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Fair Share AF Load Share
draft-kvk-trill-fair-share-af-load-share-01

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Author Kesava Vijaya Krupakaran
Last updated 2012-09-18
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draft-kvk-trill-fair-share-af-load-share-01
INTERNET-DRAFT                                  Kesava Vijaya Krupakaran
Intended Status: Proposed Standard       Janardhanan Pathangi Narasimhan
Expires: March 22, 2013                                             Dell
                                                      September 18, 2012

                        Fair Share AF Load Share
              draft-kvk-trill-fair-share-af-load-share-01

Abstract

   In an access LAN of a TRILL campus, the DRB can choose to load share
   the AF responsibility among other RBridges in the LAN. This document
   throws light on one such approach where the AF appointment is fair
   share scheduled among the RBridges.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

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Copyright and License Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
 

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   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document. Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  Shares  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3  AF Affinity VLAN Set  . . . . . . . . . . . . . . . . . . . . .  4
   4  AF Affinity VLAN Set Overlap  . . . . . . . . . . . . . . . . .  5
   5  AF Distribution Among Heterogeneous RBridges  . . . . . . . . .  6
   6  AF Computation at DRB . . . . . . . . . . . . . . . . . . . . .  6
   7  AF and VLAN Mapping . . . . . . . . . . . . . . . . . . . . . .  7
   8  AF and Multiple ports on a link . . . . . . . . . . . . . . . .  7
   9  Multi-Topology-Aware Port Capability Sub-TLVs . . . . . . . . .  7
     9.1  Fair Share Sub-TLV  . . . . . . . . . . . . . . . . . . . .  7
     9.2  AF Affinity VLAN Set Sub-TLV  . . . . . . . . . . . . . . .  7
     9.3  Partial VLANs Appointing Sub-TLV  . . . . . . . . . . . . .  8
   10  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   11  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   12  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     12.1  Normative References . . . . . . . . . . . . . . . . . . .  9
     12.2  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

 

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1  Introduction

   In a shared access LAN, the appointed forwarder for a VLAN is
   responsible for encapsulating and decapsulating native traffic on
   that VLAN. Other non-AF RBridges in the LAN discard the native
   traffic for that VLAN.

   The DRB can choose to be the AF for all VLANs or load share the AF
   responsibility among other RBRidges in the LAN. This ensures better
   utilization of resources like hardware tables and buffers. The VLAN
   partitioning scheme suggested in [RFC6439] section 2.2.1 is static
   and requires careful configuration. Another simple protocol would be
   to allocate VLANs in a round-robin fashion among all RBridges in the
   LAN. However this doesn't leave scope for schemes like retaining 50%
   of VLANs with the DRB and distribute only the rest among others.

   Fair share scheduling of AF allows for the flexibility of assigning
   certain RBridges (say with higher switching capability) AF for higher
   proportion of VLANs than others.

1.1  Terminology

   This document uses the acronyms defined in [RFC6439].

   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].

2  Shares

   Each RBridge is configured with certain quantity of shares. A share
   is the proportion of VLANs which would be allocated to the RBridge in
   comparison with other RBridges. The face value of the shares is a
   relative quantity and makes sense only when taken in conjunction with
   total shares allocated in the LAN.

   These shares are advertised by each RBridge in its hello. The DRB
   load shares the AF among RBridges based on the relative value of
   shares.

   For instance, let A, B and C be three RBridges with S(A) = 2, S(B) =
   1 and S(C) = 1. Then A is assigned the AF for 1/2 of the VLANs while
   B and C the AF for 1/4th of the VLANs each.

   Even when the number of VLANs for which the RBridge is to be AF
   calculates to a non integer value, it should be made sure that there
 

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   is only one AF for a VLAN in a multi-access LAN.

3  AF Affinity VLAN Set

   Fair share scheduling distributes VLANs among RBridges according to
   proportion of shares allocated. This allows allocation of higher
   proportion of VLANs to certain RBridges (with higher switching
   capability). However, this does not guarantee that these RBridges
   would handle larger share of the native traffic.

   Following the previous example, even though A is appointed AF for 50%
   of the VLANs while B only 25% of the VLANs, the traffic load of VLANs
   for which B is AF could be considerably higher that those in A.

   In order to overcome this conundrum, each RBriged in access LAN is
   configured with an AF Affinity VLAN Set apart form the share
   proportion. This RBridge has AF affinity to the set of configured
   VLANs. Thus when the DRB appoints an RBridge AF for a set of VLANs,
   the members of the set are chosen from the AF Affinity VLAN Set
   advertised.

   Expanding on the previous example, if X denotes an RBridge, let S(X)
   be the shares assigned to X, V(X) be the AF Affinity VLAN Set and
   AF(X) denote the set of VLANs for which X is assigned AF. Let the
   access LAN encompass ten shared VLANs [11, 20]. In this case the AF
   assignment with just the shares configured could be as in Table 1. If
   RBridge A has higher switching capability and VLANs [16, 20] are
   heavily loaded, this AF appointment defeats the purpose.

      +----------------------------------------------------+
      |Table 1: AF appointment using fair share scheduling |
      +--------+-------------+-----------------------------+
      |   X    |     S(X)    |            AF(X)            |
      +--------+-------------+-----------------------------+
      |   A    |     2       |     {11, 12, 13, 14, 15}    |
      +--------+-------------+-----------------------------+
      |   B    |     1       |         {16, 17, 18}        |
      +--------+-------------+-----------------------------+
      |   C    |     1       |          {19, 20}           |
      +--------+-------------+-----------------------------+

   By configuring AF Affinity VLAN set in each RBridge, this difficulty
   can be overcome. Such a configuration is shown in Table 2. How the AF
   Affinity VLAN set is arrived at is beyond the scope of this document.
   Long term traffic planning tools could be helpful in extrapolating a
   decent configuration.
 

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   +----------------------------------------------------------+
   |Table 2: Fair share scheduling with AF Affinity VLAN set  |
   +--+-------+-----------------------+-----------------------+
   |X |  S(X) |          V(X)         |         AF(X)         |
   +--+-------+-----------------------+-----------------------+
   |A |   2   |  {16, 17, 18, 19, 20} |  {16, 17, 18, 19, 20} |
   +--+-------+-----------------------+-----------------------+
   |B |   1   |    {11, 12, 13, 14}   |      {11, 12, 13}     |
   +--+-------+-----------------------+-----------------------+
   |C |   1   |      {12, 13, 14}     |        {14, 15}       |
   +--+-------+-----------------------+-----------------------+

4  AF Affinity VLAN Set Overlap

   If the AF Affinity VLAN sets advertised by the RBridges overlap, the
   RBridge with higher share has priority over the affinity of common
   VLANs. In case the RBridges advertise same share with conflicting AF
   Affinity VLAN sets, then the one with higher system ID gets more AF
   affinity over the common VLANs.

   +-------------------------------------------------------------------+
   | Table 3: Fair share scheduling with AF Affinity VLAN set overlap  |
   +-+--------------+----+------------------------+--------------------+
   |X|    ID(X)     |S(X)|          V(X)          |       AF(X)        |
   +-+--------------+----+------------------------+--------------------+
   |A|0000.0000.000a| 2  |{15, 16, 17, 18, 19, 20}|{15, 16, 17, 18, 19}|
   +-+--------------+----+------------------------+--------------------+
   |B|0000.0000.000b| 1  |  {11, 12, 13, 14, 15}  |    {11, 12, 13}    |
   +-+--------------+----+------------------------+--------------------+
   |C|0000.0000.000c| 1  |    {11, 12, 13, 14}    |      {14, 20}      |
   +-+--------------+----+------------------------+--------------------+

   Consider the previous examples with the LAN comprising of three
   RBridges A, B and C coloured for VLANs [11, 20]. As shown in table 3,
   the AF Affinity VLAN sets overlap in RBridges {A, C} as well as {B,
   C}. A, having the highest share has the most affinity over the VLANs
   configured there. In this example, A has higher AF affinity to VLAN
   15 than C. Similarly, C has greater the AF affinity of VLANs [11, 14]
   than B on virtue of its higher system ID.

   It is possible to calculate a better AF distribution by examining
   common VLANs in AF Affinity VLAN sets when they overlap. Such
   algorithms have been avoided to keep the computation at DRB simple.

 

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5  AF Distribution Among Heterogeneous RBridges

   An access LAN could constitute motley set of RBridges with some that
   support fair share AF scheduling and some that doesn't. If the DRB
   doesn't support fair share AF scheduling, it ignores the sub-TLVs
   advertised by other RBridges and continue to distribute AF as it did
   previously.

   If the DRB does support fair share AF scheduling and it receives
   hello form an RBridge without Fair Share Sub-TLV, it is assigned a
   default share equal to average of all shares advertised in the LAN
   during AF computation. If the AF Affinity VLAN Set Sub-TLV was not
   advertised, it is taken to be a NULL set. In case an RBridge
   advertises AF Affinity Sub-TLV without saying the shares, such TLV is
   ignored and the behaviour follows as though it had not advertised AF
   Affinity VLAN set.

   In particular, if DRB is the only RBridge supporting the feature, all
   the RBridges get equal shares (equal to the one configured at DRB,
   consistent with the average rule discussed).

   For instance, in an access LAN with RBridges A, B and C where S(A) =
   7, S(B) = 3, and C doesn't support fair share AF scheduling, the DRB
   assigns it a default of 5 shares.

   As a special case, if DRB supports fair share AF load share and none
   of the RBridges advertise any share and no share is configured in
   DRB, then DRB assigns a share value of 1 to all RBridges and load
   shares VLANs equally among all the RBridges.

6  AF Computation at DRB

   DRB runs through all RBridges, in descending order of shares
   configured and assigns the AF based on Affinity VLAN set. If the
   shares advertised are equal, then RBridges are ordered based on
   system ID. If RBridges don't advertise shares, they are assigned
   default shares and are placed below RBridges who advertise shares in
   the ordered list of RBridges. If there are multiple such non
   congruous RBridges, they are again ordered based on system ID.

   The DRB also monitors hellos for any change from previously
   advertised shares or AF Affinity VLAN set. If it detects a change,
   the AF assignment is recomputed for all RBridges. Any addition or
   deletion of adjacency also triggers fresh AF assignment. This
   simplifies the computation at DRB.

 

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7  AF and VLAN Mapping

   If the DRB detects VLAN mapping, it appoints one RBridge (possibly
   itself) as the AF for all VLANs as suggested in [RFC6439] section 2.4
   to prevent loops.

8  AF and Multiple ports on a link

   The shares configured represents the whole RBridge's proportion of AF
   sought. Further load sharing of AF among multiple ports on same link
   in an RBridge is a local decision.

9  Multi-Topology-Aware Port Capability Sub-TLVs

   Two new Multi-Topology-Aware Port Capability Sub-TLVs are required
   for the purpose of fair share AF appointment - Fair Share Sub-TLV and
   AF Affinity VLAN Set Sub-TLV.

9.1  Fair Share Sub-TLV

   Fair Share Sub-TLV is used to advertise the number of shares
   configured in the RBridge. Number of shares is a two octet value.
   When an RBridge advertises zero shares, it is not assigned any AF.

   +-+-+-+-+-+-+-+-+
   | Type          |                  (1 byte)
   +-+-+-+-+-+-+-+-+
   | Length        |                  (1 byte)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Number of Shares            |    (2 bytes)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9.2  AF Affinity VLAN Set Sub-TLV

   AF Affinity VLAN Set Sub-TLV is used to advertise the AF Affinity
   VLAN set configured in an RBridge. It is a facsimile of the Enabled-
   VLANs sub-TLV.

   +-+-+-+-+-+-+-+-+
   |     Type      |                  (1 byte)
   +-+-+-+-+-+-+-+-+
   |   Length      |                  (1 byte)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | RESV  |  Start VLAN ID        |  (2 bytes)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | VLAN bit-map....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 

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9.3  Partial VLANs Appointing Sub-TLV

   As discussed in [RFC6439] section 2.2.3, the size of hello imposes a
   limit on the distribution of AF info in AF Sub-TLV by the DRB. The
   nature of the algorithm means that the AF appointment information
   could be disjoint. If the number of VLANs on a shared link is too
   high, all AF appointments cannot be accommodated in a single hello
   using the start end mechanism of AF Sub-TLV. In such case, the DRB
   should appoint one RBridge (possibly itself) as AF for all VLANs. 

   Alternatively, the AF information can be sent in a bitmap rather than
   start-end mechanism as suggested in AF Sub-TLV. For this purpose the
   Partial VLANs Appointing Sub-TLV suggested in Adaptive VLAN
   Assignment draft [VlanAsn] can be used.

 

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10  Security Considerations

   This document raises no new security issues for IS-IS.

11  IANA Considerations

   This document suggests two additional Sub-TLV to Multi-Topology-Aware
   Port Capability TLV apart from the reuse of Partial VLANs Appointing
   Sub-TLV from Adaptive VLAN Assignment draft.

   o  Fair Share Sub-TLV
   o  AF Affinity VLAN Set Sub-TLV

12  References

12.1  Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6325] R. Perlman, D. Eastlake, et al, "RBridges: Base Protocol
             Specification", RFC 6325, July 2011.

   [RFC6326] D. Eastlake, A. Banerjee, et al, "Transparent
             Interconnection of Lots of Links (TRILL) Use of IS-IS", RFC
             6326, July 2011.

   [RFC6439] D. Eastlake, R. Perlman, et al, "Routing Bridges
             (RBridges): Appointed Forwarders", RFC 6439, November 2011.

   [RBisisb] D. Eastlake, A. Banerjee, et al, "Transparent
             Interconnection of Lots of Links (TRILL) Use of IS-IS",
             draft-eastlake-isis-rfc6326bis-09.txt, work in progress.

12.2  Informative References

   [VlanAsn] M.Zhang and D.Zhang, "Adaptive VLAN Assignment for Data
             Center RBridges", draft-zhang-trill-vlan-assign-04.txt,
             work in progress.

 

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Authors' Addresses

             Kesava Vijaya Krupakaran
             Dell
             Olympia Technology Park,
             Guindy Chennai 600 032

             Phone: +91 44 4220 8496
             Email: Kesava_Vijaya_Krupak@Dell.com

             Janardhanan Pathangi
             Dell
             Olympia Technology Park,
             Guindy Chennai 600 032

             Phone: +91 44 4220 8459
             Email: Pathangi_Janardhanan@Dell.com

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