Which statement about MPLS label encapsulation format is true?

The MPLS label encapsulation format indeed specifies that the MPLS label length is 4 bytes. This format consists of a fixed structure where the main components are the label, the 3-bit Experimental (Exp) field, the 1-bit Stack Bottom (S) field, and the 20-bit Depth (Label) field itself. The encapsulation allows for efficient packet forwarding by enabling routers to make forwarding decisions based on the label rather than the IP headers, which directly contributes to improved performance in traffic engineering.

The other statements do not hold true in the context of MPLS. The TL field is not designed to prevent packet loss; it is part of a different protocol operation. The S field indeed identifies whether a label is the last in a stack of labels but does not relate to the significance of the labels in the MPLS context. Lastly, MPLS label encapsulation can apply to a variety of protocols, not just TCP packets, making the claim about exclusivity false.