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CIDR and ICMP

CIDR and ICMP. NETS3303/3603 Week 5. Proxy ARP Clarifications. 192.168.10.0/24. I do!. Two broadcast domains sharing network address through proxy ARP Someone else could masquerade too. A. B. Who has B’s MAC?. Outcome. CIDR addressing and forwarding Learn about role of ICMP

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CIDR and ICMP

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  1. CIDR and ICMP NETS3303/3603 Week 5

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

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

  4. Classless Inter-DomainRouting (CIDR) • Problem • Continued exponential Internet growth • Subnetting insufficient • Limited IP addresses (esp. Class B)

  5. 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. 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. 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. 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. 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. Example of /30 CIDR Block • 128.211.176.212/30 Examples of CIDR Blocks • The corresponding CIDR block: • 128.211.168/21

  11. 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. 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. Path for prefix 0101 Example Of Unique Prefixes & Trie

  14. 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. 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. Protocols

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

  18. Header checksum covers icmp header/data

  19. ICMP messages (not all)

  20. continued…

  21. 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. Redirect Illustration

  23. 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. 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. 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. 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. 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. ping • ping adds identifier/sequence number fields to packets • sequence # allows you to see if packets lost • ping will also do roundtrip timing

  29. 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. 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. 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. 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. 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. 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. Router solicitation • Host wants to learn about network topology issues ICMP RS message (type 10) • Routers reply with a router advertisement (type 9)

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

  37. 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. Packet-pair

  39. 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. 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. 3 2 1 MIP HA Foreign Network Home Network FA

  42. IP IP IP Tunnelling IP Router

  43. IP IP Tunnelling IP IP FA

  44. 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

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