Re: thoughts on draft-bryant-shand-ipfrr-notvia-addresses-00.txt

Mike,

At 10:35 AM 4/26/2005, mike shand wrote:
>At 15:07 25/03/2005 -0500, Alia Atlas wrote:
>
>>Second is the list of downsides with the approach.  The main concern is 
>>that the mechanism becomes too complex such that the trade-off between 
>>its complexity and the full coverage is not desirable.
>>1.      This requires a large number of additional IP addresses in the 
>>IGP.  The same number of additional FECs is required to support LDP.
>
>Yes, it does. In the simplest case of link and node protection, and 
>ignoring LANs it requires 2 addresses per protected link. It is expected 
>that these would come out of a "private" address space, and hence wouldn't 
>consume real addresses. Indeed for security reasons it is preferable that 
>they are private addresses.
>
>I don't think this number is "too many". The question is how does this 
>number increase when we add LANs and SRLGs.

It would be useful to hear some additional opinions on the impact of adding 
a large number of addresses.  The other question is what is the boundary 
when it becomes a serious concern.

>>2.      Explicit tunnels are needed, which means that targeted LDP 
>>sessions are necessary to have this support LDP traffic.
>
>Yes. In the case of node protection we could also using Naiming's scheme 
>of next-next hop LDP advertisement.

True - but I'd want to think about the implications in terms of additional 
communication & periods of instability/inaccuracy of knowledge.    It also 
doesn't handle the multi-homed prefix case for the case when the path isn't 
via the next-next-hop.

>>  This is a particular concern for multi-homed prefixes; I'll describe my 
>> concerns on this later.
>
>Yes. This is a concern for LDP. I don't like the idea of targeted LDP 
>sessions. Two possibilities come to mind
>
>a) each node with an attached MHP distributes an additional label for that 
>prefix which has the semantics that when you pop that address you MUST 
>forward the underlying IP packet "directly".
>
>b) an alternative which doesn't require additional labels, but DOES 
>require a new "well known" label with the above semantics.
>
>Neither are very attractive, but perhaps more attractive than the directed 
>LDP sessions.

Both of these presume the ability to route based on the nested addresses of 
the packet.  In general, I don't think that this is a valid 
assumption.  Consider, for instance, the case of a BGP-free core.  Traffic 
is directed towards an ASBR in a different area (that is multi-homed to the 
one being considered).  In that case, the ABR may not have the BGP routes 
to be able to correctly forward the packet based on its IP address.  There 
are also a number of scenarios where what is underneath the top LDP label 
is another MPLs label & not routable at all.

>>3.      Substantial IGP changes are required to handle the additional 
>>Notvia addresses.
>
>Substantial is perhaps a bit strong. We need to advertise the not-via 
>address and its association. For IS-IS its pretty straightforward. OSPF, 
>by its very nature, may be a little more tricky.

More substantial than a few bits :-)  The main issue here is just the 
interop and migration concerns.

>>5.      The management of the Notvia addresses & of the tunnels can 
>>create longer time periods where protection isn't available for a part of 
>>the network (the new link or node, etc.).
>
>I don't think the tunnels add to the time at all. They are after all just 
>FIB entries. Distributing the notvia addresses for a new node/link will 
>occur at the same time as distributing the information about the link/node 
>in the first place. I don't think it significantly increases the delay.
>
>There is of course the time it takes to recompute notvia routes, but we 
>think this will be well under a second.
>
>These aspects certainly need thinking about, but they don't seem to pose 
>insurmountable issues.

I agree that they're not insurmountable - but just require consideration.

>>Third, there are a number of issues that I feel need considerable 
>>discussion to try and resolve.  I will try to go through each in turn and 
>>explain what I think the various aspects of each are.  Each of these 
>>issues has the possibility to resolve in such a way that the Notvia 
>>Addresses approach becomes overly complex.
>
>Yes. That is a temptation we need to resist!

It's frequently a coverage versus complexity trade-off, where each decision 
is along the slippery slope - alas!

>>b.      It is desirable to have some dampening on the withdrawal of 
>>Notvia addresses to minimize thrashing.
>
>The allocation of notvia addresses to links certainly shouldn't be changed 
>as a result of not "needing" the notvia address when the object with which 
>it is associated goes away. It should also get back the same notvia 
>address when it comes back. But I don't think there are any particular 
>issues associated with them disappearing and reappearing in the LSPs.
>
>Do you have any specific issues in mind?

Only keeping the notvia addresses around until after the network has 
converged...  If the notvia address is withdrawn with the link that's 
failed, then traffic may still be using that alternate.

>>c.      If configured in blocks, it would be extremely desirable to have 
>>the same Notvia address mean the same thing through multiple reboots, 
>>etc.  It'd be good to have some means of consistent association.  This is 
>>for easy manageability.
>
>Yes, definitely.

For the case where the notvia address is for a neighbor, it's not always 
that straightforward - unless one ends up advertising multiple notvia 
addresses for the same neighbor, depending on the number of parallel 
links.  This is mostly engineering, I think.

>>d.      When a new link or neighbor comes up, there will be a longer 
>>period of time when an alternate isn't available because the Notvia 
>>address hasn't been advertised yet.  These periods without protection 
>>need to be clearly understood and minimized.
>
>Yes. I'm not convinced there is a particular problem here, but it does 
>need thinking through carefully.

Agreed.

>>e.      There may be scalability concerns based on the number of Notvia 
>>addresses and LDP FECs required.  For instance, as described in the 
>>draft, it is basically the number of uni-directional links in the 
>>topology.  This is ignoring the extras for broadcast links.  To fully & 
>>certainly provide SRLG protection if at all feasible, would require that 
>>each router advertise a Notvia address for every uni-directional link 
>>into every neighbor of that router.  This would result in K*L additional 
>>addresses, where K is the average number of neighbors & L is the number 
>>of uni-directional links in the topology.
>
>Yes. This is a major concern, and we need to devise ways of solving SRLGs 
>etc. which minimize the potential proliferation of addresses.
>We need to get the right tradeoff here between optimal solutions and 
>complexity.

Agreed.  We need to understand the impact of additional addresses to know 
the complexity cost of that versus the reduced coverage of selecting a less 
complete approach to broadcast link and SRLG protection.

>>2.      Insufficiently diverse topology:  It is possible that a network 
>>topology cannot provide an alternate that suffices for link, node and 
>>SRLG protection.  It isn't clear to me how to compute a "best-available" 
>>alternate using this approach.  For instance, if one can get link 
>>protection, but not node protection, how would that be determined, 
>>computed and assigned?  This becomes much more of a concern for SRLG 
>>protection & for topologies where failures have already occurred and the 
>>network has converged for those & needs protection in the event of an 
>>additional failure.
>
>Clearly it is always possible to create a topology which contains single 
>points of failure and is inherently irreparable. This is part of the 
>tradeoff we need to address when thinking about SRLGs, since taking a 
>simple but pessimistic approach to SRLG can result in this sort of 
>failure. This seems to be a property of the problem rather than any 
>particular solution.

Let me try to explain this a bit better.  Say there's a topology that, for 
a particular next-hop & next-next-hop, can only provide an alternate that 
gives link and node protection but not SRLG protection.  Now, how does the 
notvia addresses method compute an alternate?  If the method is pruning the 
topology of the relevant link, node & SRLGs, no alternate will be 
found.  However, it was possible to compute & use an alternate that gives 
the link & node protection.  The similar case can easily occur with link & 
node protection.  Say S has two parallel links to E; if the first fails, S 
could use the other to get link protection  - but there is no 
node-protecting alternate.  How does S determine this?  What is the 
fall-back strategy in the case that no "full-protection" alternate is 
available?

>>3.      Failure Diagnosis versus Pessimism:  As written, the draft 
>>discusses the idea of doing failure diagnosis using BFD.  As Stewart, 
>>Mike & I have discussed, this isn't possible for SRLG failures, although 
>>it is possible for broadcast links.
>
>Yes, and this relates to (2) above.
>
>>a.      I am concerned about adding the failure diagnosis.  This is yet 
>>another level of complexity for implementation.  It also has 
>>ramifications for the forwarding plane, because of the need to store 
>>multiple alternates to use & have multiple states to check to decide what 
>>to use.
>
>Yes. It would be nice not to have to do it, but that is back to the 
>tradeoff above.

Complexity vs. coverage?  I'm very fond, unsurprisingly, of options that 
don't require hardware changes... so I judge that the complexity is rather 
high to support this one - as well as more error-prone (see comment on 
unreliable diagnosis).

>>b.      An example of a concern with the BFD diagnosis is that all 
>>interfaces on a node that has failed are not certain to fail exactly 
>>simultaneously or even within a sub-50ms bounded window.  It is entirely 
>>possible that BFD sessions are terminated on different line-cards, that 
>>detect the router failure at slightly different times and stop forwarding 
>>traffic, therefore, at slightly different times.
>
>Yes. There is the possibility of misdiagnosis in this case if the second 
>failure occurs too long after the first. I suppose this then looks like 
>two separate failures. Clearly an unreliable diagnosis is probably worse 
>than no diagnosis at all. We need to get some handle on how realistic or 
>not this scenario is.

Well, I think it is exceedingly realistic :-)
For a non-power related failure, routers with separate forwarding & control 
planes may take varying amounts of time for the line-cards to all realize 
that the route controller is down.

>>c.      The other approach is to pessimistically eliminate all routers 
>>connected to the broadcast link as well as the broadcast link; this may 
>>not provide an alternate.
>
>Yes. While simple, it runs into the problem of being a single (albeit 
>large) point of failure. Its the same trade-off as above.

Don't they all reduce to that?

>>   It also needs to be thought through what issues might exist if the 
>> topologies used for the SPF vary slightly for each router that is on the 
>> broadcast link, since each will, as described, not prune itself out when 
>> doing the computation; of course, there could be an approach where the 
>> same topology can be used everywhere.
>
>I'm not really sure what you mean here.

Let me try and explain it a bit.  Perhaps I'm missing something.   In the 
case where a notvia topology results in pruning the router doing the 
computation, what forms the root of the SPT?   Say routers A, B and C are 
all connected to a broadcast link X and want to compute a notvia X address 
as described in (c) by pruning the pseudo-node related to X as well as A, 
B, and C.   Now, router A prunes the pseudo-node, A, B and C from the 
topology; what does A use as the root?  IF A only prunes the pseudo-node, B 
and C to compute notvia X, B only prunes the pseudo-node, A, and C, and C 
only prunes the pseudo-node, A and B,  and all other routers prune the 
pseudo-node, A, B, and C, can there be any issues with a consistently 
computed & non-looping path for notvia X?

I think it may not be an issue - b/c once the traffic leaves A, B or C, it 
will never return - but it at least needs some thought, since this is a bit 
different from what's traditionally been done.

>>  It isn't clear to me what Notvia addresses would be needed to express 
>> "don't go through this pseudo-node or any nodes attached to it"; I don't 
>> think that it is simply the Notvia address for avoiding a particular node.
>
>No, it would need a specific notvia address bound to the LAN interface.

Agree

>>4.      Multi-homed Prefixes:  I am quite concerned about the mechanisms 
>>suggested in the draft.
>>a.      First, I really do not like the idea of having separate 
>>forwarding for "local" prefixes that come out of a tunnel.  What is a 
>>local prefix?  For instance, does this mean that an ABR has to forward 
>>traffic different depending on which area traffic from the tunnel has 
>>come from?  I am concerned about how this would scale; maybe only 2 FIBs 
>>are needed (one for backbone & one for other), but it may be worse to 
>>handle AS external routes.  I know that Stewart, Mike, Joel, Albert and I 
>>had discussed/agreed to put this idea out of scope  at least for the moment.
>
>Clearly the problem needs solving, especially since prefixes which are 
>multihomed are frequently the most important prefixes (which is WHY they 
>are multihomed in the first place).

inconvenient that!

>>b.      I am quite concerned about having tunnels to the advertisers of 
>>the prefixes.
>>i.      There needs to be a mechanism to determine whether the advertiser 
>>of a prefix will forward the packet in a loop-free fashion to avoid the 
>>failure point.  The separate forwarding for "local" prefixes avoided the 
>>need for this determination, but at more substantial cost.
>
>There seem to be two aspects to this.
>
>a) we need the ability to get the packet to the "second-best" attachment 
>point for the prefix without it being "sucked back" to the failure. This 
>in general requires a tunnel, except for the cases where a neighbor of the 
>node detecting the failure has an LFA to the second best attachment point. 
>Clearly this could be used in preference to a tunnel where available, but 
>at the expense of additional complexity. However this is really just an 
>extension of the general principle that we should use "basic" (i.e. LFA) 
>repair to cream off traffic which doesn't NEED to be tunnelled.

Yes - though the tunnels bring out the LDP issues with targeted sessions - 
of course.

>b) we need (in a very limited number of cases), the ability to force the 
>packet to the locally attached prefix. This only occurs where the local 
>cost is high compared to the cost back to the failed attachment point. But 
>when we DO need it, the use of a tunnel is a convenient means of 
>signalling this. I'm not sure how else to do it, other than using a label.
>
>Of course ONE "solution" would be to REQUIRE the costs to be set sensibly :-)

I like that one :-)  but you knew I would!  We may need to define what that 
means better or have a way of determining that it is the case.

>>ii.     To support LDP, every tunnel requires a targeted LDP session.  If 
>>multi-homed prefixes are common, then this becomes a full mesh for 
>>LDP.  That isn't acceptable.
>
>Agreed.
>
>>  Of course, multi-homed prefixes may be much more infrequent for LDP 
>> than for IP; for example, there is no reason to advertise a separate FEC 
>> for the subnet of a link.  However, multi-homed prefixes are a concern 
>> for LDP for at least the inter-area, AS External, and BGP routes.
>>iii.    If traffic is encapsulated to a node's regular address, because 
>>that traffic is destined to a prefix advertised by the node, how does the 
>>receiving node know to remove the encapsulation and forward the packet 
>>inside  all in the fast path?  Is this a just a question of different 
>>handling based on the header type inside the outer encapsulation (for GRE)?
>
>Yes.

OK.  The traffic wouldn't be directed up to the control plane because it 
was GRE encapsulated??  And had a special header type for this 
purpose?  Certainly I can see something like this working with an LDP LSP, 
b/c the label would just get it to that router & then be popped & the 
packet forwarded based on what's underneath.

>>iv.     Perhaps these issues could be handled by determining a 
>>next-next-hop that avoids the failure to reach an appropriate 
>>advertiser.   Of course, this is a different set/type of computation.
>
>Could you explain that suggestion please?

Well, if there is a neighbor's neighbor whose path to the multi-homed 
prefix doesn't go through the failure & this can be determined, then the 
traffic could be tunneled to that neighbor's neighbor & then normally 
forwarded from there.

Basically, if one knows the SPT from each neighbor's neighbor & can reach 
all of those neighbor's neighbors without going through the failure, then 
it might provide an alternate.  The issue there is first that the path to a 
neighbor's neighbor might go via the failed element & not have an 
appropriate notvia  & second the effort of computing and considering the 
different SPTs.

Does that make more sense?

>>5.      SRLGs and Broadcast Links:  There seem to be a number of possible 
>>ways to handle SRLGs and broadcast links, each of which provides a 
>>different trade-off in terms of  coverage, computation, and extra Notvia 
>>addresses.
>
>Yes.
>
>>   There are basically 4 approaches at this point.
>>a.      First, In order to compute a notvia alternate that avoids a link, 
>>the primary neighbor, and all SRLGs that the link is part of, it is 
>>necessary to have a separate topology and associated SPF computation for 
>>each link that is a member of an SRLG or a broadcast link.   This 
>>requires also a substantially larger number of Notvia addresses and the 
>>corresponding mechanisms to determine how and when to allocate and 
>>de-allocate them.
>
>.. and could potentially result in a combinatorial explosion if we weren't 
>very careful.

I do think of this as having the highest potential to be the "too 
complicated & run-away" scenario.

>>b.      Second, one could use a topology that removed the primary 
>>neighbor and see whether SRLG protection can be obtained either along S's 
>>path or along any path of a neighbor of S that is also loop-free.
>
>Could you explain that a bit more please?

This is the concept of looking for a loop-free neighbor to the notvia 
address whose path there happens to give SRLG protection.  We'd discussed 
this one the last day at IETF.

>>c.      Third, when a Notvia address indicates to avoid a node, one could 
>>remove not merely the node & the uni-directional links to and from that 
>>node, but also any other links that are in a common SRLG with any of the 
>>links to or from the removed node.  This is pessimistic  but allows some 
>>SRLG protection without increased computation or Notvia addresses.
>
>Yes. This is nice and simple, but as you have pointed out above, could 
>easily result in an inability to find a viable repair.

At the risk of additional complication, one could have it configurable as 
to the specific handling of SRLGs.  For instance, most link/node/SRLG would 
be handled with a single notvia per node  - and then there could be 
configured specific links that required their own notvia.   Of course, that 
adds substantial extra complexity.  Is it necessary?

>>d.      Fourth, one could simply track the SRLGs encountered along the 
>>Notvia path; this just reports whether the alternate provides SRLG 
>>protection without any effort to obtain it.
>
>Yes. Interesting. I wonder how useful this would be.

Well, it could feedback to a network design at least - to give an 
indication of coverage.  Also, not all SRLGs have the same likelihood to 
fail.  So, if avoiding all doesn't work, perhaps one can avoid the most 
risky ones - and then report the protection against the failures of the 
others.

This is part of my concern/question about what notvia alternate is computed 
if the best protection isn't possible.

>>6.      Implementability:  Clearly, the draft describes the basic idea 
>>for Notvia addresses, but there are a fair number of 
>>implementation/protocol decisions that need to be made before this can 
>>become anything more than an interesting idea.
>
>Sure. There are quite a few design decisions and tradeoffs as indicated 
>above that need tieing down.
>
>>7.      There is a definite need to describe the convergence case 
>>better.   This is how the transition from using the alternate to the 
>>network being converged happens, such that the alternate remains functional.
>>a.      For instance, if the node E fails, then the Notvia address E_!S 
>>will no longer be advertised.  If S was getting link protection (because 
>>that was all that was possible, for instance) by tunneling traffic to 
>>E_!S, it is important that this traffic be properly discarded when E's 
>>addresses go away.   This implies that there needs to be a default 
>>blackhole for Notvia addresses.
>
>I don't quite understand your concern here. If E goes away and S is 
>sending to E_!S, then the neighbors of E will drop the packets because we 
>don't repair a notvia address.

I'm thinking of this as the more specific prefix goes away.   Without a 
specific blackhole for the group of prefixes, why wouldn't the packets take 
that instead?  I.e., if the notvia address is 10.1.1.1 and there's a 
default route for 10.1/16  (or for 0.0.0.0/0), then the packet would pick 
up the latter when the notvia address is removed.

>Or are you concerned that after convergence, there will be nodes which 
>don't even have a forwarding entry forE_!S. By this time I don't think 
>that S (or anyone else) should still be using that address, but even if it 
>were, the absence of a forwarding entry would (SHOULD) cause the packet to 
>be dropped. Is this all you are saying?

I was also worrying about the above in reference to the notvia 
addresses.  While it is possible to say that changes to notvia 
addresses  shouldn't be installed until after the network has otherwise 
converged, that sort of detail needs to be clarified.

>>b.      Another example is when node E fails, the next-next-hop B must 
>>continue to advertise the Notvia address B_!E until the network converges 
>>so that S can continue to tunnel traffic to B_!E as the alternate.
>
>Yes. Our view was that no changes would be made to notvia advertisement or 
>more specifically notvia FIB entries until after convergence is over. Of 
>course there is an issue as to how you tell when that has happened, but 
>the timers associated with loop free convergence probably give a good 
>indication.

Conceptually, I agree.  Having a primary topology to forward the traffic on 
while the backup one reconverges helps.

>>c.      It is possible to get a micro-forwarding loop affecting a Notvia 
>>address as a result of a less severe failure than anticipated.  For 
>>instance, consider the following topology.
>>              [D]
>>               |
>>           1   |
>>      [E]-----[F]-\
>>       |       |   \ 10
>>     1 |R    1 |R   \
>>       |   5   |     \
>>      [S]-----[H]----[I]
>>                  2
>>
>>      Link S->E and Link H->F are in SRLG R
>>
>>When node E fails, if I converges before H, there will be a loop 
>>affecting the Notvia address being used to reach F without going through 
>>any of Link S->E, E or SRLG R.
>
>We discussed this privately, and I still don't see how loops could arrise 
>even if the notvia FIB were recomputed before normal convergence is 
>complete. But I think it is better to delay the notvia FIB changes anyway.

Just for clarity (hopefully),  before the failure, H computes the path for 
F_!E, the address of F's that is notvia E, to go via I and then to 
F.  After the failure of E, if H installs the changed notvia address F_!E 
the path is directly to F, b/c node E no longer has SRLG R associated with 
any of E's up links.

>>d.      How do exceptions work?  Particularly in regards to an IP-in-IP 
>>encapsulation such as GRE, it doesn't seem like MTU exceeded cases can be 
>>handled cleanly  either by use of DF or by doing IP fragmentation and 
>>then the reassembly at the end of the tunnel.  This seems like a problem 
>>for all ICMP packets; how could a source understand the header inside for 
>>a TTL expired, for instance.
>
>I'll leave this for Stewart (tunnel) Bryant!

For LDP, there are mechanisms (layer violations though they are) to handle 
exceptions generating ICMP packets.

>>e.      For IP-in-IP tunnels, another concern is flow diversity.  The IP 
>>source and destination addresses are used to determine a flow; this flow 
>>identification may then be used for a variety of purposes, including 
>>ECMP.  By putting all the traffic to a variety of destinations inside the 
>>same header, the ability to take advantage of flow diversity appears to 
>>have disappeared.   This could possibly be solved by putting the original 
>>source address into the encapsulating header?  Are there other approaches?
>
>and this.

Again, for an LDP tunnel, many routers can look under the label and 
consider the IP packet inside for flow identification.

Alia

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