Hi,
Hello Fan and team,
I think I'm not a big fan of the pending_guard and pending_dir_guard concept. To me it seems like a quick hack that tries to address fundamental issues with our algorithm that appeared when we tried to adapt the proposal to the tor codebase.
Yeah agree, this pending_guard hack was trying to avoid some implementation problem, we need to redesign. I haven't got any good idea about this, that will be nice if you already got some thoughts.
I think one of the main issues with the current algorithm structure is that _one run of the algorithm_ can be asked to _setup multiple circuits_, and each of those circuits has different requirements for guards. That is, since we do filtering on START based on the requirements of circuit #1, this means that any other circuits that appear before END is called, also have to adapt to the requirements of circuit #1. This is obvious in the code since we use guard_selection->for_directory throughout the whole algorithm run, even though for_directory was just the restriction of circuit #1.
Specifically about the pending_guard trick, I feel that it interacts in unpredictable ways with other features of the algorithm. For example, consider how it interacts with the primary guards heuristic. It could be time for the algorithm to reenter the primary guards state and retry the top guards in the list, but because of the pending_guard thing we actually return the 15th guard to the circuit.
IMO we should revisit the algorithm so that one run of the algorithm can accomodate multiple circuits by design and without the need for hacks. Here is an idea towards that direction:
I think one very important change that we can do to simplify things is to remove the need to filter guards based on whether they are dirguards, fast, or stable. My suggestion here would be to *only* consider guards that are dirguards _and_ fast _and_ stable. This way, any circuit that appears will be happy with all the guards in our list and there is no need to do the pending_dir_guard trick. See [0] on why I think that's safe to do.
This is easy to do in the current codebase. You just need to call entry_is_live() with need_uptime, need_capacity and for_directory all enabled (instead of flags being 0).
If you do the above, your sampled guard set will be able to accomodate any circuit that comes its way and that should simplify logic considerably.
Sounds great, that can simplify the logic a lot, I've done the change, no more pending_dir_guard.
Let me know if the above does not make sense.
Here are some more comments:
- So the above idea addresses a large part of the filtering logic that
happens on START. The rest of the filtering logic has to do with ClientUsesIPv6, ReachableAddreses, etc. . I think it's fine to conduct that filtering on START as well.
- I tried to run the branch as of bb3237d, but it segfaulted. Here is
where it crashed:
#1 0x000055555567eb25 in guards_update_state (next=0x5555559c3f40,next@entry=0x5555559c35e8, guards=guards@entry=0x5555559c4620, config_name=config_name@entry=0x55555570c47e "UsedGuard") at src/or/prop259.c:1137 1137 !strchr(e->chosen_by_version, ' ')) {
Let me know if you need more info here.
Never saw this before, will look into it.
- There is a memleak on 'extended' in filter_set().
In general, I feel that logic in that function is a bit weird. The function is called filter_set() but it can actually end up adding guards to the list. Maybe it can be renamed?
Split it up to filter_set & expand_set probably can make this clear.
- What's up with the each_remaining_by_bandwidth() function name?
I guess it should be iterate_remaining_guards_by_bandwidth.
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[0]: I think that's OK to do and here is why:
All Guards are Fast. About 95% of Guards are Stable (this will become 100% with #18624) About 80% of Guards are V2Dir/dirguards (this will become 100%with #12538)
#12538 got merged in 0.2.8, so if prop259 gets merged in 0.2.9, by the time prop259 becomes active, almost all guards will be dirguards. So I think it's fine to only consider guards that are dirguards &&fast && stable now, since by the time prop259 gets deployed that will be the case for almost 100% of guards.