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RFC3034 - Use of Label Switching on Frame Relay Networks Specification

王朝other·作者佚名  2008-05-31
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Network Working Group A. Conta

Request for Comments: 3034 Transwitch Corporation

Category: Standards Track P. Doolan

Ennovate

A. Malis

Vivace Networks, Inc.

January 2001

Use of Label Switching on Frame Relay Networks

Specification

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

This document defines the model and generic mechanisms for

Multiprotocol Label Switching on Frame Relay networks. Furthermore,

it extends and clarifies portions of the Multiprotocol Label

Switching Architecture described in [ARCH] and the Label Distribution

Protocol (LDP) described in [LDP] relative to Frame Relay Networks.

MPLS enables the use of Frame Relay Switches as Label Switching

Routers (LSRs).

Table of Contents

1. IntrodUCtion................................................2

2. Terminology.................................................3

3. Special Characteristics of Frame Relay Switches.............4

4. Label Encapsulation.........................................5

5. Frame Relay Label Switching Processing......................6

5.1 Use of DLCIs..............................................6

5.2 Homogeneous LSPs..........................................7

5.3 Heterogeneous LSPs........................................7

5.4 Frame Relay Label Switching Loop Prevention and Control...7

5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............7

5.4.2 Performing MPLS TTL calculations.......................8

5.5 Label Processing by Ingress FR-LSRs......................12

5.6 Label Processing by Core FR-LSRs.........................12

5.7 Label Processing by Egress FR-LSRs.......................13

6. Label Switching Control Component for Frame Relay.........13

6.1 Hybrid Switches (Ships in the Night) ...................14

7. Label Allocation and Maintenance Procedures ..............15

7.1 Edge LSR Behavior........................................15

7.2 Efficient use of label space-Merging FR-LSRs.............18

7.3 LDP message fields specific to Frame Relay...............19

8. Security Considerations .................................21

9. Acknowledgments .........................................21

10. References ..............................................22

11. Authors' Addresses ......................................23

12. Full Copyright Statement ................................24

1. Introduction

The Multiprotocol Label Switching Architecture is described in

[ARCH]. It is possible to use Frame Relay switches as Label

Switching Routers. Such Frame Relay switches run network layer

routing algorithms (such as OSPF, IS-IS, etc.), and their forwarding

is based on the results of these routing algorithms. No specific

Frame Relay routing is needed.

When a Frame Relay switch is used for label switching, the top

(current) label, on which forwarding decisions are based, is carried

in the DLCI field of the Frame Relay data link layer header of a

frame. Additional information carried along with the top (current)

label, but not processed by Frame Relay switching, along with other

labels, if the packet is multiply labeled, are carried in the generic

MPLS encapsulation defined in [STACK].

Frame Relay permanent virtual circuits (PVCs) could be configured to

carry label switching based traffic. The DLCIs would be used as MPLS

Labels and the Frame Relay switches would become Frame Relay Label

Switching Routers, while the MPLS traffic would be encapsulated

according to this specification, and would be forwarded based on

network layer routing information.

The keyWords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,

SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined

in RFC2119.

This document is a companion document to [STACK] and [ATM].

2. Terminology

LSR

A Label Switching Router (LSR) is a device which implements the

label switching control and forwarding components described in

[ARCH].

LC-FR

A label switching controlled Frame Relay (LC-FR) interface is a

Frame Relay interface controlled by the label switching control

component. Packets traversing such an interface carry labels in

the DLCI field.

FR-LSR

A FR-LSR is an LSR with one or more LC-FR interfaces which

forwards frames between two such interfaces using labels carried

in the DLCI field.

FR-LSR domain

A FR-LSR domain is a set of FR-LSRs, which are mutually

interconnected by LC-FR interfaces.

Edge Set

The Edge Set of an FR-LSR domain is the set of LSRs, which are

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.

Additionally, this document uses terminology from [ARCH].

3. Special characteristics of Frame Relay Switches

While the label switching architecture permits considerable

flexibility in LSR implementation, a FR-LSR is constrained by the

capabilities of the (possibly pre-existing) hardware and the

restrictions on such matters as frame format imposed by the

Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay

standards [FRF], etc.... Because of these constraints, some special

procedures are required for FR-LSRs.

Some of the key features of Frame Relay switches that affect their

behavior as LSRs are:

- the label swapping function is performed on fields (DLCI) in the

frame's Frame Relay data link header; this dictates the size and

placement of the label(s) in a packet. The size of the DLCI field

can be 10 (default) or 23 bits, and it can span two or four bytes

in the header.

- there is generally no capability to perform a 'TTL-decrement'

function as is performed on IP headers in routers.

- congestion control is performed by each node based on parameters

that are passed at circuit creation. Flags in the frame headers

may be set as a consequence of congestion, or exceeding the

contractual parameters of the circuit.

- although in a standard switch it may be possible to configure

multiple input DLCIs to one output DLCI resulting in a

multipoint-to-point circuit, multipoint-to-multipoint VCs are

generally not fully supported.

This document describes ways of applying label switching to Frame

Relay switches, which work within these constraints.

4. Label Encapsulation

By default, all labeled packets should be transmitted with the

generic label encapsulation as defined in [STACK], using the frame

relay null encapsulation mechanism:

0 1 (Octets)

+-----------------------+-----------------------+

(Octets)0

/ Q.922 Address /

/ (length 'n' equals 2 or 4) /

+-----------------------+-----------------------+

n .

/ . /

/ MPLS packet /

.

+-----------------------+-----------------------+

"n" is the length of the Q.922 Address which can be 2 or 4 octets.

The Q.922 [ITU] representation of a DLCI (in canonical order -

the first bit is stored in the least significant, i.e., the

right-most bit of a byte in memory) [CANON] is the following:

7 6 5 4 3 2 1 0 (bit order)

+-----+-----+-----+-----+-----+-----+-----+-----+

(octet) 0 DLCI(high order) 0 0

+-----+-----+-----+-----+-----+-----+-----+-----+

1 DLCI(low order) 0 0 0 1

+-----+-----+-----+-----+-----+-----+-----+-----+

10 bits DLCI

7 6 5 4 3 2 1 0 (bit order)

+-----+-----+-----+-----+-----+-----+-----+-----00

(octet) 0 DLCI(high order) 0 0

+-----+-----+-----+-----+-----+-----+-----+-----

1 DLCI 0 0 0 0

+-----+-----+-----+-----+-----+-----+-----+-----+

2 DLCI 0

+-----+-----+-----+-----+-----+-----+-----+-----+

3 DLCI (low order) 0 1

+-----+-----+-----+-----+-----+-----+-----+-----+

23 bits DLCI

The use of the frame relay null encapsulation implies that labels

implicitly encode the network protocol type.

Rules regarding the construction of the label stack, and error

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

5.1 Use of DLCIs

Label switching is accomplished by associating labels with routes and

using the label value to forward packets, including determining the

value of any replacement label. See [ARCH] for further details. In

a 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 carries implicitly information about the network protocol type.

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

configuration, similar to configuring the DLCI used to run IP routing

protocols between the switches.

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

two independent spaces, i.e., VCs may be half-duplex, each direction

with its own DLCI.

The allowable ranges of DLCIs, the size of DLCIs, and the support for

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

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

packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is

homogeneous.

5.3 Heterogeneous LSPs

If <LSR1, LSR2, LSR3> is an LSP, it is possible that LSR1 will use

one encoding of the label stack when transmitting packet P to LSR2,

but LSR2 will use a different encoding when transmitting a packet P

to LSR3. In general, the MPLS architecture supports LSPs with

different label stack encodings on different hops. When a labeled

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

determine the new label value of the stack, and then encode the new

value appropriately before transmitting the labeled packet to its

next hop.

Naturally there will be MPLS networks which contain a combination of

Frame Relay switches operating as LSRs, and other LSRs, which operate

using other MPLS encapsulations, such as the Generic (MPLS shim

header), or ATM encapsulation. In such networks there may be some

LSRs, which have Frame Relay interfaces as well as MPLS Generic

("MPLS Shim") interfaces. This is one example of an LSR with

different label stack encodings on different hops of the same LSP.

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

(MPLS shim) header on the outgoing interface.

5.4 Frame Relay Label Switching Loop Prevention and Control

FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay

segments. Therefore, FR-LSRs SHOULD use loop detection and MAY use

loop prevention mechanisms as described in [ARCH], and [LDP].

5.4.1 FR-LSRs Loop Control - MPLS TTL processing

The MPLS TTL encoded in the MPLS label stack is a mechanism used to:

(a) suppress loops;

(b) limit the scope of a packet.

When a packet travels along an LSP, it should emerge with the same

TTL value that it would have had if it had traversed the same

sequence of routers without having been label switched. If the

packet travels along a hierarchy of LSPs, the total number of LSR-

hops traversed should be reflected in its TTL value when it emerges

from the hierarchy of LSPs [ARCH].

The initial value of the MPLS TTL is loaded into a newly pushed label

stack entry from the previous TTL value, whether that is from the

network layer header when no previous label stack existed, or from a

pre-existent lower level label stack entry.

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".

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

unicast case, this can be achieved by propagating a meaningful LSP

length or LSP Frame Relay segment length to the FR-LSR ingress nodes,

enabling the ingress to decrement the TTL value before forwarding

packets into a non-TTL LSP segment [ARCH].

When an ingress FR-LSR determines upon decrementing the MPLS TTL that

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

the packet, but rather follow the specifications in [STACK] in an

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

is propagated to the FR-LSR egress node, enabling the egress to

decrement the TTL value before forwarding packets out of the non-TTL

LSP segment.

5.4.2 Performing MPLS TTL calculations

The calculation applied to the "input TTL" that yields the "output

TTL" depends on (i)the "input encapsulation", (ii)the "forwarding

encapsulation", and (iii)the "output encapsulation". The

relationship among (i),(ii), and (iii), can be defined as a function

"D" of "input encapsulation" (ie), "forwarding encapsulation" (fe),

and "output encapsulation" (oe). Subsequently the calculation

applied to the "input TTL" to yield the "output TTL" can be described

as:

output TTL = input TTL - D(ie, fe, oe)

or in a brief notation:

output TTL = input TTL - d

where "d" has three possible values: "0","1", or "the number of hops

of the LSP segment":

For unicast transmission:

+================+=================+=================+=================+

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)

+================+=================+=================+=================+

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.

When sending a label binding upstream, the "hop count" associated

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].

For multicast transmission:

+================+=================+=================+=================+

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)

+================+=================+=================+=================+

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:

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

"iIf" is "ingress LSR" in Frame Relay LSP and

calculates: mpls_ttl = IP_TTL - number of hops = n-5

"fIi" is "egress LSR" from Frame Relay LSP, and

calculates: IP_ttl = mpls_ttl-1 = n-6

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

"iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1

"gGf" is "egress LSR" from Generic MPLS segment, and

"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6

"fGa" "egress LSR" from Frame Relay segment, and

"ingress LSR" in ATM segment and calculates: mpls_ttl=n-9

"gGf" is "egress LSR" from Generic MPLS segment, and

"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13

"fGg" is "egress LSR" from Frame Relay segment, and

ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14

"gIi" is "egress LSR" from LSP and calculates: IP_ttl=n-15

And further examples:

Frame Relay Unicast -- TTL calculated at ingress

(ingress LSR) 1 2 3 4

x--->---+--->---+--->>--+-->>---x (egress LSR)

o.ttl=i.ttl-4 2 3

^

hops 1

x (ingress LSR)

o.ttl=i.ttl-3

Frame Relay Multicast -- TTL calculated at egress

(egress LSR)x o.ttl=i.ttl-3

hops

^3

(ingress LSR) o.ttl=i.ttl-4

x--->---+--->---+--->---+--->---x (egress LSR)

1 2 3 4

5.5 Label Processing by Ingress FR-LSRs

When a packet first enters an MPLS domain, the packet is forwarded by

normal network layer forwarding operations with the exception that

the outgoing encapsulation will include an MPLS label stack [STACK]

with at least one entry. The frame relay null encapsulation will

carry information about the network layer protocol implicitly in the

label, which MUST be associated only with that network protocol. The

TTL field in the top label stack entry is filled with the network

layer TTL (or hop limit) resulted after network layer forwarding

[STACK]. The further FR-LSR processing is similar in both possible

cases:

(a) the LSP is homogeneous -- Frame Relay only -- and the FR-LSR is

the ingress.

(b) the LSP is heterogeneous -- Frame Relay, PPP, Ethernet, ATM,

etc... segments form the LSP -- and the FR-LSR is the ingress into a

Frame Relay segment.

For unicast packets, the MPLS TTL SHOULD be decremented with the

number of hops of the Frame Relay LSP (homogeneous), or Frame Relay

segment of the LSP (heterogeneous). An LDP constructing the LSP

SHOULD pass meaningful information to the ingress FR-LSR regarding

the number of hops of the "non-TTL segment".

For multicast packets, the MPLS TTL SHOULD be decremented by 1. An

LDP constructing the LSP SHOULD pass meaningful information to the

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

data link driver with the top label as output DLCI. The Frame Relay

frame carrying the MPLS encapsulated packet is forwarded onto the

Frame Relay VC to the next LSR.

5.6 Label Processing by Core FR-LSRs

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 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 Information Base, replaced with the correspondent output

DLCI, and transmitted on the outgoing interface (forwarded to the

next hop node).

The current label information is also carried in the top of the label

stack. In the top-level entry, all fields except the label

information, which is carried and switched in the Frame Relay frame

data link-layer header, are of current significance.

5.7 Label Processing by Egress FR-LSRs

When reaching the end of a Frame Relay LSP, the FR-LSR pops the label

stack [ARCH]. If the label popped is the last label, it is necessary

to determine the particular network layer protocol which is being

carried. The label stack carries no explicit information to identify

the network layer protocol. This must be inferred from the value of

the label which is popped from the stack.

If the label popped is not the last label, the previous top level

MPLS TTL is propagated to the new top label stack entry.

If the FR-LSR is the egress switch of a Frame Relay segment of a

hybrid LSP, and the end of the Frame Relay segment is not the end of

the LSP, the MPLS packet will be processed for forwarding onto the

next segment of the LSP based on the information held in the Next Hop

Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to

the value from the NHLFE, and the MPLS TTL is decremented by the

appropriate value depending the type of the output interface and the

type of transmit operation (see section 6.3). Further, the MPLS

packet is forwarded according to the MPLS specifications for the

particular link of the next segment of the LSP.

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

next ATM segment of the LSP (heterogeneous), if the output interface

is an ATM (non-TTL) interface.

For multicast packets, the MPLS TTL SHOULD be decremented by the

number of hops of the FR segment being exited. An LDP constructing

the LSP SHOULD pass meaningful information to the egress FR-LSR

regarding the number of hops of the FR "non-TTL segment".

6. Label Switching Control Component for Frame Relay

To support label switching a Frame Relay Switch MUST implement the

control component of label switching, which consists primarily of

label allocation and maintenance procedures. Label binding

information MAY be communicated by several mechanisms, one of which

is the Label Distribution Protocol (LDP) [LDP].

Since the label switching control component uses information learned

directly from network layer routing protocols, this implies that the

switch MUST participate as a peer in these protocols (e.g., OSPF,

IS-IS).

In some cases, LSRs may use other protocols (e.g., RSVP, PIM, BGP) to

distribute label bindings. In these cases, a Frame Relay LSR should

participate in these protocols.

In the case where Frame Relay circuits are established via LDP, or

RSVP, or others, with no involvement from traditional Frame Relay

mechanisms, it is assumed that circuit establishing contractual

information such as input/output maximum frame size,

incoming/outgoing requested/agreed throughput, incoming/outgoing

acceptable throughput, incoming/outgoing burst size,

incoming/outgoing frame rate, used in transmitting, and congestion

control MAY be passed to the FR-LSRs through RSVP, or can be

statically configured. It is also assumed that congestion control

and frame header flagging as a consequence of congestion, would be

done by the FR-LSRs in a similar fashion as for traditional Frame

Relay circuits. With the goal of emulating a best-effort router as

default, the default VC parameters, in the absence of LDP, RSVP, or

other mechanisms participation to setting such parameters, should be

zero CIR, so that input policing will set the DE bit in incoming

frames, but no frames are dropped.

Control and state information for the circuits based on MPLS MAY be

communicated through LDP.

Support of label switching on a Frame Relay switch requires

conformance only to [FRF] (framing, bit-stuffing, headers, FCS)

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

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

discussed in the next section.

6.1 Hybrid Switches (Ships in the Night)

The existence of the label switching control component on a Frame

Relay switch does not preclude the ability to support the Frame Relay

control component defined by the ITU and Frame Relay Forum on the

same switch and the same interfaces (NICs). The two control

components, label switching and those defined by ITU/Frame Relay

Forum, would operate independently.

Definition of how such a device operates is beyond the scope of this

document. However, only a small amount of information needs to be

consistent between the two control components, such as the portions

of the DLCI space which are available to each component.

7. Label Allocation and Maintenance Procedures

The mechanisms and message formats of a Label Distribution Protocol

are documented in [ARCH] and [LDP]. The "downstream-on-demand" label

allocation and maintenance mechanism discussed in this section MUST

be used by FR-LSRs that do not support VC merging, and it MAY also be

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

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 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 "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 LSR receives in response from the downstream LSR the label

binding information in an LDP "mapping" message, the label is stored

in the Label Information Base (LIB) as an outgoing label for that

FEC. The "mapping" message may contain the "hop count" object, which

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

may be stored for TTL calculation. Once this is done, the LSR may

use MPLS forwarding to transmit packets in that FEC.

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

Egress-FR-LSR. It allocates a label, creates a new entry in its

Label Information Base (LIB), places that label in the incoming label

component of the entry, and returns (via LDP) a "mapping" message

containing the allocated label back upstream to the LDP peer that

originated the request. The "mapping" message contains the "hop

count" object value set to 1.

When a routing calculation causes an Edge LSR to change the next hop

for a route, and the former next hop was in the FR-LSR domain, the

Edge LSR should notify the former next hop (via an LDP "release"

message) that the label binding associated with the route is no

longer needed.

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

LC-FR interface, the FR-LSR takes the following actions:

- it allocates a label, creates a new entry in its Label

Information Base (LIB), and places that label in the incoming

label component of the entry;

- it propagates the "request", by sending an LDP "request"

message to the next hop LSR, downstream for that route (FEC);

In the "ordered control" mode [ARCH], the FR-LSR will wait for its

"request" to be responded from downstream with a "mapping" message

before returning the "mapping" upstream in response to a "request"

("ordered control" approach [ARCH]). In this case, the FR-LSR

increments the hop count it received from downstream and uses this

value in the "mapping" it returns upstream.

Alternatively, the FR-LSR may return the binding upstream without

waiting for a binding from downstream ("independent control" approach

[ARCH]). In this case, it uses a reserved value for hop count in the

"mapping", indicating that it is 'unknown'. The correct value for

hop count will be returned later, as described below.

Since both the "ordered" and "independent" control has advantages and

disadvantages, this is left as an implementation, or configuration

choice.

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

outgoing label component of the LIB entry.

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

the same FR-LSR. It must generate a new "mapping" for each "request"

(assuming adequate resources to do so), and retain any existing

mapping(s). For each "request" received, a FR-LSR should also

generate a new binding "request" toward the next hop for the route

(FEC).

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

LDP "release" message) that the label binding associated with the

route is no longer needed.

When a LSR receives a notification that a particular label binding is

no longer needed, the LSR may deallocate the label associated with

the binding, and destroy the binding. This mode is the "conservative

label retention mode" [ARCH]. In the case where a FR-LSR receives

such notification and destroys the binding, it should notify the next

hop for the route that the label binding is no longer needed. If a

LSR does not destroy the binding (the FR-LSR is configured in

"liberal label retention mode" [ARCH]), it may re-use the binding

only if it receives a request for the same route with the same hop

count as the request that originally caused the binding to be

created.

When a route changes, the label bindings are re-established from the

point where the route diverges from the previous route. LSRs

upstream of that point are (with one exception, noted below)

oblivious to the change. Whenever a LSR changes its next hop for a

particular route, if the new next hop is a FR-LSR or a member of the

edge set reachable via a LC-FR interface, then for each entry in its

LIB associated with the route the LSR should request (via LDP) a

binding from the new next hop.

When a FR-LSR receives a label binding from a downstream neighbor, it

may already have provided a corresponding label binding for this

route to an upstream neighbor, either because it is using

"independent control" or because the new binding from downstream is

the result of a routing change. In this case, it should extract the

hop count from the new binding and increment it by one. If the new

hop count is different from that which was previously conveyed to the

upstream neighbor (including the case where the upstream neighbor was

given the value 'unknown') the FR-LSR must notify the upstream

neighbor of the change. Each FR-LSR in turn increments the hop count

and passes it upstream until it reaches the ingress Edge LSR.

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

(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

received request, and notify the requester (via an LDP "withdraw"

message).

When an LSR determines that it has lost its LDP session with another

LSR, the following actions are taken:

- MUST discard any binding information learned via this

connection;

- For any label bindings that were created as a result of

receiving label binding requests from the peer, the LSR may

destroy these bindings (and deallocate labels associated with

these binding).

7.2 Efficient use of label space - Merging FR-LSRs

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-

LSR domain. In fact, it is possible to significantly reduce the

number of labels needed by having the edge LSR request instead one

label for several routes. Use of many-to-one mappings between routes

(address prefixes) and labels using the notion of Forwarding

Equivalence Classes (as described in [ARCH]) provides a mechanism to

conserve the number of labels.

Note that conserving label space (VC merging) may be restricted in

case the frame traffic requires Frame Relay fragmentation. The issue

is that Frame Relay fragments must be transmitted in sequence, i.e.,

fragments of distinct frames must not be interleaved. If the

fragmenting FR-LSR ensures the transmission in sequence of all

fragments of a frame, without interleaving with fragments of other

frames, then label conservation (VC merging) can be performed.

When label conservation is used, when a FR-LSR receives a binding

request from an upstream LSR for a certain FEC, and it does already

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 FR-LSR to select a new next hop for one of its FECs, it MAY

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

The Label Distribution Protocol [LDP] messages exchanged between two

Frame Relay "LDP-peer" LSRs may contain Frame Relay specific

information such as:

"Frame Relay Label Range":

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved Len Minimum DLCI

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved Maximum DLCI

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

with the following fields:

Reserved

This fields are reserved. They must be set to zero on

transmission and must be ignored on receipt.

Len

This field specifies the number of bits of the DLCI. The

following values are supported:

Len DLCI bits

0 10

2 23

Len values 1 and 3 are reserved for future use.

Minimum DLCI

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 the originating FR-LSR. The Minimum DLCI should be

right justified in this field and the preceding bits should be set

to 0.

Maximum DLCI

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 the originating FR-LSR. The Maximum DLCI should be

right justified 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":

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved Len DLCI

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

with the following fields:

Reserved

This field is reserved. It must be set to zero on transmission and

must be ignored on receipt.

Len

This field specifies the number of bits of the DLCI. The following

values are supported:

Len DLCI bits

0 10

2 23

Len values 1 and 3 are reserved for future use.

DLCI

The binary value of the Frame Relay Label. The significant number

of bits (10 or 23) of the label value are to be encoded into the

Data Link Connection Identifier (DLCI) field when part of the

Frame Relay data link header (see Section 4.).

8. Security Considerations

This section looks at the security ASPects of:

(a) frame traffic,

(b) label distribution.

MPLS encapsulation has no effect on authenticated or encrypted

network layer packets, that is IP packets that are authenticated or

encrypted will incur no change.

The MPLS protocol has no mechanisms of its own to protect against

misdirection of packets or the impersonation of an LSR by accident or

malicious intent.

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

fields in a potentially following label stack affects the forwarding

of that frame.

The label distribution mechanism can be secured by applying the

appropriate level of security to the underlying protocol carrying

label information - authentication or encryption - see [LDP].

9. Acknowledgments

The initial version of this document was derived from the Label

Switching over ATM document [ATM].

Thanks for the extensive reviewing and constructive comments from (in

alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller,

Eric Rosen. Also thanks to George Swallow for the suggestion to use

null encapsulation, and to Eric Gray for his reviewing.

Also thanks to Nancy Feldman and Bob Thomas for their collaboration

in including the LDP messages specific to Frame Relay LSRs.

10. References

[MIFR] Bradley, T., Brown, C. and A. Malis, "Multiprotocol

Interconnect over Frame Relay", RFC2427, September 1998.

[ARCH] Rosen, E., Callon, R. and A. Vishwanathan, "Multi-Protocol

Label Switching Architecture", RFC3031, January 2001.

[LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and R.

Thomas, "Label Distribution Protocol", RFC3036, January

2001.

[STACK] Rosen, E., Rehter, Y., Tappan, D., Farinacci, D., Fedorkow,

G., Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC

3032, January 2001.

[ATM] Davie, B., Lawrence, J., McCloghrie, M., Rosen, E., Swallow,

G., Rekhter, Y., and P. Doolan, "Use of Label Switching with

ATM", RFC3035, January 2001.

[ITU] International Telecommunications Union, "ISDN Data Link Layer

Specification for Frame Mode Bearer Services", ITU-T

Recommendation Q.922, 1992.

[FRF] Frame Relay Forum, User-to-Network Implementation Agreement

(UNI), FRF 1.1, January 19, 1996.

11. Authors' Addresses

Alex Conta

Transwitch Corporation

3 Enterprise Drive

Shelton, CT 06484

Phone: 1-203-929-8810

EMail: aconta@txc.com

Paul Doolan

Ennovate Networks

60 Codman Hill Rd

Boxborough MA 01719

Phone: 1-978-263-2002

EMail: pdoolan@ennovatenetworks.com

Andrew G. Malis

Vivace Networks, Inc.

2730 Orchard Parkway

San Jose, CA 95134

USA

Phone: 1-408-383-7223

Fax: 1-408-904-4748

EMail: Andy.Malis@vivacenetworks.com

12. Full Copyright Statement

Copyright (C) The Internet Society (2001). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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