lecture6:IPv4 Header

IPv4 Header Format
The IPv4 packet header consists of 14 fields, of which 13 are required. The 14th field is optional and aptly named: options. The most significant bits are considered to come first (MSB 0 bit numbering). The most significant bit is numbered 0, so the version field is actually found in the four most significant bits of the first byte, for example.
• Version - The first header field in an IP packet is the four-bit version field. For IPv4, this is always equal to 4, for IPv6 is equal to 6.

• Internet Header Length (IHL)
The Internet Header Length (IHL) field has 4 bits, which is the number of 32-bit words. Since an IPv4 header may contain a variable number of options, this field specifies the size of the header (this also coincides with the offset to the data). The minimum value for this field is 5, which indicates a length of 5 × 32 bits = 160 bits = 20 bytes. As a 4-bit field, the maximum value is 15 words (15 × 32 bits, or 480 bits = 60 bytes).

• Type of Service (TOS) - Defines delay, throughput and reliability requirement of the IP packet.
Differentiated Services Code Point (DSCP)
Originally defined as the Type of service (ToS) field. This field is now defined by RFC 2474 (updated by RFC 3168 and RFC 3260) for Differentiated services (DiffServ). New technologies are emerging that require real-time data streaming and therefore make use of the DSCP field. An example is Voice over IP (VoIP), which is used for interactive data voice exchange.
Explicit Congestion Notification (ECN)
This field is defined in RFC 3168 and allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature that is only used when both endpoints support it and are willing to use it. It is only effective when supported by the underlying network.

• Total Length- Indicates the entire packet size, including header and data, in bytes.
This 16-bit field defines the entire packet size in bytes, including header and data. The minimum size is 20 bytes (header without data) and the maximum is 65,535 bytes. All hosts are required to be able to reassemble datagrams of size up to 576 bytes, but most modern hosts handle much larger packets. Sometimes links impose further restrictions on the packet size, in which case datagrams must be fragmented. Fragmentation in IPv4 is handled in either the host or in routers.

• Identification- Used for uniquely identifying fragments of an original IP datagram.