Welcome!

@CloudExpo Authors: Yeshim Deniz, Liz McMillan, Carmen Gonzalez, Leon Fayer, Elizabeth White

Related Topics: Microservices Expo, Java IoT, Linux Containers, Containers Expo Blog, @CloudExpo, @BigDataExpo, SDN Journal

Microservices Expo: Article

Understanding APM on the Network

TCP Window Size

In Part 6, we dove into the Nagle algorithm - perhaps (or hopefully) something you'll never see. In Part VII, we get back to "pure" network and TCP roots as we examine how the TCP receive window interacts with WAN links.

TCP Window Size
Each node participating in a TCP connection advertises its available buffer space using the TCP window size field. This value identifies the maximum amount of data a sender can transmit without receiving a window update via a TCP acknowledgement; in other words, this is the maximum number of "bytes in flight" - bytes that have been sent, are traversing the network, but remain unacknowledged. Once the sender has reached this limit and exhausted the receive window, the sender must stop and wait for a window update.

The sender transmits a full window then waits for window updates before continuing. As these window updates arrive, the sender advances the window and may transmit more data.

Long Fat Networks
High-speed, high-latency networks, sometimes referred to as Long Fat Networks (LFNs), can carry a lot of data. On these networks, small receive window sizes can limit throughput to a fraction of the available bandwidth. These two factors - bandwidth and latency - combine to influence the potential impact of a given TCP window size. LFNs networks make it possible - common, even - for a sender to transmit very fast (high bandwidth) an entire TCP window's worth of data, having then to wait until the packets reach the distant remote site (high latency) so that acknowledgements can be returned, informing the sender of successful data delivery and available receive buffer space.

The math (and physics) concepts are straightforward. As the network speed increases, data can be clocked out onto the network medium more quickly; the bits are literally closer together. As latency increases, these bits take longer to traverse the network from sender to receiver. As a result, more bits can fit on the wire. As LFNs become more common, exhausting a receiver's TCP window becomes increasingly problematic for some types of applications.

Bandwidth Delay Product
The Bandwidth Delay Product (BDP) is a simple formula used to calculate the maximum amount of data that can exist on the network (referred to as bits or bytes in flight) based on a link's characteristics:

  • Bandwidth (bps) x RTT (seconds) = bits in flight
  • Divide the result by 8 for bytes in flight

If the BDP (in bytes) for a given network link exceeds the value of a session's TCP window, then the TCP session will not be able to use all of the available bandwidth; instead, throughput will be limited by the receive window (assuming no other constraints, of course).

The BDP can also be used to calculate the maximum throughput ("bandwidth") of a TCP connection given a fixed receive window size:

  • Bandwidth = (window size *8)/RTT

In the not-too-distant past, the TCP window had a maximum value of 65535 bytes. While today's TCP implementations now generally include a TCP window scaling option that allows for negotiated window sizes to reach 1GB, many factors limit its practical utility. For example, firewalls, load balancers and server configurations may purposely disable the feature. The reality is that we often still need to pay attention to the TCP window size when considering the performance of applications that transfer large amounts of data, particularly on enterprise LFNs.

As an example, consider a company with offices in New York and San Francisco; they need to replicate a large database each night, and have secured a 20Mbps network connection with 85 milliseconds of round-trip delay. Our BDP calculation tells us that the BDP is 212,500 (20,000,000 x .085 *8); in other words, a single TCP connection would require a 212KB window in order to take advantage of all of the bandwidth. The BDP calculation also tells us that the configured TCP window size of 65535 will permit approximately 6Mbps throughput (65535*8/.085), less than 1/3 of the link's capacity.

A link's BDP and a receiver's TCP window size are two factors that help us to identify the potential throughput of an operation. The remaining factor is the operation itself, specifically the size of individual request or reply flows. Only flows that exceed the receiver's TCP window size will benefit from, or be impacted by, these TCP window size constraints. Two common scenarios help illustrate this. Let's say a user needs to transfer a 1GB file:

  • Using FTP (in stream mode) will cause the entire file to be sent in a single flow; this operation could be severely limited by the receive window.
  • Using SMB (at least older versions of the protocol) will cause the file to be sent in many smaller write commands, as SMB used to limit write messages to under 64KB; this operation would not be able to take advantage of a TCP receive window of greater than 64K. (Instead, the operation would more likely be limited by application turns and link latency; we discuss chattiness in Part 8.)

For more networking tips, click here for the full article.

More Stories By Gary Kaiser

Gary Kaiser is a Subject Matter Expert in Network Performance Analytics at Dynatrace, responsible for DC RUM’s technical marketing programs. He is a co-inventor of multiple performance analysis features, and continues to champion the value of network performance analytics. He is the author of Network Application Performance Analysis (WalrusInk, 2014).

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@CloudExpo Stories
The speed of software changes in growing and large scale rapid-paced DevOps environments presents a challenge for continuous testing. Many organizations struggle to get this right. Practices that work for small scale continuous testing may not be sufficient as the requirements grow. In his session at DevOps Summit, Marc Hornbeek, Sr. Solutions Architect of DevOps continuous test solutions at Spirent Communications, explained the best practices of continuous testing at high scale, which is rele...
SYS-CON Events announced today that Catchpoint Systems, Inc., a provider of innovative web and infrastructure monitoring solutions, has been named “Silver Sponsor” of SYS-CON's DevOps Summit at 18th Cloud Expo New York, which will take place June 7-9, 2016, at the Javits Center in New York City, NY. Catchpoint is a leading Digital Performance Analytics company that provides unparalleled insight into customer-critical services to help consistently deliver an amazing customer experience. Designed ...
Internet of @ThingsExpo, taking place June 6-8, 2017 at the Javits Center in New York City, New York, is co-located with the 20th International Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. @ThingsExpo New York Call for Papers is now open.
Every successful software product evolves from an idea to an enterprise system. Notably, the same way is passed by the product owner's company. In his session at 20th Cloud Expo, Oleg Lola, CEO of MobiDev, will provide a generalized overview of the evolution of a software product, the product owner, the needs that arise at various stages of this process, and the value brought by a software development partner to the product owner as a response to these needs.
Containers have changed the mind of IT in DevOps. They enable developers to work with dev, test, stage and production environments identically. Containers provide the right abstraction for microservices and many cloud platforms have integrated them into deployment pipelines. DevOps and Containers together help companies to achieve their business goals faster and more effectively. In his session at DevOps Summit, Ruslan Synytsky, CEO and Co-founder of Jelastic, reviewed the current landscape of D...
Smart Cities are here to stay, but for their promise to be delivered, the data they produce must not be put in new siloes. In his session at @ThingsExpo, Mathias Herberts, Co-founder and CTO of Cityzen Data, discussed the best practices that will ensure a successful smart city journey.
"We provide DevOps solutions. We also partner with some key players in the DevOps space and we use the technology that we partner with to engineer custom solutions for different organizations," stated Himanshu Chhetri, CTO of Addteq, in this SYS-CON.tv interview at DevOps at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
Hardware virtualization and cloud computing allowed us to increase resource utilization and increase our flexibility to respond to business demand. Docker Containers are the next quantum leap - Are they?! Databases always represented an additional set of challenges unique to running workloads requiring a maximum of I/O, network, CPU resources combined with data locality.
@ThingsExpo has been named the ‘Top WebRTC Influencer' by iTrend. iTrend processes millions of conversations, tweets, interactions, news articles, press releases, blog posts - and extract meaning form them and analyzes mobile and desktop software platforms used to communicate, various metadata (such as geo location), and automation tools. In overall placement, @ThingsExpo ranked as the number one ‘WebRTC Influencer' followed by @DevOpsSummit at 55th.
In 2014, Amazon announced a new form of compute called Lambda. We didn't know it at the time, but this represented a fundamental shift in what we expect from cloud computing. Now, all of the major cloud computing vendors want to take part in this disruptive technology. In his session at 20th Cloud Expo, John Jelinek IV, a web developer at Linux Academy, will discuss why major players like AWS, Microsoft Azure, IBM Bluemix, and Google Cloud Platform are all trying to sidestep VMs and containers...
SYS-CON Events announced today that Enzu will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY, and the 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Enzu’s mission is to be the leading provider of enterprise cloud solutions worldwide. Enzu enables online businesses to use its IT infrastructure to their competitive ad...
SYS-CON Events announced today that MobiDev, a client-oriented software development company, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place June 6-8, 2017, at the Javits Center in New York City, NY, and the 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. MobiDev is a software company that develops and delivers turn-key mobile apps, websites, web services, and complex softw...
DevOps tends to focus on the relationship between Dev and Ops, putting an emphasis on the ops and application infrastructure. But that’s changing with microservices architectures. In her session at DevOps Summit, Lori MacVittie, Evangelist for F5 Networks, will focus on how microservices are changing the underlying architectures needed to scale, secure and deliver applications based on highly distributed (micro) services and why that means an expansion into “the network” for DevOps.
In his session at 19th Cloud Expo, Claude Remillard, Principal Program Manager in Developer Division at Microsoft, contrasted how his team used config as code and immutable patterns for continuous delivery of microservices and apps to the cloud. He showed how the immutable patterns helps developers do away with most of the complexity of config as code-enabling scenarios such as rollback, zero downtime upgrades with far greater simplicity. He also demoed building immutable pipelines in the cloud ...
Using new techniques of information modeling, indexing, and processing, new cloud-based systems can support cloud-based workloads previously not possible for high-throughput insurance, banking, and case-based applications. In his session at 18th Cloud Expo, John Newton, CTO, Founder and Chairman of Alfresco, described how to scale cloud-based content management repositories to store, manage, and retrieve billions of documents and related information with fast and linear scalability. He addres...
Due of the rise of Hadoop, many enterprises are now deploying their first small clusters of 10 to 20 servers. At this small scale, the complexity of operating the cluster looks and feels like general data center servers. It is not until the clusters scale, as they inevitably do, when the pain caused by the exponential complexity becomes apparent. We've seen this problem occur time and time again. In his session at Big Data Expo, Greg Bruno, Vice President of Engineering and co-founder of StackIQ...
The cloud market growth today is largely in public clouds. While there is a lot of spend in IT departments in virtualization, these aren’t yet translating into a true “cloud” experience within the enterprise. What is stopping the growth of the “private cloud” market? In his general session at 18th Cloud Expo, Nara Rajagopalan, CEO of Accelerite, explored the challenges in deploying, managing, and getting adoption for a private cloud within an enterprise. What are the key differences between wh...
Security, data privacy, reliability, and regulatory compliance are critical factors when evaluating whether to move business applications from in-house, client-hosted environments to a cloud platform. Quality assurance plays a vital role in ensuring that the appropriate level of risk assessment, verification, and validation takes place to ensure business continuity during the migration to a new cloud platform.
"Tintri was started in 2008 with the express purpose of building a storage appliance that is ideal for virtualized environments. We support a lot of different hypervisor platforms from VMware to OpenStack to Hyper-V," explained Dan Florea, Director of Product Management at Tintri, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
Containers have changed the mind of IT in DevOps. They enable developers to work with dev, test, stage and production environments identically. Containers provide the right abstraction for microservices and many cloud platforms have integrated them into deployment pipelines. DevOps and containers together help companies achieve their business goals faster and more effectively. In his session at DevOps Summit, Ruslan Synytsky, CEO and Co-founder of Jelastic, reviewed the current landscape of Dev...