Transcript MPLS (Lecture 11)

MPLS
• Some notations:
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LSP: Label Switched Path
LSR: Label Switched Router
Ingress: first LSR
Egress: last LSR
Upstream/downstream
FEC: forwarding equivalency classes
• 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
• Key concepts in MPLS:
– Label:
• 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
• 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.
• 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
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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).
• 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.
• 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)
• 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.