QoS Markings

Welcome back from the holidays! I hope everyone had a very joyous holiday season and got some rest. Over the past two weeks, I have celebrated Christmas, New Years, and my two son’s birthdays as follows:

John J. Hartmann (Johnny) – 4 years old (December 20th)
Dennis E. Hartmann (Buddy) – 7 years old (January 3rd)

Let’s get right back into it and talk about marking at layer 3 of the OSI reference model. The Internet Protocol was developed with prioritization in mind. The first Internet Engineering Task Force (IETF) specification for the Internet Protocol was published in September of 1981. IETF request for comment (RFC) 791 included an 8 bit field (one byte or octet) with prioritization in mind. The eight bit prioritization field is known as the type of service (ToS) byte. The ToS byte field is examined in page 12 of RFC791 (see hyperlink below). We will examine this original ToS byte usage and subsequent RFCs which modified the ToS byte usage. The ToS byte is broken down as follows

| IPP | D | T | R | 0 | 0 |

IPP = IP Precedence (3 bits)
D = Delay (1 bit)
T = Throughput (1 bit)
R = Reliability (1 bit)
0 = Currently Unused
0 = Currently Unused

The three most significant bits (MSB) of the ToS field (left most bits) are used to specify the priority level (precedence) of the IP packet. The three most significant bits together are known as the IP precedence (IPP) or IP prec. Since computers use binary math, there will be much coverage of binary math in the explanations of the usage of the ToS byte fields. The IP Precedence has three bits available or 8 different markings because 2 to the 3rd power equal 8. The binary values, their decimal equivalent, and their special RFC791 naming conventions are as follows:

000 0 Routine
001 1 Priority
010 2 Immediate
011 3 Flash
100 4 Flash Override
101 5 Critical
110 6 Internetwork Control
111 7 Network Control

The prioritization scheme used for the IP Precedence is quite easy to understand because it follows a linear model. The model allows 8 different markings (0 through 7) with a value of zero representing the lowest priority and a value of 7 representing the highest priority. The IP Precedence model was limited because it did not provide service providers with the granularity they needed to provide a robust QoS model. An IP Precedence value of zero represents best effort (BE) traffic and most traffic is sent as best effort by default. IP Precedence values 6 and 7 are not used for end user traffic. IP Precedence 6 is used for layer 3 control protocol traffic (routing protocols, telnet, etc.). Most routing protocol packets and telnet sessions sourced from a Cisco router are marked as IP Precedence 6 by default. IP Precedence 7 is reserved for layer 2 control protocol traffic. Why is layer 2 control protocol traffic more important than layer 3 control protocol traffic? If you cannot reach the default gateway of your layer 2 network, layer 3 connectivity across the WAN doesn’t matter very much. Layer 2 control protocols include Spanning Tree Protocol (IEEE 802.1d), Rapid Spanning Tree Protocol (IEEE 802.1w), Cisco Discover Protocol (CDP), etc. We will talk about the expansion of the 3 IPP bits with the differentiated services model and the differentiated services code point (DSCP) in future blogs.

The ToS byte includes three bits after the IP precedence field that allows devices to react to network conditions. These fields were removed in subsequent RFCs because they were very rarely used in practical applications. We will not spend much time here.

The next blog will introduced the differentiated services code point definitions and continue our conversation on the evolution of the ToS field.

REFERENCES:

Internet Protocol (DARPA Internet Program) – RFC791:
http://www.ietf.org/rfc/rfc0791.txt?number=791

Learn To Subnet (binary math converstions):
http://www.learntosubnet.com/

• Careers
• Cisco
• CCVP QoS