Subject: Re: [Lsr] draft-xie-lsr-isis-sr-vtn-mt-02

From: "Xie Chongfeng" <[email-masked]>
Date: Tue, 20 Oct 2020 20:17:25 +0800

Hi Huaimo,
Thanks for your review and comments. Please see some replies inline:

From: Lsr [mailto:[email-masked]] On Behalf Of Huaimo Chen
Sent: Monday, October 19, 2020 11:09 PM
To: [email-masked]
Subject: [Lsr] draft-xie-lsr-isis-sr-vtn-mt-02
 
Hi Authors,
 
    I have the following comments/questions:
 
In a network, can we use other methods to define VTNs while MTs are used as VTNs? 
 
[Chongfeng] As described in this document, in some network scenarios, each VTN can have an independent topology and a set of dedicated network resources. In a network which meets this scenario, using MTs to identify VTNs is a practical approach. Other approaches may also be possible, while they are out of the scope of this document. Using more than one approach in a network is not necessary thus not recommended.
 
If so, other methods may be used to define VTNs in addition to using MTs as VTNs in the same network. The draft says that MT-ID could be used as VTN-ID. In this case, a block of VTN-IDs must be allocated/reserved specially for using MT-IDs as VTN-IDs. It seems better to state something about this in the draft.
 
[Chongfeng] I assume this question is based on the “true” answer to the comment above. Since it is considered to only use one approach in a network, and this document actually does not introduce “VTN-ID” to IS-IS, IMO reserving a block of VTN-ID seems not relevant to this document.
     
An IS-IS MT has the attributes of a VTN, including the specific topology and resource. An IS-IS MT can be used as a VTN and MT-ID can be used as a VTN ID. Is it possible for an IS-IS MT to be used/shared by two or more VTNs? If so, it seems better to have some details about how the multiple VTNs use/share the resource in the same one MT. 
 
[Chongfeng] The presumption of using MT-ID to identify VTNs is that different VTNs are identified by different MT-IDs, thus sharing of one MT between different VTNs is not allowed in this document. For such sharing mechanism you may refer to other documents in the WG. 
 
In section 5 "Scalability Considerations", the draft says that while multiple VTNs share the same topology attribute, the same topology is identified by multiple different MT-IDs. This seems giving a way to let two or more VTNs to use/share one MT if IS-IS MT allows multiple MT-IDs to be used for the same one MT. It seems better to move the related text from section 5 to some of the previous sections.
 
[Chongfeng] The scalability considerations section describes the potential limitations of the MT based approach in scalability. Since each VTN is identified by a unique MT-ID, independent path computation would be needed for each VTN, even if multiple VTNs has the same topology attribute (they are still identified by different MT-IDs). For example, MT-x and MT-y are used to identify two VTNs, the topology of them may be the same, but the path computation still needs to be done for MT-X and MT-Y separately. Sharing the computation overhead among multiple VTNs is discussed in other document, which requires some protocol extensions and is out of the scope of this document.
 
Hope this helps to clarify your questions. Thanks. 
 
Best Regards,
Huaimo