Information Technology Reference
In-Depth Information
The fields in the ToS byte (RFC 1349) can be described as follows:
■
(IP) Precedence:
This 3-bit field is used to specify the relative priority or importance of a packet.
■
Type of Service (ToS):
A 4-bit field that defines how the network should make trade-offs between throughput, delay,
reliability, and cost.
■
MBZ:
Must be zero.
The use of the ToS byte has, to a great extent, been superseded in most networks by the DS field, which consists of a 6-bit
differentiated code point (DSCP) field and a 2-bit explicit congestion notification (ECN) field.
The 6-bit DSCP field (described in RFC 2474) defines the per-hop behavior (PHB). A PHB is an externally observable forwarding
behavior or QoS treatment performed by a network device, such as a router or a switch.
The four different DiffServ PHBs are Best Effort (BE), Class Selector (CS), Assured Forwarding (AF), and Expedited Forwarding (EF):
■
BE is indicated when all 6 bits of the DS field are zero, and it has no specific QoS treatment.
■
CS is used for backward compatibility with IP Precedence, and when using this PHB, the last 3 bits of the DSCP field are
zero.
■
AF (defined in RFC 2597) specifies four different classes, along with three different drop precedences.
When using AF, the first 3 bits of the DS field define the queuing class (1 to 4), and the last 3 bits define the drop precedence
(the likelihood of the packet being dropped [1 to 3]). AF PHB names are often written in the AF
xy
format, where
x
is the
queuing class and
y
is the drop precedence.
■
EF (RFC 3246) specifies a low-delay, low-jitter, and low-packet-loss QoS treatment with a bandwidth guarantee.
The second (and final) field in the DS field is Explicit Congestion Notification (ECN, RFC 3168). This 2-bit field signals network
congestion to hosts.
