Robert, Assuming C and E provide access to the same set of destinations, that are closer of further away from C and E. B (which is fast), after it notifies A that it can’t reach C directly will cause A to send traffic to D. D - dependent on total cost would start happily sending some traffic towards destinations behind C to B so LFA on B wouldn’t really help.
Cheers, Jeff On May 5, 2020, 5:04 PM -0700, Robert Raszuk <rob...@raszuk.net>, wrote: > Hi Les, > > A side comment but your example shows another - one may say even much more > serious issue. > > Assume we have LFA/TI-LFA enabled in the network and precomputed on B which > get's activated and shifts traffic to E when detects that C is down. > Detection is fast .. 10s-100s of milliseconds. > > Now if B converges fast and recomputes topology much faster then D it may > remove protection and send packets to D natively. Well clearly as we > established D is slow and will loop it back. > > That is why I mentioned the other day that a fast control plane is not always > a good thing (I am sure many will say the opposite - but it is ok ;). > > But this proves that consistent convergence time in a domain is rather a good > thing regardless if it takes 2 sec or 50 sec on all nodes. > > Best, > Robert. > > > > > On Wed, May 6, 2020 at 1:35 AM Les Ginsberg (ginsberg) > > <ginsberg=40cisco....@dmarc.ietf.org> wrote: > > > Bruno - > > > > > > Seems like it was not too long ago that we were discussing this in > > > person. Ahhh...the good old days... > > > > > > First, let's agree that the interesting case does not involve 1 or even a > > > small number of LSPs. For those cases flooding speed does not matter. > > > The interesting cases involve a large number of LSPs (hundreds or > > > thousands). And in such cases LFA/microloop avoidance techniques are not > > > applicable. > > > > > > Take the following simple topology: > > > > > > | | ... | | > > > +---+ +---+ > > > | C | | E | > > > +---+ +---+ > > > | | 1000 > > > +---+ +---+ > > > | B |-------------| D | > > > +---+ 1000 +---+ > > > | | > > > | | > > > \ / > > > \ / > > > \ / > > > \ / > > > +---+ > > > | A | > > > +---+ > > > > > > There is a topology northbound of C and E (not shown) and a topology > > > southbound of A (not shown). > > > Cost on all links is 10 except B-D and D-E where cost is high. > > > > > > C is a node with 1000 neighbors. > > > When all links are up, shortest path for all northbound destinations is > > > via C. > > > All nodes in the network support fast flooding except for Node D. > > > Let’s say fast flooding is 500 LSPs/second and slow flooding (Node D) is > > > 20 LSPs/seconds. > > > If Node C fails we have 1000 LSPs to flood. > > > All nodes except for D can receive these in 2 seconds (plus internode > > > delay time). > > > D can receive LSPs in 50 seconds. > > > > > > When A and B and all southbound nodes receive/process the LSP updates > > > they will start sending traffic to Northbound destinations via D. > > > But for the better part of 50 seconds, Node D has yet to receive all LSP > > > updates and still believes that shortest path is via B-C. It will loop > > > traffic. > > > > > > Had all nodes used slow flooding, it still would have taken 50 seconds to > > > converge, but there would be significantly less looping. There could be a > > > good amount of blackholing, but this is preferable to looping. > > > > > > One can always come up with examples – based on a specific topology and a > > > specific failure - where things might be better/worse/unchanged in the > > > face of inconsistent flooding speed support. > > > But I hope this simple example illustrates the pitfalls. > > > > > > Les > > > > > > > -----Original Message----- > > > > From: bruno.decra...@orange.com <bruno.decra...@orange.com> > > > > Sent: Tuesday, May 05, 2020 8:28 AM > > > > To: Les Ginsberg (ginsberg) <ginsb...@cisco.com>; lsr@ietf.org > > > > Subject: Flooding across a network > > > > > > > > Les, > > > > > > > > > From: Lsr [mailto:lsr-boun...@ietf.org] On Behalf Of Les Ginsberg > > > > (ginsberg) > > > > > Sent: Monday, May 4, 2020 4:39 PM > > > > [...] > > > > > when only some nodes in the network support faster flooding the > > > > > behavior > > > > of the whole network may not be "better" when faster flooding is enabled > > > > because it prolongs the period of LSDB inconsistency. > > > > > > > > 1) Do you have data on this? > > > > > > > > 2) If not, can you provide an example where increasing the flooding > > > > rate on > > > > one adjacency prolongs the period of LSDB inconsistency across the > > > > network? > > > > > > > > 3) In the meantime, let's try the theoretical analysis on a simple > > > > scenario > > > > where a single LSP needs to be flooded across the network. > > > > > > > > - Let's call Dij the time needed to flood the LSP from node i to the > > > > adjacent > > > > node j. Clearly Dij>0. > > > > - Let's call k the node originating this LSP at t0=0s > > > > > > > > >From t0, the LSDB is inconsistent across the network as all nodes but > > > > >k are > > > > missing the LSP and hence only know about the 'old' topology. > > > > > > > > Let's call SPT(k) the SPT rooted on k, using Dij as the metric between > > > > adjacent nodes i and j. Let's call SP(k,i) the shortest path from k to > > > > i; and > > > > D(k,i) the shortest distance between k and i.. > > > > > > > > It seems that the time needed: > > > > - for node j to learn about the LSP, and get in sync with k, is D(k,j) > > > > - for all nodes across the network to learn about the LSP, and get in > > > > sync with > > > > k, is Max[for all j] D(k,j) > > > > > > > > Then how can reducing the flooding delay on one adjacency could prolongs > > > > the period of LSDB inconsistency? > > > > It seems to me that it can only improve/decrease it. Otherwise, this > > > > would > > > > mean that decreasing the cost on a link can increase the cost of the > > > > shortest > > > > path. > > > > > > > > Note: I agree that there are other cases, such as multiple LSPs > > > > originated by > > > > the same node, and multiple LSPs originated by multiple nodes, but > > > > let's start > > > > with the simple case. > > > > > > > > Thanks, > > > > --Bruno > > > > > > > > > -----Original Message----- > > > > > From: Lsr [mailto:lsr-boun...@ietf.org] On Behalf Of Les Ginsberg > > > > (ginsberg) > > > > > Sent: Monday, May 4, 2020 4:39 PM > > > > > > > > > > Henk - > > > > > > > > > > Thanx for your thoughtful posts. > > > > > I have read your later posts on this thread as well - but decided to > > > > > reply to > > > > this one. > > > > > Top posting for better readability. > > > > > > > > > > There is broad agreement that faster flooding is desirable. > > > > > There are now two proposals as to how to address the issue - neither > > > > > of > > > > which is proposing to use TCP (or equivalent). > > > > > > > > > > I have commented on why IS-IS flooding requirements are significantly > > > > different than that for which TCP is used. > > > > > I think it is also useful to note that even the simple test case > > > > > which Bruno > > > > reported on in last week's interim meeting demonstrated that without any > > > > changes to the protocol at all IS-IS was able to flood an order of > > > > magnitude > > > > faster than it commonly does today. > > > > > This gives me hope that we are looking at the problem correctly and > > > > > will not > > > > need "TCP". > > > > > > > > > > Introducing a TCP based solution requires: > > > > > > > > > > a)A major change to the adjacency formation logic > > > > > > > > > > b)Removal of the independence of the IS-IS protocol from the address > > > > families whose reachability advertisements it supports - something > > > > which I > > > > think is a great strength of the protocol - particularly in > > > > environments where > > > > multiple address family support is needed > > > > > > > > > > I really don't want to do either of the above. > > > > > > > > > > Your comments regarding PSNP response times are quite correct - and > > > > both of the draft proposals discuss this - though I agree more detail > > > > will be > > > > required. > > > > > It is intuitive that if you want to flood faster you also need to ACK > > > > > faster - > > > > and probably even retransmit faster when that is needed. > > > > > The basic relationship between retransmit interval and PSNP interval > > > > > is > > > > expressed in ISO 10589: > > > > > > > > > > " partialSNPInterval - This is the amount of time between periodic > > > > > action for transmission of Partial Sequence Number PDUs. > > > > > It shall be less than minimumLSPTransmission-Interval." > > > > > > > > > > Of course ISO 10589 recommended values (2 seconds and 5 seconds > > > > respectively) associated with a much slower flooding rate and > > > > implementations I am aware of use values in this order of magnitude.. > > > > These > > > > numbers need to be reduced if we are to flood faster, but the > > > > relationship > > > > between the two needs to remain the same. > > > > > > > > > > It is also true - as you state - that sending ACKs more quickly will > > > > > result in > > > > additional PDUs which need to be received/processed by IS-IS - and this > > > > has > > > > some impact. But I think it is reasonable to expect that an > > > > implementation > > > > which can support sending and receiving LSPs at a faster rate should > > > > also be > > > > able to send/receive PSNPs at a faster rate. But we still need to be > > > > smarter > > > > than sending one PSNP/one LSP in cases where we have a burst. > > > > > > > > > > LANs are a more difficult problem than P2P - and thus far > > > > > draft-ginsberg-lsr- > > > > isis-flooding-scale has been silent on this - but not because we aren't > > > > aware > > > > of this - just have focused on the P2P behavior first. > > > > > What the best behavior on a LAN may be is something I am still > > > > > considering. > > > > Slowing flooding down to the speed at which the slowest IS on the LAN > > > > can > > > > support may not be the best strategy - as it also slows down the > > > > propagation > > > > rate for systems downstream from the nodes on the LAN which can handle > > > > faster flooding - thereby having an impact on flooding speed throughout > > > > the > > > > network in a way which may be out of proportion. This is a smaller > > > > example > > > > of the larger issue that when only some nodes in the network support > > > > faster > > > > flooding the behavior of the whole network may not be "better" when > > > > faster > > > > flooding is enabled because it prolongs the period of LSDB > > > > inconsistency. > > > > More work needs to be done here... > > > > > > > > > > In summary, I don't expect to have to "reinvent TCP" - but I do think > > > > > you > > > > have provided a useful perspective for us to consider as we progress on > > > > this > > > > topic, > > > > > > > > > > Thanx. > > > > > > > > > > Les > > > > > > > > > > > > > > > > -----Original Message----- > > > > > > From: Lsr <lsr-boun...@ietf.org> On Behalf Of Henk Smit > > > > > > Sent: Thursday, April 30, 2020 6:58 AM > > > > > > To: lsr@ietf.org > > > > > > Subject: [Lsr] Why only a congestion-avoidance algorithm on the > > > > > > sender > > > > isn't > > > > > > enough > > > > > > > > > > > > > > > > > > Hello all, > > > > > > > > > > > > Two years ago, Gunter Van de Velde and myself published this draft: > > > > > > https://tools.ietf.org/html/draft-hsmit-lsr-isis-flooding-over-tcp-00 > > > > > > That started this discussion about flow/congestion control and ISIS > > > > > > flooding. > > > > > > > > > > > > My thoughts were that once we start implementing new algorithms to > > > > > > optimize ISIS flooding speed, we'll end up with our own version of > > > > > > TCP. > > > > > > I think most people here have a good general understanding of TCP. > > > > > > But if not, this is a good overview how TCP does it: > > > > > > https://en.wikipedia.org/wiki/TCP_congestion_control > > > > > > > > > > > > > > > > > > What does TCP do: > > > > > > ==== > > > > > > TCP does 2 things: flow control and congestion control. > > > > > > > > > > > > 1) Flow control is: the receiver trying to prevent itself from being > > > > > > overloaded. The receiver indicates, through the receiver-window-size > > > > > > in the TCP acks, how much data it can or wants to receive.. > > > > > > 2) Congestion control is: the sender trying to prevent the links > > > > > > between > > > > > > sender and receiver from being overloaded. The sender makes an > > > > educated > > > > > > guess at what speed it can send. > > > > > > > > > > > > > > > > > > The part we seem to be missing: > > > > > > ==== > > > > > > For the sender to make a guess at what speed it can send, it looks > > > > > > at > > > > > > how the transmission is behaving. Are there drops ? What is the RTT > > > > > > ? > > > > > > Do drop-percentage and RTT change ? Do acks come in at the same rate > > > > > > as the sender sends segments ? Are there duplicate acks ? To be able > > > > > > to do this, the sender must know what to expect. How acks behave. > > > > > > > > > > > > If you want an ISIS sender to make a guess at what speed it can > > > > > > send, > > > > > > without changing the protocol, the only thing the sender can do is > > > > > > look > > > > > > at the PSNPs that come back from the receiver. But the RTT of PSNPs > > > > > > can > > > > > > not be predicted. Because a good ISIS implementation does not > > > > > > immediately > > > > > > send a PSNP when it receives a LSP.. 1) the receiver should jitter > > > > > > the > > > > > > PSNP, > > > > > > like it should jitter all packets. And 2) the receiver should wait a > > > > > > little > > > > > > to see if it can combine multiple acks into a single PSNP packet. > > > > > > > > > > > > In TCP, if a single segment gets lost, each new segment will cause > > > > > > the > > > > > > receiver to send an ack with the seqnr of the last received byte. > > > > > > This > > > > > > is called "duplicate acks". This triggers the sender to do > > > > > > fast-retransmission. In ISIS, this can't be be done. The information > > > > > > a sender can get from looking at incoming PSNPs is a lot less than > > > > > > what > > > > > > TCP can learn from incoming acks. > > > > > > > > > > > > > > > > > > The problem with sender-side congestion control: > > > > > > ==== > > > > > > In ISIS, all we know is that the default retransmit-interval is 5 > > > > > > seconds. > > > > > > And I think most implementations use that as the default. This means > > > > > > that > > > > > > the receiver of an LSP has one requirement: send a PSNP within 5 > > > > > > seconds. > > > > > > For the rest, implementations are free to send PSNPs however and > > > > > > whenever > > > > > > they want. This means a sender can not really make conclusions about > > > > > > flooding speed, dropped LSPs, capacity of the receiver, etc. > > > > > > There is no ordering when flooding LSPs, or sending PSNPs.. This > > > > > > makes > > > > > > a sender-side algorithm for ISIS a lot harder. > > > > > > > > > > > > When you think about it, you realize that a sender should wait the > > > > > > full 5 seconds before it can make any real conclusions about dropped > > > > > > LSPs. > > > > > > If a sender looks at PSNPs to determine its flooding speed, it will > > > > > > probably > > > > > > not be able to react without a delay of a few seconds. A sender > > > > > > might > > > > > > send > > > > > > hunderds or thousands of LSPs in those 5 seconds, which might all or > > > > > > partially be dropped, complicating matters even further. > > > > > > > > > > > > > > > > > > A sender-sider algorithm should specify how to do PSNPs. > > > > > > ==== > > > > > > So imho a sender-side only algorithm can't work just like that in a > > > > > > multi-vendor environment. We must not only specify a congestion- > > > > control > > > > > > algorithm for the sender. We must also specify for the receiver a > > > > > > more > > > > > > specific algorithm how and when to send PSNPs. At least how to do > > > > PSNPs > > > > > > under load. > > > > > > > > > > > > Note that this might result in the receiver sending more (and > > > > > > smaller) > > > > > > PSNPs. > > > > > > More packets might mean more congestion (inside routers). > > > > > > > > > > > > > > > > > > Will receiver-side flow-control work ? > > > > > > ==== > > > > > > I don't know if that's enough. It will certainly help. > > > > > > > > > > > > I think to tackle this problem, we need 3 parts: > > > > > > 1) sender-side congestion-control algorithm > > > > > > 2) more detailed algorithm on receiver when and how to send PSNPs > > > > > > 3) receiver-side flow-control mechanism > > > > > > > > > > > > As discussed at length, I don't know if the ISIS process on the > > > > > > receiving > > > > > > router can actually know if its running out of resources (buffers on > > > > > > interfaces, linecards, etc). That's implementation dependent. A > > > > > > receiver > > > > > > can definitely advertise a fixed value. So the sender has an upper > > > > > > bound > > > > > > to use when doing congestion-control. Just like TCP has both a > > > > > > flow-control > > > > > > window and a congestion-control window, and a sender uses both. > > > > Maybe > > > > > > the > > > > > > receiver can even advertise a dynamic value. Maybe now, maybe only > > > > > > in > > > > > > the > > > > > > future. An advertised upper limit seems useful to me today. > > > > > > > > > > > > > > > > > > What I didn't like about our own proposal (flooding over TCP): > > > > > > ==== > > > > > > The problem I saw with flooding over TCP concerns multi-point > > > > > > networks > > > > > > (LANs). > > > > > > > > > > > > When flooding over a multi-point network, setting up TCP connections > > > > > > introduces serious challenges. Who are the endpoints of the TCP > > > > > > connections ? > > > > > > Full mesh ? Or do all ISes on a LAN create a TCP-connection to the > > > > > > DIS ? > > > > > > There is no backup DIS in ISIS (unlike OSPF). Things get messy > > > > > > quickly. > > > > > > > > > > > > However, the other two proposals do not solve this problem either. > > > > > > How will a sender-side congestion-avoidence algorithm determine > > > > whether > > > > > > there were drops ? There are no acks (PSNPs) on a LAN. We assume > > > > > > most > > > > > > LSPs > > > > > > that are broadcasted are received by all other ISes on the LAN. > > > > > > There > > > > > > are > > > > > > no acks. Only after the DIS has sent its periodic CSNPs, ISes can > > > > > > send > > > > > > PSNPs to request retransmissions. It seems impossible (or very > > > > > > hard) to > > > > > > me for all ISes on a LAN to keep track of dropped LSPs and adjust > > > > > > their > > > > > > sending speed accordingly. > > > > > > > > > > > > When flooding on a LAN, the receiver-side algorithm seems best. > > > > Because > > > > > > all ISes can see what the lowest advertised sending-speed is. And > > > > > > make > > > > > > sure they send slow enough to not overload the slowest IS.. I'm not > > > > > > sure > > > > > > this is a good solution, but is seems easier and more realistic than > > > > > > ISIS-flooding-over-TCP or sender-side congestion-avoidance. > > > > > > > > > > > > > > > > > > My conclusion: > > > > > > ==== > > > > > > Sender-side congestion-control won't work without specifying in more > > > > > > detail how and when to send PSNPs. > > > > > > Receiver-side flow-control will certainly help. I dont' know if it's > > > > > > good enough. I don't know if advertising a static value is good > > > > > > enough. > > > > > > But it's a start. > > > > > > > > > > > > I still think we'll end up re-implementing a new (and weaker) TCP. > > > > > > > > > > > > > > > > > > henk. > > > > > > > > > > > > _______________________________________________ > > > > > > Lsr mailing list > > > > > > Lsr@ietf.org > > > > > > https://www.ietf.org/mailman/listinfo/lsr > > > > > > > > > > _______________________________________________ > > > > > Lsr mailing list > > > > > Lsr@ietf.org > > > > > https://www.ietf.org/mailman/listinfo/lsr > > > > > > > > __________________________________________________________ > > > > __________________________________________________________ > > > > _____ > > > > > > > > Ce message et ses pieces jointes peuvent contenir des informations > > > > confidentielles ou privilegiees et ne doivent donc > > > > pas etre diffuses, exploites ou copies sans autorisation. 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