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CIS 81 Fundamentals of Networking Chapter 6: Network Layer Part 1 of 2

CIS 81 Fundamentals of Networking Chapter 6: Network Layer Part 1 of 2. CCNA Introduction to Networking 5.0 Rick Graziani Cabrillo College graziani@cabrillo.edu Fall 2013. Chapter 6: Objectives. Explain how network layer protocols and services support communications across data networks.

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CIS 81 Fundamentals of Networking Chapter 6: Network Layer Part 1 of 2

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  1. CIS 81 Fundamentals of NetworkingChapter 6: Network LayerPart 1 of 2 CCNA Introduction to Networking 5.0 Rick Graziani Cabrillo College graziani@cabrillo.edu Fall 2013

  2. Chapter 6: Objectives • Explain how network layer protocols and services support communications across data networks. • Explain how routers enable end-to-end connectivity in a small to medium-sized business network. • Determine the appropriate device to route traffic in a small to medium-sized business network. • Configure a router with basic configurations.

  3. The Network Layer

  4. Encapsulation and Decapsulation Data Link Trailer Data Link Header IP Header TCP Header HTTP Header Data Data Link Trailer Data Link Trailer Data Link Header Data Link Header IP Packet IP Packet Data Link Trailer Data Link Trailer Data Link Header Data Link Header IP Packet IP Packet Data Link Trailer Data Link Trailer Data Link Header Data Link Header IP Packet IP Packet Data Link Trailer Data Link Header IP Header TCP Header HTTP Header Data

  5. Encapsulation SEGMENT PACKET FRAME

  6. IP Functions of the Network Layer IP • The network layer, or OSI Layer 3, provides services to allow end devices to exchange data across the network. • The network layer uses four basic processes: • Addressing end devices • Encapsulation • Routing • De-encapsulation

  7. Network Layer Protocols • Common Network Layer Protocols • Internet Protocol version 4 (IPv4) • Internet Protocol version 6 (IPv6) • Legacy Network Layer Protocols • Novell Internetwork Packet Exchange (IPX) • AppleTalk • Connectionless Network Service (CLNS/DECNet)

  8. Characteristics of IPv4 • Connectionless: • No connection is established before sending data packets. • Best effort delivery: • No additional overhead is used to guarantee packet delivery. • Makes it unreliable …? • Media independent: • Operates independently of the medium carrying the data.

  9. Connectionless Service = Postal Service

  10. Connectionless Service

  11. Best Effort Delivery = Unreliable

  12. Best Effort Delivery = Unreliable • IP is unreliable because it doesn’t have the capability to manage, and recover from, undelivered or corrupt packets. • TCP (if used) will manage the transmission reliability. • It also makes for a smaller IP header. • Less overhead = less delay in delivery = very fast.

  13. IPv4 Media Independent • IP doesn’t care what type of media the packet is carried on.

  14. It is my job to reconstruct the packets. The outgoing link has a large enough MTU but I don’t reconstruct packets. MTU The outgoing link has a smaller MTU so I have to fragment the packets. IP Packet IP Packet IP Packet IP Packet IP Packet Network link with larger MTU Network link with smaller MTU Network link with larger MTU IP Packet IP Packet IP Packet IP Packet • The Network layer does consider the maximum size of PDU that each medium can transport. • This is referred to as the Maximum Transmission Unit (MTU). • The Network layer determines how large to create the packets. • Routers may need to split up a packet when forwarding it from one media to a media with a smaller MTU. • This process is called fragmenting the packet or fragmentation. • This is similar to segmenting at the Transport layer but happens at the Network layer. IP Packet IP Packet

  15. IPv4 Packet

  16. IPv4 Packet IP Header Data (Payload) • IPv4 has been in use since 1983 when it was deployed on the Advanced Research Projects Agency Network (ARPANET). • An IPv4 packet has two parts: • IP Header - Identifies the packet characteristics. • Payload - Contains the Layer 4 segment information and the actual data.

  17. IPv4 Header – Significant Fields Byte 1 Byte 2 Byte 3 Byte 4

  18. IPv4 Header – Validation Fields Byte 1 Byte 2 Byte 3 Byte 4

  19. Sample IPv4 Packet

  20. Version (4 bits) • Indicates the version of IP currently used. • 0100 = 4 and therefore IPv4 • 0110 = 6 and therefore IPv6

  21. IP Header Length (4 bits) • Identifies the number of 32-bit words in the header. • The IHL value varies due to the Options and Padding fields. • The minimum value for this field is 5 (i.e., 5×32 = 160 bits = 20 bytes) and the maximum value is 15 (i.e., 15×32 = 480 bits = 60 bytes).

  22. Differentiated Services (8 bits) • Formerly called the Type of Service (ToS) field. • The field is used to determine the priority of each packet. • First 6 bits identify the Differentiated Services Code Point (DSCP) value for QoS. • Last 2 bits identify the explicit congestion notification (ECN) value used to prevent dropped packets during times of network congestion.

  23. Total Length (16 bits) • Sometimes referred to as the Packet Length. • Defines the entire packet (fragment) size, including header and data, in bytes. • The minimum length packet is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes. .

  24. A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU. When this happens, fragmentation occurs and the IPv4 packet uses the following 3 fields to keep track of the fragments

  25. Identification (16 bits) • Field uniquely identifies the fragment of an original IP packet.

  26. Flag (3 bits) • This 3-bit field identifies how the packet is fragmented. • It is used with the Fragment Offset and Identification fields to help reconstruct the fragment into the original packet.

  27. Fragment Offset (13 bits) • Field identifies the order in which to place the packet fragment in the reconstruction of the original unfragmented packet.

  28. Time-to-Live (TTL) (8 bits) • Used to limit the lifetime of a packet. • It is specified in seconds but is commonly referred to as hop count. • The packet sender sets the initial TTL value and is decreased by one each time the packet is processed by a router, or hop. • If the TTL field decrements to zero, the router discards the packet and sends an ICMP Time Exceeded message to the source IP address. • Thetraceroute command uses this field to identify the routers used between the source and destination.

  29. Protocol (8 bits) • Field indicates the data payload type that the packet is carrying, which enables the network layer to pass the data to the appropriate upper-layer protocol. • Common values include ICMP (1), TCP (6), and UDP (17). • Others: GRE (47), ESP (50), EIGRP (88), OSPF (89) • http://www.iana.org/assignments/protocol-numbers/

  30. Header Checksum (8 bits) • Field is used for error checking of the IP header. • The checksum of the header is recalculated and compared to the value in the checksum field. • If the values do not match, the packet is discarded.

  31. Source IP Address (32 bits) • Contains a 32-bit binary value that represents the source IP address of the packet.

  32. Destination IP Address (32 bits) • Contains a 32-bit binary value that represents the destination IP address of the packet.

  33. Sample IPv4 Headers

  34. Sample IPv4 Headers

  35. Sample IPv4 Headers

  36. IPv6 Packet IPv4

  37. Limitations of IPv4 • Since 1983, IPv4 has been updated to address new challenges. • However, even with changes, IPv4 still has three major issues: • IP address depletion • Internet routing table expansion • Lack of end-to-end connectivity

  38. IP Address Depletion • IPv4 has a limited number of unique public IP addresses available. • Although there are approximately 4 billion IPv4 addresses, the increasing number of new IP-enabled devices, always-on connections, and the potential growth of less-developed regions have increased the need for more addresses.

  39. Blocks Assigned in 1993

  40. Blocks Assigned in 2000

  41. Blocks Assigned in 2007

  42. Blocks Assigned in 2010

  43. IPv4 Address Depletion • In October 2010, less than 5% of the public IPv4 addresses remained unallocated.

  44. November 30, 2010Available Blocks: 7

  45. IANA Runs out of IPv4 • Monday, January 31, 2011 IANA allocated two blocks of IPv4 address space to APNIC, the RIR for the Asia Pacific region (39/8 and 106/8) • This triggered a global policy to allocate the remaining IANA pool of 5 /8’s equally between the five RIRs. • So, basically…

  46. Internet Routing Table Expansion • A routing table is used by routers to make best path determinations. • As the number of servers (nodes) connected to the Internet increases, so too does the number of network routes. • These IPv4 routes consume a great deal of memory and processor resources on Internet routers.

  47. Lack of End-to-End Connectivity NAT 192.168.1.0/24 RFC 1918 Private Address Public IPv4 Address • Network Address Translation (NAT) is a technology commonly implemented within IPv4 networks. • NAT provides a way for multiple devices to share a single public IP address. • However, because the public IP address is shared, the IP address of an internal network host is hidden. • This can be problematic for technologies that require end-to-end connectivity.

  48. IETF To The Rescue • To address these problems, the IETF it implemented solutions to solve these problems. • Short Term solutions included: • Subnetting • Variable-length subnet masking (VLSM) • Classless interdomain routing (CIDR) • Supernetting • Network Address Translation (NAT) • Private Addresses • However, its long term solution was IP version 6 (IPv6)

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