rfc2105.txt

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Network Working Group                                         Y. Rekhter
Request for Comments: 2105                                      B. Davie
Category: Informational                                          D. Katz
                                                                E. Rosen
                                                              G. Swallow
                                                     Cisco Systems, Inc.
                                                           February 1997


           Cisco Systems' Tag Switching Architecture Overview

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

IESG Note:

   This protocol is NOT the product of an IETF working group nor is it a
   standards track document.  It has not necessarily benefited from the
   widespread and in depth community review that standards track
   documents receive.

Abstract

   This document provides an overview of a novel approach to network
   layer packet forwarding, called tag switching. The two main
   components of  the tag switching architecture - forwarding and
   control - are described.  Forwarding is accomplished using simple
   label-swapping techniques, while the existing network layer routing
   protocols plus mechanisms for binding and distributing tags are used
   for control. Tag switching can retain the scaling properties of IP,
   and can help improve the scalability of IP networks. While tag
   switching does not rely on ATM, it can straightforwardly be applied
   to ATM switches. A range of tag switching applications and deployment
   scenarios are described.

Table of Contents

   1      Introduction  ...........................................   2
   2      Tag Switching components  ...............................   3
   3      Forwarding component  ...................................   3
   3.1    Tag encapsulation  ......................................   4
   4      Control component  ......................................   4
   4.1    Destination-based routing  ..............................   5
   4.2    Hierarchy of routing knowledge  .........................   7
   4.3    Multicast  ..............................................   8



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   4.4    Flexible routing (explicit routes)  .....................   9
   5      Tag switching with ATM  .................................   9
   6      Quality of service  .....................................  11
   7      Tag switching migration strategies  .....................  11
   8      Summary  ................................................  12
   9      Security Considerations  ................................  12
   10     Intellectual Property Considerations  ...................  12
   11     Acknowledgments  ........................................  12
   12     Authors' Addresses  .....................................  13

1. Introduction

   Continuous growth of the Internet demands higher bandwidth within the
   Internet Service Providers (ISPs). However, growth of the Internet is
   not the only driving factor for higher bandwidth - demand for higher
   bandwidth also comes from emerging multimedia applications.  Demand
   for higher bandwidth, in turn, requires higher forwarding performance
   (packets per second) by routers, for both multicast and unicast
   traffic.

   The growth of the Internet also demands improved scaling properties
   of the Internet routing system. The ability to contain the volume of
   routing information maintained by individual routers and the ability
   to build a hierarchy of routing knowledge are essential to support a
   high quality, scalable routing system.

   We see the need to improve forwarding performance while at the same
   time adding routing functionality to support multicast, allowing more
   flexible control over how traffic is routed, and providing the
   ability to build a hierarchy of routing knowledge. Moreover, it
   becomes more and more crucial to have a routing system that can
   support graceful evolution to accommodate new and emerging
   requirements.

   Tag switching is a technology that provides an efficient solution to
   these challenges. Tag switching blends the flexibility and rich
   functionality provided by Network Layer routing with the simplicity
   provided by the label swapping forwarding paradigm.  The simplicity
   of the tag switching forwarding paradigm (label swapping) enables
   improved forwarding performance, while maintaining competitive
   price/performance.  By associating a wide range of forwarding
   granularities with a tag, the same forwarding paradigm can be used to
   support a wide variety of routing functions, such as destination-
   based routing, multicast, hierarchy of routing knowledge, and
   flexible routing control. Finally, a combination of simple
   forwarding, a wide range of forwarding granularities, and the ability
   to evolve routing functionality while preserving the same forwarding
   paradigm enables a routing system that can gracefully evolve to



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   accommodate new and emerging requirements.

   The rest of the document is organized as follows. Section 2
   introduces the main components of tag switching, forwarding and
   control. Section 3 describes the forwarding component.  Section 4
   describes the control component. Section 5 describes how tag
   switching could be used with ATM. Section 6 describes the use of tag
   switching to help provide a range of qualities of service.  Section 7
   briefly describes possible deployment scenarios. Section 8 summarizes
   the results.

2. Tag Switching components

   Tag switching consists of two components: forwarding and control.
   The forwarding component uses the tag information (tags) carried by
   packets and the tag forwarding information maintained by a tag switch
   to perform packet forwarding. The control component is responsible
   for maintaining correct tag forwarding information among a group of
   interconnected tag switches.

3. Forwarding component

   The fundamental forwarding paradigm employed by tag switching is
   based on the notion of label swapping. When a packet with a tag is
   received by a tag switch, the switch uses the tag as an index in its
   Tag Information Base (TIB). Each entry in the TIB consists of an
   incoming tag, and one or more sub-entries of the form (outgoing tag,
   outgoing interface, outgoing link level information). If the switch
   finds an entry with the incoming tag equal to the tag carried in the
   packet, then for each (outgoing tag, outgoing interface, outgoing
   link level information) in the entry the switch replaces the tag in
   the packet with the outgoing tag, replaces the link level information
   (e.g MAC address) in the packet with the outgoing link level
   information, and forwards the packet over the outgoing interface.

   From the above description of the forwarding component we can make
   several observations. First, the forwarding decision is based on the
   exact match algorithm using a fixed length, fairly short tag as an
   index. This enables a simplified forwarding procedure, relative to
   longest match forwarding traditionally used at the network layer.
   This in turn enables higher forwarding performance (higher packets
   per second). The forwarding procedure is simple enough to allow a
   straightforward hardware implementation.

   A second observation is that the forwarding decision is independent
   of the tag's forwarding granularity. For example, the same forwarding
   algorithm applies to both unicast and multicast - a unicast entry
   would just have a single (outgoing tag, outgoing interface, outgoing



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   link level information) sub-entry, while a multicast entry may have
   one or more (outgoing tag, outgoing interface, outgoing link level
   information) sub-entries. (For multi-access links, the outgoing link
   level information in this case would include a multicast MAC
   address.) This illustrates how with tag switching the same forwarding
   paradigm can be used to support different routing functions (e.g.,
   unicast, multicast, etc...)

   The simple forwarding procedure is thus essentially decoupled from
   the control component of tag switching. New routing (control)
   functions can readily be deployed without disturbing the forwarding
   paradigm.  This means that it is not necessary to re-optimize
   forwarding performance (by modifying either hardware or software) as
   new routing functionality is added.

3.1. Tag encapsulation

   Tag information can be carried in a packet in a variety of ways:

      - as a small "shim" tag header inserted between the layer 2 and
      the Network Layer headers;

      - as part of the layer 2 header, if the layer 2 header provides
      adequate semantics (e.g., ATM, as discussed below);

      - as part of the Network Layer header (e.g., using the Flow Label
      field in IPv6 with appropriately modified semantics).

   It is therefore possible to implement tag switching over virtually
   any media type including point-to-point links, multi-access links,
   and ATM.

   Observe also that the tag forwarding component is Network Layer
   independent. Use of control component(s) specific to a particular
   Network Layer protocol enables the use of tag switching with
   different Network Layer protocols.

4. Control component

   Essential to tag switching is the notion of binding between a tag and
   Network Layer routing (routes). To provide good scaling
   characteristics, while also accommodating diverse routing
   functionality, tag switching supports a wide range of forwarding
   granularities. At one extreme a tag could be associated (bound) to a
   group of routes (more specifically to the Network Layer Reachability
   Information of the routes in the group). At the other extreme a tag
   could be bound to an individual application flow (e.g., an RSVP
   flow). A tag could also be bound to a multicast tree.



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   The control component is responsible for creating tag bindings, and
   then distributing the tag binding information among tag switches.
   The control component is organized as a collection of modules, each
   designed to support a particular routing function. To support new
   routing functions, new modules can be added. The following describes
   some of the modules.

4.1. Destination-based routing

   In this section we describe how tag switching can support
   destination-based routing. Recall that with destination-based routing
   a router makes a forwarding decision based on the destination address
   carried in a packet and the information stored in the Forwarding
   Information Base (FIB) maintained by the router. A router constructs