Internet Protocol: Connectionless Datagram Delivery (IPv4)

1. Internet Protocol:Connectionless Datagram Delivery (IPv4) Chapter 6

2. Have looked at HW & SW that make internet communication possible • Now begin looking at IP • Internet Protocol • Provides connectionless delivery • IP datagrams form basis for all internet communication

3. Internet Philosophy • Focus: interface internet provides to users • Not on the interconnection technology • User sees single virtual network • Underlying architecture is hidden and irrelevant • Conceptually, TCP/IP provides three sets of services:

4. Protocol SW can be associated with each • Instead, consider them conceptual internet parts • Embody philosophical underpinnings of design • Internet SW designed around the conceptual services • Surprisingly robust and adaptable architecture • Adv of conceptual separation • Can replace one service without disturbing others • R&D can proceed concurrently on all three

5. Connectionless Delivery System • Most fundamental internet service: • Packet delivery system • Technically, the service is: • Unreliable • Packet can be lost, duplicated, delayed, out-of-order • No notification of such problems • Best-effort • Makes earnest attempt to deliver • Connectionless • Packets treated independently

6. Purpose of the IP • IP: protocol that defines delivery service • Specifies basic unit of transfer • Exact format of data • Performs the routing function • Chooses the paths for packets • Includes rules for unreliable packet delivery • How hosts and routers process packets • How and when error messages are generated • When packets can be discarded

7. Internet Datagram • Physical network: • Unit of transfer is frame • Contains header and data • Internet • Unit of transfer is Internet datagram • IP datagram or datagram • Contains header and data • Header difference: • IP addresses versus physical addresses

8. IP Datagram • Datagram format

9. Service Type field: • Originally • Precedence 0-7 • Routers use 6 or 7 (info goes thru during congestion) • D: low delay • T: high throughput • R: high reliability

10. Later • Differential Services interpretation • Have 8 ordered classes when of form: xxx000 • Just like previous precedence • 6 or 7 goes to high priority class of service • Codepoint values divided into 3 groups: xxxxx0 : assigned by Standards organization xxxx11 : local or experimental xxxx01 : local or experimental for now

11. Regardless of the interpretation: • Service type specification is a hint to the routing algorithm • Chose among various paths based on: • Local policies • Knowledge of technologies available on the paths • No guarantee to provide a type of service

12. Datagram Encapsulation • How long can a datagram be? • Handled by SW (not HW) • Any length protocol designers want • IPv4 has 16 bits for total length field • Limit is 65,535 octets • But, want efficient transportation • Map abstract physical packet to real packet

13. Encapsulation: • Idea to carry 1 datagram in 1 network frame • Underlying HW not concerned with datagram • One machine to another: datagram is in the data portion of a frame

14. Ideal: entire IP datagram in one frame • Would need maximum datagram size • What would that be? • Look at network hardware: • MTU: maximum transfer unit • Ethernet: 1500 octet MTU • FDDI: 4470 octet MTU • Some hardware: 128 octets or less • Limit to smallest: inefficient • If bigger than MTU: need multiple frames

15. Design goal: convenience for user • Not worry about physical network constraints • Solution: • Pick convenient initial datagram size • Have way to divide up for small MTU • Pieces of divided datagram: fragments • Process of dividing: fragmentation

16. Fragmentation usually occurs along the path Host A Host B Net 1 Net 3 MTU=1500 MTU=1500 Net 2MTU=620 R1 R2

17. Fragments sized for one per frame • Size is a multiple of eight • Last piece may be shorter than rest • Fragments must be reassembled • Datagram must be rebuilt before processing • IP does not limit datagrams to small size • Source can choose any size • Fragmentation and reassembly are automatic • Routers must accept datagrams up to max size of MTU’s of attached networks • Routers must handle datagrams up to 576 octets • Each piece formatted like original datagram

18. Fragment header mostly the same as datagram header • Bit in the FLAGS field; Value in TOTAL LENGTH field; checksum

19. Reassembly of Fragments • Reassembly after each hop or at end? • TCP/IP: once fragmented, stays that way • Reassemble at ultimate destination (host) • Two disadvantages: • Inefficient if other networks have higher MTU • Probability of datagram loss increases with more fragments • Advantages: • Fragments can be routed independently • Intermediate routers do not have to store or reassemble

20. Fragmentation Control • Three datagram header fields control fragmentation and reassembly: • Identification • Unique integer to ID the datagram • Fragment Offset • Offset in original datagram of data being carried • Flags • Use two low-order bits of 3-bit field • 1st bit: if set, means do not fragment • Low bit: more fragments bit

21. Time to Live (TTL) • TTL specifies how long, in seconds, datagram is allowed to remain in the internet system • Router & hosts that process must decrement TTL • Remove when time expires • Each router decrements TTL by 1 • If long delay, decrement by number seconds there • When TTL = 0, discard and send error message • Guarantees datagram not be around forever • Mostly, TTL acts as hop limit • Rather than estimate of delay

22. Other Datagram Header Fields • PROTOCOL • Tells which high-level protocol used in creation • Specifies format of data area • HEADER CHECKSUM • Ensures integrity of header values • Only applies to header, not data • Adv: • Header smaller; routers only worry about headers • Higher level protocols choose own data checksum scheme • Disadv: • Higher level protocols must add their own data checksum

23. SOURCE IP ADDRESS DESTINATION IP ADDRESS • Contain 32-bit IP addresses of sender & recipient • Never change when going through routers • IP OPTIONS • Variable length • PADDING field depends upon options selected

24. Datagram Options • IP OPTIONS field not required • Used mostly for network testing and debugging • Option processing is integral part of IP protocol • Field length varies based on options selected • Options appear contiguously; no separators • Each option: • Consists of single octet option code • Followed by single octet length & set of data octets

25. Option code octet divided into three fields • COPYflag controls how routers treat options during fragmentation • Set to 1: copy options to all fragments • Set to 0: only copy into first fragment • CLASS & NUMBERfields specify general option class and a specific option in the class • Class 0: datagram or network control • Class 1: Reserved for future use • Class 2: Debugging and measurement • Class 3: Reserved for future use

27. Record Route Option • Source creates empty list of IP addresses • Each router adds its IP address to the list • Format:

28. Original source must allocate enough space for the addresses • When a machine handles the datagram: • Compare pointer and length fields • Pointer > length: list full (not add address) • Otherwise: put 4-octet IP address at pointer position and increment pointer • Source and destination must agree to use • Source enable option • Destination agree to process resulting list

29. Source Route Option • Sender can dictate path through the internet • Format: • Test throughput over particular network • Average user would not know topology

30. Strict source routing: • Addresses specify exact path • Path between addresses must be a single network • Loose source routing • Datagram must follow sequence of IP addresses • May be multiple hops between addresses • Processing similar to record route option • When router follows an IP address, it replaces the IP address with its own address

31. Timestamp Option • Initially empty list • Each router adds: • 32-bit IP address • 32-bit integer timestamp

32. Oflow (4-bits) • Integer count of routers that could not timestamp • Flags (4-bits) • Controls format of the option • 0: Record timestamp only; omit IP addresses • 1: Precede each timestamp by an IP address • 3: IP addresses are specified by sender; a router only records a timestamp if the next IP address in the list matches the router’s IP address

33. Timestamps tell when router handled the datagram • Expressed as milliseconds since midnight • Based on Universal Time (Greenwich Mean Time) • All computer clocks not necessarily synchronized • Local clocks may differ • Should be treated as estimates • Why not just use record route option? • Eliminates ambiguity • Receiver knows exactly which path the datagram followed

34. Processing Options During Fragmentation • COPY bit in CODE field • Replicates some options in all fragments • Places some in only one fragment • Ex: Recording the datagram route • Not all fragments will follow the same route • Reassembly would produce conflicting lists • Only put in one fragment • Ex: Source route option • Must be replicated for all fragments to follow same route

35. Summary • Fundamental TCP/IP service is • Connectionless • Unreliable • Best-effort • Packet delivery • IP formally specifies internet packet format • Called datagram

36. Like physical frame, datagram has header and data • Header contains: • Source and destination IP addresses • Fragmentation control • Precedence • Checksum • Options field • Variable in length • Intended to help monitor and control an internet