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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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].
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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.
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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
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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.
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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
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RFC, FYI, BCP