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.
January 2001
MPLS using LDP and ATM VC Switching
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
The Multiprotocol Label Switching (MPLS) Architecture [1] discusses a
way in which Asynchronous Transfer Mode (ATM) switches may be used as
Label Switching Routers. The ATM switches run network layer routing
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
this way are known as ATM-LSRs (Label Switching Routers).
This document extends and clarifies the relevant portions of [1] and
[2] by specifying in more detail the procedures which to be used when
distributing labels to or from ATM-LSRs, when those labels represent
Forwarding Equivalence Classes (FECs, see [1]) for which the routes
are determined on a hop-by-hop basis by network layer routing
algorithms.
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
companion document to [3].
Table of Contents
1 Introduction ........................................... 2
2 Specification of Requirements .......................... 3
3 Definitions ............................................ 3
4 Special Characteristics of ATM Switches ................ 4
5 Label Switching Control Component for ATM .............. 5
6 Hybrid Switches (Ships in the Night) ................... 5
7 Use of VPI/VCIs ....................................... 5
7.1 Direct Connections ..................................... 6
7.2 Connections via an ATM VP .............................. 7
7.3 Connections via an ATM SVC ............................. 7
8 Label Distribution and Maintenance Procedures .......... 7
8.1 Edge LSR Behavior ...................................... 8
8.2 Conventional ATM Switches (non-VC-merge) ............... 9
8.3 VC-merge-capable ATM Switches .......................... 11
9 Encapsulation .......................................... 12
10 TTL Manipulation ....................................... 13
11 Optional Loop Detection: Distributing Path Vectors ..... 15
11.1 When to Send Path Vectors Downstream ................... 15
11.2 When to Send Path Vectors Upstream ..................... 16
12 Security Considerations ................................ 17
13 Intellectual Property Considerations ................... 17
14 References ............................................. 18
15 Acknowledgments ........................................ 18
16 Authors' Addresses ..................................... 18
17 Full Copyright Statement ............................... 20
1. Introduction
The MPLS Architecture [1] discusses the way in which ATM switches may
be used 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. ATM switches used in
this way are known as ATM-LSRs.
This document extends and clarifies the relevant portions of [1] and
[2] by specifying in more detail the procedures which are to be used
for distributing labels to or from ATM-LSRs, when those labels
represent Forwarding Equivalence Classes (FECs, see [1]) for which
the routes are determined on a hop-by-hop basis by network layer
routing algorithms. The label distribution technique described here
is referred to in [1] as "downstream-on-demand". This label
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
that are capable of VC merge.
This document does NOT specify the label distribution techniques to
be used in the following cases:
- the routes are eXPlicitly chosen before label distribution
begins, instead of being chosen on a hop-by-hop basis as label
distribution proceeds,
- the routes are intended to diverge in any way from the routes
chosen by the conventional hop-by-hop routing at any time,
- the labels represent FECs that consist of multicast packets,
- the LSRs use "VP merge".
Further statements made in this document about ATM-LSR label
distribution do not necessarily apply in these cases.
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
companion document to [3]. The specified encapsulation is to be used
for multicast or explicitly routed labeled packets as well.
This document uses terminology from [1].
2. Specification of Requirements
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119.
3. 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. When
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
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
interface will use LDP negotiations to determine which of these cases
is applicable to that interface.
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
VPI/VCI field, without reassembling the cells into frames before
forwarding.
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.
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
respect to other pairs. For example, an ATM switch with an ethernet
interface may function as an ATM-LSR when forwarding cells between
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.
In such cases, one can consider the two functions (ATM-LSR and
frame-based LSR) as being coresident in a single box.
It is intended that an LC-ATM interface be used to connect two ATM-
LSRs, or to connect an ATM-LSR to a frame-based LSR. The use of an
LC-ATM interface to connect two frame-based LSRs is not considered in
this document.
An ATM-LSR domain is a set of ATM-LSRs which are mutually
interconnected by LC-ATM interfaces.
The Edge Set of an ATM-LSR domain is the set of frame-based LSRs
which are connected to members of the domain by LC-ATM interfaces. A
frame-based LSR which is a member of an Edge Set of an ATM-LSR domain
may be called an Edge LSR.
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.
4. Special Characteristics of ATM Switches
While the MPLS 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 affect their behavior
as LSRs are:
- 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.
5. 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, distribution, and maintenance procedures. Label binding
information is communicated by several mechanisms, notably the Label
Distribution Protocol (LDP) [2]. This document imposes certain
requirements on the LDP.
This document considers only the case where the label switching
control component uses information learned directly from network
layer routing protocols. It is presupposed that the switch
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) to distribute label bindings. In these cases, an ATM-LSR would
need to participate in these protocols. However, these are not
explicitly considered in this document.
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.
6. 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.
7. Use of VPI/VCIs
Label switching is accomplished by associating labels with Forwarding
Equivalence Classes, 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/VCI field, or, when two ATM-LSRs are connected via an ATM
"Virtual Path", in the VCI field.
Labeled packets MUST be transmitted using the null encapsulation, as
defined in Section 6.1 of RFC2684 [5].
In addition, if two LDP peers are connected via an LC-ATM interface,
a non-MPLS connection, capable of carrying unlabelled IP packets,
MUST be available. This non-MPLS connection is used to carry LDP
packets between the two peers, and MAY also be used (but is not
required to be used) for other unlabeled packets (such as OSPF
packets, etc.). The LLC/SNAP encapsulation of RFC2684 [5] MUST be
used on the non-MPLS connection.
It SHOULD be possible to configure an LC-ATM interface with
additional VPI/VCIs that are used to carry control information or
non-labelled packets. In that case, the VCI values MUST NOT be in
the 0-32 range. These may use either the null encapsulation, as
defined in Section 6.1 of RFC2684 [5], or the LLC/SNAP
encapsulation, as defined in Section 5.1 of RFC2684 [5].
7.1. Direct Connections
We say that two LSRs are "directly connected" over an LC-ATM
interface if all cells transmitted out that interface by one LSR will
reach the other, and there are no ATM switches between the two LSRs.
When two LSRs are directly connected via an LC-ATM interface, they
jointly control the allocation of VPIs/VCIs on the interface
connecting them. They may agree to use the VPI/VCI field to encode a
single label.
The default VPI/VCI value for the non-MPLS connection is VPI 0, VCI
32. Other values can be configured, as long as both parties are
aware of the configured value.
A VPI/VCI value whose VCI part is in the range 0-32 inclusive MUST
NOT be used as the encoding of a label.
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 VCIs are communicated through LDP.
7.2. Connections via an ATM VP
Sometimes it can be useful to treat two LSRs as adjacent (in some
LSP) across an LC-ATM interface, even though the connection between
them is made through an ATM "cloud" via an ATM Virtual Path. In this
case, the VPI field is not available to MPLS, and the label MUST be
encoded entirely within the VCI field.
In this case, the default VCI value of the non-MPLS connection
between the LSRs is 32. Other values can be configured, as long as
both parties are aware of the configured value. The VPI is set to
whatever is required to make use of the Virtual Path.
A VPI/VCI value whose VCI part is in the range 0-32 inclusive MUST
NOT be used as the encoding of a label.
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 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.
7.3. Connections via an ATM SVC
Sometimes it may be useful to treat two LSRs as adjacent (in some
LSP) across an LC-ATM interface, even though the connection between
them is made through an ATM "cloud" via a set of ATM Switched Virtual
Circuits.
The current document does not specify the procedure for handling this
case. Such procedures can be found in [4]. The procedures described
in [4] allow a VCID to be assigned to each such VC, and specify how
LDP can be used used to bind a VCID to a FEC. The top label of a
received packet would then be inferred (via a one-to-one mapping)
from the virtual circuit on which the packet arrived. There would
not be a default VPI or VCI value for the non-MPLS connection.
8. Label Distribution and Maintenance Procedures
This document discusses the use of "downstream-on-demand" label
distribution (see [1]) by ATM-LSRs. These label distribution
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
procedures differ somewhat in the two cases, however. We therefore
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
LSRs" are not themselves ATM-LSRs, and their behavior is the same
whether the domain contains VC-merge capable LSRs or not.
8.1. Edge LSR Behavior
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
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
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
the label binding information, it may use MPLS forwarding procedures
to transmit packets in the specified FEC, using the specified label
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
being used.)
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
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
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 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
domain when using this label. If there is a hop count associated
with the binding, the ATM-LSR SHOULD adjust a data packet's TTL by
this amount before transmitting the packet. In any event, it MUST
adjust a data packet's TTL by at least one before transmitting it.
The procedures for doing so (in the case of IP packets) are specified
in section 10. The procedures for encapsulating the packets are
specified in section 9.
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
(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 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
the label binding associated with the FEC is no longer needed.
8.2. Conventional ATM Switches (non-VC-merge)
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
interface, the ATM-LSR takes the following actions:
- it allocates a label,
- it requests (via LDP) a label binding from the next hop for
that FEC;
- it returns (via LDP) a binding containing the allocated
incoming label back to the peer that originated the request.
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
a different value.
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
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,
and the ATM-LSR MUST notify the upstream neighbor that its binding
request cannot be satisfied.
Otherwise, once the ATM-LSR receives the binding from the next hop,
it begins using that label.
The ATM-LSR MAY choose to wait for the request to be satisfied from
downstream before returning the binding upstream. This is a form of
"ordered control" (as defined in [1] and [2]), in particular
"ingress-initiated ordered control". In this case, as long as the
ATM-LSR receives from downstream a hop count which is greater than 0
and less than MAXHOP, it MUST increment the hop count it receives
from downstream and MUST include the result in the binding it returns
upstream. However, if the hop count exceeds MAXHOP, a label binding
MUST NOT be passed upstream. Rather, the upstream LDP peer MUST be
informed that the requested label binding cannot be satisfied. If
the hop count received from downstream is 0, the hop count passed
upstream should also be 0; this indicates that the actual hop count
is unknown.
Alternatively, the ATM-LSR MAY return the binding upstream without
waiting for a binding from downstream ("independent" control, as
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
correct value for hop count will be returned later, as described
below.
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
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 MUST also generate
a new binding request toward the next hop for the FEC.
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
the label binding associated with the FEC 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 MUST notify
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 it receives a request for the same FEC 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 FEC, if the new
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
request a new label binding from the new next hop.
When an ATM-LSR receives a label binding for a particular FEC from a
downstream neighbor, it may already have provided a corresponding
label binding for this FEC to an upstream neighbor, either because it
is using independent control, or because the new binding from
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
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 MUST increment the hop count and pass
it upstream until it reaches the ingress Edge LSR. If at any point
the value of the hop count equals MAXHOP, the ATM-LSR SHOULD withdraw
the binding from the upstream neighbor. A hop count of 0 MUST be
passed upstream unchanged.
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
exceeds MAXHOP, it MUST not establish a binding, and it MUST return
an error to the requester.
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 FEC and the LSR making that request is,
according to this ATM-LSR, the next hop for that FEC, it should not
return a binding for that route.
It is expected that non-merging ATM-LSRs would generally use
"conservative label retention mode" [1].
8.3. 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 FEC, even if multiple
requests for label bindings to that FEC are received from upstream
neighbors.
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
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
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
to issue a downstream binding request. Instead, it may simply
allocate an incoming label, and return that label in a binding to the
upstream requester. When packets with that label as top label are
received from the requester, the top label value will be replaced
with the existing outgoing label value that corresponds to the same
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
another request.
When sending a label binding upstream, the hop count associated with
the corresponding binding from downstream MUST be incremented by 1,
and the result transmitted upstream as the hop count associated with
the new binding. However, there are two exceptions: a hop count of 0
MUST be passed upstream unchanged, and if the hop count is already at
MAXHOP, the ATM-LSR MUST NOT pass a binding upstream, but instead
MUST send an error upstream.
Note that, just like conventional ATM-LSRs and members of the edge
set of the ATM-LSR domain, 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 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 FECs, it MAY
optionally release the binding for that route from the former next
hop. If it doesn't already have a corresponding binding for the new
next hop, it must request one. (The choice between conservative and
liberal label retention mode [1] is an implementation option.)
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
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
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 at any point the hop count exceeds MAXHOP, 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.
9. Encapsulation
The procedures described in this section affect only the Edge LSRs of
the ATM-LSR domain. The ATM-LSRs themselves do not modify the
encapsulation in any way.
Labeled packets MUST be transmitted using the null encapsulation of
Section 6.1 of RFC2684 [5].
Except in certain circumstances specified below, when a labeled
packet is transmitted on an LC-ATM interface, where the VPI/VCI (or
VCID) is interpreted as the top label in the label stack, the packet
MUST also contain a "shim header" [3].
If the packet has a label stack with n entries, it MUST carry a shim
with n entries. The actual value of the top label is encoded in the
VPI/VCI field. The label value of the top entry in the shim (which
is just a "placeholder" entry) MUST be set to 0 upon transmission,
and MUST be ignored upon reception. The packet's outgoing TTL, and
its CoS, are carried in the TTL and CoS fields respectively of the
top stack entry in the shim.
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
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
be no way to determine whether it had one or not, i.e., no way to
determine whether there are additional label stack entries.
The only ways to eliminate this extra overhead are:
- through apriori knowledge that packets have only a single 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 single label, and one for those packets which have more than
one label
The second technique would require that there be some way of
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
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.
While either of these techniques is permitted, it is doubtful that
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
layer header.
10. TTL Manipulation
The procedures described in this section affect only the Edge LSRs of
the ATM-LSR domain. The ATM-LSRs themselves do not modify the TTL in
any way.
The details of the TTL adjustment procedure are as follows. If a
packet was received by the Edge LSR as an unlabeled packet, the
"incoming TTL" comes from the IP header. (Procedures for other
network layer protocols are for further study.) If a packet was
received by the Edge LSR as a labeled packet, using the encapsulation
specified in [3], the "incoming TTL" comes from the entry at the top
of the label stack.
If a hop count has been associated with the label binding that is
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
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"
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
be transmitted as a labeled packet using the specified label. The
packet can be treated in one of two ways:
- it may be treated as having expired; this may cause an ICMP
message to be transmitted;
- the packet may be forwarded, as an unlabeled packet, with a TTL
that is 1 less than the incoming TTL; such forwarding would
need to be done over a non-MPLS connection.
Of course, if the incoming TTL is 1, only the first of these two
options is applicable.
If the packet is forwarded as a labeled packet, the outgoing TTL is
carried as specified in section 9.
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
generic encapsulation, or, if that encapsulation is not present, from
the IP header.
If the packet's next hop is an ATM-LSR, the outgoing TTL is formed
using the procedures described in this section. Otherwise the
outgoing TTL is formed using the procedures described in [3].
The procedures in this section are intended to apply only to unicast
packets.
11. Optional Loop Detection: Distributing Path Vectors
Every ATM-LSR MUST implement, as a configurable option, the following
procedure for detecting forwarding loops. We refer to this as the
LDPV (Loop Detection via Path Vectors) procedure. This procedure
does not prevent the formation of forwarding loops, but does ensure
that any such loops are detected. If this option is not enabled,
loops are detected by the hop count mechanism previously described.
If this option is enabled, loops will be detected more quickly, but
at a higher cost in overhead.
11.1. When to Send Path Vectors Downstream
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-
merging, and R is configured to use the LDPV procedure:
- 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 include a path vector object with the request, and the
path vector object MUST contain only R's own address.
- If R is sending the request as a result of having received a
request from an upstream LSR, then:
* if the received request has a path vector object, R MUST add
its own address to the received path vector object, and MUST
pass the resulting path vector object to its next hop along
with the label binding request;
* if the received request does not have a path vector object,
R MUST include a path vector object with the request it
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
object in the requests that it sends to its next hop.
If an LSR receives a label binding request whose path vector object
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
it would in the case where the hop count exceeds MAXHOP (see section
8.2).
This procedure detects the case where the request messages loop
though a sequence of non-merging ATM-LSRs.
11.2. When to Send Path Vectors Upstream
As specified in section 8, there are circumstances in which an LSR R
must inform its upstream neighbors, via a label binding response
message, of a change in hop count for a particular LSP. If the
following conditions all hold:
- R is configured for the LDPV procedure,
- R supports VC-merge,
- R is not the egress for that LSP, and
- 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.
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
R included a path vector object, then if the above conditions hold, R
MUST add itself to this object and pass the result upstream.
Otherwise, if the above conditions hold, R MUST create a new object
with only its own address.
If R is configured for the LDPV procedure, and R supports VC merge,
then it MAY include a path vector object in any label binding
response message that it sends upstream. In particular, at any time
that R receives a label binding response from its next hop, if that
response contains a path vector, R MAY (if configured for the LDPV
procedure) send a response to its upstream neighbors, containing the
path vector object formed by adding its own address to the received
path vector.
If R does not support VC merge, it SHOULD NOT send a path vector
object upstream.
If an LSR receives a message from its next hop, with a path vector
object containing its own address, then LSR MUST act as it would if
it received a message with a hop count equal to MAXHOP.
LSRs which are configured for the LDPV procedure SHOULD NOT store a
path vector once the corresponding path vector object has been
transmitted.
Note that if the ATM-LSR domain consists entirely of non-merging
ATM-LSRs, path vectors need not ever be sent upstream, since any
loops will be detected by means of the path vectors traveling
downstream.
By not sending path vectors unless the hop count increases, one
avoids sending them in many situations when there is no loop. The
cost is that in some situations in which there is a loop, the time to
detect the loop may be lengthened.
12. Security Considerations
The encapsulation and procedures specified in this document do not
interfere in any way with the application of authentication and/or
encryption to network layer packets (such as the application of IPSEC
to IP datagrams).
The procedures described in this document do not protect against the
alteration (either accidental or malicious) of MPLS labels. Such
alteration could cause misforwarding.
The procedures described in this document do not enable a receiving
LSR to authenticate the transmitting LSR.
A discussion of the security considerations applicable to the label
distribution mechanism can be found in [2].
13. Intellectual Property Considerations
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
14. References
[1] Rosen, E., Viswanathan, A. and R. Callon "Multi-Protocol Label
Switching Architecture", RFC3031, January 2001.
[2] Andersson L., Doolan P., Feldman N., Fredette A. and R. Thomas,
"LDP Specification", RFC3036, January 2001.
[3] Rosen, E., Rekhter, Y., Tappan, D., Farinacci, D., Fedorkow, G.,
Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC3032,
January 2001.
[4] Nagami, K., Demizu N., Esaki H. and P. Doolan, "VCID Notification
over ATM Link for LDP", RFC3038, January 2001.
[5] Grossman, D., Heinanen, J., "Multiprotocol Encapsulation over ATM
Adaptation Layer 5", RFC2684, September 1999.
15. 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. We thank Alex Conta for his comments.
16. Authors' Addresses
Bruce Davie
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
EMail: bsd@cisco.com
Paul Doolan
Ennovate Networks Inc.
60 Codman Hill Rd
Boxborough, MA 01719
EMail: pdoolan@ennovatenetworks.com
Jeremy Lawrence
Cisco Systems, Inc.
99 Walker St.
North Sydney, NSW, Australia
EMail: jlawrenc@cisco.com
Keith McCloghrie
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
EMail: kzm@cisco.com
Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
EMail: yakov@juniper.net
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
EMail: erosen@cisco.com
George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
EMail: swallow@cisco.com
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