DDS-XTypes 1.3 RTF Avatar
  1. OMG Issue

DDSXTY13 — Clarify valid ranges of memberIDs

  • Key: DDSXTY13-27
  • Status: open  
  • Source: Real-Time Innovations ( Gerardo Pardo-Castellote)
  • Summary:

    Section 7.5.2.5 (MemberId) states that

    The type MemberId is an alias to UInt32 and is used for the purpose of representing the ID of a member of a structured type.

    However it is clear from the serialization rules in 7.4.3.4 and specifically the definition of EMHEADER1 in table 39:

    EMHEADER1 = (M_FLAG<<31) + (LC<<28) + M.id 
    
    Where:
    M_FLAG is the value of the Must Understand option for the member
    LC is the value of the Length Code for the member.
    

    That MemberId are restricted to a 28-bit range. This should be clarified in the specification.

    Furthermore the specification should make it clear that the M_FLAG is not introducing a separate "space" of member IDs. So a type cannot have two members with the same value of the 28-bit memberID even if one has M_FLAG=1 and the other M_FLAG=0.

    Additionally in section 7.4.1.2.1 (Interpretation of Parameter ID Values) where it talks about PID_EXTENDED it states:

    The four bytes following the PID_EXTENDED and length shall be a serialized UINT32 value "eMemberHeader" that is constructed by combining four 1-bit flags with by the 28-bit member ID. The flags occupy the 4 most significant bits of the UINT32 value. The flags are combined with the memberId as shown below:
    FLAG_1 = 0x80000000
    FLAG_2 = 0x40000000
    FLAG_3 = 0x20000000
    FLAG_4 = 0x10000000
    eMemberHeader = FLAG_1 + FLAG_2 + FLAG_3 + FLAG_4 + memberId

    This text does not specify the reserved bit for the "FLAG_MUST_UNDERSTAND" and also in the case this is used for RTPS Discovery, the mapping for the "FLAG_IMPL_EXTENSION".

    For compatibility with existing deployed systems the specification should state that FLAG_1 is the FLAG_IMPL_EXTENSION and FLAG_2 is FLAG_MUST_UNDERSTAND.

  • Reported: DDS-XTypes 1.2 — Fri, 6 Jul 2018 00:09 GMT
  • Updated: Mon, 1 Apr 2019 17:40 GMT