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