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Extension protocols and network applications 67 error message. This way, the IP address of the second router is known. This procedure is continued until the final host is reached. 4.4 IGMP 4.4.1 Introduction IGMP (Internet Group Management Protocol) is the protocol for IP multicast applications on TCP/IP networks. This standard is defined in RFC 1112. Besides the definition of address- and host extensions for the support of multicasting by IP hosts, this RFC also contains a definition of version 1 of IGMP. IGMP version 2 is defined in RFC 2236. Both versions of IGMP offer a protocol with which information about the membership of a host of specific multicast groups can be exchanged and updated. Multicast messages are sent to one address (multicast IP address) but are processed by several hosts. The collection of participants that listen to one specific multicast IP address is called a multicast group. Some important aspects of multicasting: • The membership of a group is dynamic: hosts can enter or leave a group at any moment. • Hosts can join multicast groups by sending IGMP messages • There is no restriction with regard to the size of groups. The different participants can be spread out over several networks on condition that the intervening routers support IGMP. • Hosts that do not belong to a certain group can send IP messages to that group. 4.4.2 IGMP messages IGMP describes how the information about the membership status is exchanged between routers and the different participants of multicast groups. Some IGMP messages: • Host membership report: is sent if a host becomes member of a multicast group and informs by way of this message all other members of the group. A router saves these reports and guarantees the maintenance of the multicast group in this way. • Host membership query: is sent by routers to periodically inform the group members in a network. All members of a group reply once again with a membership report. Routers retain all information and ensure that no multicast messages are sent on networks where no members of the group are present. • Leave group: this message is sent by a host that is the last one of a group within a certain network segment to leave a group. The IGMP protocol is a supporting protocol for the IP protocol. It uses IP packets to send messages. Figure 4.7 Shows how an IGMP message is encapsulated in a data frame.
Extension protocols and network applications 68 Figure 4.7: Encapsulation of an IGMP message 4.4.3 IGMP snooping A switch that connects a member from a multicast group with a router can read IGMP messages and evaluate by means of IGMP snooping. IGMP snooping translates multicast IP addresses in multicast MAC addresses. A switch can store multicast MAC addresses this way in its multicast filter table. A switch can send multicast messages immediately to the correct ports in this way and thus ensures that multicast messages do not put a needless strain on a network. For switches, this is also called the dynamic use of multicasting. This is opposed to the static use of multicasting whereby the groups in all switches and for all ports have to be configured manually. 4.4.4 Multicast addresses Multicast IP addresses are class D addresses that lie in the range 224.0.0.0 up to and including 239.255.255.255. For private networks it is recommended to use multicast IP addresses in the range 239.x.x.x. The addresses within the range 224.0.0.1 up to and including 224.0.0.255 are reserved for multicast applications within one network. The time to live value of such IP packets is set to 1 so that such messages continue to be available on the network. Multicast MAC addresses are also reserved. All addresses for which the first byte is equal to 01h are available for multicasting. Addresses that start with the value 01:00:5E:0 are multicast MAC addresses that are used for IP multicasting.
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Page 23 and 24: Ethernet 18 • TYPE: for the field
Page 25 and 26: Ethernet 20 a complete data frame s
Page 27 and 28: Ethernet 22 glass cabling are alway
Page 29 and 30: Ethernet 24 2.5 IEEE802.1Q tagged f
Page 31 and 32: Ethernet 26 According to the norm,
Page 33 and 34: Ethernet 28 2.7 VLAN A VLAN or Virt
Page 35 and 36: Ethernet 30 2.8 Network redundancy
Page 37 and 38: Ethernet 32 To create a network wit
Page 39 and 40: Ethernet 34 2.9.3 Link Aggregation
Page 41 and 42: Ethernet 36 Table 2.5: Environmenta
Page 43 and 44: TCP/IP 38 The second part of the an
Page 45 and 46: TCP/IP 40 Figure 3.4: Building of t
Page 47 and 48: TCP/IP 42 Table 3.2: IP addresses f
Page 49 and 50: TCP/IP 44 Table 3.4: Subnetting and
Page 51 and 52: TCP/IP 46 3.2.7 The IP packet The d
Page 53 and 54: TCP/IP 48 3.2.8 IPv6 General The mo
Page 55 and 56: TCP/IP 50 3.3.3 How reliability is
Page 57 and 58: TCP/IP 52 - the FIN bit that is use
Page 59 and 60: TCP/IP 54 Figure 3.14: Real time au
Page 61 and 62: TCP/IP 56 3.6 Communication over TC
Page 63 and 64: TCP/IP 58 number. This combination
Page 65 and 66: TCP/IP 60 3.6.5 Status of a socket
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Page 87 and 88: The switch 82 • mounted on DIN ra
Page 89 and 90: The switch 84 is indicated in the V
Page 91 and 92: Chapter 6 The router 6.1 Introducti
Page 93 and 94: The router 88 6.3 Types of routers
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Page 99 and 100: The router 94 Figure 6.8: Principle
Page 101 and 102: The firewall 96 Packet filters are
Page 103 and 104: VPN 98 • Confidentiality: the pro
Page 105 and 106: VPN 100 Figure 8.4: Host to host ga
Page 107 and 108: Automation networks & Security 102
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