Network Working Group H. Inamura (editor) Internet-Draft NTT DoCoMo, Inc. Expires: August 23, 2001 G. Montenegro Sun Microsystems, Inc. M. Hara M. Hata Fujitsu, Inc. NTT DoCoMo, Inc. W. Gilliam J. James Hewlett-Packard Company Motorola, Inc. R. Ludwig A. Hameed Ericsson Research Fujitsu FNC, Inc. P. Ford R. Garces Microsoft Metricom F. Wills Openwave Feb 22, 2001 TCP over 2.5G and 3G Wireless Networks draft-ietf-pilc-2.5g3g-00 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 August 23, 2001 [Page 1] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 This Internet-Draft will expire on August 23, 2001. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This document describes a profile for optimizing TCP over 2.5G/3G networks. We describe the link characteristics of 3G wireless by using W-CDMA (Wideband CDMA) as an example. We then recommend TCP optimization mechanisms and, finally, present examples of wireless internet services and standardization activities. These 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 MTU larger than default IP MTU . . . . . . . . . . . . . . . 6 3.1.4 Path MTU discovery . . . . . . . . . . . . . . . . . . . . . 6 3.1.5 Selective Acknowledgments . . . . . . . . . . . . . . . . . 6 3.1.6 Explicit Congestion Notification . . . . . . . . . . . . . . 7 3.1.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2.1 i-mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2.2 WAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2.3 Ricochet MCDN Network . . . . . . . . . . . . . . . . . . . 8 4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13 Full Copyright Statement . . . . . . . . . . . . . . . . . . 15 Inamura, et. al. Expires August 23, 2001 [Page 2] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 1. Introduction Recently, much development and deployment activity has centered around GPRS, UMTS and IMT-2000, also referred to as 2.5G/3G wireless networks. Since a primary motivation for these is data communication, and, in particular, Internet access, TCP performance is a key issue. A number of TCP optimization techniques have been studied to enhance the performance of TCP transmission for various wireless environments. This document proposes a profile of such techniques, derived from previous work at the IETF, particularly effective for use with 2.5G and 3G wireless networks. Inamura, et. al. Expires August 23, 2001 [Page 3] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 2. 2.5G and 3G Link Characteristics The link layer characteristics of 2.5G/3G network affects TCP performance over the link. The characteristics are layer two ARQ (L2 ARQ), FEC (forward error correction) and so on [1]. The justification for L2 ARQ is discussed in [10], [12]. For example, W-CDMA (Wideband CDMA) uses RLC (Radio Link Control) [3] protocol, that is a kind of Selective Repeat and sliding window ARQ. RLC uses protocol data units (PDUs) 336 bits long (including a 16 bit RLC header). This is the unit for retransmission. The SDU (IP packet) is fragmented into PDUs for transmission by RLC. There is also FEC and interleaving. 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. RLC uses "status report" type acknowledgments. It does not use ack-clocking as in TCP, but rather 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 invoke retransmission. The maximum number of retransmissions is a configurable RLC parameter, with a maximum value of 10. Therefore, RLC can be described as an ARQ that can be configured in either HIGH-PERSISTENCE or LOW-PERSISTENCE, not PERFECTLY-PERSISTENT, according to the terminology in [10]. In general, the L2 ARQ and FEC can provide an almost error-free packet service to the upper layer traffic, i.e. IP. The SDU (IP packet) is fragmented into PDUs for transmission. The retransmission by L2 ARQ introduces latency and jitter to the SDU flow, that results in relatively large BDP (Bandwidth-Delay Product) of the 2.5G/3G networks. Inamura, et. al. Expires August 23, 2001 [Page 4] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 3. TCP over 2.5G and 3G 3.1 Optimization Mechanisms 3.1.1 Large window size To utilize link capacity in full, a TCP sender has to send segments to "fill the pipe". To achieve the maximum link efficiency for TCP, the advertised receive window size needs to be equal to or greater than the BDP (Bandwidth Delay Product) of the end-to-end path. The wireless link capacity varies by specific technologies used. In 2.5G/3G wireless, the link BDP tends to large. If the end-to-end path contains one or more wireless link, the end-to-end BDP might be larger than the default value of receive window size on many TCP implementations. The receiver must advertise the appropriate receive window size based on the end-to-end BDP. The traditional TCP specification limits the window size to 64 KB. If the link capacity is expected to be larger than 64 KB, the window scale option [6] must be applied. TCP over 2.5G/3G SHOULD support appropriate window sizes based on the BDP of the end-to-end path. For better round trip time estimates, timestamp option [6] is recommended to be implemented when supporting window size of 64 KB and more. TCP over 2.5G/3G MAY support timestamp. If the window size is larger than or equal to 64KB, implementation SHOULD support timestamp option. 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 widely deployed, a TCP sender should have an increased initial congestion window of two segments[4]. This effectively cancels the delayed Ack by sending two segments at once in the very first slow start turn, that contributes to avoiding the overhead in connection creation. Furthermore, the increased initial window option [5] is also effective, especially for small data to be transmitted, which is commonly seen in such an application as the Internet-enabled mobile wireless devices. For large data transfer, on the other hand, the effect of this option is negligible. [7] describes evaluations of this option by simulation. Two segments of initial congestion window size is recommended in [4][5] also notes consideration on use of initial window size of more than two. Although the increased initial congestion window is Inamura, et. al. Expires August 23, 2001 [Page 5] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 experimental status, there is no impact of use of it to the majority of the Internet if the gateway architecture is deployed that terminates TCP connection between the mobile node and the gateway. Due to the fact that the delayed Ack mechanism is the standard and that the increased initial window option is especially effective for the small data transfer that is common for mobile wireless devices, TCP over 2.5G/3G MUST use cwnd = 2. It MAY use cwnd > 2 if a gateway is present. 3.1.3 MTU larger than default IP 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 link layer. The larger MTU allows TCP to grow the congestion window faster [11] , because the window is counted in unit of segments, especially when the link condition is good. In contrast, under a high BER (Bit Error Rate) situation, smaller MTU is better in terms of the chance of successful transmission. With layer two ARQ, the upper layer can enjoy larger MTU even in a relatively high BER condition. Due to this tradeoff, TCP over 2.5G/3G SHOULD allow freedom for designers to choose MTU from a small value (such as 576B) to a large value (up to 1500B). 3.1.4 Path MTU discovery Path MTU discovery allows a sender to determine the maximum end-to-end transmission unit for a given routing path. [RFC1191] and [RFC1981] describe the MTU discovery procedure for IPv4 and IPv6 respectively. This allows TCP senders to employ larger segment sizes (without causing fragmentation) instead of assuming the default MTU. Using larger segment sizes allows for a faster increase in the congestion window and a smaller ratio of header overhead to data. It should be noted that larger MTUs increase the probability of error in a given segment and also increase the packet transmission time. 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] provides recommendations that may be relevant for some router implementations. 3.1.5 Selective Acknowledgments The selective acknowledgment option (SACK) [8] is effective when multiple TCP segments are lost in a single TCP window [13] . In particular, if the link has a large BDP and a certain amount of packet loss rate, the ratio of multiple segment losses grows high. In such cases, SACK performs better than traditional and Reno TCP [9]. TCP over 2.5G/3G MUST support SACK. Inamura, et. al. Expires August 23, 2001 [Page 6] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 3.1.6 Explicit Congestion Notification Explicit Congestion Notification [RFC2481] allows a TCP receiver to inform the sender of congestion in the network by setting the ECN-Echo flag; a receiver will set this flag on receiving an IP packet marked with the CE bit. The TCP sender can then reduce its congestion window. The use of ECN is believed to provide performance benefits [RFC2884]. TCP over 2.5G/3G MAY support ECN. [RFC2481] also places requirements on intermediate routers (e.g. active queue management and setting of the CE bit in the IP header to indicate congestion). Thus the use of ECN on the TCP connections is dependent on the necessary support from the relevant routers. 3.1.7 Summary Items Qualifier Support ------------------------------------------------------------------------- Large window size SHOULD based on BDP Window scale option Window size>64KB MUST [RFC1323] Timestamp option for RTTM Window size>64KB SHOULD [RFC1323] Large initial window (cwnd = 2) MUST [RFC2581] Large initial window (cwnd > 2) MAY [RFC2414] Selective Acknowledgment option (SACK) MUST [RFC2018] Path MTU discovery SHOULD [RFC1191,RFC1981] MTU larger than default IP MTU MAY Explicit Congestion Notification(ECN) MAY [RFC2481] 3.2 Applications We introduce wireless services deploying (or capable of deploying) the recommendation we discuss here. Net-enabled portable phones and wireless ISPs are the two major applications. 3.2.1 i-mode Mobile terminal users want to enjoy the Internet experience on their handset. This market is emerging and growing rapidly. A deployment example is i-mode, a wireless Internet service. As of this writing, it is the largest single wireless internet service in the world, with 19 million subscribers in Japan. Inamura, et. al. Expires August 23, 2001 [Page 7] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 The next version of i-mode that operates over W-CDMA will be launched at the end of May 2001. It will deploy the profiled TCP that is described in this document. The browser embedded in the handset will utilize the higher speed of 3G infrastructure that can provide up to 384kbps packet mode service. From the perspective of transport layer, the underlying W-CDMA network can be viewed as a network with a relatively large BDP and jitter. The loss rate of IP packets is low due to the ARQ, but the recovery in the layer two appears as jitter to the higher layers. The i-mode infrastructure directly conveys IP packets to the gateway for accessing the Internet. In addition to the operation by the embedded browser, the i-mode handset can be connected to a computer, a PDA and the like as a wireless modem. In this mode, most of data communication facilities can be controlled via AT modem commands. The W-CDMA infrastructure, whose core network uses GPRS (General Packet Radio Service), can be viewed as a large PPP link to GGSN (Gateway GPRS Supporting Node). The other side of GGSN is connected to fixed networks of ISPs using, for example, leased lines. 3.2.2 WAP The WAP Forum [14] is an industry association that has developed standards for wireless information and telephony services on digital mobile phones and other wireless terminals. In order to address WAP functionalities for high speed networks such as 2.5G and 3G networks and to aim at convergence to the Internet standards, the WAP Forum has been addressing adoption of TCP as its transport protocol, benefiting from relevant documents and discussions within IETF and, in particular, its PILC working group. WAP Forum is about to release a new generation of specifications for public review, subject to membership approval. These specifications may include the profiled TCP that is described in this submission. The WAP forum specification profiling TCP is expected to be available for public review in the March 2001 timeframe. 3.2.3 Ricochet MCDN Network Ricochet is a wide-area wireless packet data network. The architecture for the Ricochet system is the MicroCellular Data Network (MCDN). This architecture has seven physical components: 1) wireless modems or subscriber devices; 2) a cluster of MicroCells; 3) Wired Access Points; 4) a nation-wide wired backbone; 5) a Name Service; 6) a Network Management System; 7) and gateways. The MCDN system architecture is based on a mesh of MicroCells deployed throughout a metropolitan area following the part 15.247 FCC rules and regulations for the ISM band [1]. Inamura, et. al. Expires August 23, 2001 [Page 8] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 After radios have acquired each other, they register with the Name Service. The registration process allows the Name Service to store the information required to route from the Name Server to the registering entity. The Name Server can construct a route between the two devices and return an MCDN path with enough information to allow packets to be routed between the two entities. Each device appends the MCDN path to any packet it wishes to route through the network. The customer's radio modem typically appears as a standard AT command-based modem. The modem sends a LookUp packet to the Name Service asking for the MCDN path to the logical name, typically an MCDN gateway to the Internet or an intranet. The user's device then continues to negotiate a PPP connection so that it can be assigned an IP address, an IP router, and a DNS server and become a full-fledged communicating member of the global Internet. When the user's computing device is attempting to negotiate PPP to connect to the network, the radio modem establishes a virtual connection, analogous to TCP, to the MCDN gateway, which ensures that all of the packets from the user's computing device get routed to the Internet via the MCDN gateway. The Wired Access Point places the wireless packets onto a high bandwidth wired backbone and route them to a central collection point, the Network Interface Facility (NIF.) The user's device then appears to the rest of the Internet as if it is physically located at the PPP termination point. Inamura, et. al. Expires August 23, 2001 [Page 9] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 4. Open Issues Other ideas to enhance the performance of TCP over the 2.5G/3G networks may include the use of T/TCP, ROHC for TCP, Active Queue Management, Eifel Algorithm and so on. We have been interested in T/TCP because the Web browsing on a smart phone tends to require short TCP connection duration and small amount of data transfer. The pattern of such use is more transactional rather than streaming. Because T/TCP is regarded as being weak for attacks and not widely deployed, we did not recommend T/TCP in this document. In this document, RFC2414 is treated as an experimental status. RFC2414 is now up for reconsideration to become a proposed standard. Should it get approved as a proposed standard, we can drop the restriction that CWND=3 or 4 may only be used with gateway. There are some fairly recent results on using a larger initial cwnd in our web server in [15]. Inamura, et. al. Expires August 23, 2001 [Page 10] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 5. Security Considerations Please note some of RFCs related to this discussion contains consideration to security issues for example [4]. Inamura, et. al. Expires August 23, 2001 [Page 11] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 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, "3GPP Specifications", 1999, . [3] Third Generation Partnership Project, "RLC Protocol Specification (3G TS 25.322:)", 1999. [4] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [5] Allman, M., Floyd, S. and C. Partridge, "Increased TCP's Initial Window", RFC 2414, September 1998. [6] Jacobson, V., Bdaden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [7] Allman, M., "An Evaluation of TCP with Larger Initial Windows 40th IETF Meeting -- TCP Implementations WG. December", December 1997. [8] Mathis, M., Mahdavi, J., Floyd, S. and R. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [9] Fall, K. and S. Floyd, "Simulation-based Comparisons of Tahoe, Reno, and SACK TCP", Computer Communication Review, 26(3) , July 1996. [10] Fairhurst, G. and L. Wood, "Link ARQ issues for IP traffic", Internet draft , November 2000, . [11] Dawkins, S. and G. Montenegro, "End-to-end Performance Implications of Slow Links", Internet draft , November 2000, . [12] 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, . Inamura, et. al. Expires August 23, 2001 [Page 12] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 [13] Dawkins, S., Montenegro, G., Magret, V., Vaidya, N. and M. Kojo, "End-to-end Performance Implications of Links with Errors", Internet draft , November 2000, . [14] Wireless Application Protocol, "WAP Specifications", 2000, . [15] Allman, M., "A Web Server's View of the Transport Layer", ACM Computer Communication Review 30(5), October 2000, . Authors' Addresses Hiroshi Inamura NTT DoCoMo, Inc. 3-5 Hikarinooka Yokosuka Shi, Kanagawa Ken 239-8536 Japan Phone: +81 468 40 3329 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 Inamura, et. al. Expires August 23, 2001 [Page 13] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 2001 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 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 Metricom EMail: RGarces@Metricom.com Peter Ford Microsoft EMail: peterf@Exchange.Microsoft.com Fergus Wills Openwave EMail: fergus.wills@openwave.com Inamura, et. al. Expires August 23, 2001 [Page 14] Internet-Draft TCP over 2.5G and 3G Wireless Networks Feb 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 August 23, 2001 [Page 15]