RTGWG S. Ning
Internet-Draft Tata Communications
Intended status: Informational A. Malis
Expires: December 17, 2013 D. McDysan
Verizon
L. Yong
Huawei USA
C. Villamizar
Outer Cape Cod Network Consulting
June 15, 2013

Composite Link Use Cases and Design Considerations
draft-ietf-rtgwg-cl-use-cases-03

Abstract

This document provides a set of use cases and design considerations for composite links.

Composite link is a formalization of multipath techniques currently in use in IP and MPLS networks and a set of extensions to existing multipath techniques.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on December 17, 2013.

Copyright Notice

Copyright (c) 2013 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 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

Composite link requirements are specified in [I-D.ietf-rtgwg-cl-requirement]. A composite link framework is defined in [I-D.ietf-rtgwg-cl-framework].

Multipath techniques have been widely used in IP networks for over two decades. The use of MPLS began more than a decade ago. Multipath has been widely used in IP/MPLS networks for over a decade with very little protocol support dedicated to effective use of multipath.

The state of the art in multipath prior to composite links is documented in Appendix B.

Both Ethernet Link Aggregation [IEEE-802.1AX] and MPLS link bundling [RFC4201] have been widely used in today's MPLS networks. Composite link differs in the following characteristics.

  1. A composite link allows bundling of non-homogenous links together as a single logical link.
  2. A composite link provides more information in the TE-LSDB and supports more explicit control over placement of LSP.

2. Conventions used in this document

2.1. Terminology

Terminology defined in [I-D.ietf-rtgwg-cl-requirement] is used in this document.

In addition, the following terms are used:

classic multipath:

Classic multipath refers to the most common current practice in implementation and deployment of multipath (see Appendix B). The most common current practice makes use of a hash on the MPLS label stack and if IPv4 or IPv6 are indicates under the label stack, makes use of the IP source and destination addresses [RFC4385] [RFC4928].
classic link bundling:

Classic link bundling refers to the use of [RFC4201] where the "all ones" component is not used. Where the "all ones" component is used, link bundling behaves as classic multipath does. Classic link bundling selects a single component link to carry all of the traffic for a given LSP.

Among the important distinctions between classic multipath or classic link bundling and Composite Link are:

  1. Classic multipath has no provision to retain packet order within any specific LSP. Classic link bundling retains packet order among any given LSP but as a result does a poor job of splitting load among components and therefore is rarely (if ever) deployed. Composite Link allows per LSP control of load split characteristics.
  2. Classic multipath and classic link bundling do not provide a means to put some LSP on component links with lower delay. Composite Link does.
  3. Classic multipath will provide a load balance for IP and LDP traffic. Classic link bundling will not. Neither classic multipath or classic link bundling will measure IP and LDP traffic and reduce the advertised "Available Bandwidth" as a result of that measurement. Composite Link better supports RSVP-TE used with significant traffic levels of native IP and native LDP.
  4. Classic link bundling cannot support an LSP that is greater in capacity than any single component link. Classic multipath supports this capability but may reorder traffic on such an LSP. Composite Link can retain order of an LSP that is carried within an LSP that is greater in capacity than any single component link if the contained LSP has such a requirement.

None of these techniques, classic multipath, classic link bundling, or Composite Link, will reorder traffic among IP microflows. None of these techniques will reorder traffic among PW, if a PWE3 Control Word is used [RFC4385].

3. Composite Link Foundation Use Cases

A simple composite link composed entirely of physical links is illustrated in Figure 1, where a composite link is configured between LSR1 and LSR2. This composite link has three component links. Individual component links in a composite link may be supported by different transport technologies such as SONET, OTN, Ethernet, etc. Even if the transport technology implementing the component links is identical, the characteristics (e.g., bandwidth, latency) of the component links may differ.

The composite link in Figure 1 may carry LSP traffic flows and control plane packets. Control plane packets may appear as IP packets or may be carried within a generic associated channel (G-Ach) [RFC5586]. A LSP may be established over the link by either RSVP-TE [RFC3209] or LDP [RFC5036] signaling protocols. All component links in a composite link are summarized in the same forwarding adjacency LSP (FA-LSP) routing advertisement [RFC3945]. The composite link is summarized as one TE-Link advertised into the IGP by the composite link end points. This information is used in path computation when a full MPLS control plane is in use. The individual component links or groups of component links may optionally be advertised into the IGP as sub-TLV of the composite link advertisement to indicate capacity available with various characteristics, such as a delay range.