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Network Working Group                                          M. AllmanRequest for Comments: 2581                  NASA Glenn/Sterling SoftwareObsoletes:2001                                                V. PaxsonCategory: Standards Track                                   ACIRI / ICSI                                                              W. Stevens                                                              Consultant                                                              April 1999TCP Congestion ControlStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.Abstract   This document defines TCP's four intertwined congestion control   algorithms: slow start, congestion avoidance, fast retransmit, and   fast recovery.  In addition, the document specifies how TCP should   begin transmission after a relatively long idle period, as well as   discussing various acknowledgment generation methods.1. Introduction   This document specifies four TCP [Pos81] congestion control   algorithms: slow start, congestion avoidance, fast retransmit and   fast recovery.  These algorithms were devised in [Jac88] and [Jac90].   Their use with TCP is standardized in [Bra89].   This document is an update of [Ste97].  In addition to specifying the   congestion control algorithms, this document specifies what TCP   connections should do after a relatively long idle period, as well as   specifying and clarifying some of the issues pertaining to TCP ACK   generation.   Note that [Ste94] provides examples of these algorithms in action and   [WS95] provides an explanation of the source code for the BSD   implementation of these algorithms.Allman, et. al.             Standards Track                     [Page 1]

RFC 2581                 TCP Congestion Control               April 1999   This document is organized as follows.Section 2 provides various   definitions which will be used throughout the document.Section 3   provides a specification of the congestion control algorithms.Section 4 outlines concerns related to the congestion control   algorithms and finally,section 5 outlines security considerations.   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 [Bra97].2. Definitions   This section provides the definition of several terms that will be   used throughout the remainder of this document.   SEGMENT:      A segment is ANY TCP/IP data or acknowledgment packet (or both).   SENDER MAXIMUM SEGMENT SIZE (SMSS):  The SMSS is the size of the      largest segment that the sender can transmit.  This value can be      based on the maximum transmission unit of the network, the path      MTU discovery [MD90] algorithm, RMSS (see next item), or other      factors.  The size does not include the TCP/IP headers and      options.   RECEIVER MAXIMUM SEGMENT SIZE (RMSS):  The RMSS is the size of the      largest segment the receiver is willing to accept.  This is the      value specified in the MSS option sent by the receiver during      connection startup.  Or, if the MSS option is not used, 536 bytes      [Bra89].  The size does not include the TCP/IP headers and      options.   FULL-SIZED SEGMENT: A segment that contains the maximum number of      data bytes permitted (i.e., a segment containing SMSS bytes of      data).   RECEIVER WINDOW (rwnd) The most recently advertised receiver window.   CONGESTION WINDOW (cwnd):  A TCP state variable that limits the      amount of data a TCP can send.  At any given time, a TCP MUST NOT      send data with a sequence number higher than the sum of the      highest acknowledged sequence number and the minimum of cwnd and      rwnd.   INITIAL WINDOW (IW):  The initial window is the size of the sender's      congestion window after the three-way handshake is completed.Allman, et. al.             Standards Track                     [Page 2]

RFC 2581                 TCP Congestion Control               April 1999   LOSS WINDOW (LW):  The loss window is the size of the congestion      window after a TCP sender detects loss using its retransmission      timer.   RESTART WINDOW (RW):  The restart window is the size of the      congestion window after a TCP restarts transmission after an idle      period (if the slow start algorithm is used; seesection 4.1 for      more discussion).   FLIGHT SIZE:  The amount of data that has been sent but not yet      acknowledged.3. Congestion Control Algorithms   This section defines the four congestion control algorithms: slow   start, congestion avoidance, fast retransmit and fast recovery,   developed in [Jac88] and [Jac90].  In some situations it may be   beneficial for a TCP sender to be more conservative than the   algorithms allow, however a TCP MUST NOT be more aggressive than the   following algorithms allow (that is, MUST NOT send data when the   value of cwnd computed by the following algorithms would not allow   the data to be sent).3.1 Slow Start and Congestion Avoidance   The slow start and congestion avoidance algorithms MUST be used by a   TCP sender to control the amount of outstanding data being injected   into the network.  To implement these algorithms, two variables are   added to the TCP per-connection state.  The congestion window (cwnd)   is a sender-side limit on the amount of data the sender can transmit   into the network before receiving an acknowledgment (ACK), while the   receiver's advertised window (rwnd) is a receiver-side limit on the   amount of outstanding data.  The minimum of cwnd and rwnd governs   data transmission.   Another state variable, the slow start threshold (ssthresh), is used   to determine whether the slow start or congestion avoidance algorithm   is used to control data transmission, as discussed below.   Beginning transmission into a network with unknown conditions   requires TCP to slowly probe the network to determine the available   capacity, in order to avoid congesting the network with an   inappropriately large burst of data.  The slow start algorithm is   used for this purpose at the beginning of a transfer, or after   repairing loss detected by the retransmission timer.Allman, et. al.             Standards Track                     [Page 3]

RFC 2581                 TCP Congestion Control               April 1999   IW, the initial value of cwnd, MUST be less than or equal to 2*SMSS   bytes and MUST NOT be more than 2 segments.   We note that a non-standard, experimental TCP extension allows that a   TCP MAY use a larger initial window (IW), as defined in equation 1   [AFP98]:      IW = min (4*SMSS, max (2*SMSS, 4380 bytes))           (1)   With this extension, a TCP sender MAY use a 3 or 4 segment initial   window, provided the combined size of the segments does not exceed   4380 bytes.  We do NOT allow this change as part of the standard   defined by this document.  However, we include discussion of (1) in   the remainder of this document as a guideline for those experimenting   with the change, rather than conforming to the present standards for   TCP congestion control.   The initial value of ssthresh MAY be arbitrarily high (for example,   some implementations use the size of the advertised window), but it   may be reduced in response to congestion.  The slow start algorithm   is used when cwnd < ssthresh, while the congestion avoidance   algorithm is used when cwnd > ssthresh.  When cwnd and ssthresh are   equal the sender may use either slow start or congestion avoidance.   During slow start, a TCP increments cwnd by at most SMSS bytes for   each ACK received that acknowledges new data.  Slow start ends when   cwnd exceeds ssthresh (or, optionally, when it reaches it, as noted   above) or when congestion is observed.   During congestion avoidance, cwnd is incremented by 1 full-sized   segment per round-trip time (RTT).  Congestion avoidance continues   until congestion is detected.  One formula commonly used to update   cwnd during congestion avoidance is given in equation 2:      cwnd += SMSS*SMSS/cwnd                     (2)   This adjustment is executed on every incoming non-duplicate ACK.   Equation (2) provides an acceptable approximation to the underlying   principle of increasing cwnd by 1 full-sized segment per RTT.  (Note   that for a connection in which the receiver acknowledges every data   segment, (2) proves slightly more aggressive than 1 segment per RTT,   and for a receiver acknowledging every-other packet, (2) is less   aggressive.)Allman, et. al.             Standards Track                     [Page 4]

RFC 2581                 TCP Congestion Control               April 1999   Implementation Note: Since integer arithmetic is usually used in TCP   implementations, the formula given in equation 2 can fail to increase   cwnd when the congestion window is very large (larger than   SMSS*SMSS).  If the above formula yields 0, the result SHOULD be   rounded up to 1 byte.   Implementation Note: older implementations have an additional   additive constant on the right-hand side of equation (2).  This is   incorrect and can actually lead to diminished performance [PAD+98].   Another acceptable way to increase cwnd during congestion avoidance   is to count the number of bytes that have been acknowledged by ACKs   for new data.  (A drawback of this implementation is that it requires   maintaining an additional state variable.)  When the number of bytes   acknowledged reaches cwnd, then cwnd can be incremented by up to SMSS   bytes.  Note that during congestion avoidance, cwnd MUST NOT be   increased by more than the larger of either 1 full-sized segment per   RTT, or the value computed using equation 2.   Implementation Note: some implementations maintain cwnd in units of   bytes, while others in units of full-sized segments.  The latter will   find equation (2) difficult to use, and may prefer to use the   counting approach discussed in the previous paragraph.   When a TCP sender detects segment loss using the retransmission   timer, the value of ssthresh MUST be set to no more than the value   given in equation 3:      ssthresh = max (FlightSize / 2, 2*SMSS)            (3)   As discussed above, FlightSize is the amount of outstanding data in   the network.   Implementation Note: an easy mistake to make is to simply use cwnd,   rather than FlightSize, which in some implementations may   incidentally increase well beyond rwnd.   Furthermore, upon a timeout cwnd MUST be set to no more than the loss   window, LW, which equals 1 full-sized segment (regardless of the   value of IW).  Therefore, after retransmitting the dropped segment   the TCP sender uses the slow start algorithm to increase the window   from 1 full-sized segment to the new value of ssthresh, at which   point congestion avoidance again takes over.Allman, et. al.             Standards Track                     [Page 5]

RFC 2581                 TCP Congestion Control               April 19993.2 Fast Retransmit/Fast Recovery   A TCP receiver SHOULD send an immediate duplicate ACK when an out-   of-order segment arrives.  The purpose of this ACK is to inform the   sender that a segment was received out-of-order and which sequence   number is expected.  From the sender's perspective, duplicate ACKs   can be caused by a number of network problems.  First, they can be   caused by dropped segments.  In this case, all segments after the   dropped segment will trigger duplicate ACKs.  Second, duplicate ACKs   can be caused by the re-ordering of data segments by the network (not   a rare event along some network paths [Pax97]).  Finally, duplicate   ACKs can be caused by replication of ACK or data segments by the   network.  In addition, a TCP receiver SHOULD send an immediate ACK   when the incoming segment fills in all or part of a gap in the   sequence space.  This will generate more timely information for a   sender recovering from a loss through a retransmission timeout, a   fast retransmit, or an experimental loss recovery algorithm, such as   NewReno [FH98].   The TCP sender SHOULD use the "fast retransmit" algorithm to detect   and repair loss, based on incoming duplicate ACKs.  The fast   retransmit algorithm uses the arrival of 3 duplicate ACKs (4   identical ACKs without the arrival of any other intervening packets)   as an indication that a segment has been lost.  After receiving 3   duplicate ACKs, TCP performs a retransmission of what appears to be   the missing segment, without waiting for the retransmission timer to   expire.   After the fast retransmit algorithm sends what appears to be the   missing segment, the "fast recovery" algorithm governs the   transmission of new data until a non-duplicate ACK arrives.  The   reason for not performing slow start is that the receipt of the   duplicate ACKs not only indicates that a segment has been lost, but   also that segments are most likely leaving the network (although a   massive segment duplication by the network can invalidate this   conclusion).  In other words, since the receiver can only generate a   duplicate ACK when a segment has arrived, that segment has left the   network and is in the receiver's buffer, so we know it is no longer   consuming network resources.  Furthermore, since the ACK "clock"   [Jac88] is preserved, the TCP sender can continue to transmit new   segments (although transmission must continue using a reduced cwnd).   The fast retransmit and fast recovery algorithms are usually   implemented together as follows.   1.  When the third duplicate ACK is received, set ssthresh to no more       than the value given in equation 3.Allman, et. al.             Standards Track                     [Page 6]

RFC 2581                 TCP Congestion Control               April 1999   2.  Retransmit the lost segment and set cwnd to ssthresh plus 3*SMSS.       This artificially "inflates" the congestion window by the number       of segments (three) that have left the network and which the       receiver has buffered.   3.  For each additional duplicate ACK received, increment cwnd by       SMSS.  This artificially inflates the congestion window in order       to reflect the additional segment that has left the network.   4.  Transmit a segment, if allowed by the new value of cwnd and the       receiver's advertised window.   5.  When the next ACK arrives that acknowledges new data, set cwnd to       ssthresh (the value set in step 1).  This is termed "deflating"       the window.       This ACK should be the acknowledgment elicited by the       retransmission from step 1, one RTT after the retransmission       (though it may arrive sooner in the presence of significant out-       of-order delivery of data segments at the receiver).       Additionally, this ACK should acknowledge all the intermediate       segments sent between the lost segment and the receipt of the       third duplicate ACK, if none of these were lost.   Note: This algorithm is known to generally not recover very   efficiently from multiple losses in a single flight of packets   [FF96].  One proposed set of modifications to address this problem   can be found in [FH98].4. Additional Considerations4.1 Re-starting Idle Connections   A known problem with the TCP congestion control algorithms described   above is that they allow a potentially inappropriate burst of traffic   to be transmitted after TCP has been idle for a relatively long   period of time.  After an idle period, TCP cannot use the ACK clock   to strobe new segments into the network, as all the ACKs have drained   from the network.  Therefore, as specified above, TCP can potentially   send a cwnd-size line-rate burst into the network after an idle   period.   [Jac88] recommends that a TCP use slow start to restart transmission   after a relatively long idle period.  Slow start serves to restart   the ACK clock, just as it does at the beginning of a transfer.  This   mechanism has been widely deployed in the following manner.  When TCP   has not received a segment for more than one retransmission timeout,   cwnd is reduced to the value of the restart window (RW) beforeAllman, et. al.             Standards Track                     [Page 7]

RFC 2581                 TCP Congestion Control               April 1999   transmission begins.   For the purposes of this standard, we define RW = IW.   We note that the non-standard experimental extension to TCP defined   in [AFP98] defines RW = min(IW, cwnd), with the definition of IW   adjusted per equation (1) above.   Using the last time a segment was received to determine whether or   not to decrease cwnd fails to deflate cwnd in the common case of   persistent HTTP connections [HTH98].  In this case, a WWW server   receives a request before transmitting data to the WWW browser.  The   reception of the request makes the test for an idle connection fail,   and allows the TCP to begin transmission with a possibly   inappropriately large cwnd.   Therefore, a TCP SHOULD set cwnd to no more than RW before beginning   transmission if the TCP has not sent data in an interval exceeding   the retransmission timeout.4.2 Generating Acknowledgments   The delayed ACK algorithm specified in [Bra89] SHOULD be used by a   TCP receiver.  When used, a TCP receiver MUST NOT excessively delay   acknowledgments.  Specifically, an ACK SHOULD be generated for at   least every second full-sized segment, and MUST be generated within   500 ms of the arrival of the first unacknowledged packet.   The requirement that an ACK "SHOULD" be generated for at least every   second full-sized segment is listed in [Bra89] in one place as a   SHOULD and another as a MUST.  Here we unambiguously state it is a   SHOULD.  We also emphasize that this is a SHOULD, meaning that an   implementor should indeed only deviate from this requirement after   careful consideration of the implications.  See the discussion of   "Stretch ACK violation" in [PAD+98] and the references therein for a   discussion of the possible performance problems with generating ACKs   less frequently than every second full-sized segment.   In some cases, the sender and receiver may not agree on what   constitutes a full-sized segment.  An implementation is deemed to   comply with this requirement if it sends at least one acknowledgment   every time it receives 2*RMSS bytes of new data from the sender,   where RMSS is the Maximum Segment Size specified by the receiver to   the sender (or the default value of 536 bytes, per [Bra89], if the   receiver does not specify an MSS option during connection   establishment).  The sender may be forced to use a segment size less   than RMSS due to the maximum transmission unit (MTU), the path MTU   discovery algorithm or other factors.  For instance, consider theAllman, et. al.             Standards Track                     [Page 8]

RFC 2581                 TCP Congestion Control               April 1999   case when the receiver announces an RMSS of X bytes but the sender   ends up using a segment size of Y bytes (Y < X) due to path MTU   discovery (or the sender's MTU size).  The receiver will generate   stretch ACKs if it waits for 2*X bytes to arrive before an ACK is   sent.  Clearly this will take more than 2 segments of size Y bytes.   Therefore, while a specific algorithm is not defined, it is desirable   for receivers to attempt to prevent this situation, for example by   acknowledging at least every second segment, regardless of size.   Finally, we repeat that an ACK MUST NOT be delayed for more than 500   ms waiting on a second full-sized segment to arrive.   Out-of-order data segments SHOULD be acknowledged immediately, in   order to accelerate loss recovery.  To trigger the fast retransmit   algorithm, the receiver SHOULD send an immediate duplicate ACK when   it receives a data segment above a gap in the sequence space.  To   provide feedback to senders recovering from losses, the receiver   SHOULD send an immediate ACK when it receives a data segment that   fills in all or part of a gap in the sequence space.   A TCP receiver MUST NOT generate more than one ACK for every incoming   segment, other than to update the offered window as the receiving   application consumes new data [page 42, Pos81][Cla82].4.3 Loss Recovery Mechanisms   A number of loss recovery algorithms that augment fast retransmit and   fast recovery have been suggested by TCP researchers.  While some of   these algorithms are based on the TCP selective acknowledgment (SACK)   option [MMFR96], such as [FF96,MM96a,MM96b], others do not require   SACKs [Hoe96,FF96,FH98].  The non-SACK algorithms use "partial   acknowledgments" (ACKs which cover new data, but not all the data   outstanding when loss was detected) to trigger retransmissions.   While this document does not standardize any of the specific   algorithms that may improve fast retransmit/fast recovery, these   enhanced algorithms are implicitly allowed, as long as they follow   the general principles of the basic four algorithms outlined above.   Therefore, when the first loss in a window of data is detected,   ssthresh MUST be set to no more than the value given by equation (3).   Second, until all lost segments in the window of data in question are   repaired, the number of segments transmitted in each RTT MUST be no   more than half the number of outstanding segments when the loss was   detected.  Finally, after all loss in the given window of segments   has been successfully retransmitted, cwnd MUST be set to no more than   ssthresh and congestion avoidance MUST be used to further increase   cwnd.  Loss in two successive windows of data, or the loss of a   retransmission, should be taken as two indications of congestion and,   therefore, cwnd (and ssthresh) MUST be lowered twice in this case.Allman, et. al.             Standards Track                     [Page 9]

RFC 2581                 TCP Congestion Control               April 1999   The algorithms outlined in [Hoe96,FF96,MM96a,MM6b] follow the   principles of the basic four congestion control algorithms outlined   in this document.5.  Security Considerations   This document requires a TCP to diminish its sending rate in the   presence of retransmission timeouts and the arrival of duplicate   acknowledgments.  An attacker can therefore impair the performance of   a TCP connection by either causing data packets or their   acknowledgments to be lost, or by forging excessive duplicate   acknowledgments.  Causing two congestion control events back-to-back   will often cut ssthresh to its minimum value of 2*SMSS, causing the   connection to immediately enter the slower-performing congestion   avoidance phase.   The Internet to a considerable degree relies on the correct   implementation of these algorithms in order to preserve network   stability and avoid congestion collapse.  An attacker could cause TCP   endpoints to respond more aggressively in the face of congestion by   forging excessive duplicate acknowledgments or excessive   acknowledgments for new data.  Conceivably, such an attack could   drive a portion of the network into congestion collapse.6.  Changes Relative toRFC 2001   This document has been extensively rewritten editorially and it is   not feasible to itemize the list of changes between the two   documents. The intention of this document is not to change any of the   recommendations given inRFC 2001, but to further clarify cases that   were not discussed in detail in 2001. Specifically, this document   suggests what TCP connections should do after a relatively long idle   period, as well as specifying and clarifying some of the issues   pertaining to TCP ACK generation.  Finally, the allowable upper bound   for the initial congestion window has also been raised from one to   two segments.Acknowledgments   The four algorithms that are described were developed by Van   Jacobson.   Some of the text from this document is taken from "TCP/IP   Illustrated, Volume 1: The Protocols" by W. Richard Stevens   (Addison-Wesley, 1994) and "TCP/IP Illustrated, Volume 2: The   Implementation" by Gary R. Wright and W.  Richard Stevens (Addison-   Wesley, 1995).  This material is used with the permission of   Addison-Wesley.Allman, et. al.             Standards Track                    [Page 10]

RFC 2581                 TCP Congestion Control               April 1999   Neal Cardwell, Sally Floyd, Craig Partridge and Joe Touch contributed   a number of helpful suggestions.References   [AFP98]  Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's            Initial Window Size,RFC 2414, September 1998.   [Bra89]  Braden, R., "Requirements for Internet Hosts --            Communication Layers", STD 3,RFC 1122, October 1989.   [Bra97]  Bradner, S., "Key words for use in RFCs to Indicate            Requirement Levels",BCP 14,RFC 2119, March 1997.   [Cla82]  Clark, D., "Window and Acknowledgment Strategy in TCP",RFC813, July 1982.   [FF96]   Fall, K. and S. Floyd, "Simulation-based Comparisons of            Tahoe, Reno and SACK TCP", Computer Communication Review,            July 1996.ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z.   [FH98]   Floyd, S. and T. Henderson, "The NewReno Modification to            TCP's Fast Recovery Algorithm",RFC 2582, April 1999.   [Flo94]  Floyd, S., "TCP and Successive Fast Retransmits. Technical            report", October 1994.ftp://ftp.ee.lbl.gov/papers/fastretrans.ps.   [Hoe96]  Hoe, J., "Improving the Start-up Behavior of a Congestion            Control Scheme for TCP", In ACM SIGCOMM, August 1996.   [HTH98]  Hughes, A., Touch, J. and J. Heidemann, "Issues in TCP            Slow-Start Restart After Idle", Work in Progress.   [Jac88]  Jacobson, V., "Congestion Avoidance and Control", Computer            Communication Review, vol. 18, no. 4, pp. 314-329, Aug.            1988.ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.   [Jac90]  Jacobson, V., "Modified TCP Congestion Avoidance Algorithm",            end2end-interest mailing list, April 30, 1990.ftp://ftp.isi.edu/end2end/end2end-interest-1990.mail.   [MD90]   Mogul, J. and S. Deering, "Path MTU Discovery",RFC 1191,            November 1990.Allman, et. al.             Standards Track                    [Page 11]

RFC 2581                 TCP Congestion Control               April 1999   [MM96a]  Mathis, M. and J. Mahdavi, "Forward Acknowledgment: Refining            TCP Congestion Control", Proceedings of SIGCOMM'96, August,            1996, Stanford, CA.  Available            fromhttp://www.psc.edu/networking/papers/papers.html   [MM96b]  Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding            Parameters", Technical report.  Available fromhttp://www.psc.edu/networking/papers/FACKnotes/current.   [MMFR96] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP            Selective Acknowledgement Options",RFC 2018, October 1996.   [PAD+98] 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.   [Pax97]  Paxson, V., "End-to-End Internet Packet Dynamics",            Proceedings of SIGCOMM '97, Cannes, France, Sep. 1997.   [Pos81]  Postel, J., "Transmission Control Protocol", STD 7,RFC 793,            September 1981.   [Ste94]  Stevens, W., "TCP/IP Illustrated, Volume 1: The Protocols",            Addison-Wesley, 1994.   [Ste97]  Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast            Retransmit, and Fast Recovery Algorithms",RFC 2001, January            1997.   [WS95]   Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:            The Implementation", Addison-Wesley, 1995.Allman, et. al.             Standards Track                    [Page 12]

RFC 2581                 TCP Congestion Control               April 1999Authors' Addresses   Mark Allman   NASA Glenn Research Center/Sterling Software   Lewis Field   21000 Brookpark Rd.  MS 54-2   Cleveland, OH  44135   216-433-6586   EMail: mallman@grc.nasa.govhttp://roland.grc.nasa.gov/~mallman   Vern Paxson   ACIRI / ICSI   1947 Center Street   Suite 600   Berkeley, CA 94704-1198   Phone: +1 510/642-4274 x302   EMail: vern@aciri.org   W. Richard Stevens   1202 E. Paseo del Zorro   Tucson, AZ  85718   520-297-9416   EMail: rstevens@kohala.comhttp://www.kohala.com/~rstevensAllman, et. al.             Standards Track                    [Page 13]

RFC 2581                 TCP Congestion Control               April 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  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.Allman, et. al.             Standards Track                    [Page 14]