Basics of communication. Grundlagen der Rechnernetze Introduction 31

Transcription

1 Basics of communication Grundlagen der Rechnernetze Introduction 31

2 Types of communication H9 H8 H1 H7 R1 N3 H2 N1 R3 H3 R2 N2 H6 H5 H4 Unicast communication where a piece of information is sent from one point to another point. In this case there is just one sender, and one receiver. Multicast describe communication where a piece of information is sent from one or more points to a set of other points. In this case there is may be one or more senders, and the information is distributed to a set of receivers (theer may be no receivers, or any other number of receivers). Broadcast communication where a piece of information is sent from one point to all other points. In this case there is just one sender, but the information is sent to all connected receivers. Grundlagen der Rechnernetze Introduction 32

3 Types of communication H9 H1 R1 N3 H2 N1 R2 H3 N2 H4 Forwarding Packet (frame ) forwarding is the relaying of packets from one network segment to another by nodes in a computer network. Usually refers to the effective transfer of a packet (frame...) Routing process of selecting a path for traffic in a network, or between or across multiple networks. Path actual path used for transmission between two hosts H8 R3 H5 H7 H6 Grundlagen der Rechnernetze Introduction 33

4 Forwarding table Destination Next hop R Grundlagen der Rechnernetze Introduction 34

5 Timeouts and acknowledgements H9 H1 R1 N3 H2 N1 R2 H3 N2 H4 Timer Timeout Acknowledgement ACK H8 R3 H5 H7 H6 Grundlagen der Rechnernetze Introduction 35

6 Connection oriented and connectionless communication H1 H2 H3 N1 Connection oriented Telephone, File transfer R1 R2 H4 Connectionless VoIP, Post H9 N3 N2 H8 R3 H5 H7 H6 Grundlagen der Rechnernetze Introduction 36

7 Client Server principle H N S Client Server Grundlagen der Rechnernetze Introduction 37

8 Client Server principle H N S Client Request Server Grundlagen der Rechnernetze Introduction 38

9 Client Server principle H N Response S Client Server Grundlagen der Rechnernetze Introduction 39

10 Client Server principle H N S Client Server stateful server remembers client data (state) from one request to the next. stateless server does not keep state information. Using a stateless file server, the client must specify complete file names in each request, specify location for reading or writing. Grundlagen der Rechnernetze Introduction 40

11 Adressing Grundlagen der Rechnernetze Introduction 41

12 Motivation H1 H2 H3 N1 How do we transfer message from H8 to H4? H9 R1 R2 H4 Which path do we use? Can we always reach the destination? N3 N2 H8 R3 H5 H7 H6 Grundlagen der Rechnernetze Introduction 42

13 Physical address Ethernet example : 00 : 2B : E4 : B1 : 02 Broadcast FF:FF:FF:FF:FF:FF Multicast 1XXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX {8X,,FX}:XX:XX:XX:XX:XX Grundlagen der Rechnernetze Introduction 43

14 Address space H1 H2 H3 H7 H8 H H R R R H H6 Grundlagen der Rechnernetze Introduction 44

15 Forwarding table and address space H1 H2 H3 H7 H8 H H H R H6 R R3 4.4 Dest H1 H2 H3 H4 H5 H6 H7 H8 Next Hop After R1 After R1 After R1 Direct Direct Direct After R3 After R3 Dest Next Hop 1.X After R1 2.X Direct 4.X After R3 H9 After R3 Grundlagen der Rechnernetze Introduction 45

16 IP addresses 32 bits approximately 4 billion addresses Binary representation 4 groups of 8 bits Dot notation 4 decimal numbers representing 4 groups of bits Example: Grundlagen der Rechnernetze Introduction 46

17 Classful IP addresses What does it mean classful? Which are different classes? Network address Host address Broadcast address Source: William Stallings Data and Computer Communications, Eight Edition Grundlagen der Rechnernetze Introduction 47

18 Need for additional hierarchical layer H1 H2 H3 H7 H8 H H R R R H H6 Entrance to the University network Grundlagen der Rechnernetze Introduction 48

19 Subnetworks For example class B Address Network Host Subnet mask ( ) Solution Network number Subnet Host Grundlagen der Rechnernetze Introduction 49

20 Subnetting example Example: Using one class B network: X.X = = XXXXXXXX XXXXXXXX Subnet number : = Subnet mask : = H = R = = = = = H2 Grundlagen der Rechnernetze Introduction 50

21 Changes in forwarding tables Subnet number : Subnet mask : H Interface 1 R Interface 2 Subnet number Subnet mask Next hop direct (if 1) direct (if 2) after R2 (if 2) R3 Network number Next Hop H R Grundlagen der Rechnernetze Introduction 51

22 Address resolution IP address Physical address IP address Physical address :FF:AA:36:AB: :48:A4:28:AA: ??? :48:A4:28:AA: :35:FE:36:42:55 H :FF:AA:36:AB:11 85:48:A4:28:AA:18 R1 H2 Grundlagen der Rechnernetze Introduction 52

23 Supernetting motivation Lets assume, for example, the IT department of a university campus, which "autonomously" uses a lot of IP addresses. With subnetting, we can efficiently use given set of IP addresses. The problem is that the IT department still has to request / manage IP addressing in the granularities Class A, B, or C network. Grundlagen der Rechnernetze Introduction 53

24 Supernetting motivation What happens when for example we need 257 hosts? 1. We can apply for Class B network address. The problem is efficiency Grundlagen der Rechnernetze Introduction 54

25 Supernetting motivation What happens when for example we need 257 hosts? 1. We can apply for Class B network address. The problem is efficiency Grundlagen der Rechnernetze Introduction 55

26 Supernetting motivation What happens when for example we need 257 hosts? 1. We can apply for Class B network address. The problem is efficiency 2. We can also consider 2 class C networks. Grundlagen der Rechnernetze Introduction 56

27 Supernetting motivation What happens when for example we need 257 hosts? 1. We can apply for Class B network address. The problem is efficiency 2. We can also consider 2 class C networks. This means that we have 2 routing entries in each internet router Grundlagen der Rechnernetze Introduction 57

28 Solution: Classless Inter Domain Routing (CIDR) We can aggregate network addresses. Example: Lets assume that we have 16 * hosts. We use 16 addresses of Class C networks. Not arbitrary addresses, but consecutive, e.g.: Grundlagen der Rechnernetze Introduction 58

29 Solution: Classless Inter Domain Routing (CIDR) We can aggregate network addresses. Example: Lets assume that we have 16 * hosts. We use 16 addresses of Class C networks. But not arbitrary addresses, but consecutive, e.g.: Now we can observe following: all addresses begin with the same 20 bits: Grundlagen der Rechnernetze Introduction 59

30 Solution: Classless Inter Domain Routing (CIDR) Observation: all addresses begin with the same 20 bits: That means that we need a 20 bit network address This is between Class C (24 bit) and Class B (16 bit) Required output of 2 ^ 4 = 16 Class C addresses General question: How many class C networks requires i bit network address? Grundlagen der Rechnernetze Introduction 60

31 Solution: Classless Inter Domain Routing (CIDR) Observation: all addresses begin with the same 20 bits: That means that we need a 20 bit network address This is between Class C (24 bit) and Class B (16 bit) Required output of 2 ^ 4 = 16 Class C addresses General question: How many class C networks requires i bit network address? Grundlagen der Rechnernetze Introduction 61

32 Solution: Classless Inter Domain Routing (CIDR) We need a notation for the scheme. In our example: Notation can be summarized as: / 20 So this additional number / 20 means network address consists of first 20 bits and summarizes the 2 ^ 4 = 16 successive class C networks beginning with Grundlagen der Rechnernetze Introduction 62

33 Quiz How to represent the class C networks from to using / X notation? Grundlagen der Rechnernetze Introduction 63

34 Quiz How to represent the class C networks from to using / X notation? / 19 Grundlagen der Rechnernetze Introduction 64

35 Quiz How to represent the class C networks from to using / X notation? / 19 How to represent the single class C network in / X notation? Grundlagen der Rechnernetze Introduction 65

36 Quiz How to represent the class C networks from to using / X notation? / 19 How to represent the single class C network in / X notation? / 24 Grundlagen der Rechnernetze Introduction 66

37 Solution: Classless Inter Domain Routing (CIDR) How are aggregated addresses handled in the router: Addresses in the routing tables are pair <length, value> This is comparable to the pair <mask, value> in subnetting if the mask consists of successive 1 bit values Grundlagen der Rechnernetze Introduction 67

38 Solution: Classless Inter Domain Routing (CIDR) CIDR allows further route aggregation. For example: Client networks Advertise / /24 Internet provider /24 We don t even need to use 8 consecutive addresses Grundlagen der Rechnernetze Introduction 68

39 Solution: Classless Inter Domain Routing (CIDR) What happens with CIDR and routing table entries? Prefixes may overlap. Lets consider following routing table: Where do we route the message for ? Where do we route the message ? Network address Next hop /16 if /24 if Grundlagen der Rechnernetze Introduction 69

40 Solution: Classless Inter Domain Routing (CIDR) What happens with CIDR and routing table entries? Prefixes may overlap. Lets consider following routing table: Where do we route the message for ? if2 Where do we route the message ? Network address Next hop /16 if /24 if if1 Grundlagen der Rechnernetze Introduction 70

41 Solution: Classless Inter Domain Routing (CIDR) What happens with CIDR and routing table entries? Prefixes may overlap. Lets consider following routing table: Where do we route the message for ? if2 Where do we route the message ? if1 Network address Next hop /16 if /24 if In general: Longest Prefix Match (requires efficient algorithms / data structures to find the longest matching prefix.) Grundlagen der Rechnernetze Introduction 71

42 Subnetting vs CIDR Subnetting allows splitting a network address into subnets Distribution almost anywhere; everything that can be expressed with the subnet mask CIDR is used to aggregate network addresses in a single address Aggregation not arbitrary; network addresses must be consecutive; only 2^i sized networks can be aggregated Certain flexibility using "dummy networks" Grundlagen der Rechnernetze Introduction 72