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14.39 Server
The Server response-header field contains information about the
software used by the origin server to handle the request. The field
can contain multiple product tokens (section 3.8) and comments
identifying the server and any significant subproducts. The product
tokens are listed in order of their significance for identifying the
application.
Server = "Server" ":" 1*( product | comment )
Example:
Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the proxy
application MUST NOT modify the Server response-header. Instead, it
SHOULD include a Via field (as described in section 14.44).
Note: Revealing the specific software version of the server may
allow the server machine to become more vulnerable to attacks
against software that is known to contain security holes. Server
implementers are encouraged to make this field a configurable
option.
14.40 Transfer-Encoding
The Transfer-Encoding general-header field indicates what (if any)
type of transformation has been applied to the message body in order
to safely transfer it between the sender and the recipient. This
differs from the Content-Encoding in that the transfer coding is a
property of the message, not of the entity.
Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-
coding
Transfer codings are defined in section 3.6. An example is:
Transfer-Encoding: chunked
Many older HTTP/1.0 applications do not understand the Transfer-
Encoding header.
14.41 Upgrade
The Upgrade general-header allows the client to specify what
additional communication protocols it supports and would like to use
if the server finds it appropriate to switch protocols. The server
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MUST use the Upgrade header field within a 101 (Switching Protocols)
response to indicate which protocol(s) are being switched.
Upgrade = "Upgrade" ":" 1#product
For example,
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
The Upgrade header field is intended to provide a simple mechanism
for transition from HTTP/1.1 to some other, incompatible protocol. It
does so by allowing the client to advertise its desire to use another
protocol, such as a later version of HTTP with a higher major version
number, even though the current request has been made using HTTP/1.1.
This eases the difficult transition between incompatible protocols by
allowing the client to initiate a request in the more commonly
supported protocol while indicating to the server that it would like
to use a "better" protocol if available (where "better" is determined
by the server, possibly according to the nature of the method and/or
resource being requested).
The Upgrade header field only applies to switching application-layer
protocols upon the existing transport-layer connection. Upgrade
cannot be used to insist on a protocol change; its acceptance and use
by the server is optional. The capabilities and nature of the
application-layer communication after the protocol change is entirely
dependent upon the new protocol chosen, although the first action
after changing the protocol MUST be a response to the initial HTTP
request containing the Upgrade header field.
The Upgrade header field only applies to the immediate connection.
Therefore, the upgrade keyword MUST be supplied within a Connection
header field (section 14.10) whenever Upgrade is present in an
HTTP/1.1 message.
The Upgrade header field cannot be used to indicate a switch to a
protocol on a different connection. For that purpose, it is more
appropriate to use a 301, 302, 303, or 305 redirection response.
This specification only defines the protocol name "HTTP" for use by
the family of Hypertext Transfer Protocols, as defined by the HTTP
version rules of section 3.1 and future updates to this
specification. Any token can be used as a protocol name; however, it
will only be useful if both the client and server associate the name
with the same protocol.
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14.42 User-Agent
The User-Agent request-header field contains information about the
user agent originating the request. This is for statistical purposes,
the tracing of protocol violations, and automated recognition of user
agents for the sake of tailoring responses to avoid particular user
agent limitations. User agents SHOULD include this field with
requests. The field can contain multiple product tokens (section 3.8)
and comments identifying the agent and any subproducts which form a
significant part of the user agent. By convention, the product tokens
are listed in order of their significance for identifying the
application.
User-Agent = "User-Agent" ":" 1*( product | comment )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
14.43 Vary
The Vary response-header field is used by a server to signal that the
response entity was selected from the available representations of
the response using server-driven negotiation (section 12). Field-
names listed in Vary headers are those of request-headers. The Vary
field value indicates either that the given set of header fields
encompass the dimensions over which the representation might vary, or
that the dimensions of variance are unspecified ("*") and thus may
vary over any aspect of future requests.
Vary = "Vary" ":" ( "*" | 1#field-name )
An HTTP/1.1 server MUST include an appropriate Vary header field with
any cachable response that is subject to server-driven negotiation.
Doing so allows a cache to properly interpret future requests on that
resource and informs the user agent about the presence of negotiation
on that resource. A server SHOULD include an appropriate Vary header
field with a non-cachable response that is subject to server-driven
negotiation, since this might provide the user agent with useful
information about the dimensions over which the response might vary.
The set of header fields named by the Vary field value is known as
the "selecting" request-headers.
When the cache receives a subsequent request whose Request-URI
specifies one or more cache entries including a Vary header, the
cache MUST NOT use such a cache entry to construct a response to the
new request unless all of the headers named in the cached Vary header
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are present in the new request, and all of the stored selecting
request-headers from the previous request match the corresponding
headers in the new request.
The selecting request-headers from two requests are defined to match
if and only if the selecting request-headers in the first request can
be transformed to the selecting request-headers in the second request
by adding or removing linear whitespace (LWS) at places where this is
allowed by the corresponding BNF, and/or combining multiple message-
header fields with the same field name following the rules about
message headers in section 4.2.
A Vary field value of "*" signals that unspecified parameters,
possibly other than the contents of request-header fields (e.g., the
network address of the client), play a role in the selection of the
response representation. Subsequent requests on that resource can
only be properly interpreted by the origin server, and thus a cache
MUST forward a (possibly conditional) request even when it has a
fresh response cached for the resource. See section 13.6 for use of
the Vary header by caches.
A Vary field value consisting of a list of field-names signals that
the representation selected for the response is based on a selection
algorithm which considers ONLY the listed request-header field values
in selecting the most appropriate representation. A cache MAY assume
that the same selection will be made for future requests with the
same values for the listed field names, for the duration of time in
which the response is fresh.
The field-names given are not limited to the set of standard
request-header fields defined by this specification. Field names are
case-insensitive.
14.44 Via
The Via general-header field MUST be used by gateways and proxies to
indicate the intermediate protocols and recipients between the user
agent and the server on requests, and between the origin server and
the client on responses. It is analogous to the "Received" field of
RFC 822 and is intended to be used for tracking message forwards,
avoiding request loops, and identifying the protocol capabilities of
all senders along the request/response chain.
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Via = "Via" ":" 1#( received-protocol received-by [ comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
protocol-name = token
protocol-version = token
received-by = ( host [ ":" port ] ) | pseudonym
pseudonym = token
The received-protocol indicates the protocol version of the message
received by the server or client along each segment of the
request/response chain. The received-protocol version is appended to
the Via field value when the message is forwarded so that information
about the protocol capabilities of upstream applications remains
visible to all recipients.
The protocol-name is optional if and only if it would be "HTTP". The
received-by field is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message.
However, if the real host is considered to be sensitive information,
it MAY be replaced by a pseudonym. If the port is not given, it MAY
be assumed to be the default port of the received-protocol.
Multiple Via field values represent each proxy or gateway that has
forwarded the message. Each recipient MUST append its information
such that the end result is ordered according to the sequence of
forwarding applications.
Comments MAY be used in the Via header field to identify the software
of the recipient proxy or gateway, analogous to the User-Agent and
Server header fields. However, all comments in the Via field are
optional and MAY be removed by any recipient prior to forwarding the
message.
For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at nowhere.com, which completes
the request by forwarding it to the origin server at www.ics.uci.edu.
The request received by www.ics.uci.edu would then have the following
Via header field:
Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
Proxies and gateways used as a portal through a network firewall
SHOULD NOT, by default, forward the names and ports of hosts within
the firewall region. This information SHOULD only be propagated if
explicitly enabled. If not enabled, the received-by host of any host
behind the firewall SHOULD be replaced by an appropriate pseudonym
for that host.
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For organizations that have strong privacy requirements for hiding
internal structures, a proxy MAY combine an ordered subsequence of
Via header field entries with identical received-protocol values into
a single such entry. For example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
Applications SHOULD NOT combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. Applications MUST NOT combine entries which
have different received-protocol values.
14.45 Warning
The Warning response-header field is used to carry additional
information about the status of a response which may not be reflected
by the response status code. This information is typically, though
not exclusively, used to warn about a possible lack of semantic
transparency from caching operations.
Warning headers are sent with responses using:
Warning = "Warning" ":" 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
warn-code = 2DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding
; the Warning header, for use in debugging
warn-text = quoted-string
A response may carry more than one Warning header.
The warn-text should be in a natural language and character set that
is most likely to be intelligible to the human user receiving the
response. This decision may be based on any available knowledge,
such as the location of the cache or user, the Accept-Language field
in a request, the Content-Language field in a response, etc. The
default language is English and the default character set is ISO-
8859-1.
If a character set other than ISO-8859-1 is used, it MUST be encoded
in the warn-text using the method described in RFC 1522 [14].
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Any server or cache may add Warning headers to a response. New
Warning headers should be added after any existing Warning headers. A
cache MUST NOT delete any Warning header that it received with a
response. However, if a cache successfully validates a cache entry,
it SHOULD remove any Warning headers previously attached to that
entry except as specified for specific Warning codes. It MUST then
add any Warning headers received in the validating response. In other
words, Warning headers are those that would be attached to the most
recent relevant response.
When multiple Warning headers are attached to a response, the user
agent SHOULD display as many of them as possible, in the order that
they appear in the response. If it is not possible to display all of
the warnings, the user agent should follow these heuristics:
o Warnings that appear early in the response take priority over those
appearing later in the response.
o Warnings in the user's preferred character set take priority over
warnings in other character sets but with identical warn-codes and
warn-agents.
Systems that generate multiple Warning headers should order them with
this user agent behavior in mind.
This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.
10 Response is stale
MUST be included whenever the returned response is stale. A cache may
add this warning to any response, but may never remove it until the
response is known to be fresh.
11 Revalidation failed
MUST be included if a cache returns a stale response because an
attempt to revalidate the response failed, due to an inability to
reach the server. A cache may add this warning to any response, but
may never remove it until the response is successfully revalidated.
12 Disconnected operation
SHOULD be included if the cache is intentionally disconnected from
the rest of the network for a period of time.
13 Heuristic expiration
MUST be included if the cache heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than
24 hours.
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14 Transformation applied
MUST be added by an intermediate cache or proxy if it applies any
transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, unless this Warning code
already appears in the response. MUST NOT be deleted from a response
even after revalidation.
99 Miscellaneous warning
The warning text may include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT
take any automated action.
14.46 WWW-Authenticate
The WWW-Authenticate response-header field MUST be included in 401
(Unauthorized) response messages. The field value consists of at
least one challenge that indicates the authentication scheme(s) and
parameters applicable to the Request-URI.
WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge
The HTTP access authentication process is described in section 11.
User agents MUST take special care in parsing the WWW-Authenticate
field value if it contains more than one challenge, or if more than
one WWW-Authenticate header field is provided, since the contents of
a challenge may itself contain a comma-separated list of
authentication parameters.
15 Security Considerations
This section is meant to inform application developers, information
providers, and users of the security limitations in HTTP/1.1 as
described by this document. The discussion does not include
definitive solutions to the problems revealed, though it does make
some suggestions for reducing security risks.
15.1 Authentication of Clients
The Basic authentication scheme is not a secure method of user
authentication, nor does it in any way protect the entity, which is
transmitted in clear text across the physical network used as the
carrier. HTTP does not prevent additional authentication schemes and
encryption mechanisms from being employed to increase security or the
addition of enhancements (such as schemes to use one-time passwords)
to Basic authentication.
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The most serious flaw in Basic authentication is that it results in
the essentially clear text transmission of the user's password over
the physical network. It is this problem which Digest Authentication
attempts to address.
Because Basic authentication involves the clear text transmission of
passwords it SHOULD never be used (without enhancements) to protect
sensitive or valuable information.
A common use of Basic authentication is for identification purposes
-- requiring the user to provide a user name and password as a means
of identification, for example, for purposes of gathering accurate
usage statistics on a server. When used in this way it is tempting to
think that there is no danger in its use if illicit access to the
protected documents is not a major concern. This is only correct if
the server issues both user name and password to the users and in
particular does not allow the user to choose his or her own password.
The danger arises because naive users frequently reuse a single
password to avoid the task of maintaining multiple passwords.
If a server permits users to select their own passwords, then the
threat is not only illicit access to documents on the server but also
illicit access to the accounts of all users who have chosen to use
their account password. If users are allowed to choose their own
password that also means the server must maintain files containing
the (presumably encrypted) passwords. Many of these may be the
account passwords of users perhaps at distant sites. The owner or
administrator of such a system could conceivably incur liability if
this information is not maintained in a secure fashion.
Basic Authentication is also vulnerable to spoofing by counterfeit
servers. If a user can be led to believe that he is connecting to a
host containing information protected by basic authentication when in
fact he is connecting to a hostile server or gateway then the
attacker can request a password, store it for later use, and feign an
error. This type of attack is not possible with Digest Authentication
[32]. Server implementers SHOULD guard against the possibility of
this sort of counterfeiting by gateways or CGI scripts. In particular
it is very dangerous for a server to simply turn over a connection to
a gateway since that gateway can then use the persistent connection
mechanism to engage in multiple transactions with the client while
impersonating the original server in a way that is not detectable by
the client.
15.2 Offering a Choice of Authentication Schemes
An HTTP/1.1 server may return multiple challenges with a 401
(Authenticate) response, and each challenge may use a different
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scheme. The order of the challenges returned to the user agent is in
the order that the server would prefer they be chosen. The server
should order its challenges with the "most secure" authentication
scheme first. A user agent should choose as the challenge to be made
to the user the first one that the user agent understands.
When the server offers choices of authentication schemes using the
WWW-Authenticate header, the "security" of the authentication is only
as malicious user could capture the set of challenges and try to
authenticate him/herself using the weakest of the authentication
schemes. Thus, the ordering serves more to protect the user's
credentials than the server's information.
A possible man-in-the-middle (MITM) attack would be to add a weak
authentication scheme to the set of choices, hoping that the client
will use one that exposes the user's credentials (e.g. password). For
this reason, the client should always use the strongest scheme that
it understands from the choices accepted.
An even better MITM attack would be to remove all offered choices,
and to insert a challenge that requests Basic authentication. For
this reason, user agents that are concerned about this kind of attack
could remember the strongest authentication scheme ever requested by
a server and produce a warning message that requires user
confirmation before using a weaker one. A particularly insidious way
to mount such a MITM attack would be to offer a "free" proxy caching
service to gullible users.
15.3 Abuse of Server Log Information
A server is in the position to save personal data about a user's
requests which may identify their reading patterns or subjects of
interest. This information is clearly confidential in nature and its
handling may be constrained by law in certain countries. People using
the HTTP protocol to provide data are responsible for ensuring that
such material is not distributed without the permission of any
individuals that are identifiable by the published results.
15.4 Transfer of Sensitive Information
Like any generic data transfer protocol, HTTP cannot regulate the
content of the data that is transferred, nor is there any a priori
method of determining the sensitivity of any particular piece of
information within the context of any given request. Therefore,
applications SHOULD supply as much control over this information as
possible to the provider of that information. Four header fields are
worth special mention in this context: Server, Via, Referer and From.
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Revealing the specific software version of the server may allow the
server machine to become more vulnerable to attacks against software
that is known to contain security holes. Implementers SHOULD make the
Server header field a configurable option.
Proxies which serve as a portal through a network firewall SHOULD
take special precautions regarding the transfer of header information
that identifies the hosts behind the firewall. In particular, they
SHOULD remove, or replace with sanitized versions, any Via fields
generated behind the firewall.
The Referer field allows reading patterns to be studied and reverse
links drawn. Although it can be very useful, its power can be abused
if user details are not separated from the information contained in
the Referer. Even when the personal information has been removed, the
Referer field may indicate a private document's URI whose publication
would be inappropriate.
The information sent in the From field might conflict with the user's
privacy interests or their site's security policy, and hence it
SHOULD NOT be transmitted without the user being able to disable,
enable, and modify the contents of the field. The user MUST be able
to set the contents of this field within a user preference or
application defaults configuration.
We suggest, though do not require, that a convenient toggle interface
be provided for the user to enable or disable the sending of From and
Referer information.
15.5 Attacks Based On File and Path Names
Implementations of HTTP origin servers SHOULD be careful to restrict
the documents returned by HTTP requests to be only those that were
intended by the server administrators. If an HTTP server translates
HTTP URIs directly into file system calls, the server MUST take
special care not to serve files that were not intended to be
delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
other operating systems use ".." as a path component to indicate a
directory level above the current one. On such a system, an HTTP
server MUST disallow any such construct in the Request-URI if it
would otherwise allow access to a resource outside those intended to
be accessible via the HTTP server. Similarly, files intended for
reference only internally to the server (such as access control
files, configuration files, and script code) MUST be protected from
inappropriate retrieval, since they might contain sensitive
information. Experience has shown that minor bugs in such HTTP server
implementations have turned into security risks.
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15.6 Personal Information
HTTP clients are often privy to large amounts of personal information
(e.g. the user's name, location, mail address, passwords, encryption
keys, etc.), and SHOULD be very careful to prevent unintentional
leakage of this information via the HTTP protocol to other sources.
We very strongly recommend that a convenient interface be provided
for the user to control dissemination of such information, and that
designers and implementers be particularly careful in this area.
History shows that errors in this area are often both serious
security and/or privacy problems, and often generate highly adverse
publicity for the implementer's company.
15.7 Privacy Issues Connected to Accept Headers
Accept request-headers can reveal information about the user to all
servers which are accessed. The Accept-Language header in particular
can reveal information the user would consider to be of a private
nature, because the understanding of particular languages is often
strongly correlated to the membership of a particular ethnic group.
User agents which offer the option to configure the contents of an
Accept-Language header to be sent in every request are strongly
encouraged to let the configuration process include a message which
makes the user aware of the loss of privacy involved.
An approach that limits the loss of privacy would be for a user agent
to omit the sending of Accept-Language headers by default, and to ask
the user whether it should start sending Accept-Language headers to a
server if it detects, by looking for any Vary response-header fields
generated by the server, that such sending could improve the quality
of service.
Elaborate user-customized accept header fields sent in every request,
in particular if these include quality values, can be used by servers
as relatively reliable and long-lived user identifiers. Such user
identifiers would allow content providers to do click-trail tracking,
and would allow collaborating content providers to match cross-server
click-trails or form submissions of individual users. Note that for
many users not behind a proxy, the network address of the host
running the user agent will also serve as a long-lived user
identifier. In environments where proxies are used to enhance
privacy, user agents should be conservative in offering accept header
configuration options to end users. As an extreme privacy measure,
proxies could filter the accept headers in relayed requests. General
purpose user agents which provide a high degree of header
configurability should warn users about the loss of privacy which can
be involved.
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15.8 DNS Spoofing
Clients using HTTP rely heavily on the Domain Name Service, and are
thus generally prone to security attacks based on the deliberate
mis-association of IP addresses and DNS names. Clients need to be
cautious in assuming the continuing validity of an IP number/DNS name
association.
In particular, HTTP clients SHOULD rely on their name resolver for
confirmation of an IP number/DNS name association, rather than
caching the result of previous host name lookups. Many platforms
already can cache host name lookups locally when appropriate, and
they SHOULD be configured to do so. These lookups should be cached,
however, only when the TTL (Time To Live) information reported by the
name server makes it likely that the cached information will remain
useful.
If HTTP clients cache the results of host name lookups in order to
achieve a performance improvement, they MUST observe the TTL
information reported by DNS.
If HTTP clients do not observe this rule, they could be spoofed when
a previously-accessed server's IP address changes. As network
renumbering is expected to become increasingly common, the
possibility of this form of attack will grow. Observing this
requirement thus reduces this potential security vulnerability.
This requirement also improves the load-balancing behavior of clients
for replicated servers using the same DNS name and reduces the
likelihood of a user's experiencing failure in accessing sites which
use that strategy.
15.9 Location Headers and Spoofing
If a single server supports multiple organizations that do not trust
one another, then it must check the values of Location and Content-
Location headers in responses that are generated under control of
said organizations to make sure that they do not attempt to
invalidate resources over which they have no authority.
16 Acknowledgments
This specification makes heavy use of the augmented BNF and generic
constructs defined by David H. Crocker for RFC 822. Similarly, it
reuses many of the definitions provided by Nathaniel Borenstein and
Ned Freed for MIME. We hope that their inclusion in this
specification will help reduce past confusion over the relationship
between HTTP and Internet mail message formats.
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The HTTP protocol has evolved considerably over the past four years.
It has benefited from a large and active developer community--the
many people who have participated on the www-talk mailing list--and
it is that community which has been most responsible for the success
of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
VanHeyningen deserve special recognition for their efforts in
defining early aspects of the protocol.
This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification:
Gary Adams Albert Lunde
Harald Tveit Alvestrand John C. Mallery
Keith Ball Jean-Philippe Martin-Flatin
Brian Behlendorf Larry Masinter
Paul Burchard Mitra
Maurizio Codogno David Morris
Mike Cowlishaw Gavin Nicol
Roman Czyborra Bill Perry
Michael A. Dolan Jeffrey Perry
David J. Fiander Scott Powers
Alan Freier Owen Rees
Marc Hedlund Luigi Rizzo
Greg Herlihy David Robinson
Koen Holtman Marc Salomon
Alex Hopmann Rich Salz
Bob Jernigan Allan M. Schiffman
Shel Kaphan Jim Seidman
Rohit Khare Chuck Shotton
John Klensin Eric W. Sink
Martijn Koster Simon E. Spero
Alexei Kosut Richard N. Taylor
David M. Kristol Robert S. Thau
Daniel LaLiberte Bill (BearHeart) Weinman
Ben Laurie Francois Yergeau
Paul J. Leach Mary Ellen Zurko
Daniel DuBois
Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan,
Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
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