Internet Protocol overview for my own reference, based on:
Communication Layers: rfc-1122
IP protocol: rfc-791
ARP: rfc-826
IP encapsulation – MUST: RFC-894
IEEE 802 encapsulation SHOULD:  RFC-1042
PPP: rfc2516
In many places this is just copy-paste from RFC.

  1. Protocols
Application layer combine application & presentation of OSI model user protocols:
– telnet
– FTP
– SMTP
support protocols:
SNMP (Simple Network Management Protocol)
– BOOTP
– RARP
DNS (Domain Name Server)
DHCP (Dynamic Host Configuration Protocol)
Transport layer provides end-to-end communication services for applications TCP (Transmission Control Protocol)
UDP (User Datagram Protocol)
Internet layer Network protocol of OSI model
IP is a connectionless or datagram internetwork service:
– carry data from source host to destination host
– no end-to-end delivery guarantees (dublicates, damaged, etc. IP datagrams)
– IP: IPv4, IPv6
ICMP (Internet Control Message Protocol)
IGMP (Internet Group Management Protoc
ol)
 Link layer  Media-access layer PPP (Point to Point Protocol)
ARP (Address Resolution Protocol)

2. Terminology

Segment A segment is the unit of end-to-end transmission in the TCP protocol.  A segment consists of a TCP header followed by application data.  A segment is transmitted by encapsulation inside an IP datagram.
Message In this description of the lower-layer protocols, a message is the unit of transmission in a transport layer protocol.  In particular, a TCP segment is a message.  A message consists of a transport protocol header followed by application protocol data.  To be transmitted end-to- end through the Internet, a message must be encapsulated inside a datagram.
IP Datagram An IP datagram is the unit of end-to-end transmission in the IP protocol.  An IP datagram consists of an IP header followed by transport layer data, i.e., of an IP header followed by a message. In the description of the internet layer (Section 3), the unqualified term “datagram” should be understood to refer to an IP datagram.
Packet A packet is the unit of data passed across the interface between the internet layer and the link layer.  It includes an IP header and data.  A packet may be a complete IP datagram or a fragment of an IP datagram.
Frame  A frame is the unit of transmission in a link layer protocol, and consists of a link-layer header followed by a packet.
Multihomed A host is said to be multihomed if it has multiple IP addresses.
Physical network interface  This is a physical interface to a connected network and has a (possibly unique) link-layer address.  Multiple physical network interfaces on a single host may share the same link-layer address, but the address must be unique for different hosts on the same physical network.
Logical [network] interface  We define a logical [network] interface to be a logical path, distinguished by a unique IP address, to a connected network.
Specific-destination address  This is the effective destination address of a datagram, even if it is broadcast or multicast
Path  At a given moment, all the IP datagrams from a particular source host to a particular destination host will typically traverse the same sequence of gateways.  We use the term “path” for this sequence.  Note that a path is uni-directional; it is not unusual to have different paths  in the two directions between a given host pair
MTU  The maximum transmission unit, i.e., the size of the  largest packet that can be transmitted.

The terms frame, packet, datagram, message, and segment are
illustrated by the following schematic diagrams:

eth1

2.5  LINK LAYER REQUIREMENTS SUMMARY rfc-1122

3.