WebSockets is a stream, not a message based protocol(lenholgate.com)
lenholgate.com
WebSockets is a stream, not a message based protocol
http://www.lenholgate.com/blog/2011/07/websockets-is-a-stream-not-a-message-based-protocol.html
12 comments
I've no problem with the lack of a max message size in the RFC, what could cause problems is the fact that it needs to be passed between client and server "out of band", i.e. at the application protocol layer rather than at the websocket protocol layer. Also bear in mind that this blog entry was written based on Draft HyBi 09 and not the final RFC; the wording has changed somewhat since then.
The draft in question suggested that providing a message based interface to application code was possible and that the parser could/should deliver only complete messages to the application code. That's hard to do if you also want to allow for the 'endless streaming' scenario that others on the working group were fond of. The result was a bit of a mess.
The final RFC addresses some of this, but there's no getting around the fact that the websocket protocol itself can't tell you how big a message is until you get the final frame.
Sure you can work around all of this even for a generic parser but the initial wording in the draft in question could lead you towards the wrong design if you're not careful.
The draft in question suggested that providing a message based interface to application code was possible and that the parser could/should deliver only complete messages to the application code. That's hard to do if you also want to allow for the 'endless streaming' scenario that others on the working group were fond of. The result was a bit of a mess.
The final RFC addresses some of this, but there's no getting around the fact that the websocket protocol itself can't tell you how big a message is until you get the final frame.
Sure you can work around all of this even for a generic parser but the initial wording in the draft in question could lead you towards the wrong design if you're not careful.
As I mentioned above, I agree that that needing to pass maximum message size out of band makes some things more difficult. Whether that was the right tradeoff in terms of convenience vs protocol complexity I think has yet to be seen. At any rate, an extension to perform this in band should be trivial. Perhaps I will try writing one to test out my extension handling code.
I do see your point on the "endless streaming" section of the RFC. Stating that "(section 5.4) The primary purpose of fragmentation is to allow sending a message that is of unknown size when the message is started without having to buffer that message." implies that a web socket implementation should support this sort of operation. Indeed, if you want to support sending messages of unknown size you must expose an interface more complicated than the default message based one.
That said, a message only implementation that does not allow sending unknown sized messages is 100% compliant with both the spec and receiving such messages. The RFC probably could have made this fact more clear. I believe that endless streaming mode will not be a common use case and have not implemented it in my generic WebSocket library. I do believe, however, that fragmentation of messages provides important benefits even without unknown size sends. Once you have message fragmentation there is no additional protocol cost to allow unknown size sends.
I do see your point on the "endless streaming" section of the RFC. Stating that "(section 5.4) The primary purpose of fragmentation is to allow sending a message that is of unknown size when the message is started without having to buffer that message." implies that a web socket implementation should support this sort of operation. Indeed, if you want to support sending messages of unknown size you must expose an interface more complicated than the default message based one.
That said, a message only implementation that does not allow sending unknown sized messages is 100% compliant with both the spec and receiving such messages. The RFC probably could have made this fact more clear. I believe that endless streaming mode will not be a common use case and have not implemented it in my generic WebSocket library. I do believe, however, that fragmentation of messages provides important benefits even without unknown size sends. Once you have message fragmentation there is no additional protocol cost to allow unknown size sends.
I agree with all you're saying.
The wording of the RFC has improved since that draft and the flexibility could be useful in some scenarios.
I ended up with an API which can be asked to deliver complete messages 'if possible' given the buffers provided by the client of the API. If it's not possible and the buffer becomes full then the API simply gives you the fragment of data and tells you if it knows how much more there is to come or not.
The wording of the RFC has improved since that draft and the flexibility could be useful in some scenarios.
I ended up with an API which can be asked to deliver complete messages 'if possible' given the buffers provided by the client of the API. If it's not possible and the buffer becomes full then the API simply gives you the fragment of data and tells you if it knows how much more there is to come or not.
The WebSocket protocol design was hijacked by architecture astronauts who decided that it must have all of these extra features added, instead of remaining a simple, easily implementable and understandable protocol. The original WebSocket protocol was a simple stream of delimited messages, with the only complexity being in the handshake that was necessary to ensure that JavaScript apps couldn't send arbitrary data to arbitrary ports without permission.
The problem is that the original handshake wasn't good enough (there were still security vulnerabilities despite he handshake), and when Ian Hickson decided to hand over control to the IETF, the architecture astronauts took over, adding complex framing with six different frame types, subprotocols, extensions, versions, complex bit twiddling required to parse frame headers, fragmentation of messages into smaller frames (which is what this article is complaining about), control frames interleaved with fragmented messages, numeric status codes and textual close reason strings that "MUST NOT" be shown to the user, masking of data by xor'ing with a random value that changes for each frame, but only for one direction (client->server), a two-way closing handshake on top the existing TCP mechanisms for closing the connection, pings to test the connection for liveness, and so on. There are six registries defined for IANA to keep track of http://www.iana.org/assignments/websocket/websocket.xml; extensions, subprotocols, version numbers, close codes, opcodes, and framing bits.
And despite all of this over-engineering and attempt at extensibility, all extensions must know about each other, because there is no standard method for delimiting different extensions' data (or even specifying how much data an extension uses), and there are three header bits and 10 frame types that all extensions must share. And I don't really know why there's a need for subprotocols on top of the ability to just encode that information in the URL.
It's kind of sad how what could have been a relatively simple and easy to implement protocol has been taken over by architecture astronauts. Yes, a few of these features are actually required to securely deploy websockets (the handshake and masking). Most of them are people making up features that would be nice in theory, instead of implementing something simple that works. Ian Hickson's original protocol wasn't perfect; it still needed some work by the time he left. But it was simple, and easy to implement, and didn't impose restrictions that couldn't be worked around at a higher level.
The problem is that the original handshake wasn't good enough (there were still security vulnerabilities despite he handshake), and when Ian Hickson decided to hand over control to the IETF, the architecture astronauts took over, adding complex framing with six different frame types, subprotocols, extensions, versions, complex bit twiddling required to parse frame headers, fragmentation of messages into smaller frames (which is what this article is complaining about), control frames interleaved with fragmented messages, numeric status codes and textual close reason strings that "MUST NOT" be shown to the user, masking of data by xor'ing with a random value that changes for each frame, but only for one direction (client->server), a two-way closing handshake on top the existing TCP mechanisms for closing the connection, pings to test the connection for liveness, and so on. There are six registries defined for IANA to keep track of http://www.iana.org/assignments/websocket/websocket.xml; extensions, subprotocols, version numbers, close codes, opcodes, and framing bits.
And despite all of this over-engineering and attempt at extensibility, all extensions must know about each other, because there is no standard method for delimiting different extensions' data (or even specifying how much data an extension uses), and there are three header bits and 10 frame types that all extensions must share. And I don't really know why there's a need for subprotocols on top of the ability to just encode that information in the URL.
It's kind of sad how what could have been a relatively simple and easy to implement protocol has been taken over by architecture astronauts. Yes, a few of these features are actually required to securely deploy websockets (the handshake and masking). Most of them are people making up features that would be nice in theory, instead of implementing something simple that works. Ian Hickson's original protocol wasn't perfect; it still needed some work by the time he left. But it was simple, and easy to implement, and didn't impose restrictions that couldn't be worked around at a higher level.
Thanks for an extremely illuminating explanation. I may be wrong, but it almost sounds like long polling and other alternatives are preferable to Websockets because of the added complexity. Stories like this also make me feel fortunate that my favorite beacon of simplicity, JSON, didn't get handed over to a "task force".
I agree.
It seems to me that the hype around Web Sockets has overshadowed the Server Sent Events API (http://dev.w3.org/html5/eventsource/) which for most situations where you don't need a continues stream of data is a more sensible system. It is purely a message sending system by design.
The really nice thing about SSE is that you can fall back to long polling very easily with exactly the same back end and as it runs over vanilla http without the upgrade protocol system is much easer to implement, you just don't close the connection after sending a message. Obviously its only one way but we have a well established way of sending messages in the other direction with http POST.
The really nice thing about SSE is that you can fall back to long polling very easily with exactly the same back end and as it runs over vanilla http without the upgrade protocol system is much easer to implement, you just don't close the connection after sending a message. Obviously its only one way but we have a well established way of sending messages in the other direction with http POST.
Of course, the limitation of SSE is that it's not bidirectional - as the name implies, only the server can send messages to the client. SSE is definitely useful, especially when you don't need the full overhead of a WebSockets server, but the "hype around WebSockets" is largely because you can implement two-way connections with it. Yesterday there was actually a HN post where someone created a full TCP proxy using WebSockets.
I completely agree that WebSockets are a very good innovation and have their place but I do think that there are a lot of situations where you can achieve the same result easer with SSE and using a HTTP PUT to send messages back to the server.
For example a collaborative project management tool like Asana where saves are silent and update live on other peoples screens is a better place for SSE and a REST api than WebSockets, where as a real time game is probably a good place to the latter.
For example a collaborative project management tool like Asana where saves are silent and update live on other peoples screens is a better place for SSE and a REST api than WebSockets, where as a real time game is probably a good place to the latter.
The TCP proxy use case, which has been my primary motivation for WebSocket since day one, points to another limitation of SSE. SSE cannot handle binary data without encoding the data into text. WebSocket can.
Agree, Server Sent Events is an incredibly simple API, has none of the problems with proxies, security and complexity that websockets has run into and is perfect for the typical case where Ajax is fine for sending events to the server, but you need a way to stream events in the other direction.
I just launched the beta of a small service for making Server Sent Events painless at http://eventsourcehq.com
It's currently only available for members of Heroku's beta program, but I plan on launching it as a stand alone service as well. In any case all the code behind it available on Github...
I just launched the beta of a small service for making Server Sent Events painless at http://eventsourcehq.com
It's currently only available for members of Heroku's beta program, but I plan on launching it as a stand alone service as well. In any case all the code behind it available on Github...
Which browsers have implemented SSE/EventSource?
All except IE and Android, as usual.
http://www.caniuse.com/#search=eventsource
However, since SSE is HTTP-compatible you can easily implement fallback for these, e.g.
https://github.com/Yaffle/EventSource
http://www.caniuse.com/#search=eventsource
However, since SSE is HTTP-compatible you can easily implement fallback for these, e.g.
https://github.com/Yaffle/EventSource
Wow, I seem to have missed the boat on caniuse.com =)
Thanks for the info, everyone.
Thanks for the info, everyone.
http://caniuse.com/#search=eventsource
Firefox, Chrome, Safari and Opera all have it. I believe IE10 will have it but I cant find a reference right now.
This seems to be the nicest shim to let you use EventSource on older browsers https://github.com/Yaffle/EventSource, I did make something similar myself but this is much more complete.
Firefox, Chrome, Safari and Opera all have it. I believe IE10 will have it but I cant find a reference right now.
This seems to be the nicest shim to let you use EventSource on older browsers https://github.com/Yaffle/EventSource, I did make something similar myself but this is much more complete.
Chrome 13+, Firefox 6+, Safari 5+, Opera 11+. No IE support on the horizon, though.
http://caniuse.com/#feat=eventsource
http://caniuse.com/#feat=eventsource
This is some screwed up stuff, as nearly every WebSocket library and tutorial really encourages treating them as discreet messages. This should be fixed post haste, because very few people really want a stream based protocol for web sockets.
I don't think it'll be a problem in practicality. Most implementations primary APIs will be message based and not intended for this streaming case. I'm planning on refactoring my implementation to have a low level streaming API that's used internally and is exposed if you really need it, but on top of that build the message based API that 99.5% of people will use.
Too late! WebSocket has been finalized.
Hey, this article is better than a rant, if you don't know much about what's going on with the RFC, but are using websockets anyway. :D
Everything you send up or down is a message, or a packet, and the size of that cannot exceed the size of pipe (with bottlenecks or intermediary restrictions). Call 'em Quantum Packets, or a stream, or a message. The websocket protocol, as imagined by this developer, is meant to allow a continuous lot of Quantum Packs to "flow", without the application level overhead of parsing a bunch of protocol, headers, wackness. I want to get the data into my applications AFAP, cuz I still have to transcode it, analyze it, and all else to make the baby dance.
What we need as developers are minimum-for-reliability standards. No two people in different locations will have the same pipe. As a developer, I consider it my domain to write software on top of, or using, the socket layer to determine the potential through-put of the given socket, and to test such as needed through-out the simulcast. I don't even want the socket-layer-wrapper writers (may God shower them with blessings) intervening at this level, until everybody on Earth has unrestricted 10mbps/s up and down.
If that control is hidden from me, or not an option, or is nullified by protocol, then my app or media could break in ways I could not predict or understand, and so I would have to design my app using the socket layer in a lowest-common-reliability kind of way.
These are not the opinions of a WebSocket RFC acquainted developer.
Everything you send up or down is a message, or a packet, and the size of that cannot exceed the size of pipe (with bottlenecks or intermediary restrictions). Call 'em Quantum Packets, or a stream, or a message. The websocket protocol, as imagined by this developer, is meant to allow a continuous lot of Quantum Packs to "flow", without the application level overhead of parsing a bunch of protocol, headers, wackness. I want to get the data into my applications AFAP, cuz I still have to transcode it, analyze it, and all else to make the baby dance.
What we need as developers are minimum-for-reliability standards. No two people in different locations will have the same pipe. As a developer, I consider it my domain to write software on top of, or using, the socket layer to determine the potential through-put of the given socket, and to test such as needed through-out the simulcast. I don't even want the socket-layer-wrapper writers (may God shower them with blessings) intervening at this level, until everybody on Earth has unrestricted 10mbps/s up and down.
If that control is hidden from me, or not an option, or is nullified by protocol, then my app or media could break in ways I could not predict or understand, and so I would have to design my app using the socket layer in a lowest-common-reliability kind of way.
These are not the opinions of a WebSocket RFC acquainted developer.
The WebSocket protocol works for both small and large messages in a single frame (message based), and also small and large frames in multiple fragments (stream based)... It's capable of being used for both. It's a good idea to restrict frame sizes on your application if you know what your limits are.
It's only capable of being used for messages if there's something to guarantee that all hops along the way are going to preserve the message boundaries in ways expected by the application layer. Can the protocol split single messages? Can the protocol merge adjacent messages without reordering?
Unless the protocol specifically guarantees certain behavior and commonly-used systems regularly exercise this guarantee, it's just not going to work reliably when it's needed.
Hearing some of the "works for me" discussion from developers suggests that we're heading for that magic situation where it works 99.9% of the time. I.e., the system looks fine in testing and then fails in mysterious ways (that require deep protocol fixes) in production.
Ideally, implementations of such a protocol would intentionally fragment the messages somewhat if they were not going to guarantee they were atomic. But there are very few developers (and code reviewing managers) enlightened enough to let that kind of thing ship.
Unless the protocol specifically guarantees certain behavior and commonly-used systems regularly exercise this guarantee, it's just not going to work reliably when it's needed.
Hearing some of the "works for me" discussion from developers suggests that we're heading for that magic situation where it works 99.9% of the time. I.e., the system looks fine in testing and then fails in mysterious ways (that require deep protocol fixes) in production.
Ideally, implementations of such a protocol would intentionally fragment the messages somewhat if they were not going to guarantee they were atomic. But there are very few developers (and code reviewing managers) enlightened enough to let that kind of thing ship.
The protocol preserves message boundaries but not fragment boundaries. You may send a message of, say, 100 bytes and get 100 x 1 byte fragments arrive, or you may send 100 x 1 byte fragments and get 100 bytes in a single frame. The main issue, for me, at the time, was that when you get that first 1 byte frame there's no way to know how big the resulting message will be.
Luckily there's a rather excellent compliance test suite, here: http://www.tavendo.de/autobahn/testsuite.html which should go a long way to help nail interop issues.
Luckily there's a rather excellent compliance test suite, here: http://www.tavendo.de/autobahn/testsuite.html which should go a long way to help nail interop issues.
when you get that first 1 byte frame there's no way to know how big the resulting message will be
So that's a design tradeoff. I've implemented protocols that did it both ways and it's definitely easier on the guy trying to implement a library or other receiving application if he can get a reasonable upper limit on the size of the messages.
But on the other hand, by not requiring the total message length be known in advance, it eases the logical (and memory) burden on the sender. Often the sender will be an overloaded server.
Nothing can prevent a higher-level protocol on top of WS from negotiating its own max message size.
So the design choice that was made would seem to allow optimizations for the overloaded server case without prohibiting other optimizations. This is typical of W3C protocols.
So that's a design tradeoff. I've implemented protocols that did it both ways and it's definitely easier on the guy trying to implement a library or other receiving application if he can get a reasonable upper limit on the size of the messages.
But on the other hand, by not requiring the total message length be known in advance, it eases the logical (and memory) burden on the sender. Often the sender will be an overloaded server.
Nothing can prevent a higher-level protocol on top of WS from negotiating its own max message size.
So the design choice that was made would seem to allow optimizations for the overloaded server case without prohibiting other optimizations. This is typical of W3C protocols.
Well, all frames actually do have length in the header. It's just not the first byte... it's the lowest 7 bits of the second byte, and possibly more (check the spec).
So, if I can read 2 bytes from my buffer, I can get a good idea of the message size. If I only have 1 byte, I rewind and wait until I receive another.
So, if I can read 2 bytes from my buffer, I can get a good idea of the message size. If I only have 1 byte, I rewind and wait until I receive another.
But we're talking about messages, not frames. http://tools.ietf.org/html/rfc6455 "frames have no semantic meaning"
If Websocket developers begin making unwarranted assumptions about message framing and fragmentation you will regret it. BELIEVE ME.
/me gets back to bugfixing
If Websocket developers begin making unwarranted assumptions about message framing and fragmentation you will regret it. BELIEVE ME.
/me gets back to bugfixing
[deleted]
The WebSocket protocol guarantees that all hops along the way preserve message boundaries except in the case of explicitly negotiated extensions that dictate otherwise.
Even in cases where your WebSocket implementation exposes incomplete message fragments, the boundaries of the message are still clearly and accurately preserved.
Even in cases where your WebSocket implementation exposes incomplete message fragments, the boundaries of the message are still clearly and accurately preserved.
Don't worry, its not as bad as all that. WebSockets doesn't actually break message boundaries, it just might fragment a message without telling you the full size up front. The basic "problem" is that if you receive a 1kb fragment with no size indication, and you expose an API that is entirely based on messages and events, then you have to start buffering. But what if the message is a 5gb .iso file of your favorite DVD? Well, there's no way to predict that so you might have to dynamically allocate a monstrous buffer, which is just painful.
There's two ways to respond to this: 1) claim that websockets is broken and all client APIs should be streaming or 2) not unexpectedly send multi-gigabyte messages.
There's two ways to respond to this: 1) claim that websockets is broken and all client APIs should be streaming or 2) not unexpectedly send multi-gigabyte messages.
You mean:
2) Trust everyone on the internet to not unexpectedly send your server multi-gigabyte messages
Since this post seems to actually have been made in July of this year, does anyone who has been following WebSocket details have any comments on how this situation has changed?
My impression of WebSockets is that it's not actually a "finished" high level protocol. They could have just brought a basic socket style interface into JavaScript and left it at that. (And based on its name, that's what you'd expect at first.) But they decided to add various features, (for better or worse, I don't know yet) on top of that. (I guess part of it is the challenge of working not just on TCP, but sort of within HTTP as well). Just as you wouldn't just pick up TCP and start blowing "data" through it without some additional application specific structure, you're going to need to add your own structure inside WebSocket's framework.
My impression of WebSockets is that it's not actually a "finished" high level protocol. They could have just brought a basic socket style interface into JavaScript and left it at that. (And based on its name, that's what you'd expect at first.) But they decided to add various features, (for better or worse, I don't know yet) on top of that. (I guess part of it is the challenge of working not just on TCP, but sort of within HTTP as well). Just as you wouldn't just pick up TCP and start blowing "data" through it without some additional application specific structure, you're going to need to add your own structure inside WebSocket's framework.
I posted an update when HyBi 13 came out; http://www.lenholgate.com/blog/2011/09/the-websocket-protoco...
The wording was improved around the suggestion to provide only a message based API.
I think the WebSockets protocol ended up being a little more than it should have been. You have to understand that it was being pulled in all sorts of directions by the working group members and that there are good reasons for all of the parts of the protocol (though some of those parts could work better with other parts IMHO). It had to be finished at some point though and I think the working group did a good job in the end.
Personally I think it would have been better had it been explicitly stream based from a user's perspective, but then I don't have the javascript/browser background to know how foolish that probably sounds.
The wording was improved around the suggestion to provide only a message based API.
I think the WebSockets protocol ended up being a little more than it should have been. You have to understand that it was being pulled in all sorts of directions by the working group members and that there are good reasons for all of the parts of the protocol (though some of those parts could work better with other parts IMHO). It had to be finished at some point though and I think the working group did a good job in the end.
Personally I think it would have been better had it been explicitly stream based from a user's perspective, but then I don't have the javascript/browser background to know how foolish that probably sounds.
Looks like we should be implementing UUCP over WebSockets.
You don't sound smart for mocking the idea that a 8-exabyte message in a communication protocol is "big enough", you sound like you're mindlessly parroting ideas you don't fully understand. Yes, 8 exabytes is enough for a single message, and always will be. TCP works on "messages" (packets) in the kilobyte range, for comparison. Communication protocol packet sizes aren't equal to the amount of data the communication protocol can send.
The argument FOR 63 bit message sizes was that you could effectively turn the message based protocol into a stream, except, unfortunately, the "stream" has a limit even if it seems plenty big enough now.
Personally I wouldn't have included the 63 bit message size.
Personally I wouldn't have included the 63 bit message size.
What's the point? What's the difference between a stream of messages, and a stream of bytes? Nothing.
The difference is that with a stream of websocket messages every so often you need to deal with the framing. With a stream of bytes you don't. This precludes the use case that was the basis of the argument for 63 byte messages if you ever have to 'stream' a 'message' that needs to be longer... Sure you can send it as multiple fragments but then you can't do the 'here's a file handle, read the stream' thing that was proposed.
Does socket.io handle these issues? Does Now.js?
Out of curiosity, and forgive my ignorance here, but since everyone seems to prefer using event-driven methods in JS, why was a message-based protocol passed over in favor of this stream solution?
It's elsewhere in the comments or the article: a feature of the protocol allows transmitting partial messages, where the message size is unknown. One example might be the result of a slow, unbuffered SQL query, where it's more useful for the server to pass the result to the client incrementally, rather than buffer the full message ahead of time.
Why you'd want to do that is another question entirely. Introducing roundtrips by feeding tiny chunks to TCP is generally a horrible idea, however, it does prevent the server from dedicating a potentially huge chunk of RAM to buffer the result ahead of time.
Because of this feature, and the author's desire to model this feature as part of some client library API (a mistake? you decide), he's concluded that it's in fact a stream-oriented protocol. That's like concluding it's a byte-oriented protocol because TCP can/will further fragment the partial frames due to segment size constraints, etc. (i.e. it's a silly conclusion).
Why you'd want to do that is another question entirely. Introducing roundtrips by feeding tiny chunks to TCP is generally a horrible idea, however, it does prevent the server from dedicating a potentially huge chunk of RAM to buffer the result ahead of time.
Because of this feature, and the author's desire to model this feature as part of some client library API (a mistake? you decide), he's concluded that it's in fact a stream-oriented protocol. That's like concluding it's a byte-oriented protocol because TCP can/will further fragment the partial frames due to segment size constraints, etc. (i.e. it's a silly conclusion).
I'm not sure I follow you to get to it being a "silly conclusion".
As I said, the draft at the time suggests presenting whole messages to the application layer. The parser can't know it has a whole message until it gets the final frame... This could lead to interesting memory usage ;)
The protocol provides for a series of infinitely long messages, each separated by a terminator. I don't have a problem with that in itself, but the draft at the time was misleading to suggest otherwise...
As I said, the draft at the time suggests presenting whole messages to the application layer. The parser can't know it has a whole message until it gets the final frame... This could lead to interesting memory usage ;)
The protocol provides for a series of infinitely long messages, each separated by a terminator. I don't have a problem with that in itself, but the draft at the time was misleading to suggest otherwise...
Because generally people want to pass discrete messages rather than an undifferentiated stream of bytes. And APIs implement even streams as discrete chunks of data to process; you don't process a character at a time, or the whole stream at once, but are instead given a buffer of data to process. Now, if you do that, lazy programmers (and many JavaScript and web app programmers are lazy, or don't know any better) may just decide that one buffer is one message, an assumption which generally works. If the client sends a small chunk of data to the server, that will generally be received on the server and delivered to whatever is listening as that same chunk. But occasionally, that assumption can be broken; an IP packet can be fragmented, or a buffer may fill up and need to be dealt with immediately, or something of the sort. This will cause applications which make such assumptions to break.
If, instead, you just explicitly say that it's a message oriented protocol, then the software that implements it (both on the client and server side) can just provide an API that delivers a message at a time, and if anything happens to be fragmented, they deal with buffering and reassembling it, rather than depending on the application author to get that right.
If, instead, you just explicitly say that it's a message oriented protocol, then the software that implements it (both on the client and server side) can just provide an API that delivers a message at a time, and if anything happens to be fragmented, they deal with buffering and reassembling it, rather than depending on the application author to get that right.
A rose by any other name would smell as sweet.
Time and time again it has been demonstrated that we are bad at choosing a maximum allowed value for all applications and all future considerations (see: ethernet frame sizes, IP address lengths, operating system address spaces, file system block sizes/counts, etc).
In some cases (many of those previously listed) there were hardware, cost, or technical concerns that led to nailing down a number in an RFC. For WebSocket there is no clear benefit to forever encoding a specific numeric maximum message size. It is a high enough level protocol that there is no technical or cost benefit to make message sizes limited by anything other than individual application needs.
As such, the WebSocket RFC leaves maximum message size implementation defined, and specifically says that an implementation SHOULD implement a reasonable maximum message size for its purpose. A chat application that knows it will only be moving small text messages can set its maximum message threshold small to improve buffer performance and catch invalid messages sooner. An application that finds a business case for sending a large file in one large message can set itself up accordingly. Generic WebSocket parsers should expose a method of setting the maximum message size the application wishes to receive.
I definitely agree that not requiring implementations to return their maximum message size along with the "Message too big" error will make some sorts of interoperability more difficult. However, it also prevents exposing implementation security details and simplifies the core spec (the author has already complained that the spec is too complicated already). It is relatively simple for an application to negotiate a maximum message size privately if necessary and the WebSocket extension mechanism allows a method for standardizing a way of doing so if this turns out to be a serious issue in the future.