Side_1_360

based solely on the DSCP value. This can take
place in every DiffServ-capable router.

Which functions that are activated in a router
would depend on where the router is located,
e.g. ingress, egress or interior. An example of
use of functions is given in Table 1.

One is faced with quite a few challenges when
deciding to mark packets:

• Applications may use transient ports or source
  multiple traffic flows on the same port, where
  the flows may require different service levels.


• Users’ IP addresses change as a result of
  DHCP. Multi-user machines use the same
  IP address for multiple users.


• IPSec encryption encrypts port identities, leav-
  ing them less useful as classification criteria.

Basically, the end system may mark the packets,
or the edge router may do the marking. Allowing
end systems to mark the packets would likely
make it easier to meet the challenges listed above.

5 IntServ

The Integrated Services (IntServ) architecture
was defined to allow separate treatment to indi-
vidual or groups of traffic flows, [RFC1633].
Two sets of capabilities are necessary to enable
this: i) functions in individual network elements
along the path; and ii) ways to communicate the
requests between the network elements.

In [RFC1633] a flow is defined as a distinguish-
able stream of related datagrams that results
from a single user activity and requires the same
QoS.That is, it is the finest granularity of packet
stream that can be identified. A flow is unidirec-
tional (from a single source to a set of destina-
tions). In order to identify a flow, an MF filter
can be applied, as described in the previous
chapter.

Box A Some DiffServ Terms

Flow– A stream of packets with the same source IP address, source port

number, destination IP address, destination port number, and protocol identity

(packets not separated by a time longer than a threshold).

Service Level Agreement (SLA)– A service contract between a customer and a

service provider that specifies the forwarding service a customer should receive.

A customer may be a user organisation or another provider domain (upstream

domain).

Traffic profile– A description of the properties of a traffic flow such as rate and

burst size.

Precedence Field– The three leftmost bits in the TOS octet of an IPv4 header.

Note that in DiffServ, these bits may or may not be used to denote the prece-

dence of the IP packet.

TOS field– Bits 3–6 in the TOS octet of the IPv4 header.

Differentiated Services field (DS field)– The TOS octet of an IPv4 header, or the

traffic class octet of an IPv6 header is renamed the differentiated services field

by DiffServ. It is the field where service classes are encoded.

Admission Control– The decision process of whether to accept a request for

resources (link bandwidth plus buffer space).

Classification– The process of sorting packets based on the content of packet

headers according to defined rules.

Behaviour Aggregate (BA) classification– The process of sorting packets based

only on the content of the DS field.

Multi-Field (MF) classification– The process of classifying packets based on the

content of multiple fields such as source address, destination address, TOS

octet, protocol identity, source port number, and destination port number.

Marking– The process of setting the DS fields of packets.

Policing– The process of handling out-of-profile traffic, e.g. discarding excess

packets.

Shaping– The process of delaying packets within the traffic flow to make it

conform to some defined traffic profile.

Scheduling– The process of deciding which packet to send first in a system of

multiple queues.

Queue management– Controlling the length of packet queues by dropping

packets when necessary or appropriate.

MF Classifier

Marker

BA Classifier

Meter

Dropper

Shaper

Remarker

Remark local

domain DSCPs

when checking

marking

DSCP has

been set

Figure 18 Logical view

of traffic conditioning in

a DiffServ node