On the basis of the definitions of different services in various RFCs, traffic with Expedited
Forwarding per-hop behavior should be marked as which of these?
A.
IP ToS of 0xEF
B.
IP experimental ECN
C.
DSCP decimal 5
D.
Binary value of 101110
Explanation:
The assured forwarding (AF) model is used to provide priority values to different data applications.
The Expedited Forwarding (EF) model is used to provide resources to latency (delay) sensitive
real-time, interactive traffic. The EF model uses one marking — DSCP 46. DSCP 46 is backward
compatible with an IP Precedence value of 5 as seen in the following binary pattern:
101110 = DSCP 46
The EF marking of 46 does NOT follow the drop preference rules of the assured forwarding model.
Please do
NOT think that the 11 means high drop preference. The EF model is used for voice over IP media
traffic (RTP) by default in most vendors phones. Cisco IP Phones mark signaling packets (SCCP
or SIP) to CS3 (24), while media (RTP) is marked to EF (DSCP 46) by default. All EF traffic is
normally mapped to the priority queue (PQ) on Cisco switches and routers. The priority queue
guarantees three critical services:
Packet Loss
Delay
Jitter (delay variation)
The three most significant bits of 101 are only considered if IP Precedence was being used. The
binary digits of 4 2 1 are used to factor the 101 binary pattern when only three digits are under
consideration. The DSCP binary pattern of 101110 (46) uses six digits or binary values-32 16 8 4
2 1.
It is good to know how to convert a DSCP decimal value to an entire ToS octet (byte) values as
well. The ToS byte uses all eight bits, while the DSCP is only using the leading six digits. The EF
pattern discussed above will become 10111000 when considering the entire octet. Notice the two
least significant zeros that were added to the 101110 binary pattern. Many network management
utilities will only allow administrators to configure or display the entire ToS byte. A ping V from a
Microsoft operating system requires setting the entire ToS byte. An extended ping from a Cisco
router will also allow administrators to see the entire ToS byte. Sniffer Pro LAN and Wire Shark
sniffers show the entire ToS field as well. IP accounting shows the entire ToS byte, while Netflow
shows the ToS byte in hexadecimal format. The ToS byte value for EF is as follows:
128 64 32 16 8 4 2 1
1 0 1 1 1 0 0 0
A DSCP value of 46 results in a ToS byte value of 184. Although you can mark a ping with a ToS
value of 184, the ICMP (ping) traffic will probably not be mapped to the proper application class. Inthe next blog, we will learn QoS models for using markings for different application classes.