Chapter 5 Network Layer

Transcription

1 Chapter 5 Network Layer

2 Network Layer IPv4 2

3 IP Header Application Header + data 3

4 IP IP IP IP 4

5 Focus on Transport Layer IP IP 5

6 Network Layer The Network layer (Layer 3) provides services to exchange the data over the network between identified end devices. Layer 3 uses four basic processes: Addressing Encapsulation Routing Decapsulation 6

7 Addressing Source IP = Destination IP = Source IP = Destination IP = What would be the Source IP Address and Destination IP Address of a Packet from the client to the server? What would be the Source IP Address and Destination IP Address of a Packet from the server to the client? More later! 7

8 Reminder of encapsulation/decapsulation Data Link Header IP Header TCP Header HTTP Header Data Data Link Trailer Data Link Header IP Packet Data Link Trailer Data Link Header IP Packet Data Link Trailer Data Link Header IP Packet Data Link Trailer Data Link Header IP Header TCP Header HTTP Header Data Data Link Trailer 8

9 Decapsulation Is the Destination IP Address of this packet my IP Address? Destination Arrival packet processed at Layer 3. Destination address examined. If the address is correct segment is passed up to the appropriate service at Transport layer. 9

10 Routing Source IP = Destination IP = Routers examine Layer 3 Destination IP addresses to forward packets. Search their routing tables. Send the packet to the next-hop router or host if on that network 10

11 Network Layer Protocols The Internet Protocol (IPv4 and IPv6) is the most widely-used Layer 3 data carrying protocol and will be the focus of this course. 11

12 IPv4 basic characteristics 12

13 Connectionless IP does not notify the destination host. Which layer 4 protocol on the sending host will establish a connection? TCP: A connection-oriented protocol. Which layer 4 protocol on the sending host will not establish a connection? UDP: A connectionless protocol. 13

14 Best Effort Service (unreliable) Layer 3 (IP) Speed over reliability Unreliable: Does not have the capability or responsibility to manage, and recover from, undelivered or corrupt packets. Who does? TCP at the end-to-end hosts 14

15 Media Independent Responsibility of the OSI Data Link layer to take an IP packet and prepare it for transmission over the communications medium. Transport of IP packets is not limited to any particular medium. May need to fragment the packet if it is too many bits (later). 15

16 IP Header Where I came from. IP Destination Address 32-bit binary value that represents the packet destination Network layer host address. IP Source Address Where I am going. 32-bit binary value that represents the packet source Network layer host address. 16

17 IP s TTL Time To Live field Sending hosts generates the value for TTL. Common operating system TTL values are: UNIX: 255 Linux: 64 or 255 depending upon vendor and version Microsoft Windows 95: 32 Microsoft Vista:

18 IP s TTL Time To Live field Decrement by 1, if 0 drop the packet. Decremented by each router. If the router decrements the TTL field to 0, it will then drop the packet. What is the advantage to decrementing the TTL by each router and dropping the packet if it is 0? So IP packets can not travel around the Internet forever, from router to router. 18

19 IP s Protocol Field Protocol = 06 TCP Protocol field enables the Network layer to pass the data to the appropriate upper-layer protocol. Example values are: 01 ICMP 06 TCP 17 UDP 19

20 IP s ToS Field Type-of-Service is used to determine the priority of each packet. Enables Quality-of-Service (QoS) mechanism for high priority traffic. What types of traffic might a network administrator need to give priority to? Traffic that cannot accept any delays. VoIP Streaming video 20

21 IP Fragmentation TCP MSS defines the maximum size of the data in the TCP segment. 20 bytes 20 bytes 1460 bytes Ethernet MTU defines the maximum size of the data in the Ethernet frame. TCP MSS = 1460 Data = 1460 bytes 1500 bytes The host using Ethernet, MTU of 1500 octets so I will set my MSS to Determining TCP MTU The default Ethernet MTU value for a PC is 1500 bytes. (curriculum says MSS) Typical Maximum Segment Size (MSS) of a TCP segment is 1460 bytes. The number of bytes of data. How much is enough? 21

22 IP Fragmentation Original IP Packet IP IP Header = 20 bytes Data = 1480 bytes IP Packet Fragments IP IP IP Data = 500 Data = 500 Data = 480 L2 L2 Data = 520 Data = 520 L2 L2 This packet is too big to go over my serial link all at once. I need to break it into smaller fragments L2 Data = 500 L2 A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU. If Don t Fragment flag set, it will not fragment packet, but discard it. 22

23 IP Packet IP Fragmentation The outgoing link has a smaller large enough MTU so MTU I have but to I fragment don t reconstruct the packets. packets. It is my job to reconstruct the packets. 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 IP Packet IP Packet When fragmentation occurs, it does not get reconstructed until it reaches the host. This takes processing time. Fragment Offset field identifies the order 23

24 Path MTU Discovery Path MTU Discovery (Not discussed here, but is important) RFC 1191 (RFC1191) Path MTU Discovery and Filtering ICMP Marc Slemko 24

25 Other IPv4 fields Version - Contains the IP version number (4) Header Length (IHL) - Specifies the size of the packet header. Packet Length - This field gives the entire packet size, including header and data, in bytes. Identification - This field is primarily used for uniquely identifying fragments of an original IP packet Header Checksum - The checksum field is used for error checking the packet header. Options - There is provision for additional fields in the IPv4 header to provide other services but these are rarely used. 25

26 Host and Network Addresses

27 IP Addresses First look Kiwi Airliners - Network Address / / / / / / / / / / / / / /16 Host IP addresses are IP addresses assigned to end devices such as: Client computers Server computers Network Printers Router interfaces Note: the /16 refers to the subnet mask, which will be discussed later. Note: Intermediary devices such as a switch may have an IP address to allow the network administrator to Telnet to the device for remote management. 27

28 IP Addresses First look Kiwi Airliners - Network Address / / / / / / / / / / / / / /16 Host IP addresses are members of a group of addresses call the Network Address IANA (Internet Assigned Numbers Authority) have the responsibility to allocate network addresses. A company or individual needing a network addresses typically goes to their ISP ISPs then allocate network addresses to their customers. More detail in the next chapter. 28

29 IP Addresses First look Network Address /16 ISP Network Address / /16 Internet / / / /16 Host IP Address Unique host IP address Default Gateway A router which is used to forward packets out of the network. This is a host IP address on the router. Host IP address on the same network as the host. The host only has to be aware of: Its own network address Default gateway IP address to reach all devices outside its own network 29

30 IP Addresses First look Network Address /16 Gateway: ISP Network Address / /16 Gateway: Internet / / / /16 Gateway: All hosts in the same network will typically have the same default gateway IP address. 30

31 Confirming IP Address, Default Gateway C:\> ipconfig Windows IP Configuration Ethernet adapter Local Area Connection: Connection-specific DNS Suffix. : IP Address : Subnet Mask : Default Gateway : Root# ifconfig eth0 Link encap:ethernet HWaddr 00:0F:20:CF:8B:42 inet addr: Bcast: Mask: UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets: errors:1 dropped:0 overruns:0 frame:0 TX packets: errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes: ( Mb) TX bytes: ( Mb) Interrupt:28 Linux: netstat rn for default gateway information. 31

32 Subnets Kiwi Airliners - Network Address / / / / / / / / / / / / / / / / / / / / / /24 Networks can be subdivided into subnets. This provides for several benefits which we will discuss later. Networks can be grouped based on factors that include: Geographic location, Purpose, Ownership 32

33 A Quick Look at Routing

34 Routing First Look Network / /24 Network /24 C /24 is direction connected, FastEthernet0/1 Routers know about: Directly connected networks (C): Network addresses of its interfaces Remote networks 34

35 Routing First Look Network / /24 Network /24 C /24 is direction connected, FastEthernet0/1 Routers know about: Directly connected networks (C): Network addresses of its interfaces When a router is configured with the IP address/mask on an interface the router knows that it has an interface which is part of that network. This is just like a host that is configured with an IP address/mask. (coming) 35

36 Routing First Look Network / /24 Network /24 C /24 is direction connected, FastEthernet0/1 Routers learn about remote networks using: Static routes Dynamic Routing Protocol (R = RIP) Routes in a routing table have three main features: Destination network Next-hop Metric 36

37 Routing First Look Network / /24 Network /24 C /24 is direction connected, FastEthernet0/1 Static routes Manually entered by the administrator Dynamic Routing protocols Routers automatically learn about remote networks Ex: RIP, EIGRP, OSPF, IS-IS, BGP 37

38 Host Routing Table netstat r or route print Hosts also have a local routing table. Usually only contains: Its own network address (directly connected network) Default gateway IP address Hosts usually do not have remote networks in their routing tables 38