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

"Jakob Heitz (jheitz)" <jheitz@cisco.com> Wed, 16 December 2020 16:22 UTC

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From: "Jakob Heitz (jheitz)" <jheitz@cisco.com>
To: Claudio Jeker <cjeker@diehard.n-r-g.com>
CC: Job Snijders <job@sobornost.net>, "idr@ietf.org" <idr@ietf.org>
Thread-Topic: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0
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Date: Wed, 16 Dec 2020 16:21:53 +0000
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Subject: Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0
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No. It's not closed with a NOTIFICATION. The send queue is frozen. No data, not even a NOTIFICATION is going to get to rtr-A. The only thing that will get there is a TCP RST and/or a new TCP SYN.

Regards,
Jakob.


> On Dec 16, 2020, at 1:36 AM, Claudio Jeker <cjeker@diehard.n-r-g.com> wrote:
> 
> On Tue, Dec 15, 2020 at 11:39:52PM +0000, Jakob Heitz (jheitz) wrote:
>> If you tell the socket to shutdown and then close, it will attempt to
>> send everything in the queue with the FIN at the end.
>> Then wait for the FIN ACK and all manner of nonsense to bore rtr-B to tears.
>> So, to get on with it, send the RST.
>> 
>> Next question is what to do if GR is in effect.
>> rtr-A will dutifully retain all the routes from rtr-B and Job's beloved WITHDRAW
>> will still not happen.
>> The new session will come up (maybe), rtr-B will send all its routes again and
>> (if it doesn't get stuck again) will send its EOR. Only now can Job breathe easy.
>> 
>> Might we need a new bit in the GR capability in the OPEN message?
>> "WITHDRAW ALL MY ROUTES NOW"
> 
> GR should not be an issue since the connection is closed with a
> NOTIFICATION. At least the system detecting the stuck session will flush
> and WITHDRAW all routes. In the next OPEN message this system will neither
> set the R flag nor the F flag and so the stuck system will WITHDRAW
> all routes as well.
> 
> The per AF "Forwarding State" bit already acts as a withdraw all my routes
> now indicator.
> 
> Cheers,
> -- 
> :wq Claudio
> 
>> Regards,
>> Jakob.
>> 
>> From: Idr <idr-bounces@ietf.org> On Behalf Of Robert Raszuk
>> Sent: Tuesday, December 15, 2020 3:19 PM
>> To: Job Snijders <job@sobornost.net>
>> Cc: idr@ietf.org
>> Subject: Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0
>> 
>> Hi Job,
>> 
>> Putting all other concerns aside I have few questions ...
>> 
>> 1. Is this BGP which should trigger the session RST or FIN or TCP ?
>> 
>> 2. If this is BGP (TCP would not be aware of HOLD_SEND) how exactly do we know that peer's window is 0 for HOLD_SEND TIME ?
>> 
>> 3. Which TCP socket option will return BGP an error that for the duration of X sec window for a given peer was 0 ? I presumed even if it jumped for 100 ms above 0 the timer would be reset indicating peer is still alive ?
>> 
>> From your bgpd example you are not checking anything other then BGP's ability to write to out queue. So is this the suggestion now forgetting all about TCP layer ? Simply if I can not write anything to a peer for over X sec RST the session ?
>> 
>> Hi John,
>> 
>> I think the suggestion is to add a second HOLD_SEND TIME different from normal HOLD TIME.
>> 
>> Also there could be lost of different type of peers so unless HOLD_SEND would be say 5 x HOLD putting all peers under same time value may be suboptimal.
>> 
>> Thx,
>> R.
>> 
>> 
>> On Tue, Dec 15, 2020 at 10:54 PM Job Snijders <job@sobornost.net<mailto:job@sobornost.net>> wrote:
>>> On Tue, Dec 15, 2020 at 09:57:47PM +0100, Christoph Loibl wrote:
>>> Thanks for answering my question in more detail. Maybe I was unclear
>>> (but reading your email I think we are talking about the same).
>>>> On 15.12.2020, at 21:00, John Scudder <jgs@juniper.net<mailto:jgs@juniper.net>> wrote:
>>>> 
>>>> I think you are talking about this scenario. I’ll copy the example
>>>> from Rob’s message cited above:
>>>> 
>>>>  rtr-A                   rtr-B
>>>>  (congested c-p)         (uncongested c-p)
>>>>  send window: >0         send window: 0
>>>>  recv window: 0          recv window: >0
>>>> 
>>>> In this case we expect:
>>>> a) rtr-B does not send any BGP packet (KEEPALIVE/UPDATE/NOTIFICATION)
>>>> to rtr-A in normal operating circumstances.
>>>> b) rtr-A does not expect any KEEPALIVE/UPDATE packets from rtr-B. The
>>>> session remains established even if no packet is received in the
>>>> holdtime.
>>>> c) rtr-A continues to send KEEPALIVE packets to rtr-B.
>>> 
>>> The part I have a problem to understand is b). It is clear that rtr-A
>>> will not receive any packets from rtr-B because rtr-B cannot send them
>>> (send window: 0). But does "rtr-A does not expect any KEEPALIVE/UPDATE
>>> packets from rtr-B” mean that rtr-A has essentially suspended its
>>> hold-timer until it is ready to receive new messages and opens up its
>>> recv window? If yes, why? I would expect timers to run independently
>>> of the transport protocol.
>> 
>> Yeah, I'd expect that too. We've seen congested BGP implementations
>> continue to send KEEPALIVEs but not accept (or send!) other BGP
>> messages. And rtr-B's attempts at KEEPALIVE just be TCP ACked with zero
>> window.
>> 
>> I'd argue in the above scenario rtr-A is simply broken and rtr-B MUST
>> proceed to close down the session towards rtr-A, rtr-B must cleanup and
>> generate WITHDRAWs for any routes pointing to rtr-A. By doing the
>> clean-up rtr-B does both itself and rtr-A a favor. If the issue was
>> transcient rtr-A and rtr-B will re-establish a few minutes later
>> (IdleHoldTimer, right?) and things will normalize.
>> 
>> Arguably and measurably, rtr-A is operating its Loc-RIB (forwarding)
>> based on stale routing information (assuming rtr-A is working at all!):
>> rtr-A has not received any WITHDRAWs, UPDATEs (or somewhat less
>> importantly KEEPALIVEs) from rtr-B.
>> 
>> Rtr-B is fully aware of this stale situation, because rtr-B was not able
>> to write these BGP messages to the network: the messages are still in
>> OutQ. Rtr-A didn't accept any KEEPALIVE (or UPDATE/WITHDRAW) from
>> rtr-B.
>> 
>> How to solve this? Claudio Jeker took a look at what it would take in
>> OpenBGPD and came up with the (tiny!) following patch, should be
>> readable to most: https://marc.info/?l=openbsd-tech&m=160796802508185&w=2
>> 
>> Ben Cox helped me create a 'EBGP peer from hell': a publicly accessible
>> EBGP multihop instance which can reliably produce the undesirable
>> TCP/BGP behavior we're discussing here. This 'peer from hell' will do
>> the OPEN exchange but then manipulates the TCP recvwindow towards zero.
>> 
>> All BGP implementations tested so far (5 famous ones) appear vulnerable
>> because they continue to consider the BGP session healthy & stable
>> (meanwhile OutQ keeps growing endlessly and zero BGP messages go across
>> the wire).
>> 
>> One network operator (with thousands of EBGP sessions in the DFZ)
>> reported to me the above stalled-TCP scenario is *not* a common case on
>> the Internet. On a normal day, a network operator will see no (zero)
>> sessions stuck this way, which leads me to believe 'recvwind=0' ...
>> *for the duration of the hold timer* is a very strong indicator for a
>> really broken situation which should be attempted to automatically
>> resolve.
>> 
>> I believe BGP implementations are not helping any known deployment
>> scenarios by *not* disconnecting a stuck peer, however on the other we
>> now know about various operational examples where honoring recvwind=0
>> for (hours, days) longer than $holdtimer led to global scale problems.
>> 
>> As the 'not-at-all progressing OutQ' situation seems somewhat rare in
>> the wild (yet continues to happen from time to time) I think it is worth
>> discussing & documenting how implementers can attempt to avoid this
>> state from happening. It might help make the Internet 1% more robust.
>> 
>> BGP implementers (or operators wanting to test their equipment) feel
>> free to contact me off-list if you'd like to set up an EBGP multihop
>> session towards the 'peer from hell' testbed. Testing potential
>> solutions this way is quite easy, the behavior can be triggered within a
>> few seconds.
>> 
>> Kind regards,
>> 
>> Job
>> 
>> ps. At this moment we have (1) an attempt at problem description, (2) a
>> demonstration BGP-4 implementation of a 'problem causer', and (3) a
>> different BGP-4 implementation with a 'solution'. This enables IDR to
>> test interopability & (potentially revised) protocol compliance,
>> hopefully moving the problem a bit from theoretical to practical
>> reality? :)
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