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| 1.7.- Label
Distribution Protocol (LDP) |
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| Labels have to be distributed to build
and maintain the LSR-databases. This is done by using the Label
Distribution Protocol (LDP). This protocol is a kind of protocol
known as hard-state protocol. When one FEC is bound to one
label and this information is flooded using LDP, that bind
stays until a new call tear down the bound. Then, on the contrary to RSVP,
for example, LDP doesn't require refreshing and provide implicit
routing. |
| Some other routing protocol have been
used as LDP to implement MPLS networks. Some of them are
OSPF, IS-IS and BGP. Problem, however, is that
neither of these protocols do anything to implement good traffic engineering
practices, because traffic is always redirected to the high priority LSP
causing, finally, congestion. |
| To approach this problem some signalling
protocols have been used to create traffic tunnels using explicit routing,
allowing this way for better traffic engineering. Some of these protocols
are: Constraint Route Label Distribution Protocol (CR-LDP),
and Resource Reservation Protocol (RSVP-TE). OSPF has
also been modified to handle traffic engineering information (OSPF-TE). |
| RSVP-TE and CR-LDP give MPLS
the possibility to police traffic and control load in a fashion similar as
the Integrated Service and Differentiated Service
architectures do. |
| RSVP-TE |
| RSVP was designed to request bandwidth
and traffic conditions on a defined path. If the bandwidth requested is
available and the other traffic conditions can be meet the connection can be
established. This process is known as Admission Control. |
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| The Integrated Service architecture
defines three types of traffic to be requested using RSVP:
guaranteed load, controled load and best-effort load. When
the RSVP protocol is modify to accomodate MPLS traffic
requirements, the modify protocol is called RSVP-TE. |
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| With this scheme a path is set up between two
end points as follows: the client requests a specific path with
detailed traffic conditions and treatment parameters. The path message
is received by the server and a reservation message, reserving
bandwidth on the network is sent back to the client. After the
reservation message is received at the client, it can start to send data
throughout the explicit path previously reserved. |
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| This processs requires to be refreshed
regulary following some refreshing timers. The signalling is
called soft-state because of the refreshing requeriment. |
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| One of the problem (the worst, really) with
this scheme is scalability. Each path maintains independent
state in each router in the LSP. The finer granularity
requeriment implies one state for each connection. This, alone, poses a
great burden to the LSP router resources. Additionaly, a periodic
refreshing has to be done for each state, increasing more and more the
network overhead. Then, the only possible solution to this problem should be
become less granularity by aggregating flows in paths, just like the
differentiated service architecture does. |
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| CR-LDP |
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| CR-LDP modifies the original LDP
design to allow for traffic specification. This protocol adds new fields to
the original LDP protocol; these are: Committed Information Rate
(CIR), Peak Information Rate (PIR), Committed Burst
Size (CBS), Peak Burst Size (PBS) and Excess
Burst Size (EBS). |
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| The call setup process for CR-LDP is a
simple two-step process: a request and a map. Because
CR-LDP is a hard-state protocol, this means that once the path is
established, it will not be broken down until an specific request to do so
is received. The protocol is then, more scalable. |
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