Network Working Group J. Border
Request for Comments: 3135 Hughes Network Systems
Category: Informational M. Kojo
University of Helsinki
J. Griner
NASA Glenn Research Center
G. Montenegro
Sun Microsystems, Inc.
Z. Shelby
University of Oulu
June 2001
Performance Enhancing Proxies Intended to Mitigate Link-Related
Degradations
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 (2001). All Rights Reserved.
Abstract
This document is a survey of Performance Enhancing Proxies (PEPs)
often employed to improve degraded TCP performance caused by
characteristics of specific link environments, for example, in
satellite, wireless WAN, and wireless LAN environments. Different
types of Performance Enhancing Proxies are described as well as the
mechanisms used to improve performance. Emphasis is put on proxies
operating with TCP. In addition, motivations for their development
and use are described along with some of the consequences of using
them, especially in the context of the Internet.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Types of Performance Enhancing Proxies . . . . . . . . . . . . 4
2.1 Layering . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 Transport Layer PEPs . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Application Layer PEPs . . . . . . . . . . . . . . . . . . . 5
2.2 Distribution . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Implementation Symmetry . . . . . . . . . . . . . . . . . . . 6
2.4 Split Connections . . . . . . . . . . . . . . . . . . . . . . 7
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2.5 Transparency . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. PEP Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1 TCP ACK Handling . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 TCP ACK Spacing . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2 Local TCP Acknowledgements . . . . . . . . . . . . . . . . . 9
3.1.3 Local TCP Retransmissions . . . . . . . . . . . . . . . . . 9
3.1.4 TCP ACK Filtering and Reconstruction . . . . . . . . . . . . 10
3.2 Tunneling . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 Compression . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4 Handling Periods of Link Disconnection with TCP . . . . . . . 11
3.5 Priority-based Multiplexing . . . . . . . . . . . . . . . . . 12
3.6 Protocol Booster Mechanisms . . . . . . . . . . . . . . . . . 13
4. Implications of Using PEPs . . . . . . . . . . . . . . . . . . 14
4.1 The End-to-end Argument . . . . . . . . . . . . . . . . . . . 14
4.1.1 Security . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.1.1 Security Implications . . . . . . . . . . . . . . . . . . 15
4.1.1.2 Security Implication Mitigations . . . . . . . . . . . . . 16
4.1.1.3 Security Research Related to PEPs . . . . . . . . . . . . 16
4.1.2 Fate Sharing . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1.3 End-to-end Reliability . . . . . . . . . . . . . . . . . . . 17
4.1.4 End-to-end Failure Diagnostics . . . . . . . . . . . . . . . 19
4.2 Asymmetric Routing . . . . . . . . . . . . . . . . . . . . . . 19
4.3 Mobile Hosts . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.4 Scalability . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.5 Other Implications of Using PEPs . . . . . . . . . . . . . . . 21
5. PEP Environment Examples . . . . . . . . . . . . . . . . . . . 21
5.1 VSAT Environments . . . . . . . . . . . . . . . . . . . . . . 21
5.1.1 VSAT Network Characteristics . . . . . . . . . . . . . . . . 22
5.1.2 VSAT Network PEP Implementations . . . . . . . . . . . . . . 23
5.1.3 VSAT Network PEP Motivation . . . . . . . . . . . . . . . . 24
5.2 W-WAN Environments . . . . . . . . . . . . . . . . . . . . . . 25
5.2.1 W-WAN Network Characteristics . . . . . . . . . . . . . . . 25
5.2.2 W-WAN PEP Implementations . . . . . . . . . . . . . . . . . 26
5.2.2.1 Mowgli System . . . . . . . . . . . . . . . . . . . . . . 26
5.2.2.2 Wireless Application Protocol (WAP) . . . . . . . . . . . 28
5.2.3 W-WAN PEP Motivation . . . . . . . . . . . . . . . . . . . . 29
5.3 W-LAN Environments . . . . . . . . . . . . . . . . . . . . . . 30
5.3.1 W-LAN Network Characteristics . . . . . . . . . . . . . . . 30
5.3.2 W-LAN PEP Implementations: Snoop . . . . . . . . . . . . . . 31
5.3.3 W-LAN PEP Motivation . . . . . . . . . . . . . . . . . . . . 33
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 34
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 34
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 39
Appendix A - PEP Terminology Summary . . . . . . . . . . . . . . . 41
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 45
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1. Introduction
The Transmission Control Protocol [RFC0793] (TCP) is used as the
transport layer protocol by many Internet and intranet applications.
However, in certain environments, TCP and other higher layer protocol
performance is limited by the link characteristics of the
environment.
This document is a survey of Performance Enhancing Proxy (PEP)
performance migitigation techniques. A PEP is used to improve the
performance of the Internet protocols on network paths where native
performance suffers due to characteristics of a link or subnetwork on
the path. This document is informational and does not make
recommendations about using PEPs or not using them. Distinct
standards track recommendations for the performance mitigation of TCP
over links with high error rates, links with low bandwidth, and so
on, have been developed or are in development by the Performance
Implications of Link Characteristics WG (PILC) [PILCWEB].
Link design choices may have a significant influence on the
performance and efficiency of the Internet. However, not all link
characteristics, for example, high latency, can be compensated for by
choices in the link layer design. And, the cost of compensating for
some link characteristics may be prohibitive for some technologies.
The techniques surveyed here are applied to existing link
technologies. When new link technologies are designed, they should
be designed so that these techniques are not required, if at all
possible.
This document does not advocate the use of PEPs in any general case.
On the contrary, we believe that the end-to-end principle in
designing Internet protocols should be retained as the prevailing
approach and PEPs should be used only in specific environments and
circumstances where end-to-end mechanisms providing similar
performance enhancements are not available. In any environment where
one might consider employing a PEP for improved performance, an end
user (or, in some cases, the responsible network administrator)
should be aware of the PEP and the choice of employing PEP
functionality should be under the control of the end user, especially
if employing the PEP would interfere with end-to-end usage of IP
layer security mechanisms or otherwise have undesirable implications
in some circumstances. This would allow the user to choose end-to-
end IP at all times but, of course, without the performance
enhancements that employing the PEP may yield.
This survey does not make recommendations, for or against, with
respect to using PEPs. Standards track recommendations have been or
are being developed within the IETF for individual link
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characteristics, e.g., links with high error rates, links with low
bandwidth, links with asymmetric bandwidth, etc., by the Performance
Implications of Link Characteristics WG (PILC) [PILCWEB].
The remainder of this document is organized as follows. Section 2
provides an overview of different kinds of PEP implementations.
Section 3 discusses some of the mechanisms which PEPs may employ in
order to improve performance. Section 4 discusses some of the
implications with respect to using PEPs, especially in the context of
the global Internet. Finally, Section 5 discusses some example
environments where PEPs are used: satellite very small aperture
terminal (VSAT) environments, mobile wireless WAN (W-WAN)
environments and wireless LAN (W-LAN) environments. A summary of PEP
terminology is included in an appendix (Appendix A).
2. Types of Performance Enhancing Proxies
There are many types of Performance Enhancing Proxies. Different
types of PEPs are used in different environments to overcome
different link characteristics which affect protocol performance.
Note that enhancing performance is not necessarily limited in scope
to throughput. Other performance related aspects, like usability of
a link, may also be addressed. For example, [M-TCP] addresses the
issue of keeping TCP connections alive during periods of
disconnection in wireless networks.
The following sections describe some of the key characteristics which
differentiate different types of PEPs.
2.1 Layering
In principle, a PEP implementation may function at any protocol layer
but typically it functions at one or two layers only. In this
document we focus on PEP implementations that function at the
transport layer or at the application layer as such PEPs are most
commonly used to enhance performance over links with problematic
characteristics. A PEP implementation may also operate below the
network layer, that is, at the link layer, but this document pays
only little attention to such PEPs as link layer mechanisms can be
and typically are implemented transparently to network and higher
layers, requiring no modifications to protocol operation above the
link layer. It should also be noted that some PEP implementations
operate across several protocol layers by exploiting the protocol
information and possibly modifying the protocol operation at more
than one layer. For such a PEP it may be difficult to define at
which layer(s) it exactly operates on.
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2.1.1 Transport Layer PEPs
Transport layer PEPs operate at the transport level. They may be
aware of the type of application being carried by the transport layer
but, at most, only use this information to influence their behavior
with respect to the transport protocol; they do not modify the
application protocol in any way, but let the application protocol
operate end-to-end. Most transport layer PEP implementations
interact with TCP. Such an implementation is called a TCP
Performance Enhancing Proxy (TCP PEP). For example, in an
environment where ACKs may bunch together causing undesirable data
segment bursts, a TCP PEP may be used to simply modify the ACK
spacing in order to improve performance. On the other hand, in an
environment with a large bandwidth*delay product, a TCP PEP may be
used to alter the behavior of the TCP connection by generating local
acknowledgments to TCP data segments in order to improve the
connection's throughput.
The term TCP spoofing is sometimes used synonymously for TCP PEP
functionality. However, the term TCP spoofing more accurately
describes the characteristic of intercepting a TCP connection in the
middle and terminating the connection as if the interceptor is the
intended destination. While this is a characteristic of many TCP PEP
implementations, it is not a characteristic of all TCP PEP
implementations.
2.1.2 Application Layer PEPs
Application layer PEPs operate above the transport layer. Today,
different kinds of application layer proxies are widely used in the
Internet. Such proxies include Web caches and relay Mail Transfer
Agents (MTA) and they typically try to improve performance or service
availability and reliability in general and in a way which is
applicable in any environment but they do not necessarily include any
optimizations that are specific to certain link characteristics.
Application layer PEPs, on the other hand, can be implemented to
improve application protocol as well as transport layer performance
with respect to a particular application being used with a particular
type of link. An application layer PEP may have the same
functionality as the corresponding regular proxy for the same
application (e.g., relay MTA or Web caching proxy) but extended with
link-specific optimizations of the application protocol operation.
Some application protocols employ extraneous round trips, overly
verbose headers and/or inefficient header encoding which may have a
significant impact on performance, in particular, with long delay and
slow links. This unnecessary overhead can be reduced, in general or
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for a particular type of link, by using an application layer PEP in
an intermediate node. Some examples of application layer PEPs which
have been shown to improve performance on slow wireless WAN links are
described in [LHKR96] and [CTC+97].
2.2 Distribution
A PEP implementation may be integrated, i.e., it comprises a single
PEP component implemented within a single node, or distributed, i.e.,
it comprises two or more PEP components, typically implemented in
multiple nodes. An integrated PEP implementation represents a single
point at which performance enhancement is applied. For example, a
single PEP component might be implemented to provide impedance
matching at the point where wired and wireless links meet.
A distributed PEP implementation is generally used to surround a
particular link for which performance enhancement is desired. For
example, a PEP implementation for a satellite connection may be
distributed between two PEPs located at each end of the satellite
link.
2.3 Implementation Symmetry
A PEP implementation may be symmetric or asymmetric. Symmetric PEPs
use identical behavior in both directions, i.e., the actions taken by
the PEP occur independent from which interface a packet is received.
Asymmetric PEPs operate differently in each direction. The direction
can be defined in terms of the link (e.g., from a central site to a
remote site) or in terms of protocol traffic (e.g., the direction of
TCP data flow, often called the TCP data channel, or the direction of
TCP ACK flow, often called the TCP ACK channel). An asymmetric PEP
implementation is generally used at a point where the characteristics
of the links on each side of the PEP differ or with asymmetric
protocol traffic. For example, an asymmetric PEP might be placed at
the intersection of wired and wireless networks or an asymmetric
application layer PEP might be used for the request-reply type of
HTTP traffic. A PEP implementation may also be both symmetric and
asymmetric at the same time with regard to different mechanisms it
employs. (PEP mechanisms are described in Section 3.)
Whether a PEP implementation is symmetric or asymmetric is
independent of whether the PEP implementation is integrated or
distributed. In other words, a distributed PEP implementation might
operate symmetrically at each end of a link (i.e., the two PEPs
function identically). On the other hand, a distributed PEP
implementation might operate asymmetrically, with a different PEP
implementation at each end of the link. Again, this usually is used
with asymmetric links. For example, for a link with an asymmetric
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amount of bandwidth available in each direction, the PEP on the end
of the link forwarding traffic in the direction with a large amount
of bandwidth might focus on locally acknowledging TCP traffic in
order to use the available bandwidth. At the same time, the PEP on
the end of the link forwarding traffic in the direction with very
little bandwidth might focus on reducing the amount of TCP
acknowledgement traffic being forwarded across the link (to keep the
link from congesting).
2.4 Split Connections
A split connection TCP implementation terminates the TCP connection
received from an end system and establishes a corresponding TCP
connection to the other end system. In a distributed PEP
implementation, this is typically done to allow the use of a third
connection between two PEPs optimized for the link. This might be a
TCP connection optimized for the link or it might be another
protocol, for example, a proprietary protocol running on top of UDP.
Also, the distributed implementation might use a separate connection
between the proxies for each TCP connection or it might multiplex the
data from multiple TCP connections across a single connection between
the PEPs.
In an integrated PEP split connection TCP implementation, the PEP
again terminates the connection from one end system and originates a
separate connection to the other end system. [I-TCP] documents an
example of a single PEP split connection implementation.
Many integrated PEPs use a split connection implementation in order
to address a mismatch in TCP capabilities between two end systems.
For example, the TCP window scaling option [RFC1323] can be used to
extend the maximum amount of TCP data which can be "in flight" (i.e.,
sent and awaiting acknowledgement). This is useful for filling a
link which has a high bandwidth*delay product. If one end system is
capable of using scaled TCP windows but the other is not, the end
system which is not capable can set up its connection with a PEP on
its side of the high bandwidth*delay link. The split connection PEP
then sets up a TCP connection with window scaling over the link to
the other end system.
Split connection TCP implementations can effectively leverage TCP
performance enhancements optimal for a particular link but which
cannot necessarily be employed safely over the global Internet.
Note that using split connection PEPs does not necessarily exclude
simultaneous use of IP for end-to-end connectivity. If a split
connection is managed per application or per connection and is under
the control of the end user, the user can decide whether a particular
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TCP connection or application makes use of the split connection PEP
or whether it operates end-to-end. When a PEP is employed on a last
hop link, the end user control is relatively easy to implement.
In effect, application layer proxies for TCP-based applications are
split connection TCP implementations with end systems using PEPs as a
service related to a particular application. Therefore, all
transport (TCP) layer enhancements that are available with split
connection TCP implementations can also be employed with application
layer PEPs in conjunction with application layer enhancements.
2.5 Transparency
Another key characteristic of a PEP is its degree of transparency.
PEPs may operate totally transparently to the end systems, transport
endpoints, and/or applications involved (in a connection), requiring
no modifications to the end systems, transport endpoints, or
applications.
On the other hand, a PEP implementation may require modifications to
both ends in order to be used. In between, a PEP implementation may
require modifications to only one of the ends involved. Either of
these kind of PEP implementations is non-transparent, at least to the
layer requiring modification.
It is sometimes useful to think of the degree of transparency of a
PEP implementation at four levels, transparency with respect to the
end systems (network-layer transparent PEP), transparency with
respect to the transport endpoints (transport-layer transparent PEP),
transparency with respect to the applications (application-layer
transparent PEP) and transparency with respect to the users. For
example, a user who subscribes to a satellite Internet access service
may be aware that the satellite terminal is providing a performance
enhancing service even though the TCP/IP stack and the applications
in the user's PC are not aware of the PEP which implements it.
Note that the issue of transparency is not the same as the issue of
maintaining end-to-end semantics. For example, a PEP implementation
which simply uses a TCP ACK spacing mechanism maintains the end-to-
end semantics of the TCP connection while a split connection TCP PEP
implementation may not. Yet, both can be implemented transparently
to the transport endpoints at both ends. The implications of not
maintaining the end-to-end semantics, in particular the end-to-end
semantics of TCP connections, are discussed in Section 4.
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3. PEP Mechanisms
An obvious key characteristic of a PEP implementation is the
mechanism(s) it uses to improve performance. Some examples of PEP
mechanisms are described in the following subsections. A PEP
implementation might implement more than one of these mechanisms.
3.1 TCP ACK Handling
Many TCP PEP implementations are based on TCP ACK manipulation. The
handling of TCP acknowledgments can differ significantly between
different TCP PEP implementations. The following subsections
describe various TCP ACK handling mechanisms. Many implementations
combine some of these mechanisms and possibly employ some additional
mechanisms as well.
3.1.1 TCP ACK Spacing
In environments where ACKs tend to bunch together, ACK spacing is
used to smooth out the flow of TCP acknowledgments traversing a link.
This improves performance by eliminating bursts of TCP data segments
that the TCP sender would send due to back-to-back arriving TCP
acknowledgments [BPK97].
3.1.2 Local TCP Acknowledgements
In some PEP implementations, TCP data segments received by the PEP
are locally acknowledged by the PEP. This is very useful over
network paths with a large bandwidth*delay product as it speeds up
TCP slow start and allows the sending TCP to quickly open up its
congestion window. Local (negative) acknowledgments are often also
employed to trigger local (and faster) error recovery on links with
significant error rates. (See Section 3.1.3.)
Local acknowledgments are automatically employed with split
connection TCP implementations. When local acknowledgments are used,
the burden falls upon the TCP PEP to recover any data which is
dropped after the PEP acknowledges it.
3.1.3 Local TCP Retransmissions
A TCP PEP may locally retransmit data segments lost on the path
between the TCP PEP and the receiving end system, thus aiming at
faster recovery from lost data. In order to achieve this the TCP PEP
may use acknowledgments arriving from the end system that receives
the TCP data segments, along with appropriate timeouts, to determine
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when to locally retransmit lost data. TCP PEPs sending local
acknowledgments to the sending end system are required to employ
local retransmissions towards the receiving end system.
Some PEP implementations perform local retransmissions even though
they do not use local acknowledgments to alter TCP connection
performance. Basic Snoop [SNOOP] is a well know example of such a
PEP implementation. Snoop caches TCP data segments it receives and
forwards and then monitors the end-to-end acknowledgments coming from
the receiving TCP end system for duplicate acknowledgments (DUPACKs).
When DUPACKs are received, Snoop locally retransmits the lost TCP
data segments from its cache, suppressing the DUPACKs flowing to the
sending TCP end system until acknowledgments for new data are
received. The Snoop system also implements an option to employ local
negative acknowledgments to trigger local TCP retransmissions. This
can be achieved, for example, by applying TCP selective
acknowledgments locally on the error-prone link. (See Section 5.3
for details.)
3.1.4 TCP ACK Filtering and Reconstruction
On paths with highly asymmetric bandwidth the TCP ACKs flowing in the
low-speed direction may get congested if the asymmetry ratio is high
enough. The ACK filtering and reconstruction mechanism addresses
this by filtering the ACKs on one side of the link and reconstructing
the deleted ACKs on the other side of the link. The mechanism and
the issue of dealing with TCP ACK congestion with highly asymmetric
links are discussed in detail in [RFC2760] and in [BPK97].
3.2 Tunneling
A Performance Enhancing Proxy may encapsulate messages to carry the
messages across a particular link or to force messages to traverse a
particular path. A PEP at the other end of the encapsulation tunnel
removes the tunnel wrappers before final delivery to the receiving
end system. A tunnel might be used by a distributed split connection
TCP implementation as the means for carrying the connection between
the distributed PEPs. A tunnel might also be used to support forcing
TCP connections which use asymmetric routing to go through the end
points of a distributed PEP implementation.
3.3 Compression
Many PEP implementations include support for one or more forms of
compression. In some PEP implementations, compression may even be
the only mechanism used for performance improvement. Compression
reduces the number of bytes which need to be sent across a link.
This is useful in general and can be very important for bandwidth
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limited links. Benefits of using compression include improved link
efficiency and higher effective link utilization, reduced latency and
improved interactive response time, decreased overhead and reduced
packet loss rate over lossy links.
Where appropriate, link layer compression is used. TCP and IP header
compression are also frequently used with PEP implementations.
[RFC1144] describes a widely deployed method for compressing TCP
headers. Other header compression algorithms are described in
[RFC2507], [RFC2508] and [RFC2509].
Payload compression is also desirable and is increasing in importance
with today's increased emphasis on Internet security. Network (IP)
layer (and above) security mechanisms convert IP payloads into random
bit streams which defeat applicable link layer compression mechanisms
by removing or hiding redundant "information." Therefore,
compression of the payload needs to be applied before security
mechanisms are applied. [RFC2393] defines a framework where common
compression algorithms can be applied to arbitrary IP segment
payloads. However, [RFC2393] compression is not always applicable.
Many types of IP payloads (e.g., images, audio, video and "zipped"
files being transferred) are already compressed. And, when security
mechanisms such as TLS [RFC2246] are applied above the network (IP)
layer, the data is already encrypted (and possibly also compressed),
again removing or hiding any redundancy in the payload. The
resulting additional transport or network layer compression will
compact only headers, which are small, and possibly already covered
by separate compression algorithms of their own.
With application layer PEPs one can employ application-specific
compression. Typically an application-specific (or content-specific)
compression mechanism is much more efficient than any generic
compression mechanism. For example, a distributed Web PEP
implementation may implement more efficient binary encoding of HTTP
headers, or a PEP can employ lossy compression that reduces the image
quality of online-images on Web pages according to end user
instructions, thus reducing the number of bytes transferred over a
slow link and consequently the response time perceived by the user
[LHKR96].
3.4 Handling Periods of Link Disconnection with TCP
Periods of link disconnection or link outages are very common with
some wireless links. During these periods, a TCP sender does not
receive the expected acknowledgments. Upon expiration of the
retransmit timer, this causes TCP to close its congestion window with
all of the related drawbacks. A TCP PEP may monitor the traffic
coming from the TCP sender towards the TCP receiver behind the
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disconnected link. The TCP PEP retains the last ACK, so that it can
shut down the TCP sender's window by sending the last ACK with a
window set to zero. Thus, the TCP sender will go into persist mode.
To make this work in both directions with an integrated TCP PEP
implementation, the TCP receiver behind the disconnected link must be
aware of the current state of the connection and, in the event of a
disconnection, it must be capable of freezing all timers. [M-TCP]
implements such operation. Another possibility is that the
disconnected link is surrounded by a distributed PEP pair.
In split connection TCP implementations, a period of link
disconnection can easily be hidden from the end host on the other
side of the PEP thus precluding the TCP connection from breaking even
if the period of link disconnection lasts a very long time; if the
TCP PEP cannot forward data due to link disconnection, it stops
receiving data. Normal TCP flow control then prevents the TCP sender
from sending more than the TCP advertised window allowed by the PEP.
Consequently, the PEP and its counterpart behind the disconnected
link can employ a modified TCP version which retains the state and
all unacknowledged data segments across the period of disconnection
and then performs local recovery as the link is reconnected. The
period of link disconnection may or may not be hidden from the
application and user, depending upon what application the user is
using the TCP connection for.
3.5 Priority-based Multiplexing
Implementing priority-based multiplexing of data over a slow and
expensive link may significantly improve the performance and
usability of the link for selected applications or connections.
A user behind a slow link would experience the link more feasible to
use in case of simultaneous data transfers, if urgent data transfers
(e.g., interactive connections) could have shorter response time
(better performance) than less urgent background transfers. If the
interactive connections transmit enough data to keep the slow link
fully utilized, it might be necessary to fully suspend the background
transfers for awhile to ensure timely delivery for the interactive
connections.
In flight TCP segments of an end-to-end TCP connection (with low
priority) cannot be delayed for a long time. Otherwise, the TCP
timer at the sending end would expire, resulting in suboptimal
performance. However, this kind of operation can be controlled in
conjunction with a split connection TCP PEP by assigning different
priorities for different connections (or applications). A split
connection PEP implementation allows the PEP in an intermediate node
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to delay the data delivery of a lower-priority TCP flow for an
unlimited period of time by simply rescheduling the order in which it
forwards data of different flows to the destination host behind the
slow link. This does not have a negative impact on the delayed TCP
flow as normal TCP flow control takes care of suspending the flow
between the TCP sender and the PEP, when the PEP is not forwarding
data for the flow, and resumes it once the PEP decides to continue
forwarding data for the flow. This can further be assisted, if the
protocol stacks on both sides of the slow link implement priority
based scheduling of connections.
With such a PEP implementation, along with user-controlled
priorities, the user can assign higher priority for selected
interactive connection(s) and have much shorter response time for the
selected connection(s), even if there are simultaneous low priority
bulk data transfers which in regular end-to-end operation would
otherwise eat the available bandwidth of the slow link almost
completely. These low priority bulk data transfers would then
proceed nicely during the idle periods of interactive connections,
allowing the user to keep the slow and expensive link (e.g., wireless
WAN) fully utilized.
Other priority-based mechanisms may be applied on shared wireless
links with more than two terminals. With shared wireless mediums
becoming a weak link in Internet QoS architectures, many may turn to
PEPs to provide extra priority levels across a shared wireless medium
[SHEL00]. These PEPs are distributed on all nodes of the shared
wireless medium. For example, in an 802.11 WLAN this PEP is
implemented in the access point (base station) and each mobile host.
One PEP then uses distributed queuing techniques to coordinate
traffic classes of all nodes. This is also sometimes called subnet
bandwidth management. See [BBKT97] for an example of queuing
techniques which can be used to achieve this. This technique can be
implemented either above or below the IP layer. Priority treatment
can typically be specified either by the user or by marking the
(IPv4) ToS or (IPv6) Traffic Class IP header field.
3.6 Protocol Booster Mechanisms
Work in [FMSBMR98] shows a range of other possible PEP mechanisms
called protocol boosters. Some of these mechanisms are specific to
UDP flows. For example, a PEP may apply asymmetrical methods such as
extra UDP error detection. Since the 16 bit UDP checksum is
optional, it is typically not computed. However, for links with
errors, the checksum could be beneficial. This checksum can be added
to outgoing UDP packets by a PEP.
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Symmetrical mechanisms have also been developed. A Forward Erasure
Correction (FZC) mechanism can be used with real-time and multicast
traffic. The encoding PEP adds a parity packet over a block of
packets. Upon reception, the parity is removed and missing data is
regenerated. A jitter control mechanism can be implemented at the
expense of extra latency. A sending PEP can add a timestamp to
outgoing packets. The receiving PEP then delays packets in order to
reproduce the correct interval.
4. Implications of Using PEPs
The following sections describe some of the implications of using
Performance Enhancing Proxies.
4.1 The End-to-end Argument
As indicated in [RFC1958], the end-to-end argument [SRC84] is one of
the architectural principles of the Internet. The basic argument is
that, as a first principle, certain required end-to-end functions can
only be correctly performed by the end systems themselves. Most of
the potential negative implications associated with using PEPs are
related to the possibility of breaking the end-to-end semantics of
connections. This is one of the main reasons why PEPs are not
recommended for general use.
As indicated in Section 2.5, not all PEP implementations break the
end-to-end semantics of connections. Correctly designed PEPs do not
attempt to replace any application level end-to-end function, but
only attempt to add performance optimizations to a subpath of the
end-to-end path between the application endpoints. Doing this can be
consistent with the end-to-end argument. However, a user or network
administrator adding a PEP to his network configuration should be
aware of the potential end-to-end implications related to the
mechanisms being used by the particular PEP implementation.
4.1.1 Security
In most cases, security applied above the transport layer can be used
with PEPs, especially transport layer PEPs. However, today, only a
limited number of applications include support for the use of
transport (or higher) layer security. Network (IP) layer security
(IPsec) [RFC2401], on the other hand, can generally be used by any
application, transparently to the application.
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4.1.1.1 Security Implications
The most detrimental negative implication of breaking the end-to-end
semantics of a connection is that it disables end-to-end use of
IPsec. In general, a user or network administrator must choose
between using PEPs and using IPsec. If IPsec is employed end-to-end,
PEPs that are implemented on intermediate nodes in the network cannot
examine the transport or application headers of IP packets because
encryption of IP packets via IPsec's ESP header (in either transport
or tunnel mode) renders the TCP header and payload unintelligible to
the PEPs. Without being able to examine the transport or application
headers, a PEP may not function optimally or at all.
If a PEP implementation is non-transparent to the users and the users
trust the PEP in the middle, IPsec can be used separately between
each end system and PEP. However, in most cases this is an
undesirable or unacceptable alternative as the end systems cannot
trust PEPs in general. In addition, this is not as secure as end-
to-end security. (For example, the traffic is exposed in the PEP
when it is decrypted to be processed.) And, it can lead to
potentially misleading security level assumptions by the end systems.
If the two end systems negotiate different levels of security with
the PEP, the end system which negotiated the stronger level of
security may not be aware that a lower level of security is being
provided for part of the connection. The PEP could be implemented to
prevent this from happening by being smart enough to force the same
level of security to each end system but this increases the
complexity of the PEP implementation (and still is not as secure as
end-to-end security).
With a transparent PEP implementation, it is difficult for the end
systems to trust the PEP because they may not be aware of its
existence. Even if the user is aware of the PEP, setting up
acceptable security associations with the PEP while maintaining the
PEP's transparent nature is problematic (if not impossible).
Note that even when a PEP implementation does not break the end-to-
end semantics of a connection, the PEP implementation may not be able
to function in the presence of IPsec. For example, it is difficult
to do ACK spacing if the PEP cannot reliably determine which IP
packets contain ACKs of interest. In any case, the authors are
currently not aware of any PEP implementations, transparent or non-
transparent, which provide support for end-to-end IPsec, except in a
case where the PEPs are implemented on the end hosts.
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4.1.1.2 Security Implication Mitigations
There are some steps which can be taken to allow the use of IPsec and
PEPs to coexist. If an end user can select the use of IPsec for some
traffic and not for other traffic, PEP processing can be applied to
the traffic sent without IPsec. Of course, the user must then do
without security for this traffic or provide security for the traffic
via other means (for example, by using transport layer security).
However, even when this is possible, significant complexity may need
to be added to the configuration of the end system.
Another alternative is to implement IPsec between the two PEPs of a
distributed PEP implementation. This at least protects the traffic
between the two PEPs. (The issue of trusting the PEPs does not
change.) In the case where the PEP implementation is not transparent
to the user, (assuming that the user trusts the PEPs,) the user can
configure his end system to use the PEPs as the end points of an
IPsec tunnel. And, an IPsec tunnel could even potentially be used
between the end system and a PEP to protect traffic on this part of
the path. But, all of this adds complexity. And, it still does not
eliminate the risk of the traffic being exposed in the PEP itself as
the traffic is received from one IPsec tunnel, processed and then
forwarded (even if forwarded through another IPsec tunnel).
4.1.1.3 Security Research Related to PEPs
There is research underway investigating the possibility of changing
the implementation of IPsec to be more friendly to the use of PEPs.
One approach being actively looked at is the use of multi-layer IP
security. [Zhang00] describes a method which allows TCP headers to
be encrypted as one layer (with the PEPs in the path of the TCP
connections included in the security associations used to encrypt the
TCP headers) while the TCP payload is encrypted end-to-end as a
separate layer. This still involves trusting the PEP, but to a much
lesser extent. However, a drawback to this approach is that it adds
a significant amount of complexity to the IP security implementation.
Given the existing complexity of IPsec, this drawback is a serious
impediment to the standardization of the multi-layer IP security idea
and it is very unlikely that this approach will be adopted as a
standard any time soon. Therefore, relying on this type of approach
will likely involve the use of non-standard protocols (and the
associated risk of doing so).
4.1.2 Fate Sharing
Another important aspect of the end-to-end argument is fate sharing.
If a failure occurs in the network, the ability of the connection to
survive the failure depends upon how much state is being maintained
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on behalf of the connection in the network and whether the state is
self-healing. If no connection specific state resides in the network
or such state is self-healing as in case of regular end-to-end
operation, then a failure in the network will break the connection
only if there is no alternate path through the network between the
end systems. And, if there is no path, both end systems can detect
this. However, if the connection depends upon some state being
stored in the network (e.g., in a PEP), then a failure in the network
(e.g., the node containing a PEP crashes) causes this state to be
lost, forcing the connection to terminate even if an alternate path
through the network exists.
The importance of this aspect of the end-to-end argument with respect
to PEPs is dependent upon both the PEP implementation and upon the
types of applications being used. Sometimes coincidentally but more
often by design, PEPs are used in environments where there is no
alternate path between the end systems and, therefore, a failure of
the intermediate node containing a PEP would result in the
termination of the connection in any case. And, even when this is
not the case, the risk of losing the connection in the case of
regular end-to-end operation may exist as the connection could break
for some other reason, for example, a long enough link outage of a
last-hop wireless link to the end host. Therefore, users may choose
to accept the risk of a PEP crashing in order to take advantage of
the performance gains offered by the PEP implementation. The
important thing is that accepting the risk should be under the
control of the user (i.e., the user should always have the option to
choose end-to-end operation) and, if the user chooses to use the PEP,
the user should be aware of the implications that a PEP failure has
with respect to the applications being used.
4.1.3 End-to-end Reliability
Another aspect of the end-to-end argument is that of acknowledging
the receipt of data end-to-end in order to achieve reliable end-to-
end delivery of data. An application aiming at reliable end-to-end
delivery must implement an end-to-end check and recovery at the
application level. According to the end-to-end argument, this is the
only possibility to correctly implement reliable end-to-end
operation. Otherwise the application violates the end-to-end
argument. This also means that a correctly designed application can
never fully rely on the transport layer (e.g., TCP) or any other
communication subsystem to provide reliable end-to-end delivery.
First, a TCP connection may break down for some reason and result in
lost data that must be recovered at the application level. Second,
the checksum provided by TCP may be considered inadequate, resulting
in undetected (by TCP) data corruption [Pax99] and requiring an
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application level check for data corruption. Third, a TCP
acknowledgement only indicates that data was delivered to the TCP
implementation on the other end system. It does not guarantee that
the data was delivered to the application layer on the other end
system. Therefore, a well designed application must use an
application layer acknowledgement to ensure end-to-end delivery of
application layer data. Note that this does not diminish the value
of a reliable transport protocol (i.e., TCP) as such a protocol
allows efficient implementation of several essential functions (e.g.,
congestion control) for an application.
If a PEP implementation acknowledges application data prematurely
(before the PEP receives an application ACK from the other endpoint),
end-to-end reliability cannot be guaranteed. Typically, application
layer PEPs do not acknowledge data prematurely, i.e., the PEP does
not send an application ACK to the sender until it receives an
application ACK from the receiver. And, transport layer PEP
implementations, including TCP PEPs, generally do not interfere with
end-to-end application layer acknowledgments as they let applications
operate end-to-end. However, the user and/or network administrator
employing the PEP must understand how it operates in order to
understand the risks related to end-to-end reliability.
Some Internet applications do not necessarily operate end-to-end in
their regular operation, thus abandoning any end-to-end reliability
guarantee. For example, Internet email delivery often operates via
relay Mail Transfer Agents, that is, relay Simple Mail Transfer
Protocol (SMTP) servers. An originating MTA (SMTP server) sends the
mail message to a relay MTA that receives the mail message, stores it
in non-volatile storage (e.g., on disk) and then sends an application
level acknowledgement. The relay MTA then takes "full
responsibility" for delivering the mail message to the destination
SMTP server (maybe via another relay MTA); it tries to forward the
message for a relatively long time (typically around 5 days). This
scheme does not give a 100% guarantee of email delivery, but
reliability is considered "good enough".
An application layer PEP for this kind of an application may
acknowledge application data (e.g., mail message) without essentially
decreasing reliability, as long as the PEP operates according to the
same procedure as the regular proxy (e.g., relay MTA). Again, as
indicated above, the user and/or network administrator employing such
a PEP needs to understand how it operates in order to understand the
reliability risks associated with doing so.
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4.1.4 End-to-end Failure Diagnostics
Another aspect of the end-to-end argument is the ability to support
end-to-end failure diagnostics when problems are encountered. If a
network problem occurs which breaks a connection, the end points of
the connection will detect the failure via timeouts. However, the
existence of a PEP in between the two end points could delay
(sometimes significantly) the detection of the failure by one or both
of the end points. (Of course, some PEPs are intentionally designed
to hide these types of failures as described in Section 3.4.) The
implications of delayed detection of a failed connection depend on
the applications being used. Possibilities range from no impact at
all (or just minor annoyance to the end user) all the way up to
impacting mission critical business functions by delaying switchovers
to alternate communications paths.
In addition, tools used to debug connection failures may be affected
by the use of a PEP. For example, PING (described in [RFC792] and
[RFC2151]) is often used to test for connectivity. But, because PING
is based on ICMP instead of TCP (i.e., it is implemented using ICMP
Echo and Reply commands at the network layer), it is possible that
the configuration of the network might route PING traffic around the
PEP. Thus, PING could indicate that an end-to-end path exists
between two hosts when it does not actually exist for TCP traffic.
Even when the PING traffic does go through the PEP, the diagnostics
indications provided by the PING traffic are altered. For example,
if the PING traffic goes transparently through the PEP, PING does not
provide any indication that the PEP exists and since the PING traffic
is not being subjected to the same processing as TCP traffic, it may
not necessarily provide an accurate indication of the network delay
being experienced by TCP traffic. On the other hand, if the PEP
terminates the PING and responds to it on behalf of the end host,
then the PING provides information only on the connectivity to the
PEP. Traceroute (also described in [RFC2151]) is similarly affected
by the presence of the PEP.
4.2 Asymmetric Routing
Deploying a PEP implementation usually requires that traffic to and
from the end hosts is routed through the intermediate node(s) where
PEPs reside. With some networks, this cannot be accomplished, or it
might require that the intermediate node is located several hops away
from the target link edge which in turn is impractical in many cases
and may result in non-optimal routing.
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Note that this restriction does not apply to all PEP implementations.
For example, a PEP which is simply doing ACK spacing only needs to
see one direction of the traffic flow (the direction in which the
ACKs are flowing). ACK spacing can be done without seeing the actual
flow of data.
4.3 Mobile Hosts
In environments where a PEP implementation is used to serve mobile
hosts, additional problems may be encountered because PEP related
state information may need to be transferred to a new PEP node during
a handoff.
When a mobile host moves, it is subject to handovers. If the
intermediate node and home for the serving PEP changes due to
handover, any state information that the PEP maintains and is
required for continuous operation must be transferred to the new
intermediate node to ensure continued operation of the connection.
This requires extra work and overhead and may not be possible to
perform fast enough, especially if the host moves frequently over
cell boundaries of a wireless network. If the mobile host moves to
another IP network, routing to and from the mobile host may need to
be changed to traverse a new PEP node.
Today, mobility implications with respect to using PEPs are more
significant to W-LAN networks than to W-WAN networks. Currently, a
W-WAN base station typically does not provide the mobile host with
the connection point to the wireline Internet. (A W-WAN base station
may not even have an IP stack.) Instead, the W-WAN network takes
care of mobility with the connection point to the wireline Internet
remaining unchanged while the mobile host moves. Thus, PEP state
handover is not currently required in most W-WAN networks when the
host moves. However, this is generally not true in W-LAN networks
and, even in the case of W-WAN networks, the user and/or network
administrator using a PEP needs to be cognizant of how the W-WAN base
stations and the PEP work in case W-WAN PEP state handoff becomes
necessary in the future.
4.4 Scalability
Because a PEP typically processes packet information above the IP
layer, a PEP requires more processing power per packet than a router.
Therefore, PEPs will always be (at least) one step behind routers in
terms of the total throughput they can support. (Processing above
the IP layer is also more difficult to implement in hardware.) In
addition, since most PEP implementations require per connection
state, PEP memory requirements are generally significantly higher
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than with a router. Therefore, a PEP implementation may have a limit
on the number of connections which it can support whereas a router
has no such limitation.
Increased processing power and memory requirements introduce
scalability issues with respect to the use of PEPs. Placement of a
PEP on a high speed link or a link which supports a large number of
connections may require network topology changes beyond just
inserting the PEP into the path of the traffic. For example, if a
PEP can only handle half of the traffic on a link, multiple PEPs may
need to be used in parallel, adding complexity to the network
configuration to divide the traffic between the PEPs.
4.5 Other Implications of Using PEPs
This document describes some significant implications with respect to
using Performance Enhancing Proxies. However, the list of
implications provided in this document is not necessarily exhaustive.
Some examples of other potential implications related to using PEPs
include the use of PEPs in multi-homing environments and the use of
PEPs with respect to Quality of Service (QoS) transparency. For
example, there may be potential interaction with the priority-based
multiplexing mechanism described in Section 3.5 and the use of
differentiated services [RFC2475]. Therefore, users and network
administrators who wish to deploy a PEP should look not only at the
implications described in this document but also at the overall
impact (positive and negative) that the PEP will have on their
applications and network infrastructure, both initially and in the
future when new applications are added and/or changes in the network
infrastructure are required.
5. PEP Environment Examples
The following sections describe examples of environments where PEP is
currently used to improve performance. The examples are provided to
illustrate the use of the various PEP types and PEP mechanisms
described earlier in the document and to help illustrate the
motivation for their development and use.
5.1 VSAT Environments
Today, VSAT networks are implemented with geosynchronous satellites.
VSAT data networks are typically implemented using a star topology.
A large hub earth station is located at the center of the star with
VSATs used at the remote sites of the network. Data is sent from the
hub to the remote sites via an outroute. Data is sent from the
remote sites to the hub via one or more inroutes. VSATs represent an
environment with highly asymmetric links, with an outroute typically
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much larger than an inroute. (Multiple inroutes can be used with
each outroute but any particular VSAT only has access to a single
inroute at a time, making the link asymmetric.)
VSAT networks are generally used to implement private networks (i.e.,
intranets) for enterprises (e.g., corporations) with geographically
dispersed sites. VSAT networks are rarely, if ever, used to
implement Internet connectivity except at the edge of the Internet
(i.e., as the last hop). Connection to the Internet for the VSAT
network is usually implemented at the VSAT network hub site using
appropriate firewall and (when necessary) NAT [RFC2663] devices.
5.1.1 VSAT Network Characteristics
With respect to TCP performance, VSAT networks exhibit the following
subset of the satellite characteristics documented in [RFC2488]:
Long feedback loops
Propagation delay from a sender to a receiver in a geosynchronous
satellite network can range from 240 to 280 milliseconds,
depending on where the sending and receiving sites are in the
satellite footprint. This makes the round trip time just due to
propagation delay at least 480 milliseconds. Queueing delay and
delay due to shared channel access methods can sometimes increase
the total delay up to on the order of a few seconds.
Large bandwidth*delay products
VSAT networks can support capacity ranging from a few kilobits per
second up to multiple megabits per second. When combined with the
relatively long round trip time, TCP needs to keep a large number
of packets "in flight" in order to fully utilize the satellite
link.
Asymmetric capacity
As indicated above, the outroute of a VSAT network is usually
significantly larger than an inroute. Even though multiple
inroutes can be used within a network, a given VSAT can only
access one inroute at a time. Therefore, the incoming (outroute)
and outgoing (inroute) capacity for a VSAT is often very
asymmetric. As outroute capacity has increased in recent years,
ratios of 400 to 1 or greater are becoming more and more common.
With a TCP maximum segment size of 1460 bytes and delayed
acknowledgments [RFC1122] in use, the ratio of IP packet bytes for
data to IP packet bytes for ACKs is only (3000 to 40) 75 to 1.
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Thus, inroute capacity for carrying ACKs can have a significant
impact on TCP performance. (The issue of asymmetric link impact
on TCP performance is described in more detail in [BPK97].)
With respect to the other satellite characteristics listed in
[RFC2488], VSAT networks typically do not suffer from intermittent
connectivity or variable round trip times. Also, VSAT networks
generally include a significant amount of error correction coding.
This makes the bit error rate very low during clear sky conditions,
approaching the bit error rate of a typical terrestrial network. In
severe weather, the bit error rate may increase significantly but
such conditions are rare (when looked at from an overall network
availability point of view) and VSAT networks are generally
engineered to work during these conditions but not to optimize
performance during these conditions.
5.1.2 VSAT Network PEP Implementations
Performance Enhancing Proxies implemented for VSAT networks generally
focus on improving throughput (for applications such as FTP and HTTP
web page retrievals). To a lesser degree, PEP implementations also
work to improve interactive response time for small transactions.
There is not a dominant PEP implementation used with VSAT networks.
Each VSAT network vendor tends to implement their own version of PEP
functionality, integrated with the other features of their VSAT
product. [HNS] and [SPACENET] describe VSAT products with integrated
PEP capabilities. There are also third party PEP implementations
designed to be used with VSAT networks. These products run on nodes
external to the VSAT network at the hub and remote sites. NettGain
[FLASH] and Venturi [FOURELLE] are examples of such products. VSAT
network PEP implementations generally share the following
characteristics:
- They focus on improving TCP performance;
- They use an asymmetric distributed implementation;
- They use a split connection approach with local acknowledgments
and local retransmissions;
- They support some form of compression to reduce the amount of
bandwidth required (with emphasis on saving inroute bandwidth).
The key differentiators between VSAT network PEP implementations are:
- The maximum throughput they attempt to support (mainly a
function of the amount of buffer space they use);
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- The protocol used over the satellite link. Some implementations
use a modified version of TCP while others use a proprietary
protocol running on top of UDP;
- The type of compression used. Third party VSAT network PEP
implementations generally focus on application (e.g., HTTP)
specific compression algorithms while PEP implementations
integrated into the VSAT network generally focus on link
specific compression.
PEP implementations integrated into a VSAT product are generally
transparent to the end systems. Third party PEP implementations used
with VSAT networks usually require configuration changes in the
remote site end systems to route TCP packets to the remote site
proxies but do not require changes to the hub site end systems. In
some cases, the PEP implementation is actually integrated
transparently into the end system node itself, using a "bump in the
stack" approach. In all cases, the use of a PEP is non-transparent
to the user, i.e., the user is aware when a PEP implementation is
being used to boost performance.
5.1.3 VSAT Network PEP Motivation
VSAT networks, since the early stages of their deployment, have
supported the use of local termination of a protocol (e.g., SDLC and
X.25) on each side of the satellite link to hide the satellite link
from the applications using the protocol. Therefore, when LAN
capabilities were added to VSAT networks, VSAT customers expected
and, in fact, demanded, the use of similar techniques for improving
the performance of IP based traffic, in particular TCP traffic.
As indicated in Section 5.1, VSAT networks are primarily used to
implement intranets with Internet connectivity limited to and closely
controlled at the hub site of the VSAT network. Therefore, VSAT
customers are not as affected (or at least perceive that they are not
as affected) by the Internet related implications of using PEPs as
are other technologies. Instead, what is more important to VSAT
customers is the optimization of the network. And, VSAT customers,
in general, prefer that the optimization of the network be done by
the network itself rather than by implementing changes (such as
enabling the TCP scaled window option) to their own equipment. VSAT
customers prefer to optimize their end system configuration for local
communications related to their local mission critical functions and
let the VSAT network hide the presence of the satellite link as much
as possible. VSAT network vendors have also been able to use PEP
functionality to provide value added "services" to their customers
such as extending the useful of life of older equipment which
includes older, "non-modern" TCP stacks.
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Of course, as the line between intranets and the Internet continues
to fade, the implications of using PEPs start to become more
significant for VSAT networks. For example, twelve years ago
security was not a major concern because the equipment cost related
to being able to intercept VSAT traffic was relatively high. Now, as
technology has advanced, the cost is much less prohibitive.
Therefore, because the use of PEP functionality in VSAT networks
prevents the use of IPsec, customers must rely on the use of higher
layer security mechanisms such as TLS or on proprietary security
mechanisms implemented in the VSAT networks themselves (since
currently many applications are incapable of making (or simply don't
make) use of the standardized higher layer security mechanisms).
This, in turn, affects the cost of the VSAT network as well as
affects the ability of the customers to make use of Internet based
capabilities.
5.2 W-WAN Environments
In mobile wireless WAN (W-WAN) environments the wireless link is
typically used as the last-hop link to the end user. W-WANs include
such networks as GSM [GSM], GPRS [GPRS],[BW97], CDPD [CDPD], IS-95
[CDMA], RichoNet, and PHS. Many of these networks, but not all, have
been designed to provide mobile telephone voice service in the first
place but include data services as well or they evolve from a mobile
telephone network.
5.2.1 W-WAN Network Characteristics
W-WAN links typically exhibit some combination of the following link
characteristics:
- low bandwidth (with some links the available bandwidth might be
as low as a few hundred bits/sec)
- high latency (minimum round-trip delay close to one second is
not exceptional)
- high BER resulting in frame or packet losses, or long variable
delays due to local link-layer error recovery
- some W-WAN links have a lot of internal buffer space which tend
to accumulate data, thus resulting in increased round-trip
delay due to long (and variable) queuing delays
- on some W-WAN links the users may share common channels for
their data packet delivery which, in turn, may cause unexpected
delays to the packet delivery of a user due to simultaneous use
of the same channel resources by the other users
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- unexpected link disconnections (or intermittent link outages)
may occur frequently and the period of disconnection may last a
very long time
- (re)setting the link-connection up may take a long time
(several tens of seconds or even minutes)
- the W-WAN network typically takes care of terminal mobility:
the connection point to the Internet is retained while the user
moves with the mobile host
- the use of most W-WAN links is expensive. Many of the service
providers apply time-based charging.
5.2.2 W-WAN PEP Implementations
Performance Enhancing Proxies implemented for W-WAN environments
generally focus on improving the interactive response time but at the
same time aim at improving throughput, mainly by reducing the
transfer volume over the inherently slow link in various ways. To
achieve this, typically enhancements are applied at almost all
protocol layers.
5.2.2.1 Mowgli System
The Mowgli system [KRA94] is one of the early approaches to address
the challenges induced by the problematic characteristics of low
bandwidth W-WAN links.
The indirect approach used in Mowgli is not limited to a single layer
as in many other split connection approaches, but it involves all
protocol layers. The basic architecture is based on split TCP (UDP
is also supported) together with full support for application layer
proxies with a distributed PEP approach. An application layer proxy
pair may be added between a client and server, the agent (local
proxy) on a mobile host and the proxy on an intermediate node that
provides the mobile host with the connection to the wireline
Internet. Such a pair may be either explicit or fully transparent to
the applications, but it is, at all times, under end-user control
thus allowing the user to select the traffic that traverses through
the PEP implementation and choose end-to-end IP for other traffic.
In order to allow running legacy applications unmodified and without
recompilation, the socket layer implementation on the mobile host is
slightly modified to connect the applications, which are configured
to traverse through the PEP, to a local agent while retaining the
original TCP/IP socket semantics. Two types of application layer
agent-proxy pairs can be configured for mobile host application use.
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A generic pair can be used with any application and it simply
provides split transport service with some optional generic
enhancements like compression. An application-specific pair can be
retailed for any application or a group of applications that are able
to take leverage on the same kind of enhancements. A good example of
enhancements achieved with an application-specific proxy pair is the
Mowgli WWW system that improves significantly the user perceived
response time of Web browsing mainly by reducing the transfer volume
and the number of round trips over the wireless link [LAKLR95],
[LHKR96].
Mowgli provides also an option to replace the TCP/IP core protocols
on the last-hop link with a custom protocol that is tuned for low-
bandwidth W-WAN links [KRLKA97]. This protocol was designed to
provide the same transport service with similar semantics as regular
TCP and UDP provide, but use a different protocol implementation that
can freely apply any appropriate protocol mechanisms without being
constrained by the current TCP/IP packet format or protocol
operation. As this protocol is required to operate over a single
logical link only, it could partially combine the protocol control
information and protocol operation of the link, network, and
transport layers. In addition, the protocol can operate on top of
various link services, for example on top of different raw link
services, on top of PPP, on top of IP, or even on top of a single TCP
connection using it as a link service and implementing "TCP
multiplexing" over it. In all other cases, except when the protocol
is configured to operate on top of raw (wireless) link service, IP
may co-exist with the custom protocol allowing simultaneous end-to-
end IP delivery for the traffic not traversing through the PEP
implementation.
Furthermore, the custom protocol can be run in different operation
modes which turn on or off certain protocol functions depending on
the underlying link service. For example, if the underlying link
service provides reliable data delivery, the checksum and the
window-based error recovery can be turned off, thus reducing the
protocol overhead; only a very simple recovery mechanism is needed to
allow recovery from an unexpected link disconnection. Therefore, the
protocol design was able to use extremely efficient header encoding
(only 1-3 bytes per packet in a typical case), reduce the number of
round trips significantly, and various features that are useful with
low-bandwidth W-WAN links were easy to add. Such features include
suspending the protocol operation over the periods of link
disconnection or link outage together with fast start once the link
becomes operational again, priority-based multiplexing of user data
over the W-WAN link thus offering link capacity to interactive
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applications in a timely manner even in presence of bandwidth-
intensive background transfers, and link-level flow control to
prevent data from accumulating into the W-WAN link internal buffers.
If desired, regular TCP/IP transport, possibly with corresponding
protocol modifications in TCP (and UDP) that would tune it more
suitable for W-WAN links, can be employed on the last-hop link.
5.2.2.2 Wireless Application Protocol (WAP)
The Mowgli system was designed to support mobile hosts that are
attached to the Internet over constrained links, but did not address
the specific challenges with low-end mobile devices. Many mobile
wireless devices are power, memory, and processing constrained, and
the communication links to these devices have lower bandwidth and
less stable connections. These limitations led designers to develop
the Wireless Application Protocol (WAP) that specifies an application
framework and network protocols intended to work across differing
narrowband wireless network technologies bringing Internet content
and advanced data services to low-end digital cellular phones and
other mobile wireless terminals, such as pagers and PDAs.
The WAP model consists of a WAP client (mobile terminal), a WAP
proxy, and an origin server. It requires a WAP proxy between the WAP
client and the server on the Internet. WAP uses a layered, scalable
architecture [WAPARCH], specifying the following five protocol layers
to be used between the terminal and the proxy: Application Layer
(WAE) [WAPWAE], Session Layer (WSP) [WAPWSP], Transaction Layer (WTP)
[WAPWTP], Security Layer (WTLS) [WAPWTLS], and Transport Layer (WDP)
[WAPWDP]. Standard Internet protocols are used between the proxy and
the origin server. If the origin server includes WAP proxy
functionality, it is called a WAP Server.
In a typical scenario, a WAP client sends an encoded WAP request to a
WAP proxy. The WAP proxy translates the WAP request into a WWW
(HTTP) request, performing the required protocol conversions, and
submits this request to a standard web server on the Internet. After
the web server responds to the WAP proxy, the response is encoded
into a more compact binary format to decrease the size of the data
over the air. This encoded response is forwarded to the WAP client
[WAPPROXY].
WAP operates over a variety of bearer datagram services. When
communicating over these bearer services, the WAP transport layer
(WDP) is always used between the WAP client and WAP proxy and it
provides port addressed datagram service to the higher WAP layers.
If the bearer service supports IP (e.g., GSM-CSD, GSM-GPRS, IS-136,
CDPD), UDP is used as the datagram protocol. However, if the bearer
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service does not support IP (e.g., GSM-SMS, GSM-USSD, GSM Cell
Broadcast, CDMS-SMS, TETRA-SDS), WDP implements the required datagram
protocol as an adaptation layer between the bearer network and the
protocol stack.
The use of the other layers depends on the port number. WAP has
registered a set of well-known ports with IANA. The port number
selected by the application for communication between a WAP client
and proxy defines the other layers to be used at each end. The
security layer, WTLS, provides privacy, data integrity and
authentication. Its functionality is similar to TLS 1.0 [RFC2246]
extended with datagram support, optimized handshake and dynamic key
refreshing. If the origin server includes WAP proxy functionality,
it might be used to facilitate the end-to-end security solutions,
otherwise it provides security between the mobile terminal and the
proxy.
The transaction layer, WTP, is message based without connection
establishment and tear down. It supports three types of transaction
classes: an unconfirmed request (unidirectional), a reliable
(confirmed) request (unidirectional), and a reliable (confirmed)
request-reply transaction. Data is carried in the first packet and
3-way handshake is eliminated to reduce latencies. In addition
acknowledgments, retransmission, and flow control are provided. It
allows more than one outstanding transaction at a time. It handles
the bearer dependence of a transfer, e.g., selects timeout values and
packet sizes according to the bearer. Unfortunately, WTP uses fixed
retransmission timers and does not include congestion control, which
is a potential problem area as the use of WAP increases [RFC3002].
The session layer, WSP, supports binary encoded HTTP 1.1 with some
extensions such as long living session with suspend/resume facility
and state handling, header caching, and push facility. On top of the
architecture is the application environment (WAE).
5.2.3 W-WAN PEP Motivation
As indicated in Section 5.2.1, W-WAN networks typically offer very
low bandwidth connections with high latency and relatively frequent
periods of link disconnection and they usually are expensive to use.
Therefore, the transfer volume and extra round-trips, such as those
associated with TCP connection setup and teardown, must be reduced
and the slow W-WAN link should be efficiently shielded from excess
traffic and global (wired) Internet congestion to make Internet
access usable and economical. Furthermore, interactive traffic must
be transmitted in a timely manner even if there are other
simultaneous bandwidth intensive (background) transfers and during
the periods with connectivity the link must be kept fully utilized
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due to expensive use. In addition, the (long) periods of link
disconnection must not abort active (bulk data) transfers, if an
end-user so desires.
As (all) applications cannot be made mobility/W-WAN aware in short
time frame or maybe ever, support for mobile W-WAN use should be
implemented in a way which allows most applications, at least those
running on fixed Internet hosts, to continue their operation
unmodified.
5.3 W-LAN Environments
Wireless LANs (W-LAN) are typically organized in a cellular topology
where an access point with a W-LAN transceiver controls a single
cell. A cell is defined in terms of the coverage area of the base
station. The access points are directly connected to the wired
network. The access point in each of the cells is responsible for
forwarding packets to and from the hosts located in the cell. Often
the hosts with W-LAN transceivers are mobile. When such a mobile
host moves from one cell to another cell, the responsibility for
forwarding packets between the wired network and the mobile host must
be transferred to the access point of the new cell. This is known as
a handoff. Many W-LAN systems also support an operation mode
enabling ad-hoc networking. In this mode access points are not
necessarily needed, but hosts with W-LAN transceiver can communicate
directly with the other hosts within the transceiver's transmission
range.
5.3.1 W-LAN Network Characteristics
Current wireless LANs typically provide link bandwidth from 1 Mbps to
11 Mbps. In the future, wide deployment of higher bandwidths up to
54 Mbps or even higher can be expected. The round-trip delay with
wireless LANs is on the order of a few milliseconds or tens of
milliseconds. Examples of W-LANs include IEEE 802.11, HomeRF, and
Hiperlan. Wireless personal area networks (WPAN) such as Bluethooth
can use the same PEP techniques.
Wireless LANs are error-prone due to bit errors, collisions and link
outages. In addition, consecutive packet losses may also occur
during handoffs. Most W-LAN MAC protocols perform low level
retransmissions. This feature shields upper layers from most losses.
However, unavoidable losses, retransmission latency and link outages
still affect upper layers. TCP performance over W-LANs or a network
path involving a W-LAN link is likely to suffer from these effects.
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As TCP wrongly interprets these packet losses to be network
congestion, the TCP sender reduces its congestion window and is often
forced to timeout in order to recover from the consecutive losses.
The result is often unacceptably poor end-to-end performance.
5.3.2 W-LAN PEP Implementations: Snoop
Berkeley's Snoop protocol [SNOOP] is a TCP-specific approach in which
a TCP-aware module, a Snoop agent, is deployed at the W-LAN base
station that acts as the last-hop router to the mobile host. Snoop
aims at retaining the TCP end-to-end semantics. The Snoop agent
monitors every packet that passes through the base station in either
direction and maintains soft state for each TCP connection. The
Snoop agent is an asymmetric PEP implementation as it operates
differently on TCP data and ACK channels as well as on the uplink
(from the mobile host) and downlink (to the mobile host) TCP
segments.
For a data transfer to a mobile host, the Snoop agent caches
unacknowledged TCP data segments which it forwards to the TCP
receiver and monitors the corresponding ACKs. It does two things:
1. Retransmits any lost data segments locally by using local timers
and TCP duplicate ACKs to identify packet loss, instead of waiting
for the TCP sender to do so end-to-end.
2. Suppresses the duplicate ACKs on their way from the mobile host
back to the sender, thus avoiding fast retransmit and congestion
avoidance at the latter.
Suppressing the duplicate ACKs is required to avoid unnecessary fast
retransmits by the TCP sender as the Snoop agent retransmits a packet
locally. Consider a system that employs the Snoop agent and a TCP
sender S that sends packets to receiver R via a base station BS.
Assume that S sends packets A, B, C, D, E (in that order) which are
forwarded by BS to the wireless receiver R. Assume the first
transmission of packet B is lost due to errors on the wireless link.
In this case, R receives packets A, C, D, E and B (in that order).
Receipt of packets C, D and E trigger duplicate ACKs. When S
receives three duplicate ACKs, it triggers fast retransmit (which
results in a retransmission, as well as reduction of the congestion
window). The Snoop agent also retransmits B locally, when it
receives three duplicate ACKs. The fast retransmit at S occurs
despite the local retransmit on the wireless link, degrading
throughput. Snoop deals with this problem by dropping TCP duplicate
ACKs appropriately at BS.
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For a data transfer from a mobile host, the Snoop agent detects the
packet losses on the wireless link by monitoring the data segments it
forwards. It then employs either Negative Acknowledgements (NAK)
locally or Explicit Loss Notifications (ELN) to inform the mobile
sender that the packet loss was not related to congestion, thus
allowing the sender to retransmit without triggering normal
congestion control procedures. To implement this, changes at the
mobile host are required.
When a Snoop agent uses NAKs to inform the TCP sender of the packet
losses on the wireless link, one possibility to implement them is
using the Selective Acknowledgment (SACK) option of TCP [RFC2018].
This requires enabling SACK processing at the mobile host. The Snoop
agent sends a TCP SACK, when it detects a hole in the transmission
sequence from the mobile host or when it has not received any new
packets from the mobile host for a certain time period. This
approach relies on the advisory nature of the SACKs: the mobile
sender is advised to retransmit the missing segments indicated by
SACK, but it must not assume successful end-to-end delivery of the
segments acknowledged with SACK as these segments might get lost
later in the path to the receiver. Instead, the sender must wait for
a cumulative ACK to arrive.
When the ELN mechanism is used to inform the mobile sender of the
packet losses, Snoop uses one of the 'unreserved' bits in the TCP
header for ELN [SNOOPELN]. The Snoop agent keeps track of the holes
that correspond to segments lost over the wireless link. When a
(duplicate) ACK corresponding to a hole in the sequence space arrives
from the TCP receiver, the Snoop agent sets the ELN bit on the ACK to
indicate that the loss is unrelated to congestion and then forwards
the ACK to the TCP sender. When the sender receives a certain number
of (duplicate) ACKs with ELN (a configurable variable at the mobile
host, e.g., two), it retransmit the missing segment without
performing any congestion control measures.
The ELN mechanism using one of the six bits reserved for future use
in the TCP header is dangerous as it exercises checks that might not
be correctly implemented in TCP stacks, and may expose bugs.
A scheme such as Snoop is needed only if the possibility of a fast
retransmit due to wireless errors is non-negligible. In particular,
if the wireless link uses link-layer recovery for lost data, then
this scheme is not beneficial. Also, if the TCP window tends to stay
smaller than four segments, for example, due to congestion related
losses on the wired network, the probability that the Snoop agent
will have an opportunity to locally retransmit a lost packet is
small. This is because at least three duplicate ACKs are needed to
trigger the local retransmission, but due to small window the Snoop
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agent may not be able to forward three new packets after the lost
packet and thus induce the required three duplicate ACKs.
Conversely, when the TCP window is large enough, Snoop can provide
significant performance improvement (compared with standard TCP).
In order to alleviate the problem with small TCP windows, Snoop
proposes a solution in which a TCP sender is allowed to transmit a
new data segment for each duplicate ACK it receives as long as the
number of duplicate ACKs is less than the threshold for TCP fast
retransmission (three duplicate ACKs). If the new segment reaches
the receiver, it will generate another duplicate ACK which, in turn,
allows the sender to transmit yet another data segment. This
continues until enough duplicate ACKs have accumulated to trigger TCP
fast retransmission. This proposal is the same as the "Limited
Transfer" proposal [RFC3042] that has recently been forwarded to the
standards track. However, to be able to benefit from this solution,
it needs to be deployed on TCP senders and therefore it is not ready
for use in a short time frame.
Snoop requires the intermediate node (base station) to examine and
operate on the traffic between the mobile host and the other end host
on the wired Internet. Hence, Snoop does not work if the IP traffic
is encrypted. Possible solutions involve:
- making the Snoop agent a party to the security association
between the client and the server;
- IPsec tunneling mode, terminated at the Snooping base station.
However, these techniques require that users trust base stations.
Snoop also requires that both the data and the corresponding ACKs
traverse the same base station. Furthermore, the Snoop agent may
duplicate efforts by the link layer as it retransmits the TCP data
segments "at the transport layer" across the wireless link. (Snoop
has been described by its designers as a TCP-aware link layer. This
is the right approach: the link and network layers can be much more
aware of each other than strict layering suggests.)
5.3.3 W-LAN PEP Motivation
Wireless LANs suffer from an error prone wireless channel. Errors
can typically be considered bursty and channel conditions may change
rapidly from mobility and environmental changes. Packets are dropped
from bit errors or during handovers. Periods of link outage can also
be experienced. Although the typical MAC performs retransmissions,
dropped packets, outages and retransmission latency still can have
serious performance implications for IP performance, especially TCP.
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PEPs can be used to alleviate problems caused by packet losses,
protect TCP from link outages, and to add priority multiplexing.
Techniques such as Snoop are integrally implemented in access points,
while priority and compression schemes are distributed across the W-
LAN.
6. Security Considerations
The use of Performance Enhancing Proxies introduces several issues
which impact security. First, (as described in detail in Section
4.1.1,) using PEPs and using IPsec is generally mutually exclusive.
Unless the PEP is also both capable and trusted to be the endpoint of
an IPsec tunnel (and the use of an IPsec tunnel is deemed good enough
security for the applicable threat model), a user or network
administrator must choose between improved performance and network
layer security. In some cases, transport (or higher) layer security
can be used in conjunction with a PEP to mitigate the impact of not
having network layer security. But, support by applications for the
use of transport (or higher) layer security is far from ubiquitous.
Additionally, the PEP itself needs to be protected from attack.
First, even when IPsec tunnels are used with the PEP, the PEP
represents a point in the network where traffic is exposed. And, the
placement of a PEP in the network makes it an ideal platform from
which to launch a denial of service or man in the middle attack.
(Also, taking the PEP out of action is a potential denial of service
attack itself.) Therefore, the PEP must be protected (e.g., by a
firewall) or must protect itself from improper access by an attacker
just like any other device which resides in a network.
7. IANA Considerations
This document is an informational overview document and, as such,
does not introduce new nor modify existing name or number spaces
managed by IANA.
8. Acknowledgements
This document grew out of the Internet-Draft "TCP Performance
Enhancing Proxy Terminology", RFC 2757 "Long Thin Networks", and work
done in the IETF TCPSAT working group. The authors are indebted to
the active members of the PILC working group. In particular, Joe
Touch and Mark Allman gave us invaluable feedback on various aspects
of the document and Magdolna Gerendai provided us with essential help
on the WAP example.
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9. References
[BBKT97] P. Bhagwat, P. Bhattacharya, A. Krishma, S.K. Tripathi,
"Using channel state dependent packet scheduling to
improve TCP throughput over wireless LANs," ACM Wireless
Networks, March 1997, pp. 91 - 102. Available at:
http://www.acm.org/pubs
/articles/journals/wireless/1997-3-1/p91-bhagwat/p91-
bhagwat.pdf
[BPK97] H. Balakrishnan, V.N. Padmanabhan, R.H. Katz, "The
Effects of Asymmetry on TCP Performance," Proc. ACM/IEEE
Mobicom, Budapest, Hungary, September 1997.
[BW97] G. Brasche, B. Walke, "Concepts, Services, and Protocols
of the New GSM Phase 2+ general Packet Radio Service,"
IEEE Communications Magazine, Vol. 35, No. 8, August
1997.
[CDMA] Electronic Industry Alliance (EIA)/Telecommunications
Industry Association (TIA), IS-95: Mobile Station-Base
Station Compatibility Standard for Dual-Mode Wideband
Spread Spectrum Cellular System, 1993.
[CDPD] Wireless Data Forum, CDPD System Specification, Release
1.1, 1995.
[CTC+97] H. Chang, C. Tait, N. Cohen, M. Shapiro, S. Mastrianni,
R. Floyd, B. Housel, D. Lindquist, "Web Browsing in a
Wireless Environment: Disconnected and Asynchronous
Operation in ARTour Web Express," Proc. MobiCom'97,
Budapest, Hungary, September 1997.
[FMSBMR98] D.C. Feldmeier, A.J. McAuley, J.M. Smith, D.S. Bakin,
W.S. Marcus, T.M. Raleigh, "Protocol Boosters," IEEE
Journal on Selected Areas of Communication, Vol. 16, No.
3, April 1998.
[FLASH] Flash Networks Ltd., performance boosting products
technology vendor based in Holmdel, New Jersey. Website
at http://www.flashnetworks.com.
[FOURELLE] Fourelle Systems, performance boosting products
technology vendor based in Santa Clara, California.
Website at http://www.fourelle.com.
[GPRS] ETSI, "General Packet Radio Service (GPRS): Service
Description, Stage 2," GSM03.60, v.6.1.1, August 1998.
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[GSM] M. Rahnema, "Overview of the GSM system and protocol
architecture," IEEE Communications Magazine, Vol. 31, No.
4, pp. 92-100, April 1993.
[HNS] Hughes Network Systems, Inc., VSAT technology vendor
based in Germantown, Maryland. Website at
http://www.hns.com.
[I-TCP] A. Bakre, B.R. Badrinath, "I-TCP: Indirect TCP for Mobile
Hosts," Proc. 15th International Conference on
Distributed Computing Systems (ICDCS), May 1995.
[KRA94] M. Kojo, K. Raatikainen, T. Alanko, "Connecting Mobile
Workstations to the Internet over a Digital Cellular
Telephone Network," Proc. Workshop on Mobile and Wireless
Information Systems (MOBIDATA), Rutgers University, NJ,
November 1994. Revised version published in Mobile
Computing, pp. 253-270, Kluwer, 1996.
[KRLKA97] M. Kojo, K. Raatikainen, M. Liljeberg, J. Kiiskinen, T.
Alanko, "An Efficient Transport Service for Slow Wireless
Telephone Links," IEEE Journal on Selected Areas of
Communication, Vol. 15, No. 7, September 1997.
[LAKLR95] M. Liljeberg, T. Alanko, M. Kojo, H. Laamanen, K.
Raatikainen, "Optimizing World-Wide Web for Weakly-
Connected Mobile Workstations: An Indirect Approach,"
Proc. of the 2nd Int. Workshop on Services in Distributed
and Networked Environments, Whistler, Canada, pp. 132-
139, June 1995.
[LHKR96] M. Liljeberg, H. Helin, M. Kojo, K. Raatikainen, "Mowgli
WWW Software: Improved Usability of WWW in Mobile WAN
Environments," Proc. IEEE Global Internet 1996
Conference, London, UK, November 1996.
[M-TCP] K. Brown, S. Singh, "M-TCP: TCP for Mobile Cellular
Networks," ACM Computer Communications Review Volume
27(5), 1997. Available at
ftp://ftp.ece.orst.edu/pub/singh/papers/mtcp.ps.gz.
[Pax99] V. Paxson, "End-to-End Internet Packet Dynamics,"
IEEE/ACM Transactions on Networking, Vol. 7, No. 3, 1999,
pp. 277-292.
[PILCWEB] http://pilc.grc.nasa.gov.
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RFC 3135 PILC - Performance Enhancing Proxies June 2001
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communications Layers", STD 3, RFC 1122, October 1989.
[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.
[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC1958] Carpenter, B., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October
1996.
[RFC2151] Kessler, G. and S. Shepard, "A Primer On Internet and
TCP/IP Tools and Utilities", FYI 30, RFC 2151, June 1997.
[RFC2246] Dierk, T. and E. Allen, "TLS Protocol Version 1," RFC
2246, January 1999.
[RFC2393] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP
Payload Compression Protocol (IPcomp)", RFC 2393,
December 1998.
[RFC2401] Kent, S., and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2488] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP
Over Satellite Channels using Standard Mechanisms", BCP
28, RFC 2488, January 1999.
[RFC2507] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
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[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February
1999.
[RFC2509] Engan, M., Casner, S. and C. Bormann, "IP Header
Compression over PPP", RFC 2509, February 1999.
[RFC2663] Srisuresh, P. and Y. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J.,
Henderson, T., Heidemann, J., Kruse, H., Ostermann, S.,
Scott, K., Semke, J., Touch, J. and D. Tran, "Ongoing TCP
Research Related to Satellites", RFC 2760, February 2000.
[RFC3002] Mitzel, D., "Overview of 2000 IAB Wireless
Internetworking Workshop", RFC 3002, December 2000.
[RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing
TCP's Loss Recovery Using Limited Transmit", RFC 3042,
January 2001.
[SHEL00] Z. Shelby, T. Saarinen, P. Mahonen, D. Melpignano, A.
Marshall, L. Munoz, "Wireless IPv6 Networks - WINE," IST
Mobile Summit, Ireland, October 2000.
[SNOOP] H. Balakrishnan, S. Seshan, E. Amir, R. Katz, "Improving
TCP/IP Performance over Wireless Networks," Proc. 1st ACM
Conference on Mobile Communications and Networking
(Mobicom), Berkeley, California, November 1995.
[SNOOPELN] H. Balakrishnan, R. Katz, "Explicit Loss Notification and
Wireless Web Performance," Proc. IEEE Globecom 1998,
Internet Mini-Conference, Sydney, Australia, November
1998.
[SPACENET] Spacenet, VSAT technology vendor based in Mclean,
Virginia. Website at http://www.spacenet.com.
[SRC84] J.H. Saltzer, D.P. Reed, D.D. Clark, "End-To-End
Arguments in System Design," ACM TOCS, Vol. 2, No. 4, pp.
277-288, November 1984.
[WAPARCH] Wireless Application Protocol Architecture Specification,
April 1998, http://www.wapforum.org.
Border, et al. Informational [Page 38]
RFC 3135 PILC - Performance Enhancing Proxies June 2001
[WAPPROXY] Wireless Application Protocol Push Proxy Gateway Service
Specification, August 1999, http://www.wapforum.org.
[WAPWAE] Wireless Application Protocol Wireless Application
Environment Overview, March 2000,
http://www.wapforum.org.
[WAPWDP] Wireless Application Protocol Wireless Datagram Protocol
Specification, February 2000, http://www.wapforum.org.
[WAPWSP] Wireless Application Protocol Wireless Session Protocol
Specification, May 2000, http://www.wapforum.org.
[WAPWTLS] Wireless Application Protocol Wireless Transport Layer
Security Specification, February 2000,
http://www.wapforum.org.
[WAPWTP] Wireless Application Protocol Wireless Transaction
Protocol Specification, February 2000,
http://www.wapforum.org.
[Zhang00] Y. Zhang, B. Singh, "A Multi-Layer IPsec Protocol," Proc.
proceedings of 9th USENIX Security Symposium, Denver,
Colorado, August 2000. Available at
http://www.wins.hrl.com/people/ygz/papers/usenix00.html.
10. Authors' Addresses
Questions about this document may be directed to:
John Border
Hughes Network Systems
11717 Exploration Lane
Germantown, Maryland 20876
Phone: +1-301-548-6819
Fax: +1-301-548-1196
EMail: border@hns.com
Border, et al. Informational [Page 39]
RFC 3135 PILC - Performance Enhancing Proxies June 2001
Markku Kojo
Department of Computer Science
University of Helsinki
P.O. Box 26 (Teollisuuskatu 23)
FIN-00014 HELSINKI
Finland
Phone: +358-9-1914-4179
Fax: +358-9-1914-4441
EMail: kojo@cs.helsinki.fi
Jim Griner
NASA Glenn Research Center
MS: 54-5
21000 Brookpark Orad
Cleveland, Ohio 44135-3191
Phone: +1-216-433-5787
Fax: +1-216-433-8705
EMail: jgriner@grc.nasa.gov
Gabriel Montenegro
Sun Microsystems Laboratories, Europe
29, chemin du Vieux Chene
38240 Meylan, FRANCE
Phone: +33 476 18 80 45
EMail: gab@sun.com
Zach Shelby
University of Oulu
Center for Wireless Communications
PO Box 4500
FIN-90014
Finland
Phone: +358-40-779-6297
EMail: zach.shelby@ee.oulu.fi
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Appendix A - PEP Terminology Summary
This appendix provides a summary of terminology frequently used
during discussion of Performance Enhancing Proxies. (In some cases,
these terms have different meanings from their non-PEP related
usage.)
ACK filtering
Removing acknowledgments to prevent congestion of a low speed
link, usually used with paths which include a highly asymmetric
link. Sometimes also called ACK reduction. See Section 3.1.4.
ACK spacing
Delayed forwarding of acknowledgments in order to space them
appropriately, for example, to help minimize the burstiness of
TCP data. See Section 3.1.1.
application layer PEP
A Performance Enhancing Proxy operating above the transport
layer. May be aimed at improving application or transport
protocol performance (or both). Described in detail in Section
2.1.2.
asymmetric link
A link which has different rates for the forward channel (used for
data segments) and the back (or return) channel (used for ACKs).
available bandwidth
The total capacity of a link available to carry information at any
given time. May be lower than the raw bandwidth due to competing
traffic.
bandwidth utilization
The actual amount of information delivered over a link in a given
period, usually expressed as a percent of the raw bandwidth of
the link.
gateway
Has several meanings with respect to PEPs, depending on context:
- An access point to a particular link;
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- A device capable of initiating and terminating connections
on
behalf of a user or end system (e.g., a firewall or proxy).
Not necessarily, but could be, a router.
in flight (data)
Data sent but not yet acknowledged. More precisely, data sent for
which the sender has not yet received the acknowledgement.
link layer PEP
A Performance Enhancing Proxy operating below the network layer.
local acknowledgement
The generation of acknowledgments by an entity in the path
between two end systems in order to allow the sending system to
transmit more data without waiting for end-to-end
acknowledgments. Described (in the context of TCP) in Section
3.1.2.
performance enhancing proxy
An entity in the network acting on behalf of an end system or user
(with or without the knowledge of the end system or user) in order
to enhance protocol performance. Section 2 describes various
types of performance enhancing proxies. Section 3 describes the
mechanisms performance enhancing proxies use to improve
performance.
raw bandwidth
The total capacity of an unloaded link available to carry
information.
Snoop
A TCP-aware link layer developed for wireless packet radio and
cellular networks. It works by caching segments at a wireless
base station. If the base station sees duplicate acknowledgments
for a segment that it has cached, it retransmits the missing
segment while suppressing the duplicate acknowledgement stream
being forwarded back to the sender until the wireless receiver
starts to acknowledge new data. Described in detail in Section
5.3.2 and [SNOOP].
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split connection
A connection that has been terminated before reaching the intended
destination end system in order to initiate another connection
towards the end system. This allows the use of different
connection characteristics for different parts of the path of
the originally intended connection. See Section 2.4.
TCP PEP
A Performance Enhancing Proxy operating at the transport layer
with TCP. Aimed at improving TCP performance.
TCP splitting
Using one or more split TCP connections to improve TCP
performance.
TCP spoofing
Sometimes used as a synonym for TCP PEP. More accurately, TCP
spoofing refers to using transparent (to the TCP stacks in the
end systems) mechanisms to improve TCP performance. See Section
2.1.1.
transparent
In the context of a PEP, transparent refers to not requiring
changes to be made to the end systems, transport endpoints
and/or applications involved in a connection. See Section 2.5
for a more detailed explanation.
transport layer PEP
A Performance Enhancing Proxy operating at the transport layer.
Described in detail in Section 2.1.1.
tunneling
In the context of PEPs, tunneling refers to the process of
wrapping a packet for transmission over a particular link
between two PEPs. See Section 3.2.
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WAP
The Wireless Application Protocol specifies an application
framework and network protocols intended to work across
differing narrow-band wireless network technologies. See
Section 5.2.2.2.
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Full Copyright Statement
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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