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.It is a directed offer (forwarded by a DHCP relay agent), not sent as abroadcast, and it is directed to the MAC address of the client, and to theproposed IP address for the client.The source address is from a differentsubnet (199.199.41) than the subnet that the client is attached to, indicatingthat both the discover and the offer must have traversed a relay agent.Thiscan be verified by checking the DHCP “giaddr” field, that is set to199.199.40.1.As you might suspect, a router is configured to forward DHCPbroadcasts from this subnet to the one where the DHCP server is located.DHCPforwarding is discussed in RFC1542, and routers used for this purpose mustexplicitly support the RFC and be configured accordingly [17 Although BOOTP andDHCP are similar, the Microsoft DHCP server does not support BOOTP.It willsilently ignore BOOTP requests.].Next, the client accepts the offer:********************************************************************Time Source IP Dest IP Prot Description0.172 0.0 255.255.255.255 DHCP Request (xid=08186BD1)+ FRAME: Base frame properties+ ETHERNET: ETYPE = 0x0800 : Protocol = IP: DOD Internet Protocol+ IP: ID = 0x100; Proto = UDP; Len: 328+ UDP: IP Multicast: Src Port: BOOTP Client, (68); Dst Port: BOOTPServer (67); Length = 308 (0x134)DHCP: Request (xid=08186BD1)DHCP: Op Code (op) = 1 (0x1)DHCP: Hardware Type (htype) = 1 (0x1) 10Mb EthernetDHCP: Hardware Address Length (hlen) = 6 (0x6)DHCP: Hops (hops) = 0 (0x0)DHCP: Transaction ID (xid) = 135818193 (0x8186BD1)DHCP: Seconds (secs) = 0 (0x0)DHCP: Flags (flags) = 0 (0x0)DHCP: 0.= No BroadcastDHCP: Client IP Address (ciaddr) = 0.0DHCP: Your IP Address (yiaddr) = 0.0DHCP: Server IP Address (siaddr) = 0.0DHCP: Relay IP Address (giaddr) = 0.0DHCP: Client Ethernet Address (chaddr) = 00DD01075715DHCP: Server Host Name (sname) =DHCP: Boot File Name (file) =DHCP: Magic Cookie = [OK]DHCP: Option Field (options)DHCP: DHCP Message Type = DHCP RequestDHCP: Client-identifier = (Type: 1) 00 dd 01 07 57 15DHCP: Requested Address = 199.199.40.13DHCP: Server Identifier = 199.199.41.254DHCP: Host Name = DAVEMAC4DHCP: Parameter Request List = (Length: 7) 01 0f 03 2c 2e 2f 06DHCP: End of this option fieldThe request was again broadcast, and the proposed IP address from the server isreferenced.The request is broadcast for a reason–the client could havereceived more than one offer and, by broadcasting its request, it allows theother DHCP servers to see that it isn’t going to use their offers.Finally, the server acknowledges the client lease:********************************************************************Time Source IP Dest IP Prot Description0.061 199.199.41.254 199.199.40.13 DHCP ACK (xid=08186BD1)+ FRAME: Base frame properties+ ETHERNET: ETYPE = 0x0800 : Protocol = IP: DOD Internet Protocol+ IP: ID = 0xA06; Proto = UDP; Len: 576+ UDP: Src Port: BOOTP Server, (67); Dst Port: BOOTP Client (68);Length = 556 (0x22C)DHCP: ACK (xid=08186BD1)DHCP: Op Code (op) = 2 (0x2hop )DHCP: Hardware Type (htype) = 1 (0x1) 10Mb EthernetDHCP: Hardware Address Length (hlen) = 6 (0x6)DHCP: Hops (hops) = 0 (0x0)DHCP: Transaction ID (xid) = 135818193 (0x8186BD1)DHCP: Seconds (secs) = 0 (0x0)DHCP: Flags (flags) = 0 (0x0)DHCP: 0.= No BroadcastDHCP: Client IP Address (ciaddr) = 0.0DHCP: Your IP Address (yiaddr) = 199.199.40.13DHCP: Server IP Address (siaddr) = 0.0DHCP: Relay IP Address (giaddr) = 199.199.40.1DHCP: Client Ethernet Address (chaddr) = 00DD01075715DHCP: Server Host Name (sname) =DHCP: Boot File Name (file) =DHCP: Magic Cookie = [OK]DHCP: Option Field (options)DHCP: DHCP Message Type = DHCP ACKDHCP: Renewal Time Value (T1) = 1 Days, 12:00:00DHCP: Rebinding Time Value (T2) = 2 Days, 15:00:00DHCP: IP Address Lease Time = 3 Days, 0:00:00DHCP: Server Identifier = 199.199.41.254DHCP: Subnet Mask = 255.255.255.0DHCP: Domain Name = (Length: 22) 63 73 77 61 74 63 70 2e 6d69 63 72 6f 73 6f 66.DHCP: Router = 199.199.40.1DHCP: NetBIOS Name Service = 199.199.41.254DHCP: NetBIOS Node Type = (Length: 1) 08DHCP: End of this option fieldThe acknowledgment is the final packet of the transaction, and it contains allof the configuration parameters that the client will use.Lease Expiration and RenewalDHCP-supplied configurations are “leased” from the server.Periodically, theclient will contact the server to renew the lease.The protocol andimplementation are very robust and configurable and short-term server ornetwork outages do not generally affect lease renewal.For instance, DHCPclients start to try to renew their lease when 50% of the lease time hasexpired.Repeated attempts are made to contact the DHCP server and renew thelease, until 87.5% of the lease time has expired.At this point, the clientattempts to get a new lease from any available DHCP server.When a DHCP client is rebooted, it attempts to verify that the lease it holdsis valid for the current subnet.If it is moved to another subnet and rebooted,the following sequence takes place:Source MAC Dest MAC Source IP Dest IP Pro Descriptiondavemacp *BROADCAST 0.0 255.255.255.255 DHCP Request (xid=6E3A2E74)router *BROADCAST 10.57.8.1 255.255.255.255 DHCP NACK (xid=6E3A2E74)davemacp *BROADCAST 0.0 255.255.255.255 DHCP Discover (xid=51CA7FED)router davemacp 10.57.8.1 10.57.13.152 DHCP Offer (xid=51CA7FED)davemacp *BROADCAST 0.0 255.255.255.255 DHCP Request (xid=2081237D)router davemacp 10.57.8.1 10.57.13.152 DHCP ACK (xid=2081237D)In this example the portable computer “davemacp” was moved to a new subnet andrestarted.It broadcasted a DHCP request for renewal of its old parameters, butthe DHCP server responsible for the new subnet recognized that these wereinvalid for the subnet and nak’d them.The DHCP client software automaticallywent through a normal discovery process to get reconfigured with parametersthat are valid for the new location.Windows Internet Name Service (WINS)WINS is a NetBIOS name service as described in RFC1001/RFC1002 [18 TheMicrosoft WINS server is discussed in more detail in a separate White Paper.].When a Windows NT system is configured as an h-node (default for WINS clients),it attempts to use a WINS server for name registration and resolution firstand, if that fails, it resorts to subnet broadcasts.WINS Name Registration and ResolutionUsing WINS for name services dramatically reduces the number of IP broadcastsused by Microsoft network clients [ Pobierz caÅ‚ość w formacie PDF ]

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