at the destination. In connection-oriented
systems, a connection is established between the sender and the
recipient before any data is transferred. An example of a
connection-oriented network is the telephone system. The caller
places the call, a connection is established, and then
communication occurs. Connectionless network processes are
often referred to as packet switched processes. As the packets
pass from source to destination, packets can switch to
different paths, and possibly arrive out of order. Devices make
the path determination for each packet based on a variety of
criteria. Some of the criteria, such as available bandwidth,
may differ from packet to packet. Connection-oriented network
processes are often referred to as circuit switched processes.
A connection with the recipient is first established, and then
data transfer begins. All packets travel sequentially across
the same physical or virtual circuit. The Internet is a
gigantic, connectionless network in which all packet deliveries
are handled by IP. TCP adds Layer 4, connection-oriented
reliability services to IP. Web Links The IP Protocol
http://ironbark.ucnv.edu.au/courses/ subjects/c202/1998/
lectures/Lect19.html
Content 10.1 Routed
Protocol 10.1.5 Anatomy of an IP packet IP
packets consist of the data from upper layers plus an IP
header. The IP header consists of the following:
- Version – Indicates the version of IP currently
used; four bits. If the version field is different than the IP
version of the receiving device, that device will reject the
packets.
- IP header length (HLEN) – Indicates
the datagram header length in 32-bit words. This is the total
length of all header information, accounting for the two
variable-length header fields.
- Type-of-service (TOS) – Specifies the level
of importance that has been assigned by a particular
upper-layer protocol, eight bits.
- Total
length – Specifies the length of the entire packet in
bytes, including data and header, 16 bits. To get the length of
the data payload subtract the HLEN from the total length.
- Identification – Contains an integer that
identifies the current datagram, 16 bits. This is the sequence
number.
- Flags – A three-bit field in which the
two low-order bits control fragmentation. One bit specifies
whether the packet can be fragmented, and the other specifies
whether the packet is the last fragment in a series of
fragmented packets.
- Fragment offset – Used to
help piece together datagram fragments, 13 bits. This field
allows the previous field to end on a 16-bit boundary.
- Time-to-live (TTL) – A field that specifies the
number of hops a packet may travel. This number is decreased by
one as the packet travels through a router. When the counter
reaches zero the packet is discarded. This prevents packets
from looping endlessly.
- Protocol – indicates
which upper-layer protocol, such as TCP or UDP, receives
incoming packets after IP processing has been completed, eight
bits.
- Header checksum – helps ensure IP header
integrity, 16 bits.
- Source address – specifies
the sending node IP address, 32 bits.
- Destination
address – specifies the receiving node IP address, 32 bits.
- Options – allows IP to support various
options, such as security, variable length.
- Padding – extra zeros are added to this field to
ensure that the IP header is always a multiple of 32 bits.
- Data – contains upper-layer information,
variable length up to 64 Kb.
While the IP source
and destination addresses are important, the other header
fields have made IP very flexible. The header fields are the
information that is provided to the upper layer protocols
defining the data in the packet. Web Links IP Packet
Header http://www.erg.abdn.ac.uk/users/gorry/ course/
inet-pages/ ip-packet.htm
Content 10.2 IP
Routing Protocols 10.2.1 Routing overview
Routing is an OSI Layer 3 function. Routing is a hierarchical
organizational scheme that allows individual addresses to be
grouped together. These individual addresses are treated as a
single unit until the destination address is needed for final
delivery of the data. Routing is the process of finding the
most efficient path from one device to another. The primary
device that performs the routing process is the router. The
following are the two key functions of a router:
- Routers must maintain routing tables and make sure other
routers know of changes in the network topology. This function
is performed using a routing protocol to communicate network
information with other routers.
- When packets arrive
at an interface, the router must use the routing table to
determine where to send them. The router switches the packets
to the appropriate interface, adds the necessary framing
information for the interface, and then transmits the frame.
A router is a network layer device that uses one or
more routing metrics to determine the optimal path along which
network traffic should be forwarded. Routing metrics are values
used in determining the advantage of one route over another.
Routing protocols use various combinations of metrics for
determining the best path for data. Routers interconnect
network segments or entire networks. Routers pass data frames
between networks based on Layer 3 information. Routers make
logical decisions regarding the best path for the delivery of
data. Routers then direct packets to the appropriate output
port to be encapsulated for transmission. The encapsulation and
de-encapsulation process occurs each time a packet transfers
through a router. As shown in Figure 4, the process of sending
data from one device to another involves the process of
encapsulation and de-encapsulation. This process breaks up the
data stream into segments, adds the appropriate headers and
trailers then transmits the data. The de-encapsulation process
is the opposite process, removing the headers and trailers,
then recombining the data into a seamless stream. This course
focuses on the most common routable protocol, which is the
Internet Protocol (IP). Other examples of routable protocols
include IPX/SPX and AppleTalk. These protocols provide Layer 3
support. Non-routable protocols do not provide Layer 3 support.
The most common non-routable protocol is NetBEUI. NetBEUI is a
small, fast, and efficient protocol that is limited to frame
delivery within one segment. Web Links Routing Basics
http://www.cisco.com/univercd/cc/ td/doc/cisintwk/
ito_doc/routing.htm
Content 10.2
IP Routing Protocols 10.2.2 Routing versus
switching Routing is often contrasted with switching.
Routing and switching might seem to perform the same function
to the inexperienced observer. The primary difference is that
switching occurs at Layer 2, the data link layer, of the OSI
model and routing occurs at Layer 3. This distinction means
routing and switching use different information in the process
of moving data from source to destination. The relationship
between switching and routing parallels that of telephone local
and long distance calls. When a telephone call is made to a
number within the same area code, a local switch handles the
call. However, the local switch can only keep track of its own
local numbers. The local switch cannot handle all the telephone
numbers in the world. When the switch receives a request for a
call outside of its area code, it switches the call to a
higher-level switch that recognizes area codes. The
higher-level switch then switches the call so that it
eventually gets to the local switch for the area code dialed.
The router performs a function similar to that of the
higher-level switch in the telephone example. Figure shows the
ARP tables for Layer 2 addressing and routing tables for Layer
3 addressing. Each computer and router interface maintains an
ARP table for Layer 2 communication. The ARP table is only