Side_1_360

the same packet may be classified differently

by different routers along the path.

• Admission controlimplements the decision
  algorithm that is used to decide whether or not
  a new flow can be accepted without violating
  performance guarantees to it or existing flows.


• Reservation setup protocolis used to convey
  information between the network elements
  along the path. In order for an application to
  state its requirements, the protocol has to be
  able to carry the corresponding information
  elements (parameters).

For reservation of resources, the messages used

in the setup protocol carry some fields describ-

ing the traffic flows. These are described in

Chapter 7.

As mentioned, the Guaranteed Service ensures

bounds on delay and throughput. In order to con-

figure the resources along the path accordingly,

the parameters conveyed by the setup protocols

have to be properly quantified. Commonly,

Resource reSerVation Protocol (RSVP) is used

as an example of such a setup protocol. The rele-

vant parameters are then:

• r; rate of leaky bucket (measured in bytes per
  second: 1 byte/sec – 40 Tbyte/sec);


• b; depth of leaky bucket (measured in bytes:
  1 byte – 250 Gbyte);


• M; maximum packet size;


• m; minimum policed unit (packets shorter than
  mare counted as having size m);


• p; peak rate (measured in bytes per second –
  same as for r);


• R; service (link) rate (measured in bytes per
  second – same as for r);


• S; slack term (measured in μs).

The two latter are part of the RSpecfield while

the 5 first parameters are carried in the TSpec

field, see Figure 20.

An estimate for the end-to-end delay bound is

then given as, ref. [RFC2212]:

e – t – e delay bound =

Ctotand Dtotare “correction” terms influenced

by the way the packets are treated along the path.

Crefers to a rate-dependent term, i.e. it repre-

sents the delay a packet may experience because

of variations in the service rate, which depend

on the rate itself. An example is time for frag-

menting packets into cells (e.g. ATM cells),

which depends on the rate of sending ATM

cells. The term Cis measured in units of bytes.

Dis a rate-independent term given per node. It

captures the variation in transition time through

the node (worst case). The term Dis measured

in units of μs.

Ctotand Dtotare found by adding contributions

to the terms Cand Dalong the path.

The basis for the estimate of end-to-end delay

bound is a fluid flow model. Then, the terms C

and Dmay be considered as capturing deviates

of the node compared to a fluid flow model.

Note that the Guaranteed Service does not con-

trol minimal or average delay, nor the propaga-

tion delay, only the maximal queueing delay.

Neither does the Guaranteed Service give an

estimate of the jitter as such.

When aggregating and merging flows, a way of

handling the traffic parameters is asked for, ref.

[RFC2212]:

• TSpec for a merged flow may be calculated
  by: i) taking the largest token bucket rate; ii)
  the largest bucket size; iii) the largest peak
  rate; iv) the smallest minimum policed unit;
  and v) the smallest maximum packet size,
  across all flows in the merged flow. A merged
  TSpec is one that is adequate to describe the
  traffic from any one of the constituent TSpecs.

b−RM⋅(p−R)

p−r

+M+Ctot

R

+Dtot ,ifp≥R≥r

M+Ctot

R

+Dtot ,ifr≤p≤R

⎧

⎨

⎪⎪

⎩

⎪

⎪

Figure 20 Messages for RSVP

source destination

PATH(.., TSpec, ..)

RESV(.., TSpec, RSpec,..)

PATH(.., TSpec, ..)

RESV(.., TSpec, RSpec,..)