Cs553 st7 ch18-internet_protocols
- 1. William Stallings Data and Computer Communications 7th Edition Chapter 18 Internet Protocols
- 2. Protocol Functions • Small set of functions that form basis of all protocols • Not all protocols have all functions — Reduce duplication of effort — May have same type of function in protocols at different levels • Encapsulation • Fragmentation and reassembly • Connection control • Ordered delivery • Flow control • Error control • Addressing • Multiplexing • Transmission services
- 3. Encapsulation • Data usually transferred in blocks — Protocol data units (PDUs) — Each PDU contains data and control information — Some PDUs only control • Three categories of control • Address — Of sender and/or receiver • Error-detecting code — E.g. frame check sequence • Protocol control — Additional information to implement protocol functions • Addition of control information to data is encapsulation • Data accepted or generated by entity and encapsulated into PDU — Containing data plus control information — e.g. TFTP, HDLC, frame relay, ATM, AAL5 (Figure 11.15), LLC, IEEE 802.3, IEEE 802.11
- 4. Fragmentation and Reassembly (Segmentation – OSI) • Exchange data between two entities • Characterized as sequence of PDUs of some bounded size — Application level message • Lower-level protocols may need to break data up into smaller blocks • Communications network may only accept blocks of up to a certain size — ATM 53 octets — Ethernet 1526 octets • More efficient error control — Smaller retransmission • Fairer — Prevent station monopolizing medium • Smaller buffers • Provision of checkpoint and restart/recovery operations
- 5. Disadvantages of Fragmentation • Make PDUs as large as possible because — PDU contains some control information — Smaller block, larger overhead • PDU arrival generates interrupt — Smaller blocks, more interrupts • More time processing smaller, more numerous PDUs •
- 6. Reassembly • Segmented data must be reassembled into messages • More complex if PDUs out of order
- 7. PDUS and Fragmentation (Copied from chapter 2 fig 2.4)
- 8. Connection Control • Connectionless data transfer — Each PDU treated independently — E.g. datagram • Connection-oriented data transfer — E.g. virtual circuit • Connection-oriented preferred (even required) for lengthy exchange of data • Or if protocol details must be worked out dynamically • Logical association, or connection, established between entities • Three phases occur — Connection establishment — Data transfer — Connection termination — May be interrupt and recovery phases to handle errors
- 9. Phases of Connection Oriented Transfer
- 10. Connection Establishment • Entities agree to exchange data • Typically, one station issues connection request — In connectionless fashion • May involve central authority • Receiving entity accepts or rejects (simple) • May include negotiation • Syntax, semantics, and timing • Both entities must use same protocol • May allow optional features • Must be agreed • E.g. protocol may specify max PDU size 8000 octets; one station may wish to restrict to 1000 octets
- 11. Data Transfer and Termination • Both data and control information exchanged —e.g. flow control, error control • Data flow and acknowledgements may be in one or both directions • One side may send termination request • Or central authority might terminate
- 12. Sequencing • Many connection-oriented protocols use sequencing — e.g. HDLC, IEEE 802.11 • PDUs numbered sequentially • Each side keeps track of outgoing and incoming numbers • Supports three main functions — Ordered delivery — Flow control — Error control • Not found in all connection-oriented protocols — E.g.frame relay and ATM • All connection-oriented protocols include some way of identifying connection — Unique connection identifier — Combination of source and destination addresses
- 13. Ordered Delivery • PDUs may arrive out of order — Different paths through network • PDU order must be maintained • Number PDUs sequentially • Easy to reorder received PDUs • Finite sequence number field — Numbers repeat modulo maximum number — Maximum sequence number greater than maximum number of PDUs that could be outstanding — In fact, maximum number may need to be twice maximum number of PDUs that could be outstanding • e.g. selective-repeat ARQ
- 14. Flow Control • Performed by receiving entity to limit amount or rate of data sent • Stop-and-wait — Each PDU must be acknowledged before next sent • Credit — Amount of data that can be sent without acknowledgment — E.g. HDLC sliding-window • Must be implemented in several protocols — Network traffic control — Buffer space — Application overflow • E.g. waiting for disk access
- 15. Error Control • Guard against loss or damage • Error detection and retransmission — Sender inserts error-detecting code in PDU • Function of other bits in PDU — Receiver checks code on incoming PDU — If error, discard — If transmitter doesn’t get acknowledgment in reasonable time, retransmit • Error-correction code • Enables receiver to detect and possibly correct errors • Error control is performed at various layers of protocol — Between station and network — Inside network
- 16. Addressing • Addressing level • Addressing scope • Connection identifiers • Addressing mode
- 17. TCP/IP Concepts
- 18. Addressing Level • Level in comms architecture at which entity is named • Unique address for each end system — e.g. workstation or server • And each intermediate system — (e.g., router) • Network-level address — IP address or internet address — OSI - network service access point (NSAP) — Used to route PDU through network • At destination data must routed to some process — Each process assigned an identifier — TCP/IP port — Service access point (SAP) in OSI
- 19. Addressing Scope • Global address — Global nonambiguity — Identifies unique system — Synonyms permitted — System may have more than one global address — Global applicability — Possible at any global address to identify any other global address, in any system, by means of global address of other system — Enables internet to route data between any two systems • Need unique address for each device interface on network — MAC address on IEEE 802 network and ATM host address — Enables network to route data units through network and deliver to intended system — Network attachment point address • Addressing scope only relevant for network-level addresses • Port or SAP above network level is unique within system — Need not be globally unique — E.g port 80 web server listening port in TCP/IP
- 20. Connection Identifiers • Entity 1 on system A requests connection to entity 2 on system B, using global address B.2. • B.2 accepts connection • Connection identifier used by both entities for future transmissions • Reduced overhead — Generally shorter than global identifiers • Routing — Fixed route may be defined — Connection identifier identifies route to intermediate systems • Multiplexing — Entity may wish more than one connection simultaneously — PDUs must be identified by connection identifier • Use of state information • Once connection established, end systems can maintain state information about connection — Flow and error control using sequence numbers
- 21. Addressing Mode • Usually address refers to single system or port —Individual or unicast address • Address can refer to more than one entity or port —Multiple simultaneous recipients for data —Broadcast for all entities within domain —Multicast for specific subset of entities
- 22. Multiplexing • Multiple connections into single system —E.g. frame relay, can have multiple data link connections terminating in single end system —Connections multiplexed over single physical interface • Can also be accomplished via port names —Also permit multiple simultaneous connections —E.g. multiple TCP connections to given system • Each connection on different pair of ports
- 23. Multiplexing Between Levels • Upward or inward multiplexing —Multiple higher-level connections share single lower- level connection • More efficient use of lower-level service • Provides several higher-level connections where only single lower-level connection exists • Downward multiplexing, or splitting —Higher-level connection built on top of multiple lower- level connections —Traffic on higher connection divided among lower connections • Reliability, performance, or efficiency.
- 24. Transmission Services • Protocol may provide additional services to entities • E.g.: • Priority — Connection basis — On message basis • E.g. terminate-connection request • Quality of service — E.g. minimum throughput or maximum delay threshold • Security — Security mechanisms, restricting access • These services depend on underlying transmission system and lower-level entities
- 25. Internetworking Terms (1) • Communications Network — Facility that provides data transfer service • An internet — Collection of communications networks interconnected by bridges and/or routers • The Internet - note upper case I — The global collection of thousands of individual machines and networks • Intranet — Corporate internet operating within the organization — Uses Internet (TCP/IP and http)technology to deliver documents and resources
- 26. Internetworking Terms (2) • End System (ES) —Device attached to one of the networks of an internet —Supports end-user applications or services • Intermediate System (IS) —Device used to connect two networks —Permits communication between end systems attached to different networks
- 27. Internetworking Terms (3) • Bridge —IS used to connect two LANs using similar LAN protocols —Address filter passing on packets to the required network only —OSI layer 2 (Data Link) • Router —Connects two (possibly dissimilar) networks —Uses internet protocol present in each router and end system —OSI Layer 3 (Network)
- 28. Requirements of Internetworking • Link between networks —Minimum physical and link layer • Routing and delivery of data between processes on different networks • Accounting services and status info • Independent of network architectures
- 29. Network Architecture Features • Addressing • Packet size • Access mechanism • Timeouts • Error recovery • Status reporting • Routing • User access control • Connection based or connectionless
- 30. Architectural Approaches • Connection oriented • Connectionless
- 31. Connection Oriented • Assume that each network is connection oriented • IS connect two or more networks —IS appear as ES to each network —Logical connection set up between ESs • Concatenation of logical connections across networks —Individual network virtual circuits joined by IS • May require enhancement of local network services —802, FDDI are datagram services
- 32. Connection Oriented IS Functions • Relaying • Routing • e.g. X.75 used to interconnect X.25 packet switched networks • Connection oriented not often used —(IP dominant)
- 33. Connectionless Operation • Corresponds to datagram mechanism in packet switched network • Each NPDU treated separately • Network layer protocol common to all DTEs and routers — Known generically as the internet protocol • Internet Protocol — One such internet protocol developed for ARPANET — RFC 791 (Get it and study it) • Lower layer protocol needed to access particular network
- 34. Connectionless Internetworking • Advantages —Flexibility —Robust —No unnecessary overhead • Unreliable —Not guaranteed delivery —Not guaranteed order of delivery • Packets can take different routes —Reliability is responsibility of next layer up (e.g. TCP)
- 35. IP Operation
- 36. Design Issues • Routing • Datagram lifetime • Fragmentation and re-assembly • Error control • Flow control
- 37. The Internet as a Network
- 38. Routing • End systems and routers maintain routing tables — Indicate next router to which datagram should be sent — Static • May contain alternative routes — Dynamic • Flexible response to congestion and errors • Source routing — Source specifies route as sequential list of routers to be followed — Security — Priority • Route recording
- 39. Datagram Lifetime • Datagrams could loop indefinitely — Consumes resources — Transport protocol may need upper bound on datagram life • Datagram marked with lifetime — Time To Live field in IP — Once lifetime expires, datagram discarded (not forwarded) — Hop count • Decrement time to live on passing through a each router — Time count • Need to know how long since last router • (Aside: compare with Logan’s Run)
- 40. Fragmentation and Re-assembly • Different packet sizes • When to re-assemble —At destination • Results in packets getting smaller as data traverses internet —Intermediate re-assembly • Need large buffers at routers • Buffers may fill with fragments • All fragments must go through same router – Inhibits dynamic routing
- 41. IP Fragmentation (1) • IP re-assembles at destination only • Uses fields in header —Data Unit Identifier (ID) • Identifies end system originated datagram – Source and destination address – Protocol layer generating data (e.g. TCP) – Identification supplied by that layer —Data length • Length of user data in octets
- 42. IP Fragmentation (2) —Offset • Position of fragment of user data in original datagram • In multiples of 64 bits (8 octets) —More flag • Indicates that this is not the last fragment
- 44. Dealing with Failure • Re-assembly may fail if some fragments get lost • Need to detect failure • Re-assembly time out —Assigned to first fragment to arrive —If timeout expires before all fragments arrive, discard partial data • Use packet lifetime (time to live in IP) —If time to live runs out, kill partial data
- 45. Error Control • Not guaranteed delivery • Router should attempt to inform source if packet discarded —e.g. for time to live expiring • Source may modify transmission strategy • May inform high layer protocol • Datagram identification needed • (Look up ICMP)
- 46. Flow Control • Allows routers and/or stations to limit rate of incoming data • Limited in connectionless systems • Send flow control packets —Requesting reduced flow • e.g. ICMP
- 47. Internet Protocol (IP) Version 4 • Part of TCP/IP —Used by the Internet • Specifies interface with higher layer —e.g. TCP • Specifies protocol format and mechanisms • RFC 791 —Get it and study it! —www.rfc-editor.org • Will (eventually) be replaced by IPv6 (see later)
- 48. IP Services • Primitives —Functions to be performed —Form of primitive implementation dependent • e.g. subroutine call —Send • Request transmission of data unit —Deliver • Notify user of arrival of data unit • Parameters —Used to pass data and control info
- 49. Parameters (1) • Source address • Destination address • Protocol — Recipient e.g. TCP • Type of Service — Specify treatment of data unit during transmission through networks • Identification — Source, destination address and user protocol — Uniquely identifies PDU — Needed for re-assembly and error reporting — Send only
- 50. Parameters (2) • Don’t fragment indicator —Can IP fragment data —If not, may not be possible to deliver —Send only • Time to live —Send only • Data length • Option data • User data
- 51. Options • Security • Source routing • Route recording • Stream identification • Timestamping
- 52. IPv4 Header
- 53. Header Fields (1) • Version —Currently 4 —IP v6 - see later • Internet header length —In 32 bit words —Including options • Type of service • Total length —Of datagram, in octets
- 54. Header Fields (2) • Identification — Sequence number — Used with addresses and user protocol to identify datagram uniquely • Flags — More bit — Don’t fragment • Fragmentation offset • Time to live • Protocol — Next higher layer to receive data field at destination
- 55. Header Fields (3) • Header checksum —Reverified and recomputed at each router —16 bit ones complement sum of all 16 bit words in header —Set to zero during calculation • Source address • Destination address • Options • Padding —To fill to multiple of 32 bits long
- 56. Data Field • Carries user data from next layer up • Integer multiple of 8 bits long (octet) • Max length of datagram (header plus data) 65,535 octets
- 58. IP Addresses - Class A • 32 bit global internet address • Network part and host part • Class A —Start with binary 0 —All 0 reserved —01111111 (127) reserved for loopback —Range 1.x.x.x to 126.x.x.x —All allocated
- 59. IP Addresses - Class B • Start 10 • Range 128.x.x.x to 191.x.x.x • Second Octet also included in network address • 214 = 16,384 class B addresses • All allocated
- 60. IP Addresses - Class C • Start 110 • Range 192.x.x.x to 223.x.x.x • Second and third octet also part of network address • 221 = 2,097,152 addresses • Nearly all allocated —See IPv6
- 61. Subnets and Subnet Masks • Allow arbitrary complexity of internetworked LANs within organization • Insulate overall internet from growth of network numbers and routing complexity • Site looks to rest of internet like single network • Each LAN assigned subnet number • Host portion of address partitioned into subnet number and host number • Local routers route within subnetted network • Subnet mask indicates which bits are subnet number and which are host number
- 63. ICMP • Internet Control Message Protocol • RFC 792 (get it and study it) • Transfer of (control) messages from routers and hosts to hosts • Feedback about problems —e.g. time to live expired • Encapsulated in IP datagram —Not reliable
- 65. IP v6 - Version Number • IP v 1-3 defined and replaced • IP v4 - current version • IP v5 - streams protocol • IP v6 - replacement for IP v4 —During development it was called IPng —Next Generation
- 66. Why Change IP? • Address space exhaustion —Two level addressing (network and host) wastes space —Network addresses used even if not connected to Internet —Growth of networks and the Internet —Extended use of TCP/IP —Single address per host • Requirements for new types of service
- 67. IPv6 RFCs • 1752 - Recommendations for the IP Next Generation Protocol • 2460 - Overall specification • 2373 - addressing structure • others (find them) • www.rfc-editor.org
- 68. IPv6 Enhancements (1) • Expanded address space —128 bit • Improved option mechanism —Separate optional headers between IPv6 header and transport layer header —Most are not examined by intermediate routes • Improved speed and simplified router processing • Easier to extend options • Address autoconfiguration —Dynamic assignment of addresses
- 69. IPv6 Enhancements (2) • Increased addressing flexibility —Anycast - delivered to one of a set of nodes —Improved scalability of multicast addresses • Support for resource allocation —Replaces type of service —Labeling of packets to particular traffic flow —Allows special handling —e.g. real time video
- 70. IPv6 Structure
- 71. Extension Headers • Hop-by-Hop Options —Require processing at each router • Routing —Similar to v4 source routing • Fragment • Authentication • Encapsulating security payload • Destination options —For destination node
- 72. IP v6 Header
- 73. IP v6 Header Fields (1) • Version —6 • Traffic Class —Classes or priorities of packet —Still under development —See RFC 2460 • Flow Label —Used by hosts requesting special handling • Payload length —Includes all extension headers plus user data
- 74. IP v6 Header Fields (2) • Next Header —Identifies type of header • Extension or next layer up • Source Address • Destination address
- 75. IPv6 Addresses • 128 bits long • Assigned to interface • Single interface may have multiple unicast addresses • Three types of address
- 76. Types of address • Unicast —Single interface • Anycast —Set of interfaces (typically different nodes) —Delivered to any one interface —the “nearest” • Multicast —Set of interfaces —Delivered to all interfaces identified
- 78. Hop-by-Hop Options • Next header • Header extension length • Options — Pad1 • Insert one byte of padding into Options area of header — PadN • Insert N (2) bytes of padding into Options area of header • Ensure header is multiple of 8 bytes — Jumbo payload • Over 216 = 65,535 octets — Router alert • Tells router that contents of packet is of interest to router • Provides support for RSPV (chapter 16)
- 79. Fragmentation Header • Fragmentation only allowed at source • No fragmentation at intermediate routers • Node must perform path discovery to find smallest MTU of intermediate networks • Source fragments to match MTU • Otherwise limit to 1280 octets
- 80. Fragmentation Header Fields • Next Header • Reserved • Fragmentation offset • Reserved • More flag • Identification
- 81. Routing Header • List of one or more intermediate nodes to be visited • Next Header • Header extension length • Routing type • Segments left —i.e. number of nodes still to be visited
- 82. Destination Options • Same format as Hop-by-Hop options header
- 83. Required Reading • Stallings chapter 18 • Comer, S. Internetworking with TCP/IP, volume 1, Prentice-Hall • All RFCs mentioned plus any others connected with these topics —www.rfc-editor.org • Loads of Web sites on TCP/IP and IP version 6