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rfc:rfc5640

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.

Filsfils, et al. Standards Track [Page 1] RFC 5640 Load-Balancing for Mesh Softwires August 2009

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].

Filsfils, et al. Standards Track [Page 2] RFC 5640 Load-Balancing for Mesh Softwires August 2009

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.

Filsfils, et al. Standards Track [Page 3] RFC 5640 Load-Balancing for Mesh Softwires August 2009

 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

Filsfils, et al. Standards Track [Page 4] RFC 5640 Load-Balancing for Mesh Softwires August 2009

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.

Filsfils, et al. Standards Track [Page 5] RFC 5640 Load-Balancing for Mesh Softwires August 2009

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

Filsfils, et al. Standards Track [Page 6]

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