Westermo RedFox Series - Manual (Page 695)

Westermo OS Management Guide
Version 4.17.0-0
31.1.3.3 Modification of the DSCP field
31.1.3.3.1 DSCP Introduction
DSCP, Differentiated Services Code Point (or Diffserv Code Point), is a standard-
ised method for marking IP packets that they belong to a specific class of traffic.
Its use in the IP header is specified in RFC 2474[
26 ].
Octet 0 Octet 1 Octet 2 Octet 3 Octet 4 Octet 5 Octet 6 Octet 7
Octet 8 Octet 10 Octet 11 Octet 12 Octet 13 Octet 14 Octet 15
Octet 16 Octet 17 Octet 18 Octet 19 Octet ... Octet 20 ... ...
Octet 9
Version IHL Type of Service
Time to Live Protocol Header Checksum
Identification Flags Fragment Offset
Options, padding, payload data ... Destination Address
Total Length
Source Address
Figure 31.2: The IPv4 header
For the IPv4 header (RFC 791[ 29 ]), the ”Type of service” (or ToS) octet on offset
1 is used for carrying this kind of data. See fig. 31.2 .
The IPv4 ToS octet has historically been used in different ways.
0 1 2 3 4 5 6 7
Precedence 0 R T D M
Figure 31.3: ToS bits according to RFC 791 + RFC 1349
The original definition of ToS in RFC 791 has 3 precedence bits, and bits 3-5 as
flags for ”cost” aspects: ”Delay”, ”Throughput” and ”Reliability”. RFC 1349[
2 ]
updated ToS by adding the utilisation of bit 6 for ”Monetary cost”. See fig. 31.3 .
0 1 2 3 4 5 6 7
DSCP ECN
Figure 31.4: ToS bits according to RFC 2474 + RFC 3168
Later on, RFC 2474 redefined the use of the octet to carry DSCP information in
the first 6 bits. RFC 2481[
30 ] and its replacement RFC 3168[ 31 ] complement this
by defining bits 6-7 for ”Enhanced Congestion Notification” (ECN), see fig. 31.4 .
Both these conflicting interpretations are still in use today confusingly enough.
The DSCP modification and the Layer-2 prioritising mechanisms (
section 8.1.4 ) in
WeOS are adapted to the RFC 2474 use.
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