RFC 3226 (rfc3226) - Page 2 of 6

DNSSEC and IPv6 A6 aware server/resolver message size requirements

RFC 3226            DNSSEC and IPv6 A6 requirements        December 2001


   server also needs to keep a state of the connection during this
   transaction.  Many DNS servers answer thousands of queries per
   second, requiring them to use TCP will cause significant overhead and
   delays.

1.1.  Requirements

   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
   in this document are to be interpreted as described in RFC 2119.

2.  Motivating factors

2.1.  DNSSEC motivations

   DNSSEC [RFC 2535] secures DNS by adding a Public Key signature on each
   RR set.  These signatures range in size from about 80 octets to 800
   octets, most are going to be in the range of 80 to 200 octets.  The
   addition of signatures on each or most RR sets in an answer
   significantly increases the size of DNS answers from secure zones.

   For performance reasons and to reduce load on DNS servers, it is
   important that security aware servers and resolvers get all the data
   in Answer and Authority section in one query without truncation.
   Sending Additional Data in the same query is helpful when the server
   is authoritative for the data, and this reduces round trips.

   DNSSEC OK[OK] specifies how a client can, using EDNS0, indicate that
   it is interested in receiving DNSSEC records.  The OK bit does not
   eliminate the need for large answers for DNSSEC capable clients.

2.1.1.  Message authentication or TSIG motivation

   TSIG [RFC 2845] allows for the light weight authentication of DNS
   messages, but increases the size of the messages by at least 70
   octets.  DNSSEC specifies for computationally expensive message
   authentication SIG(0) using a standard public key signature.  As only
   one TSIG or SIG(0) can be attached to each DNS answer the size
   increase of message authentication is not significant, but may still
   lead to a truncation.

2.2.  IPv6 Motivations

   IPv6 addresses [RFC 2874] are 128 bits and can be represented in the
   DNS by multiple A6 records, each consisting of a domain name and a
   bit field.  The domain name refers to an address prefix that may
   require additional A6 RRs to be included in the answer.  Answers
   where the queried name has multiple A6 addresses may overflow a 512-
   octet UDP packet size.



Gudmundsson                 Standards Track