|By Gary Kaiser||
|July 13, 2014 01:00 PM EDT||
We know that losing packets is not a good thing; retransmissions cause delays. We also know that TCP ensures reliable data delivery, masking the impact of packet loss. So why are some applications seemingly unaffected by the same packet loss rate that seems to cripple others? From a performance analysis perspective, how do you understand the relevance of packet loss and avoid chasing red herrings?
In Part II, we examined two closely related constraints - bandwidth and congestion. In Part III, we discussed TCP slow-start and introduced the Congestion Window (CWD). In Part IV, we'll focus on packet loss, continuing the concepts from these two previous entries.
TCP ensures reliable delivery of data through its sliding window approach to managing byte sequences and acknowledgements; among other things, this sequencing allows a receiver to inform the sender of missing data caused by packet loss in multi-packet flows. Independently, a sender may detect packet loss through the expiration of its retransmission timer. We will look at the behavior and performance penalty associated with each of these cases; generally, the impact of packet loss will depend on both the characteristics of the flow and the position of the dropped packet within the flow.
The Retransmission Timer
Each packet a node sends is associated with a retransmission timer; if the timer expires before the sent data has been acknowledged, it is considered lost and retransmitted. There are two important characteristics of the retransmission timer that relate to performance. First, the default value for the initial retransmission timeout (RTO) is almost always 3000 milliseconds; this is adjusted to a more reasonable value as TCP observes actual path round-trip times. Second, the timeout value is doubled for subsequent retransmissions of a packet.
In small flows (a common characteristic of chatty operations - like web pages), the retransmission timer is the method used to detect packet loss. Consider a request or reply message of just 1000 bytes, sent in a single packet; if this packet is dropped, there will of course be no acknowledgement; the receiver has no idea the packet was sent. If the packet is dropped early in the life of a TCP connection - perhaps one of the SYN packets during the TCP 3-way handshake, or an initial GET request or a 304 Not Modified response - the dropped packet will be retransmitted only after 3 seconds have elapsed.
Triple Duplicate ACK
Within larger flows, a dropped packet may be detected before the retransmission time expires if the sender receives three duplicate ACKs; this is generally more efficient (faster) than waiting for the retransmission timer to expire. As the receiving node receives packets that are out of sequence (i.e., after the missing packet data should have been seen), it sends duplicate ACKs, the acknowledgement number repeatedly referencing the expected (missing) packet data. When the sending node receives the third duplicate ACK, it assumes the packet was in fact lost (not just delayed) and retransmits it. This event causes the sender to assume network congestion, reducing its congestion window by 50% to allow congestion to subside. Slow-start begins to increase the CWD from that new value, using a relatively conservative congestion avoidance ramp.
As an example, consider a server sending a large file to a client; the sending node is ramping up through slow-start. As the CWD reaches 24, earlier packet loss is detected via a triple duplicate ACK; the lost data is retransmitted, and the CWD is reduced to 12. Slow-start resumes from this point in its congestion avoidance mode.
While arguments abound about the inefficiency of existing congestion avoidance approaches, especially on high-speed networks, you can expect to see this behavior in today's networks.
Transaction Trace Illustration
Identifying retransmission timeouts using merged trace files is generally quite straightforward; we have proof the packet has been lost (because we see it on the sending side and not on the receiving side), and we know the delay between the dropped and retransmitted packets at the sending node. The Delta column in the Error Table indicates the retransmission delay.
Error Table entry showing a 3-second retransmission delay caused by a retransmission timeout (RTO)
For larger flows, you can illustrate the effect of dropped packets on the sender's Congestion Window by using the Time Plot view. For Series 1, graph the sender's Frames in Transit; this is essentially the CWD. For Series 2, graph the Cumulative Error Count in both directions. As errors (retransmitted packets or out-of-sequence packets) occur, the CWD will be reduced by about 50%.
Time Plot view showing the impact of packet loss (blue plot) on the Congestion Window (brown plot)
For more networking tips click here for the full article
Apixio Inc. has raised $19.3 million in Series D venture capital funding led by SSM Partners with participation from First Analysis, Bain Capital Ventures and Apixio’s largest angel investor. Apixio will dedicate the proceeds toward advancing and scaling products powered by its cognitive computing platform, further enabling insights for optimal patient care. The Series D funding comes as Apixio experiences strong momentum and increasing demand for its HCC Profiler solution, which mines unstruc...
Jun. 28, 2016 08:56 AM EDT
In his general session at 18th Cloud Expo, Lee Atchison, Principal Cloud Architect and Advocate at New Relic, discussed cloud as a ‘better data center’ and how it adds new capacity (faster) and improves application availability (redundancy). The cloud is a ‘Dynamic Tool for Dynamic Apps’ and resource allocation is an integral part of your application architecture, so use only the resources you need and allocate /de-allocate resources on the fly.
Jun. 28, 2016 08:30 AM EDT Reads: 1,184
Presidio has received the 2015 EMC Partner Services Quality Award from EMC Corporation for achieving outstanding service excellence and customer satisfaction as measured by the EMC Partner Services Quality (PSQ) program. Presidio was also honored as the 2015 EMC Americas Marketing Excellence Partner of the Year and 2015 Mid-Market East Partner of the Year. The EMC PSQ program is a project-specific survey program designed for partners with Service Partner designations to solicit customer feedbac...
Jun. 28, 2016 08:00 AM EDT Reads: 397
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, provided an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life sett...
Jun. 28, 2016 08:00 AM EDT Reads: 1,285
When people aren’t talking about VMs and containers, they’re talking about serverless architecture. Serverless is about no maintenance. It means you are not worried about low-level infrastructural and operational details. An event-driven serverless platform is a great use case for IoT. In his session at @ThingsExpo, Animesh Singh, an STSM and Lead for IBM Cloud Platform and Infrastructure, will detail how to build a distributed serverless, polyglot, microservices framework using open source tec...
Jun. 27, 2016 08:30 PM EDT Reads: 388
IoT offers a value of almost $4 trillion to the manufacturing industry through platforms that can improve margins, optimize operations & drive high performance work teams. By using IoT technologies as a foundation, manufacturing customers are integrating worker safety with manufacturing systems, driving deep collaboration and utilizing analytics to exponentially increased per-unit margins. However, as Benoit Lheureux, the VP for Research at Gartner points out, “IoT project implementers often ...
Jun. 27, 2016 07:45 PM EDT Reads: 393
Basho Technologies has announced the latest release of Basho Riak TS, version 1.3. Riak TS is an enterprise-grade NoSQL database optimized for Internet of Things (IoT). The open source version enables developers to download the software for free and use it in production as well as make contributions to the code and develop applications around Riak TS. Enhancements to Riak TS make it quick, easy and cost-effective to spin up an instance to test new ideas and build IoT applications. In addition to...
Jun. 27, 2016 05:15 PM EDT Reads: 391
Machine Learning helps make complex systems more efficient. By applying advanced Machine Learning techniques such as Cognitive Fingerprinting, wind project operators can utilize these tools to learn from collected data, detect regular patterns, and optimize their own operations. In his session at 18th Cloud Expo, Stuart Gillen, Director of Business Development at SparkCognition, discussed how research has demonstrated the value of Machine Learning in delivering next generation analytics to imp...
Jun. 27, 2016 03:00 PM EDT Reads: 776
It is one thing to build single industrial IoT applications, but what will it take to build the Smart Cities and truly society changing applications of the future? The technology won’t be the problem, it will be the number of parties that need to work together and be aligned in their motivation to succeed. In his Day 2 Keynote at @ThingsExpo, Henrik Kenani Dahlgren, Portfolio Marketing Manager at Ericsson, discussed how to plan to cooperate, partner, and form lasting all-star teams to change t...
Jun. 27, 2016 02:45 PM EDT Reads: 1,243
There are several IoTs: the Industrial Internet, Consumer Wearables, Wearables and Healthcare, Supply Chains, and the movement toward Smart Grids, Cities, Regions, and Nations. There are competing communications standards every step of the way, a bewildering array of sensors and devices, and an entire world of competing data analytics platforms. To some this appears to be chaos. In this power panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, Bradley Holt, Developer Advocate a...
Jun. 27, 2016 12:00 PM EDT Reads: 867
Connected devices and the industrial internet are growing exponentially every year with Cisco expecting 50 billion devices to be in operation by 2020. In this period of growth, location-based insights are becoming invaluable to many businesses as they adopt new connected technologies. Knowing when and where these devices connect from is critical for a number of scenarios in supply chain management, disaster management, emergency response, M2M, location marketing and more. In his session at @Th...
Jun. 27, 2016 10:00 AM EDT Reads: 1,071
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...
Jun. 27, 2016 09:30 AM EDT Reads: 917
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, wh...
Jun. 27, 2016 07:30 AM EDT Reads: 1,093
Cloud computing is being adopted in one form or another by 94% of enterprises today. Tens of billions of new devices are being connected to The Internet of Things. And Big Data is driving this bus. An exponential increase is expected in the amount of information being processed, managed, analyzed, and acted upon by enterprise IT. This amazing is not part of some distant future - it is happening today. One report shows a 650% increase in enterprise data by 2020. Other estimates are even higher....
Jun. 26, 2016 05:00 PM EDT Reads: 1,337
SYS-CON Events announced today that Bsquare has been named “Silver Sponsor” of SYS-CON's @ThingsExpo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. For more than two decades, Bsquare has helped its customers extract business value from a broad array of physical assets by making them intelligent, connecting them, and using the data they generate to optimize business processes.
Jun. 26, 2016 05:00 PM EDT Reads: 1,250
Internet of @ThingsExpo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 19th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago. All major researchers estimate there will be tens of billions devices - comp...
Jun. 26, 2016 04:00 PM EDT Reads: 1,303
19th Cloud Expo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Meanwhile, 94% of enterpri...
Jun. 26, 2016 04:00 PM EDT Reads: 1,379
The 19th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Digital Transformation, Microservices and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportuni...
Jun. 26, 2016 12:00 PM EDT Reads: 1,325
Internet of @ThingsExpo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with the 19th International Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world and ThingsExpo Silicon Valley Call for Papers is now open.
Jun. 26, 2016 12:00 PM EDT Reads: 1,160
There is little doubt that Big Data solutions will have an increasing role in the Enterprise IT mainstream over time. Big Data at Cloud Expo - to be held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA - has announced its Call for Papers is open. Cloud computing is being adopted in one form or another by 94% of enterprises today. Tens of billions of new devices are being connected to The Internet of Things. And Big Data is driving this bus. An exponential increase is...
Jun. 26, 2016 12:00 PM EDT Reads: 1,381