What is the Difference Between IP Precedence and DSCP

The main difference between IP precedence and DSCP is that IP precedence is a 3-bit field in TOC to give priorities to the IP packets, while DSCP is 6 bits in Differential Services (DS) field for packet classification.

Generally, a computer network is a collection of networking devices connected together to exchange data. Data goes through several layers when reaching the destination from the source. In the network layer of the IP model, the data is divided into packets. An IPv4 header has a field called Type of Service field (TOS). It helps to define a priority and request a route for high throughput, low delay, and highly reliable service. RFC (Request for Comments) define TOS. It is a type of publication from the technology community. On the other hand, there is a Differentiated Services Field (DS field) instead of TOS in IPv4 header. DSCP is a set of bits in the DS field.

Key Areas Covered

1. What is IP Precedence
     -Definition, Functionality
2. What is DSCP
     -Definition, Functionality
3. Difference Between IP Precedence and DSCP
     -Comparison of key differences

Key Terms

DSCP, IP Precedence, IPv4

What is IP Precedence

The RFC 791 defines that the first three bits of TOS represent precedence. The higher values indicate that the IP packet has higher priority. When there is traffic in the network, the router drops the low priority packet firstly. The other bits are types of service bits. They help to define reliability, delay, and throughput.

The first three bits can have combinations such as 000, 001, 010, etc.  Furthermore, the 0 for 3rd bit specifies normal delay while 1 denotes low delay. Similarly, 0 for bit 4 denotes normal throughput while 1 denotes high throughput and 0 for bit 5 specifies normal reliability while 1 specifies higher reliability. Additionally, bit 6 and 7 are reserved for future use.

Furthermore, the RFC 1349 has 8 bits to represent the TOS field. First three bits represent the precedence. There are only 4 bits for the service bits. The last bit is MBZ (Must Be Zero), which is for experimental tasks. The router ignores this bit.  Likewise, the precedence bits and type of service bits are used to assign a priority of an IP packet.

What is DSCP

RFC 2474 defines TOS differently than above. It is called the DS field or Differentiated Services field, which is 8 bits long. In this, the first 6 bits of the DS field for Differentiated Service Code Point (DSCP).  It helps in packet classification. Further, this codepoint also affects the PHB (Per Hop Behavior) at each node. Moreover, DSCP defines a Class Sector (CS) naming each value it defines, mirroring what must have interpreted as the IP Precedence if one follows the older specification.

Difference Between IP Precedence and DSCP

Definition

The IP precedence is a 3-bit field in TOS that threats high priority packets as more important than other packets. In contrast, DSCP is 6 bits of the Differentiated Services Field (DS Field) in the IP header for packet classification purposes.

Number of bits

First 3 bites in TOS represent IP precedence while first 6 bits represent DSCP.

Availability

Furthermore, IP precedence is in TOS while DSCP is in the DS field.

Conclusion

In brief, the DS field replaces the outdated TOS field in the IPv4 header. The main difference between IP precedence and DSCP is that IP precedence is a 3-bit field in TOS to give priorities to the IP packets while DSCP is 6 bits in Differential Services (DS) field for packet classification.

References:

1.“IP Precedence and DSCP Values.” NetworkLessons.com, 26 Feb. 2019, Available here.
2.“Type of Service.” Wikipedia, Wikimedia Foundation, 5 Feb. 2019, Available here.
3.“Differentiated Services.” Wikipedia, Wikimedia Foundation, 14 May 2019, Available here.

Image Courtesy:

1.”A diagram detailing local and remote network addresses” By Milesjpool, – Own work (CC BY-SA 4.0) via Commons Wikimedia

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