Network Theory Part II
ARP
Each network interface card manufactured has a worldwide unique
hardware address. The ARP (address resolution protocol) provides
a mechanism for retrieving the hardware address from an unknown
host based upon the IP address. If the target host is not in the
local network a router is used for the transport of the request.
ARP Request: To whom belongs the address 172.16.231.64 ?
Example:
Request to ALL:
| Transmitter hardware address |
0x00C03D002AD9 |
| Transmitter IP address |
172.16.231.12 |
| Target hardware address |
0xFFFFFFFF |
| Target IP address |
172.16.231.64 |
Response to Transmitter
| Transmitter hardware address |
0x00C03D004A05 |
| Transmitter IP address |
172.16.231.64 |
| Target hardware
address |
0x00C03D002AD9 |
| Target IP address |
172.16.231.12 |
The relation between IP address and physical address is stored in
a table in the NIC and will be deleted after a timeout.
Transport Layer
The IP address has the only purpose to address a host system.
Each host in turn could run multiple applications, e.g. a web browser,
TELNET or FTP. In order to address a certain application, TCP and
UDP provide mechanisms to do so.
For known applications so called well known ports are used. A complete
list of those service could be obtained from:
ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers
or
http://sunsite.auc.dk/RFC
The following is just a short example:
| Application |
Port
|
Protocol |
Description |
| ftp |
21 |
UDP/TCP |
File-Transfer |
| telnet |
23 |
UDP/TCP |
Telnet |
| smtp |
25 |
UDP/TCP |
Simple Mail Transfer |
| domain |
53 |
UDP/TCP |
Domain Name Server |
| tftp |
69 |
UDP/TCP |
Trivial File Transfer |
| www-http |
80 |
UDP/TCP |
Worl Wide Web HTTP |
| sftp |
115 |
UDP/TCP |
Simple File Transfer Protocol |
| snmp |
161 |
UDP/TCP |
SNMP |
Format of UDP
The features of UDP are limited to the recognition of the port
number. The delivery of a datagram via the network is not guaranteed.
The protocol does not guarantee the correct sequence of fragments
via the network. All this has to be done by a higher level layer.
Remark of the author: This is not true with
UDP driver implemented in EC32. This driver has a feature which guarantees
a secure delivery via the network!
On the other hand, UDP has a much lower overhead than TCP
and provides a higher throughput. In addition all mechanisms for
establishing and terminating a connection as they are used with
TCP have not to be used.
| 0 |
16 |
31 |
| UDP Source port |
UDP destination
port |
| UDP message length |
UDP checksum |
| Data |
TCP Transport control protocol
TCP removes the burden of security measures from the higher level
application layers.
TCP is connection oriented, which means that prior to sending any
data a virtual connection between the two end-points has be established.
An end-point is made up by an IP address and a port-number.
The communication is full-duplex, e.g. both partners are allowed to
send at the same time. The protocol is transparent, data delivered
in a certain form to the TCP interface are presented in the same way
at the adjacent side. The packet size is adjustable in wide ranges
of 1 byte to a couple of megabytes as long as the hardware being used
allows this.
Format of a TCP packet
| 0 |
4 |
10 |
16 |
24 |
31
|
|
Source Port
|
Destinationport
|
| Sequence number |
| ACKNOWLEDGMENT
NUMBER |
| HLEN |
RESERVED |
CODE BITS |
WINDOW |
| CHECKSUM |
URGENT POINTER |
|
OPTIONS
|
Padding |
| Data |
| Source port |
port number
of the application of the transmitter |
| destination
port |
port number of the application
of the receiver |
| Sequence
No. |
Offset of the
first data bytes relative to the beginning of the data |
| Acknow. No. |
sequence number to be expected
in the next packet |
| HLEN |
length of the
TCP header in DWORDS (32 bit) |
| Code Bits |
| 1 |
URG |
urgent data, see urgent
pointer |
| 2 |
ACK |
segment contains ackn. |
| 3 |
PSH |
deliver received data immediately |
| 4 |
RST |
reset connection |
| 5 |
SYN |
establish connection and
sync the sequence numbers |
| 6 |
FIN |
no more data, close connection |
|
| Window |
Number of bytes the receiver is
able to handle. |
| Checksum |
Checksum over
the TCP datagram |
| Urgent pointer |
if the flag URG is set, this value
specifies the offset of the first byte after the urgent data |
| Options |
if necessary:
most important: maximum segment size |
 Next page:
Socket Interface 1
|
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