IPP>PRO Magic Numbers in ipp-messages

IPP>PRO Magic Numbers in ipp-messages

IPP>PRO Magic Numbers in ipp-messages

Carl Kugler kugler at us.ibm.com
Thu Oct 16 12:22:47 EDT 1997


I'd like to propose a change to the IPP encoding that would allow more =
robust
implementations in a world of buggy HTTP implementations.  Specifically=
, I
propose adding a magic number at the beginning of the ipp-message:


  ipp-message =3D ipp-request / ipp-response
  ipp-request =3D magic-number version operation
           *(xxx-attributes-tag  xxx-attribute-sequence) data-tag data
  ipp-response =3D magic-number version status-code
           *(xxx-attributes-tag xxx-attribute-sequence)  data-tag  data=




  magic-number =3D %x3f2c59d1  ;Literal 4 byte value to be
                             ;defined by the protocol specification


Why would this be a useful addition to the protocol?  Take the example =
of an
IPP client reading a ipp-response.  The client can check the beginning =
of the
entity-body for the magic-number.  If there is a mismatch, it's not saf=
e to
proceed with parsing the rest of the entity body, and the client should=
 raise
some kind of protocol exception.  It might be worth scanning a few byte=
s along
in the data stream looking for the magic-number in case a buggy HTTP tr=
ansport
layer has, say, inserted an extra CRLF after the message-header.


This kind of sanity check is difficult to apply to the protocol stream =
as it's
defined today, for example (pulled from the INTERNET-DRAFT IPP/1.0: Pro=
tocol
Specification   September 29, 1997):


The following is the beginning of an example of a Print-Job request:


Octets                Symbolic Value       Protocol field


0x0100                1.0                  version
0x0002                PrintJob             operation
0x01                  start operation-     operation-attributes tag
                      attributes
...


Instead, I'd like to see this:


Octets                Symbolic Value       Protocol field


0x3f2c59d1                           magic-number
0x0100                1.0                  version
0x0002                PrintJob             operation
0x01                  start operation-     operation-attributes tag
                      attributes
...


I've seen this kind of thing done before, for example, in graphics inte=
rchange
formats and various APIs. Unnecessary in theory, but a practical way to=
 add
some robustness.


Carl Kugler
IBM
kugler at us.ibm.net
303-924-5060
=



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