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RFC 7872

Observations on the Dropping of Packets with IPv6 Extension Headers in the Real World

Pages: 15
Informational
Errata

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Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 7872                        SI6 Networks / UTN-FRH
Category: Informational                                       J. Linkova
ISSN: 2070-1721                                                   Google
                                                                T. Chown
                                                                    Jisc
                                                                  W. Liu
                                                     Huawei Technologies
                                                               June 2016


                Observations on the Dropping of Packets
             with IPv6 Extension Headers in the Real World

Abstract

This document presents real-world data regarding the extent to which packets with IPv6 Extension Headers (EHs) are dropped in the Internet (as originally measured in August 2014 and later in June 2015, with similar results) and where in the network such dropping occurs. The aforementioned results serve as a problem statement that is expected to trigger operational advice on the filtering of IPv6 packets carrying IPv6 EHs so that the situation improves over time. This document also explains how the results were obtained, such that the corresponding measurements can be reproduced by other members of the community and repeated over time to observe changes in the handling of packets with IPv6 EHs. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7872.
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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Support of IPv6 Extension Headers in the Internet . . . . . . 3 3. Security Considerations . . . . . . . . . . . . . . . . . . . 6 4. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Normative References . . . . . . . . . . . . . . . . . . 6 4.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. Reproducing Our Experiment . . . . . . . . . . . . . 8 A.1. Obtaining the List of Domain Names . . . . . . . . . . . 8 A.2. Obtaining AAAA Resource Records . . . . . . . . . . . . . 8 A.3. Filtering the IPv6 Address Datasets . . . . . . . . . . . 9 A.4. Performing Measurements with Each IPv6 Address Dataset . 9 A.5. Obtaining Statistics from Our Measurements . . . . . . . 10 Appendix B. Measurements Caveats . . . . . . . . . . . . . . . . 12 B.1. Isolating the Dropping Node . . . . . . . . . . . . . . . 12 B.2. Obtaining the Responsible Organization for the Packet Drops . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix C. Troubleshooting Packet Drops Due to IPv6 Extension Headers . . . . . . . . . . . . . . . . . . . . . . 14 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction

IPv6 Extension Headers (EHs) allow for the extension of the IPv6 protocol and provide support for core functionality such as IPv6 fragmentation. While packets employing IPv6 EHs have been suspected to be dropped in some IPv6 deployments, there was not much concrete data on the topic. Some preliminary measurements have been presented in [PMTUD-Blackholes], [Gont-IEPG88], and [Gont-Chown-IEPG89], whereas [Linkova-Gont-IEPG90] presents more comprehensive results on which this document is based. This document presents real-world data regarding the extent to which packets containing IPv6 EHs are dropped in the Internet, as measured in August 2014 and later in June 2015 with similar results (pending operational advice in this area). The results presented in this document indicate that in the scenarios where the corresponding measurements were performed, the use of IPv6 EHs can lead to packet drops. We note that, in particular, packet drops occurring at transit networks are undesirable, and it is hoped and expected that this situation will improve over time.

2. Support of IPv6 Extension Headers in the Internet

This section summarizes the results obtained when measuring the support of IPv6 EHs on the path towards different types of public IPv6 servers. Two sources of information were employed for the list of public IPv6 servers: the "World IPv6 Launch" site <http://www.worldipv6launch.org> and Alexa's list of the Top 1-Million Web Sites <http://www.alexa.com>. For each list of domain names, the following datasets were obtained: o Web servers (AAAA records of the aforementioned list) o Mail servers (MX -> AAAA records of the aforementioned list) o Name servers (NS -> AAAA records of the aforementioned list) Duplicate addresses and IPv6 addresses other than global unicast addresses were eliminated from each of those lists prior to obtaining the results included in this document. Additionally, addresses that were found to be unreachable were discarded from the dataset (please see Appendix B for further details). For each of the aforementioned address sets, three different types of probes were employed: o IPv6 packets with a Destination Options header of 8 bytes;
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   o  IPv6 packets resulting in two IPv6 fragments of 512 bytes each
      (approximately); and

   o  IPv6 packets with a Hop-by-Hop Options header of 8 bytes.

   In the case of packets with a Destination Options header and the case
   of packets with a Hop-by-Hop Options header, the desired EH size was
   achieved by means of PadN options [RFC2460].  The upper-layer
   protocol of the probe packets was, in all cases, TCP [RFC793] with
   the Destination Port set to the service port [IANA-PORT-NUMBERS] of
   the corresponding dataset.  For example, the probe packets for all
   the measurements involving web servers were TCP segments with the
   Destination Port set to 80.

   Besides obtaining the packet drop rate when employing the
   aforementioned IPv6 EHs, we tried to identify whether the Autonomous
   System (AS) dropping the packets was the same as the AS of the
   destination/target address.  This is of particular interest since it
   essentially reveals whether the packet drops are under the control of
   the intended destination of the packets.  Packets dropped by the
   destination AS are less of a concern since the device dropping the
   packets is under the control of the same organization as that to
   which the packets are destined (hence, it is probably easier to
   update the filtering policy if deemed necessary).  On the other hand,
   packets dropped by transit ASes are more of a concern since they
   affect the deployability and usability of IPv6 EHs (including IPv6
   fragmentation) by a third party (the destination AS).  In any case,
   we note that it is impossible to tell whether, in those cases where
   IPv6 packets with EHs get dropped, the packet drops are the result of
   an explicit and intended policy or the result of improper device
   configuration defaults, buggy devices, etc.  Thus, packet drops that
   occur at the destination AS might still prove to be problematic.

   Since there is some ambiguity when identifying the AS to which a
   specific router belongs (see Appendix B.2), each of our measurements
   results in two different values: one corresponding to the "best-case
   scenario" and one corresponding to the "worst-case scenario".  The
   "best-case scenario" is that in which, when in doubt, the packets are
   assumed to be dropped by the destination AS, whereas the "worst-case
   scenario" is that in which, when in doubt, the packets are assumed to
   be dropped by a transit AS (please see Appendix B.2 for details).  In
   the following tables, the values shown within parentheses represent
   the possibility that, when a packet is dropped, the packet drop
   occurs in an AS other than the destination AS (considering both the
   best-case scenario and the worst-case scenario).
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   +----------+------------------+------------------+------------------+
   | Dataset  |       DO8        |       HBH8       |      FH512       |
   +----------+------------------+------------------+------------------+
   |   Web    |      11.88%      |      40.70%      |      30.51%      |
   | servers  | (17.60%/20.80%)  | (31.43%/40.00%)  |  (5.08%/6.78%)   |
   +----------+------------------+------------------+------------------+
   |   Mail   |      17.07%      |      48.86%      |      39.17%      |
   | servers  |  (6.35%/26.98%)  | (40.50%/65.42%)  |  (2.91%/12.73%)  |
   +----------+------------------+------------------+------------------+
   |   Name   |      15.37%      |      43.25%      |      38.55%      |
   | servers  | (14.29%/33.46%)  | (42.49%/72.07%)  |  (3.90%/13.96%)  |
   +----------+------------------+------------------+------------------+

   Table 1: WIPv6LD Dataset: Packet Drop Rate for Different Destination
    Types, and Estimated (Best-Case / Worst-Case) Percentage of Packets
                    That Were Dropped in a Different AS

      NOTE: In the tables above and below, "HBH8" stands for "packets
      with a Hop-By-Hop Options extension header of 8 bytes", "DO8"
      stands for "packets with a Destination Options extension header of
      8 bytes", and "FH512" stands for "IPv6 packets with a Fragment
      Header of 512 bytes".

      NOTE: As an example, we note that the cell describing the support
      of IPv6 packets with DO8 for web servers (containing the value
      "11.88% (17.60%/20.80%)") should be read as: "when sending IPv6
      packets with DO8 to public web servers, 11.88% of such packets get
      dropped.  Among those packets that get dropped, 17.60%/20.80%
      (best case / worst case) of them get dropped at an AS other than
      the destination AS".

   +----------+------------------+------------------+------------------+
   | Dataset  |       DO8        |       HBH8       |      FH512       |
   +----------+------------------+------------------+------------------+
   |   Web    |      10.91%      |      39.03%      |      28.26%      |
   | servers  | (46.52%/53.23%)  | (36.90%/46.35%)  | (53.64%/61.43%)  |
   +----------+------------------+------------------+------------------+
   |   Mail   |      11.54%      |      45.45%      |      35.68%      |
   | servers  |  (2.41%/21.08%)  | (41.27%/61.13%)  |  (3.15%/10.92%)  |
   +----------+------------------+------------------+------------------+
   |   Name   |      21.33%      |      54.12%      |      55.23%      |
   | servers  | (10.27%/56.80%)  | (50.64%/81.00%)  |  (5.66%/32.23%)  |
   +----------+------------------+------------------+------------------+

   Table 2: Alexa's Top 1M Sites Dataset: Packet Drop Rate for Different
   Destination Types, and Estimated (Best-Case / Worst-Case) Percentage
              of Packets That Were Dropped in a Different AS
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   There are a number of observations to be made based on the results
   presented above.  Firstly, while it has been generally assumed that
   it is IPv6 fragments that are dropped by operators, our results
   indicate that it is IPv6 EHs in general that result in packet drops.
   Secondly, our results indicate that a significant percentage of such
   packet drops occurs in transit ASes; that is, the packet drops are
   not under the control of the same organization as the final
   destination.

3. Security Considerations

This document presents real-world data regarding the extent to which IPv6 packets employing EHs are dropped in the Internet. As such, this document does not introduce any new security issues.

4. References

4.1. Normative References

[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <http://www.rfc-editor.org/info/rfc793>. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, <http://www.rfc-editor.org/info/rfc2460>.

4.2. Informative References

[Gont-Chown-IEPG89] Gont, F. and T. Chown, "A Small Update on the Use of IPv6 Extension Headers", IEPG meeting before IETF 89, March 2014, <http://www.iepg.org/2014-03-02-ietf89/ fgont-iepg-ietf89-eh-update.pdf>. [Gont-IEPG88] Gont, F., "Fragmentation and Extension Header Support in the IPv6 Internet", IEPG meeting before IETF 88, November 2013, <http://www.iepg.org/2013-11-ietf88/ fgont-iepg-ietf88-ipv6-frag-and-eh.pdf>. [IANA-PORT-NUMBERS] IANA, "Service Name and Transport Protocol Port Number Registry", <http://www.iana.org/assignments/ service-names-port-numbers>.
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   [IPv6-Toolkit]
              SI6 Networks, "SI6 Networks' IPv6 Toolkit v2.0 (Guille)",
              <http://www.si6networks.com/tools/ipv6toolkit>.

   [Linkova-Gont-IEPG90]
              Linkova, J. and F. Gont, "IPv6 Extension Headers in the
              Real World v2.0", IEPG Meeting before IETF 90, July 2014,
              <http://www.iepg.org/2014-07-20-ietf90/
              iepg-ietf90-ipv6-ehs-in-the-real-world-v2.0.pdf>.

   [PMTUD-Blackholes]
              De Boer, M. and J. Bosma, "Discovering Path MTU black
              holes on the Internet using RIPE Atlas", July 2012,
              <http://www.nlnetlabs.nl/downloads/publications/
              pmtu-black-holes-msc-thesis.pdf>.
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Appendix A. Reproducing Our Experiment

This section describes, step by step, how to reproduce the experiment with which we obtained the results presented in this document. Each subsection represents one step in the experiment. The tools employed for the experiment are traditional UNIX-like tools (such as gunzip) and the SI6 Networks' IPv6 Toolkit v2.0 (Guille) [IPv6-Toolkit]. Throughout this appendix, "#" denotes the command-line prompt for commands that require superuser privileges, whereas "$" denotes the prompt for commands that do not require superuser privileges.

A.1. Obtaining the List of Domain Names

The primary data source employed was Alexa's Top 1M web sites, available at: <http://s3.amazonaws.com/alexa-static/top-1m.csv.zip>. The file is a zipped file containing the list of the most popular web sites, in Comma-Separated Value (CSV) format. The aforementioned file can be extracted with $ gunzip < top-1m.csv.zip > top-1m.csv A list of domain names (i.e., with other data stripped) can be obtained with the following command [IPv6-Toolkit]: $ cat top-1m.csv | script6 get-alexa-domains > top-1m.txt This command will create a "top-1m.txt" file containing one domain name per line. NOTE: The domain names corresponding to the WIPv6LD dataset is available at <http://www.si6networks.com/datasets/wipv6day-domains.txt>. Since the corresponding file is a text file containing one domain name per line, the steps produced in this subsection need not be performed. The WIPv6LD dataset should be processed in the same way as the Alexa dataset, starting from Appendix A.2.

A.2. Obtaining AAAA Resource Records

The file obtained in the previous subsection contains a list of domain names that correspond to web sites. The AAAA records for such domain names can be obtained with: $ cat top-1m.txt | script6 get-aaaa > top-1m-web-aaaa.txt
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   The AAAA records corresponding to the mail servers of each of the
   aforementioned domain names can be obtained with:

   $ cat top-1m.txt | script6 get-mx | script6 get-aaaa >
   top-1m-mail-aaaa.txt

   The AAAA records corresponding to the name servers of each of the
   aforementioned domain names can be obtained with:

   $ cat top-1m.txt | script6 get-ns | script6 get-aaaa >
   top-1m-dns-aaaa.txt

A.3. Filtering the IPv6 Address Datasets

The lists of IPv6 addresses obtained in the previous step could possibly contain undesired addresses (e.g., non-global unicast addresses) and/or duplicate addresses. In order to remove both undesired and duplicate addresses, each of the three files from the previous section should be filtered accordingly: $ cat top-1m-web-aaaa.txt | addr6 -i -q -B multicast -B unspec -k global > top-1m-web-aaaa-unique.txt $ cat top-1m-mail-aaaa.txt | addr6 -i -q -B multicast -B unspec -k global > top-1m-mail-aaaa-unique.txt $ cat top-1m-dns-aaaa.txt | addr6 -i -q -B multicast -B unspec -k global > top-1m-dns-aaaa-unique.txt

A.4. Performing Measurements with Each IPv6 Address Dataset

A.4.1. Measurements with Web Servers

In order to measure DO8 with the list of web servers: # cat top-1m-web-aaaa-unique.txt | script6 trace6 do8 tcp 80 > top-1m-web-aaaa-do8-m.txt In order to measure HBH8 with the list of web servers: # cat top-1m-web-aaaa-unique.txt | script6 trace6 hbh8 tcp 80 > top-1m-web-aaaa-hbh8-m.txt In order to measure FH512 with the list of web servers: # cat top-1m-web-aaaa-unique.txt | script6 trace6 fh512 tcp 80 > top-1m-web-aaaa-fh512-m.txt
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A.4.2. Measurements with Mail Servers

In order to measure DO8 with the list of mail servers: # cat top-1m-mail-aaaa-unique.txt | script6 trace6 do8 tcp 25 > top-1m-mail-aaaa-do8-m.txt In order to measure HBH8 with the list of mail servers: # cat top-1m-mail-aaaa-unique.txt | script6 trace6 hbh8 tcp 25 > top-1m-mail-aaaa-hbh8-m.txt In order to measure FH512 with the list of mail servers: # cat top-1m-mail-aaaa-unique.txt | script6 trace6 fh512 tcp 25 > top-1m-mail-aaaa-fh512-m.txt

A.4.3. Measurements with Name Servers

In order to measure DO8 with the list of name servers: # cat top-1m-dns-aaaa-unique.txt | script6 trace6 do8 tcp 53 > top-1m-dns-aaaa-do8-m.txt In order to measure HBH8 with the list of name servers: # cat top-1m-dns-aaaa-unique.txt | script6 trace6 hbh8 tcp 53 > top-1m-dns-aaaa-hbh8-m.txt In order to measure FH512 with the list of name servers: # cat top-1m-dns-aaaa-unique.txt | script6 trace6 fh512 tcp 53 > top-1m-dns-aaaa-fh512-m.txt

A.5. Obtaining Statistics from Our Measurements

A.5.1. Statistics for Web Servers

In order to compute the statistics corresponding to our measurements of DO8 with the list of web servers: $ cat top-1m-web-aaaa-do8-m.txt | script6 get-trace6-stats > top-1m-web-aaaa-do8-stats.txt
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   In order to compute the statistics corresponding to our measurements
   of HBH8 with the list of web servers:

   $ cat top-1m-web-aaaa-hbh8-m.txt | script6 get-trace6-stats >
   top-1m-web-aaaa-hbh8-stats.txt

   In order to compute the statistics corresponding to our measurements
   of FH512 with the list of web servers:

   $ cat top-1m-web-aaaa-fh512-m.txt | script6 get-trace6-stats >
   top-1m-web-aaaa-fh512-stats.txt

A.5.2. Statistics for Mail Servers

In order to compute the statistics corresponding to our measurements of DO8 with the list of mail servers: $ cat top-1m-mail-aaaa-do8-m.txt | script6 get-trace6-stats > top-1m-mail-aaaa-do8-stats.txt In order to compute the statistics corresponding to our measurements of HBH8 with the list of mail servers: $ cat top-1m-mail-aaaa-hbh8-m.txt | script6 get-trace6-stats > top-1m-mail-aaaa-hbh8-stats.txt In order to compute the statistics corresponding to our measurements of FH512 with the list of mail servers: $ cat top-1m-mail-aaaa-fh512-m.txt | script6 get-trace6-stats > top-1m-mail-aaaa-fh512-stats.txt

A.5.3. Statistics for Name Servers

In order to compute the statistics corresponding to our measurements of DO8 with the list of name servers: $ cat top-1m-dns-aaaa-do8-m.txt | script6 get-trace6-stats > top-1m-dns-aaaa-do8-stats.txt In order to compute the statistics corresponding to our measurements of HBH8 with the list of mail servers: $ cat top-1m-dns-aaaa-hbh8-m.txt | script6 get-trace6-stats > top-1m-dns-aaaa-hbh8-stats.txt
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   In order to compute the statistics corresponding to our measurements
   of FH512 with the list of mail servers:

   $ cat top-1m-dns-aaaa-fh512-m.txt | script6 get-trace6-stats >
   top-1m-dns-aaaa-fh512-stats.txt

Appendix B. Measurements Caveats

A number of issues have needed some consideration when producing the results presented in this document. These same issues should be considered when troubleshooting connectivity problems resulting from the use of IPv6 EHs.

B.1. Isolating the Dropping Node

Let us assume that we find that IPv6 packets with EHs are being dropped on their way to the destination system 2001:db8:d::1 and that the output of running traceroute towards such destination is: 1. 2001:db8:1:1000::1 2. 2001:db8:2:4000::1 3. 2001:db8:3:4000::1 4. 2001:db8:3:1000::1 5. 2001:db8:4:4000::1 6. 2001:db8:4:1000::1 7. 2001:db8:5:5000::1 8. 2001:db8:5:6000::1 9. 2001:db8:d::1 Additionally, let us assume that the output of EH-enabled traceroute to the same destination is: 1. 2001:db8:1:1000::1 2. 2001:db8:2:4000::1 3. 2001:db8:3:4000::1 4. 2001:db8:3:1000::1 5. 2001:db8:4:4000::1 For the sake of brevity, let us refer to the last-responding node in the EH-enabled traceroute ("2001:db8:4:4000::1" in this case) as "M". Assuming that packets in both traceroutes employ the same path, we'll refer to "the node following the last responding node in the EH-enabled traceroute" ("2001:db8:4:1000::1" in our case), as "M+1", etc. Based on traceroute information above, which node is the one actually dropping the EH-enabled packets will depend on whether the dropping node filters packets before making the forwarding decision or after
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   making the forwarding decision.  If the former, the dropping node
   will be M+1.  If the latter, the dropping node will be "M".

   Throughout this document (and our measurements), we assume that those
   nodes dropping packets that carry IPv6 EHs apply their filtering
   policy, and only then, if necessary, forward the packets.  Thus, in
   our example above, the last responding node to the EH-enabled
   traceroute ("M") is "2001:db8:4:4000::1", and we assume the dropping
   node to be "2001:db8:4:1000::1" ("M+1").

   Additionally, we note that when isolating the dropping node we assume
   that both the EH-enabled and the EH-free traceroutes result in the
   same paths.  However, this might not be the case.

B.2. Obtaining the Responsible Organization for the Packet Drops

In order to identify the organization operating the dropping node, one would be tempted to lookup the Autonomous System Numbers (ASNs) corresponding to the dropping node. However, assuming that M and M+1 are two peering routers, any of these two organizations could be providing the address space employed for such peering. Or, in the case of an Internet Exchange Point (IXP), the address space could correspond to the IXP AS rather than to any of the participating ASes. Thus, the organization operating the dropping node (M+1) could be the AS for M+1, but it might as well be the AS for M+2. Only when the ASN for M+1 is the same as the ASN for M+2 do we have certainty about who the responsible organization for the packet drops is (see slides 21-23 of [Linkova-Gont-IEPG90]). In the measurement results presented in Section 2, the aforementioned ambiguity results in a "best-case" and a "worst-case" scenario (rather than a single value): the lowest percentage value means that, when in doubt, we assume the packet drops occur in the same AS as the destination; on the other hand, the highest percentage value means that, when in doubt, we assume the packet drops occur at a different AS than the destination AS. We note that the aforementioned ambiguity should also be considered when troubleshooting and reporting IPv6 packet drops since identifying the organization responsible for the packet drops might prove to be a non-trivial task. Finally, we note that a specific organization might be operating more than one AS. However, our measurements assume that different ASNs imply different organizations.
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Appendix C. Troubleshooting Packet Drops Due to IPv6 Extension Headers

Isolating IPv6 blackholes essentially involves performing IPv6 traceroute for a destination system with and without IPv6 EHs. The EH-free traceroute would provide the full working path towards a destination while the EH-enabled traceroute would provide the address of the last-responding node for EH-enabled packets (say, "M"). In principle, one could isolate the dropping node by looking-up "M" in the EH-free traceroute with the dropping node being "M+1" (see Appendix B.1 for caveats). At the time of this writing, most traceroute implementations do not support IPv6 EHs. However, the path6 tool of [IPv6-Toolkit] provides such support. Additionally, the blackhole6 tool of [IPv6-Toolkit] automates the troubleshooting process and can readily provide information such as: dropping node's IPv6 address, dropping node's AS, etc.

Acknowledgements

The authors would like to thank (in alphabetical order) Mikael Abrahamsson, Mark Andrews, Fred Baker, Brian Carpenter, Gert Doering, C. M. Heard, Nick Hilliard, Joel Jaeggli, Tatuya Jinmei, Merike Kaeo, Warren Kumari, Ted Lemon, Mark Smith, Ole Troan, and Eric Vyncke for providing valuable comments on draft versions of this document. Additionally, the authors would like to thank participants of the V6OPS and OPSEC working groups for their valuable input on the topics discussed in this document. The authors would like to thank Fred Baker for his guidance in improving this document. Fernando Gont would like to thank Jan Zorz of Go6 Lab <http://go6lab.si/> and Jared Mauch of NTT America for providing access to systems and networks that were employed to produce some of the measurement results presented in this document. Additionally, he would like to thank SixXS <https://www.sixxs.net> for providing IPv6 connectivity. Fernando Gont would like to thank Nelida Garcia and Guillermo Gont for their love and support.
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Authors' Addresses

Fernando Gont SI6 Networks / UTN-FRH Evaristo Carriego 2644 Haedo, Provincia de Buenos Aires 1706 Argentina Phone: +54 11 4650 8472 Email: fgont@si6networks.com URI: http://www.si6networks.com J. Linkova Google 1600 Amphitheatre Parkway Mountain View, CA 94043 United States Email: furry@google.com Tim Chown Jisc Lumen House, Library Avenue Harwell Oxford, Didcot OX11 0SG United Kingdom Email: tim.chown@jisc.ac.uk Will (Shucheng) Liu Huawei Technologies Bantian, Longgang District Shenzhen 518129 China Email: liushucheng@huawei.com