Change:
This update removes the constraint in the section titled "Host Description Instruction" that the Host Description instruction must be pointed to by at least one Service Description instruction. It also removes some duplicate constraints from the section titled "Valid SRP Update requirements" and moves the mention of subtypes to the "Service Discovery Instruction" section.

Discussion:
Note that there can be more than one Service Discovery Instruction for the same name if the SRP requestor is advertising more than one <del> service of the same type, or is changing the target of a PTR RR.  For each such PTR RR add or remove, the above constraints must be met. </del> <ins> service of the same type, or is changing the target of a PTR RR. This is also true for SRP subtypes (RFC6763).  For each such PTR RR add or remove, the above constraints must be met. </ins> 2.3.1.2.

2.3.2. <del> An SRP Update MUST include zero or more Service Discovery Instructions.  For each Service Discovery Instruction, there MUST be at least one Service Description Instruction.  Note that in the case of SRP subtypes (RFC6763), it's quite possible that two Service Discovery Instructions might reference the same Service Description Instruction.  For each Service Description Instruction there MUST be at least one Service Discovery Instruction with its service instance name as the target of its PTR record.  There MUST be exactly one Host Description Instruction.  Every Service Description Instruction must have that Host Description Instruction as the target of its SRV record.  A DNS Update that does not meet these constraints is not an SRP Update. A DNS Update that contains any additional adds or deletes that cannot be identified as Service Discovery, Service Description or Host Description Instructions is not an SRP Update.  A DNS update that contains any prerequisites is not an SRP Update.  Such messages should either be processed as regular RFC2136 updates, including access control checks and constraint checks, if supported, or else rejected with RCODE=REFUSED. In addition, in order for an update to be a valid SRP Update, the target of every Service Discovery Instruction MUST be a Service Description Instruction that is present in the SRP Update.  There MUST NOT be any Service Description Instruction to which no Service Discovery Instruction points.  The target of the SRV record in every Service Description Instruction MUST be the single Host Description Instruction. </del> <ins> An SRP Update MUST contain exactly one Host Description Instruction. In addition, there MUST NOT be any Service Description Instruction to which no Service Discovery Instruction points.  A DNS Update that contains any additional adds or deletes that cannot be identified as Service Discovery, Service Description or Host Description Instructions is not an SRP Update.  A DNS update that contains any prerequisites is not an SRP Update.  An SRP registrar MAY either process such messages are either processed as regular RFC2136 updates, including access control checks and constraint checks, if supported, or MAY reject them with RCODE=REFUSED. </ins> If the definitions of each of these instructions are followed carefully and the update requirements are validated correctly, many