rsz_1mp-tcp_now

Multipath TCP

Multipath TCP

In today's interconnected world, a seamless and reliable internet connection is paramount. Traditional TCP/IP protocols have served us well, but they face challenges in handling modern network demands. Enter Multipath TCP (MPTCP), a groundbreaking technology that has the potential to revolutionize internet connections.

In this blog post, we will explore the intricacies of MPTCP, its benefits, and its implications for the future of networking.

MPTCP, as the name suggests, allows a single data stream to be transmitted across multiple paths simultaneously. Unlike traditional TCP, which relies on a single path, MPTCP splits the data into subflows, distributing them across different routes. This enables the utilization of multiple network interfaces, such as Wi-Fi, cellular, or wired connections, to enhance performance, resilience, and overall user experience.

Table of Contents

Highlights: Multipath TCP

Multipath TCP and TCP

Multipath TCP (MPTCP) is a protocol extension that allows for the simultaneous use of multiple network paths between two endpoints. Traditionally, TCP (Transmission Control Protocol) relies on a single path for data transmission, which can limit performance and reliability.

With MPTCP, multiple paths can be established between the sender and receiver, enabling the distribution of traffic across these paths. This offers several advantages, including increased throughput, better load balancing, and improved resilience against network failures.

Automatically Set Up Multiple Paths.

It is designed to automatically set up multiple paths between two endpoints and use those paths to send and receive data efficiently. It also provides a mechanism for detecting and recovering from packet loss and for providing low-latency communication.

MPTCP is used in applications that require high throughput and low latency, such as streaming media, virtual private networks (VPNs), and networked gaming. MPTCP is an extension to the standard TCP protocol and is supported by most modern operating systems, including Windows, macOS, iOS, and Linux.

High Throughput & Low Latency

MPTCP is an attractive option for applications that require high throughput and low latency, as it can provide both. Additionally, it can provide fault tolerance and redundancy, allowing an application to remain operational even if one or more of its paths fail. This makes it useful for applications such as streaming media, where high throughput and low latency are essential, and reliability is critical.

Before you proceed, you may find the following helpful:

  1. Software Defined Perimeter
  2. Event Stream Processing
  3. Application Aware Networking



TCP Multipath.

Key Multipath TCP Discussion points:


  • Introduction of multiple paths for a single TCP session.

  • Discussion on TCP Subflow.

  • MP-TCP setup. 

  • Multipath networking use cases

  • The issues with TCP congestion control

Back to Basic: Reliability in Multipath TCP

Reliable byte streams

To start the discussion on multipath TCP, we must understand the basics of Transmission Control Protocol (TCP) and its effect on IP Forwarding. TCP applications offer reliable byte streams with congestion control mechanisms adjusting flows to the current network load. Designed in the 70s, TCP is the most widely used protocol and remains unchanged, unlike the networks it operates within. In those days, the designers understood there could be link failure and decided to decouple the network layer (IP) from the transport layer (TCP).

This enables the routing with IP around link failures without breaking the end-to-end TCP connection. Dynamic routing protocols such as BGP Multipath do this automatically without the need for transport layer knowledge. Even though it has wide adoption, it does not fully align with the multipath networking requirements of today’s networks, driving the need for MP-TCP.

TCP delivers reliability using distinct variations of the techniques. Because it provides a byte stream interface, TCP must convert a sending application’s stream of bytes into a set of packets that IP can carry. This is called packetization. These packets contain sequence numbers, which in TCP represent the byte offsets of the first byte in each packet in the overall data stream rather than packet numbers. This allows packets to be of variable size during a transfer and may also allow them to be combined, called repacketization.

Diagram: The need for MP-TCP.

TCP’s main drawback is that it’s a single path per connection protocol. A single path means once the stream is placed on a path ( endpoints of the connection), it can not be moved to another path even though multiple paths may exist between peers. This characteristic is suboptimal as most of today’s networks and end hosts have multipath characteristics for better performance and robustness.

What is Multipath TCP?

Multipath TCP, also known as MPTCP, is an extension to the traditional TCP protocol that allows a single TCP connection to utilize multiple network paths simultaneously. Unlike conventional TCP, which operates on a single path, MPTCP offers the ability to distribute the traffic across multiple paths, enabling more efficient resource utilization and increased overall network capacity.

Critical Benefits of Multipath TCP:

1. Improved Performance: MPTCP can distribute the data traffic using multiple paths, enabling faster transmission rates and reducing latency. This enhanced performance is particularly beneficial for bandwidth-intensive applications such as streaming, file transfers, and video conferencing, where higher throughput and reduced latency are crucial.

2. Increased Resilience: MPTCP enhances network resilience by providing seamless failover capabilities. In traditional TCP, if a network path fails, the connection is disrupted, resulting in a delay or even a complete loss of service. However, with MPTCP, if one path becomes unavailable, the connection can automatically switch to an alternative path, ensuring uninterrupted communication.

3. Efficient Resource Utilization: MPTCP allows for better utilization of available network resources. Distributing traffic across multiple paths prevents congestion on a single path and optimizes the usage of available bandwidth. This results in more efficient utilization of network resources and improved overall performance.

4. Seamless Transition between Networks: MPTCP is particularly useful in scenarios where devices need to switch between different networks seamlessly. For example, when a mobile device moves from a Wi-Fi network to a cellular network, MPTCP can maintain the connection and seamlessly transfer the ongoing data traffic to the new network without interruption.

5. Compatibility with Existing Infrastructure: MPTCP is designed to be backward compatible with traditional TCP, making it easy to deploy and integrate into existing network infrastructure. It can coexist with legacy TCP connections and gradually adapt to MPTCP capabilities as more devices and networks support the protocol.

Multipath TCP

Main Multipath TCP Components

Multipath TCP

  • Allows a single transmission control protocol (TCP) connection to use multiple network paths simultaneously.

  • Automatically set up multiple paths between two endpoints.

  • TCP’s main drawback is that it’s a single path per connection protocol.

  • MPTCP enhances network resilience by providing seamless failover capabilities.

Multiple Paths for a Single TCP Session

Using multiple paths for a single TCP session increases resource usage and resilience for TCP optimization. All this is achieved with additional extensions added to regular TCP, simultaneously enabling connection transport across multiple links.

The core aim of Multipath TCP (MP-TCP) is to allow a single TCP connection to use multiple paths simultaneously by using abstractions at the transport layer. As it operates at the transport layer, the upper and lower layers are transparent to its operation. No network or link-layer modifications are needed.

There is no need to change the network or the end hosts. The end hosts use the same socket API call, and the network continues to operate as before. No unique configurations are required as it’s a capability exchange between hosts. Multipath TCP enabling multipath networking is 100% backward compatible with regular TCP.

Multipath TCP
Diagram: Multipath TCP. Source is Cisco

 

TCP sub flows

MPTCP achieves its goals through sub-flows of individual TCP connections that collectively form an MPTCP session. These sub-flows can be established over different network paths, allowing for parallel data transmission. MPTCP also includes mechanisms for congestion control and sequencing of data across the sub-flows, ensuring reliable delivery of packets.

MP-TCP binds a TCP connection between two hosts, not two interfaces, like regular TCP. Regular TCP connects two IP endpoints by establishing a source/destination by IP address and port number. The application has to choose a single link for the connection. However, MPTCP creates new TCP connections known as sub-flows, allowing the application to take different links for each subflow. 

Subflows are set up the same as regular TCP connections. They consist of a flow of TCP segments operating over individual paths but are still part of the overall MPTCP connection. Subflows are never fixed and may fluctuate in number during the lifetime of the parent Multipath TCP connection.

mp-tcp
Diagram: MP-TCP.

 

Multipath TCP uses cases.

The deployment of MPTCP has the potential to benefit various applications and use cases. For example, MPTCP can enable seamless handovers between cellular towers or Wi-Fi access points in mobile networks, providing uninterrupted connectivity. MPTCP can improve server-to-server communications in data centers by utilizing multiple links and avoiding congestion.

Multipath TCP is beneficial in multipath data centers and mobile phone environments. All mobiles allow you to connect via wifi and a 3G network. MP-TCP enables the combined throughput and the switching of interfaces (wifi / 3G ) without disrupting the end-to-end TCP connection.

For example, if you are currently on a 3G network with an active TCP stream, the TCP stream is bound to that interface. If you want to move to the wifi network, you need to reset the connection, and all ongoing TCP connections will reset. With MP-TCP, the swapping of interfaces is transparent.

Multipath networking: leaf-spine data center

Next-generation leaf and spine data center networks are built with Equal-Cost Multipath (ECMP). Within the data center, any two endpoints are equidistant. For one endpoint to communicate with another, a TCP flow is placed on a single link, not spread over multiple links. As a result, single-path TCP collisions may occur, reducing the throughput available to that flow.

what is spine and leaf architecture
Diagram: What is spine and leaf architecture? 2-Tier Spine Leaf Design

This is commonly seen for large flows, and not small mice flows. When a server starts a TCP connection in a data center, it gets placed on a path and stays there. With MP-TCP, you could use many sub-flows per connection instead of a single path per connection. Then, if some of those sub-flows get congested, you don’t send over that subflow, improving traffic fairness and bandwidth optimizations.

Video: Mice and Elephant flows.

The following is a video discussing mice and elephant flows. There are two types of flows in data center environments. We have large, elephant, and smaller mice flows. Elephant flows might only represent a low proportion of the number of flows but consume most of the total data volume. Mice flow, for example, control and alarm/control messages are usually pretty significant.

As a result, they should be given priority over more significant elephant flows, but this is sometimes not the case with simple buffer types that don’t distinguish between flow types. Priority can be given by somehow regulating the elephant flows with intelligent switch buffers.

Tech Brief Video Series – Enterprise Networking | Mice & Elephant Flows
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Hash-based distribution

The default behavior of spreading traffic through a LAG or ECMP next hops is based on the hash-based distribution of packets. First, an array of buckets is created, and each outbound link is assigned to one or more buckets. Next, fields are taken from the outgoing packet header, such as source-destination IP address / MAC address, and hashed based on this endpoint identification. Finally, the hash selects a bucket, and the packet is queued to the interface assigned to that bucket. 

redundant links
Diagram: Redundant links with EtherChannel. Source is jmcritobal

The issue is that the load-balancing algorithm does not consider interface congestions or packet drops. With all mice flows, this is fine, but once you mix mice and elephant flows together, your performance will suffer. An algorithm is needed to identify congested links and then reshuffle the traffic.

A good use for MPTCP is a mix of mice and elephant flows. Generally, MP-TCP does not improve performance for environments with only mice flows.

Small files say 50KB MPTCP offers the same performance as regular TCP. Multipath networking usually has the same results as link bonding as the file size increases. The benefits of MP-TCP come into play when files are enormous (300 KB ). MP-TCP outperforms link bonding at this level as the congestion control can better balance the load over the links.

MP-TCP connection setup

The connection aims to have a single TCP connection with many sub-flows. The two endpoints using MPTCP are synchronized and have connection identifiers for each sub-flow. MPTCP starts the same as regular TCP. Additional TCP subflow sessions are combined into the existing TCP session if different paths are available. The original TCP and other subflow sessions appear as one to the application, and the primary Multipath TCP connection seems like a regular TCP connection. Identifying additional paths boils down to the number of IP addresses on the hosts. 

tcp multipath
Diagram: TCP Multipath.

The TCP handshake starts as expected, but within the first SYN is a new MP_CAPABLE option ( value 0x0 ) and a unique connection identifier. This allows the client to indicate they want to do MPTCP. At this stage, the application layer creates a standard TCP socket with additional variables telling it intends to do MPTCP.

If the receiving server end is MP_CAPABLE, it will reply with the SYN/ACK MP_CAPABLE and its connection identifier. Once the connection is agreed upon, the client and server will set up the upstate. Inside the kernel, a Meta socket is the layer between the application and all the TCP sub-flows.

Under a multipath condition and when multiple paths are detected (based on IP addresses), the client starts a regular TCP handshake with the MP_JOIN option (value 0x1) and uses the connection identifier for the server. The server then replies with a subflow setup. New sub-flows are created, and the scheduler will schedule over each sub-flow as the data is sent from the application to the meta socket.

 

TCP sequence numbers 

Regular TCP uses sequence numbers, enabling the receiving side to put packets back in the correct order before sending them to the application. The sender can determine which packets are lost by looking at the ACK.

For MP-TCP, packets must go multiple paths, so you first need sequence numbers to put packets back in order before they are passed to the application. You also need the sequence numbers to inform the sender of any packet loss on a path. When an application sends, the segment is assigned a data sequence number.

TCP looks at the sub-flows to see where to send this segment. When it ships on a subflow, it uses a sequence number and puts it in the TCP header, and the other data sequence number gets set in the TCP options. 

The sequence number on the TCP header informs the client of any packet loss. In addition, the recipient uses the data sequence number to reorder packets before sending them to the application.

 

Congestion control

Congestion control was never a problem in circuit switching. Resources are reserved at call setup to prevent congestion during data transfer, resulting in a lot of bandwidth underutilization due to the reservation of circuits. We then moved to packet switching, where we had a single link with no reservation, but the flows could use as much of the link as they wanted. This increases the utilization of the link and also the possibility of congestion.

To help this situation, congestion control mechanisms were added to TCP. Similar TCP congestion control mechanisms are employed for MP-TCP. Standard TCP congestion control maintains a congestion window for each connection, and you increase the window size on each ACK. With a drop, you half the window. 

MP-TCP operates similarly. You maintain one congestion window for each subflow path. Similar to standard TCP, when you have a drop on a subflow, you have half the window for that subflow. However, the increased rules are different from expected TCP behavior.

It gives more of an increase for sub-flows with a larger window. A larger window means it has a lower loss. As a result, traffic moves from congested to uncongested links dynamically.

Summary: Multipath TCP

The networking world is constantly evolving, with new technologies and protocols being developed to meet the growing demands of our interconnected world. One such protocol that has gained significant attention recently is Multipath TCP (MPTCP). In this blog post, we dived into the fascinating world of MPTCP, its benefits, and its potential applications.

Section 1: Understanding Multipath TCP

Multipath TCP, often called MPTCP, is an extension of the traditional TCP protocol that allows for simultaneous data transmission across multiple paths. Unlike conventional TCP, which operates on a single path, MPTCP leverages multiple network interfaces, such as Wi-Fi and cellular connections, to improve overall network performance and reliability.

Section 2: Benefits of Multipath TCP

By utilizing multiple paths, MPTCP offers several key advantages. Firstly, it enhances throughput by aggregating the bandwidth of multiple network interfaces, resulting in faster data transfer speeds. Additionally, MPTCP improves resilience by providing seamless failover between different paths, ensuring uninterrupted connectivity even if one path becomes congested or unavailable.

Section 3: Applications of Multipath TCP

The versatility of MPTCP opens the door to a wide range of applications. One notable application is in mobile devices, where MPTCP can intelligently combine Wi-Fi and cellular connections to provide users with a more stable and faster internet experience. MPTCP also finds utility in data centers, enabling efficient load balancing and reducing network congestion by distributing traffic across multiple paths.

Section 4: Challenges and Future Developments

While MPTCP brings many benefits, it also presents challenges. One such challenge is ensuring compatibility with existing infrastructure and devices that may not support MPTCP. Additionally, optimizing MPTCP’s congestion control mechanisms and addressing security concerns are ongoing research and development areas.

Conclusion:

Multipath TCP is a groundbreaking protocol that has the potential to revolutionize the way we experience network connectivity. With its ability to enhance throughput, improve resilience, and enable new applications, MPTCP holds great promise for the future of networking. As researchers continue to address challenges and refine the protocol, we can expect even greater advancements in this exciting field.