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

[Enke Chen <[e-mail]>]

Correction to 2), it should be:

2)  tcp_probe_timer()
             For zero-window event: when there is buffered data, but no
outstanding un-acked data.

---
The case with outstanding un-acked data would be taken care of by
tcp_retransmit_timer().

Thanks.   -- Enke

On Mon, Dec 21, 2020 at 7:20 PM Enke Chen <[e-mail]>
wrote:

> 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
>
>