Network Layer (continuation)

Transcrição

1 Network Layer (continuation)

2 PRIVATE ADDRESSING 2

3 NAT: Network Address Translation rest of Internet local network (e.g., home network) / All datagrams leaving local network have same single source NAT IP address: , different source port numbers Datagrams with source or destination in this network have /24 address for source, destination (as usual) 3

4 NAT: Network Address Translation Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one IP address for all devices can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network devices inside local net not explicitly addressable, visible by outside world (a security plus).

5 Blocks of private addresses Prefixo Endereço mais baixo Endereço mais alto 10/ / / /

6 NAT: Network Address Translation Implementation: NAT router must: outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)... remote clients/servers will respond using (NAT IP address, new port #) as destination addr. remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

7 NAT: Network Address Translation 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 3345 D: , : host sends datagram to , S: , 80 D: , : Reply arrives dest. address: , 5001 S: , 80 D: , : NAT router changes datagram dest addr from , 5001 to ,

8 NAT: Network Address Translation More than one public address may available (NAT versus PAT) 16-bit port-number field: 60,000 simultaneous connections with a single LANside address! NAT is controversial: routers should only process up to layer 3 violates end-to-end argument NAT possibility must be taken into account by app designers, eg, P2P applications address shortage should instead be solved by IPv6

9 Example (I) Estação A Rede Privada Eth0 Eth1 Router Rede Pública Acesso das estações A e B a servidor FTP na estação D: Estação B Estação D Servidor FTP Estação C rede privada rede pública 9

10 Example (II) Estação A Rede Privada Eth0 Eth1 Router Rede Pública Estação B Estação D Servidor FTP Acesso das estações A e B a servidor FTP na estação D: Estação C Router#show ip nat translation verbose Pro Inside global Inside local Outside local Outside global tcp : : : :21 create 00:00:35, use 00:00:24, left 23:59:35, flags: extended, use_count: 0 tcp : : : :21 create 00:00:12, use 00:00:06, left 23:59:53, flags: extended, use_count: 0 10

11 Example (III) Segundo acesso da estação B a servidor FTP na estação D: Estação A Estação B Rede Privada Eth0 Eth1 Router Rede Pública Estação D Servidor FTP Estação C rede privada Router#show ip nat translation verbose Pro Inside global Inside local Outside local Outside global tcp : : : :21 create 00:00:49, use 00:00:42, left 23:59:17, flags: extended, use_count: 0 tcp : : : :21 create 00:02:42, use 00:02:31, left 23:57:28, flags: extended, use_count: 0 tcp : : : :21 create 00:02:18, use 00:02:13, left 23:57:46, flags: extended, use_count: 0 rede pública 11

12 Example (IV) Cliente de FTP Eth0 Eth Router Servidor de FTP Tradução de endereços IP no interior de pacotes FTP: rede privada rede pública 12

13 NAT traversal problem client wants to connect to server with address server address local to LAN (client can t use it as destination addr) only one externally visible NATted address: solution 1: statically configure NAT to forward incoming connection requests at given port to server e.g., ( , port 2500) always forwarded to port Client? NAT router

14 NAT traversal problem solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to: Application running on NATted host requests mapping between (private IP address, private port number) and (public IP address, public port number) Application can advertise (public IP address, public port number) NAT router IGD i.e., automate static NAT port map configuration 14

15 NAT traversal problem solution 3: relaying (used in Skype) NATed server establishes connection to relay External client connects to relay relay bridges packets between two connections Client 2. connection to relay initiated by client 3. relaying established 1. connection to relay initiated by NATted host NAT router

16 DHCP

17 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an on Support for mobile users who want to join network (more shortly) Also allows client to learn subnet mask, default gateway, local DNS server DHCP overview: host broadcasts DHCP discover msg DHCP server responds with DHCP offer msg host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg

18 Configuration of a DHCP server Gama de endereços Conjunto de endereços IP definido por um endereço inicial e um endereço final Gama(s) de exclusão Conjuntos de endereços IP que se querem excluir Endereços reservados Endereços IP atribuídos de uma forma permanente a endereços MAC Duração dos alugueres

19 DHCP client-server scenario A DHCP server B E arriving DHCP client needs address in this network

20 DHCP client-server scenario DHCP server: DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 arriving client time DHCP request src: , 68 dest: , 67 yiaddrr: transaction ID: 655 Lifetime: 3600 secs DHCP offer src: , 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs

21 DHCP versus BOOTP Extensão Bootstrap Protocol, BOOTP, (RFC 1542) Corre sobre o UDP Número de porto do Servidor: 67 Número de porto do Cliente: 68 O BOOTP permite que um terminal sem disco descubra o seu endereço IP, um endereço de um servidor e o nome de um ficheiro a pedir ao servidor para ser copiado para memória e executado localmente.

22 Example Rede Servidor DHCP Labcom D4.9D.48 Labcom B.4E.68.7D 22

23 DHCP Discover A mensagem DHCP Discover é encapsulada num pacote BootP Request. Serve para descobrir os servidores de DHCP existentes na rede. O cliente pode também indicar qual o endereço IP que pretende alugar. DHCP Discover DHCP Offer DHCP Request DHCP Ack 23

24 DHCP Discover 24

25 The BROADCAST (B) bit If a client has the capability to receive unicast IP datagrams before its protocol software has been configured with an IP address, it should inform the servers by clearing the BROADCAST bit to 0, in both DHCP Discover and DHCP Request messages. op (1) htype (1) hlen (1) hops (1) xid (4) secs (2) B ciaddr (4) yiaddr (4) siaddr (4) giaddr (4) chaddr (16) sname (64) file (128) options (variable) 25

26 DHCP Offer A mensagem DHCP Offer é encapsulada num pacote BootP Reply. Cada servidor indica um endereço IP para ser alugado (se possível, os servidores respeitam a preferência do cliente). DHCP Discover DHCP Offer DHCP Request DHCP Ack 26

27 DHCP Offer 27

28 DHCP Request A mensagem DHCP Request é encapsulada num pacote BootP Request. Após escolha entre as possíveis diferentes ofertas recebidas, o cliente indica qual o endereço IP pretendido. DHCP Discover DHCP Offer DHCP Request DHCP Ack 28

29 DHCP Request 29

30 DHCP Ack A mensagem DHCP Ack é encapsulada num pacote BootP Reply. O servidor identifica positivamente o aluguer do endereço IP indicado fornecendo simultaneamente outras informações de interesse. DHCP Discover DHCP Offer DHCP Request DHCP Ack 30

31 DHCP Ack 31

32 Address renting Aluguer de endereços T1 Time (50% do Lease Time) tempo ao fim do qual o terminal deve tentar renovar o aluguer T2 Time (85% do Lease Time) tempo ao fim do qual o terminal deve tentar novamente renovar o aluguer se a primeira tentativa não for bem sucedida Lease Time tempo ao fim do qual o terminal deve deixar de usar o endereço IP se o aluguer não for renovado

33 Other DHCP messages DHCP Decline: O cliente rejeita a oferta que lhe foi feita e reinicia o processo de aluguer de endereço DHCP Nack: O servidor informa que não pode satisfazer o pedido que este lhe fez, através da mensagem DHCP Request DHCP Release: O cliente informa que pretende terminar o aluguer DHCP Inform: O cliente solicita apenas alguns parâmetros (neste caso, o cliente já tem um endereço IP, mas pretende solicitar, por exemplo, o endereço de um servidor DNS)

34 Client and server in different subnets É necessário activar nos routers um BootP Relay Agent Os routers reencaminham os pacotes BootP Request para o endereço do servidor DHCP, colocando no campo Gateway IP address o endereço da interface receptora (e1) O servidor DHCP envia os BootP Reply para o Gateway IP address BootP Relay Agent 34

35 Gateway IP address field 35