Chapter 4 Network Layer: The Data Plane

Transcription

1 Chapter 4 Network Layer: The Data Plane A note on the use of these Powerpoint slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) that you mention their source (after all, we d like people to use our book!) If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 7 th edition Jim Kurose, Keith Ross Pearson/Addison Wesley April 2016 Network Layer: Data Plane 4-1

2 Chapter 4: outline 4.1 Overview of Network layer data plane control plane 4.2 What s inside a router 4.3 IP: Internet Protocol datagram format fragmentation IPv4 addressing network address translation IPv6 4.4 Generalized Forward and SDN match action OpenFlow examples of match-plus-action in action Network Layer: Data Plane 4-2

3 Chapter 4: network layer chapter goals: understand principles behind network layer services, focusing on data plane: network layer service models forwarding versus routing how a router works generalized forwarding instantiation, implementation in the Internet Network Layer: Data Plane 4-3

4 Network layer transport segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical Network Layer: Data Plane 4-4

5 Two key network-layer functions network-layer functions: forwarding: move packets from router s input to appropriate router output routing: determine the path taken by packets from source to destination (a sender to a receiver) routing algorithms: run at routers to determine paths ; Routers have a forwarding table Destination address-based in Datagram networks Virtual circuit number-based in VC Networks analogy: taking a trip forwarding: process of getting through single interchange routing: process of planning trip from source to destination Network Layer: Data Plane 4-5

6 Interplay between routing and forwarding routing algorithm local forwarding table header value output link value in arriving packet s header

7 What does the Network layer consist of? Host, router network layer functions: Transport layer: TCP, UDP Network Routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions datagram format packet handling conventions forwarding table ICMP protocol error reporting router signaling Link layer physical layer ICMP: Internet Control Message Protocol: s an error-reporting protocol network devices like routers use to generate error messages to the source IP address when network problems prevent delivery of IP packets. ICMP creates and sends messages to the source IP address indicating that a gateway to the Internet that a router, service or host cannot be reached for packet delivery. 7 Any IP network device has the capability to send, receive or process ICMP messages.

8 The Internet Protocol (IP) 8

9 IP datagram format (IPv4) IP protocol version number header length (bytes) type of data max number remaining hops (decremen ted at each router) upper layer protocol to deliver payload to 32 bits ver head. type of len service length fragment -16bit identifier flgs offset time to upper live Internet checksum layer 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of routers to visit. 9

10 1 0 IP datagram format (IPv4) IP protocol version number header length data (bytes) type of max number remaining hops (decremente d at each router) upper layer protocol to deliver payload to how much overhead with TCP??? bytes of TCP?? bytes of IP 32 bits ver head. type of len service length fragment -16bit identifier flgs offset time to upper live Internet checksum layer 32 bit source IP address 32bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of routers to visit.

11 IP datagram format (IPv)4 IP protocol version number header length data (bytes) type of max number remaining hops (decremente d at each router) upper layer protocol to deliver payload to how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver head. type of len service 32 bits 16-bit identifier flgs time to upper live layer length offset Internet checksum 32 bit source IP address data (variable length, typically a TCP or UDP segment) fragment 32bit destination IP address Options (if any) total datagram length (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of routers to visit. 10

12 Network layer: data plane, control plane Data plane local, per-router function determines how datagram arriving on router input port is forwarded to router output port forwarding function values in arriving packet header Control plane network-wide logic determines how datagram is routed among routers along end-end path from source host to destination host two control-plane approaches: traditional routing algorithms: implemented in routers software-defined networking (SDN): implemented in (remote) servers Network Layer: Data Plane 4-12

13 Per-router control plane Individual routing algorithm components in each and every router interact in the control plane Routing Algorithm control plane data plane values in arriving packet header Network Layer: Control Plane 5-13

14 Logically centralized control plane A distinct (typically remote) controller interacts with local control agents (CAs) Remote Controller CA control plane data plane values in arriving packet header CA CA CA CA Network Layer: Control Plane 5-14

15 Network layer service models: Network Architecture Service Model Bandwidth Guarantees? Loss Order Timing Congestion feedback Internet ATM ATM ATM ATM best effort CBR (constant bitrate) VBR (variable bitrate ABR UBR none constant rate guaranteed rate guaranteed minimum none no yes yes no no no yes yes yes yes no yes yes no no no (inferred via loss) no congestion no congestion yes no Network Layer: Data Plane 4-15

16 Network layer service models: CBR stands for constant bitrate. During CBR encoding, the bitrate or the number of bits per second is kept the same throughout the encoding process. Constant bit rate (CBR) encoding persists the set data rate to your setting over the whole video clip.cbr encoding does not optimize media files for quality but will save you storage space. Use CBR only if your clip contains a similar motion level across the entire duration. CBR is most commonly used for streaming video content using the Flash Media Server (rtmp).our video encoding API will support CBR encoding for the special occasions in which you need to use CBR. VBR stands for variable bitrate. Variable bit rate (VBR) encoding adjusts the data rate down and to the upper limit you set, based on the data required by the compressor. This means that during a VBR encoding process the bitrate of the media file will dynamically increase or decrease depending on the media files bitrate needs. VBR takes longer to encode but produces the most favorable results as the quality of the media file is superior. VBR is most commonly used for http delivery if video content (http progressive) Network Layer: Data Plane 4-16

17 Network layer service models: Area Border Router (ABR) mean ABR is a kind of router that is located near the border between one or more Open Shortest Path First (OSPF) areas. It is used to establish a connection between backbone networks and the OSPF areas. It is a member of both the main backbone network and the specific areas to which it connects, so it stores and maintains separate routing information or routing tables regarding the backbone and the topologies of the area to which it is connected. Unspecified Bit Rate, a traffic contract used to guarantee quality of service for networks Network Layer: Data Plane 4-17

18 Network service model Q: What service model for channel transporting datagrams from sender to receiver? example services for individual datagrams: guaranteed delivery guaranteed delivery with less than 40 msec delay example services for a flow of datagrams: in-order datagram delivery guaranteed minimum bandwidth to flow restrictions on changes in inter-packet spacing Network Layer: Data Plane 4-18

19 IP fragmentation, reassembly network links have MTU (max.transfer size) - largest possible link-level frame different link types, different MTUs large IP datagram divided ( fragmented ) within the network (internetworking) one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams Network Layer: Data Plane 4-19

20 Network Layer 4-20

21 Network Layer 4-21

22 Network Layer 4-22

23 Network Layer 4-23

24 Network Layer 4-24

25 Network Layer 4-25

26 Network Layer 4-26

27 Network Layer 4-27

28 Network Layer 4-28

29 Network Layer 4-29

30 Network Layer 4-30

31 Network Layer 4-31

32 Network Layer 4-32

33 Network Layer 4-33

34 Network Layer 4-34

35 Network Layer 4-35

36 Network Layer 4-36

37 IP fragmentation, reassembly example: 4000 byte datagram MTU = 1500 bytes length =4000 ID =x fragflag =0 offset =0 one large datagram becomes several smaller datagrams 1480 bytes in data field length =1500 ID =x fragflag =1 offset =0 offset = 1480/8 length =1500 ID =x fragflag =1 offset =185 length =1040 ID =x fragflag =0 offset =370 Network Layer: Data Plane 4-37

38 Fragmenting A Fragment Network Layer 4-38

39 Network Layer 4-39

40 Example The device performing the fragmentation follows a specific algorithm to divide the message into fragments for transmission. The exact implementation of the fragmentation process depends on the device. Let's consider the following example, an IP message 12,000 bytes wide (including the 20-byte IP header) that needs to be sent over a link with an MTU of 3,300. Network Layer 4-40

41 Create First Fragment: The first fragment is created by taking the first 3,300 bytes of the 12,000-byte IP datagram. This includes the original header, which becomes the IP header of the first fragment (with certain fields changed as described below). So, 3,280 bytes of data are in the first fragment. This leaves 8,700 bytes to encapsulate (11,980 minus 3,280) Create Second Fragment: The next 3,280 bytes of data are taken from the 8,700 bytes that remain after the first fragment was built, and paired with a new header to create fragment #2. This leaves 5,420 bytes. Create Third Fragment: The third fragment is created from the next 3,280 bytes of data, with a 20-byte header. This leaves 2,140 bytes of data. Network Layer 4-41

42 Create Fourth Fragment: The remaining 2,140 bytes are placed into the fourth fragment, with a 20-byte header of course. two important points should be illustrated here. First, IP fragmentation does not work by fully encapsulating the original IP message into the Data fields of the fragments. If this were done, the first 20 bytes of the Data field of the first fragment would contain the original IP header. The original IP header is transformed into the IP header of the first fragment. Second, note that the total number of bytes transmitted increases: we are sending 12,060 bytes (3,300 times three plus 2,160) instead of 12,000. The extra 60 bytes are from the additional headers in the second, third and fourth fragments. (The increase in size could theoretically be even larger if the headers contain options.) Network Layer 4-42

43 IP addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host typically has one or two interfaces (e.g., wired Ethernet, wireless ) IP addresses associated with each interface = Network Layer: Data Plane 4-43

44 Class-based Addressing IP addresses consist of: Network part Host part IP addresses are divided into five classes: A, B, C, D, and E. Problems?? 0 Network (7 bits) 1 0 Network (14 bits) Class A Class B Host (24 bits) Host (16 bits) 110 Network (21 bits) Host (8 bits) Class C 1110 Multicast address 1111 Future use addresses Class D Class E 44

45 Class-based Addressing (coont) The number of networks and the number of hosts per class can be derived by this formula: Number of NW =2^network_bits Number of Hosts/NW=2^host_bit -2 When calculating hosts' IP addresses, 2 IP addresses are decreased because they cannot be assigned to hosts, i.e. the first IP of a network is network number and the last IP is reserved for Broadcast IP Network Layer 4-45

46 Class A Address The first bit of the first octet is always set to 0 (zero). Thus the first octet ranges from 1 127, i.e =1-127 Class A addresses only include IP starting from 1.x.x.x to 126.x.x.x only. The IP range 127.x.x.x is reserved for loopback IP addresses The default subnet mask for Class A IP address is which implies that Class A addressing can have 126 networks (2 7-2) and hosts (2 24-2). Class A IP address format is thus: 0NNNNNNN.HHHHHHHH.HHHHHHH H.HHHHHHHH Network Layer 4-46

47 Class B Address An IP address which belongs to class B has the first two bits in the first octet set to 10, i.e Class B IP Addresses range from x.x to x.x. The default subnet mask for Class B is x.x. Class B has (2 14 ) Network addresses and (2 16-2) Host addresses. Class B IP address format is: 10NNNNNN.NNNNNNNN.HHHHHHHH. HHHHHHHH Network Layer 4-47

48 Class C Address The first octet of Class C IP address has its first 3 bits set to 110, that is: Class C IP addresses range from x to x. The default subnet mask for Class C is x. Class C gives (2 21 ) Network addresses and 254 (2 8-2) Host addresses Class C IP address format is: 110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH Network Layer 4-48

49 Class D Address first four bits of the first octet in Class D IP addresses are set to 1110, giving a range of: Class D has IP address rage from to Class D is reserved for Multicasting. In multicasting data is not destined for a particular host, that is why there is no need to extract host address from the IP address, and Class D does not have any subnet mask. Network Layer 4-49

50 Class E Address This IP Class is reserved for experimental purposes only for R&D or Study. IP addresses in this class ranges from to Like Class D, this class too is not equipped with any subnet mask. Network Layer 4-50

51 Subnets: Motivation The classful addressing scheme proposes that the network portion of a IP address uniquely identifies one physical network. Any network with more than 255 hosts needs a class B address. Class B addresses can get exhausted before we have 4 billion hosts! Take bits from the host number part to create a subnet number ) right sizing (. 51

52 Subnets IP address: subnet part - high order bits host part - low order bits what s a subnet? device interfaces with same subnet part of IP address can physically reach each other without intervening router subnet network consisting of 3 subnets Network Layer: Data Plane 4-52

53 Subnets recipe to determine the subnets, detach each interface from its host or router, creating islands of isolated networks each isolated network is called a subnet / / subnet /24 subnet mask: /24 Network Layer: Data Plane 4-53

54 Subnets how many? Network Layer: Data Plane 4-54

55 Addressing in the Internet CIDR: Classless InterDomain Routing provides the flexibility of borrowing bits of Host part of the IP address and using them as Network in Network, called Subnet. By using subnetting, one single Class A IP address can be used to have smaller sub-networks which provides better network management capabilities. subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet portion of address Before CIDR, Internet used a class-based addressing scheme where x could be 8, 16, or 24 bits. These corresponding to classes A, B, and C resp. subnet part /23 host part 55

56 IP addresses: how to get one? Q: How does a host get IP address? hard-coded by system admin in a file Windows: control-panel->network->configuration- >tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play Network Layer: Data Plane 4-56

57 Class A Subnets In Class A, only the first octet is used as Network identifier and rest of three octets are used to be assigned to Hosts (i.e Hosts per Network). To make more subnet in Class A, bits from Host part are borrowed and the subnet mask is changed accordingly. For example, if one MSB (Most Significant Bit) is borrowed from host bits of second octet and added to Network address, it creates two Subnets (2 1 =2) with (2 23-2) Hosts per Subnet. The Subnet mask is changed accordingly to reflect subnetting. Next slide shows a list of all possible combination of Class A subnets: Network Layer 4-57

58 The first and last IP address of every subnet is used for Subnet Number and Subnet Broadcast IP address respectively. Because these two IP addresses cannot be assigned to hosts, subnetting cannot be implemented by using more than 30 bits as Network Bits, which provides less than two hosts per subnet. 4-58

59 Class B Subnets By default, using Classful Networking, 14 bits are used as Network bits providing (2 14 ) Networks and (2 16-2) Hosts. Class B IP Addresses can be subnetted the same way as Class A addresses, by borrowing bits from Host bits. Next slide gives all possible combination of Class B subnetting: Network Layer 4-59

60 Network Layer 4-60

61 Class C Subnets Class C IP addresses are normally assigned to a very small size network because it can only have 254 hosts in a network. Given below is a list of all possible combination of subnetted Class C IP address Network Layer 4-61

62 IP addressing: introduction Q: how are interfaces actually connected? A: we ll learn about that in chapter 5, 6. A: wired Ethernet interfaces connected by Ethernet switches For now: don t need to worry about how one interface is connected to another (with no intervening router) A: wireless WiFi interfaces connected by WiFi base station Network Layer: Data Plane 4-62