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 © The Internet Society (2003). All Rights Reserved.

Abstract

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.

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

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.

Two example applications of the recommendations in this document are:

• 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.

• 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.

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.

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.

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 [51].

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 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 (3GPP).

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 correction).

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 4.4.)

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- PERSISTENCE or LOW-PERSISTENCE, not PERFECT-PERSISTENCE, according to 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 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 frame.

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:

• at the data receiver (frequently a stack at or near the wireless device),

• at the data sender (frequently a host in the Internet or possibly a gateway or proxy at the edge of a wireless network), or

• 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))

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].

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 [15].

4.7 Explicit Congestion Notification (Sender, Receiver & Intermediate

Routers)

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

   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:

• 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%.

• 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 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].

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)
   [RFC3042]
   
   IP MTU larger than              more applicable to 3G
   Default
   
   Path MTU Discovery              (sender & intermediate routers)
   [RFC1191,RFC1981]
   
   Selective Acknowledgment
   option (SACK)
   [RFC2018]                       (sender & receiver)
   
   Explicit Congestion
   Notification(ECN)
   [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.

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 below.

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 [38].

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 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 2988.

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.

8. Acknowledgements

The authors would like to acknowledge contributions to the text from the following individuals:

      Max Hata, NTT DoCoMo, Inc.  (hata@mml.yrp.nttdocomo.co.jp)
      
      Masahiro Hara, Fujitsu, Inc.  (mhara@FLAB.FUJITSU.CO.JP)
      
      Joby James, Motorola, Inc.  (joby@MIEL.MOT.COM)
      
      William Gilliam, Hewlett-Packard Company (wag@cup.hp.com)
      
      Alan Hameed, Fujitsu FNC, Inc. (Alan.Hameed@fnc.fujitsu.com)

Rodrigo Garces, Mobility Network Systems

                             (rodrigo.garces@mobilitynetworks.com)
      
      Peter Ford, Microsoft (peterf@Exchange.Microsoft.com)
      
      Fergus Wills, Openwave (fergus.wills@openwave.com)
      
      Michael Meyer (Michael.Meyer@eed.ericsson.se)

The authors gratefully acknowledge the valuable advice from the following individuals:

      Gorry Fairhurst (gorry@erg.abdn.ac.uk)
      
      Mark Allman (mallman@grc.nasa.gov)
      
      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.
   
   [5]  Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-to-end
        Performance Implications of Slow Links", BCP 48, RFC 3150, July
        2001.
   
   [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.

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
        1996.
  
  [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
        Progress.
  
  [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,
        <http://www.wapforum.org>.
  
  [26]  Open Mobile Alliance, "Open Mobile Alliance", 2002,
        <http://www.openmobilealliance.org/>.
  
  [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,
        <http://www.ietf.org/html.charters/rohc-charter.html>.
  
  [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,
        <http://www.wapforum.com/what/technical.htm>.
  
  [32]  Hadi Salim, J. and U. Ahmed, "Performance Evaluation of Explicit
        Congestion Notification (ECN) in IP Networks", RFC 2884, July
        2000.
  
  [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, <http://roland.grc.nasa.gov/~mallman/papers/tcp-reorder-
        ccr.ps>.
  
  [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.
  
  [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,
        <http://www.cs.helsinki.fi/u/gurtov/papers/report01.html>.
  
  [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,
        1999.
  
  [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.
  
  [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.

11. 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 Laboratories, Europe
Avenue de l'Europe
ZIRST de Montbonnot
38334 Saint Ismier CEDEX
France

EMail:

          gab@sun.com

Reiner Ludwig
Ericsson Research
Ericsson Allee 1
52134 Herzogenrath
Germany

   EMail: Reiner.Ludwig@Ericsson.com
   
   Andrei Gurtov
   Sonera
   P.O. Box 970, FIN-00051
   Helsinki,
   Finland
   
   EMail: andrei.gurtov@sonera.com
   URI:   http://www.cs.helsinki.fi/u/gurtov/

Farid Khafizov
Nortel Networks
2201 Lakeside Blvd
Richardson, TX 75082,
USA

EMail:

          faridk@nortelnetworks.com

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