MPLS General view
Enabling technology for the new broadband public network.
Introduces connection-oriented paradigm in ip networks
Forwards packets according to a label
- Faster lookup
- Traffic engineering
Key elements:
- Header containing label
- Protocols for label distribution (signaling)
- Enhanced routing protocols: contraints in choosing paths
Fault recovery is a lot faster.
MPLS header
LSP setup and control plane
Label and path selection: glimpse in the control plane
Actions taken by Label Switching routers:
- Label binding:
- Made by downstream node, that chooses label for the FEC
- Unsolicited
- On-demand: upstream node requests a fixed label.
- Made by downstream node, that chooses label for the FEC
- Label mapping: upstream node creates new entry assigning a input packets, with a specific label and a port to output packets having specific label and port.
- Label distribution: downstream node signals chosen label.
FEC
Forwarding equivalence class
Packets that are treated the same way by each LSR, that follow the same path in the MPLS network, receiving the same label
Label binding: ways
Static
- Through management
- non-scalable
- no interoperability
- impossible to establish LSPs through different networks.
Dynamic:
- Data/traffic driven: triggered by data packets
- Control driver: triggered by routing and signaling
Control driven label binding
Topology based
- Creation of LSPs is linked to the discovery of routes towards destinations
Expolicit creation of LSPs:
- Explicit signaling
- Initiated by label edge routers
Label distribution protocols
BGP: topology based
LDP (Label distribution protocol): designed for the purpose
RSVP (Resource reservation protocol): designed to allocation in integrated service networks
Routing protocols
Used to determine LSP routing
Impact label mapping phase
Existing protocols, like OSPF, IS-IS, BGP, are enhanced to carry information to contraint routing decisions (constraint data):
- capacity of links
- link utilization
- dependecies among links (fault recovery)
Enhanced routing protocols are fundamental to support traffic engineering:
- OSPF-TE
- IS-IS-TE
MPLS routing strategies
Hop-by-hop
- same as with ip routing, each LSR chooses best path
- Problem: if all routers choose the same path the probability of congestion increases.
Explicit
- centralized constrained routing, egress LSR are aware of which links are more used and loaded, they choose entire routes for packet.
- To suport it we need base protocols for lable distribution:
- CR-LDP
- RSVP-TE
Traffic engineering
We want to choose paths according to the load of the links used.
But if we are not careful we will just load other links and this leads to instability.
Traffic engineeeing without MPLS
- ATM has so far been used for TE on IP networks
- Two control plans: routers are unaware of ATM
- A lot of adjacencies: limited scalability
With MPLS:
- Is IP aware.
- Only one control plan operating on physical topology
- Simpler, greater scalability
MPLS extensions
MPƛS: MPLambdaS, control plans in optical networks
GMPLS (Generalized MPLS): MPLS control palne in any network (packet, circuit, optics)
(CoS)
Class of service
- Relative priority among different FECs
- it does offer absolute gruaranties
- Support DiffServe model
- per-hop behavior
- Per class traffic engineering
QoS MPLS
Quality of service
Guranaties:
- Bandwidth
- delay
- burst size
Advantages:
- Support of QoS and real-time services on IP is not ready
- A lot of multi-service networks now have paradigm (ships-in-the-night)
- MPLS control plan for IP services
- ATm protocols are ATM typical services
Label stack hierarchy and scalability in MPLS
- MPLS labels introduce hierarchy:
- There are as many levels as needed for scalability purposes.
- Routing tables of transit routers do not have to be comprehensive thanks to the LSP between edge routers.
- Simpler and faster exact match of labels rather than longes prefix matching
PHP
Penultimate hop popping
The last but one node on the path of an LSP pops the label
The LER routes the packed based on IP address.
LER distributes Label number 3 has a special meaning: don’t swap, pop (implicit PHP)
