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MPLS

MPLS. A single forwarding paradigm (label swapping), multiple routing paradigms Multiple link-specific realizations of the label swapping forwarding paradigm. Label Swapping. Every MPLS packet carries a label.

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MPLS

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  1. MPLS • A single forwarding paradigm (label swapping), multiple routing paradigms • Multiple link-specific realizations of the label swapping forwarding paradigm

  2. Label Swapping • Every MPLS packet carries a label. • An MPLS forwarding device (Label Switch Router – LSR) has a forwarding table. For each label it has two entries – next hop and next label. • The old label is swapped out with the next label and sent to the next hop. • Labels may be stacked on top of each other.

  3. Label Switched Path (LSP) • Concatenating label swapping between a series of LSRs creates a label switched path (LSP) • All MPLS packets that are switched along a common LSP are said to belong to the same forwarding equivalence class (FEC). • Label stacking enables aggregation of multiple LSPs into an LSP with a coarser FEC. • Labels are pushed and popped at the head and tail of an LSP.

  4. LSP setup – IP routing • Ordered approach: An egress LSR decides to advertise a FEC to a subnet 172.17/16 C A B Label = 10 FEC = 172.17/16 D Label = 21 FEC = 172.17/16

  5. LSP setup – IP routing • Independent Control: Each LSR decides on its own an FEC/Label binding to advertise. 172.17/16 C A B Label = 10 FEC = 172.17/16 D Label = 21 FEC = 172.17/16

  6. LSP setup – Constraint Based Routing • Goal: create an LSP that satisfies some constraint – e.g. minimum average throughput. • Link state protocol advertises unreserved capacity per link. • Pick shortest path on the remaining topology • Use RSVP to reserve resources and fill label switch table.

  7. Example • Route from A to Z with a bandwidth of 75 Mbps C 75 75 A B 75 Y Z 75 75 25 X PATH RESV

  8. Multiple encapsulations • MPLS violates layering by maintaining different interfaces with different link layers • Frames that do not have space for a label – like ethernet – use a “shim” header Ethernet Header Shim IP Packet Label Exp Stack TTL 1 bit 8 bits 20 bits 3 bits

  9. Another Encapsulation • MPLS puts label information in the ATM VCI/VPI header field and in the AAL5 PDU payload AAL5 PDU Label Stack AAL5 trailer IP Packet 48 bytes 48 bytes 48 bytes ATM Cell ATM Cell ATM Cell

  10. Two levels • In the OSI stack, each layer may have both a forwarding and control component. • Under MPLS there is one forwarding paradigm – label swapping. • A single forwarding paradigm implies all MPLS peers may share the same control plane. • MPLS “flattens” the OSI stack. Thus forwarding devices that belong to different layers in the OSI model may peer directly. • ATM switches, Ethernet Switches, OXC, IP routers all peer directly with each other.

  11. GMPLS • How to efficiently use bandwidth and label space? • There is a natural hierarchy of label stacking. PSC TDM LSC FSC LSC TDM PCS

  12. Generalized MPLS (GMPLS) • Support for the following link layers • Packet Switch Capable (PSC) – ATM/Ethernet/Frame Relay • TDM Capable (TDM) – SONET/SDH • Lambda Switching Capable (LSC) – Optical Switch • Fiber Switch Capable (FSC) – OXC • The last three link layers are fundamentally different from the first • It is impossible to stack lambdas • There are typically few lambdas, ports, and time slots compared to the number of possible packet labels • Bandwidth comes in discrete values (OC-12,OC-48,OC-192)

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