121 lines
7.3 KiB
HTML
121 lines
7.3 KiB
HTML
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta content="text/html; charset=UTF-8" http-equiv="content-type">
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<style>
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body {
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max-width: 30em;
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margin-left: 2em;
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p.center {
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text-align:center;
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}
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</style>
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<link rel="shortcut icon" href="../rho.ico">
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<title>Protocol Negotiation</title>
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</head>
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<body>
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<p><a href="./index.html"> To Home page</a> </p>
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<h1>Protocol Negotiation</h1>
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<p> Once a protocol is in use, it becomes very hard to change. If one person
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updates the server, and the client is not updated, everything breaks. </p>
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<p> And so, we are stuck with a lot of frozen protocols, many of which are
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in urgent need of change, but to change, requires wide consensus, which
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requires a big bunch of people showing up at a meeting, but at such
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meetings very little gets done, and what gets done is stupid.</p>
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<p> If a standard is successful, more and more people want to be in the
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committee, many of whom represent business profit centers and government
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special interests, and who really do not understand much about the
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technology, except that any change might be adverse to the very important
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people who sent them there.</p>
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<p> As the committee gets larger, it gets more unworkable, and as it
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represents more and more special interests, it gets more unworkable</p>
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<p> In order to have to have a system where the internet’s protocols can be
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upgraded, and new protocols introduced, without central coordination,
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protocol negotiation, where client and server first discuss what protocol
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version they will be using, has to be part of every protocol, all the way
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down to the level of TCP and UDP.</p>
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<p>These days everyone builds in protocol negotiation, often on top of SSL, which is on top of TCP, resulting in three additional round trips.</p>
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<p>And then a widely distributed client or server breaks the protocol negotiation, which no one notices because it interorperates with all existing implementations, until someone tries to introduce a new protocol, whereupon the new code implementing the new protocol is blamed for its failure to interoperate with the existing clients and/or servers, and then we get another layer of protocol negotiation on top of all the existing layers of protocol negotiation.</p>
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<p>TCP has built in protocol negotiation, eight bits worth, which turned
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out, unsurprisingly, to be inadequate.</p>
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<p> For the content of the internet to be free from central control, we need
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to ensure that the address spaces and protocols are free from central
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control.</p>
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<p> When an old protocol is broken, clients and servers that have not
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upgraded to a new improved protocol will remain forever, so the old
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defective protocol has to be supported forever – without, however,
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allowing an attacker a downgrade attack. </p>
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<p>To prevent a downgrade attack, there has to be some way of disabling
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protocols in the field, where the signed ban on certain protocols flood
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fills from one program to the next.</p>
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<p> Often, it is impossible to support the old clients, because protocol
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negotiation was never adequately designed in, or because it was designed
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in but was designed vulnerable to a downgrade attack.</p>
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<p>But let us suppose the protocol negotiation was well designed: The
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committee has to assign a code. And of course, they will only assign
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this code to a protocol that they agree is right – and nothing gets done,
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for there is always some vested interest that for some strange and obscure
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reason does not want this protocol to exist.</p>
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<p>One solution is to have quite large protocol identifiers, or arbitrarily
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large variable length protocol identifiers, so that anyone can whip up a
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protocol and assign it an identifier, and hack a client and server to use
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it, without having to walk it past three dozen members of the committee. </p>
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<p>But then, of course, we would probably wind up with a lot of protocols.
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This could potentially lead to a lot of protocol negotiation round
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trips </p>
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<blockquote>
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<p>Do you speak protocol A? No.</p>
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<p>Do you speak protocol B? No.</p>
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<p>Do you speak protocol C? No.</p>
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<p>Do you speak protocol D? No.</p>
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<p>Do you speak protocol E? Yes. </p>
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</blockquote>
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<p>One solution to this problem is to have complete lists of protocols, call
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it a protocol dictionary, which dictionary maps the long probabilistically
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globally unique protocol names to short deterministically unique local
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protocol names, and gives an order of preference. If the client
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names a dictionary that it supports, and/or the server names a dictionary
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that it supports, then they can usually come to immediate agreement. <br/>
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</p>
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<p>If, for example, the client wants to talk protocol X, it proposes one or
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more dictionaries of updates to protocol X, implying that it can talk all
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the updates listed in each dictionary, and an order of preference among
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dictionaries</p>
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<p>If the server recognizes one or more of the dictionaries, it then
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responds with one of the protocols listed in the first dictionary that it
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recognizes, by its short dictionary name, and the conversation proceeds.</p>
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<p>An ordered list of dictionaries is identified by a public key and a short
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human readable type name. The typename is only unique with respect
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to the dictionaries signed by this public key, thus ftp version 1, ftp
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version 2, ftp version 4 ... </p>
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<p>The globally unique identifier of a dictionary is the hash of the rule
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identifying its public key, plus its typename and version number.</p>
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<p>If the server recognizes the hash of the rule identifying the dictionary
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public key, but not the version number, it responds with the highest
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version number that it does recognize, and the most favored protocol in
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that dictionary. Thus if the client requests a protocol of
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dictionary version n, it has to know dictionaries versions 1 to n, and be
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able to deal with all protocols in versions 1 to n, if only to the extent
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that it is able to fail the protocol gracefully. </p>
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<h3>The one true ciphersuite</h3>
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<p>Why would you want multiple ciphers?</p>
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<p>In case one turns out to be weak. </p>
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<p>OK, suppose one turns out to be weak. Oops, Malloc can now launch a
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downgrade attack.</p>
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<p>So, if supporting multiple ciphers, you need a floodfill mechanism where
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you can disable the bad ciphersuite in the field.</p>
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<p>Each program supporting a set of ciphersuits has a set of signatures it
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recognizes as authoritative. If another program that it talks to has
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a revocation of ciphersuite, and it recognizes one of the signatures on the
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revocation, the revocation floodfills.</p>
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<p>So, ideally you should support multiple ciphersuites – but if you do,
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have a mechanism for field revocation.</p>
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<p style="background-color : #ccffcc; font-size:80%">These documents are
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licensed under the <a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/">Creative
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Commons
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Attribution-Share Alike 3.0 License</a></p>
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</body>
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</html>
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