On 2011-07-17 19.24, Bob Armstrong wrote:
Bob, endnodes in different areas can only talk to each other
if there's no router present. So your scheme only works if
all of us shut down all our L1 and L2 routers.
We don't have to shut down all the routers on HECnet, right? Just
shutting down the router for my area (or the area of anyone else who cares
about the issue) would be sufficient.
Yes... As long as you don't want to talk to anything that is not on the same segment. But since there are machines further away, this sounds like a sad limitation.
Once an endnode sees a router in its area ...
Exactly... If there are no routers in the end node's area, then
communications is direct. Routers for other areas are moot.
No. Communication are only direct to destinations which are on the same segment. The endnode will have no idea what to do if the destination is not on the same segment, and there is no router. And so those packets will be tossed.
Johnny
On 2011-07-17 17.30, hvlems at zonnet.nl wrote:
Bob, endnodes in different areas can only talk to each other if there's no router present. So your scheme only works if all of us shut down all our L1 and L2 routers. Once an endnode sees a router in its area it will send all its off-area datagrams to it. If that router is an L1 router then the other area is seen as unreachable. If it is an L2 router then it must see the other area router.
No, endnodes will be clever and directly talk to other nodes that are directly connected, even in the presence of a router. But the router is needed to talk to anything not directly connected.
Johnny
I do need L1 routers (for DDCMP and DSSI circuits) and possibly others do too.
Show net/old is neat and useful when systems are in different rooms !
Hans
Verzonden vanaf mijn draadloze BlackBerry -toestel
-----Original Message-----
From: "Bob Armstrong"<bob at jfcl.com>
Sender: owner-hecnet at Update.UU.SE
Date: Sun, 17 Jul 2011 08:21:12
To:<hecnet at Update.UU.SE>
Reply-To: hecnet at Update.UU.SESubject: RE: [HECnet] DECnet et al
Yes. But any L1 routers need help from L2 routers to get out of area.
Yeah, but given that the routers aren't actually needed for two nodes to
talk (under HECnet circumstances) then we don't actually need the routing
nodes at all. Sounds like their only real use is to be able to do a "SHOW
NETWORK/OLD" and see an nice list of active nodes (which is undeniably
neat)...
So a node configured as an end node in area 'n' can actually communicate
directly, over Ethernet, with another end node in area 'm'? But a L1
routing node in area 'n', in the same physical network topology, would
actually relay its packets for a node in area 'm' via an L2 router? And
does this actually take two L2 routers to hand off the packets, one for area
'n' and one for 'm'?
So (again, in the HECnet situation only) having an L1 router is a real
penalty - where as two end nodes could talk directly, just changing the same
machines to L1 routers would require the same traffic to be handled by two
intermediate nodes.
Or an I confused? That's a bit bizarre.
Bob
On 2011-07-17 16.55, Bob Armstrong wrote:
Paul Koning wrote:
It is legal to have multiple areas on a single Ethernet;
the routing spec covers that case explicitly. If that is done,
end nodes will communicate directly to a destination on the
Ethernet even if off-area ...
Given that Johnny's bridge program effectively creates a big Ethernet,
does that imply that HECnet end nodes in different areas can communicate
without an area router?
Yes, at least partially.
They can comminucate directly with machines sitting on the same ethernet segment, even if those machines are in another area.
However, I think that an endnode will not pick any router in another area as its designated router.
Johnny
It's the return path I'm worried about.
Say your area has just endnodes. My area has at least one L1 router up and running. How does a node in my area return the traffic since its router won't have a path towards your area. If mu area contains an L2 router then I think it will return an error like "destination area unreachable".
So, yes Johnny's bridge program makes HECnet behave like we're running DECnet (and LAT) on a large flat LAN. But the rules must observed by all: either we're running routers or neither of us do.
At least that's how I read Paul's messages.
Hans
------Origineel bericht------
Van: Bob Armstrong
Afzender: owner-hecnet at Update.UU.SE
Aan: hecnet at Update.UU.SE
Beantwoorden: hecnet at Update.UU.SE
Onderwerp: RE: [HECnet] DECnet et al
Verzonden: 17 juli 2011 19:24
Bob, endnodes in different areas can only talk to each other
if there's no router present. So your scheme only works if
all of us shut down all our L1 and L2 routers.
We don't have to shut down all the routers on HECnet, right? Just
shutting down the router for my area (or the area of anyone else who cares
about the issue) would be sufficient.
Once an endnode sees a router in its area ...
Exactly... If there are no routers in the end node's area, then
communications is direct. Routers for other areas are moot.
Bob
Verzonden vanaf mijn draadloze BlackBerry -toestel
Bob, endnodes in different areas can only talk to each other
if there's no router present. So your scheme only works if
all of us shut down all our L1 and L2 routers.
We don't have to shut down all the routers on HECnet, right? Just
shutting down the router for my area (or the area of anyone else who cares
about the issue) would be sufficient.
Once an endnode sees a router in its area ...
Exactly... If there are no routers in the end node's area, then
communications is direct. Routers for other areas are moot.
Bob
Bob, endnodes in different areas can only talk to each other if there's no router present. So your scheme only works if all of us shut down all our L1 and L2 routers. Once an endnode sees a router in its area it will send all its off-area datagrams to it. If that router is an L1 router then the other area is seen as unreachable. If it is an L2 router then it must see the other area router.
I do need L1 routers (for DDCMP and DSSI circuits) and possibly others do too.
Show net/old is neat and useful when systems are in different rooms !
Hans
Verzonden vanaf mijn draadloze BlackBerry -toestel
-----Original Message-----
From: "Bob Armstrong" <bob at jfcl.com>
Sender: owner-hecnet at Update.UU.SE
Date: Sun, 17 Jul 2011 08:21:12
To: <hecnet at Update.UU.SE>
Reply-To: hecnet at Update.UU.SESubject: RE: [HECnet] DECnet et al
Yes. But any L1 routers need help from L2 routers to get out of area.
Yeah, but given that the routers aren't actually needed for two nodes to
talk (under HECnet circumstances) then we don't actually need the routing
nodes at all. Sounds like their only real use is to be able to do a "SHOW
NETWORK/OLD" and see an nice list of active nodes (which is undeniably
neat)...
So a node configured as an end node in area 'n' can actually communicate
directly, over Ethernet, with another end node in area 'm'? But a L1
routing node in area 'n', in the same physical network topology, would
actually relay its packets for a node in area 'm' via an L2 router? And
does this actually take two L2 routers to hand off the packets, one for area
'n' and one for 'm'?
So (again, in the HECnet situation only) having an L1 router is a real
penalty - where as two end nodes could talk directly, just changing the same
machines to L1 routers would require the same traffic to be handled by two
intermediate nodes.
Or an I confused? That's a bit bizarre.
Bob
Yes. But any L1 routers need help from L2 routers to get out of area.
Yeah, but given that the routers aren't actually needed for two nodes to
talk (under HECnet circumstances) then we don't actually need the routing
nodes at all. Sounds like their only real use is to be able to do a "SHOW
NETWORK/OLD" and see an nice list of active nodes (which is undeniably
neat)...
So a node configured as an end node in area 'n' can actually communicate
directly, over Ethernet, with another end node in area 'm'? But a L1
routing node in area 'n', in the same physical network topology, would
actually relay its packets for a node in area 'm' via an L2 router? And
does this actually take two L2 routers to hand off the packets, one for area
'n' and one for 'm'?
So (again, in the HECnet situation only) having an L1 router is a real
penalty - where as two end nodes could talk directly, just changing the same
machines to L1 routers would require the same traffic to be handled by two
intermediate nodes.
Or an I confused? That's a bit bizarre.
Bob
On Jul 17, 2011, at 10:55 AM, Bob Armstrong wrote:
Paul Koning wrote:
It is legal to have multiple areas on a single Ethernet;
the routing spec covers that case explicitly. If that is done,
end nodes will communicate directly to a destination on the
Ethernet even if off-area ...
Given that Johnny's bridge program effectively creates a big Ethernet,
does that imply that HECnet end nodes in different areas can communicate
without an area router?
Yes. But any L1 routers need help from L2 routers to get out of area.
paul
Paul Koning wrote:
It is legal to have multiple areas on a single Ethernet;
the routing spec covers that case explicitly. If that is done,
end nodes will communicate directly to a destination on the
Ethernet even if off-area ...
Given that Johnny's bridge program effectively creates a big Ethernet,
does that imply that HECnet end nodes in different areas can communicate
without an area router?
Bob
For all the details, the routing spec (at http://h71000.www7.hp.com/wizard/decnet/) is the place to look. The only problem is that this is the original Phase IV spec (routing 2.0) not the IV-plus (routing 2.1) spec which added things like the previous hop cache. It doesn't look like that spec, or for that matter the Phase V specs, were ever put on-line though the intent was to do so.
Anyway...
The purpose of hierarchical routing (two levels in DECnet; more than than in things like ATM P-NNI) is to be able to handle networks larger than what can be sensibly handled by a single global routing algorithm. How large a network can be handled without hierarchy depends on the algorithm; for example, the RIP-style ("hot potato") routing algorithm in DECnet phase III and IV is more limited than the distance-vector (Dijkstra) routing algorithm in DECnet phase V, IS-IS, and OSPF.
Once you introduce hierarchy, that unavoidably means you lose information, because the knowledge about distant nodes is abstracted (abbreviated) to knowledge about the area they are in. So the routing is somewhat less efficient. How much less depends on the topology. The design rule for a hierarchical network should be that in-area traffic is predominant, and the area routing topology is such that routing by area number is not a whole lot less optimal than routing directly to the destination node (if you knew how).
Associating areas with geography can be a good way to do that, but it does not need to be. Similarly, associating areas with administrative subdivisions may be suitable, or it may not. I remember debates within DEC on the area layout of the internal network ("Easynet"), where both approaches were used, not necessarily always sensibly.
It is legal to have multiple areas on a single Ethernet; the routing spec covers that case explicitly. If that is done, end nodes will communicate directly to a destination on the Ethernet even if off-area, but L1 routers will go via an L2 router. Basically, the rule is that L1 routers ignore any packets, including hello and routing table packets, from routers in other areas.
Some other points that were raised.
The rule that node address zero means "nearest L2" applies only to the L1 routing message; in other places, as was pointed out, it might not mean anything, or it might be "local node".
Nodes know the node type of adjacent nodes, since node type is encoded in the hello message. They don't know the type of non-adjacent nodes.
Yes, the rule for hierarchical topologies is that each level must be intact. So an area must be in one piece; if it is partitioned then one piece can't get to the other (DECnet Phase IV does not route around the break via L2 routing, though some other routing protocols can do this). Similarly, the L2 network must not be partitioned.
The rules for how to transmit a packet depend on node type. They are:
1. For an end node: if the destination address is in the on-Ethernet cache (Phase IV) or previous hop cache (IV+) send it there. Otherwise send to the designated router.
2. For an L1 router: if the destination is in-area, send it to the adjacency which is the best path to that address (according to the L1 routing algorithm); otherwise send it to the adjacency for the nearest L2 router, according to entry 0 in the L1 routing table.
3. For an L2 router: if the destination is in-area, proceed as for an L1 router. Otherwise, send to the adjacency which is the best path to that area, according to the L2 routing algorithm.
On Jul 17, 2011, at 3:44 AM, Johnny Billquist wrote:
Well, there is really no reason why areas would be tied to geographical separation. You can spread out nodes that are in one area over the whole world as well.
There are both L1 hello messages, and L2 hello messages.
These are used to build the connection topography matrix for the routers.
What areas really brought to the table was a hierarchy where you could also more easily separate administration of the network.
All L1 routers within one are must be able to communicate with all other L1 routers within the area, by just talking directly, or via other L1 routers in the area.
All L2 routers must be able to talk to all other L2 routers, either directly or using only other L2 routers.
Both are obvious requirements when you think of the hello messages, which are not routed, but broadcast on all interfaces. So a scenarion of
A(L2) - B(L1) - C(L2)
would mean that A and C could not exchange L2 hello messages, which means they will not know of each other, or what connectivity the other node have.
Also worth mentioning is that all L2 routers also are L1 routers.
Johnny
On 2011-07-17 00.03, hvlems at zonnet.nl wrote:
Exactly my point Johnny. The original function for areas was to identify different geograhical locations. Same LAN area routing just wasn't part of the design. The rule is that area routers must always be adjacent to make the connection. In those days (30 years ago) that always meant poiint-to-point connections.
ISTR that there is a L2 hello message for that purpose.
Verzonden vanaf mijn draadloze BlackBerry -toestel
-----Original Message-----
From: Johnny Billquist<bqt at softjar.se>
Sender: owner-hecnet at Update.UU.SE
Date: Sat, 16 Jul 2011 23:52:42
To:<hecnet at Update.UU.SE>
Reply-To: hecnet at Update.UU.SESubject: Re: [HECnet] DECnet et al
On 2011-07-16 23:46, Paul Koning wrote:
On Jul 16, 2011, at 5:40 PM,<hvlems at zonnet.nl> wrote:
Ok, so the convention that the output of the SHO NET command always lists the area router with the highest address is just a display rule. It has nothing to do with an "active" area router then?
I can't find the rule for what L2 adjacent router to pick if you're an L1 router going out of area. It may be that this is where "highest" kicks in.
For non-adjacent L2 routers, the L1 router doesn't have any idea which router is represented by the "nearest L2 router" pseudo-address zero. It can't know that.
I don't know all the details here, and instead of starting to read
through documentation, perhaps you know, Paul, if L1 routers knows
*which* nodes in the area are L2 routers?
Because L1 routers knows the shortest paths to all nodes within it's
area, if I don't remember wrong.
However, an L1 router can never know which L2 router is the better for
reaching a specific area, since L1 routers have no idea of the area
topology.
But I *think* that L1 routers knows the type of nodes of all nodes in
the local area, so they should easily be able to figure out where the
closest L2 router is. But that is based on the assumption that they know
the type of each node in the area.
Johnny
paul
------Origineel bericht------
Van: Paul Koning
Afzender: owner-hecnet at Update.UU.SE
Aan: hecnet at Update.UU.SE
Beantwoorden: hecnet at Update.UU.SE
Onderwerp: Re: [HECnet] DECnet et al
Verzonden: 16 juli 2011 23:25
On Jul 16, 2011, at 3:42 PM, Johnny Billquist wrote:
On 2011-07-16 19.18, hvlems at zonnet.nl wrote:
I'm not sure what the router rules are. There are 63 areas, each with
one actove area router. There may be more routers configured as an area
router in one area; the one with the highest (?) DECnet address is
selected as the active one.
As far as I know, there can be more than one active area router. Just look at what the next hop are for different nodes in your node list... :-)
The way it works is that address 0 in the level 1 routing data corresponds to "nearest L2 router". Any L2 router contributes to that. The L1 routers don't know or care who is the nearest L2 router, they only care which direction to send to get there.
paul
Verzonden vanaf mijn draadloze BlackBerry -toestel
