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rfc:rfc5143

Network Working Group A. Malis Request for Comments: 5143 Verizon Communications Obsoleted by: 4842 J. Brayley Category: Historic J. Shirron

                                                      ECI Telecom Inc.
                                                            L. Martini
                                                   Cisco Systems, Inc.
                                                          S. Vogelsang
                                                        Alcatel-Lucent
                                                         February 2008

Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH)

     Circuit Emulation Service over MPLS (CEM) Encapsulation

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.

IESG Note

 The IESG thinks that this work is related to IETF work done in WG
 PWE3, but this does not prevent publishing.

Abstract

 This document describes a historical method for encapsulating
 Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH)
 Path signals for transport across packet-switched networks (PSNs).
 The PSNs explicitly supported by this document include MPLS and IP.
 Note that RFC 4842 describes the standards-track protocol for this
 functionality, and new implementations must use RFC 4842 rather than
 this document except when interoperability with older implementations
 is desired.

Malis, et al. Historic [Page 1] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

Table of Contents

 1. Introduction ....................................................3
 2. Conventions Used in This Document ...............................3
 3. Scope ...........................................................3
 4. CEM Encapsulation Format ........................................4
    4.1. Transport Encapsulation ....................................6
         4.1.1. MPLS Transport ......................................6
         4.1.2. IP Transport ........................................7
 5. CEM Operation ...................................................8
    5.1. Introduction and Terminology ...............................8
         5.1.1. CEM Packetizer and De-Packetizer ....................9
         5.1.2. CEM DBA .............................................9
    5.2. Description of Normal CEM Operation .......................10
    5.3. Description of CEM Operation during DBA ...................10
    5.4. Packet Synchronization ....................................11
         5.4.1. Acquisition of Packet Synchronization ..............11
         5.4.2. Loss of Packet Synchronization .....................11
 6. SONET/SDH Maintenance Signals ..................................12
    6.1. SONET/SDH to PSN ..........................................12
         6.1.1. AIS-P Indication ...................................13
         6.1.2. STS SPE Unequipped Indication ......................14
         6.1.3. CEM-RDI ............................................14
    6.2. PSN to SONET/SDH ..........................................15
         6.2.1. AIS-P Indication ...................................15
         6.2.2. STS SPE Unequipped Indication ......................15
 7. Clocking Modes .................................................16
    7.1. Synchronous ...............................................16
         7.1.1. Synchronous Unstructured CEM .......................16
         7.1.2. Synchronous Structured CEM .........................16
    7.2. Asynchronous ..............................................17
 8. CEM LSP Signaling ..............................................17
 9. Security Considerations ........................................18
 10. IANA Considerations ...........................................18
 11. References ....................................................18
    11.1. Normative References .....................................18
    11.2. Informative References ...................................19
 Appendix A. SONET/SDH Rates and Formats ...........................20
 Appendix B. ECC-6 Definition ......................................21

Malis, et al. Historic [Page 2] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

1. Introduction

 This document describes a historical method for encapsulating
 SONET/SDH Path signals for transport across packet-switched networks
 (PSNs).
 The native transmission system for circuit-oriented Time Division
 Multiplexing (TDM) signals is the Synchronous Optical Network (SONET)
 [T1.105], [GR-253]/Synchronous Digital Hierarchy (SDH) [G.707].  To
 support TDM traffic (which includes voice, data, and private leased
 line services), PSNs 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) function.  The MPLS encapsulation may be
 further encapsulated in IP for carriage across IP PSNs [RFC4023].
 This document also describes an optional extension to CEM called
 Dynamic Bandwidth Allocation (DBA).  This is a method for dynamically
 reducing the bandwidth utilized by emulated SONET/SDH circuits in the
 packet network.  This bandwidth reduction is accomplished by not
 sending the SONET/SDH payload through the packet network under
 certain conditions, such as Alarm Indication Signal - Path (AIS-P) or
 Synchronous Transport Signal Synchronous Payload Envelope (STS SPE)
 Unequipped.

2. Conventions Used in This Document

 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 [RFC2119].

3. Scope

 This document describes how to provide CEM for the following digital
 signals:
 1. SONET STS-1 synchronous payload envelope (SPE)/SDH VC-3
 2. STS-Nc SPE (N = 3, 12, or 48)/SDH VC-4, VC-4-4c, VC-4-16c
 3. Unstructured SONET Emulation, where the entire SONET bit-stream
    (including the transport overhead) is packetized and transported
    across the PSN.
 For the remainder of this document, these constructs will be referred
 to as SONET/SDH channels.

Malis, et al. Historic [Page 3] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 Other SONET/SDH signals, such as virtual tributary (VT) structured
 sub-rate mapping, are not explicitly discussed in this document;
 however, it can be extended in the future to support VT and lower
 speed non-SONET/SDH services.  OC-192c SPE/VC-4-64c are also not
 included at this point, since most PSNs use OC-192c or slower trunks,
 and thus would not have sufficient capacity.  As trunk capacities
 increase in the future, the scope of this document can be accordingly
 extended.

4. CEM Encapsulation Format

 In order to transport SONET/SDH SPEs through a packet-oriented
 network, the SPE is broken into fragments.  A 32-bit CEM header is
 pre-pended to each fragment.  The Basic CEM packet appears in Figure
 1.
 +-----------------------------------+
 |            CEM Header             |
 +-----------------------------------+
 |                                   |
 |                                   |
 |        SONET/SDH SPE Fragment     |
 |                                   |
 |                                   |
 +-----------------------------------+
 Figure 1.  Basic CEM Packet
 The 32-bit CEM header has the following format:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |D|R|Rvd|   Sequence Num    | Structure Pointer |N|P|   ECC-6   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 2.  CEM Header Format
 The above fields are defined as follows:
 D-bit: This bit signals DBA Mode.  It MUST be set to zero for normal
 operation, and it MUST be set to one if CEM is currently in DBA mode.
 DBA is an optional mode during which trivial SPEs are not transmitted
 into the packet network.  See Table 1 and sections 7 and 8 for
 further details.
    Note: for unstructured CEM, the D-bit MUST be set to zero.

Malis, et al. Historic [Page 4] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 R bit: CEM-RDI (Remote Defect Indicator).  This bit is set to one to
 signal to the remote CEM function that a loss of packet
 synchronization has occurred.
 Rvd: These bits are reserved for future use, and MUST be set to zero.
 Sequence Number:  This is a packet sequence number, which MUST
 continuously cycle from 0 to 1023.  It SHOULD begin at zero when a
 CEM LSP (Label Switched Path) is created.
 Structure Pointer: The Structure Pointer MUST contain the offset of
 the J1 byte within the CEM payload.  The value is from 0 to 1,022,
 where 0 means the first byte after the CEM header.  The Structure
 Pointer MUST be set to 0x3FF (1,023) if a packet does not carry the
 J1 byte.  See [T1.105], [G.707], and [GR-253] for more information On
 the J1 byte and the SONET/SDH payload pointer.
    Note: for unstructured CEM, the Structure Pointer field MUST be
    set to 0x3FF.
 The N and P bits: These bits indicate negative and positive pointer
 adjustment events.  They are also used to relay SONET/SDH maintenance
 signals, such as AIS-P.  See Table 1 and sections 7 and 8 for more
 details.
    Note: for unstructured CEM, the N and P bits MUST both be set to
    zero.
 +---+---+---+----------------------------------------------+
 | D | N | P |         Interpretation                       |
 +---+---+---+-------------+--------------------------------+
 | 0 | 0 | 0 | Normal Mode | No Ptr Adjustment              |
 | 0 | 0 | 1 | Normal Mode | Positive Ptr Adjustment        |
 | 0 | 1 | 0 | Normal Mode | Negative Ptr Adjustment        |
 | 0 | 1 | 1 | Normal Mode | AIS-P                          |
 |   |   |   |             |                                |
 | 1 | 0 | 0 | DBA Mode    | STS SPE Unequipped             |
 | 1 | 0 | 1 | DBA Mode    | STS SPE Unequipped Pos Ptr Adj |
 | 1 | 1 | 0 | DBA Mode    | STS SPE Unequipped Neg Ptr Adj |
 | 1 | 1 | 1 | DBA Mode    | AIS-P                          |
 +---+---+---+-------------+--------------------------------+
 Table 1.  Interpretation of D, N, and P bits

Malis, et al. Historic [Page 5] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 ECC-6: An Error Correction Code to protect the CEM header.  This
 offers the ability to correct single bit errors and detect up to two
 bit errors.  The ECC algorithm is described in Appendix B.  The ECC-6
 can be optionally disabled at provisioning time.  If the ECC-6 is not
 utilized, it MUST be set to zero.
    Note: Normal CEM packets are fixed in length for all of the
    packets of a particular emulated TDM stream.  This length is
    signaled using the CEM Payload Bytes parameter defined in
    [RFC4447], or is statically provisioned for each TDM stream.
    Therefore, the length of each CEM packet does not need to be
    carried in the CEM header.
    Note: Setting the D-bit to 0 and the R bit to 1 violates the Best
    Current Practice defined in [RFC4928] when operating on MPLS
    networks.  In this situation, MPLS networks could mistake a CEM
    payload as the first nibble of an IPv4 packet, potentially causing
    mis-ordering of packets on the pseudowire in the presence of IP
    Equal Cost Multi-Path (ECMP) in the MPLS network.  The use of this
    CEM header preceded the use of MPLS ECMP.  As stated earlier,
    [RFC4842] describes the standards-track protocol for this
    functionality, and it does not share this violation.

4.1. Transport Encapsulation

 In principle, CEM packets can be transported over any packet-oriented
 network.  The following sections describe specifically how CEM
 packets MUST be encapsulated for transport over MPLS or IP networks.

4.1.1. MPLS Transport

 To transport a CEM packet over an MPLS network, an MPLS label stack
 MUST be pushed on top of the CEM packet.
 The last two labels prior to the CEM header are referred to as the
 Tunnel and Virtual Circuit (VC) labels.
 The VC label is required, and is the last label prior to the CEM
 Header.  The VC label MUST be used to identify the CEM connection
 within the MPLS tunnel.
 The optional tunnel label is immediately above the VC label on the
 label stack.  If present, the tunnel label MUST be used to identify
 the MPLS LSP used to tunnel the TDM packets through the MPLS network
 (the tunnel LSP).
 This is similar to the label stack usage defined in [RFC4447].

Malis, et al. Historic [Page 6] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 +-----------------------------------+
 | Additional MPLS Labels (Optional) |
 +-----------------------------------+
 |       Tunnel Label (Optional)     |
 +-----------------------------------+
 |             VC Label              |
 +-----------------------------------+
 |            CEM Header             |
 +-----------------------------------+
 |                                   |
 |                                   |
 |       SONET/SDH SPE Fragment      |
 |                                   |
 |                                   |
 +-----------------------------------+
 Figure 3.  Typical MPLS Transport Encapsulation

4.1.2. IP Transport

 It is highly desirable to define a single encapsulation format that
 will work for both IP and MPLS.  Furthermore, it is desirable that
 the encapsulation mechanism be as efficient as possible.
 One way to achieve these goals is to map CEM directly onto IP by
 mapping the previously described MPLS packets onto IP.
 A mechanism for carrying MPLS over IP is described in [RFC4023].
 Using this encapsulation scheme would result in the packet format
 illustrated in Figure 4.

Malis, et al. Historic [Page 7] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 +-----------------------------------+
 |                                   |
 |    IPv6/v4 Header [RFC4023]       |
 |                                   |
 +-----------------------------------+
 |      Tunnel Label (Optional)      |
 +-----------------------------------+
 |             VC Label              |
 +-----------------------------------+
 |            CEM Header             |
 +-----------------------------------+
 |                                   |
 |                                   |
 |       SONET/SDH SPE Fragment      |
 |                                   |
 |                                   |
 +-----------------------------------+
 Figure 4.  MPLS Transport Encapsulation

5. CEM Operation

 The following sections describe CEM operation.

5.1. Introduction and Terminology

 There are two types of CEM: structured and unstructured.
 Unstructured CEM packetizes the entire SONET/SDH bit-stream
 (including transport overhead).
 Structured CEM terminates the transport overhead and packetizes
 individual channels (STS-1/Nc) within the SONET/SDH frame.  Because
 structured CEM terminates the transport overhead, structured CEM
 implementations SHOULD meet the generic requirements for SONET/SDH
 Line Terminating Equipment as defined in [T1.105], [G.707], and
 [GR-253].
 Implementations MUST support structured CEM and MAY support
 unstructured CEM.
 Structured CEM MUST support a normal mode of operation and MAY
 support an optional extension called Dynamic Bandwidth Allocation
 (DBA).  During normal operation, SONET/SDH payloads are fragmented,
 pre-pended with the CEM header, the VC label, and the PSN header, and

Malis, et al. Historic [Page 8] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 then transmitted into the packet network.  During DBA mode, only the
 CEM header, the VC label, and PSN header are transmitted.  This is
 done to conserve bandwidth when meaningful user data is not present
 in the SPE, such as during AIS-P or STS SPE Unequipped.

5.1.1. CEM Packetizer and De-Packetizer

 As with all adaptation functions, CEM has two distinct components:
 adapting TDM SONET/SDH into a CEM packet stream, and converting the
 CEM packet stream back into a TDM SONET/SDH.  The first function will
 be referred to as CEM packetizer and the second as CEM de-packetizer.
 This terminology is illustrated in Figure 5.
           +------------+              +---------------+
           |            |              |               |
 SONET --> |    CEM     | --> PSN  --> |      CEM      | --> SONET
  SDH      | Packetizer |              | De-Packetizer |      SDH
           |            |              |               |
           +------------+              +---------------+
 Figure 5.  CEM Terminology
 Note: the CEM de-packetizer requires a buffering mechanism to account
 for delay variation in the CEM packet stream.  This buffering
 mechanism will be generically referred to as the CEM jitter buffer.

5.1.2. CEM DBA

 DBA is an optional mode of operation for structured CEM that only
 transmits the CEM header, the VC label, and PSN header into the
 packet network under certain circumstances, such as AIS-P or STS SPE
 Unequipped.
 If DBA is supported by a CEM implementation, the user SHOULD be able
 to configure if DBA will be triggered by AIS-P, STS SPE Unequipped,
 both, or neither on a per channel basis.
 If DBA is supported, the determination of AIS-P and STS SPE
 Unequipped MUST be based on the state of SONET/SDH Section, Line, and
 Path Overhead bytes.  DBA based on pattern detection within the SPE
 (i.e., all zeros, 7Es, or ATM idle cells) is for further study.
 During AIS-P, there is no valid payload pointer, so pointer
 adjustments cannot occur.  During STS SPE Unequipped, the SONET/SDH
 payload pointer is valid, and therefore pointer adjustments MUST be
 supported even during DBA.  See Table 1 for details.

Malis, et al. Historic [Page 9] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

5.2. Description of Normal CEM Operation

 During normal operation, the CEM packetizer will receive a fixed rate
 byte stream from a SONET/SDH interface.  When a packet's worth of
 data has been received from a SONET/SDH channel, the CEM header, the
 VC Label, and PSN header are pre-pended to the SPE fragment and the
 resulting CEM packet is transmitted into the packet network.  Because
 all normal CEM packets associated with a specific SONET/SDH channel
 will have the same length, the transmission of CEM packets for that
 channel SHOULD occur at regular intervals.
 At the far-end of the packet network, the CEM de-packetizer will
 receive packets into a jitter buffer and then play out the received
 byte stream at a fixed rate onto the corresponding SONET/SDH channel.
 The jitter buffer SHOULD be adjustable in length to account for
 varying network delay behavior.  The received packet rate from the
 packet network should be exactly balanced by the transmission rate
 onto the SONET/SDH channel, on average.  The time over which this
 average is taken corresponds to the depth of the jitter buffer for a
 specific CEM channel.
 The CEM sequence numbers provide a mechanism to detect lost and/or
 mis-ordered packets.  The CEM de-packetizer MUST detect lost or
 mis-ordered packets.  The CEM de-packetizer MUST play out a
 programmable byte pattern in place of any dropped packets.  The CEM
 de-packetizer MAY re-order packets received out of order.  If the CEM
 de-packetizer does not support re-ordering, it MUST drop mis-ordered
 packets.

5.3. Description of CEM Operation during DBA

 (Note: DBA is only applicable to structured CEM.)
 There are several issues that should be addressed by a workable CEM
 DBA mechanism.  First, when DBA is invoked, there should be a
 substantial savings in bandwidth utilization in the packet network.
 The second issue is that the transition in and out of DBA should be
 tightly coordinated between the local CEM packetizer and CEM
 de-packetizer at the far side of the packet network.  A third is that
 the transition in and out of DBA should be accomplished with minimal
 disruption to the adapted data stream.
 Another goal is that the reduction of CEM traffic due to DBA should
 not be mistaken for a fault in the packet network or vice-versa.
 Finally, the implementation of DBA should require minimal
 modifications beyond what is necessary for the nominal CEM case.  The
 mechanism described below is a reasonable balance of these goals.

Malis, et al. Historic [Page 10] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 During DBA, packets MUST be emitted at exactly the same rate as they
 would be during normal operation.  This SHOULD be accomplished by
 transmitting each DBA packet after a complete packet of data has been
 received from the SONET/SDH channel.  The only change from normal
 operation is that the CEM packets during DBA MUST only carry the CEM
 header, the VC label, and the PSN header.
 Because some links have a minimum supported packet size, the CEM
 packetizer MAY append a configurable number of bytes immediately
 after the CEM header to pad out the CEM packet to reach the minimum
 supported packet size.  The value of the padding bytes is
 implementation specific.  The D-bit MUST be set to one, to indicate
 that DBA is active.
 The CEM de-packetizer MUST assume that each packet received with the
 D-bit set represents a normal-sized packet containing an AIS-P or STS
 SPE Unequipped payload as noted by N and P, (see Table 1).  The CEM
 de-packetizer MUST accept DBA packets with or without padding.
 This allows the CEM packetization and de-packetization logic during
 DBA to be similar to the nominal case.  It insures that the correct
 SONET/SDH indication is reliably transmitted between CEM adaptation
 points.  It minimizes the risk of under or over running the jitter
 buffer during the transition in and out of DBA.  And, it guarantees
 that faults in the packet network are recognized as distinctly
 different from line conditioning on the SONET/SDH interfaces.

5.4. Packet Synchronization

 A key component in declaring the state of a CEM service is whether or
 not the CEM de-packetizer is in or out of packet synchronization.
 The following paragraphs describe how that determination is made.

5.4.1. Acquisition of Packet Synchronization

 At startup, a CEM de-packetizer will be out of packet synchronization
 by default.  To declare packet synchronization at startup or after a
 loss of packet synchronization, the CEM de-packetizer must receive a
 configurable number of CEM packets with sequential sequence numbers.

5.4.2. Loss of Packet Synchronization

 Once a CEM de-packetizer is in packet sync, it may encounter a set of
 events that will cause it to lose packet synchronization.
 As discussed in section 5.2, a CEM de-packetizer MAY support the
 re-ordering of mis-ordered packets.

Malis, et al. Historic [Page 11] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 If a CEM de-packetizer supports re-ordering, then the determination
 that packet synchronization has been lost cannot be made at the time
 the packets are received from the PSN.  Instead, the determination
 MUST be made as the packets are being played out onto the SONET/SDH
 interface.  This is because it is only at play-out time that the
 determination can be made as to whether the entire emulated SONET/SDH
 stream was received from the PSN.
 If a CEM de-packetizer does not support re-ordering, a number of
 approaches may be used to minimize the impact of mis-ordered or lost
 packets on the final re-assembled SONET/SDH stream.  For example, ATM
 Adaptation Layer 1 (AAL1) [I.363.1] uses a simple state-machine to
 re-order packets in a subset of possible cases.  The algorithm for
 these state-machines is outside of the scope of CEM.  However, the
 final determination as to whether or not to declare loss of packet
 synchronization MUST be based on the same criteria as for
 implementations that do support re-ordering.
 Whether or not a CEM implementation supports re-ordering, the
 declaration of loss of packet synchronization MUST be based on the
 following criteria.
 As packets are played out towards the SONET/SDH interface, the CEM
 de-packetizer will encounter empty packets in the place of packets
 that were dropped by the PSN, or effectively dropped due to
 limitations of the CEM implementation.  If the CEM de-packetizer
 encounters more than a configurable number of sequential dropped
 packets, the CEM de-packetizer MUST declare loss of packet
 synchronization.

6. SONET/SDH Maintenance Signals

 There are several issues that must be considered in the mapping of
 maintenance signals between SONET/SDH and a PSN.  A description of
 how these signals and conditions are mapped between the two domains
 is given below.
 For clarity, the mappings are split into two groups: SONET/SDH to PSN
 and PSN to SONET/SDH.

6.1. SONET/SDH to PSN

 The following sections describe how SONET/SDH Maintenance Signals and
 Alarm conditions are mapped into a Packet-Switched Network.

Malis, et al. Historic [Page 12] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

6.1.1. AIS-P Indication

 In a SONET/SDH network, SONET/SDH Path outages are signaled using
 maintenance alarms, such as Path AIS (AIS-P).  In particular, AIS-P
 indicates that the SONET/SDH Path is not currently transmitting valid
 end-user data, and the SPE contains all ones.
 It should be noted that for structured CEM, nearly every type of
 service-effecting section or line defect will result in an AIS-P
 condition.
 The SONET/SDH hierarchy is illustrated below.
                            +----------+
                            |   PATH   |
                            +----------+
                                 ^
                                 |
                               AIS-P
                                 |
                                 |
                            +----------+
                            |   LINE   |
                            + ---------+
                               ^     ^
                               |     |
                             AIS-L   +------ LOP
                               |
                               |
                            +----------+
                            | SECTION  |
                            +----------+
                               ^    ^
                               |    |
                               |    |
                              LOS  LOF
                  Figure 6.  SONET/SDH Fault Hierarchy
 Should the Section Layer detect a Loss of Signal (LOS) or Loss of
 Frame (LOF) condition, it sends AIS-L up to the Line Layer.  If the
 Line Layer detects AIS-L or Loss of Path (LOP), it sends AIS-P to the
 Path Layer.
 In normal mode during AIS-P, structured CEM packets are generated as
 usual.  The N and P bits MUST be set to 11 binary to signal AIS-P
 explicitly through the packet network.  The D-bit MUST be set to zero

Malis, et al. Historic [Page 13] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 to indicate that the SPE is being carried through the packet network.
 Normal CEM packets with the SPE fragment, CEM header, the VC label,
 and PSN header MUST be transmitted into the packet network.
 However, to conserve network bandwidth during AIS-P, DBA MAY be
 employed.  If DBA has been enabled for AIS-P and AIS-P is currently
 occurring, the N and P bits MUST be set to 11 binary to signal AIS,
 and the D-bit MUST be set to one to indicate that the SPE is not
 being carried through the packet network.  Only the CEM header, the
 VC label, and the PSN header MUST be transmitted into the packet
 network.
 Also note that this differs from the outage mechanism in [RFC4447],
 which withdraws the VC label as a result of an endpoint outage.  TDM
 circuit emulation requires the ability to distinguish between the
 de-provisioning of a circuit (which causes the VC label to be
 withdrawn), and temporary outages (which are signaled using AIS-P).

6.1.2. STS SPE Unequipped Indication

 The STS SPE Unequipped Indication is a slightly different case than
 AIS-P.  When byte C2 of the Path Overhead (STS path signal label) is
 00h and Byte B3 (STS Path BIP-8) is valid, it indicates that the STS
 SPE is unequipped.  Note: this is typically signaled by setting the
 entire SPE to zeros.
 For normal structured CEM operation during STS SPE Unequipped, the N
 and P bits MUST be interpreted as usual.  The SPE MUST be transmitted
 into the packet network along with the CEM header, the VC label, and
 PSN header, and the D-Bit MUST be set to zero.
 If DBA has been enabled for STS SPE Unequipped and the Unequipped
 condition is occurring on the SONET/SDH channel, the D-bit MUST be
 set to one to indicate DBA is active.  Only the CEM header, the VC
 Label, and PSN header MUST be transmitted into the packet network.
 The N and P bits MUST be used to signal pointer adjustments as
 normal.  See Table 1 and section 8 for details.

6.1.3. CEM-RDI

 The CEM function MUST send CEM-RDI towards the packet network during
 loss of packet synchronization.  This MUST be accomplished by setting
 the R bit to one in the CEM header.  This applies for both structured
 and unstructured CEM.

Malis, et al. Historic [Page 14] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

6.2. PSN to SONET/SDH

 The following sections discuss how the various conditions on the
 packet network are converted into SONET/SDH indications.

6.2.1. AIS-P Indication

 There are several conditions in the packet network that will cause
 the structured CEM de-packetization function to send an AIS-P
 indication onto a SONET/SDH channel.
 The first of these is the receipt of structured CEM packets with the
 N and P bits set to one, and the D-bit set to zero.  This is an
 explicit indication of AIS-P being received at the far-end of the
 packet network, with DBA disabled for AIS-P.  The CEM de-packetizer
 MUST play out the received SPE fragment (which will incidentally be
 carrying all ones), and MUST configure the SONET/SDH Overhead to
 signal AIS-P as defined in [T1.105], [G.707], and [GR-253].
 The second case is the receipt of structured CEM packets with the N
 and P bits set to one, and the D-bit set to one.  This is an explicit
 indication of AIS-P being received at the far-end of the packet
 network, with DBA enabled for AIS-P.  The CEM de-packetizer MUST play
 out one packet's worth of all ones for each packet received, and MUST
 configure the SONET/SDH Overhead to signal AIS-P as defined in
 [T1.105], [G.707], and [GR-253].
 A third case that will cause a structured CEM de-packetization
 function to send an AIS-P indication onto a SONET/SDH channel is loss
 of packet synchronization.

6.2.2. STS SPE Unequipped Indication

 There are three conditions in the packet network that will cause the
 CEM function to transmit STS SPE Unequipped Indications onto the
 SONET/SDH channel.
 The first, which is transparent to CEM, is the receipt of regular CEM
 packets that happen to be carrying an SPE that contains the
 appropriate Path Overhead to signal STS SPE Unequipped.  This case
 does not require any special processing on the part of the CEM
 de-packetizer.
 The second case is the receipt of structured CEM packets that have
 the D-bit set to one to indicate that DBA is active and the N and P
 bits set to 00 binary, 01 binary, or 10 binary to indicate STS SPE

Malis, et al. Historic [Page 15] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 Unequipped with or without pointer adjustments.  The CEM
 de-packetizer MUST use this information to transmit a packet of all
 zeros onto the SONET/SDH interface, and adjust the payload pointer as
 necessary.
 The third case when a structured CEM de-packetizer MUST send an STS
 SPE Unequipped Indication towards the SONET/SDH interface is when the
 VC-label has been withdrawn due to de-provisioning of the circuit.

7. Clocking Modes

 It is necessary to be able to regenerate the input service clock at
 the output interface.  Two clocking modes are supported: synchronous
 and asynchronous.  Selection of the clocking mode is made as part of
 service provisioning.  Both ends of the emulated circuit must be
 configured with the same clocking mode.

7.1. Synchronous

 When synchronous SONET/SDH timing is available at both ends of the
 circuit, the issue of clock recovery becomes much simpler.

7.1.1. Synchronous Unstructured CEM

 For unstructured CEM, the external clock is used to clock each bit
 onto the optical carrier.

7.1.2. Synchronous Structured CEM

 For structured CEM, the external clock is used to clock the SONET/SDH
 carrier.  The N and P bits are used to signal negative or positive
 pointer adjustment events between structured CEM endpoints.
 If there is a frequency offset between the frame rate of the
 transport overhead and that of the SONET/SDH SPE, then the alignment
 of the SPE shall periodically slip back or advance in time through
 positive or negative stuffing.  The N and P bits are used to replay
 the pointer adjustment events and eliminate transport jitter.
 During a negative pointer adjustment event, the H3 byte from the
 SONET/SDH stream is incorporated into the CEM packet payload in order
 with the rest of the SPE.  During a positive pointer adjustment
 event, the stuff byte is not included in the CEM packet payload.
 The pointer adjustment event MUST be transmitted in three consecutive
 packets by the packetizer.  The de-packetizer MUST play out the
 pointer adjustment event when the first packet of the three with the
 N/P bits set is received.

Malis, et al. Historic [Page 16] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 The CEM de-packetizer MUST utilize the CEM sequence numbers to insure
 that SONET/SDH pointer adjustment events are not played any more
 frequently than once per every three CEM packets transmitted by the
 remote CEM packetizer.
 References [T1.105], [G.707], and [GR-253] specify that pointer
 adjustment events MUST be separated by three SONET/SDH frames without
 a pointer adjustment event.  In order to relay all legal pointer
 adjustment events, the packet size for a specific circuit MUST be no
 larger than (783 * 4 * N)/3, where N is the STS-Nc multiplier.
 However, some SONET/SDH equipment allows pointer adjustments to occur
 in back-to-back SONET/SDH frames.  In order to support this
 possibility, the packet size for a particular circuit SHOULD be no
 larger than (783*N)/3, where N is the STS-Nc multiplier.
 Since the minimum value of N is one, CEM implementations SHOULD
 support a minimum payload length of 783/3 or 261 bytes.  Smaller
 payload lengths MAY be supported as an option.

7.2. Asynchronous

 If synchronous timing is not available, other methods MAY be employed
 to regenerate the circuit timing.
 For structured CEM, the CEM packetizer SHOULD generate the N and P
 bits as usual.  However, without external synchronization, this
 information is not sufficient to reliably justify the SPE within the
 SONET/SDH transport framing at the CEM de-packetizer.  The
 de-packetizer MAY employ an adaptive algorithm to introduce pointer
 adjustment events to map the CEM SPE to the SONET/SDH transport
 framing.  Regardless of whether the N and P bits are used by the
 de-packetizer as part of the adaptive clock recovery algorithm, they
 MUST still be used in conjunction with the D-bit to signal AIS-P, STS
 SPE Unequipped, and DBA.
 For unstructured CEM, the CEM de-packetizer MAY use an adaptive clock
 recovery technique to regenerate the SONET/SDH transport clock.
 An example adaptive clock recovery method can be found in section
 3.4.2 of [VTOA].

8. CEM LSP Signaling

 For maximum network scaling in MPLS applications, CEM LSP signaling
 may be performed using the Label Distribution Protocol (LDP) Extended
 Discovery mechanism as augmented by the Pseudo-Wire id Forward Error
 Correction (PWid FEC) Element defined in [RFC4447].  MPLS traffic

Malis, et al. Historic [Page 17] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 tunnels may be dedicated to CEM, or shared with other MPLS-based
 services.  The value 0x8008 is used for the PWE3 PW Type in the PWid
 FEC Element in order to signify that the LSP being signaled is to
 carry CEM.  Note that the generic control word defined in [GR-253] is
 not used, as its functionality is included in the CEM encapsulation
 header.
 Alternatively, static label assignment may be used, or a dedicated
 traffic engineered LSP may be used for each CEM service.
 Normal CEM packets are fixed in length for all of the packets of a
 particular emulated TDM stream.  This length is signaled using the
 CEM Payload Bytes parameter defined in [RFC4447], or it is statically
 provisioned for each CEM service.
 At this time, other aspects of the CEM service MUST be statically
 provisioned.

9. Security Considerations

 The CEM encapsulation is subject to all of the general security
 considerations discussed in [RFC3985] and [RFC4447].  In addition,
 this document specifies only encapsulations, and not the protocols
 used to carry the encapsulated packets across the PSN.  Each such
 protocol may have its own set of security issues, but those issues
 are not affected by the encapsulations specified herein.  Note that
 the security of the transported CEM service will only be as good as
 the security of the PSN.  This level of security may be less rigorous
 then that available from a native TDM service due to the inherent
 differences between circuit-switched and packet-switched public
 networks.

10. IANA Considerations

 IANA has already allocated the PWE3 PW Type value 0x0008 for this
 specification.  No further actions are required.

11. References

11.1. Normative References

 [G.707]     ITU Recommendation G.707, "Network Node Interface For The
             Synchronous Digital Hierarchy", 1996.
 [GR-253]    Telcordia Technologies, "Synchronous Optical Network
             (SONET) Transport Systems: Common Generic Criteria", GR-
             253-CORE, Issue 3, September 2000.

Malis, et al. Historic [Page 18] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 [I.363.1]   ITU-T, "Recommendation I.363.1, B-ISDN Adaptation Layer
             Specification: Type AAL1", Appendix III, August 1996.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4023]   Worster, T., Rekhter, Y., and E. Rosen, Ed.,
             "Encapsulating MPLS in IP or Generic Routing
             Encapsulation (GRE)", RFC 4023, March 2005.
 [RFC4447]   Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
             G. Heron, "Pseudowire Setup and Maintenance Using the
             Label Distribution Protocol (LDP)", RFC 4447, April 2006.
 [RFC4842]   Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
             "Synchronous Optical Network/Synchronous Digital
             Hierarchy (SONET/SDH) Circuit Emulation over Packet
             (CEP)", RFC 4842, April 2007.
 [RFC4928]   Swallow, G., Bryant, S., and L. Andersson, "Avoiding
             Equal Cost Multipath Treatment in MPLS Networks", BCP
             128, RFC 4928, June 2007.
 [T1.105]    American National Standards Institute, "Synchronous
             Optical Network (SONET) - Basic Description including
             Multiplex Structure, Rates and Formats," ANSI T1.105-
             1995.
 [VTOA]      ATM Forum, "Circuit Emulation Service Interoperability
             Specification Version 2.0", af-vtoa-0078.000, January
             1997.

11.2. Informative References

 [RFC3985]   Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
             Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.

Malis, et al. Historic [Page 19] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

Appendix A. SONET/SDH Rates and Formats

 For simplicity, the discussion in this section uses SONET
 terminology, but it applies equally to SDH as well.  SDH-equivalent
 terminology is shown in the tables.
 The basic SONET modular signal is the synchronous transport
 signal-level 1 (STS-1).  A number of STS-1s may be multiplexed into
 higher-level signals denoted as STS-N, with N synchronous payload
 envelopes (SPEs).  The optical counterpart of the STS-N is the
 Optical Carrier-level N, or OC-N.  Table 2 lists standard SONET line
 rates discussed in this document.
 OC Level          OC-1    OC-3    OC-12      OC-48     OC-192
 SDH Term             -   STM-1    STM-4     STM-16     STM-64
 Line Rate(Mb/s) 51.840 155.520  622.080  2,488.320  9,953.280
 Table 2.  Standard SONET Line Rates
 Each SONET frame is 125 us and consists of nine rows.  An STS-N frame
 has nine rows and N*90 columns.  Of the N*90 columns, the first N*3
 columns are transport overhead and the other N*87 columns are SPEs.
 A number of STS-1s may also be linked together to form a super-rate
 signal with only one SPE.  The optical super-rate signal is denoted
 as OC-Nc, which has a higher payload capacity than OC-N.
 The first 9-byte column of each SPE is the Path Overhead (POH) and
 the remaining columns form the payload capacity with fixed stuff
 (STS-Nc only).  The fixed stuff, which is purely overhead, is N/3-1
 columns for STS-Nc.  Thus, STS-1 and STS-3c do not have any fixed
 stuff, STS-12c has three columns of fixed stuff, and so on.
 The POH of an STS-1 or STS-Nc is always nine bytes in nine rows.  The
 payload capacity of an STS-1 is 86 columns (774 bytes) per frame.
 The payload capacity of an STS-Nc is (N*87)-(N/3) columns per frame.
 Thus, the payload capacity of an STS-3c is (3*87 - 1)*9 = 2,340 bytes
 per frame.  As another example, the payload capacity of an STS-192c
 is 149,760 bytes, which is exactly 64 times larger than the STS-3c.
 There are 8,000 SONET frames per second.  Therefore, the SPE size,
 (POH plus payload capacity) of an STS-1 is 783*8*8,000 = 50.112 Mb/s.
 The SPE size of a concatenated STS-3c is 2,349 bytes per frame or
 150.336 Mb/s.  The payload capacity of an STS-192c is 149,760 bytes
 per frame, which is equivalent to 9,584.640 Mb/s.  Table 3 lists the
 SPE and payload rates supported.

Malis, et al. Historic [Page 20] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 SONET STS Level     STS-1   STS-3c  STS-12c    STS-48c   STS-192c
 SDH VC Level            -     VC-4  VC-4-4c   VC-4-16c   VC-4-64c
 Payload Size(Bytes)   774    2,340    9,360     37,440    149,760
 Payload Rate(Mb/s) 49.536  149.760  599.040  2,396.160  9,584.640
 SPE Size(Bytes)       783    2,349    9,396     37,584    150,336
 SPE Rate(Mb/s)     50.112  150.336  601.344  2,405.376  9,621.504
 Table 3.  Payload Size and Rate
 To support circuit emulation, the entire SPE of a SONET STS or SDH VC
 level is encapsulated into packets, using the encapsulation defined
 in section 5, for carriage across packet-switched networks.

Appendix B. ECC-6 Definition

 ECC-6 is an Error Correction Code to protect the CEM header.  This
 provides single bit correction and the ability to detect up to two
 bit errors.
 Error Correction Code:
 |---------------Header bits 0-25 -------------------| ECC-6 code|
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 1 1 1 1 0 0 0 1 0 0 0 1 1 1 1 1 0 1 0 0 0 1 0 1 1|1 0 0 0 0 0|
 |1 1 1 1 0 1 0 0 0 1 0 0 1 0 0 0 0 1 0 1 1 1 1 1 1 1|0 1 0 0 0 0|
 |1 0 0 0 1 1 1 1 0 0 1 0 1 1 1 0 0 0 1 1 1 1 0 0 1 1|0 0 1 0 0 0|
 |0 1 0 0 1 1 1 1 0 0 0 1 1 0 0 1 1 1 1 1 0 0 1 1 0 1|0 0 0 1 0 0|
 |0 0 1 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 1 1 1 0 1 0 1 0|0 0 0 0 1 0|
 |0 0 0 1 0 0 0 1 1 1 1 1 0 0 1 1 0 0 1 1 0 1 1 1 1 1|0 0 0 0 0 1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 7.  ECC-6 Check Matrix X
 The ECC-6 code protects the 32-bit CEM header as follows:
 The encoder generates the 6-bit ECC using the matrix shown in Figure
 7.  In brief, the encoder builds another 26 column by 6 row matrix
 and calculates even parity over the rows.  The matrix columns
 represent CEM header bits 0 through 25.
 Denote each column of the ECC-6 check matrix by X[], and each column
 of the intermediate encoder matrix as Y[].  CEM[] denotes the CEM
 header and ECC[] is the error correction code that is inserted into
 CEM header bits 26 through 31.

Malis, et al. Historic [Page 21] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

 for i = 0 to 25 {
      if CEM[i] = 0 {
              Y[i] = 0;
      } else {
              Y[i] = X[i];
      }
 }
 In other words, for each CEM header bit (i) set to one, set the
 resulting matrix column Y[i] according to Figure 7.
 The final ECC-6 code is calculated as even parity of each row in Y
 (i.e., ECC[k]=CEM[25+k]=even parity of row k).
 The receiver also uses matrix X to calculate an intermediate matrix
 Y' based on all 32 bits of the CEM header.  Therefore, Y' is 32
 columns wide and includes the ECC-6 code.
 for i = 0 to 31 {
      if CEM[i] = 0 {
              Y'[i] = 0;
      } else {
              Y'[i] = X[i];
      }
 }
 The receiver then appends the incoming ECC-6 code to Y as column 32
 (ECC[0] should align with row 0) and calculates even parity for each
 row.  The result is a single 6-bit column Z.  If all 6 bits are 0,
 there are no bit errors (or at least no detectable errors).
 Otherwise, it uses Z to perform a reverse lookup on X[] from Figure
 7.  If Z matches column X[i], then there is a single bit error.  The
 receiver should invert bit CEM[i] to correct the header.  If Z fails
 to match any column of X, then the CEM header contains more than one
 bit error and the CEM packet MUST be discarded.
 Note that the ECC-6 code provides single-bit correction and 2-bit
 detection of errors within the received ECC-6 code itself.

Malis, et al. Historic [Page 22] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

Acknowledgments

 The authors would like to thank Mitri Halabi, Bob Colvin, and Ken
 Hsu, all formerly of Vivace Networks and Tellabs; Tom Johnson,
 Marlene Drost, Ed Hallman, and Dave Danenberg, all formerly of
 Litchfield Communications, for their contributions to the document.

Authors' Addresses

 Andrew G. Malis
 Verizon Communications
 40 Sylvan Road
 Waltham, MA 02451
 EMail: andrew.g.malis@verizon.com
 Jeremy Brayley
 ECI Telecom Inc.
 Omega Corporate Center
 1300 Omega Drive
 Pittsburgh, PA 15205
 EMail: jeremy.brayley@ecitele.com
 John Shirron
 ECI Telecom Inc.
 Omega Corporate Center
 1300 Omega Drive
 Pittsburgh, PA 15205
 EMail: john.shirron@ecitele.com
 Luca Martini
 Cisco Systems, Inc.
 9155 East Nichols Avenue, Suite 400
 Englewood, CO, 80112
 EMail: lmartini@cisco.com
 Steve Vogelsang
 Alcatel-Lucent
 600 March Road
 Kanata, ON K2K 2T6
 Canada
 EMail: steve.vogelsang@alcatel-lucent.com

Malis, et al. Historic [Page 23] RFC 5143 SONET/SDH Circuit Emulation over MPLS February 2008

Full Copyright Statement

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 contained in BCP 78 and at www.rfc-editor.org/copyright.html, and
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Malis, et al. Historic [Page 24]

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