RE: Size of CR in CRH

[Ron Bonica <[e-mail]>]

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





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From: Robert Raszuk <[e-mail]>
Sent: Thursday, May 21, 2020 11:25 AM
To: Ron Bonica <[e-mail]>
Cc: 6man <[e-mail]>
Subject: Re: Size of CR in CRH

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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 <[e-mail]<mailto:[e-mail]>> 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




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From: Robert Raszuk <[e-mail]<mailto:[e-mail]>>
Sent: Thursday, May 21, 2020 10:35 AM
To: Ron Bonica <[e-mail]<mailto:[e-mail]>>
Cc: 6man <[e-mail]<mailto:[e-mail]>>
Subject: Re: Size of CR in CRH

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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 <[e-mail]<mailto:[e-mail]>> 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 <[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





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From: Robert Raszuk <[e-mail]<mailto:[e-mail]>>
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To: Ron Bonica <[e-mail]<mailto:[e-mail]>>
Cc: 6man <[e-mail]<mailto:[e-mail]>>
Subject: RE: Size of CR in CRH

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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.