Private Line Emulation over Packet Switched Networks

This document describes a method for encapsulating high-speed bit-streams as VPWS over packet switched networks (PSN). This emulation suits applications where signal transparency is required and data or framing structure interpretation of the PE would be counter productive. One example is two ethernet connected CEs and the need for synchronous ethernet operation between them without the intermediate PEs interfering or addressing concerns about ethernet control protocol transparency for carrier ethernet services, beyond the behavior definitions of MEF specifications. Another example would be a Storage Area Networking (SAN) extension between two data centers. Operating at a bit-stream level allows for a connection between Fibre Channel switches without interfering with any of the Fibre Channel protocol mechanisms. Also SONET/SDH add/drop multiplexers or cross-connects can be interconnected without interfering with the multiplexing structures and networks mechanisms. This is a key distinction to CEP defined in where demultiplexing and multiplexing is desired in order to operate per SONET Synchronous Payload Envelope (SPE) and Virtual Tributary (VT) or SDH Virtual Container (VC). Said in another way, PLE does provide an independent layer network underneath the SONET/SDH layer network, whereas CEP does operate at the same level and peer with the SONET/SDH layer network. The mechanisms described in this document follow principals similar to but expanding the applicability beyond the narrow set of PDH interfaces (T1, E1, T3 and E3) and allow the transport of signals from many different technologies such as Ethernet, Fibre Channel, SONET/SDH / and OTN at gigabit speeds by treating them as bit-stream payload defined in Section 3.3.3 of .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

ACH - Associated Channel Header AIS - Alarm Indication Signal CBR - Constant Bit Rate CE - Customer Edge CSRC - Contributing SouRCe ES - Errored Second FEC - Forward Error Correction IWF - InterWorking Function LDP - Label Distribution Protocol LF - Local Fault MPLS - Multi Protocol Label Switching NSP - Native Service Processor ODUk - Optical Data Unit k OTN - Optical Transport Network OTUk - Optical Transport Unit k PCS - Physical Coding Sublayer PE - Provider Edge PLE - Private Line Emulation PLOS - Packet Loss Of Signal PSN - Packet Switched Network P2P - Point-to-Point QOS - Quality Of Service RSVP-TE - Resource Reservation Protocol Traffic Engineering RTCP - RTP Control Protocol RTP - Realtime Transport Protocol SAN - Storage Area Network SES - Severely Errored Seconds SDH - Synchronous Digital Hierarchy SPE - Synchronous Payload Envelope SRTP - Secure Realtime Transport Protocol SRv6 - Segment Routing over IPv6 Dataplane SSRC - Synchronization SouRCe SONET - Synchronous Optical Network TCP - Transmission Control Protocol UAS - Unavailable Seconds VPWS - Virtual Private Wire Service VC - Virtual Circuit VT - Virtual Tributary Similar to and the term Interworking Function (IWF) is used to describe the functional block that encapsulates bit streams into PLE packets and in the reverse direction decapsulates PLE packets and reconstructs bit streams.

The generic models defined in are applicable to PLE. PLE embraces the minimum intervention principle outlined in section 3.3.5 of whereas the data is flowing through the PLE encapsulation layer as received without modifications. For some applications the NSP function is responsible for performing operations on the native data received from the CE. Examples are terminating FEC in case of 100GE or terminating the OTUk layer for OTN. After the NSP the IWF is generating the payload of the VPWS which carried via a PSN tunnel.

| | | | |<-PSN tunnel->| | v v v v +---------+ +---------+ | PE1 |==============| PE2 | +---+-----+ +-----+---+ +-----+ | N | | | | N | +-----+ | CE1 |-----| S | IWF |.....VPWS.....| IWF | S |-----| CE2 | +-----+ ^ | P | | | | P | ^ +-----+ | +---+-----+ +-----+---+ | CE1 physical ^ ^ CE2 physical interface | | interface |<--- emulated service --->| | | attachment attachment circuit circuit ]]> To allow the clock of the transported signal to be carried across the PLE domain in a transparent way the network synchronization reference model and deployment scenario outlined in section 4.3.2 of is applicable.

|.............................|--------->| | +-----+ |- - -|=================|- - -| +-----+ ^ +-----+<-------+------->+-----+ | | ^ A +-+ | |I| E +-+ ]]> The attachment circuit clock E is generated by PE2 via a differential clock recovery method in reference to a common clock I. For this to work the difference between clock I and clock A MUST be explicitly transferred between the PE1 and PE2 using the timestamp inside the RTP header. For the reverse direction PE1 does generate the clock J in reference to clock I and the clock difference between I and G. The way the common clock I is implemented is out of scope of this document. Well established concepts for achieving frequency synchronization in a PSN have already been defined in and can be applied here as well.

The basic packet format used by PLE is shown in the .

This document does not imply any specific technology to be used for implementing the VPWS demultiplexing and PSN layers. When a MPLS PSN layer is used. A VPWS label provides the demultiplexing mechanism as described in section 5.4.2 of . The PSN tunnel can be a simple best path Label Switched Path (LSP) established using LDP or Segment Routing or a traffic engineered LSP established using RSVP-TE or SR-TE . When PLE is applied to a SRv6 based PSN, the mechanisms defined in and the End.DX2 endpoint behavior defined in do apply.

The PLE header MUST contain the PLE control word (4 bytes) and MUST include a fixed size RTP header . The RTP header MUST immediately follow the PLE control word.

The format of the PLE control word is in line with the guidance in and as shown in :

The first nibble is used to differentiate if it is a control word or Associated Channel Header (ACH). The first nibble MUST be set to 0000b to indicate that this header is a control word as defined in section 3 of . The other fields in the control word are used as defined below: L Set by the PE to indicate that data carried in the payload is invalid due to an attachment circuit fault (client signal failure). The downstream PE MUST play out an appropriate replacement data. The NSP MAY inject an appropriate native fault propagation signal. R Set by the downstream PE to indicate that the IWF experiences packet loss from the PSN or a server layer backward fault indication is present in the NSP. The R bit MUST be cleared by the PE once the packet loss state or fault indication has cleared. RSV These bits are reserved for future use. This field MUST be set to zero by the sender and ignored by the receiver. FRG These bits MUST be set to zero by the sender and ignored by the receiver. LEN In accordance to section 3 the length field MUST always be set to zero as there is no padding added to the PLE packet. To detect malformed packets the default, preconfigured or signaled payload size MUST be assumed. Sequence Number The sequence number field is used to provide a common PW sequencing function as well as detection of lost packets. It MUST be generated in accordance with the rules defined in Section 5.1 of for the RTP sequence number and MUST be incremented with every PLE packet being sent.

The RTP header MUST be included and is used for explicit transfer of timing information. The RTP header is purely a formal reuse and RTP mechanisms, such as header extensions, contributing source (CSRC) list, padding, RTP Control Protocol (RTCP), RTP header compression, Secure Realtime Transport Protocol (SRTP), etc., are not applicable to PLE VPWS. The format of the RTP header is as shown in :

V: Version The version field MUST be set to 2. P: Padding The padding flag MUST be set to zero by the sender and ignored by the receiver. X: Header Extension The X bit MUST be set to zero by sender and ignored by receiver. CC: CSRC Count The CC field MUST be set to zero by the sender and ignored by the receiver. M: Marker The M bit MUST be set to zero by sender and ignored by receiver. PT: Payload Type A PT value MUST be allocated from the range of dynamic values define by for each direction of the VPWS. The same PT value MAY be reused both for direction and between different PLE VPWS. Sequence Number The packet sequence number MUST continuously cycle from 0 to 0xFFFF. It is generated and processed in accordance with the rules established in . The PLE receiver MUST sequence packets according to the Sequence Number field of the PLE control word and MAY verify correct sequencing using RTP Sequence Number field. Timestamp Timestamp values are used in accordance with the rules established in . For bit-streams up to 200 Gbps the frequency of the clock used for generating timestamps MUST be 125 MHz based on a the common clock I. For bit-streams above 200 Gbps the frequency MUST be 250 MHz. SSRC: Synchronization Source The SSRC field MAY be used for detection of misconnections.

A bit-stream is mapped into a PLE packet with a fixed payload size which MUST be defined during VPWS setup, MUST be the same in both directions of the VPWS and MUST remain unchanged for the lifetime of the VPWS. All PLE implementations MUST be capable of supporting the default payload size of 1024 bytes.

The PLE payload is filled with incoming bits of the bit-stream starting from the most significant to the least significant bit without considering any structure of the bit-stream. For PCS based attachment circuits supporting FEC the NSP function MUST terminate the FEC and pass the PCS encoded signal to the IWF function. For PCS based attachment circuits supporting virtual lanes (i.e. 100GE) a PLE payload MUST carry information from all virtual lanes in a bit interleaved manner after the NSP function has performed PCS layer de-skew and re-ordering. A PLE implementation MUST support the structure agnostic payload for all bit-streams except the following: OTN 200GBASE-R ethernet 400GBASE-R ethernet

In case of OTN bit-streams, the NSP function MUST present to the IWF an extended ODUk including a valid frame alignment overhead. The IWF is performing byte-aligned mapping into PLE packets. The egress NSP function will recover the ODUk by searching for the frame alignment overhead. For byte aligned payloads PLE uses the following order for packetization: The order of the payload bytes corresponds to their order on the attachment circuit. Consecutive bits coming from the attachment circuit fill each payload byte starting from most significant bit to least significant. All PLE implementations MUST support the transport of OTN bit-streams using the byte aligned payload.

In IEEE 802.3BS the PCS layer for 200GBASE-R and 400GBASE-R is defined with the functions shown in .

For 200GBASE-R and 400GBASE-R bit-streams, on ingress the NSP function will perform alignment lock and lane de-skew, lane order and de-interleave, FEC decode and post-FEC interleave as shown in . After the post-FEC interleave the NSP function will create a stream of 10280 bit blocks (comprising of two 5140 code blocks). On the egress the IWF sends a stream of 10280 bit blocks to the NSP function and which performs pre-FEC distribution, FEC encode and distribute and interleave functions as shown in . In the 10280 bit block stream, alignment markers exist every 4096, 10280 bit blocks (8192 code blocks) for 400GBASE-R and every 2048, 10280 bit blocks (4096 code blocks) for 200GBASE-R. On ingress the NSP must indicate to the IWF when a code word carries an alignment marker (or every n-th alignment marker where n is a multiple of 2). The IWF will create a PLE packet with the alignment marker bits at the beginning of the PLE payload. Considering the default PLE payload size of 1024 bytes, the PLE payload will contain the first 8096 bits (1024 bytes) of the 10280 bit block in the first packet. The following PLE packets will contain the remaining bits followed by the next 10280 bits. The egress NSP will recover the 10280 bit block by searching for the alignment markers at the beginning of PLE packets and recover the 10280 bit block stream. For the 10280 bit data streams the NSP will use the following order of packetization. The first alignment bit of a 10280 bit block is always mapped to the first bit of a PLE payload The order of the bits corresponds to their order in the attached circuit Consecutive bits from the attached circuit are mapped directly into the PLE packet With the default payload size of 1024 bytes the alignment markers will be present at the start of every 5140-th PLE packet for 400GBASE-R and every 2570-th PLE packet for 200GBASE-R. Non-default payload sizes must be chosen so that alignment markers will always be at the start of every N-th packet. Alignment of the signal may use the alignment marker state machine defined in IEEE802.3BS.

A PLE VPWS can be established using manual configuration or leveraging mechanisms of a signaling protocol Furthermore emulation of bit-stream signals using PLE is only possible when the two attachment circuits of the VPWS are of the same type (OC192, 10GBASE-R, ODU2, etc) and are using the same PLE payload type and payload size. This can be ensured via manual configuration or via a signaling protocol Extensions to the PWE3 and EVPN-VPWS control protocols are described in a separate document .

After the VPWS is set up, the PSN-bound IWF does perform the following steps: Packetize the data received from the CE is into a fixed size PLE payloads Add PLE control word and RTP header with sequence numbers, flags and timestamps properly set Add the VPWS demultiplexer and PSN headers Transmit the resulting packets over the PSN Set L bit in the PLE control word whenever attachment circuit detects a fault Set R bit in the PLE control word whenever the local CE-bound IWF is in packet loss state

The CE-bound IWF is responsible for removing the PSN and VPWS demultiplexing headers, PLE control word and RTP header from the received packet stream and play-out of the bit-stream to the local attachment circuit. A de-jitter buffer MUST be implemented where the PLE packets are stored upon arrival. The size of this buffer SHOULD be locally configurable to allow accommodation of specific PSN packet delay variation expected. The CE-bound IWF SHOULD use the sequence number in the control word to detect lost and mis-ordered packets. It MAY use the sequence number in the RTP header for the same purposes. The payload of a lost packet MUST be replaced with equivalent amount of replacement data. The contents of the replacement data MAY be locally configurable. All PLE implementations MUST support generation of "0xAA" as replacement data. The alternating sequence of 0s and 1s of the "0xAA" pattern does ensure clock synchronization is maintained. While playing out the replacement data, the IWF will apply a holdover mechanism to maintain the clock. Whenever the VPWS is not operationally up, the CE-bound NSP function MUST inject the appropriate native downstream fault indication signal (for example ODUk-AIS or ethernet LF). Whenever a VPWS comes up, the CE-bound IWF enters the intermediate state, will start receiving PLE packets and will store them in the jitter buffer. The CE-bound NSP function will continue to inject the appropriate native downstream fault indication signal until a pre-configured amount of payloads is stored in the jitter buffer. After the pre-configured amount of payload is present in the jitter buffer the CE-bound IWF transitions to the normal operation state and the content of the jitter buffer is played out to the CE in accordance with the required clock. In this state the CE-bound IWF MUST perform egress clock recovery. The recovered clock MUST comply with the jitter and wander requirements applicable to the type of attachment circuit, specified in: and for SDH for SONET for synchronous ethernet for OTN Whenever the L bit is set in the PLE control word of a received PLE packet the CE-bound NSP function SHOULD inject the appropriate native downstream fault indication signal instead of playing out the payload. If the CE-bound IWF detects loss of consecutive packets for a pre-configured amount of time (default is 1 millisecond), it enters packet loss (PLOS) state and a corresponding defect is declared. If the CE-bound IWF detects a packet loss ratio (PLR) above a configurable signal-degrade (SD) threshold for a configurable amount of consecutive 1-second intervals, it enters the degradation (DEG) state and a corresponding defect is declared. Possible values for the SD-PLR threshold are between 1..100% with the default being 15%. Possible values for consecutive intervals are 2..10 with the default 7. While either a PLOS or DEG defect is declared the CE-bound NSP function SHOULD inject the appropriate native downstream fault indication signal. Also the PSN-bound IWF SHOULD set the R bit in the PLE control word of every packet transmitted. The CE-bound IWF does change from the PLOS to normal state after the pre-configured amount of payload has been received similarly to the transition from intermediate to normal state. Whenever the R bit is set in the PLE control word of a received PLE packet the PLE performance monitoring statistics SHOULD get updated.

PLE SHOULD provide the following functions to monitor the network performance to be inline with expectations of transport network operators. The near-end performance monitors defined for PLE are as follows: ES-PLE : PLE Errored Seconds SES-PLE : PLE Severely Errored Seconds UAS-PLE : PLE Unavailable Seconds Each second with at least one packet lost or a PLOS/DEG defect SHALL be counted as ES-PLE. Each second with a PLR greater than 15% or a PLOS/DEG defect SHALL be counted as SES-PLE. UAS-PLE SHALL be counted after configurable number of consecutive SES-PLE have been observed, and no longer counted after a configurable number of consecutive seconds without SES-PLE have been observed. Default value for each is 10 seconds. Once unavailability is detected, ES and SES counts SHALL be inhibited up to the point where the unavailability was started. Once unavailability is removed, ES and SES that occurred along the clearing period SHALL be added to the ES and SES counts. A PLE far-end performance monitor is providing insight into the CE-bound IWF at the far end of the PSN. The statistics are based on the PLE-RDI indication carried in the PLE control word via the R bit. The PLE VPWS performance monitors are derived from the definitions in accordance with

The PSN carrying PLE VPWS may be subject to congestion, but PLE VPWS representing constant bit-rate (CBR) flows cannot respond to congestion in a TCP-friendly manner as described in . Hence the PSN providing connectivity for the PLE VPWS between PE devices MUST be Diffserv enabled and MUST provide a per domain behavior that guarantees low jitter and low loss. To achieve the desired per domain behavior PLE VPWS SHOULD be carried over traffic-engineering paths through the PSN with bandwidth reservation and admission control applied.

As PLE is leveraging VPWS as transport mechanism the security considerations described in and are applicable.

Applicable signaling extensions are out of the scope of this document. PLE does not introduce additional requirements from IANA.

The authors would like to thank Andreas Burk for reviewing this document and providing useful comments and suggestions.