Ip header ethernet- header – Siemens HIPATH 8000 User Manual
Page 209
Alphabetical Reference
209
Assured forwarding is used for signaling messages by default (AF31) and
less stringent than EF in a multiple dropping system.
The AF values are containing two digits X and Y (AFXY), where X is describ-
ing the priority class and Y the drop level.
Four classes X are reserved for AFXY: AF1Y (High Priority), AF2Y, AF3Y and
AF4Y (Low Priority).
Three drop levels Y are reserved for AFXY: AFX1 (low drop level), AFX2 and
AFX3 (High drop level). In the case of low drop level, packets are buffered
over an extended period in the case of high drop level, packets are prompt-
ly rejected if they cannot be forwarded.
Layer 3 / IP Precedence (old recommendation, replaced by Diffserv)
IP Precedence is classifying traffic flows into 8 different precedence levels.
These 3 Bits are the same as the priority Bits from Diffserv. Value 5 (BIN:
101) is used for voice by default. And value 3 (BIN: 011) is used for signaling.
These values are already configured by configuring the DiffServ values.
If a routing partner can only work with one of the two standards (DiffServ
or IP precedence, for example an older router that only works with IP pre-
cedence), than the router can translate the ToS field accordingly. This can
be set for each PSTN peer or LAN interface.
Standard IPV4: Three MSB Called IP Precedence
(DiffServ Uses Six MSB Bits)
Version
Length
ToS
1 Byte Len ID Offset TTL Proto FCS IP-SA IP-DA Data
QoS
IP header
Ethernet-
Header
DiffServ
versus
IP Precedence
IEEE802.1p
Diffserv-
Codepoint
Binary
Decimal
Hex (8Bit)
Drop level
Binary
Decimal
6-Bit
8-Bit
ToS Field
Priority
high
med
low
EF
101110
46
184
B8
very high
x
<->
101
101
5
AF 43
100110
38
152
98
high
x
<->
100
100
4
AF 42
100100
36
144
90
x
<->
100
100
4
AF 41
100010
34
136
88
x
<->
100
100
4
AF 33
011110
30
120
78
medium
x
<->
011
011
3
AF 32
011100
28
112
70
x
<->
011
011
3
AF 31
011010
26
104
68
x
<->
011
011
3
AF 23
010110
22
88
58
low
x
<->
010
010
2
AF 22
010100
20
80
50
x
<->
010
010
2
AF 21
010010
18
72
48
x
<->
010
010
2
AF 13
001110
16
64
38
very low
x
<->
001
001
1
AF 12
001100
12
48
30
x
<->
001
001
1
AF 11
001010
10
40
28
x
<->
001
001
1