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 18:51 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 18:50:51 +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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The restarting speaker in this case did not actually restart.
It just restarted this one session. There is no reason for it
to delete any forwarding state. There is no evidence of any
problem with its received routes, only with the routes it sent
to the stuck peer. It may still set the (F) bit to force the
stuck peer to WITHDRAW.

Regards,
Jakob.

-----Original Message-----
From: Claudio Jeker <cjeker@diehard.n-r-g.com> 
Sent: Wednesday, December 16, 2020 9:43 AM
To: Jakob Heitz (jheitz) <jheitz@cisco.com>
Cc: Job Snijders <job@sobornost.net>et>; idr@ietf.org
Subject: Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0

On Wed, Dec 16, 2020 at 04:21:53PM +0000, Jakob Heitz (jheitz) wrote:
> 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.

While true the sending system doing the reset will consider it as if it
sent a NOTIFICATION and will therefor not do a GR. Once the new session is
established rtr-A will flush its table because rtr-B will not include the
F flags. Considering that rtr-A is probably not handling any route updates
it is more important that rtr-B routes traffic away from rtr-A and this
happens.
 
> 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? :)
> > 
> >> _______________________________________________
> >> Idr mailing list
> >> Idr@ietf.org
> >> https://www.ietf.org/mailman/listinfo/idr
> > 

-- 
:wq Claudio