1. School of Information Technologies CIDR and ICMP NETS3303/3603 Week 5

2. School of Information Technologies Proxy ARP Clarifications Two broadcast domains sharing network address through proxy ARP Someone else could masquerade too AB 192.168.10.0/24 Who has B’s MAC? I do!

3. School of Information Technologies Outcome CIDR addressing and forwarding Learn about role of ICMP Protocol specifics Know about applications that use ICMP

4. School of Information Technologies Classless Inter-Domain Routing (CIDR) Problem –Continued exponential Internet growth –Subnetting insufficient –Limited IP addresses (esp. Class B)

5. School of Information Technologies CIDR Addressing Solution to problem –Temporary fix until next generation of IP –Backward compatible with classful addressing –Permits network prefix to be arbitrary length CIDR was predicted to work ‘‘for a few years’’ –Extremely successful! –Will work for at least 20 years!

6. School of Information Technologies One Motivation For CIDR: Class C Fewer than seventeen thousand Class B numbers (total) More than two million Class C network numbers No one wants Class C (too small) CIDR allows –Merging Class C numbers into a single prefix –Splitting a Class B along power of two boundaries

7. School of Information Technologies CIDR and Supernetting CIDR’s first use Supernetting allows to assign a block of class C addresses E.g.: A medium-sized org expects to have 200 networks (assign class B is wasteful!) So, an ISP can assign a block of contiguous class C numbers –(192.100.0.0 – 192.100.255.0)

8. School of Information Technologies CIDR Notation Addresses written NUMBER/m –NUMBER is IP prefix –m is ‘‘address mask’’ length Example:214.5.48.0/20 –Prefix occupies 20 bits –Suffix occupies 12 bits Mask values must be converted to dotted decimal when configuring a router (and binary internally) –255.255.240.0

9. School of Information Technologies CIDR & Routing Table CIDR involves changing forwarding as well as addressing Store address mask with each route Send pair of (address, mask) whenever exchanging routing information Known as a CIDR block

10. School of Information Technologies Examples of CIDR Blocks The corresponding CIDR block: –128.211.168/21 Example of /30 CIDR Block –128.211.176.212/30

11. School of Information Technologies Implementation Of CIDR Route Lookup Each entry in routing table has address plus mask Search is organized from most-specific to least-specific (i.e., entry with longest mask is tested first) Known as longest-prefix lookup or longest- prefix search

12. School of Information Technologies Implementing Longest-Prefix Matching Unlike classful addressing, the prefix boundary is not obvious So, cannot easily use hashing A data structure of choice is binary trie –An ordered tree structure –Support fast lookup for variable key-length Identifies unique prefix needed to match route

13. School of Information Technologies Example Of Unique Prefixes & Trie Path for prefix 0101

14. School of Information Technologies Internet Control Message Protocol considered part of IP functionality includes: –error messages (ttl exceeded, destination unreachable, router is out of memory, can’t fragment packet) –network management (ping/traceroute) –end host configuration (router advert, netmask) error messages go from router/end host to original sender, not between intermediate hops –why? => don’t know route

15. School of Information Technologies ICMP Ideas error messages typically sent to IP sender –later may forward to application ICMP error messages never generated due to: –ICMP error message (creates loop!) –broadcasts/multicasts packets Why? prevent broadcast storms ICMP error messages contains offending IP header + 1st 8 bytes of IP data (contains tcp/udp ports)

16. School of Information Technologies Protocols

17. School of Information Technologies Encapsulation ICMP transmitted within IP datagram so that it is routable (unlike arp)

18. School of Information Technologies Header checksum covers icmp header/data

19. School of Information Technologies ICMP messages (not all)

20. School of Information Technologies continued…

21. School of Information Technologies ICMP redirect limited form of dynamic routing technique only done on same link situation: –1. assume dumb host with 1 default routing table entry –2. two routers on same link, one is default, one is route to net X –3. dumb host sends pkt to net X via default router –4. default router sends ICMP redirect with correct router address to dumb host

22. School of Information Technologies Redirect Illustration

23. School of Information Technologies Dest Unreachable Host or router cannot deliver a datagram Return IP header & first 8 bytes of payload Codes –0 Network unreachable –1 Host unreachable –2 Protocol unreachable –3 Port unreachable –Etc.

24. School of Information Technologies Source Quench Primitive flow control mechanism –No flow control in IP itself (data rate) Source quench alerts sender –A packet was discarded –Slow down transmission rate Returned is IP header plus 8 bytes of data

25. School of Information Technologies Time Exceeded If TTL value 0, discard packet and issue ICMP time exceeded, code 0 If all fragments not received within a certain time limit, discard fragments and issue ICMP time exceeded, code 1

26. School of Information Technologies Parameter problem If the IP header format wrong –Issue ICMP parameter problem Code 0 faulty header field, pointer field in ICMP addresses start byte of problem in IP header Code 1 required part of option is missing

27. School of Information Technologies ping - ICMP echo request/reply ping program, useful diagnostic tool, uses ICMP echo request/reply packets Linux implementation uses “raw” sockets - i/f directly to ip layer, bypass transports ping would send 1 pkt per second some newer pings require -s to do that and only do one ping with this output: –hostname is alive

28. School of Information Technologies ping ping adds identifier/sequence number fields to packets sequence # allows you to see if packets lost ping will also do roundtrip timing

29. School of Information Technologies More ping so what do you learn using ping? –timing info, does it take too long ? –are packets being lost (why? didn’t tell you) –you can route –end system’s tcp/ip stack is working at least echo reply sent by end system’s ICMP module

30. School of Information Technologies ping example $ ping cse.ogi.edu PING cse.ogi.edu (129.95.20.2): 56 data bytes 64 bytes from 129.95.20.2 icmp_seq=0 time=8ms 64 bytes from 129.95.20.2 icmp_seq=1 time=8ms 64 bytes from 129.95.20.2 icmp_seq=2 time=20ms ---cse.ogi.edu PING statistics --- 3 packets transmitted, 3 packets received, 0% loss round-trip (ms) min/avg/max = 8/12/20

31. School of Information Technologies traceroute traceroute (a command) allows you to determine the routers from one end to another % traceroute north.pole.com uses ICMP ttl exceeded and UDP port unreachable (or ICMP echo reply) messages to do the job

32. School of Information Technologies traceroute example % traceroute cse.ogi.edu (from sirius.cs.pdx.edu) traceroute to cse.ogi.edu (129.95.20.2), 30 hops max... 1. pdx-gwy (131.252.20.1) 3 ms 4 ms 3 ms 2. 198.104.197.58 (198.104.197.58) 7 ms 4 ms 8 ms 3. portland1-gw.nwnet.net (198.104.196.193) 6 ms 5 ms 5 ms 4. ogi-gw-nwnet.net (198.104.196.129) 8 ms 7 ms 7 ms 5. cse.ogi.edu (129.95.20.2) 14 ms 7 ms 9 ms

33. School of Information Technologies traceroute algorithm ttl = 1 (to 1st router) while we haven’t got UDP port unreachable send raw/ip packet with ttl == 1 get response if rcv ttl error compute roundtrip time else if UDP port unreachable quit print output ttl++ end

34. School of Information Technologies Address mask If host does not know its netmask, issue ICMP Address mask request (Type 17) Router on network replies with mask Can be unicast or broadcast Often used at bootstrapping

35. School of Information Technologies Router solicitation Host wants to learn about network topology issues ICMP RS message (type 10) Routers reply with a router advertisement (type 9)

36. School of Information Technologies Packet-pair Problem, what is the bandwidth of the links between my machine and destination? Use packet-pair to find bottleneck link speed

37. School of Information Technologies Bandwidth Estimation Algorithm Issue two packets back-to- back Each link will space packets Remote host might compact again Read time between packets on return path

38. School of Information Technologies Packet-pair

39. School of Information Technologies IP Final frontier, Mobility Once a socket opened, address cannot change –Change address – lose connection –Change place of attachment – lose routing One solution MobileIP - RFC2002

40. School of Information Technologies MobileIP MIP allows hosts to migrate to foreign networks Communication handled by Home Agent in home network Helper router in foreign network Foreign Agent

41. School of Information Technologies MIP Home Network Foreign Network HA FA 1 2 3

42. School of Information Technologies IP Tunnelling IP Router IP

43. School of Information Technologies IP Tunnelling IP FA IP

44. School of Information Technologies Summary CIDR substantially overcame IPv4 address issues –Required routing table and prefix-lookup change too ICMP provides error and management support Many useful network tools exploit ICMP to help check network health Next: Autonomous System and Routing architecture