QoS fields

The QoS fields used by the network device depend on the capabilities of the device and the QoS deployments of the network. Below is a little info about a few of these fields. This post will be edited over time with more info added.

A table showing some of the QoS fields available:

Field Location Length Description
Discard Eligible (DE) Frame relay header 1bit Frames marked with DE will be discarded to avoid having to drop non DE marked frames. Can be used when policing.
Cell Loss Priority (CLP) ATM cell header 1bit The ATM equivalent of the frame realy DE bit
Class of Service (CoS) ISL and 802.1Q/P headers 3bits Provides up to seven different classes for frames crossing layer 2 trunk links.
MPLS Experimental MPLS headers 3bits Map IP precedence etc onto the MPLS label to allow for QoS without looking past the label.
QoS Group Internal to a Cisco router N/A A value between 1 and 99 is used to mark traffic internally to the router i.e. doe not modify the IP packet in any way. Only specific Cisco routers support this.
IP Precedence IP Packet header 3bits First three (most significant) bits of the ToS byte in an IP packet header. Offers eight different classes.
DSCP IP Packet header 6bits First six (most significant) bits of the DS byte in an IP packet header.
ToS byte IP Packet header 1byte (8bits) A one byte field in an IP Packet header. (replaced by the DS byte)
DS byte IP Packet header 1byte (8bits) A one byte field in an IP Packet header when Diffserv is in use this replaces the ToS byte. Defined under RFC 2475

Lets have a look at the Layer 2 and 3 I’m interested in at the moment.

Layer 2

CoS:

Table showing various CoS info:

CoS Decimal Value Binary Value
Routine 0 000
Priority 1 001
Immediate 2 010
Flash 3 011
Flash override 4 100
Critical 5 101
Internetwork control 6 110
Network control 7 111

Cos is a three bit field that can be marked in the user_priority field on an ethernet frame crossing a 802.1q (or ISL ) trunk. It is important to emphasise again that this works at Layer 2. These fields can be mapped to either DSCP or IP Precedence classes so the traffic will receive QoS across the network. Once a frame is received from a 802.1q trunk this information is stripped so it is imporatnt to provide some mapping. Voice call traffic is usually classed as Critical (5).

Layer 3

At Layer 3 there is the DSCP and IP Precedence fields that can be used.

DSCP:

A table showing the various DSCPs, PHBs and a short description of the QoS each class should expect:

DSCPs QoS to expect PHB
BE (default) No specific treatment. Default when traffic is not marked. Best Effort (BE)
CS1,CS2,CS3,CS4, CS5,CS6,CS7 The general rule for CS is bigger is better. High the DSCP the better the QoS allowance. Class Selector (CS)
AF11,AF12,AF13,AF21,AF22,AF23,AF31,AF32,AF33,AF41,AF42,AF43 Provides A minimum amount of “guaranteed” bandwidth to each of the four queues and within each of the queues three different drop thresholds. Does NOT always follow bigger is better rule. Assured Forwarding (AF)
EF Provides a low jitter ,delay and loss queue. Also provides a guaranteed amount of bandwidth but this should be policed to avoid other queues being starved Expedited Forwarding (EF)

Since RFC 2474 the DS byte has replaced the ToS byte in IP headers. This change allows the use of the 6 bit DSCP field. DSCP uses the first (most significant bits) of the DS field. Offering A much more granular approach to QoS than IP Precedence. DSCP offers the class selector PHBs as a means of backward compatibility with IP precedence (the3 most significant  bits in the header are those same bits used for IP precedence).

Each of the four Assured Forwarding PHBs has three levels of drop threshold meaning that even within an AF PHB traffic can be differentiated.

EF offers the very best QoS can offer, usually reserved for voice traffic (not including voice signalling). Should always be policed to ensure other queues are not starved!

IP Precedence:

A table showing various IP Precedence info:

IP Precedence Decimal Value Binary Value
Routine 0 000
Priority 1 001
Immediate 2 010
Flash 3 011
Flash override 4 100
Critical 5 101
Internetwork control 6 110
Network control 7 111

IP Precedence is a three bit field that is mapped to the ToS byte of an IP packet header. As you can see by the CoS table above the IP precedence and CoS values map directly but importantly IP Precedence is a layer 3 QoS classification. Voice call traffic is usually classed as Critical (5).

More info about mapping and relationships can be seen here

m00nie 🙂

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