Re: Size of CR in CRH

Brian E Carpenter <brian.e.carpenter@gmail.com> Fri, 22 May 2020 03:04 UTC

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Subject: Re: Size of CR in CRH
To: "Joel M. Halpern" <jmh@joelhalpern.com>, Robert Raszuk <robert@raszuk.net>
Cc: 6man <6man@ietf.org>
References: <CAOj+MMFsy=dDciY=TMwSf75CZCr_i1Mfv6oUiPs5U6hT2Bq94w@mail.gmail.com> <DM6PR05MB6348D0DB381145F1A4C53450AEB70@DM6PR05MB6348.namprd05.prod.outlook.com> <CAOj+MMHT=TWqf=A71PhvCcrFggCQ=okRrP=sGaO4hrcbmsCvGw@mail.gmail.com> <CAOj+MMGYbw83c-T9GWCs_cLDWWbGi1dZ_Xfc8tS6TV6EfvWsDw@mail.gmail.com> <DM6PR05MB63484502B4CFCB745DFCED3EAEB70@DM6PR05MB6348.namprd05.prod.outlook.com> <CAOj+MMEfkenHmSLje62wNRw3OrxBzJJq_MwesozK-ABeLXbZ2Q@mail.gmail.com> <DM6PR05MB634807B4AAB6452B6FDA535CAEB70@DM6PR05MB6348.namprd05.prod.outlook.com> <CAOj+MMEX3qxQw0WHt3b69-KL5w+Ozufh_2eod-VO6Bt-ojSf9A@mail.gmail.com> <DM6PR05MB6348382D4496673BA96140C3AEB70@DM6PR05MB6348.namprd05.prod.outlook.com> <CAOj+MMFB3fYuYn5euzUzPpZbxr81eN5zfa2ATyHhC3RJbtch=A@mail.gmail.com> <DM6PR05MB634817EB3CB574C5A7D0BA77AEB70@DM6PR05MB6348.namprd05.prod.outlook.com> <CAOj+MMF9SsxSMXuVVQJmrQQGdsGN=RMeb2Kxu88+bjH__7r=Lg@mail.gmail.com> <05b32199-e282-f5e6-6ca8-fea5acb0101a@joelhalpern.com>
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
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Date: Fri, 22 May 2020 15:04:36 +1200
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Joel,
On 22-May-20 12:21, Joel M. Halpern wrote:
> Robert, local signifance has a very clear meaning in this space.  We 
> understand it in MPLS.  We understand it in SRv6.
> And in neither case does it mean that nodes can make up any old label, 
> and use it any old way, without suitable communication / coordination.
> 
> However, equally, the exact coordination depends upon the control 
> mechanism and indications one wants to use.  For example, if local 
> labels always follow global labels which indicate their presence, then 
> sure, you can use almost any value.  But if the disciple or pattern is 
> different, then different constraints apply.
> 
> So just what are you trying to ask?

I'm not sure what Robert's question is. Mine is something like this:

If the local domain contains routers from vendor A, routers from
vendor B, and a management system from C, what are the minimum
rules about the label's format and semantics that must be followed
to guarantee interoperability? In other words, C needs to be able
to tell both A routers and B routers the same thing in order to
set up their CRH-FIBs.

A supplementary question is: Should those rules be specified in
the basic CRH draft, or in a separate document?

(I have a similar question about the SRH work, but it probably
doesn't belong here.)

Regards
   Brian

> 
> Yours,
> Joel
> 
> On 5/21/2020 6:29 PM, Robert Raszuk wrote:
>> Hi Ron,
>>
>> I don't think we need to go through a tutorial here what the FIB, 
>> CRH-FIB or LFIB is.
>>
>> I asked specific question on which you have not provided any answer:
>>
>> If I have part of the network non CRH aware and each node is free to 
>> allocate their own SID - as you are claiming SIDs are locally 
>> significant - how would the CRH look like in case of SID conflict 
>> between local node and remote node SID collision.
>>
>> Now rest of your answer is rather vague at best. And this is not just a 
>> detail. This is fundamental frame to the proposal we are discussing 
>> adoption of.
>>
>> Sure once document becomes a WG a collective brains can paint it well - 
>> but if it does not even have solid frames it may be a pretty hard task.
>>
>> Just my own little side input. Others may see it different way,
>>
>> Many thx,
>> R.
>>
>>
>> On Fri, May 22, 2020 at 12:22 AM Ron Bonica <rbonica@juniper.net 
>> <mailto:rbonica@juniper.net>> wrote:
>>
>>     Robert,____
>>
>>     __ __
>>
>>     I think that you are confusing two data structures. The CRH-FIB is
>>     just that, a FIB. I contains enough information to resolve an
>>     incoming identifier to an IPv6 address and a forwarding method. Each
>>     node maintains a unique CRH-FIB and there is no requirement for
>>     nodes to share their CRH-FIBs with one another. The CRH-FIB lives on
>>     the forwarding plane and is an appropriate topic for 6man.____
>>
>>     __ __
>>
>>     Somewhere in the network, there is an entity constructs the CRH and
>>     the list that it contains. That entity needs access to another data
>>     structure, that includes a global view of each node’s CRH-FIB. That
>>     entity might be:____
>>
>>     __ __
>>
>>       * A human, manually constructing forwarding policy____
>>       * A controller____
>>       * Path computation software on a router.. ____
>>
>>     __ __
>>
>>                                                                                                             Ron____
>>
>>     __ __
>>
>>     __ __
>>
>>     __ __
>>
>>     Juniper Business Use Only____
>>
>>     *From:* Robert Raszuk <robert@raszuk.net <mailto:robert@raszuk.net>>
>>     *Sent:* Thursday, May 21, 2020 4:05 PM
>>     *To:* Ron Bonica <rbonica@juniper.net <mailto:rbonica@juniper.net>>
>>     *Cc:* 6man <6man@ietf.org <mailto:6man@ietf.org>>
>>     *Subject:* Re: Size of CR in CRH____
>>
>>     __ __
>>
>>     *[External Email. Be cautious of content]____*
>>
>>     __ __
>>
>>     Hello Ron,____
>>
>>     __ __
>>
>>      > Why should it? It isn’t attached to link X->Y. So it couldn’t use
>>     that entry even if it had it. ____
>>
>>     __ __
>>
>>     This question I think exposes or uncovers (at least for me) the crux
>>     of your proposal ... perhaps even fatal one. ____
>>
>>     __ __
>>
>>     You are assuming that only locally allocated SIDs are in CRH-FIB -
>>     that is fatal assumption for bunch of reasons ... one swapping DA to
>>     some node N hops away. How are you going to accomplish that if such
>>     entries are not even in CRH-FIB ? ____
>>
>>     __ __
>>
>>     I guess it is very clear now why the other day you stated that "all
>>     nodes in the domain must support CRH". ____
>>
>>     __ __
>>
>>     What seems you are doing here ... and of course this is not written
>>     anywhere in any document ... so this is pure acceptance call
>>     guessing - is a forward referencing SIDs against the peers. ____
>>
>>     __ __
>>
>>     So on any node you are allocating SID per interface -
>>     strictly speaking per forwarding adjacency. Clearly you can not
>>     build such construct for remote nodes based on the above. ____
>>
>>     __ __
>>
>>     Furthermore you are building forwarding chain on the basis of
>>     ordered forwarding list of SIDs just hoping that the peer will
>>     accept the packet if his DA address is in the IPv6 header. Then it
>>     will look up his own SID and continue. ____
>>
>>     __ __
>>
>>     One thing I must agree with you that this is not Segment Routing ...
>>     In fact I am not sure how to call this architecture. Maybe forward
>>     referenced source routing ? ____
>>
>>     __ __
>>
>>     You can not do TI-LFA with this approach unless you pre-program any
>>     possible alternative paths to all nodes in the network. ____
>>
>>     __ __
>>
>>     Sure you can demo this in the lab or even on a network just like you
>>     could demo static mpls labels. Yes it is very simple and you got
>>     attention of few folks with that. And yes you could perhaps even
>>     show that if you just add few lines of xml config you could tunnel
>>     it across non CRH capable nodes ... But is this solution for any
>>     production network ? ____
>>
>>     __ __
>>
>>     I think and I was told by unicast emails that I am not alone - we
>>     are just guessing what the vehicle looks like after seeing the first
>>     wheel. So far it does not even look like a car ... maybe bike or
>>     scooter. Who knows .... ____
>>
>>     __ __
>>
>>     If I may recommend next action without dismissing your proposal a
>>     wise thing to do would be to get from you set of slides or perhaps
>>     youtube recording showing exactly not only all mapping distribution,
>>     but more over illustrating exact packet's header including CRH in
>>     all various cases I and others asked when packet is
>>     traversing throughout a controlled domain.____
>>
>>     __ __
>>
>>     Only after that we could start a new adoption call when more folks
>>     actually has a clear picture what it is being adopted here. Is it a
>>     brilliant and cool solution or is it some form of wild animal which
>>     can bite. ____
>>
>>     __ __
>>
>>     Many thx,____
>>
>>     Robert.____
>>
>>     __ __
>>
>>     __ __
>>
>>     On Thu, May 21, 2020 at 9:46 PM Ron Bonica <rbonica@juniper.net
>>     <mailto:rbonica@juniper.net>> wrote:____
>>
>>         Robert,____
>>
>>         ____
>>
>>         I am assuming that B is attached to Z. When I say, it isn’t
>>         attached, I mean that B isn’t attached to Link X->Y. Link X->Y
>>         is attached to Z.____
>>
>>         ____
>>
>>                                                                                                        Ron____
>>
>>         ____
>>
>>         ____
>>
>>         ____
>>
>>         Juniper Business Use Only____
>>
>>         *From:* Robert Raszuk <robert@raszuk.net
>>         <mailto:robert@raszuk.net>>
>>         *Sent:* Thursday, May 21, 2020 3:14 PM
>>         *To:* Ron Bonica <rbonica@juniper.net <mailto:rbonica@juniper.net>>
>>         *Cc:* 6man <6man@ietf.org <mailto:6man@ietf.org>>
>>         *Subject:* Re: Size of CR in CRH____
>>
>>         ____
>>
>>         *[External Email. Be cautious of content]*____
>>
>>         ____
>>
>>          > It isn’t attached to link X->Y. ____
>>
>>         ____
>>
>>         Please assume it is attached. ____
>>
>>         ____
>>
>>         I stated very clearly: "(or maybe even connected to B)"____
>>
>>         ____
>>
>>         Thx,
>>         R.____
>>
>>         ____
>>
>>         ____
>>
>>         ____
>>
>>         On Thu, May 21, 2020 at 8:45 PM Ron Bonica <rbonica@juniper.net
>>         <mailto:rbonica@juniper.net>> wrote:____
>>
>>             Robert,____
>>
>>             ____
>>
>>             Identifiers have node local scope. This means:____
>>
>>             ____
>>
>>               * One a single node, there is a one-to-one mapping between
>>                 identifiers and the CRH-FIB entries that they identify____
>>               * Nodes A through Z can all have a CRH-FIB entry that is
>>                 identified by N. However, all of those CRH-FIB entries
>>                 do not need to contain the same information.____
>>
>>             ____
>>
>>             Referring back to your example, Node B will never have the
>>             following entry in its CRH-FIB:____
>>
>>             ____
>>
>>               * Identifier = 15, IPv6 Address = Node Z, Method = strict,
>>                 Link = X->Y____
>>
>>             ____
>>
>>             Why should it? It isn’t attached to link X->Y. So it
>>             couldn’t use that entry even if it had it.____
>>
>>             ____
>>
>>                                         
>>                                           Ron____
>>
>>             ____
>>
>>             ____
>>
>>             ____
>>
>>             ____
>>
>>             Juniper Business Use Only____
>>
>>             *From:* Robert Raszuk <robert@raszuk.net
>>             <mailto:robert@raszuk.net>>
>>             *Sent:* Thursday, May 21, 2020 11:25 AM
>>             *To:* Ron Bonica <rbonica@juniper.net
>>             <mailto:rbonica@juniper.net>>
>>             *Cc:* 6man <6man@ietf.org <mailto:6man@ietf.org>>
>>             *Subject:* Re: Size of CR in CRH____
>>
>>             ____
>>
>>             *[External Email. Be cautious of content]*____
>>
>>             ____
>>
>>             Hi Ron,____
>>
>>             ____
>>
>>              > Node B decrements Segments Left and looks for entry 15 in
>>             **its** CRH-FIB. If finds:____
>>
>>              > ____
>>
>>              >     On Node B:  Identifier = 15, IPv6 Address = Node C,
>>             Method = strict, Link = B->C____
>>
>>             ____
>>
>>             Your example works when the entire network has a single
>>             segment routed path :) ____
>>
>>             ____
>>
>>             What happens if also Node Z somewhere in the domain (or
>>             maybe even connected to B) advertised SID 15 with some
>>             different outbound link ? ____
>>
>>             ____
>>
>>             So Node B will have two FIB entries: ____
>>
>>             ____
>>
>>                Identifier = 15, IPv6 Address = Node C, Method = strict,
>>             Link = B->C ____
>>
>>                Identifier = 15, IPv6 Address = Node Z, Method = strict,
>>             Link = X->Y ____
>>
>>             ____
>>
>>             So how will B decided which one to use ? ____
>>
>>             ____
>>
>>             Best,____
>>
>>             R.____
>>
>>             ____
>>
>>             ____
>>
>>             ____
>>
>>             ____
>>
>>             On Thu, May 21, 2020 at 5:11 PM Ron Bonica
>>             <rbonica@juniper.net <mailto:rbonica@juniper.net>> wrote:____
>>
>>                 Robert,____
>>
>>                 ____
>>
>>                 Let’s address your question with an example. Assume that
>>                 Node A is sending a packet to Node D. The delivery path
>>                 includes the following strictly routed hops:____
>>
>>                 ____
>>
>>                   * Node A to Node B over link A->B____
>>                   * Node B to Node C over link B->C____
>>                   * Node C to Node D over link C->D____
>>
>>                 ____
>>
>>                 Now we populate the CRH-FIB on Nodes B and C as follows:____
>>
>>                 ____
>>
>>                   * On Node B:  Identifier = 15, IPv6 Address = Node C,
>>                     Method = strict, Link = B->C____
>>                   * On Node C:  Identifier = 15, IPv6 Address = Node D,
>>                     Method = strict, Link = C->D____
>>
>>                 ____
>>
>>                 Now, Node A formats a packet as follows:____
>>
>>                 ____
>>
>>                   * IPv6 Destination Address = Node B____
>>                   * CRH Segments Left = 2____
>>                   * Identifier list = [15,15]____
>>
>>                 ____
>>
>>                 Node A sends this packet to Node B over link A->B.. Node
>>                 B decrements Segments Left and looks for entry 15 in
>>                 **its** CRH-FIB. If finds:____
>>
>>                 ____
>>
>>                   * On Node B:  Identifier = 15, IPv6 Address = Node C,
>>                     Method = strict, Link = B->C____
>>
>>                 ____
>>
>>                 So, Node B updates the IPv6 address and sends the packet
>>                 to Node C over link B->C. Node C decrements Segments
>>                 Left and looks for entry 15 in **its** CRH-FIB. If
>>                 finds:____
>>
>>                 ____
>>
>>                   * On Node C:  Identifier = 15, IPv6 Address = Node D,
>>                     Method = strict, Link = C->D____
>>
>>                 ____
>>
>>                 So, Node C updates the IPv6 address and sends the packet
>>                 to Node D over link C->D.____
>>
>>                 ____
>>
>>                                                                            Ron____
>>
>>                 ____
>>
>>                 ____
>>
>>                 ____
>>
>>                 Juniper Business Use Only____
>>
>>                 *From:* Robert Raszuk <robert@raszuk.net
>>                 <mailto:robert@raszuk.net>>
>>                 *Sent:* Thursday, May 21, 2020 10:35 AM
>>                 *To:* Ron Bonica <rbonica@juniper.net
>>                 <mailto:rbonica@juniper.net>>
>>                 *Cc:* 6man <6man@ietf.org <mailto:6man@ietf.org>>
>>                 *Subject:* Re: Size of CR in CRH____
>>
>>                 ____
>>
>>                 *[External Email. Be cautious of content]*____
>>
>>                 ____
>>
>>                 Ron, ____
>>
>>                 ____
>>
>>                 While we are at the local vs global significance of SIDs
>>                 can you please elaborate how do you resolve the conflict
>>                 where given SID value is advertised by more then one
>>                 node ? In fact imagine that all nodes in a domain choose
>>                 to advertise the same SID value "15" to forward the
>>                 traffic to their respective peers. So packet arrives at
>>                 segment endpoint node A with CRH consisting of SID list
>>                 15, 15, 15, 15 ... where each value 15 means different
>>                 behaviour on different node. ____
>>
>>                 ____
>>
>>                 How do you even know which way to forward the packet ? ____
>>
>>                 ____
>>
>>                 See in this case your mapping plane will contain
>>                 different functions on different nodes signalled with
>>                 the same SID. ____
>>
>>                 ____
>>
>>                 I understand that you are trying to silently borrow
>>                 set of procedures from SR-MPLS here as documented in
>>                 RFC8660. But if you just open this RFC you will see
>>                 section 2.5 or 2.6 without which you just can not simply
>>                 propose to treat SID as locally significant in any form
>>                 of segment routing. Of course unless you would consume
>>                 two SIDs per node. ____
>>
>>                 ____
>>
>>                 Thx,
>>                 Robert.____
>>
>>                 ____
>>
>>                 ____
>>
>>                 On Thu, May 21, 2020 at 10:34 AM Robert Raszuk
>>                 <robert@raszuk.net <mailto:robert@raszuk.net>> wrote:____
>>
>>                     Ron, ____
>>
>>                     ____
>>
>>                      > Now recall that identifiers have node local
>>                     significance. ____
>>
>>                     ____
>>
>>                     I was talking about case described in yr draft
>>                     section 7: ____
>>
>>                     ____
>>
>>                     "Applications can:____
>>
>>                     ____
>>
>>                     o Allocate SIDs so that they have *domain-wide
>>                     significance*."____
>>
>>                     ____
>>
>>                     While not a must - it is an option. So I believe my
>>                     observation stays valid till draft either removes
>>                     that option or describes scaling properties
>>                     differences between both domain wide and local
>>                     significance of the SIDs.____
>>
>>                     ____
>>
>>                     Thx,____
>>
>>                     R.____
>>
>>                     ____
>>
>>                     ____
>>
>>                     On Thu, May 21, 2020 at 4:01 AM Ron Bonica
>>                     <rbonica@juniper.net <mailto:rbonica@juniper.net>>
>>                     wrote:____
>>
>>                         Robert,____
>>
>>                         ____
>>
>>                         Consider the following network:____
>>
>>                         ____
>>
>>                           * Contains 65,000 routers____
>>                           * Each router has 500 directly connected
>>                             neighbors or fewer____
>>                           * Uses 16-bit CRH____
>>
>>                         ____
>>
>>                         In this network, each node might have 65,499
>>                         CRH-FIB entries:____
>>
>>                         ____
>>
>>                           * 64,999 CRH-FIB entries cause packets to
>>                             follow the least-cost path to another node
>>                             in the domain____
>>                           * 500 CRH-FIB entries cause packets to
>>                             traverse a specific link to a specific
>>                             neighbor.____
>>
>>                         ____
>>
>>                         As a mnemonic device, an operator might assign
>>                         identifiers as follows:____
>>
>>                         ____
>>
>>                           * 0-65,000 identify CRH-FIB entries that cause
>>                             packets to follow the least-cost path to
>>                             another node in the domain____
>>                           * 65,001 – 65,565 identify CRH-FIB entries
>>                             that that cause packets to traverse a
>>                             specific link to a specific neighbor.____
>>
>>                         ____
>>
>>                         Now recall that identifiers have node local
>>                         significance. So, Node A and Node B might both
>>                         have a CRH-FIB entry that is identified by the
>>                         value 65,001. However:____
>>
>>                         ____
>>
>>                           * The CRH-FIB entry on Node A causes packets
>>                             to traverse a particular link towards Node X____
>>                           * The CRH-FIB entry on Node B causes packets
>>                             to traverse a different link towards Node Y.____
>>
>>                         ____
>>
>>                         I think that this example refutes the premise of
>>                         your argument, so there is not further need to
>>                         address the conclusion.____
>>
>>                         ____
>>
>>                                                                                                                   Ron____
>>
>>                         ____
>>
>>                         ____
>>
>>                         ____
>>
>>                         ____
>>
>>                         Juniper Business Use Only____
>>
>>                         *From:* Robert Raszuk <robert@raszuk.net
>>                         <mailto:robert@raszuk.net>>
>>                         *Sent:* Wednesday, May 20, 2020 6:20 PM
>>                         *To:* Ron Bonica <rbonica@juniper.net
>>                         <mailto:rbonica@juniper.net>>
>>                         *Cc:* 6man <6man@ietf.org <mailto:6man@ietf.org>>
>>                         *Subject:* RE: Size of CR in CRH____
>>
>>                         ____
>>
>>                         *[External Email. Be cautious of content]*____
>>
>>                         ____
>>
>>                         HI, ____
>>
>>                         ____
>>
>>                         So just to make sure I understand this analogy
>>                         of 16 bit -- 2^16 = 65536 nodes. I think this is
>>                         only on paper. ____
>>
>>                         ____
>>
>>                         Imagine I have 1000 routers so if I divide the
>>                         16 bit space by 1000 I get at most 65 local node
>>                         behaviours if anyone would like to embed such
>>                         into the SID. ____
>>
>>                         ____
>>
>>                         That means that if my router have more then 65
>>                         interfaces I am not able to steer packets by src
>>                         route out of my router ... I must always depend
>>                         on the lookup of next SID how to forward the
>>                         packets. ____
>>
>>                         ____
>>
>>                         That also means that if I want to apply any form
>>                         of NP in segment endpoint I am quite limited to
>>                         the number of local functions I could use. ____
>>
>>                         ____
>>
>>                         To conclude - Let me restate to what I and
>>                         others already said - flat SID space domain wide
>>                         in mapping plane is a mistake. Yes this is like
>>                         MPLS, but this does not make it great again due
>>                         to that legacy. ____
>>
>>                         ____
>>
>>                         Many thx,
>>                         R.____
>>
>>
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