draft-ietf-mpls-fr-03.txt   draft-ietf-mpls-fr-04.txt 
MPLS Working Group A. Conta (Lucent) MPLS Working Group A. Conta (3COM)
INTERNET-DRAFT P. Doolan (Ennovate) INTERNET-DRAFT P. Doolan (Ennovate)
A. Malis (Ascend) A. Malis (Lucent)
16 November 1998 1 May 2000
Expires 1 November 2000
Use of Label Switching on Frame Relay Networks Use of Label Switching on Frame Relay Networks
Specification Specification
draft-ietf-mpls-fr-03.txt draft-ietf-mpls-fr-04.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft and is in full conformance
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Abstract Abstract
This document defines the model and generic mechanisms for This document defines the model and generic mechanisms for
Multiprotocol Label Switching on Frame Relay networks. Furthermore, Multiprotocol Label Switching on Frame Relay networks. Furthermore,
it extends and clarifies portions of the Multiprotocol Label it extends and clarifies portions of the Multiprotocol Label
Switching Architecture described in [ARCH] and the Label Distribution Switching Architecture described in [ARCH] and the Label Distribution
Protocol (LDP) described in [LDP] relative to Frame Relay Networks. Protocol (LDP) described in [LDP] relative to Frame Relay Networks.
MPLS enables the use of Frame Relay Switches as Label Switching MPLS enables the use of Frame Relay Switches as Label Switching
Routers (LSRs). Routers (LSRs).
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Table of Contents Table of Contents
Status of this Memo.........................................1 Status of this Memo.........................................1
Table of Contents...........................................2 Table of Contents...........................................2
1. Introduction................................................3 1. Introduction................................................3
2. Terminology.................................................3 2. Terminology.................................................3
3. Special Characteristics of Frame Relay Switches.............5 3. Special Characteristics of Frame Relay Switches.............5
4. Label Encapsulation.........................................5 4. Label Encapsulation.........................................5
5. Frame Relay Label Switching Processing......................7 5. Frame Relay Label Switching Processing......................7
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7 Label Allocation and Maintenance Procedures ...............16 7 Label Allocation and Maintenance Procedures ...............16
7.1 Edge LSR Behavior........................................16 7.1 Edge LSR Behavior........................................16
7.2 Efficient use of label space-Merging FR-LSRs.............19 7.2 Efficient use of label space-Merging FR-LSRs.............19
7.3 LDP message fields specific to Frame Relay...............20 7.3 LDP message fields specific to Frame Relay...............20
8 Security Considerations ..................................22 8 Security Considerations ..................................22
9 Acknowledgments ..........................................23 9 Acknowledgments ..........................................23
10 References ...............................................23 10 References ...............................................23
11 Authors' Addresses .......................................24 11 Authors' Addresses .......................................24
Appendix A - changes since previous versions..................25 Appendix A - changes since previous versions..................25
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1. Introduction 1. Introduction
The Multiprotocol Label Switching Architecture is described in The Multiprotocol Label Switching Architecture is described in
[ARCH]. It is possible to use Frame Relay switches as Label Switching [ARCH]. It is possible to use Frame Relay switches as Label Switching
Routers. Such Frame Relay switches run network layer routing Routers. Such Frame Relay switches run network layer routing
algorithms (such as OSPF, IS-IS, etc.), and their forwarding is based algorithms (such as OSPF, IS-IS, etc.), and their forwarding is based
on the results of these routing algorithms. No specific Frame Relay on the results of these routing algorithms. No specific Frame Relay
routing is needed. routing is needed.
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LC-FR LC-FR
A label switching controlled Frame Relay (LC-FR) interface is a A label switching controlled Frame Relay (LC-FR) interface is a
Frame Relay interface controlled by the label switching control Frame Relay interface controlled by the label switching control
component. Packets traversing such an interface carry labels in component. Packets traversing such an interface carry labels in
the DLCI field. the DLCI field.
FR-LSR FR-LSR
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A FR-LSR is an LSR with one or more LC-FR interfaces which A FR-LSR is an LSR with one or more LC-FR interfaces which
forwards frames between two such interfaces using labels carried forwards frames between two such interfaces using labels carried
in the DLCI field. in the DLCI field.
FR-LSR domain FR-LSR domain
A FR-LSR domain is a set of FR-LSRs, which are mutually A FR-LSR domain is a set of FR-LSRs, which are mutually
interconnected by LC-FR interfaces. interconnected by LC-FR interfaces.
Edge Set Edge Set
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The Input TTL is the MPLS TTL of the top of the stack when a The Input TTL is the MPLS TTL of the top of the stack when a
labeled packet is received on an LSR interface, or the network labeled packet is received on an LSR interface, or the network
layer (IP) TTL if the packet is not labeled. layer (IP) TTL if the packet is not labeled.
Output TTL Output TTL
The Output TTL is the MPLS TTL of the top of the stack when a The Output TTL is the MPLS TTL of the top of the stack when a
labeled packet is transmitted on an LSR interface, or the labeled packet is transmitted on an LSR interface, or the
network layer (IP) TTL if the packet is not labeled. network layer (IP) TTL if the packet is not labeled.
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Additionally, this document uses terminology from [ARCH]. Additionally, this document uses terminology from [ARCH].
3. Special characteristics of Frame Relay Switches 3. Special characteristics of Frame Relay Switches
While the label switching architecture permits considerable While the label switching architecture permits considerable
flexibility in LSR implementation, a FR-LSR is constrained by the flexibility in LSR implementation, a FR-LSR is constrained by the
capabilities of the (possibly pre-existing) hardware and the capabilities of the (possibly pre-existing) hardware and the
restrictions on such matters as frame format imposed by the restrictions on such matters as frame format imposed by the
Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay
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This document describes ways of applying label switching to Frame This document describes ways of applying label switching to Frame
Relay switches, which work within these constraints. Relay switches, which work within these constraints.
4. Label Encapsulation 4. Label Encapsulation
By default, all labeled packets should be transmitted with the By default, all labeled packets should be transmitted with the
generic label encapsulation as defined in [STACK], using the frame generic label encapsulation as defined in [STACK], using the frame
relay null encapsulation mechanism: relay null encapsulation mechanism:
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0 1 (Octets) 0 1 (Octets)
+-----------------------+-----------------------+ +-----------------------+-----------------------+
(Octets)0 | | (Octets)0 | |
/ Q.922 Address / / Q.922 Address /
/ (length 'n' equals 2 or 4) / / (length 'n' equals 2 or 4) /
| | | |
+-----------------------+-----------------------+ +-----------------------+-----------------------+
n | . | n | . |
/ . / / . /
/ MPLS packet / / MPLS packet /
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+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
1 | DLCI | 0 | 0 | 0 | 0 | 1 | DLCI | 0 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
2 | DLCI(low order) | 0 | 2 | DLCI(low order) | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
3 | unused (set to 0) | 1 | 1 | 3 | unused (set to 0) | 1 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
17 bits DLCI 17 bits DLCI
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7 6 5 4 3 2 1 0 (bit order) 7 6 5 4 3 2 1 0 (bit order)
+-----+-----+-----+-----+-----+-----+-----+-----00 +-----+-----+-----+-----+-----+-----+-----+-----00
(octet) 0 | DLCI(high order) | 0 | 0 | (octet) 0 | DLCI(high order) | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+----- +-----+-----+-----+-----+-----+-----+-----+-----
1 | DLCI | 0 | 0 | 0 | 0 | 1 | DLCI | 0 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
2 | DLCI | 0 | 2 | DLCI | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
3 | DLCI (low order) | 0 | 1 | 3 | DLCI (low order) | 0 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
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For two connected FR-LSRs, a full-duplex connection must be available For two connected FR-LSRs, a full-duplex connection must be available
for LDP. The DLCI for the LDP VC is assigned a value by way of for LDP. The DLCI for the LDP VC is assigned a value by way of
configuration, similar to configuring the DLCI used to run IP routing configuration, similar to configuring the DLCI used to run IP routing
protocols between the switches. protocols between the switches.
With the exception of this configured value, the DLCI values used for With the exception of this configured value, the DLCI values used for
MPLS in the two directions of the link may be treated as belonging to MPLS in the two directions of the link may be treated as belonging to
two independent spaces, i.e. VCs may be half-duplex, each direction two independent spaces, i.e. VCs may be half-duplex, each direction
with its own DLCI. with its own DLCI.
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The allowable ranges of DLCIs, the size of DLCIs, and the support for The allowable ranges of DLCIs, the size of DLCIs, and the support for
VC merging MUST be communicated through LDP messages. Note that the VC merging MUST be communicated through LDP messages. Note that the
range of DLCIs used for labels depends on the size of the DLCI field. range of DLCIs used for labels depends on the size of the DLCI field.
5.2 Homogeneous LSPs 5.2 Homogeneous LSPs
If <LSR1, LSR2, LSR3> is an LSP, it is possible that LSR1, LSR2, and If <LSR1, LSR2, LSR3> is an LSP, it is possible that LSR1, LSR2, and
LSR3 will use the same encoding of the label stack when transmitting LSR3 will use the same encoding of the label stack when transmitting
packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is
homogeneous. homogeneous.
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Such an LSR may swap off a Frame Relay encoded label on an incoming Such an LSR may swap off a Frame Relay encoded label on an incoming
interface and replace it with a label encoded into a Generic MPLS interface and replace it with a label encoded into a Generic MPLS
(MPLS shim) header on the outgoing interface. (MPLS shim) header on the outgoing interface.
5.4 Frame Relay Label Switching Loop Prevention and Control 5.4 Frame Relay Label Switching Loop Prevention and Control
FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay
segments. Therefore, FR-LSRs SHOULD use loop detection and MAY use segments. Therefore, FR-LSRs SHOULD use loop detection and MAY use
loop prevention mechanisms as described in [ARCH], and [LDP]. loop prevention mechanisms as described in [ARCH], and [LDP].
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5.4.1 FR-LSRs Loop Control - MPLS TTL processing 5.4.1 FR-LSRs Loop Control - MPLS TTL processing
The MPLS TTL encoded in the MPLS label stack is a mechanism used to: The MPLS TTL encoded in the MPLS label stack is a mechanism used to:
(a) suppress loops; (a) suppress loops;
(b) limit the scope of a packet. (b) limit the scope of a packet.
When a packet travels along an LSP, it should emerge with the same When a packet travels along an LSP, it should emerge with the same
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message to its source. message to its source.
- it forwards the packet, as an unlabeled packet, with a TTL - it forwards the packet, as an unlabeled packet, with a TTL
that reflects the IP (network layer) forwarding. that reflects the IP (network layer) forwarding.
If the incoming TTL is 1, only the first option applies. If the incoming TTL is 1, only the first option applies.
In the multicast case, a meaningful LSP length or LSP segment length In the multicast case, a meaningful LSP length or LSP segment length
is propagated to the FR-LSR egress node, enabling the egress to is propagated to the FR-LSR egress node, enabling the egress to
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decrement the TTL value before forwarding packets out of the non-TTL decrement the TTL value before forwarding packets out of the non-TTL
LSP segment. LSP segment.
5.4.2 Performing MPLS TTL calculations 5.4.2 Performing MPLS TTL calculations
The calculation applied to the "input TTL" that yields the "output The calculation applied to the "input TTL" that yields the "output
TTL" depends on (i)the "input encapsulation", (ii)the "forwarding TTL" depends on (i)the "input encapsulation", (ii)the "forwarding
encapsulation", and (iii)the "output encapsulation". The encapsulation", and (iii)the "output encapsulation". The
relationship among (i),(ii), and (iii), can be defined as a function relationship among (i),(ii), and (iii), can be defined as a function
"D" of "input encapsulation" (ie), "forwarding encapsulation" (fe), "D" of "input encapsulation" (ie), "forwarding encapsulation" (fe),
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The "number of hops of the LSP segment" is the value of the "hop The "number of hops of the LSP segment" is the value of the "hop
count" that is attached with the label used when the packet is count" that is attached with the label used when the packet is
forwarded, if LDP [LDP] has provided such a "hop count" value when it forwarded, if LDP [LDP] has provided such a "hop count" value when it
distributed the label for the LSP, that is the LDP message had a "hop distributed the label for the LSP, that is the LDP message had a "hop
count object". If LDP didn't provide a "hop count", or it provided an count object". If LDP didn't provide a "hop count", or it provided an
"unknown" value, the default value of the "number of hops of the "unknown" value, the default value of the "number of hops of the
segment" is 1. segment" is 1.
When sending a label binding upstream, the "hop count" associated When sending a label binding upstream, the "hop count" associated
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with the corresponding binding from downstream, if different than the with the corresponding binding from downstream, if different than the
"unknown" value, MUST be incremented by 1, and the result transmitted "unknown" value, MUST be incremented by 1, and the result transmitted
upstream as the hop count associated with the new binding (the upstream as the hop count associated with the new binding (the
"unknown" value is transmitted unchanged). If the new "hop count" "unknown" value is transmitted unchanged). If the new "hop count"
value exceeds the "maximum" value, the FR-LSR MUST NOT pass the value exceeds the "maximum" value, the FR-LSR MUST NOT pass the
binding upstream, but instead MUST send an error upstream binding upstream, but instead MUST send an error upstream
[LDP][ARCH]. [LDP][ARCH].
For multicast transmission: For multicast transmission:
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--------------- ---------------
IP_ttl = n IP_ttl=mpls_ttl-1 = n-6 IP_ttl = n IP_ttl=mpls_ttl-1 = n-6
--------->iIf fIi---------> --------->iIf fIi--------->
| mpls_ttl = n-5 ^ | mpls_ttl = n-5 ^
| | | |
number of hops 1| Frame Relay |5 number of hops 1| Frame Relay |5
| | | |
V 2 3 4 | V 2 3 4 |
fFf--->fFf--->fFf--->fFf fFf--->fFf--->fFf--->fFf
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"iIf" is "ingress LSR" in Frame Relay LSP and "iIf" is "ingress LSR" in Frame Relay LSP and
calculates: mpls_ttl = IP_TTL - number of hops = n-5 calculates: mpls_ttl = IP_TTL - number of hops = n-5
"fIi" is "egress LSR" from Frame Relay LSP, and "fIi" is "egress LSR" from Frame Relay LSP, and
calculates: IP_ttl = mpls_ttl-1 = n-6 calculates: IP_ttl = mpls_ttl-1 = n-6
Heterogeneous LSP Heterogeneous LSP
----------------- -----------------
ingress LSR egress LSR ingress LSR egress LSR
IP_ttl = n IP_ttl = n - 15 IP_ttl = n IP_ttl = n - 15
links LAN PPP FR ATM PPP FR LAN links LAN PPP FR ATM PPP FR LAN
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(ingress LSR) 1 2 3 4 (ingress LSR) 1 2 3 4
x--->---+--->---+--->>--+-->>---x (egress LSR) x--->---+--->---+--->>--+-->>---x (egress LSR)
o.ttl=i.ttl-4 | 2 3 o.ttl=i.ttl-4 | 2 3
^ ^
hops 1| hops 1|
| |
x (ingress LSR) x (ingress LSR)
o.ttl=i.ttl-3 o.ttl=i.ttl-3
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Frame Relay Multicast -- TTL calculated at egress Frame Relay Multicast -- TTL calculated at egress
(egress LSR)x o.ttl=i.ttl-3 (egress LSR)x o.ttl=i.ttl-3
hops | hops |
^3 ^3
(ingress LSR) | o.ttl=i.ttl-4 (ingress LSR) | o.ttl=i.ttl-4
x--->---+--->---+--->---+--->---x (egress LSR) x--->---+--->---+--->---+--->---x (egress LSR)
1 2 3 4 1 2 3 4
5.5 Label Processing by Ingress FR-LSRs 5.5 Label Processing by Ingress FR-LSRs
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For multicast packets, the MPLS TTL SHOULD be decremented by 1. An For multicast packets, the MPLS TTL SHOULD be decremented by 1. An
LDP constructing the LSP SHOULD pass meaningful information to the LDP constructing the LSP SHOULD pass meaningful information to the
egress FR-LSR regarding the number of hops of the "non-TTL segment". egress FR-LSR regarding the number of hops of the "non-TTL segment".
Next, the MPLS encapsulated packet is passed down to the Frame Relay Next, the MPLS encapsulated packet is passed down to the Frame Relay
data link driver with the top label as output DLCI. The Frame Relay data link driver with the top label as output DLCI. The Frame Relay
frame carrying the MPLS encapsulated packet is forwarded onto the frame carrying the MPLS encapsulated packet is forwarded onto the
Frame Relay VC to the next LSR. Frame Relay VC to the next LSR.
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5.6 Label Processing by Core FR-LSRs 5.6 Label Processing by Core FR-LSRs
In a FR-LSR, the current (top) MPLS label is carried in the DLCI In a FR-LSR, the current (top) MPLS label is carried in the DLCI
field of the Frame Relay data link layer header of the frame. Just as field of the Frame Relay data link layer header of the frame. Just as
in conventional Frame Relay, for a frame arriving at an interface, in conventional Frame Relay, for a frame arriving at an interface,
the DLCI carried by the Frame Relay data link header is looked up in the DLCI carried by the Frame Relay data link header is looked up in
the DLCI Information Base, replaced with the correspondent output the DLCI Information Base, replaced with the correspondent output
DLCI, and transmitted on the outgoing interface (forwarded to the DLCI, and transmitted on the outgoing interface (forwarded to the
next hop node). next hop node).
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For unicast packets, the MPLS TTL SHOULD be decremented by one if the For unicast packets, the MPLS TTL SHOULD be decremented by one if the
output interface is a generic one, or with the number of hops of the output interface is a generic one, or with the number of hops of the
next ATM segment of the LSP (heterogeneous), if the output interface next ATM segment of the LSP (heterogeneous), if the output interface
is an ATM (non-TTL) interface. is an ATM (non-TTL) interface.
For multicast packets, the MPLS TTL SHOULD be decremented by the For multicast packets, the MPLS TTL SHOULD be decremented by the
number of hops of the FR segment being exited. An LDP constructing number of hops of the FR segment being exited. An LDP constructing
the LSP SHOULD pass meaningful information to the egress FR-LSR the LSP SHOULD pass meaningful information to the egress FR-LSR
regarding the number of hops of the FR "non-TTL segment". regarding the number of hops of the FR "non-TTL segment".
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6. Label Switching Control Component for Frame Relay 6. Label Switching Control Component for Frame Relay
To support label switching a Frame Relay Switch MUST implement the To support label switching a Frame Relay Switch MUST implement the
control component of label switching, which consists primarily of control component of label switching, which consists primarily of
label allocation and maintenance procedures. Label binding label allocation and maintenance procedures. Label binding
information MAY be communicated by several mechanisms, one of which information MAY be communicated by several mechanisms, one of which
is the Label Distribution Protocol (LDP) [LDP]. is the Label Distribution Protocol (LDP) [LDP].
Since the label switching control component uses information learned Since the label switching control component uses information learned
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communicated through LDP. communicated through LDP.
Support of label switching on a Frame Relay switch requires Support of label switching on a Frame Relay switch requires
conformance only to [FRF] (framing, bit-stuffing, headers, FCS) conformance only to [FRF] (framing, bit-stuffing, headers, FCS)
except for section 2.3 (PVC control signaling procedures, aka LMI). except for section 2.3 (PVC control signaling procedures, aka LMI).
Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC
signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or
SVCs when both are running on the same interface as MPLS, as SVCs when both are running on the same interface as MPLS, as
discussed in the next section. discussed in the next section.
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6.1 Hybrid Switches (Ships in the Night) 6.1 Hybrid Switches (Ships in the Night)
The existence of the label switching control component on a Frame The existence of the label switching control component on a Frame
Relay switch does not preclude the ability to support the Frame Relay Relay switch does not preclude the ability to support the Frame Relay
control component defined by the ITU and Frame Relay Forum on the control component defined by the ITU and Frame Relay Forum on the
same switch and the same interfaces (NICs). The two control same switch and the same interfaces (NICs). The two control
components, label switching and those defined by ITU/Frame Relay components, label switching and those defined by ITU/Frame Relay
Forum, would operate independently. Forum, would operate independently.
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represents the number of hops a packet will take to cross the FR-LSR represents the number of hops a packet will take to cross the FR-LSR
domain to the Egress FR-LSR when using this label. This information domain to the Egress FR-LSR when using this label. This information
may be stored for TTL calculation. Once this is done, the LSR may use may be stored for TTL calculation. Once this is done, the LSR may use
MPLS forwarding to transmit packets in that FEC. MPLS forwarding to transmit packets in that FEC.
When a member of the Edge Set of the FR-LSR domain receives an LDP When a member of the Edge Set of the FR-LSR domain receives an LDP
"request" message from a FR-LSR for a FEC, it means it is the "request" message from a FR-LSR for a FEC, it means it is the
Egress-FR-LSR. It allocates a label, creates a new entry in its Label Egress-FR-LSR. It allocates a label, creates a new entry in its Label
Information Base (LIB), places that label in the incoming label Information Base (LIB), places that label in the incoming label
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component of the entry, and returns (via LDP) a "mapping" message component of the entry, and returns (via LDP) a "mapping" message
containing the allocated label back upstream to the LDP peer that containing the allocated label back upstream to the LDP peer that
originated the request. The "mapping" message contains the "hop originated the request. The "mapping" message contains the "hop
count" object value set to 1. count" object value set to 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 route, and the former next hop was in the FR-LSR domain, the for a route, and the former next hop was in the FR-LSR domain, the
Edge LSR should notify the former next hop (via an LDP "release" Edge LSR should notify the former next hop (via an LDP "release"
message) that the label binding associated with the route is no message) that the label binding associated with the route is no
longer needed. longer needed.
skipping to change at line 778 skipping to change at line 781
Once the FR-LSR receives in response the label binding in an LDP Once the FR-LSR receives in response the label binding in an LDP
"mapping" message from the next hop, it places the label into the "mapping" message from the next hop, it places the label into the
outgoing label component of the LIB entry. outgoing label component of the LIB entry.
Note that a FR-LSR, or a member of the edge set of a FR-LSR domain, Note that a FR-LSR, or a member of the edge set of a FR-LSR domain,
may receive multiple binding requests for the same route (FEC) from may receive multiple binding requests for the same route (FEC) from
the same FR-LSR. It must generate a new "mapping" for each "request" the same FR-LSR. It must generate a new "mapping" for each "request"
(assuming adequate resources to do so), and retain any existing (assuming adequate resources to do so), and retain any existing
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mapping(s). For each "request" received, a FR-LSR should also mapping(s). For each "request" received, a FR-LSR should also
generate a new binding "request" toward the next hop for the route generate a new binding "request" toward the next hop for the route
(FEC). (FEC).
When a routing calculation causes a FR-LSR to change the next hop for When a routing calculation causes a FR-LSR to change the next hop for
a route (FEC), the FR-LSR should notify the former next hop (via an a route (FEC), the FR-LSR should notify the former next hop (via an
LDP "release" message) that the label binding associated with the LDP "release" message) that the label binding associated with the
route is no longer needed. route 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
skipping to change at line 828 skipping to change at line 831
neighbor of the change. Each FR-LSR in turn increments the hop count neighbor of the change. Each FR-LSR in turn increments the hop count
and passes it upstream until it reaches the ingress Edge LSR. and passes it upstream until it reaches the ingress Edge LSR.
Whenever a FR-LSR originates a label binding request to its next hop Whenever a FR-LSR originates a label binding request to its next hop
LSR as a result of receiving a label binding request from another 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 FR-LSR should destroy the binding created in response to the the FR-LSR should destroy the binding created in response to the
received request, and notify the requester (via an LDP "withdraw" received request, and notify the requester (via an LDP "withdraw"
message). message).
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When an LSR determines that it has lost its LDP session with another When an LSR determines that it has lost its LDP session with another
LSR, the following actions are taken: LSR, the following actions are taken:
- MUST discard any binding information learned via this - MUST discard any binding information learned via this
connection; connection;
- For any label bindings that were created as a result of - For any label bindings that were created as a result of
receiving label binding requests from the peer, the LSR may receiving label binding requests from the peer, the LSR may
destroy these bindings (and deallocate labels associated destroy these bindings (and deallocate labels associated
with these binding). with these binding).
skipping to change at line 876 skipping to change at line 879
existing outgoing label for that FEC. If the FR-LSR does not have an existing outgoing label for that FEC. If the FR-LSR does not have an
outgoing label binding for that FEC, but does have an outstanding outgoing label binding for that FEC, but does have an outstanding
request for one, it need not issue another request. This means that request for one, it need not issue another request. This means that
in a label conservation case, a FR-LSR must respond with a new in a label conservation case, a FR-LSR must respond with a new
binding for every upstream request, but it may need to send one binding for every upstream request, but it may need to send one
binding request downstream. binding request downstream.
In case of label conservation, if a change in the routing table In case of label conservation, if a change in the routing table
causes a FR-LSR to select a new next hop for one of its FECs, it MAY causes a FR-LSR to select a new next hop for one of its FECs, it MAY
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release the binding for that route from the former next hop. If it 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 doesn't already have a corresponding binding for the new next hop, it
must request one (note that the choice depends on the label retention must request one (note that the choice depends on the label retention
mode [ARCH]). mode [ARCH]).
If a new binding is obtained, which contain a hop count that differs If a new binding is obtained, which contain a hop count that differs
from that of the old binding, the FR-LSR must process the new hop from that of the old binding, the FR-LSR must process the new hop
count: increment by 1, if different than "unknown", and notify the count: increment by 1, if different than "unknown", and notify the
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. To ensure that loops will be detected, if the new hop count value. To ensure that loops will be detected, if the new hop count
skipping to change at line 923 skipping to change at line 926
Len Len
This field specifies the number of bits of the DLCI. The following This field specifies the number of bits of the DLCI. The following
values are supported: values are supported:
Len DLCI bits Len DLCI bits
0 10 0 10
1 17 1 17
2 23 2 23
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Minimum DLCI Minimum DLCI
This 23 bit field is the binary value of the lower bound of a block This 23 bit field is the binary value of the lower bound of a block
of Data Link Connection Identifiers (DLCIs) that is supported by of Data Link Connection Identifiers (DLCIs) that is supported by
the originating FR-LSR. The Minimum DLCI should be right justified the originating FR-LSR. The Minimum DLCI should be right justified
in this field and the preceding bits should be set to 0. in this field and the preceding bits should be set to 0.
Maximum DLCI Maximum DLCI
This 23 bit field is the binary value of the upper bound of a block This 23 bit field is the binary value of the upper bound of a block
of Data Link Connection Identifiers (DLCIs) that is supported by of Data Link Connection Identifiers (DLCIs) that is supported by
the originating FR-LSR. The Maximum DLCI should be right justified the originating FR-LSR. The Maximum DLCI should be right justified
skipping to change at line 971 skipping to change at line 974
and "Frame Relay Label": and "Frame Relay Label":
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| DLCI | | Reserved |Len| DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
with the following fields: with the following fields:
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Reserved Reserved
This field is reserved. It must be set to zero on transmission and This field is reserved. It must be set to zero on transmission and
must be ignored on receipt. must be ignored on receipt.
Len Len
This field specifies the number of bits of the DLCI. The following This field specifies the number of bits of the DLCI. The following
values are supported: values are supported:
Len DLCI bits Len DLCI bits
skipping to change at line 1017 skipping to change at line 1020
Altering by accident or forgery an existent label in the DLCI field Altering by accident or forgery an existent label in the DLCI field
of the Frame Relay data link layer header of a frame or one or more of the Frame Relay data link layer header of a frame or one or more
fields in a potentially following label stack affects the forwarding fields in a potentially following label stack affects the forwarding
of that frame. of that frame.
The label distribution mechanism can be secured by applying the The label distribution mechanism can be secured by applying the
appropriate level of security to the underlying protocol carrying appropriate level of security to the underlying protocol carrying
label information - authentication or encryption - see [LDP]. label information - authentication or encryption - see [LDP].
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9. Acknowledgments 9. Acknowledgments
The initial version of this document was derived from the Label The initial version of this document was derived from the Label
Switching over ATM document [ATM]. Switching over ATM document [ATM].
Thanks for the extensive reviewing and constructive comments from (in Thanks for the extensive reviewing and constructive comments from (in
alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller, alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller,
Eric Rosen. Also thanks to George Swallow for the suggestion to use Eric Rosen. Also thanks to George Swallow for the suggestion to use
null encapsulation, and to Eric Gray for his reviewing. null encapsulation, and to Eric Gray for his reviewing.
skipping to change at line 1056 skipping to change at line 1059
[ATM] B. Davie et al. "Use of Label Switching with ATM", Work in [ATM] B. Davie et al. "Use of Label Switching with ATM", Work in
Progress, July 1998. Progress, July 1998.
[ITU] International Telecommunications Union, "ISDN Data Link Layer [ITU] International Telecommunications Union, "ISDN Data Link Layer
Specification for Frame Mode Bearer Services", ITU-T Recommendation Specification for Frame Mode Bearer Services", ITU-T Recommendation
Q.922, 1992. Q.922, 1992.
[FRF] Frame Relay Forum, User-to-Network Implementation Agreement [FRF] Frame Relay Forum, User-to-Network Implementation Agreement
(UNI), FRF 1.1, January 19, 1996 (UNI), FRF 1.1, January 19, 1996
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11.Authors' Addresses 11.Authors' Addresses
Alex Conta Alex Conta
Lucent Technologies Inc. 3COM
300 Baker Ave, Suite 100 100 3COM Drive
Concord, MA 01742 Marlborough, MA 01752
+1-978-287-2842 +1 508 323-2297
E-mail: aconta@lucent.com E-mail: Alex_Conta@ne.3com.com
Paul Doolan Paul Doolan
Ennovate Networks Ennovate Networks
330 Codman Hill Rd 60 Codman Hill Rd
Boxborough MA 01719 Boxborough MA 01719
+1-978-263-2002 +1 978 263-2002
E-mail: pdoolan@ennovatenetworks.com E-mail: pdoolan@ennovatenetworks.com
Andrew Malis Andrew Malis
Ascend Communications, Inc Lucent Technologies
1 Robbins Rd 1 Robbins Rd
Westford, MA 01886 Westford, MA 01886
+1-978-952-7414 +1 978 952-7414
E-mail: malis@ascend.com E-mail: amalis@lucent.com
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Appendix A - Changes since previous versions Appendix A - Changes since previous versions
From "version 02 to 03" From "version 02 to 03"
- Replace "cloud" with "domain", - Replace "cloud" with "domain",
- Update references to other documents, - Update references to other documents,
- Change definitions in "Terminology" section, - Change definitions in "Terminology" section,
- Add more definitions to "Terminology" section, - Add more definitions to "Terminology" section,
- Make editorial changes to text and figures, - Make editorial changes to text and figures,
- Change "Performing TTL calculations" section, - Change "Performing TTL calculations" section,
- Add more reviewers in "Acknowledgments" section, - Add more reviewers in "Acknowledgments" section,
- Add Appendix A - changes. - Add Appendix A - changes.
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 End of changes. 

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