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Lecture 2 - Weeks 2 3.pptx
Lecture_2_-_Weeks_2___3.pptx
Showing 23-27 out of 70
Lecture 2 - Weeks 2 3.pptx-Data Communication an...
Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
Lecture 2 - Weeks 2 3.pptx-Data C...
Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
Page 23
Application Layer A 2-23
HTTP connections
non-persistent HTTP
❖
at most one
object sent over
TCP connection
▪
connection is
then closed
❖
downloading
multiple objects
requires multiple
connections
persistent HTTP
❖
multiple objects
can be sent over
single TCP
connection
between client,
server
❖
HTTP default
mode uses
persistent
connection, but
this can be re-
configured
Page 24
Application Layer A 2-24
Non-persistent HTTP
suppose user enters URL:
1a
.
HTTP client initiates TCP
connection to HTTP server
(process) at
www.someSchool.edu on
port 80
2
.
HTTP client sends
HTTP
request
message
(containing
URL) into TCP
connection socket.
Message indicates
that client wants
object
someDepartment/ho
1b
.
HTTP server at host
www.someSchool.ed
u waiting for TCP
connection at port
80.
“accepts”
connection, notifying
client
3
.
HTTP server receives
request message,
forms
response
message
containing
requested object,
and sends message
into its socket
tim
e
(contains text,
references to 10
jpeg images)
www.someSchool.edu/someDepartment/home.index
Page 25
Application Layer A 2-25
Non-persistent HTTP (cont.)
5
.
HTTP client receives
response message
containing html file,
displays html.
Parsing html
file, finds 10 referenced jpeg
objects
6.
Steps 1-5 repeated
for each of 10 jpeg
objects
4.
HTTP server closes
TCP connection.
ti
m
e
Q:
how these 10 jpeg objects were obtained? Have
they been retrieved in a serial fashion, or using
parallel connections?
Page 26
Application Layer A 2-26
Non-persistent HTTP: response
time
RTT (definition):
time for
a small packet to
travel from client to
server and back
HTTP response time:
❖
one RTT to initiate TCP
connection
❖
one RTT for HTTP
request and first few
bytes of HTTP
response to return
❖
file transmission time
❖
non-persistent HTTP
response time
=
2RTT
+
file
transmission
time
time to
transmit
file
initiate TCP
connection
RTT
request
file
RTT
file
receive
d
time
time
Page 27
Application Layer A 2-27
Persistent HTTP
non-persistent HTTP
issues:
❖
requires 2 RTTs per
object
❖
OS overhead for
each
TCP connection
❖
browsers often open
parallel TCP
connections to fetch
referenced objects
persistent
HTTP:
❖
server leaves
connection open after
sending response
(connection is only
closed when it is not
used for certain,
configurable, amount
of time)
❖
subsequent HTTP
messages
between
same client/server sent
over open connection
❖
client sends requests
as soon as it
encounters a
referenced object
as little as one RTT for
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