Re: [Isis-wg] draft-hegde-isis-advertising-te-protocols-02 and draft-ginsberg-isis-te-app-00

"Les Ginsberg (ginsberg)" <ginsberg@cisco.com> Mon, 15 May 2017 23:54 UTC

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From: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
To: Chris Bowers <cbowers@juniper.net>, "isis-wg@ietf.org" <isis-wg@ietf.org>
Thread-Topic: draft-hegde-isis-advertising-te-protocols-02 and draft-ginsberg-isis-te-app-00
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Date: Mon, 15 May 2017 23:52:11 +0000
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Subject: Re: [Isis-wg] draft-hegde-isis-advertising-te-protocols-02 and draft-ginsberg-isis-te-app-00
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Chris -

Thanx for the detailed write up regarding your proposed encoding for
advertising link attribute information for multiple applications.

My primary takeaway is that we are now in agreement regarding the need
to support application specific advertisement of link attribute information.
This is the major difference between the proposals in

draft-ginsberg-isis-te-app/ppsenak-ospf-te-link-attr-reuse

vs

hegde-isis-advertising-te-protocols/hegde-ospf-advertising-te-protocols

This means we have resolved the stalemate and that the respective WGs should
now be able to begin work on the proposals based on the ginsberg/ppsenak drafts.

This is a major step forward and I think achieves the task you and I were
assigned in Chicago WG meetings.

The remainder of my comments are specific to your encoding proposals -
but it is worth emphasizing that we are no longer debating the requirements -
we are simply discussing alternative encodings.

Regarding Attribute Set Identifier
----------------------------------

Your proposal is to define dynamically - via configuration on every router - a numeric
identifier which represents a set of applications. Each identifier is
associated with one or more applications - and that identifier is then
advertised with a set of link specific attribute sub-sub-TLVs.

As this is based on configuration, for correct operation the operator MUST
configure consistent numeric value/application set mappings on EVERY router.
To cover all possible combinations the operator would have to configure
up to (2**N)-1 identifiers where N is the number of applications supported.

3 applications: up to 7 identifiers
4 applications: up to 15 identifiers
5 applications: up to 31 identifiers

And the correct identifier(s) have to be associated with the appropriate sets of attributes
on every link on each router.

This seems both onerous and error prone.

The stated benefit of this vs the IANA assigned bit mappings proposed
by the ginsberg/ppsenak drafts is that a new application could be introduced
without requiring a bit assignment by IANA. If we look at the existing
applications (RSVP-TE, SR-TE, LFA) we note that all of these applications
required IETF drafts be written to define interoperable behavior. I would
expect the same would be required of any new application. Given that a
draft is required, the inclusion of an IANA request for an application bit
identifier in such a draft is trivial. By doing so we avoid the additional configuration
and its risks of inconsistency.

If the intent is to allow introduction of a proprietary or experimental
application in a network prior to developing any standards I think there
is a much easier way to support that. draft-ginsberg-isis-te-app currently
defines:

        Bit Mask Length: Non-zero (1 octet)
        Application Bit Mask: Size is (Bit Mask Length+7)/8
        The following bits are assigned:

             0 1 2 3 4 5 6 7
            +-+-+-+-+-+-+-+-+
            |L|R|S|F|       |
            +-+-+-+-+-+-+-+-+

       L-bit: Applications listed MUST use the legacy
          advertisements for the corresponding link
          found in TLVs 22, 23, 141, 222, and 223 or
          TLV 138 or TLV 139 as appropriate.

       R-bit: RSVP-TE

       S-bit: Segment Routing Traffic Engineering

       F-bit: Loop Free Alternate


We could reserve some bits (I think two would be enough) for non-standards
use. For example

             0 1 2 3 4 5 6 7
            +-+-+-+-+-+-+-+-+
            |L|R|S|F|   |P|X|
            +-+-+-+-+-+-+-+-+

       P-Bit: Reserved for proprietary application

       X-bit: Reserved for experimental (pre-standard)
               application

Regarding your proposal for: Traffic-engineering Protocol sub-TLV
-------------------------------------------------------------------

I think what you are trying to address here are the backwards compatibility
concerns. Today, because we lack the ability to advertise application specific
attributes, implementations have been forced to overload the use of the legacy
advertisements even though such advertisements were never intended to be used in this way.
One of the issues which the ginsberg/ppsenak drafts are addressing is the inappropriate use
of legacy advertisements. While we do recognize that until we have full deployment of the
extensions we need to support backwards compatibility with the existing overloaded use
of legacy advertisements, we do NOT want to standardize this behavior.

draft-ginsberg-isis-te-app provides backwards compatibility by using the L-bit
as described above. With partial deployment we then (using the example of SR-TE)
advertise an application bit mask with L and S bits set. This indicates that
SR-TE application is using the legacy advertisements. Even after full deployment
of the extensions this can be used to avoid unnecessary duplication when SR-TE
and RSVP-TE share the same attributes on a given link.

However, because you are proposing to use a numeric identifier, you have no way to
indicate when SR-TE (for example) should use legacy advertisements. In order to do so you have
to introduce another sub-TLV which uses the equivalent of the bit mask which the
ginsberg/ppsenak drafts already utilize. And, since IANA allocations for the bits in this
sub-TLV are still required, you have not actually eliminated the need for IANA bit allocations -
which is one of the goals of your proposed dynamically assigned identifiers.

For the proposals defined in the ginsberg/ppsenak drafts this information has already
been conveyed via the bit mask advertised as part of the link attribute advertisements -
so there is no need for this additional advertisement.

Also note that the concept of "application enabled on a link" is not what is required.
What is required is to identify what sets of applications can use a set of link attribute
advertisements - which is completely captured by the new application specific link
attribute advertisements defined in the ginsberg/ppsenak drafts.

There is no need for this additional sub-TLV.

   Les




From: Isis-wg [mailto:isis-wg-bounces@ietf.org] On Behalf Of Chris Bowers
Sent: Friday, May 12, 2017 10:47 AM
To: isis-wg@ietf.org
Subject: [Isis-wg] draft-hegde-isis-advertising-te-protocols-02 and draft-ginsberg-isis-te-app-00

ISIS-WG,

As I said at the microphone at the WG meeting in Chicago, I think there
may be some common ground that can address the general goals of both
draft-hegde-isis-advertising-te-protocols-02 and
draft-ginsberg-isis-te-app-00.

The text below describes proposed encodings that I think reflect
potential common ground. The main idea is to decouple the advertisement
of what protocols are enabled on a link and the advertisement of
different sets of attributes on a link, and then allow applications to
choose how to use that information as they see fit. This takes into
account input from networks operators regarding the desire for a
flexible mapping between attribute sets and the applications that use
them.

I look forward to feedback from the WG on these proposed encodings.

The text below borrows liberally from the existing text in
draft-hegde-isis-advertising-te-protocols-02 and
draft-ginsberg-isis-te-app-00 with some important differences.

Chris

======
Attribute Set Identifier

The new Attribute Set Identifier is a 32-bit value that identifies a set
of attributes.  All of the attributes advertised with a given value of
the Attribute Set Identifier are considered to be part of the attribute
set.  This allows different applications to use different attribute sets,
if desired.

The Attribute Set Identifier with a value of zero is special.  Existing
encodings for advertising attributes that do not explicitly support the
inclusion of the Attribute Set Identifier are now understood to implicitly
advertise attributes with the Attribute Set Identifier set to zero.
In this framework, existing implementations using the existing encodings
already support the advertisement of attributes with the Attribute Set
Identifier = 0.

In order to ensure a consistent view of the attribute set scoped attributes,
for encodings that explicitly support the Attribute Set Identifier,
advertising an attribute with Attribute Set Identifier set to
zero is not allowed.

>From a standardization perspective, there is not intended to be any
fixed mapping between a given Attribute Set Identifier and a given
application. A network operator wishing to advertise different attribute
sets could configure the network equipment to advertise attributes with
different values of the Attribute Set Identifier based on their
objectives. The different applications (be they controller-based
applications or distributed applications) would make use of the
different attribute sets based on convention within that network.

As an example, a network operator might choose to advertise
four different attribute sets, in support of five different applications
with the following mapping.

Application                                           Attribute Set Identifier
===========================              ========================
Distributed RSVP-based                           0 (implicit)
auto-bandwidth

Centralized SR-based TE                          0 (implicit)

Distributed SR-based FRR                         100

Centralized RSVP-based                           200
diverse low-latency paths

Potential new application                        300
that uses both SR and RSVP
to build LSPs

Below are descriptions of proposed encodings that allow attributes to
be advertised with non-zero values of the Attribute Set Identifier.
The Traffic-engineering Protocol sub-TLV is described as well, since it is
needed to indicate what protocols are enabled on a link.

======
Link Attribute Set sub-TLV

The Link Attribute Set sub-TLV is a new sub-TLV for TLVs 22, 23, 141,
222, and 223. It allows different sets of link attributes to be
advertised for the same link. This allows different applications to use
different sets of attributes.

        Type: to be assigned by IANA (suggested value 101 )
        Length: Variable (1 octet)
        Value:

                Attribute Set Identifier - a 32-bit value containing the non-zero
                Attribute Set Identifier that identifies a set of attributes. The Link
                Attribute Set sub-TLV MUST be ignored if the Attribute Set Identifier is
                zero. This ensures a consistent view of the attribute set scoped link
                attributes, where the Link Attribute sub-TLVs advertised directly
                in TLV#22 are now understood to be implicitly advertised with the
                Attribute Set Identifier equal to zero.

                Link Attribute sub-sub-TLVs - the format of these Link Attribute
                sub-sub-TLVs matches the existing formats for the Link Attribute
                sub-TLVs defined in [RFC5305] and [RFC7810]. Each Link Attribute
                sub-sub-TLV advertised in a given Link Attribute Set sub-TLV is
                associated with the Attribute Set Identifier in the Link Attribute Set
                sub-TLV.

=======
Attribute Set Scoped SRLG TLV

A new TLV is defined to allow SRLGs to be advertised for a
given link and associated with a specific attribute set identifier.
Although similar in functionality to TLV 138 (defined by
[RFC5307]) and TLV 139 (defined by [RFC6119] a single TLV provides
support for IPv4, IPv6, and unnumbered identifiers for a link.
Unlike TLVs 138/139 it utilizes sub-TLVs to encode the link
identifiers in order to provide the flexible formatting required to
support multiple link identifier types.

        Type: to be assigned by IANA (suggested value 238)
        Length: Number of octets in the value field (1 octet)
        Value:
                Neighbor System-ID + pseudo-node ID (7 octets)

                Attribute Set Identifier - a 32-bit value containing the non-zero
                Attribute Set Identifier that identifies a set of attributes. The
                Attribute Set Scoped SRLG TLV MUST be ignored if the Attribute Set Identifier is
                zero. This ensures a consistent view of the attribute set scoped link
                attributes, where the SRLGs advertised directly in TLV#138 and TLV#139
                are now understood to be implicitly advertised with the
                Attribute Set Identifier equal to zero.

                Length of sub-TLVs (1 octet)
                Link Identifier sub-TLVs (variable)
                0 or more SRLG Values (Each value is 4 octets)

        The following Link Identifier sub-TLVs are defined. The type
        values are suggested and will be assigned by IANA - but as
        the formats are identical to existing sub-TLVs defined for
        TLVs 22, 23, 141, 222, and 223 the use of the suggested sub-TLV
        types is strongly encouraged.

                   Type    Description
                        4      Link Local/Remote Identifiers (see [RFC5307])
                        6      IPv4 interface address (see [RFC5305])
                        8      IPv4 neighbor address (see [RFC5305])
                   12      IPv6 Interface Address (see [RFC6119])
                   13      IPv6 Neighbor Address (see [RFC6119])

   At least one set of link identifiers (IPv4, IPv6, or unnumbered) MUST
   be present.  TLVs which do not meet this requirement MUST be ignored.

   Multiple TLVs for the same link MAY be advertised.


=======
Traffic-engineering Protocol sub-TLV

A new Traffic-engineering protocol sub-TLV is a new sub-TLV for TLVs 22,
23, 141, 222, and 223. The sub-TLV indicates the protocols enabled on
the link. The sub-TLV has flags in the value field to indicate the
protocol enabled on the link. The length field is variable to allow the
flags field to grow for future requirements.

    Type  : to be assigned by IANA (suggested value 102)
    Length: Variable (1 octet)
    Value:

           The value field consists of bits indicating the protocols
           enabled on the link.  This document defines the two protocol values
           below.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Flags                                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               +----------+-------------------------------+
               | Value    | Protocol Name                 |
               +----------+-------------------------------+
               |0x01      | RSVP                          |
               +----------+-------------------------------+
               |0x02      | Segment Routing               |
               +----------+-------------------------------+

        The RSVP flag is set to one to indicate that RSVP-TE is enabled on a
        link.  The RSVP flag is set to zero to indicate that RSVP-TE is not
        enabled on a link.

        The Segment Routing flag is set to one to indicate that Segment
        Routing is enabled on a link.  The Segment Routing flag is set to
        zero to indicate that Segment Routing is not enabled on a link

========