Networks & Communications CS-4513, D-Term 20071 Networks & Communication (continued) CS-4513 Distributed Computing Systems (Slides include materials from

Networks & Communications

CS-4513, D-Term 2007 1

Networks & Communication(continued)

CS-4513Distributed Computing 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)


CS-4513, D-Term 2007 2

Review — OSI 7-layer model

Physical Layer

Data LinkLayer

NetworkLayer

TransportLayer

SessionLayer

PresentationLayer

ApplicationLayer

SendingProcess

Physical Layer

Data LinkLayer

NetworkLayer

TransportLayer

SessionLayer

PresentationLayer

ApplicationLayer

ReceivingProcess

Bits

DH Data DT

NH Data

TH Data

SH Data

PH Data

AH Data

Data


CS-4513, D-Term 2007 3

Layered Protocols

• OSI 7-layer model was intended to be a foundation of a family of international standard protocols

• Those protocols never gained much acceptance

• Role of Session and Presentation layers is murky, at best.

• Internet protocols (TCP/IP, etc.) are dominant


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The TCP/IP Protocol LayersTCP/IP


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The TCP/IP Protocol LayersTCP/IP

Defined by manufacturers, industry sub-groups, and separate standards bodies

Subsumed by middleware


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Modified Layers


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Examples of Middleware

• Authentication protocols

• Commit protocols for atomic transactions

• Multimedia protocols

• Remote procedure protocols

• …


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Styles of Communication

• Message-oriented

• Remote Procedure Call

• Streaming


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Some Terms

• Packet:– A unit of communication at Data Link layer– aka datagram

• IP Address:– A four-part “number” used by Network Layer to route a packet

from one computer to another

• Port:– A 16-bit number used within one computer to identify who/where

to send packet to

• Well-known port:– A port with number < 1024, used by agreement for standard

services – e.g.,• telnet (23), ftp (21), smtp (25), pop (110)


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More Terms

• Socket:– End point of a communication– 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


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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 has no state information about other side


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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.


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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


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Notes

• Responder to request for connection does not have to be the original server machine

• Delegate workload to other server systems

• Some systems 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


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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 seq. # 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”


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Reliable Connections (continued)

Packet i

Packet i+1

Packet i+2

Packet i+3

Packet i+k

…

time

rec’d i


CS-4513, D-Term 2007 17


Packet i

Packet i+1

Packet i+2

Packet i+3

Packet i+k

…

time

rec’d i

rec’d i


CS-4513, D-Term 2007 18


Packet i

Packet i+1

Packet i+2

Packet i+3

Packet i+k

…

time

rec’d i

rec’d i

rec’d i+2


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Packet i

Packet i+1

Packet i+2

Packet i+3

Packet i+k

…

time

rec’d i

rec’d i

rec’d i+2

rec’d i+2

…

lost


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• 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


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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


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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– Layered on top of telnet protocol

• FTP (File Transfer Protocol)– For transmitting ASCII or binary files– Binary data transmission not layered on telnet protocol

• …


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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

– Does not send acknowledgement of correct receipt


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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– If client does not get response, retries request

• RPC (Remote Procedure Call)– Next topic

• …


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Reading Assignment

• Silbershatz – rest of Chapter 16

• Tanenbaum & Van Steen – §4.1-§4.3

• Tanenbaum (Modern Operating System):• This topic is not covered


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Break

Documents

Networks & Communications CS-4513, D-Term 20071 Networks & Communication (continued) CS-4513 Distributed Computing Systems (Slides include materials from