Network Working Group                                        C. Filsfils
Internet-Draft                                              P. Mohapatra
Intended status: Standards Track                            C. Pignataro
Expires: June 19, 2009                                     Cisco Systems
                                                       December 16, 2008

                   Load Balancing for Mesh Softwires

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   Payloads carried over a Softwire mesh service as defined by BGP

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   Encapsulation Subsequent Address Family Identifier (SAFI) information
   exchange often carry a number of identifiable, distinct flows.  It
   can in some circumstances be desirable to distribute these flows over
   the equal cost multiple paths (ECMPs) that exist in the packet
   switched network.  Currently, the payload of a packet entering the
   Softwire can only be interpreted by the ingress and egress routers.
   Thus the load balancing decision of a core router is only based on
   the encapsulating header, presenting much less entropy than available
   in the payload or the encapsulated header since the Softwire
   encapsulation acts in a tunneling fashion.  This document describes a
   method for achieving comparable load balancing efficiency in a
   network carrying Softwire mesh service over Layer Two Tunneling
   Protocol - Version 3 (L2TPv3) over IP or Generic Routing
   Encapsulation (GRE) encapsulation to what would be achieved without
   such encapsulation.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  Load Balancing Block sub-TLV  . . . . . . . . . . . . . . . . . 3
     2.1.  Applicability to Tunnel Types . . . . . . . . . . . . . . . 4
     2.2.  Encapsulation Considerations  . . . . . . . . . . . . . . . 5
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   Consider the case of a router R1 which encapsulates a packet P into a
   Softwire bound to router R3.  R2 is a router on the shortest path
   from R1 to R3.  R2's shortest path to R3 involves equal cost multiple
   paths (ECMPs).  The goal is for R2 to be able to choose which path to
   use on the basis of the full entropy of packet P.

   This is achieved by carrying in the encapsulation header a signature
   of the inner header, hence enhancing the entropy of the flows as seen
   by the core routers.  The signature is carried as part of one of the
   fields of the encapsulation header.  To aid with better description
   in the document, we define the generic term "load balancing field" to
   mean such a value that is specific to an encapsulation type.  For
   example, for L2TPv3-over-IP [RFC3931] encapsulation, the load
   balancing field is the Session Identifier (Session ID).  For GRE
   [RFC2784] encapsulation, the key field [RFC2890], if present,
   represents the load balancing field.  This mechanism assumes that
   core routers base their load-balancing decisions on a flow definition
   that includes the load balancing field.  This is an obvious and
   generic functionality as, for example, for L2TPv3-over-IP tunnels,
   the Session ID is at the same well-known constant offset as the TCP/
   UDP ports in the encapsulating header.

   The "Encapsulation SAFI" [I-D.ietf-softwire-encaps-safi] is extended
   such that a contiguous block of the load balancing field is bound to
   the Softwire advertised by a BGP next-hop.  On a per-inner flow
   basis, the ingress PE selects one value of the load balancing field
   from the block to preserve per-flow ordering, and at the same time to
   enhance the entropy across flows.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Load Balancing Block sub-TLV

   This document defines a new sub-TLV for use with the Tunnel
   Encapsulation Attribute defined in [I-D.ietf-softwire-encaps-safi].
   The new sub-TLV is referred to as the "Load Balancing Block sub-TLV"
   and MAY be included in any Encapsulation SAFI UPDATE message where
   load balancing is desired.

   The sub-TLV type of the Load Balancing Block sub-TLV is 5.  The sub-
   TLV length is 2 octets.  The value represents the length of the block

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   in bits and it MUST NOT exceed the size of the load balancing field.
   This format is very similar to the variable-length subnet masking
   (VLSM) used in IP addresses to allow arbitrary length prefixes.  The
   block is determined by extracting the initial sequence of 'block
   size' bits from the load balancing field.

   As an example, Assume that there is a Softwire set up between R1 and
   R3 with L2TPv3-over-IP tunnel type.  Assume that R3 encodes the
   Session ID with value 0x1234ABCD in the encapsulation sub-TLV.  It
   also includes the load balancing block sub-TLV and encodes the value
   24.  This should be interpreted as follows:

   o  If an ingress router does not understand Load Balancing block sub-
      TLV, it continues to use the Session ID 0x1234ABCD and
      encapsulates all packets with that Session ID,

   o  If an ingress router understands Load Balancing Block sub-TLV, it
      picks the first 24 bits out of the Session ID (0x1234AB) to be
      used as the block and fills in the lower-order 8 bits with a per-
      flow identifier (e.g. it can be determined based on the inner
      packet's source, destination addresses and TCP/UDP ports).  This
      selection preserves per-flow ordering of packets.

   This requirement and solution applies equally to GRE where the key
   plays the same role as the Session ID in L2TPv3.

   Needless to say, if an egress router does not support load balancing
   block sub-TLV, the Softwire continues to operate with a single load
   balancing field that all ingress routers encapsulate with.

2.1.  Applicability to Tunnel Types

   The load balancing block sub-TLV is applicable to Tunnel types that
   define a load balancing field.  This document defines load balancing
   fields for tunnel types 1 (L2TPv3 over IP) and 2 (GRE) as follows:

   o  L2TPv3 over IP - Session ID.  Special care needs to be taken to
      always create a non-zero Session ID.  When an egress router
      includes a load balancing sub-TLV, it MUST encode the Session ID
      field of the Encapsulation sub-TLV in a way that ensures that the
      most significant bits of the Session ID after extracting the block
      are non-zero.

   o  GRE - GRE key

   Future tunnel types that desire to use the load balancing sub-TLV
   MUST define a load balancing field that is part of the encapsulating

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2.2.  Encapsulation Considerations

   Fields included in the encapsulation header besides the load
   balancing field are not affected by the load balancing block sub-TLV.
   All other encapsulation fields are shared between variations of the
   load balancing field.  For example, for L2TPv3-over-IP tunnel type,
   if the optional cookie is included in the Encapsulation sub-TLV by
   the egress router during Softwire signaling, it applies to all the
   "Session ID" values derived at the ingress router after applying the
   load balancing block as described in this document.

3.  IANA Considerations

   IANA is requested to assign the type of 5 for the Load Balancing
   Block sub-TLV, in the tunnel sub-TLV types of the Tunnel
   Encapsulation attribute registry (number space created as part of the
   publication of [I-D.ietf-softwire-encaps-safi]):

       Sub-TLV name                            Type
       -------------                           -----
       Load Balancing Block                      5

4.  Security Considerations

   There are no additional security risks introduced by this design.

5.  Acknowledgements

   The authors would like to thank Stewart Bryant, Mark Townsley, and
   Rajiv Asati for their review and comments.

6.  Normative References

              Mohapatra, P. and E. Rosen, "BGP Encapsulation SAFI and
              BGP Tunnel Encapsulation Attribute",
              draft-ietf-softwire-encaps-safi-03 (work in progress),
              June 2008.

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

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,

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              March 2000.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, September 2000.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

Authors' Addresses

   Clarence Filsfils
   Cisco Systems


   Pradosh Mohapatra
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134


   Carlos Pignataro
   Cisco Systems
   7200 Kit Creek Road, PO Box 14987
   Research Triangle Park, NC  27709


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