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.To provide database description packet service, one router is set as the master, and the other is the slave.The master sends the database description packets, and the slave acknowledges them with database description responses.The format of the database description packet is shown in Figure 5.16.After the OSPF header is a set of unused bits, followed by three 1-bit flags.When the I (initial) bit is set to 0, it indicates that this packet is the first in a series of packets.The M (more) bit, when set to 1, means that more database description packets follow this one.The MS (master/slave) bit indicates the master/slave relationship.When it has a value of 1 it means that the router that sent the packet is the master.A 0 indicates that the sending machine is the slave.The Data Descriptor Sequence Number is an incrementing counter.The rest of the packet contains Link State advertisements as seen in Figure 5.14.Figure 5.16.The database description packet layout.Link State Request and Update PacketsThe Link State Request packet asks for information about a topological table from a database, whereas the Update packet can provide topological information of the types shown in Table 5.11.The Request packet is usually sent when an entry in the router's topological table is corrupted, missing, or out of date.The format of the Link State Request packet is shown in Figure 5.17.The Link State Request packet contains the OSPF header and a block of three repeating fields for the Link State Type, Link State ID, and Advertising Router.Figure 5.17.OSPF Link State Request packet format.The Link State Update packet has four formats, depending on the link state type: router links, network links, summary links, or autonomous systems external links.The Router Links advertisement packet is sent to neighbors periodically and contains fields for each router link and the type of service provided in each link, as shown in Figure 5.18.Figure 5.18.OSPF Router Links advertisement packet format.After the OSPF header and the Link State advertisement header are two single bit flags surrounded by 6- and 8-bit unused fields.The E (external) flag, when set to 1, indicates that the router is an autonomous systems (AS) boundary router.The B (border) flag, when set to 1, indicates that the router is an area border router.Following the unused 8-bit area is a field for the number of links (advertisements) in the message.Following this, the links are provided in sequence, one link to a block.Each Link State advertisement block in the Router Links advertisement packet has a field for the Link ID (the type of router, although the value is dependent on the Type field later in the block), the Link Data (whose value is an IP address or a network mask, depending on the Type field's setting), the Type field (a value of 1 indicates a connection to another router, 2 a connection to a transit network, and 3 a connection to a stub network), and the Number of TOS field, which shows the number of metrics for the link (at least one must be provided, which is called TOS 0).Then, a repeating block is appended for each TOS, providing the type and the metric.The other three formats available are the Network Links advertisement, Summary Links advertisement, and Autonomous Systems (AS) External Links advertisement.The formats of these advertisements are shown in Figure 5.19.The fields have all been described earlier in this section.Figure 5.19.OSPF Network, Summary, and AS Links advertisement layouts.The last packet involved in OSPF is the Link State acknowledgment packet, which is required when a Link State advertisement has been received correctly.The layout of the acknowledgment packet is shown in Figure 5.20.The fields following the OSPF header are for the Link State Type, Link ID, Advertising Router ID, Link State Sequence Number, Link State Checksum value, and Link State Age, all of which have been mentioned earlier.Figure 5.20.Link State acknowledgment packet layout.SummaryToday I looked at the gateway protocols used within the TCP/IP family specifically, as well as those in general use on the Internet and most networks.Gateways are a critical component for forwarding information from one network to another.Without gateways, each machine on the network would require a full map of every other machine on the internetwork.As I have shown, there are several protocols of importance, depending on the role of the gateway.I also looked at the use of bridges, routers, and brouters in a network, and the role that each of these can play.With this material, I can leave the subject of gateways.Except for some message passing and administration material, you now know all you need about gateway protocols used with TCP/IP.Q&AWhat is a boundary gateway?A boundary gateway sits between two networks within a larger internetwork, as would be found in a large corporation.The boundary gateways mark the edges (or boundaries) of each LAN, passing message to other LANs within the larger internetwork.Boundary gateways do not communicate with the networks outside the organization.This task is performed by exterior gateways.How are sequence numbers used to control status messages within GGP? Explain for both the sending and receiving gateways.The sending gateway sends packets with an incrementing sequence number.The destination gateway receives each packet and echoes back the sequence number in a message.If the destination gateway receives the next packet with a sequence number that does not follow the one last received, an error message is returned to the sender with the sequence number of the last packet in it.If the sequence number is correct, an acknowledgment is sent [ Pobierz całość w formacie PDF ]
