Reviewer: Al Morton
Review result: Has Nits
Recommendations for RSVP-TE and Segment Routing LSP co-existence
draft-ietf-teas-sr-rsvp-coexistence-rec-02.txt
Overall:
This memo considers coexistence of SR LSPs and RSVP-TE LSPs,
or migration to SR, and lists 5 methods as solutions to the
problems posed. Although only one of the solutions (3.5) is
developed in detail, the text never makes a clear recommendation
among the 5 solutions. At least two seem viable (3.1 and 3.5).
Operators would probably appreciate a clear recommendation from
the authors, if possible.
Edits/nits follow, see [acm]:
1. Introduction
The problem space can be generalized as a dark bandwidth problem to
cases where any other service exists in the network that runs in
parallel across common links and whose bandwidth is not reflected in
the available and reserved values in the TED. The general problem is
management of dark bandwidth pools and can be generalized to cases
where any other service exists in the network that runs in parallel
across common links and whose bandwidth is not reflected in the
available and reserved values in the TED.
[acm]
You've written two sentences above that essentially say the same thing.
Although it appears in the Abstract, TED should be spelled-out in the
body text.
In most practical
instances given the static nature of the traffic demands, limiting
the available reservable bandwidth available to RSVP-TE has been an
acceptable solution. However, in the case of SR traffic, there is
assumed to be very dynamic traffic demands and there is considerable
risk associated with stranding capacity or overbooking service
traffic resulting in traffic drops.
The high level requirements or assumptions to consider are:
1. Placement of SR LSPs in the same domain as RSVP-TE LSPs MUST NOT
introduce inaccuracies in the TED used by distributed or
centralized path computation engines.
2. Engines that compute RSVP-TE paths MAY have no knowledge of the
existence of the SR paths in the same domain.
[acm] suggest
s/2.../Knowledge of the
existence of the SR paths in the same domain is OPTIONAL for
engines that compute RSVP-TE paths./
or, s/MAY/may/ since this is only an assumption?
3. Engines that compute RSVP-TE paths SHOULD NOT require a software
upgrade or change to their path computation logic.
4. Protocol extensions SHOULD be avoided or be minimal as in many
cases this co-existence of RSVP-TE and SR MAY be needed only
during a transition phase.
5. Placement of SR LSPs in the same domain as RSVP-TE LSPs that are
computed in a distributed fashion MUST NOT require migration to a
central controller architecture for the RSVP-TE LSPs.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Solution options
3.1. Static partitioning of bandwidth
In this model, the static reservable bandwidth of an interface can be
statically partitioned between SR and RSVP-TE and each can operate
within that bandwidth allocation and SHOULD NOT preempt each other.
While it is possible to configure RSVP-TE to only reserve up to a
certain maximum link bandwidth and manage the remaining link
bandwidth for other services, this is a deployment where SR and RSVP-
TE are separated in the same network (ships in the night) and can
lead to suboptimal link bandwidth utilization not allowing each to
consume more, if required and constraining the respective
deployments.
The downside of this approach is the inability to use the reservable
bandwidth effectively and inability to use bandwidth left unused by
the other protocol.
[acm]
.... so this option satisfies all requirements and assumptions?
3.2. Centralized management of available capacity
In this model, a central controller performs path placement for both
RSVP-TE and SR LSPs. The controller manages and updates its own view
of the in-use and the available capacity. As the controller is a
single common entity managing the network it can have a unified and
consistent view of the available capacity at all times.
[acm]
Comment and a question:
This also makes the central controller a single point of failure.
What is the Recovery & Restoration Strategy? (care to cite a reference?)
A practical drawback of this model is that it requires the
introduction of a central controller managing the RSVP-TE LSPs as a
prerequisite to the deployment of any SR LSPs. Therefore, this
approach is not practical for networks where distributed TE with
RSVP-TE LSPs is already deployed, as it requires a redesign of the
network and is not backwards compatible. This does not satisfy
requirement 5.
Note that it is not enough for the controller to just maintain the
unified view of the available capacity, it must also perform the path
computation for the RSVP-TE LSPs, as the reservations for the SR LSPs
are not reflected in the TED. This does not fit with assumption 2
mentioned earlier.
[acm]
.... So, this option fails to satisfy key requirements and assumptions.
(suggest to state that)
....
3.5. TED consistency by reflecting SR traffic
....
The following methodology can be used at every TE node for this
solution:
[acm]
sugest to identify this list as Parameters for the methodology.
s/solution:/solution, using the following parameters:/
o T: Traffic statistics collection time interval
o N: Traffic averaging calculation (adjustment) interval such that N
= k * T, where k is a constant integer multiplier greater or equal
to 1. Its purpose is to provide a smoothing function to the
statistics collection.
[acm]
k should be a separate list item, preceding N.
