Network Working Group                                     B. Constantine
Internet-Draft                                                      JDSU
Intended status: Informational
Expires: April 19, 2010
                                                           Oct. 19, 2009

                    TCP Throughput Testing Methodology

Status of this Memo

     This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts. Creation date October 19, 2009.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   This memo describes a methodology for measuring TCP throughput
   performance in an end-end managed network environment.  This memo is
   intended to provide a practical approach to help users validate the
   TCP layer performance of a managed network, which should provide a
   better indication of end-user application level experience.  In the
   methodology, various TCP and network parameters are identified that
   should be tested as part of the network verification at the TCP layer.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Goals of this Methodology. . . . . . . . . . . . . . . . . . .  5
   3.  TCP Throughput Testing Methodology . . . . . . . . . . . . . .  6
     3.1.  Baseline Round-trip Delay and Bandwidth. . . . . . . . . .  6
         3.1.1  Techniques to Measure Round Trip Delay. . . . . . . .  6
         3.1.2  Techniques to Measure End-end Bandwidth . . . . . . .  7
     3.2.  Single TCP Connection Throughput Tests . . . . . . . . . .  7
     3.3.  Multiple TCP Connection Throughput Tests . . . . . . . . .  8
         3.3.1  Multiple TCP Connections - below Link Capacity. . . .  9
         3.3.2  Multiple TCP Connections - over Link Capacity . . . .  9
     3.4. Varying MSS per Connection. . . . . . . . . . . . . . . . . 10
     3.5. TCP Sessions with Stateless Background traffic. . . . . . . 10
         3.5.1. TCP Foreground Traffic Control. . . . . . . . . . . . 11
         3.5.2  Background Traffic Control. . . . . . . . . . . . . . 11
         3.5.3. Test Methodology for TCP + Background Traffic . . . . 11
     Prioritized Stateful TCP Traffic Test. . . . . 12
     Prioritized Stateless Traffic Test . . . . . . 12
     Other Traffic Test Cases . . . . . . . . . . . 13
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14

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1. Introduction

   Even though RFC2544 was meant to benchmark network equipment and
   used by network equipment manufacturers (NEMs), network providers
   have used it to benchmark operational networks in order to
   provide SLAs (Service Level Agreements) to their business customers.
   Ultimately, network providers have come to the realization that a
   successful RFC2544 test result does not guarantee end-user

   Therefore, the network provider community desires to measure network
   throughput performance at the TCP layer. Measuring TCP throughput
   will provide a much more meaningful measure that the network can meet
   the end user's application SLA (and ultimately reach some level of
   TCP testing interoperability which does not exist today).

   The complexity of the network grows and the various queuing
   mechanisms in the network greatly affect TCP layer performance (i.e.
   improper default router settings for queuing, etc.) and devices such
   as firewalls, proxies, load-balancers can actively alter the TCP
   settings as a TCP session traverses the network (such as window size,
   MSS, etc.).  These are all very complex topics to the general network
   community, and there is a strong interest to standardize a test
   methodology at the TCP layer (especially from an end-end

   Before RFC2544 testing existed, network providers and NEMs deployed
   a variety of ad hoc test techniques to verify the Layer 2/3
   performance of the network.  RFC2544 was a huge step forward in the
   network test world, standardizing the Layer 2/3 test methodology
   which greatly improved the quality of the network and reduced
   operational test expenses.  These managed networks are intended to be
   predictable, but therein lies the problem.  It is difficult if not
   impossible, to extrapolate end user application layer performance
   from RFC2544 results and the goal of RFC2544 was never intended
   to do so.

   So the intent behind this draft TCP throughput work is to define
   a methodology for testing TCP layer performance, and guidelines for
   expected TCP throughput results that should be observed in the
   network under test.  Network providers (and NEMs) are wrestling with
   end-end complexities of the above (queuing, active proxy devices,
   etc.); they desire to standardize the methodology to validate end-end
   TCP performance, as this is the precursor to acceptable end-user
   application performance.

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2. Goals of this Methodology

   Before defining the goals of this methodology, it is important to
   clearly define the areas that are not intended to be measured or
   analyzed by such a methodology.  This is important to note, since
   this methodology clearly does not intend to benchmark underlying
   mechanisms within various flavors of TCP OS implementations:

   - The methodology is not intended to definitively benchmark TCP
   implementations of one OS to another (although some users may find
   some value in conducting qualitative experiments)

   - The methodology is not intended to provide detailed diagnosis
   of problems within end-points or the network itself as related to
   non-optimal TCP performance

   - This methodology will not drill into the detailed inner behavior
   of TCP implementations nor dissect the various timers and state
   machines that can affect TCP performance

   Concerning the goals of this methodology, it is clearly from the
   perspective of a user that needs to conduct a structured, end-end
   assessment of TCP performance within a managed business class IP
   network.  A key goal is that with the collective minds of this
   working group, to establish a set of "best practices" that a user
   should apply when validating the ability of a managed network to
   carry end-user TCP applications.  Some specific goals are to:

   - Provide the logical, next-step testing methodology so that a
   provider can test the network at Layer 4 (beyond the current Layer
   2/3 RFC2544 testing approach)

   - Provide a practical test approach that specifies the more well
   understood (and end-user configurable) TCP parameters such as Window
   size, MSS, # connections, and how these affect the outcome of TCP
   performance over a network

   - For networks that have been "tuned" with proper shaping and
   queuing control mechanisms, it is desirable to define a TCP layer
   test condition that can validate if the end-end network is tuned
   as expected.

   - Testing end-end prioritization of services is a key goal.  This
   draft proposes the use of stateful TCP connections in the midst of
   stateless background traffic such as UDP, which is a very common
   service condition in managed provider networks. The ability to test
   stateful TCP with stateless traffic (with proper prioritization for
   each), is a more thorough and realistic test than simply testing with
   all stateless traffic.  Further more, many networks will not tolerate
   TCP "traffic blasting" to emulate the TCP application traffic.

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3. TCP Throughput Testing Methodology

   This section summarizes the specific test methodology to achieve the
   goals listed in Section 2.

3.1. Baseline Round-trip Delay and Bandwidth

   Before stateful TCP testing can begin, it is important to baseline
   the round trip delay and bandwidth of the network to be tested.
   These measurements provide estimates of the ideal TCP window size,
   which will be used in subsequent test steps.

   These latency and bandwidth tests should be run over a long enough
   period of time to characterize the performance of the network over
   the course of a meaningful time period.  One example would
   be to take samples during various times of the work day. The goal
   would be to determine a representative minimum, average, and maximum
   RTD and bandwidth for the network under test.  Topology changes are
   to be avoided during this time of initial convergence (e.g. in
   crossing BGP4 boundaries).

   In some cases, baselining bandwidth may not be required, since a
   network provider's end-end topology may be well enough defined.

3.1.1 Techniques to Measure Round Trip Delay

   This is not meant to provide an exhaustive list, but summarizes some
   of the more common ways to determine round trip delay (RTD) through
   the network. The desired resolution of the measurement (i.e. msec
   versus usec) may dictate whether the RTD measurement can be achieved
   with standard tools such as ICMP ping techniques or whether
   specialized test equipment would be required with high precision
   timers.  The attempt in this section is to list several techniques in
   order of decreasing accuracy.

   - Use test equipment on each end of the network, "looping" the
   far-end tester so that a packet stream can be measured end-end.  This
   test equipment RTD measurement may be compatible with delay
   measurement protocols specified in RFC5357.

   - Conduct packet captures of TCP test applications using for example
  "iperf" or FTP, etc.  By running multiple experiments, the packet
   captures can be studied to estimate RTD based upon the SYN -> SYN-ACK
   handshakes within the TCP connection set-up.

  - ICMP Pings may also be adequate to provide round trip delay
   estimations.  Some limitations of ICMP Ping are the msec resolution
   and whether the network elements / end points respond to pings (or
   block them).

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3.1.2 Techniques to Measure End-end Bandwidth

   There are many well established techniques available to provide
   estimated measures of bandwidth over a network.  This measurement
   should be conducted in both directions of the network, especially for
   access networks which are inherently asymmetrical.  Some of the
   asymmetric implications to TCP performance are documented in RFC-3449
   and the results of this work will be further studied to determine
   relevance to this draft.

   The bandwidth measurement test must be run with stateless IP streams
   (not stateful TCP) in order to determine the available bandwidth in
   each direction.  And this test should obviously be performed at
   various intervals throughout a business day (or even across a week).
   Ideally, the bandwidth test should produce a log output of the
   bandwidth achieved across the test interval AND the round trip delay.

   And during the actual TCP level performance measurements (Sections
   3.2 - 3.5), the test tool should also be able to track round trip
   delay of the TCP connection(s) during the test.  This can provide
   insight into the potential effects of congestive delay to the
   throughput achieved for the TCP layer test.

3.2. Single TCP Connection Throughput Tests

   With a reasonable representation of round trip delay and bandwidth
   from section 3.1, a series of single connection TCP throughput tests
   can be conducted to baseline the performance of the network against
   expectations.  The optimum TCP window size can be calculated from
   the bandwidth delay product (BDP), which is:

   BDP = RTD x Bandwidth

   By dividing the BDP by 8, the "ideal" TCP window size is calculated.
   An example would be a T3 link with 25 msec RTD.  The BDP would equal
   ~1,125,000 bits and the ideal TCP window would equal ~140,000 bytes.

   There are several TCP tools that are commonly used in the network
   provider world and one of the most common is the "iperf" tool.  With
   this tool, hosts are installed at each end of the network segment;
   one as client and the other as server.  The TCP Window size of both
   the client and the server can be set and the achieved throughput is
   measured, either uni-directionally or bi-directionally.  For higher
   BDP situations in lossy networks (satellite links, etc.), TCP options
   such as Selective Acknowledgment should be considered and also become
   part of the window size / throughput characterization.

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   Ideally, the single connection TCP throughput test should be run over a
   a long duration and results logged at the desired interval.  The
   test should record RTD and TCP retransmissions at each interval.  The
   combination of throughput, RTD, and retransmissions per interval
   can provide valuable insight into the resulting TCP throughput

   One important aspect of this test relates to the capabilities of the
   end test tools or hosts.  Variation in host hardware performance can
   cause the results to be erroneous.  Underperforming host hardware may
   not be able to transfer payload fast enough to the NIC card, and
   create the unintended bottleneck to TCP throughput performance.

   Host hardware performance must be well understood before conducting
   this TCP single connection test and other tests in this section.
   Dedicated test equipment may be required, especially for line rates
   of GigE and 10 GigE.

   At the end of this step, the user will document the theoretical BDP
   and a set of Window size experiments with measured TCP throughput for
   each TCP window size setting.  If network conditions (RTD and
   retransmissions) are determined to be acceptable during the test
   interval and the TCP throughput is not, then this MAY point to active
   devices within the network that are altering window size (or other)
   TCP parameters.

3.3. Multiple TCP Connection Throughput Tests

   After baselining the network under test with a single TCP connection
   (Section 3.2), the notional capacity of the network has been
   determined.  The capacity measured in section 3.2 may be a capacity
   range and it is reasonable that some level of tuning may have been
   required (i.e. router shaping techniques employed, intermediary
   proxy like devices tuned, etc.).

   Single connection TCP testing is a useful first step to measure
   expected versus actual TCP performance and as a means to diagnose
   / tune issues in the network and active elements.  However, the
   ultimate goal of this methodology is to more closely emulate customer
   traffic, which will be many TCP connections over a network link.
   This methodology inevitably seeks to provide the framework for
   testing stateful TCP connections in concurrence with stateless
   traffic streams, and this is described in Section 3.5.

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3.3.1 Multiple TCP Connections - below Link Capacity

   First, the ability of the network to carry multiple TCP connections
   to full network capacity should be tested.  Prioritization and QoS
   settings are not considered during this step, since the network
   capacity is not to be exceeded by the test traffic (section 3.3.2
   covers the over capacity test case).

   For this multiple connection TCP throughput test, the number of
   connections will more than likely be limited by the test tool (host
   vs. dedicated test equipment).  As an example, for a GigE link with
   1 msec RTD, the optimum TCP window would equal ~128 KBytes. So under
   this condition, 8 concurrent connections with window size equal to
   16KB would fill the GigE link.  For 10G, 80 connections would be
   required to accomplish the same.

   Just as in section 3.2, the end host or test tool can not be the
   processing bottleneck or the throughput measurements will not be
   valid.  The test tool must be benchmarked in ideal lab conditions to
   verify it's ability to transfer stateful TCP traffic at the given
   network line rate.

   For this test step, it should be conducted over a reasonable test
   duration and results should be logged per interval such as throughput
   per connection, RTD, and retransmissions.

   Since the network is not to be driven into over capacity (by nature
   of the BDP allocated evenly to each connection), this test verifies
   the ability of the network to carry multiple TCP connections up to
   the link speed of the network.

3.3.2 Multiple TCP Connections - over Link Capacity

   In this step, the network bandwidth is intentionally exceeded with
   multiple TCP connections to test expected prioritization and queuing
   within the network.

   All conditions related to Section 3.3 set-up apply, especially the
   ability of the test hosts to transfer stateful TCP traffic at network
   line rates.

   Using the same example from Section 3.2, a GigE link with 1 msec
   RTD would require a window size of 128 KB to fill the link (with
   one TCP connection).  Assuming a 16KB window, 8 concurrent
   connections would fill the GigE link capacity and values higher than
   8 would over-subscribe the network capacity.  The user would select
   values to over-subscribe the network (i.e. possibly 10 15, 20, etc.)
   to conduct experiments to verify proper prioritization and queuing
   within the network.

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   Without any prioritization in the network, the over subscribed test
   results could assist in the queuing studies.  With proper queuing,
   the bandwidth should be shared in a reasonable manner.  The author
   understands that the term "reasonable" is too wide open, and future
   draft versions of this memo would attempt to quantify this sharing
   in more tangible terms.  It is known that if a network element
   is not set for proper queuing (i.e. FIFO), then an oversubscribed
   TCP connection test will generally show a very uneven distribution of

   With prioritization in the network, different TCP connections can be
   assigned various QoS settings via the various mechanisms (i.e. per
   VLAN, DSCP, etc.), and the higher priority connections must be
   verified to achieve the expected throughput.

3.4. Varying MSS per Connection

   This test step can be run either on a single TCP connection test
   (Section 3.2) or a multiple TCP connection test (section 3.3).

   By varying the MSS size of the TCP connection(s), the ability of the
   network to sustain expected TCP throughput can be verified.  This is
   similar to frame and packet size techniques within RFC2-2544, which
   aim to determine the ability of the routing/switching devices to
   handle loads in term of packets/frames per second at various frame
   and packet sizes.  This test can also further characterize the
   performance of a network in the presence of active TCP elements
   (proxies, etc.), devices that fragment IP packets, and the actual
   end hosts themselves (servers, etc.).

   The single connection testing listed in Section 3.2 should be
   repeated first, using the appropriate window size and collecting
   throughput measurements per various MSS sizes. It would be reasonable
   to run single TCP connection tests with MSS sizes of 24, 88, 216,
   472, 984, and 1460 bytes.  These tests should also be run over a
   predetermined test interval and the throughput, retransmissions, and
   RTD logged during the entire test interval.

3.5. TCP Sessions with Stateless Background Traffic

   The ultimate intent of this methodology is to more accurately assess
   the ability of the network to carry end user TCP application traffic
   in the midst of stateless background traffic.  The background traffic
   may be of a lower priority than the TCP traffic (i.e. background is
   best effort Internet), or the background traffic may be of higher
   priority (i.e.UDP representing voice).  The prioritization must be
   configured properly throughout the network to allow either the TCP
   foreground traffic or the stateless background traffic to receive
   the expected priority.

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   For this test, each stateful TCP connection must be able to have a
   unique prioritization.  Depending upon the network prioritization
   scheme (i.e. VLAN, DSCP, MPLS, etc.), the test system must allow for
   unique identification of each connection.  The same applies for the
   stateless background streams.  The individual background streams must
   also be tagged or identified based upon the prioritization mechanism.

3.5.1. TCP Foreground Traffic Control

   In addition to the prioritization of each individual TCP connection,
   the desired bandwidth for each TCP connection must be configurable.
   Depending upon the capabilities of the test system, this bandwidth
   rate may be discretely controlled (or shaped) by the test system or
   may also be controlled by limiting the window size of each
   connection.  The sophistication of the test system will dictate which
   bandwidth control mechanism is used for the TCP connections.  The
   window based approach allows the TCP traffic to reach the maximum
   achievable within the bandwidth capacity of the link (BDP), which
   may be the intended test configuration.  Each TCP foreground
   connection should also have configurable MSS sizes as well.

3.5.2  Stateless Background Traffic Control

   Each stateless background traffic stream must be configurable in
   terms of the offered network bandwidth. The frame / packet sizes
   of the background traffic streams should be individually
   configurable.  Ideally, the test system should allow for the
   stateless background traffic to ramp up from a configurable starting
   bandwidth to the final bandwidth setting (i.e. start at 10 Mbps,
   incrementing by 5 Mbps, until reaching 50 Mbps).  The time step of
   the ramping function should also be programmable.  Ramping of the
   background traffic facilitates the study of the effect of stateless
   background traffic on foreground TCP traffic.

3.5.3. Test Methodology for TCP + Stateless Background Traffic

   Depending upon the prioritization within the provider's network,
   there are many permutations of prioritization between TCP sessions,
   background streams, and combinations between.  This memo will
   summarize two common use cases: 1) higher priority TCP stateful
   traffic in the midst of best effort background traffic; 2) higher
   priority stateless traffic (i.e. VoIP) in the midst of lower
   priority TCP stateful traffic.

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   In this test, the intent is to verify that a business class data
   service (i.e. thin client, web-based application traffic) is given
   proper priority in times of high utilization.  For this test
   case, the TCP connections are given priority via the prioritization
   mechanism used in the network (VLAN, DSCP, etc..) and the stateless
   background traffic is given lower priority (or best effort).

   By one of the traffic control techniques listed in Section 3.5.1, the
   stateful TCP connections are allocated bandwidth within the test
   system and ideally the background traffic will perform a ramp traffic
   function.  The results of this test should show that the
   prioritized stateful TCP traffic reaches the designated throughput
   and is not disturbed when the best effort background traffic exceeds
   the link capacity.  The test results should be logged during the test
   interval, with each TCP connection's throughput, retransmissions, and
   RTD recorded (along with the background traffic levels). Prioritized Stateless Traffic Test

   The corollary to section is the case where the stateless
   traffic is higher priority than the stateful TCP traffic.  This may
   be the case where a network provider is offering VoIP services in
   addition to regular IP data service.  Even though the TCP traffic is
   prioritized lower than the stateless traffic, it is important to
   determine how the TCP traffic reacts in the presence of over
   subscription (this can again point to non-optimized queuing
   techniques in the network for the TCP traffic as discussed in
   Section 3.3.2.).

   With the TCP offered bandwidth set by one of the traffic control
   mechanisms listed in Section 3.5.1., the higher priority stateless
   traffic should be ramped up to exceed the link capacity.  The results
   of this test should show that the higher priority stateless traffic
   achieves the designated bandwidth and that the TCP connection
   bandwidth is reduced.  By studying the logged TCP and stateless
   traffic throughput over the test interval (and the retransmissions
   + RTD for the TCP traffic), the manner in which the TCP connections
   shared the remaining bandwidth may provide insight into possible
   queuing optimizations in the network.

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   Sections and lay out the basic foundation for
   testing the prioritization effects of TCP traffic and background
   stateless traffic.  There are many hybrids that can also be
   pertinent, dependant upon the network provider's offering.  An
   example would be strictly an IP service type test.  In this case,
   the network provider seeks to test various prioritizations of each
   stateful TCP connection and verify that the higher priority TCP
   connection(s) achieve the SLA bandwidth while the others do not.
   Regardless of the prioritization profile of the TCP connections
   and the background streams, the same test results should be
   recorded across the entire test interval (as specified in Section and

4.  Acknowledgements

   The author would like to thank Gilles Forget, Mike Hamilton,
   and Reinhard Schrage for technical review and contributions to this
   draft-00 memo.

   Also thanks to Matt Mathis and Matt Zekauskas for many good comments
   through email exchange and for pointing me to great sources of
   information pertaining to past works in the TCP capacity area.

5.  References

   [RFC2581]  Allman, M., Paxson, V., Stevens W., "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC3148]  Mathis M., Allman, M., "A Framework for Defining
              Empirical Bulk Transfer Capacity Metrics", RFC 3148, July

   [RFC2544]  Bradner, S., McQuaid, J., "Benchmarking Methodology for
              Network Interconnect Devices", RFC 2544, March 1999

   [RFC3449]  Balakrishnan, H., Padmanabhan, V. N., Fairhurst, G.,
              Sooriyabandara, M., "TCP Performance Implications of
              Network Path Asymmetry", RFC 3449, December 2002

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., Babiarz,
              J., "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, October 2008

   draft-ietf-ippm-btc-cap-00.txt Allman, M., "A Bulk Transfer Capacity
              Methodology for Cooperating Hosts", August 2001

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Author's Address

   Barry Constantine
   JDSU, Test and Measurement Division
   One Milesone Center Court
   Germantown, MD 20876-7100

   Phone: +1 240 404 2227

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