RFC3205 - On the use of HTTP as a Substrate

Network Working Group K. Moore

Request for Comments: 3205 University of Tennessee

BCP: 56 February 2002

Category: Best Current Practice

On the use of HTTP as a Substrate

Status of this Memo

This document specifies an Internet Best Current Practices for the

Internet Community, and requests discussion and suggestions for

improvements. Distribution of this memo is unlimited.

Abstract

Recently there has been widespread interest in using Hypertext

Transfer Protocol (HTTP) as a substrate for other applications-level

protocols. This document recommends technical particulars of sUCh

use, including use of default ports, URL schemes, and HTTP security

mechanisms.

1. Introduction

Recently there has been widespread interest in using Hypertext

Transfer Protocol (HTTP) [1] as a substrate for other applications-

level protocols. Various reasons cited for this interest have

included:

o familiarity and mindshare,

o compatibility with widely deployed browsers,

o ability to reuse existing servers and client libraries,

o ease of prototyping servers using CGI scripts and similar

extension mechanisms,

o ability to use existing security mechanisms such as HTTP digest

authentication [2] and SSL or TLS [3],

o the ability of HTTP to traverse firewalls, and

o cases where a server often needs to support HTTP anyway.

The Internet community has a long tradition of protocol reuse, dating

back to the use of Telnet [4] as a substrate for FTP [5] and SMTP

[6]. However, the recent interest in layering new protocols over

HTTP has raised a number of questions when such use is appropriate,

and the proper way to use HTTP in contexts where it is appropriate.

Specifically, for a given application that is layered on top of HTTP:

o Should the application use a different port than the HTTP default

of 80?

o Should the application use traditional HTTP methods (GET, POST,

etc.) or should it define new methods?

o Should the application use http: URLs or define its own prefix?

o Should the application define its own MIME-types, or use something

that already exists (like registering a new type of MIME-Directory

structure)?

This memo recommends certain design decisions in answer to these

questions.

This memo is intended as advice and recommendation for protocol

designers, working groups, implementors, and IESG, rather than as a

strict set of rules which must be adhered to in all cases.

Accordingly, the capitalized key Words defined in RFC2119, which are

intended to indicate conformance to a specification, are not used in

this memo.

2. Issues Regarding the Design Choice to use HTTP

Despite the advantages listed above, it's worth aSKINg the question

as to whether HTTP should be used at all, or whether the entire HTTP

protocol should be used.

2.1 Complexity

HTTP started out as a simple protocol, but quickly became much more

complex due to the addition of several features unanticipated by its

original design. These features include persistent connections, byte

ranges, content negotiation, and cache support. All of these are

useful for traditional web applications but may not be useful for the

layered application. The need to support (or circumvent) these

features can add additional complexity to the design and

implementation of a protocol layered on top of HTTP. Even when HTTP

can be "profiled" to minimize implementation overhead, the effort of

specifying such a profile might be more than the effort of specifying

a purpose-built protocol which is better suited to the task at hand.

Even if existing HTTP client and server code can often be re-used,

the additional complexity of layering something over HTTP vs. using a

purpose-built protocol can increase the number of interoperability

problems.

2.2 Overhead

Further, although HTTP can be used as the transport for a "remote

procedure call" paradigm, HTTP's protocol overhead, along with the

connection setup overhead of TCP, can make HTTP a poor choice. A

protocol based on UDP, or with both UDP and TCP variants, should be

considered if the payloads are very likely to be small (less than a

few hundred bytes) for the foreseeable future. This is especially

true if the protocol might be heavily used, or if it might be used

over slow or eXPensive links.

On the other hand, the connection setup overhead can become

negligible if the layered protocol can utilize HTTP/1.1's persistent

connections, and if the same client and server are likely to perform

several transactions during the time the HTTP connection is open.

2.3 Security

Although HTTP appears at first glance to be one of the few "mature"

Internet protocols that can provide good security, there are many

applications for which neither HTTP's digest authentication nor TLS

are sufficient by themselves.

Digest authentication requires a secret (e.g., a password) to be

shared between client and server. This further requires that each

client know the secret to be used with each server, but it does not

provide any means of securely transmitting such secrets between the

parties. Shared secrets can work fine for small groups where

everyone is physically co-located; they don't work as well for large

or dispersed communities of users. Further, if the server is

compromised a large number of secrets may be exposed, which is

especially dangerous if the same secret (or password) is used for

several applications. (Similar concerns exist with TLS based clients

or servers - if a private key is compromised then the attacker can

impersonate the party whose key it has.)

TLS and its predecessor SSL were originally designed to authenticate

web servers to clients, so that a user could be assured (for example)

that his credit card number was not being sent to an imposter.

However, many applications need to authenticate clients to servers,

or to provide mutual authentication of client and server. TLS does

have a capability to provide authentication in each direction, but

such authentication may or may not be suitable for a particular

application.

Web browsers which support TLS or SSL are typically shipped with the

public keys of several certificate authorities (CAs) "wired in" so

that they can verify the identity of any server whose public key was

signed by one of those CAs. For this to work well, every secure web

server's public key has to be signed by one of the CAs whose keys are

wired into popular browsers. This deployment model works when there

are a (relatively) small number of servers whose identities can be

verified, and their public keys signed, by the small number of CAs

whose keys are included in a small number of different browsers.

This scheme does not work as well to authenticate millions of

potential clients to servers. It would take a much larger number of

CAs to do the job, each of which would need to be widely trusted by

servers. Those CAs would also have a more difficult time verifying

the identities of (large numbers of) ordinary users than they do in

verifying the identities of (a smaller number of) commercial and

other enterprises that need to run secure web servers.

Also, in a situation where there were a large number of clients

authenticating with TLS, it seems unlikely that there would be a set

of CAs whose keys were trusted by every server. A client that

potentially needed to authenticate to multiple servers would

therefore need to be configured as to which key to use with which

server when attempting to establish a secure connection to the

server.

For the reasons stated above, client authentication is rarely used

with TLS. A common technique is to use TLS to authenticate the

server to the client and to establish a private channel, and for the

client to authenticate to the server using some other means - for

example, a username and password using HTTP basic or digest

authentication.

For any application that requires privacy, the 40-bit ciphersuites

provided by some SSL implementations (to conform to outdated US

export regulations or to regulations on the use or export of

cryptography in other countries) are unsuitable. Even 56-bit DES

encryption, which is required of conforming TLS implementations, has

been broken in a matter of days with a modest investment in

resources. So if TLS is chosen it may be necessary to discourage use

of small key lengths, or of weak ciphersuites, in order to provide

adequate privacy assurance. If TLS is used to provide privacy for

passwords sent by clients then it is especially important to support

longer keys.

None of the above should be taken to mean that either digest

authentication or TLS are generally inferior to other authentication

systems, or that they are unsuitable for use in other applications

besides HTTP. Many of the limitations of TLS and digest

authentication also apply to other authentication and privacy

systems. The point here is that neither TLS nor digest

authentication is a "magic pixie dust" solution to authentication or

privacy. In every case, an application's designers must carefully

determine the application's users' requirements for authentication

and privacy before choosing an authentication or privacy mechanism.

Note also that TLS can be used with other TCP-based protocols, and

there are SASL [7] mechanisms similar to HTTP's digest

authentication. So it is not necessary to use HTTP in order to

benefit from either TLS or digest-like authentication. However, HTTP

APIs may already support TLS and/or digest.

2.4 Compatibility with Proxies, Firewalls, and NATs

One oft-cited reason for the use of HTTP is its ability to pass

through proxies, firewalls, or network address translators (NATs).

One unfortunate consequence of firewalls and NATs is that they make

it harder to deploy new Internet applications, by requiring explicit

permission (or even a software upgrade of the firewall or NAT) to

accommodate each new protocol. The existence of firewalls and NATs

creates a strong incentive for protocol designers to layer new

applications on top of existing protocols, including HTTP.

However, if a site's firewall prevents the use of unknown protocols,

this is presumably a conscious policy decision on the part of the

firewall administrator. While it is arguable that such policies are

of limited value in enhancing security, this is beside the point -

well-known port numbers are quite useful for a variety of purposes,

and the overloading of port numbers erodes this utility. Attempting

to circumvent a site's security policy is not an acceptable

justification for doing so.

It would be useful to establish guidelines for "firewall-friendly"

protocols, to make it easier for existing firewalls to be compatible

with new protocols.

2.5 Questions to be asked when considering use of HTTP

o When considering payload size and traffic patterns, is HTTP an

appropriate transport for the anticipated use of this protocol?

(In other words: will the payload size be worth the overhead

associated with TCP and HTTP? Or will the application be able to

make use of HTTP persistent connections to amortize the cost of

that overhead over several requests?)

o Is this new protocol usable by existing web browsers without

modification?

(For example: Is the request transmitted as if it were a filled-in

Html form? Is the response which is returned viewable from a web

browser, say as HTML?)

o Are the existing HTTP security mechanisms appropriate for the new

application?

o Are HTTP status codes and the HTTP status code paradigm suitable

for this application? (see section 8)

o Does the server for this application need to support HTTP anyway?

3. Issues Regarding Reuse of Port 80

IANA has reserved TCP port number 80 for use by HTTP. It would not

be appropriate for a substantially new service, even one which uses

HTTP as a substrate, to usurp port 80 from its traditional use. A

new use of HTTP might be considered a "substantially new service",

thus requiring a new port, if any of the following are true:

o The "new service" and traditional HTTP service are likely to

reference different sets of data, even when they both operate on

the same host.

o There is a good reason for the "new service" to be implemented by

a separate server process, or separate code, than traditional HTTP

service on the same host, at least on some platforms.

o There is a good reason to want to easily distinguish the traffic

of the "new service" from traditional HTTP, e.g., for the purposes

of firewall Access control or traffic analysis.

o If none of the above are true, it is arguable that the new use of

HTTP is an "extension" to traditional HTTP, rather than a "new

service". Extensions to HTTP which share data with traditional

HTTP services should probably define new HTTP methods to describe

those extensions, rather than using separate ports. If separate

ports are used, there is no way for a client to know whether they

are separate services or different ways of accessing the same

underlying service.

4. Issues Regarding Reuse of the http: Scheme in URLs

A number of different URL schemes are in widespread use and many more

are in the process of being standardized. In practice, the URL

scheme not only serves as a "tag" to govern the interpretation of the

remaining portion of the URL, it also provides coarse identification

of the kind of resource or service which is being accessed. For

example, web browsers typically provide a different response when a

user mouse-clicks on an "http" URL, than when the user clicks on a

"mailto" URL.

Some criteria that might be used in making this determination are:

o Whether this URL scheme is likely to become widely used, versus

used only in limited communities or by private agreement.

o Whether a new "default port" is needed. If reuse of port 80 is

not appropriate (see above), a new "default port" is needed. A

new default port in turn requires that a new URL scheme be

registered if that URL scheme is expected to be widely used.

Explicit port numbers in URLs are regarded as an "escape hatch",

not something for use in ordinary circumstances.

o Whether use of the new service is likely to require a

substantially different setup or protocol interaction with the

server, than ordinary HTTP service. This could include the need

to request a different type of service from the network, or to

reserve bandwidth, or to present different TLS authentication

credentials to the server, or different kind of server

provisioning, or any number of other needs.

o Whether user interfaces (such as web browsers) are likely to be

able to exploit the difference in the URL prefix to produce a

significant improvement in usability.

According to the rules in [8] the "http:" URI is part of the "IETF

Tree" for URL scheme names, and IETF is the maintainer of the "IETF

Tree". Since IESG is the decision-making body for IETF, IESG has the

authority to determine whether a resource accessed by a protocol that

is layered on top of HTTP, should use http: or some other URL prefix.

Note that the convention of appending an "s" to the URL scheme to

mean "use TLS or SSL" (as in "http:" vs "https:") is nonstandard and

of limited value. For most applications, a single "use TLS or SSL"

bit is not sufficient to adequately convey the information that a

client needs to authenticate itself to a server, even if it has the

proper credentials. For instance, in order to ensure that adequate

security is provided with TLS an application may need to be

configured with a list of acceptable ciphersuites, or with the client

certificate to be used to authenticate to a particular server. When

it is necessary to specify authentication or other connection setup

information in a URL these should be communicated in URL parameters,

rather than in the URL prefix.

5. Issues regarding use of MIME media types

Since HTTP uses the MIME media type system [9] to label its payload,

many applications which layer on HTTP will need to define, or select,

MIME media types for use by that application. Especially when using

a multipart structure, the choice of media types requires careful

consideration. In particular:

o Should some existing framework be used, such as text/directory

[10], or XML [11,12], or should the new content-types be built

from scratch? Just as with HTTP, it's useful if code can be

reused, but protocol designers should not be over-eager to

incorporate a general but complex framework into a new protocol.

Experience with ASN.1, for example, suggests that the advantage of

using a general framework may not be worth the cost.

o Should MIME multipart or message types be allowed? This can be an

advantage if it is desirable to incorporate (for example) the

multipart/alternative construct or the MIME security framework.

On the other hand, these constructs were designed specifically for

use in store-and-forward electronic mail systems, and other

mechanisms may be more appropriate for the application being

considered.

The point here is that a decision to use MIME content-type names

to describe protocol payloads (which is generally desirable if the

same payloads may appear in other applications) does not imply

that the application must accept arbitrary MIME content-types,

including MIME multipart or security mechanisms. Nor does it

imply that the application must use MIME syntax or that it must

recognize or even tolerate existing MIME header fields.

o If the same payload is likely to be sent over electronic mail, the

differences between HTTP encoding of the payload and email

encoding of the payload should be minimized. Ideally, there

should be no differences in the "canonical form" used in the two

environments. Text/* media types can be problematic in this

regard because MIME email requires CRLF for line endings of text/*

body parts, where HTTP traditionally uses LF only.

o A MIME content-type label describes the nature of the object being

labeled. It does not describe, and should not be used to

describe, the semantics which should be applied when the object is

received. For instance, the transmission of an object with a

particular content-type using HTTP POST, should not be taken as a

request for some operation based solely on the type. The request

should be separate from the content-type label and it should be

explicit.

When it is necessary for a protocol layered on HTTP to allow

different operations on the same type of object, this can be

communicated in a number of different ways: HTTP methods, HTTP

request-URI, HTTP request headers, the MIME Content-Disposition

header field, or as part of the payload.

6. Issues Regarding Existing vs. New HTTP Methods

It has been suggested that a new service layered on top of HTTP

should define one or more new HTTP methods, rather than allocating a

new port. The use of new methods may be appropriate, but is not

sufficient in all cases. The definition of one or more new methods

for use in a new protocol, does not by itself alleviate the need for

use of a new port, or a new URL type.

7. Issues regarding reuse of HTTP client, server, and proxy code

As mentioned earlier, one of the primary reasons for the use of HTTP

as a substrate for new protocols, is to allow reuse of existing HTTP

client, server, or proxy code. However, HTTP was not designed for

such layering. Existing HTTP client and code may have "http"

assumptions wired into them. For instance, client libraries and

proxies may expect "http:" URLs, and clients and servers may send

(and expect) "HTTP/1.1", in requests and responses, as opposed to the

name of the layered protocol and its version number.

Existing client libraries may not understand new URL types. In order

to get a new HTTP-layered application client to work with an existing

client library, it may be necessary for the application to convert

its URLs to an "http equivalent" form. For instance, if service

"xyz" is layered on top of HTTP using port ###, the xyz client may

need, when invoking an HTTP client library, to translate its URLs

from "xyz://host/something" format to "http://host:###/something" for

the purpose of calling that library. This should be done ONLY when

calling the HTTP client library - such URLs should not be used in

other parts of the protocol, nor should they be exposed to users.

Note that when a client is sending requests directly to an origin

server, the URL prefix ("http:") is not normally sent. So

translating xyz: URLs to http: URLs when calling the client library

should not actually cause http: URLs to be sent over the wire. But

when the same client is sending requests to a proxy server, the

client will normally send the entire URL (including the http: prefix)

in those requests. The proxy will remove the http: prefix when the

request is communicated to the origin server.

Existing HTTP client libraries and servers will transmit "HTTP/1.1"

(or a different version) in requests and responses. To facilitate

reuse of such libraries and servers by a new protocol, such a

protocol may therefore need to transmit and accept "HTTP/1.1" rather

than its own protocol name and version number. Designers of

protocols which are layered on top of HTTP should explicitly choose

whether or not to accept "HTTP/1.1" in protocol exchanges.

For certain applications it may be necessary to require or limit use

of certain HTTP features, for example, to defeat caching of responses

by proxies. Each protocol layered on HTTP must therefore specify the

specific way that HTTP will be used, and in particular, how the

client and server should interact with HTTP proxies.

8. Issues regarding use of HTTP status codes

HTTP's three-digit status codes were designed for use with

traditional HTTP applications (e.g., document retrieval, forms-based

queries), and are unlikely to be suitable to communicate the

specifics of errors encountered in dissimilar applications. Even

when it seems like there is a close match between HTTP status codes

and the codes needed by the application, experience with reuse of

other protocols indicates that suBTle variations in usage are likely;

and that this is likely to degrade interoperability of both the

original protocol (in this case HTTP) and any layered applications.

HTTP status codes therefore should not be used to indicate subtle

errors of layered applications. At most, the "generic" HTTP codes

200 (for complete success) and 500 (for complete failure) should be

used to indicate errors resulting from the content of the request

message-body. Under certain circumstances, additional detail about

the nature of the error can then be included in the response

message-body. Other status codes than 200 or 500 should only appear

if the error was detected by the HTTP server or by an intermediary.

A layered application should not define new HTTP status codes. The

set of available status codes is small, conflicts in code assignment

between different layered applications are likely, and they may be

needed by future versions of, or extensions to, mainstream HTTP.

Use of HTTP's error codes is problematic when the layered application

does not share same notion of success or failure as HTTP. The

problem exists when the client does not connect directly to the

origin server, but via one or more HTTP caches or proxies. (Since

the ability of HTTP to communicate through intermediaries is often

the primary motivation for reusing HTTP, the ability of the

application to operate in the presence of such intermediaries is

considered very important.) Such caches and proxies will interpret

HTTP's error codes and may take additional action based on those

codes. For instance, on receipt of a 200 error code from an origin

server (and under other appropriate conditions) a proxy may cache the

response and re-issue it in response to a similar request. Or a

proxy may modify the result of a request which returns a 500 error

code in order to add a "helpful" error message. Other response codes

may produce other behaviors.

A few guidelines are therefore in order:

o A layered application should use appropriate HTTP error codes to

report errors resulting from information in the HTTP request-line

and header fields associated with the request. This request

information is part of the HTTP protocol and errors which are

associated with that information should therefore be reported

using HTTP protocol mechanisms.

o A layered application for which all errors resulting from the

message-body can be classified as either "complete success" or

"complete failure" may use 200 and 500 for those conditions,

respectively. However, the specification for such an application

must define the mechanism which ensures that its successful (200)

responses are not cached by intermediaries, or demonstrate that

such caching will do no harm; and it must be able to operate even

if the message-body of an error (500) response is not transmitted

back to the client intact.

o A layered application may return a 200 response code for both

successfully processed requests and errors (or other exceptional

conditions) resulting from the request message-body (but not from

the request headers). Such an application must return its error

code as part of the response message body, and the specification

for that application protocol must define the mechanism by which

the application ensures that its responses are not cached by

intermediaries. In this case a response other than 200 should be

used only to indicate errors with, or the status of, the HTTP

protocol layer (including the request headers), or to indicate the

inability of the HTTP server to communicate with the application

server.

o A layered application which cannot operate in the presence of

intermediaries or proxies that cache and/or alter error responses,

should not use HTTP as a substrate.

9. Summary of recommendations regarding reuse of HTTP

1. All protocols should provide adequate security. The security

needs of a particular application will vary widely depending on

the application and its anticipated use environment. Merely using

HTTP and/or TLS as a substrate for a protocol does not

automatically provide adequate security for all environments, nor

does it relieve the protocol developers of the need to analyze

security considerations for their particular application.

2. New protocols - including but not limited to those using HTTP -

should not attempt to circumvent users' firewall policies,

particularly by masquerading as existing protocols.

"Substantially new services" should not reuse existing ports.

3. In general, new protocols or services should not reuse http: or

other URL schemes.

4. Each new protocol specification that uses HTTP as a substrate

should describe the specific way that HTTP is to be used by that

protocol, including how the client and server interact with

proxies.

5. New services should follow the guidelines in section 8 regarding

use of HTTP status codes.

10. Security Considerations

Much of this document is about security. Section 2.3 discusses

whether HTTP security is adequate for the needs of a particular

application, section 2.4 discusses interactions between new HTTP-

based protocols and firewalls, section 3 discusses use of separate

ports so that firewalls are not circumvented, and section 4 discusses

the inadequacy of the "s" suffix of a URL prefix for specifying

security levels.

11. References

[1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,

Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --

HTTP/1.1", RFC2616, June 1999.

[2] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,

Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:

Basic and Digest Access Authentication", RFC2617, June 1999.

[3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC

2246, January 1999.

[4] Postel, J. and J. Reynolds, "Telnet Protocol Specification",

STD 8, RFC854, May 1983.

[5] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,

RFC959, October 1985.

[6] Klensin, J., "Simple Mail Transfer Protocol", RFC2821, April

2001.

[7] Myers, J., "Simple Authentication and Security Layer (SASL)",

RFC2222, October 1997.

[8] Petke, R. and I. King, "Registration Procedures for URL Scheme

Names", BCP 35, RFC2717, November 1999.

[9] Freed, N. and N. Borenstein, "Multipurpose Internet Mail

Extensions (MIME) Part Two: Media Types", RFC2046, November

1996.

[10] Howes, T., Smith, M. and F. Dawson, "A MIME Content-Type for

Directory Information", RFC2425, September 1998.

[11] Bray, T., Paoli, J. and C. Sperberg-McQueen, "Extensible Markup

Language (XML)" World Wide Web Consortium Recommendation REC-

xml-19980210, February 1998. http://www.w3.org/TR/1998/REC-

xml-19980210.

[12] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC

3023, January 2001.

12. Author's Address

Keith Moore

University of Tennessee

Computer Science Department

1122 Volunteer Blvd, Suite 203

Knoxville TN, 37996-3450

USA

EMail: moore@cs.utk.edu

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