Internet DRAFT - draft-song-atr-large-resp
draft-song-atr-large-resp
Internet Engineering Task Force L. Song
Internet-Draft Beijing Internet Institute
Intended status: Informational S. Wang
Expires: September 9, 2019 Beijing Normal University
March 8, 2019
ATR: Additional Truncation Response for Large DNS Response
draft-song-atr-large-resp-03
Abstract
As the increasing use of DNSSEC and IPv6, there are more public
evidence and concerns on IPv6 fragmentation issues due to larger DNS
payloads over IPv6. This memo introduces an simple improvement on
DNS server by replying an additional truncated response just after
the normal fragmented response. It can be used to relieve users
suffering on DNS latency and failures due to large DNS response. An
ATR Experiment was done to show how well it works and some
operational issues are discussed in this memo as well.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 9, 2019.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The ATR mechanism . . . . . . . . . . . . . . . . . . . . . . 3
3. Experiment on how well ATR works . . . . . . . . . . . . . . 5
4. Operational considerations . . . . . . . . . . . . . . . . . 6
4.1. ATR timer . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. ATR payload size . . . . . . . . . . . . . . . . . . . . 7
4.3. Less aggressiveness of ATR . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Considerations on Resolver awareness of ATR . . . . 11
Appendix B. Revision history of this document . . . . . . . . . 11
B.1. draft-song-atr-large-resp-01 . . . . . . . . . . . . . . 11
B.2. draft-song-atr-large-resp-02 . . . . . . . . . . . . . . 12
B.3. draft-song-atr-large-resp-03 . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Large DNS response is identified as a issue for a long time. There
is an inherent mechanism defined in [RFC1035] to handle large DNS
response (larger than 512 octets) by indicating (set TrunCation bit)
the resolver to fall back to query via TCP. Due to the fear of cost
of TCP, EDNS(0) [RFC6891] was proposed which encourages server to
response larger response instead of falling back to TCP. However, as
the increasing use of DNSSEC and IPv6, there are more public
evidence[DNSSEC-impact] and concerns on user's suffering due to
packets dropping caused by IPv6 fragmentation in DNS due to large DNS
response.
It is observed that some IPv6 network devices like firewalls
intentionally choose to drop the IPv6 packets with fragmentation
Headers[I-D.taylor-v6ops-fragdrop]. [RFC7872] reported more than 30%
drop rates for sending fragmented packets. Regarding IPv6
fragmentation issue due to larger DNS payloads in response, one
measurement [IPv6-frag-DNS] reported 35% of endpoints using
IPv6-capable DNS resolver can not receive a fragmented IPv6 response
over UDP. Depending on retry model, the resolver's failing to
receive fragmented response may experience long latency or failure
due to timeout and reties.And, most of the underlying issues with
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fragments are unrevealed due to good redundancy and resilience of DNS
and dual-stack network.
Generally speaking there two approaches for this issue. One is to
make the DNS response as small as possible, for example, using ECC
instead of RSA to shorten the size of Key and signature. However,
few zones are signed by ECC for the time being. In addition there is
an uncertainty in the algorithm rollover from RSA to ECC. Another
approach is to fall back to TCP by setting on either server side or
client side. For resolver it is to set EDNS0 buffsize below a
certain number Server. For authoritative servers it is to set their
maximum UDP response size small enough.
However, one study [Not-speak-TCP] shows that about 17% of resolvers
in the samples can not ask a query in TCP when they receive truncated
response. It seems a dilemma to choose hurting either the users who
can not receive fragments or the users without TCP fallback
capacity.There is also some voice of "moving all DNS over TCP". But
It is generally desired that DNS can keep the efficiency and high
performance by using DNS UDP in most of time and fallback as soon as
possible to TCP if necessary for some case.
To relieve the problem, this memo introduces an small improvement on
DNS responding process by replying an Additional Truncated Response
(ATR) just after a normal large response which is to be fragmented.
It is a hybrid approach of using UDP when we can, and TCP only when
we must. It does not require any changes on resolver and has a
deploy-and-gain feature to encourage operators to implement it to
benefit their resolvers.
[REMOVE BEFORE PUBLICATION] Note that ATR is not just a proposed
idea. Some advocates of ATR implemented it based on BIND9
(https://gitlab.isc.org/isc-projects/bind9/merge_requests/158). And
Some verify it based on an large-scale experiment platform of APNIC
lab Section 3 which is introduced in this memo.
2. The ATR mechanism
The ATR mechanism is very simple that it involves a ATR module in the
responding process of current DNS implementation . As show in the
following diagram the ATR module is right after truncation loop if
the packet is not going to be fragmented.
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A DNS +-------------+ +-------------+ Normal
query | | No | | response
+------> Truncation +--------> ATR +--------->
| loop | | Module |
| truncation? | | truncation? |
+-------------+ +-------------+
yes| yes| +-----+
| +-----+timer+-->
| +-----+
| Truncated Response
+--------------->
Truncated Response
Figure 1: High-Level Testbed Components
The ATR responding process goes as follows:
o When an authoritative server receives a query and enters the
responding process, it first go through the normal truncation loop
to see whether the size of response surpasses the EDNS0 payload
size. If yes, it ends up with responding a truncated packets. If
no, it enters the ATR module.
o In ATR module, similar like truncation loop, the size of response
is compared with a value called ATR payload size. If the response
of a query is larger than ATR payload size, the server firstly
sends the normal response and then coin a truncated response with
the same ID of the query.
o The server can reply the coined truncated response in no time.
But considering the possible impact of network reordering, it is
suggested a timer to delay the second truncated response, for
example 10~50 millisecond which can be configured by local
operation.
Note that the choice of ATR payload size and timer SHOULD be
configured locally. And the operational consideration and guidance
is discussed in Section 4.2 and Section 4.1 respectively.
There are three typical cases of ATR-unaware resolver behavior when a
resolver send query to an ATR server in which the server will
generate a large response with fragments:
o Case 1: a resolver (or sub-resolver) will receive both the large
response and a very small truncated response in sequence. It will
happily accepts the first response and drop the second one because
the transaction is over.
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o Case 2: In case a fragment is dropped in the middle, the resolver
will end up with only receiving the small truncated response. It
will retry using TCP in no time.
o Case 3: For those (probably 30%*17% of them) who can not speak TCP
and sitting behind a firewall stubbornly dropping fragments. Just
say good luck to them!
In the case authoritative server truncated all response surpass
certain value , for example setting IPv6-edns-size to 1220 octets,
ATR will helpful for resolver with TCP capacity, because the resolver
still has a fair chance to receive the large response.
3. Experiment on how well ATR works
It is worth of mentioning APNIC report[How-ATR-Work] on "How well
does ATR actually work?" done by Geoff Huston and Joao Damas after 00
version of ATR draft. It was reported firstly in IEPG meeting before
IETF 101 and then posted in APNIC Blog later.
It is said the test was performed over 55 million endpoints, using an
on-line ad distribution network to deliver the test script across the
Internet. The result is positive that ATR works! From the end
users' perspective, in some 9% of IPv4 cases the use of ATR by the
server will improve the speed of resolution of a fragmented UDP
response by signaling to the client an immediate switch to TCP to
perform a re-query. The IPv6 behavior would improve the resolution
times in 15% of cases.
It also analyzed the pros and cons of ATR. On one hand, It is said
that ATR certainly looks attractive if the objective is to improve
the speed of DNS resolution when passing large DNS responses. And
ATR is incrementally deployable in favor of decision made by each
server operator. On another hand, ATR also has some negative or
unanswered factors. One is adding another DNS DDoS attack vector due
to the additional packet sent by ATR, (author's note : very small
adding actually.) Another issue is risk of RO by the choice of the
delay timer which is discussed fully in Section 4.1. It is also
founded that the trailing UDP packet may generate ICMP Port
Unreachable messages back to the server as a kind of noise (a rate of
approximately 1 in 5 responses in our experiments). Note that in
author's argument, it is not a big issue and the server can simply
ignore it if it decides to adopt ATR.
As a conclusion, it is said that "ATR does not completely fix the
large response issue. If a resolver cannot receive fragmented UDP
responses and cannot use TCP to perform DNS queries, then ATR is not
going to help. But where there are issues with IP fragment
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filtering, ATR can make the inevitable shift of the query to TCP a
lot faster than it is today. But it does so at a cost of additional
packets and additional DNS functionality". "If a faster DNS service
is your highest priority, then ATR is worth considering", said at the
end of this report
4. Operational considerations
There are some operational consideration on ATR, such as the
parameter of the ATR timer and ATR payload size, and policies on when
ATR is triggered to avoid side-effect.
4.1. ATR timer
As introduced in Section 2 ATR timer is a way to avoid the impact of
network reordering(RO). The value of the timer is critical, because
if the delay is too short, the ATR response may be received earlier
than the fragmented response (the first piece), the resolver will
fall back to TCP bearing the cost which should have been avoided. If
the delay is too long, the client may timeout and retry which negates
the incremental benefit of ATR. Generally speaking, the delay of the
timer should be "long enough, but not too long".
To the best knowledge of author, the nature of RO is characterized as
follows hopefully helping ATR users understand RO and how to operate
ATR appropriately in RO context.
o RO is mainly caused by the parallelism in Internet components and
links other than network anomaly [Bennett]. It was observed that
RO is highly related to the traffic load of Internet components.
So RO will long exists as long as the traffic load continue
increase and the parallelism is used to enhance network
throughput.
o The probability of RO varies largely depending on the different
tests samples. Some work shown RO probability below 2% [Paxson]
[Tinta] and another work was above 90% [Bennett]. But it is
agreed that RO is site-dependent and path-dependent. It is
observed in that when RO happens, it is mostly exhibited
consistently in a small percentages of the paths. It is also
observed that higher rates smaller packets were more prone to RO
because the sending inter-spacing time was small.
o It was reported that the inter-arrival time of RO varies from a
few milliseconds to multiple tens of milliseconds [Tinta]. And
the larger the packet the larger the inter-arrival time, since
larger packets will take longer to be transmitted.
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Reasonably we can infer that firstly RO should be taken into account
because it long exists due to middle Internet components which can
not be avoided by end-to-end way. Secondly the mixture of larger and
small packets in ATR case will increase the inter-arrival time of RO
as well as the its probability. The good news is that the RO is
highly site specific and path specific, and persistent which means
the ATR operator is able to identify a few sites and paths, setup a
tunable timer setting for them, or just put them into a blacklist
without replying ATR response.
Based on the above analysis it is hard to provide a perfect value of
ATR timer for all ATR users due to the diversity of networks. It
seems OK to set the timer with a range from ten to hundreds ms, just
below the timeout setting of typical resolver. Is suggested that a
decision should be made as operator-specific according to the
statistic of the RTT of their users. Some measurement shown
[Brownlee][Liang] the mean of response time is below 50 ms for the
sites with lots of anycast instance like L-root, .com and .net name
servers. For that sites, delay less than 50 ms is appropriate.
4.2. ATR payload size
Regarding the operational choice for ATR payload size, there are some
good input from APNIC study [scoring-dns-root]on how to react to
large DNS payload for authoritative server. The difference in ATR is
that ATR focuses on the second response after the ordinary response.
For IPv4 DNS server, it is suggested the study that do not truncate
and fragment IPv4 UDP response with a payload up to 1472 octets which
is Ethernet MTU(1500) minus the sum of IPv4 header(20) and UDP
header(8). The reason is to avoid gratuitously fragmenting outbound
packets and TCP fallback at the source.
In the case of ATR, the first ordinary response is emitted without
knowing it be to fragmented or not on the path. If a large value is
set up to 1472 octets, payload size between 512 octets and the large
value size will probably get fragmented by aggressive firewalls which
leads losing the benefit of ATR. If ATR payload size set exactly 512
octets, in most of case ATR response and the single unfragmented
packets are under a race at the risk of RO.
Given IPv4 fragmentation issue is not so serious compared to IPv6, it
is suggested in this memo to set ATR payload size 1472 octets which
means ATR only fit large DNS response larger than 1500 octets in
IPv4.
For IPv6 DNS server, similar to IPv4, the APNIC study is suggested
that do not truncate IPv6 UDP packets with a payload up to 1,452
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octets which is Ethernet MTU(1500) minus the sum of IPv6 header(40)
and UDP header(8). 1452 octets is chosen to avoid TCP fallback in the
context that most TCP MSS in the root server is not set probably at
that time.
In the case of ATR considering the second truncated response, a
smaller size: 1232 octets, which is IPv6 MTU for most network
devices(1280) minus the sum of IPv6 header(40) and UDP header(8),
should be chosen as ATR payload size to trigger necessary TCP
fallback. As a complementary requirement with ATR, the TCP MSS
should be set 1220 octets to avoid Packet Too Big ICMP message as
suggested in the APNIC study.
In short, it is recommended that in IPv4 ATR payload size SHOULD be
1472 octets, and in IPv6 the value SHOULD be 1232 octets.
4.3. Less aggressiveness of ATR
There is a concern ATR sends TC=1 response too aggressively
especially in the beginning of adoption. ATR can be implemented as
an optional and configurable feature at the disposal of authoritative
server operator. One of the idea to mitigate this aggressiveness,
ATR may respond TC=1 responses at a low possibility, such as 10%.
Another way is to reply ATR response selectively. It is observed
that RO and IPv6 fragmentation issues are path specific and
persistent due to the Internet components and middle box. So it is
reasonable to keep a ATR "whitelist" by counting the retries and
recording the IP destination address of that large response causing
many retires. ATR only acts to those queries from the IP address in
the white list.
5. Security Considerations
There may be concerns on DDoS attack problem due to the fact that the
ATR introduces multiple responses from authoritative server. The
extra packet is pretty small. In the worst case, it's 50% more
packets and they are small
DNS cookies [RFC7873] and RRL on authoritative may be possible
solutions
6. IANA considerations
No IANA considerations for this memo
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7. Acknowledgments
Many thanks to reviewers and their comments. Geoff Huston and Joao
Damas did a testing on the question "How well does ATR actually
work?". Alexander Dupuy proposed the idea to distinguish ATR
responses from normal ones. Akira Kato contributed ideas on
operational consideration. Shane Kerr help author with the security
consideration. Stephane Bortzmeyer gave thought of happyeyeballs on
resolver side.
Acknowledgments are also give to Mukund Sivaraman, Evan Hunt and Mark
Andrews who implement it and maintained it in a brunch in BIND9 code
base.
8. References
[ATR-Github]
"XML source file and test script of DNS ATR", September
2017, <https://github.com/songlinjian/DNS_ATR>.
[Bennett] Bennett, J. C. R., "Packet Reordering is Not Pathological
Network Behavior", December 1999,
<http://citeseerx.ist.psu.edu/viewdoc/
download?doi=10.1.1.461.7629&rep=rep1&type=pdf>.
[Brownlee]
Brownlee, N., "Response time distributions for global name
servers", 2002,
<http://www.caida.org/publications/papers/2002/nsrtd/
nsrtd.pdf>.
[DNSSEC-impact]
Broek, G. V. D., "DNSSEC meets real world: dealing with
unreachability caused by fragmentation", April 2014,
<https://repository.ubn.ru.nl/bitstream/
handle/2066/132796/132796.pdf?sequence=1>.
[How-ATR-Work]
Huston, G., "How well does ATR actually work?", April
2018, <https://blog.apnic.net/2018/04/16/
how-well-does-atr-actually-work/>.
[I-D.taylor-v6ops-fragdrop]
Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
progress), December 2013.
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[IPv6-frag-DNS]
Huston, G., "Dealing with IPv6 fragmentation in the DNS",
August 2017, <https://blog.apnic.net/2017/08/22/
dealing-ipv6-fragmentation-dns>.
[Liang] Liang, J., "Measuring Query Latency of Top Level DNS
Servers", February 2013,
<https://netsec.ccert.edu.cn/duanhx/files/2013/02/
latency.pdf>.
[Not-speak-TCP]
Huston, G., "A Question of DNS Protocols", August 2013,
<https://labs.ripe.net/Members/gih/
a-question-of-dns-protocols>.
[Paxson] Paxson, V., "End-to-End Internet Packet Dynamics", August
1999, <https://cseweb.ucsd.edu/classes/fa01/cse222/papers/
paxson-e2e-packets-sigcomm97.pdf>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.
[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", RFC 7872,
DOI 10.17487/RFC7872, June 2016,
<https://www.rfc-editor.org/info/rfc7872>.
[RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
<https://www.rfc-editor.org/info/rfc7873>.
[scoring-dns-root]
Huston, G., "Scoring the DNS Root Server System", November
2016, <https://blog.apnic.net/2016/11/15/
scoring-dns-root-server-system/>.
[Tinta] Tinta, S. P., "Characterizing End-to-End Packet Reordering
with UDP Traffic", August 2009, <https://static.googleuser
content.com/media/research.google.com/en//pubs/
archive/35247.pdf>.
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Appendix A. Considerations on Resolver awareness of ATR
ATR proposed in this memo is a server-side function which requires no
change in resolver, so it is not required that resolver MUST
recognized ATR and react accordingly. But it may helpful for some
cases where a resolver is able to recognized ATR response, for
example by checking the large edns0 payload size and TrunCation bit.
One case is use ATR is used as troubleshooting tool by which resolver
operators are able to flag problematic name servers. The resolver
operators is enable to log cases where ATR responses is received
without a (reassembled) UDP response to a query. In the case of
receiving a ATR, RDNS can choose to restrict maximum EDNS to a lower
value than the default 4096 that currently used.
Another case is that when receiving a ATR response a ATR-aware
resolver can adopt a "happyeyeballs" strategy by opening a separate
transaction sending the query via TCP instead of falling back to TCP
and closing the original UDP transaction. Listen to port 53 on both
TCP and UDP port 53 will enhance the availability and reduce the
latency. It will add more tolerance to network reordering issue as
well. However, it should be taken into account about the balance of
resolver's resource. Less priority should be given to that function
when the resolver is "busy".
The awareness of ATR on resolver can also avoid sending ICMP Port
Unreachable messages back to the server. In some implementations,
reusing the same UDP sockets for multiple queries will not generating
that ICMP noise.
However resolver use case of ATR is currently outside of the scope of
server-ATR proposal. It needs further discussion.
Appendix B. Revision history of this document
B.1. draft-song-atr-large-resp-01
After receiving reviews and comments, changes of 01 version are shown
as belows:
o Rewrite introduction and add another goal of ATR as a measuring
tool;
o Add section 3 indicating a ATR response. An bit in the EDNS0 OPT
header is defined as a indicator of ATR response. The flag bit is
called "ATR Response" (AT) bit;
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o Add Section 4 Operation considerations, which discuss ATR timer ,
ATR payload size, and less aggressiveness of ATR;
o Add IANA consideration to register the AT bit;
o Add section 7 Acknowledgments;
o Append a list of references regarding Network reordering, and
APNIC's study on IPv6 and DNS;
o Add Appendix A, An introduce of APNIC testing work and author's
comments;
o Appendix B. Considerations on Resolver awareness of ATR;
o Change the category="std" . It is said in RFC6891 IETF Standards
Action is required for assignments of new EDNS(0) flags. So the
draft should be categorized as standard track if registering AT
bit is desired in this document.
Change history is also available in the public GitHub repository
where this document is maintained: <https://github.com/songlinjian/
DNS_ATR>.
B.2. draft-song-atr-large-resp-02
Changes in 02 version of ATR draft:
o Remove the section of introduction of AT bit as well as
requirement of IANA registration of that bit;
o Change the category of this document to experimental and move the
introduction of APNIC's experiment from Appendix A to section 3;
o Add more names in Acknowledgments part after IETF102;
B.3. draft-song-atr-large-resp-03
Changes in 03 version of ATR draft:
o Add related work in the introduction session;
o Introduce ICMP noise as a finding of APNIC's experiment and
propose how to avoid it in Appendix A;
o Change the category from "exp" to "info";
o Move to ISE for review.
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o Add one author S. Wang
Authors' Addresses
Linjian Song
Beijing Internet Institute
2nd Floor, Building 5, No.58 Jing Hai Wu Lu, BDA
Beijing 100176
P. R. China
Email: songlinjian@gmail.com
URI: http://www.biigroup.com/
Shengling Wang
Beijing Normal University
Beijing Normal University, No. 19, XinJieKouWai St., HaiDian District
Beijing 100875
P. R. China
Email: wangshengling@bnu.edu.cn
URI: https://cist.bnu.edu.cn/
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