Transcript Lecture 2: IP addresses, TCP and UDP

Lecture 2: IP addresses, TCP and
UDP
 This lecture will cover:
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The “whole thing” (how your email gets to you)
More about IP addresses.
How names become IP addresses.
TCP and UDP
ICMP
 More Information
 Bertsekas/Gallager: Section 2.8-2.9
 Tanenbaum: Section 6.1-6.4
Reminder from last lecture
 IP sends data from place to place. TCP or UDP sit
above it at either end.
 When you use the internet you use addresses like
http://manor.york.ac.uk or [email protected]
 These addresses must then be converted to an IP
address e.g. 144.32.100.24
 This means that data (packets) can get from A to B.
 But what happens if data is lost, how do we know
where they are going to and how can we put packets
back together into data?
The Internet - emailing a friend
your computer
university of
york
G/169
router
JANET
transatlantic
cable
US backbone
LAN
your friend's
computer
Domain Name System (DNS)
 DNS takes the human readable name and
converts it to octets.
 On a unix machine you can try this using
nslookup. (Linux users may prefer dig).
manor.york.ac.uk 1% nslookup www.ntk.net
Server: castle2.york.ac.uk
Address: 144.32.128.5
Question
Non-authoritative answer:
Name:
vwww.flirble.org
Address: 195.40.6.34
Aliases: www.ntk.net
Answer
DNS(2)
generic/US
national
TLDs (Top Level Domains)
com org gov mil
jp
uk
net
sun
eng
ac
vnvnation
www
york
www
nl
co
org
musicnonstop
ic
manor
doc
src
www
DNS (3)
Routing Tables
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How do packets know where to go?
This problem is known as routing.
The oldest (and easiest) solution is static routing.
Each computer has a table saying where to go to get to
each other computer.
 On a Local Area Network (LAN) list all machines on
your subnet and the address of the external router for
everything else.
 Most machines only need to know how to get to their
nearest router. Much more will be said about routing
later in the course
TCP and UDP
 Once we’ve got our IP packet safely to its destination
what happens next?
 Having stripped off the header, the first thing we find
is another header.
 The second header provides information on which port
to enter the machine on and where to send the reply.
 It also provides a checksum to check the data is valid.
 UDP will do nothing else. TCP will ensure that the
connection is lossless.
What are ports?
 Ports are conceptual “points of entry” into a host
computer.
 They do not correspond with real hardware but are
an abstraction for convenience.
 Usually a service is associated with a port (e.g. http
on port 80).
 Servers “listen on a port” for connection attempts.
 Ports provide one level of internet security.
 Generally, low number ports (< 100) are reserved
for special services.
Common Services and Ports
Service
ftp
telnet
smtp (mail)
finger
http
Listens on Port
21
23
25
79
80
User configured services (your Half-Life server?)
will listen on high numbered ports which are
usually left open to all users.
UDP data
 User Datagram Protocol – the header is shown
below.
 Length and checksum are as for IP.
About UDP
 Provides a lossy connection (data may vanish).
 Does not guarantee packets are delivered in
order.
 Useful for real time applications. (It is no use
having your Quake III information arriving
correctly but ten seconds late).
 UDP applications can implement their own
packet loss checking but it is best to use TCP for
this.
The TCP header
 The TCP header is shown below
About the TCP header
 Sequence number (what is the “order” of this
packet) incremented by 1 for every packet.
 Acknowledgement number (what packet
sequence number does this acknowledge).
 Header length (how many 32 bit words are in
options).
 Flags: SYN = start connection, ACK =
acknowledge packet, FIN= finish connection.
 (Three other flags, URG, RST, PSH).
TCP header (2)
 Window size will be described in more detail later (it
sets how many unacknowledged packets may exist).
 Checksum – is as for IP and UDP.
 Urgent Pointer – points to part of the data that must be
looked at by the receiver before the TCP session (rarely
used).
 Offsets says how long the options field is (the options
field can contain “other things” – extra facilities that
TCP might implement).
About TCP
 TCP provides a lossless connection (or flags an
error when losses occur).
 Data packets are given an order and can be
reassembled.
 TCP provides some limited congestion control.
 TCP is most useful for applications where data
validity is important but real-time is not critical
(email, www, ftp).
 TCP packets are part of a TCP session.
TCP connections
 This diagram shows the start of a TCP connection.
A sends packet X with
SYN. “Hello I would
like to talk”.
B sends a SYN, ACK
pair “I got your
message. I would
also like to talk”
A sends an ACK (and
some data) “I
got your message,
here is some data.”
TCP mechanisms
 The window size is the number of outstanding
(unacknowledged) packets that that a TCP
session can send.
 The window size provides a crude method for
congestion control.
 The window size increases to allow more
packets to be sent (it increases throughput).
 If a packet is lost then the window is reduced
again.
TCP lost packets
 When a packet is received out of sequence the
receiver sends an ACK with the same number as
the previous.
 If the sender receives three duplicate ACKs then
it assumes the packet has been lost and resends.
 If the sender has not received an ACK for a
packet within a certain amount of time then it
times out and assumes the packet lost.
 Packet loss causes the packet to be resent and the
congestion window to be reduced.
TCP Window Increase/Decrease
Congestion window
Timeout
Threshold
The initial doubling of the
window size is called “slow
start”.
Threshold
Transmission no
Closing a TCP/session – an
interesting dilemma (aside)
 How can we close a TCP session and stop
listening?
ICMP
 Internet Control Message Protocol packets are
used for various control purposes. Here are
some common ones:
 Time exceeded: TTL hit 0.
 Echo request: Can you hear me out there?
 Echo reply: Yes I can hear you.
 Source Quench: Stop sending so much data.
 Timestamp request/reply (as echo but with
times).
The story of ping
 Ping is a handy utility for checking if a computer is alive
using ICMP echo request/reply (or timestamp if we want).
manor.york.ac.uk 20% ping -s castle.york.ac.uk
PING castle2.york.ac.uk: 56 data bytes
64 bytes from castle2.york.ac.uk (144.32.128.5):
64 bytes from castle2.york.ac.uk (144.32.128.5):
64 bytes from castle2.york.ac.uk (144.32.128.5):
64 bytes from castle2.york.ac.uk (144.32.128.5):
icmp_seq=0.
icmp_seq=1.
icmp_seq=2.
icmp_seq=3.
time=1.
time=1.
time=1.
time=1.
 Ping is a first test if a computer is networked.
 We can even measure the speed of light using ping.
http://xxx.lanl.gov/abs/physics/0201053
 Hacking makes it increasingly unused.
ms
ms
ms
ms
Traceroute
 Traceroute neatly combines ping and the TTL
flag to get a “route” to a computer.
 If the TTL is one the the packet will “die” after
one hop.
 ICMP will return a Time exceeded flag.
 This will tell us where the first “hop” of our
journey is.
 Increase the TTL by one to find the next “hop”.
ICMP tourism (with traceroute)
This shows the trip from Estonia to my flat in Fulford
via my Internet Service Provider (ISP) – V21 in Rochdale
traceroute to host213-121-67-224: (213.121.67.224): 2-20 hops, 38 byte packets
2 213.180.11.162
tondi-CR.online.ee
1.62 ms (ttl=127)
3 213.180.25.1
liiva-CR.online.ee
1.82 ms (ttl=126)
4 213.180.11.189
tix-CR.online.ee
2.16 ms (ttl=125)
5 212.47.215.6
r1-Fa4-0-80-Tln-TIX.EE.KPNQwest.net
2.28 ms (ttl=251)
6 134.222.224.5
r5-AT3-1.105.sthm-KPN1.SE.kpnqwest.net
12.2 ms (ttl=250)
7 134.222.119.226 r2-Ge0-2-0-0.Sthm-KQ1.SE.KPNQwest.net
34.3 ms (ttl=246!)
8 134.222.230.157 r2-Se0-3-0.hmbg-KQ2.DE.KPNQwest.net
33.4 ms (ttl=247!)
9 134.222.230.117 r2-Se0-2-0.0.ffm-KQ1.DE.kpnqwest.net
34.1 ms (ttl=249!)
10 134.222.230.29
r2-Se0-3-0.0.ledn-KQ1.NL.kpnqwest.net
39.6 ms (ttl=248!)
11 134.222.230.169 r1-Se0-0-0.0.ldn-KQ1.UK.kpnqwest.net
43.7 ms (ttl=246!)
12 134.222.231.14
r1-Se0-0-0.0.Ldn-KQ4.UK.KPNQwest.net
44.9 ms (ttl=245!)
13 134.222.109.241 r13-Gi5-0.200.ldn-KQ4.UK.kpnqwest.net
45.4 ms (ttl=245!)
14 195.66.225.10
linx-l1.ukcore.bt.net
45.2 ms (ttl=244!)
15 194.74.65.126
core2-pos14-0.ilford.ukcore.bt.net
45.3 ms (ttl=243!)
16 194.74.65.222
core2-pos5-0.reading.ukcore.bt.net
46.7 ms (ttl=242!)
17 62.6.196.109
core2-pos8-0.birmingham.ukcore.bt.net
54.3 ms (ttl=241!)
18 194.74.16.194
core2-pos9-0.rochdale.ukcore.bt.net
51.0 ms (ttl=240!)
19 217.32.168.5
vhsaccess1-gig1-0.rochdale.fixed.bt.net
51.1 ms (ttl=239!)
20 213.121.156.22
ugint0066-p.vhsaccess1.rochdale.fixed-nte.bt.net
51.3 ms
(ttl=238!)
The journey of email
To: [email protected]
From: richard@manor
Look up IP
name for
distant.com
Dave,
Great to see you
the other day...
Dav
e, Gr
eat
to s
Packetise
the data
Dav
Get first
Add IP
hop from
header to
routing table front of that
Send the
first packet
to its first
hop
And so on
for further
hops.
SYN
SYN,ACK
ACK
Set up
the TCP
connection
Dav
Add TCP
header to
first packet
Destination gets packet
and returns ACK
Start sending rest of data