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RFC 3135

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Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations

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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

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.


   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 Security Implications  . . . . . . . . . . . . . . . . . . 15 Security Implication Mitigations . . . . . . . . . . . . . 16 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 Mowgli System  . . . . . . . . . . . . . . . . . . . . . . 26 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

   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

   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

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

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

   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

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

   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

Top      ToC       Page 13 
   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.

Top      ToC       Page 15 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.

Top      ToC       Page 16 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). 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

Top      ToC       Page 18 
   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.

(page 21 continued on part 2)

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