Transcript NETS 3007

CIDR and ICMP
NETS3303/3603
Week 5
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Proxy ARP Clarifications
192.168.10.0/24
I do!
A
B
Who has B’s MAC?
• Two broadcast domains sharing network
address through proxy ARP
• Someone else could masquerade too
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Outcome
•
•
•
•
CIDR addressing and forwarding
Learn about role of ICMP
Protocol specifics
Know about applications that use ICMP
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Classless Inter-Domain
Routing (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
• 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)
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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
• Example of /30 CIDR Block
– 128.211.176.212/30
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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 longestprefix 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
Path for prefix 0101
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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, 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
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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
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–
–
–
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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? 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
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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_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
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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 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
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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-toback
• Each link will space
packets
• Remote host might
compact again
• Read time between
packets on return path
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BW 
probesize
t end 2 t end 1
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
Foreign
Network
HA
1
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Home
Network
2
3
FA
IP Tunnelling
IP
IP
Router
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IP
IP Tunnelling
IP
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
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