| MPTCP Working Group | O. Bonaventure |
| Internet-Draft | C. Paasch |
| Intended status: Informational | UCLouvain |
| Expires: September 9, 2015 | G. Detal |
| UCLouvain and Tessares | |
| March 08, 2015 |
Experience with Multipath TCP
draft-ietf-mptcp-experience-01
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.
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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.
This document is organized as follows. We explain in Section 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 {{usecases}. Section {{congestion} summarises the MPTCP specific congestion controls that have been implemented. Sections Section 5 and Section 6 discuss heuristics and issues with respect to subflow management as well as the scheduling across the subflows. Section Section 7 explains some problems that occurred with subflows having different MSS values. Section Section 8 presents issues with respect to content delivery networks and suggests a solution to this issue. Finally, Section Section 9 shows an issue with captive portals where MPTCP will behave suboptimal.
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 :
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].
Users of the Multipath TCP implementation have reported some experience with middlebox interference. The strangest scenario has been a middlebox that accepts the Multipath TCP options in the SYN segment but later replaces Multipath TCP options with a TCP EOL option [StrangeMbox]. This causes Multipath TCP to perform a fallback to regular TCP without any impact on the application.
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 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 [SIGCOMM11].
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.
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 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 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.
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].
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 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.
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 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. The N subflows are created by the client and differ only in the source port selected by the client.
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 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.
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 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.
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.
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.
When an application performs a write/send system call, the kernel allocates a packet buffer (sk_buff in Linux) to store the data the application wants to send. The kernel will store at most one MSS (Maximum Segment Size) of data per buffer. As MSS can differ amongst subflows, an MPTCP implementation must select carefully the MSS used to generate application data. The Linux kernel implementation had various ways of selecting the MSS: minimum or maximum amongst the different subflows. However, these heuristics of MSS selection can cause significant performances issues in some environment. Consider the following example. An MPTCP connection has two established subflows that respectively use a MSS of 1420 and 1428 bytes. If MPTCP selects the maximum, then the application will generate segments of 1428 bytes of data. An MPTCP implementation will have to split the segment in two (a 1420-byte and 8-byte segments) when pushing on the subflow with the smallest MSS. The latter segment will introduce a large overhead as for a single data segment 2 slots will be used in the congestion window (in packets) therefore reducing by ~2 the potential throughput (in bytes/s) of this subflow. Taking the smallest MSS does not solve the issue as there might be a case where the sublow with the smallest MSS will only participate marginally to the overall performance therefore reducing the potential throughput of the other subflows.
The Linux implementation recently took another approach [DetalMSS]. Instead of selecting the minimum and maximum values, it now dynamically adapts the MSS based on the contribution of all the subflows to the connection’s throughput. For this it computes, for each subflow, the potential throughput achieved by selecting each MSS value and by taking into account the lost space in the cwnd. It then selects the MSS that allows to achieve the highest potential throughput.
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).
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.
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.
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.