[rrd-users] Re: [smokeping-users] [OT] Using ping with a set ToS value

black at galaxy.silvren.com black at galaxy.silvren.com
Thu Mar 27 15:55:27 MET 2003

Hi. DSCP values are partly comprised of IP precedence values and partly of
the remaining ToS bits. From what I understand, ToS is deprecated and
those bits are now being used for other things, like DSCP.

"The Internet Engineering Task Force (IETF) has defined the siz most
significant bits of the 1-byte ToS field as the DSCP."

DSCP is backwards compatible with IP precedence.

The IP precedence bits are the 3 MOST SIGNIFICANT bits in the ToS field.
The DSCP bits are the IP precedence bits + the 3 next most significant
bits (total of 6 bits).

So the mapping looks like:

Binary 000000 = IP precedence 0, DSCP 0
Binary 001000 = IP precedence 1, DSCP 8
Binary 010000 = IP precedence 2, DSCP 16

Or you could mix it up and have:

Binary 001010 = IP precedence 1, DSCP 10

Based on that you should be able to figure out the entire mapping. The
last 2 bits in the ToS field are used for Early Congestion Notification
(ECN), so you can effectively set those to 0.

So to ping with a DSCP value of 8, you would set the tos field to be
00100000 or decimal 32.

Hope it helps.

A good reference point is to go to Cisco's web site and search for
"Implementing Quality of Service Policies with DSCP", they have a good
section on the ToS byte and how it all breaks down.

On Wed, 26 Mar 2003, Edson Manners wrote:

> I currently nee to use ping with certain QoS values. We currently use dscp
> values that are multiples of 8. eg. (0,8,16,24,32,40,48,56)
> The sprint man page though has a slightly different convention for nameing
> these values and I cannot tell if there is some equivalent in there for sure.
> Can someone please point me to where I can get more info or explain to m ehow
> I can get to ping these different dscp values?
> Thanks.
> P.S. This was copied from the ping man page.
> -Q tos  Set Quality of Service -related bits in ICMP datagrams.  tos can
>              be either decimal or hex number.  Traditionally (RFC1349), these
>              have been interpreted as: 0 for reserved (currently being rede­
>              fined as congestion control), 1-4 for Type of Service and 5-7 for
>              Precendence.  Possible settings for Type of Service are: minimal
>              cost: 0x02, reliability: 0x04, throughput: 0x08, low delay: 0x10.
>              Multiple TOS bits should not be set simultaneously.  Possible
>              settings for special Precedence range from priority (0x20) to net
>              control (0xe0).  You must be root (CAP_NET_ADMIN capability) to
>              use Critical or higher precendence value.  You cannot set bit
>              0x01 (reserved) unless ECN has been enabled in the kernel.  In
>              RFC2474, these fields has been redefined as 8-bit Differentiated
>              Services (DS), consisting of: bits 0-1 of separate data (ECN will
>              be used, here), and bits 2-7 of Differentiated Services Codepoint
>              (DSCP).
> --
> Education's purpose is to replace an empty
> mind with an open one. - Unknown
> Edson Manners
> Academic Computing and Network Systems
> Florida State University
> --
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