This is a bit offtopic, but anyway... On 2024-05-28 05:02, Mouse wrote:
Where do we attach 3 priority levels to data?[I]n the context of poll itself, it's undefined. But it's easy to think that the TCP urgent data would be something usable in this context. But as you note, the urgent data is a somewhat broken thing that noone ever really figured out how it was meant to be used or anything about it at all.TCP's urgent pointer is well defined. It is not, however, an out-of-band data stream, nor, despite Berkeley's attempts, can it really be twisted and bent into one, unless you are on a network which is high bandwidth, low latency, and low loss (as compared to the "out-of-band" data rate). Even then, the receiving process has to handle data promptly. Which, probably not coincidentally, describes Berkeley's network and most of their network programs at the time. However, the urgent pointer is close to useless in today's network, in that there are few-to-no use cases that it is actually useful for.
It was always useless. The original design clearly had an idea that they wanted to get something, but it was never clear exactly what that something was, and even less clear how the urgent pointer would provide it.
It's not a BSD problem, but a problem with the whole design. Which is why there is an RFC that basically says that it should all go away (RFC 6093). But then BSD (and others) did try to make it somehow OOB, which makes it even more strange.
Oh well. As for poll, it tries to be generic, but at the same time, why did it then divide incoming data into three priority brackets? SysV stuff was never something that I got into.
Johnny
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