RFC2105 - Cisco Systems Tag Switching Architecture Overview

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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

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

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

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.

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
its FIB by using the information the router receives from routing
protocols (e.g., OSPF, BGP).

To support destination-based routing with tag switching, a tag
switch, just like a router, participates in routing protocols (e.g.,
OSPF, BGP), and constructs its FIB using the information it receives
from these protocols.

There are three permitted methods for tag allocation and Tag
Information Base (TIB) management: (a) downstream tag allocation, (b)
downstream tag allocation on demand, and (c) upstream tag allocation.
In all cases, a switch allocates tags and binds them to address
prefixes in its FIB. In downstream allocation, the tag that is
carried in a packet is generated and bound to a prefix by the switch
at the downstream end of the link (with respect to the direction of
data flow). In upstream allocation, tags are allocated and bound at
the upstream end of the link. `On demand' allocation means that tags
will only be allocated and distributed by the downstream switch when
it is requested to do so by the upstream switch. Methods (b) and (c)
are most useful in ATM networks (see Section 5). Note that in
downstream allocation, a switch is responsible for creating tag
bindings that apply to incoming data packets, and receives tag
bindings for outgoing packets from its neighbors. In upstream
allocation, a switch is responsible for creating tag bindings for
outgoing tags, i.e. tags that are applied to data packets leaving the
switch, and receives bindings for incoming tags from its neighbors.

The downstream tag allocation scheme operates as follows: for each
route in its FIB the switch allocates a tag, creates an entry in its
Tag Information Base (TIB) with the incoming tag set to the allocated
tag, and then advertises the binding between the (incoming) tag and
the route to other adjacent tag switches. The advertisement could be
accomplished by either piggybacking the binding on top of the
existing routing protocols, or by using a separate Tag Distribution

Protocol [TDP]. When a tag switch receives tag binding information
for a route, and that information was originated by the next hop for
that route, the switch places the tag (carried as part of the binding
information) into the outgoing tag of the TIB entry associated with
the route. This creates the binding between the outgoing tag and the
route.

With the downstream tag allocation on demand scheme, operation is as
follows. For each route in its FIB, the switch identifies the next
hop for that route. It then issues a request (via TDP) to the next
hop for a tag binding for that route. When the next hop receives the
request, it allocates a tag, creates an entry in its TIB with the
incoming tag set to the allocated tag, and then returns the binding
between the (incoming) tag and the route to the switch that sent the
original request. When the switch receives the binding information,
the switch creates an entry in its TIB, and sets the outgoing tag in
the entry to the value received from the next hop.

The upstream tag allocation scheme is used as follows. If a tag
switch has one or more point-to-point interfaces, then for each
route in its FIB whose next hop is reachable via one of these
interfaces, the switch allocates a tag, creates an entry in its TIB
with the outgoing tag set to the allocated tag, and then advertises
to the next hop (via TDP) the binding between the (outgoing) tag and
the route. When a tag switch that is the next hop receives the tag
binding information, the switch places the tag (carried as part of
the binding information) into the incoming tag of the TIB entry
associated with the route.

Once a TIB entry is populated with both incoming and outgoing tags,
the tag switch can forward packets for routes bound to the tags by
using the tag switching forwarding algorithm (as described in Section
3).

When a tag switch creates a binding between an outgoing tag and a
route, the switch, in addition to populating its TIB, also updates
its FIB with the binding information. This enables the switch to add
tags to previously untagged packets.

To understand the scaling properties of tag switching in conjunction
with destination-based routing, observe that the total number of tags
that a tag switch has to maintain can not be greater than the number
of routes in the switch's FIB. Moreover, in some cases a single tag
could be associated with a group of routes, rather than with a single
route. Thus, much less state is required than would be the case if
tags were allocated to individual flows.

In general, a tag switch will try to populate its TIB with incoming
and outgoing tags for all routes to which it has reachability, so
that all packets can be forwarded by simple label swapping. Tag
allocation is thus driven by topology (routing), not traffic - it is
the existence of a FIB entry that causes tag allocations, not the
arrival of data packets.

Use of tags associated with routes, rather than flows, also means
that there is no need to perform flow classification procedures for
all the flows to determine whether to assign a tag to a flow. That,
in turn, simplifies the overall scheme, and makes it more robust and
stable in the presence of changing traffic patterns.

Note that when tag switching is used to support destination-based
routing, tag switching does not completely eliminate the need to
perform normal Network Layer forwarding. First of all, to add a tag
to a previously untagged packet requires normal Network Layer
forwarding. This function could be performed by the first hop router,
or by the first router on the path that is able to participate in tag
switching. In addition, whenever a tag switch aggregates a set of
routes (e.g., by using the technique of hierarchical routing), into a
single tag, and the routes do not share a common next hop, the switch
needs to perform Network Layer forwarding for packets carrying that
tag. However, one could observe that the number of places where
routes get aggregated is smaller than the total number of places
where forwarding decisions have to be made. Moreover, quite often
aggregation is applied to only a subset of the routes maintained by a
tag switch. As a result, on average a packet can be forwarded most of
the time using the tag switching algorithm.

4.2. Hierarchy of routing knowledge

The IP routing architecture models a network as a collection of
routing domains. Within a domain, routing is provided via interior
routing (e.g., OSPF), while routing across domains is provided via
exterior routing (e.g., BGP). However, all routers within domains
that carry transit traffic (e.g., domains formed by Internet Service
Providers) have to maintain information provided by not just interior
routing, but exterior routing as well. That creates certain problems.
First of all, the amount of this information is not insignificant.
Thus it places additional demand on the resources required by the
routers. Moreover, increase in the volume of routing information
quite often increases routing convergence time. This, in turn,
degrades the overall performance of the system.

Tag switching allows the decoupling of interior and exterior routing,
so that only tag switches at the border of a domain would be required
to maintain routing information provided by exterior routing, while

all other switches within the domain would just maintain routing
information provided by the domain's interior routing (which is
usually significantly smaller than the exterior routing information).
This, in turn, reduces the routing load on non-border switches, and
shortens routing convergence time.

To support this functionality, tag switching allows a packet to carry
not one but a set of tags, organized as a stack. A tag switch could
either swap the tag at the top of the stack, or pop the stack, or
swap the tag and push one or more tags into the stack.

When a packet is forwarded between two (border) tag switches in
different domains, the tag stack in the packet contains just one tag.
However, when a packet is forwarded within a domain, the tag stack in
the packet contains not one, but two tags (the second tag is pushed
by the domain's ingress border tag switch). The tag at the top of
the stack provides packet forwarding to an appropriate egress border
tag switch, while the next tag in the stack provides correct packet
forwarding at the egress switch. The stack is popped by either the
egress switch or by the penultimate (with respect to the egress
switch) switch.

The control component used in this scenario is fairly similar to the
one used with destination-based routing. In fact, the only essential
difference is that in this scenario the tag binding information is
distributed both among physically adjacent tag switches, and among
border tag switches within a single domain. One could also observe
that the latter (distribution among border switches) could be
trivially accommodated by very minor extensions to BGP (via a
separate Tag Binding BGP attribute).

4.3. Multicast

Essential to multicast routing is the notion of spanning trees.
Multicast routing procedures (e.g., PIM) are responsible for
constructing such trees (with receivers as leafs), while multicast
forwarding is responsible for forwarding multicast packets along such
trees.

To support a multicast forwarding function with tag switching, each
tag switch associates a tag with a multicast tree as follows. When a
tag switch creates a multicast forwarding entry (either for a shared
or for a source-specific tree), and the list of outgoing interfaces
for the entry, the switch also creates local tags (one per outgoing
interface). The switch creates an entry in its TIB and populates
(outgoing tag, outgoing interface, outgoing MAC header) with this
information for each outgoing interface, placing a locally generated
tag in the outgoing tag field. This creates a binding between a

multicast tree and the tags. The switch then advertises over each
outgoing interface associated with the entry the binding between the
tag (associated with this interface) and the tree.

When a tag switch receives a binding between a multicast tree and a
tag from another tag switch, if the other switch is the upstream
neighbor (with respect to the multicast tree), the local switch
places the tag carried in the binding into the incoming tag component
of the TIB entry associated with the tree.

When a set of tag switches are interconnected via a multiple-access
subnetwork, the tag allocation procedure for multicast has to be
coordinated among the switches. In all other cases tag allocation
procedure for multicast could be the same as for tags used with
destination-based routing.

4.4. Flexible routing (explicit routes)

One of the fundamental properties of destination-based routing is
that the only information from a packet that is used to forward the
packet is the destination address. While this property enables highly
scalable routing, it also limits the ability to influence the actual
paths taken by packets. This, in turn, limits the ability to evenly
distribute traffic among multiple links, taking the load off highly
utilized links, and shifting it towards less utilized links. For
Internet Service Providers (ISPs) who support different classes of
service, destination-based routing also limits their ability to
segregate different classes with respect to the links used by these
classes. Some of the ISPs today use Frame Relay or ATM to overcome
the limitations imposed by destination-based routing. Tag switching,
because of the flexible granularity of tags, is able to overcome
these limitations without using either Frame Relay or ATM.

To provide forwarding along the paths that are different from the
paths determined by the destination-based routing, the control
component of tag switching allows installation of tag bindings in tag
switches that do not correspond to the destination-based routing
paths.

5. Tag switching with ATM

Since the tag switching forwarding paradigm is based on label
swapping, and since ATM forwarding is also based on label swapping,
tag switching technology can readily be applied to ATM switches by
implementing the control component of tag switching.

The tag information needed for tag switching can be carried in the
VCI field. If two levels of tagging are needed, then the VPI field
could be used as well, although the size of the VPI field limits the
size of networks in which this would be practical. However, for most
applications of one level of tagging the VCI field is adequate.

To obtain the necessary control information, the switch should be
able (at a minimum) to participate as a peer in Network Layer routing
protocols (e.g., OSPF, BGP). Moreover, if the switch has to perform
routing information aggregation, then to support destination-based
unicast routing the switch should be able to perform Network Layer
forwarding for some fraction of the traffic as well.

Supporting the destination-based routing function with tag switching
on an ATM switch may require the switch to maintain not one, but
several tags associated with a route (or a group of routes with the
same next hop). This is necessary to avoid the interleaving of
packets which arrive from different upstream tag switches, but are
sent concurrently to the same next hop. Either the downstream tag
allocation on demand or the upstream tag allocation scheme could be
used for the tag allocation and TIB maintenance procedures with ATM
switches.

Therefore, an ATM switch can support tag switching, but at the
minimum it needs to implement Network Layer routing protocols, and
the tag switching control component on the switch. It may also need
to support some network layer forwarding.

Implementing tag switching on an ATM switch would simplify
integration of ATM switches and routers - an ATM switch capable of
tag switching would appear as a router to an adjacent router. That
could provide a viable, more scalable alternative to the overlay
model. It also removes the necessity for ATM addressing, routing and
signalling schemes. Because the destination-based forwarding approach
described in section 4.1 is topology driven rather than traffic
driven, application of this approach to ATM switches does not high
call setup rates, nor does it depend on the longevity of flows.

Implementing tag switching on an ATM switch does not preclude the
ability to support a traditional ATM control plane (e.g., PNNI) on
the same switch. The two components, tag switching and the ATM
control plane, would operate in a Ships In the Night mode (with
VPI/VCI space and other resources partitioned so that the components
do not interact).

6. Quality of service

Two mechanisms are needed for providing a range of qualities of
service to packets passing through a router or a tag switch. First,
we need to classify packets into different classes. Second, we need
to ensure that the handling of packets is such that the appropriate
QOS characteristics (bandwidth, loss, etc.) are provided to each
class.

Tag switching provides an easy way to mark packets as belonging to a
particular class after they have been classified the first time.
Initial classification would be done using information carried in the
network layer or higher layer headers. A tag corresponding to the
resultant class would then be applied to the packet. Tagged packets
can then be efficiently handled by the tag switching routers in their
path without needing to be reclassified. The actual packet scheduling
and queueing is largely orthogonal - the key point here is that tag
switching enables simple logic to be used to find the state that
identifies how the packet should be scheduled.

The exact use of tag switching for QOS purposes depends a great deal
on how QOS is deployed. If RSVP is used to request a certain QOS for
a class of packets, then it would be necessary to allocate a tag
corresponding to each RSVP session for which state is installed at a
tag switch. This might be done by TDP or by extension of RSVP.

7. Tag switching migration strategies

Since tag switching is performed between a pair of adjacent tag
switches, and since the tag binding information could be distributed
on a pairwise basis, tag switching could be introduced in a fairly
simple, incremental fashion. For example, once a pair of adjacent
routers are converted into tag switches, each of the switches would
tag packets destined to the other, thus enabling the other switch to
use tag switching. Since tag switches use the same routing protocols
as routers, the introduction of tag switches has no impact on
routers. In fact, a tag switch connected to a router acts just as a
router from the router's perspective.

As more and more routers are upgraded to enable tag switching, the
scope of functionality provided by tag switching widens. For example,
once all the routers within a domain are upgraded to support tag
switching, in becomes possible to start using the hierarchy of
routing knowledge function.

8. Summary

In this document we described the tag switching technology. Tag
switching is not constrained to a particular Network Layer protocol -
it is a multiprotocol solution. The forwarding component of tag
switching is simple enough to facilitate high performance forwarding,
and may be implemented on high performance forwarding hardware such
as ATM switches. The control component is flexible enough to support
a wide variety of routing functions, such as destination-based
routing, multicast routing, hierarchy of routing knowledge, and
explicitly defined routes. By allowing a wide range of forwarding
granularities that could be associated with a tag, we provide both
scalable and functionally rich routing. A combination of a wide range
of forwarding granularities and the ability to evolve the control
component fairly independently from the forwarding component results
in a solution that enables graceful introduction of new routing
functionality to meet the demands of a rapidly evolving computer
networking environment.

9. Security Considerations

Security issues are not discussed in this memo.

10. Intellectual Property Considerations

Cisco Systems may seek patent or other intellectual property
protection for some or all of the technologies disclosed in this
document. If any standards arising from this document are or become
protected by one or more patents assigned to Cisco Systems, Cisco
intends to disclose those patents and license them on reasonable and
non-discriminatory terms.

11. Acknowledgments

Significant contributions to this work have been made by Anthony
Alles, Fred Baker, Paul Doolan, Dino Farinacci, Guy Fedorkow, Jeremy
Lawrence, Arthur Lin, Morgan Littlewood, Keith McCloghrie, and Dan
Tappan.

12. Authors' Addresses

Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134

EMail: yakov@cisco.com

Bruce Davie
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824

EMail: bsd@cisco.com

Dave Katz
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134

EMail: dkatz@cisco.com

Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824

EMail: erosen@cisco.com

George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824

EMail: swallow@cisco.com

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