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Re: Last Call: Routing Extensions in Support of Generalized MPLS vto Proposed Standard
Hi Jonathan,
On Sun, 23 Feb 2003, Jonathan Sadler wrote:
> Please consider the following comments on these drafts:
> draft-ietf-ccamp-gmpls-routing-05.txt
> draft-ietf-ccamp-ospf-gmpls-extensions-09.txt
> Many of the comments are based on implementation experience. These comments are
> marked with a (*).
Thanks for your comments. Most of these would have more appropriate to
raise in the WG Last Call, but it is certainly better to bring them to
light now than never.
> ==========
>
> 1. In section 4.4.2 of draft-ietf-ccamp-gmpls-routing-05.txt, the operations for
> Packet Switch Capable (PSC) are defined. Reference is made to Minimum LSP bandwidth
> for SDH encoding. None of the examples in section 5 of
> draft-ietf-ccamp-gmpls-routing-05.txt show how this field should filled.
Note that all possible permutations cannot possibly be addressed; however,
we will consider adding some examples here.
> 2. The mechanism for showing relationships between server and client layers is not
> generalized*. Specifically:
> - Relationships between SONET/SDH layers (ISC type: TDM) are
> implicitly stated based on the Min and Max LSP bandwidth
> advertised*. To quote draft-ietf-ccamp-gmpls-routing-05.txt:
>
> "On an interface having Standard SDH multiplexing, an LSP
> at priority p could reserve any bandwidth allowed by the
> branch of the SDH hierarchy, with the leaf and the root
> of the branch being defined by the Minimum LSP Bandwidth
> and the Maximum LSP Bandwidth at priority p."
>
> This requires node doing the route calculation to understand
> the G.707 multiplexing hierarchy. Since LSP routing is source
> routed, it could be the node doing the route calculation
> doesn't understand G.707.
This document does *not* cover how path computation is done. Of
course, the information is less than useful if it cannot be used
effectively. However, it has been pointed out implicitly and
explicitly in several documents that the head end does *not* have
to do the entire route computation -- this is one reason why there
are loose hops in the ERO. See also the various drafts on multi-area
TE computation.
> - Relationships between PSC-n (for IP switching) and SONET/SDH are
> explicitly specified on the encoding type specified in the PSC-n
> announcement*. However, SONET/SDH is not a single layer. It
> must be possible to identify if the PSC-n layer can be carried
> by a VC-11 trail, a VC-12 trail, a VC-3 trail, a VC-4 trail,
> or a VC-4-nc trail.
>
> Section 4.4.2 of draft-ietf-ccamp-gmpls-routing-05.txt tries
> to deal with this in the following paragraph:
>
> "On a PSC interface that supports Standard SDH encoding, an
> LSP at priority p could reserve any bandwidth allowed by
> the branch of the SDH hierarchy, with the leaf and the root
> of the branch being defined by the Minimum LSP Bandwidth
> and the Maximum LSP Bandwidth at priority p."
>
> The problem is this contradicts the following paragraph:
>
> "The Maximum LSP Bandwidth takes the place of the Maximum
> Bandwidth ([ISIS-TE], [OSPF-TE]). However, while Maximum
> Bandwidth is a single fixed value (usually simply the link
> capacity), Maximum LSP Bandwidth is carried per priority,
> and may vary as LSPs are set up and torn down."
> Specifically, how does a completely available OC-48 interface
> with VC-11 over VC-3, VC-3, and VC-4 get encoded? Based on
> the first paragraph, the encoding would be MinLSPBW=1.5M, and
> MaxLSPBW[p]=155M. Ignoring the issue that VC-11 over VC-4
> ends up being shown as supported, the second paragraph states
> that the MaxLSPBW[p] should be 2.5G.
>
> Knowing the OC-48 is completely available is useful information,
> as multiple VC-4s could be used in an LCAS group to support
> connections that need data rates over 155M*.
It is not the intent of this document to support all possible info
about a link, but to support a useful *and* scalable subset. We will
be guided by implementation and deployment experience to add
selectively to the information. Note that the Max Reservable bandwidth
field remains, and in the case that the link is not a bundle, will be
set to ~2.5G.
> - The mechanism does not support describing common muli-layer
> relationships such as IP over DS1 over VC-11 or IP over DS3
> over VC-3*.
It is also not the intent of this document to provide a full description
of routing info for SDH. This is a *general* document. The intent is
to provide a code point for SDH to be expanded by another document. This
was the model used for signaling as well: a *general* func spec, a
*general* doc for each protocol, and *SDH-specific* docs. There is an
SDH specific routing doc; detailed comments are better directed there.
> - Sometimes layer relationships are described in an "inverted"
> manner*. Section 5.1 of draft-ietf-ccamp-gmpls-routing-05.txt
> states:
> "STM-16 POS Interface on a LSR
>
> Interface Switching Capability Descriptor:
> Interface Switching Capability = PSC-1
> Encoding = SDH
> Max LSP Bandwidth[p] = 2.5 Gbps, for all p"
>
> Where PSC-1 is the client of an SDH (sic) server.
>
> Section 5.5 states:
> "Interface of an opaque OXC (SDH framed) with support for
> one lambda per port/interface"
>
> " Interface Switching Capability Descriptor:
> Interface Switching Capability = LSC
> Encoding = SDH"
>
> In this case, SDH is a client of a wavelength server (LSC).
> However, unlike in section 5.1, the layer relationship is
> inverted.
Is this pointed out as a curiosity, or is there a question that needs
to be addressed?
> 3. Layer specific attributes are not supported*. Specifically:
Good points. Please raise this with the SDH routing doc.
> 4. The "TDM" Interface Switching Capability presumably includes
> layers other than SONET/SDH, such as PDH* (DS1, DS3, E1, E3) and
> G.709. The interaction with these layers needs to be defined.
Ditto.
> 5. In many cases, 8 levels of priority are not necessary*. A more
> compact encoding that has a bitfield stating the priority levels
> being announced would reduce the size of the announcement.
This has been discussed elsewhere. This is the model in the base
TE document; it has proven reasonable in practice. If deployment
proves otherwise, this is easy to fix. For now, though, I would
leave it as is.
Thanks again for your comments,
Kireeti.