slides27 Flashcards
other char encodings
Indian Script Code for Information Interchange (ISCII)
Mac OS Roman (extends ASCII)
what does big endian format imply when storing an integer
de facto order used on most networks is big endian
this stores the most significant byte of an integer (the big end) at the lowest machine address, less significant bytes at increasing addresses
00 00 00 2A, for integer 42 hex 0000002A
what does little endian format imply when storing an integer
the least significant byte (little end) is stored at the lowest machine address, more significant bytes at increasing addresses
00 00 00 2A, for integer 42: 704643072 (hex 2A000000)
wat does lil endian do wen he communicates
A little endian machine must reverse the order of the bytes as it sends or receives
A little endian machine always converts, even when connected to another little endian machine
This is simpler than having a protocol to negotiate endianness and having separate chunks of code for each combination
how to agree on presentation
The External Data Representation (XDR) package is one approach to the presentation problem
what is XDR
It is a collection of functions that convert data in and out of a standard network format
how Multipurpose Internet Mail Extension (MIME) allowed more freedom in emails
Early email systems only supported 7-bit ASCII text and MIME was developed to allow emailing of pictures, sounds and so on, by converting everything to a common, 7-bit-based format
how MIME encoding works
MIME regards data as a sequence of 8 bit bytes and encodes them in one of a variety of ways
• 7bit: no transformation, only useful if the data were already ASCII
• 8bit: no transformation, for some of the basic extensions to ASCII; don’t regard the top bit being set as an error
• quoted-printable: bytes with values less than 128 represent themselves. Values over 128 are represented by an ASCII ’=’ followed by a two digit hex value, written in ASCII
base64 (review)
the input is transformed into a 65 character subset of ASCII, namely A-Z a-z 0-9 + / plus = as a special pad character
The 64 non-pad characters can be represented in 6 bits
ISO-4489-15 character set
a simple 8-bit extension to ASCII
how to represent the end of a line in a text file?
- Unix-derived systems use a linefeed (LF, character 10 in ASCII)
- Windows systems use a carriage return (CR, ASCII 13) followed by a LF
- Pre-MacOS X used a single CR
Telnet
Logging in to a remote machine for interactive use: first appear in 1969. Uses TCP
Telnet was one of the earliest applications developed for the Internet as telnet’s main purpose is to give remote access to machines and the original purpose of the Internet was to give remote access to machines
This connection is insecure in that data — including the login password — is readable by machines that route the packets as they travel
An eavesdropper could read confidential information as it goes past
So telnet is no longer recommended and has been replaced by the more sophisticated secure shell (ssh)
File Transfer Protocol (FTP)
A basic way of transferring files. TCP
SMTP, DNS
- Simple Mail Transfer Protocol (SMTP). Email. TCP
* Domain Name System (DNS). Already seen, mostly UDP
NFS
• Network File System (NFS). Allows remote disks to appear as a local file system. UDP and TCP
