Classification and Marking at Layer 3 51
Classification and Marking at Layer 3
RFC 791 first defined Layer 3 packet marking in terms of IP precedence and type of service
(ToS), whereas RFC 795 expanded on the service mappings of the TOS bits. The discussion
of these RFCs provided here is for historical purposes, and it should be noted that both of
these RFCs are obsolete, due to various RFCs that are part of the DiffServ architecture.
Refer to Chapter 1 for a discussion of current RFCs that are part of the DiffServ archi-
tecture.
IP Precedence and the Type of Service Byte
RFC 791 defines the three most significant bits of the ToS byte as the IP precedence bits,
and the next 3 bits as delay, throughput, and reliability. Table 2-3 shows the original
definition of the ToS byte.
Keep in mind that the bits are numbered left to right, with the left bits (0–2) being the most
significant. Table 2-4 shows the values assigned to the 3 bits used for IP precedence.
Table 2-3 Definition of ToS Byte
Bits Precedence
Bits 0–2 Precedence
Bit 3 0 = normal delay, 1 = low delay
Bit 4 0 = normal throughput, 1 = high throughput
Bit 5 0 = normal reliability, 1 = high reliability
Bits 6–7 Reserved for future use
Table 2-4 IP Precedence Bits and Their Definitions
Bits IP Precedence
111 Network control (precedence 7)
110 Internetwork control (precedence 6)
101 CRITIC/ECP (precedence 5)
100 Flash override (precedence 4)
011 Flash (precedence 3)
010 Immediate (precedence 2)
001 Priority (precedence 1)
000 Routine (precedence 0)
52 Chapter 2: End-to-End QoS: Quality of Service at Layer 3 and Layer 2
The concept of IP precedence became wildly popular and is in use in many networks today.
The use of the next three most significant bits (defined for delay, throughput, and reliability)
was not as well received for a variety of reasons. For the most part, these bits were not used
due to confusion over the proper implementation to deliver the service levels specified.
RFC 1349 redefined these 3 bytes and made use of 1 additional bit (bit 6). Table 2-5 shows
how RFC 1349 defined what were now the four types of service bits.
Unfortunately, this new definition was not implemented much more than the previous
definition of these bits. Because these bits are not commonly used, and have been made
obsolete due to later RFCs, they are not discussed here. You can find more information
about the service mappings in RFCs 795 and 1349.
The Differentiated Services Field (DS Field)
As stated in RFC 2474, DiffServ enhancements to IP are designed to allow service differ-
entiation on the Internet in a scalable manner, primarily because per-flow classification will
not be necessary at each hop. There is a great deal more explained in RFC 2474 than is
covered here. The purpose of this section is only to discuss packet marking, using the DS field.
The idea behind DiffServ is that packets will be marked (much like with IP precedence) at
the edge of a network and those markings will then be read by each router (hop) that a
packet passes through. Based on the specific rules that have been defined within that hop,
the packet is assigned to a traffic class and that traffic class will have been given some
amount of access to scheduling and other resources.
With IP precedence, only the 3 left-most bits were used for IP precedence, and the next 3
bits were rarely used. So RFC 2474 redefines the ToS byte to make use of the 6 left-most
bits (bits 0–5) for packet marking. Doing so increases the number of possible packet
markings from 8 to 64 and eliminates “wasted” bits (those that were rarely used). The 2
least significant bits of the ToS byte are not defined by RFC 2474, but have since been
defined by RFC 2481. RFC 2481 is beyond the scope of this discussion, but is an interesting
concept called explicit congestion notification (ECN). You can view RFC 2481 (and all
other RFCs) at www.ietf.org, under the RFC Pages section.
Table 2-5 Definition of ToS Bits in RFC 1349
Bits Precedence
1000 Minimize delay
0100 Maximize throughput
0010 Maximize reliability
0001 Minimize monetary cost
0000 Normal service
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