wdiff draft-andrews-dns-more-00.txt draft-andrews-dns-more-01.txt

Mark Andrews (CSIRO)
INTERNET-DRAFT Paul Vixie (ISC)
<draft-andrews-dns-more-00.txt>
<draft-andrews-dns-more-01.txt> June 1996

Amends:

Updates: RFC 1035

Large Responses to DNS Queries (DNS MORE)

Status of This Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
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Abstract

DNS messages are limited to 64 kilobytes in size. At times it is
necessary to send a message that is greater that 64 kilobytes.
This is currently not possible. AXFR is the one exception. This
document describes how to send a sequence of messages, the total
length which may be greater than 64 kilobytes, by extending the
protocol.

In addition average message sizes are increasing and the 512
byte payload limit for UDP is now too small. This document
describes how servers can identify when they can send bigger
messages without necessarily resorting to TCP.

1 - Protocol

This extension uses one of the RESERVED flags bits from DNS header
[RFC1035 4.1.1] to indicate when a message is one of a sequence of mes-
sages. server can send the extended
response. This flag bit shall be known as MORE.

The MORE flag's semantics depend upon the underlying transport protocol.

This document only defines the use of the MORE flag with the opcode
QUERY.

1.1 - TCP Usage

When using TCP a resolver sets the MORE flag to indicate that it is
capable of receiving a multi message response (which we call a ``message
sequence'').

To indicate that the message sequence is not complete, complete the server
inserts an additional record with name = QNAME, class = QCLASS, TTL = 0,
type = ANY and dlen = 0, shall
set the RCODE to CONTINUED (TBA) in all but the last message of the mes-
sage sequence.
This record shall be known as the Message Continuation Indicator (MCI
RR).

The order of resource records in a multi message response MUST be the
same as if the response could have been sent is a single response, after
the MCI RR has been removed. response. The
Questions first followed by, the Answer RRs, Authority RRs and Addi-
tional RRs.

Each message in a sequence will contain a header with the same ID value,
flags, opcode and response code. opcode. Only the count fields and the rcode are permitted to
change. The counts shall represent the number of resource records in
this message. TC MUST be set, and MORE MUST cleared in the response.

1.1.1 - TCP Example

The following example show how to send an answer with one question, 10
answer records, 14 authority records and 5 additional records. The
answer is split up across 3 messages.

MESSAGE 1: QCOUNT=1, ANCOUNT=10, AUCOUNT=0,
ADCOUNT=0, RCODE=CONTINUED
MESSAGE 2: QCOUNT=0, ANCOUNT=0, AUCOUNT=11,
ADCOUNT=0. RCODE=CONTINUED
MESSAGE 3: QCOUNT=0, ANCOUNT=0, AUCOUNT=3,
ADCOUNT=5, RCODE=NOERROR

1.2 - UDP Usage

When using UDP, a resolver may set the MORE flag in a QUERY request to
indicate that its receive buffer is 8192 greater than 512 bytes in size,
rather than the 512 byte size given in [RFC1035 3.2.4]. The resolver is
expected to set this flag only if it knows that the host's reassembly
buffer is large enough to accomadate accommodate datagrams of that the size indicated.

The new size is indicated by the RCODE is the query. The receive buffer
is 512 * (2 ^ RCODE) bytes in size.

RCODE SIZE
0 512
1 1024
2 2048
3 4096
4 8192
5 16384
6 32768
7 65536
8 131072
9 262144
10 524288
11 1048576
12 2097152
13 4194304
14 8388608
15 16777216

A server receiving a QUERY request with the MORE flag set is allowed to
transmit a response of up to 8192 bytes. the size indicated. If the response will
not fit in 8192 bytes, size indicated, then the rules given in [RFC1035 4.1.1,
4.2.1, 6.2] apply.

The server MUST clear the MORE flag in the response.

The server SHOULD disable path MTU discovery on the UDP response packet
resulting in host fragmentation.

2 - Header Format

The header format is that described in [RFC1035 4.1.1] with the MORE
flag added:

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode |AA|TC|RD|RA|MO| Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Where MO is the MORE flag.

3 - Security Considerations

Though DNS is related to several security problems, no attempt is made
to fix them in this document.

This document is believed to introduce no additional security problems
to the current DNS protocol.

References

[RFC1035]P. Mockapetris, ``Domain Names - Implementation and Specifica-
tion,'' RFC 1035, USC/Information Sciences Institute, November
1987.

Authors' Addresses

Mark Andrews
CSIRO - Division of Mathematics and Statistics
Locked Bag 17
North Ryde NSW 2113
AUSTRALIA
+61 2 325 3148
<Mark.Andrews@dms.csiro.au>

Paul Vixie
Internet Software Consortium
Star Route Box 159A
Woodside, CA 94062
USA
+1 415 747 0204
<paul@vix.com>