Private Key Cryptography:Introduction

Encryption can be viewed as a process of locking the confidential message inside a box.
Decryption can be viewed just inverse of encryption, process of unlocking the message for reading by one who is authorized.

Encryption and decryption done using the same key is Private Key Cryptography aka Symmetric Key Cryptography. The ciphers developed by such encryption are called as symmetric or secret ciphers. Here the secret key is shared among sender and receiver. 
Consider Alice and Bob as sender and receiver respectively and plain-text is the original message to be sent to Bob. This plaintext is first converted into ciphertext by applying encryption algorithm and is then transmitted towards Bob. Alice, while encryption uses a secret key which is also shared with Bob who uses it for decrypting the message received from Alice. This means that both Alice and Bob need a secure channel for exchanging the key. Traditional ciphers were generated by using this technique of Private Key Cryptography. 

Internet Control Message Protocol

The internet is managed and monitored by routers. Any unexpected is reported by a protocol called Internet Control Message Protocol (ICMP). As it goes with its name, this protocol is used as a control and to test the internet.
It must be noted that there are a number of messages associated with this protocol, but only important ones are discussed here in brief.

DESTINATION UNREACHABLE
Used when a packet fails to reach the destination or when the router/subnet can not locate the destination.

TIME EXCEEDED
Used when the packet is dropped as its counter reaches zero. This may signify heavy traffic and/or congestion.

PARAMETER PROBLEM
Used when an illegal/incorrect parameter is inserted header field. 

SOURCE QUENCH
Used for flow control and congestion control. When a packet is discarded, this message is sent to the source informing about the congestion is the path. 

REDIRECT
Used when the router judges that the packet is wrongly routed. 

ECHO and ECHO REPLY
These messages are used to confirm the availability of the destination. ECHO message is transmitted towards destination and ECHO REPLY message from destination to source confirms presence of destination. 

Other messages along with their formats are available here.