Welcome!

Microservices Expo Authors: Liz McMillan, Pat Romanski, Elizabeth White, Derek Weeks, Mehdi Daoudi

Related Topics: Microservices Expo, Java IoT, Linux Containers, Containers Expo Blog, @CloudExpo, @DXWorldExpo, 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.


@MicroservicesExpo Stories
As many know, the first generation of Cloud Management Platform (CMP) solutions were designed for managing virtual infrastructure (IaaS) and traditional applications. But that's no longer enough to satisfy evolving and complex business requirements. In his session at 21st Cloud Expo, Scott Davis, Embotics CTO, explored how next-generation CMPs ensure organizations can manage cloud-native and microservice-based application architectures, while also facilitating agile DevOps methodology. He expla...
SYS-CON Events announced today that Synametrics Technologies will exhibit at SYS-CON's 22nd International Cloud Expo®, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Synametrics Technologies is a privately held company based in Plainsboro, New Jersey that has been providing solutions for the developer community since 1997. Based on the success of its initial product offerings such as WinSQL, Xeams, SynaMan and Syncrify, Synametrics continues to create and hone in...
DevOps promotes continuous improvement through a culture of collaboration. But in real terms, how do you: Integrate activities across diverse teams and services? Make objective decisions with system-wide visibility? Use feedback loops to enable learning and improvement? With technology insights and real-world examples, in his general session at @DevOpsSummit, at 21st Cloud Expo, Andi Mann, Chief Technology Advocate at Splunk, explored how leading organizations use data-driven DevOps to clos...
"I focus on what we are calling CAST Highlight, which is our SaaS application portfolio analysis tool. It is an extremely lightweight tool that can integrate with pretty much any build process right now," explained Andrew Siegmund, Application Migration Specialist for CAST, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
The dynamic nature of the cloud means that change is a constant when it comes to modern cloud-based infrastructure. Delivering modern applications to end users, therefore, is a constantly shifting challenge. Delivery automation helps IT Ops teams ensure that apps are providing an optimal end user experience over hybrid-cloud and multi-cloud environments, no matter what the current state of the infrastructure is. To employ a delivery automation strategy that reflects your business rules, making r...
The past few years have brought a sea change in the way applications are architected, developed, and consumed—increasing both the complexity of testing and the business impact of software failures. How can software testing professionals keep pace with modern application delivery, given the trends that impact both architectures (cloud, microservices, and APIs) and processes (DevOps, agile, and continuous delivery)? This is where continuous testing comes in. D
Modern software design has fundamentally changed how we manage applications, causing many to turn to containers as the new virtual machine for resource management. As container adoption grows beyond stateless applications to stateful workloads, the need for persistent storage is foundational - something customers routinely cite as a top pain point. In his session at @DevOpsSummit at 21st Cloud Expo, Bill Borsari, Head of Systems Engineering at Datera, explored how organizations can reap the bene...
Admiral Calcote - also known as Lee Calcote (@lcalcote) or the Ginger Geek to his friends - gave a presentation entitled Characterizing and Contrasting Container Orchestrators at the 2016 All Day DevOps conference. Okay, he isn't really an admiral - nor does anyone call him that - but he used the title admiral to describe what container orchestrators do, relating it to an admiral directing a fleet of container ships. You could also say that they are like the conductor of an orchestra, directing...
The past few years have seen a huge increase in the amount of critical IT services that companies outsource to SaaS/IaaS/PaaS providers, be it security, storage, monitoring, or operations. Of course, along with any outsourcing to a service provider comes a Service Level Agreement (SLA) to ensure that the vendor is held financially responsible for any lapses in their service which affect the customer’s end users, and ultimately, their bottom line. SLAs can be very tricky to manage for a number ...
Our work, both with clients and with tools, has lead us to wonder how it is that organizations are handling compliance issues in the cloud. The big cloud vendors offer compliance for their infrastructure, but the shared responsibility model requires that you take certain steps to meet compliance requirements. Which lead us to start poking around a little more. We wanted to get a picture of what was available, and how it was being used. There is a lot of fluidity in this space, as in all things c...
Gaining visibility in today’s sprawling cloud infrastructure is complex and laborious, involving drilling down into tools offered by various cloud services providers. Enterprise IT organizations need smarter and effective tools at their disposal in order to address this pertinent problem. Gaining a 360 - degree view of the cloud costs requires collection and analysis of the cost data across all cloud infrastructures used inside an enterprise.
Some people are directors, managers, and administrators. Others are disrupters. Eddie Webb (@edwardawebb) is an IT Disrupter for Software Development Platforms at Liberty Mutual and was a presenter at the 2016 All Day DevOps conference. His talk, Organically DevOps: Building Quality and Security into the Software Supply Chain at Liberty Mutual, looked at Liberty Mutual's transformation to Continuous Integration, Continuous Delivery, and DevOps. For a large, heavily regulated industry, this task...
The goal of Microservices is to improve software delivery speed and increase system safety as scale increases. Microservices being modular these are faster to change and enables an evolutionary architecture where systems can change, as the business needs change. Microservices can scale elastically and by being service oriented can enable APIs natively. Microservices also reduce implementation and release cycle time and enables continuous delivery. This paper provides a logical overview of the Mi...
The notion of improving operational efficiency is conspicuously absent from the healthcare debate - neither Obamacare nor the newly proposed GOP plan discusses the impact that a step-function improvement in efficiency could have on access to healthcare (through more capacity), quality of healthcare services (through reduced wait times for patients) or cost (through better utilization of scarce, expensive assets).
Gone are the days when application development was the daunting task of the highly skilled developers backed with strong IT skills, low code application development has democratized app development and empowered a new generation of citizen developers. There was a time when app development was in the domain of people with complex coding and technical skills. We called these people by various names like programmers, coders, techies, and they usually worked in a world oblivious of the everyday pri...
The “Digital Era” is forcing us to engage with new methods to build, operate and maintain applications. This transformation also implies an evolution to more and more intelligent applications to better engage with the customers, while creating significant market differentiators. In both cases, the cloud has become a key enabler to embrace this digital revolution. So, moving to the cloud is no longer the question; the new questions are HOW and WHEN. To make this equation even more complex, most ...
Some journey to cloud on a mission, others, a deadline. Change management is useful when migrating to public, private or hybrid cloud environments in either case. For most, stakeholder engagement peaks during the planning and post migration phases of a project. Legacy engagements are fairly direct: projects follow a linear progression of activities (the “waterfall” approach) – change managers and application coders work from the same functional and technical requirements. Enablement and develo...
Kubernetes is an open source system for automating deployment, scaling, and management of containerized applications. Kubernetes was originally built by Google, leveraging years of experience with managing container workloads, and is now a Cloud Native Compute Foundation (CNCF) project. Kubernetes has been widely adopted by the community, supported on all major public and private cloud providers, and is gaining rapid adoption in enterprises. However, Kubernetes may seem intimidating and complex ...
You know you need the cloud, but you’re hesitant to simply dump everything at Amazon since you know that not all workloads are suitable for cloud. You know that you want the kind of ease of use and scalability that you get with public cloud, but your applications are architected in a way that makes the public cloud a non-starter. You’re looking at private cloud solutions based on hyperconverged infrastructure, but you’re concerned with the limits inherent in those technologies.
For DevOps teams, the concepts behind service-oriented architecture (SOA) are nothing new. A style of software design initially made popular in the 1990s, SOA was an alternative to a monolithic application; essentially a collection of coarse-grained components that communicated with each other. Communication would involve either simple data passing or two or more services coordinating some activity. SOA served as a valid approach to solving many architectural problems faced by businesses, as app...