Hacker News Reader

Comment by loeg | Hacker News Reader

I agree that is super important and maybe worth mentioning but the point of the quiz is to demonstrate that Linux's implementation is actually more constrained than the traditional "unique src addr/port dst addr/port 4-tuple" (for TCP).

View on HN

Yeah I feel like people are missing the point that it's not a 4-tuple thing, it's an ordering issue. Since the source port (sometimes) gets picked before the interface or destination is, you can get an EADDRNOTAVAIL result even when there's technically a potential for it to end-up as a unique 4-tuple. Doing the assignment in a different order or more explicitly can allow it to work by making sure that the kernel always knows it will be unique.

It's interesting design decision to preemptively fail when the connect might later be ambiguous. However in memory allocation, Linux will gladly allocate memory it doesn't have, return success and then later when you attempt to use it, it will kill the process with OOM killer.

> However in memory allocation, Linux will gladly allocate memory it doesn't have, return success and then later when you attempt to use it, it will kill the process with OOM killer.

In userspace, with overcommit enabled, yes.

In the kernel, you will often do the opposite -- speculatively allocate memory first, then take locks or other serialization primitives, then attempt to insert the object into some container, but fail if there was a collision. (The idea is to keep the relatively slow memory allocation outside of the locked region.)

Very late on this one :P But I think the difference is the fact that ports are unique, so if the kernel assigns you a bad source port by accident then it can't change it later. You would need to know this is what happened and bind to a new source port (and do you attempt automatic again?). Where-as with overcommit, your program doesn't know or care what particular memory page you're using, and the kernel can easily fix things up in the background (Ex. use swap, or kill some other process) without you ever knowing about it.

Reply on news.ycombinator.com