Network Working Group I. Cooper
Internet-Draft Mirror Image
Expires: January 11, 2001 J. Dilley
Akamai
July 13, 2000
Known HTTP Proxy/Caching Problems
draft-ietf-wrec-known-prob-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 11, 2001.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This memo catalogs a number of known problems with World Wide Web
(caching) proxies and cache servers. The goal of the document is to
provide a discussion of the problems and proposed workarounds, and
ultimately to improve conditions by illustrating problems. The
construction of this document is a joint effort of the web caching
community. It is being done under the auspices of the IETF Web
Replication and Caching working group. We gratefully acknowledge
RFC2525[1], which helped define the initial format for this known
problems list.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Problem Template . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Document Template . . . . . . . . . . . . . . . . . . . . . 5
2. Known Problems . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1 Interception proxies break client cache directives . . . . . 6
2.1.2 Interception proxies prevent introduction of new HTTP
methods . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3 Cannot specify multiple URIs for replicated resources . . . 8
2.1.4 Replica distance is unknown . . . . . . . . . . . . . . . . 8
2.1.5 Proxy resource location . . . . . . . . . . . . . . . . . . 9
2.1.6 Caching proxy peer selection in heterogeneous networks . . . 10
2.1.7 ICP performance . . . . . . . . . . . . . . . . . . . . . . 11
2.1.8 Caching proxy meshes can break HTTP serialization of content 12
2.1.9 Interception proxies break IP address-based authentication . 13
2.2 Implementation . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1 Use of Cache-Control headers . . . . . . . . . . . . . . . . 14
2.2.2 Lack of HTTP/1.1 compliance for caching proxies . . . . . . 15
2.2.3 ETag support . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.4 User agent/proxy failover . . . . . . . . . . . . . . . . . 17
2.2.5 Servers and content should be optimized for caching . . . . 17
2.2.6 Some servers send bad Content-Length header files that
contain CR . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3 Administration . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.1 Lack of fine-grained, standardized hierarchy controls . . . 19
2.3.2 Proxy/Server exhaustive log format standard for analysis . . 20
2.3.3 Trace log timestamps . . . . . . . . . . . . . . . . . . . . 21
2.3.4 Exchange format for log summaries . . . . . . . . . . . . . 22
3. Security Considerations . . . . . . . . . . . . . . . . . . 24
References . . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 25
A. Text Template . . . . . . . . . . . . . . . . . . . . . . . 26
B. RFC2629 XML Template . . . . . . . . . . . . . . . . . . . . 27
Full Copyright Statement . . . . . . . . . . . . . . . . . . 28
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1. Introduction
This memo discusses problems with both proxies, which act as
application-level intermediaries for Web requests, and caching
proxies which additionally hold copies of previously requested
resources in the hope of improving overall quality of service by
serving the content more locally. Commonly used terminology in this
memo can be found in the companion document "Internet Web
Replication and Caching Taxonomy"[2].
No individual or organization has complete knowledge of the known
problems in web caching. If you know of a problem that is not
documented on this list you are encouraged to send it to the WREC
mailing list, <wrec@cs.utk.edu> for discussion or to the memo's
editors, <jad@akamai.com> & <ian.cooper@mirror-image.com>, for
review and inclusion in the list.
1.1 Problem Template
Each problem is defined in a common format, summarized in the
following table and described below.
Name: short, descriptive name of the problem (3-5 words)
Classification: classifies the problem: performance, security, etc
Description: describes the problem succinctly
Significance: magnitude of problem, environments where it exists
Implications: the impact of the problem on systems and networks
See Also: a reference to a related known problem
Indications: states how to detect the presence of this problem
Solution(s): describe the solution(s) to this problem, if any
Workaround: practical workaround for the problem
References: information about the problem or solution
Contact: contact name and email address for this section
Name
A short, descriptive name (3-5 words) name associated with the
problem.
Classification
Problems are grouped into categories of similar problems for ease
of reading of this memo. Choose the category that best describes
the problem. The suggested categories include three general
categories and several more specific categories.
* Architecture: the fundamental design is incomplete, or
incorrect
* Specification: the spec is ambiguous, incomplete, or incorrect.
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* Implementation: the implementation of the spec is incorrect.
* Performance: perceived page response at the client is
excessive; network bandwidth consumption is excessive; demand
on origin or proxy servers exceed reasonable bounds.
* Administration: care and feeding of caches is or causes a
problem.
* Security: privacy, integrity, or authentication concerns. This
is the first draft of this memo. The classification structure
is in revision. In the published drafts of the memo the
classification structure should be fixed but may be revised
from time to time.
Description
A definition of the problem, succinct but including necessary
background information.
Significance (High, Medium, Low)
May include a brief summary of the environments for which the
problem is significant.
Implications
Why the problem is viewed as a problem. What inappropriate
behavior results from it? This section should substantiate the
magnitude of any problem indicated with High significance.
See Also
Optional. List of other known problems that are related to this
one.
Indications
How to detect the presence of the problem. This may include
references to one or more substantiating documents that
demonstrate the problem. This should include the network
configuration that led to the problem such that it can be
reproduced. Problems that are not reproduceable will not appear
in this memo.
Solution(s)
Solutions that permanently fix the problem, if such are known.
For example, what version of the software does not exhibit the
problem? Indicate if the solution is accepted by the community,
one of several solutions pending agreement, or open possibly with
experimental solutions.
Workaround
Practical workaround if no solution is available or usable. The
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workaround should have sufficient detail for someone experiencing
the problem to get around it.
References
References to related information in technical publications or on
the web. Where can someone interested in learning more go to find
out more about this problem, its solution, or workarounds?
Contact
Contact name and email address of the person who supplied the
information for this section. If you would prefer to remain
anonymous the editor's name will appear here instead, but we
believe in credit where credit is due.
1.2 Document Template
Text and RFC2629 style XML templates for the submission of known
problems can be found in Appendix A and Appendix B.
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2. Known Problems
The remaining sections present the currently documented known
problems. The problems are ordered by classification and
significance. Issues with protocol specification or architecture
are first, followed by implementation issues. Issues of high
significance are first, followed by lower significance. A full list
of the problems is available in the table of contents.
2.1 Architecture
2.1.1 Interception proxies break client cache directives
Name
Interception proxies break client cache directives
Classification
Architecture
Description
HTTP[3] is designed for the user agent to be aware if it is
connected to an origin server or to a proxy. User agents
believing they are transacting with an origin server but which
are really in a connection with an interception proxy may fail to
send critical cache-control information they would have otherwise
included in their request.
Significance
High
Implications
Clients may receive data that is not synchronized with the origin
even when they request an end to end refresh because of the lack
of inclusion of either a cache-control: no-cache or
must-revalidate header. These headers have no impact on origin
server behavior so may not be included by the browser if it
believes it is connected to that resource. Other related data
implications are possible as well. For instance data security may
be compromised by the lack of inclusion of private or no-store
clauses of the cache-control header under similar conditions.
Indications
Easily detected by placing fresh (un-expired) content on a proxy
while changing the authoritative copy and requesting an end to
end reload of the data through a proxy in both interception and
explicit modes.
Solution(s)
Eliminate the need for interception proxies and IP spoofing which
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will return correct context awareness to the client.
Workaround
Include relevant cache-control: directives in every request at
the cost of increased bandwidth and CPU requirements.
Contact
Patrick McManus <mcmanus@AppliedTheory.com>
2.1.2 Interception proxies prevent introduction of new HTTP methods
Name
Interception proxies prevent introduction of new HTTP methods
Classification
Architecture
Description
A proxy that receives a request with a method unknown to it is
required to generate an HTTP 501 Error as a response. HTTP
methods are designed to be extensible so there may be
applications deployed with initial support just for the user
agent and origin server. An interception proxy that hijacks
requests which include new methods destined for servers that have
implemented those methods creates a de-facto firewall where none
may be intended.
Significance
Medium within interception proxy environments.
Implications
Renders new compliant applications useless unless modifications
are made to proxy software. Because new methods are not required
to be globally standardized it is impossible to keep up to date
in the general case.
Solution(s)
Eliminate the need for interception proxies. A client receiving a
501 in a traditional HTTP environment may either choose to repeat
the request to the origin server directly, or perhaps be
configured to use a different cache.
Workaround
Level 5 switches (sometimes called Level 7 or application layer
switches) can be used to keep HTTP traffic with unknown methods
out of the proxy. However, these devices have heavy buffering
responsibilities, still require TCP sequence number spoofing, and
do not interact well with persistent connections.
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The HTTP/1.1 specification allows a proxy to switch over to
tunnel mode when it receives a request with a method or HTTP
version it does not understand how to handle.
Contact
Patrick McManus <mcmanus@AppliedTheory.com>
Henrik Nordstrom <hno@hem.passagen.se> (HTTP/1.1 clarification)
2.1.3 Cannot specify multiple URIs for replicated resources
Name
Cannot specify multiple URIs for replicated resources
Classification
Architecture
Description
There is no way to specify that multiple URIs may be used for a
single resource, one for each replica of the resource. Similarly,
there is no way to say that some set of proxies (each identified
by a URI) may be used to resolve a URI.
Significance
Medium
Implications
Forces users to understand the replication model and mechanism.
Makes it difficult to create a replication framework without
protocol support for replication and naming.
Indications
Inherent in HTTP 1.0, HTTP 1.1.
Solution(s)
Architectural - protocol design is necessary.
Workaround
Replication mechanisms force users to locate a replica or mirror
site for replicated content.
Contact
Daniel LaLiberte <liberte@w3.org>
2.1.4 Replica distance is unknown
Name
Replica distance is unknown
Classification
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Architecture
Description
There is no recommended way to find out which of several servers
or proxies is closer either to the requesting client or to
another machine, either geographically or in the network topology.
Significance
Medium
Implications
Clients must guess which replica is closer to them when
requesting a copy of a document that may be served from multiple
locations. Users must know the set of servers that can serve a
particular object. This in general is hard to determine and
maintain. Users must understand network topology in order to
choose the closest copy. Note that the closest copy is not always
the one that will result in quickest service. A nearby but
heavily loaded server may be slower than a more distant but
lightly loaded server.
Indications
Inherent in HTTP 1.0, HTTP 1.1.
Solution(s)
Architectural - protocol work is necessary. This is a specific
instance of a general problem in widely distributed systems. A
general solution is unlikely, however a specific solution in the
web context is possible.
Workaround
Servers can (many do) provide location hints in a replica
selection web page. Users choose one based upon their location.
Users can learn which replica server gives them best performance.
Note that the closest replica geographically is not necessarily
the closest in terms of network topology. Expecting users to
understand network topology is unreasonable.
Contact
Daniel LaLiberte <liberte@w3.org>
2.1.5 Proxy resource location
Name
Proxy resource location
Classification
Architecture
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Description
There is no way to tell a proxy that it may request a resource
from an alternate location; the receiver may need to check the
authenticity of the resource if redirected.
Significance
Medium
Implications
Proxies have no systematic way to locate resources within other
proxies or origin servers. This makes it more difficult to share
information among proxies. Information sharing would improve
global efficiency.
Indications
Inherent in HTTP 1.0, HTTP 1.1.
Solution(s)
Architectural - protocol design is necessary.
Workaround
Certain proxies share location hints in the form of summary
digests of their contents (e.g., Squid). Certain proxy protocols
enable a proxy query another for its contents (e.g., ICP). (See
however "ICP Performance" issue.)
Contact
Daniel LaLiberte <liberte@w3.org>
2.1.6 Caching proxy peer selection in heterogeneous networks
Name
Caching proxy peer selection in heterogeneous networks
Classification
Architecture
Description
Caching proxy peer selection in networks with large variance in
latency and bandwidth between peers can lead to non-optimal peer
selection. For example take Proxy C with two siblings, Sib1 and
Sib2, and the following network topology (summarized).
* Cache C's link to Sib1, 2 Mbit/sec with 300 msec latency
* Cache C's link to Sib2, 64 Kbit/sec with 10 msec latency.
ICP does not work well in this context. If a user submits a
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request to Proxy C for page P that results in a miss. C will send
an ICP request to Sib1 and Sib2. Assume both siblings have the
requested object P. The ICP-HIT reply will always come from Sib2
before Sib1. However, for large objects it is clear that the
retrieval will be faster from Sib1 rather than Sib2.
In fact, the problem is more complex because Sib1 and Sib2 can't
have a 100% hit ratio. With a hit rate of 10%, it is more
efficient to use Sib1 with resources larger than 48K. The best
choice depends on at least the hit rate and link characteristics;
maybe other parameters as well.
Significance
Medium
Implications
By selecting the first peer to respond, peer selection algorithms
are not optimizing retrieval latency to end users. Furthermore
they are causing more work for the high-latency peer since it
must respond to such requests but will never be chosen to serve
content if the lower latency peer has a copy.
Indications
Inherent in design of ICP v1, ICP v2, and any proxy mesh protocol
that selects peer based upon first response.
This problem is not exhibited by cache digest or other protocols
which (attempt to) maintain knowledge of peer contents and only
hit peers that are believed to have a copy of the requested page.
Solution(s)
This problem is architectural with the peer selection protocol.
Workaround
Proxy mesh design when using such a protocol should be done in
such a way that there is not a high latency variance among peers.
In the example presented in the above description the high
latency high bandwidth peer could be used as a parent, but should
not be used as a sibling.
Contact
Ivan LOVRIC <ivan.lovric@cnet.francetelecom.fr>
John Dilley <jad@akamai.com>
2.1.7 ICP performance
Name
ICP performance
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Classification
Architecture(ICP), Performance
Description
The ICP protocol exhibits O(n^2) scaling properties, where n is
the number of peer proxies participating in the protocol. This
can lead ICP traffic to dominate HTTP traffic within a network.
Significance
Medium
Implications
If a proxy has many ICP peers the bandwidth demand of ICP can be
excessive. System managers must carefully regulate ICP peering.
ICP also leads proxies to become heterogeneous in what they
serve. This means if your proxy does not have a document it is
unlikely your peers will have it either. Therefore, ICP traffic
requests are largely unable to locate a local copy of an object
[credit to Ingrid Melve's 3WCW talk for this].
Indications
Inherent in design of ICP v1, ICP v2.
Solution(s)
This problem is architectural - protocol redesign or replacement
are required to solve it if ICP is to continue to be used.
Workaround
Implementation workarounds exist, for example to turn off use of
ICP, to carefully regulate peering, or to use another mechanism
if available, such as cache digests. A cache digest protocol
shares a summary of cache contents using a Bloom Filter
technique. This allows a cache to estimate whether a peer has a
document. Filters are updated regularly but are not always
up-to-date so cannot help when a spike in popularity occurs. They
also increase traffic but not as much as ICP.
Proxy clustering protocols organize proxies into a mesh provide
another alternative solution. There is ongoing research on this
topic.
Contact
John Dilley <jad@akamai.com>
2.1.8 Caching proxy meshes can break HTTP serialization of content
Name
Caching proxy meshes can break HTTP serialization of content
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Classification
Architecture (HTTP protocol)
Description
A caching proxy mesh where a request may travel different paths
depending on the state of the mesh and associated caches can
break HTTP content serialization, possibly causing the end user
to receive older content than seen on an earlier request where
the request traveled another path in the mesh.
Significance
Medium
Implications
Can cause end user confusion. May in some situations (sibling
cache hit, object has changed state from cacheable to
uncacheable) be close to impossible to get the caches properly
updated with the new content.
Indications
Older content is unexpectedly returned from a caching proxy mesh
after some time.
Solutions(s)
Work with caching proxy vendors and researchers to find a
suitable protocol for maintaining proxy relations and object
state in a mesh.
Workaround
When designing a hierarchy/mesh, make sure that for each
end-user/URL combination there is only one single path in the
mesh during normal operation.
Contact
Henrik Nordstrom <hno@hem.passagen.se>
2.1.9 Interception proxies break IP address-based authentication
Name
Interception proxies break IP address-based authentication
Classification
Architecture
Description
Some web servers are not open for public access, but restrict
themselves to accept only certain IP address ranges for security
reasons. Using interception proxies at the ISP level, for
example, will alter the source (client) IP addresses to that of
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the proxy itself. This will break such authentication mechanisms
and prohibit the otherwise allowed clients access to the servers.
Significance
Medium
Implications
This creates end user confusion and frustration.
Indications
Users may start to see refused connections to servers after
interception proxies are deployed.
Solution(s)
Use user-based authentication instead of (IP) address-based
authentication.
Workaround
By using IP filters at the intercepting device (L4 switch) and
bypass all requests to such servers concerned.
Contact
Keith K. Chau <keithc@unitechnetworks.com>
2.2 Implementation
2.2.1 Use of Cache-Control headers
Name
Use of Cache-Control headers
Classification
Implementation
Description
Many (if not most) implementations incorrectly interpret
Cache-Control response headers.
Significance
High
Implications
CC headers will be spurned by end users if there are conflicting
or non-standard implementations.
Indications
Check: Squid, NetCache, Cache Engine, HTTP State Management draft
for use of CC: no-cache and must-revalidate against HTTP/1.1rev6.
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Solution(s)
Work with vendors and others to assure proper application
Workaround
None
Contact
Mark Nottingham <mnot@mnot.net>
2.2.2 Lack of HTTP/1.1 compliance for caching proxies
Name
Lack of HTTP/1.1 compliance for caching proxies
Classification
Implementation
Description
Although performance benchmarking of caches is starting to be
explored, protocol compliance is just as important.
Significance
High
Implications
Caching proxy vendors implement their interpretation of the
specification; because the specification is very large, sometimes
vague and ambiguous, this can lead to inconsistent behavior
between caching proxies.
Caching proxies need to comply to the specification (or the
specification needs to change).
Indications
There is no currently known compliance test being used.
There is work underway to quantify how closely servers comply
with the current specification. A joint technical report between
AT&T (#990803-05-TM, available at
<URL:http://www.research.att.com/~bala/papers/procow-1.ps.gz> and
HP Labs (to be published) describes the compliance testing. This
report examines how well each of a set of top traffic-producing
sites support certain HTTP/1.1 features.
The IRCache group is working to develop protocol compliance
testing software. Running such a conformance test suite against
proxy cache products would measure compliance and ultimately
would help assure they comply to the specification.
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Solution(s)
Testing should commence and be reported in an open industry
forum. Proxy implementations should conform to the specification.
Workaround
There is no workaround for non-compliance.
Contact
Mark Nottingham <mnot@mnot.net>
IRCache: Duane Wessels <wessels@ircache.net>
Glenn Chisholm <glenn@ircache.net>
2.2.3 ETag support
Name
ETag support
Classification
Implementation
Description
No currently released caching proxy implements ETag (strong)
validation.
Significance
Medium
Implications
LM/IMS validation is inappropriate for many requirements, both
because of its weakness and its use of dates. Lack of a usable,
strong coherency protocol leads developers and end users not to
trust caches.
Indications
-
Solution(s)
Work with vendors to implement ETags; work for better validation
protocols.
Workaround
use LM/IMS validation.
Contact
Mark Nottingham <mnot@mnot.net>
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2.2.4 User agent/proxy failover
Name
User agent/proxy failover
Classification
Implementation
Description
Failover between proxies at the user agent (using a proxy.pac
file) is erratic and no standard behavior is defined.
Additionally, behavior is hard-coded into the browser, so that
proxy administrators cannot use failover at the user agent
effectively.
Significance
Medium
Implications
Architects are forced to implement failover at the proxy itself,
when it may be more appropriate and economical to do it within
the user agent.
Indications
If a browser detects that its primary proxy is down, it will wait
n minutes before trying the next one it is configured to use. It
will then wait y minutes before asking the user if they'd like to
try the original proxy again. This is very confusing for end
users.
Solution(s)
Work with browser vendors to establish standard extensions to
JavaScript proxy.pac libraries that will allow configuration of
these timeouts.
Workaround
User education; redundancy at the proxy level.
Contact
Mark Nottingham <mnot@mnot.net>
2.2.5 Servers and content should be optimized for caching
Name
Servers and content should be optimized for caching
Classification
Implementation (Performance)
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Description
Many web servers and much web content could be implemented to be
more conducive to caching, reducing bandwidth demand and page
load delay.
Significance
Medium
Implications
By making poor use of caches, origin servers encourage longer
load times, greater load on cache servers, and increased network
demand.
Indications
The problem is most apparent for pages that have low or zero
expires time, yet do not change.
Solution(s)
...
Workaround
For example servers could start using unique object identifiers
for write-only content: if an object changes it gets a new name,
otherwise is is considered to be immutable and therefore have an
infinite expire age. Certain hosting providers do this already.
Contact
Peter Danzig <danzig@west.akamai.com>
2.2.6 Some servers send bad Content-Length header files that contain CR
Name
Some servers send bad Content-Length header files that contain CR.
Classification
Implementation
Description
Certain web servers send a Content-length value that is larger
than number of bytes in the HTTP message body. This happens when
the server strips off CR characters from text files with lines
terminated with CRLF as the file is written to the client. The
server probably uses the stat() system call to get the file size
for the Content-Length header. Servers that exhibit this behavior
include the GN Web server (version 2.14 at least)
(http://gopher.unicom.com/gn-info/).
Significance
Low. Surveys indicate only a small number of sites run faulty
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servers.
Implications
In this case, an HTTP client (e.g. user agent or proxy) may
believe it received a partial response. HTTP/1.1 (RFC 2616)
advises that caches MAY store partial responses.
Indications
Count the number of bytes in the message body and comparing it to
the Content-length value. If they differ the server exhibits this
problem.
Solutions
Upgrade or replace the buggy server.
Workaround
Some browsers and proxies use one TCP connection per object and
ignore the Content-Length. The document end of file is identified
by the close of the TCP socket.
Contact
Duane Wessels <wessels@ircache.net>
2.3 Administration
2.3.1 Lack of fine-grained, standardized hierarchy controls
Name
Lack of fine-grained, standardized hierarchy controls
Classification
Administration
Description
There is no standard for instructing a proxy as to how it should
resolve what parent to fetch a given object from. Because of
this, implementations vary greatly, and it can be difficult to
make them interoperate correctly in a complex environment.
Significance
Medium
Implications
Complications in deployment of caches in a complex network (esp.
corporate networks)
Indications
Inability of some proxies to be configured to direct traffic
based on domain name, reverse lookup IP address, raw IP address,
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in normal operation and in failover mode. Inability in some
proxies to set a preferred parent / backup parent configuration.
Solution(s)
?
Workaround
Work with vendors to establish an acceptable configuration within
the limits of their product; standardize on one product.
Contact
Mark Nottingham <mnot@mnot.net>
2.3.2 Proxy/Server exhaustive log format standard for analysis
Name
Proxy/Server exhaustive log format standard for analysis
Classification
Administration
Description
Most proxy or origin server logs used for characterization or
evaluation do not provide sufficient detail to determine
cacheability of responses.
Significance
Low (for operationality; high significance for research efforts)
Implications
Characterizations and simulations are based on non-representative
workloads.
See Also
W3C Web Characterization Activity (http://www.w3.org/WCA/) since
they are are also concerned with collecting high quality logs and
building characterizations from them.
Indications
-
Solution(s)
To properly clean and to accurately determine cacheability of
responses, a complete log is required (including all request
headers as well as all response headers such as User-agent [for
removal of spiders] and Expires, max-age, set-cookie, no-cache,
etc.)
Workaround
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-
References
See "Web Traffic Logs: An Imperfect Resource for Evaluation" in
INET99 <URL:http://www.cs.rutgers.edu/~davison/pubs/inet99/> for
some discussion of this.
Contact
Brian D. Davison <davison@cs.rutgers.edu>
Terence Kelly <tpkelly@eecs.umich.edu>
2.3.3 Trace log timestamps
Name
Trace log timestamps
Classification
Administration
Description
Some proxies/servers log requests without sufficient timing
detail. Millisecond resolution is often too small to preserve
request ordering and either the servers should record request
reception time in addition to completion time, or elapsed time
plus either one.
Significance
Low (for operationality; medium significance for research efforts)
Implications
Characterization and simulation fidelity is improved with
accurate timing and ordering information. Since logs are
generally written in order of request completion, these logs
cannot be re-played without knowing request generation times and
reordering accordingly.
See Also
-
Indications
Timestamps can be identical for multiple entries (when only
millisecond resolution is used). Request orderings can be jumbled
when clients open additional connections for embedded objects
while still receiving the container object.
Solution(s)
Since request completion time is common (e.g. Squid), recommend
continuing to use it (with microsecond resolution if possible)
plus recording elapsed time since request reception.
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Workaround
-
References
See "Web Traffic Logs: An Imperfect Resource for Evaluation" in
INET99 <URL:http://www.cs.rutgers.edu/~davison/pubs/inet99/> for
some discussion of this.
Contact
Brian D. Davison <davison@cs.rutgers.edu>
2.3.4 Exchange format for log summaries
Name
Exchange format for log summaries
Classification
Administration/Analysis?
Description
Although we have (more or less) a standard log file format for
proxies (plain vanilla Common Logfile and Squid), there isn't a
commonly accepted format for summaries of those log files.
Summaries could be generated by the cache itself, or by
post-processing existing log file formats such as Squid's.
Significance
High, since it means that each log file summarizing/analysis tool
is essentially reinventing the wheel (un-necessary repetition of
code), and the cost of processing a large number of large log
files through a variety of analysis tools is (again for no good
reason) excessive.
Implications
In order to perform a meaningful analysis (e.g. to measure
performance in relation to loading/configuration over time) the
access logs from multiple busy caches, it's often necessary to
run first one tool then another, each against the entire log file
(or a significantly large subset of the log). With log files
running into hundreds of MB even after compression (for a cache
dealing with millions of transactions per day) this is a
non-trivial task.
See Also
IP packet/header sniffing - it may be that individual
transactions are at a level of granularity which simply isn't
sensible to be attempting on extremely busy caches. There may
also be legal implications in some countries, e.g. if this
analysis identifies individuals.
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Indications
Disks/memory full(!) Stats (using multiple programs) take too
long to run. Stats crunching must be distributed out to multiple
machines because of its high computational cost.
Solution(s)
Have the proxy produce a standardized summary of its activity
either automatically or via an external (e.g. third party) tool,
in a commonly agreed format. The format could be something like
XML or the Extended Common Logfile, but the format and contents
are subjects for discussion. Ideally this approach would permit
individual cache server products to supply subsets of the
possible summary info, since it may not be feasible for all
servers to provide all of the information which people would like
to see.
Workaround
Devise a private summary format for your own personal use - but
this complicates or even precludes the exchange of summary info
with other interested parties.
References
See the web pages for the commonly used cache stats analysis
programs, e.g. Calamaris, squidtimes, squidclients, ...
Contact
Martin Hamilton <martin@wwwcache.ja.net>
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3. Security Considerations
See individual submissions within this memo.
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References
[1] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner, J.,
Heavens, I., Lahey, K., Semke, J. and B. Volz, "Known TCP
Implementation Problems", RFC 2525, March 1999,
<URL:http://www.rfc-editor/rfc/rfc2525.txt>.
[2] Cooper, I., Melve, I. and G. Tomlinson, "Internet Web
Replication and Caching Taxonomy",
draft-ietf-wrec-taxonomy-05.txt (work in progress), July 2000,
<URL:http://www.ietf.org/internet-drafts/draft-ietf-wrec-taxonom
y-05.txt>.
[3] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999,
<URL:http://www.ietf.org/rfc/rfc2616.txt>.
Authors' Addresses
Ian Cooper
Mirror Image Internet, Inc.
49 Dragon Court
Woburn, MA 01801
USA
Phone: +1 781 376 1109
EMail: ian.cooper@mirror-image.com
John Dilley
Akamai Technologies, Inc.
1400 Fashion Island Blvd
Suite 703
San Mateo, CA 94404
USA
Phone: +1 650 627-5244
EMail: jad@akamai.com
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Appendix A. Text Template
Name
A short, descriptive name (3-5 words) name associated with the
problem.
Classification
Architecture, Specification, Implementation, Performance,
Administration, or Security
Description
A definition of the problem, succinct but including necessary
background information.
Significance
High, Medium, or Low
Implications
Why the problem is viewed as a problem
See Also
Optional. List of other known problems that are related to this one.
Indications
How to detect the presence of the problem.
Solution(s)
Solutions that permanently fix the problem, if such are known.
Workaround
Practical workaround if no solution is available or usable.
The workaround should have sufficient detail for someone experiencing
the problem to get around it.
References
References to related information in technical publications
or on the web.
Contact
Contact name and <email address> of the person who
supplied the information for this section. If you would prefer
to remain anonymous the editor's name will appear here instead,
but we believe in credit where credit is due.
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Appendix B. RFC2629 XML Template
<section title="Name">
<t><list style="hanging">
<t hangText="Name"><vspace/>
A short, descriptive name (3-5 words) name associated with the
problem.</t>
<t hangText="Classification"><vspace/>
Architecture, Specification, Implementation, Performance,
Administration, or Security</t>
<t hangText="Description"><vspace/>
A definition of the problem, succinct but including necessary
background information.</t>
<t hangText="Significance"><vspace/>
High, Medium, or Low</t>
<t hangText="Implications"><vspace/>
Why the problem is viewed as a problem</t>
<t hangText="See Also"><vspace/>
Optional. List of other known problems that are related to
this one.</t>
<t hangText="Indications"><vspace/>
How to detect the presence of the problem.</t>
<t hangText="Solution(s)">
Solutions that permanently fix the problem, if such are known.</t>
<t hangText="Workaround"><vspace/>
Practical workaround if no solution is available or usable.
The workaround should have sufficient detail for someone
experiencing the problem to get around it.</t>
<t hangText="References"><vspace/>
References to related information in technical publications
or on the web.</t>
<t hangText="Contact"><vspace/>
Contact name and <email address> of the person who
supplied the information for this section. If you would prefer
to remain anonymous the editor's name will appear here instead,
but we believe in credit where credit is due.</t>
</list></t>
</section>
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Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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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
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement
Funding for the RFC editor function is currently provided by the
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Cooper & Dilley Expires January 11, 2001 [Page 28]
