Tài liệu Mob 6 addressing routing 2010

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Mobile Communications   Anne Fladenmuller Network Layer   DHCP   IP mobility (IPv4 and V6)   Ad hoc routing Internet & Mobile Communications 2007 Network Layer Goal: routing and IP packets control. Operations taken into account by IP: • QoS management Identification of each • Triggering of address network resolution mechanisms   Identification of networks’ • Information about control nodes and links state   Packets redirection   Fragmentation and reassembly Mobility requires modifications mainly in packets redirection functions   2 Internet & Mobile Communications - 2007 Network Layer Classical routing 132.227.xx 98.217.15.xx …. E A B 98.217.15.xx A D E 132.227.61.xx C From: To: 3 Internet & Mobile Communications - 2007 Network Layer Terminals mobility 98.217.15.xx WAN 132.227.61.xx 129.210.112.xx 4 Internet & Mobile Communications - 2007 Network Layer Terminals mobility ? WAN 98.217.15.xx From: To: 132.227.61.xx 129.210.112.xx 5 Internet & Mobile Communications - 2007 Network Layer   Different mobility aspects –  Unfrequent moves from the user with his laptop or PDA. IP address modification: DHCP.   Macro mobility management.   –  Frequent moves from the user: micro mobility management. –  Independant moves of all network devices: Ad Hoc networks. 6 Internet & Mobile Communications - 2007 DHCP   Allocation of a new IP address with DHCP (Dynamic Host Configuration Protocol). –  Main goal: Simplification of network administration.   Nomadism management: user’s move to a new network with his computer   –  It allows obtaining a network configuration dynamically: DHCP is mainly used for IP addresses distribution   It comes from an evolution of BOOTP (Bootstrap Protocol) used to set up machines accross a network.   A DHCP server can return BOOTP parameters or configuration parameters specific to a given host.   7   RFCs: 1541 Internet & Mobile Communications - 2007 DHCP   DHCP protocols principles –  1 DHCP server distributes IP addresses. The server is a base for all DHCP requests (1 server with a fixed IP address per network).   The fundamental communication mechanism is BOOTP   –  –  –  When a machine gets started, it has no information about its network configuration. To find the DHCP server and dialog with it, the machine will send a particular broadcast packet on the LAN. When the DHCP server receives this packet, it answers with another broadcast packet (the client does not necessarily have his IP address and is thus unreacheable directly) which contains all necessary information for the client. 8 Internet & Mobile Communications - 2007 DHCP   Allocation of a static or dynamic address –  A DHCP server generally provides dynamic addresses   –  A same computer can thus receive 2 different addresses one successively But it can also provide a fixed IP address to a specific client.   This must be used reasonably, otherwise the DHCP server is more or less useless 9 Internet & Mobile Communications - 2007 DHCP   Dialog with the server –  –  –  DHCP messages are transmitted through UDP. DHCP thus works in an unconnected mode. Ports’ numbers:     –  The client only uses port 68 to send and receive its messages The server sends and receives its messages on a single port, port 67. BOOTP/DHCP frame format     The DHCP frame is indeed the same as BOOTP Parameters exchange (machine’s name...) is done through options. –  Options are described in RFC2132. They are all identified by a number. For example,   option 15: provides the client with the network’s domain name.   option 53: DHCPACK 10 Internet & Mobile Communications - 2007 DHCP   Frame format 
op: equals 1 for BOOTREQUEST (client request), 2 for BOOTREPLY (server answer) 
htype: type of hardware address 
hlen: length of hardware address (in bytes). It is 6 for a MAC address 
hops: can be used by DHCP relays 
xid: random number chosen by the client and used to recognize the client 
secs: time spent (in seconds) since the client started its request 
flags: various flags 11 Internet & Mobile Communications - 2007 DHCP ciaddr: client’s IP address, when he already has one yiaddr: the (future) client’s IP address siaddr: IP address of the (next) server to use giaddr: relay’s IP address (e.g. gateway) when the direct client/server connection is not possible chaddr: client’s hardware address sname: optional field. Server’s name file: name of the file that should be used to boot options: Reserved field for options. A DHCP client must be ready to receive at least 576 bytes, but it can ask the server to limit the size of its messages. 12 Internet & Mobile Communications - 2007 DHCP –  DHCP messages types:                 13 DHCPDISCOVER (1) to locate available DHCP servers and ask a first configuration DHCPOFFER (2) answer of a server to a DHCPDISCOVER message, which contains the first parameters DHCPREQUEST (3) client request e.g. to extend its lease DHCPDECLINE (4) the client tells the server that the address is already used DHCPACK (5) answer from the server which contains the client’s parameters and address DHCPNAK (6) server’s answer to tell the client that its lease has expired or if the client announces a worng network configuration DHCPRELEASE (7) the client releases its IP address DHCPINFORM (8) the client asks for local parameters, it already has its IP address Internet & Mobile Communications - 2007 DHCP –  Operation: The first packet sent by the client is a DHCPDISCOVER packet .   The server answers with a DHCPOFFER packet to propose an IP address to the client (among others).   The client sets up its configuration, then makes a DHCPREQUEST to validate its IP address (broadcast request because DHCPOFFER does not contain its IP address).   The server simply answers with a DHCPACK with the IP address to confirm the allocation.   –  14 Normally, it is enough for a client to get an efficient network configuration, but it can take more or less time depending on whether the client accepts the IP address or not... Internet & Mobile Communications - 2007 DHCP 15 Internet & Mobile Communications - 2007 DHCP –  In order to optimize network resources, IP addresses are allocated with a starting and an end validity date: a lease.   In the lease, there is not only an IP address for the client, with a validity period, but also other configuration information like: –  –  –    DNS address (Name resolution) Default Gateway address (to get out of the network where the DHCP server has set up the client). The DHCP server address. The lease may be extended on the client’s request or on the server’s proposal. If the server does not receive any valid answer, it makes the IP address available. 16 Internet & Mobile Communications - 2007 DHCP   Optimization of IP addresses allocation by modifying the leases’ duration. –  The problem is here:     17 If all addresses are allocated and none is released after a certain time, no more request will be satisfied. –  In a network to which many computers connect frequently, it is interesting to propose short leases. But be careful not to block the bandwidth on the small and very busy networks. –  In a network mostly constituted of fixed machines very rarely rebooted, long leases are sufficient. It is recommended not to create unnecessarily short leases, which leads to a significant increase in the network. A compromise must be found between the mean users’ connection suration, the server’s number of IP addresses left, the number of subscribers... Internet & Mobile Communications - 2007 DHCP   Lease expiration –  DHCP is preventive: it waits until 50% of the lease’s duration to ask for its extension     –  To do this the client contact the original server through a (DHCPREQUEST) message. If the leas eextension is accepted by the server, it send a (DHCPACK) message so that the client can be updated with this new duration. If the client does not get any answer il will wait 7/8th of the lease’s duration and will ask its DHCP server, if it can extend its lease’s duration.     DHCP servers can answer with a (DHCPACK) to extend the client’s lease. If a server cannot extend its lease, it will send a (DHCPNACK) message to the client who will have to restart the whole IP lease request procedure. 18 Internet & Mobile Communications - 2007 DHCP 19 Internet & Mobile Communications - 2007 DHCP   Server’s set up –  –  –  There is no need to set up one DHCP server per network The server’s IP address does not necessarily belong to the same class as those in its addresses range. Negociation is done as follows: DHCP requests must reach the server which is located on another nework, they must thus cross routers (theoretically impossible).   Installation on one or several routers of a relay agent who will intercept bradcast requests and will forward them to a DHCP server known by this agent.   20 Internet & Mobile Communications - 2007
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