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Historically, IP networks have been layered on top of connection-orientated Layer 1 or Layer 2 technologies for example, SDH/SONET virtual containers or ATM virtual circuits which have then adapted the traffic onto the underlying transport infrastructure (fibres or wavelengths) As IP (and especially Internet) traffic grew exponentially in the mid-1990s, it became an ever greater consumer of the SDH/SONET or ATM infrastructure, but with routers using general-purpose microprocessors to forward traffic, there was still a speed gap between the fastest IP interface and that of the long-haul transport systems However, by the late 1990s, routers became available with ASICbased forwarding able to fill the 25 Gbps and 10 Gbps links provided by transport systems, making it possible to bypass the ATM and SDH layers New entrant carriers with all-IP networks now faced a challenge in offering ATM and Frame Relay services over their networks because IP networks supported connectionless Layer 3 service rather than connection-orientated Layer 2 service However MPLS, with its mix of connectionless and connection-orientated characteristics, was increasingly being used both to offer IP VPN services and to enable traffic engineering for IP core networks and could easily be extended to offer Layer 2 services One early solution to offering Layer 2 services over MPLS was Juniper s Circuit Cross Connect (CCC) With CCC, an LSR associated a pair of RSVP-TE signalled LSPs (one for which it was ingress and one for which it was egress) with an attachment circuit (either a physical interface or a logical interface such as a port and VLAN) Any frame received on the attachment circuit was sent over the outbound LSP, and any frame received from the inbound LSP was sent over the attachment circuit The problem with such a solution, however, is that if a carrier wished to offer Layer 2 services to its customers, then for each Layer 2 circuit, there was a pair of RSVP-TE LSPs through the core of the IP/MPLS network, resulting in poor scaling properties for the service A more scalable approach was invented by a team working at Level(3) Communications, and published in IETF as draft-martini (after its inventor, Luca Martini) In the draftmartini approach, each attachment circuit is associated with a pair of MPLS labels one for each direction however these labels are then carried within MPLS LSPs
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(using MPLS label stacking), so that a single MPLS LSP can carry a large number of customer services (and, in fact, the LSPs may be the same ones that are used to carry IP traffic) The draft-martini approach was eventually standardized in the IETF PWE3 ( Pseudowire Emulation Edge to Edge ) Working Group and forms the basis of the pseudowire architecture One key to the draft-martini approach was to maximise reuse of existing hardware and software technology Using MPLS labels as the forwarding plane identifiers for pseudowires and extending LDP signalling to establish pseudowires made it relatively straightforward to implement pseudowires on the IP/MPLS router platforms available at the time Some minor compromises were, in fact, made to make it easier for existing hardware to implement the technology Layer 2 pseudowires have been defined to carry Ethernet, ATM, Frame Relay, HDLC, and PPP traffic Layer 1 pseudowires have also been defined to enable service providers to transport TDM signals and SONET/SDH frames across IP/MPLS networks IP pseudowires have been defined to enable interconnection of IP endpoints running different Layer 2 protocols but using Layer 2 forwarding (the IP pseudowire encapsulation removes all Layer 2 framing and carries only the IP payload) In the pseudowire architecture, as in the Layer 2 and Layer 3 VPN architectures, the LER is known as a provider edge or PE device (core LSRs are known as provider or P devices), and the device at the far end of the attachment circuit to the LER is known as the customer edge or CE device As just discussed, the draft-martini architecture relies on MPLS LSPs to transport Layer 2 (or Layer 1) information across IP/MPLS networks In order to send a Layer 2 frame across the network, an ingress PE prepends a label stack to the frame and sends the resulting packet into an MPLS LSP The bottom label in the stack is known as the PW label This label is used to identify the pseudowire and the corresponding egress port at the egress PE and is assigned by the egress PE If the payload of the MPLS packet is, for example, an Ethernet frame, the PW label will correspond to an Ethernet port or port and VLAN at the egress PE So when the egress PE receives the packet, it will remove the PW label and switch the payload to the corresponding port or port and VLAN This process is unidirectional and will be repeated independently for bidirectional operation Any labels above the PW label in the stack are used simply to transport the packet from the ingress PE to the egress PE along the selected MPLS LSP In general, only one such label is required, and it is known as the tunnel label Note that the tunnel label, if present, must be immediately above the PW label (except for the case where an intermediate label is used for router alert VCCV, as described in the pseudowire OAM section below) The tunnel label may be removed before the packet reaches the egress PE if PHP is used, and if the ingress and egress PE are adjacent, may never be pushed on So when the ingress PE wants to send a frame along a pseudowire to the egress PE, it first pushes on the PW label assigned by the egress PE and then (in the general case) adds a tunnel label corresponding to the selected tunnel LSP and possibly additional labels
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