Download IP address.

TCP/IP
Transmission Control Protocol / Internet
Protocol
Netprog 2002 TCP/IP
1
TCP/IP & OSI
• In OSI reference model terminology the TCP/IP protocol suite covers the
network and transport layers.
• TCP/IP can be used on many datalink layers (can support many
network hardware implementations).
Netprog 2002 TCP/IP
2
Ethernet - A Real Data-Link Layer
• It will be useful to discuss a real
•
•
data-link layer.
Ethernet (really IEEE 802.3) is
widely used.
Supported by a variety of physical
layer implementations.
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3
Ethernet
• Multi-access (shared medium).
• Every Ethernet interface has a unique
•
•
•
48 bit address (a.k.a. hardware
address).
Example: C0:B3:44:17:21:17
The broadcast address is all 1’s.
Addresses are assigned to vendors by
a central authority.
Netprog 2002 TCP/IP
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CSMA/CD
Carrier Sense Multiple Access
with
Collision Detection
• Carrier Sense: can tell when another
host is transmitting
• Multiple Access: many hosts on 1
wire
• Collision Detection: can tell when
another host transmits at the same
time.
Netprog 2002 TCP/IP
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An Ethernet Frame
Preamble
8 bytes
Destination Source
Address
Address
6
6
Len
DATA
CRC
2
0-1500
4
• The preamble is a sequence of alternating 1s
•
and 0s used for synchronization.
CRC is Cyclic Redundency Check
Netprog 2002 TCP/IP
6
Ethernet Addressing
• Each interface looks at every frame and
•
inspects the destination address. If the
address does not match the hardware
address of the interface or the
broadcast address, the frame is
discarded.
Some interfaces can also be
programmed to recognize multicast
addresses.
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Internet Protocol
The IP in TCP/IP
• IP is the network layer
• packet delivery service (host-to-host).
• translation between different data-link
protocols.
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IP Datagrams
• IP provides connectionless,
•
•
unreliable delivery of IP datagrams.
Connectionless: each datagram is
independent of all others.
Unreliable: there is no guarantee
that datagrams are delivered
correctly or at all.
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IP Addresses
• IP addresses are not the
same as the underlying
data-link (MAC)
addresses.
Why ?
Netprog 2002 TCP/IP
10
IP Addresses
• IP is a network layer - it must be
•
capable of providing communication
between hosts on different kinds of
networks (different data-link
implementations).
The address must include
information about what network the
receiving host is on. This makes
routing feasible.
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IP Addresses
• IP addresses are logical addresses (not
physical)
• 32 bits.
• Includes a network ID and a host ID.
• Every host must have a unique IP
address.
• IP addresses are assigned by a central
authority (American Registry for Internet
Numbers)
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The four formats of IP Addresses
Class
A 0 NetID
B
10
C
110
D
1110
HostID
NetID
8 bits
HostID
HostID
NetID
Multicast Address
8 bits
8 bits
8 bits
Netprog 2002 TCP/IP
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

Class A
128 possible network IDs
over 4 million host IDs per network ID
Class B
 16K possible network IDs
 64K host IDs per network ID
Class C
 over 2 million possible network IDs
 about 256 host IDs per network ID
Netprog 2002 TCP/IP
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Network and Host IDs
• A Network ID is assigned to an
organization by a global authority.
• Host IDs are assigned locally by a
system administrator.
• Both the Network ID and the Host
ID are used for routing.
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IP Addresses
• IP Addresses are usually shown in
dotted decimal notation:
1.2.3.4
00000001 00000010 00000011
cs.rpi.edu is 128.213.1.1
•
00000100
10000000 11010101 00000001 00000001
CS has a class B network
Netprog 2002 TCP/IP
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Host and Network
Addresses
• A single network interface is
•
•
assigned a single IP address called
the host address.
A host may have multiple interfaces,
and therefore multiple host
addresses.
Hosts that share a network all have
the same IP network address (the
network ID).
Netprog 2002 TCP/IP
17
IP Broadcast and Network
Addresses
• An IP broadcast addresses has a
host ID of all 1s.
• IP broadcasting is not necessarily a
•
true broadcast, it relies on the
underlying hardware technology.
An IP address that has a host ID
of all 0s is called a network
address and refers to an entire
network.
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Subnet Addresses
• An organization can subdivide it’s host
•
address space into groups called
subnets.
The subnet ID is generally used to group
hosts based on the physical network
topology.
10
NetID
SubnetID HostID
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19
Subnetting
router
Subnet 1
128.213.1.x
Subnet 2
128.213.2.x
Subnet 3
128.213.3.x
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20
Subnetting
• Subnets can simplify routing.
• IP subnet broadcasts have a hostID of
•
all 1s.
It is possible to have a single wire
network with multiple subnets.
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21
Mapping IP Addresses to
Hardware Addresses
• IP Addresses are not recognized
by hardware.
• If we know the IP address of a
•
host, how do we find out the
hardware address ?
The process of finding the
hardware address of a host given
the IP address is called
Address Resolution
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22
Reverse Address Resolution
• The process of finding out the IP
address of a host given a hardware
address is called
Reverse Address Resolution
• Reverse address resolution is
needed by diskless workstations
when booting.
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23
Arp Arp!
ARP
• The Address Resolution Protocol is
•
•
used by a sending host when it
knows the IP address of the
destination but needs the Ethernet
address.
ARP is a broadcast protocol - every
host on the network receives the
request.
Each host checks the request
against it’s IP address - the right one
responds.
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24
ARP (cont.)
• ARP does not need to be done every
•
time an IP datagram is sent - hosts
remember the hardware addresses of
each other.
Part of the ARP protocol specifies
that the receiving host should also
remember the IP and hardware
addresses of the sending host.
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ARP conversation
HEY - Everyone please listen!
Will 128.213.1.5 please send me
his/her Ethernet address?
not me
Hi Green! I’m 128.213.1.5, and
my Ethernet address is
87:A2:15:35:02:C3
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26
RARP conversation
HEY - Everyone please listen!
My Ethernet address is
22:BC:66:17:01:75.
Does anyone know my IP address ?
not me
Hi Green! Your IP address is
128.213.1.17.
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27
Services provided by IP
• Connectionless Delivery (each
datagram is treated individually).
• Unreliable (delivery is not
guaranteed).
• Fragmentation / Reassembly
(based on hardware MTU).
• Routing.
• Error detection.
Netprog 2002 TCP/IP
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IP Datagram
1 byte
1 byte
1 byte
1 byte
VERS
HL
Service
Fragment Length
Datagram ID
FLAG
Fragment Offset
TTL
Protocol
Header Checksum
Source Address
Destination Address
Options (if any)
Data
Netprog 2002 TCP/IP
29
IP Datagram Fragmentation
• Each fragment (packet) has the
same structure as the IP datagram.
• IP specifies that datagram
•
reassembly is done only at the
destination (not on a hop-by-hop
basis).
If any of the fragments are lost - the
entire datagram is discarded (and an
ICMP message is sent to the
sender).
Netprog 2002 TCP/IP
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IP Flow Control & Error
Detection
• If packets arrive too fast - the
•
receiver discards excessive packets
and sends an ICMP message to the
sender (SOURCE QUENCH).
If an error is found (header
checksum problem) the packet is
discarded and an ICMP message is
sent to the sender.
Netprog 2002 TCP/IP
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ICMP
Internet Control Message Protocol
• ICMP is a protocol used for
•
•
exchanging control messages.
ICMP uses IP to deliver messages.
ICMP messages are usually
generated and processed by the IP
software, not the user process.
Netprog 2002 TCP/IP
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ICMP Message Types
• Echo Request
• Echo Response
• Destination Unreachable
• Redirect
• Time Exceeded
• Redirect (route change)
• there are more ...
Netprog 2002 TCP/IP
33
Transport Layer & TCP/IP
Q: We know that IP is the network
layer - so TCP must be the transport
layer, right ?
A: No… well, almost.
TCP is only part of the TCP/IP
transport layer - the other part is
UDP (User Datagram Protocol).
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34
Process
Process
TCP
UDP
ICMP, ARP
&
RARP
Process Layer
Transport Layer
Network Layer
IP
Data-Link Layer
802.3
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35
UDP User Datagram Protocol
• UDP is a transport protocol
• communication between processes
• UDP uses IP to deliver datagrams
•
to the right host.
UDP uses ports to provide
communication services to
individual processes.
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36
Ports
• TCP/IP uses an abstract
•
•
destination point called a protocol
port.
Ports are identified by a positive
integer.
Operating systems provide some
mechanism that processes use
to specify a port.
Netprog 2002 TCP/IP
37
UDP
• Datagram Delivery
• Connectionless
• Unreliable
• Minimal
UDP Datagram Format
Source Port
Destination Port
Length
Checksum
Data
Netprog 2002 TCP/IP
38
TCP
Transmission Control Protocol
• TCP is an alternative transport layer
•
protocol supported by TCP/IP.
TCP provides:
Connection-oriented
Reliable
Full-duplex
Byte-Stream
•
•
•
•
Netprog 2002 TCP/IP
39
Connection-Oriented
• Connection oriented means that a
•
•
virtual connection is established
before any user data is
transferred.
If the connection cannot be
established - the user program is
notified.
If the connection is ever
interrupted - the user program(s)
is notified.
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40
Reliable
• Reliable means that every
•
transmission of data is
acknowledged by the receiver.
If the sender does not receive
acknowledgement within a
specified amount of time, the
sender retransmits the data.
Netprog 2002 TCP/IP
41
Byte Stream
• Stream means that the connection
is treated as a stream of bytes.
• The user application does not
need to package data in individual
datagrams (as with UDP).
Netprog 2002 TCP/IP
42
Buffering
• TCP is responsible for buffering
data and determining when it is
time to send a datagram.
• It is possible for an application to
tell TCP to send the data it has
buffered without waiting for a buffer
to fill up.
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43
Full Duplex
• TCP provides transfer in both
directions.
• Piggybacking
Netprog 2002 TCP/IP
44
TCP Ports
• Interprocess communication via
TCP is achieved with the use of
ports (just like UDP).
• UDP ports have no relation to TCP
ports (different name spaces).
Netprog 2002 TCP/IP
45
TCP Segments
• The chunk of data that TCP asks IP
to deliver is called a TCP segment.
• Each segment contains:
• data bytes from the byte stream
• control information that identifies the
data bytes
Netprog 2002 TCP/IP
46
TCP Segment Format
1 byte
1 byte
1 byte
1 byte
Source Port
Destination Port
Sequence Number
Request Number
offset Reser.
Control
Window
Checksum
Urgent Pointer
Options (if any)
Data
Netprog 2002 TCP/IP
47
If the SYN flag is set, this is the initial sequence
If the SYN flag is NOT number.
set, this is the sequence number
of theoffirst
byte
The sequence number
thedata
actual
first data byte will
then be this sequence number plus 1.
Netprog 2002 TCP/IP
48
if the ACK flag is set then the value of this field is the
next expected sequence number that the receiver is
expecting.
Netprog 2002 TCP/IP
49
The size of the TCP header in 32-bit words. The
minimum size header is 5 words and the maximum is
15 words thus giving the minimum size of 20 bytes and
maximum of 60 bytes. This field gets its name from the
fact that it is also the offset from the start of the TCP
segment to the actual data.
Netprog 2002 TCP/IP
50
For future use and should be set to 0s.
Netprog 2002 TCP/IP
51
Congestion Window Reduced (CWR) flag is set by the
sending host to indicate that it received a TCP segment
with the ECE flag set and had responded in congestion
control mechanism.
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52
indicates (1) that the TCP peer is ECN capable during
3-way handshake, and (2) that a packet with
Congestion Experienced flag in IP header set is
received during normal transmission.
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53
indicates that the URGent pointer field is significant.
Netprog 2002 TCP/IP
54
indicates that the ACKnowledgment field is significant.
Netprog 2002 TCP/IP
55
Push function. The set ensures that data will be
delivered immediately to the application layer by the
receiving transport layer
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56
Reset the connection. Tells receiver to tear down
connection immediately
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57
Synchronize sequence numbers.
Netprog 2002 TCP/IP
58
No more data from sender.
Netprog 2002 TCP/IP
59
the size of the receive window, which specifies the
number of bytes (beyond the sequence number in the
acknowledgment field) that the receiver is currently
willing to receive .
Netprog 2002 TCP/IP
60
The 16-bit checksum field is used for error-checking of
the header and data.
Netprog 2002 TCP/IP
61
if the URG flag is set, then this 16-bit field is an offset
from the sequence number indicating the last urgent
data byte.
The Urgent Pointer is used when some information
has to reach the server ASAP. When the TCP/IP stack
at the other end sees a packet using the Urgent
Pointer, it is duty bound to stop all it's doing and
immediately send this packet to the relevant server
Netprog 2002 TCP/IP
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Example
•
Lets assume we've got this data to send across to the
guy at the other end.
ABCDEFGHIJ
Now for some reason or another, we're going to send
the bytes across only four bytes at a time.
The First Packet:
ABCD
The Second Packet: EFGH
The Third Packet:
IJ
Netprog 2002 TCP/IP
63
Example
•
In the very first packet we send across we set the
four byte sequence number to 1 i.e. the number of
the first byte in the packet and the acknowledgement
number as 0.
ABCD 1 2 3 4
The computer across the wire will respond with an
ACK packet (an acknowledgement packet with the
ACK flag on in the TCP header) holding an
acknowledgement number of ?.
Netprog 2002 TCP/IP
64
Example
•
The next packet we send will have a sequence
number of 5 i.e. the number of the first byte in the
packet relative to the start of the data stream. The
acknowledgment number will be the other guys
sequence number + 1.
EFGH 5 6 7 8
Netprog 2002 TCP/IP
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Example
•
We will then receive an ACK with the
acknowledgement number set to 9; the byte we have
to start our next packet with.
We then shot off the last two bytes and wait for the
ACK and when that comes, we know that all the
bytes we've sent across has reached the computer at
the other end.
IJ 9 10
Netprog 2002 TCP/IP
66
Three-way Handshake
•
Before a client attempts to connect with a server, the
server must first bind to a port to open it up for
connections: this is called a passive open. Once the
passive open is established, a client may initiate an
active open. To establish a connection, the three-way
(or 3-step) handshake occurs:
The active open is performed by the client sending a
SYN to the server. It sets the segment's sequence
number to a random value.
Netprog 2002 TCP/IP
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Three-way Handshake
In response, the server replies with a SYN-ACK. The
acknowledgment number is set to one more than the
received sequence number, and the sequence
number is random.
Finally, the client sends an ACK back to the server.
The sequence number is set to the received
acknowledgement value, and the acknowledgement
number is set to one more than the received
sequence number.
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Three-way Handshake
At this point, both the client and server have received
an acknowledgment of the connection.
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Connection Termination
• A four-way handshake, with each side of the
•
•
connection terminating independently
When an endpoint wishes to stop its half of
the connection, it transmits a FIN packet,
which the other end acknowledges with an
ACK.
A typical tear-down requires a pair of FIN and
ACK segments from each TCP endpoint.
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70
TCP : Connection
Client
Host
Client
Send SYN seq=x
Receive SYN
+ACK segment
Host
Send FIN seq=x
Receive SYN segment
Send SYN seq=y,
ACK x+1
Receive ACK segment
Receive FIN
+ ACK segment
Send ACK y+1
Send ACK y+1
Receive ACK segment
Establishing a TCP Connection
Receive FIN segment
Send ACK x+1
Send FIN seq=y,
ACK x+1
Receive ACK segment
Closing a TCP Connection
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TCP : Data transfer
Client
Send Packet 1
Start Timer
Host
Packet Lost
Timer
ACK would normally
Arrive at this time
Packet should arrive
ACK should be sent
Time Expires
Timer
Retransmit Packet1
Start Timer
Receive Packet 1
Send ACK 1
Receive ACK 1
Cancel Timer
Netprog 2002 TCP/IP
72
TCP vs. UDP
Q: Which protocol is better ?
A: It depends on the application.
TCP provides a connection-oriented, reliable
byte stream service (lots of overhead).
UDP offers minimal datagram delivery service
(as little overhead as possible).
Netprog 2002 TCP/IP
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TCP/IP Summary
• IP: network layer protocol
• unreliable datagram delivery between
hosts.
• UDP: transport layer protocol
• unreliable datagram delivery between
processes.
• TCP: transport layer protocol
• reliable, byte-stream delivery
between processes.
Netprog 2002 TCP/IP
74
Hmmmmm. TCP or UDP ?
• Internet commerce ?
• Video server?
• File transfer?
• Email ?
• Chat groups?
• Robotic surgery controlled remotely
over a network?
Netprog 2002 TCP/IP
75
Example 1: Server Sends IP datagram to PC
•
How to routing, i e., why server knows to send the IP
packet to the router first ?
Look up routing table, in detail,
•
• by complete destination IP address, if not found
• by network ID of destination IP address, if not found
• the default router is selected. (In this example, we
assume the router r is the default router).
•
The IP address of a home computer connected to the
Internet through modem is dynamically assigned
(DHCP) .
Netprog 2002 TCP/IP
76
S sends a packet to R:
1.
2.
3.
4.
5.
Find R’s IP address by DNS.
Check its routing table for R, if find (next hop), send to it.
Otherwise, send to default router
Needs to find the physical address of the next hop router.
The router checks its routing table for the next hop and send to it.
s
net 3
G
net 1
G
G
G
net 2
net 5
G
net 4
G
R
6. continue until the packet reaches the router in the same LAN with R.
7. The router finds R’s physical address and sends to it.
Netprog 2002 TCP/IP
Figure 2.8
77
Big picture: web document browsing
•
•
Suppose a user on PC clicks a link of a document
contained in the server, and HTTP client passes a
request to TCP layer asking for setting up a TCP
connection, and the TCP connection between the PC
and the server has been established .
The http client then passes http request message
(such as GET /….) to TCP layer.
Netprog 2002 TCP/IP
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Big picture: HTTP request is passed down
HTTP Request
c, 80
Header contains source
and destination port
numbers
TCP
Header
TCP
Header contains source and
destination IP addresses;
transport protocol type
Header contains
source and destination
physical addresses;
network protocol type
IP
Header
Frame
Check
Sequence
ppp
Header
Netprog 2002 TCP/IP
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Big picture: web document browsing
•
•
•
•
The ppp driver (data link entity) in PC forms a PPP frame and
sends the frame to the other end of the PPP link, i.e., router
The router extracts IP packet (from the PPP frame), makes
routing decision according on destination IP address, forms
an Ethernet frame (encapsulating the IP packet) and
broadcasts it onto Ethernet
The server NIC captures the frame, extracts the IP packet and
passes it to IP entity, then to TCP entity and then to HTTP
server
Finally the server retrieves the document and puts it in HTTP
response packet and sends back to PC.
Netprog 2002 TCP/IP
80
Sever processes multiple requests
•
Q: there is one http server, there may be several http clients
which sends http requests to the http server simultaneously,so
there are several connections at the same with the same
destination IP address, same port number: 80, and the same
protocol type: TCP. How does the server distinguish these
connections and process them separately?
http client
http client
http server
http client
http client
Netprog 2002 TCP/IP
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Sever processes multiple requests
•
Answer: the way to specify the end-to-end process-toprocess connection.
•
•
•
•
Socket address: port number + IP address + protocol type
Sender socket address: sender port number + sender IP
address + protocol type
Receiver socket address: receiver port number + receiver IP
address + protocol type.
Connection = sender socket address + receiver socket address
http client
http client
c2,m1; s, 80, TCP
http server
m1
http client
cc,m3; s, 80,TCP
c1,m1; s, 80, TCP
m2
http client
m3
Netprog 2002 TCP/IP
82
Application protocols and TCP/IP utilities
• telnet: remote login. Also a tool to test other
protocols.
• FTP: File Transfer Protocols.
• Ping: determine whether a host is reachable
• Traceroute: determine the route that a packet
will take to another host
• Netstate: provide information about the
network status of a local host
• TCPdump: capture and observe packet
exchange in a link.
Netprog 2002 TCP/IP
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A simple TCP/IP Example
•
A user on host argon.tcpip-lab.edu (“Argon”) makes a
web access to URL
http://neon.tcpip-lab.edu/index.html.
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HTTP Request and HTTP response
• Web browser runs an HTTP client program
• Web server runs an HTTP server program
• HTTP client sends an HTTP request to HTTP server
• HTTP server responds with HTTP response
Argon
HTTP client
Neon
HTTP request
HTTP server
HTTP response
Netprog 2002 TCP/IP
85
From HTTP to TCP
• To send request, HTTP client program establishes an
•
TCP connection to the HTTP server Neon.
The HTTP server at Neon has a TCP server running
Argon
Neon
HTTP client
HTTP request / HTTP response
HTTP server
TCP client
TCP connection
TCP server
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86
Resolving hostnames and port
numbers
• Since TCP does not work with hostnames and also
would not know how to find the HTTP server program
at Neon, two things must happen:
1. The name “neon.tcpip-lab.edu” must be
translated into a 32-bit IP address.
2. The HTTP server at Neon must be
identified by a 16-bit port number.
Netprog 2002 TCP/IP
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Translating a hostname into an IP
address
•
The translation of the hostname neon.tcpip-lab.edu into an IP
address is done via a database lookup
neon.tcpip-lab.edu
HTTP client
128.143.71.21
argon.tcpip-lab.edu
•
•
DNS Server
128.143.136.15
The distributed database used is called the Domain Name
System (DNS)
All machines on the Internet have an IP address:
argon.tcpip-lab.edu
128.143.137.144
neon.tcpip-lab.edu
128.143.71.21
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88
Finding the port number
• Note: Most services on the Internet are reachable via
•
well-known ports. E.g. All HTTP servers on the
Internet can be reached at port number “80”.
So: Argon simply knows the port number of the HTTP
server at a remote machine.
• The well-known port numbers of some of the most
popular services are:
ftp
21
telnet 23
smtp
25
finger 79
http
80
nntp 119
Netprog 2002 TCP/IP
89
Requesting a TCP Connection
• The HTTP client at argon.tcpip-lab.edu requests the TCP
client to establish a connection to port 80 of the machine with
address 128.141.71.21
argon.tcpip-lab.edu
HTTP client
Establish a TCP connection
to port 80 of 128.143.71.21
TCP client
Netprog 2002 TCP/IP
90
Invoking the IP Protocol
• The TCP client at Argon
argon.tcpip-lab.edu
sends a request to establish
a connection to port 80 at
Neon
TCP client
Send an IP datagram to
128.143.71.21
• This is done by asking its
local IP module to send an
IP datagram to
128.143.71.21
IP
Netprog 2002 TCP/IP
91
Sending the IP datagram to an IP
router
•
•
•
•
•
Argon (128.143.137.144) can deliver the IP datagram directly to
Neon (128.143.71.21), only if it is on the same local network
(“subnet”)
But Argon and Neon are not on the same local network
(Q: How does Argon know this?)
So, Argon sends the IP datagram to its default gateway
The default gateway is an IP router
The default gateway for Argon is Router137.tcpip-lab.edu
(128.143.137.1).
Netprog 2002 TCP/IP
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The route from Argon to Neon
•
Note that the gateway has a different name for each of its
interfaces.
Netprog 2002 TCP/IP
93
Finding the MAC address of the gateway
•
•
•
•
To send an IP datagram to Router137, Argon puts the IP
datagram in an Ethernet frame, and transmits the frame.
However, Ethernet uses different addresses, so-called Media
Access Control (MAC) addresses (also called: physical address,
hardware address).
Therefore, Argon must first translate the IP address
128.143.137.1 into a MAC address.
The translation of addressed is performed via the Address
Resolution Protocol (ARP).
Netprog 2002 TCP/IP
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Address resolution with ARP
Netprog 2002 TCP/IP
95
Invoking the device driver
• The IP module at Argon, tells its Ethernet device
driver to send an Ethernet frame to address
00:e0:f9:23:a8:20
argon.tcpip-lab.edu
IP module
Send an Ethernet frame
to 00:e0:f9:23:a8:20
Ethernet
Netprog 2002 TCP/IP
96
Sending an Ethernet frame
• The Ethernet device driver of Argon sends the
•
Ethernet frame to the Ethernet network interface card
(NIC)
The NIC sends the frame onto the wire
Netprog 2002 TCP/IP
97
Forwarding the IP datagram
• The IP router receives the Ethernet frame at interface
•
128.143.137.1, recovers the IP datagram and determines that
the IP datagram should be forwarded to the interface with
name 128.143.71.1
The IP router determines that it can deliver the IP datagram
directly
Netprog 2002 TCP/IP
98
Another lookup of a MAC address
•
•
The router needs to find the MAC address of Neon.
Again, ARP is invoked, to translate the IP address of Neon
(128.143.71.21) into the MAC address of neon
(00:20:af:03:98:28).
Netprog 2002 TCP/IP
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Invoking the device driver at the router
•
The IP protocol at Router71, tells its Ethernet device driver to
send an Ethernet frame to address 00:20:af:03:98:28
router71.tcpip-lab.edu
IP module
Send a frame to
00:20:af:03:98:28
Ethernet
Netprog 2002 TCP/IP
100
Sending another Ethernet frame
• The Ethernet device driver of Router71 sends the
Ethernet frame to the Ethernet NIC, which transmits
the frame onto the wire.
Netprog 2002 TCP/IP
101
Data has arrived at Neon
•
•
•
•
Neon receives the Ethernet frame
The payload of the Ethernet frame is an IP
datagram which is passed to the IP protocol.
The payload of the IP datagram is a TCP
segment, which is passed to the TCP server
Note: Since the TCP segment is a connection
request (SYN), the TCP protocol does not pass
data to the HTTP program for this packet.
Instead, the TCP protocol at neon will respond
with a SYN segment to Argon.
Neon.cerf.edu
HTTP server
TCP server
IP module
Ethernet
Netprog 2002 TCP/IP
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Wrapping-up the example
•
•
•
So far, Neon has only obtained a single packet
Much more work is required to establish an actual TCP
connection and the transfer of the HTTP Request
The example was simplified in several ways:
No transmission errors
The route between Argon and Neon is short
(only one IP router)
Argon knew how to contact the DNS server (without
routing or address resolution)
….
•
•
•
•
Netprog 2002 TCP/IP
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How many packets were really sent?
tcpdump: listening on fxp0
16:54:51.340712
16:54:51.341749
16:54:51.342539
16:54:51.343436
16:54:51.344147
16:54:51.345220
128.143.137.144.1555 > 128.143.137.11.53:
128.143.137.11.53 > 128.143.137.144.1555:
128.143.137.144.1556 > 128.143.137.11.53:
128.143.137.11.53 > 128.143.137.144.1556:
128.143.137.144.1557 > 128.143.137.11.53:
128.143.137.11.53 > 128.143.137.144.1557:
1+ A? neon.cs. (25)
1 NXDomain* 0/1/0 (98) (DF)
2+ (41)
2 NXDomain* 0/1/0 (109) (DF)
3+ (38)
3* 1/1/2 (122) (DF)
16:54:51.350996 arp who-has 128.143.137.1 tell 128.143.137.144
16:54:51.351614 arp reply 128.143.137.1 is-at 0:e0:f9:23:a8:20
16:54:51.351712 128.143.137.144.1558 > 128.143.71.21.21: S 607568:607568(0) win 8192
<mss 1460> (DF)
16:54:51.352895 128.143.71.21.80 > 128.143.137.144.1558: S 3964010655:3964010655(0)
ack 607569 win 17520 <mss
1460> (DF)
16:54:51.353007 128.143.137.144.1558 > 128.143.71.21.80: . ack 1 win 8760 (DF)
16:54:51.365603 128.143.71.21.80 > 128.143.137.144.1558: P 1:60(59)
ack 1 win 17520 (DF) [tos
0x10]
16:54:51.507399 128.143.137.144.1558 > 128.143.71.21.80: . ack 60 win 8701 (DF)
Netprog 2002 TCP/IP
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