Internet Engineering Task Force J. Border
INTERNET-DRAFT Hughes Network Systems
M. Kojo
University of Helsinki
Jim Griner
NASA Glenn Research Center
G. Montenegro
Sun Microsystems, Inc.
March 10, 2000
Performance Enhancing Proxies
draft-ietf-pilc-pep-02.txt
Status of This Memo
The document is an Internet-Draft and is in full conformance with all
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Abstract
This document provides a high level overview of Performance Enhancing
Proxies. Different types of Performance Enhancing Proxies are
described as well as the mechanisms used to improve performance. In
addition, motivations for their development and use are described
along with some the consequences of using them, especially in the
context of the Internet.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Types of Performance Enhancing Proxies . . . . . . . . . . . . . . 5
2.1 Layering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Transport Layer PEPs . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Application Layer PEPs . . . . . . . . . . . . . . . . . . . . 6
2.2 Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Implementation Symmetry . . . . . . . . . . . . . . . . . . . . 7
2.4 Split Connections . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 Transparency . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. PEP Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1 TCP ACK Handling . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 TCP ACK Spacing . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2 Local TCP Acknowledgements . . . . . . . . . . . . . . . . . . 10
3.1.3 Local TCP Retransmissions . . . . . . . . . . . . . . . . . . 10
3.2 Tunneling . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3 Compression . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Handling Periods of Link Disconnection with TCP . . . . . . . . 12
3.5 Priority-based Multiplexing . . . . . . . . . . . . . . . . . . 13
3.6 Other Link Specific Enhancements . . . . . . . . . . . . . . . . 13
3.6.1 Protocol Booster Mechanisms . . . . . . . . . . . . . . . . . 13
3.6.2 TCP ACK Filtering and Reconstruction . . . . . . . . . . . . . 14
3.6.3 Other Possible Mechanisms . . . . . . . . . . . . . . . . . . 14
4 Implications of Using PEPs . . . . . . . . . . . . . . . . . . . . 14
4.1 The End-to-end Argument . . . . . . . . . . . . . . . . . . . . 14
4.1.1 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.2 Fate Sharing . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1.3 End-to-end Reliability . . . . . . . . . . . . . . . . . . . . 16
4.1.4 End-to-end Failure Diagnostics . . . . . . . . . . . . . . . . 18
4.2 Asymmetric Routing . . . . . . . . . . . . . . . . . . . . . . . 18
4.3 Mobile Hosts . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.4 Other Implications . . . . . . . . . . . . . . . . . . . . . . . 18
4.4.1 Scalability . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.4.2 Multi-Homing Environments . . . . . . . . . . . . . . . . . . 19
4.4.3 QoS Transparency . . . . . . . . . . . . . . . . . . . . . . . 19
4.4.4 Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5 PEP Environment Examples . . . . . . . . . . . . . . . . . . . . . 19
5.1 VSAT Environments . . . . . . . . . . . . . . . . . . . . . . . 19
5.1.1 VSAT Network Characteristics . . . . . . . . . . . . . . . . . 20
5.1.2 VSAT Network PEP Implementations . . . . . . . . . . . . . . . 21
5.1.3 VSAT Network PEP Motivation . . . . . . . . . . . . . . . . . 22
5.2 W-WAN Environments . . . . . . . . . . . . . . . . . . . . . . . 23
5.2.1 W-WAN Network Characteristics . . . . . . . . . . . . . . . . 23
5.2.2 W-WAN PEP Implementations . . . . . . . . . . . . . . . . . . 24
5.2.2.1 Mowgli System . . . . . . . . . . . . . . . . . . . . . . . 24
5.2.2.2 Wireless Application Protocol (WAP) . . . . . . . . . . . . 26
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5.3 W-LAN Environments . . . . . . . . . . . . . . . . . . . . . . . 27
5.3.1 W-LAN Network Characteristics . . . . . . . . . . . . . . . . 27
5.3.2 W-LAN PEP Implementations: Snoop . . . . . . . . . . . . . . . 28
6 Security Considerations . . . . . . . . . . . . . . . . . . . . . 30
7 Appendix - PEP Terminology Summary . . . . . . . . . . . . . . . . 31
7.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 33
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
11 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 38
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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. [Karn99] discusses various link layer design
considerations that should be taken into account when designing a
link layer service that is intended to support the Internet
protocols. Such 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. A
Performance Enhancing Proxy (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 does not intend to advocate use of PEPs in general. 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 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 performance enhancements that employing the PEP may
yield.
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 (Section 7).
NOTE: This is a working draft and it may fail to cover many important
aspects related to PEPs. In particular, this version does not
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necessarily list all the possible implications of using PEPs
nor does the included text on each of the implications cover
all the aspects related to the particular implication.
Suggestions to improve the text are solicited.
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. 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.
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 behaviour
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, a TCP proxy 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 proxy may be
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used to alter the behaviour of the TCP connection by generating local
acknowledgements 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
applies to only a subset of 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
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
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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 behaviour 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 HTTP
traffic.
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
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
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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, 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
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
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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 the 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 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.
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 acknowledgements 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.
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3.1.1 TCP ACK Spacing
Some TCP PEP implementations are concerned only with manipulating TCP
acknowledgements. ACK spacing is used to smooth out the flow of TCP
acknowledgements traversing a link in order to improve performance by
eliminating bursts of TCP data segments [BPK97], [Part98].
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 acknowledgements are automatically employed with split
connection TCP implementations.
When local acknowledgements 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 acknowledgements arriving from the end system that receives
the TCP data segments, along with appropriate timeouts, to determine
when to locally retransmit lost data. TCP PEPs sending local
acknowledgements 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 acknowledgements 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 acknowledgements coming from the
receiving TCP end system for duplicate acknowledgements (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 acknowledgements for new data are
received. (See Section 5.3 for details.)
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3.2 Tunneling
<Text in this subsection is subject to change>
A Performance Enhancing Proxy may encapsulate messages to carry the
messages across a particular link. 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
connecting split connection 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 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. These benefits are described in more detail in
[DMKM99].
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
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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. In particular, with slow links any compression that
effectively reduces transfer volume is tremendously useful. 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 inline-images on Web
pages according to end user instructions, thus reducing the number of
bytes transferred over the 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 outage are very common with
some wireless links. During these periods, a TCP sender does not
receive the expected acknowledgements. 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
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 PEP thus precluding the TCP connection from breaking even
if the period of link disconnection lasts a very long time.
Consequently, the proxy 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.
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3.5 Priority-based Multiplexing
Implementing priority-based multiplexing of data over a slow
(expensive) link may improve the usability of the link and
performance for selected applications or connectios.
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 transfers. This kind of
operation can be controlled by assigning different priorities for
different connections (or applications).
In flight TCP segments of an end-to-end TCP connection (with low
priority) can not be delayed for a long time. Otherwise, the TCP
timer at the sending end would expire, resulting in suboptimal
performance. A split connection PEP implementation allows a PEP in an
intermediate node to reschedule freely the order in which it forwards
data of different connections to the destination host behind the slow
link. 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 together with user-controlled
priorities the user can assign higher priority for some interactive
connection(s) and in this way have much shorter response time for
selected connections, even if there are simultaneous low priority
bulk data transfers (which would in regular end-to-end operation eat
almost all available bandwidth of the slow link). 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.
3.6 Other Link Specific Enhancements
< Editor's comment: the following subsections provide placeholders
for describing other link specific enhancements. Any help is
appreciated and contributions on these subjects are solicited. >
3.6.1 Protocol Booster Mechanisms
A number of possible protocol booster mechanisms are described
in [FMSBMR98].
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3.6.2 TCP ACK Filtering and Reconstruction
< Editor's note: the upcoming text for this subsection is to be moved
under the section 3.1. >
On paths with highly asymmetric bandwidth the TCP ACKs flowing on the
low-speed direction may get congested if the asymmetry ratio is high
enough. This issue is discussed in [RFC2760] and in a companion PILC
document on Implications of Network Asymmetry [BaPa99].
3.6.3 Other Possible Mechanisms
< Editor's note: contributions describing other mechanisms are
solicited. >
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 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.
4.1.1 Security
The most detrimental negative implication of breaking the end-to-end
semantics of a connection is that it disables end-to-end use of
network (IP) layer security (IPsec) [RFC2401]. If, on the other hand,
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IPsec is employed end-to-end, it precludes PEPs from working because
they need to examine transport or application headers but encryption
of IP packets via IPsec's ESP header (in either transport or tunnel
mode) renders the TCP header and payload unintelligible to
intermediate PEPs. However, if an end user can select end-to-end IP
for the IPsec traffic and use a PEP for other traffic, the problem is
considerably alleviated although the encrypted traffic is not subject
to possible performance enhancements while the other traffic is.
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. 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. But, 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.
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. However, IPsec can be implemented between the two PEPs of
a distributed PEP implementation. And, if the PEP implementation is
non-transparent to the users, the users could configure their end
systems to use PEPs as the end points of an IPsec tunnel.
There is also research underway investigating the possibility of
using multi-layer IP security. [Zhang99] 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.
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
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non-transparent, which provide support for end-to-end IPsec.
In most cases, security applied above the transport layer can be used
with PEPs, especially transport layer PEPs.
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
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 very implementation dependent. 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, the 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. Note that
accepting the risk must be under the control of the user and the
user must always have the option to choose end-to-end operation.
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
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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
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.
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 MTAs (relay 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).
Transport layer PEP implementations, including TCP PEPs, generally do
not interfere with end-to-end application layer acknowledgements as
they let applications operate end-to-end.
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4.1.4 End-to-end Failure Diagnostics
- Implications due to PEPs breaking the end-to-end failure
diagnostics.
< Editor's note: contributions providing text are solicited >
4.2 Asymmetric Routing
Deploying a PEP implementation requires that traffic to and from the
end hosts be 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 unpractical in many cases and
may result in non-optimal routing.
4.3 Mobile Hosts
In mobile host environments where a PEP implementation is used to
serve mobile hosts, additional problems are encountered as the PEP
related state information should be transferred to the new PEP
node during a handoff.
When a mobile host moves, it is subject to handovers by the
serving base station. If the base station acts as the intermediate
node and home for the serving PEP, 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 causes
overhead. If the mobile host moves to another IP network, routing
to and from the mobile host may need to be changed to traverse the
new PEP node.
In most W-WAN wireless networks today, unlike W-LANs, the W-WAN base
station does not provide the mobile host with the connection point
to the wireline Internet (such base stations may not even have an
IP stack). Instead, the W-WAN network takes care of the mobility
and retains the connection point to the wireline Internet unchanged
while the mobile host moves. Thus, PEP state handover is not required
in most W-WANs when the host moves.
4.4 Other Implications
The following subsections describe other implications of using PEPs.
< Editor's note: text for the subsections to be added in later
versions. >
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4.4.1 Scalability
- PEPs require more work and therefore will always be (at least)
one step behind routers. The higher the link bandwidth and
the number of connections (packets) traversing through PEP
is, more likely it is that performance becomes an issue.
4.4.2 Multi-Homing Environments
- the effect of multi-homing environments
< Editor's note: contributions providing text are solicited >
4.4.3 QoS Transparency
- QoS transparency implications
< Editor's note: contributions providing text are solicited >
4.4.4 Others
- other possible implications
< Editor's note: contributions addressing other implications and
providing text are solicited >
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
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.)
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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 acknowledgements [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. 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
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more detail in [BaPa99].)
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. SatBooster
[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 acknowledgements
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
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includes older, "non-modern" TCP stacks.
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], CDMA
[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
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- 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
- unexpected link disconnections (or intermittent link outages) may
occur frequently and the pariod of disconnection may last a very
long time
- (re)setting link-connection up may take a long time (several tens
of seconds or even minutes
- 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
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.
<How/What>
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 (also
UDP is supported) together with full support for application layer
proxies with 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 the end-user
control. In order to allow running legacy applications unmodified
and without recompilation, the socket layer on the mobile host is
slightly modified to connect the application 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. A generic pair can be used with any
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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
application group that can take leverage on the same enhancements. A
good example of enhancements achieved with an application-specific
proxy pair is the Mowgli WWW system [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 a
raw link, on top of PPP, on top of IP, or even on top of a single TCP
connection. Furthermore, the 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 a 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 after
the link becomes operational again, priority-based multiplexing of
user data over the W-WAN link thus offering link capacity to
interactive 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, the 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.
<Why>
- transfer volume must be reduced to make the Internet access usable,
(long) periods of link disconnection must not abort active (bulk
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data) transfers, slow W-WAN link should be efficiently shielded
from excess traffic and the global (wired) Internet congestion,
(all) applications can not be made mobility/W-WAN aware in short
time frame or maybe ever, interactive traffic must be transmitted
in a timely manner even if there are other simultaneous bandwidth
intensive (background) transfers, during the periods connection
the link must be kept fully utilized due to expensive use, ...
5.2.2.2 Wireless Application Protocol (WAP)
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 narrow-band 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, specifying the following five protocol layers to be
used between the terminal and the proxy: Application Layer (WAE),
Session Layer (WSP), Transaction Layer (WTP) [WAPWTP], Security Layer
(WTLS), and Transport Layer (WDP) [WAPWDP]. The Internet protocols
are used between the proxy and the origin server. If the origin
server includes WAP proxy functionality, it is called 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
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
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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 extended with
datagram support, optimised handshake and dynamic key refreshing. If
the origin server includes WAP proxy functionality, it migth 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 used to provide necessary retransmissions
and acknowledgements. 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, asynchronous transactions
usage, header caching, etc.
5.3 W-LAN Environments
Wireless LANs (W-LAN) are typically organized in a cellular topology
where a base station with a W-LAN transceiver controls a single
cell. A cell is defined in terms of the coverage area of the base
station. The base stations are directly connected to the wired
network. The base station 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 tranceivers 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 base station 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 base stations are not necessarily
needed, but hosts with W-LAN tranceiver can communicate directly with
the other hosts within the tranceiver's transmission range.
5.3.1 W-LAN Network Characteristics
Current wireless LANs typically provide link bandwidth from 1 Mbps to
10 Mbps, most typically bandwidth being 1 or 2 Mbps. In the future,
wide deployment of higher bandwidths up to 20 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 ... <[TBD>.
Wireless LANs are error-prone due to wireless link corruption. TCP
performance over W-LANs or a network path involving a W-LAN link
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suffers as packet losses due to wireless bit errors tend to occur
in bursts. In addition, consecutive packet losses may occur also
during handoffs.
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
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deals with this problem by dropping TCP duplicate ACKs appropriately
at BS.
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
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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
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).
< TBD: some text how Snoop tries to alleviate the problem with small
windows >
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.)
<Why>
- to alleviate local link pkt drops due to high-BER (wireless) link
6 Security Considerations
The security implications of using PEP are discussed in Section
4.1.1.
<Editor's note: are there other security considerations which need
mentioning?>
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7 Appendix - 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.)
7.1 Definitions
ACK spacing
Delayed forwarding of acknowledgements in order to space them
appropriately, for example, to help minimize the burstiness of
TCP data.
application layer PEP
Performance enhancement operating above the transport layer.
May be aimed at improving application or transport protocol
performance (or both).
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, expressed as a percent of the raw bandwidth of the link.
gateway
Has several meanings depending on context:
- An access point to a particular link;
- 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.
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in flight (data)
Data sent but not yet acknowledged. More precisely, data sent for
which the sender has not yet received the acknowledgement.
local acknowledgement
The generation of acknowledgements 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
acknowledgements.
performance enhancing proxy
<the definition is subject to change>
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.
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 acknowledgements
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 [SNOOP].
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.
TCP PEP
Performance enhancement operating at the transport layer with TCP.
Aimed at improving TCP performance.
TCP splitting
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Using one or more split connections to improve TCP performance.
TCP spoofing
<the definition is subject to change>
( Sometimes used as a synonym for TCP PEP but more accurately refers
to using transparent mechanisms to improve TCP performance. )
transparent
<the definition is subject to change: refine per layer>
( Requires no changes to be made to either end system involved in a
connection.)
tunneling
<the definition is subject to change>
The process of wrapping a packet for transmission over a
particular link.
8 Acknowledgements
This document grew out of the Internet-Draft "TCP Performance
Enhancing Proxy Terminology" and RFC 2757 "Long Thin Networks" and
the work done in the IETF TCPSAT working group.
9 References
[BaPa99] H. Balakrishnan, V.N. Padmanabhan, "TCP Performance
Implications of Network Asymmetry," Internet Draft
(draft-ietf-pilc-asym-00.txt), Work in progress, September
1999.
[BPK97] H. Balakrishnan, V.N. Padmanabhan, and R.H. Katz.
"The Effects of Asymmetry on TCP Performance. In Proceedings
of the ACM/IEEE Mobicom, Budapest, Hungary, ACM. 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
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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," in proceedings of MobiCom'97, Budapest, Hungary,
September 1997.
[DMKM99] S. Dawkins, G. Montenegro, M. Kojo, V. Magret, "End-to-end
Performance Implications of Slow Links," Internet Draft
(draft-ietf-pilc-slow-02.txt), Work in progress, October 1999.
[FMSBMR98] D.C. Feldmeier, A.J. McAuley, J.M. Smith, D.S. Bakin,
W.S. Marcus, T.M. Raleigh, "Protocol Boosters," in IEEE Journal
on Selected Areas of Communication, volume 16, number 3, April
1998.
[FLASH] Flash Networks Ltd., performance boosting products technology
vendor based in Kerselia, Israel. Website at
http://www.flash-networks.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.
[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," in proceedings of the 15th International Conference on
Distributed Computing Systems (ICDCS), May 1995.
[Karn99] P. Karn, A. Falk, J. Touch, M-J. Montpetit, J. Mahdavi,
G., Montenegro, "Advice for Internet Subnetwork Designers,"
Internet Draft (draft-ietf-pilc-link-design-01.txt), Work in
progress, October 1999.
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[KRA94] M. Kojo, K. Raatikainen, T. Alanko, "Connecting Mobile
Workstations to the Internet over a Digital Cellular Telephone
Network," in 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," in IEEE Journal on Selected Areas of
Communication, volume 15, number 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," in Proc. 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," in proceedings of 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.
[Part98] C. Partridge, "ACK Spacing for High Delay-Bandwidth Paths
with Insufficient Buffering," Internet-Draft
(draft-rfced-info-partridge-01.txt), Work in progress, September
1998.
[Pax99] V. Paxson, "End-to-End Internet Packet Dynamics," IEEE/ACM
Transactions on Networking, Vol 7, Number 3, 1999, pp 277-292.
[RFC0793] J. Postel, "Transmission Control Protocol," STD 7,
RFC 793, September 1981.
[RFC1122] R. Braden, "Requirements for Internet Hosts --
Communications Layers," STD 3, RFC 1122, October 1989.
[RFC1144] V. Jacobson, "Compressiing TCP/IP Headers for Low-Speed
Serial Links," RFC 1144, February 1990.
[RFC1323] V. Jacobson, R. Braden, D. Borman, "TCP Extensions for
High Performance," RFC 1323, May 1992.
[RFC1958] B. Carpenter, "Architectural Principles of the Internet,"
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RFC 1958, June 1996.
[RFC2018] M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow,
"TCP Selective Acknowledgment Options," RFC 2018, October, 1996.
[RFC2246] T. Dierk, E. Allen, "TLS Protocol Version 1", RFC
2246, January 1999.
[RFC2393] A. Shacham, R. Monsour, R. Pereira, M. Thomas, "IP Payload
Compression Protocol (IPcomp)," RFC 2393, December 1998.
[RFC2401] S. Kent, R. Atkinson, "Security Architecture for the
Internet Protocol," RFC 2401, November 1998.
[RFC2488] M. Allman, D. Glover, L. Sanchez, "Enhancing TCP Over
Satellite Channels using Standard Mechanisms," BCP 28, RFC 2488,
January 1999.
[RFC2507] M. Degermark, B. Nordgren, S. Pink, "IP Header
Compression," RFC 2507, February 1999.
[RFC2508] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers
for Low-Speed Serial Links," RFC 2508, February 1999.
[RFC2509] M. Engan, S. Casner, C. Bormann, "IP Header Compression
over PPP," RFC 2509, February 1999.
[RFC2663] P. Srisuresh, M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations," RFC 2663, August 1999.
[RFC2760] M. Allman, S. Dawkins, D. Glover, J. Griner, T. Henderson,
J. Heidemann, H. Kruse, S. Ostermann, K. Scott, J. Semke,
J. Touch, D. Tran, "Ongoing TCP Research Related to Satellites,"
RFC 2760, February 2000.
[SNOOP] H. Balakrishnan, S. Seshan, E. Amir, R. Katz, "Improving
TCP/IP Performance over Wireless Networks," in proceedings of the
1st ACM Conference on Mobile Communications and Networking
(Mobicom), Berkeley, CA, November 1995.
[SNOOPELN] H. Balakrishnan, R. Katz, "Explicit Loss Notification
and Wireless Web Performance," In 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
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in System Design," ACM TOCS, Vol 2, Number 4, November 1984,
pp 277-288.
[WAPARCH] Wireless Application Protocol Architecture Specification,
April 1998, http://www.wapforum.org
[WAPPROXY] Wireless Application Protocol Push Proxy Gateway Service
Specification, August 1999, http://www.wapforum.org
[WAPWDP] Wireless Application Protocol Wireless Datagram Protocol
Specification, November 1999, http://www.wapforum.org
[WAPWTP] Wireless Application Protocol Wireless Transaction Protocol
Specification, June 1999, http://www.wapforum.org
[Zhang99] Y. Zhang, "Multi-Layer Protection Scheme for IPSEC,"
Internet Draft (draft-zhang-ipsec-mlipsec-00.txt), Work in
progress, October 1999.
10 Authors' Addresses
Questions about this document may be directed to:
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John Border
Hughes Network Systems
11717 Exploration Lane
Germantown, Maryland 20876
Voice: +1-301-601-4099
Fax: +1-301-601-4275
E-Mail: border@hns.com
Markku Kojo
Department of Computer Science
University of Helsinki
P.O. Box 26 (Teollisuuskatu 23)
FIN-00014 HELSINKI
Finland
Voice: +358-9-1914-4179
Fax: +358-9-1914-4441
E-Mail: kojo@cs.helsinki.fi
Jim Griner
NASA Glenn Research Center
MS: 54-2
21000 Brookpark Orad
Cleveland, Ohio 44135-3191
Voice: +1-216-433-5787
Fax: +1-216-433-8705
E-Mail: jgriner@grc.nasa.gov
Gabriel E. Montenegro
Sun Labs Networking and Security Group
Sun Microsystems, Inc.
901 San Antonio Road
Mailstop UMPK 15-214
Mountain View, California 94303
Voice: +1-650-786-6288
Fax: +1-650-786-6445
E-Mail: gab@sun.com
11 Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
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others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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