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Technical White Paper for PWE3
Huawei Technologies Co., Ltd.
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. ihttp://datacomm.huawei.com
Table of Contents
1 Preview ........................................................................................................................................... 1
2 Overview of PWE3 Technology.................................................................................................... 1 2.1 Essential Transmission Components of PWE3 Network...................................................... 2 2.2 PW Using LDP Signaling ...................................................................................................... 4 2.3 Packet Forwarding................................................................................................................ 6
3 Critical Technologies .................................................................................................................... 7 3.1 Integrated MH-PW Networking ............................................................................................. 7 3.2 PW Backup Protection.......................................................................................................... 7 3.3 Control Word (CW) ............................................................................................................... 8 3.4 VCCV-PING.......................................................................................................................... 9
Appendix A Acronyms and Abbreviations .................................................................................. 11
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 1http://datacomm.huawei.com
Technical White Paper for PWE3
Abstract: Pseudo Wire Emulation Edge to Edge (PWE3) is a layer 2 Virtual Private Network (VPN)
protocol that provides tunnels to emulate services (such as Frame Relay (FR), ATM,
Ethernet, TDM SONET/SDH) over packet switched networks (IP, MPLS). This protocol
connects traditional networks and packet switched networks (PSNs) to share resources
and extend networks. It is the extended Martini protocol. It extends new signaling
(optimized signaling overhead) and specifies the negotiation mechanisms for MH-WP to
enrich the networking modes. This document describes the working principle, critical
technologies, advantages and defects of the PWE3 technology. It also provides
suggestion to PWE3 application and deployment.
Keywords: PWE3, VC, AC, UPE, SPE, VCCV, CW, SH-PW, MH-PW
1 Preview
The IP data network is evolving, showing powerful extensibility, scalability, compatibility
and interworking. Comparatively, the traditional communication network, limited by the
transmission mode and service types, shows a poor capability in regards to these
features. In addition, new networks are poor in sharing and difficult in interworking
management. In upgrading traditional communication networks and expanding the
applications, we are confronted with the problem: to build redundant networks or to make
full use of existing or public resources. The PWE3 technology is designed to solve the
problems of the traditional communication networks and existing PSNs.
2 Overview of PWE3 Technology
PWE3 is an edge-to-edge L2 service carrier technology. It is a point-to-point (PTP)
L2VPN. On two provider edges (PEs) on a PSN, PWE3 takes Label Distribution
Protocol/Resource Reservation Protocol (LDP/RSVP) as signaling to emulate various L2
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 2http://datacomm.huawei.com
services (such as L2 data packets, bit stream) at the customer edges (CEs) to enable the
transparent transmission of L2 data at the CEs.
2.1 Essential Transmission Components of PWE3 Network
The essential transmission components of a PWE3 network include:
1) Attachment Circuit (AC): The physical or virtual circuit attaching a CE to a PE. All user packets (L2 and L3 protocol packets) on the AC are required to be forwarded to the peer site, intact.
2) Pseudo Wire (PW): A VC with a tunnel. The tunnel can be over LSP, L2TPV3 or TE. A PW is oriented. A PW is established to manage the VC and tunnel on a PWE3 network by passing VC through LDP/RSVP signaling. A PW is a directly connected channel between the local AC and peer AC to complete transparent transmission of L2 data in the PWE3 system.
3) Forwarders: When a PE receives data frame reported by the AC, the forwarders select the PW to be used to forward the packets. The forwarders are forwarded tables of PWE3.
4) Tunnel: is used to carry PWs. A tunnel can carry multiple PWs. The tunnels are usually MPLS tunnels. A tunnel is a directly connected channel between the local PE and peer PE to complete transparent transmission of data between PEs.
5) Encapsulation: The packet in a PW is encapsulated in standard format and technology. For the encapsulation mechanisms of PWE3 packets, see the definitions in draft-ietf-pwe3-iana-allocation-x.
6) Pseudo-wire signaling protocols: PW signaling protocols are the basis of PWE3 to establish and maintain a PW. The main PW signaling protocols include LDP and RSVP.
7) QoS: The priority information in the L2 header information is mapped to and forwarded as the QoS priority, which is transmitted on a public network. This requires the applications to support MPLS QoS.
Figure 3 illustrates the locations of the essential transmission components of PWE3 on a
network.
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 3http://datacomm.huawei.com
Figure 1 Essential transmission components of PWE3
PWE3 extended the Martini protocol. The differences between the two protocols are:
1) In regards to control, LDP is used as signaling to establish pseudo wire (PE) and
notification is available. PWE3 reduces interactivity for packet control and is
compatible with the Martini model. L2TPv3 can also be used as signaling. When
RSVP is used as signaling, the established PW, RSVP-TE PW, is bandwidth
ensured.
2) Multi-hop PW (MH-PW), this function extends the networking mode and reduces the
requirements for the number of LDP connections. Multi-hop access nodes provide
the convergence of a PW.
3) Fragmentation capability negotiation is provided for control and the fragmentation
and re-organization mechanism is defined for forwarding.
4) A PW connectivity detection mechanism and means (VCCV) is provided.
5) Low-rate Time Division Multiplexing (TDM) interfaces are supported. TDM packet
sequencing, and clock extraction and synchronization are introduced by using control
word (CW) and Real-time Transport Protocol (RTP).
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 4http://datacomm.huawei.com
6) PWE3 perfects MIBs.
PW tunnels are established based on LDP (draft-ietf-pwe3-control-protocol-x) signaling
and RSVP (draft-raggarwa-rsvpte-pw-xx) signaling usually.
2.2 PW Using LDP Signaling
When LDP signaling is used, the VC information carried by LDP TLV include two new FEC
TLVs, 128 and 129. When a PW is established, LDP must be used in its downstream
unsolicited (DE) mode. LDP's liberal label retention mode should be used. The LDP used
to exchanging VC signaling must be configured to remote mode.
Figure 2 illustrates the typical procedure to establish and remove a Single-Hop PW
(SH-PW) with LDP signaling. Suppose a virtual circuit (VC) is configured to PE1, whose
peer is PE2. If an LDP session has been set up between PE1 and PE2, PE1 will assign a
label and send a mapping message to PE2. When PE2 receives the mapping message, it
checks whether a same VC is configured. If yes and the VC IDs are identical, the VCs of
the two PEs are on the same VPN. If the interface parameters are the same, the PW at
PE2 will be established. When PE1 receives the mapping message from PE2, it will do
the same.
In case the AC or tunnel of a PW is in the down state, Martini protocol will transmit
withdraw packets and detach the PW. When the AC or tunnel is in the up state, a new
negotiation process will be carried out to establish connection. In the same condition,
PWE3 protocol will send a packet to notify the peer that packet forwarding is not available,
but it does not detach the PW. When the AC or tunnel is in the up state, it sends a packet
to notify the peer that packet forwarding is available.
When PW1 does not want to forward the packets of PE2 (for example, we withdraw
specifying PE2 as the peer of PE1), it sends a withdraw packet to PE2. Upon receiving the
withdraw packet, PE2 will remove the PW and return a release packet. When PE1
receives the release packet, it releases the label and removes the PW.
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 5http://datacomm.huawei.com
Figure 2 Single hop PWE3 signaling process
Single hop PWE3 signaling can meet the requirements in most conditions. The
exceptions include:
1) The two PEs are not in the same autonomous system (AS) and establishing signaling connection or tunnel between them is not available.
2) The two PEs are not using the same type of signaling. For example, one is using LDP while the other is RSVP.
3) If the access device can run MPLS, but cannot establish a large amount of LDP sessions, we can use a User Facing Provider Device (UFPE) as the Ultimate PE (U-PE) and a PE switching point (S-PE) as the switching point of LDP sessions. The S-PE acts like a signaling reflector.
Figure 3 illustrates the typical procedure to establish a MH-PW with LDP signaling.
Comparing with establishing a SH-PW, the establishment of a MH-PW requires an SPE
between the two PEs. PE1 and PE2 are connected through the SPE. PE1 and PE2
connect to the SPE respectively and the SPE connects the two PWs. During the signaling
negotiation, PE1 sends a mapping message with parameters to the SPE (Step 10), who
will forward it to PE2 (Step 12). Similarly, PE2 sends parameters to the SPE in a mapping
message (Step 11), who will forward it to PE1 (Step 13). When the negotiation between
the parameters of the two PEs comes to an agreement, the PW is in the up state. Similar
to the mapping message, the release, withdraw and notification packets are also passed
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 6http://datacomm.huawei.com
hop by hop until packet forwarding is stopped or the PW is withdrawn or released. There
can be one or more SPEs.
Figure 3 Multi-hop PWE3 signaling process
2.3 Packet Forwarding
PWE3 can establish a PTP channel. Different channels are separated. L2 packets are
transparently transmitted in PWs. For a PE device, the mapping between user ACs and
virtual circuit PWs are determined after PWs are established. The PE completes MPLS
packet forwarding according to the MPLS label, but does not care about the L2 user
packet encapsulated in the MPLS packet.
We take the flow of forwarding the VPN1 packet from CE1 to CE3 for example to explain
the basic data flow. CE1 accesses PE1 through AC to report a L2 packet. When PE1
receives the packet, the forwarders select the PW to be used to forward the packet.
According to the forwarder information of the PW, PE1 assigns and sends a PW label (to
identify the PW) to the tunnel on an external layer. The packet will be sent to PE2 across
the tunnel on the public network. According to the PW label, PE2 forwards the packet to
the corresponding AC and to CE3 finally.
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 7http://datacomm.huawei.com
3 Critical Technologies
3.1 Integrated MH-PW Networking
An integrated MH-PW includes two ends, one static PWs and the other dynamic PWs
(LDP). Either the static or dynamic PWs can be multi-hop. A static PW alternating with a
dynamic PW is not an integrated MH-PW.
The UPEs and SPEs on static or dynamic PWs are configured in the same way, but the
SPE connecting the static PW segment and dynamic PW segment is configured
differently.
On the SPE connecting the static PW segment and dynamic PW segment, the dynamic
PW segment can be taken as the AC of the static PW segment and the changes of the
static PW segment can be taken as the AC changes of the dynamic PW segment. To
begin the signaling procedure, the SPE must has knowledge of the PW type and interface
MTU, which must be the same as the configuration of the CE interface for the static PWs.
The static PWs will be set up if a tunnel is established. The remote dynamic PWs will be
set up if the tunnel is established. If the PW type and MTU are the same as those on the
local host, the dynamic PW will also be in the up state.
3.2 PW Backup Protection
PW backup protection allows quickly switching over to the other PW when the active PW
fails (for example, the tunnel of the PW is removed) to complete fast data switchover. See
the following figure:
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 8http://datacomm.huawei.com
Figure 4 Topology of PW backup protection
Follow these steps to configure PW backup protection (multi-hop):
1) Establish two PWs on each of the two U-PEs, where a PW on a U-PE corresponds to
a PW on the other U-PE. Set a PW (PW5) on a U-PE (U-PE1) as backup PW.
Establish two PWs to each S-PE to set up a MH-PW with the PWs of the U-PEs (see
the topology of PW backup protection).
2) Configure the same signaling negotiation and processing for the active and backup
PWs as those for a MH-PW.
3) If the active PW is in the down state (for example, LDP session in down state, tunnel
withdrawn), notify the backup PW immediately. When the backup PW is in the up
state, it will become the active PW.
3.3 Control Word (CW)
A CW must be negotiated on the control layer for packet sequence detection, packet
segmentation and re-organization on the forwarder layer.
The PWE3 protocol requires supporting ATM AAL5 CW and FR CW. If the negotiation
result on the control layer supports a CW, the CW must be sent to the forwarders, which
implement packet sequence detection, packet segmentation and re-organization on the
forwarder layer.
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 9http://datacomm.huawei.com
3.4 VCCV-PING
The VCCV-PING, similar to the ICMP-PING and LSP-PING tools, is used to detect the
connection state of a PW manually. It is the extended LSP-PING. For details about the
VCCV-PING, refer to draft-ietf-pwe3-vccv-x and draft-ietf-mpls-lsp-ping-xx.
1) During signaling setup, Intf TLV of the mapping messages must contain the VCCV
parameters as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0c | 0x04 | CC Types | CV Types|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where the CC type is:
0x01 PWE3 control word with 0x0001 as first nibble 0x02 MPLS Router Alert Label 0x04 MPLS inner label TTL = 1
The CV type is:
0x01 ICMP Ping 0x02 LSP Ping 0x04 BFD
To support the VCCV-PING tool, CC should support CW or Router Alert Label (if CW
not supported), and CV should support the ICMP-PING (the PSN is an IP network,
such as GRE or L2TPv3) or LSP-PING (PSN is an MPLS network).
2) Delivering VCCV function to the forwarding layer, at the PW ingress on the forwarder
layer, the VCCV-PING packet is encapsulated in the payload of the data packet. That
is after the CW or Router Alert Label over the PW. At the PW egress, the VCCV-PING
packet is reported to the CPU, but not forwarded to the CE directly.
3) According to the requirements of the LSP-PING, the VCCV-PING packet is the UDP
packet, which includes PW FEC information.
The format of a ping packet is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Number | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Reply mode | Return Code | Return Subcode |
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 10http://datacomm.huawei.com
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's Handle |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Sent (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Sent (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Received (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Received (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The TLV structure is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type # Value Field
------ -----------
1 Target FEC Stack
The contents of the Value field is SUB_TLV (the type is 9, Length is 10, namely L2 circuit
ID). The values in SUB_TLV is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's PE Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote PE Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VC ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encapsulation Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Technical White Paper for PWE3
Copyright ©2007 Huawei Technologies Co., Ltd. All rights reserved. 11http://datacomm.huawei.com
Alert encapsulation: Tunnel Label Alert Label PW Label IP Header UDP Header Ping packet
CW encapsulation: Tunnel Label PW Label Control word IP Header UDP Header Ping packet
Appendix A Acronyms and Abbreviations
Acronym/Abbreviation Full Spelling
PWE3 Pseudo Wire Edge to Edge Emulation
CE Custom Edge
PE Provider Edge Router
UPE Ultimate Provider Edge
SPE PW Switching Point
PW Pseudo Wire
AC Attachment Circuit
CW Control word
VCCV Virtual Circuit Connectivity Verification