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IP Basics. IP. ICMP. Network. RoutingTables. ARP. Link. Physical. IP Basics. IP encapsulates TCP IP packets travel through many different routers (hops) before reaching it’s destination - PowerPoint PPT Presentation
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IP Basics
Physical
Link
Network
IP
ARP
ICMP
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
R R R1000 b 500 b 500 b 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.
R R R1000 b 500 b 500 b 1000 b
What if frames are lost?
R R250 b 250 b 250 b 250 b1234
ReceiveComputer
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 formatFragmentation Fields
VersionHeaderLength
Type ofService
Packet Length
Flags Fragment OffsetIdentification
Time To Live Protocol Checksum
Source IP Address
Destination IP Address
Options
DATA
32 bitsEachline
Fragmentation and the FrameFig 7.34
4000 byte frame
router
FragId 345FragOffset=0MoreFragments Bit=1Size->1400 byes
FragId 345FragOffset=175MoreFragments Bit=1Size-> 1400 bytes
FragId 345FragOffset=350MoreFragments Bit=0Size->1200 bytes
FragOffset should be multiplied by 8 (8x175=1400)
IP Frame format
VersionHeaderLength
Type ofService
Packet Length
Flags Fragment OffsetIdentification
Time To Live Protocol Checksum
Source IP Address
Destination IP Address
Options
DATA
32 bitsEachline
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
Physical
Link
Network
TransportTCP UDP
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
Transport
Network(IP)
Physical
Network(IP)
Physical
1000
1000
2000
Transport
Network(IP)
Physical
2501
500
2502
2503 2504
(Send 2000 bytes)
(ACK 500 bytes)
TCP handling of fragmentation
Up to destination port
Transport
Network(IP)
Physical
Network(IP)
Physical
1000
1000
2000
Transport
Network(IP)
Physical
2501
500
2502
2503 2504
(Send 2000 bytes)
(ACK 500 bytes)
TCP handling of fragmentation
Up to destination port
Transport
Network(IP)
Physical
Network(IP)
Physical
1000
1000
2000
Transport
Network(IP)
Physical
2501
500
2502
2503 2504
(Send 2000 bytes)
(ACK 500 bytes)
TCP handling of fragmentation
Up to destination port
Transport
Network(IP)
Physical
Network(IP)
Physical
1000
1000
2000
Transport
Network(IP)
Physical
2501
500
2502
2503 2504
(Send 2000 bytes)
(ACK 500 bytes)
TCP handling of fragmentation
Up to destination port
What does the TCP frame look like?
Source Port
Destination Port
Length Checksum
Data
And after TCP is encapsulated in IP?
IP Header IP Trailer
TCP
More TCP Issues
TCP handshake/setup
time
Host A Host B
Ack 0, Syn 1
Ack 1, Syn 0
Ack 1, Syn 1
Ack 1, Syn 0... setup
data
TCP Frame
Source Port Destination Port
Sequence Number
HeaderLength Flags
Acknowledgment Number
Urgent Pointer
Options
DATA
Checksum
Window
Flow Control w/TCP
Data … s=101, a=701
Data … s=201, a=701
Data … s=701, a=301, c=0
Data … s=801, a=301, c=200
Data … s=301, a=901
Data … s=401, a=901
A B
B says STOP
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
• 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)
physical
IP
TCP/UDP
ICMP
ICMP Message types
• Echo Request
• Echo Response
• Time Exceeded
• Destination Unreachable
• Redirect
IP Tunnelling
Transport
(IP)
Physical
Network(IP)
Physical
Appletalk
IntermediateRouters only
See IP
RouterCAN doAT in IP
ReceivingRouter
InsideNetwork
(AT)
ConnectedNetwork
(AT)
Transport
(IP)
Physical
Appletalk
Transport
(IP)
Physical
Appletalk
IP Tunnelling at one end
Physical
Appletalk
Physical
Appletalk
Appletalk to local Appletalk to non-local
ATIP
ATIP
Route to DestinationAs IP
Summary
