| HYBI | M. Thomson |
| Internet-Draft | Microsoft |
| Intended status: Standards Track | S. Loreto |
| Expires: December 31, 2012 | Ericsson |
| G. Wilkins | |
| Intalio | |
| July 2012 |
Hypertext Transfer Protocol (HTTP) Keep-Alive Header
A Keep-Alive header is defined for HTTP. This hop-by-hop header informs hosts about connection management policies. Parameters are defined for idle connection timeout and maximum request count.
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This Internet-Draft will expire on December 31, 2012.
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This document describes the Keep-Alive header. The Keep-Alive header provides Hypertext Transfer Protocol (HTTP) [I-D.ietf-httpbis-p1-messaging] clients, servers and intermediaries with information about the connection use policies of their peers.
The timeout header parameter indicates the time that a connection will be allowed to remain idle before it is closed.
The max header parameter indicates the maximum number of requests that will be permitted before the connection is closed.
Some HTTP implementations already provide an implementation for this header. Not all of those implementations are interoperable due to significant differences in the header format. This draft defines a single format for the header and ascribes specific semantics to the header parameters.
Management of idle HTTP connections has an impact on long-lived communications between hosts. Hosts are able to close idle connections in order to reduce resource consumption.
Many clients choose not to send non-idempotent requests on idle connections. If the intermediary or server holding the other end of the connection chooses to close the connection while a non-idempotent request is in transit, the client has no way to tell if the request has succeeded. For this reason, many clients establish a new connection for every non-idempotent request. This is inefficient if the existing connection is a usable connection: establishing a new connection adds significantly to the latency of the request.
Connection resources can be more efficiently used when an idle connection timeout is known. A client that only periodically sends request can learn about the possibility of a connection timeout and can act to create a new connection for requests or send requests that keep the connection from timing out. Alternatively, a client that knows that more requests on a connection are unlikely within the discovered timeout interval can close the connection immediately after a poll, releasing resources.
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 [RFC2119] and indicate requirement levels for compliant implementations.
The Keep-Alive header is a hop-by-hop header that provides information about a persistent connection. Both client and server are able to provide information independently.
Keep-Alive = "Keep-Alive" ":" 1#keep-alive-info
keep-alive-info = "timeout" "=" delta-seconds
/ keep-alive-extension
keep-alive-extension = token [ "=" ( token / quoted-string ) ]
This header is sent by either host participating in a persistent connection. The values might be set based on policy implemented by servers, clients and intermediaries. Values might also be set based on knowledge that a host has about lower layer intermediaries in the path of the request, such as a TCP middlebox. Such middleboxes, in particular network address translators (NATs), frequently discard mappings for idle connections, causing the connection to fail after a certain duration of inactivity.
The value of Keep-Alive parameters can change on each request or response sent on a connection. Absence of the header or any parameter implies that any previously provided value still applies.
As a hop-by-hop header, this header only applies to a single transport-level connection. If a Keep-Alive header is added to a request or response, the Connection header MUST include the tag Keep-Alive. This ensures that compliant intermediaries that do not recognize this header remove it before forwarding a request.
A host sets the value of the timeout parameter to the time that the host will allow an idle connection to remain open before it is closed. A connection is idle if no data is sent or received by a host.
The value of the timeout parameter is a single integer in seconds.
A host MAY keep an idle connection open for longer than the time that it indicates, but it SHOULD attempt to retain a connection for at least as long as indicated.
Each peer, client or server, has a different view of the time that a connection becomes idle. Packet transmission at one peer necessarily occurs before receipt, meaning that the sending peer perceives the connection as being idle earlier to the receiving peer. Similarly, the buffering or retransmission of data by lower layers of the stack, which is unlikely to be visible to the HTTP implementation, compounds this effect. Clients are advised to make allowances for delays in determining whether to reuse an idle connection.
The Keep-Alive header can be extended by adding any number of keep-alive-extension values to the header. Any extension that is not understood MUST be ignored.
The HTTP Keep-Alive Information Registry defines the namespace for Keep-Alive extensions. Section 7.2 describes this registry.
The max parameter has been used to indicate the maximum number of requests that would be made on the connection. This parameter is deprecated. Any limit on requests can be enforced by sending Connection: close and closing the connection.
The exact impact of an intermediary on an HTTP request with a Keep-Alive header depends on the type of intermediary.
An intermediary that is compliant with HTTP/1.1, but does not implement Keep-Alive, ignores and discards this header before forwarding a request. Since it is unaware of the semantics of the header it could drop an idle connection at any time (see Section 7.1.4 of [I-D.ietf-httpbis-p1-messaging]).
A non-compliant "transparent" intermediary could pass this header on to the next hop. This results in errors of the sort that are described in Section A.1.2 of [I-D.ietf-httpbis-p1-messaging]. Clients that send this header to HTTP/1.0 servers or proxies SHOULD monitor for "hung" connections and avoid sending the header if a connection appears to hang.
A network address translation (NAT) device or other middlebox might cause a connection to become unavailable prior to the advertised timeout.
A client or intermediary can revise or remove the Keep-Alive header for subsequent requests to the same resource or origin server if it detects non-compliant intermediaries or middleboxes that have shorter timeout periods.
A connection timeout can apply to a connection that is subsequently upgraded to another protocol [RFC2817], such as the websocket protocol [RFC6455].
The idle connection timeout applies to the upgraded connection, unless the upgraded protocol provides another method for indicating idle timeouts. The maximum request count does not apply to the upgraded connection; the upgrade request and subsequent exchange are regarded as a single HTTP request.
A server, client or intermediary might apply different policies to an upgraded protocol.
The example in Figure 1 shows how a Keep-Alive header could be used. All connections are independently negotiated. In this example, the client indicates a timeout of 600 seconds (10 minutes), but the proxy is prepared to retain the connection for up to 3600 seconds (1 hour). On the link between proxy and server, the proxy requests a timeout of 1200 seconds and the server indicates a lower limit of 300 seconds.
Client Proxy Server | | | +- Keep-Alive: timeout=600 -->| | | Connection: Keep-Alive | | | +- Keep-Alive: timeout=1200 -->| | | Connection: Keep-Alive | | | | | |<-- Keep-Alive: timeout=300 --+ | | Connection: Keep-Alive | |<- Keep-Alive: timeout=3600 -+ | | Connection: Keep-Alive | | | | |
Figure 1: Independent HTTP Hops
As this example shows, the timeout policies maintained by the proxy are different for each connection. Each connection hop is independent.
The example in Figure 2 shows the headers included in an upgrade from HTTP/1.1 to WebSocket [RFC6455]. With a websocket upgrade, the connections on each hop cannot have independent lifecycles on either side of an intermediary. After the upgrade, timeout policies cannot be independent for each hop. The proxy adjusts the timeout value to reflect the lower of the values set by client and the proxy policies so that the server is aware of the connection characteristics; similarly, the value from the server is provided to the client.
Client Proxy Server | | | | Upgrade: websocket | | +- Keep-Alive: timeout=600 -->| | | Connection: Keep-Alive, | | | Upgrade | | | | Upgrade: websocket | | +- Keep-Alive: timeout=600 --->| | | Connection: Keep-Alive, | | | Upgrade | | | | | | Upgrade: websocket | | |<-- Keep-Alive: timeout=300 --+ | | Connection: Keep-Alive, | | | Upgrade | | Upgrade: websocket | | |<- Keep-Alive: timeout=300 --+ | | Connection: Keep-Alive, | | | Upgrade | | | | |
Figure 2: Interdependent Connections with Upgrade
Establishing a persistent connection requires a commitment of resources at a host. The Keep-Alive header are used to express host policy that could alter the way that a host allocates connection resources. Since these policies can be enacted without this feedback, these indications have little effect on security other than exposing specifics of policy.
A host can close a non-idle connection sooner than the indicated time if necessary or as dictated by local policy (see Section 7.1.4 of [I-D.ietf-httpbis-p1-messaging]).
[[Note to IANA/RFC Editor: Please replace instance of RFCXXXX with the number of the published RFC and remove this note.]]
This document registers the HTTP Keep-Alive header in the "Permanent Message Header Fields" registry established by [RFC3864]
This document establishes a registry for Keep-Alive Information.
Registrations are subject to Specification Required [RFC5226]. The designated expert is advised to review registrations and work with the submitter to ensure that:
The registry includes the following initial values:
Jamie Lokier provided valuable contributions of experience, insight and text suggestions to this document. Roy Fielding provided information on existing implementations of the poorly documented header. Also provided useful feedback: Dave Thaler, Konstantinos Pentikousis.
Since -01:
Since -00:
Since draft-loreto-http-timeout:
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [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. |
| [I-D.ietf-httpbis-p1-messaging] | Fielding, R, Gettys, J, Mogul, J, Nielsen, H, Masinter, L, Leach, P, Berners-Lee, T, Reschke, J and Y Lafon, "HTTP/1.1, part 1: URIs, Connections, and Message Parsing", Internet-Draft draft-ietf-httpbis-p1-messaging-16, August 2011. |
| [RFC2068] | Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, January 1997. |
| [RFC2817] | Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1", RFC 2817, May 2000. |
| [RFC6455] | Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, December 2011. |