Practical TCP/IP and Ethernet Networking- P25: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 102 Practical TCP IP and Ethernet Networking SCOP is a 4-bit multicast scope value used to limit the scope of the multicast group for example to ensure that packets intended for a local videoconference are not spread across the Internet. The values are 1 Interface-local scope 2 Link-local scope 3 Subnet-local scope 4 Admin-local scope 5 Site-local scope 8 Organization-local scope GROUP ID identifies the multicast group either permanent or transient within the given scope. Permanent group IDs are assigned by IANA. The following example shows how it all fits together. The multicast address FF 08 43 points to all NTP servers in a given organization in the following way FF indicates that this is a multicast address 0 indicates that the T flag is set to 0 . this is a permanently assigned multicast address 8 points to all interfaces in the same organization as the sender see SCOPE options above Group ID 43 has been permanently assigned to network time protocol NTP servers Flow labels The 20-bit flow label field in the IPv6 header may be used by a source to label those packets for which it requests special handling by the IPv6 routers. Hosts or routers that do not support the functions of the flow label field are required to set the field to zero when originating a packet pass the field on unchanged when forwarding a packet and ignore the field when receiving a packet. The actual nature of that special handling might be conveyed to the routers by a control protocol such as a resource reservation protocol . RSVP or by information within the flow s packets themselves . in a hop-by-hop option. A flow is uniquely identified by the combination of a source IP address and a nonzero flow label. A flow label is assigned to a flow by the flow s source node. Flow labels are chosen pseudo- randomly and uniformly from the range 0x1 to 0xFFFFFF. The purpose of the random allocation is to make any set of bits within the flow label field suitable for use as a hash key by .
