Network Working Group;;;G. Montenegro
Request for Comments: 2757;;;;;Sun Microsystems, Inc.
Category: Informational;;;;S. Dawkins
Nortel Networks
M. Kojo
University of Helsinki
V. Magret
Alcatel
N. Vaidya
Texas A&M University

;January 2000

Long Thin Networks

Status of this Memo

This memo provides information for the Internet community.; It does
not specify an Internet standard of any kind.; Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2000).; All Rights Reserved.

Abstract

In view of the unpredictable and problematic nature of long thin
networks (for example, wireless WANs), arriving at an optimized
transport is a daunting task.; We have reviewed the existing
proposals along with future research items. Based on this overview,
we also recommend mechanisms for implementation in long thin
networks.

Our goal is to identify a TCP that works for all users, including
users of long thin networks. We started from the working
recommendations of the IETF TCP Over Satellite Links (tcpsat) working
group with this end in mind.

We recognize that not every tcpsat recommendation will be required
for long thin networks as well, and work toward a set of TCP
recommendations that are "benign" in environments that do not require
them.

Table of Contents

1 IntrodUCtion .................................................;3
1.1 Network Architecture ....................................;5
1.2 Assumptions about the Radio Link ........................;6
2 Should it be IP or Not?; .....................................;7
2.1 Underlying Network Error Characteristics ................;7
2.2 Non-IP Alternatives .....................................;8
2.2.1 WAP ................................................;8
2.2.2 Deploying Non-IP Alternatives ......................;9
2.3 IP-based Considerations .................................;9

2.3.1 Choosing the MTU [Stevens94, RFC1144] ..............;9
2.3.2 Path MTU Discovery [RFC1191] .......................10
2.3.3 Non-TCP Proposals ..................................10
3 The Case for TCP .............................................11
4 Candidate Optimizations ......................................12
4.1 TCP: Current Mechanisms .................................12
4.1.1 Slow Start and Congestion Avoidance ................12
4.1.2 Fast Retransmit and Fast Recovery ..................12
4.2 Connection Setup with T/TCP [RFC1397, RFC1644] ..........14
4.3 Slow Start Proposals ....................................14
4.3.1 Larger Initial Window ..............................14
4.3.2 Growing the Window during Slow Start ...............15
4.3.2.1 ACK Counting ..................................15
4.3.2.2 ACK-every-segment .............................16
4.3.3 Terminating Slow Start .............................17
4.3.4 Generating ACKs during Slow Start ..................17
4.4 ACK Spacing .............................................17
4.5 Delayed Duplicate Acknowlegements .......................18
4.6 Selective Acknowledgements [RFC2018] ....................18
4.7 Detecting Corruption Loss ...............................19
4.7.1 Without EXPlicit Notification ......................19
4.7.2 With Explicit Notifications ........................20
4.8 Active Queue Management .................................21
4.9 Scheduling Algorithms ...................................21
4.10 Split TCP and Performance-Enhancing Proxies (PEPs) .....22

4.10.1 Split TCP Approaches ..............................23
4.10.2 Application Level Proxies .........................26
4.10.3 Snoop and its Derivatives .........................27
4.10.4 PEPs to handle Periods of Disconnection ...........29
4.11 Header Compression Alternatives ........................30
4.12 Payload Compression ....................................31
4.13 TCP Control Block Interdependence [Touch97] ............32
5 Summary of Recommended Optimizations .........................33
6 Conclusion ...................................................35
7 Acknowledgements .............................................35
8 Security Considerations ......................................35

9 References ...................................................36
Authors" Addresses .............................................44
Full Copyright Statement .......................................46

1 Introduction

Optimized wireless networking is one of the major hurdles that Mobile
Computing must solve if it is to enable ubiquitous Access to
networking resources. However, current data networking protocols have
been optimized primarily for wired networks.; Wireless environments
have very different characteristics in terms of latency, jitter, and
error rate as compared to wired networks.; Accordingly, traditional
protocols are ill-suited to this medium.

Mobile Wireless networks can be grouped in W-LANs (for example,
802.11 compliant networks) and W-WANs (for example, CDPD [CDPD],
Ricochet, CDMA [CDMA], PHS, DoCoMo, GSM [GSM] to name a few).; W-WANs
present the most serious challenge, given that the length of the
wireless link (expressed as the delay*bandwidth product) is typically
4 to 5 times as long as that of its W-LAN counterparts.; For example,
for an 802.11 network, assuming the delay (round-trip time) is about
3 ms.; and the bandwidth is 1.5 Mbps, the delay*bandwidth product is
4500 bits. For a W-WAN such as Ricochet, a typical round-trip time
may be around 500 ms. (the best is about 230 ms.), and the sustained

bandwidth is about 24 Kbps. This yields a delay*bandwidth product
roughly equal to 1.5 KB. In the near future, 3rd Generation wireless
services will offer 384Kbps and more.; Assuming a 200 ms round-trip,
the delay*bandwidth product in this case is 76.8 Kbits (9.6 KB). This
value is larger than the default 8KB buffer space used by many TCP
implementations. This means that, whereas for W-LANs the default
buffer space is enough, future W-WANs will operate inefficiently
(that is, they will not be able to fill the pipe) unless they
override the default value. A 3rd Generation wireless service
offering 2 Mbps with 200-millisecond latency requires a 50 KB buffer.

Most importantly,; latency across a link adversely affects
throughput. For example,; [MSMO97] derives an upper bound on TCP
throughput. Indeed, the resultant expression is inversely related to
the round-trip time.

The long latencies also push the limits (and commonly transgress
them) for what is acceptable to users of interactive applications.

As a quick glance to our list of references will reveal, there is a
wealth of proposals that attempt to solve the wireless networking
problem. In this document, we survey the different solutions
available or under investigation, and issue the corresponding
recommendations.

There is a large body of work on the subject of improving TCP
performance over satellite links. The documents under development by
the tcpsat working group of the IETF [AGS98, ADGGHOSSTT98] are very
relevant. In both cases, it is essential to start by improving the
characteristics of the medium by using forward error correction (FEC)
at the link layer to reduce the BER (bit error rate) from values as
high as 10-3 to 10-6 or better. This makes the BER manageable. Once
in this realm, retransmission schemes like ARQ (automatic repeat
request) may be used to bring it down even further. Notice that
sometimes it may be desirable to forego ARQ because of the additional
delay it implies.; In particular, time sensitive traffic (video,
audio) must be delivered within a certain time limit beyond which the
data is obsolete. Exhaustive retransmissions in this case merely
succeed in wasting time in order to deliver data that will be
discarded once it arrives at its destination.; This indicates the
desirability of augmenting the protocol stack implementation on
devices such that the upper protocol layers can inform the link and

MAC layer when to avoid such costly retransmission schemes.

Networks that include satellite links are examples of "long fat
networks" (LFNs or "elephants"). They are "long" networks because
their round-trip time is quite high (for example, 0.5 sec and higher
for geosynchronous satellites). Not all satellite links fall within
the LFN regime. In particular, round-trip times in a low-earth
orbiting (LEO) satellite network may be as little as a few
milliseconds (and never extend beyond 160 to 200 ms). W-WANs share
the "L" with LFNs. However, satellite networks are also "fat" in the
sense that they may have high bandwidth. Satellite networks may often
have a delay*bandwidth product above 64 KBytes, in which case they
pose additional problems to TCP [TCPHP]. W-WANs do not generally
exhibit this behavior. Accordingly, this document only deals with
links that are "long thin pipes", and the networks that contain them:
"long thin networks". We call these "LTNs".

This document does not give an overview of the API used to access the
underlying transport. We believe this is an orthogonal issue, even
though some of the proposals below have been put forth assuming a
given interface.; It is possible, for example, to support the
traditional socket semantics without fully relying on TCP/IP
transport [MOWGLI].

Our focus is on the on-the-wire protocols. We try to include the most
relevant ones and briefly (given that we provide the references
needed for further study) mention their most salient points.

1.1 Network Architecture

One significant difference between LFNs and LTNs is that we assume
the W-WAN link is the last hop to the end user. This allows us to
assume that a single intermediate node sees all packets transferred
between the wireless mobile device and the rest of the Internet.
This is only one of the topologies considered by the TCP Satellite
community.

Given our focus on mobile wireless applications, we only consider a
very specific architecture that includes:

-; a wireless mobile device, connected via

-; a wireless link (which may, in fact comprise several hops at
the link layer), to

-; an intermediate node (sometimes referred to as a base station)

connected via

-; a wireline link, which in turn interfaces with

-; the landline Internet and millions of legacy servers and web
sites.

Specifically, we are not as concerned with paths that include two
wireless segments separated by a wired one. This may occur, for
example, if one mobile device connects across its immediate wireless
segment via an intermediate node to the Internet, and then via a
second wireless segment to another mobile device.; Quite often,
mobile devices connect to a legacy server on the wired Internet.

Typically, the endpoints of the wireless segment are the intermediate
node and the mobile device. However, the latter may be a wireless
router to a mobile network. This is also important and has
applications in, for example, disaster recovery.

Our target architecture has implications which concern the
deployability of candidate solutions. In particular, an important
requirement is that we cannot alter the networking stack on the
legacy servers. It would be preferable to only change the networking
stack at the intermediate node, although changing it at the mobile
devices is certainly an option and perhaps a necessity.

We envision mobile devices that can use the wireless medium very
efficiently, but overcome some of its traditional constraints.; That
is, full mobility implies that the devices have the flexibility and
agility to use whichever happens to be the best network connection

available at any given point in time or space.; Accordingly, devices
could switch from a wired Office LAN and hand over their ongoing
connections to continue on, say, a wireless WAN. This type of agility
also requires Mobile IP [RFC2002].

1.2 Assumptions about the Radio Link

The system architecture described above assumes at most one wireless
link (perhaps comprising more than one wireless hop).; However, this
is not enough to characterize a wireless link.; Additional
considerations are:

-; What are the error characteristics of the wireless medium?; The
link may present a higher BER than a wireline network due to
burst errors and disconnections. The techniques below usually

do not address all the types of errors. Accordingly, a complete
solution should combine the best of all the proposals.
Nevertheless, in this document we are more concerned with (and
give preference to solving) the most typical case: (1) higher
BER due to random errors (which implies longer and more
variable delays due to link-layer error corrections and
retransmissions) rather than (2) an interruption in service due
to a handoff or a disconnection.; The latter are also important
and we do include relevant proposals in this survey.

-; Is the wireless service datagram oriented, or is it a virtual
circuit?; Currently, switched virtual circuits are more common,
but packet networks are starting to appear, for example,
Metricom"s Starmode [CB96], CDPD [CDPD] and General Packet
Radio Service (GPRS) [GPRS],[BW97] in GSM.

-; What kind of reliability does the link provide? Wireless
services typically retransmit a packet (frame) until it has
been acknowledged by the target. They may allow the user to
turn off this behavior. For example, GSM allows RLP [RLP]
(Radio Link Protocol); to be turned off.; Metricom has a
similar "lightweight" mode. In GSM RLP, a frame is
retransmitted until the maximum number of retransmissions
(protocol parameter) is reached. What happens when this limit
is reached is determined by the telecom operator:; the physical
link connection is either disconnected or a link reset is
enforced where the sequence numbers are resynchronized and the
transmit and receive buffers are flushed resulting in lost
data. Some wireless services, like CDMA IS95-RLP [CDMA,

Karn93], limit the latency on the wireless link by
retransmitting a frame only a couple of times. This decreases
the residual frame error rate significantly, but does not
provide fully reliable link service.

-; Does the mobile device transmit and receive at the same time?
Doing so increases the cost of the electronics on the mobile
device. Typically, this is not the case. We assume in this
document that mobile devices do not transmit and receive
simultaneously.

-; Does the mobile device directly address more than one peer on
the wireless link? Packets to each different peer may traverse
spatially distinct wireless paths. Accordingly, the path to
each peer may exhibit very different characteristics.; Quite
commonly, the mobile device addresses only one peer (the
intermediate node) at any given point in time.; When this is
not the case, techniques such as Channel-State Dependent Packet
Scheduling come into play (see the section "Packet Scheduling"
below).

2 Should it be IP or Not?

The first decision is whether to use IP as the underlying network
protocol or not. In particular, some data protocols evolved from
wireless telephony are not always -- though at times they may be --
layered on top of IP [MOWGLI, WAP]. These proposals are based on the
concept of proxies that provide adaptation services between the
wireless and wireline segments.

This is a reasonable model for mobile devices that always communicate
through the proxy. However, we expect many wireless mobile devices to
utilize wireline networks whenever they are available. This model
closely follows current laptop usage patterns: devices typically
utilize LANs, and only resort to dial-up access when "out of the
office."

For these devices, an architecture that assumes IP is the best
approach, because it will be required for communications that do not

traverse the intermediate node (for example, upon reconnection to a
W-LAN or a 10BaseT network at the office).

2.1 Underlying Network Error Characteristics

Using IP as the underlying network protocol requires a certain (low)
level of link robustness that is expected of wireless links.

IP, and the protocols that are carried in IP packets, are protected
end-to-end by checksums that are relatively weak [Stevens94,
Paxson97] (and, in some cases, optional). For much of the Internet,
these checksums are sufficient; in wireless environments, the error
characteristics of the raw wireless link are much less robust than
the rest of the end-to-end path.; Hence for paths that include

wireless links, exclusively relying on end-to-end mechanisms to
detect and correct transmission errors is undesirable. These should
be complemented by local link-level mechanisms. Otherwise, damaged IP
packets are propagated through the network only to be discarded at
the destination host. For example, intermediate routers are required
to check the IP header checksum, but not the UDP or TCP checksums.
Accordingly, when the payload of an IP packet is corrupted, this is
not detected until the packet arrives at its ultimate destination.

A better approach is to use link-layer mechanisms such as FEC,
retransmissions, and so on in order to improve the characteristics of
the wireless link and present a much more reliable service to IP.
This approach has been taken by CDPD, Ricochet and CDMA.

This approach is roughly analogous to the successful deployment of
Point-to-Point Protocol (PPP), with robust framing and 16-bit
checksumming, on wireline networks as a replacement for the Serial
Line Interface Protocol (SLIP), with only a single framing byte and
no checksumming.

[AGS98] recommends the use of FEC in satellite environments.

Notice that the link-layer could adapt its frame size to the
prevalent BER.; It would perform its own fragmentation and reassembly
so that IP could still enjoy a large enough MTU size [LS98].

A common concern for using IP as a transport is the header overhead
it implies. Typically, the underlying link-layer appears as PPP
[RFC1661] to the IP layer above. This allows for header compression
schemes [IPHC, IPHC-RTP, IPHC-PPP] which greatly alleviate the
problem.

2.2 Non-IP Alternatives

A number of non-IP alternatives aimed at wireless environments have

been proposed. One representative proposal is discussed here.

2.2.1 WAP

The Wireless Application Protocol (WAP) specifies an application
framework and network protocols for wireless devices such as mobile
telephones, pagers, and PDAs [WAP]. The architecture requires a proxy
between the mobile device and the server. The WAP protocol stack is
layered over a datagram transport service.; Such a service is
provided by most wireless networks; for example, IS-136, GSM
SMS/USSD, and UDP in IP networks like CDPD and GSM GPRS. The core of

the WAP protocols is a binary HTTP/1.1 protocol with additional
features such as header caching between requests and a shared state
between client and server.

2.2.2 Deploying Non-IP Alternatives

IP is such a fundamental element of the Internet that non-IP
alternatives face substantial obstacles to deployment, because they
do not exploit the IP infrastructure. Any non-IP alternative that is
used to provide gatewayed access to the Internet must map between IP
addresses and non-IP addresses, must terminate IP-level security at a
gateway, and cannot use IP-oriented discovery protocols (Dynamic Host
Configuration Protocol, Domain Name Services, Lightweight Directory
Access Protocol, Service Location Protocol, etc.) without translation
at a gateway.

A further complexity occurs when a device supports both wireless and
wireline operation. If the device uses IP for wireless operation,
uninterrupted operation when the device is connected to a wireline
network is possible (using Mobile IP). If a non-IP alternative is
used, this switchover is more difficult to accomplish.

Non-IP alternatives face the burden of proof that IP is so ill-suited
to a wireless environment that it is not a viable technology.

2.3 IP-based Considerations

Given its worldwide deployment, IP is an obvious choice for the
underlying network technology. Optimizations implemented at this
level benefit traditional Internet application protocols as well as
new ones layered on top of IP or UDP.

2.3.1 Choosing the MTU [Stevens94, RFC1144]

In slow networks, the time required to transmit the largest possible
packet may be considerable.; Interactive response time should not
exceed the well-known human factors limit of 100 to 200 ms. This
should be considered the maximum time budget to (1) send a packet and

(2) oBTain a response. In most networking stack implementations, (1)
is highly dependent on the maximum transmission unit (MTU). In the
worst case, a small packet from an interactive application may have
to wait for a large packet from a bulk transfer application before
being sent. Hence, a good rule of thumb is to choose an MTU such that
its transmission time is less than (or not much larger than) 200 ms.

Of course, compression and type-of-service queuing (whereby
interactive data packets are given a higher priority) may alleviate
this problem. In particular, the latter may reduce the average wait
time to about half the MTU"s transmission time.

2.3.2 Path MTU Discovery [RFC1191]

Path MTU discovery benefits any protocol built on top of IP. It
allows a sender to determine what the maximum end-to-end transmission
unit is to a given destination. Without Path MTU discovery, the
default IPv4 MTU size is 576. The benefits of using a larger MTU are:

-; Smaller ratio of header overhead to data

-; Allows TCP to grow its congestion window faster, since it
increases in units of segments.

Of course, for a given BER, a larger MTU has a correspondingly larger
probability of error within any given segment. The BER may be reduced
using lower level techniques like FEC and link-layer retransmissions.
The issue is that now delays may become a problem due to the
additional retransmissions, and the fact that packet transmission
time increases with a larger MTU.

Recommendation: Path MTU discovery is recommended. [AGS98] already
recommends its use in satellite environments.

2.3.3 Non-TCP Proposals

Other proposals assume an underlying IP datagram service, and
implement an optimized transport either directly on top of IP
[NETBLT] or on top of UDP [MNCP]. Not relying on TCP is a bold move,
given the wealth of experience and research related to it.; It could
be argued that the Internet has not collapsed because its main
protocol, TCP, is very careful in how it uses the network, and
generally treats it as a black box assuming all packet losses are due
to congestion and prudently backing off. This avoids further
congestion.

However, in the wireless medium, packet losses may also be due to

corruption due to high BER, fading, and so on. Here, the right
approach is to try harder, instead of backing off. Alternative
transport protocols are:

-; NETBLT [NETBLT, RFC1986, RFC1030]

-; MNCP [MNCP]

-; ESRO [RFC2188]

-; RDP [RFC908, RFC1151]

-; VMTP [VMTP]

3 The Case for TCP

This is one of the most hotly debated issues in the wireless arena.
Here are some arguments against it:

-; It is generally recognized that TCP does not perform well in
the presence of significant levels of non-congestion loss.; TCP
detractors argue that the wireless medium is one such case, and
that it is hard enough to fix TCP. They argue that it is easier
to start from scratch.

-; TCP has too much header overhead.

-; By the time the mechanisms are in place to fix it, TCP is very
heavy, and ill-suited for use by lightweight, portable devices.

and here are some in support of TCP:

-; It is preferable to continue using the same protocol that the
rest of the Internet uses for compatibility reasons. Any
extensions specific to the wireless link may be negotiated.

-; Legacy mechanisms may be reused (for example three-way
handshake).

-; Link-layer FEC and ARQ can reduce the BER such that any losses
TCP does see are, in fact, caused by congestion (or a sustained
interruption of link connectivity). Modern W-WAN technologies
do this (CDPD, US-TDMA, CDMA, GSM), thus improving TCP
throughput.

-; Handoffs among different technologies are made possible by
Mobile IP [RFC2002], but only if the same protocols, namely
TCP/IP, are used throughout.

-; Given TCP"s wealth of research and experience, alternative

protocols are relatively immature, and the full implications of
their widespread deployment not clearly understood.

Overall, we feel that the performance of TCP over long-thin networks
can be improved significantly. Mechanisms to do so are discussed in
the next sections.

4 Candidate Optimizations

There is a large volume of work on the subject of optimizing TCP for
operation over wireless media. Even though satellite networks
generally fall in the LFN regime, our current LTN focus has much to
benefit from it.; For example, the work of the TCP-over-Satellite
working group of the IETF has been extremely helpful in preparing
this section [AGS98, ADGGHOSSTT98].

4.1 TCP: Current Mechanisms

A TCP sender adapts its use of bandwidth based on feedback from the
receiver. The high latency characteristic of LTNs implies that TCP"s
adaptation is correspondingly slower than on networks with shorter
delays.; Similarly, delayed ACKs exacerbate the perceived latency on
the link. Given that TCP grows its congestion window in units of
segments, small MTUs may slow adaptation even further.

4.1.1 Slow Start and Congestion Avoidance

Slow Start and Congestion Avoidance [RFC2581] are essential the
Internet"s stability.; However there are two reasons why the wireless
medium adversely affects them:

-; Whenever TCP"s retransmission timer expires, the sender assumes
that the network is congested and invokes slow start. This is
why it is important to minimize the losses caused by
corruption, leaving only those caused by congestion (as
expected by TCP).

-; The sender increases its window based on the number of ACKs
received. Their rate of arrival, of course, is dependent on the
RTT (round-trip-time) between sender and receiver, which
implies long ramp-up times in high latency links like LTNs. The
dependency lasts until the pipe is filled.

-; During slow start, the sender increases its window in units of
segments. This is why it is important to use an appropriately

large MTU which, in turn, requires requires link layers with
low loss.

4.1.2 Fast Retransmit and Fast Recovery

When a TCP sender receives several duplicate ACKs, fast retransmit
[RFC2581] allows it to infer that a segment was lost.; The sender
retransmits what it considers to be this lost segment without waiting
for the full timeout, thus saving time.

After a fast retransmit, a sender invokes the fast recovery [RFC2581]
algorithm. Fast recovery allows the sender to transmit at half its
previous rate (regulating the growth of its window based on
congestion avoidance), rather than having to begin a slow start. This
also saves time.

In general, TCP can increase its window beyond the delay-bandwidth
product. However, in LTN links the congestion window may remain
rather small, less than four segments, for long periods of time due
to any of the following reasons:

1. Typical "file size" to be transferred over a connection is
relatively small (Web requests, Web document objects, email
messages, files, etc.) In particular, users of LTNs are not
very willing to carry out large transfers as the response time
is so long.

2. If the link has high BER, the congestion window tends to stay
small

3. When an LTN is combined with a highly congested wireline
Internet path, congestion losses on the Internet have the same
effect as 2.

4. Commonly, ISPs/operators configure only a small number of
buffers (even as few as for 3 packets) per user in their dial-
up routers

5. Often small socket buffers are recommended with LTNs in order
to prevent the RTO from inflating and to diminish the amount of
packets with competing traffic.

A small window effectively prevents the sender from taking advantage
of Fast Retransmits. Moreover, efficient recovery from multiple
losses within a single window requires adoption of new proposals
(NewReno [RFC2582]). In addition, on slow paths with no packet

reordering waiting for three duplicate ACKs to arrive postpones
retransmission unnecessarily.

Recommendation: Implement Fast Retransmit and Fast Recovery at this
time. This is a widely-implemented optimization and is currently at
Proposed Standard level. [AGS98] recommends implementation of Fast
Retransmit/Fast Recovery in satellite environments.; NewReno
[RFC2582] apparently does help a sender better handle partial ACKs
and multiple losses in a single window, but at this point is not
recommended due to its experimental nature.; Instead, SACK [RFC2018]
is the preferred mechanism.

4.2 Connection Setup with T/TCP [RFC1397, RFC1644]

TCP engages in a "three-way handshake" whenever a new connection is
set up.; Data transfer is only possible after this phase has
completed successfully.; T/TCP allows data to be exchanged in
parallel with the connection set up, saving valuable time for short
transactions on long-latency networks.

Recommendation: T/TCP is not recommended, for these reasons:

-; It is an Experimental RFC.

-; It is not widely deployed, and it has to be deployed at both ends
of a connection.

-; Security concerns have been raised that T/TCP is more vulnerable
to address-spoofing attacks than TCP itself.

-; At least some of the benefits of T/TCP (eliminating three-way
handshake on subsequent query-response transactions, for instance)
are also available with persistent connections on HTTP/1.1, which
is more widely deployed.

[ADGGHOSSTT98] does not have a recommendation on T/TCP in satellite
environments.

4.3 Slow Start Proposals

Because slow start dominates the network response seen by interactive
users at the beginning of a TCP connection, a number of proposals
have been made to modify or eliminate slow start in long latency
environments.

Stability of the Internet is paramount, so these proposals must
demonstrate that they will not adversely affect Internet congestion
levels in significant ways.

4.3.1 Larger Initial Window

Traditional slow start, with an initial window of one segment, is a
time-consuming bandwidth adaptation procedure over LTNs. Studies on
an initial window larger than one segment [RFC2414, AHO98] resulted
in the TCP standard supporting a maximum value of 2 [RFC2581]. Higher

values are still experimental in nature.

In simulations with an increased initial window of three packets
[RFC2415], this proposal does not contribute significantly to packet
drop rates, and it has the added benefit of improving initial
response times when the peer device delays acknowledgements during
slow start (see next proposal).

[RFC2416] addresses situations where the initial window exceeds the
number of buffers available to TCP and indicates that this situation
is no different from the case where the congestion window grows
beyond the number of buffers available.

[RFC2581] now allows an initial congestion window of two segments. A
larger initial window, perhaps as many as four segments, might be
allowed in the future in environments where this significantly
improves performance (LFNs and LTNs).

Recommendation: Implement this on devices now. The research on this
optimization indicates that 3 segments is a safe initial setting, and
is centering on choosing between 2, 3, and 4. For now, use 2
(following RFC2581), which at least allows clients running query-
response applications to get an initial ACK from unmodified servers
without waiting for a typical delayed ACK timeout of 200
milliseconds, and saves two round-trips. An initial window of 3
[RFC2415] looks promising and may be adopted in the future pending
further research and experience.

4.3.2 Growing the Window during Slow Start

The sender increases its window based on the flow of ACKs coming back
from the receiver. Particularly during slow start, this flow is very
important.; A couple of the proposals that have been studied are (1)
ACK counting and (2) ACK-every-segment.

4.3.2.1 ACK Counting

The main idea behind ACK counting is:

-; Make each ACK count to its fullest by growing the window based
on the data being acknowledged (byte counting) instead of the
number of ACKs (ACK counting). This has been shown to cause
bursts which lead to congestion. [Allman98] shows that Limited
Byte Counting (LBC), in which the window growth is limited to 2
segments, does not lead to as much burstiness, and offers some
performance gains.

Recommendation: Unlimited byte counting is not recommended.; Van

Jacobson cautions against byte counting [TCPSATMIN] because it leads
to burstiness, and recommends ACK spacing [ACKSPACING] instead.

ACK spacing requires ACKs to consistently pass through a single ACK-
spacing router.; This requirement works well for W-WAN environments
if the ACK-spacing router is also the intermediate node.

Limited byte counting warrants further investigation before we can
recommend this proposal, but it shows promise.

4.3.2.2 ACK-every-segment

The main idea behind ACK-every-segment is:

-; Keep a constant stream of ACKs coming back by turning off
delayed ACKs [RFC1122] during slow start. ACK-every-segment
must be limited to slow start, in order to avoid penalizing
asymmetric-bandwidth configurations. For instance, a low
bandwidth link carrying acknowledgements back to the sender,
hinders the growth of the congestion window, even if the link
toward the client has a greater bandwidth [BPK99].

Even though simulations confirm its promise (it allows receivers to
receive the second segment from unmodified senders without waiting
for a typical delayed ACK timeout of 200 milliseconds), for this
technique to be practical the receiver must acknowledge every segment
only when the sender is in slow start.; Continuing to do so when the
sender is in congestion avoidance may have adverse effects on the
mobile device"s battery consumption and on traffic in the network.

This violates a SHOULD in [RFC2581]:; delayed acknowledgements SHOULD
be used by a TCP receiver.

"Disabling Delayed ACKs During Slow Start" is technically
unimplementable, as the receiver has no way of knowing when the
sender crosses ssthresh (the "slow start threshold") and begins using
the congestion avoidance algorithm.; If receivers follow
recommendations for increased initial windows, disabling delayed ACKs
during an increased initial window would open the TCP window more
rapidly without doubling ACK traffic in general.; However, this
scheme might double ACK traffic if most connections remain in slow-
start.

Recommendation: ACK only the first segment on a new connection with
no delay.

4.3.3 Terminating Slow Start

New mechanisms [ADGGHOSSTT98] are being proposed to improve TCP"s

adaptive properties such that the available bandwidth is better
utilized while reducing the possibility of congesting the network.
This results in the closing of the congestion window to 1 segment
(which precludes fast retransmit), and the subsequent slow start
phase.

Theoretically, an optimum value for slow-start threshold (ssthresh)
allows connection bandwidth utilization to ramp up as aggressively as
possible without "overshoot" (using so much bandwidth that packets
are lost and congestion avoidance procedures are invoked).

Recommendation: Estimating the slow start threshold is not
recommended.; Although this would be helpful if we knew how to do it,
rough consensus on the tcp-impl and tcp-sat mailing lists is that in
non-trivial operational networks there is no reliable method to probe
during TCP startup and estimate the bandwidth available.

4.3.4 Generating ACKs during Slow Start

Mitigations that inject additional ACKs (whether "ACK-first-segment"
or "ACK-every-segment-during-slow-start") beyond what today"s
conformant TCPs inject are only applicable during the slow-start
phases of a connection. After an initial exchange, the connection
usually completes slow-start, so TCPs only inject additional ACKs
when (1) the connection is closed, and a new connection is opened, or
(2) the TCPs handle idle connection restart correctly by performing
slow start.

Item (1) is typical when using HTTP/1.0, in which each request-
response transaction requires a new connection.; Persistent
connections in HTTP/1.1 help in maintaining a connection in
congestion avoidance instead of constantly reverting to slow-start.
Because of this, these optimizations which are only enabled during
slow-start do not get as much of a chance to act. Item (2), of
course, is independent of HTTP version.

4.4 ACK Spacing

During slow start, the sender responds to the incoming ACK stream by
transmitting N+1 segments for each ACK, where N is the number of new
segments acknowledged by the incoming ACK.; This results in data
being sent at twice the speed at which it can be processed by the
network.; Accordingly, queues will form, and due to insufficient
buffering at the bottleneck router, packets may get dropped before
the link"s capacity is full.

Spacing out the ACKs effectively controls the rate at which the
sender will transmit into the network, and may result in little or no

queueing at the bottleneck router [ACKSPACING].; Furthermore, ack
spacing reduces the size of the bursts.

Recommendation: No recommendation at this time. Continue monitoring
research in this area.

4.5 Delayed Duplicate Acknowlegements

As was mentioned above, link-layer retransmissions may decrease the
BER enough that congestion accounts for most of packet losses; still,
nothing can be done about interruptions due to handoffs, moving
beyond wireless coverage, etc. In this scenario, it is imperative to
prevent interaction between link-layer retransmission and TCP
retransmission as these layers duplicate each other"s efforts. In
such an environment it may make sense to delay TCP"s efforts so as to
give the link-layer a chance to recover. With this in mind, the
Delayed Dupacks [MV97, Vaidya99] scheme selectively delays duplicate
acknowledgements at the receiver.; It is preferable to allow a local
mechanism to resolve a local problem, instead of invoking TCP"s end-
to-end mechanism and incurring the associated costs, both in terms of
wasted bandwidth and in terms of its effect on TCP"s window behavior.

The Delayed Dupacks scheme can be used despite IP encryption since
the intermediate node does not need to examine the TCP headers.

Currently, it is not well understood how long the receiver should
delay the duplicate acknowledgments. In particular, the impact of
wireless medium access control (MAC) protocol on the choice of delay
parameter needs to be studied. The MAC protocol may affect the
ability to choose the appropriate delay (either statically or
dynamically). In general, significant variabilities in link-level
retransmission times can have an adverse impact on the performance of
the Delayed Dupacks scheme. Furthermore, as discussed later in
section 4.10.3, Delayed Dupacks and some other schemes (such as Snoop
[SNOOP]) are only beneficial in certain types of network links.

Recommendation: Delaying duplicate acknowledgements may be useful in
specific network topologies, but a general recommendation requires
further research and experience.

4.6 Selective Acknowledgements [RFC2018]

SACK may not be useful in many LTNs, according to Section 1.1 of
[TCPHP].; In particular, SACK is more useful in the LFN regime,
especially if large windows are being used, because there is a

considerable probability of multiple segment losses per window. In

the LTN regime, TCP windows are much smaller, and burst errors must
be much longer in duration in order to damage multiple segments.

Accordingly, the complexity of SACK may not be justifiable, unless
there is a high probability of burst errors and congestion on the
wireless link. A desire for compatibility with TCP recommendations
for non-LTN environments may dictate LTN support for SACK anyway.

[AGS98] recommends use of SACK with Large TCP Windows in satellite
environments, and notes that this implies support for PAWS
(Protection Against Wrapped Sequence space) and RTTM (Round Trip Time
Measurement) as well.

Berkeley"s SNOOP protocol research [SNOOP] indicates that SACK does
improve throughput for SNOOP when multiple segments are lost per
window [BPSK96]. SACK allows SNOOP to recover from multi-segment
losses in one round-trip. In this case, the mobile device needs to
implement some form of selective acknowledgements.; If SACK is not
used, TCP may enter congestion avoidance as the time needed to
retransmit the lost segments may be greater than the retransmission
timer.

Recommendation: Implement SACK now for compatibility with other TCPs
and improved performance with SNOOP.

4.7 Detecting Corruption Loss

4.7.1 Without Explicit Notification

In the absence of explicit notification from the network, some
researchers have suggested statistical methods for congestion
avoidance [Jain89, WC91, VEGAS]. A natural extension of these
heuristics would enable a sender to distinguish between losses caused
by congestion and other causes.; The research results on the
reliability of sender-based heuristics is unfavorable [BV97, BV98].
[BV98a] reports better results in constrained environments using
packet inter-arrival times measured at the receiver, but highly-
variable delay - of the type encountered in wireless environments
during intercell handoff - confounds these heuristics.

Recommendation: No recommendation at this time - continue to monitor
research results.

4.7.2 With Explicit Notifications

With explicit notification from the network it is possible to
determine when a loss is due to congestion. Several proposals along
these lines include:

-; Explicit Loss Notification (ELN) [BPSK96]

-; Explicit Bad State Notification (EBSN) [BBKVP96]

-; Explicit Loss Notification to the Receiver (ELNR), and Explicit

Delayed Dupack Activation Notification (EDDAN) (notifications
to mobile receiver) [MV97]

-; Explicit Congestion Notification (ECN) [ECN]

Of these proposals, Explicit Congestion Notification (ECN) seems
closest to deployment on the Internet, and will provide some benefit
for TCP connections on long thin networks (as well as for all other
TCP connections).

Recommendation: No recommendation at this time. Schemes like ELNR and
EDDAN [MV97], in which; the only systems that need to be modified are
the intermediate node and the mobile device, are slated for adoption
pending further research.; However, this solution has some
limitations. Since the intermediate node must have access to the TCP
headers, the IP payload must not be encrypted.

ECN uses the TOS byte in the IP header to carry congestion
information (ECN-capable and Congestion-encountered).; This byte is
not encrypted in IPSEC, so ECN can be used on TCP connections that
are encrypted using IPSEC.

Recommendation: Implement ECN. In spite of this, mechanisms for
explicit corruption notification are still relevant and should be
tracked.

Note: ECN provides useful information to avoid deteriorating further
a bad situation, but has some limitations for wireless applications.
Absence of packets marked with ECN should not be interpreted by ECN-
capable TCP connections as a green light for aggressive
retransmissions. On the contrary, during periods of extreme network
congestion routers may drop packets marked with explicit notification
because their buffers are exhausted - exactly the wrong time for a
host to begin retransmitting aggressively.

4.8 Active Queue Management

As has been pointed out above, TCP responds to congestion by closing
down the window and invoking slow start. Long-delay networks take a
particularly long time to recover from this condition. Accordingly,
it is imperative to avoid congestion in LTNs. To remedy this, active
queue management techniques have been proposed as enhancements to
routers throughout the Internet [RED].; The primary motivation for
deployment of these mechanisms is to prevent "congestion collapse" (a
severe degradation in service) by controlling the average queue size
at the routers. As the average queue length grows, Random Early
Detection [RED] increases the possibility of dropping packets.

The benefits are:

-; Reduce packet drops in routers. By dropping a few packets

before severe congestion sets in, RED avoids dropping bursts of
packets. In other Words, the objective is to drop m packets
early to prevent n drops later on, where m is less than n.

-; Provide lower delays. This follows from the smaller queue
sizes, and is particularly important for interactive
applications, for which the inherent delays of wireless links
already push the user experience to the limits of the non-
acceptable.

-; Avoid lock-outs. Lack of resources in a router (and the
resultant packet drops) may, in effect, obliterate throughput
on certain connections.; Because of active queue management, it
is more probable for an incoming packet to find available
buffer space at the router.

Active Queue Management has two components: (1) routers detect
congestion before exhausting their resources, and (2) they provide
some form of congestion indication. Dropping packets via RED is only
one example of the latter.; Another way to indicate congestion is to
use ECN [ECN] as discussed above under "Detecting Corruption Loss:
With Explicit Notifications."

Recommendation: RED is currently being deployed in the Internet, and
LTNs should follow suit. ECN deployment should complement RED"s.

4.9 Scheduling Algorithms

Active queue management helps control the length of the queues.
Additionally, a general solution requires replacing FIFO with other
scheduling algorithms that improve:

1. Fairness (by policing how different packet streams utilize the
available bandwidth), and

2. Throughput (by improving the transmitter"s radio channel
utilization).

For example, fairness is necessary for interactive applications (like
telnet or web browsing) to coexist with bulk transfer sessions.
Proposals here include:

- Fair Queueing (FQ) [Demers90]

- Class-based Queueing (CBQ) [Floyd95]

Even if they are only implemented over the wireless link portion of

the communication path, these proposals are attractive in wireless
LTN environments, because new connections for interactive
applications can have difficulty starting when a bulk TCP transfer
has already stabilized using all available bandwidth.

In our base architecture described above, the mobile device typically
communicates directly with only one wireless peer at a given time:
the intermediate node. In some W-WANs, it is possible to directly
address other mobiles within the same cell.; Direct communication
with each such wireless peer may traverse a spatially distinct path,
each of which may exhibit statistically independent radio link
characteristics. Channel State Dependent Packet Scheduling (CSDP)
[BBKT96] tracks the state of the various radio links (as defined by
the target devices), and gives preferential treatment to packets
destined for radio links in a "good" state. This avoids attempting to
transmit to (and expect acknowledgements from) a peer on a "bad"
radio link, thus improving throughput.

A further refinement of this idea suggests that both fairness and
throughput can be improved by combining a wireless-enhanced CBQ with
CSDP [FSS98].

Recommendation: No recommendation at this time, pending further
study.

4.10 Split TCP and Performance-Enhancing Proxies (PEPs)

Given the dramatic differences between the wired and the wireless
links, a very common approach is to provide some impedance matching
where the two different technologies meet: at the intermediate node.

The idea is to replace an end-to-end TCP connection with two clearly
distinct connections: one across the wireless link, the other across
its wireline counterpart. Each of the two resulting TCP sessions
operates under very different networking characteristics, and may
adopt the policies best suited to its particular medium.; For
example, in a specific LTN topology it may be desirable to modify TCP
Fast Retransmit to resend after the first duplicate ack and Fast
Recovery to not shrink the congestion window if the LTN link has an
extremely long RTT, is known to not reorder packets, and is not
subject to congestion. Moreover, on a long-delay link or on a link
with a relatively high bandwidth-delay product it may be desirable to
"slow-start" with a relatively large initial window, even larger than
four segments.; While these kinds of TCP modifications can be

negotiated to be employed over the LTN link, they would not be
deployed end-to-end over the global Internet. In LTN topologies where
the underlying link characteristics are known, a various similar
types of performance enhancements can be employed without endangering
operations over the global Internet.

In some proposals, in addition to a PEP mechanism at the intermediate
node, custom protocols are used on the wireless link (for example,
[WAP], [YB94] or [MOWGLI]).

Even if the gains from using non-TCP protocols are moderate or
better, the wealth of research on optimizing TCP for wireless, and
compatibility with the Internet are compelling reasons to adopt TCP
on the wireless link (enhanced as suggested in section 5 below).

4.10.1 Split TCP Approaches

Split-TCP proposals include schemes like I-TCP [ITCP] and MTCP [YB94]
which achieve performance improvements by abandoning end-to-end
semantics.

The Mowgli architecture [MOWGLI] proposes a split approach with
support for various enhancements at all the protocol layers, not only
at the transport layer. Mowgli provides an option to replace the
TCP/IP core protocols on the LTN link with a custom protocol that is
tuned for LTN links [KRLKA97].; In addition, the protocol provides
various features that are useful with LTNs. For example, it provides
priority-based multiplexing of concurrent connections together with
shared flow control, thus offering link capacity to interactive
applications in a timely manner even if there are bandwidth-intensive
background transfers.; Also with this option, Mowgli preserves the
socket semantics on the mobile device so that legacy applications can
be run unmodified.

Employing split TCP approaches have several benefits as well as
drawbacks. Benefits related to split TCP approaches include the
following:

-; Splitting the end-to-end TCP connection into two parts is a
straightforward way to shield the problems of the wireless link
from the wireline Internet path, and vice versa. Thus, a split TCP
approach enables applying local solutions to the local problems on
the wireless link.; For example, it automatically solves the
problem of distinguishing congestion related packet losses on the
wireline Internet and packet losses due to transmission error on

;;;the wireless link as these occur on separate TCP connections.
Even if both segments experience congestion, it may be of a
different nature and may be treated as such.; Moreover, temporary
disconnections of the wireless link can be effectively shielded
from the wireline Internet.

-; When one of the TCP connections crosses only a single hop wireless
link or a very limited number of hops, some or all link
characteristics for the wireless TCP path are known. For example,
with a particular link we may know that the link provides reliable
delivery of packets, packets are not delivered out of order, or
the link is not subject to congestion. Having this information for
the TCP path one could expect that defining the TCP mitigations to
be employed becomes a significantly easier task. In addition,
several mitigations that cannot be employed safely over the global
Internet, can be successfully employed over the wireless link.

-; Splitting one TCP connection into two separate ones allows much
earlier deployment of various recent proposals to improve TCP
performance over wireless links; only the TCP implementations of
the mobile device and intermediate node need to be modified, thus
allowing the vast number of Internet hosts to continue running the
legacy TCP implementations unmodified. Any mitigations that would
require modification of TCP in these wireline hosts may take far
too long to become widely deployed.

-; Allows exploitation of various application level enhancements
which may give significant performance gains (see section 4.10.2).

Drawbacks related to split TCP approaches include the following:

-; One of the main criticisms against the split TCP approaches is
that it breaks TCP end-to-end semantics. This has various
drawbacks some of which are more severe than others. The most
detrimental drawback is probably that splitting the TCP connection
disables end-to-end usage of IP layer security mechanisms,
precluding the application of IPSec to achieve end-to-end

security. Still, IPSec could be employed separately in each of the

;;;two parts, thus requiring the intermediate node to become a party
to the security association between the mobile device and the
remote host. This, however, is an undesirable or unacceptable
alternative in most cases. Other security mechanisms above the
transport layer, like TLS [RFC2246] or SOCKS [RFC1928], should be
employed for end-to-end security.

-; Another drawback of breaking end-to-end semantics is that crashes
of the intermediate node become unrecoverable resulting in
termination of the TCP connections. Whether this should be
considered a severe problem depends on the expected frequency of
such crashes.

-; In many occasions claims have been stated that if TCP end-to-end
semantics is broken, applications relying on TCP to provide
reliable data delivery become more vulnerable. This, however, is
an overstatement as a well-designed application should never fully
rely on TCP in achieving end-to-end reliability at the application
level. First, current APIs to TCP, such as the Berkeley socket
interface, do not allow applications to know when an TCP
acknowledgement for previously sent user data arrives at TCP
sender.; Second, even if the application is informed of the TCP
acknowledgements, the sending application cannot know whether the
receiving application has received the data: it only knows that
the data reached the TCP receive buffer at the receiving end.
Finally, in order to achieve end-to-end reliability at the
application level an application level acknowledgement is required
to confirm that the receiver has taken the appropriate actions on
the data it received.

-; When a mobile device moves, it is subject to handovers by the
serving base station. If the base station acts as the intermediate
node for the split TCP connection, the state of both TCP endpoints
on the previous intermediate node must be transferred to the new
intermediate node to ensure continued operation over the split TCP
connection. This requires extra work and causes overhead. However,

;;;in most of the W-WAN wireless networks, unlike in W-LANs, the W-
WAN base station does not provide the mobile device with the
connection point to the wireline Internet (such base stations may
not even have an IP stack).; Instead, the W-WAN network takes care
of the mobility and retains the connection point to the wireline
Internet unchanged while the mobile device moves.; Thus, TCP state
handover is not required in most W-WANs.

-; The packets traversing through all the protocol layers up to
transport layer and again down to the link layer result in extra
overhead at the intermediate node. In case of LTNs with low

bandwidth, this extra overhead does not cause serious additional
performance problems unlike with W-LANs that typically have much
higher bandwidth.

-; Split TCP proposals are not applicable to networks with asymmetric
routing. Deploying a split TCP approach requires that traffic to
and from the mobile device be routed through the intermediate
node. With some networks, this cannot be accomplished, or it
requires that the intermediate node is located several hops away
from the wireless network edge which in turn is unpractical in
many cases and may result in non-optimal routing.

-; Split TCP, as the name implies, does not address problems related
to UDP.

It should noted that using split TCP does not necessarily exclude
simultaneous usage of IP for end-to-end connectivity.; Correct usage
of split TCP should be managed per application or per connection and
should be under the end-user control so that the user can decide
whether a particular TCP connection or application makes use of split
TCP or whether it operates end-to-end directly over IP.

Recommendation: Split TCP proposals that alter TCP semantics are not
recommended. Deploying custom protocols on the wireless link, such as
MOWGLI proposes is not recommended, because this note gives
preference to (1) improving TCP instead of designing a custom
protocol and (2) allowing end-to-end sessions at all times.

4.10.2 Application Level Proxies

Nowadays, application level proxies are widely used in the Internet.

Such proxies include Web proxy caches, relay MTAs (Mail Transfer
Agents), and secure transport proxies (e.g., SOCKS). In effect,
employing an application level proxy results in a "split TCP
connection" with the proxy as the intermediary.; Hence, some of the
problems present with wireless links, such as combining of a
congested wide-area Internet path with a wireless LTN link, are
automatically alleviated to some extent.

The application protocols often employ plenty of (unnecessary) round
trips, lots of headers and inefficient encoding. Even unnecessary
data may get delivered over the wireless link in regular application
protocol operation. In many cases a significant amount of this
overhead can be reduced by simply running an application level proxy
on the intermediate node.; With LTN links, significant additional
improvement can be achieved by introducing application level proxies
with application-specific enhancements. Such a proxy may employ an
enhanced version of the application protocol over the wireless link.

In an LTN environment enhancements at the application layer may
provide much more notable performance improvements than any transport
level enhancements.

The Mowgli system provides full support for adding application level
agent-proxy pairs between the client and the server, the agent on the
mobile device and the proxy on the intermediate node. Such a pair may
be either explicit or fully transparent to the applications, but it
is, at all times, under the end-user control. Good examples of
enhancements achieved with application-specific proxies include
Mowgli WWW [LAKLR95], [LHKR96] and WebExpress [HL96], [CTCSM97].

Recommendation: Usage of application level proxies is conditionally
recommended: an application must be proxy enabled and the decision of
employing a proxy for an application must be under the user control
at all times.

4.10.3 Snoop and its Derivatives

Berkeley"s SNOOP protocol [SNOOP] is a hybrid scheme mixing link-
layer reliability mechanisms with the split connection approach. It
is an improvement over split TCP approaches in that end-to-end
semantics are retained. SNOOP does two things:

1. Locally (on the wireless link) retransmit lost packets, instead
of allowing TCP to do so end-to-end.

2. Suppress the duplicate acks on their way from the receiver back

to the sender, thus avoiding fast retransmit and congestion
avoidance at the latter.

Thus, the Snoop protocol is designed to avoid unnecessary fast
retransmits by the TCP sender, when the wireless link layer
retransmits a packet locally. Consider a system that does not use the
Snoop agent. Consider a TCP sender S that sends packets to receiver R
via an intermediate node IN. Assume that the sender sends packet A,
B, C, D, E (in that order) which are forwarded by IN to the wireless
receiver R. Assume that the intermediate node then retransmits B
subsequently, because the first transmission of packet B is lost due
to errors on the wireless link. In this case, receiver R receives
packets A, C, D, E and B (in that order). Receipt of packets C, D and
E triggers duplicate acknowledgements. When the TCP sender receives
three duplicate acknowledgements, it triggers fast retransmit (which
results in a retransmission, as well as reduction of congestion
window).; The fast retransmit occurs despite the link level
retransmit on the wireless link, degrading throughput.

SNOOP [SNOOP] deals with this problem by dropping TCP dupacks
appropriately (at the intermediate node). The Delayed Dupacks (see
section 4.5) attempts to approximate Snoop without requiring
modifications at the intermediate node.; Such schemes are needed only
if the possibility of a fast retransmit due to wireless errors is
non-negligible. In particular, if the wireless link use