1. MPLS
A single forwarding paradigm (label swapping), multiple routing paradigms
Multiple link-specific realizations of the label swapping forwarding paradigm
Sources: draft-many-gmpls-architecture-00.txt, MPLS Technology and Applications: Davie and Rekhter, Generalized Multiprotocol Label Switching: An Overview of Routing and Management Enhancements IEEE Comm Mag

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 = /16 D
Label = 21
FEC = /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
Pro: Faster convergence of LSPs to IP routing.
Con: If FEC granularities do not match, LSP through the local domain is not possible – hack: require all FECs to have the same granularity.
Label = 10
FEC = /16 D
Label = 21
FEC = /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.
RSVP entities maintains soft state. Each LSR maintains an explicit list of all reservations. The sources use PATH messages to refresh reservations. Thus LSRs know at all times what resources are reserved by what source/receiver pair and knows exactly how much resources to free when a reservation times out or is terminated.

7. Example Route from A to Z with a bandwidth of 75 Mbps C 75 75 A B 75 Y25 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
Exp: experimental bits. People have suggested using this field for QoS. Stack field: indicates that this label is the bottom of the stack. Note this picture shows only a single label. In general the shim maintains a stack of labels. TTL field: This is used for loop mitigation just like in IP.
Ethernet Header
Shim
IP Packet
Label
Exp
Stack
TTL
20 bits
3 bits
1 bit
8 bits

9. Another Encapsulation
MPLS puts label information in the ATM VCI/VPI header field and in the AAL5 PDU payload
AAL5 PDU
The label is contained in both ATM header and AAL5 PDU payload to enable stacking of labels. The VCI/VPI header field contain only the top most label.
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)