ToS - IPP/DSCP

Tim OBrien

I'm unclear on the bits used for DSCP and IPP, it is my understanding

Tos = 1 Byte
Bits 7,6,5,4,3,2,1,0

IPP uses Bits 7,6,5, according to QoS Part 2 Video(CCNP);it uses the 3 Most Significant Bits ( I think I understand this)
Part I am unclear

DSCP - Does it use the 6 Most Significant Bits so 7,6,5,4,3,2 and 1,0 for Congestion? Or 0,1,2,3,4,5 and 7,8 for Congestion?

I am reading additional resources (see below for link) and this seems incorrect as it is referring to IPP using 0,1,2 when it should be 6,7,8??
ToS is 1 byte of the layer 3 IP packet, which is divided and used as the following:

[0] [1] [2] [3] [4] [5] [6] [7]

For IP precedence, the bits 0, 1 & 2 are used to map the CoS 3 bits in Layer 2 QoS.
Link: https://me2learn.wordpress.com/2014/12/06/layer-3-qos-tos/

Ronnie Wong

@Tim-OBrien,

This field, the ToS byte was defined back in 1981 in RFC 791. This byte was defined where the first 3 bits (0-2) were for Precedence and the last 5 bits (3 - 7) were for type of service to assign the kind of delay, throughput and reliability. Even so, bits 3 - 7 were rarely used and the focus was mainly on bits 0 - 2.

In 1998, RFC 2474 was created to redescribe the ToS byte as now called DS field along with the change of bits too. Now 0 - 5 (6 bits) are for DSCP value. These are used similar to the IP Precedence bits. Now bits 6-7 are currently unused

This newer order also allows for older devices that cannot recognize the RFC2474 standard by allowing for bits 0 - 2 to become class-selector codepoints or (CS names).

Tim OBrien

Thanks, according to the video Anthony says all bits are used for DSCP, 0-5 and 6,7 ECN Congestion? You said 6 and 7 are currently unused so could you confirm? Thanks!

Ronnie Wong

@Tim-OBrien,

So it depends on certain devices and standards used. The latest standard it is what is called "CU." --currently unused.

The use of ECN is an optional feature that allows for the end-to-end notification of network congestion without stoping the traffic to send a signal. The CU bits can be used for this if the devices are ECN-aware .