One document matched: draft-davie-mpls-atm-00.txt
Network Working Group Bruce Davie
Internet Draft Jeremy Lawrence
Expiration Date: May 1998 Keith McCloghrie
Yakov Rekhter
Eric Rosen
George Swallow
Cisco Systems, Inc.
Paul Doolan
Ennovate Networks, Inc.
November 1997
Use of Label Switching With ATM
draft-davie-mpls-atm-00.txt
Status of this Memo
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Abstract
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A Multi Protocol Label Switching architecture is described in [1].
Label Switching enables the use of ATM Switches as Label 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 label switching architecture is
applied to ATM switches.
Contents
1 Introduction ........................................... 2
2 Definitions ............................................ 3
3 Special Characteristics of ATM Switches ................ 3
4 Label Switching Control Component for ATM .............. 4
5 Hybrid Switches (Ships in the Night) ................... 5
6 Use of VPI/VCIs ....................................... 5
7 Label Allocation and Maintenance Procedures ............ 6
7.1 Edge LSR Behavior ...................................... 6
7.2 Conventional ATM Switches (non-VC-merge) ............... 7
7.3 VC-merge-capable ATM Switches .......................... 9
7.4 Efficient use of label space ........................... 11
8 Encapsulation .......................................... 11
9 Security Considerations ................................ 12
10 Intellectual Property Considerations ................... 12
11 References ............................................. 12
12 Acknowledgments ........................................ 12
13 Authors' Addresses ..................................... 12
1. Introduction
A label switching architecture is described in [1]. It is possible to
use ATM switches as label 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 label switching, the label on which
forwarding decisions are based is carried in the VCI and/or VPI
fields. (It is possible to carry multiple labels in the VCI and/or
VPI fields, but the scope of this document is restricted to the case
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of a single label.)
The characteristics of ATM switches require some specialized
procedures and conventions to support label switching. This document
describes those aspects of label switching which are specific to ATM.
2. Definitions
A Label Switching Router (LSR) is a device which implements the label
switching control and forwarding components described in [1].
A label switching controlled ATM (LC-ATM) interface is an ATM
interface controlled by the label switching control component.
Packets traversing such an interface carry labels in the VCI and/or
VPI field.
An ATM-LSR is a LSR with a number of LC-ATM interfaces which forwards
cells between these interfaces using labels carried in the VCI and/or
VPI field.
A frame-based LSR is a LSR which forwards complete frames between its
interfaces. Note that such a LSR may have zero, one or more LC-ATM
interfaces.
An ATM-LSR cloud is a set of ATM-LSRs which are mutually
interconnected by LC-ATM interfaces.
The Edge Set of an ATM-LSR cloud is the set of frame-based LSRs which
are connected to the cloud by LC-ATM interfaces.
VC-merge is the process by which a switch receives cells on several
incoming VCIs and transmits them on a single outgoing VCI without
causing the cells of different AAL5 PDUs to become interleaved.
3. Special Characteristics of ATM Switches
While the label switching architecture permits considerable
flexibility in LSR implementation, an ATM-LSR is constrained by the
capabilities of the (possibly pre-existing) hardware and the
restrictions on such matters as cell format imposed by ATM standards.
Because of these constraints, some special procedures are required
for ATM-LSRs.
Some of the key features of ATM switches that affects their behavior
as LSRs are:
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- the label swapping function is performed on fields (the VCI
and/or VPI) in the cell header; this dictates the size and
placement of the label(s) in a packet.
- multipoint-to-point and multipoint-to-multipoint VCs are
generally not supported. This means that most switches cannot
support `VC-merge' as defined above.
- there is generally no capability to perform a `TTL-decrement'
function as is performed on IP headers in routers.
This document describes ways of applying label switching to ATM
switches which work within these constraints.
4. Label Switching Control Component for ATM
To support label switching an ATM switch must implement the control
component of label switching. This consists primarily of label
allocation and maintenance procedures. Label binding information is
communicated by several mechanisms, notably the Label Distribution
Protocol (LDP). Candidate protocols being considered by the LDP
design team are described in [2, 4]. This document imposes certain
requirements on the LDP.
Since the label 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).
In some cases, LSRs make use of other protocols (e.g. RSVP, PIM, BGP)
to distribute label bindings. In these cases, an ATM LSR would need
to participate in these protocols.
Support of label 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-LSR may optionally respond to OAM
cells.
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5. Hybrid Switches (Ships in the Night)
The existence of the label 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, label 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.
6. Use of VPI/VCIs
Label switching is accomplished by associating labels with routes and
using the label value to forward packets, including determining the
value of any replacement label. See [1] for further details. In an
ATM-LSR, the label is carried in the VPI and/or VCI field. Just as in
conventional ATM, for a cell arriving at an interface, the VPI/VCI is
looked up, replaced, and the cell is switched.
ATM-LSRs may be connected by ATM virtual paths to enable
interconnection of ATM-LSRs over a cloud of conventional ATM
switches. In this case, the label is carried in the VCI field.
For two connected ATM-LSRs, a connection must be available for LDP.
The default is for this connection to be on VPI 0, VCI 32. For ATM-
LSRs connected by a VPI of value x, the default for the LDP
connection is VPI x, VCI 32. Additionally, for all VPI values, VCIs 0
- 32 are not used as labels.
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 LDP.
If more than one VPI is used for label switching, the allowable range
of VCIs may be different for each VPI, and each range is communicated
through LDP.
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7. Label Allocation and Maintenance Procedures
ATM-LSRs use the downstream-on-demand allocation mechanism described
in [1]. The procedures for label allocation depend on whether the
switches support VC-merge or not. We therefore describe the two
scenarios in turn. We begin by describing the behavior of members of
the Edge Set of an ATM-LSR cloud; these edge LSRs are not themselves
ATM-LSRs, and their behavior is the same whether the cloud contains
VC-merge capable LSRs or not.
7.1. Edge LSR Behavior
Consider a member of the Edge Set of an ATM-LSR cloud. Assume that,
as a result of its routing calculations, it selects an ATM-LSR as the
next hop of a certain route, and that the next hop is reachable via a
LC-ATM interface. The Edge LSR uses LDP's binding request message to
request a label binding from the next hop. The hop count field in
the request is set to 1. Once the Edge LSR receives the label
binding information, the label is used as an outgoing label. The
binding received by the edge LSR may contain a hop count, which
represents the number of hops a packet will take to cross the ATM-LSR
cloud when using this label. The edge LSR may either
- use this hop count to decrement the TTL of packets before
transmitting them over the cloud
- decrement the TTL of packets by one before transmitting them
over the cloud.
The choice between these two options should be made based on local
configuration.
When a member of the Edge Set of the ATM-LSR cloud receives a label
binding request from an ATM-LSR, it allocates a label, creates a new
entry in its Label Information Base (LIB), places that label in the
incoming label component of the entry, and returns (via LDP) a
binding containing the allocated label back to the peer that
originated the request. It sets the hop count in the binding to 1.
When a routing calculation causes an Edge LSR to change the next hop
for a route, and the former next hop was in the ATM-LSR cloud, the
Edge LSR should notify the former next hop (via LDP) that the label
binding associated with the route is no longer needed.
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7.2. Conventional ATM Switches (non-VC-merge)
When an ATM-LSR receives (via LDP) a label binding request for a
certain route from a peer connected to the ATM-LSR over a LC-ATM
interface, the ATM-LSR takes the following actions:
- it allocates a label, creates a new entry in its Label
Information Base (LIB), and places that label in the incoming
label component of the entry;
- it requests (via LDP) a label binding from the next hop for
that route;
- it returns (via LDP) a binding containing the allocated
incoming label back to the peer that originated the request.
The hop count field in the request that the ATM-LSR sends (to the
next hop LSR) is set to the hop count field in the request that it
received from the upstream LSR plus one. Once the ATM-LSR receives
the binding from the next hop, it places the label from the binding
into the outgoing label component of the LIB entry.
The ATM-LSR may choose to wait for the request to be satisfied from
downstream before returning the binding upstream (a "conservative"
approach). In this case, the ATM-LSR increments the hop count it
received from downstream and uses this value in the binding it
returns upstream. If the value of the hop count equals MAX_HOP_COUNT
the ATM-LSR should notify the upstream neighbor that it could not
satisfy the binding request.
Alternatively, the ATM-LSR may return the binding upstream without
waiting for a binding from downstream (an "optimistic" approach). In
this case, it uses a reserved value for hop count in the binding,
indicating that it is unknown. The correct value for hop count will
be returned later, as described below.
Since both the conservative and the optimistic approaches have
advantages and disadvantages, this is left as an implementation
choice.
Note that an ATM-LSR, or a member of the edge set of an ATM-LSR
cloud, may receive multiple binding requests for the same route from
the same ATM-LSR. 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-LSR should also
generate a new binding request toward the next hop for the route.
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When a routing calculation causes an ATM-LSR to change the next hop
for a route, the ATM-LSR should notify the former next hop (via LDP)
that the label binding associated with the route is no longer needed.
When a LSR receives a notification that a particular label binding is
no longer needed, the LSR may deallocate the label associated with
the binding, and destroy the binding. In the case where an ATM-LSR
receives such notification and destroys the binding, it should notify
the next hop for the route that the label binding is no longer
needed. If a LSR 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 label bindings are re-established from the
point where the route diverges from the previous route. LSRs
upstream of that point are (with one exception, noted below)
oblivious to the change. Whenever a LSR changes its next hop for a
particular route, if the new next hop is an ATM-LSR or a member of
the edge set reachable via a LC-ATM interface, then for each entry in
its LIB associated with the route the LSR should request (via LDP) a
binding from the new next hop.
When an ATM-LSR receives a label binding from a downstream neighbor,
it may already have provided a corresponding label binding for this
route to an upstream neighbor, either because it is operating
optimistically or because the new binding from downstream is the
result of a routing change. In this case, it should extract the hop
count from the new binding and increment it by one. If the new hop
count is different from that which was previously conveyed to the
upstream neighbor (including the case where the upstream neighbor was
given the value `unknown') the ATM-LSR must notify the upstream
neighbor of the change. Each ATM-LSR in turn increments the hop count
and passes it upstream until it reaches the ingress Edge LSR. If at
any point the value of the hop count equals MAX_HOP_COUNT, the ATM-
LSR should withdraw the binding from the upstream neighbor.
Whenever an ATM-LSR originates a label binding request to its next
hop LSR as a result of receiving a label binding request from another
(upstream) LSR, and the request to the next hop LSR is not satisfied,
the ATM-LSR should destroy the binding created in response to the
received request, and notify the requester (via LDP).
If an ATM-LSR 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.
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When a LSR determines that it has lost its LDP session with another
LSR, the following actions are taken. Any binding information
learned via this connection must be discarded. For any label
bindings that were created as a result of receiving label binding
requests from the peer, the LSR may destroy these bindings (and
deallocate labels associated with these binding).
An ATM-LSR should use `split-horizon' when it satisfies binding
requests from its neighbors. That is, if it receives a request for a
binding to a particular route and the LSR making that request is,
according to this ATM-LSR, the next hop for that route, it should not
return a binding for that route.
Note that it is not possible for complete label-switched looping
paths to be established when VC merge is not supported. The worst
case behavior (possible only if optimistic mode is used) is that
binding request messages could travel in a loop, setting up a label
switched spiral, which would then be torn down when the hop count in
the binding request reached MAX_HOP_COUNT. In conservative mode, no
label switched path can be set up unless the path exits the ATM-LSR
cloud.
7.3. VC-merge-capable ATM Switches
Relatively minor changes are needed to accommodate ATM-LSRs which
support VC-merge. The primary difference is that a VC-merge-capable
ATM-LSR needs only one outgoing label per route, even if multiple
requests for label bindings to that route are received from upstream
neighbors.
When a VC-merge-capable ATM-LSR receives a binding request from an
upstream LSR for a certain route, and it does not already have an
outgoing label binding for that route, it issues a bind request to
its next hop just as before. If, however, it already has an outgoing
label binding for that route, it does not need to issue a downstream
binding request. Instead, it creates a new LIB entry, allocates an
incoming label for that entry and returns that label in a binding to
the upstream requester, and uses the existing outgoing label for the
outgoing label entry in the LIB. It also takes the hop count that was
provided with the label binding it received from downstream,
increments it by one, and uses this value in the binding that it
sends to the upstream requester.
Note that, just like conventional ATM-LSRs and members of the edge
set of the ATM-LSR cloud, a VC-merge-capable ATM-LSR must issue a new
binding every time it receives a request from upstream, since there
may be switches upstream which do not support VC-merge. However, it
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only needs to issue a corresponding binding request downstream if it
does not already have a label binding for the appropriate route.
When a change in the routing table of a VC-merge-capable ATM-LSR
causes it to select a new next hop for one of its routes, it releases
the binding for that route from the former next hop and requests a
new binding from the new next hop. If the new binding contains a hop
count that differs from that which was received in the old binding,
then the ATM-LSR must take the new hop count, increment it by one,
and notify any upstream neighbors who have label bindings for this
route of the new value. Just as with conventional ATM-LSRs, this
enables the new hop count to propagate back towards the ingress of
the ATM-LSR cloud. If at any point the hop count reaches
MAX_HOP_COUNT, then the label bindings for this route must be
withdrawn from all upstream neighbors to whom a binding was
previously provided. This ensures that any loops caused by routing
transients will be detected and broken.
While the choice between "conservative" and "optimistic" binding
remains for VC-merge capable LSRs, the advantages of the conservative
approach are greater in this case. This is because:
- the optimistic mode permits the formation of a looping switched
path which will not be removed until routing changes to remove the
loop;
- the conservative mode does not have the same drawbacks when VC
merge is supported, since it will often be possible to obtain a
binding by merging into an existing path, without waiting for
binding requests and responses to propagate across the entire
ATM-LSR cloud.
The use of conservative mode along with hop counts in the binding
requests and responses ensures that stable looping paths cannot be
set up. Implementations may choose to support the diffusion
algorithm described in [1] for stronger protection against transient
loops.
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7.4. Efficient use of label space
The above discussion assumes that an edge LSR will request one label
for each prefix in its routing table that has a next hop in the ATM-
LSR cloud. In fact, it is possible to significantly reduce the number
of labels needed by having the edge LSR request instead one label for
several routes. Use of many-to-one mappings between routes (address
prefixes) and labels using the notion of Forwarding Equivalence
Classes (as described in [1]) provides a mechanism to conserve the
number of labels.
8. Encapsulation
By default, all labelled packets should be transmitted with the
generic label encapsulation, as defined in [3]. A packet thus
encapsulated is then directly encapsulated within an AAL5 frame.
Since the value at the top of the label stack is determined from the
VCI and/or VPI fields, the top label in the label encapsulation is
ignored. Other fields in the top stack entry, such as TTL and COS,
may be used by LSRs at the edges of the ATM-LSR cloud that are able
to examine that entry.
For systems which are using only one level of labelling, it is
theoretically possible to use a null encapsulation. In this case, IP
packets are carried directly inside AAL5 frames, as in the null
encapsulation of RFC 1483. However, all ATM-LSRs in a cloud must be
configured to use this encapsulation to avoid reassembly and re-
encapsulation in the middle of the cloud.
The initial LDP 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.
LDP may be used to advertise additional VPI/VCIs to carry control
information or non-labelled 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.
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9. Security Considerations
Security considerations are not addressed in this document.
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 under openly
specified and non-discriminatory terms, for no fee.
11. References
[1] Rosen, E. et al. A Proposed Architecture for MPLS, Internet
Draft, draft-ietf-mpls-arch-00.txt, August, 1997
[2] Doolan, P. et al. Tag Distribution Protocol, Internet Draft,
draft-doolan-tdp-spec-01.txt, May, 1997.
[3] Rosen, E. et al. Label Switching: Label Stack Encodings, Internet
Draft, draft-rosen-tag-stack-03.txt, July, 1997.
[4] Feldman, N., and Viswanathan, A. ARIS Specification, Internet
Draft, draft-feldman-aris-spec-00.txt, March, 1997.
12. 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.
13. Authors' Addresses
Bruce Davie
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: bsd@cisco.com
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Paul Doolan
Ennovate Networks Inc.
330 Codman Hill Rd
Boxborough, MA 01719
E-mail: pdoolan@ennovatenetworks.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
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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