The type of service (ToS) field in the IPv4 header has had various purposes over the years, and has been defined in different ways by five RFCs. The modern redefinition of the ToS field is a six-bit Differentiated Services Code Point (DSCP) field and a two-bit Explicit Congestion Notification (ECN) field. While Differentiated Services is somewhat backwards compatible with ToS, ECN is not.
Contents
The ToS field could specify a datagram's priority and request a route for low-delay, high-throughput, or highly-reliable service.
Based on these ToS values, a packet would be placed in a prioritized outgoing queue, or take a route with appropriate latency, throughput, or reliability.
In practice, the ToS field never saw widespread use outside of US Department of Defense networks. However, a great deal of experimental, research, and deployment work has focused on how to make use of these eight bits, resulting in the current DS field definition.
Implementation
Prior to its deprecation, the Type of Service field was defined as follows.
Precedence was a 3 bit field which treats high priority packets as more important than other packets.If a router is congested and needs to discard some packets, it will discard packets having lowest priority first. Although precedence field was part of version 4, it was never used.
The last bit of Type of Service (bit 7) was defined as "Must Be Zero". Because the last three bits went through many definitions prior to RFC 2474 (see below), documentation and implementations may be confusing and contradictory.
DSCP and ECN
RFC 2474 (which was released in the year 1998) reserved the first six bits of DS field (or IPv4 ToS) for the DiffServ Code Points (DSCP), and RFC 3168 reserved the last two bits for Explicit Congestion Notification.
Example based on the table above :
Note: In table showed aboved, TOS is considered in Decimal Format, in many routers TOS are expresed in hex format. In this case conversion to Decimal is required.
IPP = 1 in decimal, or 001 in bits. Which gives 001 00000 for the entire ToS field, or ToS = 32. The DSCP field is : 001000 00, or DSCP = 8 (the last 2 bits are unused regarding priority, so we start counting at the third bit).
1) Traditionally, the first three IP precedence (-> RFC 791) bits were supposed to be used in TOS Application Routing (RFC 1583-> OSPF, IS-IS) but no widely deployed application really supports it.2) The TOS field has then been redefined as the Differentiated Services Code Point (DSCP-> RFC 2474) which consists of the first 6 bits and 2 bits used for a TCP mechanism called Explicit Congestion Notification (ECN) defined in RFC 3168.3) TOS significates "Type of service" and represents the 2nd byte in the IP datagram4) Summary of the concept for the 3 QoS values :IP Precedence, ToS,DSCP