Internet Engineering Task Force                             Jim Boyle
Internet Draft
draft-boyle-tewg-ds-nop-00.txt
July 4, 2001
Expires: December, 2001


         Accomplishing Diffserv TE needs with Current Specifications

STATUS OF THIS MEMO

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Abstract

       The emerging requirements for differentiated service control
       in MPLS based traffic engineered networks [DSTE-REQ] can be met
       by existing specifications.  Current implementations could be
       adapted with a relaxed perception of semantics on the syntax
       outlined in current specifications [OSPF-TE][ISIS-TE][RSVP-TE].
       This would provide for a solution that fulfills the scenarios
       and functionality requirements called for in DSTE-REQ.

       Comments should be sent to tewg@ops.ietf.org

1. Introduction

   A common theme in the scenarios spelled out in section 2 of
   DSTE-REQ is that current approaches to traffic engineering do not
   allow an operator to properly control either to what extent a class
   of traffic can use a link, or what minimal amount of traffic should
   be held in reserve for a "lower" class of traffic to prevent total
   starvation. This memo asserts that this is more a function of
   current traffic engineering implementations commonly in use today
   than it is of the current specifications governing the extensions
   of the base protocols.  Further, by rethinking the semantics
   possible with the current specifications, it would be not only
   possible to meet the requirements in DSTE-REQ, but it would also be
   the most operationally optimal approach to meeting those
   requirements.

   The word "priority" was perhaps unfortunate, as it carries a rather
   strong perception by most operators of the ability of one type of
   traffic to preempt another.  A more generalized term might have
   been desirable, and general approaches have proven useful in the
   affinities attributes where intent is not inferable from the name
   or value seen in the signaled messages or usually in the
   configuration.  A link is colored with one or more of 32 bits, and
   LSPs are told to stick with links thus marked, or avoid them, but
   the reasoning is not inferable.  With priority, it is natural to
   assume that a connection at one priority should be able to preempt
   one of lower perceived importance.  In fact, this is how at least
   two prominent implementations behave in effect today.  Nonetheless,
   even with the current objects referred to as priority, and current
   implementations leaning heavily toward preemtability of one
   priority to another (in direct relationship to numeric value of
   priority), the current specifications allow for a semantic view
   which generalizes the interrelationship of bandwidths at different
   "priorities", as will be spelled out in this memo.


2. Altered Perceptions

2.1 Review of current specifications

   The signaling specification RSVP-TE allows for use of setup and
   hold priority objects which allow for "the capability to preempt an
   established LSP tunnel under administrative policy control".
   However, in section 4.7.3 of RSVP-TE it clearly states that what
   determines the acceptance of a session is availability of bandwidth
   "at the priority specified in the Setup Priority".  This correlates
   to the less semantic definitions of what is advertised in the
   IGP. For instance section 2.5.8 of OSPF-TE states that the
   unreserved bandwidth "not yet reserved at each of the eight
   priorities" is advertised.  There is no language in either which
   states that the bandwidth at the highest priority (0) must be
   higher than that at the lowest priority (7).  Nor is there language
   in RSVP-TE that states that in path computation if one can not find
   a path at the setup priority of a to-be-established LSP, it should
   then try lower priorities to see if it might benefit by preempting
   traffic.  Thus, what is advertised by the IGP could easily be used
   to control what is available to higher priorities or to provide a
   limited amount of resources to lower priority sessions.

2.2 Current Basis

   Most current implementations in fact do advertise bandwidths
   available in a manner which has a relationship of:

             {Effect 1}

             bw-link[n] >= bw-link[n+1]  where n is the numeric priority

   To the best knowledge of the public, current implementations
   also follow rules during setup:

             {Rule 1}

             topology for bw-lsp[n] is the topology consisting of all
                       bw-link[n], not that of max(bw-link[n..7]).

   And during call admission control:

             {Rule 2}

             succeed if (bw-lsp[n] <= bw-link[n])

   The latter is likely followed by a bandwidth update procedure which
   propagates through bandwidths n to 7, thus causing {Effect 1} above
   as seen by inspection of the IGP's TE database.

   This is where some modification would be required in
   implementations (though not in protocols or even specifications).
   Priorities would need to be implemented in a way that doesn't
   necessarily involve relationships with other priorities except as
   inferred by default or direct configuration.

2.3 Pseudo configuration

   In the below pseudo configurations, I hope the intent is clear. I
   know it falls short of professional cli definition.  Syntax is
   intended to be unix shell/man like.

            []     denotes an optional configuration
            <>     a list where selection of one parameter is
                   mandatory.  For optional commands the first in the
                   list is the default.


2.3.1   Pseudo configuration of current implementations

   In most network elements that have traffic engineering
   capabilities, there is the ability to configure the maximum
   reservable bandwidth of a link, which then propagates into the
   unreserved bandwidths based on current state of reservations.
   LSP configuration usually allows a wide variety of parameters
   including the setup priority and the bandwidth.  Some even allow
   one to configure whether preemption is supported or not.

   For discussion, suppose that there exist the capability to get the
   desired traffic into an LSP, and that these are configured as
   follows:

            lsp foo priority 2 bandwidth 64k

            link sonet1 bandwidth 2.5G

            [mpls preempt <yes|no>]

2.3.1   Pseudo configuration of proposed implementations

   It is probably desirable, though not necessarily necessary, to
   decouple the numeric priority from the class of traffic.  It is
   necessary to allow more control on what limits are now placed on a
   link.

            class voice use priority 2
            class data use priority 4
            [mpls preempt <yes|limited|map|no>]
            [preempt map voice over data]
            [class mute <list>]

            lsp foo class voice bandwidth 64k

            link sonet1 bandwidth 2.5G
            [link sonet1 class-bandwidth voice max 1G]
            [link sonet1 class-bandwidth voice max-percent 40]
            [link sonet1 class-bandwidth data min 500M]

3. Scenario Rundown

   The following scenarios are from DSTE-REQ.

3.1 Scenario 1: High Proportion of Voice

   The scenario is one in which voice is a class of traffic at a high
   priority.  The operator would like for it to use the shortest
   administrative path possible under the constraint that no link
   exceed a certain threshold (ratio wise) of voice traffic.

            class voice use priority 2
            class data use priority 4
            class mute 0-1,3,5-7
            mpls preempt limited

            lsp foo class voice bandwidth 10M
            lsp foo class data bandwidth 40M

            link sonet1 bandwidth 2.5G
            link sonet1 class-bandwidth voice max-percent 50

   At initial advertisement, the link sonet1 will advertise that it
   has the following available:

            [0, 0, 1.25G, 0, 2.5G, 0, 0, 0] for priorities [0..7]

   As the voice traffic fills this link, the bandwidth advertisements
   will be debited from voice and data, so that should no data LSPs be
   established, the voice LSPs will be limited to 1.25G and the final
   advertisement would have been:

            [0, 0, 0, 0, 1.25G, 0, 0, 0]

   One might wonder what would have been the advertisements should
   three waves of LSPs come, in (1) 1G data, (2) another 1G data and
   (3) 1G voice.

            [0, 0, 1.25G, 0, 2.5G, 0, 0, 0] initially
            [0, 0, 1.25G, 0, 1.5G, 0, 0, 0] wave (1)
            [0, 0, 1.25G, 0, 500M, 0, 0, 0] wave (2)
            [0, 0, 250M, 0, 0, 0, 0, 0] wave (3)

   Wave 3 requires preemption of 500 Mbs of data LSPs.

3.2 Scenario 2:  Rerouting on Lower Speed facilities

   Appears to be very similar to scenario 1, except perhaps that
   scenario 1 applies more to non-failure and scenario 2 is directly
   in context of a failure response.  In that case, the discussion
   section 3.1 applies here as well.

3.3 Scenario 3:  Maintain relative proportion of traffic classes

   Or not.  This scenario points out that many router implementations
   of MPLS TE don't correlate their queuing configurations to the
   bandwidth "reserved" on the link.  The scenario states that this
   being the case, it might be good to attempt to keep the traffic
   proportions similar to the queuing proportions.  In the case of the
   scenario, one would have the following configuration.

            class one use priority 1
            class two use priority 2
            class three use priority 3
            class mute 4-7
            mpls preempt limited

            lsp foo class one bandwidth 10M
            lsp foo class two bandwidth 20M
            lsp foo class three bandwidth 30M

            link sonet1 bandwidth 2.5G
            link sonet1 class-bandwidth one max-percent 45
            link sonet1 class-bandwidth two max-percent 35
            link sonet1 class-bandwidth three max-percent 20

   Initial link advertisement would be:

            [1125M, 875M, 500M, 0, 0, 0, 0, 0]

   This is likely to not be the most efficient way to operate a
   network so it would be probable that one would notch up the
   efficiency by systematically overstating the link bandwidths or
   understating the LSP bandwidths.

   This scenario hints at directly coupling the bandwidth reserved at
   a given priority/class to the parameters used in queuing.  That's
   probably worth a try.  A configuration might look like:

            class one use priority 1
            class two use priority 2
            class three use priority 3

            queues 1 by mpls-class one  # queue parameters associated with
            queues 2 by mpls-class two  # reserved bandwidth
            queues 3 by mpls-class three

            [mpls queue by <cos|class>] # queue by cos bits, or by lsp class

            lsp foo class one bandwidth 10M
            lsp foo class two bandwidth 20M
            lsp foo class three bandwidth 30M

            link sonet1 bandwidth 2.5G

3.4 Scenario 4:  Guaranteed Bandwidth Services

   This appears to be related to the above scenarios, particularly in
   that it requires limiting the amount of traffic ultimately
   available at a given class (as in Scenario 1) and the interaction
   this has with queuing (scenario 3).  The additional pieces include
   that this class is likely best served with some form of priority
   queuing and that traffic entering an LSP would be policed.  So:

            queues 1 by mpls-class 1
            queues 1 priority

            lsp foo class one bandwidth 10M police

4. Functionality Rundown

   These correlate to the functionality requirements in DSTE-REQ section 2.

4.1 DS-TE Compatibility

   If there is a network with a mix of routers where some have the
   legacy implementations and some the implementations outlined in
   this memo, there is a potential for implementation compatibility
   problems.  Areas of concern might include the following:

   - a router's inability to accept TE IGP advertisements that do not
     follow {Effect 1} of section 2.2 above.  This is not specified in
     OSPF-TE nor ISIS-TE.  Any "sanity checks" of this sort should not
     be done.

   - a router which does not follow {Rule 1} above might attempt to
     signal an bw-lsp[n] against an insufficient bw-link[n] because it
     found sufficient bw-link[n+1..7] This should fail for routers
     which follow {Rule 2} above, in accordance with RSVP-TE.

   Implementation nuances aside, there should be no compatibility
   issues between routers with legacy and new implementations.

   The most difficult part of the transition would be that of router
   syntax reconfiguration.  However, as the on-the-wire signaling
   would be indistinguishable between the two implementations, it
   would be possible to even migrate a small portion of a network and
   try out the reconfiguration and code stability.  If successful,
   migration through the rest of the network could proceed.

   This approach carries no additional information in the IGP nor are
   any new or additional objects required in the signaling or routing
   protocol.  It is suspected that the allowed variability of
   configuration is not without any additional implementation
   complexity, and that single solution approaches can be coded more
   optimally than general approaches.  So it is assumed that there
   would likely be some impact to stability or scalability with this
   approach.  However, assuming some typical configurations, it is
   likely that coding and testing could be optimized for speed and
   robustness around those configurations.

   That said, current implementations can not accomplish the scenarios
   outlined in DSTE-REQ, and this memo is an attempt to meet those in
   a way that is with minimal impact on coding, scalability, stability
   and operations.

4.2 Separate Bandwidth Constraints

   This is achievable with current specifications, as shown in the
   above examples.

4.3 Number of Class-Types

   Eight classes are currently supported, and these may be grouped
   together in sets, or Class-Types, as required and supported by
   implementations.

4.4 Number of Classes

   See 4.3.

4.5 Preemption

4.5.1 Preemption Within a Class-Type

   This is achievable.  It is expected that implementations would
   allow one to follow current (original) approach which can be
   thought of as 8 classes in one Class-Type.  Additional approaches
   would include original approach with limits on higher priorities as
   well as arbitrary associations as shown above.

4.5.2 Preemption across Class-Types

   This is achievable.  Also see 4.5.1, though this might be a
   non-requirement.

4.6 Resource Class Affinity

   This appears to be a requirement for no new requirement.  Current
   encoding specification allows for Resource Class Affinity.

4.7 Traffic Mapping

   This is achievable.  It is not precluded by current specifications.

4.8 Dynamic Adjustment of Diff-serv PHBs

   This is achievable.  This could potentially be a key approach in
   diffserv enabled networks taking advantage of traffic engineering
   technologies.

4.9 Multiple TE Metrics

   No requirements are specified.

5. Acknowledgments

   This idea has benefitted from private discussions with many of the
   usual suspects.  Namely Francois, William, Don, Luca, Vijay,
   Kireeti and most especially Dave.  Thanks.

6. References

  [DSTE-REQ]    draft-ietf-tewg-diff-te-reqts-01.txt

  [ISIS-TE]     draft-ietf-isis-traffic-03.txt

  [OSPF-TE]     draft-katz-yeung-ospf-traffic-05.txt

  [RSVP-TE]     draft-ietf-mpls-rsvp-lsp-tunnel-08.txt

7. Author's Address

  Jim Boyle
  email: jimpb@nc.rr.com