Struct IPv4Header 

pub struct IPv4Header<Span> {

    pub version: u8,
    pub ihl: u8,
    pub tos: u8,
    pub length: u16,
    pub id: u16,
    pub flags: u8,
    pub fragment_offset: u16,
    pub ttl: u8,
    pub protocol: IPProtocol,
    pub chksum: u16,
    pub source_addr: Ipv4Addr,
    pub dest_addr: Ipv4Addr,
    pub options: Span,
}

https://en.wikipedia.org/wiki/Internet_Protocol_version_4

Fields

version: u8

The first header field in an IP packet is the four-bit version field. For IPv4, this is always equal to 4.

ihl: u8

The IPv4 header is variable in size due to the optional 14th field (options). The IHL field contains the size of the IPv4 header; it has 4 bits that specify the number of 32-bit words in the header. 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; this means that the maximum size of the IPv4 header is 15 × 32 bits = 480 bits = 60 bytes.

tos: u8

Originally defined as the type of service (ToS), this field specifies differentiated services (DiffServ) per RFC 2474. Real-time data streaming makes use of the DSCP field. An example is Voice over IP (VoIP), which is used for interactive voice services.

length: u16

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. Links may impose further restrictions on the packet size, in which case datagrams must be fragmented. Fragmentation in IPv4 is performed in either the sending host or in routers. Reassembly is performed at the receiving host.

id: u16

This field is an identification field and is primarily used for uniquely identifying the group of fragments of a single IP datagram. Some experimental work has suggested using the ID field for other purposes, such as for adding packet-tracing information to help trace datagrams with spoofed source addresses, but RFC 6864 now prohibits any such use.

flags: u8

A three-bit field follows and is used to control or identify fragments. They are (in order, from most significant to least significant):

bit 0: Reserved; must be zero. bit 1: Don’t Fragment (DF) bit 2: More Fragments (MF)

If the DF flag is set, and fragmentation is required to route the packet, then the packet is dropped. This can be used when sending packets to a host that does not have resources to perform reassembly of fragments. It can also be used for path MTU discovery, either automatically by the host IP software, or manually using diagnostic tools such as ping or traceroute. For unfragmented packets, the MF flag is cleared. For fragmented packets, all fragments except the last have the MF flag set. The last fragment has a non-zero Fragment Offset field, differentiating it from an unfragmented packet.

fragment_offset: u16

This field specifies the offset of a particular fragment relative to the beginning of the original unfragmented IP datagram. The fragmentation offset value for the first fragment is always 0. The field is 13 bits wide, so that the offset can be from 0 to 8191 (from (20 –1) to (213 – 1)). Fragments are specified in units of 8 bytes, which is why fragment length must be a multiple of 8. Therefore, the 13-bit field allows a maximum offset of (213 – 1) × 8 = 65,528 bytes, with the header length included (65,528 + 20 = 65,548 bytes), supporting fragmentation of packets exceeding the maximum IP length of 65,535 bytes.

ttl: u8

An eight-bit time to live field limits a datagram’s lifetime to prevent network failure in the event of a routing loop. It is specified in seconds, but time intervals less than 1 second are rounded up to 1. In practice, the field is used as a hop count—when the datagram arrives at a router, the router decrements the TTL field by one. When the TTL field hits zero, the router discards the packet and typically sends an ICMP time exceeded message to the sender. The program traceroute sends messages with adjusted TTL values and uses these ICMP time exceeded messages to identify the routers traversed by packets from the source to the destination.

protocol: IPProtocol

This field defines the protocol used in the data portion of the IP datagram. IANA maintains a list of IP protocol numbers as directed by RFC 790.

chksum: u16

The 16-bit IPv4 header checksum field is used for error-checking of the header. When a packet arrives at a router, the router calculates the checksum of the header and compares it to the checksum field. If the values do not match, the router discards the packet. Errors in the data field must be handled by the encapsulated protocol. Both UDP and TCP have separate checksums that apply to their data. When a packet arrives at a router, the router decreases the TTL field in the header. Consequently, the router must calculate a new header checksum. The checksum field is the 16 bit one’s complement of the one’s complement sum of all 16 bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero.

source_addr: Ipv4Addr

This 32-bit field is the IPv4 address of the sender of the packet. Note that this address may be changed in transit by a network address translation device.

dest_addr: Ipv4Addr

This 32-bit field is the IPv4 address of the receiver of the packet. As with the source address, this may be changed in transit by a network address translation device.

options: Span

The options field is not often used. Packets containing some options may be considered as dangerous by some routers and be blocked. Note that the value in the IHL field must include enough extra 32-bit words to hold all the options plus any padding needed to ensure that the header contains an integer number of 32-bit words. If IHL is greater than 5 (i.e., it is from 6 to 15) it means that the options field is present and must be considered. The list of options may be terminated with an EOOL (End of Options List, 0x00) option; this is only necessary if the end of the options would not otherwise coincide with the end of the header.

Trait Implementations

impl<Span: Clone> Clone for IPv4Header<Span>

fn clone(&self) -> IPv4Header<Span>

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<Span: Debug> Debug for IPv4Header<Span>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Span: PartialEq> PartialEq for IPv4Header<Span>

fn eq(&self, other: &IPv4Header<Span>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Span: Eq> Eq for IPv4Header<Span>

impl<Span> StructuralPartialEq for IPv4Header<Span>