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Document: draft-ietf-ospf-ospfv3-segment-routing-extensions-18
Reviewer: Pete Resnick
Review Date: 2018-11-16
IETF LC End Date: 2018-11-16
IESG Telechat date: Not scheduled for a telechat

Summary: One serious concern, one minor issue, and some nits.

Major issues:

In 3.1:

I get worried when I see this:

      SID/Label: If length is set to 3, then the 20 rightmost bits
      represent a label.

So, the Length is not a length, but rather a flag. This seems like it has the potential for an interop problem. If a general implementation treats it as a length, it's going to get the left-most 24 bits when the value is 3. Even if the implementation chooses the right-most 24 bits, it's only supposed to take the right-most 20 bits and mask off the extra 4 bits. Or are you going to specify that implementations must always set the extra 4 bits to 0?

Maybe this sort of thing is the way things have always been done for TLVs, and if so feel free to ignore me, but from an code implementation perspective, this seems like an accident waiting to happen. (Known sometimes as a "foot-gun".)

Minor issues:

In 4.4:

   The SRMS Preference TLV MAY only be advertised once in the OSPFv3
   Router Information Opaque LSA and has the following format:

You mean MUST, not MAY there.

In 7.1 and 7.2:

If SID/Index/Label is a label, is it using the low 20 bits of the first 3 bytes of the field (i.e., bits 4-23)? Is there a requirement that the high 4 bits and the low 8 bits must be cleared by the implementation? Some clarification would be useful.

In 8.1:

You talk about setting the IA-flag, but this version of the document doesn't define the IA-flag anymore. Is it defined elsewhere?

Nits/editorial comments: 

In 3.1:

Ignoring the issue stated above, I also found this text a bit confusing:

      Length: Variable, 3 or 4 octets
   
Obviously you mean that the value of Length is either 3 or 4. At first I read it as the value of Length was variable, and that it took up 3 or 4 octets in the Sub-TLV. If this is the way you've always written these things, fine, but it seems to me it would be clearer to say.

      Length: Either 3 or 4
	
In 5:

   s/we need a single advertisement/a single advertisement is needed

Just being pedantic. If you like it, use it. If not, don't.