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User Agent
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... HTTP is also used as a generic protocol for communication between
user agents and proxies/gateways to other Internet ...
... requests.
user agent
The client which initiates a request. These are often browsers,
...
...
Most HTTP communication is initiated by a user agent and consists of
a request to be applied to a resource on some origin server. In the
simplest case, this may be accomplished via a single connection ...
... simplest case, this may be accomplished via a single connection (v)
between the user agent (UA) and the origin server (O).
...
...
The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections ...
... semantics of the parameter name. Linear white space
(LWS) MUST NOT be used between the type and subtype, nor between an
attribute and its value. User agents that recognize the media-type
MUST process (or arrange to be processed by any external applications
...
... charset label provided by the sender; and those user agents that have
a provision to "guess" a charset MUST use the charset ...
...
In general, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent ...
... user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type.
If an application receives an unrecognized multipart subtype, the
application MUST treat it as being equivalent to "multipart/mixed ...
... the message-body. HTTP/1.1 user agents MUST notify the user when an
invalid length is received and detected.
...
... treat the response as if it had received a 400 status code. In such
cases, user agents SHOULD present to the user the entity returned
with the response, since that entity ...
... 9.1.2); other methods MUST NOT be automatically retried, although
user agents MAY offer a human operator the choice of retrying the
request.
...
... idempotent (see section 9.1.2); other methods MUST NOT be
automatically retried, although user agents MAY offer a human
operator the choice of retrying the request.. If the client does
...
... other than retrieval. These methods should be considered "safe." This
allows user agents to represent other methods, such as POST, PUT and
DELETE ...
... Cache-Control or Expires header fields. However,
the 303 (See Other) response can be used to direct the user agent to
retrieve a cachable resource.
...
... Request-URI does not point to an existing resource, and that URI is
capable of being defined as a new resource by the requesting user
agent, the origin server can create the resource with that URI. If a
...
... URI. If a
new resource is created, the origin server MUST inform the user agent
via the 201 (Created) response. If an existing resource is modified,
...
... URI in a PUT request identifies the entity enclosed
with the request -- the user agent knows what URI is intended and the
server MUST NOT attempt to apply the request to some other resource.
...
... If the server desires that the request be applied to a different URI,
it MUST send a 301 (Moved Permanently) response; the user agent MAY
then make its own decision regarding whether or not to redirect the
request.
...
... allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist
until the process is completed. The entity ...
... The server has fulfilled the request but there is no new information
to send back. If the client is a user agent, it SHOULD NOT change its
document view from that which caused the request to be sent. This
...
...
response is primarily intended to allow input for actions to take
place without causing a change to the user agent's active document
view. The response MAY include new metainformation in the form of
...
... entity-headers, which SHOULD apply to the document currently in the
user agent's active view.
...
...
The server has fulfilled the request and the user agent SHOULD reset
the document view which caused the request to be sent. This response
is primarily intended to allow input for actions to take place via
...
... with the user if and only if the method used in the second request is
GET or HEAD. A user agent SHOULD NOT automatically redirect a request
more than 5 times, since such redirections usually indicate an
infinite loop.
...
... driven negotiation information (section 12) is being provided so that
the user (or user agent) can select a preferred representation and
redirect its request to that location.
...
... entity
containing a list of resource characteristics and location(s) from
which the user or user agent can choose the one most appropriate. The
entity format is specified by the media type ...
... Type header field. Depending upon the format and the capabilities of
the user agent, selection of the most appropriate choice may be
performed automatically. However, this specification does not define
any standard for such automatic selection.
...
... include the specific URL for that representation in the Location
field; user agents MAY use the Location field value for automatic
redirection. This response is cachable unless indicated otherwise.
...
... If the 301 status code is received in response to a request other
than GET or HEAD, the user agent MUST NOT automatically redirect the
request unless it can be confirmed by the user, since this might
...
... a 301 status code, some existing HTTP/1.0 user agents will
erroneously change it into a GET request.
...
... If the 302 status code is received in response to a request other
than GET or HEAD, the user agent MUST NOT automatically redirect the
request unless it can be confirmed by the user, since this might
...
... a 302 status code, some existing HTTP/1.0 user agents will
erroneously change it into a GET request.
...
... method
exists primarily to allow the output of a POST-activated script to
redirect the user agent to a selected resource. The new URI is not a
substitute reference for the originally requested resource. The 303
...
... status codes are applicable to any request method.
User agents SHOULD display any included entity to the user.
...
... credentials. If the 401 response contains the same challenge as the
prior response, and the user agent has already attempted
authentication at least once, then the user SHOULD be presented the
...
... containing a list of available entity characteristics and location(s)
from which the user or user agent can choose the one most
appropriate. The entity format is specified by the media type ...
... Content-Type header field. Depending upon the format and the
capabilities of the user agent, selection of the most appropriate
choice may be performed automatically. However, this specification
does not define any standard for such automatic selection.
...
... headers sent in the request.
In some cases, this may even be preferable to sending a 406
response. User agents are encouraged to inspect the headers of an
incoming response to determine if it is acceptable. If the response
...
... headers of an
incoming response to determine if it is acceptable. If the response
could be unacceptable, a user agent SHOULD temporarily stop receipt
of more data and query the user for a decision on further actions.
...
... Ideally, the response entity would include enough information for the
user or user agent to fix the problem; however, that may not be
possible and is not required.
...
... entity containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. User agents SHOULD display any included entity to the
user. These response codes ...
... response message is used by an origin server
to challenge the authorization of a user agent. This response MUST
include a WWW-Authenticate header field ...
... authentication scheme.
A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving ...
...
containing the authentication information of the user agent for the
realm of the resource being requested.
...
... domain over which credentials can be automatically applied by a
user agent is determined by the protection space. If a prior request
has been authorized, the same credentials MAY be reused for all other
...
...
Proxies MUST be completely transparent regarding user agent
authentication. That is, they MUST forward the WWW-Authenticate ...
...
The "basic" authentication scheme is based on the model that the user
agent must authenticate itself with a user-ID and a password for each
...
... Userids might be case sensitive.
If the user agent wishes to send the userid "Aladdin" and password
"open sesame", it would use the following header field ...
... request. Unfortunately for servers and caches, not all users have
the same preferences for what is "best," and not all user agents are
equally capable of rendering all entity types. For that reason, HTTP ...
... algorithm for
selecting from among the available representations is difficult to
describe to the user agent, or when the server desires to send its
"best guess" to the client along with the first response (hoping to
...
... round-trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the server's
guess, the user agent MAY include request header fields (Accept,
Accept-Language ...
... 1. It is impossible for the server to accurately determine what might be
"best" for any given user, since that would require complete
knowledge of both the capabilities of the user agent and the intended
use for the response (e.g., does the user want to view it on screen
or print it on paper?).
...
... or print it on paper?).
2. Having the user agent describe its capabilities in every request can
be both very inefficient (given that only a small percentage of
responses have multiple representations) and a potential violation of
...
... request-header fields for enabling
server-driven negotiation through description of user agent
capabilities and user preferences: Accept (section 14.1), Accept-
Charset ...
... URI.
Selection from among the representations may be performed
automatically (if the user agent is capable of doing so) or manually
by the user selecting from a generated (possibly hypertext) menu.
...
... language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches ...
... is required by all parties.
2. Protocol features that allow an origin server or user agent to
explicitly request and control non-transparent operation.
...
... SHOULD NOT attempt to revalidate a response simply because that
response became stale in transit; this might lead to an infinite
loop. An user agent that receives a stale response without a Warning
MAY display a warning indication to the user.
...
... Explicit User Agent Warnings ...
...
Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent may allow the user to
...
... Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent may allow the user to
specify that cached entities (even explicitly stale ones) are never
validated ...
... specify that cached entities (even explicitly stale ones) are never
validated. Or the user agent might habitually add "Cache-Control:
max-stale=3600" to every request. The user should have to explicitly
...
... abnormally ineffective caching.
If the user has overridden the basic caching mechanisms, the user
agent should explicitly indicate to the user whenever this results in
the display of information that might not meet the server's
transparency requirements ...
... requirements (in particular, if the displayed entity is
known to be stale). Since the protocol normally allows the user agent
to determine if responses are stale or not, this indication need only
be displayed when this actually happens. The indication need not be a
...
... If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent should
continually display an indication (for example, a picture of currency
...
... potential problem.
It also allows the user agent to take steps to obtain a first-hand or
fresh response. For this reason, a cache SHOULD NOT return a stale
...
... header.
An expiration time cannot be used to force a user agent to refresh
its display or reload a resource; its semantics ...
... origin server, MAY use that value in subrange cache-conditional
requests (using If-Unmodified-Since:). The user agent should
provide a way to disable this, in case of difficulty.
o If both an entity ...
...
User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved
...
... range
parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that
media-range, using the qvalue scale from 0 to 1 (section 3.9). The
...
... text/html;level=3 = 0.7
Note: A user agent may be provided with a default set of quality
values for certain media ranges. However, unless the user agent ...
... user agent may be provided with a default set of quality
values for certain media ranges. However, unless the user agent is
a closed system which cannot interact with other rendering agents,
...
... ISO-
8859-1 character set can be assumed to be acceptable to all user
agents.
Accept-Charset ...
... header field even if it does not
understand all the methods specified, since the user agent MAY have
other means of communicating with the origin server.
...
...
A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving ...
... credentials
containing the authentication information of the user agent for the
realm of the resource being requested.
...
... o Restrictions on what may be stored by a cache; these may be imposed
by either the origin server or the user agent.
o Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent ...
... user agent.
o Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent.
o Controls over cache revalidation and reload; these may only be
...
... o Controls over cache revalidation and reload; these may only be
imposed by a user agent.
o Control over transformation of entities.
o Extensions to the caching system.
...
... observe the max-age directive.
Other directives allow an user agent to modify the basic expiration
mechanism. These directives may be specified on a request:
...
... entity if revalidation fails.
Although this is not recommended, user agents operating under severe
connectivity constraints may violate this directive but, if so, MUST
...
... proxy-revalidate directive has the same meaning as the must-
revalidate directive, except that it does not apply to non-shared
user agent caches. It can be used on a response to an authenticated
...
...
If a message is received via direct connection with the user agent
(in the case of requests) or the origin server (in the case of
responses), then the date can be assumed to be the current date at
...
... cache (either a proxy cache or an user
agent cache) unless it is first validated with the origin server (or
...
... e-mail address for the human user who controls the requesting user
agent. The address SHOULD be machine-usable, as defined by mailbox
...
... credentials containing the authentication information of the user
agent for the proxy and/or realm of the resource being requested.
...
... 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 ...
... 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
...
... 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 ...
... 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
...
... 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 ...
... subject to server-driven
negotiation, since this might provide the user agent with useful
information about the dimensions over which the response might vary.
...
... 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
...
... 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
...
...
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 ...
... 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:
...
... 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
...
... 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
...
... Authenticate) response, and each challenge may use a different
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 ...
... 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.
...
... 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 ...
... 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 ...
... 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 ...
...
An approach that limits the loss of privacy would be for a user agent
to omit the sending of Accept-Language headers ...
... 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
...
... proxies are used to enhance
privacy, user agents should be conservative in offering accept header
configuration options to end users. As an extreme privacy ...
... 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 ...
... representations of the requested resource, along with their
distinguishing attributes, and thus providing a more reliable means
for a user agent to perform subsequent selection of another
representation which better fits the desires of its user (described
as agent ...