Every so often I hear the claim that some service or other has deliberately chosen not to support IPv6, and the reason cited is not because of some technical issue, or some cost or business issue, but simply because the service operator is of the view that IPv6 offers an inferior level service as compared to IPv4, and by offering the service over IPv6 they would be exposing their clients to an inferior level of performance of the service.

But is this really the case? Is IPv6 an inferior cousin of IPv4 in terms of service performance?

Let's try and answer that question by using measurement. In this article I'd like to report the results of a large scale measurement of IPv4 and IPv6 performance looking at the relativities of IPv6 and IPv4 performance.

(This is an update of an article originally published in November 2015. The update describes a new reporting tool that can show the performance of IPv6 relative to IPv4 at both a country-by-country level and a network-by-network level.)

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What is "Performance"?

In networking the term "performance" can have many aspects. We might characterize a network's performance by its sustained carrying capacity, or bandwidth, or by its end-to-end delay, or its level of delay variation or jitter. Each of these parameters could affect an application's performance. A large scale data transfer is affected by carrying capacity and by end-to-end delay, while a raw encoding of a voice or video stream could be more sensitive to jitter than to the end-to-end delay.

That's a large collection of elements that can affect the network transaction. We can reduce this somewhat by refining the question. If we have two sessions running between the same endpoints, using the same end-to-end transport protocol and using the same applications at each end, performing the same transaction at much the same time, and we vary only the underlying IP protocol used by these two sessions, then to what extent to we see differences in performance?

There appear to be two readily measurable aspects of performance that can vary between the two protocols with the same endpoints that will impact on the end result.

The first is the reliability of the protocol in the face of active middleware. Are all connection attempts successful? Do we observe a higher connection failure rate in one protocol or the other? Is there middleware deployed on the network path that is dropping IP packets based on the IP protocol?

The second is path variation. A change in IP protocol may produce a change in the network path used by the packet.



As an illustration of this protocol-related path change, I've reproduced below two traceroute logs between the same two endpoints, taken at much the same time of day, varying only the IP protocol used between versions 4 and 6. The first traceroute starts from a system located in Singapore, and the target is a host located in Canberra, Australia, and the second performs the same operation in the opposite direction. Both hosts are dual stack, and there are a number of dual stack network paths between the two hosts. The IPv4 paths from Singapore to Canberra, and back to Singapore are shown in the following two traceroute logs: $ traceroute from Singapore to Canberra, IPv4

traceroute to 202.158.xxxx.yyy, 30 hops max, 60 byte packets

1 103.3.60.3 0.672ms 0.796ms 0.899ms

2 139.162.0.9 0.754ms 0.708ms 0.732ms

3 te0-1-0-21.br03.sin02.pccwbtn.net 1.697ms 0.760ms 0.726ms

4 ntt.fe3-18.br01.sin02.pccwbtn.net 69.526ms 69.644ms 69.754ms

5 ae-10.r20.sngpsi05.sg.bb.gin.ntt.net 60.702ms 68.474ms 68.469ms

6 ae-8.r22.snjsca04.us.bb.gin.ntt.net 168.447ms 168.532ms 168.138ms

7 ae-19.r01.snjsca04.us.bb.gin.ntt.net 167.489ms 170.665ms 178.832ms

8 xe-0-8-0-21.r01.snjsca04.us.ce.gin.ntt.net 330.084ms 323.556ms 329.772ms

9 xe-1-0-1.pe1.msct.nsw.aarnet.net.au 330.020ms 323.738ms 334.474ms

10 et-3-3-0.pe1.rsby.nsw.aarnet.net.au 327.788ms 334.157ms 328.199ms

11 138.44.161.6 323.644ms 319.455ms 323.563ms

12 202.158.xxx.yyy 319.885ms 333.933ms 325.014ms $ traceroute from Canberra to Singapore, IPv4

traceroute to 139.162.xxx.yyy, 64 hops max, 52 byte packets

1 202.158.x.y 0.682ms 0.388ms 0.313ms

2 xe-5-0-4-205.pe1.actn.act.aarnet.net.a 0.721ms 0.828ms 0.674ms

3 et-0-3-0.pe1.rsby.nsw.aarnet.net.au 4.548ms 4.733ms 4.533ms

4 et-7-1-0.pe1.brwy.nsw.aarnet.net.au 4.734ms 5.418ms 4.745ms

5 et-0-3-0.pe1.bkvl.nsw.aarnet.net.au 5.117ms 5.512ms 5.524ms

6 xe-0-0-0.bb1.b.sea.aarnet.net.au 148.017ms 148.019ms 148.131ms

7 ge3-0.cr02.sea01.pccwbtn.net (206.81.80.13) 148.469ms 148.059ms 148.429ms

8 tenge0-2-0-14.br03.sin02.pccwbtn.net 319.435ms 325.053ms 319.117ms

9 tenge0-2-0-15.br03.sin02.pccwbtn.net 319.257ms 332.560ms 323.415ms

10 linode.te0-1-0-21.br03.sin02.pccwbtn.net 323.723ms 323.627ms 323.587ms

11 139.162.aaa.bbb 334.609ms 347.243ms 347.220ms

12 139.162.xxx.yyy 325.186ms 338.209ms 325.603ms The comparable IPv6 paths from Singapore to Canberra, and back to Singapore are shown in the following two traceroute logs: $ traceroute from Singapore to Canberra, IPv6

traceroute6 to 2001:388:1000:110:e4d:e9ff:x:y, 30 hops max, 80 byte packets

1 2400:8901::5287:89ff:fe40:9fc1 0.897ms 0.912ms 1.051ms

2 2400:8901:1111::1 0.851ms 0.827ms 0.792ms

3 2001:cb0:2102:2:f::1 0.364ms 0.333ms 0.516ms

4 2001:cb0:2102:2:f::1 0.502ms 0.461ms 0.431ms

5 2001:cb0:21f0:1:17::2 2.512ms 2.176ms 3.445ms

6 2001:cb0:21f0:1:17::2 2.354ms 2.382ms 1.238ms

7 10gigabitethernet3-5.core1.sin1.he.net 1.080ms 1.034ms 1.020ms

8 10ge1-5.core1.tyo1.he.net 88.053ms

10ge1-16.core1.hkg1.he.net 39.369ms

10ge1-5.core1.tyo1.he.net 88.084ms

9 10ge1-5.core1.tyo1.he.net 88.157ms

100ge8-1.core1.sea1.he.net 192.408ms 192.642ms

10 100ge8-1.core1.sea1.he.net 192.631ms 192.608ms 196.154ms

11 xe-1-0-1.pe2.brwy.nsw.aarnet.net.au 214.176ms 186.238ms 213.061ms

12 et-3-1-0.pe1.brwy.nsw.aarnet.net.au 211.298ms 211.300ms 214.200ms

13 et-1-1-0.pe1.rsby.nsw.aarnet.net.au 211.492ms 211.359ms 211.427ms

14 et-0-3-0.pe1.actn.act.aarnet.net.au 213.332ms 211.458ms 211.476ms

15 2001:388:1000:110:e4d:e9ff:x.y 213.274ms 213.199ms 213.169ms $ traceroute from Canberra to Singapore, IPv6

traceroute6 to 2400:8901::f03c:91ff:a:b) 64 hops max, 12 byte packets

1 2001:388:1000:110::x:y 0.808ms 0.899ms 1.586ms

2 xe-5-0-4-205.pe1.actn.act.aarnet.net.au 1.633ms 0.646ms 0.578ms

3 et-0-1-0.pe1.dksn.act.aarnet.net.au 0.682ms 0.649ms 0.694ms

4 et-5-3-0.pe1.crlt.vic.aarnet.net.au 8.072ms 8.086ms 8.049ms

5 et-5-1-0.pe1.wmlb.vic.aarnet.net.au 8.116ms 8.055ms 8.073ms

6 et-0-3-0.pe1.adel.sa.aarnet.net.au 17.790ms 16.984ms 17.036ms

7 et-1-1-0.pe1.prka.sa.aarnet.net.au 17.080ms 17.152ms

et-0-3-0.pe1.eper.wa.aarnet.net.au 43.319ms

8 et-0-3-0.pe1.knsg.wa.aarnet.net.au 43.357ms 43.443ms 43.353ms

9 gigabitethernet-5-1-0.bb1.b.per.aarnet.net.au 43.849ms 43.919ms 43.850ms

10 so-0-0-0.bb1.a.sin.aarnet.net.au 92.219ms 92.275ms 92.189ms

11 as6939.singapore.megaport.com 212.347ms 212.426ms 212.471ms

12 * * *

13 2400:8901:1110::2 213.924ms 213.904ms 213.717ms

14 2400:8901::f03c:91ff:a:b 213.954ms 213.393ms 213.726ms The data shows that in this particular case the IPv4 round trip time is some 325ms, while the IPv6 time is considerably shorter, at 213ms. The traceroute data also shows why this is the case. In the case of IPv4, the packet path to and from Singapore involves crossing the Pacific Ocean 4 times, as the forward and return paths involve directing the traffic through a router located on the west coast of the United States. In the case of IPv6 there is an asymmetric path where the outbound packets from Canberra to Singapore transit across a submarine cable system that runs North-South across the Indian ocean between Perth and Singapore. This path difference is shown on the map below. I will note that this is not a generic problem per se, and both asymmetric paths and paths that differ between IPv4 and IPv6 are not particularly unusual. The specific paths taken here are the result of the particular source and destination addresses and the various routing policies of the access and transit providers that lie on the transit paths. This example is illustrative of the observation that while the dual stack end hosts may remain constant, changing the IP protocol from IPv4 to IPv6 may also trigger a significant change of the transit path through the network, which, in turn, will have performance implications for any application that is attempting to send and receive data on these paths.

This leads to the observation that for the purposes of comparing the performance of these two versions of IP, what we would like to look at is the relative levels of reliability of connection establishment and relative round trip times across a series of experiments where the other factors are held constant. To what extent does changing just the IP protocol from IPv4 to IPv6 have an impact on the anticipated performance of the network transaction?

Measuring Performance

This measurement is based on data collected from end user measurements. The measurement technique uses an advertisement distribution network to load a measurement script on a user's browser. The script fetches three invisible pixels, one using a URL that is only accessible using IPv4, the second with IPv6 and the third is accessible using either protocol. All of these pixels are loaded from the experiment's servers, and a full packet log is recorded at each server. The actual measurements are performed through subsequent analysis of the server's packet logs.

Connection failure is challenging to measure if the only available data is coming from the server, as the task has some aspects of attempting to measure something that does not in fact exist. However, we are using TCP in this measurement, and one aspect of connection failure is directly visible to the server. TCP uses a three-way handshake to start a session. Normally this handshake looks like Figure 1. The server receives an opening TCP packet with the TCP SYN flag set. The servers will respond with a TCP packet with the SYN and ACK flags set, and the connection is complete when the server receives an ACK packet.

Figure 1 – TCP Handshake

One visible form of failure is a hung connection, where the server sees and responds to the opening SYN, but fails to receive the subsequent ACK to complete the connection. What we can measure for each protocol is the relative ratio of hung connections to successful connections. What percentage of all connection attempts fail to complete, and is this number different when considering IPv4 and IPv6 connections? This is not a complete picture of connection failures, as we may not even see the initial SYN of an attempted connection, but it is a useful comparative metric, in that it allows us to directly compare one aspect of connection robustness between IPv4 and IPv6.

The second measurable performance attribute is the end-to-end delay, measured as a round trip time. A log of a TCP session provides a constant record of round trip time measurements by looking in the server's logs at the time when data was sent and the time when that data was acknowledged. This implicit RTT estimate can be noisy, with factors such as delayed ACKs in the TCP protocol, and client side issues, such as internal job scheduling both in the host operating system and with the browser itself, and similar server-side issues. One way to avoid much of this added noise component to the underlying signal of RTT measurements is to look at the time taken to complete the initial TCP connection in Figure 1, from reception to the initial SYN to the reception of the ensuing ACK. This TCP connection operation is normally performed within the operating system as a kernel function and is less prone to imposed jitter from the external factors, so this measurement offers us one of the clearer ways to view the end-to-end round trip delay.

Measurement Results

The major update from the previous report of work on this topic is the creation of an online tool that reports on performance on an ongoing basis. The tool is accessible at: http://stats.apnic.net/v6perf.

The home page of this report is a world map, and a set of tables. There are two generic families of reports: a comparison of RTT measurements, comparing IPv4 and IPv6, and an absolute value of IPv6 connection robustness.

RTT Comparison

The "home" screen of the RTT comparison reports is the world map, shown in Figure 2.

Figure 2 – World Map of IPv6 - IPv4 RTT differences per country (Click to Enlarge)

Each country is coloured based on a relative RTT metric. This metric is computed by looking at the two RTT times for each dual stack client (in milliseconds) and subtracting the IPv4 RTT from the IPv6 RTT. In the example shown at the start of this article, the IPv6 RTT is 213ms, while the IPv4 RTT is 315ms. The result of the subtraction is -102ms, indicating that in this particular case the IPv6 RTT is 102ms faster than IPv4. These individual measurements are collected for each country (the country is determined by geo-locating the IPv4 part of the dual stack address pair), and the last 30 days measurements are used to generate an average value for each country.

The data used to colour the map is also reproduced in the tables in the second half of the page. This table is helpful in so far as often a single average value can conceal a wealth of information. What the table shows that the map cannot is the number of sample points on which the map was based. For example, while the average RTT difference in the Dominican Republic was 93.4ms in favour of IPv6 being faster, this result is based on the measurement of a single dual stack host. A more reliable number is drawn from Vietnam, where averaged across 4,479 values, IPv6 was on average 36.07ms faster.

Control variables in the screen allow variation of the end date (the default is the most recently processed date, typically yesterday), and the averaging window (the default is 30 days). The threshold sample rate can also be raised, to remove from the map and the report those countries whose total sample count in the window is less than the threshold.

Connection Robustness

The other form of world map is obtained by selecting the Plot Type of "Failure of V6 TCP connections". Here the data reflects the relative ratio of failed IPv6 connections as compared to all IPv6 connection attempts. The map shows the average connection failure rate expressed as a percentage of all seen connection attempts over the specified time window for users who are geolocated to a particular country. An example screen shot of this view is shown in Figure 3.

Figure 3 – World Map of IPv6 Connection Failure Rate (Click to Enlarge)

There are many potential causes of IPv6 connection failures. A common reason is overly restrictive filters applied on the customer site, where incoming IPv6 packets are refused. It is also possible that there are asymmetric routing issues that allow the client to see the relevant experiment server, but not the reverse.

Other Data

As well as the data relating to the mean difference in measured RTT values, there are three additional columns in the tabular reports: "Dual Stack", "Dual Stack (300ms)" and "V6 Use Rate".

Let's look at the Dual Stack figure first. As well as RTT times, we can also collect data on which protocol was used by the client to retrieve the dual stack object from the server. We can match this with the different the RTT times recorded against this host to ask the question: "Did the client end up picking the fastest protocol to retrieve the dual stack object?" In the tabular report the percentage value is a variant of this question, namely: "What percentage of the time for all clients did the client use the fastest protocol to retrieve the dual stack object?" Where the sample size is relatively high (more than 1,000 individual samples over a 30 day window), the 'accuracy' of the dual stack selection outcome is shown as a percentage, of the total fetch. This is the "Dual Stack" column.

However, this Dual Stack selection figure probably misrepresents the true objective of the protocol selection algorithm in dual stack hosts. The aim of the "Happy Eyeballs" protocol selection algorithm is not to select the fastest protocol in all cases. The algorithms will prefer IPv6, and it does so by selecting IPv6 as long as it is not too slow as compared to IPv4. The RFC that describes this procedure, RFC6555 does not specify how slow is "too slow", but it does note that Firefox and Chrome implementations of this procedure use a 300ms timer. The second column reports of the Dual Stack selection process, using a modified selection question: "What percentage of the time did the client use the fastest protocol, allowing IPv6 to have a 300ms handicap advantage, to retrieve the dual stack object?"

There are some changes evident between these two questions. For dual stack users in Greece, for example (136,299 samples over 30 days) IPv6 is, on average 3.85ms faster than IPv4. Users picked the fastest protocol 63% of the time. However, when IPv6 is allowed a 300ms "Happy Eyeballs" advantage, the result is 98% accuracy in selecting the fastest protocol.

The final column in the tabular reports in the V6 use rate as a percentage, showing to what extent there is IPv6 use as a percentage of all samples for IPv6.

Per-Country and Per-AS Reports

There are also reports for each country and each access network service provider, providing a similar report at the level of individual countries and individual networks.

The per-country reports also include a table of the network service providers who provide services in that country. The table provides the same details of the average RTT difference between IPv4 and IPv6, the IPv6 TCP connection failure rate, and the Dual Stack selection outcomes for each access network in the country that has recorded IPv6 activity.

It is possible to add a number of plots to the country and network reports. Three plots are added when the radio button for "Add Plot of Timeseries RTT Profiles to RTT Report" is ticked.

The first plot is the actual distribution of the individual RTT difference measurements across the same time window used in the table. This shows more information that is shown in a single average figure, sowing both the distribution of measurements and the cumulative distribution of measurements, as well as showing the average and the mean standard deviation values.

The distributions shown in these plots show that it is rare to see a smooth distribution. More common is a distribution such as that shown in Figure 4 (data for AS1221, Telstra), where there are a number of local peaks of difference in RTT values.

Figure 4 – Distribution of RTT values (AS1221) (Click to Enlarge)

This distribution shows a level of variation in RTT measurements that is not visible in the average value, nor in the mean standard deviation. In this case shown in Figure 4, there is a local peak of some 1,000 sample points where the IPv4 path is some 40ms faster than the equivalent IPv6 path. In this case slightly more than 50% of the samples show a faster path using IPv6, with some clustering at 10ms and 24ms.

AS7018 in the United States (AT&T Services) shows an interesting distribution (Figure 5). Slightly over one half of the samples show IPv6 being no slower than 10ms than the accompanying IPv4 RTT value. However, there are three local clusters where IPv6 is respectively 15ms, 25ms and 75ms slower.

Figure 5 – Distribution of RTT values (AS7018) (Click to Enlarge)

The second plot is a time series plot of daily values, showing the average and the mean standard deviation of the daily data set for each day. An example of this plot is shown in Figure 6.

Figure 6 – Daily RTT values: average and mean standard deviation (Click to Enlarge)

The third plot shows the number of measurement sample points that have been gathered per day. (Figure 7). The blue data points represent the number of IPv6 addresses see per day that have completed a connection and are able to be associated with an IPv4 address in a dual stack scenario. The red data points are the total of all unique IPv6 addresses seen per day, including those addresses that fail to complete a connection. Teredo and 6to4 IPv6 addresses are not included in this count.

Figure 7 – Daily Sample Count (Click to Enlarge)

Conclusion

What can we conclude about relative performance of IPv6 to IPv4 from these measurements?

These measurements show that in a large set of individual comparisons where the IPv4 and IPv6 paths between the same two dual stack endpoints are examined, the two protocols, as measured by the TCP SYN round trip time, are roughly equivalent on average, but with some significant outliers. At a finer level of detail, there are discernible differences between geographic regions. IPv6 appears to be faster on average in Europe, Africa and Oceania, while IPv4 is faster in Asia, and the Americas. At a further level of detail these differences between IPv4 and IPv6 extend to individual access networks where some networks appear to handle IPv6 packets in an entirely different manner to IPv4 packets.

While the TCP connection performance is roughly equivalent once the connection is established, the probability of establishing the connection is not the same. The current connection failure rate for IPv4 connections was seen to be some 0.2% of all connection attempts, while the equivalent connection failure rate for unicast IPv6 is eight times higher, at 1.6% of all connection attempts.

So, with respect to the performance of IPv6, there is still some scope for improvement here.

These reports are available from http://stats.labs.apnic.net/v6perf.