CIDR and ICMP

School of Information Technologies

CIDR and ICMP

NETS3303/3603

Week 5

School of Information Technologies

Proxy ARP Clarifications

• Two broadcast domains sharing network address through proxy ARP

• Someone else could masquerade too

A B

192.168.10.0/24

Who has B’s MAC?

I do!

School of Information Technologies

Outcome

• CIDR addressing and forwarding

• Learn about role of ICMP

• Protocol specifics

• Know about applications that use ICMP

School of Information Technologies

Classless Inter-DomainRouting (CIDR)

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

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!

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

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)

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

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

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

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

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

School of Information Technologies

Example Of Unique Prefixes & Trie

Path for prefix 0101

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

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)

School of Information Technologies

Protocols

School of Information Technologies

Encapsulation

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

School of Information Technologies

Header

checksum covers icmp header/data

School of Information Technologies

ICMP messages (not all)

School of Information Technologies

continued…

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

School of Information Technologies

Redirect Illustration

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.

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

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

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

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

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

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

School of Information Technologies

ping example

• $ ping cse.ogi.eduPING cse.ogi.edu (129.95.20.2): 56 data bytes64 bytes from 129.95.20.2 icmp_seq=0 time=8ms64 bytes from 129.95.20.2 icmp_seq=1 time=8ms64 bytes from 129.95.20.2 icmp_seq=2 time=20ms---cse.ogi.edu PING statistics ---3 packets transmitted, 3 packets received, 0% lossround-trip (ms) min/avg/max = 8/12/20

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

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 ms2. 198.104.197.58 (198.104.197.58) 7 ms 4 ms 8 ms3. portland1-gw.nwnet.net (198.104.196.193) 6 ms 5 ms 5

ms4. ogi-gw-nwnet.net (198.104.196.129) 8 ms 7 ms 7 ms5. cse.ogi.edu (129.95.20.2) 14 ms 7 ms 9 ms

School of Information Technologies

traceroute algorithm• ttl = 1 (to 1st router)

while we haven’t got UDP port unreachablesend raw/ip packet with ttl == 1get responseif rcv ttl error

compute roundtrip timeelse if UDP port unreachable quit

print output ttl++end

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

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)

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

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

tt

probesizeBW

endend 12

School of Information Technologies

Packet-pair

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

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

School of Information Technologies

MIP

Home Network

Home Network

Foreign Network

Foreign Network

HA

FA12

3

School of Information Technologies

IP Tunnelling

IP

Router

IP IP

School of Information Technologies

IP Tunnelling

IP

FA

IP IP

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