lecture 3: hardware and physical links chap 1.4, 2 of [pd] based partly on lecture notes by xiaowei...
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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
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
IPs V. Ports : Server V. App.
Server has ..34.232.23.99
Client has ..12.32.43.23
Server has ..12.32.43.23
Gmail: 23
Plus: 43
Xbox: 23
Bing: 43
The Internet Google
microsoft
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
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
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
Client-Server Model with TCP
Server–Passive open–Prepares to accept connection, does not
actually establish a connection
Server invokesint 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)
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
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
Client-Server Model with TCP
Client– Application performs active open– It says who it wants to communicate with
Client invokesint 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
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,
int flags)
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
Network architectures
• Layering is an abstraction that captures important aspects of the system, provides service interfaces, and hides implementation details
Protocols
• 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
Layer N
Layer N-1Layer N-1
Layer N+1
Layer N
Layer N-1Layer N-1
Layer N+1
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
The protocol graph of Internet
• 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
Link layer
Network layer
Transport layer
Applicatoin layer
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
Multiplexing & Demultiplexing
• Same ideas apply up and down the protocol graph
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
An Example
• A user on host argon.tcpip-lab.edu (“Argon”) makes
web access to URL
http://neon. tcpip-lab.edu/index.html.
• What actually happens in the network?
A simple TCP/IP Example
argon.tcpip-lab.edu("Argon")
neon.tcpip-lab.edu("Neon")
Web request
Web page
Web client Web server
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
HTTP client
Argon
HTTP server
Neon
HTTP request
HTTP response
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: 192.168.123.144
Connection: Keep-Alive
HTTP Response
HTTP/1.1 200 OK
Date: Sat, 25 May 2002 21:10:32 GMT
Server: Apache/1.3.19 (Unix)
Last-Modified: Sat, 25 May 2002 20:51:33 GMT
ETag: "56497-51-3ceff955"
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="http://www.tcpip-lab.net/index.html">here</a> for the Internet Lab webpage.
</BODY>
</HTML>
• How does the HTTP request get from Argon to Neon?
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
HTTP client
TCP client
Argon
HTTP server
TCP server
Neon
HTTP request / HTTP response
TCP connection
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.
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 128.143.137.144neon.tcpip-lab.edu 128.143.71.21
HTTP client DNS Server
argon.tcpip-lab.edu 128.143.136.15
neon.tcpip-lab.edu
128.143.71.21
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 79telnet 23 http 80smtp 25 nntp 119
Requesting a TCP Connection
• The HTTP client at argon.tcpip-lab.edu requests the TCP client to establish a connection to port 80 of the machine with address 128.141.71.21
HTTP client
TCP client
argon.tcpip-lab.edu
Establish a TCP connectionto port 80 of 128.143.71.21
connect(s, (struct sockaddr*)&sin, sizeof(sin))
Invoking the IP Protocol
• 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 128.143.71.21
• (The data portion of the IP datagram contains the request to open a connection)
TCP client
argon.tcpip-lab.edu
IP
Send an IP datagram to128.143.71.21
ip_output()
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 (128.143.137.1).
Invoking the device driver
• 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
argon.tcpip-lab.edu
IP module
Ethernet
Send an Ethernet frameto 00:e0:f9:23:a8:20
ether_output
The route from Argon to Neon
• Note that the router has a different name for each of its interfaces.
neon.tcpip-lab.edu"Neon"
128.143.71.21
argon.tcpip-lab.edu"Argon"128.143.137.144
router137.tcpip-lab.edu"Router137"
128.143.137.1
router71.tcpip-lab.edu"Router71"128.143.71.1
Ethernet NetworkEthernet Network
Router
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
argon.tcpip-lab.edu128.143.137.14400:a0:24:71:e4:44
IP Datagram for Neon
router137.tcpip-lab.edu128.143.137.100:e0:f9:23:a8:20
Forwarding the IP datagram
• The IP router receives the Ethernet frame at interface 128.143.137.11. recovers the IP datagram2. determines that the IP datagram should be forwarded to the interface
with name 128.143.71.1• The IP router determines that it can deliver the IP datagram directly
neon.tcpip-lab.edu"Neon"
128.143.71.21
argon.tcpip-lab.edu"Argon"128.143.137.144
router137.tcpip-lab.edu"Router137"
128.143.137.1
router71.tcpip-lab.edu"Router71"128.143.71.1
Ethernet NetworkEthernet Network
Router
• The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28
router71.tcpip-lab.edu
IP module
Ethernet
Send a frame to00:20:af:03:98:28
Invoking the Device Driver at the Router
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.
IP Datagram for Neon
neon.tcpip-lab.edu128.143.71.21
00:20:af:03:98:28
router71.tcpip-lab.edu128.143.71.1
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
HTTP server
neon.tcpip-lab.edu
TCP server
IP module
Ethernet
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
Layers, Services, Protocols
Network
Link
Physical
Transport
Application
Service: move bits to other node across link
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
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
Components of a Square Wave
Graphs from Dr. David Alciatore, Colorado State University
Graphs from Dr. David Alciatore, Colorado State University
Approximation of a Square Wave
Idea: Use Carriers• Only use frequencies that transmit well
• Modulate the signal to encode bits
OOK: On-Off Keying
ASK: Amplitude 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
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)
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?
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?
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)
The channel is noisy!
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
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)
0 0 1 0 1 0 1 1 0
NRZ(non-return to zero)
Clock
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
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 1s0 0 1 0 1 0 1 1 0
Clock
NRZI(non-return to zero
intverted)
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
0 0 1 0 1 0 1 1 0
Clock
Manchester
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
4B/5B Table
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
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
Questions so far?
Photo: Lewis Hine
Layers, Services, Protocols
Network
Link
Physical
Transport
Application
Service: move bits to other node across link
Service: move frames to other node across link.May add reliability, medium access control
Service: move packets to any other node in the networkIP: Unreliable, best-effort service model
Service: multiplexing applicationsReliable byte stream to other node (TCP), Unreliable datagram (UDP)
Service: user-facing application.Application-defined messages
Framing
• Given a stream of bits, how can we represent boundaries?
• Break sequence of bits into a frame
• Typically done by network adaptor
Representing Boundaries
Approaches
• Sentinels
• Length counts
• Clock-based
Characteristics
• Bit- or byte oriented
• Fixed or variable length
• Data-dependent or independent
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
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 01111110
• Use bit stuffing to escape 01111110– 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.
Representing Boundaries
Approaches
• Sentinels
• Length counts
• Clock-based
Characteristics
• Bit- or byte oriented
• Fixed or variable length
• Data-dependent or independent
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
Representing Boundaries
Approaches
• Sentinels
• Length counts
• Clock-based
Characteristics
• Bit- or byte oriented
• Fixed or variable length
• Data-dependent or independent
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
Representing Boundaries
Approaches
• Sentinels
• Length counts
• Clock-based
Characteristics
• Bit- or byte oriented
• Fixed or variable length
• Data-dependent or independent
Error Detection
• Basic idea: use a checksum– 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
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