Network Working Group                                       B. Constantine
Internet-Draft                                                 	      JDSU
Intended status: Informational                                   G. Forget
Expires: December 18, 2010 January 9, 2011                     Bell Canada (Ext. Consultant)
                                                              L. Jorgenson
                                                        Apparent Networks
                                                          Reinhard Schrage
                                                        Schrage Consulting
                                                             June 8,
                                                              July 9, 2010

                    TCP Throughput Testing Methodology


   This memo describes a methodology for measuring sustained TCP
   throughput performance in an end-to-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

Status of this Memo

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Goals of this Methodology. . . . . . . . . . . . . . . . . . .  4
     2.1   TCP Equilibrium State Throughput . . . . . . . . . . . . .  5
     2.2   Metric   Metrics for TCP Throughput Tests . . . . . . . . . . . . .  6
   3.  TCP Throughput Testing Methodology . . . . . . . . . . . . . .  7  6
     3.1   Determine Network Path MTU . . . . . . . . . . . . . . . .  8
     3.2.  Baseline Round-trip Delay and Bandwidth. . . . . . . . . .  9
         3.2.1  Techniques to Measure Round Trip Time . . . . . . . . 10  9
         3.2.2  Techniques to Measure End-end Bandwidth . . . . . . . 10
     3.3.  Single  TCP Connection Throughput Tests . . . . . . . . . . 11 . . . . . . . . . 10
         3.3.1 Interpretation of Calculate Optimum TCP Window Size. . . . . . . . . . . 11
         3.3.2 Conducting the Single Connection TCP Throughput Results Tests. . . . . . . . . . 14
         3.3.3 Single vs. Multiple TCP Connection Testing . . . . . . 14
         3.3.4 Interpretation of the TCP Throughput Results . . . . . 14 15
     3.4. Traffic Management Testing Tests .  . . . . . . . . . . . . . . . 14 . 15
         3.4.1 Multiple TCP Connections - below Link Capacity Traffic Shaping Tests. . . . . 14
         3.4.2 Multiple TCP Connections - over Link Capacity. . . . . 15
         3.4.3 . . . . . . . . 16
 Interpretation of Multiple TCP Connection Traffic Shaping Test Results. . . 16 17
         3.4.2 RED Tests. . . . . . . . . . . . . . . . . . . . . . . 17
 Interpretation of RED Results . . . . . . . . . . . 18
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 18
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 20

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
   verify SLAs (Service Level Agreements) before turning on a service
   to their business customers.  Testing an operational network prior to
   customer activation is referred to as "turn-up" testing and the SLA
   is generally Layer 2/3 packet throughput, delay, loss and

   Network providers are coming to the realization that RFC2544 Layer 2/3 testing
   and TCP layer testing are required to more adequately ensure end-user
   satisfaction. Therefore, the network provider community desires to
   measure network throughput performance at the TCP layer. Measuring
   TCP throughput provides a meaningful measure with respect to 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

   Additionally, end-users (business enterprises) seek to conduct
   repeatable TCP session traverses throughput tests between enterprise locations.  Since
   these enterprises rely on the network (such as window size,
   MSS, etc.).  Network providers (and NEMs) are wrestling with end-end
   complexities networks of the above and there is a strong interest in the
   standardization of providers, a common test
   methodology (and metrics) would be equally beneficial to validate end-to-end TCP
   performance (as this is the precursor to acceptable end-user
   application performance). both parties.

   So the intent behind this draft TCP throughput work is to define
   a methodology for testing sustained TCP layer performance.  In this
   document, sustained TCP throughput is that amount of data per unit
   time that TCP transports during equilibrium (steady state), i.e.
   after the initial slow start phase. We refer to this state as TCP
   Equilibrium, and that the equalibrium throughput is the maximum
   achievable for the TCP connection(s).

   One other important note; the precursor to conducting the TCP tests
   test methodlogy is to perform "network stress tests" such as RFC2544
   Layer 2/3 tests or other conventional tests (OWAMP, etc.). tests.  Examples include
   OWAMP or manual packet layer test techniques where packet throughput,
   loss, and delay measurements are conducted.  It is  highly recommended
   to run traditional Layer 2/3 type test to verify the integrity of the
   network before conducting TCP testing. tests.

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.

   - The methodology is not intended to predict TCP throughput
   behavior during the transient stages of a TCP connection, such
   as initial slow start.
   - 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, although a results interpretation
   section for each test step may provide insight into potential
   issues within the network

   In contrast to the above exclusions, the goals of this methodology
   are to define a method to conduct a structured, end-to-end
   assessment of sustained TCP performance within a managed business
   class IP network.  A key goal is to establish a set of "best
   practices" that an engineer should apply when validating the
   ability of a managed network to carry end-user TCP applications.

   Some specific goals are to:

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

   - Provide specific test conditions (link speed, RTT, window size,
   etc.) and maximum achievable TCP throughput under TCP Equilbrium
   conditions.  For guideline purposes, provide examples of these test
   conditions and the maximum achievable TCP throughput during the
   equilbrium state.  Section 2.1 provides specific details concerning
   the definition of TCP Equilibrium within the context of this draft.

   - In test situations where the Define two (2) basic metrics that can be used to compare the
   performance of TCP connections under various network conditions

   - In test situations where the recommended procedure does not yield
   the maximum achievable TCP throughput result, this draft provides some
   possible areas within the end host or network that should be
   considered for investigation (although again, this draft is not
   intended to provide a detailed diagnosis of these issues)

2.1 TCP Equilibrium State Throughput

   TCP connections have three (3) fundamental congestion window phases
   as documented in RFC2581.  These states are:

   - Slow Start, which occurs during the beginning of a TCP transmission
   or after a retransmission time out event

   - Congestion avoidance, which is the phase during which TCP ramps up
   to establish the maximum attainable throughput on an end-end network
   path.  Retransmissions are a natural by-product of the TCP congestion
   avoidance algorithm as it seeks to achieve maximum throughput on
   the network path.

   - Retransmission phase, which include Fast Retransmit (Tahoe) and Fast
   Recovery (Reno and New Reno).  When a packet is lost, the Congestion
   avoidance phase transitions to a Fast Retransmission or Recovery
   Phase dependent upon the TCP implementation.

   The following diagram depicts these states.

            |        ssthresh
   TCP      |           |
   Through- |           |       Equilibrium
   put      |           |\      /\/\/\/\/\  Retransmit          /\/\ ...
            |           | \    /         |  Time-out           /
            |           |  \  /          |  _______          _/
            |  Slow   _/    |/           | /       | Slow  _/
            | Start _/      Congestion   |/        |Start_/   Congestion
            |     _/         Avoidance   Loss      |   _/     Avoidance
            |   _/                       Event     | _/
            | _/                                   |/

   This TCP methodology provides guidelines to measure the equilibrium
   throughput which refers to the maximum sustained rate obtained by
   congestion avoidance before packet loss conditions occur (which would
   cause the state change from congestion avoidance to a retransmission
   phase). All maximum achievable throughputs specified in Section 3 are
   with respect to this Equilibrium state.

2.2 Metrics for TCP Throughput Tests

   This draft focuses on a TCP throughtput methodology and also
   provides two basic metrics to compare results of various throughput
   tests.  It is recognized that the complexity and unpredictability of
   TCP makes it impossible to develop a complete set of metrics that
   account for the myriad of variables (i.e. RTT variation, loss
   conditions, TCP implementation, etc.).  However, these two basic
   metrics are useful to
   compare faciliate TCP throughput comparisons under varying network
   conditions and between network traffic management techniques, especially in section
   3.4 of this document (Traffic Management Tests). techniques.

   The TCP Efficiency metric is the percentage of bytes that were not
   retransmitted and is defined as:

                Transmitted Bytes - Retransmitted Bytes
                ---------------------------------------  x 100
                          Transmitted Bytes

   This metric is easy to understand and provides a comparative measure between various QoS
   mechanisms such as traffic management, congestion avoidance, and also
   various TCP implementations (i.e. Reno, Vegas, etc.).

   As an example, if 1000 TCP segments were sent and 20 had to be
   retransmitted, the TCP Efficiency would be calculated as:

                   1000 - 20
                   ---------  x 100 = 98%

   The second measure is also basic and metric is the TCP Transfer Time, which is simply the time is
   it takes to transfer a block of data across simultaneous TCP
   connections.  The concept is useful to benchmark when benchmarking traffic
   management tests, techniques, where multiple connections are required and
   it simplifies comparing results of different approaches. generally
   required.  An example would be the bulk transfer of 10 MB upon 8
   separate TCP connections (each connection uploading 10 MB).  Each
   connection may achieve different throughputs during a test and the
   overall throughput rate is not always easy to determine (especially as
   the number of connections increases).  But by defining the TCP Transfer
   Time as that of the successful total transfer time of 10MB over all 8 connections, the
   single transfer time metric is very a useful means to rate compare various
   traffic management techniques (i.e. FiFO, WFQ queuing, WRED, etc.).

3. TCP Throughput Testing Methodology

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

   As stated in Section 1, it is considered best practice to verify
   the integrity of the network by conducting Layer2/3 stress tests
   such as RFC2544 or (or other methods of network stress tests. tests).  If the
   network is not performing properly in terms of packet loss, jitter,
   etc. then the TCP layer testing will not be meaningful since the
   equalibrium throughput would be very difficult to achieve (in a
   "dysfunctional" network).

   The following provides represents the sequential order of steps to conduct the
   TCP throughput testing methodology:

   1. Identify the Path MTU.  Packetization Layer Path MTU Discovery
   or PLPMTUD (RFC4821) should be conducted to verify the minimum network
   path MTU.  Conducting PLPMTUD establishes the upper limit for the MSS
   to be used in subsequent steps.

   2. Baseline Round-trip Delay and Bandwidth. These measurements provide
   estimates of the ideal TCP window size, which will be used in
   subsequent test steps.

   3. Single TCP Connection Throughput Tests.  With baseline measurements
   of round trip delay and bandwidth, a series of single connection and multiple TCP
   connection throughput tests can be conducted to baseline the performance of the network against
   performance expectations.

   4. Traffic Management Tests.  Various traffic management and queuing
   techniques are tested in this step, using multiple TCP connections.
   Multiple connection testing can verify that the network is configured
   properly for traffic shaping versus policing, various queuing
   implementations, and RED.

   Important to note are some of the key characteristics and
   considerations for the TCP test instrument.  The test host may be a
   standard computer or dedicated communications test instrument
   and these TCP test hosts be capable of emulating both a client and a
   server.  As a general rule of thumb, testing TCP throughput at rates
   greater than 250-500 Mbit/sec generally requires high performance
   server hardware or dedicated hardware based test tools.

   Whether the TCP test host is a standard computer or dedicated test
   instrument, the following areas should be considered when selecting
   a test host:

   - TCP implementation used by the test host OS, i.e. Linux OS kernel
   using TCP Reno, TCP options supported, etc.  This will obviously be
   more important when using custom test equipment where the TCP
   implementation may be customized or tuned to run in higher
   performance hardware
   - Most importantly, the TCP test host must be capable of generating
   and receiving stateful TCP test traffic at the full link speed of the
   network under test. This requirement is very serious and may require
   custom As a general rule of thumb, testing TCP throughput
   at rates greater than 100 Mbit/sec generally requires high
   performance server hardware or dedicated hardware based test equipment, especially on 1 GigE and 10 GigE networks. tools.

3.1. Determine Network Path MTU

   TCP implementations should use Path MTU Discovery techniques (PMTUD).
   PMTUD relies on ICMP 'need to frag' messages to learn the path MTU.
   When a device has a packet to send which has the Don't Fragment (DF)
   bit in the IP header set and the packet is larger than the Maximum
   Transmission Unit (MTU) of the next hop link, the packet is dropped
   and the device sends an ICMP 'need to frag' message back to the host
   that originated the packet. The ICMP 'need to frag' message includes
   the next hop MTU which PMTUD uses to tune the TCP Maximum Segment
   Size (MSS). Unfortunately, because many network managers completely
   disable ICMP, this technique does not always prove reliable in real
   world situations.

   Packetization Layer Path MTU Discovery or PLPMTUD (RFC4821) should
   be conducted to verify the minimum network path MTU.  PLPMTUD can
   be used with or without ICMP. The following sections provide a
   summary of the PLPMTUD approach and an example using the TCP
   protocol. RFC4821 specifies a search_high and search_low parameter
   for the MTU.  As specified in RFC4821, a value of 1024 is a generally
   safe value to choose for search_low in modern networks.

   It is important to determine the overhead of the links in the path,
   and then to select a TCP MSS size corresponding to the Layer 3 MTU.
   For example, if the MTU is 1024 bytes and the TCP/IP headers are 40
   bytes, then the MSS would be set to 984 bytes.

   An example scenario is a network where the actual path MTU is 1240
   bytes.  The TCP client probe MUST be capable of setting the MSS for
   the probe packets and could start at MSS = 984 (which corresponds
   to an MTU size of 1024 bytes).

   The TCP client probe would open a TCP connection and advertise the
   MSS as 984.  Note that the client probe MUST generate these packets
   with the DF bit set. The TCP client probe then sends test traffic
   per a nominal window size (8KB, etc.).  The window size should be
   kept small to minimize the possibility of congesting the network,
   which could induce congestive loss.  The duration of the test should
   also be short (10-30 seconds), again to minimize congestive effects
   during the test.

   In the example of a 1240 byte path MTU, probing with an MSS equal to
   984 would yield a successful probe and the test client packets would
   be successfully transferred to the test server.

   Also note that the test client MUST verify that the MSS advertised
   is indeed negotiated.  Network devices with built-in Layer 4
   capabilities can intercede during the connection establishment
   process and reduce the advertised MSS to avoid fragmentation.  This
   is certainly a desirable feature from a network perspective, but
   can yield erroneous test results if the client test probe does not
   confirm the negotiated MSS.

   The next test probe would use the search_high value and this would
   be set to MSS = 1460 to correspond to a 1500 byte MTU.  In this
   example, the test client would retransmit based upon time-outs (since
   no ACKs will be received from the test server).  This test probe is
   marked as a conclusive failure if none of the test packets are
   ACK'ed.  If any of the test packets are ACK'ed, congestive network
   may be the cause and the test probe is not conclusive.  Re-testing
   at other times of the day is recommended to further isolate.

   The test is repeated until the desired granularity of the MTU is
   discovered.  The method can yield precise results at the expense of
   probing time.  One approach would be to reduce the probe size to
   half between the unsuccessful search_high and successful search_low
   value, and increase by increments of 1/2 when seeking the upper

3.2. 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-to-end topology may be well enough defined.

   3.2.1 Techniques to Measure Round Trip Time

   We follow in

   Following the definitions used in the references of the appendix;
   Round Trip Time (RTT) is the time elapsed between the clocking in of
   the first bit of a payload packet to the receipt of the last bit of the
   corresponding acknowledgement.  Round Trip Delay (RTD) is used
   synonymously to twice the Link Latency.

   In any method used to baseline round trip delay between network
   end-points, it is important to realize that network latency is the
   sum of inherent network delay and congestion.  The RTT should be
   baselined during "off-peak" hours to obtain a reliable figure for
   network latency (versus additional delay caused by congestion).

   During the actual sustained TCP throughput tests, it is critical
   to measure RTT along with measured TCP throughput. Congestive
   effects can be isolated if RTT is concurrently measured measured.

   This is not meant to provide an exhaustive list, but summarizes some
   of the more common ways to determine round trip time (RTT) through
   the network. The desired resolution of the measurement (i.e. msec
   versus usec) may dictate whether the RTT 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 objective 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 RTT 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 RTT based upon the SYN -> SYN-ACK
   handshakes within the TCP connection set-up.

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

   3.2.2 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
   3.3 - 3.5), the test tool must be able to track round trip time
   of the TCP connection(s) during the test.  Measuring round trip time
   variation (aka "jitter") provides insight into effects of congestive
   delay on the sustained throughput achieved for the TCP layer test.

3.3. Single TCP Connection Throughput Tests

   This draft specifically defines TCP throughput techniques to verify
   sustained TCP performance in a managed business network.  Defined
   in section 2.1, the equalibrium throughput reflects the maximum

   rate achieved by a TCP connection within the congestion avoidance
   phase on a end-end network path.  This section and others will define
   the method to conduct these sustained throughput tests and guidelines
   of the predicted results.

   With baseline measurements of round trip time and bandwidth
   from section 3.2, a series of single connection and multiple TCP connection
   throughput tests can be conducted to baseline the performance of the network performance
   against expectations.

3.3.1 Calculate Optimum TCP Window Size

   The optimum TCP window size can be calculated from the bandwidth delay
   product (BDP), which is:

   BDP (bits) = RTT (sec) x Bandwidth (bps)

   By dividing the BDP by 8, the "ideal" TCP window size is calculated.
   An example would be a T3 link with 25 msec RTT.  The BDP would equal
   ~1,105,000 bits and the ideal TCP window would equal ~138,000 bytes.

   The following table provides some representative network link speeds,
   latency, BDP, and associated "optimum" TCP window size.  Sustained
   TCP transfers should reach nearly 100% throughput, minus the overhead
   of Layers 1-3 and the divisor of the MSS into the window.

   For this single connection baseline test, the MSS size will effect
   the achieved throughput (especially for smaller TCP window sizes).
   Table 3.2 provides the achievable, equalibrium TCP throughput (at
   Layer 4) using 1000 1460 byte MSS.  Also in this table, the case of 58 byte
   L1-L4 overhead including the Ethernet CRC32 is used for simplicity.

   Table 3.2: Link Speed, RTT and calculated BDP, TCP Throughput

   Link                               Ideal TCP      Maximum Achievable
   Speed*    RTT (ms)  BDP (bits)  Window (kbytes)  TCP Throughput(Mbps)
    T1         20        30,720          3.84              1.20              1.17
    T1         50        76,800          9.60 	           1.44 	           1.40
    T1        100       153,600         19.20              1.44              1.40
    T3         10       442,100         55.26             41.60             42.05
    T3         15       663,150         82.89             41.13             42.05
    T3         25     1,105,250        138.16             41.92             41.52
    T3(ATM)    10       407,040         50.88             32.44             36.50
    T3(ATM)    15       610,560         76.32             32.44             36.23
    T3(ATM)    25     1,017,600        127.20             32.44             36.27
    100M        1       100,000         12.50             90.699             91.98
    100M        2       200,000         25.00             92.815

   Link                               Ideal TCP      Maximum Achievable
   Speed*    RTT (ms)  BDP (bits)  Window (kbytes)  TCP Throughput (Mbps)
   ----------------------------------------------------------------------             93.44
    100M        5       500,000         62.50             90.699             93.44
    1Gig      0.1       100,000         12.50            906.991            919.82
    1Gig      0.5       500,000         62.50            906.991            934.47
    1Gig        1     1,000,000        125.00            906.991            934.47
    10Gig     0.05      500,000         62.50          9,069.912          9,344.67
    10Gig     0.3     3,000,000        375.00          9,069.912          9,344.67

   * Note that link speed is the minimum link speed throughput a network;
   i.e. WAN with T1 link, etc.

   Also, the following link speeds (available payload bandwidth) were
   used for the WAN entries:

   - T1 = 1.536 Mbits/sec (B8ZS line encoding facility)
   - T3 = 44.21 Mbits/sec (C-Bit Framing)
   - T3(ATM) = 36.86 Mbits/sec (C-Bit Framing & PLCP, 96000 Cells per

   The calculation method used in this document is a 3 step process :

   1 - We determine what should be the optimal TCP Window size value
       based on the optimal quantity of "in-flight" octets discovered by
       the BDP calculation. We take into consideration that the TCP
       Window size has to be an exact multiple value of the MSS.
   2 - Then we calculate the achievable layer 2 throughput by multiplying
       the value determined in step 1 with the MSS & (MSS + L2 + L3 + L4
       Overheads) divided by the RTT.
   3 - Finally, we multiply the calculated value of step 2 by the MSS
       versus (MSS + L2 + L3 + L4 Overheads) ratio.

   This gives us the achievable TCP Throughput value.  Sometimes, the
   maximum achievable throughput is limited by the maximum achievable
   quantity of Ethernet Frames per second on the physical media. Then
   this value is used in step 2 instead of the calculated one.

   There are several

  The following diagram compares achievable TCP tools that are commonly used in the network
   provider world and one throughputs on a T3 link
  with Windows 2000/XP TCP window sizes 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 maunally set and the achieved
   throughput is measured, either uni-directionally or bi-directionally.
   For higher BDP situations in lossy networks (long fat networks or
   satellite links, etc.), TCP options such as Selective Acknowledgment
   should be considered and also become part of the window
   size / throughput characterization.

   The following diagram shows the achievable TCP throughput on a T3 with
   the default Windows2000/XP TCP Window size of 17520 Bytes. 16KB versus 64KB.

             |          _____42.1M
           40|          |64K|
TCP          |          |   |
Throughput 35|          |   |           _____34.3M
in Mbps      |          |   |           |64K|
           30|          |   |           |   |
             |          |   |           |   |
           25|          |   |           |   |
             |          |   |           |   |
           20|          |   |           |   |           _____20.5M
             |          |   |           |   |           |64K|
           15|         _______ 14.48M 14.5M____|   |           |   |           |   |
             |      |16K|   |           |   |           |   |
           10|      |   |         +-----+ 9.65M   |   9.6M+---+   |           |   |
             |        _______ 5.79M      |   |   |       |16K|   |   5.8M____+   |
            5|      |   |   |       |   |   |
             |_________+_____+_________+_____+________+____ +___________       |16K|   |
             |______+___+___+_______+___+___+_______+__ +___+_______
                        10              15              25
                                RTT in milliseconds

   The following diagram shows the achievable TCP throughput on a 25ms T3
   when the TCP Window size is increased and with the RFC1323 TCP Window
   scaling option.

             |                                              +-----+42.47M
           40|                                              |     |
TCP          |                                              |     |
Throughput 35|                                              |     |
in Mbps      |                                              |     |
           30|                                              |     |
             |                                              |     |
           25|                                              |     |
             |                                ______ 21.23M |     |
           20|                                |    |        |     |
             |                                |    |        |     |
           15|                                |    |        |     |
             |                                |    |        |     |
           10|               +----+10.62M     |    |        |     |
             |  _______5.31M |    |           |    |        |     |
            5|  |     |      |    |           |    |        |     |
                   16           32           64              128
                               TCP Window size in Kilo Bytes

   The single connection KBytes

3.3.2 Conducting the TCP throughput test must be run over a
   a long duration and results must be logged at Throughput Tests

   There are several TCP tools that are commonly used in the desired interval.
   The test must record RTT network
   world and TCP retransmissions one of the most common is the "iperf" tool.  With this tool,
   hosts are installed at each interval.

   This correlation end of retransmissions the network segment; one as client
   and RTT over the course other as server.  The TCP Window size of both the
   test will clearly identify which portions of client and
   the transfer reached
   TCP Equilbrium state server can be maunally set and to what effect increased RTT (congestive
   effects) may have been the cause of reduced equilibrium performance. achieved throughput is measured,
   either uni-directionally or bi-directionally.  For higher BDP
   situations in lossy networks (long fat networks or satellite links,
   etc.), TCP options such as Selective Acknowledgment should be
   considered and also become part of the window size / throughput

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

3.3.1 Interpretation

   The TCP throughput test should be run over a a long enough duration
   to properly exercise network buffers and also characterize performance
   during different time periods of the Single Connection day.  The results must be logged
   at the desired interval and the test must record RTT and TCP Throughput Results

   retransmissions at each interval.

   This correlation of retransmissions and RTT over the end course of this step, the user
   test will document clearly identify which portions of the theoretical BDP transfer reached
   TCP Equilbrium state and a set to what effect increased RTT (congestive
   effects) may have been the cause of Window size experiments with measured reduced equilibrium performance.

   Additionally, the TCP throughput for
   each Efficiency and TCP Transfer time metrics should
   be logged in order to further characterize the window size setting.  For cases where the sustained tests.

3.3.3 Single vs. Multiple TCP
   throughput does not equal the predicted value, some possible causes
   are listed:

   - Network congestion causing packet loss
   - Network congestion not causing packet loss, but effectively
   increasing Connection Testing

   The decision whether to conduct single or multiple TCP connection
   tests depends upon the size of the required TCP window during the transfer
   - Intermediate network devices which actively regenerate BDP in relation to the TCP
   connection and can alter window size, MSS, etc.

3.4. Traffic Management Tests

   After baselining sizes
   configured in the network under end-user environment.  For example, if the BDP for a
   long-fat pipe turns out to be 2MB, then it is probably more realistic
   to test this pipe with a single TCP connection
   (Section 3.3), multiple connections. Assuming typical host
   computer window settings of 64 KB, using 32 connections would
   realistically test this pipe.

   The following table is provided to illustrate the nominal capacity relationship of the network has been
   determined.  The capacity measured in section 3.3 may be a capacity
   BDP, window size, and it is reasonable that some level the number of tuning may have been connections required (i.e. router shaping techniques employed, intermediary
   proxy like devices tuned, etc.).

   Single connection to utilize the
   the available capacity.  For this example, the network bandwidth is
   500 Mbps, RTT is equal to 5 ms, and the BDP equates to 312 KBytes.

    Window    to Fill Link
    16KB          20
    32KB          10
    64KB           5
    128KB          3

   The TCP testing Transfer Time metric is a useful first step for conducting multiple
   connection tests.  Each connection should be configured to measure
   expected versus actual TCP performance transfer
   a certain payload (i.e. 100 MB), and as the TCP Transfer time provides
   a means simple metric to diagnose
   / tune issues in verify the network and active elements.  However, actual versus expected results.

   Note that the
   ultimate goal of this methodology is to more closely emulate customer
   traffic, which comprise many TCP transfer time is the time for all connections over a network link.

3.4.1 Multiple TCP Connections - below Link Capacity

   First, to
   complete the ability transfer of the network to carry multiple TCP configured payload size.  From the
   example table listed above, the 64KB window is considered.  Each of
   the 5 connections
   to full network capacity should would be tested.  Prioritization configured to transfer 100MB, and QoS
   settings are not considered during each
   TCP should obtain a maximum of 100 Mb/sec per connection.  So for this step, since
   example, the network
   capacity is not to 100MB payload should be exceeded by transferred across the connections
   in approximately 8 seconds (which would be the ideal TCP transfer time
   for these conditions).

   Additionally, the TCP Efficiency metric should be computed for each
   connection tested (defined in section 2.2).

3.3.4 Interpretation of the TCP Throughput Results

   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.  For cases where the sustained TCP
   throughput does not equal the predicted value, some possible causes
   are listed:

   - Network congestion causing packet loss; the TCP Efficiency metric
   is a useful gauge to compare network performance
   - Network congestion not causing packet loss but increasing RTT
   - Intermediate network devices which actively regenerate the TCP
   connection and can alter window size, MSS, etc.
   - Over utilization of available link or rate limiting (policing). More
   discussion of traffic management tests follows in section 3.4

3.4. Traffic Management Tests

   In most cases, the network connection between two geographic locations
   (branch offices, etc.) is lower than the network connection of the
   host computers.  An example would be LAN connectivity of GigE and
   WAN connectivity of 100 Mbps.  The WAN connectivity may be physically
   100 Mbps or logically 100 Mbps (over a GigE WAN connection).  In the
   later case, rate limiting is used to provide the WAN bandwidth per the

   Traffic management techniques are employed to provide various forms of
   QoS, the more common include:

   - Traffic Shaping
   - Priority Queuing
   - Random Early Discard (RED, etc.)

   Configuring the end-end network with these various traffic management
   mechanisms is a complex under-taking.  For traffic shaping and RED
   techniques, the end goal is to provide better performance for bursty
   traffic such as TCP (RED is specifically intended for TCP).

   This section of the methodology provides guidelines to test traffic
   shaping and RED implementations.  As in section 3.3, host hardware
   performance must be well understood before conducting the traffic
   shaping and RED tests. Dedicated test equipment will generally be
   required, especially for line rates of GigE and 10 GigE.

3.4.1 Traffic Shaping Tests

   For services where the available bandwidth is rate limited, there are
   two (2) techniques used to implement rate limiting: traffic policing
   and traffic shaping.

   Simply stated, traffic policing marks and/or drops packets which
   exceed the SLA bandwidth (in most cases, excess traffic is dropped).
   Traffic shaping employs the use of queues to smooth the bursty
   traffic and then send out within the SLA bandwidth limit (without
   dropping packets unless the traffic shaping queue is exceeded).

   Traffic shaping is generally configured for TCP data services and
   can provide improved TCP performance since the retransmissions are
   reduced, which in turn optimizes TCP throughput for the given
   available bandwidth.  Through this section, the available rate-limited
   bandwidth shall be referred to as the "bottleneck bandwidth".

   The ability to detect proper traffic shaping is more easily diagnosed
   when conducting a multiple TCP connection test.  Proper shaping will
   provide a fair distribution of the test available bottleneck bandwidth,
   while traffic (section 3.5.2
   covers policing will not.

   The traffic shaping tests build upon the over capacity test case).

   For this concepts of multiple
   connection TCP throughput test, testing as defined in section 3.3.3.  Calculating the BDP
   for the bottleneck bandwidth is first required and then selecting
   the number of connections will more than likely be limited by / window size per connection.

   Similar to the test tool (host
   vs. dedicated test equipment).  As an example, for example in section 3.3, a typical test scenario might
   be:  GigE link LAN with
   1 a 100Mbps bottleneck bandwidth (rate limited logical
   interface), and 5 msec RTT, the optimum TCP window RTT.  This would equal ~128 KBytes. So under
   this condition, 8 concurrent require five (5) TCP
   connections with of 64 KB window size equal to
   16KB would evenly fill the GigE link.  For 10G, 80 connections would be
   required to accomplish the same.

   Just as in section 3.3, the end host or test tool can not be the
   processing bottleneck or the throughput measurements will not be
   valid. bandwidth
   (about 100 Mbps per connection).

   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 shaping should be conducted run over a reasonable test long enough duration to
   properly exercise network buffers and results should also characterize performance
   during different time periods of the day.  The throughput of each
   connection must be logged during the entire test, along with the TCP
   Efficiency and TCP Transfer time metric. Additionally, it is
   recommended to log RTT and retransmissions per interval such as connection over the test
   interval. Interpretation of Traffic Shaping Test Restults

   By plotting the throughput
   per achieved by each TCP connection, RTT, and retransmissions.

   Since the network is fair
   sharing of the bandwidth is generally very obvious when traffic shaping
   is properly configured for the bottleneck interface.  For the previous
   example of 5 connections sharing 500 Mbps, each connection would
   consume ~100 Mbps with a smooth variation.  If traffic policing was
   present on the bottleneck interface, the bandwidth sharing would not to
   be driven into over capacity (by nature fair and the resulting throughput plot would reveal "spikey"
   connection throughput consumption of the BDP allocated evenly competing TCP connections
   (due to each connection), this test verifies the ability of retransmissions).

3.4.2 RED Tests

   Random Early Discard techniques are specifically targeted to provide
   congestion avoidance for TCP traffic.  Before the network to carry multiple element queue
   "fills" and enters the tail drop state, RED drops packets at
   configurable queue depth thresholds.  This action causes TCP
   connections up to back-off which helps to prevent tail drop, which in
   turn helps to prevent global TCP synchronization.

   Again, rate limited interfaces can benefit greatly from RED based
   techniques.  Without RED, TCP is generally not able to achieve the link speed full
   bandwidth of the network.

3.4.2 Multiple bottleneck interface.  With RED enabled, TCP Connections - over Link Capacity

   In this step,
   congestion avoidance throttles the network connections on the higher speed
   interface (i.e. LAN) and can reach equalibrium with the bottleneck
   bandwidth (achieving closer to full throughput).

   The ability to detect proper RED configuration is intentionally exceeded with more easily diagnosed
   when conducting a multiple TCP connection test.  Multiple TCP
   connections to test expected prioritization and queuing
   within provide the network.

   All multiple bursty sources that emulate the
   real-world conditions related to Section 3.3 set-up apply, especially for which RED was intended.

   The RED tests also build upon the
   ability concepts of multiple connection
   testing as defined in secion 3.3.3.  Calculating the test hosts BDP for the
   bottleneck bandwidth is first required and then selecting the number of
   connections / window size per connection.

   For RED testing, the desired effect is to transfer stateful cause the TCP traffic at network
   line rates. connections to
   burst beyond the bottleneck bandwidth so that queue drops will occur.
   Using the same example from Section 3.3, a GigE link with 1 msec
   RTT would require a section 3.4.1 (traffic shaping), the
   500 Mbps bottleneck bandwidth requires 5 TCP connections (with window
   size of 128 KB 64Kb) to fill the link (with
   one capacity.  Some experimentation is required,
   but it is recommended to start with double the number of connections
   to stress the network element buffers / queues.  In this example, 10
   connections would produce TCP bursts of 64KB for each connection.
   If the timing of the TCP tester permits, these TCP bursts could stress
   queue sizes in the 512KB range.  Again experimentation will be required
   and the proper number of TCP connection).  Assuming a 16KB window, 8 concurrent connections would fill / window size will be dictated
   by the GigE link capacity and values higher than
   8 would over-subscribe size the network capacity. element queue. Interpretation of RED Results

   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 default queuing
   within technique for most network devices is FIFO based.
   Without RED, the network.

3.4.3 Interpretation FIFO based queue will cause excessive loss to all of Multiple
   the TCP Connection Test Restults

   Without any prioritization connections and in the network, worst case global TCP synchronization.

   By plotting the over subscribed test
   results could assist in aggregate throughput achieved on the queuing studies.  With bottleneck
   interface, proper queuing,
   the bandwidth should RED operation can be shared in a reasonable manner.  The author
   understands that determined if the term "reasonable" bottleneck
   bandwidth is too wide open, and future
   draft versions fully utilized.  For the previous example of this memo would attempt to quantify this 10
   connections (window = 64 KB) sharing
   in more tangible terms.  It is known that if a network element
   is 500 Mbps, each connection should
   consume ~50 Mbps.  If RED was not set for proper queuing (i.e. FIFO), properly enabled on the interface,
   then an oversubscribed the TCP connection test connections will generally show retransmit at a very uneven distribution of

   With prioritization in higher rate and the net
   effect is that the bottleneck bandwidth is not fully utilized.

   Another means to study non-RED versus RED implementation is to use
   the network, different TCP Transfer Time metric for all of the connections.  In this
   example, a 100 MB payload transfer should take ideally 16 seconds
   across all 10 connections can be
   assigned various QoS settings via (with RED enabled).  With RED not enabled,
   the various mechanisms (i.e. per
   VLAN, DSCP, etc.), throughput across the bottleneck bandwidth would be greatly reduced
   (generally 20-40%) and the higher priority connections must TCP Transfer time would be
   verified to achieve proportionally
   longer then the expected throughput. ideal transfer time.

   Additionally, the TCP Transfer Efficiency metric is useful, since
   non-RED implementations will exhibit a lower TCP Tranfer Efficiency
   than RED implementations.

4.  Acknowledgements

   The author would like to thank Gilles Forget, Loki Jorgenson,
   and Reinhard Schrage for technical review and contributions to this
   draft-03 memo.

   Also thanks to Matt Mathis and Matt Zekauskas for many good comments
   through email exchange and for pointing us 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, June 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, June 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

   [RFC4821]  Mathis, M., Heffner, J., "Packetization Layer Path MTU
              Discovery", RFC 4821, June 2007

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

   [MSMO]     The Macroscopic Behavior of the TCP Congestion Avoidance
              Algorithm Mathis, M.,Semke, J, Mahdavi, J, Ott, T
              July 1997 SIGCOMM Computer Communication Review,
              Volume 27 Issue 3

   [Stevens Vol1]  TCP/IP Illustrated, Vol1, The Protocols

Authors' Addresses

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

   Phone: +1 240 404 2227

   Gilles Forget
   Independent Consultant to Bell Canada.
   308, rue de Monaco, St-Eustache
   Qc. CANADA, Postal Code : J7P-4T5

   Phone: (514) 895-8212

   Loki Jorgenson
   Apparent Networks

   Phone: (604) 433-2333 ext 105 908-5833

   Reinhard Schrage
   Schrage Consulting

   Phone: +49 (0) 5137 909540