PWE3 has three packet formats: MPLS+PW, IP+PW, and IP+UDP. The MA5600T/
MA5603T currently supports MPLS+PW and IP+PW.
l MPLS+PW: In this packet format, the combination of PWE3 outer label and inner label is MPLS over MPLS. It is applicable to MPLS network transmission.
Figure 8-8 shows the format of an MPLS+PW PWE3 packet.
Figure 8-8 Format of an MPLS+PW PWE3 packet
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l IP+PW: In this packet format, the combination of PWE3 outer label and inner label is MPLS over IP. It is applicable to MPLS over IP network transmission. Different from MPLS+PW packets, IP+PW packets are forwarded at IP Layer 3 when transmitted over the PSN network.
Figure 8-9 shows the format of an IP+PW PWE3 packet.
Module
Feature Description 8 Layer 2 VPN
Figure 8-9 Format of an IP+PW PWE3 packet
l IP+UDP: In this packet format, the combination of PWE3 outer label and inner label is UDP over IP. It is applicable to IP network transmission. Different from MPLS+PW packets, IP+UDP packets are forwarded at IP Layer 3 and meanwhile forwarded at Layer 2 by UDP port redirection when transmitted over the PSN network.
Figure 8-10 shows the format of an IP+UDP PWE3 packet.
Figure 8-10 Format of an IP+UDP PWE3 packet
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-8.1.6.2 Principle of TDM PWE3 Packet Format
When the TDM service is transmitted over the PSN, there are two packet encapsulation formats, namely, aware packet format and agnostic packet format. The structure-aware packet, which is also called CESoPSN packet, is defined in RFC5086 and RFC5687. The structure-agnostic packet, which is also called SAToP packet, is defined in RFC4553.
Module
Feature Description 8 Layer 2 VPN
Circuit emulation service over packet switched network (CESoPSN) is a generic term for circuit emulation service and also refers to structure-aware circuit emulation.
1. CESoPSN packet format (structure-aware packet format)
The CESoPSN standard provides the channelized TDM service with emulation and transmission functions, and can identify the TDM frame structure and in-frame signaling.
Therefore, if the customer needs to provide services based on the timeslot, the CESoPSN packet format can meet this requirement.
Figure 8-11 shows the format of the CESoPSN packet.
Figure 8-11 CESoPSN packet format
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PW Label EXP 1 TTL
MPLS Label EXP S TTL
Time slot 1 Time slot 2 Time slot 3 Time slot 4 Time slot n (Frame 1#)……
Time slot 1 Time slot 2 Time slot 3 Time slot 4 Time slot n (Frame 2#)……
Time slot n (Frame m#)……
Time slot 5
Time slot 5
Sequence Number 0 0 0 0 L R RSV FRG LEN
RTP header
l A CESoPSN packet contains a four-byte MPLS header and a four-byte PW header. The length of the CESoPSN control word is 4 bytes, including fields as shown in Table 8-3.
l The length of the Real-Time Transport Protocol (RTP) header is 12 bytes, including the version number, padding flag, and time stamp fields. The time stamp field, whose length is 32 bits, is used for clock synchronization. For format of the RTP header, see RFC3550.
l Time slot indicates the TS in the TDM frame. Each TS occupies 8 bits. All TSs comprise the encapsulated TDM data payload, which does not include the CRC bit. The number of encapsulated frames and the number of TSs in each frame can be set by users according to conditions.
Table 8-3 Fields of the control word
Field Description
0000 This field is generally all 0s, with the length of 4 bits.
When the virtual circuit connectivity verification (VCCV) is needed to help to monitor the SAToP PW status, these four bits are used to identify the start of the associated channel header (ACH).
Module
Feature Description 8 Layer 2 VPN
Field Description
L Indicates whether the TDM data in the packet is valid. Its length is 1 bit. When it is set to 1, it indicates that the TDM data in the packet is invalid; that is, the TDM data in the packet can be neglected to save bandwidth resources.
R Indicates whether the interconnection function of the local customer edge (CE) is in the packet loss state. When it is set to 0, it indicates that consecutive packets have been received and will no longer be lost.
RSV Indicates the reserved bit. Its length is 2 bits.
FRG Indicates the fragmentation status of the packet. Its length is 2 bits. Its values are as follows:
l 00: Indicates that the packet encapsulates the entire TDM data.
l 01: Indicates that the packet encapsulates the first fragmentation of the TDM data.
l 10: Indicates that the packet encapsulates the last fragmentation of the TDM data.
l 11: Indicates that the packet encapsulates the intermediate fragmentation of the TDM data.
LEN Indicates the length of the entire CESoPSN packet (the size of the CESoPSN header and TDM data.) When the length is shorter than 64 bytes, LEN is a specific length value. When the length is equal to or longer than 64 bytes, LEN is 0.
Sequence Number Indicates the sequence number for transmitting the CESoPSN packet. Its length is 16 bits. Its initial value should be random.
It must be incremented by 1 with each CESoPSN data packet sent in the specific PW.
The CESoPSN packet structure has the following characteristics:
l CESoPSN provides emulation and transmission of structure-aware TDM service. That is, CESoPSN can identify the TDM frame structure and in-frame signaling and transmit the frames.
For example, a structure-aware E1 link consists of 32 timeslots. Except timeslot 0, the other 31 timeslots can each carry a channel of 64 kbit/s voice service. Timeslot 0 is used only for transmitting signaling and frame delimiter.
l CESoP can identify the TDM frame structure. Therefore, idle timeslots are not transmitted, and only the data in the timeslots that are useful for the CE devices is retrieved from the E1 service stream and transmitted after being encapsulated into CESoPSN frames.
l CESoPSN can identify and transmit the CAS and CCS signaling of E1 service streams.
2. SAToP packet format (structure-agnostic packet format)
The structure-agnostic TDM over PSN (SAToP) standard provides the non-channelized TDM service with the emulation and transmission functions. The protocol need not be Module
Feature Description 8 Layer 2 VPN
aware of the structure of the TDM packets and transparently transmits the packets.
Therefore, if the customer only needs to provide services based on E1, SAToP (unstructured packet format) can meet this requirement.
Figure 8-12 shows the format of the SAToP packet.
Figure 8-12 SAToP packet format
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l An SAToP packet also contains a four-byte MPLS header and a four-byte PW header.
The length of the SAToP control word is 4 bytes, including fields as shown in Table 8-3.
l The SAToP protocol treats the TDM service as serial data code stream for segmentation, and transmits the service over PWs after encapsulation. SAToP can transmit the synchronization timing information although it is unaware of the structure of the TDM frame.