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Network Working Group                                          M. MathisRequest for Comments: 3148              Pittsburgh Supercomputing CenterCategory: Informational                                        M. Allman                                                          BBN/NASA Glenn                                                               July 2001A Framework for Defining Empirical Bulk Transfer Capacity MetricsStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This document defines a framework for standardizing multiple BTC   (Bulk Transport Capacity) metrics that parallel the permitted   transport diversity.1   Introduction   Bulk Transport Capacity (BTC) is a measure of a network's ability to   transfer significant quantities of data with a single congestion-   aware transport connection (e.g., TCP).  The intuitive definition of   BTC is the expected long term average data rate (bits per second) of   a single ideal TCP implementation over the path in question.   However, there are many congestion control algorithms (and hence   transport implementations) permitted by IETF standards.  This   diversity in transport algorithms creates a difficulty for   standardizing BTC metrics because the allowed diversity is sufficient   to lead to situations where different implementations will yield   non-comparable measures -- and potentially fail the formal tests for   being a metric.   Two approaches are used.  First, each BTC metric must be much more   tightly specified than the typical IETF protocol.  Second, each BTC   methodology is expected to collect some ancillary metrics which are   potentially useful to support analytical models of BTC.   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].  AlthoughMathis, et al.               Informational                      [Page 1]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   [RFC2119] was written with protocols in mind, the key words are used   in this document for similar reasons.  They are used to ensure that   each BTC methodology defined contains specific pieces of information.   Bulk Transport Capacity (BTC) is a measure of a network's ability to   transfer significant quantities of data with a single congestion-   aware transport connection (e.g., TCP).  For many applications the   BTC of the underlying network dominates the overall elapsed time for   the application to run and thus dominates the performance as   perceived by a user.  Examples of such applications include FTP, and   the world wide web when delivering large images or documents.  The   intuitive definition of BTC is the expected long term average data   rate (bits per second) of a single ideal TCP implementation over the   path in question.  The specific definition of the bulk transfer   capacity that MUST be reported by a BTC tool is:      BTC = data_sent / elapsed_time   where "data_sent" represents the unique "data" bits transfered (i.e.,   not including header bits or emulated header bits).  Also note that   the amount of data sent should only include the unique number of bits   transmitted (i.e., if a particular packet is retransmitted the data   it contains should be counted only once).   Central to the notion of bulk transport capacity is the idea that all   transport protocols should have similar responses to congestion in   the Internet.  Indeed the only form of equity significantly deployed   in the Internet today is that the vast majority of all traffic is   carried by TCP implementations sharing common congestion control   algorithms largely due to a shared developmental heritage.   [RFC2581] specifies the standard congestion control algorithms used   by TCP implementations.  Even though this document is a (proposed)   standard, it permits considerable latitude in implementation.  This   latitude is by design, to encourage ongoing evolution in congestion   control algorithms.   This legal diversity in congestion control algorithms creates a   difficulty for standardizing BTC metrics because the allowed   diversity is sufficient to lead to situations where different   implementations will yield non-comparable measures -- and potentially   fail the formal tests for being a metric.   There is also evidence that most TCP implementations exhibit non-   linear performance over some portion of their operating region.  It   is possible to construct simple simulation examples where incremental   improvements to a path (such as raising the link data rate) results   in lower overall TCP throughput (or BTC) [Mat98].Mathis, et al.               Informational                      [Page 2]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   We believe that such non-linearity reflects weakness in our current   understanding of congestion control and is present to some extent in   all TCP implementations and BTC metrics.  Note that such non-   linearity (in either TCP or a BTC metric) is potentially problematic   in the market because investment in capacity might actually reduce   the perceived quality of the network.  Ongoing research in congestion   dynamics has some hope of mitigating or modeling the these non-   linearities.   Related areas, including integrated services [RFC1633,RFC2216],   differentiated services [RFC2475] and Internet traffic analysis   [MSMO97,PFTK98,Pax97b,LM97] are all currently receiving significant   attention from the research community.  It is likely that we will see   new experimental congestion control algorithms in the near future.   In addition, Explicit Congestion Notification (ECN) [RFC2481] is   being tested for Internet deployment.  We do not yet know how any of   these developments might affect BTC metrics, and thus the BTC   framework and metrics may need to be revisited in the future.   This document defines a framework for standardizing multiple BTC   metrics that parallel the permitted transport diversity.  Two   approaches are used.  First, each BTC metric must be much more   tightly specified than the typical IETF transport protocol.  Second,   each BTC methodology is expected to collect some ancillary metrics   which are potentially useful to support analytical models of BTC.  If   a BTC methodology does not collect these ancillary metrics, it should   collect enough information such that these metrics can be derived   (for instance a segment trace file).   As an example, the models in [PFTK98,MSMO97, OKM96a, Lak94] all   predict bulk transfer performance based on path properties such as   loss rate and round trip time.  A BTC methodology that also provides   ancillary measures of these properties is stronger because agreement   with the analytical models can be used to corroborate the direct BTC   measurement results.   More importantly the ancillary metrics are expected to be useful for   resolving disparity between different BTC methodologies.  For   example, a path that predominantly experiences clustered packet   losses is likely to exhibit vastly different measures from BTC   metrics that mimic Tahoe, Reno, NewReno, and SACK TCP algorithms   [FF96].  The differences in the BTC metrics over such a path might be   diagnosed by an ancillary measure of loss clustering.Mathis, et al.               Informational                      [Page 3]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   There are some path properties which are best measured as ancillary   metrics to a transport protocol.  Examples of such properties include   bottleneck queue limits or the tendency to reorder packets.  These   are difficult or impossible to measure at low rates and unsafe to   measure at rates higher than the bulk transport capacity of the path.   It is expected that at some point in the future there will exist an   A-frame [RFC2330] which will unify all simple path metrics (e.g.,   segment loss rates, round trip time) and BTC ancillary metrics (e.g.,   queue size and packet reordering) with different versions of BTC   metrics (e.g., that parallel Reno or SACK TCP).2   Congestion Control Algorithms   Nearly all TCP implementations in use today utilize the congestion   control algorithms published in [Jac88] and further refined in   [RFC2581].  In addition to using the basic notion of using an ACK   clock, TCP (and therefore BTC) implements five standard congestion   control algorithms: Congestion Avoidance, Retransmission timeouts,   Slow-start, Fast Retransmit and Fast Recovery.  All BTC   implementations MUST implement slow start and congestion avoidance,   as specified in [RFC2581] (with extra details also specified, as   outlined below).  All BTC methodologies SHOULD implement fast   retransmit and fast recovery as outlined in [RFC2581].  Finally, all   BTC methodologies MUST implement a retransmission timeout.   The algorithms specified in [RFC2581] give implementers some choices   in the details of the implementation.  The following is a list of   details about the congestion control algorithms that are either   underspecified in [RFC2581] or very important to define when   constructing a BTC methodology.  These details MUST be specifically   defined in each BTC methodology.      *  [RFC2581] does not standardize a specific algorithm for         increasing cwnd during congestion avoidance.  Several candidate         algorithms are given in [RFC2581].  The algorithm used in a         particular BTC methodology MUST be defined.      *  [RFC2581] does not specify which cwnd increase algorithm (slow         start or congestion avoidance) should be used when cwnd equals         ssthresh.  This MUST be specified for each BTC methodology.      *  [RFC2581] allows TCPs to use advanced loss recovery mechanism         such as NewReno [RFC2582,FF96,Hoe96] and SACK-based algorithms         [FF96,MM96a,MM96b].  If used in a BTC implementation, such an         algorithm MUST be fully defined.Mathis, et al.               Informational                      [Page 4]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001      *  The actual segment size, or method of choosing a segment size         (e.g., path MTU discovery [RFC1191]) and the number of header         bytes assumed to be prepended to each segment MUST be         specified.  In addition, if the segment size is artificially         limited to less than the path MTU this MUST be indicated.      *  TCP includes a retransmission timeout (RTO) to trigger         retransmissions of segments that have not been acknowledged         within an appropriate amount of time and have not been         retransmitted via some more advanced loss recovery algorithm.         A BTC implementation MUST include a retransmission timer.         Calculating the RTO is subject to a number of details that MUST         be defined for each BTC metric.  In addition, a BTC metric MUST         define when the clock is set and the granularity of the clock.         [RFC2988] specifies the behavior of the retransmission timer.         However, there are several details left to the implementer         which MUST be specified for each BTC metric defined.   Note that as new congestion control algorithms are placed on the   standards track they may be incorporated into BTC metrics (e.g., the   Limited Transmit algorithm [ABF00]).  However, any implementation   decisions provided by the relevant RFCs SHOULD be fully specified in   the particular BTC metric.3   Ancillary Metrics   The following ancillary metrics can provide additional information   about the network and the behavior of the implemented congestion   control algorithms in response to the behavior of the network path.   It is RECOMMENDED that these metrics be built into each BTC   methodology.  Alternatively, it is RECOMMENDED that the BTC   implementation provide enough information such that the ancillary   metrics can be derived via post-processing (e.g., by providing a   segment trace of the connection).3.1 Congestion Avoidance Capacity   The "Congestion Avoidance Capacity" (CAC) metric is the data rate   (bits per second) of a fully specified implementation of the   Congestion Avoidance algorithm, subject to the restriction that the   Retransmission Timeout and Slow-Start algorithms are not invoked.   The CAC metric is defined to have no meaning across Retransmission   Timeouts or Slow-Start periods (except the single segment Slow-Start   that is permitted to follow recovery, as discussed insection 2).Mathis, et al.               Informational                      [Page 5]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   In principle a CAC metric would be an ideal BTC metric, as it   captures what should be TCP's steady state behavior.  But, there is a   rather substantial difficulty with using it as such.  The Self-   Clocking of the Congestion Avoidance algorithm can be very fragile,   depending on the specific details of the Fast Retransmit, Fast   Recovery or advanced recovery algorithms chosen.  It has been found   that timeouts and periods of slow start loss recovery are prevalent   in traffic on the Internet [LK98,BPS+97] and therefore these should   be captured by the BTC metric.   When TCP loses Self-Clock it is re-established through a   retransmission timeout and Slow-Start.  These algorithms nearly   always require more time than Congestion Avoidance would have taken.   It is easily observed that unless the network loses an entire window   of data (which would clearly require a retransmit timeout) TCP likely   missed some opportunity to safely transmit data.  That is, if TCP   experiences a timeout after losing a partial window of data, it must   have received at least one ACK that was generated after some of the   partial data was delivered, but did not trigger the transmission of   new data.  Recent research in congestion control (e.g., FACK [MM96a],   NewReno [FF96,RFC2582], rate-halving [MSML99]) can be characterized   as making TCP's Self-Clock more tenacious, while preserving fairness   under adverse conditions.  This work is motivated by how poorly   current TCP implementations perform under some conditions, often due   to repeated clock loss.  Since this is an active research area,   different TCP implementations have rather considerable differences in   their ability to preserve Self-Clock.3.2 Preservation of Self-Clock   Losing the ACK clock can have a large effect on the overall BTC, and   the clock is itself fragile in ways that are dependent on the loss   recovery algorithm.  Therefore, the transition between timer driven   and Self-Clocked operation SHOULD be instrumented.3.2.1 Lost Transmission Opportunities   If the last event before a timeout was the receipt of an ACK that did   not trigger a transmission, the possibility exists that an alternate   congestion control algorithm would have successfully preserved the   Self-Clock.  A BTC SHOULD instrument key items in the BTC state (such   as the congestion window) in the hopes that this may lead to further   improvements in congestion control algorithms.Mathis, et al.               Informational                      [Page 6]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   Note that in the absence of knowledge about the future, it is not   possible to design an algorithm that never misses transmission   opportunities.  However, there are ever more subtle ways to gauge   network state, and to estimate if a given ACK is likely to be the   last.3.2.2 Loosing an Entire Window   If an entire window of data (or ACKs) is lost, there will be no   returning ACKs to clock out additional data.  This condition can be   detected if the last event before a timeout was a data transmission   triggered by an ACK.  The loss of an entire window of data/ACKs   forces recovery to be via a Retransmission Timeout and Slow-Start.   Losing an entire window of data implies an outage with a duration at   least as long as a round trip time.  Such an outage can not be   diagnosed with low rate metrics and is unsafe to diagnose at higher   rates than the BTC.  Therefore all BTC metrics SHOULD instrument and   report losses of an entire window of data.   Note that there are some conditions, such as when operating with a   very small window, in which there is a significant probability that   an entire window can be lost through individual random losses (again   highlighting the importance of instrumenting cwnd).3.2.3 Heroic Clock Preservation   All algorithms that permit a given BTC to sustain Self-Clock when   other algorithms might not, SHOULD be instrumented.  Furthermore, the   details of the algorithms used MUST be fully documented (as discussed   insection 2).   BTC metrics that can sustain Self-Clock in the presence of multiple   losses within one round trip SHOULD instrument the loss distribution,   such that the performance of alternate congestion control algorithms   may be estimated (e.g., Reno style).3.2.4  False Timeouts   All false timeouts, (where the retransmission timer expires before   the ACK for some previously transmitted data arrives) SHOULD be   instrumented when possible.  Note that depending upon how the BTC   metric implements sequence numbers, this may be difficult to detect.Mathis, et al.               Informational                      [Page 7]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 20013.3 Ancillary Metrics Relating to Flow Based Path Properties   All BTC metrics provide unique vantage points for observing certain   path properties relating to closely spaced packets.  As in the case   of RTT duration outages, these can be impossible to diagnose at low   rates (less than 1 packet per RTT) and inappropriate to test at rates   above the BTC of the network path.   All BTC metrics SHOULD instrument packet reordering.  The frequency   and distance out-of-sequence SHOULD be instrumented for all out-of-   order packets.  The severity of the reordering can be classified as   one of three different cases, each of which SHOULD be reported.      Segments that are only slightly out-of-order should not trigger      the fast retransmit algorithm, but they may affect the window      calculation.  BTC metrics SHOULD document how slightly out-of-      order segments affect the congestion window calculation.      If segments are sufficiently out-of-order, the Fast Retransmit      algorithm will be invoked in advance of the delayed packet's late      arrival.  These events SHOULD be instrumented.  Even though the      the late arriving packet will complete recovery, the the window      will still be reduced by half.      Under some rare conditions segments have been observed that are      far out of order - sometimes many seconds late [Pax97b].  These      SHOULD always be instrumented.   BTC implementations SHOULD instrument the maximum cwnd observed   during congestion avoidance and slow start.  A TCP running over the   same path as the BTC metric must have sufficient sender buffer space   and receiver window (and window shift [RFC1323]) to cover this cwnd   in order to expect the same performance.   There are several other path properties that one might measure within   a BTC metric.  For example, with an embedded one-way delay metric it   may be possible to measure how queuing delay and and (RED) drop   probabilities are correlated to window size.  These are open research   questions.3.4 Ancillary Metrics as Calibration Checks   Unlike low rate metrics, BTC SHOULD include explicit checks that the   test platform is not the bottleneck.   Any detected dropped packets within the sending host MUST be   reported.  Unless the sending interface is the path bottleneck, any   dropped packets probably indicates a measurement failure.Mathis, et al.               Informational                      [Page 8]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   The maximum queue lengths within the sending host SHOULD be   instrumented.  Any significant queue may indicate that the sending   host has insufficient burst data rate, and is smoothing the data   being transmitted into the network.3.5 Ancillary Metrics Relating to the Need for Advanced TCP Features   If TCP would require advanced TCP extensions to match BTC performance   (such asRFC 1323 orRFC 2018 features), it SHOULD be reported.3.6 Validate Reverse Path Load   To the extent possible, the BTC metric SHOULD distinguish between the   properties of the forward and reverse paths.   BTC methodologies which rely on non-cooperating receivers may only be   able to measure round trip path properties and may not be able to   independently differentiate between the properties of the forward and   reverse paths.  In this case the load on the reverse path contributed   by the BTC metric SHOULD be instrumented (or computed) to permit   other means of gauge the proportion of the round trip path properties   attributed to the the forward and reverse paths.   To the extent possible, BTC methodologies that rely on cooperating   receivers SHOULD support separate ancillary metrics for the forward   and reverse paths.4   Security Considerations   Conducting Internet measurements raises security concerns.  This memo   does not specify a particular implementation of a metric, so it does   not directly affect the security of the Internet nor of applications   which run on the Internet.  However, metrics produced within this   framework, and in particular implementations of the metrics may   create security issues.4.1 Denial of Service Attacks   Bulk Transport Capacity metrics, as defined in this document,   naturally attempt to fill a bottleneck link.  The BTC metrics based   on this specification will be as "network friendly" as current well-   tuned TCP connections.  However, since the "connection" may not be   using TCP packets, a BTC test may appear to network operators as a   denial of service attack.Mathis, et al.               Informational                      [Page 9]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   Administrators of the source host of a test, the destination host of   a test, and the intervening network(s) may wish to establish   bilateral or multi-lateral agreements regarding the timing, size, and   frequency of collection of BTC metrics.4.2 User data confidentiality   Metrics within this framework generate packets for a sample, rather   than taking samples based on user data.  Thus, a BTC metric does not   threaten user data confidentiality.4.3 Interference with metrics   It may be possible to identify that a certain packet or stream of   packets are part of a BTC metric.  With that knowledge at the   destination and/or the intervening networks, it is possible to change   the processing of the packets (e.g., increasing or decreasing delay,   introducing or heroically preventing loss) that may distort the   measured performance.  It may also be possible to generate additional   packets that appear to be part of a BTC metric.  These additional   packets are likely to perturb the results of the sample measurement.   To discourage the kind of interference mentioned above, packet   interference checks, such as cryptographic hash, may be used.5   IANA Considerations   Since this metric framework does not define a specific protocol, nor   does it define any well-known values, there are no IANA   considerations for this document.  However, a bulk transport capacity   metric within this framework, and in particular protocols that   implement a metric may have IANA considerations that need to be   addressed.6   Acknowledgments   Thanks to Wil Leland, Jeff Semke, Matt Zekauskas and the IPPM working   group for numerous clarifications.   Matt Mathis's work was supported by the National Science Foundation   under Grant Numbers 9415552 and 9870758.Mathis, et al.               Informational                     [Page 10]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 20017   References   [BPS+97]     Hari Balakrishnan, Venkata Padmanabhan, Srinivasan                Seshan, Mark Stemm, and Randy Katz.  TCP Behavior of a                Busy Web Server:  Analysis and Improvements.  Technical                Report UCB/CSD-97-966, August 1997.  Available fromhttp://nms.lcs.mit.edu/~hari/papers/csd-97-966.ps.                (Also in Proc. IEEE INFOCOM Conf., San Francisco, CA,                March 1998.)   [FF96]       Fall, K., Floyd, S..  "Simulation-based Comparisons of                Tahoe, Reno and SACK TCP".  Computer Communication                Review, July 1996.ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z.   [Flo95]      Floyd, S., "TCP and successive fast retransmits", March                1995, Obtain viaftp://ftp.ee.lbl.gov/papers/fastretrans.ps.   [Hoe96]      Hoe, J., "Improving the start-up behavior of a                congestion control scheme for TCP, Proceedings of ACM                SIGCOMM '96, August 1996.   [Hoe95]      Hoe, J., "Startup dynamics of TCP's congestion control                and avoidance schemes".  Master's thesis, Massachusetts                Institute of Technology, June 1995.   [Jac88]      Jacobson, V., "Congestion Avoidance and Control",                Proceedings of SIGCOMM '88, Stanford, CA., August 1988.   [Lak94]      V. T. Lakshman and U. Madhow. The Performance of TCP/IP                for Networks with High Bandwidth-Delay Products and                Random Loss. IFIP Transactions C-26, High Performance                Networking, pages 135--150, 1994.   [LK98]       Lin, D. and Kung, H.T., "TCP Fast Recovery Strategies:                Analysis and Improvements", Proceedings of InfoCom,                March 1998.   [LM97]       T.V.Lakshman and U.Madhow.  "The Performance of TCP/IP                for Networks with High Bandwidth-Delay Products and                Random Loss".  IEEE/ACM Transactions on Networking, Vol.                5, No. 3, June 1997, pp.336-350.Mathis, et al.               Informational                     [Page 11]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   [Mat98]      Mathis, M., "Empirical Bulk Transfer Capacity", IP                Performance Metrics Working Group report in Proceedings                of the Forty Third Internet Engineering Task Force,                Orlando, FL, December 1988.  Available fromhttp://www.ietf.org/proceedings/98dec/43rd-ietf-98dec-122.html andhttp://www.ietf.org/proceedings/98dec/slides/ippm-mathis-98dec.pdf.   [MM96a]      Mathis, M. and Mahdavi, J. "Forward acknowledgment:                Refining TCP congestion control", Proceedings of ACM                SIGCOMM '96, Stanford, CA., August 1996.   [MM96b]      M. Mathis, J. Mahdavi, "TCP Rate-Halving with Bounding                Parameters".  Available fromhttp://www.psc.edu/networking/papers/FACKnotes/current.   [MSML99]     Mathis, M., Semke, J., Mahdavi, J., Lahey, K., "The                Rate-Halving Algorithm for TCP Congestion Control", June                1999, Work in Progress.   [MSMO97]     Mathis, M., Semke, J., Mahdavi, J., Ott, T., "The                Macroscopic Behavior of the TCP Congestion Avoidance                Algorithm", Computer Communications Review, 27(3), July                1997.   [OKM96a],    Ott, T., Kemperman, J., Mathis, M., "The Stationary                Behavior of Ideal TCP Congestion Avoidance", In                progress, August 1996. Obtain via pub/tjo/TCPwindow.ps                using anonymous ftp to ftp.bellcore.com   [OKM96b],    Ott, T., Kemperman, J., Mathis, M., "Window Size                Behavior in TCP/IP with Constant Loss Probability",                DIMACS Special Year on Networks, Workshop on Performance                of Real-Time Applications on the Internet, Nov 1996.   [Pax97a]     Paxson, V., "Automated Packet Trace Analysis of TCP                Implementations", Proceedings of ACM SIGCOMM '97, August                1997.   [Pax97b]     Paxson, V., "End-to-End Internet Packet Dynamics,"                Proceedings of SIGCOMM '97, Cannes, France, Sep. 1997.   [PFTK98]     Padhye, J., Firoiu. V., Towsley, D., and Kurose, J.,                "TCP Throughput: A Simple Model and its Empirical                Validation", Proceedings of ACM SIGCOMM '98, August                1998.Mathis, et al.               Informational                     [Page 12]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   [RFC793]     Postel, J., "Transmission Control Protocol", STD 7,RFC793, September 1981.  Obtain via:http://www.rfc-editor.org/rfc/rfc793.txt   [RFC1191]    Mogul, J. and S. Deering, "Path MTU Discovery",RFC1191, November 1990.  Obtain via:http://www.rfc-editor.org/rfc/rfc1191.txt   [RFC1323]    Jacobson, V., Braden, R. and D. Borman, "TCP Extensions                for High Performance", May 1992.  Obtain via:http://www.rfc-editor.org/rfc/rfc1323.txt   [RFC1633]    Braden R., Clark D. and S. Shenker, "Integrated Services                in the Internet Architecture: an Overview",RFC 1633,                June 1994.  Obtain via:http://www.rfc-editor.org/rfc/rfc1633.txt   [RFC2001]    Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast                Retransmit, and Fast Recovery Algorithms",RFC 2001,                January 1997.  Obtain via:http://www.rfc-editor.org/rfc/rfc2001.txt   [RFC2018]    Mathis, M., Mahdavi, J. Floyd, S., Romanow, A., "TCP                Selective Acknowledgment Options",RFC 2018, October                1996.  Obtain via:http://www.rfc-editor.org/rfc/rfc2018.txt   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997.                Obtain via:http://www.rfc-editor.org/rfc/rfc2119.txt   [RFC2216]    Shenker, S. and J. Wroclawski, "Network Element Service                Specification Template",RFC 2216, September 1997.                Obtain via:http://www.rfc-editor.org/rfc/rfc2216.txt   [RFC2330]    Paxson, V., Almes, G., Mahdavi, J. and M. Mathis,                "Framework for IP Performance Metrics",RFC 2330, April                1998.  Obtain via:http://www.rfc-editor.org/rfc/rfc2330.txt   [RFC2475]    Black D., Blake S., Carlson M., Davies E., Wang Z. and                W. Weiss, "An Architecture for Differentiated Services",RFC 2475, December 1998.  Obtain via:http://www.rfc-editor.org/rfc/rfc2475.txtMathis, et al.               Informational                     [Page 13]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001   [RFC2481]    Ramakrishnan, K. and S. Floyd, "A Proposal to add                Explicit Congestion Notification (ECN) to IP",RFC 2481,                January 1999.  Obtain via:http://www.rfc-editor.org/rfc/rfc2481.txt   [RFC2525]    Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,                J., Heavens, I., Lahey, K., Semke, J. and B. Volz,                "Known TCP Implementation Problems",RFC 2525, March                1999.  Obtain via:http://www.rfc-editor.org/rfc/rfc2525.txt   [RFC2581]    Allman, M., Paxson, V. and W. Stevens, "TCP Congestion                Control",RFC 2581, April 1999.  Obtain via:http://www.rfc-editor.org/rfc/rfc2581.txt   [RFC2582]    Floyd, S. and T. Henderson, "The NewReno Modification to                TCP's Fast Recovery Algorithm",RFC 2582, April 1999.                Obtain via:http://www.rfc-editor.org/rfc/rfc2582.txt   [RFC2988]    Paxson, V. and M. Allman, "Computing TCP's                Retransmission Timer",RFC 2988, November 2000.  Obtain                via:http://www.rfc-editor.org/rfc/rfc2988.txt   [RFC3042]    Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing                TCP's Loss Recovery Using Limited Transmit",RFC 3042,                January 2001.  Obtain via:http://www.rfc-editor.org/rfc/rfc3042.txt   [Ste94]      Stevens, W., "TCP/IP Illustrated, Volume 1: The                Protocols", Addison-Wesley, 1994.   [WS95]       Wright, G., Stevens, W., "TCP/IP Illustrated Volume II:                The Implementation", Addison-Wesley, 1995.Mathis, et al.               Informational                     [Page 14]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001Author's Addresses   Matt Mathis   Pittsburgh Supercomputing Center   4400 Fifth Ave.   Pittsburgh PA 15213   EMail: mathis@psc.eduhttp://www.psc.edu/~mathis   Mark Allman   BBN Technologies/NASA Glenn Research Center   Lewis Field   21000 Brookpark Rd.  MS 54-2   Cleveland, OH  44135   Phone: 216-433-6586   EMail: mallman@bbn.comhttp://roland.grc.nasa.gov/~mallmanMathis, et al.               Informational                     [Page 15]

RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Mathis, et al.               Informational                     [Page 16]