Load-Balancing for Mesh Softwires
RFC 5640

Network Working Group                                        C. Filsfils
Request for Comments: 5640                                  P. Mohapatra
Category: Standards Track                                   C. Pignataro
                                                           Cisco Systems
                                                             August 2009

                   Load-Balancing for Mesh Softwires

Abstract

   Payloads transported over a Softwire mesh service (as defined by BGP
   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.

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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Table of Contents

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

1.  Introduction

   Consider the case of a router R1 that 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 [RFC5512] 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 Provider
   Edge (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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.  Load-Balancing Block sub-TLV

   This document defines a new sub-TLV for use with the Tunnel
   Encapsulation Attribute defined in [RFC5512].  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
   in bits and 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.

   If a load-balancing field is not signaled (e.g., if the encapsulation
   sub-TLV is not included in an advertisement as in the case of GRE
   without a Key), then the Load-Balancing Block sub-TLV MUST NOT be
   included.

   The smaller the value field of the Load-Balancing Block sub-TLV, the
   larger the space for per-flow identification, and hence the better
   entropy for potential load-balancing in the core, as well as, the
   lower the polarization when mapping flows to ECMP paths.  However,
   reducing the load-balancing block size consumes more L2TPv3 Session
   IDs or GRE Keys, resulting in potentially less numbers of supported
   services.  A typical deployment would need to arbitrate between this
   trade-off.

   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 the 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 the 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 the per-flow ordering of packets.

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   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 the Load-
   Balancing Block sub-TLV, the Softwire continues to operate with a
   single load-balancing field with which all ingress routers
   encapsulate.

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 Block 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

   This document does not define a load-balancing field for the IP-in-IP
   tunnel type (tunnel types 7).  Future tunnel types that desire to use
   the Load-Balancing Block sub-TLV MUST define a load-balancing field
   that is part of the encapsulating header.

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 the 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 has assigned the value 5 for the Load-Balancing Block sub-TLV,
   in the BGP Tunnel Encapsulation Attribute Sub-TLVs registry (number
   space created as part of the publication of [RFC5512]):

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

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4.  Security Considerations

   This document defines a new sub-TLV for the BGP Tunnel Encapsulation
   Attribute.  Security considerations for the BGP Encapsulation SAFI
   and the BGP Tunnel Encapsulation Attribute are covered in [RFC5512].
   There are no additional security risks introduced by this design.

5.  Acknowledgements

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

6.  Normative References

   [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,
              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.

   [RFC5512]  Mohapatra, P. and E. Rosen, "The BGP Encapsulation
              Subsequent Address Family Identifier (SAFI) and the BGP
              Tunnel Encapsulation Attribute", RFC 5512, April 2009.

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Authors' Addresses

   Clarence Filsfils
   Cisco Systems
   Brussels,
   Belgium

   EMail: cfilsfil@cisco.com

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

   EMail: pmohapat@cisco.com

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

   EMail: cpignata@cisco.com

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