From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pj1-x102b.google.com (mail-pj1-x102b.google.com [IPv6:2607:f8b0:4864:20::102b]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id CA19C3B2A4 for ; Sat, 29 Oct 2022 09:48:26 -0400 (EDT) Received: by mail-pj1-x102b.google.com with SMTP id h14so6893330pjv.4 for ; Sat, 29 Oct 2022 06:48:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=XPChn2XivfzzKdTUN3SXcL8cGwJmiDkZmHa6zhDb4XA=; b=iUV1bkHpWirtqg2aQIwx+yQB8Cx3FHNJnir5yre4fkYHYTxp90TJzRCbGyJob6abSs xLGFuYUpi1G8DXM52GTWxxHxzHTYRdBl739Dq9r35NBtRD0d1E+jW9Srk+XeepbX634I P4/EXZzVfkPNSXhQ6eFnuLtcEWypTCJ3rwFgdDBU0i6AVVzE30CclajB2TRqif/x5nQA gW+ANsYJe1s4Uy9fMPyX4xPVIBAfFcuuhon5MNJnExF0Q7NZCTDmgiLrpp/S9eo0MBGJ bK/ZYElwY4xMdUXmMVNBpvlZR5AAXsxqyB8SX4Es5TK2v3YjMtXsmPEbIAeAwahNRvmE ZUZA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=XPChn2XivfzzKdTUN3SXcL8cGwJmiDkZmHa6zhDb4XA=; b=78OtiN4pho5wyDQE92RyZSAeV0yvms/FVG1QD+VA62v9fD/dcPXaOUNKPjyk1fLfOe taT+6dOYg87iAoHXjqaY51Ni/lpQME4OkDpjepluqbObRMSZsgNGVJGc4QIhQHCInlto J5pLFiWMj6YEDvwmyLGV7G2rHr+g0BVUh6o4hTziW9eR3JfhlMDxlYFQCbcIVbVH/FH0 /sPvb0GQPOcaYV/6JsxBAGAzh63YFBab8nvvQTXmW1hi2Nk/+8YVzeIAKZhAZnxYjnTs 9Rn4AhnNxqUNu4pq4hYddhQCbOauIazVVSTgtb/EJfJRknSShawRgagDzGRpKvbQvrqd XGHA== X-Gm-Message-State: ACrzQf03nBO1vVgQutgbH83UGazLMBwE8tlSamvp+gELCssZtxfuGPQh E9zQckJ68JhVybG0sl9DfezrpjXNXIClCK+l6zQ= X-Google-Smtp-Source: AMsMyM4ZHURIghoYe5XRhIwtT8u0JGgWxbVE/6UuWDT4iIKhQMTEj0pqOwT0j2753S7BpKpNmJ5bNoOo57mbbU0DJdE= X-Received: by 2002:a17:90b:4b90:b0:213:6a84:b4e9 with SMTP id lr16-20020a17090b4b9000b002136a84b4e9mr16793951pjb.212.1667051305696; Sat, 29 Oct 2022 06:48:25 -0700 (PDT) MIME-Version: 1.0 References: <87sfj7vczj.wl-jch@irif.fr> <87leozuh1s.wl-jch@irif.fr> In-Reply-To: <87leozuh1s.wl-jch@irif.fr> From: Herbert Wolverson Date: Sat, 29 Oct 2022 08:48:14 -0500 Message-ID: To: Juliusz Chroboczek Cc: Dave Taht , libreqos@lists.bufferbloat.net Content-Type: multipart/alternative; boundary="000000000000e6015e05ec2ca067" Subject: Re: [LibreQoS] routing protocols and daemons X-BeenThere: libreqos@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: Many ISPs need the kinds of quality shaping cake can do List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 29 Oct 2022 13:48:27 -0000 --000000000000e6015e05ec2ca067 Content-Type: text/plain; charset="UTF-8" Juliusz, it's a pleasure to meet you. I've seen your name quite often in the async/await world. Admittedly, usually in the detailed "how things work" part - while I tend to be on the "teaching how to use an implementation" side of things. > > Dijkstra's algorithm remains a very natural approach to mapping a > > graph > I'm not sure what that means. Dijkstra's is a shortest path algorithm, > it's not in the business of mapping. I guess the author meant that > representing a graph as an adjacency list (the LSDB) is natural, which is > certainly true, but in no way specific to OSPF. Absolutely. Most of my development background is in game development, I also do a lot of GIS. In both fields, Dijkstra's algorithm - and its adaptations (A*, weighted flow maps, etc.) refer to mapping in the spatial sense; and converting a map to a node graph (whether grid, waypoint, etc.) and then working with cost-based adjacency (not raw adjacency) is a very natural way to resolve the issue of "how do I get from X to Y" on a map. It's in no way specific to OSPF (although specific adjacency cost specification was one of many reasons OSPF outperforms RIP). OSPF is where it is now because it's "good enough (for now)" and just about everything supports it. Sure, an implementation that spits out bad LSAs is going to break everything - you're going to get some pretty nasty results from sending out broken destination-distance-vector data, too. Garbage-in, garbage-out is one of the few truly universal rules! I agree, though - I wouldn't hand out large-scale OSPF administration to the new guy (although "here's the standard router config, plug in the numbers for the locally attached networks here" does work). I'd love to see good support for dynamic capacity analysis, unequal cost multipath and similar. Babel looks very promising, but the chicken-egg problem is very real; I can't put it to use until it's widely available, but it won't become widely available until enough people put it to use. (It seems like wireless vendors are busy trying to reinvent it at layer 2 with proprietary meshing that doesn't talk to other proprietary meshing; ugh) On Sat, Oct 29, 2022 at 4:15 AM Juliusz Chroboczek wrote: > > our toasts to the builders of Notre-Dame. > > ...which then burnt down :-/ > > > Dijkstra's algorithm remains a very natural approach to mapping a > > graph > > I'm not sure what that means. Dijkstra's is a shortest path algorithm, > it's not in the business of mapping. I guess the author meant that > representing a graph as an adjacency list (the LSDB) is natural, which is > certainly true, but in no way specific to OSPF. > > > I don't suppose you have ever had any ideas to how to improve things? > > Modern OSPF and IS-IS have pretty much reached a local optimum: all the > low-hanging fruit has been picked, I doubt there's much that can still be > done to improve them without a complete redesign. Well-implemented OSPF > and IS-IS work beautifully in a well-administered network, any other > protocol is going to converge slower and give less visibility into the > network. > > On the other hand, OSPF is extremely fragile in the presence of bad > implementation. If two routers have the same id, OSPF is going to create > routing pathologies. If a router corrupts its LSDB (for example due to > bad RAM), OSPF will create routing pathologies which will only go away > once the faulty LSA expires (30 minutes worst case). If a router runs out > of memory for its LSDB, it needs to stop participating in the protocol, > lest it cause routing pathologies (IS-IS has the overload bit to deal with > this case, which causes the router to become a stub router). Compare this > with distance vector, where a corrupt routing table entry will only > interfere with the traffic to that particular destination, and where it is > perfectly correct to run with a partial routing table. > > OSPF also requires a skilled administrator. Splitting a network into > areas without causing suboptimal routing takes significant skill, route > filtering can only happen on area boundaries, and there are multiple > different ways of redistributing routes into OSPF (external LSAs). > > In my opinion, you want to be running OSPF in parts of your network that > are implemented with reliable gear and are managed by a competent > administrator, but you'll prefer a modern distance-vector protocol > (somebody mentioned Babel) where the hardware is cheap and the > administator is busy with other things. Fortunately, due to the > flexibility of route redistribution in distance-vector protocols, you can > do both: a stable backbone using OSPF, and unadministered Babel bits at > the edges. > > -- Juliusz > --000000000000e6015e05ec2ca067 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Juliusz, it's a pleasure to meet you. I've se= en your name quite
often in the async/await world. Admittedly, us= ually in the detailed
"how things work" part - while I = tend to be on the "teaching how to
use an implementation&quo= t; side of things.

> > Dijkstra's algorithm remains a very natural approach to mapping= a
> > graph
> I'm not sure what that means.=C2=A0 Dijkstra's is a sho= rtest path algorithm,
> it's not in the business of mapping.=C2= =A0 I guess the author meant that
> representing a graph as an adjace= ncy list (the LSDB) is natural, which is
> certainly true, but i= n no way specific to OSPF.

Absol= utely. Most of my development background is in game development,
I also do a lot of GIS. In both fields, Dij= kstra's algorithm - and its adaptations
(A*, weighted flow maps, etc.) refer to mapping in the spati= al sense; and converting
a map to= a node graph (whether grid, waypoint, etc.) and then working with
cost-based adjacency (not raw adjace= ncy) is a very natural way to
res= olve the issue of "how do I get from X to Y" on a map. It's i= n no way
specific to OSPF (althou= gh specific adjacency cost specification was
one of many reasons OSPF outperforms RIP).

OSPF is where it is now because it's "good enoug= h (for now)" and just
about everything supports it. Sure, an= implementation that spits out bad
LSAs is going to break everyth= ing - you're going to get some pretty nasty
results from send= ing out broken destination-distance-vector data, too.
Garbage-in,= garbage-out is one of the few truly universal rules! I agree,
th= ough - I wouldn't hand out large-scale OSPF administration to the new
guy (although "here's the standard router config, plug in= the numbers for
the locally attached networks here" does wo= rk).

I'd love to see good support for dyn= amic capacity analysis, unequal
cost multipath and similar. = Babel looks very promising, but the chicken-egg
problem is very r= eal; I can't put it to use until it's widely available, but
it won't become widely available until enough people put it to use. = (It
seems like wireless vendors are busy trying to reinvent it at= layer 2 with
proprietary meshing that doesn't talk to other = proprietary meshing; ugh)



<= /div>

= On Sat, Oct 29, 2022 at 4:15 AM Juliusz Chroboczek <jch@irif.fr> wrote:
> our toasts to the builders of Notre-Dame.

...which then burnt down :-/

> Dijkstra's algorithm remains a very natural approach to mapping a<= br> > graph

I'm not sure what that means.=C2=A0 Dijkstra's is a shortest path a= lgorithm,
it's not in the business of mapping.=C2=A0 I guess the author meant tha= t
representing a graph as an adjacency list (the LSDB) is natural, which is certainly true, but in no way specific to OSPF.

> I don't suppose you have ever had any ideas to how to improve thin= gs?

Modern OSPF and IS-IS have pretty much reached a local optimum: all the
low-hanging fruit has been picked, I doubt there's much that can still = be
done to improve them without a complete redesign.=C2=A0 Well-implemented OS= PF
and IS-IS work beautifully in a well-administered network, any other
protocol is going to converge slower and give less visibility into the
network.

On the other hand, OSPF is extremely fragile in the presence of bad
implementation.=C2=A0 If two routers have the same id, OSPF is going to cre= ate
routing pathologies.=C2=A0 If a router corrupts its LSDB (for example due t= o
bad RAM), OSPF will create routing pathologies which will only go away
once the faulty LSA expires (30 minutes worst case).=C2=A0 If a router runs= out
of memory for its LSDB, it needs to stop participating in the protocol,
lest it cause routing pathologies (IS-IS has the overload bit to deal with<= br> this case, which causes the router to become a stub router).=C2=A0 Compare = this
with distance vector, where a corrupt routing table entry will only
interfere with the traffic to that particular destination, and where it is<= br> perfectly correct to run with a partial routing table.

OSPF also requires a skilled administrator.=C2=A0 Splitting a network into<= br> areas without causing suboptimal routing takes significant skill, route
filtering can only happen on area boundaries, and there are multiple
different ways of redistributing routes into OSPF (external LSAs).

In my opinion, you want to be running OSPF in parts of your network that are implemented with reliable gear and are managed by a competent
administrator, but you'll prefer a modern distance-vector protocol
(somebody mentioned Babel) where the hardware is cheap and the
administator is busy with other things.=C2=A0 Fortunately, due to the
flexibility of route redistribution in distance-vector protocols, you can do both: a stable backbone using OSPF, and unadministered Babel bits at
the edges.

-- Juliusz
--000000000000e6015e05ec2ca067--