Request for more intelligent local port allocation algorithm

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Request for more intelligent local port allocation algorithm

Paul-8
Hi dev team,

It's not a secret that when application is trying to establish new TCP connection, without
first binding a socket to specific local interface address, OS handles that automatically.
Unfortunately there is a catch, that lies in a different logic of local port allocation:
(1) when socket is bound before connect() vs (2) when it is not. When allocating the port  
in in_pcb_lport() by checking whether different ports are free, using in_pcblookup_local(),
the behaviour is following:

(1) Bound, ie laddr is assigned with specific address:
    Port is considered occupied only if there is a PCBs that matches both laddr and lport

(2) Not bound, ie laddr == INADDR_ANY:
    Port is considered occupied if there is any PCBs that only matches lport. What this  
    means is that in order to allocate a port none of the all available local addresses  
    should have it allocated, even though this requirement is ridiculous, since we are
    allocating only one PCB

Looking though the code, it seems that (2) is due to the fact that tcp_connect() first
allocates the port, indirectly through the call to in_pcbbind() and only then allocates
the actual local address, also indirectly, though the call to in_pcbconnect_setup(), that
in turn calls in_pcbladdr(). So, probably, in order to guarantee that in_pcbconnect_setup()
will not fail we make sure that all range of local addresses are available, no matter
which one of them is actually selected by in_pcbladdr()?

In real world, this creates serious problems for servers that have a lot of outgoing
connections, for example nginx proxy with a lot of open HTTP2 connections. In order to
avoid this limitation we have created workarounds within the nginx config as well as
within our  own software, basically by having 50 local addresses and only following the
scenario (1). Alas, all of the built-in Unix utilities as well as other software always  
follow scenario (2). As the result given large number of connections there may be points
in time, when whole range of ports is occupied by at least one local address. Even worse is  
the outcome of such condition: when in_pcb_lport() travels over the range of possible port
numbers, making myriad of calls to in_pcblookup_local(), some  kind of important lock is
being held withing the kernel. So important that it leads to a complete lock of the system.
Even the direct terminal access is not available: it is not responsive. The more calls to
connect through scenario (2) there are the longer it takes the system to unfreeze. Given
some circumstances, the only option is hard reset.

Is it possible to somehow update the code that does connect via scenario (2) to enable
more intelligent port allocation, like for example allocating local address and port simultaneously  

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Re: Request for more intelligent local port allocation algorithm

freebsd-stable mailing list
Just to add to this, if anyone is doing some work on the outbound tcp
connection, could they also have a look at the bug here
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210726

Thanks!

On Wed, 6 Feb 2019 at 15:15, Paul <[hidden email]> wrote:

> Hi dev team,
>
> It's not a secret that when application is trying to establish new TCP
> connection, without
> first binding a socket to specific local interface address, OS handles
> that automatically.
> Unfortunately there is a catch, that lies in a different logic of local
> port allocation:
> (1) when socket is bound before connect() vs (2) when it is not. When
> allocating the port
> in in_pcb_lport() by checking whether different ports are free, using
> in_pcblookup_local(),
> the behaviour is following:
>
> (1) Bound, ie laddr is assigned with specific address:
>     Port is considered occupied only if there is a PCBs that matches both
> laddr and lport
>
> (2) Not bound, ie laddr == INADDR_ANY:
>     Port is considered occupied if there is any PCBs that only matches
> lport. What this
>     means is that in order to allocate a port none of the all available
> local addresses
>     should have it allocated, even though this requirement is ridiculous,
> since we are
>     allocating only one PCB
>
> Looking though the code, it seems that (2) is due to the fact that
> tcp_connect() first
> allocates the port, indirectly through the call to in_pcbbind() and only
> then allocates
> the actual local address, also indirectly, though the call to
> in_pcbconnect_setup(), that
> in turn calls in_pcbladdr(). So, probably, in order to guarantee that
> in_pcbconnect_setup()
> will not fail we make sure that all range of local addresses are
> available, no matter
> which one of them is actually selected by in_pcbladdr()?
>
> In real world, this creates serious problems for servers that have a lot
> of outgoing
> connections, for example nginx proxy with a lot of open HTTP2 connections.
> In order to
> avoid this limitation we have created workarounds within the nginx config
> as well as
> within our  own software, basically by having 50 local addresses and only
> following the
> scenario (1). Alas, all of the built-in Unix utilities as well as other
> software always
> follow scenario (2). As the result given large number of connections there
> may be points
> in time, when whole range of ports is occupied by at least one local
> address. Even worse is
> the outcome of such condition: when in_pcb_lport() travels over the range
> of possible port
> numbers, making myriad of calls to in_pcblookup_local(), some  kind of
> important lock is
> being held withing the kernel. So important that it leads to a complete
> lock of the system.
> Even the direct terminal access is not available: it is not responsive.
> The more calls to
> connect through scenario (2) there are the longer it takes the system to
> unfreeze. Given
> some circumstances, the only option is hard reset.
>
> Is it possible to somehow update the code that does connect via scenario
> (2) to enable
> more intelligent port allocation, like for example allocating local
> address and port simultaneously
>
> _______________________________________________
> [hidden email] mailing list
> https://lists.freebsd.org/mailman/listinfo/freebsd-stable
> To unsubscribe, send any mail to "[hidden email]"
>
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