Subject: [OSPF] Notes on draft-acee-ospfv3-lsa-extend-02.txt

From: Anton Smirnov <[email-masked]>
Date: Tue, 17 Sep 2013 15:23:33 +0200

Hi,
    a few notes on current revision of ELSA draft (starting from 
editorialgoing to more important).

Cosmetic notes:

> 6.  OSPFv3 E-Inter-Area-Prefix-LSA
> ...
>     All LSA Header fields are the same as defined for the Network-LSA.

Should be IAP LSA.

> Lindem, et al.           Expires March 14, 2014                [Page 20]
> 
> Internet-Draft          OSPFv3 LSA Extendibility          September 2013
>
>
>     families as defined in [OSPFV3-AF].  The IPv6 Link-Local Address TLV
>     is only applicable to the E-Link-LSA.  Inclusion in other Extended
>     LSAs MUST be ignored.  Only a single instance of the IPv6 Link-Local
>     Address family SHOULD be included in the E-Link-LSA.  Instances
>     preceding the first MUST be ignored.

It is probably meant to be "instances following the first ..."

Page 21:
>     Address family SHOULD be included in the E-Link-LSA.  Instances
>     preceding the first MUST be ignored.  For IPv6 address families as
>     defined in [OSPFV3-AF].
Incomplete sentence.

>     For simplicity and to avoid the scaling impact of maintaining both
>     TLV and non-TLV based versions of the same LSA within a routing
>     domain, the basic backward compatibility mode will not allow mixing
>     of LSA formats. Different formats could still be supported with
>     multiple OSPFv3 instances and separate OSPFv3 routing domains.
"Basic compatibility mode" is confusing name for a mode when instance 
enabled with new functionality will not even talk to instance not 
enabled for (or not supporting) it.

Notes to functionality:

Tag field in E-ASE-LSAs is made optional.
Subject of propagating tags together with Intra- and Inter- area routes 
was raised more than once. It is very likely that sooner or later tag 
sub-TLVs will be proposed for Intra- and Inter- Area E-LSA. In this case 
implementations will have to deal with optional tag field in E-ASE LSA 
and tag sub-TLV in Intra-/Inter- Prefix TLV. This is inconvenient. It is 
better to avoid optional field in E-ASE-LSA and define tag sub-TLV for 
External-Prefix TLV from the beginning.

>     In order to retain compatibility and semantics with the current
>     OSPFv3 specification, each LSA MUST contain a single Inter-Area
>     Prefix TLV.  This will facilitate migration and avoid changes to
>     functions such as incremental SPF computation.
I appreciate ease of migration. OTOH, for better or worse but 
E-Intra-Area-Prefix LSA can propagate multiple prefixes in single LSA. 
So well-developed implementation will have to be able to work with an 
LSA advertising multiple prefixes. It is a pity to restrict once and 
forever Inter- and Ex- Prefix TLVs if Intra- does not have such limitation.
    To give implementations possibility of future optimizations 
(advertising multiple prefixes in single LSA) and still keep advantage 
of faster standardization and deployment I propose:
- Allow Inter- and Ex- TLVs to be present more than once in respective LSAs
- Stipulate that routers running in any Migration mode MUST advertise 
TLV only once per LSA.

Thanks,

Anton