CIDR and ICMP

1
CIDR and ICMP

• NETS3303/3603
• Week 5

2
Proxy ARP Clarifications
192.168.10.0/24
I do!
A
B
Who has Bs MAC?

• Two broadcast domains sharing network address
  through proxy ARP
• Someone else could masquerade too

3
Outcome

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

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Classless Inter-DomainRouting (CIDR)

• Problem
• Continued exponential Internet growth
• Subnetting insufficient
• Limited IP addresses (esp. Class B)

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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!

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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

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CIDR and Supernetting

• CIDRs 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)

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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

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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

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Examples of CIDR Blocks

• The corresponding CIDR block
• 128.211.168/21

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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

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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

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Example Of Unique Prefixes Trie
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Internet Control Message Protocol

• considered part of IP
• functionality includes
• error messages (ttl exceeded, destination
  unreachable, router is out of memory, cant
  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? gt dont know route

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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)

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Protocols
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Encapsulation
ICMP transmitted within IP datagram so that it is
routable (unlike arp)
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Header
checksum covers icmp header/data
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ICMP messages (not all)
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continued
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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

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Redirect Illustration
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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.

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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

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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

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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

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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

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ping

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

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More ping

• so what do you learn using ping?
• timing info, does it take too long ?
• are packets being lost (why? didnt tell you)
• you can route
• end systems tcp/ip stack is working at least
• echo reply sent by end systems ICMP module

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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_seq0 time8ms
• 64 bytes from 129.95.20.2 icmp_seq1 time8ms
• 64 bytes from 129.95.20.2 icmp_seq2 time20ms
• ---cse.ogi.edu PING statistics ---
• 3 packets transmitted, 3 packets received, 0
  loss
• round-trip (ms) min/avg/max 8/12/20

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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

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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

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traceroute algorithm

• ttl 1 (to 1st router)
• while we havent 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

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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

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Router solicitation

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

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Packet-pair

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

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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

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Packet-pair
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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

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MobileIP

• MIP allows hosts to migrate to foreign networks
• Communication handled by Home Agent in home
  network
• Helper router in foreign network Foreign Agent

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MIP
HA
Foreign Network
Home Network
FA
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IP Tunnelling
IP
Router
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IP Tunnelling
IP
IP
FA
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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