requirements connectivity resource sharing support for common services performance
TRANSCRIPT
Requirements
• Connectivity
• Resource Sharing
• Support for Common Services
• Performance
Performance Metrics• Bandwidth (throughput)
– data transmitted per time unit– link versus end-to-end– notation
• KB = 210 bytes• Mbps = 106 bits per second
• Latency (delay)– time to send message from point A to point B– one-way versus round-trip time (RTT)– components
Latency = Propagation + Transmit + QueuePropagation = Distance / cTransmit = Size / Bandwidth
– Examples of RTT: LAN, Cross-country link, Satellite
Delay x Bandwidth Product
• Amount of data “in flight” or “in the pipe”• Example: 100ms x 45Mbps = 560KB
• Why is it important to know Delay x Bandwidth product?
Bandwidth
Delay
Bandwidth versus Latency• Relative importance
– 1-byte: 1ms vs 100ms dominates 1Mbps vs 100Mbps– 25MB: 1Mbps vs 100Mbps dominates 1ms vs 100ms
• Infinite bandwidth– RTT dominates
• Throughput = TransferSize / TransferTime• TransferTime = RTT + TransferSize /Bandwidth
• 1-MB file to 1-Gbps link as 1-KB packet to 1-Mbps link
Network Architecture
• Layering and Protocols
• ISO Architecture
• Internet architecture
Layering• Use abstractions to hide complexity• Abstraction naturally lead to layering• Alternative abstractions at each layer
Request/replychannel
Message streamchannel
Application programs
Hardware
Host-to-host connectivity
Advantages:• Solve small problems vs. monolithic software• Modularity: easily add new servicesDrawback:• May hide important information
Protocols
• Building blocks of a network architecture• Each protocol object has two different interfaces
– service interface: operations on this protocol (SAP)– peer-to-peer interface: messages exchanged with peer
• Term “protocol” is overloaded– specification of peer-to-peer interface (rules)– module that implements this interface
• Protocol stack: set of consecutive layers• Interoperability problems
Host 1
Protocol
Host 2
Protocol
High-level
object
High-levelobject
Service
interface
Peer-to-peer
interface
Interfaces
9
Protocol Machinery• Protocol Graph
– most peer-to-peer communication is indirect– peer-to-peer is direct only at hardware level
Fileapplication
Digitallibrary
application
Videoapplication
RRP MSP
HHP
Host 1
Fileapplication
Digitallibrary
application
Videoapplication
RRP MSP
HHP
Host 2
10
Machinery (cont)• Multiplexing and Demultiplexing (demux
key)• Encapsulation (header/body)
RRP DataHHP
Applicationprogram
Applicationprogram
Host 1 Host 2
Data
RRP
RRP Data
HHP
Data
RRP
RRP Data
HHP
OSI Architecture:Reference Model
Application
Presentation
Session
Transport
End host
One or more nodeswithin the network
Network
Data link
Physical
Network
Data link
Physical
Network
Data link
Physical
Application
Presentation
Session
Transport
End host
Network
Data link
Physical
Physical Layer
• Function: provides a “virtual bit pipe”• How: maps bits into electrical/electromagnetic
signals appropriate for the channel• The physical layer module is called a modem
(modulator/demodulator)• Important issues:
– Timing: synchronous, intermittent synchronous, asynchronous (characters)
– Interfacing the physical layer and DLC (e.g., RS-232, X.21)
Data Link Control Layer (DLC)
• Receives packets from the network layer and transforms then into bits transmitted by the physical layer. Generally guarantees order and correctness.
• Mechanisms of the DLC:– Framing: header, trailer to separate packets, detect errors…– Multiple access schemes: when the link is shared by
several nodes there is a need for addressing and controlling the access (this entity is called MAC sublayer)
– Error detection and retransmission (LLC sublayer)
Network Layer
• Provides naming/addressing, routing, flow control, and scheduling/queuing in a multi-hop network
• Makes decisions based on packet header (e.g., destination address) and module stored information (e.g., routing tables)
• General comment: each layer looks only at its corresponding header (here packet header)
• Routing is different on virtual circuit networks than on datagram networks
Transport Layer
• Provides a reliable mechanism to transmit messages between two end-nodes through:– Message fragmentation into packets– Packets reassembly in original order– Retransmission of lost packets– End-to-end flow control– Congestion control
Session Layer
• Was intended to handle the interaction between two end points in setting up a session:– multiple connections– Service location (e.g., would achieve load sharing)– Control of access rights
• For example, managing an audio and video stream in a teleconferencing application
• In many networks these functionalities are inexistent or spread over other layers
Presentation Layer
• Concerned with the format of the data exchanged
• Provides data encryption, data compression, and code conversion.
Application Layer
• What’s left over…
• Examples:WWW, Email, Telnet, …
Internet Architecture• Defined by Internet Engineering Task Force (IETF)• IETF requires working implementations for standard
adoption• Application vs. Application Protocol (FTP, HTTP)
…
FTP HTTP NV TFTP
TCP UDP
IP
NET1 NET2 NETn
Internet Architecture• Not quite layered
TCP UDP
IP
Network
Application
Implementing Network Software
• Success of the Internet is partially due to:– Minimal functionality within the network– Most of the functionality running is software
over general-purpose computers
• Simple Application Programming Interface
• Efficient protocol implementation
Application Programming Interface
• Each OS can have a special interface exported by the network to the applications developer
• Most widely used network API is: socket interface– Initially developed by the Berkeley Distribution
of Unix and today ported to almost all OS
Creating a Socket
• int socket( int domain, /* PF_INET, PF_UNIX */
int type, /* SOCK_STREAM,
SOCK_DGRAM */
int protocol)
Active Open
• Clients perform an active open operation using the connect routine
• int connect (int socket, struct sockaddr *address, int addr_len)
Passive Open
• Servers perform a passive open– bind, listen, accept
• 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)
Sending and Receiving Data
• Once the connection is established, use send/recv functions to exchange data
• int send (int socket, char *message, int msg_len, int flags)
• int recv (int socket, char *buffer, int buf_len, int flags)
Closing a Socket
• void close (int socket)
Client/Server Sockets
• Client:– socket, connect, (send, recv)*, close
• Server:– socket, bind, listen, (accept, (recv,
send)*, close)*
Exercise
• Write the simplex-talk client and server programs given in Section 1.3.2 of text– Given in C
• Run a client and server on same machine and test
• Run client and server on different machines
• Do Exercises 28, 29, and 30 of text
Protocol Implementation Issues
• The socket API is an interface between the application and the network subsystem (TCP, UDP, IP, etc.)
• Could potentially be used as a protocol-to-protocol interface
• Turns out that the socket interface, while clean, introduces a number of inefficiencies
• Different protocol-to-protocol interfaces designed
Process Models
Process-per-protocol Process-per-message
Context Switches
• Inefficiency of process-per-protocol model:– Too many context switches, as the message is
handed from one protocol to the next
• Context switches become procedure calls in the process-per-message model– Sending a message: high-level protocol calls send on the low-level protocol
– Receiving a message: high-level protocol calls receive on the low-level protocol
Protocol-to-Protocol Interface
• The receive operation replaced by deliver
IP
TCP
send(IP, message) deliver(TCP, message)
Message Buffer Handling• Copying messages between buffers expensive• Shared message abstraction
– Adding and stripping headers– Fragmenting and reassembling messages
send()deliver()
Topmost protocol
Application process
Other Common Library Routines
• Event library: commonly need to schedule an event in the future– Examples: timeout, garbage collection– A common event & event manager abstraction
• Map library: For maintaining bindings between identifiers– Example: Mapping incoming messages to the
connection that will process the message