Network Working Group S. Loreto
Internet-Draft Ericsson
Intended status: Informational P. Saint-Andre
Expires: August 27, 2010 Cisco
S. Salsano
Univ. of Rome "Tor Vergata"
G. Wilkins
Webtide
Feb 23, 2010
Best Practices for the Use of Long Polling and Streaming in
Bidirectional HTTP
draft-loreto-http-bidirectional-02
Abstract
There is widespread interest in using the Hypertext Transfer Protocol
(HTTP) to enable asynchronous or server-initiated communication from
a server to a client as well as from a client to a server. This
document describes how to better use HTTP, as it exists today, to
enable such "bidirectional HTTP" using "long polling" and "HTTP
streaming" mechanisms.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 27, 2010.
Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Long Polling . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Long Polling Issues . . . . . . . . . . . . . . . . . . . 5
3. HTTP Streaming . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. HTTP Streaming Issues . . . . . . . . . . . . . . . . . . 8
4. Overview of Technologies . . . . . . . . . . . . . . . . . . . 9
4.1. Bayeux . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. BOSH . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Server-Sent Events . . . . . . . . . . . . . . . . . . . . 12
5. HTTP Best Practices . . . . . . . . . . . . . . . . . . . . . 12
5.1. Two Connection Limit . . . . . . . . . . . . . . . . . . . 12
5.2. Pipelined Connections . . . . . . . . . . . . . . . . . . 13
5.3. Proxies . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.4. HTTP Responses . . . . . . . . . . . . . . . . . . . . . . 14
5.5. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.6. Impact on intermediary entities . . . . . . . . . . . . . 15
6. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. Informative References . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
The Hypertext Transfer Protocol [HTTP-1.1] 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 the 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.
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 open and close connections necessary
to periodically poll for data.
The two most common server push mechanisms are "Long Polling" and
"HTTP Streaming":
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
available to use for 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
bidirectional HTTP technologies that work within the current scope of
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HTTP as defined in [HTTP-1.1] and [HTTP-1.0].
The remainder of this document is organized as follows. Section 2
analyzes the "long polling" technique. Section 3 analyzes the "HTTP
streaming" technique. Section 4 provides an overview of the specific
technologies that use server-push technique. Section 5 lists best
practices for bidirectional HTTP using existing technologies.
The preferred venue for discussion of this document is the
apps-discuss@ietf.org; visit
<https://www.ietf.org/mailman/listinfo/apps-discuss> for further
information.
2. 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 a period before sending another poll request. The frequency
depends on the latency which can be tolerated 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 frequency of the poll request can cause an
unacceptable burden on the server, the network, or both.
By contrast with such "short polling", "long polling" attempts to
minimize both latency in server-client message delivery and the
processing/network resource as compared to normal polling techniques.
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.
The basic life cycle of an application using "long polling" is as
follows:
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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 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 or after a pause to allow an acceptable latency
period.
The 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
TCP/IP connection for every long poll.
2.2. Long Polling Issues
The long polling mechanism introduces the following issues.
Header Overhead: With the 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. This does not introduce
significant technical issues if the network MTU allows all the
information (including the HTTP header) to fit within a single IP
packet. On the other hand, it can introduce business issues
related to data cost, as the amount of transferred data can be
significantly larger than the real payload carried by HTTP.
Maximal Latency: After a long polling response is sent to a client,
the server must wait for the next long polling 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 on TCP/IP, packet loss and
retransmission can occur, so maximal latency for any TCP/IP
protocol will be more than three network transits (lost packet,
next packet, negative ack, retransmit).
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
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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 and network appliances will
allocate resources to TCP/IP connections and to HTTP requests
outstanding on those requests. The 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 a 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 clients will
queue (waiting either for a long poll request or for available CPU
to use a held long poll request). If multiple messages are queued
for a client, then they may be delivered in a batch within a
single long poll response. This can significantly reduce the per-
message overhead and thus ease the work load of the client or
server for the given message load.
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 a particular status or timeout has occurred.
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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 on 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.
A HTTP response content length can be defined using 3 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 be break into chunks of known size.
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 still possible to use EOF by setting the 'Connection:close'
header in either the request or the response, to indicate that the
connection should not be considered 'persistent' after the 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. By
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. It does so by including the header
"Transfer-Encoding: chunked" at the beginning of the response, which
enables it 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).
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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
0
Figure 1: Transfer-Encoding response
A HTTP/1.0 server will omit the Content-Length header in the response
to achieve the same result, so it will be able to send the following
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.
Network Intermediaries: The HTTP protocol allows for intermediaries
(proxies, transparent proxies, gateways, etc.) to be involved in
the transmission of a response from server to the client. There
is no requirement for an intermediary to immediately forward a
partial response and it is legal for it 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 elements that will grow in size for every
message received. Thus in order to avoid unlimited memory growth
in the client, an HTTP streaming implementation must occasionally
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 on TCP/IP, packet loss
and retransmission can occur, so maximal latency for any TCP/IP
protocol will be more than three network transits (lost packet,
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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, but some require a buffer overflow to trigger
execution, so 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 perfomed 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 long
polling technique, which provides a canonical framing technique:
each application message can be sent in a different HTTP response.
4. Overview of Technologies
This section provides an overview of how the specific technologies
that implement server-push mechanisms employ HTTP 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 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
pipeling 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
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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 by non-Bayeux JavaScript
simultaneously issuing 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 "text/json; charset=utf-8" MIME
content type.
For browsers that are operating in cross-domain mode, Bayeux clients
use the "script src Ajax" ("AJAST") mechanism as described at
<http://en.wikipedia.org/wiki/AJAST_(programming)>.
Client-to-server messages are sent as encoded JSON on the URL query
parameters.
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 [XMPP]). BOSH
employs the 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.
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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. 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, respecting at the same time the two-
connection limit discussed here under Section 5.1.
The number of long polling 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 to use more than two pairs.
The roles of the two response-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 to the blocked request on the other TCP
connection, thus freeing it; in this way, in a scenario where only
the client sends data, all the even requests are sent over one
connection and the odd ones are sent over the other connection.
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 relay 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 realise 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 it is able to push again.
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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.
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).
4.3. Server-Sent Events
W3C Server-Sent Events specification [W3C.WD-eventsource-20090423]
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 a
HTTP streaming mechanism, but the specification suggests to disable
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 here under Section 3.2.
However it is not clear the benefit of using EOF rather than chunking
with regards to intermediaries, unless they are HTTP/1.0.
5. HTTP Best Practices
5.1. Two Connection Limit
HTTP [HTTP-1.1] section 8.1.4 specified that a single user client
should not maintain more than two connections to any server or proxy,
to prevent the server from being overloaded and to avoid unexpected
side effects in congested networks. The new specification
[I-D.ietf-httpbis-p1-messaging] removes the limitation, encouraging
clients to be conservative when opening multiple connections.
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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 possible best practice is for a server to use cookies 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.
5.2. Pipelined Connections
HTTP [HTTP-1.1] permits optional request pipelining over persistent
connections. Multiple requests can be enqueued before the responses
arrive.
There is a possible open 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.
5.3. Proxies
Most proxies work well with long polling, because a complete HTTP
response must be sent either on an event or a timeout. Proxies
should 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 under 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 short polling or long polling
Comet solution should work fine with this, as will most streaming
Comet connections. The main downside is performance, since most
proxies are not designed to hold many open connections (as a
dedicated Comet server is).
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
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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.
Much of the "badness" of both long polling and HTTP streaming for
servers and proxies results from using 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 Comet long polls with few resources
above that of 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
The server responds to a request 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.
5.5. Timeouts
The 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 polling request timeout should be set to a high value.
However, the value timeout value has to be chosen carefully; indeed,
there can be problem 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 have proxies
and server that do not have such a long timeout.
Several experiments have shown success with timeouts as high as 120
seconds, but generally 30 seconds is a safer value. Therefore it is
recommended that all network equipment wishing to be compatible with
the long polling mechanism should implement a timeout substantially
greater than 30 seconds (where "substantially" means several times
more than the medium network transit time).
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5.6. Impact on intermediary entities
There is no way for an end client or host to give HTTP intermediaries
the hints that long-polling is in use, therefore long-poll requests
are completely transparent for intermediary entities and are handled
as normal requests. This may have an impact for intermediary
entities that perform operations that are not useful in case of long-
polling. However any capabilities that may interfere with
bidirectional flow (e.g. caching) can be controlled with standard
headers or cookies.
The caching is always intentionally suppressed in a long-poll request
or response: the "Cache-Control" header is set to "no-cache".
OPEN ISSUE 1: Any experience on different headers or on the usage of
cookies?
OPEN ISSUE 2: Any well known impact on Firewall(s) and NATs?
6. Future Work
This document focuses on best practices for bidirectional HTTP in the
context of HTTP as it exists today. Future documents might define
additions to HTTP that could enable improved mechanisms for
bidirectional HTTP. Examples include:
o An HTTP extension for long polling, including request tracking,
duplication, and retry methods.
o A method for monitoring the state of multiple resources.
o A request header to determine timeouts.
o A request header to determine the longest acceptable polling
interval.
o Improved rendezvous logic between the user agent, a proxy /
connection manager, and the backend application server.
o Improved addressing for the entities involved in bidirectional
HTTP, possibly including the use of URI templates.
o Possible improvements/extensions to XMLHttpRequest (XHR) API
[W3C.WD-XMLHttpRequest2-20090820] to expose connection-handling
details (e.g., use of pconns, pipelining, etc.)
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7. Acknowledgments
Thanks to Joe Hildebrand, Mark Nottingham, Martin Thomson and Martin
Tyler for their feedback.
8. IANA Considerations
This document does not require any actions by the IANA.
9. 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 expexted behaviour of http
entities. Therefore this document does not introduce new security
concerns into existing http infrastructure.
OPEN ISSUE 3: the number of open connections in long polling and http
streaming could more easily lead to DOS attacks [RFC4732].
OPEN ISSUE 4: a possible slight security concern with cross site long
polling is related to the fact that often it is implemented by
executing the javascript returned from the long-poll request. If the
server is prone to an injection attacks, then it could be far easier
to trick a browser into executing the code [W3C.WD-cors-20090317].
10. Informative References
[BAYEUX] Russell, A., Wilkins, G., Davis, D., and M. Nesbitt,
"Bidirectional-streams Over Synchronous HTTP (BOSH)",
2007.
[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-1.0]
Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[HTTP-1.1]
Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
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Internet-Draft Bidirectional HTTP Feb 2010
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-08 (work in
progress), October 2009.
[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[W3C.WD-XMLHttpRequest2-20090820]
Kesteren, A., "XMLHttpRequest Level 2", World Wide Web
Consortium WD WD-XMLHttpRequest2-20090820, August 2009,
<http://www.w3.org/TR/2009/WD-XMLHttpRequest2-20090820>.
[W3C.WD-cors-20090317]
Kesteren, A., "Cross-Origin Resource Sharing", World Wide
Web Consortium WD WD-cors-20090317, March 2009,
<http://www.w3.org/TR/2009/WD-cors-20090317>.
[W3C.WD-eventsource-20090423]
Hickson, I., "Server-Sent Events", World Wide Web
Consortium WD WD-eventsource-20090423, April 2009,
<http://www.w3.org/TR/2009/WD-eventsource-20090423>.
[XMPP] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
Authors' Addresses
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: salvatore.loreto@ericsson.com
Peter Saint-Andre
Cisco
Email: psaintan@cisco.com
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Stefano Salsano
Univ. 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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