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Network Working Group H. Inamura, Ed. Request for Comments: 3481 NTT DoCoMo, Inc. BCP: 71 G. Montenegro, Ed. Category: Best Current Practice Sun Microsystems Laboratories

                                                             R. Ludwig
                                                     Ericsson Research
                                                             A. Gurtov
                                                           F. Khafizov
                                                       Nortel Networks
                                                         February 2003
 TCP over Second (2.5G) and Third (3G) Generation Wireless Networks

Status of this Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.


 This document describes a profile for optimizing TCP to adapt so that
 it handles paths including second (2.5G) and third (3G) generation
 wireless networks.  It describes the relevant characteristics of 2.5G
 and 3G networks, and specific features of example deployments of such
 networks.  It then recommends TCP algorithm choices for nodes known
 to be starting or ending on such paths, and it also discusses open
 issues.  The configuration options recommended in this document are
 commonly found in modern TCP stacks, and are widely available
 standards-track mechanisms that the community considers safe for use
 on the general Internet.

Inamura, et al. Best Current Practice [Page 1] RFC 3481 TCP over 2.5G/3G February 2003

Table of Contents

 1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  2.5G and 3G Link Characteristics. . . . . . . . . . . . . . .   4
     2.1  Latency. . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2  Data Rates . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3  Asymmetry  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4  Delay Spikes . . . . . . . . . . . . . . . . . . . . . .   6
     2.5  Packet Loss Due to Corruption. . . . . . . . . . . . . .   7
     2.6  Intersystem Handovers. . . . . . . . . . . . . . . . . .   7
     2.7  Bandwidth Oscillation. . . . . . . . . . . . . . . . . .   7
 3.  Example 2.5G and 3G Deployments . . . . . . . . . . . . . . .   8
     3.1  2.5G Technologies: GPRS, HSCSD and CDMA2000 1XRTT. . . .   8
     3.2  A 3G Technology: W-CDMA. . . . . . . . . . . . . . . . .   8
     3.3  A 3G Technology: CDMA2000 1X-EV. . . . . . . . . . . . .  10
 4.  TCP over 2.5G and 3G. . . . . . . . . . . . . . . . . . . . .  10
     4.1  Appropriate Window Size (Sender & Receiver). . . . . . .  11
     4.2  Increased Initial Window (Sender). . . . . . . . . . . .  11
     4.3  Limited Transmit (Sender). . . . . . . . . . . . . . . .  12
     4.4  IP MTU Larger than Default . . . . . . . . . . . . . . .  12
     4.5  Path MTU Discovery (Sender & Intermediate Routers) . . .  13
     4.6  Selective Acknowledgments (Sender & Receiver). . . . . .  13
     4.7  Explicit Congestion Notification (Sender, Receiver &
          Intermediate Routers). . . . . . . . . . . . . . . . . .  13
     4.8  TCP Timestamps Option (Sender & Receiver). . . . . . . .  13
     4.9  Disabling RFC 1144 TCP/IP Header Compression (Wireless
          Host)   . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . 16
 5.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 16
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
 9.  Normative References . . . . . . . . . . . . . . . . . . . . . 19
 10. Informative References . . . . . . . . . . . . . . . . . . . . 21
 11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
 12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 26

Inamura, et al. Best Current Practice [Page 2] RFC 3481 TCP over 2.5G/3G February 2003

1. Introduction

 The second generation cellular systems are commonly referred to as
 2G.  The 2G phase began in the 1990s when digital voice encoding had
 replaced analog systems (1G).  2G systems are based on various radio
 technologies including frequency-, code- and time- division multiple
 access.  Examples of 2G systems include GSM (Europe), PDC (Japan),
 and IS-95 (USA).  Data links provided by 2G systems are mostly
 circuit-switched and have transmission speeds of 10-20 kbps uplink
 and downlink.  Demand for higher data rates, instant availability and
 data volume-based charging, as well as lack of radio spectrum
 allocated for 2G led to the introduction of 2.5G (for example, GPRS
 and PDC-P) and 3G (for example, Wideband CDMA and cdma2000) systems.
 Radio technology for both Wideband CDMA (W-CDMA) (adopted, for
 example, in Europe, Japan, etc) and cdma2000 (adopted, for example,
 in US, South Korea, etc) is based on code division multiple access
 allowing for higher data rates and more efficient spectrum
 utilization than 2G systems.  3G systems provide both packet-switched
 and circuit-switched connectivity in order to address the quality of
 service requirements of conversational, interactive, streaming, and
 bulk transfer applications.  The transition to 3G is expected to be a
 gradual process.  Initially, 3G will be deployed to introduce high
 capacity and high speed access in densely populated areas.  Mobile
 users with multimode terminals will be able to utilize existing
 coverage of 2.5G systems on the rest of territory.
 Much development and deployment activity has centered around 2.5G and
 3G technologies.  Along with objectives like increased capacity for
 voice channels, 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 [19].
 This document proposes a profile of such techniques, (particularly
 effective for use with 2.5G and 3G wireless networks).  The
 configuration options in this document are commonly found in modern
 TCP stacks, and are widely available IETF standards-track mechanisms
 that the community has judged to be safe on the general Internet
 (that is, even in predominantly non-wireless scenarios).
 Furthermore, this document makes one set of recommendations that
 covers both 2.5G and 3G networks.  Since both generations of wireless
 technologies exhibit similar challenges to TCP performance (see
 Section 2), one common set is warranted.

Inamura, et al. Best Current Practice [Page 3] RFC 3481 TCP over 2.5G/3G February 2003

 Two example applications of the recommendations in this document are:
 o  The WAP Forum [25] (part of the Open Mobile Alliance [26] as of
    June 2002) 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 higher 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 [31] adopts TCP as its
    transport protocol, and recommends TCP optimization mechanisms
    closely aligned with those described in this document.
 o  I-mode [33] is a wireless Internet service deployed on handsets in
    Japan.  The newer version of i-mode runs on FOMA [34], an
    implementation of W-CDMA.  I-mode over FOMA deploys the profile of
    TCP described in this document.
 This document is structured as follows: Section 2 reviews the link
 layer characteristics of 2.5G/3G networks; Section 3 gives a brief
 overview of some representative 2.5G/3G technologies like W-CDMA,
 cdma2000 and GPRS; Section 4 recommends mechanisms and configuration
 options for TCP implementations used in 2.5G/3G networks, including a
 summary in chart form at the end of the section; finally, Section 5
 discusses some open issues.

2. 2.5G and 3G Link Characteristics

 Link layer characteristics of 2.5G/3G networks have significant
 effects on TCP performance.  In this section we present various
 aspects of link characteristics unique to the 2.5G/3G networks.

2.1 Latency

 The latency of 2.5G/3G links is high mostly due to the extensive
 processing required at the physical layer of those networks, e.g.,
 for FEC and interleaving, and due to transmission delays in the radio
 access network [58] (including link-level retransmissions).  A
 typical RTT varies between a few hundred milliseconds and one second.
 The associated radio channels suffer from difficult propagation
 environments.  Hence, powerful but complex physical layer techniques
 need to be applied to provide high capacity in a wide coverage area
 in a resource efficient way.  Hopefully, rapid improvements in all
 areas of wireless networks ranging from radio layer techniques over
 signal processing to system architecture will ultimately also lead to
 reduced delays in 3G wireless systems.

Inamura, et al. Best Current Practice [Page 4] RFC 3481 TCP over 2.5G/3G February 2003

2.2 Data Rates

 The main incentives for transition from 2G to 2.5G to 3G are the
 increase in voice capacity and in data rates for the users.  2.5G
 systems have data rates of 10-20 kbps in uplink and 10-40 kbps in
 downlink.  Initial 3G systems are expected to have bit rates around
 64 kbps in uplink and 384 kbps in downlink.  Considering the
 resulting bandwidth-delay product (BDP) of around 1-5 KB for 2.5G and
 8-50 KB for 3G, 2.5G links can be considered LTNs (Long Thin Networks
 [19]), and 3G links approach LFNs (Long Fat Networks [2], as
 exemplified by some satellite networks [48]).  Accordingly,
 interested readers might find related and potentially relevant issues
 discussed in RFC 2488 [49].  For good TCP performance both LFNs and
 LTNs require maintaining a large enough window of outstanding data.
 For LFNs, utilizing the available network bandwidth is of particular
 concern.   LTNs need a sufficiently large window for efficient loss
 recovery.  In particular, the fast retransmit algorithm cannot be
 triggered if the window is less than four segments.  This leads to a
 lengthy recovery through retransmission timeouts.  The Limited
 Transmit algorithm RFC 3042 [10] helps avoid the deleterious effects
 of timeouts on connections with small windows.  Nevertheless, making
 full use of the SACK RFC 2018 [3] information for loss recovery in
 both LFNs and LTNs may require twice the window otherwise sufficient
 to utilize the available bandwidth.
 This document recommends only standard mechanisms suitable both for
 LTNs and LFNs, and to any network in general.  However, experimental
 mechanisms suggested in Section 5 can be targeted either for LTNs
 [19] or LFNs [48].
 Data rates are dynamic due to effects from other users and from
 mobility.  Arriving and departing users can reduce or increase the
 available bandwidth in a cell.  Increasing the distance from the base
 station decreases the link bandwidth due to reduced link quality.
 Finally, by simply moving into another cell the user can experience a
 sudden change in available bandwidth.  For example, if upon changing
 cells a connection experiences a sudden increase in available
 bandwidth, it can underutilize it, because during congestion
 avoidance TCP increases the sending rate slowly.  Changing from a
 fast to a slow cell normally is handled well by TCP due to the self-
 clocking property.  However, a sudden increase in RTT in this case
 can cause a spurious TCP timeout as described in Section 2.7.  In
 addition, a large TCP window used in the fast cell can create
 congestion resulting in overbuffering in the slow cell.

Inamura, et al. Best Current Practice [Page 5] RFC 3481 TCP over 2.5G/3G February 2003

2.3 Asymmetry

 2.5G/3G systems may run asymmetric uplink and downlink data rates.
 The uplink data rate is limited by battery power consumption and
 complexity limitations of mobile terminals.  However, the asymmetry
 does not exceed 3-6 times, and can be tolerated by TCP without the
 need for techniques like ACK congestion control or ACK filtering
 [50].  Accordingly, this document does not include recommendations
 meant for such highly asymmetric networks.

2.4 Delay Spikes

 A delay spike is a sudden increase in the latency of the
 communication path.  2.5G/3G links are likely to experience delay
 spikes exceeding the typical RTT by several times due to the
 following reasons.
 1. A long delay spike can occur during link layer recovery from a
    link outage due to temporal loss of radio coverage, for example,
    while driving into a tunnel or within an elevator.
 2. During a handover the mobile terminal and the new base station
    must exchange messages and perform some other time-consuming
    actions before data can be transmitted in a new cell.
 3. Many wide area wireless networks provide seamless mobility by
    internally re-routing packets from the old to the new base station
    which may cause extra delay.
 4. Blocking by high-priority traffic may occur when an arriving
    circuit-switched call or higher priority data temporarily preempts
    the radio channel.  This happens because most current terminals
    are not able to handle a voice call and a data connection
    simultaneously and suspend the data connection in this case.
 5. Additionally, a scheduler in the radio network can suspend a low-
    priority data transfer to give the radio channel to higher
    priority users.
 Delay spikes can cause spurious TCP timeouts, unnecessary
 retransmissions and a multiplicative decrease in the congestion
 window size.

Inamura, et al. Best Current Practice [Page 6] RFC 3481 TCP over 2.5G/3G February 2003

2.5 Packet Loss Due to Corruption

 Even in the face of a high probability of physical layer frame
 errors, 2.5G/3G systems have a low rate of packet losses thanks to
 link-level retransmissions.  Justification for link layer ARQ is
 discussed in [23], [22], [44].  In general, link layer ARQ and FEC
 can provide a packet service with a negligibly small probability of
 undetected errors (failures of the link CRC), and a low level of loss
 (non-delivery) for the upper layer traffic, e.g., IP.  The loss rate
 of IP packets is low due to the ARQ, but the recovery at the link
 layer appears as delay jitter to the higher layers lengthening the
 computed RTO value.

2.6 Intersystem Handovers

 In the initial phase of deployment, 3G systems will be used as a 'hot
 spot' technology in high population areas, while 2.5G systems will
 provide lower speed data service elsewhere.  This creates an
 environment where a mobile user can roam between 2.5G and 3G networks
 while keeping ongoing TCP connections.  The inter-system handover is
 likely to trigger a high delay spike (Section 2.4), and can result in
 data loss.  Additional problems arise because of context transfer,
 which is out of scope of this document, but is being addressed
 elsewhere in the IETF in activities addressing seamless mobility
 Intersystem handovers can adversely affect ongoing TCP connections
 since features may only be negotiated at connection establishment and
 cannot be changed later.  After an intersystem handover, the network
 characteristics may be radically different, and, in fact, may be
 negatively affected by the initial configuration.  This point argues
 against premature optimization by the TCP implementation.

2.7 Bandwidth Oscillation

 Given the limited RF spectrum, satisfying the high data rate needs of
 2.5G/3G wireless systems requires dynamic resource sharing among
 concurrent data users.  Various scheduling mechanisms can be deployed
 in order to maximize resource utilization.  If multiple users wish to
 transfer large amounts of data at the same time, the scheduler may
 have to repeatedly allocate and de-allocate resources for each user.
 We refer to periodic allocation and release of high-speed channels as
 Bandwidth Oscillation.  Bandwidth Oscillation effects such as
 spurious retransmissions were identified elsewhere (e.g., [30]) as
 factors that degrade throughput.  There are research studies [52],
 [54], which show that in some cases Bandwidth Oscillation can be the
 single most important factor in reducing throughput.  For fixed TCP
 parameters the achievable throughput depends on the pattern of

Inamura, et al. Best Current Practice [Page 7] RFC 3481 TCP over 2.5G/3G February 2003

 resource allocation.  When the frequency of resource allocation and
 de-allocation is sufficiently high, there is no throughput
 degradation.  However, increasing the frequency of resource
 allocation/de-allocation may come at the expense of increased
 signaling, and, therefore, may not be desirable.  Standards for 3G
 wireless technologies provide mechanisms that can be used to combat
 the adverse effects of Bandwidth Oscillation.  It is the consensus of
 the PILC Working Group that the best approach for avoiding adverse
 effects of Bandwidth Oscillation is proper wireless sub-network
 design [23].

3. Example 2.5G and 3G Deployments

 This section provides further details on a few example 2.5G/3G
 technologies.  The objective is not completeness, but merely to
 discuss some representative technologies and the issues that may
 arise with TCP performance.  Other documents discuss the underlying
 technologies in more detail.  For example, ARQ and FEC are discussed
 in [23], while further justification for link layer ARQ is discussed
 in [22], [44].

3.1 2.5G Technologies: GPRS, HSCSD and CDMA2000 1XRTT

 High Speed Circuit-Switched Data (HSCSD) and General Packet Radio
 Service (GPRS) are extensions of GSM providing high data rates for a
 user.  Both extensions were developed first by ETSI and later by
 3GPP.  In GSM, a user is assigned one timeslot downlink and one
 uplink.  HSCSD allocates multiple timeslots to a user creating a fast
 circuit-switched link.  GPRS is based on packet-switched technology
 that allows efficient sharing of radio resources among users and
 always-on capability.  Several terminals can share timeslots.  A GPRS
 network uses an updated base station subsystem of GSM as the access
 network; the GPRS core network includes Serving GPRS Support Nodes
 (SGSN) and Gateway GPRS Support Nodes (GGSN).  The RLC protocol
 operating between a base station controller and a terminal provides
 ARQ capability over the radio link.  The Logical Link Control (LLC)
 protocol between the SGSN and the terminal also has an ARQ capability
 utilized during handovers.

3.2 A 3G Technology: W-CDMA

 The International Telecommunication Union (ITU) has selected Wideband
 Code Division Multiple Access (W-CDMA) as one of the global telecom
 systems for the IMT-2000 3G mobile communications standard.  W-CDMA
 specifications are created in the 3rd Generation Partnership Project

Inamura, et al. Best Current Practice [Page 8] RFC 3481 TCP over 2.5G/3G February 2003

 The link layer characteristics of the 3G network which have the
 largest effect on TCP performance over the link are error controlling
 schemes such as layer two ARQ (L2 ARQ) and FEC (forward error
 W-CDMA uses RLC (Radio Link Control) [20], a Selective Repeat and
 sliding window ARQ.  RLC uses protocol data units (PDUs) with a 16
 bit RLC header.  The size of the PDUs may vary.  Typically, 336 bit
 PDUs are implemented [34].  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
 In W-CDMA, one to twelve PDUs (RLC frames) constitute one FEC frame,
 the actual size of which depends on link conditions and bandwidth
 allocation.  The FEC frame is the unit of interleaving.  This
 accumulation of PDUs for FEC adds part of the latency mentioned in
 Section 2.1.
 For reliable transfer, RLC has an acknowledged mode for PDU
 retransmission.  RLC uses checkpoint ARQ [20] with "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 acknowledges.  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 [39] (Radio Resource Controller) through RLC
 connection initialization.  The RRC can set the maximum number of
 retransmissions (up to a maximum of 40).  Therefore, RLC can be
 described as an ARQ that can be configured for either HIGH-
 the terminology in [22].
 Since the RRC manages RLC connection state, Bandwidth Oscillation
 (Section 2.7) can be eliminated by the RRC's keeping RF resource on
 an RLC connection with data in its queue.  This avoids resource de-
 allocation in the middle of transferring data.
 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 delay jitter to the IP flow.  This is why the transport
 layer sees the underlying W-CDMA network as a network with a

Inamura, et al. Best Current Practice [Page 9] RFC 3481 TCP over 2.5G/3G February 2003

 relatively large BDP (Bandwidth-Delay Product) of up to 50 KB for the
 384 kbps radio bearer.

3.3 A 3G Technology: CDMA2000 1X-EV

 One of the Terrestrial Radio Interface standards for 3G wireless
 systems, proposed under the International Mobile Telecommunications-
 2000 umbrella, is cdma2000 [55].  It employs Multi-Carrier Code
 Division Multiple Access (CDMA) technology with a single-carrier RF
 bandwidth of 1.25 MHz.  cdma2000 evolved from IS-95 [56], a 2G
 standard based on CDMA technology.  The first phase of cdma2000
 utilizes a single carrier and is designed to double the voice
 capacity of existing CDMA (IS-95) networks and to support always-on
 data transmission speeds of up to 316.8 kbps.  As mentioned above,
 these enhanced capabilities are delivered by cdma2000 1XRTT.  3G
 speeds of 2 Mbps are offered by cdma2000 1X-EV.  At the physical
 layer, the standard allows transmission in 5,10,20,40 or 80 ms time
 frames.  Various orthogonal (Walsh) codes are used for channel
 identification and to achieve higher data rates.
 Radio Link Protocol Type 3 (RLP) [57] is used with a cdma2000 Traffic
 Channel to support CDMA data services.  RLP provides an octet stream
 transport service and is unaware of higher layer framing.  There are
 several RLP frame formats.  RLP frame formats with higher payload
 were designed for higher data rates.  Depending on the channel speed,
 one or more RLP frames can be transmitted in a single physical layer
 RLP can substantially decrease the error rate exhibited by CDMA
 traffic channels [53].  When transferring data, RLP is a pure NAK-
 based finite selective repeat protocol.  The receiver does not
 acknowledge successfully received data frames.  If one or more RLP
 data frames are missing, the receiving RLP makes several attempts
 (called NAK rounds) to recover them by sending one or more NAK
 control frames to the transmitter.  Each NAK frame must be sent in a
 separate physical layer frame.  When RLP supplies the last NAK
 control frame of a particular NAK round, a retransmission timer is
 set.  If the missing frame is not received when the timer expires,
 RLP may try another NAK round.  RLP may not recover all missing
 frames.  If after all RLP rounds, a frame is still missing, RLP
 supplies data with a missing frame to the higher layer protocols.

4. TCP over 2.5G and 3G

 What follows is a set of recommendations for configuration parameters
 for protocol stacks which will be used to support TCP connections
 over 2.5G and 3G wireless networks.  Some of these recommendations
 imply special configuration:

Inamura, et al. Best Current Practice [Page 10] RFC 3481 TCP over 2.5G/3G February 2003

 o  at the data receiver (frequently a stack at or near the wireless
 o  at the data sender (frequently a host in the Internet or possibly
    a gateway or proxy at the edge of a wireless network), or
 o  at both.
 These configuration options are commonly available IETF standards-
 track mechanisms considered safe on the general Internet.  System
 administrators are cautioned, however, that increasing the MTU size
 (Section 4.4) and disabling RFC 1144 header compression (Section 4.9)
 could affect host efficiency, and that changing such parameters
 should be done with care.

4.1 Appropriate Window Size (Sender & Receiver)

 TCP over 2.5G/3G should support appropriate window sizes based on the
 Bandwidth Delay Product (BDP) of the end-to-end path (see Section
 2.2).  The TCP specification [14] limits the receiver window size to
 64 KB.  If the end-to-end BDP is expected to be larger than 64 KB,
 the window scale option [2] can be used to overcome that limitation.
 Many operating systems by default use small TCP receive and send
 buffers around 16KB.  Therefore, even for a BDP below 64 KB, the
 default buffer size setting should be increased at the sender and at
 the receiver to allow a large enough window.

4.2 Increased Initial Window (Sender)

 TCP controls its transmit rate using the congestion window mechanism.
 The traditional initial window value of one segment, coupled with the
 delayed ACK mechanism [17] implies unnecessary idle times in the
 initial phase of the connection, including the delayed ACK timeout
 (typically 200 ms, but potentially as much as 500 ms) [4].  Senders
 can avoid this by using a larger initial window of up to four
 segments (not to exceed roughly 4 KB) [4].  Experiments with
 increased initial windows  and related measurements have shown (1)
 that it is safe to deploy this mechanism (i.e., it does not lead to
 congestion collapse), and (2) that it is especially effective for the
 transmission of a few TCP segments' worth of data (which is the
 behavior 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.
 TCP over 2.5G/3G SHOULD set the initial CWND (congestion window)
 according to Equation 1 in [4]:
                 min (4*MSS, max (2*MSS, 4380 bytes))

Inamura, et al. Best Current Practice [Page 11] RFC 3481 TCP over 2.5G/3G February 2003

 This increases the permitted initial window from one to between two
 and four segments (not to exceed approximately 4 KB).

4.3 Limited Transmit (Sender)

 RFC 3042 [10], 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 [1].  If a sender has previously unsent
 data queued for transmission, the limited transmit mechanism calls
 for sending a new data 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 avoid some
 retransmissions due to TCP timeouts.  In particular, some studies
 [10] have shown that over half of a busy server's retransmissions
 were due to RTO expiration (as opposed to Fast Retransmit), and that
 roughly 25% of those could have been avoided using Limited Transmit.
 Similar to the discussion in Section 4.2, this mechanism is useful
 for small amounts of data to be transmitted.  TCP over 2.5G/3G
 implementations SHOULD implement Limited Transmit.

4.4 IP MTU Larger than Default

 The maximum size of an IP datagram supported by a link layer is the
 MTU (Maximum Transfer Unit).  The link layer may, in turn, fragment
 IP datagrams into PDUs.  For example, on 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).  Given that the window is counted in units
 of segments, a larger MTU allows TCP to increase the congestion
 window faster [5].  Hence, designers are generally encouraged to
 choose larger values.  These may exceed the default IP MTU values of
 576 bytes for IPv4 RFC 1191 [6] and 1280 bytes for IPv6 [18].  While
 this recommendation is applicable to 3G networks, operation over 2.5G
 networks should exercise caution as per the recommendations in RFC
 3150 [5].

Inamura, et al. Best Current Practice [Page 12] RFC 3481 TCP over 2.5G/3G February 2003

4.5 Path MTU Discovery (Sender & Intermediate Routers)

 Path MTU discovery allows a sender to determine the maximum end-to-
 end transmission unit (without IP fragmentation) for a given routing
 path.  RFC 1191 [6] and RFC 1981 [8] 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.  RFC 1435 [7] provides
 recommendations that may be relevant for some router implementations.

4.6 Selective Acknowledgments (Sender & Receiver)

 The selective acknowledgment option (SACK), RFC 2018 [3], is
 effective when multiple TCP segments are lost in a single TCP window
 [24].  In particular, if the end-to-end path has a large BDP and a
 high packet loss rate, the probability of multiple segment losses in
 a single window of data increases.  In such cases, SACK provides
 robustness beyond TCP-Tahoe and TCP-Reno [21].  TCP over 2.5G/3G
 SHOULD support SACK.
 In the absence of SACK feature, the TCP should use NewReno RFC 2582

4.7 Explicit Congestion Notification (Sender, Receiver & Intermediate

 Explicit Congestion Notification, RFC 3168 [9], 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 [32], [43].  RFC
 3168 [9] 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.  TCP over 2.5G/3G SHOULD support ECN.

4.8 TCP Timestamps Option (Sender & Receiver)

 Traditionally, TCPs collect one RTT sample per window of data [14],
 [17].  This can lead to an underestimation of the RTT, and spurious
 timeouts on paths in which the packet transmission delay dominates
 the RTT.  This holds despite a conservative retransmit timer such as
 the one specified in RFC 2988 [11].  TCP connections with large
 windows may benefit from more frequent RTT samples provided with

Inamura, et al. Best Current Practice [Page 13] RFC 3481 TCP over 2.5G/3G February 2003

 timestamps by adapting quicker to changing network conditions [2].
 However, there is some empirical evidence that for TCPs with an RFC
 2988 timer [11], timestamps provide little or no benefits on backbone
 Internet paths [59].   Using the TCP Timestamps option has the
 advantage that retransmitted segments can be used for RTT
 measurement, which is otherwise forbidden by Karn's algorithm [17],
 [11].  Furthermore, the TCP Timestamps option is the basis for
 detecting spurious retransmits using the Eifel algorithm [30].
 A 2.5/3G link (layer) is dedicated to a single host.  It therefore
 only experiences a low degree of statistical multiplexing between
 different flows.  Also, the packet transmission and queuing delays of
 a 2.5/3G link often dominate the path's RTT.  This already results in
 large RTT variations as packets fill the queue while a TCP sender
 probes for more bandwidth, or as packets drain from the queue while a
 TCP sender reduces its load in response to a packet loss.  In
 addition, the delay spikes across a 2.5/3G link (see Section 2.4) may
 often exceed the end-to-end RTT.  The thus resulting large variations
 in the path's RTT may often cause spurious timeouts.
 When running TCP in such an environment, it is therefore advantageous
 to sample the path's RTT more often than only once per RTT.  This
 allows the TCP sender to track changes in the RTT more closely.  In
 particular, a TCP sender can react more quickly to sudden increases
 of the RTT by sooner updating the RTO to a more conservative value.
 The TCP Timestamps option [2] provides this capability, allowing the
 TCP sender to sample the RTT from every segment that is acknowledged.
 Using timestamps in the mentioned scenario leads to a more
 conservative TCP retransmission timer and reduces the risk of
 triggering spurious timeouts [45], [52], [54], [60].
 There are two problematic issues with using timestamps:
 o  12 bytes of overhead are introduced by carrying the TCP Timestamps
    option and padding in the TCP header.  For a small MTU size, it
    can present a considerable overhead.  For example, for an MTU of
    296 bytes the added overhead is 4%.  For an MTU of 1500 bytes, the
    added overhead is only 0.8%.
 o  Current TCP header compression schemes are limited in their
    handling of the TCP options field.  For RFC 2507 [13], any change
    in the options field (caused by timestamps or SACK, for example)
    renders the entire field uncompressible (leaving the TCP/IP header
    itself compressible, however).  Even worse, for RFC 1144 [40] such
    a change in the options field effectively disables TCP/IP header
    compression altogether.  This is the case when a connection uses
    the TCP Timestamps option.  That option field is used both in the
    data and the ACK path, and its value typically changes from one

Inamura, et al. Best Current Practice [Page 14] RFC 3481 TCP over 2.5G/3G February 2003

    packet to the next.  The IETF is currently specifying a robust
    TCP/IP header compression scheme with better support for TCP
    options [29].
 The original definition of the timestamps option [2] specifies that
 duplicate segments below cumulative ACK do not update the cached
 timestamp value at the receiver.  This may lead to overestimating of
 RTT for retransmitted segments.  A possible solution [47] allows the
 receiver to use a more recent timestamp from a duplicate segment.
 However, this suggestion allows for spoofing attacks against the TCP
 receiver.  Therefore,  careful consideration is needed in
 implementing this solution.
 Recommendation: TCP SHOULD use the TCP Timestamps option.  It allows
 for better RTT estimation and reduces the risk of spurious timeouts.

4.9 Disabling RFC 1144 TCP/IP Header Compression (Wireless Host)

 It is well known (and has been shown with experimental data) that RFC
 1144 [40] TCP header compression does not perform well in the
 presence of packet losses [43], [52].  If a wireless link error is
 not recovered, it will cause TCP segment loss between the compressor
 and decompressor, and then RFC 1144 header compression does not allow
 TCP to take advantage of Fast Retransmit Fast Recovery mechanism.
 The RFC 1144 header compression algorithm does not transmit the
 entire TCP/IP headers, but only the changes in the headers of
 consecutive segments.  Therefore, loss of a single TCP segment on the
 link causes the transmitting and receiving TCP sequence numbers to
 fall out of synchronization.   Hence, when a TCP segment is lost
 after the compressor, the decompressor will generate false TCP
 headers.  Consequently, the TCP receiver will discard all remaining
 packets in the current window because of a checksum error.  This
 continues until the compressor receives the first retransmission
 which is forwarded uncompressed to synchronize the decompressor [40].
 As previously recommended in RFC 3150 [5], RFC 1144 header
 compression SHOULD NOT be enabled unless the packet loss probability
 between the compressor and decompressor is very low.  Actually,
 enabling the Timestamps Option effectively accomplishes the same
 thing (see Section 4.8).  Other header compression schemes like RFC
 2507 [13] and Robust Header Compression [12] are meant to address
 deficiencies in RFC 1144 header compression.  At the time of this
 writing, the IETF was working on multiple extensions to Robust Header
 Compression (negotiating Robust Header Compression over PPP,
 compressing TCP options, etc) [16].

Inamura, et al. Best Current Practice [Page 15] RFC 3481 TCP over 2.5G/3G February 2003

4.10 Summary

 Items                                   Comments
 Appropriate Window Size         (sender & receiver)
                                 based on end-to-end BDP
 Window Scale Option             (sender & receiver)
 [RFC1323]                       Window size > 64KB
 Increased Initial Window        (sender)
 [RFC3390]                       CWND = min (4*MSS,
                                 max (2*MSS, 4380 bytes))
 Limited Transmit                (sender)
 IP MTU larger than              more applicable to 3G
 Path MTU Discovery              (sender & intermediate routers)
 Selective Acknowledgment
 option (SACK)
 [RFC2018]                       (sender & receiver)
 Explicit Congestion
 [RFC3168]                       (sender, receiver &
                                 intermediate routers)
 Timestamps Option               (sender & receiver)
 [RFC1323, R.T.Braden's ID]
 Disabling RFC1144
 TCP/IP Header Compression
 [RFC1144]                       (wireless host)

5. Open Issues

 This section outlines additional mechanisms and parameter settings
 that may increase 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 time of this writing.  Therefore,
 they cannot be recommended for the Internet in general.

Inamura, et al. Best Current Practice [Page 16] RFC 3481 TCP over 2.5G/3G February 2003

 Other mechanisms for increasing TCP performance include enhanced TCP/
 IP header compression schemes [29], active queue management RFC 2309
 [28], link layer retransmission schemes [23], and caching packets
 during transient link outages to retransmit them locally when the
 link is restored to operation [23].
 Shortcomings of existing TCP/IP header compression schemes (RFC 1144
 [40], RFC 2507 [13]) are that they do not compress headers of
 handshaking packets (SYNs and FINs), and that they lack proper
 handling of TCP option fields (e.g., SACK or timestamps) (see Section
 4.8).   Although RFC 3095 [12] does not yet address this issue, the
 IETF is developing improved TCP/IP header compression schemes,
 including better handling of TCP 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 RFC 2309 [28].  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 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
 Since 2.5G/3G networks are commonly characterized by high delays,
 avoiding unecessary round-trip times is particularly attractive.
 This is specially 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
 RFC 1644 [27], have not been adopted due to known security concerns
 Spurious timeouts, 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
 [30], [35], [41], and standardization work within the IETF is ongoing
 at the time of writing.  Those solutions include reverting 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 because wireless access links
 may often be subject to handovers and resource preemption, or the
 mobile transmitter may traverse through a radio coverage hole.  Such

Inamura, et al. Best Current Practice [Page 17] RFC 3481 TCP over 2.5G/3G February 2003

 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 RFC 2988 [11].  The standard specifies that the initial setting of
 the retransmission timeout value (RTO) should 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
 packet transmission delays.  In this case, the RTT for the first data
 segment may easily exceed 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 packet transmission 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.
 Some of the recommendations in RFC 2988 [11] are optional, and are
 not followed by all TCP implementations.  Specifically, some TCP
 stacks allow a minimum RTO less than the recommended value of 1
 second (section 2.4 of [11]), and some implementations do not
 implement the recommended restart of the RTO timer when an ACK is
 received (section 5.3 of [11]).  Some experiments [52], [54], have
 shown that in the face of bandwidth oscillation, using the
 recommended minimum RTO value of 1 sec (along with the also
 recommended initial RTO of 3 sec) reduces the number of spurious
 retransmissions as compared to using small minimum RTO values of 200
 or 400 ms.  Furthermore, TCP stacks that restart the retransmission
 timer when an ACK is received experience far less spurious
 retransmissions than implementations that do not restart the RTO
 timer when an ACK is received.  Therefore, at the time of this
 writing, it seems preferable for TCP implementations used in 3G
 wireless data transmission to comply with all recommendations of RFC

6. 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 RFC 2401 [37] or TLS RFC 2246 [36] can
 be deployed by user devices for end-to-end security.

7. IANA Considerations

 This specification requires no IANA actions.

Inamura, et al. Best Current Practice [Page 18] RFC 3481 TCP over 2.5G/3G February 2003

8. Acknowledgements

 The authors would like to acknowledge contributions to the text from
 the following individuals:
    Max Hata, NTT DoCoMo, Inc.  (
    Masahiro Hara, Fujitsu, Inc.  (mhara@FLAB.FUJITSU.CO.JP)
    Joby James, Motorola, Inc.  (joby@MIEL.MOT.COM)
    William Gilliam, Hewlett-Packard Company (
    Alan Hameed, Fujitsu FNC, Inc. (
    Rodrigo Garces, Mobility Network Systems
    Peter Ford, Microsoft (
    Fergus Wills, Openwave (
    Michael Meyer (
 The authors gratefully acknowledge the valuable advice from the
 following individuals:
    Gorry Fairhurst (
    Mark Allman (
    Aaron Falk (falk@ISI.EDU)

9. Normative References

 [1]  Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control",
      RFC 2581, April 1999.
 [2]  Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High
      Performance", RFC 1323, May 1992.
 [3]  Mathis, M., Mahdavi, J., Floyd, S. and R. Romanow, "TCP
      Selective Acknowledgment Options", RFC 2018, October 1996.
 [4]  Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
      Initial Window", RFC 3390, October 2002.

Inamura, et al. Best Current Practice [Page 19] RFC 3481 TCP over 2.5G/3G February 2003

 [5]  Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-to-end
      Performance Implications of Slow Links", BCP 48, RFC 3150, July
 [6]  Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
      November 1990.
 [7]  Knowles, S., "IESG Advice from Experience with Path MTU
      Discovery", RFC 1435, March 1993.
 [8]  McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP
      version 6", RFC 1981, August 1996.
 [9]  Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
      Explicit Congestion Notification (ECN) to IP", RFC 3168,
      September 2001.
[10]  Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's Loss
      Recovery Using Limited Transmit", RFC 3042, January 2001.
[11]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
      Timer", RFC 2988, November 2000.
[12]  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.
[13]  Degermark, M., Nordgren, B. and S. Pink, "IP Header
      Compression", RFC 2507, February 1999.
[14]  Postel, J., "Transmission Control Protocol - DARPA Internet
      Program Protocol Specification", STD 7, RFC 793, September 1981.
[15]  Floyd, S. and T. Henderson, "The NewReno Modification to TCP's
      Fast Recovery Algorithm", RFC 2582, April 1999.
[16]  Bormann, C., "Robust Header Compression (ROHC) over PPP", RFC
      3241, April 2002.
[17]  Braden, R., "Requirements for Internet Hosts - Communication
      Layers", STD 3, RFC 1122, October 1989.
[18]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 2460, December 1998.

Inamura, et al. Best Current Practice [Page 20] RFC 3481 TCP over 2.5G/3G February 2003

10. Informative References

[19]  Montenegro, G., Dawkins, S., Kojo, M., Magret, V. and N.
      Vaidya, "Long Thin Networks", RFC 2757, January 2000.
[20]  Third Generation Partnership Project, "RLC Protocol
      Specification (3G TS 25.322:)", 1999.
[21]  Fall, K. and S. Floyd, "Simulation-based Comparisons of Tahoe,
      Reno, and SACK TCP", Computer Communication Review, 26(3) , July
[22]  Fairhurst, G. and L. Wood, "Advice to link designers on link
      Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366, August 2002.
[23]  Karn, P., "Advice for Internet Subnetwork Designers", Work in
[24]  Dawkins, S., Montenegro, G., Magret, V., Vaidya, N. and M.
      Kojo, "End-to-end Performance Implications of Links with
      Errors", BCP 50, RFC 3135, August 2001.
[25]  Wireless Application Protocol, "WAP Specifications", 2002,
[26]  Open Mobile Alliance, "Open Mobile Alliance", 2002,
[27]  Braden, R., "T/TCP -- TCP Extensions for Transactions", RFC
      1644, July 1994.
[28]  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.
[29]  IETF, "Robust Header Compression", 2001,
[30]  Ludwig, R. and R. H. Katz, "The Eifel Algorithm: Making TCP
      Robust Against Spurious Retransmissions", ACM Computer
      Communication Review 30(1), January 2000.
[31]  Wireless Application Protocol, "WAP Wireless Profiled TCP",
      WAP-225-TCP-20010331-a, April 2001,

Inamura, et al. Best Current Practice [Page 21] RFC 3481 TCP over 2.5G/3G February 2003

[32]  Hadi Salim, J. and U. Ahmed, "Performance Evaluation of Explicit
      Congestion Notification (ECN) in IP Networks", RFC 2884, July
[33]  NTT DoCoMo Technical Journal, "Special Issue on i-mode Service",
      October 1999.
[34]  NTT DoCoMo Technical Journal, "Special Article on IMT-2000
      Services", September 2001.
[35]  Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An
      Extension to the Selective Acknowledgement (SACK) Option for
      TCP", RFC 2883, July 2000.
[36]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
      2246, January 1999.
[37]  Kent, S. and R. Atkinson, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.
[38]  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.
[39]  Third Generation Partnership Project, "RRC Protocol
      Specification (3GPP TS 25.331:)", September 2001.
[40]  Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial
      Links", RFC 1144, February 1990.
[41]  Blanton, E. and M. Allman, "On Making TCP More Robust to Packet
      Reordering", ACM Computer Communication Review 32(1), January
      2002, <>.
[42]  Karn, P. and C. Partridge, "Improving Round-Trip Time Estimates
      in Reliable Transport Protocols", ACM SIGCOMM 87, 1987.
[43]  Ludwig, R., Rathonyi, B., Konrad, A. and A. Joseph, "Multi-layer
      tracing of TCP over a reliable wireless link", ACM SIGMETRICS
      99, May 1999.
[44]  Ludwig, R., Konrad, A., Joseph, A. and R. Katz, "Optimizing the
      End-to-End Performance of Reliable Flows over Wireless Links",
      Kluwer/ACM Wireless Networks Journal Vol. 8, Nos. 2/3, pp. 289-
      299, March-May 2002.

Inamura, et al. Best Current Practice [Page 22] RFC 3481 TCP over 2.5G/3G February 2003

[45]  Gurtov, A., "Making TCP Robust Against Delay Spikes", University
      of Helsinki, Department of Computer Science, Series of
      Publications C, C-2001-53, Nov 2001,
[46]  Stevens, W., "TCP/IP Illustrated, Volume 1; The Protocols",
      Addison Wesley, 1995.
[47]  Braden, R., "TCP Extensions for High Performance: An Update",
      Work in Progress.
[48]  Allman, M., Dawkins, S., Glover, D., Griner, J., Tran, D.,
      Henderson, T., Heidemann, J., Touch, J., Kruse, H., Ostermann,
      S., Scott, K. and J. Semke, "Ongoing TCP Research Related to
      Satellites", RFC 2760, February 2000.
[49]  Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over
      Satellite Channels using Standard Mechanisms", BCP 28, RFC 2488,
      January 1999.
[50]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G. and M.
      Sooriyabandara, "TCP Performance Implications of Network
      Asymmetry", RFC 3449, December 2002.
[51]  Kempf, J., "Problem Description: Reasons For Performing Context
      Transfers Between Nodes in an IP Access Network", RFC 3374,
      September 2002.
[52]  Khafizov, F. and M. Yavuz, "Running TCP over IS-2000", Proc. of
      IEEE ICC, 2002.
[53]  Khafizov, F. and M. Yavuz, "Analytical Model of RLP in IS-2000
      CDMA Networks", Proc. of IEEE Vehicular Technology Conference,
      September 2002.
[54]  Yavuz, M. and F. Khafizov, "TCP over Wireless Links with
      Variable Bandwidth", Proc. of IEEE Vehicular Technology
      Conference, September 2002.
[55]  TIA/EIA/cdma2000, "Mobile Station - Base Station Compatibility
      Standard for Dual-Mode Wideband Spread Spectrum Cellular
      Systems", Washington: Telecommunication Industry Association,
[56]  TIA/EIA/IS-95 Rev A, "Mobile Station - Base Station
      Compatibility Standard for Dual-Mode Wideband Spread Spectrum
      Cellular Systems", Washington: Telecommunication Industry
      Association, 1995.

Inamura, et al. Best Current Practice [Page 23] RFC 3481 TCP over 2.5G/3G February 2003

[57]  TIA/EIA/IS-707-A-2.10, "Data Service Options for Spread Spectrum
      Systems: Radio Link Protocol Type 3", January 2000.
[58]  Dahlman, E., Beming, P., Knutsson, J., Ovesjo, F., Persson, M.
      and C. Roobol, "WCDMA - The Radio Interface for Future Mobile
      Multimedia Communications", IEEE Trans. on Vehicular Technology,
      vol. 47, no. 4, pp. 1105-1118, November 1998.
[59]  Allman, M. and V. Paxson, "On Estimating End-to-End Network Path
      Properties", ACM SIGCOMM 99, September 1999.
[60]  Gurtov, A. and R. Ludwig, "Responding to Spurious Timeouts in
      TCP", IEEE INFOCOM'03, March 2003.

Inamura, et al. Best Current Practice [Page 24] RFC 3481 TCP over 2.5G/3G February 2003

11. Authors' Addresses

 Hiroshi Inamura
 NTT DoCoMo, Inc.
 3-5 Hikarinooka
 Yokosuka Shi, Kanagawa Ken  239-8536
 Gabriel Montenegro
 Sun Microsystems Laboratories, Europe
 Avenue de l'Europe
 ZIRST de Montbonnot
 38334 Saint Ismier CEDEX
 Reiner Ludwig
 Ericsson Research
 Ericsson Allee 1
 52134 Herzogenrath
 Andrei Gurtov
 P.O. Box 970, FIN-00051
 Farid Khafizov
 Nortel Networks
 2201 Lakeside Blvd
 Richardson, TX 75082,

Inamura, et al. Best Current Practice [Page 25] RFC 3481 TCP over 2.5G/3G February 2003

12. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  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
 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


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Inamura, et al. Best Current Practice [Page 26]

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