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The Internet: Technology and Applications Course: 635.413.31

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  1. The Internet: Technology and ApplicationsCourse: 635.413.31 Summer 2007 Johns Hopkins University Instructor: John A. Romano

  2. Internetworking Review • The Goals of the Internet • Hide technological details from the user • Refrain from mandating a specific network interconnection technology or topology • Utilize a universal address space • Internet Architecture & Routers • The key piece of equipment in the internet are routers • Special systems that attach to two or more networks and forward packets between them • Can separate networks of different technologies • The key protocol (the ‘glue’ to the Internet) is called IP, or the Internet Protocol

  3. Internetworking • Review -- where does IP fit?

  4. The Internet Protocol • Why IP? • Creates a seamless virtual network • Provides global address space • Defines a connectionless, packet-oriented protocol • Provides “best effort” delivery; up to higher layer protocols to detect & recover from failures • Core definition in RFC 791 (with several extensions and amendment RFCs) • What we cover in this class • IP Addressing • ARP: how IP addresses translate to Hardware addresses • IP Packet (Datagram) Structure & Operation • IP Packet Forwarding • ICMP: Error & Status Reporting

  5. Classful IP Addressing • IP Addresses • Hierarchical versus Flat Addressing • IP Address Hierarchy: Host part vs. network part • Allows for smaller routing tables • Allows for distributed control and distribution of addresses • Can cause inefficient allocation of addresses • Classful Addressing Scheme: 5 different ‘classes’ • BIG Networks: Class A • Network mask is eight bits (high order address bit is zero) • 127 possible networks (actually 125) • Medium Networks: Class B • Network mask is 16 bits (high order address bits are ‘10’) • Small Networks: Class C • Network mask is 24 bits (high order address bits are ‘110’)

  6. Classful IP Addressing • Multicast Addresses: Class D • High order address bits are ‘1110’ • The rest of the address has no inherent structure like the ‘primary’ addresses; each address defines a multicast ‘group’ (think channels stations “tune” into) • Some multicast IP addresses are reserved as ‘well-known’ addresses • Experimental Addresses: Class E • High order address bits are ‘11110’ • Used for research; example -- the development of ‘Anycast’ services • The Classful Scheme has been largely replaced by a “Classless” Scheme that is much more flexible • The newer scheme requires the transmission of a ‘mask’ value to determine which part of the address is ‘network’ and which is ‘host’ • Classful & Classless Examples

  7. Classful IP Addressing • IP Address Field Details

  8. Subnetting • Allows a single network address to span multiple physical networks • Adds another hierarchical level to the IP address scheme • Instead of dividing the address into network & host parts, it is divided into network and local parts (Figure 9.3 in textbook) • A 32 bit subnet mask denotes what portion of the address is the host part • So important that support of subnetting is now a required part of the IP standard • Reasons for subnetting • Better control and security of network traffic • Allows for more efficient routing within an organization’s network (particularly a large network) • Allows for distributed control and distribution of addresses, but can contribute to inefficient address allocation if improperly used

  9. Subnetting • Variable-length Subnet Masking (VLSM) • A enhancement to subnetting that allows the flexible allocation of different size subnets to physical networks • Allows for even more efficient allocation of addresses • Requires the use & exchange of subnet masks for proper network operation (e.g. – in routing protocols) • Calculation of netmask with subnetting (Regular & VLSM)

  10. Special IP Addresses • Multicast • Allows for more efficient use of network bandwidth • Important for one-to-many services • Video • Software distribution • Newsfeeds • Used in several routing protocols • Relationship between Multicast IP and Ethernet addresses • Ethernet HW address range 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff reserved for multicast • Low order 23 bits of IP Multicast address map to an ethernet HW multicast address • Well-known Multicast Addresses (RFC 1700) • – All OSPF routers • – HSRP (Hot Standby Router Protocol)

  11. Special IP Addresses • Broadcast • Another one-to-many means of communication related to multicast • Important in many host’s initialization process • If managed carelessly can severely degrade network performance (or worse!) • Two classes of broadcast: • Local Broadcast • Local uses IP address of all ones ( • Broadcasts to the network physically connected to the host interface • Local broadcast not forwarded by routers • Directed Broadcast • Allows a host to send a broadcast to a ‘remote’ network or subnet • Network/Subnet part of address is the real address while the host part is all ones (example • CAREFUL!!! This feature may not make you many friends

  12. Special IP Addresses • Loopback • Whole Class A (127.x.y.z) allocated to this function • Allows the testing of a host’s protocol stack without affecting the network • Similar in function to addressing something to the local host’s ‘real’ IP address (though differences can be implementation dependent) • ‘Network’ & Special Host Addresses • An IP address specifying a network has all zeros in the host field • Typically see network addresses in routing tables • During startup a host may need to use a temporary IP address; typically is used for this purpose

  13. Special IP Addresses • ‘Private’ IP Addresses (Non-routable) • The IETF has declared several blocks of addresses as private or nonroutable • Internet routers should be configured to block/filter these addresses • Commonly used with DSL, Cable Modems, and behind Firewalls in conjunction with NAT (Network Address Translation) • Reserved Blocks • • • • Other Special IP Addresses (RFC 3330) • ‘Link Local’ addresses for use across a single link • Used for network benchmarking [per RFC 2544] • A ‘test network’ block of addresses

  14. Address Resolution Protocol (ARP) • What is ARP needed for? • For delivery an IP address must be ‘mapped’ to a data link layer address • ARP defines a dynamic means for mapping to occur • There are other ways for providing this functionality: table lookup & computational methods • ARP for Ethernet defined in RFC 826 • ARP packet format (for Ethernet) • Can accommodate multiple lower layer protocols (not just Ethernet) • ARP frame type is 0x0806; ARP Request type is 1 & Reply is type 2

  15. Address Resolution Protocol (ARP) • The ARP cache • Reduces network traffic by storing recently used address ARP data • Entries typically time out after 20 minutes • Newer ARP information replaces older information in the ARP cache • Automatic ARP Cache Revalidation • Minimizes the ‘jitter’ in network traffic flow after an ARP entry expires • The Address Resolution process • ARP requests are broadcast while a reply is typically unicast • ARP example

  16. Address Resolution Protocol (ARP) • Variations of ARP • Proxy ARP • Allows a router to answer ARP requests on one interface for a host on a different router interface • Proxy ARP examples • Gratuitous ARP • Denotes a host broadcasting an ARP request for its own IP address • Contains a new or updated IP to HW address binding; other hosts update their cache • Sometimes used to provide faster recovery from system outages • Not implemented on all operating system network protocol stacks

  17. Address Resolution Protocol (ARP) • ARP’s relative: RARP (the Reverse Address Resolution Protocol) • Allows a host (particularly diskless workstations) to obtain IP address automatically • RARP packet format • Same as ARP except the Ethernet frame type is 0x8035 • RARP Request =3 and Reply = 4 • There are better ways of providing this information and more (e.g. – BOOTP & DHCP) which we will learn about later!

  18. IP Packet Format & Structure • The Internet Protocol (IP) Packet

  19. IP Packet Structure – Mandatory Fields • Protocol Field • Version 4 (current) and Version 6 (future) • IP packet header length field (4 bits) • Header size is not fixed; there can be options • Field counts the number of four byte ‘words’ in the header • Maximum header size: 60 bytes • Type of Service (TOS) field (8 bits) • Original definition: 3 bits for precedence and 3 bits for TOS • TOS bits: Minimize delay, maximize throughput, & maximize reliability • The original specification has been superseded by the “Diff-Serv” specs • New definitions in RFC 2474 redefine the use of the field • Backwards compatible with older definitions • A whole new set of ‘codepoints’ defined to help apply QoS to IP networks • Finding wider use because of VoIP and other real-time streaming services

  20. IP Packet Structure – Mandatory Fields • IP packet length field (16 bits) • Some IP packets can be smaller than the minimum data link frame size • Example: minimum Ethernet frame size is 46 bytes • Tiny IP packets are padded out to the minimum frame size with zeros • Maximum packet size: 65535 bytes • IP packet identification field (16 bits) • Uniquely identifies each IP packet; very important for fragmentation • Hosts typically use an internal counter to set this field which is incremented each time an IP packet is sent • Fragmentation Flags and Offset fields • DF (Don’t Fragment) bit • MF (More Fragments) bit • Offset field (13 bits) - specifies the offset in 8 byte units of the fragment from the beginning of the original IP packet

  21. IP Packet Structure – Mandatory Fields • Time-to-Live (TTL) field (8 bits) • Used to limit the lifetime of an IP packet • Decremented every time the IP packet transits a router • TTL set by the source host; value is OS and application dependent • Protocol field (8 bits) • Identifies the higher layer protocol payload encapsulated in the IP packet • Allows IP layer to determine what higher layer process should receive the data • Header Checksum field (16 bits) • Checks for errors in the IP header ONLY • One’s complement addition used to calculate checksum • Errored IP packets are silently discarded; recovery is up to higher layers • Source & destination IP address fields (32 bits each)

  22. IP Packet Structure – Optional Fields • Header Option Fields • Header options can take up an additional 40 bytes in the IP header • Provide a variety of services used in special circumstances • First byte specifies option type – some options are only one byte while others are variable length • Generic Structure of Header Options

  23. IP Packet Structure – Optional Fields Record Route Option • Used to detect and record the path being taken by a particular IP packet • Code field: Record Route option specified by a value of 7 in this 8 bit field • Length Field: contains total length of the option header (usually 39 bytes) • At maximum length option can store nine IP addresses in the list, after that the list is full and routers ignore the option • Pointer Field: shows the router where to store the next IP address; points to the first empty byte (i.e.– ptr=4 if no IP addresses have been recorded) • Routers typically record the outgoing interface of the IP packet

  24. IP Packet Structure – Optional Fields • Timestamp Option (Code field = 44) • Allows a host to query another system for its current time • Same fields at the Record Route option plus two additional 4 bit fields • Overflow (OF) field- 4 bit counter incremented by routers after option header is full • Flags (FL) field specifies whether routers record a timestamp only or a timestamp and its IP address. • Time returned is number of milliseconds past midnight UTC • There are now better ways of time synchronization (NTP, OSF DCE, etc) • Security Options • Defined in RFC 1108; rarely used today • Allowed the labeling of IP packets with classification information • Provided no inherent protection; relied on routers to read labels and route packets through paths of the appropriate security level

  25. IP Packet Structure – Optional Fields • Source Routing Options • Allows a source host to specify the path IP packets will take through the Internet • Option header fields (code, length, pointer) and maximum size are the same as the Record Route option • Code is 0x83 for loose source routing and 0x89 for strict source routing • Two varieties: Loose and Strict • Strict Source Routing: the EXACT path is specified in the IP packet • Loose Source Routing: the IP packet contains a list of IP addresses that it must traverse but it can traverse others not listed. • Source Route Examples

  26. IP Fragmentation and Reassembly • Concept -- Maximum Transmission Unit (MTU) • Based on underlying transmission protocol • Cannot be violated (includes the frame headers & trailers) • MTU example • Fragmentation • Allows IP to deal with physical networks that have different MTUs • IP header fields and flags important during IP Fragmentation • IP Fragmentation example • Reassembly • Done at destination host • Eases processing burden on routers • Allows IP fragments to traverse different routes in the network • Example illustrating different routing of IP packet fragments • Example for reassembly at destination host

  27. IP Fragmentation and Reassembly • Concept -- Maximum Transmission Unit (MTU) • Loss of a fragment can & does occur (just like any other IP packet) • Two things that can go wrong • Fragment gets corrupted and are discarded • Upon receipt of the first fragment destination host sets a timer; if any fragment fails to make it into the reassembly buffers before the timer expires ALL fragments are discarded. • Multiple Fragmentations & Example

  28. IP Packet Forwarding • Encapsulation of an IP packet for transmission • Lower layer frame may change many times during transit • The role of routers (versus a multi-homed host) • The characteristics of IP packet forwarding • Table-driven • Next-hop • Done on a per-packet basis • The routing table • The mechanism a host uses to determine what to do with an IP packet it’s trying to send • The mechanism a router uses to determine how to forward an IP packet • In general routing tables contain routes to networks • How the tables are filled is covered in Class #4!

  29. IP Packet Forwarding • IP Forwarding example

  30. IP Packet Forwarding • Example routing table from a Cisco Router a-tserver>sh ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route Gateway of last resort is to network is variably subnetted, 126 subnets, 8 masks O E2 [110/1] via, 16:03:32, Ethernet0 O E1 [110/34] via, 16:03:32, Ethernet0 O IA [110/27] via, 16:03:32, Ethernet0 O [110/75] via, 16:03:32, Ethernet0 O IA [110/17] via, 16:03:32, Ethernet0 O [110/21] via, 16:03:32, Ethernet0 O E2 [110/20] via, 16:03:32, Ethernet0 O [110/11] via, 16:03:42, Ethernet0 C is directly connected, Ethernet0

  31. Internet Message Control Protocol (ICMP) • What is ICMP used for? • Provides rudimentary error reporting capability • Provides a basic informational and troubleshooting mechanism • ICMP Mechanics • Required part of IP • Defined in RFC 792 • Generic ICMP Message Format • Type and Code fields • Header Checksum • Additional header bytes

  32. Internet Message Control Protocol (ICMP) • ICMP Error Messages • Sent in response to a problem delivering an IP packet • Includes the IP header plus eight bytes of payload from the packet causing the error (contains the TCP or UDP port numbers so the source application can be notified) • NOT sent under the following conditions: • in response to any other Network layer protocol besides IP • in response to an errored ICMP packet • in response to an IP multicast or broadcast source

  33. Internet Message Control Protocol (ICMP) • ICMP Error Messages • Major Error Types • Destination Unreachable (Type 3) • Network Unreachable (Code 0) • Host Unreachable (Code 1) • Protocol Unreachable (Code 2) • Port Unreachable (Code 3) • Fragmentation required but the DF bit set (Code 4) • IP Redirect (Type 5) • Used by routers to ‘correct’ hosts • Time Exceeded (Type 11) • Either a TTL or a Destination Reassembly Issue • Parameter Problem (Type 12) • The ‘catch-all’ error message

  34. Internet Message Control Protocol (ICMP) • ICMP Informational & Troubleshooting Messages • Echo Request (Type 8) and Echo Reply (Type 0) • Used to tell whether a host’s network interface card is functioning • Payload typically empty but certain implementations will allow you to specify the ICMP payload • Older Messages no longer in use Timestamp Request (Type 13) and Timestamp Reply (Type 14) • Allows a host to query another for the current time • Returns the number of milliseconds past midnight UTC; stills requires the receiving host to calculate the current time • There are better ways of doing this: NTP, RPC time functions • Address Mask Request (Type 17) & (Type 18) • Allows a host to determine its address mask from it’s neighbors • Sometimes good (if the mask is right) and sometimes bad!

  35. Internet Message Control Protocol (ICMP) • PING • A fundamental troubleshooting tool based on ICMP • PING Example: > ping PING ( 56 data bytes 64 bytes from icmp_seq=0 ttl=117 time=94.168 ms 64 bytes from icmp_seq=1 ttl=117 time=73.961 ms 64 bytes from icmp_seq=2 ttl=117 time=63.667 ms 64 bytes from icmp_seq=3 ttl=117 time=57.443 ms 64 bytes from icmp_seq=4 ttl=117 time=65.453 ms 64 bytes from icmp_seq=5 ttl=117 time=85.126 ms 64 bytes from icmp_seq=6 ttl=117 time=69.730 ms 64 bytes from icmp_seq=7 ttl=117 time=67.107 ms ^C --- ping statistics --- 10 packets transmitted, 10 packets received, 0% packet loss round-trip min/avg/max/stddev = 57.004/70.505/94.168/11.062 ms

  36. Review of Class #2 • The Key Conclusions to Class #2 • The Network Interconnection ‘model’ from Class #1 is used in the Internet • The Internet Protocol is the key to internetworking; it is a flexible and feature-rich base to the family of internet protocols • ARP provides a dynamic & standard means to map between MAC and network layer addresses • IP forwarding is a datagram-based, next-hop, table-driven process • ICMP provides error reporting, informational, & troubleshooting mechanism for IP

  37. Reading and Homework • Reading • Comer: Chapters 4 through 9 (except sections 9.20 and 9.21) • First Homework Assignment is due in a week (see Class #1 slides for the problems) • Next Monday: Transport Layer (TCP & UDP) Protocols