Internet Applications Browsers, Ping, Traceroute, ARP

Transcription

1 Internet Applications Browsers, Ping, Traceroute, ARP Professor Richard Harris School of Engineering and Advanced Technology (SEAT)

2 Presentation Outline Computer Networks - 1/2

3 Learning Objectives You will be able to: To gain an understanding of Internet applications such as Web browsers, traceroute and ping utilities. To understand the reason for the existence of the Address Resolution Protocol (ARP). Computer Networks - 1/3

4 References Forouzan, Data Communications and Networking, 4 th Edition Tanenbaum, Computer Networks, 4 th Edition Cisco CCNA1 Module 10 - part 1 Stallings, William 2000 Data and Computer Communications, Prentice Hall, Sixth Edition Russell, Travis 1997 Telecommunications Protocols, McGraw Hill Computer Networks - 1/4

5 Internet Applications in Context MIME BGP FTP HTTP SMTP TELNET SNMP TCP UDP ICMP IGMP OSPF RSVP IP Computer Networks - 1/5

6 Internet Applications in Context Layer 7: Application Layer 6: Presentation Application Network Applications Layer 5: Session Layer 4: Transport Transport TCP UDP Layer 3: Network Internet ICMP IGMP IP ARP Layer 2: Logical Link Layer 1: Physical Network LAN Technologies WAN Technologies Computer Networks - 1/6

7 Browsers More commonly known as the Web browser, it is a software application used to locate and display Web pages. A web browser is an example of the client part of a distributed client / server application. It uses the Hypertext Transfer Protocol (HTTP) to make requests from a web server. There are two broad categories of browsers: Graphical (e.g., Internet Explorer, Netscape, Opera) Text-based (e.g., Lynx) Computer Networks - 1/7

8 Browsers Nowadays, the Web is no longer a static environment for displaying graphics and text, but also presents multimedia information that gives the user a more fulfilling and richer experience. This is facilitated through the use of plug-ins. Plug-ins are hardware or software components that add a specific feature or service to a larger system. Examples include RealPlayer, Macromedia Flash, Adobe Acrobat. Computer Networks - 1/8

9 Browsers Very importantly, the Web browser and its associated technologies are envisaged as the standard interface or front end for current and future applications. You may have come across terms such as intranets and extranets being implemented in many enterprises. Essentially, these organizations are establishing private versions of the Internet that uses standard Internet protocols such as TCP/IP and HTTP. They use it for facilitation of daily business activities such as delivery and presentation of information, transaction processing, , etc. Computer Networks - 1/9

10 Mozilla Browser Computer Networks - 1/10 10

11 Packet InterNet Groper (Ping) Ping is a network debugging tool used to determine whether a specific IP address is accessible. It is implemented using the ICMP Echo and and ICMP Echo Reply messages. This is documented in RFC 792. In an IP network, ping sends an ICMP Echo message (in a single packet) to the destination IP address and listens for a single packet in reply (the ICMP Echo Reply). Computer Networks - 1/11 11

12 Ping Example for Windows XP: At the command prompt, type: ping rmit.edu.au or ping Computer Networks - 1/12 12

13 Ping What ping can tell you: Determine if packets have been dropped, duplicated or reordered. Recall that an ICMP Echo/Echo Reply messages can support sequence numbers. Ping uses these sequence numbers to facilitate this. Detect some forms of damaged packets by using the checksum field in the ICMP Echo/Echo Reply messages. Compute the Round Trip Time (RTT) by placing a timestamp in each packet, which is echoed back. Can report other ICMP messages, such as if a router is declaring the target host unreachable. Computer Networks - 1/13 13

14 Ping What ping can t tell you: May not always be able to tell the reason for unanswered packets. The reason could be due to routers silently discarding undeliverable packets or a destination host chooses not to reply as in the case of Microsoft at Can t tell the reason for a damaged, delayed or duplicated packet nor where it occurred although a logical deduction can usually be made with respect to this. Can t provide a detailed description of every host/router that handled the packet and everything that happened every step of the way. Computer Networks - 1/14 14

15 Traceroute Traceroute is a network debugging tool that is used to trace a packet from the source host to a destination host. It attempts to trace the route the packet takes, i.e., it shows how many hops the packet requires in order to reach the destination host, and how long each hop takes. Traceroute works because it DEPENDS on the common router practice of sending an ICMP Time Exceeded message (defined in RFC 792). Recall the TTL (Time to Live) IP header field in an IP packet. The TTL is designed to prevent packets from running in loops in an internet. Computer Networks - 1/15 15

16 Traceroute When a TTL reaches a value of zero, the packet has expired and is discarded by the router. In doing so, the router sends an ICMP Time Exceeded Message back to the sender, and thus identifying itself to the sender. A TTL value of one should produce a message from the first router, a TTL value of two should produce a message from the second router, and so forth. Hence, by sending a series of packets and incrementing the TTL value with each successive packet, traceroute finds out who all the intermediate hosts are. Computer Networks - 1/16 16

17 Traceroute Example for Windows XP: At the command prompt, type: tracert rmit.edu.au or tracert Computer Networks - 1/17 17

18 Address Resolution Protocol (ARP) In a broadcast-based network such as Ethernet, hosts must know the hardware address (machine address) of other hosts to communicate on a network. The Address Resolution Protocol (ARP), which is part of the TCP/IP Internet layer, obtains hardware addresses of hosts on the same physical network. ARP is formally defined in RFC 826. ARP uses a local broadcast of the destination IP address to acquire the hardware address of the destination host or gateway. Computer Networks - 1/18 18

19 ARP After the ARP obtains the hardware address, both the IP address and hardware address are stored as an entry in the ARP cache. The ARP cache is consulted before initiating another ARP request broadcast. Reverse Address Resolution Protocol (RARP) obtains IP addresses given a host s hardware address. Computer Networks - 1/19 19

20 ARP Resolving a Local IP Address Prior to any communication between hosts, the IP address of each host must be resolved to the host s hardware address. The process includes an ARP request and an ARP reply. Computer Networks - 1/20

21 ARP Resolving a Local IP Address Example: Ping a local host at ping ping ARP Cache ARP Cache ARP Broadcast 3 IP Address = Hardware Address = IP Address = Hardware Address = Computer Networks - 1/21

22 ARP Resolving a Local IP Address 1. In this case, IP confirms that the IP address is for the local network. The source host then check its own ARP cache for the IP address/hardware address mapping of the destination host. 2. Finding no mapping, the source host broadcasts an ARP request. The source host s IP address and hardware address are included in the request. 3. Each host on the local network receives the broadcast and checks for a match to its own IP address. If there is no match, the request is ignored. 4. The destination host determines that the IP address matches its own IP address and sends an ARP reply containing its hardware address directly to the source host. It then updates its ARP cache with the IP address/hardware address mapping of the source host. Communication is established when the source host receives the reply. Computer Networks - 1/22

23 ARP Resolving a Remote IP Address In this situation, the ARP broadcast is for the IP address of the gateway/router rather than the IP address of the destination host. Computer Networks - 1/23

24 ARP Resolving a Remote IP Address Example: Ping a remote host at ping ping ARP Cache ARP Cache ARP Broadcast A ARP Cache Router B IP Address = Hardware Address = ARP for Router B IP Address = Hardware Address = Computer Networks - 1/24

25 ARP Resolving a Remote IP Address 1. In this case, IP confirms that the IP address is for a remote network. Next, the source host checks the local routing table for a route to the destination host or network. If no mapping is found, the source host determines the IP address of the gateway/router. The source host then check its own ARP cache for the IP address/hardware mapping of the specified gateway. 2. Finding no mapping, the source host broadcasts an ARP request to the gateway s address. The router responds to the source host s ARP request with its hardware address. The source host then sends the data packet to the router to deliver to the destination host s network, and ultimately the destination host. Computer Networks - 1/25

26 ARP Resolving a Remote IP Address 3. At the router, IP determines whether the destination is local or remote. If it is local, the router uses ARP (either cache or broadcast) to obtain its hardware address. If it is remote, the router checks its routing table for a specified gateway, and then uses ARP (either cache or broadcast) to obtain the gateway s hardware address. The packet is sent directly to the next destination host. 4. After the destination host receives the request, it formulates an ICMP echo reply. The local routing table is checked for a specified gateway to the source host s network. When it finds the gateway, ARP obtains its hardware address. 5. If the specified gateway s address is not in the ARP cache, an ARP request which is broadcasted is sent. Once it obtains the hardware address, the ICMP reply is sent to the router to be routed to the source host. Computer Networks - 1/26

27 ARP Packet Structure Bit: 28 octets Hardware Type Protocol Type HLEN PLEN Operation Sender HA (octets 0-3) Sender s HA (octets 4-5) Sender s PA (octets 0-1) Sender s PA (octets 2-3) Target s HA (octets 0-1) Target s HA (octets 2-5) Target s PA (octets 0-3) Hardware Type: the type of hardware (Network Access Layer) being used. Protocol Type: the protocol being used for the resolution process using the EtherType value. Hence the Protocol Type for IP is 0x Computer Networks - 1/27

28 ARP Packet Structure HLEN: Hardware Address Length. Length in bytes of the hardware address. For Ethernet and Token Ring, the length is 6 bytes. PLEN: Protocol Address Length. Length in bytes of the protocol address. For IP, the length is 4 bytes. Operation (Opcode): the operation field specifies the operation being performed. Sender s HA: the hardware address of the sender (the ARP requester). Sender s PA: the protocol address of the sender (the ARP requester). Target s HA: the hardware address of the target (the ARP responder). Target s PA: the protocol address of the target (the ARP responder). Computer Networks - 1/28