Re: [bess] Shepherd's review of draft-ietf-bess-mvpn-fast-failover

[<[e-mail]>]

Hi Greg,

 

I think there is still a misunderstanding on the proposed encoding 😊 I may not have been fully clear, sorry.

 

Here is what you have:

 

       0                   1                   2                   3

       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |    BFD Mode   |                  Reserved                     |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                       BFD Discriminator                       |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |              Type             |           Length              |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ~                             Value                             ~

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 

 

Here is what I expected :

 

       0                   1                   2                   3

       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |    BFD Mode   |                  Reserved                     |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                       BFD Discriminator                       |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |              Optional TLVs (variable)

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 

 

 

Associated with a text:

Optional TLVs:

The BFD Discriminator attribute MAY context optional TLVs for future extension. Each TLV consists of a sequence of:

*	1 octet of TLV type
*	1 octet of length of the value field of the TLV
*	Followed by the value 

This document does not define any TLV yet.

 

Thinking more about it, while it remains a good idea to use TLVs for future extensions, we need to define a registry to manage the TLV type in the context of the BFD attribute. Which means we will create an empty registry.

I would be happy to hear other opinions on that.

 

Brgds,

 

Stephane

 

 

 

From: Greg Mirsky <[e-mail]> 
Sent: jeudi 23 janvier 2020 01:06
To: [e-mail]
Cc: BESS <[e-mail]>; [e-mail]
Subject: Re: Shepherd's review of draft-ietf-bess-mvpn-fast-failover

 

Hi Stephane,

I've followed your suggestions and updated the working version of the draft with adding:

*	explicit TLV in the format of BFD Discriminator attribute;
*	text to define check the proper format of the BFD Discriminator attribute and the handling in case it is malformed per RFC 7606.

I greatly appreciate your feedback. Attached please find the diff to the updated working version of the draft and its copy.

 

Regards,

Greg

 

On Tue, Jan 7, 2020 at 5:59 AM <[e-mail] <mailto:[e-mail]> > wrote:

Hi Greg,

 

More inline,

 

 

From: Greg Mirsky <[e-mail] <mailto:[e-mail]> > 
Sent: mercredi 4 décembre 2019 23:22
To: [e-mail] <mailto:[e-mail]> ; BESS <[e-mail] <mailto:[e-mail]> >; [e-mail] <mailto:[e-mail]> 
Subject: RE: Shepherd's review of draft-ietf-bess-mvpn-fast-failover

 

Hi Stephane,

thank you for the review and your thoughtful comments. Please find my answers and notes in-lined under GIM>> tag.

Attached, please find the diff and copy of the working version.

 

Regards,

Greg

 

Hi,

 

Please find below my review of the document.

 

Nits:

 


Section 3.1.1: 

As the document is standard track, could you introduce normative language in the expected behavior description ?

 

GIM2>> My apologies, I've pasted the same text twice. I propose to remove "may be omitted" altogether. Hence the updated text:

   If BGP next-hop tracking is done for VPN routes and the root address
   of a given tunnel happens to be the same as the next-hop address in
   the BGP auto-discovery route advertising the tunnel, then the use of this

   mechanism for the tunnel will not bring any specific benefit. 

Do you see this version without any normative language as acceptable?

 

[SLI] Looks good thanks

 

 


Section 3.1.2:

As the document is standard track, could you introduce normative language in the expected behavior description ?

 

“This method should not be used”. Wouldn’t this be a normative statement ?

GIM>> Would the following modification of the text be acceptable:

OLD TEXT:

   This method should not be used when there is a fast restoration
   mechanism (such as MPLS FRR [RFC4090]) in place for the link.

NEW TEXT:
    Using this method when a fast restoration mechanism (such as MPLS FRR
   [RFC4090]) is in place for the link requires careful consideration
   and coordination of defect detection intervals for the link and the
   tunnel.  In many cases, it is not practical to use both methods at
   the same time.

 

[SLI] Are we strongly disencouraging the practice ? if yes, “it is not practical” is a bit too soft. I’m wondering if “is NOT RECOMMENDED” could be a good wording. But it is up to you.

GIM2>> The use of OAM in multi-layer fashion is a question I'd be interested to discuss. But I feel that it deserves a separate document and would prefer to leave the text as a note of caution for now.

 

[SLI] Ok

 



Section 3.1.4:

As the document is standard track, could you introduce normative language in the expected behavior description ?
GIM>> Updating to the normative language as follows:

OLD TEXT:

   A PE can be removed from the UMH candidate list for a given (C-S,
   C-G) if the P-tunnel (I or S, depending) for this (S, G) is leaf
   triggered (PIM, mLDP), but for some reason internal to the protocol
   the upstream one-hop branch of the tunnel from P to PE cannot be
   built.  In this case, the downstream PE can immediately update its
   UMH when the reachability condition changes.

NEW TEXT:

   A PE MAY be removed from the UMH candidate list for a given (C-S,
   C-G) if the P-tunnel (I-PMSI or S-PMSI) for this (S, G) is leaf-
   triggered (PIM, mLDP), but for some reason internal to the protocol
   the upstream one-hop branch of the tunnel from P to PE cannot be
   built.  In this case, the downstream PE MAY immediately update its
   UMH when the reachability condition changes.
 

[SLI] I understand the first “MAY” as optional feature, however the second “MAY” is more a “SHOULD” IMO. Thoughts?

GIM2>>  Thank you for the clarification. The UMH list will certainly be updated once the reachability of the downstream PE changes. In some scenarios, such an update may be immediate, i.e., ASAP, but in some, it might be better to delay it. Would you suggest adding a note about the option to delay the update?

 

[SLI] Could you check with Jeffrey Zhang on this point ? I’m not enough expert here to tell what may be the best option. On my side, I just want the text to be clear 😊

 

 

 

 


 
Section 3.1.6:

As the document is standard track, could you introduce normative language in the expected behavior description ?

GIM>> Sub-sections of 3.1.6 define the use of RFC 8562 and the new attribute. In the introduction to these sub-sections, I propose s/can/MAY/


>From a wider perspective, do you foresee other use case of signaling BFD information in BGP ? I’m just wondering if we may need something extensible for future use or not.

GIM>> Great question. BGP, and I'm speculating here, may be used to for other BFD-related scenarios. I think that we may use the Flags field.
[SLI] Is it enough or should you add some optional TLVs behind the discriminator ? (with nothing defined yet).

GIM2>> Great idea, thank you! Please see the updated figure and the text:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    BFD Mode   |                  Reserved                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       BFD Discriminator                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Reserved  TLV                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                 Format of the BFD Discriminator Attribute

   Where:

      BFD Mode is the one octet long field.  This specification defines
      the P2MP value (TBA3) Section 7.1.

      Reserved field is three octets long and the value MUST be zeroed
      on transmission and ignored on receipt.

      BFD Discriminator is four octets long field.

      Reserved TLV field is four octets long.  It MAY be used for future
      extensions of the BFD Discriminator Attribute using Type-Length-
      Value format.  This specification defines that the value in
      Reserved TLV field MUST be zeroed on transmission and ignored on

      receipt.

 

[SLI] If your field is 4-bytes long, it is not extensible, I was thinking of options encoded as TLVs.

If there is no TLV, the attribute ends on BFD discriminator, the attribute length should tell if there are TLVs or not.

 

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    BFD Mode   |                  Reserved                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       BFD Discriminator                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         optional TLVs                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Another point I have missed, you should define error handling procedures for your attribute as per RFC7606.