10.5 Server Error 5xx
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.
10.5.1 500 Internal Server Error
The server encountered an unexpected condition which prevented it
from fulfilling the request.
10.5.2 501 Not Implemented
The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for
10.5.3 502 Bad Gateway
The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to
fulfill the request.
10.5.4 503 Service Unavailable
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated after
some delay. If known, the length of the delay MAY be indicated in a
Retry-After header. If no Retry-After is given, the client SHOULD
handle the response as it would for a 500 response.
Note: The existence of the 503 status code does not imply that a
server must use it when becoming overloaded. Some servers may wish
to simply refuse the connection.
10.5.5 504 Gateway Timeout
The server, while acting as a gateway or proxy, did not receive a
timely response from the upstream server specified by the URI (e.g.
HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
to access in attempting to complete the request.
Note: Note to implementors: some deployed proxies are known to
return 400 or 500 when DNS lookups time out.
10.5.6 505 HTTP Version Not Supported
The server does not support, or refuses to support, the HTTP protocol
version that was used in the request message. The server is
indicating that it is unable or unwilling to complete the request
using the same major version as the client, as described in section
3.1, other than with this error message. The response SHOULD contain
an entity describing why that version is not supported and what other
protocols are supported by that server.
11 Access Authentication
HTTP provides several OPTIONAL challenge-response authentication
mechanisms which can be used by a server to challenge a client
request and by a client to provide authentication information. The
general framework for access authentication, and the specification of
"basic" and "digest" authentication, are specified in "HTTP
Authentication: Basic and Digest Access Authentication" . This
specification adopts the definitions of "challenge" and "credentials"
from that specification.
12 Content Negotiation
Most HTTP responses include an entity which contains information for
interpretation by a human user. Naturally, it is desirable to supply
the user with the "best available" entity corresponding to the
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
has provisions for several mechanisms for "content negotiation" --
the process of selecting the best representation for a given response
when there are multiple representations available.
Note: This is not called "format negotiation" because the
alternate representations may be of the same media type, but use
different capabilities of that type, be in different languages,
Any response containing an entity-body MAY be subject to negotiation,
including error responses.
There are two kinds of content negotiation which are possible in
HTTP: server-driven and agent-driven negotiation. These two kinds of
negotiation are orthogonal and thus may be used separately or in
combination. One method of combination, referred to as transparent
negotiation, occurs when a cache uses the agent-driven negotiation
information provided by the origin server in order to provide
server-driven negotiation for subsequent requests.
12.1 Server-driven Negotiation
If the selection of the best representation for a response is made by
an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g. language,
content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request
(such as the network address of the client).
Server-driven negotiation is advantageous when the 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
avoid the 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, Accept-Encoding, etc.) which describe its
preferences for such a response.
Server-driven negotiation has disadvantages:
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?).
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 the user's privacy.
3. It complicates the implementation of an origin server and the
algorithms for generating responses to a request.
4. It may limit a public cache's ability to use the same response
for multiple user's requests.
HTTP/1.1 includes the following request-header fields for enabling
server-driven negotiation through description of user agent
capabilities and user preferences: Accept (section 14.1), Accept-
Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
Language (section 14.4), and User-Agent (section 14.43). However, an
origin server is not limited to these dimensions and MAY vary the
response based on any aspect of the request, including information
outside the request-header fields or within extension header fields
not defined by this specification.
The Vary header field can be used to express the parameters the
server uses to select a representation that is subject to server-
driven negotiation. See section 13.6 for use of the Vary header field
by caches and section 14.44 for use of the Vary header field by
12.2 Agent-driven Negotiation
With agent-driven negotiation, selection of the best representation
for a response is performed by the user agent after receiving an
initial response from the origin server. Selection is based on a list
of the available representations of the response included within the
header fields or entity-body of the initial response, with each
representation identified by its own 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.
Agent-driven negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, 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 are used to distribute server load and reduce network usage.
Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition,
this specification does not define any mechanism for supporting
automatic selection, though it also does not prevent any such
mechanism from being developed as an extension and used within
HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
status codes for enabling agent-driven negotiation when the server is
unwilling or unable to provide a varying response using server-driven
12.3 Transparent Negotiation
Transparent negotiation is a combination of both server-driven and
agent-driven negotiation. When a cache is supplied with a form of the
list of available representations of the response (as in agent-driven
negotiation) and the dimensions of variance are completely understood
by the cache, then the cache becomes capable of performing server-
driven negotiation on behalf of the origin server for subsequent
requests on that resource.
Transparent negotiation has the advantage of distributing the
negotiation work that would otherwise be required of the origin
server and also removing the second request delay of agent-driven
negotiation when the cache is able to correctly guess the right
This specification does not define any mechanism for transparent
negotiation, though it also does not prevent any such mechanism from
being developed as an extension that could be used within HTTP/1.1.
13 Caching in HTTP
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. The
HTTP/1.1 protocol includes a number of elements intended to make
caching work as well as possible. Because these elements are
inextricable from other aspects of the protocol, and because they
interact with each other, it is useful to describe the basic caching
design of HTTP separately from the detailed descriptions of methods,
headers, response codes, etc.
Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate the need
to send requests in many cases, and to eliminate the need to send
full responses in many other cases. The former reduces the number of
network round-trips required for many operations; we use an
"expiration" mechanism for this purpose (see section 13.2). The
latter reduces network bandwidth requirements; we use a "validation"
mechanism for this purpose (see section 13.3).
Requirements for performance, availability, and disconnected
operation require us to be able to relax the goal of semantic
transparency. The HTTP/1.1 protocol allows origin servers, caches,
and clients to explicitly reduce transparency when necessary.
However, because non-transparent operation may confuse non-expert
users, and might be incompatible with certain server applications
(such as those for ordering merchandise), the protocol requires that
transparency be relaxed
- only by an explicit protocol-level request when relaxed by
client or origin server
- only with an explicit warning to the end user when relaxed by
cache or client
Therefore, the HTTP/1.1 protocol provides these important elements:
1. Protocol features that provide full semantic transparency when
this is required by all parties.
2. Protocol features that allow an origin server or user agent to
explicitly request and control non-transparent operation.
3. Protocol features that allow a cache to attach warnings to
responses that do not preserve the requested approximation of
A basic principle is that it must be possible for the clients to
detect any potential relaxation of semantic transparency.
Note: The server, cache, or client implementor might be faced with
design decisions not explicitly discussed in this specification.
If a decision might affect semantic transparency, the implementor
ought to err on the side of maintaining transparency unless a
careful and complete analysis shows significant benefits in
13.1.1 Cache Correctness
A correct cache MUST respond to a request with the most up-to-date
response held by the cache that is appropriate to the request (see
sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
1. It has been checked for equivalence with what the origin server
would have returned by revalidating the response with the
origin server (section 13.3);
2. It is "fresh enough" (see section 13.2). In the default case,
this means it meets the least restrictive freshness requirement
of the client, origin server, and cache (see section 14.9); if
the origin server so specifies, it is the freshness requirement
of the origin server alone.
If a stored response is not "fresh enough" by the most
restrictive freshness requirement of both the client and the
origin server, in carefully considered circumstances the cache
MAY still return the response with the appropriate Warning
header (see section 13.1.5 and 14.46), unless such a response
is prohibited (e.g., by a "no-store" cache-directive, or by a
"no-cache" cache-request-directive; see section 14.9).
3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
or error (4xx or 5xx) response message.
If the cache can not communicate with the origin server, then a
correct cache SHOULD respond as above if the response can be
correctly served from the cache; if not it MUST return an error or
warning indicating that there was a communication failure.
If a cache receives a response (either an entire response, or a 304
(Not Modified) response) that it would normally forward to the
requesting client, and the received response is no longer fresh, the
cache SHOULD forward it to the requesting client without adding a new
Warning (but without removing any existing Warning headers). A cache
SHOULD NOT attempt to revalidate a response simply because that
response became stale in transit; this might lead to an infinite
loop. A user agent that receives a stale response without a Warning
MAY display a warning indication to the user.
Whenever a cache returns a response that is neither first-hand nor
"fresh enough" (in the sense of condition 2 in section 13.1.1), it
MUST attach a warning to that effect, using a Warning general-header.
The Warning header and the currently defined warnings are described
in section 14.46. The warning allows clients to take appropriate
Warnings MAY be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguish these responses from true failures.
Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning MUST or MUST NOT be deleted from a
stored cache entry after a successful revalidation:
1xx Warnings that describe the freshness or revalidation status of
the response, and so MUST be deleted after a successful
revalidation. 1XX warn-codes MAY be generated by a cache only when
validating a cached entry. It MUST NOT be generated by clients.
2xx Warnings that describe some aspect of the entity body or entity
headers that is not rectified by a revalidation (for example, a
lossy compression of the entity bodies) and which MUST NOT be
deleted after a successful revalidation.
See section 14.46 for the definitions of the codes themselves.
HTTP/1.0 caches will cache all Warnings in responses, without
deleting the ones in the first category. Warnings in responses that
are passed to HTTP/1.0 caches carry an extra warning-date field,
which prevents a future HTTP/1.1 recipient from believing an
erroneously cached Warning.
Warnings also carry a warning text. The text MAY be in any
appropriate natural language (perhaps based on the client's Accept
headers), and include an OPTIONAL indication of what character set is
Multiple warnings MAY be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number. For example, a server might provide the same warning with
texts in both English and Basque.
When multiple warnings are attached to a response, it might not be
practical or reasonable to display all of them to the user. This
version of HTTP does not specify strict priority rules for deciding
which warnings to display and in what order, but does suggest some
13.1.3 Cache-control Mechanisms
The basic cache mechanisms in HTTP/1.1 (server-specified expiration
times and validators) are implicit directives to caches. In some
cases, a server or client might need to provide explicit directives
to the HTTP caches. We use the Cache-Control header for this purpose.
The Cache-Control header allows a client or server to transmit a
variety of directives in either requests or responses. These
directives typically override the default caching algorithms. As a
general rule, if there is any apparent conflict between header
values, the most restrictive interpretation is applied (that is, the
one that is most likely to preserve semantic transparency). However,
in some cases, cache-control directives are explicitly specified as
weakening the approximation of semantic transparency (for example,
"max-stale" or "public").
The cache-control directives are described in detail in section 14.9.
13.1.4 Explicit User Agent Warnings
Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent might allow the user
to 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 agent SHOULD NOT
default to either non-transparent behavior, or behavior that results
in abnormally ineffective caching, but MAY be explicitly configured
to do so by an explicit action of the user.
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 (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
dialog box; it could be an icon (for example, a picture of a rotting
fish) or some other indicator.
If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent SHOULD
continually indicate this state to the user (for example, by a
display of a picture of currency in flames) so that the user does not
inadvertently consume excess resources or suffer from excessive
13.1.5 Exceptions to the Rules and Warnings
In some cases, the operator of a cache MAY choose to configure it to
return stale responses even when not requested by clients. This
decision ought not be made lightly, but may be necessary for reasons
of availability or performance, especially when the cache is poorly
connected to the origin server. Whenever a cache returns a stale
response, it MUST mark it as such (using a Warning header) enabling
the client software to alert the user that there might be a potential
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
response if the client explicitly requests a first-hand or fresh one,
unless it is impossible to comply for technical or policy reasons.
13.1.6 Client-controlled Behavior
While the origin server (and to a lesser extent, intermediate caches,
by their contribution to the age of a response) are the primary
source of expiration information, in some cases the client might need
to control a cache's decision about whether to return a cached
response without validating it. Clients do this using several
directives of the Cache-Control header.
A client's request MAY specify the maximum age it is willing to
accept of an unvalidated response; specifying a value of zero forces
the cache(s) to revalidate all responses. A client MAY also specify
the minimum time remaining before a response expires. Both of these
options increase constraints on the behavior of caches, and so cannot
further relax the cache's approximation of semantic transparency.
A client MAY also specify that it will accept stale responses, up to
some maximum amount of staleness. This loosens the constraints on the
caches, and so might violate the origin server's specified
constraints on semantic transparency, but might be necessary to
support disconnected operation, or high availability in the face of
13.2 Expiration Model
13.2.1 Server-Specified Expiration
HTTP caching works best when caches can entirely avoid making
requests to the origin server. The primary mechanism for avoiding
requests is for an origin server to provide an explicit expiration
time in the future, indicating that a response MAY be used to satisfy
subsequent requests. In other words, a cache can return a fresh
response without first contacting the server.
Our expectation is that servers will assign future explicit
expiration times to responses in the belief that the entity is not
likely to change, in a semantically significant way, before the
expiration time is reached. This normally preserves semantic
transparency, as long as the server's expiration times are carefully
The expiration mechanism applies only to responses taken from a cache
and not to first-hand responses forwarded immediately to the
If an origin server wishes to force a semantically transparent cache
to validate every request, it MAY assign an explicit expiration time
in the past. This means that the response is always stale, and so the
cache SHOULD validate it before using it for subsequent requests. See
section 14.9.4 for a more restrictive way to force revalidation.
If an origin server wishes to force any HTTP/1.1 cache, no matter how
it is configured, to validate every request, it SHOULD use the "must-
revalidate" cache-control directive (see section 14.9).
Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control header.
An expiration time cannot be used to force a user agent to refresh
its display or reload a resource; its semantics apply only to caching
mechanisms, and such mechanisms need only check a resource's
expiration status when a new request for that resource is initiated.
See section 13.13 for an explanation of the difference between caches
and history mechanisms.
13.2.2 Heuristic Expiration
Since origin servers do not always provide explicit expiration times,
HTTP caches typically assign heuristic expiration times, employing
algorithms that use other header values (such as the Last-Modified
time) to estimate a plausible expiration time. The HTTP/1.1
specification does not provide specific algorithms, but does impose
worst-case constraints on their results. Since heuristic expiration
times might compromise semantic transparency, they ought to used
cautiously, and we encourage origin servers to provide explicit
expiration times as much as possible.
13.2.3 Age Calculations
In order to know if a cached entry is fresh, a cache needs to know if
its age exceeds its freshness lifetime. We discuss how to calculate
the latter in section 13.2.4; this section describes how to calculate
the age of a response or cache entry.
In this discussion, we use the term "now" to mean "the current value
of the clock at the host performing the calculation." Hosts that use
HTTP, but especially hosts running origin servers and caches, SHOULD
use NTP  or some similar protocol to synchronize their clocks to
a globally accurate time standard.
HTTP/1.1 requires origin servers to send a Date header, if possible,
with every response, giving the time at which the response was
generated (see section 14.18). We use the term "date_value" to denote
the value of the Date header, in a form appropriate for arithmetic
HTTP/1.1 uses the Age response-header to convey the estimated age of
the response message when obtained from a cache. The Age field value
is the cache's estimate of the amount of time since the response was
generated or revalidated by the origin server.
In essence, the Age value is the sum of the time that the response
has been resident in each of the caches along the path from the
origin server, plus the amount of time it has been in transit along
We use the term "age_value" to denote the value of the Age header, in
a form appropriate for arithmetic operations.
A response's age can be calculated in two entirely independent ways:
1. now minus date_value, if the local clock is reasonably well
synchronized to the origin server's clock. If the result is
negative, the result is replaced by zero.
2. age_value, if all of the caches along the response path
Given that we have two independent ways to compute the age of a
response when it is received, we can combine these as
corrected_received_age = max(now - date_value, age_value)
and as long as we have either nearly synchronized clocks or all-
HTTP/1.1 paths, one gets a reliable (conservative) result.
Because of network-imposed delays, some significant interval might
pass between the time that a server generates a response and the time
it is received at the next outbound cache or client. If uncorrected,
this delay could result in improperly low ages.
Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct
for delays imposed by the network by recording the time at which the
request was initiated. Then, when an Age value is received, it MUST
be interpreted relative to the time the request was initiated, not
the time that the response was received. This algorithm results in
conservative behavior no matter how much delay is experienced. So, we
corrected_initial_age = corrected_received_age
+ (now - request_time)
where "request_time" is the time (according to the local clock) when
the request that elicited this response was sent.
Summary of age calculation algorithm, when a cache receives a
* is the value of Age: header received by the cache with
* this response.
* is the value of the origin server's Date: header
* is the (local) time when the cache made the request
* that resulted in this cached response
* is the (local) time when the cache received the
* is the current (local) time
apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
The current_age of a cache entry is calculated by adding the amount
of time (in seconds) since the cache entry was last validated by the
origin server to the corrected_initial_age. When a response is
generated from a cache entry, the cache MUST include a single Age
header field in the response with a value equal to the cache entry's
The presence of an Age header field in a response implies that a
response is not first-hand. However, the converse is not true, since
the lack of an Age header field in a response does not imply that the
response is first-hand unless all caches along the request path are
compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
the Age header field).
13.2.4 Expiration Calculations
In order to decide whether a response is fresh or stale, we need to
compare its freshness lifetime to its age. The age is calculated as
described in section 13.2.3; this section describes how to calculate
the freshness lifetime, and to determine if a response has expired.
In the discussion below, the values can be represented in any form
appropriate for arithmetic operations.
We use the term "expires_value" to denote the value of the Expires
header. We use the term "max_age_value" to denote an appropriate
value of the number of seconds carried by the "max-age" directive of
the Cache-Control header in a response (see section 14.9.3).
The max-age directive takes priority over Expires, so if max-age is
present in a response, the calculation is simply:
freshness_lifetime = max_age_value
Otherwise, if Expires is present in the response, the calculation is:
freshness_lifetime = expires_value - date_value
Note that neither of these calculations is vulnerable to clock skew,
since all of the information comes from the origin server.
If none of Expires, Cache-Control: max-age, or Cache-Control: s-
maxage (see section 14.9.3) appears in the response, and the response
does not include other restrictions on caching, the cache MAY compute
a freshness lifetime using a heuristic. The cache MUST attach Warning
113 to any response whose age is more than 24 hours if such warning
has not already been added.
Also, if the response does have a Last-Modified time, the heuristic
expiration value SHOULD be no more than some fraction of the interval
since that time. A typical setting of this fraction might be 10%.
The calculation to determine if a response has expired is quite
response_is_fresh = (freshness_lifetime > current_age)
13.2.5 Disambiguating Expiration Values
Because expiration values are assigned optimistically, it is possible
for two caches to contain fresh values for the same resource that are
If a client performing a retrieval receives a non-first-hand response
for a request that was already fresh in its own cache, and the Date
header in its existing cache entry is newer than the Date on the new
response, then the client MAY ignore the response. If so, it MAY
retry the request with a "Cache-Control: max-age=0" directive (see
section 14.9), to force a check with the origin server.
If a cache has two fresh responses for the same representation with
different validators, it MUST use the one with the more recent Date
header. This situation might arise because the cache is pooling
responses from other caches, or because a client has asked for a
reload or a revalidation of an apparently fresh cache entry.
13.2.6 Disambiguating Multiple Responses
Because a client might be receiving responses via multiple paths, so
that some responses flow through one set of caches and other
responses flow through a different set of caches, a client might
receive responses in an order different from that in which the origin
server sent them. We would like the client to use the most recently
generated response, even if older responses are still apparently
Neither the entity tag nor the expiration value can impose an
ordering on responses, since it is possible that a later response
intentionally carries an earlier expiration time. The Date values are
ordered to a granularity of one second.
When a client tries to revalidate a cache entry, and the response it
receives contains a Date header that appears to be older than the one
for the existing entry, then the client SHOULD repeat the request
unconditionally, and include
to force any intermediate caches to validate their copies directly
with the origin server, or
to force any intermediate caches to obtain a new copy from the origin
If the Date values are equal, then the client MAY use either response
(or MAY, if it is being extremely prudent, request a new response).
Servers MUST NOT depend on clients being able to choose
deterministically between responses generated during the same second,
if their expiration times overlap.
13.3 Validation Model
When a cache has a stale entry that it would like to use as a
response to a client's request, it first has to check with the origin
server (or possibly an intermediate cache with a fresh response) to
see if its cached entry is still usable. We call this "validating"
the cache entry. Since we do not want to have to pay the overhead of
retransmitting the full response if the cached entry is good, and we
do not want to pay the overhead of an extra round trip if the cached
entry is invalid, the HTTP/1.1 protocol supports the use of
The key protocol features for supporting conditional methods are
those concerned with "cache validators." When an origin server
generates a full response, it attaches some sort of validator to it,
which is kept with the cache entry. When a client (user agent or
proxy cache) makes a conditional request for a resource for which it
has a cache entry, it includes the associated validator in the
The server then checks that validator against the current validator
for the entity, and, if they match (see section 13.3.3), it responds
with a special status code (usually, 304 (Not Modified)) and no
entity-body. Otherwise, it returns a full response (including
entity-body). Thus, we avoid transmitting the full response if the
validator matches, and we avoid an extra round trip if it does not
In HTTP/1.1, a conditional request looks exactly the same as a normal
request for the same resource, except that it carries a special
header (which includes the validator) that implicitly turns the
method (usually, GET) into a conditional.
The protocol includes both positive and negative senses of cache-
validating conditions. That is, it is possible to request either that
a method be performed if and only if a validator matches or if and
only if no validators match.
Note: a response that lacks a validator may still be cached, and
served from cache until it expires, unless this is explicitly
prohibited by a cache-control directive. However, a cache cannot
do a conditional retrieval if it does not have a validator for the
entity, which means it will not be refreshable after it expires.
13.3.1 Last-Modified Dates
The Last-Modified entity-header field value is often used as a cache
validator. In simple terms, a cache entry is considered to be valid
if the entity has not been modified since the Last-Modified value.
13.3.2 Entity Tag Cache Validators
The ETag response-header field value, an entity tag, provides for an
"opaque" cache validator. This might allow more reliable validation
in situations where it is inconvenient to store modification dates,
where the one-second resolution of HTTP date values is not
sufficient, or where the origin server wishes to avoid certain
paradoxes that might arise from the use of modification dates.
Entity Tags are described in section 3.11. The headers used with
entity tags are described in sections 14.19, 14.24, 14.26 and 14.44.
13.3.3 Weak and Strong Validators
Since both origin servers and caches will compare two validators to
decide if they represent the same or different entities, one normally
would expect that if the entity (the entity-body or any entity-
headers) changes in any way, then the associated validator would
change as well. If this is true, then we call this validator a
However, there might be cases when a server prefers to change the
validator only on semantically significant changes, and not when
insignificant aspects of the entity change. A validator that does not
always change when the resource changes is a "weak validator."
Entity tags are normally "strong validators," but the protocol
provides a mechanism to tag an entity tag as "weak." One can think of
a strong validator as one that changes whenever the bits of an entity
changes, while a weak value changes whenever the meaning of an entity
changes. Alternatively, one can think of a strong validator as part
of an identifier for a specific entity, while a weak validator is
part of an identifier for a set of semantically equivalent entities.
Note: One example of a strong validator is an integer that is
incremented in stable storage every time an entity is changed.
An entity's modification time, if represented with one-second
resolution, could be a weak validator, since it is possible that
the resource might be modified twice during a single second.
Support for weak validators is optional. However, weak validators
allow for more efficient caching of equivalent objects; for
example, a hit counter on a site is probably good enough if it is
updated every few days or weeks, and any value during that period
is likely "good enough" to be equivalent.
A "use" of a validator is either when a client generates a request
and includes the validator in a validating header field, or when a
server compares two validators.
Strong validators are usable in any context. Weak validators are only
usable in contexts that do not depend on exact equality of an entity.
For example, either kind is usable for a conditional GET of a full
entity. However, only a strong validator is usable for a sub-range
retrieval, since otherwise the client might end up with an internally
Clients MAY issue simple (non-subrange) GET requests with either weak
validators or strong validators. Clients MUST NOT use weak validators
in other forms of request.
The only function that the HTTP/1.1 protocol defines on validators is
comparison. There are two validator comparison functions, depending
on whether the comparison context allows the use of weak validators
- The strong comparison function: in order to be considered equal,
both validators MUST be identical in every way, and both MUST
NOT be weak.
- The weak comparison function: in order to be considered equal,
both validators MUST be identical in every way, but either or
both of them MAY be tagged as "weak" without affecting the
An entity tag is strong unless it is explicitly tagged as weak.
Section 3.11 gives the syntax for entity tags.
A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:
- The validator is being compared by an origin server to the
actual current validator for the entity and,
- That origin server reliably knows that the associated entity did
not change twice during the second covered by the presented
- The validator is about to be used by a client in an If-
Modified-Since or If-Unmodified-Since header, because the client
has a cache entry for the associated entity, and
- That cache entry includes a Date value, which gives the time
when the origin server sent the original response, and
- The presented Last-Modified time is at least 60 seconds before
the Date value.
- The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the entity, and
- That cache entry includes a Date value, which gives the time
when the origin server sent the original response, and
- The presented Last-Modified time is at least 60 seconds before
the Date value.
This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The arbitrary 60-
second limit guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at somewhat
different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.
If a client wishes to perform a sub-range retrieval on a value for
which it has only a Last-Modified time and no opaque validator, it
MAY do this only if the Last-Modified time is strong in the sense
A cache or origin server receiving a conditional request, other than
a full-body GET request, MUST use the strong comparison function to
evaluate the condition.
These rules allow HTTP/1.1 caches and clients to safely perform sub-
range retrievals on values that have been obtained from HTTP/1.0
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates
We adopt a set of rules and recommendations for origin servers,
clients, and caches regarding when various validator types ought to
be used, and for what purposes.
HTTP/1.1 origin servers:
- SHOULD send an entity tag validator unless it is not feasible to
- MAY send a weak entity tag instead of a strong entity tag, if
performance considerations support the use of weak entity tags,
or if it is unfeasible to send a strong entity tag.
- SHOULD send a Last-Modified value if it is feasible to send one,
unless the risk of a breakdown in semantic transparency that
could result from using this date in an If-Modified-Since header
would lead to serious problems.
In other words, the preferred behavior for an HTTP/1.1 origin server
is to send both a strong entity tag and a Last-Modified value.
In order to be legal, a strong entity tag MUST change whenever the
associated entity value changes in any way. A weak entity tag SHOULD
change whenever the associated entity changes in a semantically
Note: in order to provide semantically transparent caching, an
origin server must avoid reusing a specific strong entity tag
value for two different entities, or reusing a specific weak
entity tag value for two semantically different entities. Cache
entries might persist for arbitrarily long periods, regardless of
expiration times, so it might be inappropriate to expect that a
cache will never again attempt to validate an entry using a
validator that it obtained at some point in the past.
- If an entity tag has been provided by the origin server, MUST
use that entity tag in any cache-conditional request (using If-
Match or If-None-Match).
- If only a Last-Modified value has been provided by the origin
server, SHOULD use that value in non-subrange cache-conditional
requests (using If-Modified-Since).
- If only a Last-Modified value has been provided by an HTTP/1.0
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.
- If both an entity tag and a Last-Modified value have been
provided by the origin server, SHOULD use both validators in
cache-conditional requests. This allows both HTTP/1.0 and
HTTP/1.1 caches to respond appropriately.
An HTTP/1.1 origin server, upon receiving a conditional request that
includes both a Last-Modified date (e.g., in an If-Modified-Since or
If-Unmodified-Since header field) and one or more entity tags (e.g.,
in an If-Match, If-None-Match, or If-Range header field) as cache
validators, MUST NOT return a response status of 304 (Not Modified)
unless doing so is consistent with all of the conditional header
fields in the request.
An HTTP/1.1 caching proxy, upon receiving a conditional request that
includes both a Last-Modified date and one or more entity tags as
cache validators, MUST NOT return a locally cached response to the
client unless that cached response is consistent with all of the
conditional header fields in the request.
Note: The general principle behind these rules is that HTTP/1.1
servers and clients should transmit as much non-redundant
information as is available in their responses and requests.
HTTP/1.1 systems receiving this information will make the most
conservative assumptions about the validators they receive.
HTTP/1.0 clients and caches will ignore entity tags. Generally,
last-modified values received or used by these systems will
support transparent and efficient caching, and so HTTP/1.1 origin
servers should provide Last-Modified values. In those rare cases
where the use of a Last-Modified value as a validator by an
HTTP/1.0 system could result in a serious problem, then HTTP/1.1
origin servers should not provide one.
13.3.5 Non-validating Conditionals
The principle behind entity tags is that only the service author
knows the semantics of a resource well enough to select an
appropriate cache validation mechanism, and the specification of any
validator comparison function more complex than byte-equality would
open up a can of worms. Thus, comparisons of any other headers
(except Last-Modified, for compatibility with HTTP/1.0) are never
used for purposes of validating a cache entry.
13.4 Response Cacheability
Unless specifically constrained by a cache-control (section 14.9)
directive, a caching system MAY always store a successful response
(see section 13.8) as a cache entry, MAY return it without validation
if it is fresh, and MAY return it after successful validation. If
there is neither a cache validator nor an explicit expiration time
associated with a response, we do not expect it to be cached, but
certain caches MAY violate this expectation (for example, when little
or no network connectivity is available). A client can usually detect
that such a response was taken from a cache by comparing the Date
header to the current time.
Note: some HTTP/1.0 caches are known to violate this expectation
without providing any Warning.
However, in some cases it might be inappropriate for a cache to
retain an entity, or to return it in response to a subsequent
request. This might be because absolute semantic transparency is
deemed necessary by the service author, or because of security or
privacy considerations. Certain cache-control directives are
therefore provided so that the server can indicate that certain
resource entities, or portions thereof, are not to be cached
regardless of other considerations.
Note that section 14.8 normally prevents a shared cache from saving
and returning a response to a previous request if that request
included an Authorization header.
A response received with a status code of 200, 203, 206, 300, 301 or
410 MAY be stored by a cache and used in reply to a subsequent
request, subject to the expiration mechanism, unless a cache-control
directive prohibits caching. However, a cache that does not support
the Range and Content-Range headers MUST NOT cache 206 (Partial
A response received with any other status code (e.g. status codes 302
and 307) MUST NOT be returned in a reply to a subsequent request
unless there are cache-control directives or another header(s) that
explicitly allow it. For example, these include the following: an
Expires header (section 14.21); a "max-age", "s-maxage", "must-
revalidate", "proxy-revalidate", "public" or "private" cache-control
directive (section 14.9).
13.5 Constructing Responses From Caches
The purpose of an HTTP cache is to store information received in
response to requests for use in responding to future requests. In
many cases, a cache simply returns the appropriate parts of a
response to the requester. However, if the cache holds a cache entry
based on a previous response, it might have to combine parts of a new
response with what is held in the cache entry.
13.5.1 End-to-end and Hop-by-hop Headers
For the purpose of defining the behavior of caches and non-caching
proxies, we divide HTTP headers into two categories:
- End-to-end headers, which are transmitted to the ultimate
recipient of a request or response. End-to-end headers in
responses MUST be stored as part of a cache entry and MUST be
transmitted in any response formed from a cache entry.
- Hop-by-hop headers, which are meaningful only for a single
transport-level connection, and are not stored by caches or
forwarded by proxies.
The following HTTP/1.1 headers are hop-by-hop headers:
All other headers defined by HTTP/1.1 are end-to-end headers.
Other hop-by-hop headers MUST be listed in a Connection header,
(section 14.10) to be introduced into HTTP/1.1 (or later).
13.5.2 Non-modifiable Headers
Some features of the HTTP/1.1 protocol, such as Digest
Authentication, depend on the value of certain end-to-end headers. A
transparent proxy SHOULD NOT modify an end-to-end header unless the
definition of that header requires or specifically allows that.
A transparent proxy MUST NOT modify any of the following fields in a
request or response, and it MUST NOT add any of these fields if not
A transparent proxy MUST NOT modify any of the following fields in a
but it MAY add any of these fields if not already present. If an
Expires header is added, it MUST be given a field-value identical to
that of the Date header in that response.
A proxy MUST NOT modify or add any of the following fields in a
message that contains the no-transform cache-control directive, or in
A non-transparent proxy MAY modify or add these fields to a message
that does not include no-transform, but if it does so, it MUST add a
Warning 214 (Transformation applied) if one does not already appear
in the message (see section 14.46).
Warning: unnecessary modification of end-to-end headers might
cause authentication failures if stronger authentication
mechanisms are introduced in later versions of HTTP. Such
authentication mechanisms MAY rely on the values of header fields
not listed here.
The Content-Length field of a request or response is added or deleted
according to the rules in section 4.4. A transparent proxy MUST
preserve the entity-length (section 7.2.2) of the entity-body,
although it MAY change the transfer-length (section 4.4).
13.5.3 Combining Headers
When a cache makes a validating request to a server, and the server
provides a 304 (Not Modified) response or a 206 (Partial Content)
response, the cache then constructs a response to send to the
If the status code is 304 (Not Modified), the cache uses the entity-
body stored in the cache entry as the entity-body of this outgoing
response. If the status code is 206 (Partial Content) and the ETag or
Last-Modified headers match exactly, the cache MAY combine the
contents stored in the cache entry with the new contents received in
the response and use the result as the entity-body of this outgoing
response, (see 13.5.4).
The end-to-end headers stored in the cache entry are used for the
constructed response, except that
- any stored Warning headers with warn-code 1xx (see section
14.46) MUST be deleted from the cache entry and the forwarded
- any stored Warning headers with warn-code 2xx MUST be retained
in the cache entry and the forwarded response.
- any end-to-end headers provided in the 304 or 206 response MUST
replace the corresponding headers from the cache entry.
Unless the cache decides to remove the cache entry, it MUST also
replace the end-to-end headers stored with the cache entry with
corresponding headers received in the incoming response, except for
Warning headers as described immediately above. If a header field-
name in the incoming response matches more than one header in the
cache entry, all such old headers MUST be replaced.
In other words, the set of end-to-end headers received in the
incoming response overrides all corresponding end-to-end headers
stored with the cache entry (except for stored Warning headers with
warn-code 1xx, which are deleted even if not overridden).
Note: this rule allows an origin server to use a 304 (Not
Modified) or a 206 (Partial Content) response to update any header
associated with a previous response for the same entity or sub-
ranges thereof, although it might not always be meaningful or
correct to do so. This rule does not allow an origin server to use
a 304 (Not Modified) or a 206 (Partial Content) response to
entirely delete a header that it had provided with a previous
13.5.4 Combining Byte Ranges
A response might transfer only a subrange of the bytes of an entity-
body, either because the request included one or more Range
specifications, or because a connection was broken prematurely. After
several such transfers, a cache might have received several ranges of
the same entity-body.
If a cache has a stored non-empty set of subranges for an entity, and
an incoming response transfers another subrange, the cache MAY
combine the new subrange with the existing set if both the following
conditions are met:
- Both the incoming response and the cache entry have a cache
- The two cache validators match using the strong comparison
function (see section 13.3.3).
If either requirement is not met, the cache MUST use only the most
recent partial response (based on the Date values transmitted with
every response, and using the incoming response if these values are
equal or missing), and MUST discard the other partial information.
13.6 Caching Negotiated Responses
Use of server-driven content negotiation (section 12.1), as indicated
by the presence of a Vary header field in a response, alters the
conditions and procedure by which a cache can use the response for
subsequent requests. See section 14.44 for use of the Vary header
field by servers.
A server SHOULD use the Vary header field to inform a cache of what
request-header fields were used to select among multiple
representations of a cacheable response subject to server-driven
negotiation. The set of header fields named by the Vary field value
is known as the "selecting" request-headers.
When the cache receives a subsequent request whose Request-URI
specifies one or more cache entries including a Vary header field,
the cache MUST NOT use such a cache entry to construct a response to
the new request unless all of the selecting request-headers present
in the new request match the corresponding stored request-headers in
the original request.
The selecting request-headers from two requests are defined to match
if and only if the selecting request-headers in the first request can
be transformed to the selecting request-headers in the second request
by adding or removing linear white space (LWS) at places where this
is allowed by the corresponding BNF, and/or combining multiple
message-header fields with the same field name following the rules
about message headers in section 4.2.
A Vary header field-value of "*" always fails to match and subsequent
requests on that resource can only be properly interpreted by the
If the selecting request header fields for the cached entry do not
match the selecting request header fields of the new request, then
the cache MUST NOT use a cached entry to satisfy the request unless
it first relays the new request to the origin server in a conditional
request and the server responds with 304 (Not Modified), including an
entity tag or Content-Location that indicates the entity to be used.
If an entity tag was assigned to a cached representation, the
forwarded request SHOULD be conditional and include the entity tags
in an If-None-Match header field from all its cache entries for the
resource. This conveys to the server the set of entities currently
held by the cache, so that if any one of these entities matches the
requested entity, the server can use the ETag header field in its 304
(Not Modified) response to tell the cache which entry is appropriate.
If the entity-tag of the new response matches that of an existing
entry, the new response SHOULD be used to update the header fields of
the existing entry, and the result MUST be returned to the client.
If any of the existing cache entries contains only partial content
for the associated entity, its entity-tag SHOULD NOT be included in
the If-None-Match header field unless the request is for a range that
would be fully satisfied by that entry.
If a cache receives a successful response whose Content-Location
field matches that of an existing cache entry for the same Request-
]URI, whose entity-tag differs from that of the existing entry, and
whose Date is more recent than that of the existing entry, the
existing entry SHOULD NOT be returned in response to future requests
and SHOULD be deleted from the cache.
13.7 Shared and Non-Shared Caches
For reasons of security and privacy, it is necessary to make a
distinction between "shared" and "non-shared" caches. A non-shared
cache is one that is accessible only to a single user. Accessibility
in this case SHOULD be enforced by appropriate security mechanisms.
All other caches are considered to be "shared." Other sections of
this specification place certain constraints on the operation of
shared caches in order to prevent loss of privacy or failure of
13.8 Errors or Incomplete Response Cache Behavior
A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header) MAY store
the response. However, the cache MUST treat this as a partial
response. Partial responses MAY be combined as described in section
13.5.4; the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such, using the 206 (Partial
Content) status code. A cache MUST NOT return a partial response
using a status code of 200 (OK).
If a cache receives a 5xx response while attempting to revalidate an
entry, it MAY either forward this response to the requesting client,
or act as if the server failed to respond. In the latter case, it MAY
return a previously received response unless the cached entry
includes the "must-revalidate" cache-control directive (see section
13.9 Side Effects of GET and HEAD
Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They MAY still have side
effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an
origin server's explicit restrictions on caching.
We note one exception to this rule: since some applications have
traditionally used GETs and HEADs with query URLs (those containing a
"?" in the rel_path part) to perform operations with significant side
effects, caches MUST NOT treat responses to such URIs as fresh unless
the server provides an explicit expiration time. This specifically
means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
be taken from a cache. See section 9.1.1 for related information.
13.10 Invalidation After Updates or Deletions
The effect of certain methods performed on a resource at the origin
server might cause one or more existing cache entries to become non-
transparently invalid. That is, although they might continue to be
"fresh," they do not accurately reflect what the origin server would
return for a new request on that resource.
There is no way for the HTTP protocol to guarantee that all such
cache entries are marked invalid. For example, the request that
caused the change at the origin server might not have gone through
the proxy where a cache entry is stored. However, several rules help
reduce the likelihood of erroneous behavior.
In this section, the phrase "invalidate an entity" means that the
cache will either remove all instances of that entity from its
storage, or will mark these as "invalid" and in need of a mandatory
revalidation before they can be returned in response to a subsequent
Some HTTP methods MUST cause a cache to invalidate an entity. This is
either the entity referred to by the Request-URI, or by the Location
or Content-Location headers (if present). These methods are:
In order to prevent denial of service attacks, an invalidation based
on the URI in a Location or Content-Location header MUST only be
performed if the host part is the same as in the Request-URI.
A cache that passes through requests for methods it does not
understand SHOULD invalidate any entities referred to by the
13.11 Write-Through Mandatory
All methods that might be expected to cause modifications to the
origin server's resources MUST be written through to the origin
server. This currently includes all methods except for GET and HEAD.
A cache MUST NOT reply to such a request from a client before having
transmitted the request to the inbound server, and having received a
corresponding response from the inbound server. This does not prevent
a proxy cache from sending a 100 (Continue) response before the
inbound server has sent its final reply.
The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network
failure prior to write-back.
13.12 Cache Replacement
If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
response is received from a resource while any existing responses for
the same resource are cached, the cache SHOULD use the new response
to reply to the current request. It MAY insert it into cache storage
and MAY, if it meets all other requirements, use it to respond to any
future requests that would previously have caused the old response to
be returned. If it inserts the new response into cache storage the
rules in section 13.5.3 apply.
Note: a new response that has an older Date header value than
existing cached responses is not cacheable.
13.13 History Lists
User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved
earlier in a session.
History mechanisms and caches are different. In particular history
mechanisms SHOULD NOT try to show a semantically transparent view of
the current state of a resource. Rather, a history mechanism is meant
to show exactly what the user saw at the time when the resource was
By default, an expiration time does not apply to history mechanisms.
If the entity is still in storage, a history mechanism SHOULD display
it even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents.
This is not to be construed to prohibit the history mechanism from
telling the user that a view might be stale.
Note: if history list mechanisms unnecessarily prevent users from
viewing stale resources, this will tend to force service authors
to avoid using HTTP expiration controls and cache controls when
they would otherwise like to. Service authors may consider it
important that users not be presented with error messages or
warning messages when they use navigation controls (such as BACK)
to view previously fetched resources. Even though sometimes such
resources ought not to cached, or ought to expire quickly, user
interface considerations may force service authors to resort to
other means of preventing caching (e.g. "once-only" URLs) in order
not to suffer the effects of improperly functioning history