Use Cases and Operational Experience with Multipath TCP
draft-ietf-mptcp-experience-06
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| Authors | Olivier Bonaventure , Christoph Paasch , Gregory Detal | ||
| Last updated | 2016-09-15 (Latest revision 2016-08-29) | ||
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draft-ietf-mptcp-experience-06
MPTCP Working Group O. Bonaventure
Internet-Draft UCLouvain
Intended status: Informational C. Paasch
Expires: March 2, 2017 Apple, Inc.
G. Detal
Tessares
August 29, 2016
Use Cases and Operational Experience with Multipath TCP
draft-ietf-mptcp-experience-06
Abstract
This document discusses both use cases and operational experience
with Multipath TCP in real world networks. It lists several
prominent use cases for which Multipath TCP has been considered and
is being used. It also gives insight to some heuristics and
decisions that have helped to realize these use cases.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 2, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Datacenters . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Cellular/WiFi Offload . . . . . . . . . . . . . . . . . . 5
2.3. Multipath TCP proxies . . . . . . . . . . . . . . . . . . 9
3. Operational Experience . . . . . . . . . . . . . . . . . . . . 11
3.1. Middlebox interference . . . . . . . . . . . . . . . . . . 11
3.2. Congestion control . . . . . . . . . . . . . . . . . . . . 13
3.3. Subflow management . . . . . . . . . . . . . . . . . . . . 13
3.4. Implemented subflow managers . . . . . . . . . . . . . . . 14
3.5. Subflow destination port . . . . . . . . . . . . . . . . . 16
3.6. Closing subflows . . . . . . . . . . . . . . . . . . . . . 17
3.7. Packet schedulers . . . . . . . . . . . . . . . . . . . . 18
3.8. Segment size selection . . . . . . . . . . . . . . . . . . 19
3.9. Interactions with the Domain Name System . . . . . . . . . 19
3.10. Captive portals . . . . . . . . . . . . . . . . . . . . . 20
3.11. Stateless webservers . . . . . . . . . . . . . . . . . . . 21
3.12. Loadbalanced serverfarms . . . . . . . . . . . . . . . . . 22
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
5. Security Considerations . . . . . . . . . . . . . . . . . . . 24
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
7. Informative References . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
Multipath TCP was standardized in [RFC6824] and five independent
implementations have been developed. As of September 2015, Multipath
TCP has been or is being implemented on the following platforms:
o Linux kernel [MultipathTCP-Linux]
o Apple iOS and MacOS [Apple-MPTCP]
o Citrix load balancers
o FreeBSD [FreeBSD-MPTCP]
o Oracle
The first three implementations are known to interoperate. The last
two are currently being tested and improved against the Linux
implementation. Three of these implementations are open-source.
Apple's implementation is widely deployed.
Since the publication of [RFC6824], experience has been gathered by
various network researchers and users about the operational issues
that arise when Multipath TCP is used in today's Internet.
When the MPTCP working group was created, several use cases for
Multipath TCP were identified [RFC6182]. Since then, other use cases
have been proposed and some have been tested and even deployed. We
describe these use cases in Section 2.
Section 3 focuses on the operational experience with Multipath TCP.
Most of this experience comes from the utilisation of the Multipath
TCP implementation in the Linux kernel [MultipathTCP-Linux]. This
open-source implementation has been downloaded and is used by
thousands of users all over the world. Many of these users have
provided direct or indirect feedback by writing documents (scientific
articles or blog messages) or posting to the mptcp-dev mailing list
(see https://listes-2.sipr.ucl.ac.be/sympa/arc/mptcp-dev ). This
Multipath TCP implementation is actively maintained and continuously
improved. It is used on various types of hosts, ranging from
smartphones or embedded routers to high-end servers.
The Multipath TCP implementation in the Linux kernel is not, by far,
the most widespread deployment of Multipath TCP. Since September
2013, Multipath TCP is also supported on smartphones and tablets
running iOS7 [IOS7]. There are likely hundreds of millions of
Multipath TCP enabled devices. However, this particular Multipath
TCP implementation is currently only used to support a single
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application. Unfortunately, there is no public information about the
lessons learned from this large scale deployment.
Section 3 is organized as follows. Supporting the middleboxes was
one of the difficult issues in designing the Multipath TCP protocol.
We explain in Section 3.1 which types of middleboxes the Linux Kernel
implementation of Multipath TCP supports and how it reacts upon
encountering these. Section 3.2 summarises the MPTCP specific
congestion controls that have been implemented. Section 3.3 to
Section 3.7 discuss heuristics and issues with respect to subflow
management as well as the scheduling across the subflows.
Section 3.8 explains some problems that occurred with subflows having
different Maximum Segment Size (MSS) values. Section 3.9 presents
issues with respect to content delivery networks and suggests a
solution to this issue. Finally, Section 3.10 documents an issue
with captive portals where MPTCP will behave suboptimally.
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2. Use cases
Multipath TCP has been tested in several use cases. There is already
an abundant scientific literature on Multipath TCP [MPTCPBIB].
Several of the papers published in the scientific literature have
identified possible improvements that are worth being discussed here.
2.1. Datacenters
A first, although initially unexpected, documented use case for
Multipath TCP has been in datacenters [HotNets][SIGCOMM11]. Today's
datacenters are designed to provide several paths between single-
homed servers. The multiplicity of these paths comes from the
utilization of Equal Cost Multipath (ECMP) and other load balancing
techniques inside the datacenter. Most of the deployed load
balancing techniques in datacenters rely on hashes computed over the
five tuple. Thus all packets from the same TCP connection follow the
same path and so are not reordered. The results in [HotNets]
demonstrate by simulations that Multipath TCP can achieve a better
utilization of the available network by using multiple subflows for
each Multipath TCP session. Although [RFC6182] assumes that at least
one of the communicating hosts has several IP addresses, [HotNets]
demonstrates that Multipath TCP is beneficial when both hosts are
single-homed. This idea is analysed in more details in [SIGCOMM11]
where the Multipath TCP implementation in the Linux kernel is
modified to be able to use several subflows from the same IP address.
Measurements in a public datacenter show the quantitative benefits of
Multipath TCP [SIGCOMM11] in this environment.
Although ECMP is widely used inside datacenters, this is not the only
environment where there are different paths between a pair of hosts.
ECMP and other load balancing techniques such as Link Aggregation
Groups (LAG) are widely used in today's networks and having multiple
paths between a pair of single-homed hosts is becoming the norm
instead of the exception. Although these multiple paths have often
the same cost (from an IGP metrics viewpoint), they do not
necessarily have the same performance. For example, [IMC13c] reports
the results of a long measurement study showing that load balanced
Internet paths between that same pair of hosts can have huge delay
differences.
2.2. Cellular/WiFi Offload
A second use case that has been explored by several network
researchers is the cellular/WiFi offload use case. Smartphones or
other mobile devices equipped with two wireless interfaces are a very
common use case for Multipath TCP. In September 2015, this is also
the largest deployment of Multipath-TCP enabled devices [IOS7]. It
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has been briefly discussed during IETF88 [ietf88], but there is no
published paper or report that analyses this deployment. For this
reason, we only discuss published papers that have mainly used the
Multipath TCP implementation in the Linux kernel for their
experiments.
The performance of Multipath TCP in wireless networks was briefly
evaluated in [NSDI12]. One experiment analyzes the performance of
Multipath TCP on a client with two wireless interfaces. This
evaluation shows that when the receive window is large, Multipath TCP
can efficiently use the two available links. However, if the window
becomes smaller, then packets sent on a slow path can block the
transmission of packets on a faster path. In some cases, the
performance of Multipath TCP over two paths can become lower than the
performance of regular TCP over the best performing path. Two
heuristics, reinjection and penalization, are proposed in [NSDI12] to
solve this identified performance problem. These two heuristics have
since been used in the Multipath TCP implementation in the Linux
kernel. [CONEXT13] explored the problem in more detail and revealed
some other scenarios where Multipath TCP can have difficulties in
efficiently pooling the available paths. Improvements to the
Multipath TCP implementation in the Linux kernel are proposed in
[CONEXT13] to cope with some of these problems.
The first experimental analysis of Multipath TCP in a public wireless
environment was presented in [Cellnet12]. These measurements explore
the ability of Multipath TCP to use two wireless networks (real WiFi
and 3G networks). Three modes of operation are compared. The first
mode of operation is the simultaneous use of the two wireless
networks. In this mode, Multipath TCP pools the available resources
and uses both wireless interfaces. This mode provides fast handover
from WiFi to cellular or the opposite when the user moves.
Measurements presented in [CACM14] show that the handover from one
wireless network to another is not an abrupt process. When a host
moves, there are regions where the quality of one of the wireless
networks is weaker than the other, but the host considers this
wireless network to still be up. When a mobile host enters such
regions, its ability to send packets over another wireless network is
important to ensure a smooth handover. This is clearly illustrated
from the packet trace discussed in [CACM14].
Many cellular networks use volume-based pricing and users often
prefer to use unmetered WiFi networks when available instead of
metered cellular networks. [Cellnet12] implements support for the
MP_PRIO option to explore two other modes of operation.
In the backup mode, Multipath TCP opens a TCP subflow over each
interface, but the cellular interface is configured in backup mode.
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This implies that data flows only over the WiFi interface when both
interfaces are considered to be active. If the WiFi interface fails,
then the traffic switches quickly to the cellular interface, ensuring
a smooth handover from the user's viewpoint [Cellnet12]. The cost of
this approach is that the WiFi and cellular interfaces are likely to
remain active all the time since all subflows are established over
the two interfaces.
The single-path mode is slightly different. This mode benefits from
the break-before-make capability of Multipath TCP. When an MPTCP
session is established, a subflow is created over the WiFi interface.
No packet is sent over the cellular interface as long as the WiFi
interface remains up [Cellnet12]. This implies that the cellular
interface can remain idle and battery capacity is preserved. When
the WiFi interface fails, a new subflow is established over the
cellular interface in order to preserve the established Multipath TCP
sessions. Compared to the backup mode described earlier,
measurements reported in [Cellnet12] indicate that this mode of
operation is characterised by a throughput drop while the cellular
interface is brought up and the subflows are reestablished.
From a protocol viewpoint, [Cellnet12] discusses the problem posed by
the unreliability of the REMOVE_ADDR option and proposes a small
protocol extension to allow hosts to reliably exchange this option.
It would be useful to analyze packet traces to understand whether the
unreliability of the REMOVE_ADDR option poses an operational problem
in real deployments.
Another study of the performance of Multipath TCP in wireless
networks was reported in [IMC13b]. This study uses laptops connected
to various cellular ISPs and WiFi hotspots. It compares various file
transfer scenarios. [IMC13b] observes that 4-path MPTCP outperforms
2-path MPTCP, especially for larger files. However, for three
congestion control algorithms (LIA, OLIA and Reno - see Section 3.2),
there is no significant performance difference for file sizes smaller
than 4MB.
A different study of the performance of Multipath TCP with two
wireless networks is presented in [INFOCOM14]. In this study the two
networks had different qualities : a good network and a lossy
network. When using two paths with different packet loss ratios, the
Multipath TCP congestion control scheme moves traffic away from the
lossy link that is considered to be congested. However, [INFOCOM14]
documents an interesting scenario that is summarised in Figure 1.
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client ----------- path1 -------- server
| |
+--------------- path2 ------------+
Figure 1: Simple network topology
Initially, the two paths have the same quality and Multipath TCP
distributes the load over both of them. During the transfer, the
second path becomes lossy, e.g. because the client moves. Multipath
TCP detects the packet losses and they are retransmitted over the
first path. This enables the data transfer to continue over the
first path. However, the subflow over the second path is still up
and transmits one packet from time to time. Although the N packets
have been acknowledged over the first subflow (at the MPTCP level),
they have not been acknowledged at the TCP level over the second
subflow. To preserve the continuity of the sequence numbers over the
second subflow, TCP will continue to retransmit these segments until
either they are acknowledged or the maximum number of retransmissions
is reached. This behavior is clearly inefficient and may lead to
blocking since the second subflow will consume window space to be
able to retransmit these packets. [INFOCOM14] proposes a new
Multipath TCP option to solve this problem. In practice, a new TCP
option is probably not required. When the client detects that the
data transmitted over the second subflow has been acknowledged over
the first subflow, it could decide to terminate the second subflow by
sending a RST segment. If the interface associated to this subflow
is still up, a new subflow could be immediately reestablished. It
would then be immediately usable to send new data and would not be
forced to first retransmit the previously transmitted data. As of
this writing, this dynamic management of the subflows is not yet
implemented in the Multipath TCP implementation in the Linux kernel.
Some studies have started to analyse the performance of Multipath TCP
on smartphones with real applications. In contrast with the bulk
transfers that are used by many publications, real applications do
not exchange huge amounts of data and establish a large number of
small connections. [COMMAG2016] proposes a software testing
framework that allows to automate Android applications to study their
interactions with Multipath TCP. [PAM2016] analyses a one-month
packet trace of all the packets exchanged by a dozen of smartphones
used by regular users. This analysis reveals that short connections
are important on smartphones and that the main benefit of using
Multipath TCP on smartphones is the ability to perform seamless
handovers between different wireless networks. Long connections
benefit from these handovers.
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2.3. Multipath TCP proxies
As Multipath TCP is not yet widely deployed on both clients and
servers, several deployments have used various forms of proxies. Two
families of solutions are currently being used or tested.
A first use case is when a Multipath TCP enabled client wants to use
several interfaces to reach a regular TCP server. A typical use case
is a smartphone that needs to use both its WiFi and its cellular
interface to transfer data. Several types of proxies are possible
for this use case. An HTTP proxy deployed on a Multipath TCP capable
server would enable the smartphone to use Multipath TCP to access
regular web servers. Obviously, this solution only works for
applications that rely on HTTP. Another possibility is to use a
proxy that can convert any Multipath TCP connection into a regular
TCP connection. Multipath TCP-specific proxies have been proposed
[HotMiddlebox13b] [HAMPEL].
Another possibility leverages the SOCKS protocol [RFC1928]. SOCKS is
often used in enterprise networks to allow clients to reach external
servers. For this, the client opens a TCP connection to the SOCKS
server that relays it to the final destination. If both the client
and the SOCKS server use Multipath TCP, but not the final
destination, then Multipath TCP can still be used on the path between
the client and the SOCKS server. At IETF'93, Korea Telecom announced
that they have deployed in June 2015 a commercial service that uses
Multipath TCP on smartphones. These smartphones access regular TCP
servers through a SOCKS proxy. This enables them to achieve
throughputs of up to 850 Mbps [KT].
Measurements performed with Android smartphones [Mobicom15] show that
popular applications work correctly through a SOCKS proxy and
Multipath TCP enabled smartphones. Thanks to Multipath TCP, long-
lived connections can be spread over the two available interfaces.
However, for short-lived connections, most of the data is sent over
the initial subflow that is created over the interface corresponding
to the default route and the second subflow is almost not used
[PAM2016].
A second use case is when Multipath TCP is used by middleboxes,
typically inside access networks. Various network operators are
discussing and evaluating solutions for hybrid access networks
[BBF-WT348]. Such networks arise when a network operator controls
two different access network technologies, e.g. wired and cellular,
and wants to combine them to improve the bandwidth offered to the
endusers [I-D.lhwxz-hybrid-access-network-architecture]. Several
solutions are currently investigated for such networks [BBF-WT348].
Figure 2 shows the organisation of such a network. When a client
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creates a normal TCP connection, it is intercepted by the Hybrid CPE
(HPCE) that converts it in a Multipath TCP connection so that it can
use the available access networks (DSL and LTE in the example). The
Hybrid Access Gateway (HAG) does the opposite to ensure that the
regular server sees a normal TCP connection. Some of the solutions
that are currently discussed for hybrid networks use Multipath TCP on
the HCPE and the HAG. Other solutions rely on tunnels between the
HCPE and the HAG [I-D.lhwxz-gre-notifications-hybrid-access].
client --- HCPE ------ DSL ------- HAG --- internet --- server
| |
+------- LTE -----------+
Figure 2: Hybrid Access Network
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3. Operational Experience
3.1. Middlebox interference
The interference caused by various types of middleboxes has been an
important concern during the design of the Multipath TCP protocol.
Three studies on the interactions between Multipath TCP and
middleboxes are worth discussing.
The first analysis appears in [IMC11]. This paper was the main
motivation for Multipath TCP incorporating various techniques to cope
with middlebox interference. More specifically, Multipath TCP has
been designed to cope with middleboxes that :
o change source or destination addresses
o change source or destination port numbers
o change TCP sequence numbers
o split or coalesce segments
o remove TCP options
o modify the payload of TCP segments
These middlebox interferences have all been included in the MBtest
suite [MBTest]. This test suite is used in [HotMiddlebox13] to
verify the reaction of the Multipath TCP implementation in the Linux
kernel [MultipathTCP-Linux] when faced with middlebox interference.
The test environment used for this evaluation is a dual-homed client
connected to a single-homed server. The middlebox behavior can be
activated on any of the paths. The main results of this analysis are
:
o the v0.87 Multipath TCP implementation in the Linux kernel is not
affected by a middlebox that performs NAT or modifies TCP sequence
numbers
o when a middlebox removes the MP_CAPABLE option from the initial
SYN segment, the v0.87 Multipath TCP implementation in the Linux
kernel falls back correctly to regular TCP
o when a middlebox removes the DSS option from all data segments,
the v0.87 Multipath TCP implementation in the Linux kernel falls
back correctly to regular TCP
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o when a middlebox performs segment coalescing, the v0.87 Multipath
TCP implementation in the Linux kernel is still able to accurately
extract the data corresponding to the indicated mapping
o when a middlebox performs segment splitting, the v0.87 Multipath
TCP implementation in the Linux kernel correctly reassembles the
data corresponding to the indicated mapping. [HotMiddlebox13]
shows on figure 4 in section 3.3 a corner case with segment
splitting that may lead to a desynchronisation between the two
hosts.
The interactions between Multipath TCP and real deployed middleboxes
is also analyzed in [HotMiddlebox13] and a particular scenario with
the FTP application level gateway running on a NAT is described.
Middlebox interference can also be detected by analysing packet
traces on Multipath TCP enabled servers. A closer look at the
packets received on the multipath-tcp.org server [TMA2015] shows that
among the 184,000 Multipath TCP connections, only 125 of them were
falling back to regular TCP. These connections originated from 28
different client IP addresses. These include 91 HTTP connections and
34 FTP connections. The FTP interference is expected and due to
Application Level Gateways running home routers. The HTTP
interference appeared only on the direction from server to client and
could have been caused by transparent proxies deployed in cellular or
enterprise networks. A longer trace is discussed in [COMCOM2016] and
similar conclusions about the middlebox interference are provided.
From an operational viewpoint, knowing that Multipath TCP can cope
with various types of middlebox interference is important. However,
there are situations where the network operators need to gather
information about where a particular middlebox interference occurs.
The tracebox software [tracebox] described in [IMC13a] is an
extension of the popular traceroute software that enables network
operators to check at which hop a particular field of the TCP header
(including options) is modified. It has been used by several network
operators to debug various middlebox interference problems.
Experience with tracebox indicates that supporting the ICMP extension
defined in [RFC1812] makes it easier to debug middlebox problems in
IPv4 networks.
Users of the Multipath TCP implementation have reported some
experience with middlebox interference. The strangest scenario has
been a middlebox that accepts the Multipath TCP options in the SYN
segment but later replaces Multipath TCP options with a TCP EOL
option [StrangeMbox]. This causes Multipath TCP to perform a
fallback to regular TCP without any impact on the application.
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3.2. Congestion control
Congestion control has been an important challenge for Multipath TCP.
The standardised congestion control scheme for Multipath TCP is
defined in [RFC6356]. A detailed description of this algorithm is
provided in [NSDI11]. This congestion control scheme has been
implemented in the Linux implementation of Multipath TCP. Linux uses
a modular architecture to support various congestion control schemes.
This architecture is applicable for both regular TCP and Multipath
TCP. While the coupled congestion control scheme defined in
[RFC6356] is the default congestion control scheme in the Linux
implementation, other congestion control schemes have been added.
The second congestion control scheme is OLIA [CONEXT12]. This
congestion control scheme is also an adaptation of the NewReno single
path congestion control scheme to support multiple paths.
Simulations and measurements have shown that it provides some
performance benefits compared to the the default congestion control
scheme [CONEXT12]. Measurements over a wide range of parameters
reported in [CONEXT13] also indicate some benefits with the OLIA
congestion control scheme. Recently, a delay-based congestion
control scheme has been ported to the Multipath TCP implementation in
the Linux kernel. This congestion control scheme has been evaluated
by using simulations in [ICNP12]. The fourth congestion control
scheme that has been included in the Linux implementation of
Multipath TCP is the BALIA scheme that provides a better balance
between TCP friendliness, responsiveness, and window oscillation
[BALIA].
These different congestion control schemes have been compared in
several articles. [CONEXT13] and [PaaschPhD] compare these
algorithms in an emulated environment. The evaluation showed that
the delay-based congestion control scheme is less able to efficiently
use the available links than the three other schemes. Reports from
some users indicate that they seem to favor OLIA.
3.3. Subflow management
The multipath capability of Multipath TCP comes from the utilisation
of one subflow per path. The Multipath TCP architecture [RFC6182]
and the protocol specification [RFC6824] define the basic usage of
the subflows and the protocol mechanisms that are required to create
and terminate them. However, there are no guidelines on how subflows
are used during the lifetime of a Multipath TCP session. Most of the
published experiments with Multipath TCP have been performed in
controlled environments. Still, based on the experience running them
and discussions on the mptcp-dev mailing list, interesting lessons
have been learned about the management of these subflows.
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From a subflow viewpoint, the Multipath TCP protocol is completely
symmetrical. Both the clients and the server have the capability to
create subflows. However in practice the existing Multipath TCP
implementations have opted for a strategy where only the client
creates new subflows. The main motivation for this strategy is that
often the client resides behind a NAT or a firewall, preventing
passive subflow openings on the client. Although there are
environments such as datacenters where this problem does not occur,
as of this writing, no precise requirement has emerged for allowing
the server to create new subflows.
3.4. Implemented subflow managers
The Multipath TCP implementation in the Linux kernel includes several
strategies to manage the subflows that compose a Multipath TCP
session. The basic subflow manager is the full-mesh. As the name
implies, it creates a full-mesh of subflows between the communicating
hosts.
The most frequent use case for this subflow manager is a multihomed
client connected to a single-homed server. In this case, one subflow
is created for each interface on the client. The current
implementation of the full-mesh subflow manager is static. The
subflows are created immediately after the creation of the initial
subflow. If one subflow fails during the lifetime of the Multipath
TCP session (e.g. due to excessive retransmissions, or the loss of
the corresponding interface), it is not always reestablished. There
is ongoing work to enhance the full-mesh path manager to deal with
such events.
When the server is multihomed, using the full-mesh subflow manager
may lead to a large number of subflows being established. For
example, consider a dual-homed client connected to a server with
three interfaces. In this case, even if the subflows are only
created by the client, 6 subflows will be established. This may be
excessive in some environments, in particular when the client and/or
the server have a large number of interfaces. A recent draft has
proposed a Multipath TCP option to negotiate the maximum number of
subflows. However, it should be noted that there have been reports
on the mptcp-dev mailing indicating that users rely on Multipath TCP
to aggregate more than four different interfaces. Thus, there is a
need for supporting many interfaces efficiently.
Creating subflows between multihomed clients and servers may
sometimes lead to operational issues as observed by discussions on
the mptcp-dev mailing list. In some cases the network operators
would like to have a better control on how the subflows are created
by Multipath TCP [I-D.boucadair-mptcp-max-subflow]. This might
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require the definition of policy rules to control the operation of
the subflow manager. The two scenarios below illustrate some of
these requirements.
host1 ---------- switch1 ----- host2
| | |
+-------------- switch2 --------+
Figure 3: Simple switched network topology
Consider the simple network topology shown in Figure 3. From an
operational viewpoint, a network operator could want to create two
subflows between the communicating hosts. From a bandwidth
utilization viewpoint, the most natural paths are host1-switch1-host2
and host1-switch2-host2. However, a Multipath TCP implementation
running on these two hosts may sometimes have difficulties to obtain
this result.
To understand the difficulty, let us consider different allocation
strategies for the IP addresses. A first strategy is to assign two
subnets : subnetA (resp. subnetB) contains the IP addresses of
host1's interface to switch1 (resp. switch2) and host2's interface to
switch1 (resp. switch2). In this case, a Multipath TCP subflow
manager should only create one subflow per subnet. To enforce the
utilization of these paths, the network operator would have to
specify a policy that prefers the subflows in the same subnet over
subflows between addresses in different subnets. It should be noted
that the policy should probably also specify how the subflow manager
should react when an interface or subflow fails.
A second strategy is to use a single subnet for all IP addresses. In
this case, it becomes more difficult to specify a policy that
indicates which subflows should be established.
The second subflow manager that is currently supported by the
Multipath TCP implementation in the Linux kernel is the ndiffport
subflow manager. This manager was initially created to exploit the
path diversity that exists between single-homed hosts due to the
utilization of flow-based load balancing techniques [SIGCOMM11].
This subflow manager creates N subflows between the same pair of IP
addresses. The N subflows are created by the client and differ only
in the source port selected by the client. It was not designed to be
used on multihomed hosts.
A more flexible subflow manager has been proposed, implemented and
evaluated in [CONEXT15]. This subflow manager exposes various kernel
events to a user space daemon that decides when subflows need to be
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created and terminated based on various policies.
3.5. Subflow destination port
The Multipath TCP protocol relies on the token contained in the
MP_JOIN option to associate a subflow to an existing Multipath TCP
session. This implies that there is no restriction on the source
address, destination address and source or destination ports used for
the new subflow. The ability to use different source and destination
addresses is key to support multihomed servers and clients. The
ability to use different destination port numbers is worth discussing
because it has operational implications.
For illustration, consider a dual-homed client that creates a second
subflow to reach a single-homed server as illustrated in Figure 4.
client ------- r1 --- internet --- server
| |
+----------r2-------+
Figure 4: Multihomed-client connected to single-homed server
When the Multipath TCP implementation in the Linux kernel creates the
second subflow it uses the same destination port as the initial
subflow. This choice is motivated by the fact that the server might
be protected by a firewall and only accept TCP connections (including
subflows) on the official port number. Using the same destination
port for all subflows is also useful for operators that rely on the
port numbers to track application usage in their network.
There have been suggestions from Multipath TCP users to modify the
implementation to allow the client to use different destination ports
to reach the server. This suggestion seems mainly motivated by
traffic shaping middleboxes that are used in some wireless networks.
In networks where different shaping rates are associated to different
destination port numbers, this could allow Multipath TCP to reach a
higher performance. As of this writing, we are not aware of any
implementation of this kind of tweaking.
However, from an implementation point-of-view supporting different
destination ports for the same Multipath TCP connection can cause
some issues. A legacy implementation of a TCP stack creates a
listening socket to react upon incoming SYN segments. The listening
socket is handling the SYN segments that are sent on a specific port
number. Demultiplexing incoming segments can thus be done solely by
looking at the IP addresses and the port numbers. With Multipath TCP
however, incoming SYN segments may have an MP_JOIN option with a
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different destination port. This means, that all incoming segments
that did not match on an existing listening-socket or an already
established socket must be parsed for an eventual MP_JOIN option.
This imposes an additional cost on servers, previously not existent
on legacy TCP implementations.
3.6. Closing subflows
client server
| |
MPTCP: established | | MPTCP: established
Sub: established | | Sub: established
| |
| DATA_FIN |
MPTCP: close-wait | <------------------------ | close() (step 1)
Sub: established | DATA_ACK |
| ------------------------> | MPTCP: fin-wait-2
| | Sub: established
| |
| DATA_FIN + subflow-FIN |
close()/shutdown() | ------------------------> | MPTCP: time-wait
(step 2) | DATA_ACK | Sub: close-wait
MPTCP: closed | <------------------------ |
Sub: fin-wait-2 | |
| |
| subflow-FIN |
MPTCP: closed | <------------------------ | subflow-close()
Sub: time-wait | subflow-ACK |
(step 3) | ------------------------> | MPTCP: time-wait
| | Sub: closed
| |
Figure 5: Multipath TCP may not be able to avoid time-wait state on
the subflow (indicated as Sub in the drawing), even if enforced by
the application on the client-side.
Figure 5 shows a very particular issue within Multipath TCP. Many
high-performance applications try to avoid Time-Wait state by
deferring the closure of the connection until the peer has sent a
FIN. That way, the client on the left of Figure 5 does a passive
closure of the connection, transitioning from Close-Wait to Last-ACK
and finally freeing the resources after reception of the ACK of the
FIN. An application running on top of a Multipath TCP enabled Linux
kernel might also use this approach. The difference here is that the
close() of the connection (Step 1 in Figure 5) only triggers the
sending of a DATA_FIN. Nothing guarantees that the kernel is ready
to combine the DATA_FIN with a subflow-FIN. The reception of the
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DATA_FIN will make the application trigger the closure of the
connection (step 2), trying to avoid Time-Wait state with this late
closure. This time, the kernel might decide to combine the DATA_FIN
with a subflow-FIN. This decision will be fatal, as the subflow's
state machine will not transition from Close-Wait to Last-Ack, but
rather go through Fin-Wait-2 into Time-Wait state. The Time-Wait
state will consume resources on the host for at least 2 MSL (Maximum
Segment Lifetime). Thus, a smart application that tries to avoid
Time-Wait state by doing late closure of the connection actually ends
up with one of its subflows in Time-Wait state. A high-performance
Multipath TCP kernel implementation should honor the desire of the
application to do passive closure of the connection and successfully
avoid Time-Wait state - even on the subflows.
The solution to this problem lies in an optimistic assumption that a
host doing active-closure of a Multipath TCP connection by sending a
DATA_FIN will soon also send a FIN on all its subflows. Thus, the
passive closer of the connection can simply wait for the peer to send
exactly this FIN - enforcing passive closure even on the subflows.
Of course, to avoid consuming resources indefinitely, a timer must
limit the time our implementation waits for the FIN.
3.7. Packet schedulers
In a Multipath TCP implementation, the packet scheduler is the
algorithm that is executed when transmitting each packet to decide on
which subflow it needs to be transmitted. The packet scheduler
itself does not have any impact on the interoperability of Multipath
TCP implementations. However, it may clearly impact the performance
of Multipath TCP sessions. The Multipath TCP implementation in the
Linux kernel supports a pluggable architecture for the packet
scheduler [PaaschPhD]. As of this writing, two schedulers have been
implemented: round-robin and lowest-rtt-first. The second scheduler
relies on the round-trip-time (rtt) measured on each TCP subflow and
sends first segments over the subflow having the lowest round-trip-
time. They are compared in [CSWS14]. The experiments and
measurements described in [CSWS14] show that the lowest-rtt-first
scheduler appears to be the best compromise from a performance
viewpoint. Another study of the packet schedulers is presented in
[PAMS2014]. This study relies on simulations with the Multipath TCP
implementation in the Linux kernel. They compare the lowest-rtt-
first with the round-robin and a random scheduler. They show some
situations where the lowest-rtt-first scheduler does not perform as
well as the other schedulers, but there are many scenarios where the
opposite is true. [PAMS2014] notes that "it is highly likely that
the optimal scheduling strategy depends on the characteristics of the
paths being used."
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3.8. Segment size selection
When an application performs a write/send system call, the kernel
allocates a packet buffer (sk_buff in Linux) to store the data the
application wants to send. The kernel will store at most one MSS
(Maximum Segment Size) of data per buffer. As the MSS can differ
amongst subflows, an MPTCP implementation must select carefully the
MSS used to generate application data. The Linux kernel
implementation had various ways of selecting the MSS: minimum or
maximum amongst the different subflows. However, these heuristics of
MSS selection can cause significant performance issues in some
environment. Consider the following example. An MPTCP connection
has two established subflows that respectively use a MSS of 1420 and
1428 bytes. If MPTCP selects the maximum, then the application will
generate segments of 1428 bytes of data. An MPTCP implementation
will have to split the segment in two (a 1420-byte and 8-byte
segments) when pushing on the subflow with the smallest MSS. The
latter segment will introduce a large overhead as for a single data
segment 2 slots will be used in the congestion window (in packets)
therefore reducing by roughly twice the potential throughput (in
bytes/s) of this subflow. Taking the smallest MSS does not solve the
issue as there might be a case where the subflow with the smallest
MSS only sends a few packets therefore reducing the potential
throughput of the other subflows.
The Linux implementation recently took another approach [DetalMSS].
Instead of selecting the minimum and maximum values, it now
dynamically adapts the MSS based on the contribution of all the
subflows to the connection's throughput. For this it computes, for
each subflow, the potential throughput achieved by selecting each MSS
value and by taking into account the lost space in the cwnd. It then
selects the MSS that allows to achieve the highest potential
throughput.
3.9. Interactions with the Domain Name System
Multihomed clients such as smartphones can send DNS queries over any
of their interfaces. When a single-homed client performs a DNS
query, it receives from its local resolver the best answer for its
request. If the client is multihomed, the answer returned to the DNS
query may vary with the interface over which it has been sent.
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cdn1
|
client -- cellular -- internet -- cdn3
| |
+----- wifi --------+
|
cdn2
Figure 6: Simple network topology
If the client sends a DNS query over the WiFi interface, the answer
will point to the cdn2 server while the same request sent over the
cellular interface will point to the cdn1 server. This might cause
problems for CDN providers that locate their servers inside ISP
networks and have contracts that specify that the CDN server will
only be accessed from within this particular ISP. Assume now that
both the client and the CDN servers support Multipath TCP. In this
case, a Multipath TCP session from cdn1 or cdn2 would potentially use
both the cellular network and the WiFi network. Serving the client
from cdn2 over the cellular interface could violate the contract
between the CDN provider and the network operators. A similar
problem occurs with regular TCP if the client caches DNS replies.
For example the client obtains a DNS answer over the cellular
interface and then stops this interface and starts to use its WiFi
interface. If the client retrieves data from cdn1 over its WiFi
interface, this may also violate the contract between the CDN and the
network operators.
A possible solution to prevent this problem would be to modify the
DNS resolution on the client. The client subnet EDNS extension
defined in [RFC7871] could be used for this purpose. When the client
sends a DNS query from its WiFi interface, it should also send the
client subnet corresponding to the cellular interface in this
request. This would indicate to the resolver that the answer should
be valid for both the WiFi and the cellular interfaces (e.g., the
cdn3 server).
3.10. Captive portals
Multipath TCP enables a host to use different interfaces to reach a
server. In theory, this should ensure connectivity when at least one
of the interfaces is active. In practice however, there are some
particular scenarios with captive portals that may cause operational
problems. The reference environment is shown in Figure 7.
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client ----- network1
|
+------- internet ------------- server
Figure 7: Issue with captive portal
The client is attached to two networks : network1 that provides
limited connectivity and the entire Internet through the second
network interface. In practice, this scenario corresponds to an open
WiFi network with a captive portal for network1 and a cellular
service for the second interface. On many smartphones, the WiFi
interface is preferred over the cellular interface. If the
smartphone learns a default route via both interfaces, it will
typically prefer to use the WiFi interface to send its DNS request
and create the first subflow. This is not optimal with Multipath
TCP. A better approach would probably be to try a few attempts on
the WiFi interface and then try to use the second interface for the
initial subflow as well.
3.11. Stateless webservers
MPTCP has been designed to interoperate with webservers that benefit
from SYN-cookies to protect against SYN-flooding attacks [RFC4987].
MPTCP achieves this by echoing the keys negotiated during the
MP_CAPABLE handshake in the third ACK of the 3-way handshake.
Reception of this third ACK then allows the server to reconstruct the
state specific to MPTCP.
However, one caveat to this mechanism is the non-reliable nature of
the third ACK. Indeed, when the third ACK gets lost, the server will
not be able to reconstruct the MPTCP-state. MPTCP will fallback to
regular TCP in this case. This is in contrast to regular TCP. When
the client starts sending data, the first data segment also includes
the SYN-cookie, which allows the server to reconstruct the TCP-state.
Further, this data segment will be retransmitted by the client in
case it gets lost and thus is resilient against loss. MPTCP does not
include the keys in this data segment and thus the server cannot
reconstruct the MPTCP state.
This issue might be considered as a minor one for MPTCP. Losing the
third ACK should only happen when packet loss is high. However, when
packet-loss is high MPTCP provides a lot of benefits as it can move
traffic away from the lossy link. It is undesirable that MPTCP has a
higher chance to fall back to regular TCP in those lossy
environments.
[I-D.paasch-mptcp-syncookies] discusses this issue and suggests a
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modified handshake mechanism that ensures reliable delivery of the
MP_CAPABLE, following the 3-way handshake. This modification will
make MPTCP reliable, even in lossy environments when servers need to
use SYN-cookies to protect against SYN-flooding attacks.
3.12. Loadbalanced serverfarms
Large-scale serverfarms typically deploy thousands of servers behind
a single virtual IP (VIP). Steering traffic to these servers is done
through layer-4 loadbalancers that ensure that a TCP-flow will always
be routed to the same server [Presto08].
As Multipath TCP uses multiple different TCP subflows to steer the
traffic across the different paths, loadbalancers need to ensure that
all these subflows are routed to the same server. This implies that
the loadbalancers need to track the MPTCP-related state, allowing
them to parse the token in the MP_JOIN and assign those subflows to
the appropriate server. However, serverfarms typically deploy
multiple of these loadbalancers for reliability and capacity reasons.
As a TCP subflow might get routed to any of these loadbalancers, they
would need to synchronize the MPTCP-related state - a solution that
is not feasible at large scale.
The token (carried in the MP_JOIN) contains the information
indicating which MPTCP-session the subflow belongs to. As the token
is a hash of the key, servers are not able to generate the token in
such a way that the token can provide the necessary information to
the loadbalancers which would allow them to route TCP subflows to the
appropriate server. [I-D.paasch-mptcp-loadbalancer] discusses this
issue in detail and suggests two alternative MP_CAPABLE handshakes to
overcome these. As of September 2015, it is not yet clear how MPTCP
might accomodate such use-case to enable its deployment within
loadbalanced serverfarms.
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4. IANA Considerations
There are no IANA considerations in this informational document.
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5. Security Considerations
The security considerations for Multipath TCP have already been
documented in [RFC6181], [RFC6182], [RFC6824] and [RFC7430].
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6. Acknowledgements
This work was partially supported by the FP7-Trilogy2 project. We
would like to thank all the implementers and users of the Multipath
TCP implementation in the Linux kernel. This document has benefited
from the comments of John Ronan, Yoshifumi Nishida, Phil Eardley,
Jaehyun Hwang and Mirja Kuehlewind.
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7. Informative References
[Apple-MPTCP]
Apple, Inc, ., "iOS - Multipath TCP Support in iOS 7",
n.d., <https://support.apple.com/en-us/HT201373>.
[BALIA] Peng, Q., Walid, A., Hwang, J., and S. Low, "Multipath TCP
Analysis, Design, and Implementation", IEEE/ACM Trans. on
Networking, Vol. 24, No. 1 , 2016.
[BBF-WT348]
Fabregas (Ed), G., "WT-348 - Hybrid Access for Broadband
Networks", Broadband Forum, contribution bbf2014.1139.04 ,
June 2015.
[CACM14] Paasch, C. and O. Bonaventure, "Multipath TCP",
Communications of the ACM, 57(4):51-57 , April 2014,
<http://inl.info.ucl.ac.be/publications/multipath-tcp>.
[COMCOM2016]
"Observing real Multipath TCP traffic", Computer
Communications , April 2016, <http://inl.info.ucl.ac.be/
publications/observing-real-multipath-tcp-traffic>.
[COMMAG2016]
De Coninck, Q., Baerts, M., Hesmans, B., and O.
Bonaventure, "Observing Real Smartphone Applications over
Multipath TCP", IEEE Communications Magazine , March 2016,
<http://inl.info.ucl.ac.be/publications/
observing-real-smartphone-applications-over-multipath-
tcp>.
[CONEXT12]
Khalili, R., Gast, N., Popovic, M., Upadhyay, U., and J.
Leboudec, "MPTCP is not pareto-optimal performance issues
and a possible solution", Proceedings of the 8th
international conference on Emerging networking
experiments and technologies (CoNEXT12) , 2012.
[CONEXT13]
Paasch, C., Khalili, R., and O. Bonaventure, "On the
Benefits of Applying Experimental Design to Improve
Multipath TCP", Conference on emerging Networking
EXperiments and Technologies (CoNEXT) , December 2013, <ht
tp://inl.info.ucl.ac.be/publications/
benefits-applying-experimental-design-improve-multipath-
tcp>.
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[CONEXT15]
Hesmans, B., Detal, G., Barre, S., Bauduin, R., and O.
Bonaventure, "SMAPP - Towards Smart Multipath TCP-enabled
APPlications", Proc. Conext 2015, Heidelberg, Germany ,
December 2015, <http://inl.info.ucl.ac.be/publications/
smapp-towards-smart-multipath-tcp-enabled-applications>.
[CSWS14] Paasch, C., Ferlin, S., Alay, O., and O. Bonaventure,
"Experimental Evaluation of Multipath TCP Schedulers",
SIGCOMM CSWS2014 workshop , August 2014.
[Cellnet12]
Paasch, C., Detal, G., Duchene, F., Raiciu, C., and O.
Bonaventure, "Exploring Mobile/WiFi Handover with
Multipath TCP", ACM SIGCOMM workshop on Cellular Networks
(Cellnet12) , 2012, <http://inl.info.ucl.ac.be/
publications/exploring-mobilewifi-handover-multipath-tcp>.
[DetalMSS]
Detal, G., "Adaptive MSS value", Post on the mptcp-dev
mailing list , September 2014, <https://
listes-2.sipr.ucl.ac.be/sympa/arc/mptcp-dev/2014-09/
msg00130.html>.
[FreeBSD-MPTCP]
Williams, N., "Multipath TCP For FreeBSD Kernel Patch
v0.5", n.d., <http://caia.swin.edu.au/urp/newtcp/mptcp>.
[HAMPEL] Hampel, G., Rana, A., and T. Klein, "Seamless TCP mobility
using lightweight MPTCP proxy", Proceedings of the 11th
ACM international symposium on Mobility management and
wireless access (MobiWac '13). , 2013.
[HotMiddlebox13]
Hesmans, B., Duchene, F., Paasch, C., Detal, G., and O.
Bonaventure, "Are TCP Extensions Middlebox-proof?", CoNEXT
workshop HotMiddlebox , December 2013, <http://
inl.info.ucl.ac.be/publications/
are-tcp-extensions-middlebox-proof>.
[HotMiddlebox13b]
Detal, G., Paasch, C., and O. Bonaventure, "Multipath in
the Middle(Box)", HotMiddlebox'13 , December 2013, <http:/
/inl.info.ucl.ac.be/publications/multipath-middlebox>.
[HotNets] Raiciu, C., Pluntke, C., Barre, S., Greenhalgh, A.,
Wischik, D., and M. Handley, "Data center networking with
multipath TCP", Proceedings of the 9th ACM SIGCOMM
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Workshop on Hot Topics in Networks (Hotnets-IX) , 2010,
<http://doi.acm.org/10.1145/1868447.1868457>.
[I-D.boucadair-mptcp-max-subflow]
Boucadair, M. and C. Jacquenet, "Negotiating the Maximum
Number of Multipath TCP (MPTCP) Subflows",
draft-boucadair-mptcp-max-subflow-02 (work in progress),
May 2016.
[I-D.lhwxz-gre-notifications-hybrid-access]
Leymann, N., Heidemann, C., Wasserman, M., Xue, L., and M.
Zhang, "GRE Notifications for Hybrid Access",
draft-lhwxz-gre-notifications-hybrid-access-01 (work in
progress), January 2015.
[I-D.lhwxz-hybrid-access-network-architecture]
Leymann, N., Heidemann, C., Wasserman, M., Xue, L., and M.
Zhang, "Hybrid Access Network Architecture",
draft-lhwxz-hybrid-access-network-architecture-02 (work in
progress), January 2015.
[I-D.paasch-mptcp-loadbalancer]
Paasch, C., Greenway, G., and A. Ford, "Multipath TCP
behind Layer-4 loadbalancers",
draft-paasch-mptcp-loadbalancer-00 (work in progress),
September 2015.
[I-D.paasch-mptcp-syncookies]
Paasch, C., Biswas, A., and D. Haas, "Making Multipath TCP
robust for stateless webservers",
draft-paasch-mptcp-syncookies-02 (work in progress),
October 2015.
[ICNP12] Cao, Y., Xu, M., and X. Fu, "Delay-based congestion
control for multipath TCP", 20th IEEE International
Conference on Network Protocols (ICNP) , 2012.
[IMC11] Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and H. Tokuda, "Is it still possible to
extend TCP?", Proceedings of the 2011 ACM SIGCOMM
conference on Internet measurement conference (IMC '11) ,
2011, <http://doi.acm.org/10.1145/2068816.2068834>.
[IMC13a] Detal, G., Hesmans, B., Bonaventure, O., Vanaubel, Y., and
B. Donnet, "Revealing Middlebox Interference with
Tracebox", Proceedings of the 2013 ACM SIGCOMM conference
on Internet measurement conference , 2013, <http://
inl.info.ucl.ac.be/publications/
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revealing-middlebox-interference-tracebox>.
[IMC13b] Chen, Y., Lim, Y., Gibbens, R., Nahum, E., Khalili, R.,
and D. Towsley, "A measurement-based study of MultiPath
TCP performance over wireless network", Proceedings of the
2013 conference on Internet measurement conference (IMC
'13) , n.d., <http://doi.acm.org/10.1145/2504730.2504751>.
[IMC13c] Pelsser, C., Cittadini, L., Vissicchio, S., and R. Bush,
"From Paris to Tokyo on the suitability of ping to
measure latency", Proceedings of the 2013 conference on
Internet measurement conference (IMC '13) , 2013,
<http://doi.acm.org/10.1145/2504730.2504765>.
[INFOCOM14]
Lim, Y., Chen, Y., Nahum, E., Towsley, D., and K. Lee,
"Cross-Layer Path Management in Multi-path Transport
Protocol for Mobile Devices", IEEE INFOCOM'14 , 2014.
[IOS7] Apple, ., "Multipath TCP Support in iOS 7", January 2014,
<http://support.apple.com/kb/HT5977>.
[KT] Seo, S., "KT's GiGA LTE", July 2015, <https://
www.ietf.org/proceedings/93/slides/slides-93-mptcp-3.pdf>.
[MBTest] Hesmans, B., "MBTest", 2013,
<https://bitbucket.org/bhesmans/mbtest>.
[MPTCPBIB]
Bonaventure, O., "Multipath TCP - An annotated
bibliography", Technical report , April 2015,
<https://github.com/obonaventure/mptcp-bib>.
[Mobicom15]
De Coninck, Q., Baerts, M., Hesmans, B., and O.
Bonaventure, "Poster - Evaluating Android Applications
with Multipath TCP", Mobicom 2015 (Poster) ,
September 2015.
[MultipathTCP-Linux]
Paasch, C., Barre, S., and . et al, "Multipath TCP
implementation in the Linux kernel", n.d.,
<http://www.multipath-tcp.org>.
[NSDI11] Wischik, D., Raiciu, C., Greenhalgh, A., and M. Handley,
"Design, implementation and evaluation of congestion
control for Multipath TCP", In Proceedings of the 8th
USENIX conference on Networked systems design and
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implementation (NSDI11) , 2011.
[NSDI12] Raiciu, C., Paasch, C., Barre, S., Ford, A., Honda, M.,
Duchene, F., Bonaventure, O., and M. Handley, "How Hard
Can It Be? Designing and Implementing a Deployable
Multipath TCP", USENIX Symposium of Networked Systems
Design and Implementation (NSDI12) , April 2012, <http://
inl.info.ucl.ac.be/publications/
how-hard-can-it-be-designing-and-implementing-deployable-
multipath-tcp>.
[PAM2016] De Coninck, Q., Baerts, M., Hesmans, B., and O.
Bonaventure, "A First Analysis of Multipath TCP on
Smartphones", 17th International Passive and Active
Measurements Conference (PAM2016) , March 2016, <http://
inl.info.ucl.ac.be/publications/
first-analysis-multipath-tcp-smartphones>.
[PAMS2014]
Arzani, B., Gurney, A., Cheng, S., Guerin, R., and B. Loo,
"Impact of Path Selection and Scheduling Policies on MPTCP
Performance", PAMS2014 , 2014.
[PaaschPhD]
Paasch, C., "Improving Multipath TCP", Ph.D. Thesis ,
November 2014, <http://inl.info.ucl.ac.be/publications/
improving-multipath-tcp>.
[Presto08]
Greenberg, A., Lahiri, P., Maltz, D., Parveen, P., and S.
Sengupta, "Towards a Next Generation Data Center
Architecture - Scalability and Commoditization", ACM
PRESTO 2008 , August 2008,
<http://dl.acm.org/citation.cfm?id=1397732>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<http://www.rfc-editor.org/info/rfc1812>.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC1928, March 1996,
<http://www.rfc-editor.org/info/rfc1928>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<http://www.rfc-editor.org/info/rfc4987>.
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[RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for
Multipath Operation with Multiple Addresses", RFC 6181,
DOI 10.17487/RFC6181, March 2011,
<http://www.rfc-editor.org/info/rfc6181>.
[RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
Iyengar, "Architectural Guidelines for Multipath TCP
Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
<http://www.rfc-editor.org/info/rfc6182>.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<http://www.rfc-editor.org/info/rfc6356>.
[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,
<http://www.rfc-editor.org/info/rfc6824>.
[RFC7430] Bagnulo, M., Paasch, C., Gont, F., Bonaventure, O., and C.
Raiciu, "Analysis of Residual Threats and Possible Fixes
for Multipath TCP (MPTCP)", RFC 7430, DOI 10.17487/
RFC7430, July 2015,
<http://www.rfc-editor.org/info/rfc7430>.
[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<http://www.rfc-editor.org/info/rfc7871>.
[SIGCOMM11]
Raiciu, C., Barre, S., Pluntke, C., Greenhalgh, A.,
Wischik, D., and M. Handley, "Improving datacenter
performance and robustness with multipath TCP",
Proceedings of the ACM SIGCOMM 2011 conference , n.d.,
<http://doi.acm.org/10.1145/2018436.2018467>.
[StrangeMbox]
Bonaventure, O., "Multipath TCP through a strange
middlebox", Blog post , January 2015, <http://
blog.multipath-tcp.org/blog/html/2015/01/30/
multipath_tcp_through_a_strange_middlebox.html>.
[TMA2015] Hesmans, B., Tran Viet, H., Sadre, R., and O. Bonaventure,
"A First Look at Real Multipath TCP Traffic", Traffic
Monitoring and Analysis , 2015, <http://
inl.info.ucl.ac.be/publications/
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first-look-real-multipath-tcp-traffic>.
[ietf88] Stewart, L., "IETF'88 Meeting minutes of the MPTCP working
group", n.d., <http://tools.ietf.org/wg/mptcp/
minutes?item=minutes-88-mptcp.html>.
[tracebox]
Detal, G. and O. Tilmans, "tracebox", 2013,
<http://www.tracebox.org>.
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Appendix A. Changelog
This section should be removed before final publication
o initial version : September 16th, 2014 : Added section Section 3.8
that discusses some performance problems that appeared with the
Linux implementation when using subflows having different MSS
values
o update with a description of the middlebox that replaces an
unknown TCP option with EOL [StrangeMbox]
o version ietf-02 : July 2015, answer to last call comments
* Reorganised text to better separate use cases and operational
experience
* New use case on Multipath TCP proxies in Section 2.3
* Added some text on middleboxes in Section 3.1
* Removed the discussion on SDN
* Restructured text and improved writing in some parts
o version ietf-03 : September 2015, answer to comments from Phil
Eardley
* Improved introduction
* Added details about using SOCKS and Korea Telecom's use-case in
Section 2.3.
* Added issue around clients caching DNS-results in Section 3.9
* Explained issue of MPTCP with stateless webservers Section 3.11
* Added description of MPTCP's use behind layer-4 loadbalancers
Section 3.12
* Restructured text and improved writing in some parts
o version ietf-04 : April 2016, answer to last comments
* Updated text on measurements with smartphones
* Updated conclusion
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o version ietf-06 : August 2016, answer to AD's review
* removed some expired drafts
* removed conclusion
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Authors' Addresses
Olivier Bonaventure
UCLouvain
Email: Olivier.Bonaventure@uclouvain.be
Christoph Paasch
Apple, Inc.
Email: cpaasch@apple.com
Gregory Detal
Tessares
Email: Gregory.Detal@tessares.net
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