Lecture 7: File Systems Networkinglabe.felk.cvut.cz/~stepan/AE3B33OSD/OSD-Lecture-7.pdf ·...

Lecture 7: File Systems

Networking

Lecture 7 / AE3B33OSD 2011

Contents

Files & File System Interface

Directories & their Organization

File System Implementation

Disk Space Allocation

File System Efficiency & Reliability

Networking

OSI - Open System Interconnection Model

Ethernet

IP

UDP

TCP


File Systems Interface

Concept of the file Contiguous logical address space Types:

Data – numeric, character, binary Program

File Structure None - sequence of words, bytes

Simple record structure – lines, fixed length records, variable length records

Complex Structures Formatted documents, relocatable load files

Complex Structures can be simulated by simple record structures through inserting appropriate control

characters

by having special control blocks in the file (e.g., section table at the file beginning)


File Systems Interface (2)

File Attributes Name – the only information kept in human-readable form

Identifier – unique tag (number) identifies file within file system

Type – needed for systems that support different types

Location – information on file location on a device

Size – current file size

Protection – for control who can do reading, writing, executing

Time, date, and user identification – data for protection, security, and usage monitoring

Information about files is kept in the file-system structures, which are stored and maintained on the disk

File Operations – exported by the OS API (cf. e.g., POSIX)

Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory

Write, Read, Reposition within file Close(Fi) – move the content of entry Fi in memory to directory

structure on disk Delete, Truncate etc.


Directory Structure

Directory is a collection of nodes containing information about files Both the directory

structure and the files reside on disk

A Typical File-system Organization

F1 F2 F3

F4

Fn

Directory

Files


Logical Organization the Directories

Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

Organize directories to get Efficiency – locating a file quickly

The same file can have several different names

Naming – convenient to users Two users can have same name for different files

Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)


Single-Level Directory

A single directory for all users

Easy but Naming problem Grouping problem Sharing problem


Two-Level Directory

Separate directory for each user

Path name

Can have the same file name for different user

Efficient searching

No grouping capability


Tree-Structured Directories

Efficient searching Grouping Capability Current directory (working directory)

cd /spell/mail/prog type list


Acyclic-Graph Directories

Have shared subdirectories and files aliasing – an object can have

different names

Problem: When ‘joe’ deletes file

‘test’, the directory item ‘joetst’ points wrong

Solution: Each object has a counter

containing a count of references. The counter increments when a new reference is created and decrements when a reference is deleted. The object is erased when the counter drops to zero

test mail prg.c

sent inbox

text joetst mail

jeff joe /home:


File System Mounting

A file system must be mounted before it can be accessed E.g., file system on a removable media must be ‘announced’ to the

OS, i.e. must be mounted

Have prepared a mount point – a directory Anything referenced from the mount-point before mounting will be

hidden after mounting

/

home

adam mnt docs

joe

progs work

docs

/

joe

progs work

/

home

adam mnt

submnt


File Sharing

Sharing of files on multi-user systems is desirable

Sharing may be done through a protection scheme

On distributed systems, files may be shared across a network Network File System (NFS) is a common distributed file-sharing

method

User IDs identify users, allowing permissions and protections to be per-user

Group IDs allow users to be in groups, permitting group access rights POSIX rwx|rwx|rwx scheme

U G O ACL – Access Control Lists (Windows, some UNIXes)


File System Implementation Objectives

Implementation possibilities of local file systems and directory structures

File block allocation and free-block strategies, algorithms and trade-offs

File structure Logical storage unit

Collection of related information

File system resides on secondary storage (disks)

File system is organized into layers

File control block – storage structure consisting of information about a file Size, ownership, allocation info, time

stamps, ...


In-Memory File System Structures

The following figure illustrates the necessary file system structures provided by the operating systems.

opening a file

reading a file


Virtual File Systems

Virtual File Systems (VFS) provide an object-oriented way of implementing file systems.

VFS allows the same system call interface (the API) to be used for different types of file systems.

The API is to the VFS interface, rather than any specific type of file system.


Directory Implementation

Linear list of file names with pointer to the data blocks. simple to program

time-consuming to execute

Hash Table – linear list with hash data structure. decreases directory search time

collisions – situations where two file names hash to the same location

fixed size

Complex data structure – e.g., B+ tree NTFS in MS Windows


Allocation Methods for Files

An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation

Contiguous allocation

– simple to implement Each file occupies a set of

contiguous blocks on the disk Simple – only starting location

(block #) and length (number of blocks) are required

Random access Wasteful of space (dynamic

storage-allocation problem) Files cannot grow


Extent-Based Systems

Many newer file systems (e.g., Veritas File System) use a modified contiguous allocation scheme

Extent-based file systems allocate disk blocks in extents

An extent is a contiguous block of disks Extents are allocated for file growth

A file consists of one or more extents


Linked Allocation

Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.

Simple – need only starting address Free-space management system

– no waste of space Difficult random access

must go through the whole chain

pointer to next block block =


Linked Allocation with FAT

Allocation chains stored separately File-allocation table (FAT)

Disk-space allocation used by MS-DOS and OS/2.

Problems: Size of the table Access speed Reliability

All file info is concentrated in one place

FAT duplicates


Allocation block size with FAT

Allocation block, cluster group of adjacent disk sectors

Fixed size of FAT on disk Different FAT types

FAT item has 12, 16 or 32 bits Directory entry (MSDOS):

Addressing capability of different FAT types

File size 1st block Date and time Reserved Attrs Extension Name FAT-16

4 2 4 10 1 3 8 bytes

2 TB 2 GB 32 KB = 64 sectors

2 TB 1 GB 16 KB = 32 sectors

2 TB 512 MB

b)

8 KB = 16 sectors

1 TB 256 MB 16 MB 4 KB = 8 sectors

128 MB 8 MB 2 KB = 4 sectors

4 MB 1 KB = 2 sectors a)

2 MB 0.5 KB = 1 sector

FAT-32 FAT-16 FAT-12 Block size

Empty entries in the table are unused because:

a) FAT is too large compared to the disk capacity

b) losses due to internal fragmentation are to high


Indexed Allocation

Brings all pointers for one file together into an index block. Logical view

Need index table

Random access

Dynamic access without external fragmentation, but have overhead of index block. Mapping from logical to physical in a file of maximum size of 256K

words and block size of 512 words. We need only 1 block for index table

Only “small” files

index table


Multi-level Indexed Allocation

outer-index

index table file


Combined Scheme: UNIX FS Disk i-node

4K bytes per block


Free-Space Management

Bit vector (n blocks) – one bit per block Bit map requires extra space Easy to get contiguous files

Linked list (free list)

Cannot get contiguous space easily No waste of space

Need to protect:

Pointer to free list Bit map

Must be kept on disk Copy in memory and disk may differ Cannot allow for block[i] to have a

situation where bit[i] = 1 in memory and bit[i] = 0 on disk

Solution: Set bit[i] = 1 in disk Allocate block[i] Set bit[i] = 1 in memory

…

0 1 2 n-1

bit[i] =

0 block[i] free

1 block[i] occupied


Directory Implementation

Linear list of file names with pointer to the data blocks simple to implement

time-consuming to execute

directory can grow and shrink

Hash Table – linear list with hash data structure decreases directory search time

collisions – situations where two file names hash to the same location

fixed size


File System Efficiency and Performance

Efficiency dependent on: disk allocation and directory algorithms types of data kept in file’s directory entry

Performance disk cache – separate section of main memory for frequently

used blocks free-behind and read-ahead – techniques to optimize sequential

access improve PC performance by dedicating section of memory as

virtual disk, or RAM disk


Recovery from a Crash

Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies

Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape, other magnetic disk, optical)

Recover lost file or disk by restoring data from backup


Log Structured File Systems

Log structured (or journaling) file systems record each update to the file system as a transaction similar to database systems

All transactions are written to a log A transaction is considered committed once it is written to the log

However, the file system may not yet be updated

The transactions in the log are asynchronously written to the file system When the file system is modified, the transaction is removed from

the log

If the file system crashes, all remaining transactions in the log must still be performed

Used by NTFS file system


Short introduction to Networking


Network Structure - LAN

Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network

Speed 10 megabits/second, or higher

Broadcast is fast and cheap

Nodes: usually workstations and/or

personal computers

a few (usually one or two) mainframes


Network Structure - WAN

Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a

phone company) Broadcast usually requires

multiple messages Nodes:

often a high percentage of mainframes


Network Topology Sites in the system can be physically connected in a

variety of ways; they are compared with respect to the following criteria: Basic cost – How expensive is it to link various sites in the

system? Communication cost – How long does it take to send a

message from site A to site B? Reliability – If a link or a site in the

system fails, can the remaining sites still communicate with each other?

The various topologies are depicted as graphs whose nodes correspond to sites An edge from node A to

node B corresponds to a direct connection between the two sites


CESNET structure


Communication Structure

The design of a communication network must address four basic issues:

Naming and name resolution How do two processes locate each other to communicate?

Routing strategies How are messages sent through the network?

Connection strategies How do two processes send a sequence of messages?

Contention The network is a shared resource, so how do we resolve

conflicting demands for its use?


Communication Protocol

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

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

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

The communication network is partitioned into the

following multiple layers:


Communication Protocol (Cont.)

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

Session layer – implements sessions, or process-to-process communications protocols

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

Application layer – interacts directly with the users’ deals with file transfer, remote-login protocols and electronic mail, as well as schemas for distributed databases, etc.


The ISO Protocol Layers


Communication Via ISO Network Model


The TCP/IP Protocol Layers


Design Issues

Transparency – the distributed system should appear as a conventional, centralized system to the user

Fault tolerance – the distributed system should continue to function in the face of failure

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

Clusters – a collection of semi-autonomous machines that acts as a single system


Example: Networking

The transmission of a network packet between hosts on an Ethernet network

Every host has a unique IP address and a corresponding Ethernet (MAC) address

Communication requires both addresses

Domain Name Service (DNS) can be used to acquire IP addresses

Address Resolution Protocol (ARP) is used to map MAC addresses to IP addresses

If the hosts are on the same network, ARP can be used If the hosts are on different networks, the sending host will send

the packet to a router which routes the packet to the destination network


An Ethernet Packet

Preamble Destination

address Source address

Frame type

Message data Checksum

64 bits 48 bits 48 bits 16 bits 368-12000 bits 32 bits

Preamble:

7 bytes [10101010] + 1 byte [10101011] used to synchronize the transfer rate

6 byte addresses (MAC addresses) World unique addresses

If destination address = 0xFF-FF-FF-FF-FF-FF then broadcast

Frame type – indicates different standards: 0x0800 = the packet contains an IPv4 datagram 0x86DD indicates an IPv6 frame 0x0806 indicates an ARP frame

Message data usually 1500 bytes Encapsulates packet with the transferred data


Internet Architecture

Basic Internet properties Each host has its unique identification: the IP address

IP address is composed by the network address and the host address within the network

The applications and the network API behavior is independent of the LAN technology

Basic architecture of the Internet (and general internetworking)

Gateways and routers connect LAN’s

LAN’s may be based on different technologies

Gateways keep info on hosts belonging to LAN’s they connect Routers maintain knowledge about networks

Routers forward packets based on the network part of the IP address (the host part is ignored when forwarding)

Units connecting LAN’s usually merge gateway and router functionality

IP protocols consider all LAN’s as equivalent regardless of the technology

LAN 1 LAN 2 LAN 3 gateway or

router

gateway or

router


Internet addresses

Current Internet – v. 4 uses 32 bits addresses Convention: decimal numbers per 8 bits each – 147.32.85.1

Internet v. 6 uses 128 bits addresses Not treated here as still under experimental development

IP address Identifies each single network adaptor

Host can have several adaptors („multi-homed“ host)

One adaptor can even have more addresses

Is composed of two parts

Identification (address) of the network – netid (leftmost bits)

Identification (address) of the host within the network – hostid (rightmost

bits)


Internet addresses (2)

Primary IP address classes

Reserved ranges: In A 0.0.0.0 – 0.255.255.255, 127.0.0.0 – 127.255.255.255,

In B 128.0.0.0 – 128.0.255.255, 191.255.0.0 – 191.255.255.255,

In C 192.0.0.0 – 192.0.0.255, 255.255.255.0 – 255.255.255.255

0 1 2 3 4 8 16 24 31 Net Mask Address range

Class A 0 netid hostid 255.0.0.0 0.0.0.0 –

127.255.255.255

Class B 1 0 netid hostid 255.255.0.0 128.0.0.0 –

191.255.255.255

Class C 1 1 0 netid hostid 255.255.255.0 192.0.0.0 –

255.255.255.255


Internet addresses (3)

Convention: Network address netid is the full IP address with hostid = 0

Address composed by netid and the hostid part is full of "1" serves for addressing all hosts in the network (network broadcast address)

Net maska: Address „arithmetic“

Special addresses 127.0.0.1 – loopback address – a host speaks to itself

Private addresses – may not spread over Internet – routers must not forward datagrams containing these addresses 1 class A network: 10.0.0.0 – 10.255.255.255

16 class B networks: 172.16.0.0 – 172.31.255.255

256 class C networks: 192.168.0.0 – 192.168.255.255

Multicast addresses – one host sends info to many “subscribed” hosts (e.g. Internet TV)

range 224.0.0.0 – 238.255.255.255

hostidNetMaskAddressIP

netidNetMaskAddressIP

)(_

_


Internet addresses (4) CIDR addressing (= Classless Inter-Domain Routing)

Address arithmetic enables for more efficient splitting netid|hostid – the border between netid and hostid may be anywhere

Net mask may be any number composed of n (n=0 ... 32) leftmost “1” bits

CIDR notation: IP_Address/n; e.g.: 147.32.85.128 – 147.32.85.191 = 147.32.85.128/26

Reserved ranges in CIDR notation: 0.0.0.0/8, 127.0.0.0 /8, 128.0.0.0/16, 191.255.0.0/16, 192.0.0.0/24, 255.255.255.0/24

LAN 192.168.200.64/30 contains 4 addresses: 192.168.200.64 = netid, 192.168.200.65=host1, 192.168.200.66=host2, 192.168.200.67 = LAN broadcast

Saving IP addresses Using private addresses and their translation to “public” addresses

(NAT = Network Address Translation) Many private addresses is translated into 1 public Problem with publicly available servers on the private address LAN

(behind the NAT router) The principle of NAT is connected to IP protocols

Internet LAN with private

IP addresses Router

with NAT 147.32.85.27 192.168.100.1


Internet datagrams

Internet creates a virtual network and carries IP datagrams

The network is a best effort delivery system

Datagrams travel through Internet over different physicals LAN’s

Datagrams may not depend on the LAN technology

Format of an IP datagram

Datagram

header Datagram data part

Frame header

Frame data part

Encapsulating IP datagram into physical network frame


IP datagram header

Every IP datagram has a header carrying information necessary for datagram delivery

Fields VERS: IP protocol version – for IP v. 4 VERS = 4

HLEN: Header length in 32-bit words (standard 5)

TOTAL LENGTH: of the datagram in bytes (header+data) – max. 64 kB

SOURCE IP ADDRESS: sender’s IP address

DESTINATION IP ADDRESS: IP address where to deliver

IDENTIFICATION: usually sequential or a random number generated by the datagram sender

0 4 8 16 19 24 31

VERS HLEN SERVICE TYPE TOTAL LENGTH

IDENTIFICATION FLAGS FRAGMENT OFFSET

TIME TO LIVE PROTOCOL HEADER CHECKSUM

SOURCE IP ADDRESS

DESTINATION IP ADDRESS

IP OPTIONS (IF ANY) PADDING

DATA

…

IP datagram format


IP datagram header (cont.)

PROTOCOL: Identification of the protocol carried in the IP datagram (UDP=17, ICMP=1, TCP=6, ...). Defined in RFC 1060

FLAGS, FRAGMENT OFFSET: Information on datagram fragmentation

TIME TO LIVE (TTL): Determines how long the datagram may travel through Internet. Every router decrements this value; if TTL==0 the datagram is discarded and the router sends an ICMP message to the sender

SERVICE TYPE: 8-bit field commanding the datagram routing

0 1 2 3 4 5 6 7 PRECEDENCE D T R UNUSED

Datagram precedence: 0=normal,

7=network control Low

delay

High

throughput High reliability


Datagram fragmentation (1)

MTU: (Maximum Transmission Unit) defines the maximum datagram size that can be transferred in a LAN Net type Implicit MTU Net type Implicit MTU PPP 296 X.25 576 Ethernet 1 500 FDDI 4 352 TokenRing 4Mb 4 464 TokenRing 16Mb 17 914

Internet – a set of LAN’s with different MTU’s Whenever the datagram is larger than MTU it must be fragmented

Host

B

Net 3,

MTU=1500

Host

A

Net 1,

MTU=1500

Net 2,

MTU=620

G 1 G 2


Datagram fragmentation (2) Fragmentation occurs anywhere during the datagram travel

If the datagram is fragmented it is not defragmented on the way and the reconstruction is the task of the destination host

Every fragment travels as a separate datagram: The following fields are copied from the original datagram header: VERS,

HLEN, SERVICE TYPE, IDENTIFICATION, PROTOCOL, SOURCE IP ADDRESS, DESTINATION IP ADDRESS

TOTAL LENGTH is changed to the fragment size a and the field FRAGMENT OFFSET determines the offset of the fragment in the original datagram

Field FLAGS contains a bit more fragments. If this bit is 0 then the target host knows that the last fragment has been obtained. The target host reconstructs the original datagram using the FRAGMENT OFFSET and TOTAL LENGTH fields

DATAGRAM HEADER

Data_1 600 bytes

Data_2 600 bytes

Data_3 200 bytes

FRAGMENT 1 HEADER

Data_1 Fragment 1 (offset=0)

FRAGMENT 2 HEADER


FRAGMENT 3 HEADER


1400 bytes datagram fragmentation


UDP Protocol

UDP (= User Datagram Protocol), IP protocol number = 17 Very simple transport protocol defined in RFC 768 Provides connectionless and unreliable transport of user datagrams. If any kind of transmission reliability is needed, it must be

implemented in the user's application. Compared to pure IP datagrams, UDP has the ability to address

target processes on the destination host using the port field

UDP HEADER

UDP DATA AREA

IP HEADER IP DATA AREA

FRAME HEADER FRAME DATA AREA

UDP encapsulated in an IP datagram

0 16 31 UDP SOURCE PORT UDP DESTINATION PORT

UDP MESSAGE LENGTH UDP CHECKSUM

UDP Header

PORT fields are used to distinguish computing processes awaiting a

UDP datagram on the destination host. SOURCE PORT field is optional; must be 0 if unused. Otherwise it contains the port number to which a possible reply should

be sent.


UDP Protocol (cont.)

To ensure that different hosts on Internet will understand each other, IANA (= Internet Assigned Numbers Authority) issues the obligatory list of “Well Known Port Numbers”

Selected UDP port numbers are:

For the complete list see http://www.iana.org/assignments/port-numbers

Port Keyword Meaning

0 - Reserved

7 echo Echo the datagram

9 discard Discard the datagram

13 daytime Time of the day

53 dns Domain Name Service

67 bootps Bootstrap Protocol Server, DHCP Server

68 bootpc Bootstrap Protocol Client, DHCP Client

69 tftp Trivial File Transfer

137 netbios-ns NETBIOS Name Service

…

http://www.iana.org/assignments/port-numbers




TCP Protocol for reliable data streaming TCP is the most important general reliable transport service

providing a virtual bidirectional communication channel between two hosts TCP/IP is the IP implementation of this service

TCP properties Data stream

Applications communicating through a TCP connection consider the communication channel as a byte stream similarly to a file

Virtual connection Before the data transmission can start, the communicating applications have to negotiate the connection by means of the network components of their local operating systems – create and connect the sockets.

The protocol software (transport layer) in the operating systems of both hosts make an agreement on the connection using messages passed over the network. The hosts also verify that the connection can be reliably established and that both end-point systems are ready to communicate – open the sockets.

Afterwards the end-point applications are informed about the established connection and the data communication can start.

If the connection breaks, both communicating applications are informed The term virtual connection is used to create an illusion that applications are interconnected through a dedicated line.

The reliability is ensured by full hand-shake communication (everything must be acknowledged by the other party)


TCP/IP – TCP Protocol IP Implementation

Besides of the general TCP properties, the TCP/IP implementation provides: Buffered transport (streaming)

To improve the efficiency, the TCP/IP module in the OS assembles bytes from the stream into packets (datagrams) of reasonable size. If this is not desirable (e.g., TELNET ctrl-C), TCP/IP is equipped by a mechanism enforcing the priority transfer of a short datagram “out-of-order”.

Full duplex connection Application processes can see the TCP/IP link as two independent data

streams running in opposite directions without an apparent interaction. The protocol software actually acknowledges data running in one direction in the packets sent together with the data in the opposite direction.

TCP HEADER TCP DATA AREA

IP HEADER IP DATA AREA

FRAME HEADER FRAME DATA AREA

TCP/IP encapsulated in an IP datagram on a physical network


TCP reliability solution The reliable transfer is ensured by

positive acknowledging of the received data together with repeated packet transfer

Events on the sender side

Events on the receiver side Datagrams on network

Send packet 1 Get packet 1 Send ACK1

Get ACK1

Send packet 2 Get packet 2

Send ACK2 Get ACK2

Lost packet are repeated based on suitable timeouts

Packet should have arrived ACK should have been sent

Get packet 1

Send ACK1

Send packet 1 Start timer

ACK should have arrived now but it timed-out.

Resend packet 1 Start timer

Get ACK1 Stop timer


Events on the receiver side

Datagrams on network

Packet lost


TCP reliability solution (cont.)

Datagrams may get duplicated during the travel (both data and ACK). This problem is solved by sequential numbering of datagrams

The problem of positive acknowledgement efficiency The moving window method

Starting window position

1 2 3 4 5 6 7 8

The window moves

1 2 3 4 5 6 7 8

Massages on the net

Send packet 1

Send packet 2

Send packet 3

Get ACK1

Get ACK2

Get ACK3

Get packet 1 Send ACK1

Get packet 2

Send ACK2

Get packet 3

Get ACK3


Events on the receiver side


Retransmission timeouts

Constant value of timeout when to resend the TCP/IP segment is inappropriate Internet is too heterogeneous and is composed of a huge number of

LAN’s based on different HW technologies Giga-bit ethernet, 33 kbit serial line, intercontinental satellite link, etc.

TCP/IP adapts to changing timing parameters of the virtual connection A simple adaptive algorithm to adjust the timeout is used

The algorithm is based on continuous monitoring of „round trip time“ (RTT)

Time between dispatching the packet and its acknowledgement.

The real timeout is then computed as a weighted average of RTT measured in the recent history.

This strategy quickly accommodates to the speed and load changes on the intermediate networks and routers


Establishing the TCP connection

TCP uses a three-stage procedure to establish the virtual connection:

1. In the first step, the connection initiator (client) sends the other party (server) a segment containing SYN bit = 1, randomly generated SEQUENCE NUMBER = x and an empty data section.

2. The server responds by a segment with SYN and ACK set to 1, random SEQUENCE NUMBER = y and ACKNOWLEDGMENT NUMBER = x+1.

3. After receiving this segment, the client acknowledges it by sending a segment with ACK set to 1, SYN bit=0 and the ACKNOWLEDGMENT

NUMBER = y+1.

This way the initial values of SEQUENCE NUMBER and ACKNOWLEDGMENT

NUMBER fields are synchronized for the future life time of the virtual connection and are used to number the following TCP/IP segments

The sequential numbers are random

It enables to detect a failure or restart of hosts on the connection ends that can happen during a longer timeout


TCP connection termination

Connection is normally terminated on request of one of the connected applications Application tells TCP that there are no more data to be exchanged

If server – passive close If client – active close

TCP software closes the connection by sending a segment with a set FIN bit

For the final termination of the virtual connection, it is necessary also to close the opposite direction. The party that received the segment with FIN bit reacts by sending a segment with FIN bit, too.

The TCP connection can be terminated forcibly using the RST bit


Well known TCP ports

Examples of some TCP/IP ports

and many others (see http://www.iana.org/assignments/port-numbers)

Port Keyword Meaning

0 - Reserved

7 echo Echo the datagram

13 daytime Time of the day

20 ftp-data File Transfer Protocol (data stream)

21 ftp File Transfer Protocol (controls)

22 ssh Secure Terminal Connection

23 telnet Terminal Connection

25 smtp Simple Mail Transport Protocol

53 domain Domain Name Server Query

79 finger Finger service (who is logged on?)

80 http World-wide web

110 pop3 Post Office Protocol - V3 – get incoming mail from server

443 https Secured world-wide web





TCP finite state machine

Dashed arrows – server side state transitions

Solid arrows – client side state transitions

Lower left frame – active connection close

Lower right frame – passive connection close

appl: transition caused by system call from application

recv: transition caused by received segment

send: shows what is sent in reaction to state change

End of Lecture 7

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