MEHBOOB NAZIM SHEHZAD

01085

Assignment # 02

Advance Computer Networks

Submit To:

Mr.Imran Shafi

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1. There are several transport layer models proposed in the OSI model. Research and find

All of them. Explain the differences between them.

ANS:

Transport Layer:

Transport layers are contained in both the TCP/IP model(RFC 1122)which is the foundation of the Internet, and the Open Systems Interconnection(OSI) model of general networking.

Services:

There are many services that can be optionally provided by a Transport Layer protocol, and different protocols may or may not implement them.

Connection-oriented communication : Byte orientation :

Same order delivery:

Reliability :

Flow control :

Congestion avoidance

Multiplexing :

Protocol:

ATP, AppleTalk Transaction Protocol CUDP, Cyclic UDP

DCCP, Datagram Congestion Control Protocol

FCP, Fiber Channel Protocol

IL, IL Protocol

NBF, NetBIOS Frames protocol

SCTP, Stream Control Transmission Protocol

SPX, Sequenced Packet Exchange

SST, Structured Stream Transport

TCP, Transmission Control Protocol

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UDP, User Datagram Protocol

UDP Lite

µTP, Micro Transport Protocol

Analysis:

Finally, some Transport Layer protocols, for example TCP, but not UDP, provide end-to-end reliable communication, i.e. error recovery by means of error detecting code and automatic repeat request (ARQ) protocol. The ARQ protocol also provides flow control, which may be combined with congestion avoidance.

UDP is a very simple protocol, and does not provide virtual circuits, nor reliable communication, delegating these functions to the application program. UDP packets are called datagram’s, rather than segments.

TCP is used for many protocols, including HTTP web browsing and email transfer. UDP may be used for multicasting and broadcasting, since retransmissions are not possible to a large amount of hosts. UDP typically gives higher throughput and shorter latency, and is therefore often used for real-time multimedia communication where packet loss occasionally can be accepted, for example IP-TV and IP-telephony, and for online computer games.

2. There are several network layer models proposed in the OSI model. Research and find

All of them. Explain the differences between them.

ANS:

Internet Layer:

Internet Layer protocols use IP-based packets. The Internet Layer does not include the protocols that define communication between local ("on-link") network nodes which fulfill the purpose of maintaining link states between the local nodes, such as the local network topology, and that usually use protocols that are based on the framing of packets specific to the link types. Such protocols belong to the Link Layer.

Internet Layer functions:

The Internet Layer has three basic functions: For outgoing packets, select the "next hop" host (gateway) and transmit the packet to this host by passing it to the appropriate Link Layer implementation; for incoming packets, capture packets and pass the packet payload up to the appropriate Transport Layer protocol, if appropriate. In addition it provides error detection and diagnostic capability.

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In Version 4 of the Internet Protocol (IPv4), during both transmit and receive operations, IP is capable of automatic or intentional fragmentation or defragmentation of packets, based, for example, on the maximum transmission unit (MTU) of link elements. However, this feature has been dropped in IPv6, as the communications end points, the hosts, now have to perform path MTU discovery and assure that end-to-end transmissions don't exceed the minimum discovered.

In its operation, the Internet Layer is not responsible for reliable transmission. It provides only an unreliable service, and "best effort" delivery. This means that the network makes no guarantees about packets' proper arrival (see also Internet Protocol#Reliability). This was an important design principle and change from the previous protocols used on the early ARPANET. Since packet delivery across diverse networks is inherently an unreliable and failure-prone operation, the burden of providing reliability was placed with the end points of a communication path, i.e., the hosts, rather than on the network. This is one of the reasons of the resiliency of the Internet against individual link failures and its proven scalability.

The function of providing reliability of service is the duty of higher level protocols, such as the Transmission Control Protocol (TCP) in the Transport Layer.

Integrity of packets is guaranteed only in IPv4 (not in IPv6) through checksums computed for IP packets.

Internet Layer protocols:

The core protocols in the Internet Layer are:

Internet protocol (IP), it is implemented in two versions, for IPv4 and IPv6. Internet Control Message Protocol (ICMP), primarily used for error and diagnostic

functions, different implementations exist for IPv4 and IPv6.

Internet Group Management Protocol (IGMP), used by IPv4 hosts and adjacent multicast routers to establish multicast group memberships.

Read the following classical research paper to answer questions in your own words.

NO CUT PASTE PLEASE

a. In the phrase “Open System Interconnection”, what is the term “Open”

emphasizes?

Short for Open System Interconnection, an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Most of the functionality in the OSI model exists in all communications systems, although two or three OSI layers may be incorporated into one.OSI is also referred to as the OSI Reference Model or just the OSI Model.

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How the binding between (N) entities and (N-1) SAP is established?

A common service offered by all layers consists of providing associations between peer SAP’S which can be used in particular to transfer data (it can, for instance, also be used to synchronize the served entities participating in the association).

According to technology use (www.wordpedia.com) only take hint....

The (N) services are offered to the (N + 1) entities at (N) service access points, or (N) SAP3 for short, which present the logical interfaces between the (N) entities and e (N + 1) entities. An (N) SAP can be served by only one.

c. Give the justification for seven layers?

OS1 Architecture defines identifiers for entities, SAP’S, and connections as well as relations between these identifiers. trance, within the domain corresponding to the (N) layer, (N) entities are identified with (N) global titles which are unique within the (N) layer.

d. Explain the author’s view point for the development of protocols in various

layers?

Ans:

The OS1 Architecture model briefly described in this paper salts from the work of more than 100 experts from many unties and international organizations.

e. What are the differences you observe of this model with the TCP/IP implementation?

Ans:

The three top layers in the OSI model—the Application Layer, the Presentation Layer and the Session Layer—are not distinguished separately in the TCP/IP model where it is just the Application Layer. While some pure OSI protocol applications, such as X.400, also combined them, there is no requirement that a TCP/IP protocol stack needs to impose monolithic architecture above the Transport Layer. For example, the Network File System (NFS) application protocol runs over the eXternal Data Representation (XDR) presentation protocol, which, in turn, runs over a protocol with Session Layer functionality, Remote Procedure Call (RPC). RPC provides reliable record transmission, so it can run safely over the best-effort User Datagram Protocol (UDP) transport.

The Session Layer roughly corresponds to the Telnet virtual terminal functionality[citation needed], which is part of text based protocols such as the HTTP and SMTP TCP/IP model Application Layer protocols. It also corresponds to TCP and UDP port numbering, which is considered as part of the transport layer in the TCP/IP model. Some functions that would have been performed by an OSI presentation layer are realized at the Internet application layer using the MIME standard, which is used in application layer protocols such as HTTP and SMTP.

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Since the IETF protocol development effort is not concerned with strict layering, some of its protocols may not appear to fit cleanly into the OSI model. These conflicts, however, are more frequent when one only looks at the original OSI model, ISO 7498, without looking at the annexes to this model (e.g., ISO 7498/4 Management Framework), or the ISO 8648 Internal Organization of the Network Layer (IONL). When the IONL and Management Framework documents are considered, the ICMP and IGMP are neatly defined as layer management protocols for the network layer. In like manner, the IONL provides a structure for "subnetwork dependent convergence facilities" such as ARP and RARP.

IETF protocols can be encapsulated recursively, as demonstrated by tunneling protocols such as Generic Routing Encapsulation (GRE). While basic OSI documents do not consider tunneling, there is some concept of tunneling in yet another extension to the OSI architecture, specifically the transport layer gateways within the International Standardized Profile framework [11]. The associated OSI development effort, however, has been abandoned given the overwhelming adoption of TCP/IP protocols.

e. Do you agree with the conclusion of the author? Is he realistic about the model?

Ans:

It is essential that all partners in this standardization process expend their best effort so it will be successful, and benefits can be shared by all users, manufacturers of terminals and computers, and the PTT’s/common carriers.

1. What is the purpose of whereis command in Linux? Try to use this command and

show some of your results.

Ans:

NAME

whereis - locate the binary, source, and manual page files for a command  

SYNOPSIS

whereis [ -bmsu ] [ -BMS directory... -f ] filename ...  

DESCRIPTION

whereis locates source/binary and manuals sections for specified files. The supplied names are first stripped of leading pathname components and any (single) trailing extension of the form .ext, for example, .c. Prefixes of s. resulting from use of source code control are also dealt with. whereis then attempts to locate the desired program in a list of standard Linux places.  

2. Describe the function performed by touch command? Under what condition, this

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command can be helpful.

Ans:

touch, settime- change file access and modification times touch [-acm] [-r ref_file| -t time] file...touch [-acm] [date_time] file...settime [-f ref_file] [date_time] file...

The touch utility sets the access and modification times of each file. The file operand is created if it does not already exist.

The time used can be specified by -t time, by the corresponding time fields of the file referenced by -r ref_file, or by the date_time operand. If none of these are specified, touch uses the current time (the value returned by the time(2) function).

What does the traceroute command do? Use its –x and –I switches for

www.iqra.edu.pk.

I hve no linux so I don’t know confrm about it…

So soory

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