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Re: 0xc0



On Thu, 10 Sep 1998, Default Debian Reader wrote:

[ snip ]

 : >  : Anyone tell me what tos 0xc0 is in ip headers?
 : > 
 : > First, install the doc-iana package.
 : > 
 : > I'm no IP expert (I'm currently reading Stevens's IP Illustrated book)
 : That book was copyright 1994, i imagine it was printed even before that.
 : > but if you are referring to the Type Of Service field, 0xc0 is not a
 : > valid choice.
 : > 
 : That is my question, Is it valid because i have seen ip headers with a tos
 : of 0xc0 or at least it appeared.
 : 
 : > [ from /usr/doc/doc-iana/assignments/ip-parameters.gz ]
 : > 
 : > TOS Value       Description                             Reference
 : > ---------       --------------------------              ---------
 : >   0000          Default                                 [RFC1349]
 : >   0001          Minimize Monetary Cost                  [RFC1349]
 : >   0010          Maximize Reliability                    [RFC1349]
 : >   0100          Maximize Throughput                     [RFC1349]
 : >   1000          Minimize Delay                          [RFC1349]
 : >   1111          Maximize Security                       [RFC1455]
 : > 
 : > The TOS value is used to indicate "better".  Only one TOS value or
 : > property can be requested in any one IP datagram.
 : > --
 : > 
 : > Note that the TOS field is an eight-bit field composed of a three-bit
 : > precedence field, the 4-bit field described above, and one unused bit.
 : > This is confusing since a TOS "value" of 0001 is written in hex as 0x02.
 : > 
 : > 0xc0 would be 011000000, so two bits in the precedence field have been
 : > set (and none of the TOS bits).  According to Stevens, the precedence
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
 : > field is ignored, so I'm not sure what's going on here :)
     ^^^^^^^^^^^^^^^^
Uh, did you read this part?  The precedence field is ignored (according
to Stevens, and he seems to know what he's talking about).  A value of
0xc0 means that some application is setting the two most significant
precendence bit, and since these are supposedly ignored in IP-land
today, it's no big deal.  Weird, but not a huge problem.

--
Nathan Norman
MidcoNet  410 South Phillips Avenue  Sioux Falls, SD
mailto:finn@midco.net           http://www.midco.net
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