qrcoder c# Pseudowire Forwarding in Objective-C

Generation DataMatrix in Objective-C Pseudowire Forwarding

Pseudowire Forwarding
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MPLS
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Any intermediate LSRs along the LSP need only examine the tunnel label in order to forward the packet along the tunnel LSP they do not need to be aware of the existence of one or more pseudowires within the tunnel LSP Intermediate LSRs may also push or pop additional labels, for example, for fast reroute protection or in cases where LSP hierarchy is used The egress PE uses the PW label to determine which attachment circuit to forward the received Layer 2 frame on Note that for some Layer 2 protocols, such as Ethernet (see Figure 143 below), the frame header is carried across the pseudowire, whereas for others, such as Frame Relay, it is removed and a new header is added by the egress PE When carrying Layer 2 traffic over MPLS, the MPLS label stack sits between the service provider s Layer 2 header and the service user s Layer 2 frame (which may or may not include a header) When Ethernet is carried over MPLS and there is an Ethernet link between two MPLS LSRs, there will be two MAC headers on each packet, separated by an MPLS label stack
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The Control Word The Generic PW MPLS Control Word defined in RFC 4385 and shown in Figure 144 is required for some pseudowire types, such as Frame Relay, and is optional for others, including Ethernet When used, it is inserted between the label stack and the payload The fields in the control word are used as follows:
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The first four bits of the control word are set to all zeroes This enables core LSRs, which perform ECMP by inspecting the packet to identify flows, to distinguish pseudowire payloads from IP packets (where the first four bits will be 0100 for IPv4 and 0110 for IPv6) In cases where the Layer 2 header is removed, there may be a need to carry header flags such as congestion notification to the egress PE Four bits are provided for this These bits are not used for Ethernet pseudowires, as there are no flags in the Ethernet header There may be cases where large frames need to be carried over LSPs with insufficiently large Maximum Transmission Units (MTUs) In such cases, it may be necessary to fragment the frames before transmission and to reassemble the fragments at egress and hence to carry an indication of whether a packet contains a fragment, and if so, whether it contains the final fragment of the frame Two bits are provided for this Fragmentation is rarely implemented for Ethernet pseudowires Service providers generally ensure that the MTU of their LSPs is sufficient to transport the largest frame size supported by their service
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Layer 2 header Figure 143
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Ethernet frame (FCS removed)
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L2 FCS
Ethernet pseudowire encapsulation
14
Flags
Length
Sequence number
Figure 144 The Generic PW MPLS Control Word
One potential issue in carrying Layer 2 protocols over MPLS is that if the payload plus MPLS headers is smaller than the minimum Ethernet payload length (46 bytes), then padding will be added to the packet when it crosses an Ethernet segment on the LSP path Because MPLS has no length field, this padding will be retained until the packet reaches the egress PE When carrying IP over MPLS, the length field in the IP packet header can be used to determine how much padding must be removed; however, most Layer 2 protocols have no length field and instead rely on physical layer framing to detect the end of the frame The length field is 6 bits and is only valid if the frame length is less than 64 octets Since this is greater than the minimum Ethernet payload length, this enables transport of small frames over Ethernet links These bits are, of course, not used for Ethernet pseudowires since a minimum-sized Ethernet frame (the smallest frame the pseudowire will carry) is bigger than the minimum Ethernet payload length! The 16-bit sequence number may be useful when carrying Layer 2 protocols such as ATM and Frame Relay, which have strict requirements that frame order be maintained over a network that may reorder packets It is rarely implemented in the Ethernet pseudowire case because most Ethernet applications do not have strict frame order requirements; IP/MPLS networks do not generally reorder packets, and many routers have forwarding plane implementations that are unable to support sequence number processing
Given all the above, it is not surprising that the control word is rarely used for Ethernet pseudowires
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