HTTP/1.1, part 2: Message Semantics

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

An implementation is not compliant if it fails to satisfy one or more of the MUST or REQUIRED level requirements for the protocols it implements. An implementation that satisfies all the MUST or REQUIRED level and all the SHOULD level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST level requirements but not all the SHOULD level requirements for its protocols is said to be "conditionally compliant."¶

The HTTP Method Registry defines the name space for the Method token in the Request line of an HTTP request.¶

Registrations MUST include the following fields: ¶

• Method Name (see Section 3)
• Safe ("yes" or "no", see Section 8.1.1)
• Pointer to specification text

Values to be added to this name space are subject to IETF review ([RFC5226], Section 4.1). Any document registering new method names should be traceable through statuses of either 'Obsoletes' or 'Updates' to this document.¶

The registry itself is maintained at <http://www.iana.org/assignments/http-methods>.¶

The HTTP Status Code Registry defines the name space for the Status-Code token in the Status line of an HTTP response.¶

Values to be added to this name space are subject to IETF review ([RFC5226], Section 4.1). Any document registering new status codes should be traceable through statuses of either 'Obsoletes' or 'Updates' to this document.¶

The registry itself is maintained at <http://www.iana.org/assignments/http-status-codes>.¶

The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI. This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.¶

Responses to this method are not cacheable.¶

If the OPTIONS request includes an entity-body (as indicated by the presence of Content-Length or Transfer-Encoding), then the media type MUST be indicated by a Content-Type field. Although this specification does not define any use for such a body, future extensions to HTTP might use the OPTIONS body to make more detailed queries on the server.¶

If the Request-URI is an asterisk ("*"), the OPTIONS request is intended to apply to the server in general rather than to a specific resource. Since a server's communication options typically depend on the resource, the "*" request is only useful as a "ping" or "no-op" type of method; it does nothing beyond allowing the client to test the capabilities of the server. For example, this can be used to test a proxy for HTTP/1.1 compliance (or lack thereof).¶

If the Request-URI is not an asterisk, the OPTIONS request applies only to the options that are available when communicating with that resource.¶

A 200 response SHOULD include any header fields that indicate optional features implemented by the server and applicable to that resource (e.g., Allow), possibly including extensions not defined by this specification. The response body, if any, SHOULD also include information about the communication options. The format for such a body is not defined by this specification, but might be defined by future extensions to HTTP. Content negotiation MAY be used to select the appropriate response format. If no response body is included, the response MUST include a Content-Length field with a field-value of "0".¶

The Max-Forwards request-header field MAY be used to target a specific proxy in the request chain. When a proxy receives an OPTIONS request on an absoluteURI for which request forwarding is permitted, the proxy MUST check for a Max-Forwards field. If the Max-Forwards field-value is zero ("0"), the proxy MUST NOT forward the message; instead, the proxy SHOULD respond with its own communication options. If the Max-Forwards field-value is an integer greater than zero, the proxy MUST decrement the field-value when it forwards the request. If no Max-Forwards field is present in the request, then the forwarded request MUST NOT include a Max-Forwards field.¶

The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process.¶

The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.¶

The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred, as described in Section 6.4 of [Part5]. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client.¶

The response to a GET request is cacheable if and only if it meets the requirements for HTTP caching described in [Part6].¶

See Section 12.2 for security considerations when used for forms.¶

The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification.¶

The response to a HEAD request MAY be cacheable in the sense that the information contained in the response MAY be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the cache MUST treat the cache entry as stale.¶

The POST method is used to request that the origin server accept the entity enclosed in the request as data to be processed by the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions: ¶

• Annotation of existing resources;
• Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;
• Providing a block of data, such as the result of submitting a form, to a data-handling process;
• Extending a database through an append operation.

The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI.¶

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.¶

If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an entity which describes the status of the request and refers to the new resource, and a Location header (see Section 10.4).¶

Responses to this method are not cacheable, unless the response includes appropriate Cache-Control or Expires header fields. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cacheable resource.¶

The PUT method requests that the enclosed entity be stored at the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the 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 new resource is created at the Request-URI, the origin server MUST inform the user agent via the 201 (Created) response. If an existing resource is modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent to indicate successful completion of the request. If the resource could not be created or modified with the Request-URI, an appropriate error response SHOULD be given that reflects the nature of the problem. The recipient of the entity MUST NOT ignore any Content-* (e.g. Content-Range) headers that it does not understand or implement and MUST return a 501 (Not Implemented) response in such cases.¶

If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.¶

The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource might be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the 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.¶

A single resource MAY be identified by many different URIs. For example, an article might have a URI for identifying "the current version" which is separate from the URI identifying each particular version. In this case, a PUT request on a general URI might result in several other URIs being defined by the origin server.¶

HTTP/1.1 does not define how a PUT method affects the state of an origin server.¶

Unless otherwise specified for a particular entity-header, the entity-headers in the PUT request SHOULD be applied to the resource created or modified by the PUT.¶

The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD NOT indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.¶

A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the action has been enacted but the response does not include an entity.¶

If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.¶

The TRACE method is used to invoke a remote, application-layer loop-back of the request message. The final recipient of the request SHOULD reflect the message received back to the client as the entity-body of a 200 (OK) response. The final recipient is either the origin server or the first proxy or gateway to receive a Max-Forwards value of zero (0) in the request (see Section 10.5). A TRACE request MUST NOT include an entity.¶

TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (Section 8.9 of [Part1]) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.¶

If the request is valid, the response SHOULD contain the entire request message in the entity-body, with a Content-Type of "message/http" (see Section 9.3.1 of [Part1]). Responses to this method MUST NOT be cached.¶

This specification reserves the method name CONNECT for use with a proxy that can dynamically switch to being a tunnel (e.g. SSL tunneling [RFC2817]).¶

This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. There are no required headers for this class of status code. Since HTTP/1.0 did not define any 1xx status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client except under experimental conditions.¶

A client MUST be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a 100 (Continue) status message. Unexpected 1xx status responses MAY be ignored by a user agent.¶

Proxies MUST forward 1xx responses, unless the connection between the proxy and its client has been closed, or unless the proxy itself requested the generation of the 1xx response. (For example, if a proxy adds a "Expect: 100-continue" field when it forwards a request, then it need not forward the corresponding 100 (Continue) response(s).)¶

This class of status code indicates that the client's request was successfully received, understood, and accepted.¶

This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. The action required MAY be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A client SHOULD detect infinite redirection loops, since such loops generate network traffic for each redirection. ¶

• Note: previous versions of this specification recommended a maximum of five redirections. Content developers should be aware that there might be clients that implement such a fixed limitation.

The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents SHOULD display any included entity to the user.¶

If the client is sending data, a server implementation using TCP SHOULD be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.¶

Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has erred or is incapable of performing the request. Except when responding to a HEAD request, the server SHOULD include an 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 are applicable to any request method.¶

The Allow response-header field lists the set of methods advertised as supported by the resource identified by the Request-URI. The purpose of this field is strictly to inform the recipient of valid methods associated with the resource. An Allow header field MUST be present in a 405 (Method Not Allowed) response.¶

The actual set of allowed methods is defined by the origin server at the time of each request.¶

A proxy MUST NOT modify the Allow header field even if it does not understand all the methods specified, since the user agent might have other means of communicating with the origin server.¶

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 417 (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 417 (Expectation Failed) status.¶

Comparison of expectation values is case-insensitive for unquoted tokens (including the 100-continue token), and is case-sensitive for quoted-string expectation-extensions.¶

The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST return a 417 (Expectation Failed) status if it receives a request with an expectation that it cannot meet. However, the Expect request-header itself is end-to-end; it MUST be forwarded if the request is forwarded.¶

Many older HTTP/1.0 and HTTP/1.1 applications do not understand the Expect header.¶

See Section 7.2.3 of [Part1] for the use of the 100 (Continue) status.¶

This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. It SHOULD NOT be used as an insecure form of access protection. 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. For example, when a request is passed through a proxy the original issuer's address SHOULD be used.¶

The client SHOULD NOT send the From header field without the user's approval, as it might conflict with the user's privacy interests or their site's security policy. 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.¶

The Location response-header field is used for the identification of a new resource or to redirect the recipient to a location other than the Request-URI for completion of the request. For 201 (Created) responses, the Location is that of the new resource which was created by the request. For 3xx responses, the location SHOULD indicate the server's preferred URI for automatic redirection to the resource. The field value consists of a single absolute URI.¶

• Note: The Content-Location header field (Section 6.7 of [Part3]) differs from Location in that the Content-Location identifies the original location of the entity enclosed in the response. It is therefore possible for a response to contain header fields for both Location and Content-Location.

There are circumstances in which a fragment identifier in a Location URL would not be appropriate: ¶

• With a 201 Created response, because in this usage the Location header specifies the URL for the entire created resource.
• With a 300 Multiple Choices, since the choice decision is intended to be made on resource characteristics and not fragment characteristics.
• With 305 Use Proxy.

The Max-Forwards request-header field provides a mechanism with the TRACE (Section 8.8) and OPTIONS (Section 8.2) methods to limit the number of proxies or gateways that can forward the request to the next inbound server. 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.¶

Each proxy or gateway recipient of a TRACE or OPTIONS request containing a Max-Forwards header field MUST check and update its value prior to forwarding the request. If the received value is zero (0), the recipient MUST NOT forward the request; instead, it MUST respond as the final recipient. 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.¶

The Referer[sic] request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained (the "referrer", although the header field is misspelled.) The Referer request-header allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. The Referer field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard.¶

If the field value is a relative URI, it SHOULD be interpreted relative to the Request-URI. The URI MUST NOT include a fragment. See Section 12.2 for security considerations.¶

The Retry-After response-header field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client. This field MAY also be used with any 3xx (Redirection) response to indicate the minimum time the user-agent is asked wait before issuing the redirected request. The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response.¶

Time spans are non-negative decimal integers, representing time in seconds.¶

Two examples of its use are¶

In the latter example, the delay is 2 minutes.¶

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 3.5 of [Part1]) and comments identifying the server and any significant subproducts. The product tokens are listed in order of their significance for identifying the application.¶

If the response is being forwarded through a proxy, the proxy application MUST NOT modify the Server response-header. Instead, it MUST include a Via field (as described in Section 8.9 of [Part1]). ¶

• Note: Revealing the specific software version of the server might allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Server implementors are encouraged to make this field a configurable option.

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 3.5 of [Part1]) 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.¶

The registration procedure for HTTP Methods is defined by Section 3.1 of this document.¶

The registration procedure for HTTP Status Codes -- previously defined in Section 7.1 of [RFC2817] -- is now defined by Section 5.1 of this document.¶

The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".¶

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.¶

Revealing the specific software version of the server might allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Implementors 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 header 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 header might 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.¶

The User-Agent (Section 10.9) or Server (Section 10.8) header fields can sometimes be used to determine that a specific client or server have a particular security hole which might be exploited. Unfortunately, this same information is often used for other valuable purposes for which HTTP currently has no better mechanism.¶

Because the source of a link might be private information or might reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not the Referer field is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending of Referer and From information.¶

Clients SHOULD NOT include a Referer header field in a (non-secure) HTTP request if the referring page was transferred with a secure protocol.¶

Authors of services should not use GET-based forms for the submission of sensitive data because that data will be encoded in the Request-URI. Many existing servers, proxies, and user agents log or display the Request-URI in places where it might be visible to third parties. Such services can use POST-based form submission instead.¶

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.¶

14.1. Normative References

[Part1]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-04 (work in progress), August 2008.

[Part3]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 3: Message Payload and Content Negotiation”, Internet-Draft draft-ietf-httpbis-p3-payload-04 (work in progress), August 2008.

[Part4]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 4: Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-04 (work in progress), August 2008.

[Part5]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests and Partial Responses”, Internet-Draft draft-ietf-httpbis-p5-range-04 (work in progress), August 2008.

[Part6]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-04 (work in progress), August 2008.

[Part7]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 7: Authentication”, Internet-Draft draft-ietf-httpbis-p7-auth-04 (work in progress), August 2008.

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

14.2. Informative References

[RFC1945]

Berners-Lee, T., Fielding, R., and H. Nielsen, “Hypertext Transfer Protocol -- HTTP/1.0”, RFC 1945, May 1996.

[RFC2068]

Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.

[RFC2616]

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.

[RFC2817]

Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1”, RFC 2817, May 2000.

[RFC2822]

Resnick, P., “Internet Message Format”, RFC 2822, April 2001.

[RFC3864]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

[RFC5226]

Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

Clarified which error code should be used for inbound server failures (e.g. DNS failures). (Section 9.5.5).¶

201 (Created) had a race that required an Etag be sent when a resource is first created. (Section 9.2.2).¶

Rewrite of message transmission requirements to make it much harder for implementors to get it wrong, as the consequences of errors here can have significant impact on the Internet, and to deal with the following problems: ¶

1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where this was incorrectly placing a requirement on the behavior of an implementation of a future version of HTTP/1.x
2. Made it clear that user-agents should retry requests, not "clients" in general.
3. Converted requirements for clients to ignore unexpected 100 (Continue) responses, and for proxies to forward 100 responses, into a general requirement for 1xx responses.
4. Modified some TCP-specific language, to make it clearer that non-TCP transports are possible for HTTP.
5. Require that the origin server MUST NOT wait for the request body before it sends a required 100 (Continue) response.
6. Allow, rather than require, a server to omit 100 (Continue) if it has already seen some of the request body.
7. Allow servers to defend against denial-of-service attacks and broken clients.

This change adds the Expect header and 417 status code.¶

Clean up confusion between 403 and 404 responses. (Section 9.4.4, 9.4.5, and 9.4.11)¶

The PATCH, LINK, UNLINK methods were defined but not commonly implemented in previous versions of this specification. See Section 19.6.1 of [RFC2068].¶

This document takes over the Status Code Registry, previously defined in Section 7.1 of [RFC2817]. (Section 5.1)¶

Clarify definition of POST. (Section 8.5)¶

Failed to consider that there are many other request methods that are safe to automatically redirect, and further that the user agent is able to make that determination based on the request method semantics. (Sections 9.3.2, 9.3.3 and 9.3.8)¶

Deprecate 305 Use Proxy status code, because user agents did not implement it. It used to indicate that the requested resource must be accessed through the proxy given by the Location field. The Location field gave the URI of the proxy. The recipient was expected to repeat this single request via the proxy. (Section 9.3.6)¶

Reclassify Allow header as response header, removing the option to specify it in a PUT request. Relax the server requirement on the contents of the Allow header and remove requirement on clients to always trust the header value. (Section 10.1)¶

Correct syntax of Location header to allow fragment, as referred symbol wasn't what was expected, and add some clarifications as to when it would not be appropriate. (Section 10.4)¶

In the description of the Server header, the Via field was described as a SHOULD. The requirement was and is stated correctly in the description of the Via header in Section 8.9 of [Part1]. (Section 10.8)¶

Extracted relevant partitions from [RFC2616].¶

Closed issues: ¶

• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/5>: "Via is a MUST" (<http://purl.org/NET/http-errata#via-must>)
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/6>: "Fragments allowed in Location" (<http://purl.org/NET/http-errata#location-fragments>)
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods vs Redirection" (<http://purl.org/NET/http-errata#saferedirect>)
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/17>: "Revise description of the POST method" (<http://purl.org/NET/http-errata#post>)
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and Informative references"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/42>: "RFC2606 Compliance"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative references"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/84>: "Redundant cross-references"

Other changes: ¶

• Move definitions of 304 and 412 condition codes to [Part4]

Closed issues: ¶

• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/21>: "PUT side effects"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/91>: "Duplicate Host header requirements"

Ongoing work on ABNF conversion (<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Move "Product Tokens" section (back) into Part 1, as "token" is used in the definition of the Upgrade header.
• Add explicit references to BNF syntax and rules imported from other parts of the specification.
• Copy definition of delta-seconds from Part6 instead of referencing it.

Closed issues: ¶

• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/24>: "Requiring Allow in 405 responses"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/59>: "Status Code Registry"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/61>: "Redirection vs. Location"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/70>: "Cacheability of 303 response"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/76>: "305 Use Proxy"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/105>: "Classification for Allow header"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store under' vs 'store at'"

Ongoing work on IANA Message Header Registration (<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40>): ¶

• Reference RFC 3984, and update header registrations for headers defined in this document.

Ongoing work on ABNF conversion (<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Replace string literals when the string really is case-sensitive (method).

Closed issues: ¶

• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/98>: "OPTIONS request bodies"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/119>: "Description of CONNECT should refer to RFC2817"
• <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/125>: "Location Content-Location reference request/response mixup"

Ongoing work on Method Registry (<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/72>): ¶

• Added initial proposal for registration process, plus initial content (non-HTTP/1.1 methods to be added by a separate specification).