Transcript Introduction - University of Virginia

Higher Layers - for Lab 1
• IP and TCP Headers
• Subnetting
• ifconfig and netstat commands
© Jörg Liebeherr (modified by M. Veeraraghavan)
1
IP Datagram Format
• 20 bytes  Header Size  24 * 32 bit-words = 60 bytes
• 20 bytes  Total Length  216 bytes = 65536 bytes
>= five 32-bit words
version
(4 bits)
header
length
Type of Service/TOS
(8 bits)
Identification (16 bits)
TTL Tim e-to-Live
(8 bits)
Total Length (in bytes)
(16 bits)
flags
(3 bits)
Protocol
(8 bits)
Fragm e nt Offse t (13 bits)
Header Che cksum (16 bits)
Source IP address (32 bits)
Destination IP address (32 bits)
Options (if any, <40 byte s)
DATA
0
31
32-bit w ord
© Jörg Liebeherr (modified by M. Veeraraghavan)
2
TCP Header Format
• TCP packets have a 20 byte header + a data field of  0 bytes
IP header TCP header
20 bytes
TCP data
20 bytes
0
15 16
Source Port Number
31
Destination Port Number
Sequence number (32 bits)
header
length
0
Flags
TCP checksum
20 bytes
Acknowledgement number (32 bits)
window size
urgent pointer
Options (if any)
DATA
© Jörg Liebeherr (modified by M. Veeraraghavan)
3
Internet Address Classes
• IP distinguishes 5 classes of addresses.
Class A
0
network id
host
7 bits
Class B
Class C
Class D
1 0
24 bits
network id
host
14 bits
16 bits
1 1 0
1 1 1 0
network id
host
21 bits
8 bits
multicast group id
28 bits
Class E
1 1 1 1 0
(reserved for future use)
27 bits
© Jörg Liebeherr (modified by M. Veeraraghavan)
4
Subnet Masks
• Routers and hosts use a subnet mask to separate the
subnetwork id from the host id.*
Class B
1 0
network id
host
14 bits
with
subnetting
1 0
16 bits
network id
14 bits
Subnet
mask
(255.255.255.0)
*
subnet id
8 bits
111111111111111111111111
24 bits
host
8 bits
00000000
8 bits
There are different ways of subnetting. Subnetting with mask 255.255.255.00 is
most common.
© Jörg Liebeherr (modified by M. Veeraraghavan)
5
Configuring a Network Interface
• The ifconfig command is used for setting and testing
network interfaces (in Unix systems) that run TCP/IP
• Ifconfig must be run at boot time to initialize the network
interfaces.
• Ifconfig -a plots a report on all interfaces.
: aida:/ ; ifconfig -a
ed1: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,
MULTICAST> mtu 1500
inet 128.238.42.114 netmask 0xffffff00 broadcast
128.238.42.255
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet 127.0.0.1 netmask 0xff000000
© Jörg Liebeherr (modified by M. Veeraraghavan)
6
Netstat command
• The netstat command plots (a lot of) network related data
structures in the operating system.
• Netstat -i plots information on the network interfaces.
: aida:/ ; netstat -i
Name Mtu
Network
Address
Ipkts Ierrs
ed1
1500 128.238.42/24 aida
2394482 35664
lo0
16384 your-net
localhost
153
0
© Jörg Liebeherr (modified by M. Veeraraghavan)
Opkts Oerrs Coll
346880
223 119591
153
0
0
7