CCNA-1. Chapter5. OSI Network layer.

Network layer basic processes:

• Addressing (when address is added to a device, the device is then referred to as a host)
• Encapsulation (add among other information, the address of the host to which it is being sent)
• Routing (As the packet is forwarded, the Transport layer PDU, remain intact until the destination host is reached).
• Decapsulation (If  packet was addressed to this device the packet is decapsulated by the Network layer to Lv4)

Protocols implemented at the Network layer include:

• Internet Protocol version 4 (IPv4)
• Internet Protocol version 6 (IPv6)
• Novell Internetwork Packet Exchange (IPX)
• AppleTalk
• Connectionless Network Service (CLNS/DECNet)

Pv4 basic characteristics:

• Connectionless -  sending a packet without notifying the recipient.
• Best Effort (unreliable) - No overhead but no guarantee packet delivery.
• Media Independent - Operates independently of the medium carrying the data. BUT considers 1major characteristic of the media - MTU. Intermediary device need to fragment packet.when forwarding from one media to a media with a smaller MTU.

Maximum Transmission Unit (MTU) - the maximum size of PDU that each medium can transport

Fragmentation - spliting up a packet.

                                                                     IP packet

Fields:

• Version - Contains the IP version number (4).
• Header Length (IHL) - size of the packet header.
• Type-of-Service (ToS) (used to determine the priority of each packet. This field is Quality-of-Service (QoS) value)
• Packet Length - full packet size (header+data) in bytes.
• Identification - This field is primarily used for uniquely identifying fragments of an original IP packet.
• Flag (MF-fragmented and it is not the last fragment , DF-fragmentation of packet NOT permitted, need to fragment - discard)
  
  • Unfragmented packet (MF = 0, Fragment offset =0)
  • Last fragment of packet (MF = 0,Fragment offset = non-zero value)
  • Not last fragment of packet (MF=1, Fragment offset = non-zero value)
• Fragment Offset (if fragmentation occurs used with MF flag to identify the order of fragments at the destination)
• Time-to-Live (TTL) (-1 each time the packet is processed by a router, prevents packets forward indefinitely in routing loop)
• Protocol (upper-layer protocol: 01- ICMP, 06 - TCP, 17- UDP, 88- EIGRP)
• Header Checksum - used for error checking of packet header.
• IP Source Address (Enables the destination host to respond if required)
• IP Destination Address (Enables routers at each hop to forward the packet towards the destination)
• Options - provide additional services (rarely used).
• Padding - additional field is used if packet is less than minimum packet size.

Networks can be divided based on:

• Geographic location (each building or each floor of a multi-level building. Network management and operation.)
• Purpose (balance number of hosts on a network with the amount of traffic generated by the users. Traffic ballance)
• Ownership (boundary for security enforcement and management of each network. Network security management)

Reasons to divide large networks :

• Improving Performance (reduce broadcasts, prevents volumes of traffic to overwhelm bandwidth and routing capability)
• Security issues (access to and from resources outside each network can be prohibited, allowed, or monitored)
• Address Management (reduces the unnecessary overhead of all hosts needing to know all addresses, only gateway)

Routes in a routing table have three main features:

• Destination network
• Next-hop
• Metric (is used to decide which route appears on the routing table If there are two or more routes to destination)

When forwarding a packet, the router always select the most specific route!


Dynamic routing protocols - easy to manage, overhead that consumes network bandwidth, high router cpu processing.