1. CSC 600 Internetworking with TCP/IP Unit 5: IP, IP Routing, and ICMP (ch. 7, ch. 8, ch. 9, ch. 10) Dr. Cheer-Sun Yang Spring 2001

2. Internet Protocol (IP) Part of TCP/IP –Used by the Internet Specifies interface with higher layer –e.g. TCP Specifies protocol format and mechanisms

3. IP Services Unreliable Connectionless Best-effort delivery

4. IP Protocol Specification IP datagram format Routing function Fragmentation and reassembly Internet control message protocol (ICMP): network level error message handling

7. Header Fields (1) Version –Currently 4 –IP v6 - see later Internet header length –In 32 bit words –Including options Type of service(next slide) Total length –Of datagram, in octets

9. Type of Service Precedence –8 levels Reliability –Normal or high Delay –Normal or low Throughput –Normal or high

10. Type of Service In the late 1990s, the IETF redefiend the meaning of the 8-bit SERVICE TYPE field to accommodate a set of differential services (DS).

13. Header Fields (2) Total Length Identification –Sequence number –Used with addresses and user protocol to identify datagram uniquely Flags –More bit –Don’t fragment

14. Header Fields (3) Don’t fragment indicator –Can IP fragment data –If not, may not be possible to deliver –Send only Time to Live Protocol –next higher layer to receive data field at destination 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

15. Header Fields (4) Source address Destination address Options(next slides) Padding –To fill to multiple of 32 bits long

16. Options Security Source routing Route recording Stream identification Timestamping

17. 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

18. Design Issues Routing (later) Fragmentation and re-assembly Datagram lifetime Error control Flow control

22. Reassembly of Fragments Maximum Transfer Unit (MTU)

23. Fragmentation Control Identification Flags Fragment Offset

24. Time to Live (TTL) TTL – specifies how long, in seconds, a datagram is allowed to remain in the internet system.

25. Other Header Fields Protocol Header Checksum Source IP Address Destination IP Address Data

26. Internet Datagram Options Record Route Option –Use ping -R on taz.cs.wcupa.edu Source Route Options Timestamp Option Processing Options During Fragmentation

30. Record Route Option

31. Source Route Option The strict source route option specifies an exact route by giving a list of IP addresses the data gram must follow.

32. Timestamp Option

34. 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

35. 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)

36. 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

37. 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

38. 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

39. Fragmentation Example

40. 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

41. 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)

42. 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

43. Chapter 8: IP Routing Overview Router: perform IP forwarding as its main function Host: a multi-homed host also forward IP datagrams

44. Routing IP Datagrams Routing in an Internet Direct and Indirect Delivery Table-Driven IP Routing Next-Hop Routing Default Routers Host-Specific Routers The IP Routing Algorithms

46. Datagram Delivery Over a Single Network A machine can send a frame directly to another machine on the same network.

48. Datagram Delivery Over a Single Network How does a machine know if another machine is located in a directly-connected network?

50. Indirect Delivery How does a machine deliver a datagram indirectly to another host? –It encapsulate the datagram –sends it to the nearest router –The IP software on the router selects the next router towards the destination How does a router know where to send next?

52. Indirect Delivery Table-Driven IP Routing Next-Hop Routing Default Routers Host-Specific Routes

56. Indirect Delivery We ignored the routing table initialization and maintenance as network changes.

57. Chapter 9 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

59. ICMP Message Formats

72. Chapter 10 Subnetting and Routing

73. Subnetting

77. Routing in the Presence of Subnets The standard IP routing must be modified to work with subnet addresses. All hosts and routers that attach to the subnet must use the modified algorithms, called subnet routing.

79. Subnet Routing A conventional routing table contains entries of the form (network address, next hop address). A subnetting routing table consists of entries of the form (subnet mask, network address, next hop address).