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<pre>Network Working Group L. Martini, Ed.
Request for Comments: 4905 E. Rosen, Ed.
Category: Historic Cisco Systems, Inc.
N. El-Aawar, Ed.
Level 3 Communications, LLC
June 2007
<span class="h1">Encapsulation Methods for Transport of</span>
<span class="h1">Layer 2 Frames over MPLS Networks</span>
Status of This Memo
This memo defines a Historic Document for the Internet community. It
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes methods for encapsulating the Protocol Data
Units (PDUs) of layer 2 protocols such as Frame Relay, Asynchronous
Transfer Mode (ATM), or Ethernet for transport across an MPLS
network. This document describes the so-called "<a href="./draft-martini">draft-martini</a>"
protocol, which has since been superseded by the Pseudowire Emulation
Edge to Edge Working Group specifications described in <a href="./rfc4447">RFC 4447</a> and
related documents.
<span class="grey">Martini, et al. Historic [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
<a href="#section-2">2</a>. Specification of Requirements ...................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. Special Note ....................................................<a href="#page-4">4</a>
<a href="#section-4">4</a>. General Encapsulation Method ....................................<a href="#page-4">4</a>
<a href="#section-4.1">4.1</a>. The Control Word ...........................................<a href="#page-4">4</a>
<a href="#section-4.1.1">4.1.1</a>. Setting the Sequence Number .........................<a href="#page-5">5</a>
<a href="#section-4.1.2">4.1.2</a>. Processing the Sequence Number ......................<a href="#page-6">6</a>
<a href="#section-4.2">4.2</a>. MTU Requirements ...........................................<a href="#page-6">6</a>
<a href="#section-5">5</a>. Protocol-Specific Details .......................................<a href="#page-7">7</a>
<a href="#section-5.1">5.1</a>. Frame Relay ................................................<a href="#page-7">7</a>
<a href="#section-5.2">5.2</a>. ATM ........................................................<a href="#page-8">8</a>
<a href="#section-5.2.1">5.2.1</a>. ATM AAL5 CPCS-SDU Mode ..............................<a href="#page-9">9</a>
<a href="#section-5.2.2">5.2.2</a>. ATM Cell Mode ......................................<a href="#page-10">10</a>
<a href="#section-5.2.3">5.2.3</a>. OAM Cell Support ...................................<a href="#page-12">12</a>
<a href="#section-5.2.4">5.2.4</a>. CLP bit to Quality of Service Mapping ..............<a href="#page-12">12</a>
<a href="#section-5.3">5.3</a>. Ethernet VLAN .............................................<a href="#page-12">12</a>
<a href="#section-5.4">5.4</a>. Ethernet ..................................................<a href="#page-12">12</a>
<a href="#section-5.5">5.5</a>. High-Level Data Link Control (HDLC) .......................<a href="#page-13">13</a>
<a href="#section-5.6">5.6</a>. PPP .......................................................<a href="#page-13">13</a>
<a href="#section-6">6</a>. Using an MPLS Label as the Demultiplexer Field .................<a href="#page-13">13</a>
<a href="#section-6.1">6.1</a>. MPLS Shim EXP Bit Values ..................................<a href="#page-14">14</a>
<a href="#section-6.2">6.2</a>. MPLS Shim S Bit Value .....................................<a href="#page-14">14</a>
<a href="#section-6.3">6.3</a>. MPLS Shim TTL Values ......................................<a href="#page-14">14</a>
<a href="#section-7">7</a>. Security Considerations ........................................<a href="#page-14">14</a>
<a href="#section-8">8</a>. Normative References ...........................................<a href="#page-14">14</a>
<a href="#section-9">9</a>. Informative References .........................................<a href="#page-16">16</a>
<a href="#section-10">10</a>. Co-Authors ....................................................<a href="#page-16">16</a>
<span class="grey">Martini, et al. Historic [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
In an MPLS network, it is possible to use control protocols such as
those specified in [<a href="./rfc4906" title=""Transport of Layer 2 Frames Over MPLS"">RFC4906</a>] to set up "emulated virtual circuits"
that carry the Protocol Data Units of layer 2 protocols across the
network. A number of these emulated virtual circuits (VCs) may be
carried in a single tunnel. This requires, of course, that the layer
2 PDUs be encapsulated. We can distinguish three layers of this
encapsulation:
- the "tunnel header", which contains the information needed to
transport the PDU across the MPLS network; this header belongs
to the tunneling protocol, e.g., MPLS, Generic Routing
Encapsulation (GRE), and Layer 2 Tunneling Protocol (L2TP).
- the "demultiplexer field", which is used to distinguish
individual emulated virtual circuits within a single tunnel;
this field must be understood by the tunneling protocol as well;
it may be, e.g., an MPLS label or a GRE key field.
- the "emulated VC encapsulation", which contains the information
about the enclosed layer 2 PDU that is necessary in order to
properly emulate the corresponding layer 2 protocol.
This document specifies the emulated VC encapsulation for a number of
layer 2 protocols. Although different layer 2 protocols require
different information to be carried in this encapsulation, an attempt
has been made to make the encapsulation as common as possible for all
layer 2 protocols.
This document also specifies the way in which the demultiplexer field
is added to the emulated VC encapsulation when an MPLS label is used
as the demultiplexer field.
Quality of service (QoS)-related issues are not discussed in this
document.
For the purpose of this document, R1 will be defined as the ingress
router, and R2 as the egress router. A layer 2 PDU will be received
at R1, encapsulated at R1, transported, decapsulated at R2, and
transmitted out of R2.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Specification of Requirements</span>
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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="grey">Martini, et al. Historic [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Special Note</span>
This document describes the so called "<a href="./draft-martini">draft-martini</a>" protocol, which
is used in many deployed implementations. This document and its
contents have since been superseded by the Pseudowire Emulation Edge
to Edge Working Group specifications: [<a href="./rfc4447" title=""Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)"">RFC4447</a>], [<a href="./rfc4385" title=""Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN"">RFC4385</a>],
[<a href="./rfc4448" title=""Encapsulation Methods for Transport of Ethernet over MPLS Networks"">RFC4448</a>], [<a href="./rfc4717" title=""Encapsulation Methods for Transport of Asynchronous Transfer Mode (ATM) over MPLS Networks"">RFC4717</a>], [<a href="./rfc4618" title=""Encapsulation Methods for Transport of PPP/High-Level Data Link Control (HDLC) over MPLS Networks"">RFC4618</a>], [<a href="./rfc4619" title=""Encapsulation Methods for Transport of Frame Relay over Multiprotocol Label Switching (MPLS) Networks"">RFC4619</a>], [<a href="./rfc4553" title=""Structure- Agnostic Time Division Multiplexing (TDM) over Packet (SAToP)"">RFC4553</a>], [<a href="./rfc4842" title=""Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) Circuit Emulation over Packet (CEP)"">RFC4842</a>], and
related documents. This document serves as documentation of current
implementations, and MUST NOT be used for new implementations. The
PWE3 Label Distribution Protocol control protocol document [<a href="./rfc4447" title=""Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)"">RFC4447</a>],
which is backward compatible with this document, MUST be used for all
new implementations of this protocol.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. General Encapsulation Method</span>
In most cases, it is not necessary to transport the layer 2
encapsulation across the network; rather, the layer 2 header can be
stripped at R1 and reproduced at R2. This is done using information
carried in the control word (see below), as well as information that
may already have been signaled from R1 to R2.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. The Control Word</span>
There are three requirements that may need to be satisfied when
transporting layer 2 protocols over an MPLS backbone:
-i. Sequentiality may need to be preserved.
-ii. Small packets may need to be padded in order to be transmitted
on a medium where the minimum transport unit is larger than the
actual packet size.
-iii. Control bits carried in the header of the layer 2 frame may
need to be transported.
The control word defined here addresses all three of these
requirements. For some protocols, this word is REQUIRED, and for
others OPTIONAL. For protocols where the control word is OPTIONAL,
implementations MUST support sending no control word, and MAY support
sending a control word.
In all cases, the egress router must be aware of whether the ingress
router will send a control word over a specific virtual circuit.
This may be achieved by configuration of the routers or by signaling,
for example, as defined in [<a href="./rfc4906" title=""Transport of Layer 2 Frames Over MPLS"">RFC4906</a>].
<span class="grey">Martini, et al. Historic [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
The 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rsvd | Flags |0 0| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram, the first 4 bits are reserved for future use.
They MUST be set to 0 when transmitting, and MUST be ignored upon
receipt.
The next 4 bits provide space for carrying protocol-specific flags.
These are defined in the protocol-specific details below.
The next 2 bits MUST be set to 0 when transmitting.
The next 6 bits provide a length field, which is used as follows: If
the packet's length (defined as the length of the layer 2 payload
plus the length of the control word) is less than 64 bytes, the
length field MUST be set to the packet's length. Otherwise, the
length field MUST be set to 0. The value of the length field, if
non-zero, can be used to remove any padding. When the packet reaches
the service provider's egress router, it may be desirable to remove
the padding before forwarding the packet.
The next 16 bits provide a sequence number that can be used to
guarantee ordered packet delivery. The processing of the sequence
number field is OPTIONAL.
The sequence number space is a 16-bit, unsigned circular space. The
sequence number value 0 is used to indicate an unsequenced packet.
<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>. Setting the Sequence Number</span>
For a given emulated VC, and a pair of routers R1 and R2, if R1
supports packet sequencing, then the following procedures should be
used:
- The initial packet transmitted on the emulated VC MUST use
sequence number 1.
- Subsequent packets MUST increment the sequence number by 1 for
each packet.
- When the transmit sequence number reaches the maximum 16 bit
value (65535), the sequence number MUST wrap to 1.
<span class="grey">Martini, et al. Historic [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
If the transmitting router R1 does not support sequence number
processing, then the sequence number field in the control word MUST
be set to 0.
<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>. Processing the Sequence Number</span>
If a router R2 supports receive sequence number processing, then the
following procedures should be used:
When an emulated VC is initially set up, the "expected sequence
number" associated with it MUST be initialized to 1.
When a packet is received on that emulated VC, the sequence number
should be processed as follows:
- If the sequence number on the packet is 0, then the packet
passes the sequence number check.
- Else if the packet sequence number >= the expected sequence
number and the packet sequence number - the expected sequence
number < 32768, then the packet is in order.
- Else if the packet sequence number < the expected sequence
number and the expected sequence number - the packet sequence
number >= 32768, then the packet is in order.
- Otherwise, the packet is out of order.
If a packet passes the sequence number check or is in order, then it
can be delivered immediately. If the packet is in order, then the
expected sequence number should be set using the algorithm:
expected_sequence_number := packet_sequence_number + 1 mod 2**16
if (expected_sequence_number = 0) then expected_sequence_number := 1;
Packets that are received out of order MAY be dropped or reordered at
the discretion of the receiver.
If a router R2 does not support receive sequence number processing,
then the sequence number field MAY be ignored.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. MTU Requirements</span>
The network MUST be configured with an MTU that is sufficient to
transport the largest encapsulation frames. If MPLS is used as the
tunneling protocol, for example, this is likely to be 12 or more
bytes greater than the largest frame size. Other tunneling protocols
may have longer headers and require larger MTUs. If the ingress
<span class="grey">Martini, et al. Historic [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
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router determines that an encapsulated layer 2 PDU exceeds the MTU of
the tunnel through which it must be sent, the PDU MUST be dropped.
If an egress router receives an encapsulated layer 2 PDU whose
payload length (i.e., the length of the PDU itself without any of the
encapsulation headers) exceeds the MTU of the destination layer 2
interface, the PDU MUST be dropped.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Protocol-Specific Details</span>
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Frame Relay</span>
A Frame Relay PDU is transported without the Frame Relay header or
the Frame Check Sequence (FCS). The control word is REQUIRED;
however, its use is optional, although desirable. Use of the control
word means that the ingress and egress Label Switching Routers (LSRs)
follow the procedures below. If an ingress LSR chooses not to use
the control word, it MUST set the flags in the control word to 0; if
an egress LSR chooses to ignore the control word, it MUST set the
Frame Relay control bits to 0.
The BECN (Backward Explicit Congestion Notification), FECN (Forward
Explicit Congestion Notification), DE (Discard Eligibility), and C/R
(Command/Response) bits are carried across the network in the control
word. The edge routers that implement this document MAY, when either
adding or removing the encapsulation described herein, change the
BECN and/or FECN bits from 0 to 1 in order to reflect congestion in
the network that is known to the edge routers, and the D/E bit from 0
to 1 to reflect marking from edge policing of the Frame Relay
Committed Information Rate. The BECN, FECN, and D/E bits SHOULD NOT
be changed from 1 to 0.
The following is an example of a Frame Relay packet:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rsvd |B|F|D|C| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Relay PDU |
| " |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<span class="grey">Martini, et al. Historic [Page 7]</span></pre>
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* B ( BECN ) Bit
The ingress router, R1, SHOULD copy the BECN field from the
incoming Frame Relay header into this field. The egress router,
R2, MUST generate a new BECN field based on the value of the B
bit.
* F ( FECN ) Bit
The ingress router, R1, SHOULD copy the FECN field from the
incoming Frame Relay header into this field. The egress router,
R2, MUST generate a new FECN field based on the value of the F
bit.
* D ( DE ) Bit
The ingress router, R1, SHOULD copy the DE field from the
incoming Frame Relay header into this field. The egress router,
R2, MUST generate a new DE field based on the value of the D
bit.
If the tunneling protocol provides a field that can be set to
specify a Quality of Service, the ingress router, R1, MAY
consider the DE bit of the Frame Relay header when determining
the value of that field. The egress router MAY then consider
the value of this field when queuing the layer 2 PDU for egress.
Note however that frames from the same VC MUST NOT be reordered.
* C ( C/R ) Bit
The ingress router, R1, SHOULD copy the C/R bit from the
received Frame Relay PDU to the C bit of the control word. The
egress router, R2, MUST copy the C bit into the output frame.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. ATM</span>
Two encapsulations are supported for ATM transport: one for ATM
Adaption Layer 5 (AAL5) and another for ATM cells.
The AAL5 Common Part Convergence Sublayer - Service Data Unit
(CPCS-SDU) encapsulation consists of the REQUIRED control word and
the AAL5 CPCS-SDU. The ATM cell encapsulation consists of an
OPTIONAL control word, a 4-byte ATM cell header, and the ATM cell
payload.
<span class="grey">Martini, et al. Historic [Page 8]</span></pre>
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<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>. ATM AAL5 CPCS-SDU Mode</span>
In ATM AAL5 mode, the ingress router is required to reassemble AAL5
CPCS-SDUs from the incoming VC and transport each CPCS-SDU as a
single packet. No AAL5 trailer is transported. The control word is
REQUIRED; its use, however, is optional, although desirable. Use of
the control word means that the ingress and egress LSRs follow the
procedures below. If an ingress LSR chooses not to use the control
word, it MUST set the flags in the control word to 0; if an egress
LSR chooses to ignore the control word, it MUST set the ATM control
bits to 0.
The EFCI (Explicit Forward Congestion Indication) and CLP (Cell Loss
Priority) bits are carried across the network in the control word.
The edge routers that implement this document MAY, when either adding
or removing the encapsulation described herein, change the EFCI bit
from 0 to 1 in order to reflect congestion in the network that is
known to the edge routers, and the CLP bit from 0 to 1 to reflect
marking from edge policing of the ATM Sustained Cell Rate. The EFCI
and CLP bits MUST NOT be changed from 1 to 0.
The AAL5 CPCS-SDU is prepended by the following header:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rsvd |T|E|L|C| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM AAL5 CPCS-SDU |
| " |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* T (transport type) bit
Bit (T) of the control word indicates whether the packet
contains an ATM cell or an AAL5 CPCS-SDU. If set, the packet
contains an ATM cell, encapsulated according to the ATM cell
mode section below; otherwise, it contains an AAL5 CPCS-SDU.
The ability to transport an ATM cell in the AAL5 mode is
intended to provide a means of enabling Operations and
Management (OAM) functionality over the AAL5 VC.
<span class="grey">Martini, et al. Historic [Page 9]</span></pre>
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<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
* E ( EFCI ) Bit
The ingress router, R1, SHOULD set this bit to 1 if the EFCI bit
of the final cell of those that transported the AAL5 CPCS-SDU is
set to 1, or if the EFCI bit of the single ATM cell to be
transported in the packet is set to 1. Otherwise, this bit
SHOULD be set to 0. The egress router, R2, SHOULD set the EFCI
bit of all cells that transport the AAL5 CPCS-SDU to the value
contained in this field.
* L ( CLP ) Bit
The ingress router, R1, SHOULD set this bit to 1 if the CLP bit
of any of the ATM cells that transported the AAL5 CPCS-SDU is
set to 1, or if the CLP bit of the single ATM cell to be
transported in the packet is set to 1. Otherwise, this bit
SHOULD be set to 0. The egress router, R2, SHOULD set the CLP
bit of all cells that transport the AAL5 CPCS-SDU to the value
contained in this field.
* C ( Command / Response Field ) Bit
When FRF.8.1 Frame Relay / ATM PVC Service Interworking
[<a href="#ref-FRF.8.1" title=""Frame Relay / ATM PVC Service Interworking Implementation Agreement"">FRF.8.1</a>] traffic is being transported, the CPCS-UU Least
Significant Bit (LSB) of the AAL5 CPCS-SDU may contain the Frame
Relay C/R bit. The ingress router, R1, SHOULD copy this bit to
the C bit of the control word. The egress router, R2, SHOULD
copy the C bit to the CPCS-UU Least Significant Bit (LSB) of the
AAL5 CPCS PDU.
<span class="h4"><a class="selflink" id="section-5.2.2" href="#section-5.2.2">5.2.2</a>. ATM Cell Mode</span>
In this encapsulation mode, ATM cells are transported individually
without a Segmentation and Reassembly (SAR) process. The ATM cell
encapsulation consists of an OPTIONAL control word, and one or more
ATM cells - each consisting of a 4-byte ATM cell header and the 48-
byte ATM cell payload. This ATM cell header is defined in the FAST
encapsulation [<a href="#ref-FAST" title=""Frame Based ATM over SONET/SDH Transport (FAST)"">FAST</a>] <a href="#section-3.1.1">section 3.1.1</a>, but without the trailer byte.
The length of each frame, without the encapsulation headers, is a
multiple of 52 bytes long. The maximum number of ATM cells that can
be fitted in a frame, in this fashion, is limited only by the network
MTU and by the ability of the egress router to process them. The
ingress router MUST NOT send more cells than the egress router is
willing to receive. The number of cells that the egress router is
willing to receive may either be configured in the ingress router or
may be signaled, for example, using the methods described in
[<a href="./rfc4906" title=""Transport of Layer 2 Frames Over MPLS"">RFC4906</a>]. The number of cells encapsulated in a particular frame
can be inferred by the frame length. The control word is OPTIONAL.
<span class="grey">Martini, et al. Historic [Page 10]</span></pre>
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If the control word is used, then the flag bits in the control word
are not used, and MUST be set to 0 when transmitting, and MUST be
ignored upon receipt.
The EFCI and CLP bits are carried across the network in the ATM cell
header. The edge routers that implement this document MAY, when
either adding or removing the encapsulation described herein, change
the EFCI bit from 0 to 1 in order to reflect congestion in the
network that is known to the edge router, and the CLP bit from 0 to 1
to reflect marking from edge policing of the ATM Sustained Cell Rate.
The EFCI and CLP bits SHOULD NOT be changed from 1 to 0.
This diagram illustrates an encapsulation of two ATM cells:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control word ( Optional ) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI | PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Payload ( 48 bytes ) |
| " |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI | PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Payload ( 48 bytes ) |
| " |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* VPI (Virtual Path Identifier)
The ingress router MUST copy the VPI field from the incoming
cell into this field. For particular emulated VCs, the egress
router MAY generate a new VPI and ignore the VPI contained in
this field.
* VCI (Virtual Circuit Identifier)
The ingress router MUST copy the VCI field from the incoming ATM
cell header into this field. For particular emulated VCs, the
egress router MAY generate a new VCI.
<span class="grey">Martini, et al. Historic [Page 11]</span></pre>
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* PTI (Payload Type Identifier) & CLP ( C bit )
The PTI and CLP fields are the PTI and CLP fields of the
incoming ATM cells. The cell headers of the cells within the
packet are the ATM headers (without HEC) of the incoming cell.
<span class="h4"><a class="selflink" id="section-5.2.3" href="#section-5.2.3">5.2.3</a>. OAM Cell Support</span>
OAM cells MAY be transported on the VC LSP. An egress router that
does not support transport of OAM cells MUST discard frames 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 [<a href="#ref-FAST" title=""Frame Based ATM over SONET/SDH Transport (FAST)"">FAST</a>] <a href="#section-8">section 8</a> for mode 0 only, in addition
to the applicable procedures specified in [<a href="./rfc4906" title=""Transport of Layer 2 Frames Over MPLS"">RFC4906</a>].
<span class="h4"><a class="selflink" id="section-5.2.4" href="#section-5.2.4">5.2.4</a>. CLP bit to Quality of Service Mapping</span>
The ingress router MAY consider the CLP bit when determining the
value to be placed in the Quality of Service fields (e.g., the EXP
fields of the MPLS label stack) of the encapsulating protocol. This
gives the network visibility of the CLP bit. Note however that cells
from the same VC MUST NOT be reordered.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Ethernet VLAN</span>
For an Ethernet 802.1q VLAN, the entire Ethernet frame without the
preamble or FCS is transported as a single packet. The control word
is OPTIONAL. If the control word is used, then the flag bits in the
control word are not used, and MUST be set to 0 when transmitting,
and MUST be ignored upon receipt. The 4-byte VLAN tag is transported
as is, and MAY be overwritten by the egress router.
The ingress router MAY consider the user priority field [<a href="#ref-IEEE802.3ac" title=""Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements Part 3: Carrier sense multiple access with collision detection (CSMA/CD) frame extensions for Virtual Bridged Local Area Networks (VLAN) tagging on 802.3 networks"">IEEE802.3ac</a>]
of the VLAN tag header when determining the value to be placed in the
Quality of Service field of the encapsulating protocol (e.g., the EXP
fields of the MPLS label stack). In a similar way, the egress router
MAY consider the Quality of Service field of the encapsulating
protocol when queuing the packet for egress. Ethernet packets
containing hardware-level Cyclic Redundancy Check (CRC) errors,
framing errors, or runt packets MUST be discarded on input.
<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>. Ethernet</span>
For simple Ethernet port to port transport, the entire Ethernet frame
without the preamble or FCS is transported as a single packet. The
control word is OPTIONAL. If the control word is used, then the flag
bits in the control word are not used, and MUST be set to 0 when
transmitting, and MUST be ignored upon receipt. As in the Ethernet
<span class="grey">Martini, et al. Historic [Page 12]</span></pre>
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VLAN case, Ethernet packets with hardware-level CRC errors, framing
errors, and runt packets MUST be discarded on input.
<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>. High-Level Data Link Control (HDLC)</span>
HDLC mode provides port to port transport of 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/byte stuffing is undone. The control word is
OPTIONAL. If the control word is used, then the flag bits in the
control word are not used, and MUST be set to 0 when transmitting,
and MUST be ignored upon receipt.
The HDLC mode is suitable for port to port transport of Frame Relay
User-Network Interface (UNI) or Network-Network Interface (NNI)
traffic. It must be noted, however, that this mode is transparent to
the FECN, BECN, and DE bits.
<span class="h3"><a class="selflink" id="section-5.6" href="#section-5.6">5.6</a>. PPP</span>
PPP mode provides point to point transport of PPP-encapsulated
traffic, as specified in [<a href="./rfc1661" title=""The Point-to-Point Protocol (PPP)"">RFC1661</a>]. The PPP PDU is transported in
its entirety, including the protocol field (whether compressed using
PFC or not), but excluding any media-specific framing information,
such as HDLC address and control fields or FCS. Since media-specific
framing is not carried, the following options will not operate
correctly if the PPP peers attempt to negotiate them:
- Frame Check Sequence (FCS) Alternatives
- Address-and-Control-Field-Compression (ACFC)
- Asynchronous-Control-Character-Map (ACCM)
Note also that VC LSP Interface MTU negotiation as specified in
[<a href="./rfc4906" title=""Transport of Layer 2 Frames Over MPLS"">RFC4906</a>] is not affected by PPP Maximum Receive Unit (MRU)
advertisement. Thus, if a PPP peer sends a PDU with a length in
excess of that negotiated for the VC LSP, that PDU will be discarded
by the ingress router.
The control word is OPTIONAL. If the control word is used, then the
flag bits in the control word are not used, and MUST be set to 0 when
transmitting, and MUST be ignored upon receipt.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Using an MPLS Label as the Demultiplexer Field</span>
To use an MPLS label as the demultiplexer field, a 32-bit label stack
entry [<a href="./rfc3032" title=""MPLS Label Stack Encoding"">RFC3032</a>] is simply prepended to the emulated VC encapsulation,
and hence will appear as the bottom label of an MPLS label stack.
This label may be called the "VC label". The particular emulated VC
<span class="grey">Martini, et al. Historic [Page 13]</span></pre>
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identified by a particular label value must be agreed by the ingress
and egress LSRs, either by signaling (e.g., via the methods of
[<a href="./rfc4906" title=""Transport of Layer 2 Frames Over MPLS"">RFC4906</a>]) or by configuration. Other fields of the label stack
entry are set as follows.
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. MPLS Shim EXP Bit Values</span>
If it is desired to carry Quality of Service information, the Quality
of Service information SHOULD be represented in the EXP field of the
VC label. If more than one MPLS label is imposed by the ingress LSR,
the EXP field of any labels higher in the stack SHOULD also carry the
same value.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. MPLS Shim S Bit Value</span>
The ingress LSR, R1, MUST set the S bit of the VC label to a value of
1 to denote that the VC label is at the bottom of the stack.
<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>. MPLS Shim TTL Values</span>
The ingress LSR, R1, SHOULD set the TTL field of the VC label to a
value of 2.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Security Considerations</span>
This document specifies only encapsulations, and not the protocols,
used to carry the encapsulated packets across the network. Each such
protocol may have its own set of security issues, but those issues
are not affected by the encapsulations specified herein. More
detailed security considerations are also described in <a href="./rfc4447#section-8">Section 8 of
[RFC4447]</a>.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC4447">RFC4447</a>] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
and G. Heron, "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", <a href="./rfc4447">RFC 4447</a>, April
2006.
[<a id="ref-RFC4385">RFC4385</a>] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word
for Use over an MPLS PSN", <a href="./rfc4385">RFC 4385</a>, February 2006.
<span class="grey">Martini, et al. Historic [Page 14]</span></pre>
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[<a id="ref-RFC4842">RFC4842</a>] Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
"Synchronous Optical Network/Synchronous Digital
Hierarchy (SONET/SDH) Circuit Emulation over Packet
(CEP)", <a href="./rfc4842">RFC 4842</a>, April 2007.
[<a id="ref-RFC4553">RFC4553</a>] Vainshtein, A., Ed., and YJ. Stein, Ed., "Structure-
Agnostic Time Division Multiplexing (TDM) over Packet
(SAToP)", <a href="./rfc4553">RFC 4553</a>, June 2006.
[<a id="ref-RFC4619">RFC4619</a>] Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed.,
"Encapsulation Methods for Transport of Frame Relay
over Multiprotocol Label Switching (MPLS) Networks",
<a href="./rfc4619">RFC 4619</a>, September 2006.
[<a id="ref-RFC4717">RFC4717</a>] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N.,
Brayley, J., and G. Koleyni, "Encapsulation Methods for
Transport of Asynchronous Transfer Mode (ATM) over MPLS
Networks", <a href="./rfc4717">RFC 4717</a>, December 2006.
[<a id="ref-RFC4618">RFC4618</a>] Martini, L., Rosen, E., Heron, G., and A. Malis,
"Encapsulation Methods for Transport of PPP/High-Level
Data Link Control (HDLC) over MPLS Networks", <a href="./rfc4618">RFC 4618</a>,
September 2006.
[<a id="ref-RFC4448">RFC4448</a>] Martini, L., Ed., Rosen, E., El-Aawar, N., and G.
Heron, "Encapsulation Methods for Transport of Ethernet
over MPLS Networks", <a href="./rfc4448">RFC 4448</a>, April 2006.
[<a id="ref-RFC4906">RFC4906</a>] Martini, L., Ed., Rosen, E., Ed., and N. El-Aawar, Ed.,
"Transport of Layer 2 Frames Over MPLS", <a href="./rfc4906">RFC 4906</a>, June
2007.
[<a id="ref-RFC3032">RFC3032</a>] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", <a href="./rfc3032">RFC 3032</a>, January 2001.
[<a id="ref-FRF.8.1">FRF.8.1</a>] Frame Relay Forum, "Frame Relay / ATM PVC Service
Interworking Implementation Agreement", February 2000.
[<a id="ref-FAST">FAST</a>] ATM Forum, "Frame Based ATM over SONET/SDH Transport
(FAST)", af-fbatm-0151.000, July 2000.
<span class="grey">Martini, et al. Historic [Page 15]</span></pre>
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<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
[<a id="ref-IEEE802.3ac">IEEE802.3ac</a>] IEEE 802.3ac-1998, "Information technology -
Telecommunications and information exchange between
systems - Local and metropolitan area networks -
Specific requirements Part 3: Carrier sense multiple
access with collision detection (CSMA/CD) frame
extensions for Virtual Bridged Local Area Networks
(VLAN) tagging on 802.3 networks".
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Informative References</span>
[<a id="ref-RFC1661">RFC1661</a>] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
STD 51, <a href="./rfc1661">RFC 1661</a>, July 1994.
<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. Co-Authors</span>
Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
EMail: giles.heron@tellabs.com
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
EMail: d@mazunetworks.com
Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: tappan@cisco.com
Jayakumar Jayakumar
Cisco Systems Inc.
225, E.Tasman, MS-SJ3/3,
San Jose, CA 95134
EMail: jjayakum@cisco.com
<span class="grey">Martini, et al. Historic [Page 16]</span></pre>
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Alex Hamilton
Cisco Systems Inc.
285 W. Tasman, MS-SJCI/3/4,
San Jose, CA 95134
EMail: tahamilt@cisco.com
Steve Vogelsang
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: sjv@laurelnetworks.com
John Shirron
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: jshirron@laurelnetworks.com
Toby Smith
Network Appliance, Inc.
800 Cranberry Woods Drive
Suite 300
Cranberry Township, PA 16066
EMail: tob@netapp.com
Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
EMail: Andy.Malis@tellabs.com
Vinai Sirkay
Redback Networks
300 Holger Way
San Jose, CA 95134
EMail: vsirkay@redback.com
<span class="grey">Martini, et al. Historic [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica MA 01821
EMail: vasile@nortelnetworks.com
Chris Liljenstolpe
Alcatel
11600 Sallie Mae Dr.
9th Floor
Reston, VA 20193
EMail: chris.liljenstolpe@alcatel.com
Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
EMail: dcooper@gblx.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
EMail: kireeti@juniper.net
<span class="grey">Martini, et al. Historic [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
Authors' Addresses
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
EMail: lmartini@cisco.com
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO 80021
EMail: nna@level3.net
Eric Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: erosen@cisco.com
<span class="grey">Martini, et al. Historic [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc4905">RFC 4905</a> Encapsulation for L2 Frames over MPLS June 2007</span>
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.
Copies of IPR disclosures made to the IETF Secretariat and any
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such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
<a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.
The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Martini, et al. Historic [Page 20]
</pre>
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