continuity type is NON_PERSISTENT
(i.e. in SCC mode 2):
At any point in time, the current NON_PERSISTENT IP address may be
taken down by the network stack. The MPTCP implementation 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.
When the initial IP address session continuity type is
GRACEFUL_REPLACEMENT (i.e. in SCC mode 3):
At any point in time, a new GRACEFUL_REPLACEMENT IP address may be
made available by the network stack. The MPTCP implementation
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 implementation should
consider any subsequent GRACEFUL_REPLACEMENT IP address as the new
backup IP address, in replacement of the first
GRACEFUL_REPLACEMENT IP address.
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Authors' Addresses
Xavier de Foy
InterDigital Communications, LLC
1000 Sherbrooke West
Montreal H3A 3G4
Canada
Email: Xavier.Defoy@InterDigital.com
Ulises Olvera-Hernandez
InterDigital Communications, LLC
64 Great Eastern Street
London EC2A 3QR
England
Email: Ulises.Olvera-Hernandez@InterDigital.com
Uma Chunduri
Huawei USA
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: uma.chunduri@huawei.com
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