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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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PHBs will be defined in groups. A PHB group is a a set of one or more PHBs that<br />

can only be specified <strong>and</strong> implemented simultaneously because of queue<br />

servicing or queue management policies that apply to all PHBs in one group. A<br />

default PHB must be available in all DS-compliant nodes. It represents the<br />

st<strong>and</strong>ard best-effort forwarding behavior available in existing routers. When no<br />

other agreements are in place, it is assumed that packets belong to this service<br />

level. The IETF working group recommends that you use the DSCP value<br />

000000 in the DS field to define the default PHB.<br />

Another PHB that is proposed for st<strong>and</strong>ardization is the Expedited Forwarding<br />

(EF) PHB. It is a high priority behavior that is typically used for network control<br />

traffic, such as routing updates. The value 101100 in the DSCP field of the DS<br />

field is recommended for the EF PHB.<br />

8.3.2 Organization of the DSCP<br />

There are some IANA considerations concerning the DSCP. The codepoint<br />

space for the DSCP distinguishes between 64 codepoint values. The proposal is<br />

to divide the space into tree pools. Pool1 can be used for st<strong>and</strong>ard actions. The<br />

other pools can be used for experimental local usage, where one of the two pools<br />

is provided for experimental local use in the near future.<br />

The proposal is to divide the space into three pools (see Table 8-1).<br />

Table 8-1 DSCP pools<br />

Pool Codepoint space Assignment<br />

1 xxxxx0 St<strong>and</strong>ard action<br />

2 xxxx11 Experimental/local use<br />

3 xxxx01 Future experimental /local use<br />

Differentiated Services domains<br />

The setup of QoS guarantees is not made for specific end-to-end connections<br />

but for well-defined Differentiated Services domains. The IETF working group<br />

defines a Differentiated Services domain as a contiguous portion of the Internet<br />

over which a consistent set of Differentiated Services policies are administered in<br />

a coordinated fashion. It can represent different administrative domains or<br />

autonomous systems, different trust regions, <strong>and</strong> different network technologies,<br />

such as cell or frame-based techniques, hosts, <strong>and</strong> routers. A DS domain<br />

consists of boundary components that are used to connect different DS domains<br />

to each other <strong>and</strong> interior components that are only used inside of the domains.<br />

Chapter 8. Quality of service 313

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