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Viop
IP (Internet Protocol)
The Internet Protocol is the lowest level protocol considered in this document. It is responsible for the delivery of packets (or datagrams) between host computers. IP is a connectionless protocol, that is, it does not establish a virtual connection through a network prior to commencing transmission; this is the job for higher level protocols. IP makes no guarantees concerning reliability, flow control, error detection or error correction. The result is that datagrams could arrive at the destination computer out of sequence, with errors or not even arrive at all. Nevertheless, IP succeeds in making the network transparent to the upper layers involved in voice transmission through an IP based network. Any Voice over IP transmission must use IP (by definition). IP is not well suited to voice transmission. Real time applications such as voice and video require guaranteed connection with consistent delay characteristics. Higher layer protocols address these issues (to a certain extent). The diagram below shows the header that proceeds the data payload to be transmitted. In its most basic form, the header comprises 20 octets. There are optional fields which can be appended to the basic header, but these offer additional capabilities which are not necessary for VoIP transmission as described in this document.
The version of IP being used. For this format header, the version would be 4. IHL The length of the IP header in units of four octets (32 bits). For the basic header shown in this diagram, the value would be 5 (each line in the diagram represents four octets).
Type of service
Specifies the quality of service requested by the host computer sending the datagram. This is not always effectively supported by routers or Internet Service Providers.
Total length
The length of the datagram, measured in octets, including the header and payload.
Identification
As well as handling the addressing of datagrams between two computers (or hosts), IP needs to handle the splitting of data payloads into smaller packages. This process, known as fragmentation, is required because, although a single IP datagram can handle a theoretical maximum length of 65,515 octets, lower link layer protocols such as Ethernet cannot always handle these large packet sizes. This field is a unique reference number assigned by the sending host to aid in the reassembly of a fragmented datagram.
Flags
These flags indicate whether the datagram may be fragmented, and, if it has been fragmented, whether further fragments follow this one.
Fragment offset
This field indicates where in the datagram this fragment belongs. It is measured in units of 8 octets (64 bits).
Time to live
This field indicates the maximum time the datagram is permitted to remain in the internet system. This parameter ensures that a datagram which cannot reach its destination host is given a finite lifetime.
Protocol
This indicates the higher level protocol in use for this datagram. Numbers have been assigned for use with this field to represent such transport layer protocols as TCP and UDP.
Header checksum
This is a checksum covering the header only.
Source address
The IP address of the host which generated this datagram. IPv4 addresses are 32 bits in length and, when written or spoken, a dotted decimal notation is used (e.g.: 192.168.0.1).
Destination address
The IP address of the destination host.