Subject: Mutability requirements in draft-ietf-6man-segment-routing-header-20

From: Tom Herbert <[email-masked]>
Date: Mon, 10 Jun 2019 14:00:01 -0700

Hello,

I think mutability requirements are still not clear.

>From section 2:

"The mutability of the TLV value is defined by the most significant
bit in the type, as specified in Section 2.1. ... Section 4.3 defines
the mutability of the remaining fields in the SRH (Flags, Tag, Segment
List) in the context of the SID defined in this document."

>From section 4.3.1:

"The remainder of the SRH (Flags, Tag, Segment List, and TLVs that do
not change en route) are immutable while processing this SID."

Issues:

* There is no mention of Last Entry field in either section and no
mutability requirements stated for it otherwise. Presumably, Last
Entry is immutable as an invariant of the protocol (if it were mutable
then that would necessitate adding or deleting TLVs which AFAICT is
prohibited, see also issue #69). Please clarify.

* Section 2 clearly describes the mutability requirements of TLVs with
msb in type and defers discussion of mutability of other fields to
section 4.3.1. However, section 4.3.1 also mentions TLVs and seemingly
qualifies the requirements by implying Section 2.1 might apply for
"this SID" (i.e. maybe other SIDs don't have to abide by the msb type
bit?). Please clarify.

Tom