18. Datagram Transmission
• Datagram sent across conventional network
– From source host and router
– Between intermediate routers
– From final router to destination host
• Network hardware does not recognize
– Datagram format
– IP addresses
• Encapsulation needed
19. IP Encapsulation
• Entire datagram treated like data
• Frame type identifies contents as IP datagram
• Frame destination address gives next hop
20. Frames and Datagrams
• Datagram survives entire trip across
Internet
• Frame only survives one hop
• Frame address
– Hardware (MAC) address
– Next hop
• Datagram address
– IP address
– Ultimate destination
21. Frame Address For
Encapsulated Datagram
• A datagram is encapsulated in a frame for
transmission across a physical network.
• The destination address in the frame is the
address of the next hop to which the
datagram should be sent; the address is
obtained by translating the IP address of
the next hop to an equivalent hardware
address.
22. Window Advertisement
• Each acknowledgment carries new window
information window advertisement
– Can be zero closed window)
• Interpretation:
“ I have received up through X, and can take
Y more octets. ”
24. Startup and Shutdown
• Connection startup
– Must be reliable
• Connection shutdown
– Must be graceful
• Difficult WHY ?
25. Why Startup/Shutdown Difficult?
• Segments can be
–
–
–
–
–
–
Lost
Duplicated
Delayed
Delivered out of order
Either side can crash
Either side can reboot
• Need to avoid duplicate “shutdown”
message from affecting later connection
26. TCP’s Startup/Shutdown Solution
• Uses three-message exchange
• Known as 3-way handshake
• Necessary and sufficient for
– Unambiguous, reliable startup
– Unambiguous, graceful shutdown
• SYN used for startup
• FIN used for shutdown
28. TCP Segment Format (1)
• All TCP segments have same format
–
–
–
–
Data
Acknowledgment
SYN (startup)
FIN (shutdown)
• Segment divided into two parts
– Header
– Payload area (zero or more bytes of data)
29. TCP Segment Format (2)
• Header contains
– Protocol port numbers to identify
• Sending application
• Receiving application
– Bits to specify items such as
• SYN
• FIN
• ACK
– Fields for window advertisement,
acknowledgment, etc.
