Side_1_360

These mechanisms are defined as extensions

to LDP. Using CR-LDP, resources can also be

reserved along a path to guarantee service levels

and adequate handling for traffic carried on the

LSP.

To designate an explicit path that satisfies the

constraints, it is necessary to discern the re-

sources available to each link or node in the net-

work. For the collection of such resource infor-

mation, routing protocols can be extended to dis-

tribute additional state information.

Additional fields are introduced in the LDP sup-

porting constraint-based routing of LSPs. The

following features are supported:

• Strict and loose explicit routing; where the
  route is given by a list of groups of nodes.
  In case more than one router is given in the
  group a certain level of flexibility is present
  when fulfilling the explicit route.


• Specification of traffic parameters; for in-
  stance given by peak rate, committed rate and
  delay variation allowed.


• Route pinning; which can be used when it is
  undesirable to change the path followed by the
  LSP, e.g. in loosely routed segments in case a
  better route becomes available in that seg-
  ment.


• LSP pre-emption through set-up/holding pri-
  orities; set-up and holding priorities are used
  to rank existing LSPs (holding priority) and
  the new LSP (set up priority) to determine
  whether the new LSP can preempt an existing
  LSP. Priorities in the range from 0 (highest) to
  7 (lowest) are suggested.


• Failure handling.


• LSP identity.


• Resource classes; used when the network
  resources are categorised into classes to indi-

cate which types of resources an LSP can be

placed on (often called colours).

These features are reflected in a number of fields

(Type-Length-Value, ref. [Jens01]):

• Explicit route hop TLV – being a series of
  variable length TLVs where each gives the
  address of a router (or router group) in a strict
  or loose sense.


• Explicit route TLV – specifying the path to
  be taken by the LSP to be established. It is
  composed of one or more Explicit route hop
  TLVs.


• Traffic parameters TLV – lists traffic parame-
  ters: peak rate (peak token rate, PDR, and
  maximum token bucket size, PBS), committed
  rate (committed token rate, CDR, and maxi-
  mum token bucket size of this rate, CBS),
  excess burst size, EBS. As seen, a dual token
  bucket may be used, one operating on the
  peak rate and another operating on the com-
  mitted rate. A flag field is used to indicate
  which of the parameters that can be negoti-
  ated. A weight field is also included specify-
  ing the LSP’s relative share of a possible
  excess bandwidth above its committed rate.


• Pre-emption TLV – containing the set-up and
  holding priorities.


• LSPID TLV – giving the unique identifier of
  the LSP, composed of the ingress LSR iden-
  tity (or its IP address) and a locally unique
  LSP identity for that LSR.


• Resource class (colour) TLV – specifying
  which link types that are acceptable for the
  LSP given as a bit mask (32 bits).


• Route pinning TLV – indicating whether
  route pinning is requested (bit set) or not (bit
  cleared). A single bit is currently defined.

Figure 27 Illustration of LSP
set-up by use of CR-LDP
incoming traffic

ingress

egress

ingress

LER A

LSR B

LSR C LER D

LSP

label request

(B, C, D)

label mapping

(label 7)

label request

(C, D)

label mapping

(label 3)

label request

(D)

label mapping

(label 5)