Transcript R c - Northwestern Networks Group

“Real” Internet delays and routes


What do “real” Internet delay & loss look like?
Traceroute program: provides delay
measurement from source to router along end-end
Internet path towards destination. For all i:



sends three packets that will reach router i on path
towards destination
router i will return packets to sender
sender times interval between transmission and reply.
3 probes
3 probes
3 probes
“Real” Internet delays and routes
traceroute: zappa.cs.nwu.edu to www.zju.edu.cn
Three delay measements from
Zappa.cs.cs.nwu.edu to 1890mpl-idf-vln-122.northwestern.edu
1 1890mpl-idf-vln-122.northwestern.edu (129.105.100.1) 0.287 ms 0.211 ms 0.193 ms
2 lev-mdf-6-vln-54.northwestern.edu (129.105.253.53) 0.431 ms 0.315 ms 0.321 ms
3 abbt-mdf-1-vln-902.northwestern.edu (129.105.253.222) 0.991 ms 0.950 ms 1.151 ms
4 abbt-mdf-4-ge-0-1-0.northwestern.edu (129.105.253.22) 1.659 ms 1.255 ms 1.520 ms
5 starlight-lsd6509.northwestern.edu (199.249.169.6) 1.713 ms 1.368 ms 1.278 ms
6 206.220.240.154 (206.220.240.154) 1.284 ms 1.204 ms 1.279 ms
7 206.220.240.105 (206.220.240.105) 2.892 ms 2.003 ms 2.808 ms
8 202.112.61.5 (202.112.61.5) 116.475 ms 196.663 ms 241.792 ms
9 sl-gw25-stk-1-2.sprintlink.net (144.223.71.221) 145.502 ms 150.033 ms 151.715 ms
10 sl-bb21-stk-8-1.sprintlink.net (144.232.4.225) 166.762 ms 177.180 ms 166.235 ms
11 sl-bb21-hk-2-0.sprintlink.net (144.232.20.28) 331.858 ms 340.613 ms 346.332 ms
12 sl-gw10-hk-14-0.sprintlink.net (203.222.38.38) 346.842 ms 356.915 ms 366.916 ms
13 sla-cent-3-0.sprintlink.net (203.222.39.158) 482.426 ms 495.908 ms 509.712 ms
14 202.112.61.193 (202.112.61.193) 515.548 ms 501.186 ms 509.868 ms
15 202.112.36.226 (202.112.36.226) 537.994 ms 561.658 ms 541.695 ms
16 shnj4.cernet.net (202.112.46.78) 451.750 ms 263.390 ms 342.306 ms
17 hzsh3.cernet.net (202.112.46.134) 349.855 ms 366.082 ms 380.849 ms
18 zjufw.zju.edu.cn (210.32.156.130) 350.693 ms 394.553 ms 366.636 ms
19 * * *
20
***
21 www.zju.edu.cn (210.32.0.9) 353.623 ms 397.532 ms 396.326 ms
trans-oceanic
link
* means no reponse (probe lost, router not
replying)
Packet loss
queue (aka buffer) preceding link in buffer has
finite capacity
 packet arriving to full queue dropped (aka lost)
 lost packet may be retransmitted by previous
node, by source end system, or not at all

buffer
(waiting area) packet being transmitted
A
B
packet arriving to
full buffer is lost
Throughput

throughput: rate (bits/time unit) at which
bits transferred between sender/receiver


instantaneous: rate at given point in time
average: rate over longer period of time
link
capacity
that
can carry
server,
with
server
sends
bits pipe
Rs bits/sec
fluid
at rate
file of
F bits
(fluid)
into
pipe
Rs bits/sec)
to send to client
link that
capacity
pipe
can carry
Rfluid
c bits/sec
at rate
Rc bits/sec)
Throughput (more)

Rs < Rc What is average end-end throughput?
Rs bits/sec

Rc bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec
Rc bits/sec
bottleneck link
link on end-end path that constrains end-end throughput
Throughput: Internet scenario
per-connection
end-end
throughput:
min(Rc,Rs,R/10)
 in practice: Rc or
Rs is often
bottleneck

Rs
Rs
Rs
R
Rc
Rc
Rc
10 connections (fairly) share
backbone bottleneck link R bits/sec
Protocol “Layers”
Networks are complex!
 many “pieces”:
 hosts
 routers
 links of various
media
 applications
 protocols
 hardware,
software
Question:
Is there any hope of
organizing structure of
network?
Or at least our discussion
of networks?
Why layering?
Dealing with complex systems:
 Explicit structure allows identification, relationship
of complex system’s pieces
 layered reference model for discussion
 Modularization eases maintenance, updating of
system
 change of implementation of layer’s service
transparent to rest of system
 e.g., change in gate procedure doesn’t affect rest
of system
 Layering considered harmful?
Internet protocol stack
 application: supporting network applications
 FTP, SMTP, HTTP
 transport: host-host data transfer
 TCP, UDP
 network: routing of datagrams from source
to destination

IP, routing protocols
 link: data transfer between neighboring
network elements

PPP, Ethernet
 physical: bits “on the wire”
application
transport
network
link
physical
Layering: logical communication
Each layer:
 distributed
 “entities”
implement
layer functions
at each node
 entities
perform
actions,
exchange
messages with
peers
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
Layering: logical communication
E.g.: transport
 take data from app
 add addressing,
reliability check
info to form
“datagram”
 send datagram to
peer
 wait for peer to
ack receipt
 analogy: post
office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
network
link
physical
application
transport
network
link
physical
data
application
transport
transport
network
link
physical
Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
Protocol layering and data
Each layer takes data from above
 adds header information to create new data unit
 passes new data unit to layer below
M
Ht M
Hn Ht M
Hl Hn Ht M
source
destination
application
transport
network
link
physical
application
transport
network
link
physical
M
message
Ht M
Hn Ht M
Hl Hn Ht M
segment
datagram
frame
Summary
 Network access and physical media
 Internet structure and ISPs
 Delay & loss in packet-switched networks
 Protocol layers, service models
 More depth, detail to follow!
Application Layer
Our goals:
 conceptual,
implementation
aspects of network
application protocols
 transport-layer
service models
 client-server
paradigm
 peer-to-peer
paradigm
 learn about protocols
by examining popular
application-level
protocols




HTTP
FTP
SMTP / POP3 / IMAP
DNS
 programming network
applications
 socket API
Some network apps
 e-mail
 voice over IP
 web
 real-time video
 instant messaging
 remote login
 P2P file sharing
 multi-user network
games
 streaming stored video
(YouTube)
conferencing
 cloud computing
 …
 …

Creating a network app
write programs that



run on (different) end
systems
communicate over network
e.g., web server software
communicates with browser
software
No need to write software
for network-core devices


network-core devices do
not run user applications
applications on end systems
allows for rapid app
development, propagation
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
Outline
 Principles of network applications
App architectures
 App requirements

 Web and HTTP
 FTP
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Application architectures
 Client-server
 Peer-to-peer (P2P)
 Hybrid of client-server and P2P
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Client-server architecture
server:
 always-on host
 permanent IP address
 server farms for
scaling
clients:
client/server




communicate with server
may be intermittently
connected
may have dynamic IP
addresses
do not communicate
directly with each other
Pure P2P architecture
 no always-on server
 arbitrary end systems
directly communicate peer-peer
 peers are intermittently
connected and change IP
addresses
highly scalable but
difficult to manage
21
Hybrid of client-server and P2P
Skype
 voice-over-IP P2P application
 centralized server: finding address of remote
party:
 client-client connection: direct (not through
server)
Instant messaging
 chatting between two users is P2P
 centralized service: client presence
detection/location
• user registers its IP address with central
server when it comes online
• user contacts central server to find IP
addresses of buddies
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Outline
 Principles of network applications
App architectures
 App requirements

 Web and HTTP
 FTP
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Processes communicating
Process: program running
within a host.
 within same host, two
processes communicate
using inter-process
communication (defined
by OS).
 processes in different
hosts communicate by
exchanging messages
Client process: process
that initiates
communication
Server process: process
that waits to be
contacted
 Note: applications with
P2P architectures have
client processes &
server processes
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Sockets
 process sends/receives
messages to/from its
socket
 socket analogous to door


sending process shoves
message out door
sending process relies on
transport infrastructure
on other side of door which
brings message to socket
at receiving process
host or
server
host or
server
process
controlled by
app developer
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
controlled
by OS
 API: (1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)
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