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rfc:rfc6202

Internet Engineering Task Force (IETF) S. Loreto Request for Comments: 6202 Ericsson Category: Informational P. Saint-Andre ISSN: 2070-1721 Cisco

                                                            S. Salsano
                                      University of Rome "Tor Vergata"
                                                            G. Wilkins
                                                               Webtide
                                                            April 2011
                  Known Issues and Best Practices
  for the Use of Long Polling and Streaming in Bidirectional HTTP

Abstract

 On today's Internet, the Hypertext Transfer Protocol (HTTP) is often
 used (some would say abused) to enable asynchronous, "server-
 initiated" communication from a server to a client as well as
 communication from a client to a server.  This document describes
 known issues and best practices related to such "bidirectional HTTP"
 applications, focusing on the two most common mechanisms: HTTP long
 polling and HTTP streaming.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6202.

Loreto, et al. Informational [Page 1] RFC 6202 Bidirectional HTTP April 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  HTTP Long Polling  . . . . . . . . . . . . . . . . . . . . . .  4
   2.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.2.  HTTP Long Polling Issues . . . . . . . . . . . . . . . . .  5
 3.  HTTP Streaming . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.2.  HTTP Streaming Issues  . . . . . . . . . . . . . . . . . .  8
 4.  Overview of Technologies . . . . . . . . . . . . . . . . . . . 10
   4.1.  Bayeux . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.2.  BOSH . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.3.  Server-Sent Events . . . . . . . . . . . . . . . . . . . . 13
 5.  HTTP Best Practices  . . . . . . . . . . . . . . . . . . . . . 13
   5.1.  Limits to the Maximum Number of Connections  . . . . . . . 13
   5.2.  Pipelined Connections  . . . . . . . . . . . . . . . . . . 14
   5.3.  Proxies  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.4.  HTTP Responses . . . . . . . . . . . . . . . . . . . . . . 15
   5.5.  Timeouts . . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.6.  Impact on Intermediary Entities  . . . . . . . . . . . . . 16
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
 7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
   7.2.  Informative References . . . . . . . . . . . . . . . . . . 17
 8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18

Loreto, et al. Informational [Page 2] RFC 6202 Bidirectional HTTP April 2011

1. Introduction

 The Hypertext Transfer Protocol [RFC2616] is a request/response
 protocol.  HTTP defines the following entities: clients, proxies, and
 servers.  A client establishes connections to a server for the
 purpose of sending HTTP requests.  A server accepts connections from
 clients in order to service HTTP requests by sending back responses.
 Proxies are intermediate entities that can be involved in the
 delivery of requests and responses from the client to the server and
 vice versa.
 In the standard HTTP model, a server cannot initiate a connection
 with a client nor send an unrequested HTTP response to a client;
 thus, the server cannot push asynchronous events to clients.
 Therefore, in order to receive asynchronous events as soon as
 possible, the client needs to poll the server periodically for new
 content.  However, continual polling can consume significant
 bandwidth by forcing a request/response round trip when no data is
 available.  It can also be inefficient because it reduces the
 responsiveness of the application since data is queued until the
 server receives the next poll request from the client.
 In order to improve this situation, several server-push programming
 mechanisms have been implemented in recent years.  These mechanisms,
 which are often grouped under the common label "Comet" [COMET],
 enable a web server to send updates to clients without waiting for a
 poll request from the client.  Such mechanisms can deliver updates to
 clients in a more timely manner while avoiding the latency
 experienced by client applications due to the frequent opening and
 closing of connections necessary to periodically poll for data.
 The two most common server-push mechanisms are HTTP long polling and
 HTTP streaming:
 HTTP Long Polling:  The server attempts to "hold open" (not
    immediately reply to) each HTTP request, responding only when
    there are events to deliver.  In this way, there is always a
    pending request to which the server can reply for the purpose of
    delivering events as they occur, thereby minimizing the latency in
    message delivery.
 HTTP Streaming:  The server keeps a request open indefinitely; that
    is, it never terminates the request or closes the connection, even
    after it pushes data to the client.
 It is possible to define other technologies for bidirectional HTTP;
 however, such technologies typically require changes to HTTP itself
 (e.g., by defining new HTTP methods).  This document focuses only on

Loreto, et al. Informational [Page 3] RFC 6202 Bidirectional HTTP April 2011

 bidirectional HTTP technologies that work within the current scope of
 HTTP as defined in [RFC2616] (HTTP 1.1) and [RFC1945] (HTTP 1.0).
 The authors acknowledge that both the HTTP long polling and HTTP
 streaming mechanisms stretch the original semantic of HTTP and that
 the HTTP protocol was not designed for bidirectional communication.
 This document neither encourages nor discourages the use of these
 mechanisms, and takes no position on whether they provide appropriate
 solutions to the problem of providing bidirectional communication
 between clients and servers.  Instead, this document merely
 identifies technical issues with these mechanisms and suggests best
 practices for their deployment.
 The remainder of this document is organized as follows.  Section 2
 analyzes the HTTP long polling technique.  Section 3 analyzes the
 HTTP streaming technique.  Section 4 provides an overview of the
 specific technologies that use the server-push technique.  Section 5
 lists best practices for bidirectional HTTP using existing
 technologies.

2. HTTP Long Polling

2.1. Definition

 With the traditional or "short polling" technique, a client sends
 regular requests to the server and each request attempts to "pull"
 any available events or data.  If there are no events or data
 available, the server returns an empty response and the client waits
 for some time before sending another poll request.  The polling
 frequency depends on the latency that the client can tolerate in
 retrieving updated information from the server.  This mechanism has
 the drawback that the consumed resources (server processing and
 network) strongly depend on the acceptable latency in the delivery of
 updates from server to client.  If the acceptable latency is low
 (e.g., on the order of seconds), then the polling frequency can cause
 an unacceptable burden on the server, the network, or both.
 In contrast with such "short polling", "long polling" attempts to
 minimize both the latency in server-client message delivery and the
 use of processing/network resources.  The server achieves these
 efficiencies by responding to a request only when a particular event,
 status, or timeout has occurred.  Once the server sends a long poll
 response, typically the client immediately sends a new long poll
 request.  Effectively, this means that at any given time the server
 will be holding open a long poll request, to which it replies when
 new information is available for the client.  As a result, the server
 is able to asynchronously "initiate" communication.

Loreto, et al. Informational [Page 4] RFC 6202 Bidirectional HTTP April 2011

 The basic life cycle of an application using HTTP long polling is as
 follows:
 1.  The client makes an initial request and then waits for a
     response.
 2.  The server defers its response until an update is available or
     until a particular status or timeout has occurred.
 3.  When an update is available, the server sends a complete response
     to the client.
 4.  The client typically sends a new long poll request, either
     immediately upon receiving a response or after a pause to allow
     an acceptable latency period.
 The HTTP long polling mechanism can be applied to either persistent
 or non-persistent HTTP connections.  The use of persistent HTTP
 connections will avoid the additional overhead of establishing a new
 TCP/IP connection [TCP] for every long poll request.

2.2. HTTP Long Polling Issues

 The HTTP long polling mechanism introduces the following issues.
 Header Overhead:  With the HTTP long polling technique, every long
    poll request and long poll response is a complete HTTP message and
    thus contains a full set of HTTP headers in the message framing.
    For small, infrequent messages, the headers can represent a large
    percentage of the data transmitted.  If the network MTU (Maximum
    Transmission Unit) allows all the information (including the HTTP
    header) to fit within a single IP packet, this typically does not
    represent a significant increase in the burden for networking
    entities.  On the other hand, the amount of transferred data can
    be significantly larger than the real payload carried by HTTP, and
    this can have a significant impact (e.g., when volume-based
    charging is in place).
 Maximal Latency:  After a long poll response is sent to a client, the
    server needs to wait for the next long poll request before another
    message can be sent to the client.  This means that while the
    average latency of long polling is close to one network transit,
    the maximal latency is over three network transits (long poll
    response, next long poll request, long poll response).  However,
    because HTTP is carried over TCP/IP, packet loss and
    retransmission can occur; therefore, maximal latency for any
    TCP/IP protocol will be more than three network transits (lost

Loreto, et al. Informational [Page 5] RFC 6202 Bidirectional HTTP April 2011

    packet, next packet, negative ack, retransmit).  When HTTP
    pipelining (see Section 5.2) is available, the latency due to the
    server waiting for a new request can be avoided.
 Connection Establishment:  A common criticism of both short polling
    and long polling is that these mechanisms frequently open TCP/IP
    connections and then close them.  However, both polling mechanisms
    work well with persistent HTTP connections that can be reused for
    many poll requests.  Specifically, the short duration of the pause
    between a long poll response and the next long poll request avoids
    the closing of idle connections.
 Allocated Resources:  Operating systems will allocate resources to
    TCP/IP connections and to HTTP requests outstanding on those
    connections.  The HTTP long polling mechanism requires that for
    each client both a TCP/IP connection and an HTTP request are held
    open.  Thus, it is important to consider the resources related to
    both of these when sizing an HTTP long polling application.
    Typically, the resources used per TCP/IP connection are minimal
    and can scale reasonably.  Frequently, the resources allocated to
    HTTP requests can be significant, and scaling the total number of
    requests outstanding can be limited on some gateways, proxies, and
    servers.
 Graceful Degradation:  A long polling client or server that is under
    load has a natural tendency to gracefully degrade in performance
    at a cost of message latency.  If load causes either a client or
    server to run slowly, then events to be pushed to the client will
    queue (waiting either for the client to send a long poll request
    or for the server to free up CPU cycles that can be used to
    process a long poll request that is being held at the server).  If
    multiple messages are queued for a client, they might be delivered
    in a batch within a single long poll response.  This can
    significantly reduce the per-message overhead and thus ease the
    workload of the client or server for the given message load.
 Timeouts:  Long poll requests need to remain pending or "hanging"
    until the server has something to send to the client.  The timeout
    issues related to these pending requests are discussed in
    Section 5.5.
 Caching:  Caching mechanisms implemented by intermediate entities can
    interfere with long poll requests.  This issue is discussed in
    Section 5.6.

Loreto, et al. Informational [Page 6] RFC 6202 Bidirectional HTTP April 2011

3. HTTP Streaming

3.1. Definition

 The HTTP streaming mechanism keeps a request open indefinitely.  It
 never terminates the request or closes the connection, even after the
 server pushes data to the client.  This mechanism significantly
 reduces the network latency because the client and the server do not
 need to open and close the connection.
 The basic life cycle of an application using HTTP streaming is as
 follows:
 1.  The client makes an initial request and then waits for a
     response.
 2.  The server defers the response to a poll request until an update
     is available, or until a particular status or timeout has
     occurred.
 3.  Whenever an update is available, the server sends it back to the
     client as a part of the response.
 4.  The data sent by the server does not terminate the request or the
     connection.  The server returns to step 3.
 The HTTP streaming mechanism is based on the capability of the server
 to send several pieces of information in the same response, without
 terminating the request or the connection.  This result can be
 achieved by both HTTP/1.1 and HTTP/1.0 servers.
 An HTTP response content length can be defined using three options:
 Content-Length header:  This indicates the size of the entity body in
    the message, in bytes.
 Transfer-Encoding header:  The 'chunked' valued in this header
    indicates the message will break into chunks of known size if
    needed.
 End of File (EOF):  This is actually the default approach for
    HTTP/1.0 where the connections are not persistent.  Clients do not
    need to know the size of the body they are reading; instead they
    expect to read the body until the server closes the connection.
    Although with HTTP/1.1 the default is for persistent connections,
    it is still possible to use EOF by setting the 'Connection:close'
    header in either the request or the response, thereby indicating
    that the connection is not to be considered 'persistent' after the

Loreto, et al. Informational [Page 7] RFC 6202 Bidirectional HTTP April 2011

    current request/response is complete.  The client's inclusion of
    the 'Connection: close' header field in the request will also
    prevent pipelining.
    The main issue with EOF is that it is difficult to tell the
    difference between a connection terminated by a fault and one that
    is correctly terminated.
 An HTTP/1.0 server can use only EOF as a streaming mechanism.  In
 contrast, both EOF and "chunked transfer" are available to an
 HTTP/1.1 server.
 The "chunked transfer" mechanism is the one typically used by
 HTTP/1.1 servers for streaming.  This is accomplished by including
 the header "Transfer-Encoding: chunked" at the beginning of the
 response, which enables the server to send the following parts of the
 response in different "chunks" over the same connection.  Each chunk
 starts with the hexadecimal expression of the length of its data,
 followed by CR/LF (the end of the response is indicated with a chunk
 of size 0).
         HTTP/1.1 200 OK
         Content-Type: text/plain
         Transfer-Encoding: chunked
         25
         This is the data in the first chunk
         1C
         and this is the second one
                 Figure 1: Transfer-Encoding response
 To achieve the same result, an HTTP/1.0 server will omit the Content-
 Length header in the response.  Thus, it will be able to send the
 subsequent parts of the response on the same connection (in this
 case, the different parts of the response are not explicitly
 separated by HTTP protocol, and the end of the response is achieved
 by closing the connection).

3.2. HTTP Streaming Issues

 The HTTP streaming mechanism introduces the following issues.

Loreto, et al. Informational [Page 8] RFC 6202 Bidirectional HTTP April 2011

 Network Intermediaries:  The HTTP protocol allows for intermediaries
    (proxies, transparent proxies, gateways, etc.) to be involved in
    the transmission of a response from the server to the client.
    There is no requirement for an intermediary to immediately forward
    a partial response, and it is legal for the intermediary to buffer
    the entire response before sending any data to the client (e.g.,
    caching transparent proxies).  HTTP streaming will not work with
    such intermediaries.
 Maximal Latency:  Theoretically, on a perfect network, an HTTP
    streaming protocol's average and maximal latency is one network
    transit.  However, in practice, the maximal latency is higher due
    to network and browser limitations.  The browser techniques used
    to terminate HTTP streaming connections are often associated with
    JavaScript and/or DOM (Document Object Model) elements that will
    grow in size for every message received.  Thus, in order to avoid
    unlimited growth of memory usage in the client, an HTTP streaming
    implementation occasionally needs to terminate the streaming
    response and send a request to initiate a new streaming response
    (which is essentially equivalent to a long poll).  Thus, the
    maximal latency is at least three network transits.  Also, because
    HTTP is carried over TCP/IP, packet loss and retransmission can
    occur; therefore maximal latency for any TCP/IP protocol will be
    more than three network transits (lost packet, next packet,
    negative ack, retransmit).
 Client Buffering:  There is no requirement in existing HTTP
    specifications for a client library to make the data from a
    partial HTTP response available to the client application.  For
    example, if each response chunk contains a statement of
    JavaScript, there is no requirement in the browser to execute that
    JavaScript before the entire response is received.  However, in
    practice, most browsers do execute JavaScript received in partial
    responses -- although some require a buffer overflow to trigger
    execution.  In most implementations, blocks of white space can be
    sent to achieve buffer overflow.
 Framing Techniques:  Using HTTP streaming, several application
    messages can be sent within a single HTTP response.  The
    separation of the response stream into application messages needs
    to be performed at the application level and not at the HTTP
    level.  In particular, it is not possible to use the HTTP chunks
    as application message delimiters, since intermediate proxies
    might "re-chunk" the message stream (for example, by combining
    different chunks into a longer one).  This issue does not affect
    the HTTP long polling technique, which provides a canonical
    framing technique: each application message can be sent in a
    different HTTP response.

Loreto, et al. Informational [Page 9] RFC 6202 Bidirectional HTTP April 2011

4. Overview of Technologies

 This section provides an overview of existing technologies that
 implement HTTP-based server-push mechanisms to asynchronously deliver
 messages from the server to the client.

4.1. Bayeux

 The Bayeux protocol [BAYEUX] was developed in 2006-2007 by the Dojo
 Foundation.  Bayeux can use both the HTTP long polling and HTTP
 streaming mechanisms.
 In order to achieve bidirectional communications, a Bayeux client
 will use two HTTP connections to a Bayeux server so that both server-
 to-client and client-to-server messaging can occur asynchronously.
 The Bayeux specification requires that implementations control
 pipelining of HTTP requests, so that requests are not pipelined
 inappropriately (e.g., a client-to-server message pipelined behind a
 long poll request).
 In practice, for JavaScript clients, such control over pipelining is
 not possible in current browsers.  Therefore, JavaScript
 implementations of Bayeux attempt to meet this requirement by
 limiting themselves to a maximum of two outstanding HTTP requests at
 any one time, so that browser connection limits will not be applied
 and the requests will not be queued or pipelined.  While broadly
 effective, this mechanism can be disrupted if non-Bayeux JavaScript
 clients simultaneously issue requests to the same host.
 Bayeux connections are negotiated between client and server with
 handshake messages that allow the connection type, authentication
 method, and other parameters to be agreed upon between the client and
 the server.  Furthermore, during the handshake phase, the client and
 the server reveal to each other their acceptable bidirectional
 techniques, and the client selects one from the intersection of those
 sets.
 For non-browser or same-domain Bayeux, clients use HTTP POST requests
 to the server for both the long poll request and the request to send
 messages to the server.  The Bayeux protocol packets are sent as the
 body of the HTTP messages using the "application/json" Internet media
 type [RFC4627].
 For browsers that are operating in cross-domain mode, Bayeux attempts
 to use Cross-Origin Resource Sharing [CORS] checking if the browser
 and server support it, so that normal HTTP POST requests can be used.
 If this mechanism fails, Bayeux clients use the "JSONP" mechanism as

Loreto, et al. Informational [Page 10] RFC 6202 Bidirectional HTTP April 2011

 described in [JSONP].  In this last case, client-to-server messages
 are sent as encoded JSON on the URL query parameters, and server-to-
 client messages are sent as a JavaScript program that wraps the
 message JSON with a JavaScript function call to the already loaded
 Bayeux implementation.

4.2. BOSH

 BOSH, which stands for Bidirectional-streams Over Synchronous HTTP
 [BOSH], was developed by the XMPP Standards Foundation in 2003-2004.
 The purpose of BOSH is to emulate normal TCP connections over HTTP
 (TCP is the standard connection mechanism used in the Extensible
 Messaging and Presence Protocol as described in [RFC6120]).  BOSH
 employs the HTTP long polling mechanism by allowing the server
 (called a "BOSH connection manager") to defer its response to a
 request until it actually has data to send to the client from the
 application server itself (typically an XMPP server).  As soon as the
 client receives a response from the connection manager, it sends
 another request to the connection manager, thereby ensuring that the
 connection manager is (almost) always holding a request that it can
 use to "push" data to the client.
 In some situations, the client needs to send data to the server while
 it is waiting for data to be pushed from the connection manager.  To
 prevent data from being pipelined behind the long poll request that
 is on hold, the client can send its outbound data in a second HTTP
 request over a second TCP connection.  BOSH forces the server to
 respond to the request it has been holding on the first connection as
 soon as it receives a new request from the client, even if it has no
 data to send to the client.  It does so to make sure that the client
 can send more data immediately, if necessary -- even in the case
 where the client is not able to pipeline the requests -- while
 simultaneously respecting the two-connection limit discussed in
 Section 5.1.
 The number of long poll request-response pairs is negotiated during
 the first request sent from the client to the connection manager.
 Typically, BOSH clients and connection managers will negotiate the
 use of two pairs, although it is possible to use only one pair or
 more than two pairs.
 The roles of the two request-response pairs typically switch whenever
 the client sends data to the connection manager.  This means that
 when the client issues a new request, the connection manager
 immediately answers the blocked request on the other TCP connection,
 thus freeing it; in this way, in a scenario where only the client
 sends data, the even requests are sent over one connection, and the
 odd ones are sent over the other connection.

Loreto, et al. Informational [Page 11] RFC 6202 Bidirectional HTTP April 2011

 BOSH is able to work reliably both when network conditions force
 every HTTP request to be made over a different TCP connection and
 when it is possible to use HTTP/1.1 and then rely on two persistent
 TCP connections.
 If the connection manager has no data to send to the client for an
 agreed amount of time (also negotiated during the first request),
 then the connection manager will respond to the request it has been
 holding with no data, and that response immediately triggers a fresh
 client request.  The connection manager does so to ensure that if a
 network connection is broken then both parties will realize that fact
 within a reasonable amount of time.
 Moreover, BOSH defines the negotiation of an "inactivity period"
 value that specifies the longest allowable inactivity period (in
 seconds).  This enables the client to ensure that the periods with no
 requests pending are never too long.
 BOSH allows data to be pushed immediately when HTTP pipelining is
 available.  However, if HTTP pipelining is not available and one of
 the endpoints has just pushed some data, BOSH will usually need to
 wait for a network round-trip time until the server is able to again
 push data to the client.
 BOSH uses standard HTTP POST request and response bodies to encode
 all information.
 BOSH normally uses HTTP pipelining over a persistent HTTP/1.1
 connection.  However, a client can deliver its POST requests in any
 way permitted by HTTP 1.0 or HTTP 1.1.  (Although the use of HTTP
 POST with pipelining is discouraged in RFC 2616, BOSH employs various
 methods, such as request identifiers, to ensure that this usage does
 not lead to indeterminate results if the transport connection is
 terminated prematurely.)
 BOSH clients and connection managers are not allowed to use Chunked
 Transfer Coding, since intermediaries might buffer each partial HTTP
 request or response and only forward the full request or response
 once it is available.
 BOSH allows the usage of the Accept-Encoding and Content-Encoding
 headers in the request and in the response, respectively, and then
 compresses the response body accordingly.
 Each BOSH session can share the HTTP connection(s) it uses with other
 HTTP traffic, including other BOSH sessions and HTTP requests and
 responses completely unrelated to the BOSH protocol (e.g., Web page
 downloads).

Loreto, et al. Informational [Page 12] RFC 6202 Bidirectional HTTP April 2011

4.3. Server-Sent Events

 W3C Server-Sent Events specification [WD-eventsource] defines an API
 that enables servers to push data to Web pages over HTTP in the form
 of DOM events.
 The data is encoded as "text/event-stream" content and pushed using
 an HTTP streaming mechanism, but the specification suggests disabling
 HTTP chunking for serving event streams unless the rate of messages
 is high enough to avoid the possible negative effects of this
 technique as described in Section 3.2.
 However, it is not clear if there are significant benefits to using
 EOF rather than chunking with regards to intermediaries, unless they
 support only HTTP/1.0.

5. HTTP Best Practices

5.1. Limits to the Maximum Number of Connections

 HTTP [RFC2616], Section 8.1.4, recommends that a single user client
 not maintain more than two connections to any server or proxy, in
 order to prevent the server from being overloaded and to avoid
 unexpected side effects in congested networks.  Until recently, this
 limit was implemented by most commonly deployed browsers, thus making
 connections a scarce resource that needed to be shared within the
 browser.  Note that the available JavaScript APIs in the browsers
 hide the connections, and the security model inhibits the sharing of
 any resource between frames.  The new HTTP specification [HTTPBIS]
 removes the two-connection limitation, only encouraging clients to be
 conservative when opening multiple connections.  In fact, recent
 browsers have increased this limit to 6 or 8 connections; however, it
 is still not possible to discover the local limit, and usage of
 multiple frames and tabs still places 8 connections within easy
 reach.
 Web applications need to limit the number of long poll requests
 initiated, ideally to a single long poll that is shared between
 frames, tabs, or windows of the same browser.  However, the security
 constraints of the browsers make such sharing difficult.
 A best practice for a server is to use cookies [COOKIE] to detect
 multiple long poll requests from the same browser and to avoid
 deferring both requests since this might cause connection starvation
 and/or pipeline issues.

Loreto, et al. Informational [Page 13] RFC 6202 Bidirectional HTTP April 2011

5.2. Pipelined Connections

 HTTP [RFC2616] permits optional request pipelining over persistent
 connections.  Multiple requests can be enqueued before the responses
 arrive.
 In the case of HTTP long polling, the use of HTTP pipelining can
 reduce latency when multiple messages need to be sent by a server to
 a client in a short period of time.  With HTTP pipelining, the server
 can receive and enqueue a set of HTTP requests.  Therefore, the
 server does not need to receive a new HTTP request from the client
 after it has sent a message to the client within an HTTP response.
 In principle, the HTTP pipelining can be applied to HTTP GET and HTTP
 POST requests, but using HTTP POST requests is more critical.  In
 fact, the use of HTTP POST with pipelining is discouraged in RFC 2616
 and needs to be handled with special care.
 There is an issue regarding the inability to control pipelining.
 Normal requests can be pipelined behind a long poll, and are thus
 delayed until the long poll completes.
 Mechanisms for bidirectional HTTP that want to exploit HTTP
 pipelining need to verify that HTTP pipelining is available (e.g.,
 supported by the client, the intermediaries, and the server); if it's
 not available, they need to fall back to solutions without HTTP
 pipelining.

5.3. Proxies

 Most proxies work well with HTTP long polling because a complete HTTP
 response will be sent either on an event or a timeout.  Proxies are
 advised to return that response immediately to the user agent, which
 immediately acts on it.
 The HTTP streaming mechanism uses partial responses and sends some
 JavaScript in an HTTP/1.1 chunk as described in Section 3.  This
 mechanism can face problems caused by two factors: (1) it relies on
 proxies to forward each chunk (even though there is no requirement
 for them to do so, and some caching proxies do not), and (2) it
 relies on user agents to execute the chunk of JavaScript as it
 arrives (even though there is also no requirement for them to do so).
 A "reverse proxy" basically is a proxy that pretends to be the actual
 server (as far as any client or client proxy is concerned), but it
 passes on the request to the actual server that is usually sitting
 behind another layer of firewalls.  Any HTTP short polling or HTTP

Loreto, et al. Informational [Page 14] RFC 6202 Bidirectional HTTP April 2011

 long polling solution will work fine with this, as will most HTTP
 streaming solutions.  The main downside is performance, since most
 proxies are not designed to hold many open connections.
 Reverse proxies can come to grief when they try to share connections
 to the servers between multiple clients.  As an example, Apache with
 mod_jk shares a small set of connections (often 8 or 16) between all
 clients.  If long polls are sent on those shared connections, then
 the proxy can be starved of connections, which means that other
 requests (either long poll or normal) can be held up.  Thus, Comet
 mechanisms currently need to avoid any connection sharing -- either
 in the browser or in any intermediary -- because the HTTP assumption
 is that each request will complete as fast as possible.
 One of the main reasons why both HTTP long polling and HTTP streaming
 are perceived as having a negative impact on servers and proxies is
 that they use a synchronous programming model for handling requests,
 since the resources allocated to each request are held for the
 duration of the request.  Asynchronous proxies and servers can handle
 long polls using slightly more resources than normal HTTP traffic.
 Unfortunately some synchronous proxies do exist (e.g., Apache mod_jk)
 and many HTTP application servers also have a blocking model for
 their request handling (e.g., the Java servlet 2.5 specification).

5.4. HTTP Responses

 In accordance with [RFC2616], the server responds to a request it has
 successfully received by sending a 200 OK answer, but only when a
 particular event, status, or timeout has occurred.  The 200 OK body
 section contains the actual event, status, or timeout that occurred.
 This "best practice" is simply standard HTTP.

5.5. Timeouts

 The HTTP long polling mechanism allows the server to respond to a
 request only when a particular event, status, or timeout has
 occurred.  In order to minimize (as much as possible) both latency in
 server-client message delivery and the processing/network resources
 needed, the long poll request timeout ought to be set to a high
 value.
 However, the timeout value has to be chosen carefully; indeed,
 problems can occur if this value is set too high (e.g., the client
 might receive a 408 Request Timeout answer from the server or a 504
 Gateway Timeout answer from a proxy).  The default timeout value in a
 browser is 300 seconds, but most network infrastructures include
 proxies and servers whose timeouts are not that long.

Loreto, et al. Informational [Page 15] RFC 6202 Bidirectional HTTP April 2011

 Several experiments have shown success with timeouts as high as 120
 seconds, but generally 30 seconds is a safer value.  Therefore,
 vendors of network equipment wishing to be compatible with the HTTP
 long polling mechanism are advised to implement a timeout
 substantially greater than 30 seconds (where "substantially" means
 several times more than the medium network transit time).

5.6. Impact on Intermediary Entities

 There is no way for an end client or host to signal to HTTP
 intermediaries that long polling is in use; therefore, long poll
 requests are completely transparent for intermediary entities and are
 handled as normal requests.  This can have an impact on intermediary
 entities that perform operations that are not useful in case of long
 polling.  However, any capabilities that might interfere with
 bidirectional flow (e.g., caching) can be controlled with standard
 headers or cookies.
 As a best practice, caching is always intentionally suppressed in a
 long poll request or response, i.e., the "Cache-Control" header is
 set to "no-cache".

6. Security Considerations

 This document is meant to describe current usage of HTTP to enable
 asynchronous or server-initiated communication.  It does not propose
 any change to the HTTP protocol or to the expected behavior of HTTP
 entities.  Therefore this document does not introduce new security
 concerns into existing HTTP infrastructure.  The considerations
 reported hereafter refer to the solutions that are already
 implemented and deployed.
 One security concern with cross-domain HTTP long polling is related
 to the fact that often the mechanism is implemented by executing the
 JavaScript returned from the long poll request.  If the server is
 prone to injection attacks, then it could be far easier to trick a
 browser into executing the code [CORS].
 Another security concern is that the number of open connections that
 needs to be maintained by a server in HTTP long polling and HTTP
 streaming could more easily lead to denial-of-service (DoS) attacks
 [RFC4732].

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7. References

7.1. Normative References

 [RFC1945]         Berners-Lee, T., Fielding, R., and H. Nielsen,
                   "Hypertext Transfer Protocol -- HTTP/1.0",
                   RFC 1945, May 1996.
 [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.
 [RFC4732]         Handley, M., Rescorla, E., and IAB, "Internet
                   Denial-of-Service Considerations", RFC 4732,
                   December 2006.

7.2. Informative References

 [BAYEUX]          Russell, A., Wilkins, G., Davis, D., and M.
                   Nesbitt, "Bayeux Protocol -- Bayeux 1.0.0", 2007,
                   <http://svn.cometd.com/trunk/bayeux/bayeux.html>.
 [BOSH]            Paterson, I., Smith, D., and P. Saint-Andre,
                   "Bidirectional-streams Over Synchronous HTTP
                   (BOSH)", XSF XEP 0124, February 2007.
 [COMET]           Russell, A., "Comet: Low Latency Data for the
                   Browser", March 2006, <http://infrequently.org/
                   2006/03/comet-low-latency-data-for-the-browser/ >.
 [COOKIE]          Barth, A., "HTTP State Management Mechanism", Work
                   in Progress, March 2011.
 [CORS]            van Kesteren, A., "Cross-Origin Resource Sharing",
                   W3C Working Draft WD-cors-20100727, latest version
                   available at <http://www.w3.org/TR/cors/>,
                   July 2010,
                   <http://www.w3.org/TR/2010/WD-cors-20100727/>.
 [HTTPBIS]         Fielding, R., Ed., Gettys, J., Mogul, J., Nielsen,
                   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",
                   Work in Progress, March 2011.
 [JSONP]           Wikipedia, "JSON with padding",
                   <http://en.wikipedia.org/wiki/JSONP#JSONP>.

Loreto, et al. Informational [Page 17] RFC 6202 Bidirectional HTTP April 2011

 [RFC4627]         Crockford, D., "The application/json Media Type for
                   JavaScript Object Notation (JSON)", RFC 4627,
                   July 2006.
 [RFC6120]         Saint-Andre, P., "Extensible Messaging and Presence
                   Protocol (XMPP): Core", RFC 6120, March 2011.
 [TCP]             Postel, J., "Transmission Control Protocol", STD 7,
                   RFC 793, September 1981.
 [WD-eventsource]  Hickson, I., "Server-Sent Events", W3C Working
                   Draft WD-eventsource-20091222, latest version
                   available at <http://www.w3.org/TR/eventsource/>,
                   December 2009, <http://www.w3.org/TR/2009/
                   WD-eventsource-20091222/>.

8. Acknowledgments

 Thanks to Joe Hildebrand, Julien Laganier, Jack Moffitt, Subramanian
 Moonesamy, Mark Nottingham, Julian Reschke, Martin Thomson, and
 Martin Tyler for their feedback.

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Authors' Addresses

 Salvatore Loreto
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: salvatore.loreto@ericsson.com
 Peter Saint-Andre
 Cisco
 1899 Wyknoop Street, Suite 600
 Denver, CO  80202
 USA
 Phone: +1-303-308-3282
 EMail: psaintan@cisco.com
 Stefano Salsano
 University of Rome "Tor Vergata"
 Via del Politecnico, 1
 Rome  00133
 Italy
 EMail: stefano.salsano@uniroma2.it
 Greg Wilkins
 Webtide
 EMail: gregw@webtide.com

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