Does Anycast Hang up on You (UDP and TCP)?

Does Anycast Hang up on You (UDP and TCP)?. Wei, L. & Heidemann, J. IEEE Transactions on Network and Service Management, 15(2):707–717, February, 2018.

Paper abstract bibtex

Anycast-based services today are widely used commercially, with several major providers serving thousands of important websites. However, to our knowledge, there has been only limited study of how often anycast fails because routing changes interrupt connections between users and their current anycast site. While the commercial success of anycast CDNs means anycast usually works well, do some users end up shut out of anycast? In this paper we examine data from more than 9000 geographically distributed vantage points (VPs) to 11 anycast services to evaluate this question. Our contribution is the analysis of this data to provide the first quantification of this problem, and to explore where and why it occurs. We see that about 1% of VPs are \emphanycast unstable, reaching a different anycast site frequently (sometimes every query). Flips back and forth between two sites in 10 seconds are observed in selected experiments for given service and VPs. Moreover, we show that anycast instability is \emphpersistent for some VPs—a few VPs never see a stable connections to certain anycast services during a week or even longer. The vast majority of VPs only saw unstable routing towards one or two services instead of instability with all services, suggesting the cause of the instability lies somewhere in the path to the anycast sites. We point out that for highly-unstable VPs, their probability to hit a given site is constant, suggesting load balancing might be the cause to anycast routing flipping. Finally, we directly examine TCP flipping and show that it is much rarer than UDP flipping, but does occur in about 0.15% (VP, letter) combinations. Moreover, we show concrete cases in which TCP connection timeout in anycast connection due to per-packet flipping. Our findings confirm the common wisdom that anycast almost always works well, but provide evidence that a small number of locations in the Internet where specific anycast services are never stable.

@Article{Wei18a,
	author = 	"Lan Wei and John Heidemann",
	title = 	"Does Anycast Hang up on You ({UDP} and {TCP})?",
        journal =       " {IEEE} Transactions on Network and Service Management",
        year =          2018,
	jlocation = 	"johnh: pafile",
	keywords = 	"passive observation, internet census",
	url =		"https://ant.isi.edu/%7ejohnh/PAPERS/Wei18a.html",
	pdfurl =	"https://ant.isi.edu/%7ejohnh/PAPERS/Wei18a.pdf",
	xxxblogurl = "https://ant.isi.edu/blog/?p=1007-xxx",
	myorganization =	"USC/Information Sciences Institute",
	sortdate = 	"2018-02-11", 
	Project = "ant, retrofuturebridge, lacrend, researchroot, nipet",
	jsubject = "network_security",
        volume =     "15",
        number =     "2",
        pages =      "707--717",
        month =      feb,
        oldnote =       "Accepted for publication Feb. 2018",
	abstract = "Anycast-based services today are widely used commercially, with
several major providers serving thousands of important
websites. However, to our knowledge, there has been only limited study
of how often anycast fails because routing changes interrupt
connections between users and their current anycast site. While the
commercial success of anycast CDNs means anycast usually works well,
do some users end up shut out of anycast? In this paper we examine
data from more than 9000 geographically distributed vantage points
(VPs) to 11 anycast services to evaluate this question. Our
contribution is the analysis of this data to provide the first
quantification of this problem, and to explore where and why it
occurs. We see that about 1\% of VPs are \emph{anycast unstable},
reaching a different anycast site frequently (sometimes every
query). Flips back and forth between two sites in 10 seconds are
observed in selected experiments for given service and VPs. Moreover,
we show that anycast instability is \emph{persistent} for some VPs---a
few VPs never see a stable connections to certain anycast services
during a week or even longer. The vast majority of VPs only saw
unstable routing towards one or two services instead of instability
with all services, suggesting the cause of the instability lies
somewhere in the path to the anycast sites. We point out that for
highly-unstable VPs, their probability to hit a given site is
constant, suggesting load balancing might be the cause to anycast
routing flipping. Finally, we directly examine TCP flipping and show
that it is much rarer than UDP flipping, but does occur in about
0.15\% (VP, letter) combinations. Moreover, we show concrete cases in
which TCP connection timeout in anycast connection due to per-packet
flipping. Our findings confirm the common wisdom that anycast almost
always works well, but provide evidence that a small number of
locations in the Internet where specific anycast services are never
stable."
,}

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However, to our knowledge, there has been only limited study of how often anycast fails because routing changes interrupt connections between users and their current anycast site. While the commercial success of anycast CDNs means anycast usually works well, do some users end up shut out of anycast? In this paper we examine data from more than 9000 geographically distributed vantage points (VPs) to 11 anycast services to evaluate this question. Our contribution is the analysis of this data to provide the first quantification of this problem, and to explore where and why it occurs. We see that about 1% of VPs are \\emphanycast unstable, reaching a different anycast site frequently (sometimes every query). Flips back and forth between two sites in 10 seconds are observed in selected experiments for given service and VPs. Moreover, we show that anycast instability is \\emphpersistent for some VPs—a few VPs never see a stable connections to certain anycast services during a week or even longer. The vast majority of VPs only saw unstable routing towards one or two services instead of instability with all services, suggesting the cause of the instability lies somewhere in the path to the anycast sites. We point out that for highly-unstable VPs, their probability to hit a given site is constant, suggesting load balancing might be the cause to anycast routing flipping. Finally, we directly examine TCP flipping and show that it is much rarer than UDP flipping, but does occur in about 0.15% (VP, letter) combinations. Moreover, we show concrete cases in which TCP connection timeout in anycast connection due to per-packet flipping. 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However, to our knowledge, there has been only limited study\nof how often anycast fails because routing changes interrupt\nconnections between users and their current anycast site. While the\ncommercial success of anycast CDNs means anycast usually works well,\ndo some users end up shut out of anycast? In this paper we examine\ndata from more than 9000 geographically distributed vantage points\n(VPs) to 11 anycast services to evaluate this question. Our\ncontribution is the analysis of this data to provide the first\nquantification of this problem, and to explore where and why it\noccurs. We see that about 1\\% of VPs are \\emph{anycast unstable},\nreaching a different anycast site frequently (sometimes every\nquery). Flips back and forth between two sites in 10 seconds are\nobserved in selected experiments for given service and VPs. Moreover,\nwe show that anycast instability is \\emph{persistent} for some VPs---a\nfew VPs never see a stable connections to certain anycast services\nduring a week or even longer. The vast majority of VPs only saw\nunstable routing towards one or two services instead of instability\nwith all services, suggesting the cause of the instability lies\nsomewhere in the path to the anycast sites. We point out that for\nhighly-unstable VPs, their probability to hit a given site is\nconstant, suggesting load balancing might be the cause to anycast\nrouting flipping. Finally, we directly examine TCP flipping and show\nthat it is much rarer than UDP flipping, but does occur in about\n0.15\\% (VP, letter) combinations. Moreover, we show concrete cases in\nwhich TCP connection timeout in anycast connection due to per-packet\nflipping. 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