Network Working Group H. Inamura (editor) Internet-Draft NTT DoCoMo, Inc. Expires: May 21, 2002 G. Montenegro Sun Microsystems, Inc. M. Hata NTT DoCoMo, Inc. M. Hara Fujitsu, Inc. J. James Motorola, Inc. W. Gilliam Hewlett-Packard Company A. Hameed Fujitsu FNC, Inc. R. Ludwig Ericsson Research R. Garces P. Ford Microsoft F. Wills Openwave November 20, 2001 TCP over 2.5G and 3G Wireless Networks draft-ietf-pilc-2.5g3g-05 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Comments should be submitted to the PILC mailing list at pilc@grc.nasa.gov. Distribution of this memo is unlimited. Inamura, et. al. Expires May 21, 2002 [Page 1] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 This Internet-Draft will expire on May 21, 2002. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This document describes a profile for optimizing TCP over 2.5G/3G wireless networks. We describe the link characteristics of 3G wireless by using W-CDMA (Wideband CDMA) as an example. We then recommend TCP optimization mechanisms. We also present examples of wireless internet services and standardization activities that potentially will deploy the profile described in this document. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. 2.5G and 3G Link Characteristics . . . . . . . . . . . . . . 4 3. TCP over 2.5G and 3G . . . . . . . . . . . . . . . . . . . . 5 3.1 Optimization Mechanisms . . . . . . . . . . . . . . . . . . 5 3.1.1 Large window size . . . . . . . . . . . . . . . . . . . . . 5 3.1.2 Large initial window . . . . . . . . . . . . . . . . . . . . 5 3.1.3 Limited Transmit . . . . . . . . . . . . . . . . . . . . . . 5 3.1.4 Large MTU . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.5 Path MTU discovery . . . . . . . . . . . . . . . . . . . . . 6 3.1.6 Selective Acknowledgments . . . . . . . . . . . . . . . . . 6 3.1.7 Explicit Congestion Notification . . . . . . . . . . . . . . 7 3.1.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . 10 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15 Full Copyright Statement . . . . . . . . . . . . . . . . . . 17 Inamura, et. al. Expires May 21, 2002 [Page 2] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 1. Introduction Much development and deployment activity has centered around 2.5G (GPRS and EDGE) and 3G technologies (Wideband CDMA and cdma2000). A primary motivation for these is data communication, and, in particular, Internet access. Accordingly, key issues are TCP performance and the several techniques which can be applied to optimize it over different wireless environments[1]. This document proposes a profile of such techniques, (particularly effective for use with 2.5G and 3G wireless networks), derived from previous work at the IETF [30]. Two example applications of the recommendations in this document are: The WAP Forum [13] is an industry association that has developed standards for wireless information and telephony services on digital mobile phones. In order to address WAP functionality for high speed networks such as 2.5G and 3G networks, and to aim at convergence with Internet standards, the WAP Forum thoroughly revised its specifications. The resultant version 2.0[18] adopts TCP as its transport protocol, and recommends TCP optimization mechanisms closely aligned with those described in this document. I-mode[24], with a huge subscriber base, is a wireless Internet service deployed on handsets in Japan. The new version of i-mode that operates over W-CDMA is called FOMA[25]. It deploys the profile of TCP described in this document. Inamura, et. al. Expires May 21, 2002 [Page 3] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 2. 2.5G and 3G Link Characteristics Link layer characteristics of 2.5G/3G networks have significant effects on TCP performance. ARQ and FEC are discussed in[1]. Justification for link layer ARQ is discussed in [9], [11]. This section provides further details on a specific 3G technology, namely, Wideband CDMA (W-CDMA). W-CDMA (Wideband CDMA) uses RLC (Radio Link Control) [2], a Selective Repeat and sliding window ARQ. RLC uses protocol data units (PDUs) with a 16 bit RLC header. There is an implementation with a 336 bit PDU[25]. This is the unit for link layer retransmission. The IP packet is fragmented into PDUs for transmission by RLC (For more fragmentation discussion, see Section 3.1.4). In W-CDMA, one to twelve PDUs (RLC frames) constitute one FEC frame. The actual size of the FEC frame depends on the link conditions and bandwidth allocation. The FEC frame is the unit of interleaving. For reliable transfer, RLC has an acknowledge mode for PDU retransmitssion. RLC uses checkpoint ARQ [9]. using "status report" type acknowledgments: the poll bit in the header explicitly solicits the peer for a status report containing the sequence number that the peer acknowledged. The use of the poll bit is controlled by timers and by the size of available buffer space in RLC. Also, when the peer detects a gap between sequence numbers in received frames, it can issue a status report to invoke retransmission. RLC preserves the order of packet delivery. The maximum number of retransmissions is a configurable RLC parameter that is specified by RRC [33] (Radio Resource Controller) through RLC connection initialization. The RRC can give the maximum number of retransmissions up to 40. Therefore, RLC can be described as an ARQ that can be configured for either HIGH-PERSISTENCE or LOW-PERSISTENCE, not PERFECT-PERSISTENCE, according to the terminology in [9]. In summary, the link layer ARQ and FEC can provide a packet service with a negligibly small probability of undetected error (failure of the link CRC), and a low level of loss (non-delivery) for the upper layer traffic, i.e. IP. Retransmission of PDUs by ARQ introduces latency and jitter to the IP flow. This is why the transport layer sees the underlying W-CDMA network as a network with a relatively large BDP (Bandwidth-Delay Product), the typical values may range around 50KB. Inamura, et. al. Expires May 21, 2002 [Page 4] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 3. TCP over 2.5G and 3G 3.1 Optimization Mechanisms 3.1.1 Large window size The traditional TCP specification [38] limits the window size to 64 KB. If the end-to-end capacity is expected to be larger than 64 KB, the window scale option [5] can overcome that limitation. TCP over 2.5G/3G should support appropriate window sizes based on the Bandwidth Delay Product (BDP) of the end-to-end path. If the estimated path BDP is larger than 64 KB, the window scale option may be used. 3.1.2 Large initial window TCP controls its transmit rate using the congestion window mechanism. Traditionally, the initial value of the window is one segment. Because the delayed Ack mechanism is the standard, a TCP sender should have an increased initial congestion window of two segments[3]. This effectively cancels the delayed Ack by sending two segments at once in the first RTT of slow start, which helps avoid overhead in the initial phase of the connection. Furthermore, the increased initial window mechanism[4] is also effective, especially for the transmission of small amounts of data, which is the behaviour commonly seen in such applications as Internet-enabled mobile wireless devices. For large data transfers, on the other hand, the effect of this mechanism is negligible. [6] describes evaluations of this mechanism by measurements. An initial congestion window size of two segments is recommended in RFC2581[3]. RFC2414[4] also considers the use of an initial window size larger than two segments. At the time of writing RFC2414 has been proposed to become a standards track RFC. Due to the fact that the delayed Ack mechanism is the standard RFC2581[3], and that the increased initial window option is especially effective for the small data transfers that are common for mobile wireless devices, TCP over 2.5G/3G should use initial CWND (congestion window) = 2 segments. It may use a CWND > 2 segments between a internet host and a mobile node. 3.1.3 Limited Transmit RFC3042[27], Limited Transmit, extends Fast Retransmit/Fast Recovery for TCP connections with small congestion windows that are not likely to generate the three duplicate acknowledgements required to trigger Fast Retransmit. The mechanism calls for sending a new data Inamura, et. al. Expires May 21, 2002 [Page 5] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 segment in response to each of the first two duplicate acknowledgments that arrive at the sender. This mechanism is effective when the congestion window size is small or if a large number of segments in a window are lost. This may reduce the amount of retransmission due to TCP round trip timeout. Similar to the discussion in Section 3.1.2, this mechanism is useful for small amounts of data to be transmitted. TCP over 2.5G/3G implementations should implement Limited Transmit. 3.1.4 Large MTU One of the link layer parameters is MTU (Maximum Transfer Unit). In TCP, the slow start mechanism tries to find an adequate rate for the network path. A larger MTU allows TCP to increase the congestion window faster [10], because the window is counted in units of segments. In links with high error rates, a smaller link PDU size increases the chance of successful transmission. With layer two ARQ and transparent link layer fragmentation, the network layer can enjoy a larger MTU even in a relatively high BER (Bit Error Rate) condition. Without these features in the link, a smaller MTU is suggested. TCP over 2.5G/3G should allow freedom for designers to choose MTU values ranging from small values (such as 576 bytes) to a large value that is supported by the type of link in use (such as 1500 bytes for IP packets on Ethernet), designers are generally encouraged to choose large values. 3.1.5 Path MTU discovery Path MTU discovery allows a sender to determine the maximum end-to-end transmission unit (without IP fragmentation) for a given routing path. RFC1191[19] and RFC1981[21] describe the MTU discovery procedure for IPv4 and IPv6 respectively. This allows TCP senders to employ larger segment sizes (without causing IP layer fragmentation) instead of assuming the small default MTU. TCP over 2.5G/3G implementations should implement Path MTU Discovery. Path MTU Discovery requires intermediate routers to support the generation of the necessary ICMP messages. RFC1435[20] provides recommendations that may be relevant for some router implementations. 3.1.6 Selective Acknowledgments The selective acknowledgment option (SACK), RFC2018[7], is effective when multiple TCP segments are lost in a single TCP window[12]. In particular, if the end-to-end path has a large BDP and a certain packet loss rate, the probability of multiple segment losses in a single window of data increases. In such cases, SACK provides reliability beyond traditional and Reno TCP[8]. TCP over 2.5G/3G should support SACK. Inamura, et. al. Expires May 21, 2002 [Page 6] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 In the absence of SACK feature, the TCP may use NewReno RFC2582[39] as a fallback semantics. 3.1.7 Explicit Congestion Notification Explicit Congestion Notification, RFC3168[23], allows a TCP receiver to inform the sender of congestion in the network by setting the ECN-Echo flag upon receiving an IP packet marked with the CE bit(s). The TCP sender will then reduce its congestion window. Thus, the use of ECN is believed to provide performance benefits[22]. TCP over 2.5G/3G may support ECN. RFC3168[23] also places requirements on intermediate routers (e.g. active queue management and setting of the CE bit(s) in the IP header to indicate congestion). Therefore, the potential improvement in performance can only be achieved when ECN capable routers are deployed along the path. 3.1.8 Summary Items Qualifier ---------------------------------------------------------------- Large window size based on end-to-end BDP Window scale option Window size>64KB [RFC1323] Large initial window (CWND = 2 segments) [RFC2581] Large initial window (CWND > 2 segments) Behind a gateway [RFC2414] Limited Transmit [RFC3042] Selective Acknowledgment option (SACK) [RFC2018] Path MTU discovery [RFC1191,RFC1981] MTU larger than default IP MTU Explicit Congestion Notification(ECN) [RFC3168] Inamura, et. al. Expires May 21, 2002 [Page 7] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 4. Open Issues This section outlines additional mechanisms and parameter settings that may increase the end-to-end performance when running TCP across 2.5G/3G networks. Note, that apart from the discussion of the RTO's initial value, those mechanisms and parameter settings are not part of any standards track RFC at the timing of writing, and can therefore not be recommended internet in general. Link layer mechanisms for increasing TCP performance include enhanced TCP/IP header compression schemes [16], and active queue management RFC2309[15], link layer retransmission schemes [11], and holding on to packets during transient link outages [11]. Shortcomings of existing TCP/IP header compression schemes RFC1144[35], RFC2507[36] are that headers of handshaking packets (SYNs and FINs), and TCP option fields (e.g., SACK or timestamps) are not compressed. In fact, the presence of timestamps effectively disables header compression based on RFC1144[35]. Although RFC3095[34] does not yet address this issue, the ROHC working group is developing schemes to compress TCP headers, including options such as timestamps and selective acknowledgements. Especially, if many short-lived TCP connections run across the link, the compression of the handshaking packets may greatly improve the overall header compression ratio. Implementing active queue management is attractive for a number of reasons as outlined in RFC2309[15]. One important benefit for 2.5G/3G networks, is that it minimizes the amount of potentially stale data that may be queued in the network ("clicking from page to page" before the download of the previous page is complete). Avoiding the transmission of stale data across the 2.5G/3G radio link saves transmission (battery) power, and increases goodput (the ratio of useful data over total data transmitted). Another important benefit of active queue management for 2.5G/3G networks, is that it reduces the risk of a spurious timeout for the first data segment as outlined below. Finding ways to avoid the path round-trip times required for TCP's connection setup and disconnect is particularly attractive for 2.5G/3G networks since these networks are commonly characterized by high delays. This would be particularly beneficial for short-lived, transactional (request/response-style) TCP sessions that typically result from browsing the Web from a smart phone. However, existing solutions such as T/TCP RFC1644[14], have not been adopted due to known security concerns [31]. Spurious timeouts RFC3150[10], packet re-ordering, and packet duplication may reduce TCP's performance. Thus, making TCP more robust against those events is desirable. Solutions to this problem have been proposed [17], [26], [37], and standardization work within the IETF is ongoing at the time of writing. Those solutions include Inamura, et. al. Expires May 21, 2002 [Page 8] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 reversing congestion control state after such an event has been detected, and adapting the retransmission timer and duplicate acknowledgement threshold. The deployment of such solutions may be particularly beneficial when running TCP across wireless networks. This is since wireless access links may often be subject to handovers and resource preemption, or the mobile transmitter may traverse through a radio coverage hole. Such disrupting events may easily trigger a spurious timeout despite a conservative retransmission timer. Also, the mobility mechanisms of some wireless networks may cause packet duplication. The algorithm for computing TCP's retransmission timer is specified in RFC2988[32]. The standard specifies that the initial setting of the retransmission timeout value (RTO) must not be less than 3 seconds. This value might be too low when running TCP across 2.5G/3G networks. In addition to its high latencies, those networks may be run at bit rates of as low as about 10 kb/s which results in large transmission delays. In this case, the RTT for the first data segment may easily go beyond the initial TCP retransmission timer setting of 3 seconds. This would then cause a spurious timeout for that segment. Hence, in such situations it may be advisable to set TCP's initial RTO to a value larger than 3 seconds. Furthermore, due to the potentially large transmissions delays, a TCP sender might choose to refrain from initializing its RTO from the RTT measured for the SYN, but instead take the RTT measured for the first data segment. Inamura, et. al. Expires May 21, 2002 [Page 9] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 5. Security Considerations In 2.5G/3G wireless networks, data is transmitted as ciphertext over the air and as cleartext between the Radio Access Network (RAN) and the core network. IP security RFC2411[29] or TLS RFC2246[28] can be deployed by user devices for end-to-end security. The use of a transport gateway introduces conflicts with IPsec; however TLS can be used in such architectures. Inamura, et. al. Expires May 21, 2002 [Page 10] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 6. Acknowledgements The authors gratefully acknowledges the valuable advises from following individuals: Gorry Fairhurst (gorry@erg.abdn.ac.uk) Mark Allman (mallman@grc.nasa.gov) Aaron Falk (afalk@ieee.org) Inamura, et. al. Expires May 21, 2002 [Page 11] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 References [1] Montenegro, G., Dawkins, S., Kojo, M., Magret, V. and N. Vaidya, "Long Thin Networks", RFC 2757, January 2000. [2] Third Generation Partnership Project, "RLC Protocol Specification (3G TS 25.322:)", 1999. [3] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [4] Allman, M., Floyd, S. and C. Partridge, "Increased TCP's Initial Window", RFC 2414, September 1998. [5] Jacobson, V., Bdaden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [6] Allman, M., "An Evaluation of TCP with Larger Initial Windows 40th IETF Meeting -- TCP Implementations WG. December", December 1997. [7] Mathis, M., Mahdavi, J., Floyd, S. and R. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [8] Fall, K. and S. Floyd, "Simulation-based Comparisons of Tahoe, Reno, and SACK TCP", Computer Communication Review, 26(3) , July 1996. [9] Fairhurst, G. and L. Wood, "Link ARQ issues for IP traffic", Internet draft , November 2000, . [10] Dawkins, S. and G. Montenegro, "End-to-end Performance Implications of Slow Links", RFC 3150/BCP 48, July 2001. [11] Karn, P., Falk, A., Touch, J., Montpetit, M., Mahdavi, J., Montenegro, G., Grossman, D. and G. Fairhurst, "Advice for Internet Subnetwork Designers", Internet draft , November 2000, . [12] Dawkins, S., Montenegro, G., Magret, V., Vaidya, N. and M. Kojo, "End-to-end Performance Implications of Links with Errors", RFC 3135/BCP 50, August 2001. [13] Wireless Application Protocol, "WAP Specifications", 2001, . Inamura, et. al. Expires May 21, 2002 [Page 12] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 [14] Braden, R., "T/TCP -- TCP Extensions for Transactions", RFC 1644, July 1994. [15] Braden, R., Clark, D., Crowcroft, J., Davie, B., Deering, S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J. and L. Zhang, "Recommendations on Queue Management and Congestion Avoidance in the Internet", RFC 2309, April 1998. [16] IETF, "Robust Header Compression", 2001, . [17] Ludwig, R. and R. H. Katz, "The Eifel Algorithm: Making TCP Robust Against Spurious Retransmissions", ACM Computer Communication Review 30(1), January 2000, . [18] Wireless Application Protocol, "WAP Wireless Profiled TCP", WAP-225-TCP-20010331-a, April 2001, . [19] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191, November 1990. [20] Knowles, S., "IESG Advice from Experience with Path MTU Discovery", RFC 1435, March 1993. [21] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [22] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks", RFC 2884, july 2000. [23] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [24] NTT DoCoMo, Inc., "i-mode", 2001, . [25] NTT DoCoMo, Inc., "FOMA", 2001, . [26] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, July 2000. Inamura, et. al. Expires May 21, 2002 [Page 13] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 [27] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's Loss Recovery Using Limited Transmit", RFC 3042, January 2001. [28] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [29] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document Roadmap", RFC 2411, November 1998. [30] Mitzel, D., "Overview of 2000 IAB Wireless Internetworking Workshop", RFC 3002, December 2000. [31] de Vivo, M., O. de Vivo, G., Koeneke, R and G. Isern, "Internet Vulnerabilities Related to TCP/IP and T/TCP", ACM Computer Communication Review 29(1), January 1999, . [32] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. [33] Third Generation Partnership Project, "RRC Protocol Specification (3GPP TS 25.331:)", September 2001. [34] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. [35] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, Feburary 1990. [36] Degermark, M., Nordgren, B. and S. Pink, "IP Header Compression", RFC 2507, Feburary 1999. [37] Blanton, E. and M. Allman, "Using TCP DSACKs and SCTP Duplicate TSNs to Detect Spurious Retransmissions", Internet draft , August 2001, . [38] Postel, J., "Transmission Control Protocol - DARPA Internet Program Protocol Specification", RFC 793, September 1981. [39] Floyd, S. and T. Henderson, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 2582, April 1999. Inamura, et. al. Expires May 21, 2002 [Page 14] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 Authors' Addresses Hiroshi Inamura NTT DoCoMo, Inc. 3-5 Hikarinooka Yokosuka Shi, Kanagawa Ken 239-8536 Japan EMail: inamura@mml.yrp.nttdocomo.co.jp URI: http://www.nttdocomo.co.jp/ Gabriel Montenegro Sun Microsystems, Inc. EMail: gab@sun.com Max Hata NTT DoCoMo, Inc. EMail: hata@mml.yrp.nttdocomo.co.jp URI: http://www.nttdocomo.co.jp/ Masahiro Hara Fujitsu, Inc. EMail: mhara@FLAB.FUJITSU.CO.JP Joby James Motorola, Inc. 33-A, Ulsoor Road, Bangalore 560042 India EMail: joby@MIEL.MOT.COM William Gilliam Hewlett-Packard Company Cupertino, California EMail: wag@cup.hp.com Inamura, et. al. Expires May 21, 2002 [Page 15] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 Alan Hameed Fujitsu FNC, Inc. EMail: Alan.Hameed@fnc.fujitsu.com Reiner Ludwig Ericsson Research Ericsson Allee 1 52134 Herzogenrath Germany EMail: Reiner.Ludwig@Ericsson.com Rodrigo Garces EMail: rgarces2000@yahoo.com> Peter Ford Microsoft EMail: peterf@Exchange.Microsoft.com Fergus Wills Openwave EMail: fergus.wills@openwave.com Inamura, et. al. Expires May 21, 2002 [Page 16] Internet-Draft TCP over 2.5G and 3G Wireless Networks November 2001 Full 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. Inamura, et. al. Expires May 21, 2002 [Page 17]