Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0

[Enke Chen <[e-mail]>]

Hi, John:

I checked a recent Linux version (v5.10-rc7-149-g33dc961), and confirmed
that the per-session USER_TIMEOUT value (icsk->icsk_user_timeout) is applied
 in all these cases:

  1) tcp_retransmit_timer()
              tcp_write_timeout()
              tcp_clamp_rto_to_user_timeout()

     2)  tcp_probe_timer()
             For zero-window event: when there is outstanding un-acked
data, or buffered data.

     3)  tcp_keepalive_timer()

So it seems that the implementation does provide the desired semantics
described in RFC 793, but the man page is out-dated.

Thanks.  -- Enke


On Fri, Dec 18, 2020 at 2:32 PM John Scudder <[e-mail]> wrote:

> On Dec 18, 2020, at 1:09 PM, Enke Chen <[e-mail]>
> wrote:
> >
> > No, I am not assuming that packets are getting somewhere. The
> TCP_USER_TIMEOUT would work as long as there is "pending data" (either
> unacked, or locally queued). The data can be from the local BGP Keepalives
> or the TCP_KEEPALIVE.
>
> Apart from the other objections to relying on TCP_USER_TIMEOUT, which I
> think are sufficient, it’s not clear to me that implementations will
> provide the desired semantics. RFC 793 seems like it specifies the right
> semantics (“get this data to the peer within N seconds or close”):
>
>         The timeout, if present, permits the caller to set up a timeout
>         for all data submitted to TCP.  If data is not successfully
>         delivered to the destination within the timeout period, the TCP
>         will abort the connection.  The present global default is five
>         minutes.
>
> However the Linux man page documents different semantics:
>
>        TCP_USER_TIMEOUT (since Linux 2.6.37)
>               This option takes an unsigned int as an argument.  When the
>               value is greater than 0, it specifies the maximum amount of
>               time in milliseconds that transmitted data may remain
>               unacknowledged before TCP will forcibly close the
>               corresponding connection and return ETIMEDOUT to the
>               application.  If the option value is specified as 0, TCP will
>               use the system default.
>
> The important difference being that whereas 793 implies data written to
> the socket, the Linux man page says “transmitted” data, which seems like it
> must mean data TCP has written to the network. These are two very different
> things! If Linux (or another stack) implements what the man page seems to
> say, it’s not useful for our purposes.
>
> —John