Manual Last Protocol

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LOOKING AT STUDENTS' THINKING (LAST) PROTOCOL. I. Getting started (5 minutes). • The group chooses a facilitator who will make sure the group stays.
Table of contents



Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type.

The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response. This field allows clients capable of understanding more comprehensive or special- purpose character sets to signal that capability to a server which is capable of representing documents in those character sets. Character set values are described in section 3.

Each charset MAY be given an associated quality value which represents the user's preference for that charset. If no Accept-Charset header is present, the default is that any character set is acceptable. If an Accept-Charset header is present, and if the server cannot send a response which is acceptable according to the Accept-Charset header, then the server SHOULD send an error response with the not acceptable status code, though the sending of an unacceptable response is also allowed.

The Accept-Encoding request-header field is similar to Accept, but restricts the content-codings section 3. A server tests whether a content-coding is acceptable, according to an Accept-Encoding field, using these rules:. If an Accept-Encoding field is present in a request, and if the server cannot send a response which is acceptable according to the Accept-Encoding header, then the server SHOULD send an error response with the Not Acceptable status code. If no Accept-Encoding field is present in a request, the server MAY assume that the client will accept any content coding.

In this case, if "identity" is one of the available content-codings, then the server SHOULD use the "identity" content-coding, unless it has additional information that a different content-coding is meaningful to the client. The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request.

Language tags are defined in section 3. Each language-range MAY be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range. The language quality factor assigned to a language-tag by the Accept-Language field is the quality value of the longest language- range in the field that matches the language-tag. If no language- range in the field matches the tag, the language quality factor assigned is 0.

If no Accept-Language header is present in the request, the server. If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are acceptable. It might be contrary to the privacy expectations of the user to send an Accept-Language header with the complete linguistic preferences of the user in every request. For a discussion of this issue, see section As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user.

The directives specify behavior intended to prevent caches from adversely interfering with the request or response. These directives typically override the default caching algorithms. Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given in the response. It is not possible to specify a cache- directive for a specific cache. When a directive appears without any 1 field-name parameter, the directive applies to the entire request or response.

When such a directive appears with a 1 field-name parameter, it applies only to the named field or fields, and not to the rest of the request or response. By default, a response is cacheable if the requirements of the request method, request header fields, and the response status indicate that it is cacheable. The following Cache-Control response directives allow an origin server to override the default cacheability of a response:.

This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content. The expiration time of an entity MAY be specified by the origin server using the Expires header see section Alternatively, it MAY be specified using the max-age directive in a response.

When the max-age cache-control directive is present in a cached response, the response is stale if its current age is greater than the age value given in seconds at the time of a new request for that resource. The max-age directive on a response implies that the response is cacheable i. If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. An origin server might wish to use a relatively new HTTP cache control feature, such as the "private" directive, on a network including older caches that do not understand that feature.

The origin server will need to combine the new feature with an Expires field whose value is less than or equal to the Date value. This will prevent older caches from improperly caching the response. Note that most older caches, not compliant with this specification, do not implement any cache-control directives. Other directives allow a user agent to modify the basic expiration mechanism. These directives MAY be specified on a request:. If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured to override the expiration time of a response, the cache MUST attach a Warning header to the stale response, using Warning Response is stale.

A cache MAY be configured to return stale responses without validation, but only if this does not conflict with any "MUST"-level requirements concerning cache validation e. If both the new request and the cached entry include "max-age" directives, then the lesser of the two values is used for determining the freshness of the cached entry for that request. Sometimes a user agent might want or need to insist that a cache revalidate its cache entry with the origin server and not just with the next cache along the path to the origin server , or to reload its cache entry from the origin server.

End-to-end revalidation might be necessary if either the cache or the origin server has overestimated the expiration time of the cached response. End-to-end reload may be necessary if the cache entry has become corrupted for some reason. End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it "specific end-to-end revalidation.

The client can specify these three kinds of action using Cache- Control request directives:. The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional assigned value. Informational extensions those which do not require a change in cache behavior MAY be added without changing the semantics of other directives.

Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications which do not understand the new directive will default to the behavior specified by the standard directive, and those that understand the new directive will recognize it as modifying the requirements associated with the standard directive.


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  2. HTTP/ Header Field Definitions.
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In this way, extensions to the cache-control directives can be made without requiring changes to the base protocol. This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version, obeying certain extensions, and ignoring all directives that it does not understand. For example, consider a hypothetical new response directive called community which acts as a modifier to the private directive. We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members of the community named within its value may cache the response.

An origin server wishing to allow the UCI community to use an otherwise private response in their shared cache s could do so by including. A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since it will also see and understand the private directive and thus default to the safe behavior. The Connection general-header field allows the sender to specify options that are desired for that particular connection and MUST NOT be communicated by proxies over further connections.

Connection options are signaled by the presence of a connection-token in the Connection header field, not by any corresponding additional header field s , since the additional header field may not be sent if there are no parameters associated with that connection option. The Content-Encoding entity-header field is used as a modifier to the media-type. When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms must be applied in order to obtain the media-type referenced by the Content-Type header field.

Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type. Content codings are defined in section 3. An example of its use is. The content-coding is a characteristic of the entity identified by the Request-URI.

Excluding content

Typically, the entity-body is stored with this encoding and is only decoded before rendering or analogous usage. However, a non-transparent proxy MAY modify the content-coding if the new coding is known to be acceptable to the recipient, unless the "no-transform" cache-control directive is present in the message. If the content-coding of an entity is not "identity", then the response MUST include a Content-Encoding entity-header section If the content-coding of an entity in a request message is not acceptable to the origin server, the server SHOULD respond with a status code of Unsupported Media Type.

If multiple encodings have been applied to an entity, the content codings MUST be listed in the order in which they were applied. Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification.

The Content-Language entity-header field describes the natural language s of the intended audience for the enclosed entity. Note that this might not be equivalent to all the languages used within the entity-body. The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user's own preferred language.

Thus, if the body content is intended only for a Danish-literate audience, the appropriate field is. If no Content-Language is specified, the default is that the content is intended for all language audiences. This might mean that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended.

HTTP/ Header Field Definitions

Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi," presented simultaneously in the original Maori and English versions, would call for. However, just because multiple languages are present within an entity does not mean that it is intended for multiple linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin," which is clearly intended to be used by an English-literate audience. In this case, the Content-Language would properly only include "en".

Content-Language MAY be applied to any media type -- it is not limited to textual documents. Applications SHOULD use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in section 4. Any Content-Length greater than or equal to zero is a valid value.

The Content-Location entity-header field MAY be used to supply the resource location for the entity enclosed in the message when that entity is accessible from a location separate from the requested resource's URI. A server SHOULD provide a Content-Location for the variant corresponding to the response entity; especially in the case where a resource has multiple entities associated with it, and those entities actually have separate locations by which they might be individually accessed, the server SHOULD provide a Content-Location for the particular variant which is returned.

The Content-Location value is not a replacement for the original requested URI; it is only a statement of the location of the resource corresponding to this particular entity at the time of the request. However, the Content- Location can be used to differentiate between multiple entities retrieved from a single requested resource, as described in section The Content-MD5 header field MAY be generated by an origin server or client to function as an integrity check of the entity-body.

Any recipient of the entity- body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received. The MD5 digest is computed based on the content of the entity-body, including any content-coding that has been applied, but not including any transfer-encoding applied to the message-body.

If the message is received with a transfer-encoding, that encoding MUST be removed prior to checking the Content-MD5 value against the received entity. This has the result that the digest is computed on the octets of the entity-body exactly as, and in the order that, they would be sent if no transfer-encoding were being applied. There are several consequences of this.

If a body-part has a Content-Transfer- Encoding or Content-Encoding header, it is assumed that the content of the body-part has had the encoding applied, and the body-part is included in the Content-MD5 digest as is -- i. The Transfer-Encoding header field is not allowed within body-parts.


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  • The Content-Range entity-header is sent with a partial entity-body to specify where in the full entity-body the partial body should be applied. Refer to each search engine's documentation for details. Also, all URLs in a Sitemap must be from a single host, such as www. For further details, refer the Sitemap file location. The following example shows a Sitemap that contains just one URL and uses all optional tags. The optional tags are in italics.

    Encapsulates the file and references the current protocol standard. Parent tag for each URL entry. The remaining tags are children of this tag. URL of the page. This URL must begin with the protocol such as http and end with a trailing slash, if your web server requires it. This value must be less than 2, characters. The date of last modification of the file. This date should be in W3C Datetime format. Note that this tag is separate from the If-Modified-Since header the server can return, and search engines may use the information from both sources differently.

    How frequently the page is likely to change. This value provides general information to search engines and may not correlate exactly to how often they crawl the page. The value "always" should be used to describe documents that change each time they are accessed. The value "never" should be used to describe archived URLs. Please note that the value of this tag is considered a hint and not a command.

    Even though search engine crawlers may consider this information when making decisions, they may crawl pages marked "hourly" less frequently than that, and they may crawl pages marked "yearly" more frequently than that. Crawlers may periodically crawl pages marked "never" so that they can handle unexpected changes to those pages. Valid values range from 0. This value does not affect how your pages are compared to pages on other sites—it only lets the search engines know which pages you deem most important for the crawlers.

    Please note that the priority you assign to a page is not likely to influence the position of your URLs in a search engine's result pages. Search engines may use this information when selecting between URLs on the same site, so you can use this tag to increase the likelihood that your most important pages are present in a search index. Also, please note that assigning a high priority to all of the URLs on your site is not likely to help you.

    Since the priority is relative, it is only used to select between URLs on your site. Your Sitemap file must be UTF-8 encoded you can generally do this when you save the file. As with all XML files, any data values including URLs must use entity escape codes for the characters listed in the table below. However, if you are using any sort of script, tool, or log file to generate your URLs anything except typing them in by hand , this is usually already done for you.

    The following example shows a Sitemap in XML format. The Sitemap in the example contains a small number of URLs, each using a different set of optional parameters. You can provide multiple Sitemap files, but each Sitemap file that you provide must have no more than 50, URLs and must be no larger than 50MB 52,, bytes. If you would like, you may compress your Sitemap files using gzip to reduce your bandwidth requirement; however the sitemap file once uncompressed must be no larger than 50MB.

    If you want to list more than 50, URLs, you must create multiple Sitemap files. If you do provide multiple Sitemaps, you should then list each Sitemap file in a Sitemap index file. Sitemap index files may not list more than 50, Sitemaps and must be no larger than 50MB 52,, bytes and can be compressed. You can have more than one Sitemap index file. A Sitemap index file can only specify Sitemaps that are found on the same site as the Sitemap index file.

    Identifies the time that the corresponding Sitemap file was modified. It does not correspond to the time that any of the pages listed in that Sitemap were changed. The value for the lastmod tag should be in W3C Datetime format. By providing the last modification timestamp, you enable search engine crawlers to retrieve only a subset of the Sitemaps in the index i.

    This incremental Sitemap fetching mechanism allows for the rapid discovery of new URLs on very large sites. The Sitemap protocol enables you to provide details about your pages to search engines, and we encourage its use since you can provide additional information about site pages beyond just the URLs.

    In practice, the date can be generated at any time during the message origination without affecting its semantic value. Some origin server implementations might not have a clock available. An origin server without a clock MUST NOT assign Expires or Last- Modified values to a response, unless these values were associated with the resource by a system or user with a reliable clock. It MAY assign an Expires value that is known, at or before server configuration time, to be in the past this allows "pre-expiration" of responses without storing separate Expires values for each resource.

    The ETag response-header field provides the current value of the entity tag for the requested variant. The headers used with entity tags are described in sections The entity tag MAY be used for comparison with other entities from the same resource see section The Expect request-header field is used to indicate that particular server behaviors are required by the client.

    A server that does not understand or is unable to comply with any of the expectation values in the Expect field of a request MUST respond with appropriate error status. The server MUST respond with a Expectation Failed status if any of the expectations cannot be met or, if there are other problems with the request, some other 4xx status. This header field is defined with extensible syntax to allow for future extensions.

    If a server receives a request containing an Expect field that includes an expectation-extension that it does not support, it MUST respond with a Expectation Failed status. Comparison of expectation values is case-insensitive for unquoted tokens including the continue token , and is case-sensitive for quoted-string expectation-extensions. The Expect mechanism is hop-by-hop: However, the Expect request-header itself is end-to-end; it MUST be forwarded if the request is forwarded.

    A stale cache entry may not normally be returned by a cache either a proxy cache or a user agent cache unless it is first validated with the origin server or with an intermediate cache that has a fresh copy of the entity. The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time. The format is an absolute date and time as defined by HTTP-date in section 3.

    To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header value. See the rules for expiration calculations in section To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time the response is sent.

    The presence of an Expires header field with a date value of some time in the future on a response that otherwise would by default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field section This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests.

    The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed. In particular, robot agents SHOULD include this header so that the person responsible for running the robot can be contacted if problems occur on the receiving end. The Internet e-mail address in this field MAY be separate from the Internet host which issued the request.

    There was a problem providing the content you requested

    It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any time prior to a request. A "host" without any trailing port information implies the default port for the service requested e. The If-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that one of those entities is current by including a list of their associated entity tags in the If-Match header field.

    Entity tags are defined in section 3. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. A server MUST use the strong comparison function see section This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it.

    The meaning of "If-Match: A request intended to update a resource e. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.

    The If-Modified-Since request-header field is used with a method to make it conditional: A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases:. The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.

    The If-None-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that none of those entities is current by including a list of their associated entity tags in the If-None-Match header field. It is also used to prevent a method e. PUT from inadvertently modifying an existing resource when the client believes that the resource does not exist. The meaning of "If-None-Match: This feature is intended to be useful in preventing races between PUT operations.

    The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification. If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request-header with a conditional GET using either or both of If-Unmodified-Since and If-Match.

    However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body. The If-Range header allows a client to "short-circuit" the second request. The server can distinguish between a valid HTTP-date and any form of entity-tag by examining no more than two characters.

    If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server SHOULD provide the specified sub-range of the entity using a Partial content response. The If-Unmodified-Since request-header field is used with a method to make it conditional. If the requested resource has not been modified since the time specified in this field, the server SHOULD perform the requested operation as if the If-Unmodified-Since header were not present.

    If the request normally i. The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification. The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified. The exact meaning of this header field depends on the implementation of the origin server and the nature of the original resource.

    For files, it may be just the file system last-modified time. For entities with dynamically included parts, it may be the most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update time stamp of the record. For virtual objects, it may be the last time the internal state changed. In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace that date with the message origination date.

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    This allows a recipient to make an accurate assessment of the entity's modification time, especially if the entity changes near the time that the response is generated. The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource.

    For Created responses, the Location is that of the new resource which was created by the request. The field value consists of a single absolute URI. This can be useful when the client is attempting to trace a request chain which appears to be failing or looping in mid-chain. The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded.

    If the received Max-Forwards value is greater than zero, then the forwarded message MUST contain an updated Max-Forwards field with a value decremented by one 1. The Max-Forwards header field MAY be ignored for all other methods defined by this specification and for any extension methods for which it is not explicitly referred to as part of that method definition. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives.

    When the no-cache directive is present in a request message, an application SHOULD forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive has the same semantics as the no-cache cache-directive see section It is not possible to specify a pragma for a specific recipient; however, any pragma directive not relevant to a recipient SHOULD be ignored by that recipient. The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the proxy for this Request-URI.

    Basic and Digest Access Authentication" [43]. However, an intermediate proxy might need to obtain its own credentials by requesting them from the downstream client, which in some circumstances will appear as if the proxy is forwarding the Proxy-Authenticate header field. The Proxy-Authorization request-header field allows the client to identify itself or its user to a proxy which requires authentication. Unlike Authorization, the Proxy-Authorization header field applies only to the next outbound proxy that demanded authentication using the Proxy- Authenticate field. When multiple proxies are used in a chain, the.

    Proxy-Authorization header field is consumed by the first outbound proxy that was expecting to receive credentials. A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request.

    However, not all clients and servers need to support byte- range operations. Byte range specifications in HTTP apply to the sequence of bytes in the entity-body not necessarily the same as the message-body. A byte range operation MAY specify a single range of bytes, or a set of ranges within a single entity. The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero. If the last-byte-pos value is present, it MUST be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte- range-spec is syntactically invalid.

    The recipient of a byte-range- set that includes one or more syntactically invalid byte-range-spec values MUST ignore the header field that includes that byte-range- set. If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the entity-body, last-byte-pos is taken to be equal to one less than the current length of the entity- body in bytes.

    By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the entity.

    A suffix-byte-range-spec is used to specify the suffix of the entity-body, of a length given by the suffix-length value. That is, this form specifies the last N bytes of an entity-body. If the entity is shorter than the specified suffix-length, the entire entity-body is used. If a syntactically valid byte-range-set includes at least one byte- range-spec whose first-byte-pos is less than the current length of the entity-body, or at least one suffix-byte-range-spec with a non- zero suffix-length, then the byte-range-set is satisfiable. Otherwise, the byte-range-set is unsatisfiable. HTTP retrieval requests using conditional or unconditional GET methods MAY request one or more sub-ranges of the entity, instead of the entire entity, using the Range request header, which applies to the entity returned as the result of the request:.

    A server MAY ignore the Range header.