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Subject: RESEND - This IETF draft from Keith Moore touches on ebXML issues being discussed.
Network Working Group Keith Moore
Internet-Draft University of Tennessee
4 May 2000
Expires: 4 November 2000
On the use of HTTP as a Substrate for Other Protocols
draft-moore-using-http-00.txt
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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The list of current Internet-Drafts can be accessed at
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Comments regarding this internet-draft should be sent to the
author, whose address appears below.
Abstract
Recently there has been widespread interest in using Hypertext
Transport 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
Transport Protocol (HTTP) [1] as a substrate for other applications-
level protocols. Various reasons cited for this interest have included:
o familiarity and mindshare,
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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.
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
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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 HTTP over
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.
TLS is descended from SSL, which was originally designed to
authenticate servers to clients - not the other way around. Even though
TLS now provides mutual authentication, a client that needs to talk to
multiple servers must still know which credentials to present to each
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server before establishing a secure connection to the server. Client
and server must each use private keys that are trusted by the other
party - typically because they are signed by a certificate authority
(CA) known to the other. As in the digest authentication case, both
client and server need ways to protect their private keys against
exposure.
Web browsers typically are shipped with the public keys of several
CAs "wired in" so that they can verify the identity of any server whose
public key was signed by one of those CAs. This deployment model does
not necessarily work well for other applications, and it doesn't provide
any way for a server to verify a client's identity. Even if the
client's CA is recognized by the server, this doesn't necessarily convey
authorization to use the service. Existing clients and servers may
therefore lack the mechanisms needed for robust authentication using TLS
or SSL and HTTP.
For any application that requires privacy, the 40-bit ciphersuites
provided by some SSL implementations (to conform to outdated US export
regulations) are unsuitable. Even 56-bit DES encryption, which is
required by TLS, has been broken in a matter of days with only a modest
investment in resources.
None of the above should be taken to mean that digest
authentication or TLS are generally unsuitable for use in other
applications - only that they are not a "magic pixie dust" solution to
either authentication or privacy. An application's designers should
carefully determine the application's users' requirements for
authentication and privacy before automatically choosing TLS or digest
authentication.
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 even if TLS and/or digest are suitable for an application, this does
not imply that HTTP should be used.
2.4 Compatibility with Proxies and Firewalls
One oft-cited reason for the use of HTTP is its ability to pass
through proxies or firewalls. Firewalls are an unfortunate consequence
of the Internet's explosive growth, in that they decrease the
deployability of new Internet applications, by requiring explicit
permission (or even a software upgrade) to accommodate each new
protocol.
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 whether or not new
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protocols should be "firewall-friendly", they should definitely not be
"firewall-hostile". In particular, new protocols should not attempt to
circumvent a site's existing security policy.
It would be desirable 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?
o Is this new protocol usable by existing web browsers without
modification?
o Are the existing HTTP security mechanisms appropriate for the new
application?
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
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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 "type" of resource or service. This is used, for instance, by web
browsers that 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 is likely to become widely used, versus used only
in limited communities or by private agreement.
o Whether a new "default port" is needed. A new "default port"
requires a new URL type. 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, or to reserve bandwidth, or to present
different TLS authentication credentials to the server, 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
should not be propagated. 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. Authentication or other connection setup
information should be communicated in URL parameters, rather than in the
URL prefix.
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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 gen-
eral 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 If it is at all useful to be able to use the same payload over
email, 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 Different "commands" or "operations" on the same kind of object can
be communicated in a number of different ways, including different
HTTP methods, different Content-Types, different Content-Type
parameters, the Content-Disposition field, or inside the payload.
Different protocols have solved this problem in different ways.
Again, if it's desirable to provide the same services over elec-
tronic mail, the means of communicating the operation should ide-
ally be the same in both environments.
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. This may be useful, 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 prime 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
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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, the application may need 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 to translate URLs of
the form "xyz://host/something" to "http://host:###/something" for the
purpose of calling the existing HTTP client 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 URL prefix when the request is communicated to the
origin server.
Existing clients and servers will transmit "HTTP/1.1" (or a
different version) in requests and responses. To facilitate reuse of
existing code, protocols layered on top of HTTP must therefore transmit
and accept "HTTP/1.1" rather than their own protocol name and version
number. This may change in the future if client libraries and servers
gain more flexibility.
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.
HTTP's three-digit status codes were designed for use with
traditional HTTP applications, and may not be suitable to communicate
the specifics of errors encountered in other applications. HTTP status
codes should therefore not be used to indicate subtle errors of layered
applications. They should be re-used only to indicate errors with, or
the status of, the HTTP protocol layer; or to indicate the inability of
the HTTP server to communicate with the application server.
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8. 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.
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 to re-use existing ports.
3. New protocols or services should use new URL types.
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 define their own error reporting mechanisms,
and use HTTP status codes only for communicating the state of the
HTTP protocol.
9. 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.
10. Author's address
Keith Moore
University of Tennessee, Knoxville
104 Ayres Hall
Knoxville TN, 37996-1301
USA
email: moore@cs.utk.edu
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11. References
[1] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
Leach, T. Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1.
RFC 2616, June 1999.
[2] J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P. Leach, A.
Luotonen, L. Stewart. HTTP Authentication: Basic and Digest
Access Authentication. RFC 2617, June 1999.
[3] T. Dierks, C. Allen. The TLS Protocol Version 1.0. RFC 2246,
January 1999.
[4] J. Postel, J.K. Reynolds. Telnet Protocol Specification. RFC 854,
May 1983.
[5] J. Postel, J.K. Reynolds. File Transfer Protocol. RFC 959, October
1985.
[6] J. Postel. Simple Mail Transfer Protocol. RFC 821, August 1982.
[7] J. Myers. Simple Authentication and Security Layer (SASL). RFC
2222, October 1997.
[8] R. Petke, I. King. Registration Procedures for URL Scheme Names.
BCP 35, RFC 2717, November 1999.
[9] N. Freed, N. Borenstein. Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types. RFC 2046, November 1996.
[10] T. Howes, M. Smith, F. Dawson. A MIME Content-Type for Directory
Information. RFC 2425, September 1998.
[11] T. Bray, J. Paoli, C. M. 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] E. Whitehead, M. Murata. XML Media Types. RFC 2376, July 1998.
Moore Expires 4 November 2000 [Page 10]
Dick Brooks
Group 8760
110 12th Street North
Birmingham, AL 35203
dick@8760.com
205-250-8053
Fax: 205-250-8057
http://www.8760.com/
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