General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

A computer can have more than one network interface. (e.g. a router)
c○: Michael Kohlhase 341
The next layer ist he Internet Layer, it performs two parts: addressing and packing packets.
Internet Protocol and IP Addresses
Definition 496 The Internet Protocol (IP) is a protocol used for communicating data across
a packet-switched internetwork. The Internet Protocol defines addressing methods and structures
for datagram encapsulation. The Internet Protocol also routes data packets between
networks
Definition 497 An Internet Protocol (IP) address is a numerical label that is assigned to
devices participating in a computer network, that uses the Internet Protocol for communication
between its nodes.
An IP address serves two principal functions: host or network interface identification and
location addressing.
Definition 498 The global IP address space allocations are managed by the Internet Assigned
Numbers Authority (IANA), delegating allocate IP address blocks to five Regional
Internet Registries (RIRs) and further to Internet service providers (ISPs).
Definition 499 The Internet mainly uses Internet Protocol Version 4 (IPv4) [RFC80], which
uses 32-bit numbers (IPv4 addresses) for identification of network interfaces of Computers.
IPv4 was standardized in 1980, it provides 4,294,967,296 (2 32 ) possible unique addresses.
With the enormous growth of the Internet, we are fast running out of IPv4 addresses
Definition 500 Internet Protocol Version 6 (IPv6) [DH98], which uses 128-bit numbers
(IPv6 addresses) for identification.
Although IP addresses are stored as binary numbers, they are usually displayed in humanreadable
notations, such as 208.77.188.166 (for IPv4), and 2001 : db8 : 0 : 1234 : 0 : 567 :
1 : 1 (for IPv6).
c○: Michael Kohlhase 342
The Internet infrastructure is currently undergoing a dramatic retooling, because we are moving
from IPv4 to IPv6 to counter the depletion of IP addresses. Note that this means that all routers
and switches in the Internet have to be upgraded. At first glance, it would seem that that this
problem could have been avoided if we had only anticipated the need for more the 4 million
computers. But remember that TCP/IP was developed at a time, where the Internet did not exist
yet, and it’s precursor had about 100 computers. Also note that the IP addresses are part of every
packet, and thus reserving more space for them would have wasted bandwidth in a time when it
was scarce.
We will now go into the detailed structure of the IP packets as an example of how a low-level
protocol is structured. Basically, an IP packet has two parts: the “header”, whose sequence of
bytes is strictly standardized, and the “payload”, a segment of bytes about which we only know
the length, which is specified in the header.
The Structure of IP Packets
Definition 501 IP packets are composed of a 160b header and a payload. The IPv4 packet
header consists of:
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