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IP Basics
IP
ICMP
Network
ARP
Link
Physical
RoutingTables
IP Basics
• IP encapsulates TCP
• IP packets travel through many different
routers (hops) before reaching it’s destination
• MTU variation at the physical layer requires
IP to fragment the message into smaller units
along the way
• Reassembly is an option at each hop.
• IP does NOT guarantee delivery!
IP Fragmentation
1000 b
R
500 b
500 b
R
250 b
250 b
250 b 250 b
Every link has the potential to dictate adjusting size of frames.
It is possible to reassemble at any point.
1000 b
R
500 b
500 b
R
1000 b
R
R
What if frames are lost?
R
250 b4
250 b3
250 b2 250 b1
R
Receive
Computer
Receive computer will hold the first 2 frames awaiting the
3rd.
After a period of time, a timer expires and IP level passes the
500 bytes up and stops looking for the other pieces.
TCP (NOT IP) then will acknowledge receipt of 500 more
bytes to the sending TCP layer. If the first frame is lost,
NONE are passed up to TCP. More on TCP later.
IP Frame format
Fragmentation Fields
Version
32 bits
Each
line
Header
Length
Type of
Service
Packet Length
Identification
Time To Live
Flags
Fragment Offset
Checksum
Protocol
Source IP Address
Destination IP Address
Options
DATA
Fragmentation and the Frame
Fig 7.34
FragId 345
FragOffset=0
MoreFragments Bit=1
Size->1400 byes
4000 byte
frame
router
FragId 345
FragOffset=175
MoreFragments Bit=1
Size-> 1400 bytes
FragId 345
FragOffset=350
MoreFragments Bit=0
Size->1200 bytes
FragOffset should be multiplied by 8 (8x175=1400)
IP Frame format
Version
32 bits
Each
line
Header
Length
Type of
Service
Packet Length
Identification
Time To Live
Flags
Fragment Offset
Checksum
Protocol
Source IP Address
Destination IP Address
Options
DATA
IP Summary
• Fragmentation results in delivery of frames which
are potentially smaller than the original
transmission.
• Some of the frames can be lost
• If a message is fragmented and frames are lost, all
frames up to the first lost frame are passed up to
the receiving TCP and all subsequent frames are
dropped.
• TCP views this as a stream and is unaware of the
loss of frames. It just accepts the next “n” bytes,
acks the receipt, and waits for subsequent data.
TCP
End To End Delivery
Transport
TCP UDP
Network
Link
Physical
TCP basics
• Connection-oriented
– Sets up the connection prior to data transmission
• SYN and 3-way handshake
– Guarantees delivery of data
• Sender holds a copy of the data for retransmission if
necessary
• Receiver ACKS specific byte positions in the stream so
sender can resend from any byte position
• Encapsulated by IP
• Receiver tells sender it’s receive window size
to limit rate of data arrival (flow control)
Consider How TCP and IP
Work Together
TCP handling of fragmentation
(Send 2000 bytes)
Transport
2000
Up to
destination port
Transport
(ACK 500 bytes)
500
Network(IP)
Network(IP)
1000
2501 2502
1000
Physical
Network(IP)
Physical
2503 2504
Physical
TCP handling of fragmentation
(Send 2000 bytes)
Transport
2000
Up to
destination port
Transport
(ACK 500 bytes)
500
Network(IP)
Network(IP)
1000
2501 2502
1000
Physical
Network(IP)
Physical
2503 2504
Physical
TCP handling of fragmentation
(Send 2000 bytes)
Transport
2000
Up to
destination port
Transport
(ACK 500 bytes)
500
Network(IP)
Network(IP)
1000
2501 2502
1000
Physical
Network(IP)
Physical
2503 2504
Physical
TCP handling of fragmentation
(Send 2000 bytes)
Transport
2000
Up to
destination port
Transport
(ACK 500 bytes)
500
Network(IP)
Network(IP)
1000
2501 2502
1000
Physical
Network(IP)
Physical
2503 2504
Physical
What does the TCP frame
look like?
Data
Source Destination Length Checksum
Port
Port
And after TCP is
encapsulated in IP?
IP Header
IP Trailer
TCP
More TCP Issues
TCP handshake/setup
Host A
Host B
Ack 0, Syn 1
Ack 1, Syn 1
Ack 1, Syn 0
Ack 1, Syn 0
time
.
.
.
setup
data
TCP Frame
Destination Port
Source Port
Sequence Number
Acknowledgment Number
HeaderLength
Flags
Window
Checksum
Urgent Pointer
Options
DATA
Flow Control w/TCP
A
B
Data … s=101, a=701
Data … s=201, a=701
B says STOP
Data … s=701, a=301, c=0
Data … s=801, a=301, c=200
Data … s=301, a=901
Data … s=401, a=901
B says GO
(up to 200)
UDP Issues
UDP basics
•
•
•
•
•
At transport layer as alternative to TCP
No connection establishment
No Guaranteed Delivery
Practically adds nothing to IP
Application must guarantee delivery if
necessary
• TCP and timing is hard for you to
implement at the application layer
• UDP runs faster if delivery is not required
to be error-free.
ICMP
ICMP Basics
TCP/UDP
IP
ICMP
physical
•
•
•
•
•
Lower than IP
A support protocol for routing info/problems
Doesn’t use ports
Used in ping
Frequently used to deny service.. problematic
– ping of death (too large message), and
– denial of service (ping flood)
ICMP Message types
•
•
•
•
•
Echo Request
Echo Response
Time Exceeded
Destination Unreachable
Redirect
IP Tunnelling
Transport
Apple
talk
Transport
(IP)
Apple
talk
(IP)
Network(IP)
Physical
Inside
Network
(AT)
Router
CAN do
AT in IP
Physical
Intermediate
Routers only
See IP
Physical
Receiving
Router
Connected
Network
(AT)
IP Tunnelling at one end
Transport
Apple
talk
(IP)
Appletalk
Appletalk
IP AT
Physical
Physical
Physical
IP AT
Appletalk to local
Appletalk to non-local
Route to Destination
As IP
Summary