Network Working Group O. Gudmundsson

Request for Comments: 3226 December 2001

Updates: 2874, 2535

Category: Standards Track

DNSSEC and IPv6 A6 aware server/resolver message size requirements

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

This document mandates support for EDNS0 (Extension Mechanisms for

DNS) in DNS entities claiming to support either DNS Security

Extensions or A6 records. This requirement is necessary because

these new features increase the size of DNS messages. If EDNS0 is

not supported fall back to TCP will happen, having a detrimental

impact on query latency and DNS server load. This document updates

RFC 2535 and RFC 2874, by adding new requirements.

1. Introduction

Familiarity with the DNS [RFC1034, RFC1035], DNS Security Extensions

[RFC2535], EDNS0 [RFC2671] and A6 [RFC2874] is helpful.

STD 13, RFC 1035 Section 2.3.4 requires that DNS messages over UDP

have a data payload of 512 octets or less. Most DNS software today

will not accept larger UDP datagrams. Any answer that requires more

than 512 octets, results in a partial and sometimes useless reply

with the Truncation Bit set; in most cases the requester will then

retry using TCP. Furthermore, server delivery of truncated responses

varies widely and resolver handling of these responses also varies,

leading to additional inefficiencies in handling truncation.

Compared to UDP, TCP is an expensive protocol to use for a simple

transaction like DNS: a TCP connection requires 5 packets for setup

and tear down, excluding data packets, thus requiring at least 3

round trips on top of the one for the original UDP query. The DNS

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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 [RFC2535] 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 [RFC2845] 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 [RFC2874] 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.

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2.3. Root server and TLD server motivations

The current number of root servers is limited to 13 as that is the

maximum number of name servers and their address records that fit in

one 512-octet answer for a SOA record. If root servers start

advertising A6 or KEY records then the answer for the root NS records

will not fit in a single 512-octet DNS message, resulting in a large

number of TCP query connections to the root servers. Even if all

client resolver query their local name server for information, there

are millions of these servers. Each name server must periodically

update its information about the high level servers.

For redundancy, latency and load balancing reasons, large numbers of

DNS servers are required for some zones. Since the root zone is used

by the entire net, it is important to have as many servers as

possible. Large TLDs (and many high-visibility SLDs) often have

enough servers that either A6 or KEY records would cause the NS

response to overflow the 512 byte limit. Note that these zones with

large numbers of servers are often exactly those zones that are

critical to network operation and that already sustain fairly high

loads.

2.4. UDP vs TCP for DNS messages

Given all these factors, it is essential that any implementation that

supports DNSSEC and or A6 be able to use larger DNS messages than 512

octets.

The original 512 restriction was put in place to reduce the

probability of fragmentation of DNS responses. A fragmented UDP

message that suffers a loss of one of the fragments renders the

answer useless and the query must be retried. A TCP connection

requires a larger number of round trips for establishment, data

transfer and tear down, but only the lost data segments are

retransmitted.

In the early days a number of IP implementations did not handle

fragmentation well, but all modern operating systems have overcome

that issue thus sending fragmented messages is fine from that

standpoint. The open issue is the effect of losses on fragmented

messages. If connection has high loss ratio only TCP will allow

reliable transfer of DNS data, most links have low loss ratios thus

sending fragmented UDP packet in one round trip is better than

establishing a TCP connection to transfer a few thousand octets.

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RFC 3226 DNSSEC and IPv6 A6 requirements December 2001

2.5. EDNS0 and large UDP messages

EDNS0 [RFC2671] allows clients to declare the maximum size of UDP

message they are willing to handle. Thus, if the expected answer is

between 512 octets and the maximum size that the client can accept,

the additional overhead of a TCP connection can be avoided.

3. Protocol changes:

This document updates RFC 2535 and RFC 2874, by adding new

requirements.

All RFC 2535 compliant servers and resolvers MUST support EDNS0 and

advertise message size of at least 1220 octets, but SHOULD advertise

message size of 4000. This value might be too low to get full

answers for high level servers and successor of this document may

require a larger value.

All RFC 2874 compliant servers and resolver MUST support EDNS0 and

advertise message size of at least 1024 octets, but SHOULD advertise

message size of 2048. The IPv6 datagrams should be 1024 octets,

unless the MTU of the path is known. (Note that this is smaller than

the minimum IPv6 MTU to allow for some extension headers and/or

encapsulation without exceeding the minimum MTU.)

All RFC 2535 and RFC 2874 compliant entities MUST be able to handle

fragmented IPv4 and IPv6 UDP packets.

All hosts supporting both RFC 2535 and RFC 2874 MUST use the larger

required value in EDNS0 advertisements.

4. Acknowledgments

Harald Alvestrand, Rob Austein, Randy Bush, David Conrad, Andreas

Gustafsson, Jun-ichiro itojun Hagino, Bob Halley, Edward Lewis

Michael Patton and Kazu Yamamoto were instrumental in motivating and

shaping this document.

5. Security Considerations:

There are no additional security considerations other than those in

RFC 2671.

6. IANA Considerations:

None

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7. References

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",

STD 13, RFC 1034, November 1987.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and

Specification", STD 13, RFC 1035, November 1987.

[RFC2535] Eastlake, D. "Domain Name System Security Extensions", RFC

2535, March 1999.

[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC

2671, August 1999.

[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B.

Wellington, "Secret Key Transaction Authentication for DNS

(TSIG)", RFC 2845, May 2000.

[RFC2874] Crawford, M. and C. Huitema, "DNS Extensions to Support

IPv6 Address Aggregation and Renumbering", RFC 2874, July

2000.

[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC

3225, December 2001.

8. Author Address

Olafur Gudmundsson

3826 Legation Street, NW

Washington, DC 20015

USA

EMail: ogud@ogud.com

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9. Full Copyright Statement

Copyright (C) The Internet Society (2001). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the

Internet Society.

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