Re: [Bier] WGLC - draft-ietf-bier-te-arch

["Jeffrey (Zhaohui) Zhang" <[e-mail]>]

Hi Toerless,

Please see zzh> below.

> 4.3.  Leaf BFERs
> 
>    Leaf BFERs are BFERs where incoming BIER-TE packets never need to be
>    forwarded to another BFR but are only sent to the BFER to exit the
>    BIER-TE domain.  For example, in networks where PEs are spokes
>    connected to P routers, those PEs are Leaf BFIRs unless there is a
>    U-turn between two PEs.
> 
> Understand the Leaf BFER definition, but the ???U-turn??? example is hard to picture. Might as well just remove the example.

I Thought u-turn is the most simple comparison leaf vs. non-leaf BFR.

Zzh> The text in the email is seriously misaligned. Looking at the picture in the diff link, while you gave a U-turn example, though even if BFER2 is not connected to BFR2  but only connected to BFER1 (hence no U-turn), then BFER1 is still not a leaf BFER I suppose. That's why I said the first sentence of the above paragraph is enough to define Leaf BFER while the example itself is actually not needed.
Zzh> Additionally, in left part of the picture you added, if some failure leads to BFR2 to be only reachable via BFER1, then BFER1 is no longer a leaf BFER. I assume you don't reassign BPs when links go up and down.

> but subsequent polarization example confuses me. It seems that BP 0:6 is assigned to the routed adjacency BFR10 (which is actually talked about in Section 4.8).

Section 4.7 does not mention "routed" at all, so there are no routed adjacencies at all used in 4.7. So i am not sure what you are confused about.

Zzh> "The BIFT of each BFR are only populated with BPs that are adjacent to the BFR in the BIER-TE topology". Since the same 0:6 is in BIFTS of BFR1/BFR2/BFR3 (and I suppose in BFR4~BFR9 as well even though not drawn), I assumed it's for the "MP2P" routed adjacency to R10; though I then ruled that out - but I don't know what 0:6 represent now on BFR1, BFR2, and BFR3.

The whole purpose of the ECMP BPs is of course to save bits, otherwise we'd give each link a separate BP, which would be 6 BP to reach to BFR4...BFR7 from BFR1. 

Zzh> The trouble I am having is that the same 0:6 is assigned to different things and it's present on all BFR1/BFR2/BFR3. It is perhaps an intentional smart design but I have not wrapped my mind around it. It's apparently different from the link bundle case, so better separate it out and elaborate it (including the DNR flag that might be needed here - If the packet arrives on BFR1 with 0:6, would the BP reset when it is sent to BFR2/3)?

> 4.8.  Routed adjacencies
> 
> If I understand it correctly, there is a BP assigned to L1/L2/L3 
> respectively (p2p link), and then there are BPs assigned to MP2P tunnels (routed adjacency from every BFR) to the L1/L2/L3 interface addresses and loopback addresses on BFR2/3.

Ok that wasn't quite the read i expected. Let me clarify the text/picture:

                   ...............     
         ...BFR1--...           ...--L1-- BFR2...
                  ... .Routers. ...--L2--/  
         ...BFR4--...           ...------ BFR3...
                   ...............         |
                                          LO
                    Network Area 1

Assume the requirement in the above picture is to explicitly steer traffic flows that have arrived at BFR1 or BFR4 via a shortest path in the routing underlay "network area 1" to one of the following three next segments: (1) BFR2 via link L1, (2) BFR2 via link L2, (3) via BFR3.

To achieve this, both BFR1 and BFR4 are set up with a forward_routed adjacency BitPosition towards an address of BFR2 on link L1, another forward_routed BitPosition towards an address of BFR2 on link L2 and a third forward_routed Bitposition towards a node address LO of BFR3.

Does this clear ip the confusion ?

Zzh> The picture is badly misaligned. I'll wait till 4.7 questions are cleared.

> If BFR2/3 are also BFERs, then they additionally will have BFER BPs.
> On BFR1/4, the BIFT entries for the MP2P BPs for the L1/L2/L3/loopback interface addresses of BFR2/3 will use forward_routed(interface/loopback address). For a packet to be decapsulated on a BFER, there is a need for both the BFER BP and another BP (p2p/lan/hub-spoke/routed-adjacency) in the packet (the former is for decapsulation and the latter is for getting it there).

This is not discussed in this section, but you are right - unless
BFR2 or BFR3 is a leaf BFR. In that case, it would just leverage the one shared "leaf-BFR" BP, so they do not need a per-BFER BP for local_decap(). 

Zzh> Right - shared leaf-BFR BP but still need that BP (the key is that we need a BP to get packet to a BFER and then a BP for decapsulation).

> If that???s the case, it???s worth point the above out.

Hmm... The logic of BFER BPs is totally independent of the logic of forward_routed adjacency, so i would worry that repeating the explanation of BFER BPs would conflate the forward_routed explanation.

Zzh> It's just that this is a place where all kinds of BPs are used so it's good to have a summary (could be a subsection 4.9).

> Actually, the reason that I thought this is MP2P is that 0:6 is present on R1, R2, and R3 (and more I assume) in Figure 12, but now I think it can???t be MP2P (so it is not correct to have 0:6 present on those routers ??? only the p2p tunnel head/tail should have the BP present in the BIFT). The reason is that if it were MP2P, any router getting a copy will send it to the endpoint of the routed adjacency, causing lots of duplicates.
> 
> Am I getting this correct?

I think you are still explaining from the misunderstsanding that the ECMP explanations where about routed adjacencies.

I have now expanded the somewhat terse text in the BIFT table pictures, to make it clear that the ECMP is across multipe forward_connected adjacencies in the examples. For example, first BIFT picture:

  BIFT entry in BFR1:
  ------------------------------------------------------------------
  | Index |  Adjacencies                                           |
  ==================================================================
  | 0:6   |  ECMP({forward_connected(L1, BFR2),                    |
  |       |        forward_connected(L2, BFR2),                    |
  |       |        forward_connected(L3, BFR2)}, seed)             |
  ------------------------------------------------------------------

Of course, an ECMP adjacency can be across any type of adjacencies, but all the text/explanations used forward_connected, and now the pictures show that explicitly.

Zzh> I can understand the multi-link case, but the multi-hop ECMP case (from BFR1 towards BFR10) is confusing me. It would help to give an example how it can be used, WITHOUT worrying about polarization.

>    To inhibit looping in the face of such physical misconfiguration,
>    only forward_connected adjacencies are permitted to have DNR set, and
>    the link layer destination address of the adjacency (e.g.  MAC
>    address) protects against closing the loop.  Link layers without port
>    unique link layer addresses should not be used with the DNR flag set.
>
> It???s not clear how link layer address helps?

I have expanded this to
"link layer port unique unicast destination address"

Aka: MPLS or ethernet have unique link layer destination destination addresses (label or destination MAC). If you think about incorrectly plugged HDLC links (such as old T1/T3/... links), they only have 2 generic addresses, if i remember 1 or 3 in the HDLC frame. So when you misplug one of those p2p cables wrong, the packets would be incrrectly received by the wrong receiver node and then DNR could cause persistent loops only solved by TTL.

Zzh> "Consider in the ring picture that link L4 from BFR3 is plugged into the L1 interface of BFRa" - still not sure how label/mac helps here. I suppose the ring topology is discovered/verified by the control plane and when the miscalling happens then the ring will not include the BFR1/BFR2 part and BFR3 will not have the DNR set? If ring discovery/varication is not done then perhaps we should point out that RPF based on link layer address is needed - the key is RPF (which needs unique link layer address)?

> Because the forwarding is different from BIER forwarding (because of [1] above), we might as well introduce an optimization here ??? for each BIFT, calculate the F-BM of the BIFT itself (the logical ???or??? of all the BPs presented in this BIFT) and then use (packet->bitstring & BIFT.F-BM) as the input to GetFirst/NextBitPosition(). That should skip many bits.

Right. But i explicitly removed those optimizations (i had them in older draft versions) because the whole idea of this picture is solely the comparison with figure 4 of RFC8279.

Zzh> I think it's worth point that optimization out; you can mark it optional if you want to emphasize the similarity to BIER forwarding, but since BIER forwarding does do the maskoff step, it is very efficient while BIER-TE forwarding does not it the maskoff step so this optimization is important.


>    The following pseudocode is comprehensive:
> 
> The above sentence reads a bit strange (or lacks some segue).

I hope not, but maybe best left to a native english speaker (RFC-editor).

The first (RFC8279) pseudocode was simplified. The second one is comprehensive. If not comprehensive, whats a good opposite of simplified ?

Zzh> Perhaps "The above simplified pseudocode is elaborated further as following"?
Zzh> Jeffrey
   
> ________________________________________
> From: BIER [[e-mail]<mailto:[e-mail]>] on 
> behalf of Toerless Eckert [[e-mail]<mailto:[e-mail]>]
> Sent: Tuesday, July 09, 2019 23:38
> To: Mike McBride
> Cc: Greg Shepherd; BIER WG; Pascal Thubert (pthubert)
> Subject: Re: [Bier] WGLC - draft-ietf-bier-te-arch
> 
> Thanks, Mike
> 
> The authors also reviewed the document and concluded that it was 
> really hard to get into the document context because of too many 
> forward dependencies. We tried to fix this by adding two hopefully 
> good & basic examples into the Introduction section and using them to 
> also add a better definition of the term "BIER-TE Topology" in the Introduction.
> Hopefully this makes readin the rest of te document smoother.
> 
> Also improved text of Abstract and refined text compariing BIER-TE with SR.
> 
> https://urldefense.com/v3/__http://tools.ietf.org/*rfcdiff?url1=https:
> **Atools.ietf.org*id*draft-ietf-bier-te-arch-02.txt&url2=https:**Atool
> s.ietf.org*id*draft-ietf-bier-te-arch-03.txt__;Ly8vLy8vLy8v!8WoA6RjC81
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> 
> Cheers
>     Toerless
> 
> On Wed, Jun 26, 2019 at 10:39:36AM -0700, Mike McBride wrote:
> > How about three? I support.
> > mike
> >
> > On Tue, Jun 25, 2019 at 10:42 AM Greg Shepherd <[e-mail]<mailto:[e-mail]>> wrote:
> > >
> > > We cannot take two 'yes' votes and WG consensus.
> > > Please, read and respond. If you don't support, then please vote as much publicly right here.
> > >
> > > Thanks,
> > > Greg
> > >
> > > On Mon, Jun 3, 2019 at 10:05 PM Pascal Thubert (pthubert) <[e-mail]<mailto:[e-mail]>> wrote:
> > >>
> > >> Support:
> > >>
> > >> I see great value in deterministic networks as well as IOT (with RPL).
> > >>
> > >> All the best,
> > >>
> > >> Pascal
> > >>
> > >> > -----Original Message-----
> > >> > From: BIER 
> > >> > <[e-mail]<mailto:[e-mail]>> On Behalf 
> > >> > Of Toerless Eckert
> > >> > Sent: mardi 4 juin 2019 02:03
> > >> > To: Greg Shepherd <[e-mail]<mailto:[e-mail]>>
> > >> > Cc: BIER WG <[e-mail]<mailto:[e-mail]>>
> > >> > Subject: Re: [Bier] WGLC - draft-ietf-bier-te-arch
> > >> >
> > >> > +1
> > >> > Obviously support as co-author.
> > >> >
> > >> > On Wed, May 29, 2019 at 12:41:26PM -0700, Greg Shepherd wrote:
> > >> > > Please read and respond to this thread w/ or w/o support.
> > >> > >
> > >> > > https://urldefense.com/v3/__https://datatracker..ietf.org/doc
> > >> > > /draft-ietf-bier-te-arch/__;!8WoA6RjC81c!XvH4AAxfrDjFoK_sercw
> > >> > > ZMsc0O5N42eENOs4l_qdsXF0KwZD82cJLDFFNV9eClBj$ 
> > >> > > <https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/
> > >> > > draft-ietf-bier-te-arch/__;!8WoA6RjC81c!UBTGvWWpMHyeiSanxs6vI
> > >> > > b_EnBVgyg6boAAW4nrqju8UCLOgiuXc8Y_6sD40kmtH$>
> > >> > >
> > >> > > Vote ends 5 June 2019.
> > >> > >
> > >> > > Thanks,
> > >> > > Shep
> > >> > > (chairs)
> > >> >
> > >> > > _______________________________________________
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