Part 2. Network and Distributed Systems

distributed system

is a collection of loosely coupled nodes interconnected by a communications network

• processors

• computers

• machines

• hosts

Nodes variously called

Site

is location of the machine

node

refers to specific system

server

has a resource a client node at a different site wants to use

• client-server

• peer-to-peer

• hybrid configuration.

Nodes may exist in a

client-server configuration

server has a resource that a client would like to use

peer-to-peer configuration

each node shares equal responsibilities and can act as both clients and servers

message passing

Communication over a network occurs through ?

standalone system

All higher-level functions of a ______________ can be expanded to encompass a distributed system

Resource sharing

• Sharing files or printing at remote sites

• Processing information in a distributed database

• Using remote specialized hardware devices such as graphics processing units (GPUs)

Computation speedup

Distribute subcomputations among various sites to run concurrently

Load balancing

moving jobs to more lightly-loaded sites

Reliability

Detect and recover from site failure, function transfer, reintegrate failed site

Local-Area Network (LAN)

designed to cover small geographical area

Local-Area Network (LAN)

Consists of multiple computers (workstations, laptops, mobile devices), peripherals (printers, storage arrays), and routers providing access to other networks

Ethernet or Wireless (WiFi)

most common way to construct LANs

Wide-Area Network (WAN)

links geographically separated sites • Point-to-point connections via links

Wide-Area Network (WAN)

• Point-to-point connections via links

• Telephone lines, leased (dedicated data) lines, optical cable, microwave links, radio waves, and satellite channels

routers

Implemented via ______ to direct traffic from one network to another

Internet (World Wide Web) WAN

enables hosts world wide to communicate

40-100Gbps

Many backbone providers have speeds at

Local Internet Service Providers (ISPs)

may be slower

Cell phones

use radio waves to cell towers

Domain name system (DNS)

specifies the naming structure of the hosts, as well as name to address resolution (Internet)

Layer 1: Physical layer

Layer 2: Data-link layer

Layer 3: Network layer

Layer 4: Transport layer

Layer 5: Session layer

Layer 6: Presentation layer

Layer 7: Application layer

The communication network is partitioned into the following multiple layers:

Layer 1: Physical layer

handles the mechanical and electrical details of the physical transmission of a bit stream

Layer 2: Data-link layer

handles the frames, or fixed-length parts of packets, including any error detection and recovery that occurred in the physical layer

Layer 3: Network layer

provides connections and routes packets in the communication network, including handling the address of outgoing packets, decoding the address of incoming packets, and maintaining routing information for proper response to changing load levels

Layer 4: Transport layer

responsible for low-level network access and for message transfer between clients, including partitioning messages into packets, maintaining packet order, controlling flow, and generating physical addresses

Layer 5: Session layer

implements sessions, or process-toprocess communications protocols

Layer 6: Presentation layer

resolves the differences in formats among the various sites in the network, including character conversions, and half duplex/full duplex (echoing)

Layer 7: Application layer

interacts directly with the users, deals with file transfer, remote-login protocols and electronic mail, as well as schemas for distributed databases

OSI Network Model

Logical communication between two computers, with the three lowest level layers implemented in hardware

application layer

file transfer, access, and management; document and message interchange; job transfer and manipulation

presentation layer

transfer-syntax negotiation data-representation transformations

session layer

dialog and synchronization control for application entities

transport layer

end-to-end message transfer (connection management, error control, fragmentation, flow control)

OSI model

formalizes some of the earlier work done in network protocols but was developed in the late 1970s and is currently not in widespread use

TCP/IP model

The most widely adopted protocol stack is the ________, which has been adopted by virtually all Internet sites

TCP/IP protocol stack

has fewer layers than the OSI model. Theoretically, because it combines several functions in each layer, it is more difficult to implement but more efficient than OSI networking

Router

uses segmented network part of host-id to determine where to transfer packet

packet

may be complete message, or it may need to be reassembled into larger message spanning multiple packets

Media Access Control (MAC)

Every Ethernet/WiFi device has a

address resolution protocol (ARP)

If a system needs to send data to another system, it needs to discover the IP to MAC address mapping, uses __________________

broadcast

uses a special network address to signal that all hosts should receive and process the packet

true

true or false? Once a host with a specific IP address receives a packet, it must somehow pass it to the correct waiting process

port number

Transport protocols TCP and UDP identify receiving and sending processes through the use of a ?

protocols TCP and UDP

Allows host with single IP address to have multiple server/client processes sending/receiving packets

• FTP – port and 21

• ssh – port 22

• SMTP – port 25

• HTTP – port 80

Well-known port numbers are used for many services

Transport protocol

can be simple or can add reliability to network packet stream

UDP is unreliable

bare-bones extension to IP with addition of port number

• Since there are no guarantees of delivery in the lower network (IP) layer, packets may become lost

UDP is also connectionless

no connection setup at the beginning of the transmission to set up state • Also no connection tear-down at the end of transmission

datagrams

UDP packets are also called

reliable and connection-oriented

TCP is both ?

(opposite sila sa UDP)

TCP

provides abstraction to allow in-order, uninterrupted byte-stream across an unreliable network

acknowledgement packet (ACK)

Whenever host sends packet, the receiver must send an _______________. If not received before a timer expires, sender will resend.

Sequence numbers

____________ in TCP header allow receiver to put packets in order and notice missing packets

three-way handshake

Connections are initiated with series of control packets called a

cumulative ACK

Receiver can send a ____________ to acknowledge series of packets

flow control and congestion control

Flow of packets regulated through ?

Flow control

prevents sender from overrunning capacity of receiver

Congestion control

approximates congestion of the network to slow down or speed up packet sending rate

• Network Operating Systems

• Distributed Operating Systems

Two main types:

Network Operating Systems

Users are aware of multiplicity of machines

Distributed Operating Systems

Users not aware of multiplicity of machines

Data Migration

transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task

Computation Migration

transfer the computation, rather than the data, across the system

• Via remote procedure calls (RPCs)

• Via messaging system

Process Migration

execute an entire process, or parts of it, at different sites

Load balancing

distribute processes across network to even the workload

Computation speedup

subprocesses can run concurrently on different sites

Hardware preference

process execution may require specialized processor

Software preference

required software may be available at only a particular site

Data access

run process remotely, rather than transfer all data locally

Robustness

Can the distributed system withstand failures?

Transparency

Can the distributed system be transparent to the user both in terms of where files are stored and user mobility?

Scalability

Can the distributed system be scalable to allow addition of more computation power, storage, or users?

Robustness

Hardware failures can include failure of a link, failure of a site, and loss of a message

fault-tolerant system

can tolerate a certain level of failure

• Degree of fault tolerance depends on design of system and the specific fault

• The more fault tolerance, the better!

• Involves failure detection, reconfiguration, and recovery

heartbeat protocol

To detect a link failure, a ___________ can be used

User mobility

allows users to log into any machine in the environment and see his/her environment

Scalability

As demands increase, the system should easily accept the addition of new resources to accommodate the increased demand

Data compression or deduplication

can cut down on storage and network resources used

Distributed file system (DFS)

a file system whose clients, servers, and storage devices are dispersed among the machines of a distributed system

• Should appear to its clients as a conventional, centralized file system

Service

software entity running on one or more machines and providing a particular type of function to a priori unknown clients

Server

service software running on a single machine

Client

process that can invoke a service using a set of operations that forms its client interface

create, delete, read, write

A client interface for a file service is formed by a set of primitive file operations

• Naming and transparency

• Remote file access

• Caching and cache consistency

Two widely-used architectural models include client-server model and cluster-based model ▪ Challenges include:

Naming

mapping between logical and physical objects

Multilevel mapping

abstraction of a file that hides the details of how and where on the disk the file is actually stored

transparent DFS

hides the location where in the network the file is stored

Location transparency

file name does not reveal the file’s physical storage location

Location independence

file name does not need to be changed when the file’s physical storage location changes

Remote-service mechanism

is one transfer approach.

• A requests for accesses are delivered to the server, the server machine performs the accesses, and their results are forwarded back to the user

Cache-consistency problem

keeping the cached copies consistent with the master file

network virtual memory

Cache-consistency problem could be called ?

• Permit workstations to be diskless

• Data can be accessed more quickly

• Performance speedup in bigger memories

• Server caches (used to speed up disk I/O) are in main memory

Advantages of main-memory caches:

Write-through

write data through to disk as soon as they are placed on any cache

Delayed-write (write-back)

modifications are written to the cache and then written through to the server later