draft-ietf-mpls-fr-02.txt   draft-ietf-mpls-fr-03.txt 
MPLS Working Group A. Conta (Lucent) MPLS Working Group A. Conta (Lucent)
INTERNET-DRAFT P. Doolan (Ennovate) INTERNET-DRAFT P. Doolan (Ennovate)
A. Malis (Ascend) A. Malis (Ascend)
22 October 1998 16 November 1998
Use of Label Switching on Frame Relay Networks Use of Label Switching on Frame Relay Networks
Specification Specification
draft-ietf-mpls-fr-02.txt draft-ietf-mpls-fr-03.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. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute and its Working Groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted by months. Internet-Drafts may be updated, replaced, or obsoleted by
skipping to change at line 38 skipping to change at line 38
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).
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 described in [LDP] relative to Frame Relay Networks. MPLS Protocol (LDP) described in [LDP] relative to Frame Relay Networks.
enables the use of Frame Relay Switches as Label Switching Routers MPLS enables the use of Frame Relay Switches as Label Switching
(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.............4 3. Special Characteristics of Frame Relay Switches.............5
4. Label Encapsulation.........................................5 4. Label Encapsulation.........................................5
5. Frame Relay Label Switching Processing......................6 5. Frame Relay Label Switching Processing......................7
5.1 Use of DLCIs..............................................6 5.1 Use of DLCIs..............................................7
5.2 Homogeneous LSPs..........................................7 5.2 Homogeneous LSPs..........................................8
5.3 Heterogeneous LSPs........................................7 5.3 Heterogeneous LSPs........................................8
5.4 Frame Relay Label Switching Loop Prevention and Control...8 5.4 Frame Relay Label Switching Loop Prevention and Control...8
5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............8 5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............9
5.4.2 Performing MPLS TTL calculations.......................9 5.4.2 Performing MPLS TTL calculations......................10
5.5 Label Processing by Ingress FR-LSRs......................11 5.5 Label Processing by Ingress FR-LSRs......................13
5.6 Label Processing by Core FR-LSRs.........................12 5.6 Label Processing by Core FR-LSRs.........................14
5.7 Label Processing by Egress FR-LSRs.......................12 5.7 Label Processing by Egress FR-LSRs.......................14
6 Label Switching Control Component for Frame Relay..........13 6 Label Switching Control Component for Frame Relay..........15
6.1 Hybrid Switches (Ships in the Night) ...................14 6.1 Hybrid Switches (Ships in the Night) ...................16
7 Label Allocation and Maintenance Procedures ...............14 7 Label Allocation and Maintenance Procedures ...............16
7.1 Edge LSR Behavior........................................14 7.1 Edge LSR Behavior........................................16
7.2 Efficient use of label space-Merging FR-LSRs.............17 7.2 Efficient use of label space-Merging FR-LSRs.............19
7.3 LDP message fields specific to Frame Relay...............17 7.3 LDP message fields specific to Frame Relay...............20
8 Security Considerations ..................................19 8 Security Considerations ..................................22
9 Acknowledgments ..........................................20 9 Acknowledgments ..........................................23
10 References ...............................................20 10 References ...............................................23
11 Authors' Addresses .......................................21 11 Authors' Addresses .......................................24
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.
When a Frame Relay switch is used for label switching, the current When a Frame Relay switch is used for label switching, the top
label, on which forwarding decisions are based, is carried in the (current) label, on which forwarding decisions are based, is carried
DLCI field of the Frame Relay data link layer header of a frame. in the DLCI field of the Frame Relay data link layer header of a
Additional information carried along with the current label, but not frame. Additional information carried along with the top (current)
processed by Frame Relay switching, along with other labels, if the label, but not processed by Frame Relay switching, along with other
packet is multiply labeled, are carried in the generic MPLS labels, if the packet is multiply labeled, are carried in the generic
encapsulation defined in [STACK]. MPLS encapsulation defined in [STACK].
Frame Relay permanent virtual circuits (PVCs) could be configured to Frame Relay permanent virtual circuits (PVCs) could be configured to
carry label switching based traffic. The DLCIs would be used as MPLS carry label switching based traffic. The DLCIs would be used as MPLS
Labels and the Frame Relay switches would become MPLS switches while Labels and the Frame Relay switches would become Frame Relay Label
the MPLS traffic would be encapsulated according to this Switching Routers, while the MPLS traffic would be encapsulated
specification, and would be forwarded based on network layer routing according to this specification, and would be forwarded based on
information. network layer routing information.
The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED, The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as
defined in RFC 2119. defined in RFC 2119.
This document is a companion document to [STACK] and [ATM].
2. Terminology 2. Terminology
LSR LSR
A Label Switching Router (LSR) is a device which implements the A Label Switching Router (LSR) is a device which implements the
label switching control and forwarding components described in label switching control and forwarding components described in
[ARCH]. [ARCH].
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
A FR-LSR is an LSR with one or more LC-FR interfaces which
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forwards frames onto these interfaces using labels carried in A FR-LSR is an LSR with one or more LC-FR interfaces which
the DLCI field. forwards frames between two such interfaces using labels carried
in the DLCI field.
FR-LSR cloud FR-LSR domain
A FR-LSR cloud 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
The Edge Set of an FR-LSR cloud is the set of LSRs, which are The Edge Set of an FR-LSR domain is the set of LSRs, which are
connected to the cloud by LC-FR interfaces. connected to the domain by LC-FR interfaces.
Forwarding Encapsulation
The Forwarding Encapsulation is the type of MPLS encapsulation
(Frame Relay, ATM, Generic) of a packet that determines the
packet's MPLS forwarding, or the network layer encapsulation if
that packet is forwarded based on the network layer (IP,
etc...)header.
Input Encapsulation
The Input Encapsulation is the type of MPLS encapsulation (Frame
Relay, ATM, Generic) of a packet when that packet is received on
an LSR's interface, or the network layer (IP,
etc...)encapsulation if that packet has no MPLS encapsulation.
Output Encapsulation
The Output Encapsulation is the type of MPLS encapsulation
(Frame Relay, ATM, Generic) of a packet when that packet is
transmitted on an LSR's interface, or the network layer (IP,
etc...)encapsulation if that packet has no MPLS encapsulation.
Input TTL
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
layer (IP) TTL if the packet is not labeled.
Output TTL
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
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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- congestion control is performed by each node based on parameters - congestion control is performed by each node based on parameters
that are passed at circuit creation. Flags in the frame headers that are passed at circuit creation. Flags in the frame headers
may be set as a consequence of congestion, or exceeding the may be set as a consequence of congestion, or exceeding the
contractual parameters of the circuit. contractual parameters of the circuit.
- although in a standard switch it may be possible to configure - although in a standard switch it may be possible to configure
multiple input DLCIs to one output DLCI resulting in a multiple input DLCIs to one output DLCI resulting in a
multipoint-to-point circuit, multipoint-to-multipoint VCs are multipoint-to-point circuit, multipoint-to-multipoint VCs are
generally not fully supported. generally not fully supported.
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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. The labels implicitly encode the relay null encapsulation mechanism:
network protocol type, consequently those particular labels cannot be
used with other network protocols. Rules regarding the construction
of the label stack, and error messages returned to the frame source
are also described in [STACK].
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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 /
skipping to change at line 213 skipping to change at line 247
7 6 5 4 3 2 1 0 (bit order) 7 6 5 4 3 2 1 0 (bit order)
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
(octet) 0 | DLCI(high order) | 0 | 0 | (octet) 0 | DLCI(high order) | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
1 | DLCI(low order) | 0 | 0 | 0 | 1 | 1 | DLCI(low order) | 0 | 0 | 0 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
10 bits DLCI 10 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)
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
(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(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 |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
23 bits DLCI 23 bits DLCI
The generic encapsulation contains "n" labels for a label stack of depth The use of the frame relay null encapsulation implies that labels
"n" [STACK], where the top stack entry carries significant values for implicitly encode the network protocol type.
the EXP, S , and TTL fields [STACK] but not for the label, which is
rather carried in the DLCI field of the Frame Relay data link header Rules regarding the construction of the label stack, and error
encoded in Q.922 [ITU] address format. messages returned to the frame source are also described in [STACK].
The generic encapsulation contains "n" labels for a label stack of
depth "n" [STACK], where the top stack entry carries significant
values for the EXP, S , and TTL fields [STACK] but not for the label,
which is rather carried in the DLCI field of the Frame Relay data
link header encoded in Q.922 [ITU] address format.
5. Frame Relay Label Switching Processing 5. Frame Relay Label Switching Processing
5.1 Use of DLCIs 5.1 Use of DLCIs
Label switching is accomplished by associating labels with routes and Label switching is accomplished by associating labels with routes and
using the label value to forward packets, including determining the using the label value to forward packets, including determining the
value of any replacement label. See [ARCH] for further details. In a value of any replacement label. See [ARCH] for further details. In a
FR-LSR, the current (top) MPLS label is carried in the DLCI field of FR-LSR, the top (current) MPLS label is carried in the DLCI field of
the Frame Relay data link layer header of the frame. The top label the Frame Relay data link layer header of the frame. The top label
carries implicitly information about the network protocol type. carries implicitly information about the network protocol type.
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
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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. In case of DLCI aggregation (DLCI space with its own DLCI.
conservation), half-duplex (unidirectional) VCs are desired, since a
"many to few" aggregation is possible in one direction but not in
reverse.
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Note that the range of DLCIs used for labels depends on the size of The allowable ranges of DLCIs, the size of DLCIs, and the support for
the DLCI field. 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.
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.
5.3 Heterogeneous LSPs 5.3 Heterogeneous LSPs
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to LSR3. In general, the MPLS architecture supports LSPs with to LSR3. In general, the MPLS architecture supports LSPs with
different label stack encodings on different hops. When a labeled different label stack encodings on different hops. When a labeled
packet is received, the LSR must decode it to determine the current packet is received, the LSR must decode it to determine the current
value of the label stack, then must operate on the label stack to value of the label stack, then must operate on the label stack to
determine the new label value of the stack, and then encode the new determine the new label value of the stack, and then encode the new
value appropriately before transmitting the labeled packet to its value appropriately before transmitting the labeled packet to its
next hop. next hop.
Naturally there will be MPLS networks which contain a combination of Naturally there will be MPLS networks which contain a combination of
Frame Relay switches operating as LSRs, and other LSRs, which operate Frame Relay switches operating as LSRs, and other LSRs, which operate
using other MPLS encapsulations, such as the MPLS shim header, or ATM using other MPLS encapsulations, such as the Generic (MPLS shim
encapsulation. In such networks there may be some LSRs, which have header), or ATM encapsulation. In such networks there may be some
Frame Relay interfaces as well as "MPLS Shim" interfaces. This is one LSRs, which have Frame Relay interfaces as well as MPLS Generic
example of an LSR with different label stack encodings on different ("MPLS Shim") interfaces. This is one example of an LSR with
hops of the same LSP. Such an LSR may swap off a Frame Relay encoded different label stack encodings on different hops of the same LSP.
label on an incoming interface and replace it with a label encoded Such an LSR may swap off a Frame Relay encoded label on an incoming
into an MPLS shim header on the outgoing interface. interface and replace it with a label encoded into a Generic MPLS
(MPLS shim) header on the outgoing interface.
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5.4 Frame Relay Label Switching Loop Prevention and Control 5.4 Frame Relay Label Switching Loop Prevention and Control
FR-LSRs MUST use a mechanism that insures loop free FR- LSPs or LSP FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay
FR segments. Such mechanisms are described in [ARCH], and [LDP]. segments. Therefore, FR-LSRs SHOULD use loop detection and MAY use
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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stack entry from the previous TTL value, whether that is from the stack entry from the previous TTL value, whether that is from the
network layer header when no previous label stack existed, or from a network layer header when no previous label stack existed, or from a
pre-existent lower level label stack entry. pre-existent lower level label stack entry.
A FR-LSR switching same level labeled packets does not decrement the A FR-LSR switching same level labeled packets does not decrement the
MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment". MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment".
When a packet emerges from a "non-TTL LSP segment", it should however When a packet emerges from a "non-TTL LSP segment", it should however
reflect in the TTL the number of LSR-hops it traversed. In the reflect in the TTL the number of LSR-hops it traversed. In the
unicast case, this can be achieved by propagating a meaningful LSP unicast case, this can be achieved by propagating a meaningful LSP
length or LSP segment length to the FR-LSR ingress nodes, enabling length or LSP Frame Relay segment length to the FR-LSR ingress nodes,
the ingress to decrement the TTL value before forwarding packets into enabling the ingress to decrement the TTL value before forwarding
a non-TTL LSP segment [ARCH]. packets into a non-TTL LSP segment [ARCH].
When an ingress FR-LSR determines upon decrementing the MPLS TTL that When an ingress FR-LSR determines upon decrementing the MPLS TTL that
a particular packet's TTL will expire before the packet reaches the a particular packet's TTL will expire before the packet reaches the
egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch
the packet, but rather follow the specifications in [STACK] in an the packet, but rather follow the specifications in [STACK] in an
attempt to return an error message to the packet's source. attempt to return an error message to the packet's source:
- it treats the packet as an expired packet and return an ICMP
message to its source.
- it forwards the packet, as an unlabeled packet, with a TTL
that reflects the IP (network layer) forwarding.
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.
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5.4.2 Performing MPLS TTL calculations 5.4.2 Performing MPLS TTL calculations
Considering the "incoming TTL" the MPLS TTL of the top of the stack The calculation applied to the "input TTL" that yields the "output
when a labeled packet is received, and the "output TTL" the MPLS TTL TTL" depends on (i)the "input encapsulation", (ii)the "forwarding
of the top of the stack when a packet leaves a node, the relationship encapsulation", and (iii)the "output encapsulation". The
between the two is defined as a function of the type of the output relationship among (i),(ii), and (iii), can be defined as a function
interface, and the type of transmit operation done on the output "D" of "input encapsulation" (ie), "forwarding encapsulation" (fe),
interface (unicast or multicast): and "output encapsulation" (oe). Subsequently the calculation applied
to the "input TTL" to yield the "output TTL" can be described as:
output TTL = function (input TTL, output interface type, type of transmit)=
= input TTL - funct (output interface type, type of transmit) output TTL = input TTL - D(ie, fe, oe)
Considering the symbol "I" for an IP interface, the symbol "G" for a or in a brief notation:
generic MPLS encapsulating interface, the symbol "A" for a MPLS ATM
encapsulating
interface, the symbol "F" for a MPLS FR encapsulating interface, and
"G_G", "F_G", etc... LSRs with specific input and output interfaces,
and also the symbols "O.TTL" and "I.TTL" for the "output" and "input"
TTL, the following describes the possible combinations:
input,output Unicast output TTL = input TTL - d
->G_G-> O.TTL = I.TTL - 1 where "d" has three possible values: "0","1", or "the number of hops
of the LSP segment":
->F_G-> O.TTL = I.TTL - nr. of hops of starting segment (ingress F) For unicast transmission:
->G_F-> O.TTL = I.TTL - 1 (egress F)
->A_F-> O.TTL = I.TTL - nr. of hops of starting segment (ingress F) +================+=================+=================+=================+
->F_A-> O.TTL = I.TTL - 1 (egress F) | | Type of | Type of | Type of |
| d | Input | Forwarding | Output |
| | Encapsulation | Encapsulation | Encapsulation |
+================+=================+=================+=================+
| 0 | Frame Relay | Frame Relay | Frame Relay |
+----------------+-----------------+-----------------+-----------------+
| 1 | any | Generic MPLS | Generic MPLS |
+----------------+-----------------+-----------------+-----------------+
| number of hops | | Generic MPLS | |
| of | any | or | Frame Relay |
| LSP segment | |IP(network layer)| |
+================+=================+=================+=================+
->F_F-> similar to ->A A-> no TTL processing 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
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
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
segment" is 1.
input,output Multicast When sending a label binding upstream, the "hop count" associated
->G_G-> O.TTL = I.TTL - 1 (Conta & Doolan & Malis) Draft expires in six months [Page 10]
with the corresponding binding from downstream, if different than the
"unknown" value, MUST be incremented by 1, and the result transmitted
upstream as the hop count associated with the new binding (the
"unknown" value is transmitted unchanged). If the new "hop count"
value exceeds the "maximum" value, the FR-LSR MUST NOT pass the
binding upstream, but instead MUST send an error upstream
[LDP][ARCH].
->G_F-> O.TTL = I.TTL - 1 (ingress F) For multicast transmission:
->F_G-> O.TTL = I.TTL - nr. of hops of ending segment (egress F)
->A_F-> O.TTL = I.TTL - 1 (ingress F) +================+=================+=================+=================+
->F_A-> O.TTL = I.TTL - nr. of hops of ending segment (egress F) | | Type of | Type of | Type of |
| d | Input | Forwarding | Output |
| | Encapsulation | Encapsulation | Encapsulation |
+================+=================+=================+=================+
| 0 | Frame Relay | Frame Relay | Frame Relay |
+----------------+-----------------+-----------------+-----------------+
| | | Generic MPLS | |
| 1 | any | or | Frame Relay |
| | |IP(network layer)| |
+----------------+-----------------+-----------------+-----------------+
| number of hops | | Generic MPLS | |
| of | Frame Relay | or | any |
| LSP segment | |IP(network layer)| |
+================+=================+=================+=================+
->F_F-> similar to ->A A-> no TTL processing Referring to the "forwarding encapsulation" with the abbreviation "I"
for IP (network layer), "G" for Generic MPLS, and "F" for Frame
Relay MPLS, referring to an LSR interface with the abbreviation "i"
if the input or output encapsulation is IP and no MPLS encapsulation,
"g" when the input or output MPLS encapsulation is Generic MPLS, "f"
when it is Frame Relay, "a" when it is ATM, and furthermore
considering the symbols "iIf", "gGf", "fFf", etc... as LSRs with
input, forwarding and output encapsulations as referred above, the
following describes examples of TTL calculations for the Homogeneous
and Heterogeneous LSPs discussed in previous sections:
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Homogeneous LSP Homogeneous LSP
---------------
IP_ttl = n IP_ttl=mpls_ttl-1 = n-6
--------->iIf fIi--------->
| mpls_ttl = n-5 ^
| |
number of hops 1| Frame Relay |5
| |
V 2 3 4 |
fFf--->fFf--->fFf--->fFf
--->I_F Frame Relay F_I---> (Conta & Doolan & Malis) Draft expires in six months [Page 11]
hops = 5 | | "iIf" is "ingress LSR" in Frame Relay LSP and
F_F--->F_F--->F_F--->F_F calculates: mpls_ttl = IP_TTL - number of hops = n-5
loop free "fIi" is "egress LSR" from Frame Relay LSP, and
ip_ttl = n ip_ttl=n-6 calculates: IP_ttl = mpls_ttl-1 = n-6
mpls_ttl = n-5 n-5
Heterogeneous LSP Heterogeneous LSP
-----------------
ingress LSR egress LSR
IP_ttl = n IP_ttl = n - 15
links LAN PPP FR ATM PPP FR LAN
--->iIg-->gGg-->gGf fGa aGg-->gGf fGg-->gIi--->
hops 1 2 | 6 | | 9 | 10 | 13 ^ 14 15
|1 4| |1 3| |1 3|
V 2 3 | V 2 | V 2 |
fFf-->fFf-->fFf aAa-->aAa fFf-->fFf
mpls_ttl
n-1 n-2 (n-2)-4=n-6 (n-6)-3=n-9 n-10 n-13 n-14
LSP LSP "iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1
ingress egress "gGf" is "egress LSR" from Generic MPLS segment, and
"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6
LAN PPP FR ATM PPP FR LAN "fGa" "egress LSR" from Frame Relay segment, and
"ingress LSR" in ATM segment and calculates: mpls_ttl=n-9
--->I_G-->G_G-->G_F F_A A_G-->G_F F_G-->G_I---> "gGf" is "egress LSR" from Generic MPLS segment, and
| / | | | | "ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13
hops 1 1 | 4 / | 3 | 1 | 3 | 1 1 "fGg" is "egress LSR" from Frame Relay segment, and
F_F--F_F--F_F A_A--A_A F_F--F_F ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14
"gIi" is "egress LSR" from LSP and calculates: IP_ttl=n-15
loop free loop free loop free And further examples:
ip_ttl
n n-15
mpls_ttl
n-1 n-2 n-6 n-9 n-10 n-13 n-14
Unicast -- TTL calculated at ingress Frame Relay Unicast -- TTL calculated at ingress
1 2 3 4 (ingress LSR) 1 2 3 4
o-------o-------o-------o-------o x--->---+--->---+--->>--+-->>---x (egress LSR)
ttl=n-4 / 2 3 o.ttl=i.ttl-4 | 2 3
/ ^
hops 1/ hops 1|
/ |
o ttl=n-3 x (ingress LSR)
o.ttl=i.ttl-3
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Multicast -- TTL calculated at egress Frame Relay Multicast -- TTL calculated at egress
o ttl=n-3 (egress LSR)x o.ttl=i.ttl-3
hops / hops |
3/ ^3
/ ttl=n-4 (ingress LSR) | o.ttl=i.ttl-4
o-------o-------o-------o-------o 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
When a packet first enters an MPLS domain, the packet is forwarded by When a packet first enters an MPLS domain, the packet is forwarded by
normal network layer forwarding operations with the exception that normal network layer forwarding operations with the exception that
the outgoing encapsulation will include an MPLS label stack [STACK] the outgoing encapsulation will include an MPLS label stack [STACK]
with at least one entry. The frame relay null encapsulation will with at least one entry. The frame relay null encapsulation will
carry information about the network layer protocol implicitly in the carry information about the network layer protocol implicitly in the
label, which MUST be associated only with that network protocol. The label, which MUST be associated only with that network protocol. The
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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).
skipping to change at line 535 skipping to change at line 635
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
skipping to change at line 582 skipping to change at line 682
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.
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
document. However, only a small amount of information needs to be document. However, only a small amount of information needs to be
consistent between the two control components, such as the portions consistent between the two control components, such as the portions
of the DLCI space which are available to each component. of the DLCI space which are available to each component.
7. Label Allocation and Maintenance Procedures 7. Label Allocation and Maintenance Procedures
The mechanisms and message formats of a Label Distribution Protocol The mechanisms and message formats of a Label Distribution Protocol
are documented in [ARCH] and [LDP]. A possible scenario for the are documented in [ARCH] and [LDP]. The "downstream-on-demand" label
label allocation and maintenance for FR-LSRs is "downstream-on- allocation and maintenance mechanism discussed in this section MUST
demand" as it follows (note that this applies to hop-by-hop routed be used by FR-LSRs that do not support VC merging, and it MAY also be
traffic): used by FR-LSRs that do support VC merging (note that this mechanism
applies to hop-by-hop routed traffic):
7.1 Edge LSR Behavior 7.1 Edge LSR Behavior
Consider a member of the Edge Set of a FR-LSR cloud. Assume that, as Consider a member of the Edge Set of a FR-LSR domain. Assume that, as
a result of its routing calculations, it selects a FR-LSR as the next a result of its routing calculations, it selects a FR-LSR as the next
hop of a certain route (FEC), and that the next hop is reachable via hop of a certain route (FEC), and that the next hop is reachable via
a LC-Frame Relay interface. Assume that the next-hop FR-LSR is an a LC-Frame Relay interface. Assume that the next-hop FR-LSR is an
"LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message "LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message
for a label binding from the next hop, downstream LSR. When the Edge for a label binding from the next hop, downstream LSR. When the Edge
LSR receives in response from the downstream LSR the label binding LSR receives in response from the downstream LSR the label binding
information in an LDP "mapping" message, the label is stored in the information in an LDP "mapping" message, the label is stored in the
Label Information Base (LIB) as an outgoing label for that FEC. The Label Information Base (LIB) as an outgoing label for that FEC. The
"mapping" message may contain the "hop count" object, which "mapping" message may contain the "hop count" object, which
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
cloud 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 cloud 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
component of the entry, and returns (via LDP) a "mapping" message
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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 cloud, 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.
When a Frame Relay-LSR receives an LDP "request" message for a When a Frame Relay-LSR receives an LDP "request" message for a
certain route (FEC) from an LDP peer connected to the FR-LSR over a certain route (FEC) from an LDP peer connected to the FR-LSR over a
LC-FR interface, the FR-LSR takes the following actions: LC-FR interface, the FR-LSR takes the following actions:
- it allocates a label, creates a new entry in its Label - it allocates a label, creates a new entry in its Label
Information Base (LIB), and places that label in the incoming Information Base (LIB), and places that label in the incoming
skipping to change at line 672 skipping to change at line 773
count will be returned later, as described below. count will be returned later, as described below.
Since both the "ordered" and "independent" control has advantages and Since both the "ordered" and "independent" control has advantages and
disadvantages, this is left as an implementation, or configuration disadvantages, this is left as an implementation, or configuration
choice. choice.
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 cloud, 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
mapping(s). For each "request" received, a FR-LSR should also
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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
no longer needed, the LSR may deallocate the label associated with no longer needed, the LSR may deallocate the label associated with
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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).
7.2 Efficient use of label space - Merging FR-LSRs 7.2 Efficient use of label space - Merging FR-LSRs
The above discussion assumes that an edge LSR will request one label The above discussion assumes that an edge LSR will request one label
for each prefix in its routing table that has a next hop in the FR- for each prefix in its routing table that has a next hop in the FR-
LSR cloud. In fact, it is possible to significantly reduce the number LSR domain. In fact, it is possible to significantly reduce the
of labels needed by having the edge LSR request instead one label for number of labels needed by having the edge LSR request instead one
several routes. Use of many-to-one mappings between routes (address label for several routes. Use of many-to-one mappings between routes
prefixes) and labels using the notion of Forwarding Equivalence (address prefixes) and labels using the notion of Forwarding
Classes (as described in [ARCH]) provides a mechanism to conserve the Equivalence Classes (as described in [ARCH]) provides a mechanism to
number of labels. conserve the number of labels.
Note that conserving label space may be restricted in case the frame Note that conserving label space (VC merging) may be restricted in
traffic requires Frame Relay fragmentation. The issue is that Frame case the frame traffic requires Frame Relay fragmentation. The issue
Relay fragments must be transmitted in sequence, i.e. fragments of is that Frame Relay fragments must be transmitted in sequence, i.e.
distinct frames must not be interleaved. If the fragmenting FR-LSR fragments of distinct frames must not be interleaved. If the
ensures the transmission in sequence of all fragments of a frame, fragmenting FR-LSR ensures the transmission in sequence of all
without interleaving with fragments of other frames, then label fragments of a frame, without interleaving with fragments of other
conservation (aggregation) can be performed. frames, then label conservation (VC merging) can be performed.
In the case where the label space is to be conserved, it is desirable When label conservation is used, when a FR-LSR receives a binding
to use half-duplex (unidirectional) VCs, since a "many to few" request from an upstream LSR for a certain FEC, and it does already
aggregation is possible in one direction but not in reverse. have an outgoing label binding for that FEC, it does not need to
issue a downstream binding request. Instead, it may allocate an
incoming label, and return that label in a binding to the upstream
requester. Packets received from the requester, with that label as
top label, will be forwarded after replacing the label with the
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
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
binding for every upstream request, but it may need to send one
binding request downstream.
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
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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 (note that the choice depends on the label retention
mode [ARCH]).
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
count: increment by 1, if different than "unknown", and notify the
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
exceeds the "maximum" value, the label values for this FEC must be
withdrawn from all upstream neighbors to whom a binding was
previously sent.
7.3 LDP messages specific to Frame Relay 7.3 LDP messages specific to Frame Relay
The Label Distribution Protocol [LDP] messages exchanged between two The Label Distribution Protocol [LDP] messages exchanged between two
Frame Relay "LDP-peer" LSRs may contain Frame Relay specific Frame Relay "LDP-peer" LSRs may contain Frame Relay specific
information such as: information such as:
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"Frame Relay Label Range": "Frame Relay Label Range":
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| Minimum DLCI | | Reserved |Len| Minimum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Maximum DLCI | | Reserved | Maximum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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
in this field and the preceding bits should be set to 0. in this field and the preceding bits should be set to 0.
"Frame Relay Merge":
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |M|
+-+-+-+-+-+-+-+-+
with the following fields:
Merge
One bit field that specifies the merge capabilities of the FR-LSR:
Value Meaning
0 Merge NOT supported
1 Merge supported
A FR-LSR that supports VC merging MUST ensure that fragmented
frames from distinct incoming DLCIs are not interleaved on the
outgoing DLCI.
Reserved
This field is reserved. It must be set to zero on transmission and
must be ignored on receipt.
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 842 skipping to change at line 996
DLCI DLCI
The binary value of the Frame Relay Label. The significant number The binary value of the Frame Relay Label. The significant number
of bits (10, 17, or 23) of the label value are to be encoded into of bits (10, 17, or 23) of the label value are to be encoded into
the Data Link Connection Identifier (DLCI) field when part of the the Data Link Connection Identifier (DLCI) field when part of the
Frame Relay data link header (see Section 4.). Frame Relay data link header (see Section 4.).
8. Security Considerations 8. Security Considerations
This section looks at the security aspects of: This section looks at the security aspects of:
(a) frame traffic (a) frame traffic,
(b) label distribution. (b) label distribution.
MPLS encapsulation has no effect on authenticated or encrypted MPLS encapsulation has no effect on authenticated or encrypted
network layer packets, that is IP packets that are authenticated or network layer packets, that is IP packets that are authenticated or
encrypted will incur no change. encrypted will incur no change.
The MPLS protocol has no mechanisms of its own to protect against The MPLS protocol has no mechanisms of its own to protect against
misdirection of packets or the impersonation of an LSR by accident or misdirection of packets or the impersonation of an LSR by accident or
malicious intent. malicious intent.
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,
Also thanks to George Swallow for the suggestion to use null Eric Rosen. Also thanks to George Swallow for the suggestion to use
encapsulation, and to Eric Gray for his reviewing. null encapsulation, and to Eric Gray for his reviewing.
Also thanks to Nancy Feldman and Bob Thomas for their collaboration Also thanks to Nancy Feldman and Bob Thomas for their collaboration
in including the LDP messages specific to Frame Relay LSRs in including the LDP messages specific to Frame Relay LSRs.
10. References 10. References
[MIFR] T. Bradley, C. Brown, A. Malis "Multiprotocol Interconnect [MIFR] T. Bradley, C. Brown, A. Malis "Multiprotocol Interconnect
over Frame Relay" RFC 2427, September 1998. over Frame Relay" RFC 2427, September 1998.
[ARCH] E. Rosen, R. Callon, A. Vishwanathan, "Multi-Protocol Label [ARCH] E. Rosen, R. Callon, A. Vishwanathan, "Multi-Protocol Label
Switching Architecture", Work in Progress, July 1998. Switching Architecture", Work in Progress, July 1998.
[LDP] L. Anderson, P. Doolan, N. Feldman, A. Fredette, R. Thomas, [LDP] L. Anderson, P. Doolan, N. Feldman, A. Fredette, R. Thomas,
skipping to change at line 902 skipping to change at line 1056
[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. Lucent Technologies Inc.
300 Baker Ave, Suite 100 300 Baker Ave, Suite 100
Concord, MA 01742 Concord, MA 01742
+1-978-287-2842 +1-978-287-2842
E-mail: aconta@lucent.com E-mail: aconta@lucent.com
skipping to change at line 927 skipping to change at line 1081
+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 Ascend Communications, Inc
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: malis@ascend.com
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Appendix A - Changes since previous versions
From "version 02 to 03"
- Replace "cloud" with "domain",
- Update references to other documents,
- Change definitions in "Terminology" section,
- Add more definitions to "Terminology" section,
- Make editorial changes to text and figures,
- Change "Performing TTL calculations" section,
- Add more reviewers in "Acknowledgments" section,
- Add Appendix A - changes.
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 End of changes. 

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