Computer Networks. Network Layer Protocols BOOTP, DHCP, ICMP

Transcription

1 Computer Networks Network Layer Protocols BOOTP, DHCP, ICMP

2 BOOTP

3 Bootstrap Protocol (BOOTP) Bootstrap process in diskless systems requires finding out the IP address of the host. The Reverse Address Resolution Protocol (RARP) was the first attempt to resolve this bootstrap problem. RARP is capable of providing a diskless device with its IP address, using a simple client/server exchange of a request and reply between a host and an RARP server. The difficulty with RARP is: It operates at a fairly low level using hardware broadcasts, so it requires adjustments for different hardware types. An RARP server is also required on every physical network to respond to layer-two broadcasts. Each RARP server must have address assignments manually provided by an administrator. RARP only provides an IP address to a host and none of the other information a host may need.

4 Bootstrap Protocol (BOOTP) It is based on a client/server exchange, but is implemented as a higher-layer software protocol, using UDP for message transport. It is not dependent on the particular hardware of the network like RARP. It supports sending additional configuration information to a client beyond just an IP address. This extra information can usually all be sent in one message for efficiency. It can handle having the client and server on different networks of an internetwork. This allows the administration of the server providing IP addresses to be more centralized, saving money as well as administrative time.

5 Bootstrap Protocol (BOOTP) BOOTP is client/server in nature. The operation of the protocol consists of a single exchange of messages between a BOOTP client and a BOOTP server The BOOTP server maintains a special list of the clients and their IP addresses BOOTP messaging uses the User Datagram Protocol (UDP) as its layer four transport protocol. UDP is less complex and ideal for simple request/reply protocols like BOOTP. Since the client obviously doesn't know the address of a BOOTP server, the request is broadcast on its local network; UDP supports broadcasts while TCP does not. The BOOTP server can use the client's hardware address to create an ARP reply to the host.

6 DHCP

7 Dynamic Host Configuration Protocol (DHCP) The Dynamic Host Configuration Protocol (DHCP) is a network protocol used to configure devices that are connected to a network (known as hosts) so they can communicate on that network using the Internet Protocol (IP). It involves clients and a server operating in a client-server model.

8 Dynamic Host Configuration Protocol (DHCP) The clients request configuration settings using the DHCP protocol such as an IP address, a gateway one or more DNS server addresses. The client is able to communicate on that internet using these settings. The DHCP server maintains a database of available IP addresses and configuration information.

9 Dynamic Host Configuration Protocol (DHCP) DHCP operations fall into four basic phases: DHCP discovery, DHCP offer, IP request, IP lease acknowledgement.

10 DHCP Discovery The client broadcasts messages on the physical subnet to discover available DHCP servers. Network administrators can configure a gateway to forward DHCP packets to a DHCP server from a different subnet. The discovery message is created as a User Datagram Protocol (UDP) packet with the broadcast destination of or the specific subnet broadcast address. The source IP address in the packet is because the client does not yet have an IP address.

11 DHCP Offer When a DHCP server receives a Discover message, it extends an IP lease offer by sending a DHCP Offer message to the client. This message contains the client's MAC address, the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer. More than one DHCP server can respond with a DHCP Offer packet.

12 DHCP Request When a DHCP client receives a DHCP Offer packet, it responds by broadcasting a DHCP Request packet that contains the offered IP address, and shows acceptance of the offered IP address. The message is broadcast but includes the IP address of the DHCP server in the message body.

13 DHCP Acknowledge The selected DHCP server acknowledges the client DHCP Request for the IP address by sending a DHCP Acknowledge message. At this time the server also forwards any optional configuration parameters. Upon receipt of the DHCP Acknowledge, the client can participate on the TCP/IP network and complete its system startup.

14 DHCP Release A DHCP client sends a DHCP Release packet to the server to release the IP address and cancel any remaining lease.

15 IP Addresses in DHCP Messages Discover: Src= sport=68 Dest= dport=67 Offer: Src= sport=67 Dest= dport=68 Request: Src= sport=68 Dest= dport=67 Acknowledge: Src= sport=67 Dest= dport=68

16 ICMP

17 Internet Control Message Protocol (ICMP) The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is used by the operating systems of networked computers to send error messages indicating that a requested service is not available ICMP is also used to relay query messages

18 Internet Control Message Protocol (ICMP) ICMP is sent in an IP packet. ICMP message includes a header. The ICMP header starts after the IPv4 header and is identified by protocol number '1'. All ICMP packets will have an 8-byte header and variable-sized data section. The first 4 bytes of the header will be consistent.

19 Internet Control Message Protocol (ICMP) ICMP header fields are: Type : ICMP type of the operation. Code : Subtype to the given type. Checksum : Error checking data. Calculated from the ICMP header+data, with value 0 for this field. Rest of Header : Four byte field. Varies based on the ICMP type and code.

20 Internet Control Message Protocol (ICMP) ICMP types: ( Some of the most common types ) 0 Echo Reply 3 Destination Unreachable 4 Source Quench 5 Redirect Message 8 Echo Request 9 Router Advertisement 10 Router Solicitation 11 Time Exceeded 12 Parameter Problem: Bad IP header 13 Timestamp 14 Timestamp Reply 30 Traceroute

21 ICMP Code Sample code for destination unreachable type 0: Destination network unreachable 1: Destination host unreachable 2: Destination protocol unreachable 3: Destination port unreachable 4: Fragmentation required, and DF flag set 5: Source route failed 6: Destination network unknown 7: Destination host unknown 8: Source host isolated 9: Network administratively prohibited 10: Host administratively prohibited 11: Network unreachable for TOS 12: Host unreachable for TOS 13: Communication administratively prohibited 14: Host Precedence Violation 15: Precedence cutoff in effect

22 ICMP Data ICMP data field value depends on the type of the operation and the code. At least 4 bytes are used for ICMP data If ICMP does not need data field, it is left empty

23 ICMP Message Examples Query

24 ICMP Data Example Error Reporting