📄 rfc3034.txt
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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
Conta, et al. Standards Track [Page 1]
RFC 3034 Label Switching with Frame Relay January 2001
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 RFC 2119.
This document is a companion document to [STACK] and [ATM].
Conta, et al. Standards Track [Page 2]
RFC 3034 Label Switching with Frame Relay January 2001
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.
Conta, et al. Standards Track [Page 3]
RFC 3034 Label Switching with Frame Relay January 2001
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.
Conta, et al. Standards Track [Page 4]
RFC 3034 Label Switching with Frame Relay January 2001
- 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
Conta, et al. Standards Track [Page 5]
RFC 3034 Label Switching with Frame Relay January 2001
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.
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