Network Working Group Luca Martini
Internet Draft Nasser El-Aawar
Expiration Date: May 2001 Giles Heron
Level 3 Communications, LLC.
Daniel Tappan
Eric C. Rosen
Cisco Systems, Inc.
Steve Vogelsang
John Shirron
Laurel Networks, Inc.
Andrew G. Malis
Vivace Networks, Inc.
Dimitri Stratton Vlachos
Mazu Networks, Inc.
November 2000
Transport of Layer 2 Frames Over MPLS
draft-martini-l2circuit-trans-mpls-04.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document describes methods for transporting the Protocol Data
Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5,
Ethernet, and providing a SONET circuit emulation service across an
MPLS network.
Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 2
3 Tunnel Labels and VC Labels ............................ 3
4 Protocol-Specific Issues ............................... 4
4.1 Frame Relay ............................................ 4
4.2 ATM .................................................... 4
4.2.1 OAM Cell Support ....................................... 4
4.2.2 ILMI Support ........................................... 5
4.3 HDLC ( Cisco ) ......................................... 5
4.4 PPP .................................................... 5
5 LDP .................................................... 6
6 Security Considerations ................................ 9
7 References ............................................. 9
8 Author Information ..................................... 9
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
2. Introduction
In an MPLS network, it is possible to carry the Protocol Data Units
(PDUs) of layer 2 protocols by prepending an MPLS label stack to
these PDUs. This document specifies the necessary label distribution
procedures for accomplishing this using the encapsulation methods in
[7]. We restrict discussion to the case of point-to-point transport.
QoS related issues are not discussed in this draft.
An accompanying document [8] also describes a method for transporting
time division multiplexed (TDM) digital signals (TDM circuit
emulation) over a packet-oriented MPLS network. The transmission
system for circuit-oriented TDM signals is the Synchronous Optical
Network (SONET)[5]/Synchronous Digital Hierarchy (SDH) [6]. To
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support TDM traffic, which includes voice, data, and private leased
line service, the MPLS network must emulate the circuit
characteristics of SONET/SDH payloads. MPLS labels and a new circuit
emulation header are used to encapsulate TDM signals and provide the
Circuit Emulation Service over MPLS (CEM). This encapsulation method
is described in [8].
3. Tunnel Labels and VC Labels
Suppose it is desired to transport layer 2 PDUs from ingress LSR R1
to egress LSR R2, across an intervening MPLS network. We assume that
there is an LSP from R1 to R2. That is, we assume that R1 can cause a
packet to be delivered to R2 by pushing some label onto the packet
and sending the result to one of its adjacencies. Call this label the
"tunnel label", and the corresponding LSP the "tunnel LSP".
The tunnel LSP merely gets packets from R1 to R2, the corresponding
label doesn't tell R2 what to do with the payload, and in fact if
penultimate hop popping is used, R2 may never even see the
corresponding label. (If R1 itself is the penultimate hop, a tunnel
label may not even get pushed on.) Thus if the payload is not an IP
packet, there must be a label, which becomes visible to R2, that
tells R2 how to treat the received packet. Call this label the "VC
label".
So when R1 sends a layer 2 PDU to R2, it first pushes a VC label on
its label stack, and then (if R1 is not adjacent to R2) pushes on a
tunnel label. The tunnel label gets the MPLS packet from R1 to R2;
the VC label is not visible until the MPLS packet reaches R2. R2's
disposition of the packet is based on the VC label.
If the payload of the MPLS packet is, for example, an ATM AAL5 PDU,
the VC label will generally correspond to a particular ATM VC at R2.
That is, R2 needs to be able to infer from the VC label the outgoing
interface and the VPI/VCI value for the AAL5 PDU. If the payload is a
Frame Relay PDU, then R2 needs to be able to infer from the VC label
the outgoing interface and the DLCI value. If the payload is an
ethernet frame, then R2 needs to be able to infer from the VC label
the outgoing interface, and perhaps the VLAN identifier. This process
is unidirectional, and will be repeated independently for
bidirectional operation. It is REQUIRED to assign the same VC, and
Group ID for a given circuit in both directions.
Note that the VC label must always be at the bottom of the label
stack, and the tunnel label, if present, must be immediately above
the VC label. Of course, as the packet is transported across the MPLS
network, additional labels may be pushed on (and then popped off) as
needed. Even R1 itself may push on additional labels above the tunnel
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label. If R1 and R2 are directly adjacent LSRs, then it may not be
necessary to use a tunnel label at all.
This document does not specify a method for distributing the tunnel
label or any other labels that may appear above it on the stack. Any
acceptable method of MPLS label distribution will do.
This document does specify a method for assigning and distributing
the VC label. Static label assignment MAY be used, and
implementations SHOULD provide support for this. If signaling is
used, the VC label MUST be distributed from R2 to R1 using LDP in the
downstream unsolicited mode; this requires that an LDP connection be
created between R1 and R2.
Note that this technique allows an unbounded number of layer 2 "VCs"
to be carried together in a single "tunnel". Thus it scales quite
well in the network backbone.
4. Protocol-Specific Issues
4.1. Frame Relay
The MPLS edge LSR MAY provide a Frame Relay LMI to the CE device.
If the MPLS edge LSR detects a service affecting condition as defined
in [2] Q.933 Annex A.5 sited in IA FRF1.1, it MUST withdraw the label
that corresponds to the frame relay DLCI. The Egress LSR SHOULD
generate the corresponding errors and alarms as defined in [2] on the
Frame relay VC.
4.2. ATM
4.2.1. OAM Cell Support
OAM cells MAY be transported on the VC LSP. A router that does not
support transport of ATM cells MUST discard incoming MPLS frames on
an ATM VC LSP that contain a control word with the T bit set. [7] A
router that supports transport of OAM cells MUST follow the
procedures outlined in [9] section 8 for mode 0 only in addition to
the applicable procedures specified in [6].
A router that does not support transport of OAM cells across an LSP
MAY provide OAM support on ATM PVCs using the following procedures:
If an F5 end-to-end OAM cell is received from a VC by a LSR with a
loopback indication value of 1 and the LSR has a label mapping for
the VC, the LSR MUST decrement the loopback indication value and loop
back the cell on the VC. Otherwise the loopback cell MUST be
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discarded by the LSR.
The LSR MAY optionally be configured to periodically generate F5
end-to-end loopback OAM cells on a VC. In this case, the LSR must
only generate F5 end-to-end loopback cells while a label mapping
exists for the VC. If the VC label mapping is withdrawn the LSR MUST
cease generation of F5 end-to-end loopback OAM cells. If the LSR
fails to receive a response to an F5 end-to-end loopback OAM cell for
a pre-defined period of time it MUST withdraw the label mapping for
the VC.
If an ingress LSR receives an AIS F5 OAM cell, fails to receive a
pre-defined number of the End-to-End loop OAM cells, or a physical
interface goes down, it MUST withdraw the label mappings for all VCs
associated with the failure. When a VC label mapping is withdrawn,
the egress LSR SHOULD generate AIS F5 OAM cells on the VC associated
with the withdrawn label mapping.
4.2.2. ILMI Support
An MPLS edge LSR MAY provide an ATM ILMI to the CE device.
If an ingress LSR receives an ILMI message indicating that the CE has
deleted a VC, or if the physical interface goes down, it MUST
withdraw the label mappings for all VCs associated with the failure.
When a VC label mapping is withdrawn, the egress LSR SHOULD notify
its client of this failure by deleting the VC using ILMI.
4.3. HDLC ( Cisco )
If the MPLS edge LSR detects that the physical link has failed it
MUST withdraw the label that corresponds to the HDLC link. The Egress
LSR SHOULD notify the CE device of this failure by using a physical
layer mechanism to take the link out of service.
4.4. PPP
If the MPLS edge LSR detects that the physical link has failed it
MUST withdraw the label that corresponds to the PPP link. The Egress
LSR SHOULD notify the CE device of this failure by using a physical
layer mechanism to take the link out of service.
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5. LDP
The VC label bindings are distributed using the LDP downstream
unsolicited mode described in [1]. The LSRs will establish an LDP
session using the Extended Discovery mechanism described in [1,
section 2.4-2.5], for this purpose a new type of FEC TLV element is
defined. The FEC element type is 128. [note1]
The Virtual Circuit FEC TLV element, is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VC tlv |C| VC Type | VC ID len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| VC ID |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- VC Type
A 15 bit quantity containing a value which represents the type of
VC. Assigned Values are:
VC Type Description
0x0001 Frame Relay DLCI
0x0002 ATM VCC transport
0x0003 ATM VPC transport
0x0004 Ethernet VLAN
0x0005 Ethernet
0x0006 HDLC ( Cisco )
0x0007 PPP
0x8008 CEM [8]
The highest order bit is used to flag the presence of a control word as
follows:
bit 15 = 1 control word present on this VC.
bit 15 = 0 no control word present on this VC.
- VC ID length
Length of the VC ID field in octets. If this value is 0, then it
references all VCs using the specified group ID
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- Group ID
An arbitrary 32 bit value which represents a group of VCs that is
used to augment the VC space. This value MUST be user
configurable. The group ID is intended to be used as either a
port index , or a virtual tunnel index. In the latter case a
switching function at ingress will map a particular circuit from
a port to a circuit in the virtual tunnel for transport to the
egress router.
- VC ID
Identifies a particular VC. The interpretation of the identifier
depends on the VC type:
* Frame Relay
A 32-bit value representing a 16-bit DLCI value as follows:
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 | DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* ATM VCC Transport
A 32-bit value representing a 16-bit VPI, and a 16-bit VCI as
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* ATM VPC Transport
A 32-bit value containing a 16-bit VPI as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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* Ethernet VLAN
A 32 bit value representing 16bit vlan identifier as follows:
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 | VLAN ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Ethernet
A 32 bit port identifier.
* HDLC ( Cisco )
A 32-bit port identifier
* PPP
A 32-bit port identifier
* CEM[8]
A 32-bit value used follows:
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 | Circuit ID | Payload Bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Circuit ID: An assigned number for the SONET circuit being
transported.
Payload Bytes(N): the number of TDM payload bytes contained
in all packets on the CEM stream, from 48 to 1,023 bytes. All
of the packets in a given CEM stream have the same number of
payload bytes. Note that there is a possibility that the
packet size may exceed the SPE size in the case of an STS-1
SPE, which could cause two pointers to be needed in the CEM
header, since the payload may contain two J1 bytes for
consecutive SPEs. For this reason, the number of payload
bytes must be less than or equal to 783 for STS-1 SPEs. The
reserved fields in the above specifications MUST be set to 0
in the FEC TLV, and ignored when received.
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6. Security Considerations
This document does not affect the underlying security issues of MPLS.
7. References
[1] "LDP Specification", draft-ietf-mpls-ldp-11.txt ( work in
progress )
[2] ITU-T Recommendation Q.933, and Q.922 Specification for Frame
Mode Basic call control, ITU Geneva 1995
[3] "MPLS Label Stack Encoding", draft-ietf-mpls-label-encaps-08.txt
( work in progress )
[4] "IEEE 802.3ac-1998" IEEE standard specification.
[5] American National Standards Institute, "Synchronous Optical
Network Formats," ANSI T1.105-1995.
[6] ITU Recommendation G.707, "Network Node Interface For The
Synchronous Digital Hierarchy", 1996.
[7] "Encapsulation Methods for Transport of Layer 2 Frames Over
MPLS", draft-martini-l2circuit-encap-mpls-00.txt ( Work in progress )
[8] "SONET/SDH Circuit Emulation Service Over MPLS (CEM)
Encapsulation", draft-malis-sonet-ces-mpls-01.txt ( Work in progress
)
[9] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000.
[note1] FEC element type 128 is pending IANA approval.
8. Author Information
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: luca@level3.net
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Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
Giles Heron
Level 3 Communications
66 Prescot Street
London
E1 8HG
United Kingdom
e-mail: giles@level3.net
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
e-mail: d@mazunetworks.com
Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: tappan@cisco.com
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: erosen@cisco.com
Steve Vogelsang
Laurel Networks, Inc.
2607 Nicholson Rd.
Sewickley, PA 15143
e-mail: sjv@laurelnetworks.com
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John Shirron
Laurel Networks, Inc.
2607 Nicholson Rd.
Sewickley, PA 15143
e-mail: jshirron@laurelnetworks.com
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
Phone: +1 408 383 7223
Email: Andy.Malis@vivacenetworks.com
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