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Internet Engineering Task Force Yaron Y. Goland
INTERNET DRAFT Microsoft Corporation
November 9, 1999
Expires April 2000
Multicast and Unicast UDP HTTP Messages
<draft-goland-http-udp-01.txt>
Status of this Memo
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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Please send comments to the SSDP mailing list. Subscription
information for the SSDP mailing list is available at
http://www.upnp.org/resources/ssdpmail.htm.
Abstract
This document provides rules for encapsulating HTTP messages in
Multicast and Unicast UDP packets to be sent within a single
administrative scope. No provisions are made for guaranteeing
delivery beyond re-broadcasting.
1. Introduction
This document provides rules for encapsulating HTTP messages in
multicast and unicast UDP messages. No provisions are made for
guaranteeing delivery beyond re-broadcasting.
This technology is motivated by applications such as SSDP where it
is expected that messages which are primarily transmitted over TCP
HTTP need to be transmitted over Multicast or Unicast UDP in extreme
circumstances.
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This document will not specify a mechanism suitable for replacing
HTTP over TCP. Rather this document will define a limited mechanism
only suitable for extreme circumstances where the use of TCP is
impossible. Thus this mechanism will not have the robustness of
functionality and congestion control provided by TCP. It is expected
that in practice the mechanisms specified here in will only be used
as a means to get to TCP based HTTP communications.
2. Changes
2.1. Since 00
Divided each section of the spec into three parts, problem
definition, proposed solution and design rationale. When the spec is
ready for standardization the problem definition and design
rationale sections will be removed. Design rationale is presented in
question/answer form because I have found that to be very effective
in addressing design issues.
Clarified that a HTTPU/HTTPMU URI without an abs_path translates to
"*" in the request-URI.
Added the "S" header to allow request and responses to be
associated. Note that while clients aren't require to send out "S"
headers servers are required to return them.
Got rid of MM. The lower bound is always 0.
The introduction of the "S" makes proxying and caching possible so
the sections on those topics have been expanded, but they should be
considered experimental at best.
3. Terminology
Since this document describes a set of extensions to the HTTP/1.1
protocol, the augmented BNF used herein to describe protocol
elements is exactly the same as described in section 2.1 of
[RFC2616]. Since this augmented BNF uses the basic production rules
provided in section 2.2 of [RFC2616], these rules apply to this
document as well.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
4. HTTPU URL
4.1. Problem Definition
A mechanism is needed to allow for communications that are to be
sent over Unicast UDP HTTP to be identified in the URI namespace.
4.2. Proposed Solution
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The HTTPU URL specifies that the HTTP request is to be sent over
unicast UDP according to the rules laid out in this document.
HTTPU_URL = "HTTPU:" "//" host [ ":" port ] [ abs_path [ "?" query]]
The BNF productions host, port and abs_path are defined in
[RFC2616].
The syntax of the HTTPU URL is to be processed identically to the
HTTP URL with the exception of the transport.
One MUST NOT assume that if a HTTP, HTTPU or HTTPMU URL are
identical in all ways save the protocol that they necessarily point
to the same resource.
4.3. Design Rationale
4.3.1. Why would we ever need a HTTPU/HTTPMU URL?
Imagine one wants to tell a system to send responses over HTTPU. How
would one express this? If one uses a HTTP URL there is no way for
the system to understand that you really meant HTTPU.
5. HTTPMU URL
5.1. Problem Definition
A mechanism is needed to allow for communications that are to be
sent over Multicast UDP HTTP to be identified in the URI namespace.
5.2. Proposed Solution
The HTTPMU URL specifies that the HTTP request that HTTP request is
to be sent over multicast UDP according to the rules laid out in
this document.
HTTPMU_URL = "HTTPMU:" "//" host [ ":" port ] [ abs_path [ "?"
query]]
The BNF productions host, port and abs_path are defined in
[RFC2616].
The syntax of the HTTPMU URL is to be processed identically to the
HTTP URL with the exception of the transport.
One MUST NOT assume that if a HTTP, HTTPU or HTTPMU URL are
identical in all ways save the protocol that they necessarily point
to the same resource.
If a HTTPMU URL does not have an abs_path element then when the HTTP
UDP multicast request is made the request-URI MUST be "*".
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For example, HTTPU://www.foo.com would translate into a request-URI
of "*". Note, however, that HTTPU://www.foo.com/ would translate
into a request-URI of "/".
5.3. Design Rationale
5.3.1. In the HTTPMU URL a request such as http://www.foo.com is
translated to a "*" in the request-URI rather than a "/", why isn't the
same the case for HTTPU?
A HTTPU request is a point-to-point request. There is one sender and
one receiver. Thus the semantics of the URL are identical to HTTP
with the exception of the transport.
In HTTPMU a request, generally, is going to many receivers. The way
to indicate this on a HTTPMU request is by using the URI "*". Since
using "*" is probably the single most common way to send a HTTPMU
request there needed to be a way to indicate that the request-URI
should be "*". There is no way to do that today with a HTTP URL.
Therefore a mechanism had to be added.
As a side note, one could send a point-to-point request of HTTPMU.
One need only put a particular request-URI in the request. Only the
resource matching that request-URI will respond.
6. Unicast UDP HTTP Messages
6.1. Problem Definition
A mechanism is needed to send HTTP messages over the unicast UDP
transport.
6.2. Proposed Solution
HTTP messages sent over unicast UDP function identically to HTTP
messages sent over TCP as defined in [RFC2616] except as specified
below.
For brevity's sake HTTP messages sent over unicast UDP will be
referred to as HTTPU messages.
HTTPU messages MUST fit entirely in a single UDP message. If a HTTPU
message can not be fit into a single UDP message then it MUST NOT be
sent using unicast UDP. Incomplete HTTPU messages SHOULD be ignored.
The request-URI of a HTTPU message MUST always be fully qualified.
A single unicast UDP message MUST only contain a single HTTPU
message.
A HTTPU request without a "S" header MUST NOT be responded to. When
responding to a HTTPU request with a "S" header the rules for the
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proper handling of "S" headers, as specified in section 11.3 MUST be
followed.
6.3. Design Rationale
See section 11.3.3 for the design rationale of the "S" header.
6.3.1. Why can't a single HTTP message be sent over multiple UDP
messages?
The ability to send unlimited size messages across the Internet is
one of the key features of TCP. The goal of this paper is not to re-
invent TCP but rather to provide a very simple emergency back up
HTTP system that can leverage UDP where TCP can not be used. As such
features to allow a single HTTP message to span multiple UDP
messages is not provided.
6.3.2. Why are request-URIs sent over HTTPU required to be fully
qualified?
A relative URI in a HTTP message is assumed to be relative to a HTTP
URL. However this would clearly be inappropriate for a HTTPU or
HTTPMU message. The easiest solution would be to simply state that a
relative URI is relative to the type of message it was sent in. But
one of the unstated (but now stated) goals of this draft is to allow
current HTTP message processors to be able to happily munch on
HTTPU/HTTPMU messages and this would cause a change to those
processors. Besides, relative URIs were always wacky, a left over
from the early days of HTTP.
The cost of this simplification is that you repeat the host
information, once in the URI and once in the host header.
Eventually the host header will go away and we will all use fully
qualified URIs. But again, taking out the host header would make a
lot of existing HTTP message munchers very unhappy.
6.3.3. Why is the requirement for ignoring incomplete HTTPU messages
a SHOULD instead of a MUST?
Some systems use a lot of redundant data or have good mechanisms for
handling partial data. As such they could actually do something
intelligent with a partial message. A SHOULD allows them to do this
while still making it clear that in the majority case partial
HTTPU/HTTPMU messages are going to get thrown out.
6.3.4. Why aren't multiple HTTP messages allowed into a single UDP
message if they will fit?
It was easier to ban it and it didn't seem to buy us much. It was
especially worrying because it would start to convince people that
they could actually order their UDP requests in a pipelinesque
manner. It was easier to just keep things simple and ban it.
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6.3.5. Why aren't we allowed to leave off content-lengths if only a
single HTTPU message is allowed in a UDP message?
In general we try to only change from RFC 2616 when we are forced
to. Although including a content-length is annoying it makes it easy
to use HTTP/1.1 message parsing/generating systems with this spec.
6.3.6. Why might a HTTPU message choose to not have a "S" header
thus making it impossible to respond to it?
Leaving off the "S" header would be useful for throw away events. In
systems with a high event rate it is usually easier to just throw
away an event rather than re-sending it. As such there is no real
benefit to confirming that the event was received since it won't be
resent if it wasn't received.
6.3.7. Why isn't the mx header used on HTTPU messages?
As HTTPU messages are point-to-point there will be exactly one
response. Mx is only useful in cases, such as HTTPMU requests, where
there can be many potential responses from numerous different
clients. Mx helps to prevent the client from getting creamed with
responses.
6.3.8. Can I send 1xx responses over HTTPU?
Yes. Error handling is identical to RFC 2616.
7. Multicast UDP HTTP Requests
7.1. Problem Definition
A mechanism is needed to send HTTP messages over the multicast UDP
transport.
7.2. Proposed Solution
HTTP messages sent over multicast UDP MUST obey all the requirements
for HTTPU messages in addition to the requirements provided below.
For brevity's sake HTTP messages sent over multicast UDP will be
referred to as HTTPMU messages.
Resources that support receiving multicast UDP HTTP requests MUST
honor the mx header if included in the request.
Resources are required to generate a random number between 0 and mx
that represents the number of seconds the resource must wait before
sending a response. This prevents all responses from being sent at
once. HTTP clients SHOULD keep listening for responses for a
reasonable delta of time after mx. That delta will be based on the
type of network the request is being sent over. This means that if a
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server cannot respond to a request before mx then there is little
point in sending the response as the client will most likely not be
listening for it.
When used with a multicast UDP HTTP request the "*" request-URI
means "to everyone who is listening to this IP address and port."
A HTTPMU request without a mx header MUST NOT be responded to.
7.3. Design Rationale
7.3.1. Why is there a "delta" after the mx time when the client
should still be listening?
So let's say the mx value is 5 seconds. The HTTP resource generates
a number between 0 and 5 and gets 5. After 5 seconds of waiting the
HTTP resource will send its response.
Now for some math:
0.5 seconds - Time it took the client's request to reach the HTTP
resource.
5 seconds - Time the HTTP resource waited after receiving the
message to respond, based on the mx value.
0.5 seconds - Time for the response to get back to the client.
Total time elapsed - 6 seconds
If the client only waits 5 seconds, the mx value, then they would
have stopped listening for this response by the time it arrived.
Hence the need for the delta.
7.3.2. What should the "delta" after mx expires be?
Unfortunately this is an impossible question to answer. How fast is
your network? How far is the message going? Is there any congestion?
In general delta values will be set based on a combination of
heuristics and application necessity. That is, if you are displaying
information to a user any data that comes in after 20 or 30 seconds
is probably too late.
7.3.3. When would a HTTPMU request not be responded to?
When a HTTP resource is making a general announcement, such as "I am
here", it generally isn't useful to have everyone respond confirming
they received the message. This is especially the case given that
the HTTP resource probably doesn't know who should have received the
announcement so the absence of a HTTP client in the responses
wouldn't be meaningful.
7.3.4. Why do we require the mx header on HTTPMU requests that are
to be responded to?
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This is to prevent overloading the HTTP resource. If all the HTTP
clients responded simultaneously the resource would probably loose
most of the responses as its UDP buffer overflowed.
8. Retrying Requests
8.1. Problem Definition
UDP is an unreliable transport with no failure indicators, as such
some mechanism is needed to reasonably increase the chance that a
HTTPU/HTTPMU message will be delivered.
8.2. Proposed Solution
UDP is an inherently unreliable transport and subject to routers
dropping packets without notice. Applications requiring delivery
guarantees SHOULD NOT use HTTPU or HTTPMU.
In order to increase the probability that a HTTPU or HTTPMU message
is delivered the message MAY be repeated several times.
In order to prevent the network from being flooded a message SHOULD
NOT be repeated more than MAX_RETRIES time. A random period of time
between 0 and MAX_RETRY_INTERVAL SHOULD be selected between each
retry to determine how long to wait before issuing the retry.
8.3. Design Rationale
8.3.1. Why is the requirement "applications requiring delivery
guarantees should not use HTTPU or HTTPMU" only a SHOULD and not a
MUST?
Because there might come a day when it makes sense to use HTTPU or
HTTPMU for guaranteed delivery and there is no reason to completely
ban the possibility.
8.3.2. Why is the requirement that a request not be repeated more
than MAX_RETRIES times a SHOULD and not a MUST?
Local knowledge may make the limit unnecessary. For example, if one
knew that the message was being delivered using a super reliable
network then repeats are not necessary. Similarly if one knew that
the network the requests were going through were particularly
unreliable and assuming one had properly accounted for the effects
of additional messages on that congestion, one might have a good
reason to send more than MAX_RETRIES.
9. Caching UDP HTTP Requests
9.1. Problem Definition
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Caching is a feature that has demonstrated its usefulness in HTTP,
provisions need to be made so as to ensure that HTTPU/HTTPMU
messages can be cached using a consistent algorithm.
9.2. Proposed Solution
[Ed. Note: Never having tried to actually build a HTTPU/HTTPMU
generic cache we suspect there are some very serious gotchas here
that we just haven't found yet. This section should definitely be
treated as "under development."]
Caching rules for HTTPU/HTTPMU responses are no different than
normal HTTP responses. HTTPU/HTTPMU responses are matched to their
requests through the "S" header value.
9.3. Design Rationale
9.3.1. Wouldn't it be useful to be able to cache HTTPU/HTTPMU
requests if they don't have responses?
Yes, it probably would. Especially if we are talking about a client
side cache. It is probably worth investigating the use of cache
control headers on requests for this very purpose.
10. Proxying UDP HTTP Requests
10.1. Problem Definition
For security or caching reasons it is sometimes necessary to place a
proxy in a message path. Provisions need to be made so as to ensure
that HTTPU/HTTPMU messages can be proxied.
10.2. Proposed Solution
[Ed. Note: This section should be considered experimental. No one
has really had to design much less implement a HTTPU/HTTPMU proxy
yet.]
All transport independent rules for proxying, such as length of time
to cache a response, hop-by-hop header rules, etc. are the same for
HTTPU/HTTPMU as they are for HTTP messages.
[Ed. Note: I'm not sure how far to go into the "transport
independent rules". The RFC 2616 doesn't really call them out very
well but I also don't want to have to re-write RFC 2616 spec inside
this spec.]
The transport dependent rules, however, are different. For example,
using TCP any pipelined messages are guaranteed to be delivered in
order. There are no ordering guarantees of any form for HTTPU/HTTPMU
proxies.
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In general a proxy is required to forward a HTTPU/HTTPMU message
exactly once. It SHOULD NOT repeat the message. Rather the client is
expected to repeat the message and, as the proxy receives the
repeats, they will be forwarded.
The proxy is only responsible for forwarding responses to requests
that include a "S" header. As with HTTPU/HTTPMU requests, responses
SHOULD NOT be repeated.
Note that it is acceptable, if not encouraged, for proxies to
analyze network conditions and determine the likelihood, on both
incoming and outgoing connections, of UDP messages being dropped. If
the likelihood is too high then it would be expected for the proxy,
taking into consideration the possibility of making congestion even
worse, to repeat requests and responses on its own. In a sense the
proxy could be thought of as a signal regenerator. This is why the
prohibition against repeating messages is a SHOULD NOT rather than a
MUST NOT.
HTTPMU messages are sent with the assumption that the message will
only be seen by the multicast address they were sent to. Thus when a
proxy forwards the request it is expected to only do so to the
appropriate multicast channel. Note, however, that proxies may act
as multicast bridges.
Also note that proxied HTTPMU messages with a HTTPMU URL without an
absolute path are to be treated as if they were sent to the
specified multicast address with the request-URI "*".
If a HTTPMU request is sent with a host that does not resolve to a
multicast address then the request MUST be rejected with a 400 Bad
Request error.
There is no requirement that a HTTPU proxy support HTTPMU or visa
versa.
10.3. Design Rationale
10.3.1. Why would anyone proxy HTTPMU requests?
Proxying HTTPMU requests can be a neat way to create virtual
multicast channels. Just hook a bunch of proxies together with
unicast connections and tell the proxies' users that they are all on
the same multicast scope.
11. HTTP Headers
11.1. AL Header
11.1.1. Problem Definition
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There are many instances in which a system needs to provide location
information using multiple URIs. The Location header only allows a
single URI. Therefore a mechanism is needed to allow multiple
location URIs to be returned.
11.1.2. Proposed Solution
AL = "AL" ":" 1*("<" AbsoluteURI ">") ; AbsoluteURI is defined in
section 3.2.1 of [RFC2616]
The AL header is an extension of the Location header whose semantics
are the same as the Location header. That is, the AL header allows
one to return multiple locations where as the Location header allows
one to return only one. The contents of an AL header are ordered. If
both a Location header and an AL header are included in the same
request then the URI in the location header is to be treated as if
it were the first entry in the AL header. The AL header MAY be used
by itself but implementers should be aware that existing systems
will ignore the header.
11.1.3. Design Rationale
11.1.3.1. Why not just fix the BNF for the location header?
This is tempting but the goal of maintaining compatibility with RFC
2616's message format overrides the usefulness of this solution.
11.2. mx Request Header
11.2.1. Problem Definition
A mechanism is needed to ensure that responses to HTTPMU requests do
not come at a rate greater than the requestor can handle.
11.2.2. Proposed Solution
[Ed. Note: We need to put in a max for this, at least a number after
which the client isnÆt required to respond. 32 bit integer seconds
sounds like overkill.]
mx = "mx" ":" Integer
Integer = First_digit *More_digits
First_digit = "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
More_digits = "0" | First_digit
The value of the mx header indicates the maximum number of seconds
that a multicast UDP HTTP resource MUST wait before it sends a
response stimulated by a multicast request.
HTTP resources MAY treat any mx header value greater than MX_MAX as
being equal to MX_MAX.
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11.2.3. Design Rationale
11.2.3.1. Why is mx in seconds?
In practice wait periods shorter than a second proved useless and
longer proved too coarse. Of course as faster networks get deployed
finer grain times would be useful but we need a compromise
measurement that will meet everyone's needs, seconds seem to do that
quite well.
11.2.3.2. Couldn't mx still overload the requestor if there are too
many responders?
Absolutely. If there are a 100,000 clients that want to respond even
pushing them over 30 seconds on a 10 Mbps link is still going to
blow both the client and the network away. However the only way to
prevent these sorts of situations is to know the current available
network bandwidth and the total number of likely responders ahead of
time. Both generally prove between difficult to impossible to figure
out. So we are left with heuristics and the mx header.
11.3. S General Header
11.3.1. Problem Definition
A mechanism is needed to associated HTTPU/HTTPMU requests with
responses as UDP does not have any connection semantics.
11.3.2. Proposed Solution
S = "S" ":" AbsoluteURI
The S header is a URI that is unique across the entire URI namespace
for all time. When a "S" header is sent on a HTTPU/HTTPMU request it
MUST be returned, with the same value, on the response.
If a client receives multiple responses with the same "S" header
then the client MAY assume that all the responses are from the same
source and in response to the same request. If the messages differ
from each other then the client MAY either throw all the responses
away or randomly choose one to honor.
[Ed. Note: Ipv4 guarantees that the minimum MTU is 512 bytes or so
long. The UUID URI takes 41 bytes if you don't add an extension
element plus 5 extra characters for header over giving 46
characters. Assuming that the UUID is the minimum practical
mechanism to guarantee globally unique messages this means that
about 9% of every message is eaten up just by the S header. This is
a lot. On the other hand most systems do much better than 512 bytes
and Ipv6 requires (if memory serves) 4k that reduces the overhead to
1%. Note that UDP messages are still 64k long but I know lots of
folks will want to optimize for a single UDP packet.
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On the other hand, having a universally unique S header means that
the algorithm for handling headers is very easy - if you see the
same S value it is the same message. No worrying about rap arounds,
time windows, or anything else. This is very appealing.]
11.3.3. Design Rationale
11.3.3.1. Why do we need the "S" header?
Without a "S" header the only way to match requests with responses
is to ensure that there is enough information on the response to
know what request it was intended to answer. Even in that case it is
still possible to confuse which request a response goes to if it
does not have the equivalent of a "S" header.
11.3.3.2. Couldn't the "S" header be used as a cookie?
No, "S" headers are sent out by clients and returned by servers.
Cookies are sent out by servers and returned by clients.
11.3.3.3. Why aren't "S" headers mandatory on all requests with a
response?
Some systems don't need them.
11.3.3.4. Why aren't "S" headers guaranteed to be sequential so you
could do ordering?
Because HTTPU/HTTPMU is not interested in ordering. If one wants
ordering one should use TCP.
12. Security Considerations
[Ed. Note: Besides putting in a note that all the normal HTTP
security considerations apply we need to put in a discussion of the
problems associated with requests getting lost as well as over sized
request problem. We also need to talk about the fact that requests
can get randomly lost. We also need to discuss how one uses
authentication over UDP. Specifically, that one needs to assume the
challenge and send the response as part of the request.]
[Ed. Note: Talk about the danger of abusing S headers.]
13. Acknowledgements
Thanks to John Stracke for his excellent comments.
14. Constants
MAX_RETRIES - 3
MAX_RETRY_INTERVAL - 10 seconds
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MAX_MX - 120 seconds
15. References
[RFC2119] S. Bradner. Key words for use in RFCs to Indicate
Requirement Levels. RFC 2119, March 1997.
[RFC2616] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, L.
Masinter, P. Leach and T. Berners-Lee. Hypertext Transfer Protocol -
HTTP/1.1. RFC 2616, November 1998.
16. Author's Address
Yaron Y. Goland
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Email: yarong@microsoft.com
This document will expire in April 2000.
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