Internet DRAFT - draft-bonaventure-mptcp-experience
draft-bonaventure-mptcp-experience
MPTCP Working Group O. Bonaventure
Internet-Draft C. Paasch
Intended status: Informational UCLouvain
Expires: January 2, 2015 July 01, 2014
Experience with Multipath TCP
draft-bonaventure-mptcp-experience-00
Abstract
This document discusses operational experiences of using Multipath
TCP in real world networks. It lists several prominent use cases for
which Multipath TCP has been considered and is being used. It also
gives insight in some heuristics and decisions that have helped to
realize these use cases. Further, it presents several open issues
that are yet unclear on how they can be solved.
Status of This Memo
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This Internet-Draft will expire on January 2, 2015.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Middlebox interference . . . . . . . . . . . . . . . . . . . 3
3. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Congestion control . . . . . . . . . . . . . . . . . . . . . 8
5. Subflow management . . . . . . . . . . . . . . . . . . . . . 8
5.1. Implemented subflow managers . . . . . . . . . . . . . . 9
5.2. Subflow destination port . . . . . . . . . . . . . . . . 11
5.3. Closing subflows . . . . . . . . . . . . . . . . . . . . 12
6. Packet schedulers . . . . . . . . . . . . . . . . . . . . . . 13
7. Interactions with the Domain Name System . . . . . . . . . . 14
8. Captive portals . . . . . . . . . . . . . . . . . . . . . . . 15
9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
11. Informative References . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Multipath TCP was standardized in [RFC6824] and four implementations
have been developed [I-D.eardley-mptcp-implementations-survey].
Since the publication of [RFC6824], some experience has been gathered
by various network researchers and users about the issues that arise
when Multipath TCP is used in the Internet.
Most of the experience reported in this document comes from the
utilization of the Multipath TCP implementation in the Linux kernel
[MultipathTCP-Linux]. It has been downloaded and is used by
thousands of users all over the world. Many of these users have
provided direct or indirect feedback by writing documents (scientific
articles or blog messages) or posting to the mptcp-dev mailing list (
https://listes-2.sipr.ucl.ac.be/sympa/arc/mptcp-dev ) . This
Multipath TCP implementation is actively maintained and continuously
improved. It is used on various types of hosts, ranging from
smartphones or embedded systems to high-end servers.
This is not, by far, the most widespread deployment of Multipath TCP.
Since September 2013, Multipath TCP is also supported on smartphones
and tablets running iOS7 [IOS7]. There are likely hundreds of
millions of Multipath TCP enabled devices. However, this particular
Multipath TCP implementation is currently only used to support a
single application. Unfortunately, there is no public information
about the lessons learned from this large scale deployment.
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This document is organized as follows. We explain in Section 2 which
types of middleboxes the Linux Kernel implementation of Multipath TCP
supports and how it reacts upon encountering these. Next, we list
several use cases of Multipath TCP in Section 3. Section 4
summarizes the MPTCP specific congestion controls that have been
implemented. Section 5 and 6 discuss heuristics and issues with
respect to subflow management as well as the scheduling across the
subflows. Section 7 presents issues with respect to content delivery
networks and suggests a solution to this issue. Finally, Section 8
shows an issue with captive portals where MPTCP will behave
suboptimal.
2. Middlebox interference
The interference caused by various types of middleboxes has been an
important concern during the design of the Multipath TCP protocol.
Three studies on the interactions between Multipath TCP and
middleboxes are worth being discussed.
The first analysis was described in [IMC11]. This paper was the main
motivation for including inside Multipath TCP various techniques to
cope with middlebox interference. More specifically, Multipath TCP
has been designed to cope with middleboxes that : - change source or
destination addresses - change source or destination port numbers -
change TCP sequence numbers - split or coalesce segments - remove TCP
options - modify the payload of TCP segments
These middlebox interferences have all been included in the MBtest
suite [MBTest]. This test suite has been used [HotMiddlebox13] to
verify the reaction of the Multipath TCP implementation in the Linux
kernel when faced with middlebox interference. The test environment
used for this evaluation is a dual-homed client connected to a
single-homed server. The middlebox behavior can be activated on any
of the paths. The main results of this analysis are :
o the Multipath TCP implementation in the Linux kernel is not
affected by a middlebox that performs NAT or modifies TCP sequence
numbers
o when a middlebox removes the MP_CAPABLE option from the initial
SYN segment, the Multipath TCP implementation in the Linux kernel
falls back correctly to regular TCP
o when a middlebox removes the DSS option from all data segments,
the Multipath TCP implementation in the Linux kernel falls back
correctly to regular TCP
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o when a middlebox performs segment coalescing, the Multipath TCP
implementation in the Linux kernel is still able to accurately
extract the data corresponding to the indicated mapping
o when a middlebox performs segment splitting, the Multipath TCP
implementation in the Linux kernel correctly reassembles the data
corresponding to the indicated mapping. [HotMiddlebox13]
documents a corner case with segment splitting that may lead to
desynchronisation between the two hosts.
The interactions between Multipath TCP and real deployed middleboxes
is also analyzed in [HotMiddlebox13] and a particular scenario with
the FTP application level gateway running on a NAT is described.
From an operational viewpoint, knowing that Multipath TCP can cope
with various types of middlebox interference is important. However,
there are situations where the network operators need to gather
information about where a particular middlebox interference occurs.
The tracebox software [tracebox] described in [IMC13a] is an
extension of the popular traceroute software that enables network
operators to check at which hop a particular field of the TCP header
(including options) is modified. It has been used by several network
operators to debug various middlebox interference problems. tracebox
includes a scripting language that enables its user to specify
precisely which packet is sent by the source. tracebox sends packets
with an increasing TTL/HopLimit and compares the information returned
in the ICMP messages with the packet that it sends. This enables
tracebox to detect any interference caused by middleboxes on a given
path. tracebox works better when routers implement the ICMP extension
defined in [RFC1812].
3. Use cases
Multipath TCP has been tested in several use cases. Several of the
papers published in the scientific litterature have identified
possible improvements that are worth being discussed here.
A first, although initially unexpected, documented use case for
Multipath TCP has been the datacenters [HotNets][SIGCOMM11]. Today's
datacenters are designed to provide several paths between single-
homed servers. The multiplicity of these paths comes from the
utilization of Equal Cost Multipath (ECMP) and other load balancing
techniques inside the datacenter. Most of the deployed load
balancing techniques in these datacenters rely on hashes computed or
the five tuple to ensure that all packets from the same TCP
connection will follow the same path to prevent packet reordering.
The results presented in [HotNets] demonstrate by simulations that
Multipath TCP can achieve a better utilization of the available
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network by using multiple subflows for each Multipath TCP session.
Although [RFC6182] assumes that at least one of the communicating
hosts has several IP addresses, [HotNets] demonstrates that there are
also benefits when both hosts are single-homed. This idea was
pursued further in [SIGCOMM11] where the Multipath TCP implementation
in the Linux kernel was modified to be able to use several subflows
from the same IP address. Measurements performed in a public
datacenter showed performance improvements with Multipath TCP.
Although ECMP is widely used inside datacenters, this is not the only
environment where there are different paths between a pair of hosts.
ECMP and other load balancing techniques such as LAG are widely used
in today's network and having multiple paths between a pair of
single-homed hosts is becoming the norm instead of the exception.
Although these multiple paths have often the same cost (from an IGP
metrics viewpoint), they do not necessarily have the same
performance. For example, [IMC13c] reports the results of a long
measurement study showing that load balanced Internet paths between
that same pair of hosts can have huge delay differences.
A second use case that has been explored by several network
researchers is the cellular/WiFi offload use case. Smartphones or
other mobile devices equipped with two wireless interfaces are a very
common use case for Multipath TCP. As of this writing, this is also
the largest deployment of Multipath-TCP enabled devices [IOS7].
Unfortunately, as there are no public measurements about this
deployment, we can only rely on published papers that have mainly
used the Multipath TCP implementation in the Linux kernel for their
experiment.
The performance of Multipath TCP in wireless networks was briefly
evaluated in [NSDI12]. One experiment analyzes the performance of
Multipath TCP on a client with two wireless interfaces. This
evaluation shows that when the receive window is large, Multipath TCP
can efficiently use the two available links. However, if the window
becomes smaller, then packets sent on a slow path can block the
transmission of packets on a faster path. In some cases, the
performance of Multipath TCP over two paths can become lower than the
performance of regular TCP over the best performing path. Two
heuristics, reinjection and penalization, are proposed in [NSDI12] to
solve this identified performance problem. These two heuristics have
since been used in the Multipath TCP implementation in the Linux
kernel. [CONEXT13] explored the problem in more details and revealed
some other scenarios where Multipath TCP can have difficulties in
efficiently pooling the available paths. Improvements to the
Multipath TCP implementation in the Linux kernel are proposed in
[CONEXT13] to cope with some of these problems.
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The first experimental analysis of Multipath TCP in a public wireless
environment was presented in [Cellnet12]. These measurements explore
the ability of Multipath TCP to use two wireless networks (real WiFi
and 3G networks). Three modes of operation are compared. The first
mode of operation is the simultaneous use of the two wireless
networks. In this mode, Multipath TCP pools the available resources
and uses both wireless interfaces. This mode provides fast handover
from WiFi to cellular or the opposite when the user moves.
Measurements presented in [CACM14] show that the handover from one
wireless network to another is not an abrupt process. When a host
moves, it does not experience either excellent connectivity or no
connectivity at all. Instead, there are regions where the quality of
one of the wireless networks is weaker than the other, but the host
considers this wireless network to still be up. When a mobile host
enters such regions, its ability to send packets over another
wireless network is important to ensure a smooth handover. This is
clearly illustrated from the packet trace discussed in [CACM14].
Many cellular networks use volume-based pricing and users often
prefer to use unmetered WiFi networks when available instead of
metered cellular networks. [Cellnet12] implements the support for
the MP_PRIO option to explore two other modes of operation.
In the backup mode, Multipath TCP opens a TCP subflow over each
interface, but the cellular interface is configured in backup mode.
This implies that data only flows over the WiFi interface when both
interfaces are considered to be active. If the WiFi interface fails,
then the traffic switches quickly to the cellular interface, ensuring
a smooth handover from the user's viewpoint [Cellnet12]. The cost of
this approach is that the WiFi and cellular interfaces likely remain
active all the time since all subflows are established over the two
interfaces.
The single-path mode is slightly different. This mode benefits from
the break-before-make capability of Multipath TCP. When an MPTCP
session is established, a subflow is created over the WiFi interface.
No packet is sent over the cellular interface as long as the WiFi
interface remains up [Cellnet12]. This implies that the cellular
interface can remain idle and battery capacity is preserved. When
the WiFi interface fails, new subflows are established over the
cellular interface in order to preserve the established Multipath TCP
sessions. Compared to the backup mode described earlier, this mode
of operation is characterized by a throughput drop while the cellular
interface is brought up and the subflows are reestablished. During
this time, no data packet is transmitted.
From a protocol viewpoint, [Cellnet12] discusses the problem posed by
the unreliability of the ADD_ADDR option and proposes a small
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protocol extension to allow hosts to reliably exchange this option.
It would be useful to analyze packet traces to understand whether the
unreliability of the REMOVE_ADDR option poses an operational problem
in real deployments.
Another study of the performance of Multipath TCP in wireless
networks was reported in [IMC13b]. This study uses laptops connected
to various cellular ISPs and WiFi hotspots. It compares various file
transfer scenarios and concludes based on measurements with the
Multipath TCP implementation in the Linux kernel that "MPTCP provides
a robust data transport and reduces variations in download
latencies".
A different study of the performance of Multipath TCP with two
wireless networks is presented in [INFOCOM14]. In this study the two
networks had different qualities : a good network and a lossy
network. When using two paths with different packet loss ratios, the
Multipath TCP congestion control scheme moves traffic away from the
lossy link that is considered to be congested. However, [INFOCOM14]
documents an interesting scenario that is summarised in the figure
below.
client ----------- path1 -------- server
| |
+--------------- path2 ------------+
Figure 1: Simple network topology
Initially, the two paths have the same quality and Multipath TCP
distributes the load over both of them. During the transfer, the
second path becomes lossy, e.g. because the client moves. Multipath
TCP detects the packet losses and they are retransmitted over the
first path. This enables the data transfer to continue over the
first path. However, the subflow over the second path is still up
and transmits one packet from time to time. Although the N packets
have been acknowledged over the first subflow (at the MPTCP level),
they have not been acknowledged at the TCP level over the second
subflow. To preserve the continuity of the sequence numbers over the
second subflow, TCP will continue to retransmit these segments until
either they are acknowledged or the maximum number of retransmissions
is reached. This behavior is clearly inefficient and may lead to
blocking since the second subflow will consume window space to be
able to retransmit these packets. [INFOCOM14] proposes a new
Multipath TCP option to solve this problem. In practice, a new TCP
option is probably not required. When the client detects that the
data transmitted over the second subflow has been acknowledged over
the first subflow, it could decide to terminate the second subflow by
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sending a RST segment. If the interface associated to this subflow
is still up, a new subflow could be immediately reestablished. It
would then be immediately usable to send new data and would not be
forced to first retransmit the previously transmitted data. As of
this writing, this dynamic management of the subflows is not yet
implemented in the Multipath TCP implementation in the Linux kernel.
A third use case has been the coupling between software defined
networking techniques such as Openflow and Multipath TCP. Openflow
can be used to configure different paths inside a network. Using an
international network, [TNC13] demonstrates that Multipath TCP can
achieve high throughput in the wide area. An interesting point to
note about the measurements reported in [TNC13] is that the
measurement setup used four paths through the WAN. Only two of these
paths were disjoint. When Multipath TCP was used, the congestion
control scheme ensured that only two of these paths were actually
used.
4. Congestion control
Congestion control has been an important problem for Multipath TCP.
The standardised congestion control scheme for Multipath TCP is
defined in [RFC6356] and [NSDI11]. This congestion control scheme
has been implemented in the Linux implementation of Multipath TCP.
Linux uses a modular architecture to support various congestion
control schemes. This architecture is applicable for both regular
TCP and Multipath TCP. While the coupled congestion control scheme
defined in [RFC6356] is the default congestion control scheme in the
Linux implementation, other congestion control schemes have been
added. The second congestion control scheme is OLIA [CONEXT12].
This congestion control scheme is also an adaptation of the NewReno
single path congestion control scheme to support multiple paths.
Simulations and measurements have shown that it provides some
performance benefits compared to the the default congestion control
scheme [CONEXT12]. Measurement over a wide range of parameters
reported in [CONEXT13] also indicate some benefits with the OLIA
congestion control scheme. Recently, a delay-based congestion
control scheme has been ported to the Multipath TCP implementation in
the Linux kernel. This congestion control scheme has been evaluated
by using simulations in [ICNP12]. As of this writing, it has not yet
been evaluated by performing large measurement campaigns.
5. Subflow management
The multipath capability of Multipath TCP comes from the utilization
of one subflow per path. The Multipath TCP architecture [RFC6182]
and the protocol specification [RFC6824] define the basic usage of
the subflows and the protocol mechanisms that are required to create
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and terminate them. However, there are no guidelines on how subflows
are used during the lifetime of a Multipath TCP session. Most of the
experiments with Multipath TCP have been performed in controlled
environments. Still, based on the experience running them and
discussions on the mptcp-dev mailing list, interesting lessons have
been learned about the management of these subflows.
From a subflow viewpoint, the Multipath TCP protocol is completely
symmetrical. Both the clients and the server have the capability to
create subflows. However in practice the existing Multipath TCP
implementations [I-D.eardley-mptcp-implementations-survey] have opted
for a strategy where only the client creates new subflows. The main
motivation for this strategy is that often the client resides behind
a NAT or a firewall, preventing passive subflow openings on the
client. Although there are environments such as datacenters where
this problem does not occur, as of this writing, no precise
requirement has emerged for allowing the server to create new
subflows.
5.1. Implemented subflow managers
The Multipath TCP implementation in the Linux kernel includes several
strategies to manage the subflows that compose a Multipath TCP
session. The basic subflow manager is the full-mesh. As the name
implies, it creates a full-mesh of subflows between the communicating
hosts.
The most frequent use case for this subflow manager is a multihomed
client connected to a single-homed server. In this case, one subflow
is created for each interface on the client. The current
implementation of the full-mesh subflow manager is static. The
subflows are created immediately after the creation of the initial
subflow. If one subflow fails during the lifetime of the Multipath
TCP session (e.g. due to excessive retransmissions, or the loss of
the corresponding interface), it is not always reestablished. There
is ongoing work to enhance the full-mesh path manager to deal with
such events.
When the server is multihomed, using the full-mesh subflow manager
may lead to a large number of subflows being established. For
example, consider a dual-homed client connected to a server with
three interfaces. In this case, even if the subflows are only
created by the client, 6 subflows will be established. This may be
excessive in some environments, in particular when the client and/or
the server have a large number of interfaces. It should be noted
that there have been reports on the mptcp-dev mailing indicating that
users rely on Multipath TCP to aggregate more than four different
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interfaces. Thus, there is a need for supporting many interfaces
efficiently.
It should be noted that creating subflows between multihomed clients
and servers may sometimes lead to operational issues as observed by
discussions on the mptcp-dev mailing list. In some cases the network
operators would like to have a better control on how the subflows are
created by Multipath TCP. This might require the definition of
policy rules to control the operation of the subflow manager. The
two scenarios below illustrate some of these requirements.
host1 ---------- switch1 ----- host2
| | |
+-------------- switch2 --------+
Figure 2: Simple switched network topology
Consider the simple network topology shown in Figure 2. From an
operational viewpoint, a network operator could want to create two
subflows between the communicating hosts. From a bandwidth
utilization viewpoint, the most natural paths are host1-switch1-host2
and host1-switch2-host2. However, a Multipath TCP implementation
running on these two hosts may sometimes have difficulties to achieve
this result.
To understand the difficulty, let us consider different allocation
strategies for the IP addresses. A first strategy is to assign two
subnets : subnetA (resp. subnetB) contains the IP addresses of
host1's interface to switch1 (resp. switch2) and host2's interface to
switch1 (resp. switch2). In this case, a Multipath TCP subflow
manager should only create one subflow per subnet. To enforce the
utilization of these paths, the network operator would have to
specify a policy that prefers the subflows in the same subnet over
subflows between addresses in different subnets. It should be noted
that the policy should probably also specify how the subflow manager
should react when an interface or subflow fails.
A second strategy is to use a single subnet for all IP addresses. In
this case, it becomes more difficult to specify a policy that
indicates which subflows should be established.
The second subflow manager that is currently supported by the
Multipath TCP implementation in the Linux kernel is the ndiffport
subflow manager. This manager was initially created to exploit the
path diversity that exists between single-homed hosts due to the
utilization of flow-based load balancing techniques. This subflow
manager creates N subflows between the same pair of IP addresses.
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The N subflows are created by the client and differ only in the
source port selected by the client.
5.2. Subflow destination port
The Multipath TCP protocol relies on the token contained in the
MP_JOIN option to associate a subflow to an existing Multipath TCP
session. This implies that there is no restriction on the source
address, destination address and source or destination ports used for
the new subflow. The ability to use different source and destination
addresses is key to support multihomed servers and clients. The
ability to use different destination port numbers is worth being
discussed because it has operational implications.
For illustration, consider a dual-homed client that creates a second
subflow to reach a single-homed server as illustrated in the
Figure 3.
client ------- r1 --- internet --- server
| |
+----------r2-------+
Figure 3: Multihomed-client connected to single-homed server
When the Multipath TCP implementation in the Linux kernel creates the
second subflow it uses the same destination port as the initial
subflow. This choice is motivated by the fact that the server might
be protected by a firewall and only accept TCP connections (including
subflows) on the official port number. Using the same destination
port for all subflows is also useful for operators that rely on the
port numbers to track application usage in their network.
There have been suggestions from Multipath TCP users to modify the
implementation to allow the client to use different destination ports
to reach the server. This suggestion seems mainly motivated by
traffic shaping middleboxes that are used in some wireless networks.
In networks where different shaping rates are associated to different
destination port numbers, this could allow Multipath TCP to reach a
higher performance. As of this writing, we are not aware of any
implementation of this kind of tweaking.
However, from an implementation point-of-view supporting different
destination ports for the same Multipath TCP connection introduces a
new performance issue. A legacy implementation of a TCP stack
creates a listening socket to react upon incoming SYN segments. The
listening socket is handling the SYN segments that are sent on a
specific port number. Demultiplexing incoming segments can thus be
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done solely by looking at the IP addresses and the port numbers.
With Multipath TCP however, incoming SYN segments may have an MP_JOIN
option with a different destination port. This means, that all
incoming segments that did not match on an existing listening-socket
or an already established socket must be parsed for an eventual
MP_JOIN option. This imposes an additional cost on servers,
previously not existent on legacy TCP implementations.
5.3. Closing subflows
client server
| |
MPTCP: established | | MPTCP: established
Sub: established | | Sub: established
| |
| DATA_FIN |
MPTCP: close-wait | <------------------------ | close() (step 1)
Sub: established | DATA_ACK |
| ------------------------> | MPTCP: fin-wait-2
| | Sub: established
| |
| DATA_FIN + subflow-FIN |
close()/shutdown() | ------------------------> | MPTCP: time-wait
(step 2) | DATA_ACK | Sub: close-wait
MPTCP: closed | <------------------------ |
Sub: fin-wait-2 | |
| |
| subflow-FIN |
MPTCP: closed | <------------------------ | subflow-close()
Sub: time-wait | subflow-ACK |
(step 3) | ------------------------> | MPTCP: time-wait
| | Sub: closed
| |
Figure 4: Multipath TCP may not be able to avoid time-wait state
(even if enforced by the application).
Figure 4 shows a very particular issue within Multipath TCP. Many
high-performance applications try to avoid Time-Wait state by
deferring the closure of the connection until the peer has sent a
FIN. That way, the client on the left of Figure 4 does a passive
closure of the connection, transitioning from Close-Wait to Last-ACK
and finally freeing the resources after reception of the ACK of the
FIN. An application running on top of a Multipath TCP enabled Linux
kernel might also use this approach. The difference here is that the
close() of the connection (Step 1 in Figure 4) only triggers the
sending of a DATA_FIN. Nothing guarantees that the kernel is ready
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to combine the DATA_FIN with a subflow-FIN. The reception 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
closure. This time, the kernel might decide to combine the DATA_FIN
with a subflow-FIN. This decision will be fatal, as the subflow's
state machine will not transition from Close-Wait to Last-Ack, but
rather go through Fin-Wait-2 into Time-Wait state. The Time-Wait
state will consume resources on the host for at least 2 MSL (Maximum
Segment Lifetime). Thus, a smart application, that tries to avoid
Time-Wait state by doing late closure of the connection actually ends
up with one of its subflows in Time-Wait state. A high-performance
Multipath TCP kernel implementation should honor the desire of the
application to do passive closure of the connection and successfully
avoid Time-Wait state - even on the subflows.
The solution to this problem lies in an optimistic assumption that a
host doing active-closure of a Multipath TCP connection by sending a
DATA_FIN will soon also send a FIN on all its in subflows. Thus, the
passive closer of the connection can simply wait for the peer to send
exactly this FIN - enforcing passive closure even on the subflows.
Of course, to avoid consuming resources indefinitely, a timer must
limit the time our implementation waits for the FIN.
6. Packet schedulers
In a Multipath TCP implementation, the packet scheduler is the
algorithm that is executed when transmitting each packet to decide on
which subflow it needs to be transmitted. The packet scheduler
itself does not have any impact on the interoperability of Multipath
TCP implementations. However, it may clearly impact the performance
of Multipath TCP sessions. It is important to note that the problem
of scheduling Multipath TCP packets among subflows is different from
the problem of scheduling SCTP messages. SCTP implementations also
include schedulers, but these are used to schedule the different
streams. Multipath TCP uses a single data stream.
Various researchers have explored theoretically and by simulations
the problem of scheduling packets among Multipath TCP subflows
[ICC14]. Unfortunately, none of the proposed techniques have been
implemented and used in real deployment. A detailed analysis of the
impact of the packet scheduler will appear in [CSWS14]. This article
proposes a pluggable architecture for the scheduler used by the
Multipath TCP implementation in the Linux kernel. This architecture
allows researchers to experiment with different types of schedulers.
Two schedulers are compared in [CSWS14] : round-robin and lowest-rtt-
first. The experiments and measurements described in [CSWS14] show
that the lowest-rtt-first scheduler appears to be the best compromise
from a performance viewpoint.
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Another study of the packet schedulers is presented in [PAMS2014].
This study relies on simulations with the Multipath TCP
implementation in the Linux kernel. The simulation scenarios
discussed in [PAMS2014] confirm the impact of the packet scheduler on
the performance of Multipath TCP.
7. Interactions with the Domain Name System
Multihomed clients such as smartphones could lead to operational
problems when interacting with the Domain Name System. When a
single-homed client performs a DNS query, it receives from its local
resolver the best answer for its request. If the client is
multihomed, the answer returned to the DNS query may vary with the
interface over which it has been sent.
cdn1
|
client -- cellular -- internet -- cdn3
| |
+----- wifi --------+
|
cdn2
Figure 5: Simple network topology
If the client sends a DNS query over the WiFi interface, the answer
will point to the cdn2 server while the same request sent over the
cellular interface will point to the cdn1 server. This might cause
problems for CDN providers that locate their servers inside ISP
networks and have contracts that specify that the CDN server will
only be accessed from within this particular ISP. Assume now that
both the client and the CDN servers support Multipath TCP. In this
case, a Multipath TCP session from cdn1 or cdn2 would potentially use
both the cellular network and the WiFi network. This would violate
the contract between the CDN provider 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.vandergaast-edns-client-subnet] could be used for this
purpose. When the client sends a DNS query from its WiFi interface,
it should also send the client subnet corresponding to the cellular
interface in this request. This would indicate to the resolver that
the answer should be valid for both the WiFi and the cellular
interfaces (e.g., the cdn3 server).
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8. Captive portals
Multipath TCP enables a host to use different interfaces to reach a
server. In theory, this should ensure connectivity when at least one
of the interfaces is active. In practice however, there are some
particular scenarios with captive portals that may cause operational
problems. The reference environment is the following :
client ----- network1
|
+------- internet ------------- server
Figure 6: Issue with captive portal
The client is attached to two networks : network1 that provides
limited connectivity and the entire Internet through the second
network interface. In practice, this scenario corresponds to an open
WiFi network with a captive portal for network1 and a cellular
service for the second interface. On many smartphones, the WiFi
interface is preferred over the cellular interface. If the
smartphone learns a default route via both interfaces, it will
typically prefer to use the WiFi interface to send its DNS request
and create the first subflow. This is not optimal with Multipath
TCP. A better approach would probably be to try a few attempts on
the WiFi interface and then try to use the second interface for the
initial subflow as well.
9. Conclusion
In this document, we have documented a few years of experience with
Multipath TCP. The information presented in this document was
gathered from scientific publications and discussions with various
users of the Multipath TCP implementation in the Linux kernel.
10. Acknowledgements
This work was partially supported by the FP7-Trilogy2 project. We
would like to thank all the implementers and users of the Multipath
TCP implementation in the Linux kernel.
11. Informative References
[CACM14] Paasch, C. and O. Bonaventure, "Multipath TCP",
Communications of the ACM, 57(4):51-57 , April 2014,
<http://inl.info.ucl.ac.be/publications/multipath-tcp>.
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[CONEXT12]
Khalili, R., Gast, N., Popovic, M., Upadhyay, U., and J.
Leboudec, "MPTCP is not pareto-optimal performance issues
and a possible solution", Proceedings of the 8th
international conference on Emerging networking
experiments and technologies (CoNEXT12) , 2012.
[CONEXT13]
Paasch, C., Khalili, R., and O. Bonaventure, "On the
Benefits of Applying Experimental Design to Improve
Multipath TCP", Conference on emerging Networking
EXperiments and Technologies (CoNEXT) , December 2013,
<http://inl.info.ucl.ac.be/publications/benefits-applying-
experimental-design-improve-multipath-tcp>.
[CSWS14] Paasch, C., Ferlin, S., Alay, O., and O. Bonaventure,
"Experimental Evaluation of Multipath TCP Schedulers",
SIGCOMM CSWS2014 workshop , August 2014.
[Cellnet12]
Paasch, C., Detal, G., Duchene, F., Raiciu, C., and O.
Bonaventure, "Exploring Mobile/WiFi Handover with
Multipath TCP", ACM SIGCOMM workshop on Cellular Networks
(Cellnet12) , 2012, <http://inl.info.ucl.ac.be/
publications/exploring-mobilewifi-handover-multipath-tcp>.
[HotMiddlebox13]
Hesmans, B., Duchene, F., Paasch, C., Detal, G., and O.
Bonaventure, "Are TCP Extensions Middlebox-proof?", CoNEXT
workshop HotMiddlebox , December 2013,
<http://inl.info.ucl.ac.be/publications/
are-tcp-extensions-middlebox-proof>.
[HotNets] Raiciu, C., Pluntke, C., Barre, S., Greenhalgh, A.,
Wischik, D., and M. Handley, "Data center networking with
multipath TCP", Proceedings of the 9th ACM SIGCOMM
Workshop on Hot Topics in Networks (Hotnets-IX) , 2010,
<http://doi.acm.org/10.1145/1868447.1868457>.
[I-D.eardley-mptcp-implementations-survey]
Eardley, P., "Survey of MPTCP Implementations", draft-
eardley-mptcp-implementations-survey-02 (work in
progress), July 2013.
[I-D.vandergaast-edns-client-subnet]
Contavalli, C., Gaast, W., Leach, S., and E. Lewis,
"Client Subnet in DNS Requests", draft-vandergaast-edns-
client-subnet-02 (work in progress), July 2013.
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[ICC14] Kuhn, N., Lochin, E., Mifdaoui, A., Sarwar, G., Mehani,
O., and R. Boreli, "DAPS Intelligent Delay-Aware Packet
Scheduling For Multipath Transport", IEEE ICC 2014 , 2014.
[ICNP12] Cao, Y., Xu, M., and X. Fu, "Delay-based congestion
control for multipath TCP", 20th IEEE International
Conference on Network Protocols (ICNP) , 2012.
[IMC11] Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and H. Tokuda, "Is it still possible to
extend TCP?", Proceedings of the 2011 ACM SIGCOMM
conference on Internet measurement conference (IMC '11) ,
2011, <http://doi.acm.org/10.1145/2068816.2068834>.
[IMC13a] Detal, G., Hesmans, B., Bonaventure, O., Vanaubel, Y., and
B. Donnet, "Revealing Middlebox Interference with
Tracebox", Proceedings of the 2013 ACM SIGCOMM conference
on Internet measurement conference , 2013,
<http://inl.info.ucl.ac.be/publications/
revealing-middlebox-interference-tracebox>.
[IMC13b] Chen, Y., Lim, Y., Gibbens, R., Nahum, E., Khalili, R.,
and D. Towsley, "A measurement-based study of MultiPath
TCP performance over wireless network", Proceedings of the
2013 conference on Internet measurement conference (IMC
'13) , n.d., <http://doi.acm.org/10.1145/2504730.2504751>.
[IMC13c] Pelsser, C., Cittadini, L., Vissicchio, S., and R. Bush,
"From Paris to Tokyo on the suitability of ping to measure
latency", Proceedings of the 2013 conference on Internet
measurement conference (IMC '13) , 2013,
<http://doi.acm.org/10.1145/2504730.2504765>.
[INFOCOM14]
Lim, Y., Chen, Y., Nahum, E., Towsley, D., and K. Lee,
"Cross-Layer Path Management in Multi-path Transport
Protocol for Mobile Devices", IEEE INFOCOM'14 , 2014.
[IOS7] "Multipath TCP Support in iOS 7", January 2014,
<http://support.apple.com/kb/HT5977>.
[MBTest] Hesmans, B., "MBTest", 2013, <https://bitbucket.org/
bhesmans/mbtest>.
[MultipathTCP-Linux]
Paasch, C., Barre, S., and . et al, "Multipath TCP
implementation in the Linux kernel", n.d.,
<http://www.multipath-tcp.org>.
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[NSDI11] Wischik, D., Raiciu, C., Greenhalgh, A., and M. Handley,
"Design, implementation and evaluation of congestion
control for Multipath TCP", In Proceedings of the 8th
USENIX conference on Networked systems design and
implementation (NSDI11) , 2011.
[NSDI12] Raiciu, C., Paasch, C., Barre, S., Ford, A., Honda, M.,
Duchene, F., Bonaventure, O., and M. Handley, "How Hard
Can It Be? Designing and Implementing a Deployable
Multipath TCP", USENIX Symposium of Networked Systems
Design and Implementation (NSDI12) , April 2012,
<http://inl.info.ucl.ac.be/publications/how-hard-can-it-
be-designing-and-implementing-deployable-multipath-tcp>.
[PAMS2014]
Arzani, B., Gurney, A., Cheng, S., Guerin, R., and B. Loo,
"Impact of Path Selection and Scheduling Policies on MPTCP
Performance", PAMS2014 , 2014.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
Iyengar, "Architectural Guidelines for Multipath TCP
Development", RFC 6182, March 2011.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols", RFC
6356, October 2011.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, January 2013.
[SIGCOMM11]
Raiciu, C., Barre, S., Pluntke, C., Greenhalgh, A.,
Wischik, D., and M. Handley, "Improving datacenter
performance and robustness with multipath TCP",
Proceedings of the ACM SIGCOMM 2011 conference , n.d.,
<http://doi.acm.org/10.1145/2018436.2018467>.
[TNC13] van der Pol, R., Bredel, M., and A. Barczyk, "Experiences
with MPTCP in an intercontinental multipathed OpenFlow
network", TNC2013 , 2013.
[tracebox]
Detal, G., "tracebox", 2013, <http://www.tracebox.org>.
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Authors' Addresses
Olivier Bonaventure
UCLouvain
Email: Olivier.Bonaventure@uclouvain.be
Christoph Paasch
UCLouvain
Email: Christoph.Paasch@uclouvain.be
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