Network Working Group Bruce Davie
Internet Draft Paul Doolan
Expiration Date: April 1997 Jeremy Lawrence
Keith McCloghrie
Yakov Rekhter
Eric Rosen
George Swallow
Cisco Systems, Inc.
October 1996
Use of Tag Switching With ATM
draft-davie-tag-switching-atm-00.txt
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Abstract
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Internet Draft draft-davie-tag-switching-atm-00.txt October 1996
A tag switching architecture is described in [1]. Tag Switching
enables the use of ATM Switches as Tag Switching Routers. The ATM
Switches run network layer routing algorithms (such as OSPF, IS-IS,
etc.), and their data forwarding is based on the results of these
routing algorithms. No ATM-specific routing or addressing is needed.
This document describes how the tag switching architecture is applied
to ATM switches.
Contents
1 Introduction ........................................... 2
2 Definitions ............................................ 3
3 Tag Switching Control Component for ATM ................ 3
4 Hybrid Switches (Ships in the Night) ................... 4
5 Use of VPI/VCIs ....................................... 4
6 Tag Allocation and Maintenance Procedures .............. 5
7 Encapsulation .......................................... 6
8 Security Considerations ................................ 7
9 Intellectual Property Considerations ................... 7
10 References ............................................. 7
11 Acknowledgments ........................................ 8
12 Authors' Addresses ..................................... 8
1. Introduction
A tag switching architecture is described in [1]. It is possible to
use ATM switches as tag switching routers. Such ATM switches run
network layer routing algorithms (such as OSPF, IS-IS, etc.), and
their forwarding is based on the results of these routing algorithms.
No ATM-specific routing or addressing is needed.
When an ATM switch is used for tag switching, the tag on which
forwarding decisions are based is carried in the VCI and/or VPI
fields. (It is possible to carry multiple tags in the VCI and/or VPI
fields, but the scope of this document is restricted to the case of a
single tag.)
The characteristics of ATM switches require some specialized
procedures and conventions to support tag switching. This document
describes those aspects of tag switching which are specific to ATM.
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2. Definitions
A Tag Switching Router (TSR) is a device which implements the tag
switching control and forwarding components described in [1].
A tag switching controlled ATM (TC-ATM) interface is an ATM interface
controlled by the tag switching control component. Packets traversing
such an interface carry tags in the VCI and/or VPI field.
An ATM-TSR is a TSR with a number of TC-ATM interfaces which forwards
cells between these interfaces using tags carried in the VCI and/or
VPI field.
A frame-based TSR is a TSR which forwards complete frames between its
interfaces. Note that such a TSR may have zero, one or more TC-ATM
interfaces.
An ATM-TSR cloud is a set of ATM-TSRs which are mutually
interconnected by TC-ATM interfaces.
The Edge Set of an ATM-TSR cloud is the set of frame-based TSRs which
are connected to the cloud by TC-ATM interfaces.
3. Tag Switching Control Component for ATM
To support tag switching an ATM switch must implement the control
component of tag switching. This consists primarily of tag allocation
and maintenance procedures. Tag binding information is communicated
through the Tag Distribution Protocol (TDP) [2].
Since the tag switching control component uses information learned
directly from network layer routing protocols, this implies that the
switch must participate as a peer in these protocols (e.g., OSPF,
IS-IS).
Support of tag switching on an ATM switch does not require the switch
to support the ATM control component defined by the ITU and ATM Forum
(e.g., UNI, PNNI). An ATM-TSR may optionally respond to OAM cells.
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4. Hybrid Switches (Ships in the Night)
The existence of the tag switching control component on an ATM switch
does not preclude the ability to support the ATM control component
defined by the ITU and ATM Forum on the same switch and the same
interfaces. The two control components, tag switching and the
ITU/ATM Forum defined, would operate independently.
Definition of how such a device operates is beyond the scope of this
document. However, only a small amount of information needs to be
consistent between the two control components, such as the portions
of the VPI/VCI space which are available to each component.
5. Use of VPI/VCIs
Tag switching is accomplished by associating tags with routes and
using the tag value to forward packets, including determining the
value of any replacement tag. See [1] for further details. In an
ATM-TSR, the tag is carried in the VPI and/or VCI field. Just as in
ATM, for a cell arriving at an interface, the VPI/VCI is looked up,
replaced, and the cell is switched.
ATM-TSRs may be connected by ATM virtual paths to enable
interconnection of ATM-TSRs over a cloud of conventional ATM
switches. In this case, the tag is carried in the VCI field.
For two connected ATM-TSRs, a connection must be available for TDP.
The default is for this connection to be on VPI 0, VCI 32. For ATM-
TSRs connected by a VPI of value x, the default for the TDP
connection is VPI x, VCI 32. Additionally, for all VPI values, VCIs 0
- 32 are not used as tags.
With the exception of these reserved values, the VPI/VCI values used
in the two directions of the link may be treated as independent
spaces.
The allowable ranges of VPI/VCIs are always communicated through TDP.
If more than one VPI is used for tag switching, the allowable range
of VCIs may be different for each VPI, and each range is communicated
through TDP.
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6. Tag Allocation and Maintenance Procedures
ATM-TSRs use the downstream-on-demand allocation mechanism described
in [1]. The procedures for tag allocation are as follows.
Consider a member of the Edge Set of an ATM-TSR cloud. Assume that,
as a result of its routing calculations, it selects an ATM-TSR as the
next hop of a certain route, and that the next hop is reachable via a
TC-ATM interface. The Edge TSR uses TDP's BIND_REQUEST to request a
tag binding from the next hop. The hop count field in the request is
set to 1. Once the Edge TSR receives the tag binding information,
the tag is used as an outgoing tag.
When an ATM-TSR receives (via TDP) a tag binding request for a
certain route from a peer connected to the ATM-TSR over a TC-ATM
interface, the ATM-TSR allocates a tag, creates a new entry in its
Tag Information Base (TIB), places that tag in the incoming tag
component of the entry, and returns (via TDP) a binding containing
the allocated tag back to the peer that originated the request. The
ATM-TSR then requests (via TDP) a tag binding from the next hop for
that route. The hop count field in the request that the ATM-TSR sends
(to the next hop TSR) is set to the hop count field in the request
that the ATM-TSR received (from the upstream TSR) plus one. Once the
ATM-TSR receives the binding from the next hop, the ATM-TSR places
the tag from the binding into the outgoing tag component of the TIB
entry.
When a member of the Edge Set of the ATM-TSR cloud receives a tag
binding request from an ATM-TSR, it allocates a tag, creates a new
entry in its Tag Information Base (TIB), places that tag in the
incoming tag component of the entry, and returns (via TDP) a binding
containing the allocated tag back to the peer that originated the
request. It uses the hop count that it received in the tag binding
request from the previous hop to adjust the TTL of packets that
arrive from that hop carrying this tag.
Note that an ATM-TSR, or a member of the edge set of an ATM-TSR
cloud, may receive multiple binding requests for the same route from
the same ATM-TSR. It must generate a new binding for each request
(assuming adequate resources to do so), and retain any existing
binding(s). For each request received, an ATM-TSR should also
generate a new binding request toward the next hop for the route.
When a routing calculation causes an ATM-TSR or a member of the edge
set of an ATM-TSR cloud to change the next hop for a route, the ATM-
TSR should notify the former next hop (via TDP) that the tag binding
associated with the route is no longer needed.
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When a TSR receives a notification that a particular tag binding is
no longer needed, the TSR may deallocate the tag associated with the
binding, and destroy the binding. In the case where an ATM-TSR
receives such notification and destroys the binding, it should notify
the next hop for the route that the tag binding is no longer needed.
If a TSR does not destroy the binding, it may re-use the binding only
if it receives a request for the same route with the same hop count
as the request that originally caused the binding to be created.
When a route changes, the tag bindings are re-established from the
point where the route diverges from the previous route. TSRs
upstream of that point are oblivious to the change. Whenever a TSR
changes its next hop for a particular route, if the new next hop is
an ATM-TSR or a member of the edge set reachable via a TC-ATM
interface, then for each entry in its TIB associated with the route
the TSR should request (via TDP) a binding from the new next hop.
Whenever an ATM-TSR originates a tag binding request to its next hop
TSR as a result of receiving a tag binding request from another
(upstream) TSR, and the request to the next hop TSR is not satisfied,
the ATM-TSR should destroy the binding created in response to the
received request, and notify the requester (via TDP).
If an ATM-TSR receives a binding request containing a hop count that
equals MAX_HOP_COUNT, no binding should be established and an error
message should be returned to the requester.
When a TSR determines that it has lost its TDP session with another
TSR, the following actions are taken. Any binding information
learned via this connection must be discarded. For any tag bindings
that were created as a result of receiving tag binding requests from
the peer, the ATM-TSR may destroy these bindings (and deallocate tags
associated with these binding).
7. Encapsulation
By default, all tagged packets should be transmitted with the generic
tag encapsulation, as defined in [3]. Since the value at the top of
the tag stack is determined from the VCI and/or VPI fields, the
generic encapsulation contains n-1 tags for a tag stack of depth n.
This means that for one level of tags the generic encapsulation
consists of a tag leader only.
For systems which are using only one level of tagging, TDP may be
used to negotiate null encapsulation. This negotiation is done once
at TDP open and applies to all VPI/VCI values used as tags. In this
case, IP packets are carried directly inside AAL5 frames, as in the
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null encapsulation of RFC 1483.
The initial TDP connection, described in Section 5, uses the LLC/SNAP
encapsulation, as defined in Section 4.1 of RFC1483. This same VCI
(with the LLC/SNAP encapsulation) may be used to exchange Network
Layer routing information as well.
TDP may be used to advertise additional VPI/VCIs to carry control
information or non-tagged packets. These may use either the null
encapsulation, as defined in Section 5.1 of RFC1483, or the LLC/SNAP
encapsulation, as defined in Section 4.1 of RFC1483.
8. Security Considerations
Security considerations are not addressed in this document.
9. 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.
10. References
[1] Rekhter, Y. et al. Tag Switching Architecture Overview, Internet
Draft, draft-rfced-info-rekhter-00.txt, Sept. 1996.
[2] Doolan, P. et al. Tag Distribution Protocol, Internet Draft,
draft-doolan-tdp-spec-00.txt, Sept. 1996.
[3] Rosen, E. et al. Tag Switching: Tag Stack Encodings, Internet
Draft, Oct. 1996.
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11. Acknowledgments
Significant contributions to this work have been made by Anthony
Alles, Fred Baker, Dino Farinacci, Guy Fedorkow, Arthur Lin, Morgan
Littlewood and Dan Tappan.
12. Authors' Addresses
Bruce Davie
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: bsd@cisco.com
Paul Doolan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: pdoolan@cisco.com
Jeremy Lawrence
Cisco Systems, Inc.
1400 Parkmoor Ave.
San Jose, CA, 95126
E-mail: jlawrenc@cisco.com
Keith McCloghrie
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: kzm@cisco.com
Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: yakov@cisco.com
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Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: swallow@cisco.com
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