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
Europe
R. Ludwig
Ericsson Research
A. Gurtov
Sonera
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.
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.
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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
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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.
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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.
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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.
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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.
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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
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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).
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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
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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:
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o at the data receiver (frequently a stack at or near the wireless
device),
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))
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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].
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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
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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
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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].
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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.
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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
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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.
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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.
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
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.
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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
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,
.
[26] Open Mobile Alliance, "Open Mobile Alliance", 2002,
.
[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
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, .
[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.
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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,
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.
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.
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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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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
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Inamura, et al. Best Current Practice [Page 26]
RFC, FYI, BCP