draft-ietf-mptcp-experience-02.txt   draft-ietf-mptcp-experience-03.txt 
MPTCP Working Group O. Bonaventure MPTCP Working Group O. Bonaventure
Internet-Draft C. Paasch Internet-Draft UCLouvain
Intended status: Informational UCLouvain Intended status: Informational C. Paasch
Expires: January 7, 2016 G. Detal Expires: April 21, 2016 Apple, Inc.
G. Detal
UCLouvain and Tessares UCLouvain and Tessares
July 06, 2015 October 19, 2015
Use Cases and Operational Experience with Multipath TCP Use Cases and Operational Experience with Multipath TCP
draft-ietf-mptcp-experience-02 draft-ietf-mptcp-experience-03
Abstract Abstract
This document discusses both use cases and operational experience This document discusses both use cases and operational experience
with Multipath TCP in real world networks. It lists several with Multipath TCP in real world networks. It lists several
prominent use cases for which Multipath TCP has been considered and prominent use cases for which Multipath TCP has been considered and
is being used. It also gives insight to some heuristics and is being used. It also gives insight to some heuristics and
decisions that have helped to realize these use cases. decisions that have helped to realize these use cases.
Status of This Memo Status of This Memo
skipping to change at page 1, line 36 skipping to change at page 1, line 37
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2016. This Internet-Draft will expire on April 21, 2016.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Datacenters . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Datacenters . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Cellular/WiFi Offload . . . . . . . . . . . . . . . . . . 4 2.2. Cellular/WiFi Offload . . . . . . . . . . . . . . . . . . 5
2.3. Multipath TCP proxies . . . . . . . . . . . . . . . . . . 7 2.3. Multipath TCP proxies . . . . . . . . . . . . . . . . . . 7
3. Operational Experience . . . . . . . . . . . . . . . . . . . 8 3. Operational Experience . . . . . . . . . . . . . . . . . . . 9
3.1. Middlebox interference . . . . . . . . . . . . . . . . . 8 3.1. Middlebox interference . . . . . . . . . . . . . . . . . 9
3.2. Congestion control . . . . . . . . . . . . . . . . . . . 10 3.2. Congestion control . . . . . . . . . . . . . . . . . . . 11
3.3. Subflow management . . . . . . . . . . . . . . . . . . . 11 3.3. Subflow management . . . . . . . . . . . . . . . . . . . 11
3.4. Implemented subflow managers . . . . . . . . . . . . . . 11 3.4. Implemented subflow managers . . . . . . . . . . . . . . 12
3.5. Subflow destination port . . . . . . . . . . . . . . . . 13 3.5. Subflow destination port . . . . . . . . . . . . . . . . 14
3.6. Closing subflows . . . . . . . . . . . . . . . . . . . . 14 3.6. Closing subflows . . . . . . . . . . . . . . . . . . . . 15
4. Packet schedulers . . . . . . . . . . . . . . . . . . . . . . 15 3.7. Packet schedulers . . . . . . . . . . . . . . . . . . . . 16
5. Segment size selection . . . . . . . . . . . . . . . . . . . 16 3.8. Segment size selection . . . . . . . . . . . . . . . . . 17
6. Interactions with the Domain Name System . . . . . . . . . . 16 3.9. Interactions with the Domain Name System . . . . . . . . 17
7. Captive portals . . . . . . . . . . . . . . . . . . . . . . . 17 3.10. Captive portals . . . . . . . . . . . . . . . . . . . . . 18
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.11. Stateless webservers . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 3.12. Loadbalanced serverfarms . . . . . . . . . . . . . . . . 20
10. Informative References . . . . . . . . . . . . . . . . . . . 18 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . 23 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 6. Informative References . . . . . . . . . . . . . . . . . . . 21
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
Multipath TCP was standardized in [RFC6824] and four implementations Multipath TCP was standardized in [RFC6824] and five independant
have been developed [I-D.eardley-mptcp-implementations-survey]. implementations have been developed
[I-D.eardley-mptcp-implementations-survey]. 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
[I-D.eardley-mptcp-implementations-survey] 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 Since the publication of [RFC6824], experience has been gathered by
various network researchers and users about the operational issues various network researchers and users about the operational issues
that arise when Multipath TCP is used in today's Internet. that arise when Multipath TCP is used in today's Internet.
When the MPTCP working group was created, several use cases for When the MPTCP working group was created, several use cases for
Multipath TCP were identified [RFC6182]. Since then, over use cases Multipath TCP were identified [RFC6182]. Since then, other use cases
have been proposed and some have been tested and even deployed. We have been proposed and some have been tested and even deployed. We
describe these use cases in section Section 2. describe these use cases in Section 2.
The second part of the document focuses on the operational experience Section 3 focuses on the operational experience with Multipath TCP.
with Multipath TCP. Most of this experience comes from the Most of this experience comes from the utilisation of the Multipath
utilisation of the Multipath TCP implementation in the Linux kernel TCP implementation in the Linux kernel [MultipathTCP-Linux]. This
[MultipathTCP-Linux]. This open-source implementation has been open-source implementation has been downloaded and is used by
downloaded and is used by thousands of users all over the world. thousands of users all over the world. Many of these users have
Many of these users have provided direct or indirect feedback by provided direct or indirect feedback by writing documents (scientific
writing documents (scientific articles or blog messages) or posting articles or blog messages) or posting to the mptcp-dev mailing list
to the mptcp-dev mailing list (see https://listes- (see https://listes-2.sipr.ucl.ac.be/sympa/arc/mptcp-dev ). This
2.sipr.ucl.ac.be/sympa/arc/mptcp-dev ). This Multipath TCP Multipath TCP implementation is actively maintained and continuously
implementation is actively maintained and continuously improved. It improved. It is used on various types of hosts, ranging from
is used on various types of hosts, ranging from smartphones or smartphones or embedded routers to high-end servers.
embedded routers to high-end servers.
The Multipath TCP implementation in the Linux kernel is is not, by The Multipath TCP implementation in the Linux kernel is not, by far,
far, the most widespread deployment of Multipath TCP. Since the most widespread deployment of Multipath TCP. Since September
September 2013, Multipath TCP is also supported on smartphones and 2013, Multipath TCP is also supported on smartphones and tablets
tablets running iOS7 [IOS7]. There are likely hundreds of millions running iOS7 [IOS7]. There are likely hundreds of millions of
of Multipath TCP enabled devices. However, this particular Multipath Multipath TCP enabled devices. However, this particular Multipath
TCP implementation is currently only used to support a single TCP implementation is currently only used to support a single
application. Unfortunately, there is no public information about the application. Unfortunately, there is no public information about the
lessons learned from this large scale deployment. lessons learned from this large scale deployment.
The second part of this is document is organized as follows. Section 3 is organized as follows. Supporting the middleboxes was
Supporting the middleboxes was one of the difficult issues in one of the difficult issues in designing the Multipath TCP protocol.
designing the Multipath TCP protocol. We explain in section We explain in Section 3.1 which types of middleboxes the Linux Kernel
Section 3.1 which types of middleboxes the Linux Kernel
implementation of Multipath TCP supports and how it reacts upon implementation of Multipath TCP supports and how it reacts upon
encountering these. Section Section 3.2 summarises the MPTCP encountering these. Section 3.2 summarises the MPTCP specific
specific congestion controls that have been implemented. Sections congestion controls that have been implemented. Section 3.3 and
Section 3.3 and Section 4 discuss heuristics and issues with respect Section 3.7 discuss heuristics and issues with respect to subflow
to subflow management as well as the scheduling across the subflows. management as well as the scheduling across the subflows.
Section Section 5 explains some problems that occurred with subflows Section 3.8 explains some problems that occurred with subflows having
having different MSS values. Section Section 6 presents issues with different Maximum Segment Size (MSS) values. Section 3.9 presents
respect to content delivery networks and suggests a solution to this issues with respect to content delivery networks and suggests a
issue. Finally, section Section 7 documents an issue with captive solution to this issue. Finally, Section 3.10 documents an issue
portals where MPTCP will behave suboptimal. with captive portals where MPTCP will behave suboptimally.
2. Use cases 2. Use cases
Multipath TCP has been tested in several use cases. There is already Multipath TCP has been tested in several use cases. There is already
an abundant scientific literature on Multipath TCP [MPTCPBIB]. an abundant scientific literature on Multipath TCP [MPTCPBIB].
Several of the papers published in the scientific litterature have Several of the papers published in the scientific literature have
identified possible improvements that are worth being discussed here. identified possible improvements that are worth being discussed here.
2.1. Datacenters 2.1. Datacenters
A first, although initially unexpected, documented use case for A first, although initially unexpected, documented use case for
Multipath TCP has been the datacenters [HotNets][SIGCOMM11]. Today's Multipath TCP has been in datacenters [HotNets][SIGCOMM11]. Today's
datacenters are designed to provide several paths between single- datacenters are designed to provide several paths between single-
homed servers. The multiplicity of these paths comes from the homed servers. The multiplicity of these paths comes from the
utilization of Equal Cost Multipath (ECMP) and other load balancing utilization of Equal Cost Multipath (ECMP) and other load balancing
techniques inside the datacenter. Most of the deployed load techniques inside the datacenter. Most of the deployed load
balancing techniques in these datacenters rely on hashes computed or balancing techniques in datacenters rely on hashes computed over the
the five tuple to ensure that all packets from the same TCP five tuple. Thus all packets from the same TCP connection follow the
connection will follow the same path to prevent packet reordering. same path and so are not reordered. The results in [HotNets]
The results presented in [HotNets] demonstrate by simulations that demonstrate by simulations that Multipath TCP can achieve a better
Multipath TCP can achieve a better utilization of the available utilization of the available network by using multiple subflows for
network by using multiple subflows for each Multipath TCP session. each Multipath TCP session. Although [RFC6182] assumes that at least
Although [RFC6182] assumes that at least one of the communicating one of the communicating hosts has several IP addresses, [HotNets]
hosts has several IP addresses, [HotNets] demonstrates that there are demonstrates that Multipath TCP is beneficial when both hosts are
also benefits when both hosts are single-homed. This idea was single-homed. This idea is analysed in more details in [SIGCOMM11]
pursued further in [SIGCOMM11] where the Multipath TCP implementation where the Multipath TCP implementation in the Linux kernel is
in the Linux kernel was modified to be able to use several subflows modified to be able to use several subflows from the same IP address.
from the same IP address. Measurements performed in a public Measurements in a public datacenter show the quantitative benefits of
datacenter showed performance improvements with Multipath TCP Multipath TCP [SIGCOMM11] in this environment.
[SIGCOMM11].
Although ECMP is widely used inside datacenters, this is not the only Although ECMP is widely used inside datacenters, this is not the only
environment where there are different paths between a pair of hosts. environment where there are different paths between a pair of hosts.
ECMP and other load balancing techniques such as LAG are widely used ECMP and other load balancing techniques such as Link Aggregation
in today's network and having multiple paths between a pair of Groups (LAG) are widely used in today's networks and having multiple
single-homed hosts is becoming the norm instead of the exception. paths between a pair of single-homed hosts is becoming the norm
Although these multiple paths have often the same cost (from an IGP instead of the exception. Although these multiple paths have often
metrics viewpoint), they do not necessarily have the same the same cost (from an IGP metrics viewpoint), they do not
performance. For example, [IMC13c] reports the results of a long necessarily have the same performance. For example, [IMC13c] reports
measurement study showing that load balanced Internet paths between the results of a long measurement study showing that load balanced
that same pair of hosts can have huge delay differences. Internet paths between that same pair of hosts can have huge delay
differences.
2.2. Cellular/WiFi Offload 2.2. Cellular/WiFi Offload
A second use case that has been explored by several network A second use case that has been explored by several network
researchers is the cellular/WiFi offload use case. Smartphones or researchers is the cellular/WiFi offload use case. Smartphones or
other mobile devices equipped with two wireless interfaces are a very other mobile devices equipped with two wireless interfaces are a very
common use case for Multipath TCP. As of this writing, this is also common use case for Multipath TCP. In September 2015, this is also
the largest deployment of Multipath-TCP enabled devices [IOS7]. the largest deployment of Multipath-TCP enabled devices [IOS7]. It
Unfortunately, as there are no public measurements about this has been briefly discussed during IETF88 [ietf88], but there is no
deployment, we can only rely on published papers that have mainly published paper or report that analyses this deployment. For this
used the Multipath TCP implementation in the Linux kernel for their reason, we only discuss published papers that have mainly used the
Multipath TCP implementation in the Linux kernel for their
experiments. experiments.
The performance of Multipath TCP in wireless networks was briefly The performance of Multipath TCP in wireless networks was briefly
evaluated in [NSDI12]. One experiment analyzes the performance of evaluated in [NSDI12]. One experiment analyzes the performance of
Multipath TCP on a client with two wireless interfaces. This Multipath TCP on a client with two wireless interfaces. This
evaluation shows that when the receive window is large, Multipath TCP evaluation shows that when the receive window is large, Multipath TCP
can efficiently use the two available links. However, if the window can efficiently use the two available links. However, if the window
becomes smaller, then packets sent on a slow path can block the becomes smaller, then packets sent on a slow path can block the
transmission of packets on a faster path. In some cases, 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 Multipath TCP over two paths can become lower than the
performance of regular TCP over the best performing path. Two performance of regular TCP over the best performing path. Two
heuristics, reinjection and penalization, are proposed in [NSDI12] to heuristics, reinjection and penalization, are proposed in [NSDI12] to
solve this identified performance problem. These two heuristics have solve this identified performance problem. These two heuristics have
since been used in the Multipath TCP implementation in the Linux since been used in the Multipath TCP implementation in the Linux
kernel. [CONEXT13] explored the problem in more details and revealed kernel. [CONEXT13] explored the problem in more detail and revealed
some other scenarios where Multipath TCP can have difficulties in some other scenarios where Multipath TCP can have difficulties in
efficiently pooling the available paths. Improvements to the efficiently pooling the available paths. Improvements to the
Multipath TCP implementation in the Linux kernel are proposed in Multipath TCP implementation in the Linux kernel are proposed in
[CONEXT13] to cope with some of these problems. [CONEXT13] to cope with some of these problems.
The first experimental analysis of Multipath TCP in a public wireless The first experimental analysis of Multipath TCP in a public wireless
environment was presented in [Cellnet12]. These measurements explore environment was presented in [Cellnet12]. These measurements explore
the ability of Multipath TCP to use two wireless networks (real WiFi the ability of Multipath TCP to use two wireless networks (real WiFi
and 3G networks). Three modes of operation are compared. The first and 3G networks). Three modes of operation are compared. The first
mode of operation is the simultaneous use of the two wireless mode of operation is the simultaneous use of the two wireless
networks. In this mode, Multipath TCP pools the available resources networks. In this mode, Multipath TCP pools the available resources
and uses both wireless interfaces. This mode provides fast handover and uses both wireless interfaces. This mode provides fast handover
from WiFi to cellular or the opposite when the user moves. from WiFi to cellular or the opposite when the user moves.
Measurements presented in [CACM14] show that the handover from one Measurements presented in [CACM14] show that the handover from one
wireless network to another is not an abrupt process. When a host wireless network to another is not an abrupt process. When a host
moves, it does not experience either excellent connectivity or no moves, there are regions where the quality of one of the wireless
connectivity at all. Instead, there are regions where the quality of networks is weaker than the other, but the host considers this
one of the wireless networks is weaker than the other, but the host wireless network to still be up. When a mobile host enters such
considers this wireless network to still be up. When a mobile host regions, its ability to send packets over another wireless network is
enters such regions, its ability to send packets over another important to ensure a smooth handover. This is clearly illustrated
wireless network is important to ensure a smooth handover. This is from the packet trace discussed in [CACM14].
clearly illustrated from the packet trace discussed in [CACM14].
Many cellular networks use volume-based pricing and users often Many cellular networks use volume-based pricing and users often
prefer to use unmetered WiFi networks when available instead of prefer to use unmetered WiFi networks when available instead of
metered cellular networks. [Cellnet12] implements the support for metered cellular networks. [Cellnet12] implements support for the
the MP_PRIO option to explore two other modes of operation. MP_PRIO option to explore two other modes of operation.
In the backup mode, Multipath TCP opens a TCP subflow over each In the backup mode, Multipath TCP opens a TCP subflow over each
interface, but the cellular interface is configured in backup mode. interface, but the cellular interface is configured in backup mode.
This implies that data only flows over the WiFi interface when both This implies that data only flows over only the WiFi interface when
interfaces are considered to be active. If the WiFi interface fails, both interfaces are considered to be active. If the WiFi interface
then the traffic switches quickly to the cellular interface, ensuring fails, then the traffic switches quickly to the cellular interface,
a smooth handover from the user's viewpoint [Cellnet12]. The cost of ensuring a smooth handover from the user's viewpoint [Cellnet12].
this approach is that the WiFi and cellular interfaces likely remain The cost of this approach is that the WiFi and cellular interfaces
active all the time since all subflows are established over the two are likely to remain active all the time since all subflows are
interfaces. established over the two interfaces.
The single-path mode is slightly different. This mode benefits from The single-path mode is slightly different. This mode benefits from
the break-before-make capability of Multipath TCP. When an MPTCP the break-before-make capability of Multipath TCP. When an MPTCP
session is established, a subflow is created over the WiFi interface. session is established, a subflow is created over the WiFi interface.
No packet is sent over the cellular interface as long as the WiFi No packet is sent over the cellular interface as long as the WiFi
interface remains up [Cellnet12]. This implies that the cellular interface remains up [Cellnet12]. This implies that the cellular
interface can remain idle and battery capacity is preserved. When interface can remain idle and battery capacity is preserved. When
the WiFi interface fails, new subflows are established over the the WiFi interface fails, a new subflow is established over the
cellular interface in order to preserve the established Multipath TCP cellular interface in order to preserve the established Multipath TCP
sessions. Compared to the backup mode described earlier, this mode sessions. Compared to the backup mode described earlier,
of operation is characterised by a throughput drop while the cellular measurements reported in [Cellnet12] indicate that this mode of
interface is brought up and the subflows are reestablished. During operation is characterised by a throughput drop while the cellular
this time, no data packet is transmitted. interface is brought up and the subflows are reestablished.
From a protocol viewpoint, [Cellnet12] discusses the problem posed by From a protocol viewpoint, [Cellnet12] discusses the problem posed by
the unreliability of the ADD_ADDR option and proposes a small the unreliability of the ADD_ADDR option and proposes a small
protocol extension to allow hosts to reliably exchange this option. protocol extension to allow hosts to reliably exchange this option.
It would be useful to analyze packet traces to understand whether the It would be useful to analyze packet traces to understand whether the
unreliability of the REMOVE_ADDR option poses an operational problem unreliability of the REMOVE_ADDR option poses an operational problem
in real deployments. in real deployments.
Another study of the performance of Multipath TCP in wireless Another study of the performance of Multipath TCP in wireless
networks was reported in [IMC13b]. This study uses laptops connected networks was reported in [IMC13b]. This study uses laptops connected
to various cellular ISPs and WiFi hotspots. It compares various file to various cellular ISPs and WiFi hotspots. It compares various file
transfer scenarios and concludes based on measurements with the transfer scenarios. [IMC13b] observes that 4-path MPTCP outperforms
Multipath TCP implementation in the Linux kernel that "MPTCP provides 2-path MPTCP, especially for larger files. The comparison between
a robust data transport and reduces variations in download LIA, OLIA and Reno does not reveal a significant performance
latencies". difference for file sizes smaller than 4MB.
A different study of the performance of Multipath TCP with two A different study of the performance of Multipath TCP with two
wireless networks is presented in [INFOCOM14]. In this study the two wireless networks is presented in [INFOCOM14]. In this study the two
networks had different qualities : a good network and a lossy networks had different qualities : a good network and a lossy
network. When using two paths with different packet loss ratios, the network. When using two paths with different packet loss ratios, the
Multipath TCP congestion control scheme moves traffic away from the Multipath TCP congestion control scheme moves traffic away from the
lossy link that is considered to be congested. However, [INFOCOM14] lossy link that is considered to be congested. However, [INFOCOM14]
documents an interesting scenario that is summarised in the Figure 1. documents an interesting scenario that is summarised in Figure 1.
client ----------- path1 -------- server client ----------- path1 -------- server
| | | |
+--------------- path2 ------------+ +--------------- path2 ------------+
Figure 1: Simple network topology Figure 1: Simple network topology
Initially, the two paths have the same quality and Multipath TCP Initially, the two paths have the same quality and Multipath TCP
distributes the load over both of them. During the transfer, the distributes the load over both of them. During the transfer, the
second path becomes lossy, e.g. because the client moves. Multipath second path becomes lossy, e.g. because the client moves. Multipath
skipping to change at page 7, line 19 skipping to change at page 7, line 43
is still up, a new subflow could be immediately reestablished. It is still up, a new subflow could be immediately reestablished. It
would then be immediately usable to send new data and would not be would then be immediately usable to send new data and would not be
forced to first retransmit the previously transmitted data. As of forced to first retransmit the previously transmitted data. As of
this writing, this dynamic management of the subflows is not yet this writing, this dynamic management of the subflows is not yet
implemented in the Multipath TCP implementation in the Linux kernel. implemented in the Multipath TCP implementation in the Linux kernel.
2.3. Multipath TCP proxies 2.3. Multipath TCP proxies
As Multipath TCP is not yet widely deployed on both clients and As Multipath TCP is not yet widely deployed on both clients and
servers, several deployments have used various forms of proxies. Two servers, several deployments have used various forms of proxies. Two
families solutions are currently being used or tested families of solutions are currently being used or tested
[I-D.deng-mptcp-proxy]. [I-D.deng-mptcp-proxy].
A first use case is when a Multipath TCP enabled client wants to use 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 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 is a smartphone that needs to use both its WiFi and its cellular
interface to transfer data. Several types of proxies are possible interface to transfer data. Several types of proxies are possible
for this use case. An HTTP proxy deployed on a Multipath TCP capable for this use case. An HTTP proxy deployed on a Multipath TCP capable
server would enable the smartphone to use Multipath TCP to access server would enable the smartphone to use Multipath TCP to access
regular web servers. Obviously, this solution only works for regular web servers. Obviously, this solution only works for
applications that rely on HTTP. Another possibility is to use a applications that rely on HTTP. Another possibility is to use a
proxy that can convert any Multipath TCP connection into a regular proxy that can convert any Multipath TCP connection into a regular
TCP connection. The SOCKS protocol [RFC1928] is an example of such a TCP connection. Multipath TCP-specific proxies have been proposed
protocol. Other proxies have been proposed [I-D.wei-mptcp-proxy-mechanism] [HotMiddlebox13b]
[I-D.wei-mptcp-proxy-mechanism] [HotMiddlebox13b]. Measurements [I-D.hampel-mptcp-proxies-anchors].
performed with smartphones [Mobicom15] show that popular applications
work correctly through a SOCKS proxy and Multipath TCP enabled Another possibility leverages the SOCKS protocol [RFC1928]. SOCKS is
smartphones. Thanks to Multipath TCP, long connections can be spread often used in enterprise networks to allow clients to reach external
over the two available interfaces. However, for short connections, servers. For this, the client opens a TCP connection to the SOCKS
most of the data is sent over the initial subflow that is created server that relays it to the final destination. If both the client
over the interface corresponding to the default route and the second and the SOCKS server use Multipath TCP, but not the final
subflow is almost not used. 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.
A second use case is when Multipath TCP is used by middleboxes, A second use case is when Multipath TCP is used by middleboxes,
typically inside access networks. Various network operators are typically inside access networks. Various network operators are
discussing and evaluating solutions for hybrid access networks discussing and evaluating solutions for hybrid access networks
[BBF-WT348]. Such networks arise when a network operator controls [BBF-WT348]. Such networks arise when a network operator controls
two different access network technologies, e.g. DSL and LTE, and two different access network technologies, e.g. wired and cellular,
wants to combine them to improve the bandwidth offered to the and wants to combine them to improve the bandwidth offered to the
endusers [I-D.lhwxz-hybrid-access-network-architecture]. Several endusers [I-D.lhwxz-hybrid-access-network-architecture]. Several
solutions are currently investigated for such networks [BBF-WT348]. solutions are currently investigated for such networks [BBF-WT348].
Figure 2 shows the organisation of such a network. When a client Figure 2 shows the organisation of such a network. When a client
creates a normal TCP connection, it is intercepted by the Hybrid CPE 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 (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 use the available access networks (DSL and LTE in the example). The
Hybrid Access Gateway (HAG) does the opposite to ensure that the Hybrid Access Gateway (HAG) does the opposite to ensure that the
regular server see a normal TCP connection. Some of the solutions regular server sees a normal TCP connection. Some of the solutions
that are currently discussed for those hybrid networks use Multipath that are currently discussed for hybrid networks use Multipath TCP on
TCP on the HCPE and the HAG. Other solutions rely on tunnels between the HCPE and the HAG. Other solutions rely on tunnels between the
the HCPE and the HAG [I-D.lhwxz-gre-notifications-hybrid-access]. HCPE and the HAG [I-D.lhwxz-gre-notifications-hybrid-access].
client --- HCPE ------ dsl ------- HAG --- internet --- server client --- HCPE ------ DSL ------- HAG --- internet --- server
| | | |
+------- lte -----------+ +------- LTE -----------+
Figure 2: Hybrid Access Network Figure 2: Hybrid Access Network
3. Operational Experience 3. Operational Experience
3.1. Middlebox interference 3.1. Middlebox interference
The interference caused by various types of middleboxes has been an The interference caused by various types of middleboxes has been an
important concern during the design of the Multipath TCP protocol. important concern during the design of the Multipath TCP protocol.
Three studies on the interactions between Multipath TCP and Three studies on the interactions between Multipath TCP and
middleboxes are worth being discussed. middleboxes are worth discussing.
The first analysis was described in [IMC11]. This paper was the main The first analysis appears in [IMC11]. This paper was the main
motivation for including inside Multipath TCP various techniques to motivation for Multipath TCP incorporating various techniques to cope
cope with middlebox interference. More specifically, Multipath TCP with middlebox interference. More specifically, Multipath TCP has
has been designed to cope with middleboxes that : been designed to cope with middleboxes that :
o change source or destination addresses o change source or destination addresses
o change source or destination port numbers o change source or destination port numbers
o change TCP sequence numbers o change TCP sequence numbers
o split or coalesce segments o split or coalesce segments
o remove TCP options o remove TCP options
o modify the payload of TCP segments o modify the payload of TCP segments
These middlebox interferences have all been included in the MBtest These middlebox interferences have all been included in the MBtest
suite [MBTest]. This test suite has been used [HotMiddlebox13] to suite [MBTest]. This test suite is used in [HotMiddlebox13] to
verify the reaction of the Multipath TCP implementation in the Linux verify the reaction of the Multipath TCP implementation in the Linux
kernel when faced with middlebox interference. The test environment kernel when faced with middlebox interference. The test environment
used for this evaluation is a dual-homed client connected to a used for this evaluation is a dual-homed client connected to a
single-homed server. The middlebox behavior can be activated on any single-homed server. The middlebox behavior can be activated on any
of the paths. The main results of this analysis are : of the paths. The main results of this analysis are :
o the Multipath TCP implementation in the Linux kernel is not o the Multipath TCP implementation in the Linux kernel is not
affected by a middlebox that performs NAT or modifies TCP sequence affected by a middlebox that performs NAT or modifies TCP sequence
numbers numbers
skipping to change at page 9, line 31 skipping to change at page 10, line 23
o when a middlebox performs segment splitting, the Multipath TCP o when a middlebox performs segment splitting, the Multipath TCP
implementation in the Linux kernel correctly reassembles the data implementation in the Linux kernel correctly reassembles the data
corresponding to the indicated mapping. [HotMiddlebox13] shows on corresponding to the indicated mapping. [HotMiddlebox13] shows on
figure 4 in section 3.3 a corner case with segment splitting that figure 4 in section 3.3 a corner case with segment splitting that
may lead to a desynchronisation between the two hosts. may lead to a desynchronisation between the two hosts.
The interactions between Multipath TCP and real deployed middleboxes The interactions between Multipath TCP and real deployed middleboxes
is also analyzed in [HotMiddlebox13] and a particular scenario with is also analyzed in [HotMiddlebox13] and a particular scenario with
the FTP application level gateway running on a NAT is described. 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.
From an operational viewpoint, knowing that Multipath TCP can cope From an operational viewpoint, knowing that Multipath TCP can cope
with various types of middlebox interference is important. However, with various types of middlebox interference is important. However,
there are situations where the network operators need to gather there are situations where the network operators need to gather
information about where a particular middlebox interference occurs. information about where a particular middlebox interference occurs.
The tracebox software [tracebox] described in [IMC13a] is an The tracebox software [tracebox] described in [IMC13a] is an
extension of the popular traceroute software that enables network extension of the popular traceroute software that enables network
operators to check at which hop a particular field of the TCP header operators to check at which hop a particular field of the TCP header
(including options) is modified. It has been used by several network (including options) is modified. It has been used by several network
operators to debug various middlebox interference problems. tracebox operators to debug various middlebox interference problems. tracebox
includes a scripting language that enables its user to specify includes a scripting language that enables its user to specify
precisely which packet is sent by the source. tracebox sends packets precisely which packet (including IP and TCP options) is sent by the
with an increasing TTL/HopLimit and compares the information returned source. tracebox sends packets with an increasing TTL/HopLimit and
in the ICMP messages with the packet that it sends. This enables compares the information returned in the ICMP messages with the
tracebox to detect any interference caused by middleboxes on a given packet that it sent. This enables tracebox to detect any
path. tracebox works better when routers implement the ICMP extension interference caused by middleboxes on a given path. tracebox works
defined in [RFC1812]. better when routers implement the ICMP extension defined in
[RFC1812].
A closer look at the packets received on the multipath-tcp.org server
showed that among the 184 thousands Multipath TCP connections in the
trace, we observed only 125 of them falling back to regular TCP,
which happened with 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 on NAT boxes.
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.
Users of the Multipath TCP implementation have reported some Users of the Multipath TCP implementation have reported some
experience with middlebox interference. The strangest scenario has experience with middlebox interference. The strangest scenario has
been a middlebox that accepts the Multipath TCP options in the SYN been a middlebox that accepts the Multipath TCP options in the SYN
segment but later replaces Multipath TCP options with a TCP EOL segment but later replaces Multipath TCP options with a TCP EOL
option [StrangeMbox]. This causes Multipath TCP to perform a option [StrangeMbox]. This causes Multipath TCP to perform a
fallback to regular TCP without any impact on the application. fallback to regular TCP without any impact on the application.
3.2. Congestion control 3.2. Congestion control
skipping to change at page 10, line 32 skipping to change at page 11, line 28
Linux uses a modular architecture to support various congestion Linux uses a modular architecture to support various congestion
control schemes. This architecture is applicable for both regular control schemes. This architecture is applicable for both regular
TCP and Multipath TCP. While the coupled congestion control scheme TCP and Multipath TCP. While the coupled congestion control scheme
defined in [RFC6356] is the default congestion control scheme in the defined in [RFC6356] is the default congestion control scheme in the
Linux implementation, other congestion control schemes have been Linux implementation, other congestion control schemes have been
added. The second congestion control scheme is OLIA [CONEXT12]. added. The second congestion control scheme is OLIA [CONEXT12].
This congestion control scheme is also an adaptation of the NewReno This congestion control scheme is also an adaptation of the NewReno
single path congestion control scheme to support multiple paths. single path congestion control scheme to support multiple paths.
Simulations and measurements have shown that it provides some Simulations and measurements have shown that it provides some
performance benefits compared to the the default congestion control performance benefits compared to the the default congestion control
scheme [CONEXT12]. Measurement over a wide range of parameters scheme [CONEXT12]. Measurements over a wide range of parameters
reported in [CONEXT13] also indicate some benefits with the OLIA reported in [CONEXT13] also indicate some benefits with the OLIA
congestion control scheme. Recently, a delay-based congestion congestion control scheme. Recently, a delay-based congestion
control scheme has been ported to the Multipath TCP implementation in control scheme has been ported to the Multipath TCP implementation in
the Linux kernel. This congestion control scheme has been evaluated the Linux kernel. This congestion control scheme has been evaluated
by using simulations in [ICNP12]. The fourth congestion control by using simulations in [ICNP12]. The fourth congestion control
scheme that has been included in the Linux implementation of scheme that has been included in the Linux implementation of
Multipath TCP is the BALIA scheme Multipath TCP is the BALIA scheme
[I-D.walid-mptcp-congestion-control]. [I-D.walid-mptcp-congestion-control].
These different congestion control schemes have been compared in These different congestion control schemes have been compared in
several articles. [CONEXT13] and [PaaschPhD] apply an experimental several articles. [CONEXT13] and [PaaschPhD] compare these
design approach to compare these algorithms in an emulated algorithms in an emulated environment. The evaluation showed that
environment. The evaluation showed that the delay-based congestion the delay-based congestion control scheme is less able to efficiently
control scheme is less able to efficiently use the available links use the available links than the three other schemes. Reports from
than the three other schemes. Reports from some users indicate that some users indicate that they seem to favor OLIA.
they seem to favor OLIA.
3.3. Subflow management 3.3. Subflow management
The multipath capability of Multipath TCP comes from the utilisation The multipath capability of Multipath TCP comes from the utilisation
of one subflow per path. The Multipath TCP architecture [RFC6182] of one subflow per path. The Multipath TCP architecture [RFC6182]
and the protocol specification [RFC6824] define the basic usage of and the protocol specification [RFC6824] define the basic usage of
the subflows and the protocol mechanisms that are required to create the subflows and the protocol mechanisms that are required to create
and terminate them. However, there are no guidelines on how subflows and terminate them. However, there are no guidelines on how subflows
are used during the lifetime of a Multipath TCP session. Most of the are used during the lifetime of a Multipath TCP session. Most of the
experiments with Multipath TCP have been performed in controlled published experiments with Multipath TCP have been performed in
environments. Still, based on the experience running them and controlled environments. Still, based on the experience running them
discussions on the mptcp-dev mailing list, interesting lessons have and discussions on the mptcp-dev mailing list, interesting lessons
been learned about the management of these subflows. have been learned about the management of these subflows.
From a subflow viewpoint, the Multipath TCP protocol is completely From a subflow viewpoint, the Multipath TCP protocol is completely
symmetrical. Both the clients and the server have the capability to symmetrical. Both the clients and the server have the capability to
create subflows. However in practice the existing Multipath TCP create subflows. However in practice the existing Multipath TCP
implementations [I-D.eardley-mptcp-implementations-survey] have opted implementations [I-D.eardley-mptcp-implementations-survey] have opted
for a strategy where only the client creates new subflows. The main for a strategy where only the client creates new subflows. The main
motivation for this strategy is that often the client resides behind motivation for this strategy is that often the client resides behind
a NAT or a firewall, preventing passive subflow openings on the a NAT or a firewall, preventing passive subflow openings on the
client. Although there are environments such as datacenters where client. Although there are environments such as datacenters where
this problem does not occur, as of this writing, no precise this problem does not occur, as of this writing, no precise
skipping to change at page 12, line 8 skipping to change at page 12, line 48
such events. such events.
When the server is multihomed, using the full-mesh subflow manager When the server is multihomed, using the full-mesh subflow manager
may lead to a large number of subflows being established. For may lead to a large number of subflows being established. For
example, consider a dual-homed client connected to a server with example, consider a dual-homed client connected to a server with
three interfaces. In this case, even if the subflows are only three interfaces. In this case, even if the subflows are only
created by the client, 6 subflows will be established. This may be created by the client, 6 subflows will be established. This may be
excessive in some environments, in particular when the client and/or excessive in some environments, in particular when the client and/or
the server have a large number of interfaces. A recent draft has the server have a large number of interfaces. A recent draft has
proposed a Multipath TCP option to negotiate the maximum number of proposed a Multipath TCP option to negotiate the maximum number of
subflows . However, it should be noted that there have been reports subflows. However, it should be noted that there have been reports
on the mptcp-dev mailing indicating that users rely on Multipath TCP on the mptcp-dev mailing indicating that users rely on Multipath TCP
to aggregate more than four different interfaces. Thus, there is a to aggregate more than four different interfaces. Thus, there is a
need for supporting many interfaces efficiently. need for supporting many interfaces efficiently.
Creating subflows between multihomed clients and servers may Creating subflows between multihomed clients and servers may
sometimes lead to operational issues as observed by discussions on sometimes lead to operational issues as observed by discussions on
the mptcp-dev mailing list. In some cases the network operators the mptcp-dev mailing list. In some cases the network operators
would like to have a better control on how the subflows are created would like to have a better control on how the subflows are created
by Multipath TCP [I-D.boucadair-mptcp-max-subflow]. This might by Multipath TCP [I-D.boucadair-mptcp-max-subflow]. This might
require the definition of policy rules to control the operation of require the definition of policy rules to control the operation of
skipping to change at page 12, line 33 skipping to change at page 13, line 25
| | | | | |
+-------------- switch2 --------+ +-------------- switch2 --------+
Figure 3: Simple switched network topology Figure 3: Simple switched network topology
Consider the simple network topology shown in Figure 3. From an Consider the simple network topology shown in Figure 3. From an
operational viewpoint, a network operator could want to create two operational viewpoint, a network operator could want to create two
subflows between the communicating hosts. From a bandwidth subflows between the communicating hosts. From a bandwidth
utilization viewpoint, the most natural paths are host1-switch1-host2 utilization viewpoint, the most natural paths are host1-switch1-host2
and host1-switch2-host2. However, a Multipath TCP implementation and host1-switch2-host2. However, a Multipath TCP implementation
running onthese two hosts may sometimes have difficulties to obtain running on these two hosts may sometimes have difficulties to obtain
this result. this result.
To understand the difficulty, let us consider different allocation To understand the difficulty, let us consider different allocation
strategies for the IP addresses. A first strategy is to assign two strategies for the IP addresses. A first strategy is to assign two
subnets : subnetA (resp. subnetB) contains the IP addresses of subnets : subnetA (resp. subnetB) contains the IP addresses of
host1's interface to switch1 (resp. switch2) and host2's interface to host1's interface to switch1 (resp. switch2) and host2's interface to
switch1 (resp. switch2). In this case, a Multipath TCP subflow switch1 (resp. switch2). In this case, a Multipath TCP subflow
manager should only create one subflow per subnet. To enforce the manager should only create one subflow per subnet. To enforce the
utilization of these paths, the network operator would have to utilization of these paths, the network operator would have to
specify a policy that prefers the subflows in the same subnet over specify a policy that prefers the subflows in the same subnet over
skipping to change at page 13, line 9 skipping to change at page 13, line 48
should react when an interface or subflow fails. should react when an interface or subflow fails.
A second strategy is to use a single subnet for all IP addresses. In 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 this case, it becomes more difficult to specify a policy that
indicates which subflows should be established. indicates which subflows should be established.
The second subflow manager that is currently supported by the The second subflow manager that is currently supported by the
Multipath TCP implementation in the Linux kernel is the ndiffport Multipath TCP implementation in the Linux kernel is the ndiffport
subflow manager. This manager was initially created to exploit the subflow manager. This manager was initially created to exploit the
path diversity that exists between single-homed hosts due to the path diversity that exists between single-homed hosts due to the
utilization of flow-based load balancing techniques. This subflow utilization of flow-based load balancing techniques [SIGCOMM11].
manager creates N subflows between the same pair of IP addresses. This subflow manager creates N subflows between the same pair of IP
The N subflows are created by the client and differ only in the addresses. The N subflows are created by the client and differ only
source port selected by the client. It was not designed to be used in the source port selected by the client. It was not designed to be
on multihomed hosts. used on multihomed hosts.
3.5. Subflow destination port 3.5. Subflow destination port
The Multipath TCP protocol relies on the token contained in the The Multipath TCP protocol relies on the token contained in the
MP_JOIN option to associate a subflow to an existing Multipath TCP MP_JOIN option to associate a subflow to an existing Multipath TCP
session. This implies that there is no restriction on the source session. This implies that there is no restriction on the source
address, destination address and source or destination ports used for address, destination address and source or destination ports used for
the new subflow. The ability to use different source and destination the new subflow. The ability to use different source and destination
addresses is key to support multihomed servers and clients. The addresses is key to support multihomed servers and clients. The
ability to use different destination port numbers is worth being ability to use different destination port numbers is worth discussing
discussed because it has operational implications. because it has operational implications.
For illustration, consider a dual-homed client that creates a second For illustration, consider a dual-homed client that creates a second
subflow to reach a single-homed server as illustrated in Figure 4. subflow to reach a single-homed server as illustrated in Figure 4.
client ------- r1 --- internet --- server client ------- r1 --- internet --- server
| | | |
+----------r2-------+ +----------r2-------+
Figure 4: Multihomed-client connected to single-homed server Figure 4: Multihomed-client connected to single-homed server
skipping to change at page 14, line 6 skipping to change at page 14, line 45
There have been suggestions from Multipath TCP users to modify the There have been suggestions from Multipath TCP users to modify the
implementation to allow the client to use different destination ports implementation to allow the client to use different destination ports
to reach the server. This suggestion seems mainly motivated by to reach the server. This suggestion seems mainly motivated by
traffic shaping middleboxes that are used in some wireless networks. traffic shaping middleboxes that are used in some wireless networks.
In networks where different shaping rates are associated to different In networks where different shaping rates are associated to different
destination port numbers, this could allow Multipath TCP to reach a destination port numbers, this could allow Multipath TCP to reach a
higher performance. As of this writing, we are not aware of any higher performance. As of this writing, we are not aware of any
implementation of this kind of tweaking. implementation of this kind of tweaking.
However, from an implementation point-of-view supporting different However, from an implementation point-of-view supporting different
destination ports for the same Multipath TCP connection introduces a destination ports for the same Multipath TCP connection can cause
new performance issue. A legacy implementation of a TCP stack some issues. A legacy implementation of a TCP stack creates a
creates a listening socket to react upon incoming SYN segments. The listening socket to react upon incoming SYN segments. The listening
listening socket is handling the SYN segments that are sent on a socket is handling the SYN segments that are sent on a specific port
specific port number. Demultiplexing incoming segments can thus be number. Demultiplexing incoming segments can thus be done solely by
done solely by looking at the IP addresses and the port numbers. looking at the IP addresses and the port numbers. With Multipath TCP
With Multipath TCP however, incoming SYN segments may have an MP_JOIN however, incoming SYN segments may have an MP_JOIN option with a
option with a different destination port. This means, that all different destination port. This means, that all incoming segments
incoming segments that did not match on an existing listening-socket that did not match on an existing listening-socket or an already
or an already established socket must be parsed for an eventual established socket must be parsed for an eventual MP_JOIN option.
MP_JOIN option. This imposes an additional cost on servers, This imposes an additional cost on servers, previously not existent
previously not existent on legacy TCP implementations. on legacy TCP implementations.
3.6. Closing subflows 3.6. Closing subflows
client server client server
| | | |
MPTCP: established | | MPTCP: established MPTCP: established | | MPTCP: established
Sub: established | | Sub: established Sub: established | | Sub: established
| | | |
| DATA_FIN | | DATA_FIN |
MPTCP: close-wait | <------------------------ | close() (step 1) MPTCP: close-wait | <------------------------ | close() (step 1)
skipping to change at page 15, line 18 skipping to change at page 16, line 11
close() of the connection (Step 1 in Figure 5) only triggers 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 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 to combine the DATA_FIN with a subflow-FIN. The reception of the
DATA_FIN will make the application trigger the closure of the DATA_FIN will make the application trigger the closure of the
connection (step 2), trying to avoid Time-Wait state with this late connection (step 2), trying to avoid Time-Wait state with this late
closure. This time, the kernel might decide to combine the DATA_FIN 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 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 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 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 state will consume resources on the host for at least 2 MSL (Maximum
Segment Lifetime). Thus, a smart application, that tries to avoid Segment Lifetime). Thus, a smart application that tries to avoid
Time-Wait state by doing late closure of the connection actually ends 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 up with one of its subflows in Time-Wait state. A high-performance
Multipath TCP kernel implementation should honor the desire of the Multipath TCP kernel implementation should honor the desire of the
application to do passive closure of the connection and successfully application to do passive closure of the connection and successfully
avoid Time-Wait state - even on the subflows. avoid Time-Wait state - even on the subflows.
The solution to this problem lies in an optimistic assumption that a The solution to this problem lies in an optimistic assumption that a
host doing active-closure of a Multipath TCP connection by sending a host doing active-closure of a Multipath TCP connection by sending a
DATA_FIN will soon also send a FIN on all its in subflows. Thus, the 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 passive closer of the connection can simply wait for the peer to send
exactly this FIN - enforcing passive closure even on the subflows. exactly this FIN - enforcing passive closure even on the subflows.
Of course, to avoid consuming resources indefinitely, a timer must Of course, to avoid consuming resources indefinitely, a timer must
limit the time our implementation waits for the FIN. limit the time our implementation waits for the FIN.
4. Packet schedulers 3.7. Packet schedulers
In a Multipath TCP implementation, the packet scheduler is the In a Multipath TCP implementation, the packet scheduler is the
algorithm that is executed when transmitting each packet to decide on algorithm that is executed when transmitting each packet to decide on
which subflow it needs to be transmitted. The packet scheduler which subflow it needs to be transmitted. The packet scheduler
itself does not have any impact on the interoperability of Multipath itself does not have any impact on the interoperability of Multipath
TCP implementations. However, it may clearly impact the performance TCP implementations. However, it may clearly impact the performance
of Multipath TCP sessions. The Multipath TCP implementation in the of Multipath TCP sessions. The Multipath TCP implementation in the
Linux kernel supports a pluggable architecture for the packet Linux kernel supports a pluggable architecture for the packet
scheduler [PaaschPhD]. As of this writing, two schedules have been scheduler [PaaschPhD]. As of this writing, two schedulers have been
implemented: round-robin and lowest-rtt-first. They are compared in implemented: round-robin and lowest-rtt-first. The second scheduler
[CSWS14]. The experiments and measurements described in [CSWS14] relies on the round-trip-time (rtt) measured on each TCP subflow and
show that the lowest-rtt-first scheduler appears to be the best sends first segments over the subflow having the lowest round-trip-
compromise from a performance viewpoint. Another study of the packet time. They are compared in [CSWS14]. The experiments and
schedulers is presented in [PAMS2014]. This study relies on measurements described in [CSWS14] show that the lowest-rtt-first
simulations with the Multipath TCP implementation in the Linux scheduler appears to be the best compromise from a performance
kernel. These simulations confirm the impact of the packet scheduler viewpoint. Another study of the packet schedulers is presented in
on the performance of Multipath TCP. [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."
5. Segment size selection 3.8. Segment size selection
When an application performs a write/send system call, the kernel When an application performs a write/send system call, the kernel
allocates a packet buffer (sk_buff in Linux) to store the data the 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 application wants to send. The kernel will store at most one MSS
(Maximum Segment Size) of data per buffer. As MSS can differ amongst (Maximum Segment Size) of data per buffer. As the MSS can differ
subflows, an MPTCP implementation must select carefully the MSS used amongst subflows, an MPTCP implementation must select carefully the
to generate application data. The Linux kernel implementation had MSS used to generate application data. The Linux kernel
various ways of selecting the MSS: minimum or maximum amongst the implementation had various ways of selecting the MSS: minimum or
different subflows. However, these heuristics of MSS selection can maximum amongst the different subflows. However, these heuristics of
cause significant performances issues in some environment. Consider MSS selection can cause significant performance issues in some
the following example. An MPTCP connection has two established environment. Consider the following example. An MPTCP connection
subflows that respectively use a MSS of 1420 and 1428 bytes. If has two established subflows that respectively use a MSS of 1420 and
MPTCP selects the maximum, then the application will generate 1428 bytes. If MPTCP selects the maximum, then the application will
segments of 1428 bytes of data. An MPTCP implementation will have to generate segments of 1428 bytes of data. An MPTCP implementation
split the segment in two (a 1420-byte and 8-byte segments) when will have to split the segment in two (a 1420-byte and 8-byte
pushing on the subflow with the smallest MSS. The latter segment segments) when pushing on the subflow with the smallest MSS. The
will introduce a large overhead as for a single data segment 2 slots latter segment will introduce a large overhead as for a single data
will be used in the congestion window (in packets) therefore reducing segment 2 slots will be used in the congestion window (in packets)
by ~2 the potential throughput (in bytes/s) of this subflow. Taking therefore reducing by roughly twice the potential throughput (in
the smallest MSS does not solve the issue as there might be a case bytes/s) of this subflow. Taking the smallest MSS does not solve the
where the sublow with the smallest MSS will only participate issue as there might be a case where the subflow with the smallest
marginally to the overall performance therefore reducing the MSS only sends a few packets therefore reducing the potential
potential throughput of the other subflows. throughput of the other subflows.
The Linux implementation recently took another approach [DetalMSS]. The Linux implementation recently took another approach [DetalMSS].
Instead of selecting the minimum and maximum values, it now Instead of selecting the minimum and maximum values, it now
dynamically adapts the MSS based on the contribution of all the dynamically adapts the MSS based on the contribution of all the
subflows to the connection's throughput. For this it computes, for subflows to the connection's throughput. For this it computes, for
each subflow, the potential throughput achieved by selecting each MSS each subflow, the potential throughput achieved by selecting each MSS
value and by taking into account the lost space in the cwnd. It then value and by taking into account the lost space in the cwnd. It then
selects the MSS that allows to achieve the highest potential selects the MSS that allows to achieve the highest potential
throughput. throughput.
6. Interactions with the Domain Name System 3.9. Interactions with the Domain Name System
Multihomed clients such as smartphones can send DNS queries over any Multihomed clients such as smartphones can send DNS queries over any
of their interfaces. When a single-homed client performs a DNS of their interfaces. When a single-homed client performs a DNS
query, it receives from its local resolver the best answer for its query, it receives from its local resolver the best answer for its
request. If the client is multihomed, the answer returned to the DNS request. If the client is multihomed, the answer returned to the DNS
query may vary with the interface over which it has been sent. query may vary with the interface over which it has been sent.
cdn1 cdn1
| |
client -- cellular -- internet -- cdn3 client -- cellular -- internet -- cdn3
skipping to change at page 17, line 23 skipping to change at page 18, line 23
Figure 6: Simple network topology Figure 6: Simple network topology
If the client sends a DNS query over the WiFi interface, the answer 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 will point to the cdn2 server while the same request sent over the
cellular interface will point to the cdn1 server. This might cause cellular interface will point to the cdn1 server. This might cause
problems for CDN providers that locate their servers inside ISP problems for CDN providers that locate their servers inside ISP
networks and have contracts that specify that the CDN server will networks and have contracts that specify that the CDN server will
only be accessed from within this particular ISP. Assume now that only be accessed from within this particular ISP. Assume now that
both the client and the CDN servers support Multipath TCP. In this both the client and the CDN servers support Multipath TCP. In this
case, a Multipath TCP session from cdn1 or cdn2 would potentially use case, a Multipath TCP session from cdn1 or cdn2 would potentially use
both the cellular network and the WiFi network. This would violate both the cellular network and the WiFi network. Serving the client
the contract between the CDN provider and the network operators. A from cdn2 over the cellular interface could violate the contract
possible solution to prevent this problem would be to modify the DNS between the CDN provider and the network operators. A similar
resolution on the client. The client subnet EDNS extension defined problem occurs with regular TCP if the client caches DNS replies.
in [I-D.vandergaast-edns-client-subnet] could be used for this 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 [I-D.ietf-dnsop-edns-client-subnet] could be used for this
purpose. When the client sends a DNS query from its WiFi interface, purpose. When the client sends a DNS query from its WiFi interface,
it should also send the client subnet corresponding to the cellular it should also send the client subnet corresponding to the cellular
interface in this request. This would indicate to the resolver that interface in this request. This would indicate to the resolver that
the answer should be valid for both the WiFi and the cellular the answer should be valid for both the WiFi and the cellular
interfaces (e.g., the cdn3 server). interfaces (e.g., the cdn3 server).
7. Captive portals 3.10. Captive portals
Multipath TCP enables a host to use different interfaces to reach a Multipath TCP enables a host to use different interfaces to reach a
server. In theory, this should ensure connectivity when at least one server. In theory, this should ensure connectivity when at least one
of the interfaces is active. In practice however, there are some of the interfaces is active. In practice however, there are some
particular scenarios with captive portals that may cause operational particular scenarios with captive portals that may cause operational
problems. The reference environment is shown in Figure 7. problems. The reference environment is shown in Figure 7.
client ----- network1 client ----- network1
| |
+------- internet ------------- server +------- internet ------------- server
skipping to change at page 18, line 14 skipping to change at page 19, line 24
WiFi network with a captive portal for network1 and a cellular WiFi network with a captive portal for network1 and a cellular
service for the second interface. On many smartphones, the WiFi service for the second interface. On many smartphones, the WiFi
interface is preferred over the cellular interface. If the interface is preferred over the cellular interface. If the
smartphone learns a default route via both interfaces, it will smartphone learns a default route via both interfaces, it will
typically prefer to use the WiFi interface to send its DNS request typically prefer to use the WiFi interface to send its DNS request
and create the first subflow. This is not optimal with Multipath and create the first subflow. This is not optimal with Multipath
TCP. A better approach would probably be to try a few attempts on 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 the WiFi interface and then try to use the second interface for the
initial subflow as well. initial subflow as well.
8. Conclusion 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, as
clients usually start the application's transaction by sending data
to the server. This data-segment (that is sent reliably by TCP)
enables stateless servers to create the TCP-related state, even in
case the third ACK has been lost.
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
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.
4. Conclusion
In this document, we have documented a few years of experience with In this document, we have documented a few years of experience with
Multipath TCP. The information presented in this document was Multipath TCP. The information presented in this document was
gathered from scientific publications and discussions with various gathered from scientific publications and discussions with various
users of the Multipath TCP implementation in the Linux kernel. users of the Multipath TCP implementation in the Linux kernel.
9. Acknowledgements 5. Acknowledgements
This work was partially supported by the FP7-Trilogy2 project. We This work was partially supported by the FP7-Trilogy2 project. We
would like to thank all the implementers and users of the Multipath would like to thank all the implementers and users of the Multipath
TCP implementation in the Linux kernel. This document has benefited TCP implementation in the Linux kernel. This document has benefited
from the comments of John Ronan, Yoshifumi Nishida, Phil Eardley and from the comments of John Ronan, Yoshifumi Nishida, Phil Eardley and
Jaehyun Hwang. Jaehyun Hwang.
10. Informative References 6. Informative References
[Apple-MPTCP]
Apple, Inc, ., "iOS - Multipath TCP Support in iOS 7",
n.d., <https://support.apple.com/en-us/HT201373>.
[BBF-WT348] [BBF-WT348]
Fabregas (Ed), G., "WT-348 - Hybrid Access for Broadband Fabregas (Ed), G., "WT-348 - Hybrid Access for Broadband
Networks", Broadband Forum, contribution bbf2014.1139.04 , Networks", Broadband Forum, contribution bbf2014.1139.04 ,
June 2015. June 2015.
[CACM14] Paasch, C. and O. Bonaventure, "Multipath TCP", [CACM14] Paasch, C. and O. Bonaventure, "Multipath TCP",
Communications of the ACM, 57(4):51-57 , April 2014, Communications of the ACM, 57(4):51-57 , April 2014,
<http://inl.info.ucl.ac.be/publications/multipath-tcp>. <http://inl.info.ucl.ac.be/publications/multipath-tcp>.
skipping to change at page 19, line 31 skipping to change at page 22, line 5
(Cellnet12) , 2012, (Cellnet12) , 2012,
<http://inl.info.ucl.ac.be/publications/ <http://inl.info.ucl.ac.be/publications/
exploring-mobilewifi-handover-multipath-tcp>. exploring-mobilewifi-handover-multipath-tcp>.
[DetalMSS] [DetalMSS]
Detal, G., "Adaptive MSS value", Post on the mptcp-dev Detal, G., "Adaptive MSS value", Post on the mptcp-dev
mailing list , September 2014, <https://listes- mailing list , September 2014, <https://listes-
2.sipr.ucl.ac.be/sympa/arc/mptcp-dev/2014-09/ 2.sipr.ucl.ac.be/sympa/arc/mptcp-dev/2014-09/
msg00130.html>. msg00130.html>.
[FreeBSD-MPTCP]
Williams, N., "Multipath TCP For FreeBSD Kernel Patch
v0.5", n.d., <http://caia.swin.edu.au/urp/newtcp/mptcp>.
[HotMiddlebox13] [HotMiddlebox13]
Hesmans, B., Duchene, F., Paasch, C., Detal, G., and O. Hesmans, B., Duchene, F., Paasch, C., Detal, G., and O.
Bonaventure, "Are TCP Extensions Middlebox-proof?", CoNEXT Bonaventure, "Are TCP Extensions Middlebox-proof?", CoNEXT
workshop HotMiddlebox , December 2013, workshop HotMiddlebox , December 2013,
<http://inl.info.ucl.ac.be/publications/ <http://inl.info.ucl.ac.be/publications/
are-tcp-extensions-middlebox-proof>. are-tcp-extensions-middlebox-proof>.
[HotMiddlebox13b] [HotMiddlebox13b]
Detal, G., Paasch, C., and O. Bonaventure, "Multipath in Detal, G., Paasch, C., and O. Bonaventure, "Multipath in
the Middle(Box)", HotMiddlebox'13 , December 2013, the Middle(Box)", HotMiddlebox'13 , December 2013,
skipping to change at page 20, line 21 skipping to change at page 22, line 44
Lingli, D., Liu, D., Sun, T., Boucadair, M., and G. Lingli, D., Liu, D., Sun, T., Boucadair, M., and G.
Cauchie, "Use-cases and Requirements for MPTCP Proxy in Cauchie, "Use-cases and Requirements for MPTCP Proxy in
ISP Networks", draft-deng-mptcp-proxy-01 (work in ISP Networks", draft-deng-mptcp-proxy-01 (work in
progress), October 2014. progress), October 2014.
[I-D.eardley-mptcp-implementations-survey] [I-D.eardley-mptcp-implementations-survey]
Eardley, P., "Survey of MPTCP Implementations", draft- Eardley, P., "Survey of MPTCP Implementations", draft-
eardley-mptcp-implementations-survey-02 (work in eardley-mptcp-implementations-survey-02 (work in
progress), July 2013. progress), July 2013.
[I-D.hampel-mptcp-proxies-anchors]
Hampel, G. and T. Klein, "MPTCP Proxies and Anchors",
draft-hampel-mptcp-proxies-anchors-00 (work in progress),
February 2012.
[I-D.ietf-dnsop-edns-client-subnet]
Contavalli, C., Gaast, W., Lawrence, D., and W. Kumari,
"Client Subnet in DNS Queries", draft-ietf-dnsop-edns-
client-subnet-04 (work in progress), September 2015.
[I-D.lhwxz-gre-notifications-hybrid-access] [I-D.lhwxz-gre-notifications-hybrid-access]
Leymann, N., Heidemann, C., Wasserman, M., Xue, L., and M. Leymann, N., Heidemann, C., Wasserman, M., Xue, L., and M.
Zhang, "GRE Notifications for Hybrid Access", draft-lhwxz- Zhang, "GRE Notifications for Hybrid Access", draft-lhwxz-
gre-notifications-hybrid-access-01 (work in progress), gre-notifications-hybrid-access-01 (work in progress),
January 2015. January 2015.
[I-D.lhwxz-hybrid-access-network-architecture] [I-D.lhwxz-hybrid-access-network-architecture]
Leymann, N., Heidemann, C., Wasserman, M., Xue, L., and M. Leymann, N., Heidemann, C., Wasserman, M., Xue, L., and M.
Zhang, "Hybrid Access Network Architecture", draft-lhwxz- Zhang, "Hybrid Access Network Architecture", draft-lhwxz-
hybrid-access-network-architecture-02 (work in progress), hybrid-access-network-architecture-02 (work in progress),
January 2015. January 2015.
[I-D.vandergaast-edns-client-subnet] [I-D.paasch-mptcp-loadbalancer]
Contavalli, C., Gaast, W., Leach, S., and E. Lewis, Paasch, C., Greenway, G., and A. Ford, "Multipath TCP
"Client Subnet in DNS Requests", draft-vandergaast-edns- behind Layer-4 loadbalancers", draft-paasch-mptcp-
client-subnet-02 (work in progress), July 2013. 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.
[I-D.walid-mptcp-congestion-control] [I-D.walid-mptcp-congestion-control]
Walid, A., Peng, Q., Hwang, J., and S. Low, "Balanced Walid, A., Peng, Q., Hwang, J., and S. Low, "Balanced
Linked Adaptation Congestion Control Algorithm for MPTCP", Linked Adaptation Congestion Control Algorithm for MPTCP",
draft-walid-mptcp-congestion-control-02 (work in draft-walid-mptcp-congestion-control-03 (work in
progress), January 2015. progress), July 2015.
[I-D.wei-mptcp-proxy-mechanism] [I-D.wei-mptcp-proxy-mechanism]
Wei, X., Xiong, C., and E. Ed, "MPTCP proxy mechanisms", Wei, X., Xiong, C., and E. Ed, "MPTCP proxy mechanisms",
draft-wei-mptcp-proxy-mechanism-02 (work in progress), draft-wei-mptcp-proxy-mechanism-02 (work in progress),
June 2015. June 2015.
[ICNP12] Cao, Y., Xu, M., and X. Fu, "Delay-based congestion [ICNP12] Cao, Y., Xu, M., and X. Fu, "Delay-based congestion
control for multipath TCP", 20th IEEE International control for multipath TCP", 20th IEEE International
Conference on Network Protocols (ICNP) , 2012. Conference on Network Protocols (ICNP) , 2012.
skipping to change at page 21, line 38 skipping to change at page 24, line 32
<http://doi.acm.org/10.1145/2504730.2504765>. <http://doi.acm.org/10.1145/2504730.2504765>.
[INFOCOM14] [INFOCOM14]
Lim, Y., Chen, Y., Nahum, E., Towsley, D., and K. Lee, Lim, Y., Chen, Y., Nahum, E., Towsley, D., and K. Lee,
"Cross-Layer Path Management in Multi-path Transport "Cross-Layer Path Management in Multi-path Transport
Protocol for Mobile Devices", IEEE INFOCOM'14 , 2014. Protocol for Mobile Devices", IEEE INFOCOM'14 , 2014.
[IOS7] "Multipath TCP Support in iOS 7", January 2014, [IOS7] "Multipath TCP Support in iOS 7", January 2014,
<http://support.apple.com/kb/HT5977>. <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, [MBTest] Hesmans, B., "MBTest", 2013,
<https://bitbucket.org/bhesmans/mbtest>. <https://bitbucket.org/bhesmans/mbtest>.
[MPTCPBIB] [MPTCPBIB]
Bonaventure, O., "Multipath TCP - An annotated Bonaventure, O., "Multipath TCP - An annotated
bibliography", Technical report , April 2015, bibliography", Technical report , April 2015,
<https://github.com/obonaventure/mptcp-bib>. <https://github.com/obonaventure/mptcp-bib>.
[Mobicom15] [Mobicom15]
De Coninck, Q., Baerts, M., Hesmans, B., and O. De Coninck, Q., Baerts, M., Hesmans, B., and O.
skipping to change at page 22, line 34 skipping to change at page 25, line 34
[PAMS2014] [PAMS2014]
Arzani, B., Gurney, A., Cheng, S., Guerin, R., and B. Loo, Arzani, B., Gurney, A., Cheng, S., Guerin, R., and B. Loo,
"Impact of Path Selection and Scheduling Policies on MPTCP "Impact of Path Selection and Scheduling Policies on MPTCP
Performance", PAMS2014 , 2014. Performance", PAMS2014 , 2014.
[PaaschPhD] [PaaschPhD]
Paasch, C., "Improving Multipath TCP", Ph.D. Thesis , Paasch, C., "Improving Multipath TCP", Ph.D. Thesis ,
November 2014, <http://inl.info.ucl.ac.be/publications/ November 2014, <http://inl.info.ucl.ac.be/publications/
improving-multipath-tcp>. improving-multipath-tcp>.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC [Presto08]
1812, June 1995. 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 [RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928, March L. Jones, "SOCKS Protocol Version 5", RFC 1928, DOI
1996. 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>.
[RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. [RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
Iyengar, "Architectural Guidelines for Multipath TCP Iyengar, "Architectural Guidelines for Multipath TCP
Development", RFC 6182, March 2011. 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 [RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols", RFC Congestion Control for Multipath Transport Protocols", RFC
6356, October 2011. 6356, DOI 10.17487/RFC6356, October 2011,
<http://www.rfc-editor.org/info/rfc6356>.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple "TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, January 2013. Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<http://www.rfc-editor.org/info/rfc6824>.
[SIGCOMM11] [SIGCOMM11]
Raiciu, C., Barre, S., Pluntke, C., Greenhalgh, A., Raiciu, C., Barre, S., Pluntke, C., Greenhalgh, A.,
Wischik, D., and M. Handley, "Improving datacenter Wischik, D., and M. Handley, "Improving datacenter
performance and robustness with multipath TCP", performance and robustness with multipath TCP",
Proceedings of the ACM SIGCOMM 2011 conference , n.d., Proceedings of the ACM SIGCOMM 2011 conference , n.d.,
<http://doi.acm.org/10.1145/2018436.2018467>. <http://doi.acm.org/10.1145/2018436.2018467>.
[StrangeMbox] [StrangeMbox]
Bonaventure, O., "Multipath TCP through a strange Bonaventure, O., "Multipath TCP through a strange
middlebox", Blog post , January 2015, middlebox", Blog post , January 2015,
<http://blog.multipath-tcp.org/blog/html/2015/01/30/ <http://blog.multipath-tcp.org/blog/html/2015/01/30/
multipath_tcp_through_a_strange_middlebox.html>. 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/
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] [tracebox]
Detal, G., "tracebox", 2013, <http://www.tracebox.org>. Detal, G. and O. Tilmans, "tracebox", 2013,
<http://www.tracebox.org>.
Appendix A. Changelog Appendix A. Changelog
o initial version : September 16th, 2014 : Added section Section 5 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 that discusses some performance problems that appeared with the
Linux implementation when using subflows having different MSS Linux implementation when using subflows having different MSS
values values
o update with a description of the middlebox that replaces an o update with a description of the middlebox that replaces an
unknown TCP option with EOL [StrangeMbox] unknown TCP option with EOL [StrangeMbox]
o version ietf-02 : July 2015, answer to last call comments o version ietf-02 : July 2015, answer to last call comments
* Reorganised text to better separate use cases and operational * Reorganised text to better separate use cases and operational
experience experience
* New use case on Multipath TCP proxies in Section 2.3 * New use case on Multipath TCP proxies in Section 2.3
* Added some text on middleboxes in Section 3.1 * Added some text on middleboxes in Section 3.1
* Removed the discussion on SDN * Removed the discussion on SDN
* Restructured text and improved writing in some parts * 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
Authors' Addresses Authors' Addresses
Olivier Bonaventure Olivier Bonaventure
UCLouvain UCLouvain
Email: Olivier.Bonaventure@uclouvain.be Email: Olivier.Bonaventure@uclouvain.be
Christoph Paasch
UCLouvain
Email: Christoph.Paasch@gmail.com Christoph Paasch
Apple, Inc.
Email: cpaasch@apple.com
Gregory Detal Gregory Detal
UCLouvain and Tessares UCLouvain and Tessares
Email: Gregory.Detal@tessares.net Email: Gregory.Detal@tessares.net
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