Network Working Group Luca Martini
Internet Draft Nasser El-Aawar
Expiration Date: May 2001 Giles Heron
Level 3 Communications, LLC.
Dimitri Stratton Vlachos
Daniel Tappan
Eric C. Rosen
Cisco Systems, Inc.
Steve Vogelsang
John Shirron
Laurel Networks, Inc.
Andrew G. Malis
Vivace Networks, Inc.
November 2000
Encapsulation Methods for Transport of Layer 2 Frames Over MPLS
draft-martini-l2circuit-encap-mpls-00.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 encapsulating the Protocol Data
Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5,
Ethernet for transport across an MPLS network.
Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 2
3 Optional Sequencing and/or Padding ..................... 3
4 MTU Requirements ....................................... 4
5 Protocol-Specific Issues ............................... 4
5.1 Frame Relay ............................................ 4
5.2 ATM .................................................... 4
5.2.1 OAM Cell Support ....................................... 6
5.2.2 CLP Bit to EXP Bit Mapping ............................. 7
5.3 Ethernet VLAN .......................................... 7
5.4 Ethernet ............................................... 7
5.5 HDLC ( Cisco ) ......................................... 7
5.6 PPP .................................................... 8
6 Security Considerations ................................ 8
7 Intellectual Property Disclaimer ....................... 8
8 References ............................................. 8
9 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 encapsulation
procedures for accomplishing this. The control protocol methods are
described in [5]. QoS related issues are not discussed in this draft.
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3. Optional Sequencing and/or Padding
Sometimes it is important to guarantee that sequentiality is
preserved on a layer 2 virtual circuit. To accommodate this
requirement, we provide an optional control word which may appear
immediately after the label stack and immediately before the layer 2
PDU. This control word contains a sequence number. R1 and R2 both
need to be configured with the knowledge of whether a control word
will be used for a specific virtual circuit.
Sometimes it is necessary to transmit a small packet on a medium
where there is a minimum transport unit larger than the actual packet
size. In this case, padding is appended to the packet. When the VC
label is popped, it may be desirable to remove the padding before
forwarding the packet.
To facilitate this, the control word has a length field. If the
packet's length (without any padding) is less than 256 bytes, the
length field MUST be set to the packet's length (without padding).
Otherwise the length field MUST be set to zero. The value of the
length field, if non-zero, can be used to remove any padding.
The generic control word 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |T| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram the first 7 bits are reserved for future use.
They MUST be set to 0 when transmitting, and MUST be ignored upon
receipt. The T bit is used in ATM encapsulations only, and MUST be
set to zero for other encapsulations. The length byte is set as
specified above.
The next 16 bits are the sequence number that is used to guarantee
ordered packet delivery. For a given VC label, and a given pair of
LSRs, R1 and R2, where R2 has distributed that VC label to R1, the
sequence number is initialized to 0. This is incremented by one for
each successive packet carrying that VC label which R1 transmits to
R2.
The sequence number space is a 16 bit unsigned circular space. PDUs
carrying the control word MUST NOT be delivered out of order. They
may be discarded or reordered.
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4. MTU Requirements
The MPLS network should be configured with an MTU that is at least 12
bytes larger then the largest frame size that will be transported in
the LSPs. If a packet length, once it has been encapsulated on the
ingress LSR, exceeds the LSP MTU, it MUST be dropped. If an egress
LSR receives a packet on a VC LSP with a length, once the label stack
and sequencing control word have been popped, that exceeds the MTU of
the destination layer 2 interface it MUST be dropped.
5. Protocol-Specific Issues
5.1. Frame Relay
A Frame Relay PDU is transported in its entirety, including the Frame
Relay header. The sequencing control word is OPTIONAL.
The BECN and FECN signals are carried unchanged across the network in
the Frame Relay header. These signals do not appear in the MPLS
header, and are unseen by the MPLS network. The Label Edge Routers
that implement this document MAY, when either adding or removing the
encapsulation described herein, change a zero to a one in either or
both of these bits in order to reflect congestion in the MPLS network
that is known to the LERs. The BECN and FECN bits MUST NEVER be
changed from one to zero.
The ingress LSR MAY consider the DE bit of the Frame Relay header
when determining the value to be placed in the EXP fields of the MPLS
label stack. In a similar way, the egress LSR MAY consider the EXP
field of the VC label when queuing the packet for egress.
5.2. ATM
Two encapsulations are supported for ATM transport: one for AAL5
CPCS-PDUs and another for ATM cells. The AAL5 CPCS-PDU encapsulation
consists of the MPLS label stack, an optional sequencing control
word, and the AAL5 CPCS-PDU. The ATM cell encapsulation consists of
an MPLS label stack, a required generic sequencing control word, a 4
byte ATM cell header, and the ATM cell payload as shown below:
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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 |T| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI | PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Two transport modes are supported for ATM transport, VCC transport
and VPC transport.
VCC transport mode may be used to transport ATM Adaptation Layer 5
(AAL5) CPCS-PDUs, ATM cells, or both, across the VC LSP. The
sequencing control word is optional for VCC transport if only AAL5
CPCS-PDUs are to be transported. If ATM cells, or a combination of
ATM cells and AAL5 CPCS-PDUs, are to be transported the sequencing
control word is required.
VPC transport mode may only be used to transport ATM cells. The
sequencing control word is optional in VPC transport mode.
* T (transport type) bit
In VCC transport mode, bit (T) of the sequencing control word
indicates whether the MPLS packet contains an ATM cell or an AAL5
CPCS-PDU. If set the packet contains an ATM cell, otherwise it
contains an AAL5 CPCS-PDU. In VPC transport mode this bit MUST be
set to 1.
* VPI
In both modes the ingress router MAY copy the VPI field from the
incoming cell into this field. The egress router MAY generate a
new VPI based on the value of the VC label and ignore the VPI
contained in this field.
* VCI
In VCC transport mode the ingress router MAY copy the VCI field
from the incoming ATM cell header into this field and the egress
router MAY generate a new VCI based on the value of the VC label.
When in VPC transport mode the ingress LSR MUST copy the VCI
field from the incoming cell into this field and the egress LSR
MUST copy the VCI from this field into the outgoing ATM cell
header.
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* PTI & CLP
When present the ingress router SHOULD copy the PTI and CLP
fields of the outgoing frame from the ATM cell header and the
egress router SHOULD set the left-most and EFCI bits of the PTI
in all outgoing cells to that contained in the PTI field of the
FAST header and set the CLP bit of outgoing cells to the CLP bit
contained in the FAST header. [7]
5.2.1. OAM Cell Support
OAM cells MAY be transported on the VC LSP. A router that does not
support transport of OAM cells MUST discard incoming MPLS frames on
an ATM VC LSP that contain an ATM cell with the high-order bit of the
PTI field set to 1. A router that supports transport of OAM cells
MUST follow the procedures outlined in [7] section 8 for mode 0 only,
in addition to the applicable procedures specified in [5].
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 an 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
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 MUST generate AIS F5 OAM cells on the VC associated
with the withdrawn label mapping.
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5.2.2. CLP Bit to EXP Bit Mapping
The ingress LSR MAY consider the CLP bit when determining the value
to be placed in the EXP fields of the MPLS label stack.
The egress LSR MAY consider the value of the EXP field of the VC
label when determining the value of the ATM CLP bit.
5.3. Ethernet VLAN
For an ethernet 802.1q VLAN the entire ethernet frame without the
preamble or FCS is transported as a single packet. The sequencing
control word is OPTIONAL. If a packet is received out of sequence it
MUST be dropped. The 4 byte VLAN tag is transported as is, and MAY be
overwritten by the egress LSR.
The ingress LSR MAY consider the user priority field [4] of the VLAN
tag header when determining the value to be placed in the EXP fields
of the MPLS label stack. In a similar way, the egress LSR MAY
consider the EXP field of the VC label when queuing the packet for
egress. Ethernet packets containing hardware level CRC, Framing
errors, or runt packets MUST be discarded on input.
5.4. Ethernet
For simple ethernet port to port transport, the entire ethernet frame
without the preamble or FCS is transported as a single packet. The
sequencing control word is OPTIONAL. If a packet is received out of
sequence it MUST be dropped. As in the Ethernet VLAN case, ethernet
packets with hardware level CRC, framing, and runt packets MUST be
discarded on input.
5.5. HDLC ( Cisco )
HDLC (Cisco) mode provides port to port transport of Cisco HDLC
encapsulated traffic. The HDLC PDU is transported in its entirety,
including the HDLC address, control and protocol fields, but
excluding HDLC flags and the FCS. Bit stuffing is undone. The
sequencing control word is OPTIONAL.
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5.6. PPP
PPP mode provides point to point transport of PPP encapsulated
traffic, as specified in [8]. The PPP PDU is transported in its
entirety, including the protocol field, but excluding any media-
specific framing information, such as HDLC address and control fields
or FCS. The sequencing control word is OPTIONAL.
6. Security Considerations
This document does not affect the underlying security issues of MPLS.
7. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
8. 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] "Transport of Layer 2 Frames Over MPLS", draft-martini-
l2circuit-trans-mpls-04.txt. ( work in progress )
[6] ITU Recommendation I.610, "B-ISDN operation and maintenance
principles and functions", 1999.
[7] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000.
[8] "The Point-to-Point Protocol (PPP)", RFC 1661.
[note1] FEC element type 128 is pending IANA approval.
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9. Author Information
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: luca@level3.net
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
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: dvlachos@cisco.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
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Steve Vogelsang
Laurel Networks, Inc.
2607 Nicholson Rd.
Sewickley, PA 15143
e-mail: sjv@laurelnetworks.com
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
Martini, et al. [Page 10]
