1. Lecture 3: Hardware and physical links Chap 1.4, 2 of [PD]
Based partly on lecture notes by Xiaowei Yang, Rodrigo Fonseca, David Mazières, Phil Levis, John Jannotti

2. Overview Sockets Programming Revisited Network Architectures
Examples of Networking Principles
Hardware and physical layer
Nuts and bolts of networking
Nodes
Links
Bandwidth, latency, throughput, delay-bandwidth product
Physical links

3. IPs V. Ports : Server V. App.
Plus: 43
Server has ..
Gmail: 23
The
Internet
Google
Client has ..
Bing: 43
Server has ..
Xbox: 23
microsoft

4. The University of Adelaide, School of Computer Science
21 April 2017
Socket
What is a socket?
The point where a local application process attaches to the network
An interface between an application and the network
An application creates the socket
The interface defines operations for
Creating a socket
Attaching a socket to the network
Sending and receiving messages through the socket
Closing the socket
Chapter 2 — Instructions: Language of the Computer

5. The University of Adelaide, School of Computer Science
21 April 2017
Creating a Socket
int sockfd = socket(address_family, type, protocol);
The socket number returned is the socket descriptor for the newly created socket
int sockfd = socket (PF_INET, SOCK_STREAM, 0);
int sockfd = socket (PF_INET, SOCK_DGRAM, 0);
The combination of PF_INET and SOCK_STREAM implies TCP
Chapter 2 — Instructions: Language of the Computer

6. The University of Adelaide, School of Computer Science
21 April 2017
Socket
Socket Family
PF_INET denotes the Internet family
PF_UNIX denotes the Unix pipe facility
PF_PACKET denotes direct access to the network interface (i.e., it bypasses the TCP/IP protocol stack)
Socket Type
SOCK_STREAM is used to denote a byte stream
SOCK_DGRAM is an alternative that denotes a message oriented service, such as that provided by UDP
Chapter 2 — Instructions: Language of the Computer

7. Client-Server Model with TCP
The University of Adelaide, School of Computer Science
21 April 2017
Client-Server Model with TCP
Server
Passive open
Prepares to accept connection, does not actually establish a connection
Server invokes
int bind (int socket, struct sockaddr *address,
int addr_len)
int listen (int socket, int backlog)
int accept (int socket, struct sockaddr *address,
int *addr_len)
Chapter 2 — Instructions: Language of the Computer

8. Client-Server Model with TCP
The University of Adelaide, School of Computer Science
21 April 2017
Client-Server Model with TCP
Bind
Binds the newly created socket to the specified address i.e. the network address of the local participant (the server)
Address is a data structure which combines IP and port
Listen
Defines how many connections can be pending on the specified socket
Chapter 2 — Instructions: Language of the Computer

9. Client-Server Model with TCP
The University of Adelaide, School of Computer Science
21 April 2017
Client-Server Model with TCP
Accept
Carries out the passive open
Blocking operation
Does not return until a remote participant has established a connection
When it does, it returns a new socket that corresponds to the new established connection and the address argument contains the remote participant’s address
Chapter 2 — Instructions: Language of the Computer

10. Client-Server Model with TCP
The University of Adelaide, School of Computer Science
21 April 2017
Client-Server Model with TCP
Client
Application performs active open
It says who it wants to communicate with
Client invokes
int connect (int socket, struct sockaddr *address, int addr_len)
Connect
Does not return until TCP has successfully established a connection at which application is free to begin sending data
Address contains remote machine’s address
Chapter 2 — Instructions: Language of the Computer

11. Client-Serve Model with TCP
The University of Adelaide, School of Computer Science
21 April 2017
Client-Serve Model with TCP
In practice
The client usually specifies only remote participant’s address and let’s the system fill in the local information
Whereas a server usually listens for messages on a well-known port
A client does not care which port it uses for itself, the OS simply selects an unused one
Chapter 2 — Instructions: Language of the Computer

12. Client-Server Model with TCP
The University of Adelaide, School of Computer Science
21 April 2017
Client-Server Model with TCP
Once a connection is established, the application process invokes two operation
int send (int socket, char *msg, int msg_len,
int flags)
int recv (int socket, char *buff, int buff_len,
Chapter 2 — Instructions: Language of the Computer

13. Overview Sockets Programming Revisited Network Architectures
Examples of Networking Principles
Hardware and physical layer
Nuts and bolts of networking
Nodes
Links
Bandwidth, latency, throughput, delay-bandwidth product
Physical links

14. Network architectures
Layering is an abstraction that captures important aspects of the system, provides service interfaces, and hides implementation details

15. Protocols
Layer N
Layer N-1
Layer N+1
Layer N+1
Layer N
Layer N-1
The abstract objects that make up the layers of a network system are called protocols
Each protocol defines two different interfaces
Service interface
Peer interface

16. Network architectures
A protocol graph represents protocols that make up a system
Nodes are protocols
Links are depend-on relations
Set of rules governing the form and content of a protocol graph are called a network architecture
Standard bodies such as IETF govern procedures for introducing, validating, and approving protocols

17. The protocol graph of Internet
Applicatoin layer
Transport layer
Network layer
Link layer
No strict layering. One can do cross-layer design
Hourglass shaped: IP defines a common method for exchanging packets among different networks
To propose a new protocol, one must produce both a spec and one/two implementations

18. Functions of the Layers
Data Link Layer:
Service: Reliable transfer of frames over a link Media Access Control on a LAN
Functions: Framing, media access control, error checking
Network Layer:
Service: Move packets from source host to destination host
Functions: Routing, addressing
Transport Layer:
Service: Delivery of data between hosts
Functions: Connection establishment/termination, error control, flow control, congestion control
Application Layer:
Service: Application specific (delivery of , retrieval of HTML documents, reliable transfer of file)
Functions: Application specific

19. The Open Systems Interconnection (OSI) architecture
Seven-layer

20. Physical layer: handles raw bits
International Telecommunications Union (ITU) publishes protocol specs based on the OSI reference model
X dot series
Physical layer: handles raw bits
Data link layer: aggregate bits to frames. Network adaptors implement it
Network layer: handles host-to-host packet delivery. Data units are called packets
Transport: implements process channel. Data units are called messages
Session layer: handles multiple transport streams belong to the same applications
Presentation layer: data format, e.g., integer format, ASCII string or not
Application layer: application specific protocols

21. Encapsulation Upper layer sends a message using the service interface
A header, a small data structure, to add information for peer-to-peer communication, is attached to the front message
Sometimes a trailer is added to the end
Message is called payload or data
This process is called encapsulation

23. Multiplexing & Demultiplexing
Same ideas apply up and down the protocol graph

24. Overview Sockets Programming Revisited Network Architectures
Examples of Networking Principles
Hardware and physical layer
Nuts and bolts of networking
Nodes
Links
Bandwidth, latency, throughput, delay-bandwidth product
Physical links

25. An Example

26. A simple TCP/IP Example
argon.tcpip-lab.edu
("Argon")
neon.tcpip-lab.edu
("Neon")
Web request
Web page
Web client
Web server
A user on host argon.tcpip-lab.edu (“Argon”) makes web access to URL
tcpip-lab.edu/index.html.
What actually happens in the network?

27. HTTP Request and HTTP response
Web server runs an HTTP server program
HTTP client Web browser runs an HTTP client program
sends an HTTP request to HTTP server
HTTP server responds with HTTP response

28. HTTP Request GET /example.html HTTP/1.1 Accept: image/gif, */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0
Host:
Connection: Keep-Alive

29. HTTP Response How does the HTTP request get from Argon to Neon?
HTTP/ OK
Date: Sat, 25 May :10:32 GMT
Server: Apache/ (Unix)
Last-Modified: Sat, 25 May :51:33 GMT
ETag: " ceff955"
Accept-Ranges: bytes
Content-Length: 81
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
<HTML>
<BODY>
<H1>Internet Lab</H1>
Click <a href=" for the Internet Lab webpage.
</BODY>
</HTML>
How does the HTTP request get from Argon to Neon?

30. From HTTP to TCP
To send request, HTTP client program establishes an TCP connection to the HTTP server Neon.
The HTTP server at Neon has a TCP server running

31. Resolving hostnames and port numbers
Since TCP does not work with hostnames and also would not know how to find the HTTP server program at Neon, two things must happen:
1. The name “neon.tcpip-lab.edu” must be translated into a 32-bit IP address.
2. The HTTP server at Neon must be identified by a 16-bit port number.

32. Translating a hostname into an IP address
The translation of the hostname neon.tcpip-lab.edu into an IP address is done via a database lookup
gethostbyname(host)
The distributed database used is called the Domain Name System (DNS)
All machines on the Internet have an IP address: argon.tcpip-lab.edu neon.tcpip-lab.edu

33. Finding the port number
Note: Most services on the Internet are reachable via well-known ports. E.g. All HTTP servers on the Internet can be reached at port number “80”.
So: Argon simply knows the port number of the HTTP server at a remote machine.
On most Unix systems, the well-known ports are listed in a file with name /etc/services. The well-known port numbers of some of the most popular services are:
ftp 21 finger 79
telnet 23 http 80
smtp 25 nntp 119

34. Requesting a TCP Connection
connect(s, (struct sockaddr*)&sin, sizeof(sin))
The HTTP client at argon.tcpip-lab.edu requests the TCP client to establish a connection to port 80 of the machine with address

35. Invoking the IP Protocol
ip_output()
The TCP client at Argon sends a request to establish a connection to port 80 at Neon
This is done by asking its local IP module to send an IP datagram to
(The data portion of the IP datagram contains the request to open a connection)

36. Sending the IP datagram to the default router
Argon sends the IP datagram to its default router
The default gateway is an IP router
The default gateway for Argon is Router137.tcpip-lab.edu ( ).

37. Invoking the device driver
ether_output
The IP module at Argon, tells its Ethernet device driver to send an Ethernet frame to address 00:e0:f9:23:a8:20
Ethernet address of the default router is found out via ARP

38. The route from Argon to Neon
Note that the router has a different name for each of its interfaces.

39. Sending an Ethernet frame
The Ethernet device driver of Argon sends the Ethernet frame to the Ethernet network interface card (NIC)
The NIC sends the frame onto the wire

40. Forwarding the IP datagram
The IP router receives the Ethernet frame at interface
recovers the IP datagram
determines that the IP datagram should be forwarded to the interface with name
The IP router determines that it can deliver the IP datagram directly

41. Invoking the Device Driver at the Router
The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28

42. Sending another Ethernet frame
The Ethernet device driver of Router71 sends the Ethernet frame to the Ethernet NIC, which transmits the frame onto the wire.

43. Data has arrived at Neon
Neon receives the Ethernet frame
The payload of the Ethernet frame is an IP datagram which is passed to the IP protocol.
The payload of the IP datagram is a TCP segment, which is passed to the TCP server

44. Overview Sockets Programming Revisited Network Architectures
Examples of Networking Principles
Hardware and physical layer
Nuts and bolts of networking
Nodes
Links
Bandwidth, latency, throughput, delay-bandwidth product
Physical links

45. Layers, Services, Protocols
Application
Transport
Network
Link
Physical
Service: move bits to other node across link

46. Physical Layer (Layer 1)
Responsible for specifying the physical medium
Type of cable, fiber, wireless frequency
Responsible for specifying the signal (modulation)
Transmitter varies something (amplitude, frequency, phase)
Receiver samples, recovers signal
Responsible for specifying the bits (encoding)
Bits above physical layer -> chips

47. Modulation
Specifies mapping between digital signal and some variation in analog signal
Why not just a square wave (1v=1; 0v=0)?
Not square when bandwidth limited
Bandwidth – frequencies that a channel propagates well
Signals consist of many frequency components
Attenuation and delay frequency-dependent

48. Components of a Square Wave
Graphs from Dr. David Alciatore, Colorado State University

49. Approximation of a Square Wave
Graphs from Dr. David Alciatore, Colorado State University

50. ASK: Amplitude Shift Keying
Idea: Use Carriers
Only use frequencies that transmit well
Modulate the signal to encode bits
OOK: On-Off Keying
ASK: Amplitude Shift Keying

51. FSK: Frequency Shift Keying PSK: Phase Shift Keying
Idea: Use Carriers
Only use frequencies that transmit well
Modulate the signal to encode bits
FSK: Frequency Shift Keying
PSK: Phase Shift Keying

52. How Fast Can You Send?
Encode information in some varying characteristic of the signal.
If B is the maximum frequency of the signal
C = 2B bits/s
(Nyquist, 1928)
Intuition: 2B comes from Nyquist’s

53. Can we do better?
So we can only change 2B/second, what if we encode more bits per sample?
Baud is the frequency of changes to the physical channel
Not the same thing as bits!
Suppose channel passes 1KHz to 2KHz
1 bit per sample: alternate between 1KHz and 2KHz
2 bits per sample: send one of 1, 1.33, 1.66, or 2KHz
Or send at different amplitudes: A/4, A/2, 3A/4, A
n bits: choose among 2n frequencies!
What is the capacity if you can distinguish M levels?

54. Example
Amplitude
Phase
Used for some forms of digital TV

55. Hartley’s Law
C = 2B log2(M) bits/s Great. By increasing M, we can have as large a capacity as we want! Or can we?

56. The channel is noisy!

57. The channel is noisy!
Noise prevents you from increasing M arbitrarily!
This depends on the signal/noise ratio (S/N)
Shannon: C = B log2(1 + S/N)
C is the channel capacity in bits/second
B is the bandwidth of the channel in Hz
S and N are average signal and noise power
Signal-to-noise ratio is measured in dB = 10log10(S/N)

58. Putting it all together
Noise limits M!
2B log2(M) ≤ B log2(1 + S/N)
M ≤ √1+S/N
Example: Telephone Line
3KHz b/w, 30dB S/N = 10ˆ(30/10) = 1000
C = 3KHz log2(1001) ≈ 30Kbps
From Hartley and Shannon, we have ….

59. Encoding
Now assume that we can somehow modulate a signal: receiver can decode our binary stream
How do we encode binary data onto signals?
One approach: 1 as high, 0 as low!
Called Non-return to Zero (NRZ) 1
NRZ
(non-return to zero)
Clock
Low is not necessarily 0V, (or rest), could be -1, for example; hence the name.
Assume we can change the value once per clock cycle.

60. Drawbacks of NRZ No signal could be interpreted as 0 (or vice-versa)
Consecutive 1s or 0s are problematic
Baseline wander problem
How do you set the threshold?
Could compare to average, but average may drift
Clock recovery problem
For long runs of no change, could miscount periods

61. Alternative Encodings
Non-return to Zero Inverted (NRZI)
Encode 1 with transition from current signal
Encode 0 by staying at the same level
At least solve problem of consecutive 1s 1
Clock
NRZI
(non-return to zero
intverted)

62. Manchester Map 0  chips 01 Maps 1  chips 10
Transmission rate now 1 bit per two clock cycles
Solves clock recovery, baseline wander
But cuts transmission rate in half 1
Clock
Manchester
50% efficiency
Maps transition from

63. 4B/5B Can we have a more efficient encoding?
Every 4 bits encoded as 5 chips
Need 16 5-bit codes:
selected to have no more than one leading 0 and no more than two trailing 0s
Never get more than 3 consecutive 0s
Transmit chips using NRZI
Other codes used for other purposes
E.g., 11111: line idle; 00100: halt
Achieves 80% efficiency

64. 4B/5B Table

65. Encoding Goals DC Balancing (same number of 0 and 1 chips)
Clock synchronization
Can recover some chip errors
Constrain analog signal patterns to make signal more robust
Want near channel capacity with negligible errors
Shannon says it’s possible, doesn’t tell us how
Codes can get computationally expensive
In practice
More complex encoding: fewer bps, more robust
Less complex encoding: more bps, less robust

66. Last Example: 802.15.4 Standard for low-power, low-rate wireless PANs
Must tolerate high chip error rates
Uses a 4B/32B bit-to-chip encoding

67. Questions so far?
Photo: Lewis Hine

68. Layers, Services, Protocols
Service: user-facing application.
Application-defined messages
Application
Service: multiplexing applications
Reliable byte stream to other node (TCP),
Unreliable datagram (UDP)
Transport
Service: move packets to any other node in the network
IP: Unreliable, best-effort service model
Network
Service: move frames to other node across link.
May add reliability, medium access control
Link
Physical
Service: move bits to other node across link

69. Framing Given a stream of bits, how can we represent boundaries?
Break sequence of bits into a frame
Typically done by network adaptor

70. Representing Boundaries
Approaches
Characteristics
Sentinels
Length counts
Clock-based
Bit- or byte oriented
Fixed or variable length
Data-dependent or independent

71. Sentinel-based Framing
Byte-oriented protocols (e.g. BISYNC, PPP)
Place special bytes (SOH, ETX,…) in the beginning, end of messages
What if ETX appears in the body?
Escape ETX byte by prefixing DEL byte
Escape DEL byte by prefixing DEL byte
Technique known as character stuffing

72. Bit-Oriented Protocols
View message as a stream of bits, not bytes
Can use sentinel approach as well (e.g., HDLC)
HDLC begin/end sequence
Use bit stuffing to escape
Always append 0 after five consecutive 1s in data
After five 1s, receiver uses next two bits to decide if stuffed, end of frame, or error.
USB uses bit stuffing for a different reason: it uses NRZI with 0:transition, 1:stay => 1’s are a problem. USB stuffs a 0 after every six consecutive 1’s
Go through 4 cases.

73. Representing Boundaries
Approaches
Characteristics
Sentinels
Length counts
Clock-based
Bit- or byte oriented
Fixed or variable length
Data-dependent or independent

74. Length-based Framing Drawback of sentinel techniques
Length of frame depends on data
Alternative: put length in header (e.g., DDCMP)
Danger: Framing Errors
What if high bit of counter gets corrupted?
Adds 8K to length of frame, may lose many frames
CRC checksum helps detect error
DDCMP -> DECNet point-to-point, original VAX, PDP-11
Variable length.

75. Representing Boundaries
Approaches
Characteristics
Sentinels
Length counts
Clock-based
Bit- or byte oriented
Fixed or variable length
Data-dependent or independent

76. Clock-based Framing E.g., SONET (Synchronous Optical Network)
Each frame is 125μs long
Look for header every 125μs
Encode with NRZ, but first XOR payload with 127-bit string to ensure lots of transitions
Fixed length
Must have clock synch and frame synch

77. Representing Boundaries
Approaches
Characteristics
Sentinels
Length counts
Clock-based
Bit- or byte oriented
Fixed or variable length
Data-dependent or independent

78. Error Detection Basic idea: use a checksum Good checksum algorithms
Compute small checksum value, like a hash of packet
Good checksum algorithms
Want several properties, e.g., detect any single-bit error
Details in a later lecture

79. Summary Network architectures Application Programming Interface
Hardware and physical layer
Nuts and bolts of networking
Nodes
Links
Bandwidth, latency, throughput, delay-bandwidth product
Physical links