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