CRH is back to the SPRING Use-Case - Re: Size of CR in CRH

["Zafar Ali (zali)" <[e-mail]>]

Ron,

I sensed that you will propose that … hence I already asked this question in my last email,

Let me repeat. In rev 10 of the CRH document[1], the following SIDs with normative reference to SRm6 were defined but removed in Feb. 2020:
"The following are valid segment types:
   o Adjacency.
   o Node.
   o Binding. "

Your proposed text is consistent on how CRH was positioned since inception till Feb. 2020, i.e., for SPRING use-case.
However, the very premise of the current adoption call is use-case beyond SPRING.
There is a clear need for documenting a genuine use-case & architecture for CRH beyond SPRING, before adoption.

[1] https://tools.ietf.org/html/draft-bonica-6man-comp-rtg-hdr-10#section-4

Thanks

Regards … Zafar

From: ipv6 <[e-mail]> on behalf of Ron Bonica <rbonica=[e-mail]>
Date: Thursday, May 21, 2020 at 10:19 AM
To: Robert Raszuk <[e-mail]>
Cc: 6man <[e-mail]>
Subject: RE: Size of CR in CRH

Robert,

It might makes sense for an operator to:


  *   Allocate identifiers that cause packets to traverse a least-cost path as if they had domain-wide significance
  *   Allocate identifiers that cause packets to traverse specific links as strictly node local

This concept also exists in SR-MPLS, where prefix SIDs have domain-wide scope and adjacency SIDs have node-local scope.

I will make this more clear in the next draft version.

                                        Ron




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From: Robert Raszuk <[e-mail]>
Sent: Thursday, May 21, 2020 4:34 AM
To: Ron Bonica <[e-mail]>
Cc: 6man <[e-mail]>
Subject: Re: Size of CR in CRH

[External Email. Be cautious of content]

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 <[e-mail]<mailto:[e-mail]>> 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 <[e-mail]<mailto:[e-mail]>>
Sent: Wednesday, May 20, 2020 6:20 PM
To: Ron Bonica <[e-mail]<mailto:[e-mail]>>
Cc: 6man <[e-mail]<mailto:[e-mail]>>
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.