draft-ietf-mpls-atm-04.txt   rfc3035.txt 
Network Working Group Bruce Davie
Internet Draft Jeremy Lawrence
Expiration Date: December 2000 Keith McCloghrie
Yakov Rekhter
Eric Rosen
George Swallow
Cisco Systems, Inc.
Paul Doolan Network Working Group B. Davie
Request for Comments: 3035 J. Lawrence
Category: Standards Track K. McCloghrie
E. Rosen
G. Swallow
Cisco Systems, Inc.
Y. Rekhter
Juniper Networks
P. Doolan
Ennovate Networks, Inc. Ennovate Networks, Inc.
January 2001
June 2000
MPLS using LDP and ATM VC Switching MPLS using LDP and ATM VC Switching
draft-ietf-mpls-atm-04.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document specifies an Internet standards track protocol for the
all provisions of Section 10 of RFC2026. Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Internet-Drafts are working documents of the Internet Engineering Official Protocol Standards" (STD 1) for the standardization state
Task Force (IETF), its areas, and its working groups. Note that and status of this protocol. Distribution of this memo is unlimited.
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract Abstract
The MPLS Architecture [1] discusses a way in which ATM switches may The Multiprotocol Label Switching (MPLS) Architecture [1] discusses a
be used as Label Switching Routers. The ATM switches run network way in which Asynchronous Transfer Mode (ATM) switches may be used as
layer routing algorithms (such as OSPF, IS-IS, etc.), and their data Label Switching Routers. The ATM switches run network layer routing
forwarding is based on the results of these routing algorithms. No algorithms (such as Open Shortest Path First (OSPF), Intermediate
System to Intermediate System (IS-IS), etc.), and their data
forwarding is based on the results of these routing algorithms. No
ATM-specific routing or addressing is needed. ATM switches used in ATM-specific routing or addressing is needed. ATM switches used in
this way are known as ATM-LSRs. this way are known as ATM-LSRs (Label Switching Routers).
This document extends and clarifies the relevant portions of [1] and This document extends and clarifies the relevant portions of [1] and
[2] by specifying in more detail the procedures which to be used when [2] by specifying in more detail the procedures which to be used when
distributing labels to or from ATM-LSRs, when those labels represent distributing labels to or from ATM-LSRs, when those labels represent
Forwarding Equivalence Classes (FECs, see [1]) for which the routes Forwarding Equivalence Classes (FECs, see [1]) for which the routes
are determined on a hop-by-hop basis by network layer routing are determined on a hop-by-hop basis by network layer routing
algorithms. algorithms.
This document also specifies the MPLS encapsulation to be used when This document also specifies the MPLS encapsulation to be used when
sending labeled packets to or from ATM-LSRs, and in that respect is a sending labeled packets to or from ATM-LSRs, and in that respect is a
companion document to [3]. companion document to [3].
Contents Table of Contents
1 Introduction ........................................... 3 1 Introduction ........................................... 2
2 Specification of Requirements .......................... 4 2 Specification of Requirements .......................... 3
3 Definitions ............................................ 4 3 Definitions ............................................ 3
4 Special Characteristics of ATM Switches ................ 5 4 Special Characteristics of ATM Switches ................ 4
5 Label Switching Control Component for ATM .............. 6 5 Label Switching Control Component for ATM .............. 5
6 Hybrid Switches (Ships in the Night) ................... 7 6 Hybrid Switches (Ships in the Night) ................... 5
7 Use of VPI/VCIs ....................................... 7 7 Use of VPI/VCIs ....................................... 5
7.1 Direct Connections ..................................... 8 7.1 Direct Connections ..................................... 6
7.2 Connections via an ATM VP .............................. 8 7.2 Connections via an ATM VP .............................. 7
7.3 Connections via an ATM SVC ............................. 9 7.3 Connections via an ATM SVC ............................. 7
8 Label Distribution and Maintenance Procedures .......... 9 8 Label Distribution and Maintenance Procedures .......... 7
8.1 Edge LSR Behavior ...................................... 9 8.1 Edge LSR Behavior ...................................... 8
8.2 Conventional ATM Switches (non-VC-merge) ............... 10 8.2 Conventional ATM Switches (non-VC-merge) ............... 9
8.3 VC-merge-capable ATM Switches .......................... 13 8.3 VC-merge-capable ATM Switches .......................... 11
9 Encapsulation .......................................... 14 9 Encapsulation .......................................... 12
10 TTL Manipulation ....................................... 15 10 TTL Manipulation ....................................... 13
11 Optional Loop Detection: Distributing Path Vectors ..... 16 11 Optional Loop Detection: Distributing Path Vectors ..... 15
11.1 When to Send Path Vectors Downstream ................... 16 11.1 When to Send Path Vectors Downstream ................... 15
11.2 When to Send Path Vectors Upstream ..................... 17 11.2 When to Send Path Vectors Upstream ..................... 16
12 Security Considerations ................................ 18 12 Security Considerations ................................ 17
13 Intellectual Property Considerations ................... 19 13 Intellectual Property Considerations ................... 17
14 References ............................................. 19 14 References ............................................. 18
15 Acknowledgments ........................................ 20 15 Acknowledgments ........................................ 18
16 Authors' Addresses ..................................... 20 16 Authors' Addresses ..................................... 18
17 Full Copyright Statement ............................... 21 17 Full Copyright Statement ............................... 20
1. Introduction 1. Introduction
The MPLS Architecture [1] discusses the way in which ATM switches may The MPLS Architecture [1] discusses the way in which ATM switches may
be used as Label Switching Routers. The ATM switches run network be used as Label Switching Routers. The ATM switches run network
layer routing algorithms (such as OSPF, IS-IS, etc.), and their data layer routing algorithms (such as OSPF, IS-IS, etc.), and their data
forwarding is based on the results of these routing algorithms. No forwarding is based on the results of these routing algorithms. No
ATM-specific routing or addressing is needed. ATM switches used in ATM-specific routing or addressing is needed. ATM switches used in
this way are known as ATM-LSRs. this way are known as ATM-LSRs.
skipping to change at page 4, line 12 skipping to change at page 3, line 8
the routes are determined on a hop-by-hop basis by network layer the routes are determined on a hop-by-hop basis by network layer
routing algorithms. The label distribution technique described here routing algorithms. The label distribution technique described here
is referred to in [1] as "downstream-on-demand". This label is referred to in [1] as "downstream-on-demand". This label
distribution technique MUST be used by ATM-LSRs that are not capable distribution technique MUST be used by ATM-LSRs that are not capable
of "VC merge" (defined in section 3), and is OPTIONAL for ATM-LSRs of "VC merge" (defined in section 3), and is OPTIONAL for ATM-LSRs
that are capable of VC merge. that are capable of VC merge.
This document does NOT specify the label distribution techniques to This document does NOT specify the label distribution techniques to
be used in the following cases: be used in the following cases:
- the routes are explicitly chosen before label distribution - the routes are explicitly chosen before label distribution
begins, instead of being chosen on a hop-by-hop basis as label begins, instead of being chosen on a hop-by-hop basis as label
distribution proceeds, distribution proceeds,
- the routes are intended to diverge in any way from the routes - the routes are intended to diverge in any way from the routes
chosen by the conventional hop-by-hop routing at any time, chosen by the conventional hop-by-hop routing at any time,
- the labels represent FECs that consist of multicast packets, - the labels represent FECs that consist of multicast packets,
- the LSRs use "VP merge". - the LSRs use "VP merge".
Further statements made in this document about ATM-LSR label Further statements made in this document about ATM-LSR label
distribution do not necessarily apply in these cases. distribution do not necessarily apply in these cases.
This document also specifies the MPLS encapsulation to be used when This document also specifies the MPLS encapsulation to be used when
sending labeled packets to or from ATM-LSRs, and in that respect is a sending labeled packets to or from ATM-LSRs, and in that respect is a
companion document to [3]. The specified encapsulation is to be used companion document to [3]. The specified encapsulation is to be used
for multicast or explicitly routed labeled packets as well. for multicast or explicitly routed labeled packets as well.
This document uses terminology from [1]. This document uses terminology from [1].
skipping to change at page 4, line 45 skipping to change at page 3, line 41
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. document are to be interpreted as described in RFC 2119.
3. Definitions 3. Definitions
A Label Switching Router (LSR) is a device which implements the label A Label Switching Router (LSR) is a device which implements the label
switching control and forwarding components described in [1]. switching control and forwarding components described in [1].
A label switching controlled ATM (LC-ATM) interface is an ATM A label switching controlled ATM (LC-ATM) interface is an ATM
interface controlled by the label switching control component. When a interface controlled by the label switching control component. When
packet traversing such an interface is received, it is treated as a a packet traversing such an interface is received, it is treated as a
labeled packet. The packet's top label is inferred either from the labeled packet. The packet's top label is inferred either from the
contents of the VCI field or the combined contents of the VPI and VCI contents of the VCI field or the combined contents of the VPI and VCI
fields. Any two LDP peers which are connected via an LC-ATM fields. Any two LDP peers which are connected via an LC-ATM
interface will use LDP negotiations to determine which of these cases interface will use LDP negotiations to determine which of these cases
is applicable to that interface. is applicable to that interface.
An ATM-LSR is a LSR with a number of LC-ATM interfaces which forwards 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 or cells between these interfaces, using labels carried in the VCI or
VPI/VCI field, without reassembling the cells into frames before VPI/VCI field, without reassembling the cells into frames before
forwarding. forwarding.
A frame-based LSR is a LSR which forwards complete frames between its 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. Note that such a LSR may have zero, one or more LC-ATM
interfaces. interfaces.
Sometimes a single box may behave as an ATM-LSR with respect to Sometimes a single box may behave as an ATM-LSR with respect to
certain pairs of interfaces, but may behave as a frame-based LSR with certain pairs of interfaces, but may behave as a frame-based LSR with
respect to other pairs. For example, an ATM switch with an ethernet respect to other pairs. For example, an ATM switch with an ethernet
interface may function as an ATM-LSR when forwarding cells between interface may function as an ATM-LSR when forwarding cells between
its LC-ATM interfaces, but may function as a frame-based LSR when its LC-ATM interfaces, but may function as a frame-based LSR when
forwarding frames from its ethernet to one of its LC-ATM interfaces. forwarding frames from its ethernet to one of its LC-ATM interfaces.
In such cases, one can consider the two functions (ATM-LSR and In such cases, one can consider the two functions (ATM-LSR and
frame-based LSR) as being coresident in a single box. frame-based LSR) as being coresident in a single box.
skipping to change at page 5, line 47 skipping to change at page 4, line 40
VC-merge is the process by which a switch receives cells on several VC-merge is the process by which a switch receives cells on several
incoming VCIs and transmits them on a single outgoing VCI without incoming VCIs and transmits them on a single outgoing VCI without
causing the cells of different AAL5 PDUs to become interleaved. causing the cells of different AAL5 PDUs to become interleaved.
4. Special Characteristics of ATM Switches 4. Special Characteristics of ATM Switches
While the MPLS architecture permits considerable flexibility in LSR While the MPLS architecture permits considerable flexibility in LSR
implementation, an ATM-LSR is constrained by the capabilities of the implementation, an ATM-LSR is constrained by the capabilities of the
(possibly pre-existing) hardware and the restrictions on such matters (possibly pre-existing) hardware and the restrictions on such matters
as cell format imposed by ATM standards. Because of these as cell format imposed by ATM standards. Because of these
constraints, some special procedures are required for ATM-LSRs. constraints, some special procedures are required for ATM-LSRs.
Some of the key features of ATM switches that affect their behavior Some of the key features of ATM switches that affect their behavior
as LSRs are: as LSRs are:
- the label swapping function is performed on fields (the VCI - the label swapping function is performed on fields (the VCI
and/or VPI) in the cell header; this dictates the size and and/or VPI) in the cell header; this dictates the size and
placement of the label(s) in a packet. placement of the label(s) in a packet.
- multipoint-to-point and multipoint-to-multipoint VCs are - multipoint-to-point and multipoint-to-multipoint VCs are
generally not supported. This means that most switches cannot generally not supported. This means that most switches cannot
support 'VC-merge' as defined above. support 'VC-merge' as defined above.
- there is generally no capability to perform a 'TTL-decrement' - there is generally no capability to perform a 'TTL-decrement'
function as is performed on IP headers in routers. function as is performed on IP headers in routers.
This document describes ways of applying label switching to ATM This document describes ways of applying label switching to ATM
switches which work within these constraints. switches which work within these constraints.
5. Label Switching Control Component for ATM 5. Label Switching Control Component for ATM
To support label switching an ATM switch MUST implement the control To support label switching an ATM switch MUST implement the control
component of label switching. This consists primarily of label component of label switching. This consists primarily of label
allocation, distribution, and maintenance procedures. Label binding allocation, distribution, and maintenance procedures. Label binding
information is communicated by several mechanisms, notably the Label information is communicated by several mechanisms, notably the Label
Distribution Protocol (LDP) [2]. This document imposes certain Distribution Protocol (LDP) [2]. This document imposes certain
requirements on the LDP. requirements on the LDP.
This document considers only the case where the label switching This document considers only the case where the label switching
control component uses information learned directly from network control component uses information learned directly from network
layer routing protocols. It is presupposed that the switch layer routing protocols. It is presupposed that the switch
participates as a peer in these protocols (e.g., OSPF, IS-IS). participates 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) In some cases, LSRs make use of other protocols (e.g., RSVP, PIM,
to distribute label bindings. In these cases, an ATM-LSR would need BGP) to distribute label bindings. In these cases, an ATM-LSR would
to participate in these protocols. However, these are not explicitly need to participate in these protocols. However, these are not
considered in this document. explicitly considered in this document.
Support of label switching on an ATM switch does NOT require the Support of label switching on an ATM switch does NOT require the
switch to support the ATM control component defined by the ITU and 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 ATM Forum (e.g., UNI, PNNI). An ATM-LSR may OPTIONALLY respond to
cells. OAM cells.
6. Hybrid Switches (Ships in the Night) 6. Hybrid Switches (Ships in the Night)
The existence of the label switching control component on an ATM The existence of the label switching control component on an ATM
switch does not preclude the ability to support the ATM control 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 component defined by the ITU and ATM Forum on the same switch and the
same interfaces. The two control components, label switching and the same interfaces. The two control components, label switching and the
ITU/ATM Forum defined, would operate independently. ITU/ATM Forum defined, would operate independently.
Definition of how such a device operates is beyond the scope of this Definition of how such a device operates is beyond the scope of this
skipping to change at page 9, line 27 skipping to change at page 7, line 51
received packet would then be inferred (via a one-to-one mapping) received packet would then be inferred (via a one-to-one mapping)
from the virtual circuit on which the packet arrived. There would from the virtual circuit on which the packet arrived. There would
not be a default VPI or VCI value for the non-MPLS connection. not be a default VPI or VCI value for the non-MPLS connection.
8. Label Distribution and Maintenance Procedures 8. Label Distribution and Maintenance Procedures
This document discusses the use of "downstream-on-demand" label This document discusses the use of "downstream-on-demand" label
distribution (see [1]) by ATM-LSRs. These label distribution distribution (see [1]) by ATM-LSRs. These label distribution
procedures MUST be used by ATM-LSRs that do not support VC-merge, and procedures MUST be used by ATM-LSRs that do not support VC-merge, and
MAY also be used by ATM-LSRs that do support VC-merge. The MAY also be used by ATM-LSRs that do support VC-merge. The
procedures differ somewhat in the two cases, however. We therefore procedures differ somewhat in the two cases, however. We therefore
describe the two scenarios in turn. We begin by describing the describe the two scenarios in turn. We begin by describing the
behavior of members of the Edge Set of an ATM-LSR domain; these "Edge behavior of members of the Edge Set of an ATM-LSR domain; these "Edge
LSRs" are not themselves ATM-LSRs, and their behavior is the same LSRs" are not themselves ATM-LSRs, and their behavior is the same
whether the domain contains VC-merge capable LSRs or not. whether the domain contains VC-merge capable LSRs or not.
8.1. Edge LSR Behavior 8.1. Edge LSR Behavior
Consider a member of the Edge Set of an ATM-LSR domain. Assume that, Consider a member of the Edge Set of an ATM-LSR domain. Assume that,
as a result of its routing calculations, it selects an ATM-LSR as the as a result of its routing calculations, it selects an ATM-LSR as the
next hop of a certain FEC, and that the next hop is reachable via a next hop of a certain FEC, and that the next hop is reachable via a
LC-ATM interface. The Edge LSR uses LDP to request a label binding LC-ATM interface. The Edge LSR uses LDP to request a label binding
for that FEC from the next hop. The hop count field in the request for that FEC from the next hop. The hop count field in the request
is set to 1 (but see the next paragraph). Once the Edge LSR receives is set to 1 (but see the next paragraph). Once the Edge LSR receives
the label binding information, it may use MPLS forwarding procedures the label binding information, it may use MPLS forwarding procedures
to transmit packets in the specified FEC, using the specified label to transmit packets in the specified FEC, using the specified label
as an outgoing label. (Or using the VPI/VCI that corresponds to the as an outgoing label. (Or using the VPI/VCI that corresponds to the
specified VCID as the outgoing label, if the VCID technique of [4] is specified VCID as the outgoing label, if the VCID technique of [4] is
being used.) being used.)
Note: if the Edge LSR's previous hop is using downstream-on-demand Note: if the Edge LSR's previous hop is using downstream-on-demand
label distribution to request a label from the Edge LSR for a label distribution to request a label from the Edge LSR for a
particular FEC, and if the Edge LSR is not merging the LSP from that particular FEC, and if the Edge LSR is not merging the LSP from that
previous hop with any other LSP, and if the request from the previous previous hop with any other LSP, and if the request from the previous
hop has a hop count of h, then the hop count in the request issued by hop has a hop count of h, then the hop count in the request issued by
the Edge LSR should not be set to 1, but rather to h+1. the Edge LSR should not be set to 1, but rather to h+1.
The binding received by the edge LSR may contain a hop count, which 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 represents the number of hops a packet will take to cross the ATM-LSR
domain when using this label. If there is a hop count associated with domain when using this label. If there is a hop count associated
the binding, the ATM-LSR SHOULD adjust a data packet's TTL by this with the binding, the ATM-LSR SHOULD adjust a data packet's TTL by
amount before transmitting the packet. In any event, it MUST adjust this amount before transmitting the packet. In any event, it MUST
a data packet's TTL by at least one before transmitting it. The adjust a data packet's TTL by at least one before transmitting it.
procedures for doing so (in the case of IP packets) are specified in The procedures for doing so (in the case of IP packets) are specified
section 10. The procedures for encapsulating the packets are in section 10. The procedures for encapsulating the packets are
specified in section 9. specified in section 9.
When a member of the Edge Set of the ATM-LSR domain receives a label When a member of the Edge Set of the ATM-LSR domain receives a label
binding request from an ATM-LSR, it allocates a label, and returns binding request from an ATM-LSR, it allocates a label, and returns
(via LDP) a binding containing the allocated label back to the peer (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 that originated the request. It sets the hop count in the binding to
1. 1.
When a routing calculation causes an Edge LSR to change the next hop When a routing calculation causes an Edge LSR to change the next hop
for a particular FEC, and the former next hop was in the ATM-LSR for a particular FEC, and the former next hop was in the ATM-LSR
domain, the Edge LSR SHOULD notify the former next hop (via LDP) that domain, the Edge LSR SHOULD notify the former next hop (via LDP) that
the label binding associated with the FEC is no longer needed. the label binding associated with the FEC is no longer needed.
8.2. Conventional ATM Switches (non-VC-merge) 8.2. Conventional ATM Switches (non-VC-merge)
When an ATM-LSR receives (via LDP) a label binding request for a When an ATM-LSR receives (via LDP) a label binding request for a
certain FEC from a peer connected to the ATM-LSR over a LC-ATM certain FEC from a peer connected to the ATM-LSR over a LC-ATM
interface, the ATM-LSR takes the following actions: interface, the ATM-LSR takes the following actions:
- it allocates a label, - it allocates a label,
- it requests (via LDP) a label binding from the next hop for that - it requests (via LDP) a label binding from the next hop for
FEC; that FEC;
- it returns (via LDP) a binding containing the allocated incoming - it returns (via LDP) a binding containing the allocated
label back to the peer that originated the request. incoming label back to the peer that originated the request.
For purposes of this procedure, we define a maximum hop count value For purposes of this procedure, we define a maximum hop count value
MAXHOP. MAXHOP has a default value of 255, but may be configured to MAXHOP. MAXHOP has a default value of 255, but may be configured to
a different value. a different value.
The hop count field in the request that the ATM-LSR sends (to the The hop count field in the request that the ATM-LSR sends (to the
next hop LSR) MUST be set to one more than the hop count field in the next hop LSR) MUST be set to one more than the hop count field in the
request that it received from the upstream LSR. If the resulting hop request that it received from the upstream LSR. If the resulting hop
count exceeds MAXHOP, the request MUST NOT be sent to the next hop, count exceeds MAXHOP, the request MUST NOT be sent to the next hop,
and the ATM-LSR MUST notify the upstream neighbor that its binding and the ATM-LSR MUST notify the upstream neighbor that its binding
skipping to change at page 11, line 27 skipping to change at page 9, line 49
from downstream and MUST include the result in the binding it returns from downstream and MUST include the result in the binding it returns
upstream. However, if the hop count exceeds MAXHOP, a label binding upstream. However, if the hop count exceeds MAXHOP, a label binding
MUST NOT be passed upstream. Rather, the upstream LDP peer MUST be MUST NOT be passed upstream. Rather, the upstream LDP peer MUST be
informed that the requested label binding cannot be satisfied. If informed that the requested label binding cannot be satisfied. If
the hop count received from downstream is 0, the hop count passed the hop count received from downstream is 0, the hop count passed
upstream should also be 0; this indicates that the actual hop count upstream should also be 0; this indicates that the actual hop count
is unknown. is unknown.
Alternatively, the ATM-LSR MAY return the binding upstream without Alternatively, the ATM-LSR MAY return the binding upstream without
waiting for a binding from downstream ("independent" control, as waiting for a binding from downstream ("independent" control, as
defined in [1] and [2]). In this case, it specifies a hop count of 0 defined in [1] and [2]). In this case, it specifies a hop count of 0
in the binding, indicating that the true hop count is unknown. The in the binding, indicating that the true hop count is unknown. The
correct value for hop count will be returned later, as described correct value for hop count will be returned later, as described
below. below.
Note that an ATM-LSR, or a member of the edge set of an ATM-LSR Note that an ATM-LSR, or a member of the edge set of an ATM-LSR
domain, may receive multiple binding requests for the same FEC from domain, may receive multiple binding requests for the same FEC from
the same ATM-LSR. It MUST generate a new binding for each request the same ATM-LSR. It MUST generate a new binding for each request
(assuming adequate resources to do so), and retain any existing (assuming adequate resources to do so), and retain any existing
binding(s). For each request received, an ATM-LSR MUST also generate binding(s). For each request received, an ATM-LSR MUST also generate
a new binding request toward the next hop for the FEC. a new binding request toward the next hop for the FEC.
When a routing calculation causes an ATM-LSR to change the next hop When a routing calculation causes an ATM-LSR to change the next hop
for a FEC, the ATM-LSR MUST notify the former next hop (via LDP) that for a FEC, the ATM-LSR MUST notify the former next hop (via LDP) that
the label binding associated with the FEC is no longer needed. the label binding associated with the FEC is no longer needed.
When a LSR receives a notification that a particular label binding is When a LSR receives a notification that a particular label binding is
no longer needed, the LSR MAY deallocate the label associated with no longer needed, the LSR MAY deallocate the label associated with
the binding, and destroy the binding. In the case where an ATM-LSR the binding, and destroy the binding. In the case where an ATM-LSR
receives such notification and destroys the binding, it MUST notify receives such notification and destroys the binding, it MUST notify
the next hop for the FEC that the label binding is no longer needed. the next hop for the FEC that the label binding is no longer needed.
If a LSR does not destroy the binding, it may re-use the binding only If a LSR does not destroy the binding, it may re-use the binding only
if it receives a request for the same FEC with the same hop count as if it receives a request for the same FEC with the same hop count as
the request that originally caused the binding to be created. the request that originally caused the binding to be created.
When a route changes, the label bindings are re-established from the When a route changes, the label bindings are re-established from the
point where the route diverges from the previous route. LSRs point where the route diverges from the previous route. LSRs
upstream of that point are (with one exception, noted below) upstream of that point are (with one exception, noted below)
oblivious to the change. oblivious to the change.
Whenever a LSR changes its next hop for a particular FEC, if the new Whenever a LSR changes its next hop for a particular FEC, if the new
next hop is reachable via an LC-ATM interface, then for each label next hop is reachable via an LC-ATM interface, then for each label
that it has bound to that FEC, and distributed upstream, it MUST that it has bound to that FEC, and distributed upstream, it MUST
request a new label binding from the new next hop. request a new label binding from the new next hop.
When an ATM-LSR receives a label binding for a particular FEC from a When an ATM-LSR receives a label binding for a particular FEC from a
downstream neighbor, it may already have provided a corresponding downstream neighbor, it may already have provided a corresponding
label binding for this FEC to an upstream neighbor, either because it label binding for this FEC to an upstream neighbor, either because it
is using independent control, or because the new binding from is using independent control, or because the new binding from
downstream is the result of a routing change. In this case, unless downstream is the result of a routing change. In this case, unless
the hop count is 0, it MUST extract the hop count from the new the hop count is 0, it MUST extract the hop count from the new
binding and increment it by one. If the new hop count is different binding and increment it by one. If the new hop count is different
from that which was previously conveyed to the upstream neighbor from that which was previously conveyed to the upstream neighbor
(including the case where the upstream neighbor was given the value (including the case where the upstream neighbor was given the value
'unknown') the ATM-LSR MUST notify the upstream neighbor of the 'unknown') the ATM-LSR MUST notify the upstream neighbor of the
change. Each ATM-LSR in turn MUST increment the hop count and pass it change. Each ATM-LSR in turn MUST increment the hop count and pass
upstream until it reaches the ingress Edge LSR. If at any point the it upstream until it reaches the ingress Edge LSR. If at any point
value of the hop count equals MAXHOP, the ATM-LSR SHOULD withdraw the the value of the hop count equals MAXHOP, the ATM-LSR SHOULD withdraw
binding from the upstream neighbor. A hop count of 0 MUST be passed the binding from the upstream neighbor. A hop count of 0 MUST be
upstream unchanged. passed upstream unchanged.
Whenever an ATM-LSR originates a label binding request to its next 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 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, (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 the ATM-LSR SHOULD destroy the binding created in response to the
received request, and notify the requester (via LDP). received request, and notify the requester (via LDP).
If an ATM-LSR receives a binding request containing a hop count that If an ATM-LSR receives a binding request containing a hop count that
exceeds MAXHOP, it MUST not establish a binding, and it MUST return exceeds MAXHOP, it MUST not establish a binding, and it MUST return
an error to the requester. an error to the requester.
When a LSR determines that it has lost its LDP session with another When a LSR determines that it has lost its LDP session with another
LSR, the following actions are taken. Any binding information LSR, the following actions are taken. Any binding information
learned via this connection MUST be discarded. For any label learned via this connection MUST be discarded. For any label
bindings that were created as a result of receiving label binding bindings that were created as a result of receiving label binding
requests from the peer, the LSR MAY destroy these bindings (and requests from the peer, the LSR MAY destroy these bindings (and
deallocate labels associated with these binding). deallocate labels associated with these binding).
An ATM-LSR SHOULD use 'split-horizon' when it satisfies 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 requests from its neighbors. That is, if it receives a request for a
binding to a particular FEC and the LSR making that request is, binding to a particular FEC and the LSR making that request is,
according to this ATM-LSR, the next hop for that FEC, it should not according to this ATM-LSR, the next hop for that FEC, it should not
return a binding for that route. return a binding for that route.
It is expected that non-merging ATM-LSRs would generally use It is expected that non-merging ATM-LSRs would generally use
"conservative label retention mode" [1]. "conservative label retention mode" [1].
8.3. VC-merge-capable ATM Switches 8.3. VC-merge-capable ATM Switches
Relatively minor changes are needed to accommodate ATM-LSRs which Relatively minor changes are needed to accommodate ATM-LSRs which
support VC-merge. The primary difference is that a VC-merge-capable support VC-merge. The primary difference is that a VC-merge-capable
ATM-LSR needs only one outgoing label per FEC, even if multiple ATM-LSR needs only one outgoing label per FEC, even if multiple
requests for label bindings to that FEC are received from upstream requests for label bindings to that FEC are received from upstream
neighbors. neighbors.
When a VC-merge-capable ATM-LSR receives a binding request from an When a VC-merge-capable ATM-LSR receives a binding request from an
upstream LSR for a certain FEC, and it does not already have an upstream LSR for a certain FEC, and it does not already have an
outgoing label binding for that FEC (or an outstanding request for outgoing label binding for that FEC (or an outstanding request for
such a label binding), it MUST issue a bind request to its next hop such a label binding), it MUST issue a bind request to its next hop
just as it would do if it were not merge-capable. If, however, it just as it would do if it were not merge-capable. If, however, it
already has an outgoing label binding for that FEC, it does not need already has an outgoing label binding for that FEC, it does not need
to issue a downstream binding request. Instead, it may simply to issue a downstream binding request. Instead, it may simply
allocate an incoming label, and return that label in a binding to the allocate an incoming label, and return that label in a binding to the
upstream requester. When packets with that label as top label are upstream requester. When packets with that label as top label are
received from the requester, the top label value will be replaced received from the requester, the top label value will be replaced
with the existing outgoing label value that corresponds to the same with the existing outgoing label value that corresponds to the same
FEC. FEC.
If the ATM-LSR does not have an outgoing label binding for the FEC, If the ATM-LSR does not have an outgoing label binding for the FEC,
but does have an outstanding request for one, it need not issue but does have an outstanding request for one, it need not issue
another request. another request.
skipping to change at page 14, line 12 skipping to change at page 12, line 36
causes it to select a new next hop for one of its FECs, it MAY causes it to select a new next hop for one of its FECs, it MAY
optionally release the binding for that route from the former next optionally release the binding for that route from the former next
hop. If it doesn't already have a corresponding binding for the new hop. If it doesn't already have a corresponding binding for the new
next hop, it must request one. (The choice between conservative and next hop, it must request one. (The choice between conservative and
liberal label retention mode [1] is an implementation option.) liberal label retention mode [1] is an implementation option.)
If a new binding is obtained, which contains a hop count that differs If a new binding is obtained, which contains a hop count that differs
from that which was received in the old binding, then the ATM-LSR 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 must take the new hop count, increment it by one, and notify any
upstream neighbors who have label bindings for this FEC of the new upstream neighbors who have label bindings for this FEC of the new
value. Just as with conventional ATM-LSRs, this enables the new hop value. Just as with conventional ATM-LSRs, this enables the new hop
count to propagate back towards the ingress of the ATM-LSR domain. If count to propagate back towards the ingress of the ATM-LSR domain.
at any point the hop count exceeds MAXHOP, then the label bindings If at any point the hop count exceeds MAXHOP, then the label bindings
for this route must be withdrawn from all upstream neighbors to whom for this route must be withdrawn from all upstream neighbors to whom
a binding was previously provided. This ensures that any loops caused a binding was previously provided. This ensures that any loops
by routing transients will be detected and broken. caused by routing transients will be detected and broken.
9. Encapsulation 9. Encapsulation
The procedures described in this section affect only the Edge LSRs of The procedures described in this section affect only the Edge LSRs of
the ATM-LSR domain. The ATM-LSRs themselves do not modify the the ATM-LSR domain. The ATM-LSRs themselves do not modify the
encapsulation in any way. encapsulation in any way.
Labeled packets MUST be transmitted using the null encapsulation of Labeled packets MUST be transmitted using the null encapsulation of
Section 6.1 of RFC 2684 [5]. Section 6.1 of RFC 2684 [5].
skipping to change at page 15, line 5 skipping to change at page 13, line 27
Note that if a packet has a label stack with only one entry, this Note that if a packet has a label stack with only one entry, this
requires it to have a single-entry shim (4 bytes), even though the requires it to have a single-entry shim (4 bytes), even though the
actual label value is encoded into the VPI/VCI field. This is done actual label value is encoded into the VPI/VCI field. This is done
to ensure that the packet always has a shim. Otherwise, there would to ensure that the packet always has a shim. Otherwise, there would
be no way to determine whether it had one or not, i.e., no way to be no way to determine whether it had one or not, i.e., no way to
determine whether there are additional label stack entries. determine whether there are additional label stack entries.
The only ways to eliminate this extra overhead are: The only ways to eliminate this extra overhead are:
- through apriori knowledge that packets have only a single label - through apriori knowledge that packets have only a single label
(e.g., perhaps the network only supports one level of label) (e.g., perhaps the network only supports one level of label)
- by using two VCs per FEC, one for those packets which have only a - by using two VCs per FEC, one for those packets which have only
single label, and one for those packets which have more than one a single label, and one for those packets which have more than
label one label
The second technique would require that there be some way of The second technique would require that there be some way of
signalling via LDP that the VC is carrying only packets with a single signalling via LDP that the VC is carrying only packets with a single
label, and is not carrying a shim. When supporting VC merge, one label, and is not carrying a shim. When supporting VC merge, one
would also have to take care not to merge a VC on which the shim is would also have to take care not to merge a VC on which the shim is
not used into a VC on which it is used, or vice versa. not used into a VC on which it is used, or vice versa.
While either of these techniques is permitted, it is doubtful that While either of these techniques is permitted, it is doubtful that
they have any practical utility. Note that if the shim header is not they have any practical utility. Note that if the shim header is not
present, the outgoing TTL is carried in the TTL field of the network present, the outgoing TTL is carried in the TTL field of the network
layer header. layer header.
10. TTL Manipulation 10. TTL Manipulation
skipping to change at page 15, line 48 skipping to change at page 14, line 24
used when the packet is forwarded, the "outgoing TTL" is set to the used when the packet is forwarded, the "outgoing TTL" is set to the
larger of (a) 0 or (b) the difference between the incoming TTL and larger of (a) 0 or (b) the difference between the incoming TTL and
the hop count. If a hop count has not been associated with the label the hop count. If a hop count has not been associated with the label
binding that is used when the packet is forwarded, the "outgoing TTL" binding that is used when the packet is forwarded, the "outgoing TTL"
is set to the larger of (a) 0 or (b) one less than the incoming TTL. is set to the larger of (a) 0 or (b) one less than the incoming TTL.
If this causes the outgoing TTL to become zero, the packet MUST NOT If this causes the outgoing TTL to become zero, the packet MUST NOT
be transmitted as a labeled packet using the specified label. The be transmitted as a labeled packet using the specified label. The
packet can be treated in one of two ways: packet can be treated in one of two ways:
- it may be treated as having expired; this may cause an ICMP - it may be treated as having expired; this may cause an ICMP
message to be transmitted; message to be transmitted;
- the packet may be forwarded, as an unlabeled packet, with a TTL - the packet may be forwarded, as an unlabeled packet, with a TTL
that is 1 less than the incoming TTL; such forwarding would need that is 1 less than the incoming TTL; such forwarding would
to be done over a non-MPLS connection. need to be done over a non-MPLS connection.
Of course, if the incoming TTL is 1, only the first of these two Of course, if the incoming TTL is 1, only the first of these two
options is applicable. options is applicable.
If the packet is forwarded as a labeled packet, the outgoing TTL is If the packet is forwarded as a labeled packet, the outgoing TTL is
carried as specified in section 9. carried as specified in section 9.
When an Edge LSR receives a labeled packet over an LC-ATM interface, When an Edge LSR receives a labeled packet over an LC-ATM interface,
it obtains the incoming TTL from the top label stack entry of the it obtains the incoming TTL from the top label stack entry of the
generic encapsulation, or, if that encapsulation is not present, from generic encapsulation, or, if that encapsulation is not present, from
skipping to change at page 16, line 44 skipping to change at page 15, line 22
loops are detected by the hop count mechanism previously described. loops are detected by the hop count mechanism previously described.
If this option is enabled, loops will be detected more quickly, but If this option is enabled, loops will be detected more quickly, but
at a higher cost in overhead. at a higher cost in overhead.
11.1. When to Send Path Vectors Downstream 11.1. When to Send Path Vectors Downstream
Suppose an LSR R sends a request for a label binding, for a Suppose an LSR R sends a request for a label binding, for a
particular LSP, to its next hop. Then if R does not support VC- particular LSP, to its next hop. Then if R does not support VC-
merging, and R is configured to use the LDPV procedure: merging, and R is configured to use the LDPV procedure:
- If R is sending the request because it is an ingress node for - If R is sending the request because it is an ingress node for
that LSP, or because it has acquired a new next hop, then R MUST that LSP, or because it has acquired a new next hop, then R
include a path vector object with the request, and the path MUST include a path vector object with the request, and the
vector object MUST contain only R's own address. path vector object MUST contain only R's own address.
- If R is sending the request as a result of having received a - If R is sending the request as a result of having received a
request from an upstream LSR, then: request from an upstream LSR, then:
* if the received request has a path vector object, R MUST add * if the received request has a path vector object, R MUST add
its own address to the received path vector object, and MUST its own address to the received path vector object, and MUST
pass the resulting path vector object to its next hop along pass the resulting path vector object to its next hop along
with the label binding request; with the label binding request;
* if the received request does not have a path vector object, R * if the received request does not have a path vector object,
MUST include a path vector object with the request it sends, R MUST include a path vector object with the request it
and the path vector object MUST contain only R's own address. sends, and the path vector object MUST contain only R's own
address.
An LSR which supports VC-merge SHOULD NOT include a path vector An LSR which supports VC-merge SHOULD NOT include a path vector
object in the requests that it sends to its next hop. object in the requests that it sends to its next hop.
If an LSR receives a label binding request whose path vector object If an LSR receives a label binding request whose path vector object
contains the address of the node itself, the LSR concludes that the contains the address of the node itself, the LSR concludes that the
label binding requests have traveled in a loop. The LSR MUST act as label binding requests have traveled in a loop. The LSR MUST act as
it would in the case where the hop count exceeds MAXHOP (see section it would in the case where the hop count exceeds MAXHOP (see section
8.2). 8.2).
This procedure detects the case where the request messages loop This procedure detects the case where the request messages loop
though a sequence of non-merging ATM-LSRs. though a sequence of non-merging ATM-LSRs.
11.2. When to Send Path Vectors Upstream 11.2. When to Send Path Vectors Upstream
As specified in section 8, there are circumstances in which an LSR R As specified in section 8, there are circumstances in which an LSR R
must inform its upstream neighbors, via a label binding response must inform its upstream neighbors, via a label binding response
message, of a change in hop count for a particular LSP. If the message, of a change in hop count for a particular LSP. If the
following conditions all hold: following conditions all hold:
- R is configured for the LDPV procedure, - R is configured for the LDPV procedure,
- R supports VC-merge, - R supports VC-merge,
- R is not the egress for that LSP, and - R is not the egress for that LSP, and
- R is not informing its neighbors of a decrease in the hop count, - R is not informing its neighbors of a decrease in the hop
count,
then R MUST include a path vector object in the response message. then R MUST include a path vector object in the response message.
If the change in hop count is a result of R's having been informed by If the change in hop count is a result of R's having been informed by
its next hop, S, of a change in hop count, and the message from S to its next hop, S, of a change in hop count, and the message from S to
R included a path vector object, then if the above conditions hold, R R included a path vector object, then if the above conditions hold, R
MUST add itself to this object and pass the result upstream. MUST add itself to this object and pass the result upstream.
Otherwise, if the above conditions hold, R MUST create a new object Otherwise, if the above conditions hold, R MUST create a new object
with only its own address. with only its own address.
skipping to change at page 19, line 34 skipping to change at page 18, line 13
be obtained from the IETF Secretariat. be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
14. References 14. References
[1] Rosen E., Viswanathan, A., Callon R., "Multi-Protocol Label [1] Rosen, E., Viswanathan, A. and R. Callon "Multi-Protocol Label
Switching Architecture", Work in Progress, August 1999. Switching Architecture", RFC 3031, January 2001.
[2] Andersson L., Doolan P., Feldman N., Fredette A., Thomas R., "LDP [2] Andersson L., Doolan P., Feldman N., Fredette A. and R. Thomas,
Specification", Work in Progress, June 2000. "LDP Specification", RFC 3036, January 2001.
[3] Rosen, E., Rekhter, Y., Tappan, D., Farinacci, D., Fedorkow, G., [3] Rosen, E., Rekhter, Y., Tappan, D., Farinacci, D., Fedorkow, G.,
Li, T., Conta, A., "MPLS Label Stack Encoding", Work in Progress, Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032,
September 1999. January 2001.
[4] Nagami, K., Demizu N., Esaki H., Doolan P., "VCID Notification [4] Nagami, K., Demizu N., Esaki H. and P. Doolan, "VCID Notification
over ATM link", Work in Progress, July 1999. over ATM Link for LDP", RFC 3038, January 2001.
[5] Grossman, D., Heinanen, J., "Multiprotocol Encapsulation over ATM [5] Grossman, D., Heinanen, J., "Multiprotocol Encapsulation over ATM
Adaptation Layer 5", RFC 2684, September 1999 Adaptation Layer 5", RFC 2684, September 1999.
15. Acknowledgments 15. Acknowledgments
Significant contributions to this work have been made by Anthony Significant contributions to this work have been made by Anthony
Alles, Fred Baker, Dino Farinacci, Guy Fedorkow, Arthur Lin, Morgan Alles, Fred Baker, Dino Farinacci, Guy Fedorkow, Arthur Lin, Morgan
Littlewood and Dan Tappan. We thank Alex Conta for his comments. Littlewood and Dan Tappan. We thank Alex Conta for his comments.
16. Authors' Addresses 16. Authors' Addresses
Bruce Davie Bruce Davie
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Drive 250 Apollo Drive
Chelmsford, MA, 01824 Chelmsford, MA, 01824
E-mail: bsd@cisco.com EMail: bsd@cisco.com
Paul Doolan Paul Doolan
Ennovate Networks Inc. Ennovate Networks Inc.
330 Codman Hill Rd 60 Codman Hill Rd
Boxborough, MA 01719 Boxborough, MA 01719
E-mail: pdoolan@ennovatenetworks.com EMail: pdoolan@ennovatenetworks.com
Jeremy Lawrence Jeremy Lawrence
Cisco Systems, Inc. Cisco Systems, Inc.
99 Walker St. 99 Walker St.
North Sydney, NSW, Australia North Sydney, NSW, Australia
E-mail: jlawrenc@cisco.com EMail: jlawrenc@cisco.com
Keith McCloghrie Keith McCloghrie
Cisco Systems, Inc. Cisco Systems, Inc.
170 Tasman Drive 170 Tasman Drive
San Jose, CA, 95134 San Jose, CA, 95134
E-mail: kzm@cisco.com EMail: kzm@cisco.com
Yakov Rekhter Yakov Rekhter
Cisco Systems, Inc. Juniper Networks
170 Tasman Drive 1194 N. Mathilda Avenue
San Jose, CA, 95134 Sunnyvale, CA 94089
EMail: yakov@juniper.net
E-mail: yakov@cisco.com
Eric Rosen Eric Rosen
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Drive 250 Apollo Drive
Chelmsford, MA, 01824 Chelmsford, MA, 01824
E-mail: erosen@cisco.com EMail: erosen@cisco.com
George Swallow George Swallow
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Drive 250 Apollo Drive
Chelmsford, MA, 01824 Chelmsford, MA, 01824
E-mail: swallow@cisco.com EMail: swallow@cisco.com
17. Full Copyright Statement 17. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and or assist in its implementation may be prepared, copied, published
distributed, in whole or in part, without restriction of any kind, and distributed, in whole or in part, without restriction of any
provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than followed, or as required to translate it into languages other than
English. English.
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
 End of changes. 76 change blocks. 
180 lines changed or deleted 169 lines changed or added

This html diff was produced by rfcdiff 1.41. The latest version is available from http://tools.ietf.org/tools/rfcdiff/