datavow.com - Documents

1) Introduction
- 1) Purpose
- 2) Requirements
- 3) Terminology
- 4) Overall Operation
2) Notational Conventions and Generic Grammar
- 1) Augmented BNF
- 2) Basic Rules
3) Protocol Parameters
- 1) HTTP Version
- 2) Uniform Resource Identifiers
- 3) Date/Time Formats
  - 1) Full Date
  - 2) Delta Seconds
- 4) Character Sets
  - 1) Missing Charset
- 5) Content Codings
- 6) Transfer Codings
  - 1) Chunked Transfer Coding
- 7) Media Types
  - 1) Canonicalization and Text Defaults
  - 2) Multipart Types
- 8) Product Tokens
- 9) Quality Values
- 10) Language Tags
- 11) Entity Tags
- 12) Range Units
4) HTTP Message
- 1) Message Types
- 2) Message Headers
- 3) Message Body
- 4) Message Length
- 5) General Header Fields
5) Request
- 1) Request-Line
  - 1) Method
  - 2) Request-URI
- 2) The Resource Identified by a Request
- 3) Request Header Fields
6) Response
- 1) Status-Line
  - 1) Status Code and Reason Phrase
- 2) Response Header Fields
7) Entity
- 1) Entity Header Fields
- 2) Entity Body
  - 1) Type
  - 2) Entity Length
8) Connections
- 1) Persistent Connections
- 2) Message Transmission Requirements
9) Method Definitions
- 1) Safe and Idempotent Methods
  - 1) Safe Methods
  - 2) Idempotent Methods
- 2) OPTIONS
- 3) GET
- 4) HEAD
- 5) POST
- 6) PUT
- 7) DELETE
- 8) TRACE
- 9) CONNECT
10) Status Code Definitions
- 1) Informational 1xx
  - 1) Continue
  - 2) Switching Protocols
- 2) Successful 2xx
- 3) Redirection 3xx
- 4) Client Error 4xx
- 5) Server Error 5xx
11) Access Authentication
12) Content Negotiation
- 1) Server-driven Negotiation
- 2) Agent-driven Negotiation
- 3) Transparent Negotiation
13) Caching in HTTP
- 1) ..
- 2) Expiration Model
- 3) Validation Model
- 4) Response Cacheability
- 5) Constructing Responses From Caches
- 6) Caching Negotiated Responses
- 7) Shared and Non-Shared Caches
- 8) Errors or Incomplete Response Cache Behavior
- 9) Side Effects of GET and HEAD
- 10) Invalidation After Updates or Deletions
- 11) Write-Through Mandatory
- 12) Cache Replacement
- 13) History Lists
14) Header Field Definitions
- 1) Accept
- 2) Accept-Charset
- 3) Accept-Encoding
- 4) Accept-Language
- 5) Accept-Ranges
- 6) Age
- 7) Allow
- 8) Authorization
- 9) Cache-Control
  - 1) What is Cacheable
  - 2) What May be Stored by Caches
  - 3) Modifications of the Basic Expiration Mechanism
  - 4) Cache Revalidation and Reload Controls
  - 5) No-Transform Directive
  - 6) Cache Control Extensions
- 10) Connection
- 11) Content-Encoding
- 12) Content-Language
- 13) Content-Length
- 14) Content-Location
- 15) Content-MD5
- 16) Content-Range
- 17) Content-Type
- 18) Date
  - 1) Clockless Origin Server Operation
- 19) ETag
- 20) Expect
- 21) Expires
- 22) From
- 23) Host
- 24) If-Match
- 25) If-Modified-Since
- 26) If-None-Match
- 27) If-Range
- 28) If-Unmodified-Since
- 29) Last-Modified
- 30) Location
- 31) Max-Forwards
- 32) Pragma
- 33) Proxy-Authenticate
- 34) Proxy-Authorization
- 35) Range
  - 1) Byte Ranges
  - 2) Range Retrieval Requests
- 36) Referer
- 37) Retry-After
- 38) Server
- 39) TE
- 40) Trailer
- 41) Transfer-Encoding
- 42) Upgrade
- 43) User-Agent
- 44) Vary
- 45) Via
- 46) Warning
- 47) WWW-Authenticate
15) Security Considerations
- 1) Personal Information
- 2) Attacks Based On File and Path Names
- 3) DNS Spoofing
- 4) Location Headers and Spoofing
- 5) Content-Disposition Issues
- 6) Authentication Credentials and Idle Clients
- 7) Proxies and Caching
  - 1) Denial of Service Attacks on Proxies
16) Acknowledgments
17) References
18) Authors' Addresses
19) Appendices
- 1) Internet Media Type message/http and application/http
- 2) Internet Media Type multipart/byteranges
- 3) Tolerant Applications
- 4) Differences Between HTTP Entities and RFC 2045 Entities
- 5) Additional Features
  - 1) Content-Disposition
- 6) Compatibility with Previous Versions
20) Index
21) Full Copyright Statement
22) Acknowledgement

14.9.4 Cache Revalidation and Reload Controls

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:

End-to-end reload

The request includes a "no-cache" cache-control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field names MUST NOT be included with the no-cache directive in a request. The server MUST NOT use a cached copy when responding to such a request.

Specific end-to-end revalidation

The request includes a "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request includes a cache-validating conditional with the client's current validator.

Unspecified end-to-end revalidation

The request includes "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional; the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional with its current validator.

max-age

When an intermediate cache is forced, by means of a max-age=0 directive, to revalidate its own cache entry, and the client has supplied its own validator in the request, the supplied validator might differ from the validator currently stored with the cache entry. In this case, the cache MAY use either validator in making its own request without affecting semantic transparency.

However, the choice of validator might affect performance. The best approach is for the intermediate cache to use its own validator when making its request. If the server replies with 304 (Not Modified), then the cache can return its now validated copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate cache can compare the returned validator with the one provided in the client's request, using the strong comparison function. If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified). Otherwise, it returns the new entity with a 200 (OK) response.

If a request includes the no-cache directive, it SHOULD NOT include min-fresh, max-stale, or max-age.

only-if-cached

In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses that it currently has stored, and not to reload or revalidate with the origin server. To do this, the client may include the only-if-cached directive in a request. If it receives this directive, a cache SHOULD either respond using a cached entry that is consistent with the other constraints of the request, or respond with a 504 (Gateway Timeout) status. However, if a group of caches is being operated as a unified system with good internal connectivity, such a request MAY be forwarded within that group of caches.

must-revalidate

Because a cache MAY be configured to ignore a server's specified expiration time, and because a client request MAY include a max- stale directive (which has a similar effect), the protocol also includes a mechanism for the origin server to require revalidation of a cache entry on any subsequent use. When the must-revalidate directive is present in a response received by a cache, that cache MUST NOT use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server. (I.e., the cache MUST do an end-to-end revalidation every time, if, based solely on the origin server's Expires or max-age value, the cached response is stale.)

The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey the must-revalidate directive; in particular, if the cache cannot reach the origin server for any reason, it MUST generate a 504 (Gateway Timeout) response.

Servers SHOULD send the must-revalidate directive if and only if failure to revalidate a request on the entity could result in incorrect operation, such as a silently unexecuted financial transaction. Recipients MUST NOT take any automated action that violates this directive, and MUST NOT automatically provide an unvalidated copy of the entity if revalidation fails.

Although this is not recommended, user agents operating under severe connectivity constraints MAY violate this directive but, if so, MUST explicitly warn the user that an unvalidated response has been provided. The warning MUST be provided on each unvalidated access, and SHOULD require explicit user confirmation.

proxy-revalidate

The 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 request to permit the user's cache to store and later return the response without needing to revalidate it (since it has already been authenticated once by that user), while still requiring proxies that service many users to revalidate each time (in order to make sure that each user has been authenticated). Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at all.