RTT matters

RTT	matters
Matsuzaki ‘maz’	Yoshinobu
<maz@iij.ad.jp>
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 12015/11/10
Round	Trip	Time	(RTT)
RTT
t0
t1
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 22015/11/10
TCP	3	way	handshake	and	RTT
RTT
RTT
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 32015/11/10
TCP	and	RTT
RTT
window	size
=	TCP	RX	buffer
RTT
RTT
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 42015/11/10
packet	loss	and	RTT
RTO
RTO	is	computed	based	on	RTT	-- see	rfc6298
retransmission
timeout
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 52015/11/10
RTT
• the	lower,	the	better
– faster	TCP	handshake
– better	TCP	throughput
– faster	TCP	recovery	from	a	packet	loss
• There	are	various	efforts	to	improve	TCP	
performance	even	in	a	high	latency	situation
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 62015/11/10
Wi-Fi	is	getting	popular
wirelesswired
Internet
server	2
server	1
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 72015/11/10
I	faced	a	trouble
wirelesswired
• RTT	was	not	stable	from	a	wireless	client	to	
the	server	2.	L
64	bytes	time=398.122	ms
64	bytes	time=316.330	ms
64	bytes	time=337.251	ms
64	bytes	time=358.155	ms
64	bytes	time=299.480	ms
64	bytes	time=387.789	ms
server	2
server	1
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 82015/11/10
let’s	try	to	isolate	the	problem
wireless
64	bytes	time=169.588	ms
64	bytes	time=170.666	ms
64	bytes	time=169.793	ms
64	bytes	time=168.185	ms
64	bytes	time=169.783	ms
64	bytes	time=169.957	ms
server	1
64	bytes	time=3.757	ms
64	bytes	time=3.789	ms
64	bytes	time=3.768	ms
64	bytes	time=3.718	ms
64	bytes	time=3.644	ms
64	bytes	time=3.762	ms
1) the	local	wifi network	
looks	pretty	stable
2) RTT	to	server	1			
looks	also	stable
server	2
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 92015/11/10
hmmm...
wirelesswired
server	2
server	1
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 10
64	bytes	time=296.040	ms
64	bytes	time=296.105	ms
64	bytes	time=296.442	ms
64	bytes	time=296.186	ms
64	bytes	time=296.103	ms
64	bytes	time=296.070	ms
3) from	the	wired	host	in	the	
same	network,	RTT	to	the	
server	2	looks	stable
2015/11/10
so...
wireless
server	1
server	2
wired
• this	strange	behavior	happens	only	for	this	
combination	L
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 112015/11/10
RTT	distribution	to	server	1
from	wired	host	to	server	1 from	wifi host	to	server	1
wired wifi
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 122015/11/10
RTT	distribution	to	server	2
from	wired	host	to	server	2 from	wifi host	to	server	2
wired wifi
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 132015/11/10
the	wifi AP	was	buffering	packets
wirelesswired
Internet
server	2
server	1
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 14
• and	this	caused	the	unstable	RTT	L
2015/11/10
My	wifi adapter	does	sleep
• to	reduce	battery	usage
• before	sleeping,	the	client	send	a	notification	
to	the	wifi AP,	and	the	AP	keeps	packets	until	
the	client	wake	up
• so,	my	PC	was	asking	the	buffering!
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 152015/11/10
wifi AP	sends	beacon
• beacon	interval
– time	interval	between	beacon	transmissions
– usually	100msec,	but	it’s	configurable
• TIM	(Traffic	Indication	Map)
– to	tell	any	sleeping	clients	if	the	AP	has	any	buffered	
frames	present	for	it
• wifi adapter	can	sleep	between	beacons,	and	
wake	up	to	check	a	beacon	(TIM	can	indicate	if	
the	adapter	need	to	receive	data	or	not)	
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 162015/11/10
the	scenario
• My	wifi adapter	went	to	sleep	after	200msec	
of	no	traffic
– that’s	why	the	unstable	RTT	happens	only	when	I	
was	communicating	with	server	2	(average	RTT	is	
300msec)
• Based	on	the	beacon	interval	information	
(which	was	100msec	in	my	case),	it	woke	up	
and	received	a	response
– that’s	why	most	RTT	distribution	is	within	
100msec
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 172015/11/10
sleeping	and	buffering
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 18
200msec		no	traffic
goes	to	sleep	until
the	next	beacon buffered
2015/11/10
Summary
• Strange	RTT	behavior	happens	if	your	communication	
is	between:
– a	host	connected	to	a	wifi network	and
– a	far end	host	(RTT>200msec)
• Your	wifi adapter	goes	to	sleep
– “200msec	of	no	traffic”	seems	a	common	trigger
• The	sleep	duration	is	manageable	by	setting	beacon	
interval	of	your	wifi AP
– 100msec	would	be	reasonable
– You	might	be	able	to	reduce	battery	usage	by	setting	it	as	
1000msec,	but	this	could	introduce	more	RTT	penalty
Internet	Initiative	Japan	Inc.	(IIJ/AS2497) 192015/11/10
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RTT matters