Supporting Protocols and Technologies in TCP/IP Suites

Transcription

1 Supporting Protocols and Technologies in TCP/IP Suites Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya and lecture slides from Anan Phonphoem 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. The McGraw-Hill Companies, Inc.

2 Outline Address Resolution Protocol (ARP) Internet Control Message Protocol (ICMP) Dynamic Host Configuration Protocol (DHCP) Network Address Translation (NAT) 2

3 Address Resolution Protocol (ARP)

4 Address Resolution Recall the forwarding process Forwarding uses IP addresses A MAC address is needed to communicate with the next hop IP must translate the next-hop IP address to a MAC address The translation process is known as address resolution Address resolution is local to a network 4

5 Address Resolution One computer can resolve the address of another computer only if both computers attach to the same physical network Resolve Resolve 5

6 ARP RFC Address Resolution Protocol ARP maps any network level address (such as IP) to its corresponding data link address (such as Ethernet) Supporting protocol in data link layers Not data link layer protocol itself 6

7 ARP Protocol Station 1 is looking for MAC add of IP I m looking for IP ARP request 3 4 ARP request ARP request ARP request 1 2 7

8 ARP Protocol Station 3 (IP ) responses ARP response 3 4 ARP response I m IP My physical address is A-C

9 Conceptual Address Boundary 9

10 ARP Cache Sending an ARP request for each datagram is inefficient Three frames traverse the network for each datagram ARP request, ARP response, and the data datagram itself ARP cache is used to reduce network traffic ARP saves the information from a response so it can be used for subsequent packets The software does not keep the information indefinitely Instead, ARP maintains a small table of bindings in memory 10

11 ARP from Command Prompt entry in ARP table C:\> arp -a ( ) at 0:0:e8:15:cc:c $ ping garnet.cpe.ku.ac.th : C:\> arp -a router.cpe.ku.ac.th ( ) at 0:0:c:6:13:4a cc.cpe.ku.ac.th ( ) at 2:60:8c:2e:b5:8b ( ) at 0:0:e8:15:cc:c 11

12 Error Reporting Mechanisms

13 Error Reporting Mechanisms IP problems Best effort Data can be lost, duplicate, delay, out-of-order Error detection of IP checksum if error, discard frame (cannot send back error message no trust in the header) IP requires additional helpers Internet Control Message Protocol (ICMP) 13

14 ICMP RFC 792 IP supporter For error generating Transmission problem Time to live (TTL) exceed Destination unreachable etc. Serve as useful diagnostic tools ping, traceroute 14

15 ICMP ICMP error messages never generates due to: ICMP error messages themselves Broadcast/Multicast (prevent broadcast Storms) What are Broadcast Storms? A large number of broadcast frames transmitted nearly simultaneous LAN may freeze! 15

16 ICMP encapsulation frame hdr e.g. Ethernet Frame data IP hdr IP data contain protocol 0x01 (ICMP) type code... indicate error type 16

17 Ethernet Frame Containing ICMP packet DA IP SA T Type Code Other info. header Frame (Ethernet) Header IP Header ICMP Type Code Description 8 0 Echo request 0 0 Echo reply 11 0 Time exceed 3 3 Port unreachable 17

18 ICMP header type - relevant ICMP message code - more detail information checksum - covers ICMP header/data (not IP header) type:8 code:8 checksum:16 Content specific 18

19 ICMP Messages 19

20 Diagnostic Tools: ping ping request ping reply Generate an ICMP echo request Receive the ICMP echo reply All TCP/IP node is supposed to implement ICMP and respond to ICMP echo 20

21 ping command (#1) Send a single echo request / wait for a reply Resend another request if no reply (1 sec.) Repeat until receive at least one reply or stop after time out > ping iwing.cpe.ku.ac.th iwing.cpe.ku.ac.th is alive > ping happy.cpe.ku.ac.th no answer from happy.cpe.ku.ac.th 21

22 ping command (#2) Send an echo request message every seconds Records the time it takes for each reply Every echo request contains a unique sequence number to match replies and request Record round-trip timing Perform packet lost statistics 22

23 ping example $ ping iwing.cpe.ku.ac.th PING iwing.cpe.ku.ac.th ( ) from : 56(84) bytes of data. Warning: time of day goes back, taking countermeasures. 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=0 ttl=252 time=1.187 msec 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=1 ttl=252 time=601 usec 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=2 ttl=252 time=594 usec 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=3 ttl=252 time=594 usec 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=4 ttl=252 time=585 usec 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=5 ttl=252 time=590 usec 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=6 ttl=252 time=584 usec 64 bytes from iwing.cpe.ku.ac.th ( ): icmp_seq=7 ttl=252 time=587 usec --- iwing.cpe.ku.ac.th ping statistics packets transmitted, 8 packets received, 0% packet loss round-trip min/avg/max/mdev = 0.584/0.665/1.187/0.198 ms 23

24 ping as debugging tools What we get from ping? Timing information Connection reliability Destination is reachable (routable) IP layer is functional, but no guarantee for other higher layer protocols 24

25 ping results No response Target host inactive or no connection Lost packet (significant when >2-3%) Transmission error on WAN/LAN Overloading bridges/routers Varying round-trip time host/network overloading No lost and round-trip time is reasonably constant Congratulations! That s all we want. 25

26 Diagnostic Tools: traceroute Command to determine the active route to a destination address How does it work? send a UDP messages to an unused port on the target host with ttl=1 router decrease ttl to 0, it has to return an ICMP time exceed message traceroute sets ttl =2 and retransmits, this time go one more hop ttl++ until UDP messages reach the destination. the target returns an ICMP service unavailable because there is no UDP port service. 26

27 How traceroute works? UDP(TTL =1) Destination unused port # 27

28 How traceroute works? UDP(TTL = 0 =1) ICMP(time UDP(TTL =1) exceed) 28

29 How traceroute works? UDP(TTL ICMP(time = 0 =1) exceed) UDP(TTL ICMP(time =1) =2) exceed) UDP(TTL ICMP(time =2) exceed) 29

30 How traceroute works? UDP(TTL =8) UDP(TTL =9) ICMP(port UDP(TTL =10) unreachable) ICMP(port UDP(TTL unreachable) =0) 30

31 Traceroute example $ traceroute iwing.cpe.ku.ac.th traceroute to iwing.cpe.ku.ac.th ( ), 30 hops max, 38 byte packets 1 fe-cpegw2-server ( ) ms ms ms 2 gb-cpegwbb-cpegw ( ) ms ms ms 3 gb-cpec4k6-cpec6k ( ) ms ms ms 4 iwing ( ) ms ms ms 31

32 Traceroute example $ traceroute traceroute to ( ), 30 hops max, 38 byte packets 1 fe-cpegw2-server ( ) ms ms ms 2 gb-cpegwbb-cpegw ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms 9 S1-1.R00.LA-POP.uni.net.th ( ) ms ms ms ( ) ms ms ms 11 snvang-losang.abilene.ucaid.edu ( ) ms ms ms 12 dnvrng-snvang.abilene.ucaid.edu ( ) ms ms * 13 kscyng-dnvrng.abilene.ucaid.edu ( ) ms ms ms 19 nox300gw1-peer-nox-umass nox.org ( ) ms ms ms 20 lgrc-rt gw.umass.edu ( ) ms ms ms 21 lgrc-rt gw.umass.edu ( ) ms ms ms 22 * * * 23 * * * 32

33 Dynamic Host Configuration Protocol (DHCP)

34 DHCP Allows a computer to join a new network and obtain networking parameters automatically IP address Subnet mask Default router (gateway) address DNS server's address etc. The concept has been termed plug-and-play networking 34

35 DHCP Message Format 35

36 DHCP Operation Client DHCP Server Boot DHCP DISCOVER (Broadcast) DHCP OFFER DHCP REQUEST DHCP ACK 36

37 DHCP Discover Message Broadcast by clients 37

38 DHCP Offer Message Sent directly to client 38

39 Assigned Address Types We can configure a DHCP server to supply two types of addresses: Permanently assigned addresses Typically assigned to servers A pool of dynamic addresses to be allocated on demand Typically assigned to arbitrary hosts 39

40 Address Leasing DHCP issues a lease on the address for a finite period Thus allows a DHCP server to reclaim addresses When a lease expires, a host can choose to relinquish the address or renegotiate with DHCP to extend the lease If approved, a computer continues to operate without any interruption If a server denies an extension request, the host must stop using the address 40

41 DHCP Relay Agents DHCP discover messages are broadcast locally These messages are not forwarded by routers DHCP Server Router DHCP DISCOVER New client Assuming all networks are /24 41

42 Example: DHCP Relay Agents Each network may be equipped with a DHCP relay Typically built into a router DHCP Server Router & DHCP Relay DHCP DISCOVER (Unicast) DHCP DISCOVER New client Assuming all networks are /24 42

43 Example: DHCP Relay Agents Each network may be equipped with a DHCP relay Typically built into a router DHCP Server Router & DHCP Relay DHCP OFFER DHCP OFFER Client accepts IP New client Assuming all networks are /24 43

44 Example: DHCP Relay Agents Each network may be equipped with a DHCP relay Typically built into a router DHCP ACK Router & DHCP Relay DHCP Server 7 DHCP REQUEST DHCP ACK 9 6 DHCP REQUEST New client Assuming all networks are /24 44

45 Network Address Translation (NAT)

46 Network Address Translation NAT Network Address Translation A function that translates the address of datagrams into a new address Typically, original address is private (unroutable) New address is public and routable 46

47 Private Addresses Internet routers will not route packets whose destination addresses fall within these ranges 47

48 Basic NAT Operation Address Translation Table: Inside Outside NAT Router SA = DA = SA = DA = SA = DA = SA = DA =

49 Address Pooling SA = DA = NAT Router SA = DA = SA = DA = SA = DA = Address Translation Table: Inside Outside : 49

50 Advantages of Using NAT Eliminates need to reassign addresses when changing to a new ISP Protects network security Balances load SA = DA = Internet SA = DA = Preserves IP addresses 50

51 Port Translation Single public IP address is mapped to multiple hosts in a private network In this case, NAT router modifies the port numbers for outgoing traffic Known as NAPT or PAT 51

52 NAPT Operation Address Translation Table: Inside Outside : :4511 NAT Router SA = :2322 DA = :80 SA = :4511 DA = :80 SA = :80 DA = :2322 SA = :80 DA = :

53 NAT/NAPT for Home Users Wireless router has NAT/NAPT functionality built in Along with DHCP and switch functionalities Wireless Router Map all IP addresses to single routable address DSL/Cable Modem Internet

54 NAT Performance How much work does NAT do? IP Header TCP Header vsn len tos total length source port destination port identification flgs fragment offset sequence number TTL protocol header checksum acknowledgment number source IP address hlen rsv flags window size destination IP address TCP checksum urgent pointer IP option TCP option DATA DATA Some apps include IP information in the data. E.g., FTP, DNS, SNMP

55 NAT and ICMP Unlike TCP and UDP, ICMP has no port So how do things like ping work? For ICMP query (i.e., ping) Use Query ID in the echo-request and echo-reply Type Code Checksum ID Sequence For ICMP error (e.g., used by traceroute) Use embedded IP header information Type Code Checksum Empty Next-Hop MTU IP Header + First 8 bytes of original data

56 Application Layer Gateways Or ALG for short Run on top of NAT Allow NAT to support certain application layer protocols E.g., FTP, SIP, BitTorrent, IM One ALG per application

57 FTP ALG Normal operation (no NAT/ALG) Client Server PORT 15,2,10,12,7, PORT command successful RETR myfile.zip 150 Opening data connection Establish data connection; send file

58 FTP ALG Operation with NAT/ALG Client (private) (public) Server PORT 10,0,2,5,7, PORT command successful RETR myfile.zip 150 Opening data connection PORT 128,4,1,8,11, PORT command successful RETR myfile.zip 150 Opening data connection Establish data connection; send file Establish data connection; send file

59 Issues with NAT Increases resource and performance requirements for routers Not just address/port substitution Checksum, L4 header, ALG Break end-to-end transparency paradigm NAT modifies packets in route Cripples certain applications/protocols

60 More Information RFC 3022 Traditional IP Network Address Translator