Re: Size of CR in CRH

[Robert Raszuk <[e-mail]>]

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