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MPLS Frame


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MPLS Frame

Each MPLS packet/frame has a header that is either encapsulated between the link layer and the network layer, or resides within an existing header, such as the virtual path/channel identifier (VPI/VCI) pair within asynchronous transfer mode (ATM). At most, the MPLS header will contain

         A label,
         TTL field,
         Class of Service (CoS) field,
         Stack indicator,
         Next header type indicator, and


Figure 10 - MPLS label stack encoding for packet-oriented transport

Figure 10 shows the structure of the generic MPLS frame. An MPLS label stack of one or more 32-bit entries precedes the payload (e.g., an IP packet). The label is 20 bits wide, with 3 additional bits for experimentation (e.g., to indicate queuing and scheduling disciplines). An 8-bit time to live (TTL) field is defined to assist in the detection and discard of looping MPLS packets: the TTL is set to a finite value at the beginning of the LSP, decremented by one at every label switch, and discarded if the TTL reaches zero. The S bit is set to 1 to indicate the final (and possibly only) stack entry before the original packet; an LSR that pops a stack entry with S set to 1 must be prepared to deal with the original packet in its native format 


Figure 11 - Ingress LER





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