# Internet Protocol

Routable & routed protocol
Used at layer 3 (Network)
IP is the most widely used implementation of a hierarchical network-addressing scheme
IP is a connectionless, unreliable, best-effort delivery protocol. Layer
Packet includes header - addressing & other control information + actual data - whatever is passed down from the higher layers.

# Connection oriented network services

A connection is established between the sender & the recipient before any data is transfered

Circuit switched

all packets travel sequentially across the same physical circuit, or more commonly, across the same virtual circuit

Packket switched

When the packets (datagram) pass from source to destination, they can:
- switch to different paths
- arrive out of order
Devices (routers) make the path determination for each packet based on a variety of criteria. Some of the criteria may differ from packet to packet.

Anatomy

Length: 4 bits - Header length
Service type: 8 bits - how the datagram should be handled by the routers
Packet length: 16 bits => 2¹⁶ = 65535 bytes
Identification: 16 bits, used to distinguish the fragments of a datagram from those of others, help in reassembling the datagram fragments
DF: 1 bit - Dont Fragment Flag this datagram
MF: 1 bit - More Fragments Flag if 1 and 0 if the last
Fragment Offset: 13 bits - sequence of fragment from 0 => n
Time to live: counter decreases to 0, avoid datagrams from looping endlessly
Transport: 8 bits - indicates which upper-layer protocol
- 06: TCP
- 17: UDP
- 01: ICMP (Ping)
- 08: EGP
Header checksum: 16 bits, ensure header integrity
Options:
- source route
- record route + timestamp
- securtiy
Padding: fulfill 0s

Routing is an OSI layer 3 function
find the most efficient path
Primary device is the Router
- maintain routing tables & make sure other routers know of changes in the network topology
- router switches to packets to appropriate interface, adds the necessary framing infor for the interface, and then transmits the frame

Path determination choose the next hop in the path for packet to travel based on routing metrics
- bandwidth
- hop count
- delay / tick
- load
- cost
- ...
Packet switching re-encapsulates the packet in the protocol needed for the specified port then swiches the packet out that port.

routing algorithm generates a number, called the metric value
typically, the smaller metric value, the better the path
metrics based on:
- single characteristic of a path
- combination of several characteristics

Distance vector:
- hop count
- interval update
Link state: use tree / graph structure to sync data => Shortest Path First algorithm
- event trigger update
- state connection