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Today, we begin our lessons on TCP/IP protocols and advanced topics.
TCP/IP Protocol Suite, is to build a network of
networks or the Internet that can operate over multiple,
coexisting, and heterogeneous network technologies.
The goal is to provide ubiquitous connectivity through the IP packet transfer.
The TCP/IP Protocol Suite usually refers,
not only to the two most well-known protocols called Transmission Control Protocol (TCP),
and as the Internet Protocol (IP),
but also to other protocols,
such as a User Datagram Protocol (UDP),
the Internet Control Message Protocol (ICMP),
the Address Resolution Protocol (ARP),
and other basic applications.
The Protocol Data Unit of a given layer is encapsulated in
a protocol data unit of the layer below, as shown in the figure.
For example, an HTTP request is passed to the TCP layer,
which encapsulates the message into a TCP segment.
The TCP header contains the source and destination port numbers.
So TCP segment in turn is passed to
the IP layer where it is encapsulated in an IP packet.
The IP packet header contains
an IP network address for the sender and an IP network address for the destination.
The IP packet is then passed through the network interface.
Each host in the Internet is identified by a globally unique IP address.
An IP address is divided in two parts,
a network ID and a host ID.
Routing decision is done based on destination IP address.
The Internet Protocol (IP),
provides a connection-less best effort delivery service to the transport layer.
The term best effort indicates that the IP we're tryi-
it's best to forward a packet to the destination,
but it doesn't guarantee that a packet will be delivered to the destination,
nor its quality of service.
Higher layer protocols must deal with the reliability issue, if necessary.
This design decision, however,
reduce the complexity of the IP and increases its flexibility.
So IP packet contains a header port and a data port.
The format of the header port is shown in this figure.
The header has a fixed length component of 20 bytes plus a variable
length component consisting of options that it can be up to 40 bytes.
Let's briefly discuss the meaning of each field in the IP header.
The version of the header indicates the version number used by the IP packet,
4 or 6 currently.
As IP header can be of different lengths,
the field of header lengths specifies the length of the header into 32-bit words.
The type of service field traditionally specifies the quality of the packet.
The fear is not in common use.
Recently, a work in differentiated associates redefines
the field to include other services besides the best effort.
As a packet may have different amount of data,
the total length field specifies a number of
bytes of the IP packet including header and data.
Notice the maximum possible length,
64 kilobytes is rarely used.
As the internet often limits the payload length to 1,500 bytes.
The identification flex under
fragmented offset fields are used for fragmentation and reassembly.
The time-to-live field was originally defined to indicate
the amount of time in seconds the packet is allowed to remain in the network.
However, the routers interpreted this field to indicate
the number of hops the packet is allowed to traverse in the network.
If the field reaches 0,
the router discards the packet.
It can be used to limit the scale and remove the duplicates due to flooding.
The protocol field specifies
the upper layer protocol that it is to receive IP data at a destination.
The header checksum field verifies the integrity of the IP header.
Please recall the calculation of
the Internet checksum that was discussed in ella control.
Also note that the data part is not verified and is left to upper layer protocols.
Source and destination IP addresses contains the address of source and destination hosts.
The options field, which is of variable length,
allows a packet to request special features such as security level,
route to be taken by the packet,
and timestamp at each router.
As option is of variable lengths,
the padding is used to make the header,
a multiple of the 32-bit words.
When an IP packet is passed to the router,
the following processing takes place.
First, the router calculates the header checksum for
correctness and then checks that field in header.
For example, version and a log- total length contains valid values.
Next, the router identifies the next hop for
the IP packet by consulting the routing table,
then the router changes the fields such as time-to-live
and header checksum and forwards the packet along the next hop.
This concludes today's lesson.
Xiaobo ZhouProfessor, Interim Dean
