1. MPLS Some notations: LSP: Label Switched Path
LSR: Label Switched Router
Ingress: first LSR
Egress: last LSR
Upstream/downstream
FEC: forwarding equivalency classes

2. An example for a packet to pass the following MPLS domain:
LSP 1
LSR A
LSR B
LSR C
LSP 3
LSP 2
LSR D

3. Key concepts in MPLS: Label: Hierarchical label stack
a short, fixed-length, locally significant identifier.
Labels may be mapped to some fields in the layer 2 header.
Otherwise use standard MPLS encapsulation (a thin layer between layer 3 and layer 2).
Each label identifies an LSP and is associated with an FEC.
Hierarchical label stack
Each packet may have one or more labels.
Labels in a packet are organized as a stack (LSRs may push or pop a label.
This is needed for nested tunnels

4. Key concepts in MPLS (continue):
Label-switching table
Incoming label outgoing label next-hop per-label state
Also called imcoming label map (ILM)
Each entry is called next-hop label-forwarding entry (NHLFE)
May have multiple NHLFEs for an incoming label for load sharing (exploiting multi-paths).
Label Distribution Protocols
Before LSPs can be used, the label switching table must be set. This process is called LSP setup or label distribution.
LSRs learn each other’s capabilities and exchange label-mapping information through label distribution protocols.
Signaling in MPLS networks.
LDP for hop-by-hop label distribution, CR-LDP and RSVP-TE for explicitly routed LSPs.

5. Key concepts in MPLS (continue):
Label Assignment and Distribution
Label assignment is always done by the downstream LSR. Why not the upstream LSR?
Two modes: donwstream on demand and unsolicited downstream
Label merging
Two or more LSPs may be merged into one.
Label merging may lose some information about a packet.
Label merging reduce the requirement of label space
Not all LSRs support label merging. (e.g. ATM cannot support merged circuit).

6. Key concepts in MPLS (continue):
Route selection and explicit routing
Hop-by-hop routing (follow whatever IP does)
Explicit routing (need to use constraint based routing).
FEC
A set of packets that are treated identifically in the forwarding process
Packet classification: MPLS move this functionality from the data plane to the control plane.
Some common FECs in MPLS networks:
IP prefix, egress router, application flow
A matter of scalability and controllability.

7. Key concepts in MPLS (continue):
Label Stack Encoding
PPP/HDLC header MPLS header IP packet HDLC trailer
MPLS header IP packet AAL5 pad and trailer
The MPLS label stack contains one or more label stack entries, each is 32 bits long.
Label (20 bits) exp (3 bits) s (1 bit) TTL (8 bits)

8. Key concepts in MPLS (continue):
Determining the network layer protocol
Network layer protocol information is not in the MPLS header.
Loop detection.
IP routing form loops easily
TTL in the MPLS header
Some link layer may not be able to handle TTL (e.g ATM)
Path vector in the label distribution messages
Hop count in the label distribution messages.