Internet Protocol
The Internet Protocol (IP) is a data-oriented protocol used by source and
destination hosts for communicating data across a packet-switched
internetwork.
Data in an IP internetwork is sent in blocks referred to as packets or
datagrams (the terms are basically synonomous in IP). In particular, in IP
no setup is needed before a host tries to send packets to a host it has
previously not communicated with.
The Internet Protocol provides an unreliable datagram service (also called
best effort); i.e. it makes almost no guarantees about the packet. The
packet may arrived damaged, it may be out of order (compared to other
packets sent between the same hosts), it may be duplicated, or it may be
dropped entirely. If the application needs reliability, this is added by the
Transport layer.
Packet switches, or internetwork routers, are used to forward IP datagrams
across interconnected layer 2 networks. The lack of any delivery guarantees
means that the design of packet switches is made much simpler. (Note that if
the network does drop, reorder or otherwise damage a lot of packets, the
performance seen by the user will be poor, so most network elements do try
hard to not do these things - hence the best effort term. However, an
occasional error will produce no noticeable effect.)
IP is the common element found in today's public Internet. It is described
in IETF RFC 791, which was first published in September, 1981. This document
describes the current and most popular network layer protocol in use today.
This version of the protocol is assigned as version 4. IPv6 is the proposed
successor to IPv4; the Internet is slowly running out of addresses, and IPv6
has 128-bit source and destination addresses, providing more addresses than
IPv4's 32 bits. Versions 0 through 3 were either reserved or unused. Version
5 was used for an experimental stream protocol. Other version numbers have
been assigned, usually for experimental protocols, but have not been widely
used.
IPv4 Header Format
0 4 8 16 19
Version IHL Type of Service Total Length
Identification Flags Fragment Offset
Time to Live Protocol Header Checksum
Source Address
Destination Address
Options Padding
The first header field in an IPv4 datagram is the 4-bit version field.
The second field is a 4-bit Internet Header Length (IHL) telling the number
of 32-bit words in the IPv4 header. Since an IPv4 header may contain a
variable number of options, this field essentially specifies the offset to
the data portion of an IPv4 datagram. A minimum IPv4 header is 20 bytes
long, so the minimum value in decimal in the IHL field would be 5.
In RFC 791, the following 8 bits were allocated to a Type of Service (ToS)
field. The original intention was for a sending host to specify a preference
for how the datagram would be handled as it made its way through an
internetwork. For instance, one host could set its IPv4 datagrams' ToS field
value to prefer low delay, while another might prefer high reliability. In
practice, the ToS field has not been widely implemented. However, a great
deal of experimental, research and deployment work has focused on how to
make use of these eight bits. These bits have been redefined and most
recently through DiffServ working group in the IETF and the Explicit
Congestion Notification codepoints (see RFC 3168).
The next 16-bit IPv4 field defines the entire datagram size, including
header and data, in 8-bit bytes. The minimum-length datagram is 20 bytes and
the maximum is 65535. The maximum size datagram which any host is required
to be able to handle is 576 bytes, but most modern hosts handle much larger
packets. Sometimes subnetworks impose further restrictions on the size, in
which case datagrams must be fragmented. Fragmentation is handled in either
the host or packet switch in IPv4, and in the host only in IPv6.
The next 16-bit field is an identification field. This field is primarily
used for uniquely identifying fragments of an original IP datagram. Some
experimental work has suggested using the ID field for other purposes, such
as for adding packet tracing information to datagrams in order to help trace
back datagrams with spoofed source addresses.
A 3-bit field follows and is used to control or identify fragments.
The fragment offset field is 13-bits long, and allows a receiver determine
the place of a particular fragment in the original IP datagram.
An 8-bit time to live (TTL) field helps prevent datagrams from persisting
(e.g. going in circles) on an internetwork. Historically the TTL field
limited a datagram's lifetime in seconds, but has come to be a hop count
field. Each packet switch (or router) that a datagram crosses decrements the
TTL field by one. When the TTL field hits zero, the packet is no longer
forwarded by a packet switch and is discarded.
An 8-bit Protocol field follows. This field defines the next protocol used
in the data portion of the IP datagram. The Internet Assigned Numbers
Authority maintains a list of Protocol numbers. Common protocols and their
decimal values include ICMP (1), TCP (6) and UDP (17).
The following field is a 16-bit checksum field for the IPv4 datagram header.
Some values in a IPv4 datagram header may change at each packet switch hop,
so the checksum must be adjusted on its way through an internetwork.
The checksum is followed by a 32-bit source address and 32-bit destination
address respectively. (Note: IPv6 source and destination addresses are 128
bits each.)
Additional header fields (called options) may follow the destination address
field, but these are not often used. Option fields may be followed by a pad
field which ensures that the user data is aligned on a 32-bit word boundary.
(In IPv6, options move out of the standard header and are specified by a
Next Protocol field, similar in function to IPv4's Protocol field.)
IPv4 Addressing and Routing
Perhaps the most complex aspects of IP are addressing and routing.
Addressing refers to how end hosts are assigned IP addresses and how
subnetworks of IP host addresses are divided and grouped together. IP
routing is performed by all hosts, but most famously by internetwork
routers, which typically use either interior gateway protocols (IGPs) or
external gateway protocols (EGPs) to help make IP datagram forwarding
decisions across IP connected networks.
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