Network Applications

Question 1

The TCP/IP Protocol Stack

Answer 1

• Internet functionality is split among different layers
• Header encapsulation used to achieve separation of concerns between each layer
• Advantages
  
  • Helps manage complexity through modularity
• Disadvantages
  
  • Performance overheads?
  • Redundant functionality?

Question 2

Communicating processes

Answer 2

Question 3

Sockets: The interface between Processes and Transport Protocols

Answer 3

• The OS provides a so-called “socket” interface to its networking subsystem
• Processes send/receive messages to/from sockets
• Client process: Initiates communication
• Server process: Waits to be contacted

Question 4

Identifying processes remotely

Answer 4

• A process is addressed over the network using two identifiers:
  – IP address
  
  • Network Layer identifier
  • 32 bits (IPv4) or 64 bits (IPv6)
    – Port number
  • Transport Layer identifier (16 bits)
• A “5 tuple” uniquely identifies traffic between hosts:
  – Two IP addresses, two port numbers, and the underlying transport protocol (e.g., TCP or UDP)
  
  • example:
    <206.62.226.35, 21, 198.69.10.2, 1500, TCP>
• example:
  <206.62.226.35, 21, 198.69.10.2, 1499, UDP>

Question 5

Why do we need both IP addresses and port numbers?

Answer 5

• Many processes may be running on the same host
• IP address is used by routers to forward messages to the correct host
• Then, the host’s OS uses the port number to forward messages to the correct target process
  
  • Example: Request for a Web service
    
    • IP address: 128.2.194.242
    • Port: 80

Question 6

Knowing What Port Number To Use

Answer 6

• Popular applications have well-known ports
  
  • E.g., port 80 for Web and port 25 for e-mail
• Well-known ports vs. ephemeral ports
  
  • Typically servers have a well-known port (e.g., port 80)
    
    • In range 0-1023 (requires root privileges to use)
  • Client picks an unused ephemeral (i.e., temporary) port
    
    • In range 1024 - 65535

Question 7

UNIX’s Socket API

Answer 7

• In UNIX, almost every input and output, physical and virtual, is made to look like a file
  
  • FIles are represented by integer file descriptors
  • All input is like reading a file; all output is like writing a file
• So a socket is like a file
  
  • E.g., ‘standard’ system calls are used - like send(), recv(), close()
  • Plus some other socket-specific calls

Question 8

Application Layer Protocols

Answer 8

• ALP’s are defined by
  
  • Types of messages employed (e.g., request, response, add-new-user, …)
  • Message syntax and semantics
    
    • What fields messages have & how they are delineated
    • Meaning of information in fields
  • When and how processes, send & respond to messages
• Open protocols (e.g., HTTP, SMTP)
  
  • Defined in RFCs
  • Allow for interoperability
• Proprietary protocols (e.g., Skype, AppleTalk)
  
  • Tied to specific products

Question 9

What might an application (or ALP) need from a transport service?

Answer 9

• Data integrity
  – Some apps require 100% reliable data transfer (e.g. file transfer, web transactions)
  – Other apps (e.g., audio) can tolerate some loss
• Throughput
  – Some apps require a minimum level of throughput (e.g. multimedia)
  – Other apps can make use of whatever throughput they get (“elastic apps”)
• Timing
  – Some apps (e.g., Internet telephony, interactive games) require low delay
• Security
  – Encryption, data integrity, …

Question 10

Transport service requirements of popular applications

Answer 10

Question 11

Internet transport protocols (TCP service)

Answer 11

• Reliable transport between sending and receiving process
  
  • All sent messages are delivered in correct order
• Flow control: sender can’t overwhelm receiver
  -+ Congestion control: sender is throttled when network is overloaded
• Connection-oriented: initial setup is required between client and server processes
• Does not provide: timeliness, minimum throughput guarantee, security

Question 12

Internet transport protocols (UDP service)

Answer 12

• Unreliable data transfer between sending and receiving process
• Does not provide: reliability in order delivery, flow control, congestion control, timeliness, throughput guarantee, security, or connection setup
• But timeliness may be better than TCP; and less overhead may be incurred for both hosts and the network

Question 13

Internet apps: application, transport protocols

Answer 13

Question 14

Socket programming with UDP

Answer 14

• No “connection” between client & server
  – Sender explicitly attaches IP destination address and port # to each packet
  – Receiver extracts sender IP address and port number from received packet
  – Messages are called “datagrams”

Question 15

Python example: UDP echo client

Answer 15

Question 16

Python example: UDP echo server

Answer 16

Question 17

Socket programming with TCP

Answer 17

• Recall: TCP provides reliable, in-order byte-stream transfer (“pipe”) between client and server
• TCP service is connection oriented:
  – When client application creates a socket, the client’s TCP instance establishes a connection to the server’s TCP instance
• When contacted by client, server TCP creates new socket for server process to communicate with that particular client

Question 18

Python example: TCP echo client

Answer 18

Question 19

Python example: TCP echo server

Answer 19

Question 20

What about security?

Answer 20

• TCP & UDP have no native encryption
  – e.g., clear-text passwords sent into a socket traverse Internet as clear-text
• SSL – Secure Socket Layer
  – Provides encrypted TCP connections
  – Also provides end-point authentication
• SSL is at application layer
  – applications use SSL libraries that “talk” to TCP
• Now superseded by Transport Layer Security (TLS)