Modern routers commonly implement a number of queue management algorithms such as Weighted Random Early Detection (WRED) and Weighted Fair Queuing (WFQ) to manage traffic. The performance of such algorithms is tightly connected to the nature of transmission control protocol (TCP) and its dynamic behavior, especially when networks experience congestion or excessive delay. Until now, testing the performance of real implementations of these algorithms has not been practical. Test equipment could not emulate TCP traffic with sufficient realism or to the scale of the internet.

For more background on TCP and its connection to queue management, please see our latest white paper at the link below.

>> White Paper: TCP and Queue Management | Stateful TCP Webpage | Integrated L2-7 Test Webpage | Back to top

Testing router queue management implementations in a meaningful sense requires that TCP emulation is both realistic and high performance.

Realistic TCP emulation must be true to the key congestion control, flow control and timing characteristics of TCP:

• Ability to re-transmit packets for the two common cases of fast re-transmit and time-out-based re-transmit.
• Support for dynamic windowing, where the number of packets "in flight" is adjusted dynamically on the fly, growing either exponentially in slow-start mode, or linearly in congestion avoidance mode.
• Being true to TCP's timing algorithm, where successive retries occur at exponentially increasing intervals.

High performance TCP emulation must scale sufficiently in several key dimensions:

• TCP traffic must occupy at least of 80-85% of the bandwidth on common interfaces, especially gigabit Ethernet.
• The TCP connection establishment rate must be on the order of hundreds of thousands per second.
• The number of simultaneous connections must be on the order of tens of thousands.

>> Stateful TCP Webpage | Integrated L2-7 Test Webpage | Back to top

TCP emulation on the Agilent N2X is implemented in hardware in order to satisfy the above test requirements. The N5575A Stateful TCP Traffic and Measurement License, which runs on the N5551A/B 4-port 10/100/1000 XR-2 test cards, is used to measure the interaction between TCP flows and queuing mechanisms. The fact that the N2X TCP emulation is uniquely implemented in hardware means users can realistically model TCP behavior while at the same time scaling to achieve performance expectations.

Key features:

• Simulate key TCP behaviors of fast re-transmit, time-out re-transmit and dynamic windowing (slow start and congestion avoidance) with realistic TCP algorithm timing
• Load a Gigabit Ethernet pipe with TCP traffic at line rate
• Establish and tear down hundreds of thousands of TCP connections per second (even at the theoretical minimum shortest packet lengths < 100 bytes) 
• Achieve consistent results between test runs (which is critical for identifying the variables impacting performance and for completing regression tests)
• Generate and make QoS measurements on a combination of wire-rate video, voice and stateful data services in the same application, and over the same test port
• Independently turn on/off key TCP mechanisms such as fast re-transmit and slow-start/congestion avoidance
• Mix stateless UDP traffic in the same test scenarios

N2X Stateful TCP traffic and measurement is integrated with existing N2X functionality, including the ability to emulate multiple protocols and generate multiple service traffic types in the same application, and over the same test port. N2X can generate a combination of 32,000 voice, video and stateful data traffic streams per port and deliver real-time performance measurements on a per-subscriber basis.

>> N2X Integrated L2-7 Test Solution Technical Datasheet | N2X Next Generation Ethernet Test Cards | Stateful TCP Webpage | Integrated L2-7 Test Webpage | Back to top

The N5575A N2X Stateful TCP Traffic and Measurement License is available on the N2X platform from Release 6.9 (August 2007) onwards. A product video that demonstrates how to test queue management implementations with the Agilent N2X is accessible at the link below. Please contact your local Agilent representative for a live product demonstration at your site.

>> N2X Release 6.9 | Demo Video | Stateful TCP Webpage | Integrated L2-7 Test Webpage | Back to top

Click here to contact your local Agilent representation for more information.

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