MULTIPROTOCOL LABEL SWITCHING
Muhammad Abdullah Shafiq
MPLS
Multi-Protocol Label Switching
Label Based Packet Forwarding
Forwarding Equivalence Class (FEC)
Each FEC maps a Label
WHY MPLS ? TRADITIONAL IP FORWARDING VS. MPLS BASED FORWARDING
Traditional IP based Forwarding Based on Longest Prefix Match Inefficient, slow searching
MPLS BASED FORWARDING
R1 – R4 : Label Switched Path Labels : Indexes
Searching is based on labels which are indexes to forwarding table.
OTHER ADVANTAGES OF MPLS
Forwarding requires Label lookups and replacements Simple switches can do it.
Explicitly routed LSP Vs. Source Routing Label ~ Route No need to carry identity of route with packet.
LABEL ENCAPSULATION
MPLS DOMAIN Ingress Router
Handles traffic entering MPLS Domain
Egress Router Handles traffic leaving MPLS Domain
Label Switched Path (LSP) Path traversed by packet
Label Switching Router (LSR) Forwards packets based on labels.
LABEL BINDING
FEC ~ Label binding (Forwarding Equivalence Class)
Upstream & Downstream LSR (w.r.t. binding)
If Packet travels : Ru Rd FEC F ~ Label L Ru: Upstream LSR Rd: Downstream LSR
LABEL ASSIGNMENT & DISTRIBUTION
Downstream Assigned Label binding is assigned by downstream LSR
Upstream Distribution Label bindings are distributed upstream
Label Distribution Protocols (LDP) To inform the ‘peers’ about label bindings To learn each other’s MPLS capabilities May use TCP for reliability
LABEL ASSIGNMENT & DISTR. (CONTD.)
LABEL STACK
A labeled packet may carry more than one label.
Arranged as Last In First Out (LIFO) stack
Processing is based on top label.
Unlabelled packet ~ empty packet stack
Top label: level m
Bottom Label: at level 1
Uses: LSP tunnel and MPLS hierarchy
LABEL STACK OPERATIONS
PUSH Insert a label on top of stack E.g when forwarding a packet that arrives
unlabeled but is to be forwarded with label.
POP Remove the top label e.g. at egress router
SWAP Replace the top label with new label. Normal forwarding between LSRs.
THE FORWARDING TABLE
Invalid incoming labels An LSR with no binding of incoming label.
Option 1: forward as unlabeled packet Upstream and downstream forward to each other. May cause a loop.
Option 2: Discard packet.
LSP CONTROL
What about FEC that corresponds address prefixes learned via dynamic routing ?
Two solutions: Independent LSP Control
Each LSR binds independently Distributes that bindings to peers
Ordered LSP Control Only egress LSR for FEC binds and distributes Used when some properties in paths followed are
desired (e.g. explicit specified path)
LSP ROUTE SELECTION
Hop by Hop
Each node chooses next hop independently
Explicitly routed
A single LSR specifies the LSRs in LSP
AGGREGATION
Traffic in multiple FECs may go to same egress.
Combining multiple FECs to single FEC.
Use single label for union of FECs.
Aggregation reduces the number of labels.
TUNNELING
Ru acts to deliver packets to Rd when they are not consecutive routers. Tunnel from Ru to Rd Tunneled Packet
Two methods: Hop by hop routed tunnel Explicitly routed tunnel
HIERARCHY
LSP R1, R2, R3, R4
Suppose R2 and R3 are not connected directly
R2 , R21, R22, R23 R3 forms internal tunnel.
Actual sequence: R1, R2, R21, R22, R23, R3, R4
QUESTIONS
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