Here is how I think about it: ToS is a term defined in rfc791 (IP) and it describes a field that is 8 bits long in the IP header for type of service. DSCP on the other hand, is a term that describes the 6 most significant bits of that same exact byte defined by ToS. They are different names for bits defined in the same place.
So in your example, ToS = 184 from a binary perspective you would have 10111000. In hex you have 0xB8 Now for the DSCP equivalent remember that DSCP only cares about the 6 most significant bits so you would have in binary: 101110 101110 = 46 decimal , AKA DSCP "EF" HTH On Tue, May 15, 2012 at 1:56 PM, Eugene Pefti <[email protected]> wrote: > I meant to refer to this Cisco document > http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml > > From: Mike Rojas <[email protected]> > Date: Monday, May 14, 2012 8:03 PM > To: Eugene Pefti <[email protected]>, > "[email protected]" <[email protected]> > Subject: RE: [OSL | CCIE_Security] DSCP values in different formats > > > Hello Eugene, > > I can talk for the DSCP issue and understanding that you have: > > 3. Differentiated Services Field Definition > > A replacement header field, called the DS field, is defined, which is > intended to supersede the existing definitions of the IPv4 TOS octet > [RFC791] and the IPv6 Traffic Class octet [IPv6]. > > Six bits of the DS field are used as a codepoint (DSCP) to select the > PHB a packet experiences at each node. A two-bit currently unused > (CU) field is reserved and its definition and interpretation are > outside the scope of this document. The value of the CU bits are > ignored by differentiated services-compliant nodes when determining > the per-hop behavior to apply to a received packet. > > http://tools.ietf.org/html/rfc2474 > > So basically matching that ToS value is the same as matching DSCP value of > EF. ToS was the old way to do it. On that specific field the first 6 bits > are reserved for DS (Differentiated Services) That is where you can define > the code points and also on the ECN. > > Wikipedia explains it very well I guess > > 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[1]. > 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. > > > http://en.wikipedia.org/wiki/Type_of_service > > Hope this clears a bit. > > > ________________________________ > From: [email protected] > To: [email protected] > Date: Tue, 15 May 2012 00:43:35 +0000 > Subject: [OSL | CCIE_Security] DSCP values in different formats > > Hello folks, > > I ran into different ways of setting DSCP values. > > My two QoS relating tasks ask to match for VoIP traffic with DSCP value > equals to EF. > > Then the verifications use something that I have no idea how to understand > and cross-check with Cisco documentation. > > > > E.g. The first task verifies it with IP SLA setting the ToS to 184. > > The calculations of ToS value are based on the fact that the actual value of > EF is 46 and then the corresponding ToS byte is 46*4=184: > > > > Ip sla 1 > > udp-echo 10.5.5.5 32767 source-ip 10.8.8.8 source-port 32767 control > disable > > tos 184 > > timeout 1000 > > frequency 1 > > > > Another task uses telnet to mark all telnet traffic with DSCP value of EF by > setting it right away to 0XB8. > > > > ip telnet tos B8 > > > > I understand that 0xB8 equals to 184 in decimal but what are the rules of > setting DSCP in two different cases ? Is there any Cisco guide that can be > used to consolidate this knowledge and access it in case of a problem during > the lab ? > > > > Eugene > > > _______________________________________________ For more information > regarding industry leading CCIE Lab training, please visit www.ipexpert.com > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com -- Regards, Joe Astorino CCIE #24347 http://astorinonetworks.com "He not busy being born is busy dying" - Dylan _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com
