What is its MPLS’ place in layered communication?
It is a niche all its own with no other competitors, MPLS has a complete monopoly in layered communications. Often referred to as switching at layer 2.5, MPLS uses a shim header that contains information that helps move frames from hop to hop.
A shim header is a 32 bit header placed between the layer 2 header, and layer 3 payload as shown in the picture below. In certain applications like MPLS VPN and MPLS TE, the shim header can contain enough information to define a path through an entire network, not just to the next hop. The standard shim header has 4 fields, of which the functions are defined below.
The 4 fields are:
Label - 20 bits; This field stores the label value. This value can be between 0 and 2^20th - 1. The first 16 of these labels ( 0 - 15) are exempted from nthey are reserved for specific functions known as label operations.
Experimental (EXP) - 3 bits; This field is used specifically for Quality of Service implementation.
Bottom of Stack (S) - 1 bit; This identifies if the particular label in the stack is the bottom or top label. This bit is set to 0 unless it is the bottom label in a label stack, if so the bit is set to 1. A stack is a collection of labels on top of the packet. The number of labels you can have (that is, the number of 32 bit fields) on top of a packet is limitless, though you should seldom see a stack of 4 labels or more.
Time to Live (TTL) - 8 bits; This field performs the same function as the TTL field found in an IP header. Its main function is to avoid a packet being stuck in a routing loop. If a routing loop occurs and no TTL is present the packet loops forever. If the TTL reaches 0, the packet is discarded.
Understanding IP Communications: What is MPLS | Understanding IP Communications wrote,
[...] Essentially, MPLS creates a unique layer 2 (data link layer) identifier for layer 3 (network layer) network information. It does this by use of a Shim header. This is why MPLS is often referred to as switching at layer 2.5. The “network information” mentioned are often referred to as a prefixes, nothing more than the IP networks that connect to and from the MPLS switch or router. [...]
Link | May 14th, 2007 at 3:12 am
Understanding MPLS: MPLS Terminology | Understanding IP Communications wrote,
[...] PE: Provider Edge - This is where the customer routes are received. This is also where label imposition/disposition happens through the use of a Shim header. That is, this is where labels are created and/or removed for customer traffic to and from the provider network. [...]
Link | May 14th, 2007 at 1:51 pm