1. Networks & Communication (continued)
CS-502 Operating Systems
(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne, Modern Operating Systems, 2nd ed., by Tanenbaum, and Distributed Systems: Principles & Paradigms, 2nd ed. By Tanenbaum and Van Steen)
Assumptions:
Graduate level
Operating Systems
Making Choices about operation systems
Why a micro-century? …just about enough time for one concept
CS-502 Fall 2007
Networks & Communication

2. Principal Abstraction – Socket
Originally created in BSD Unix
Now, part of most operating systems
Allows opening a connection between two processes across network
Connection:
a serial conversation between two end points
e.g., processes, threads, tasks on different machines
organized as a sequence of messages or datagrams
distinct from all other connections
One of the four principal abstractions provided by most modern operating systems
CS-502 Fall 2007
Networks & Communication

3. Networks & Communication
Definition — Protocol
Formal set of rules that govern the formats, contents, and meanings of messages from computer to computer, process to process, etc.
Must be agreed to by all parties to a communication
May be defined in terms of other protocols
CS-502 Fall 2007
Networks & Communication

4. The OSI 7-layer model (in a nutshell)
Physical
Layer
Data Link
Network
Transport
Session
Presentation
Application
Silbershatz, §§
CS-502 Fall 2007
Networks & Communication

5. Annotated OSI 7-Layer Stack
Silbershatz, page 630
CS-502 Fall 2007
Networks & Communication

6. Summary — OSI 7-layer model
Physical
Layer
Data Link
Network
Transport
Session
Presentation
Application
Sending
Process
Receiving
Bits DH
Data DT NH TH SH PH AH
CS-502 Fall 2007
Networks & Communication

7. Networks & Communication
Circular Definition
Socket:
End point of a connection
Usually used in pairs, one for each direction
Comprises [IP Address: Port #]
Connection:
A logical linkage between pairs of sockets at two endpoints for purposes of a particular communication between those endpoints
i.e., a serial conversation between endpoints
Usually two-way
CS-502 Fall 2007
Networks & Communication

8. Networks & Communication
Connection
The backbone of most message-oriented communication protocols
Each party retains knowledge of the other
Each party retains information about state of the other (vis a vis the protocol itself)
Each party “knows” if connection is broken …
Note: some popular protocols are “connection-less”
one side retains no state information about other side
CS-502 Fall 2007
Networks & Communication

9. Establishing a Connection
Process a on machine m creates a socket
OS assigns a new port number q to that socket
Process a attempts to open a connection to machine n:p
p is a well-known port
Process b on machine n is listening on p
Receives request from m:q
Process b forks a process or spawns a thread c to talk with m:q, then resumes listening on p
Thread/process c
Creates a new socket r for this connection
Replies to m:q with return address n:r
a and c continue to communicate over this pair of sockets until they are finished.
CS-502 Fall 2007
Networks & Communication

10. Typical Client-Server Connection
Create socket
On server side
Bind
I.e., connect socket to port # (usually well-known port)
Listen
Sit and wait for a communication to come in
Accept
Create new socket for purpose of responding to this caller
CS-502 Fall 2007
Networks & Communication

11. Networks & Communication
Notes
Responder to request for connection does not have to be the original server machine
Delegate workload to other server systems
Systems often include a connection ID as part of request to open connection
Unique or randomly chosen
Reduces spoofing of server responses
Unix/Linux will not re-use a socket # within 30 seconds
To avoid confusion between old connection and new
CS-502 Fall 2007
Networks & Communication

12. Networks & Communication
Reliable Connections
Transport layer partitions messages into packets
TCP – Transmission Control Protocol
Sequence number of current packet
Sequence number of last packet received correctly
Receiver keeps track of sequence # of packets
Reassembles in right order
Notify sender of missing, broken packets
Sender keeps copy of each packet until receipt acknowledged
Retransmits packets if no acknowledgement
Window defines how many packet buffers to maintain for efficient transmission
Allows many packets in “flight”
CS-502 Fall 2007
Networks & Communication

13. Reliable Connections (continued)
Packet i
Packet i+1
Packet i+2
Packet i+3 …
rec’d i
Packet i+k
time
CS-502 Fall 2007
Networks & Communication

14. Reliable Connections (continued)
Packet i
Packet i+1
Packet i+2
Packet i+3 …
rec’d i
rec’d i
Packet i+k
time
CS-502 Fall 2007
Networks & Communication

15. Reliable Connections (continued)
Packet i
Packet i+1
Packet i+2
Packet i+3 …
rec’d i
rec’d i
Packet i+k
rec’d i+2
time
CS-502 Fall 2007
Networks & Communication

16. Reliable Connections (continued)
Packet i
Packet i+1
Packet i+2
Packet i+3 …
rec’d i
lost
rec’d i
Packet i+k
rec’d i+2 …
rec’d i+2
time
CS-502 Fall 2007
Networks & Communication

17. Reliable Connections (continued)
If acknowledgement received for packet i
Delete from buffer all packets  i
If no acknowledgement received within a reasonable time for packet k
Retransmit from buffer all packets  k
Result
Recovers from loss of packets
Recovers from loss of acknowledgements
Works well for reasonably reliable internet
Doesn’t work so well for noisy, unreliable networks
CS-502 Fall 2007
Networks & Communication

18. Networks & Communication
Reminder
How do we know if a packet is received correctly?
Cyclic Redundancy Check (CRC)
Polynomial computed from packet header and body
Usually 16 or 32 bits, computed by hardware
Appended to message
Recomputed on reception, compared with transmitted CRC
Equal  packet received correctly
CS-502 Fall 2007
Networks & Communication

19. Examples of Connection-based Protocols
Telnet (virtual terminal)
2-way communication by character stream
Line-by-line organization
SMTP (Simple Mail Transport Protocol)
For sending mail
Layered on top of telnet protocol
POP (Post Office Protocol)
For receiving your mail
FTP (File Transfer Protocol)
For transmitting ASCII or binary files
Binary data transmission not layered on telnet protocol …
CS-502 Fall 2007
Networks & Communication

20. Connection-less communication
Some communication protocols don’t need the overhead of reliable connections
When some number of errors can be tolerated
Where recovery from those errors is easy
UDP – User Datagram Protocol
The internet connection-less protocol (layer 4)
Breaks messages into packets
Reassembles at destination
Messages delivered completely or not at all
Does not send acknowledgement of correct receipt
CS-502 Fall 2007
Networks & Communication

21. Networks & Communication
Examples
HTTP (HyperText Transport Protocol)
Web server responds directly to requests
If client does not get response, retries request
NFS (Network File System)
For access to files on servers as if they are local
RPC (Remote Procedure Call)
Next topic …
CS-502 Fall 2007
Networks & Communication

22. Networks & Communication
Summary
Socket, connection
Network stack, 7-layer model
Establishing a connection
Reliable transmission
Reading assignment
Silbershatz Chapter 16
CS-502 Fall 2007
Networks & Communication