Transcript lecture04-arp

Routing & Switching
Umar Kalim
Dept. of Communication Systems Engineering
[email protected]
http://www.niit.edu.pk/~umarkalim
03/04/2007
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Outline
ARP, RARP
ICMP
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A packet using one logical addressing
scheme may pass through networks with
different physical layer technologies
Thus we need to consider two different and
independent addressing schemes; logical and
physical
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Problem
Whenever a host needs to send a message to
another host, it has the IP address.
However the IP datagram must be encapsulated
in a frame (by the MAC layer) so that it may be
delivered over the physical layer
Hence the need for the physical address
The only way to get the physical address is to
have a mapping scheme
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Dynamic mapping
Each time the machine knows one of the
two addresses (physical or logical) it can
obtain the other
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ARP & RARP
Protocols defined for the dynamic mapping of
logical and physical addresses
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ARP & RARP
ARP: Address Resolution Protocol
RARP: Reverse Address Resolution Protocol
Use unicast and broadcast physical addresses
Position of ARP & RARP in the TCP/IP stack
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ARP
Instead of maintaining a static table, we can
obtain the physical address at runtime
– This is where ARP comes in
ARP associates an IP address with a physical
address
On a typical network, each interface is identified
by a physical address
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ARP
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ARP packet format
16-bits: type of
network e.g.
Ethernet 1
8-bits: length of
logical address
e.g. IPv4 4
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16-bits: protocol
e.g. IPv4 080016
8-bits: length of
physical
address e.g.
Ethernet 6
Variable length
field
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Encapsulation
ARP packet is encapsulated into a data-link
frame
Ethernet frame
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Four cases
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Proxy ARP
Proxy ARP is used to create subnetting effect
Proxy ARP: An ARP that acts on behalf of a set
of hosts
– Whenever a router receives an ARP request which
asks for the physical address of any one of the hosts
it is representing, the router responds with its own
physical address
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Proxy ARP
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ARP’s Hypothetical Implementation
The software package would include five
modules:
–
–
–
–
–
Cache table
Queues
The output module
The input module and a
Cache control module
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Cache table
FREE,
PENDING and
RESOLVED
Hardware type, Protocol type, hardware length,
protocol length, interface number
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RARP
Reverse Address Resolution Protocol
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RARP
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Encapsulation
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Alternates to RARP
BOOTP
DHCP
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ICMP
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Why ICMP?
IP provides connectionless & unreliable (besteffort) datagram delivery
IP has deficiencies
– Lack of error control/correction
 What if something goes wrong?
– Router drops a packet, how to know?
– TTL is decremented by one at each hop, how will the
source know that TTL has reached 0
– Lack of host and management queries
 Host is alive or not? Misc information is required, how to
fetch?
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ICMP w.ref.t Network layer
Supporting protocol for IP
– As is ARP and RARP
– ICMP is shown above, since ICMP datagrams are
not sent to the data-link layer directly. They are
encapsulated in IP datagrams and then forwarded
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ICMP Encapsulation
IPheader.protocol = 1
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Types of ICMP Messages
Errors which a router or
a host (destination) may
encounter
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Query messages to get
specific information from
a router or a host.
Always occur in pairs
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ICMP Messages
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Message Format
An ICMP message has an 8-byte header and a
variable-size data section
The general format of the header is different for
each message type yet the first 4 bytes are
common to all
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Error Messages
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Error Reporting vs Error Correction
ICMP does not
– Provide interaction between a router and the source
of trouble
– Maintain state information (each packet is handled
independently)
When a datagram causes an error, ICMP can
only report the error condition back to the
original source of the datagram
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Error Reporting
Note:
ICMP always reports error messages
to the original source.
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Note:
The following are important points about ICMP
error messages:
❏ No ICMP error message will be generated in response
to a datagram carrying an ICMP error message.
❏ No ICMP error message will be generated for a
fragmented datagram that is not the first fragment.
❏ No ICMP error message will be generated for a
datagram having a multicast address.
❏ No ICMP error message will be generated for a
datagram having a special address such as 127.0.0.0 or
0.0.0.0.
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Error Messages
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Destination unreachable ~ 3
Generated by the router to inform the client
that the destination host is unreachable
Reasons for this message may include the
– physical connection to the host does not exist
(distance is infinite),
– the indicated protocol or port is not active, or
– the data must be fragmented but the 'don't fragment'
flag is on
 http://en.wikipedia.org/wiki/ICMP_Destination_Unreachabl
e
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Destination unreachable ~ 3
Defined codes for each type of failure
– Refer to the book for the values
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Source Quench ~ 4
Message which requests the sender to decrease
the traffic rate of messages to a router or host
Generated if the router or host does not have
sufficient buffer space to process the request,
or may occur if the router or host's buffer is
approaching its limit
 http://en.wikipedia.org/wiki/ICMP_Source_Quench
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Source Quench ~ 4
No mechanism for Flow-control in IP
A source-quench message informs the source that a
datagram has been discarded due to congestion in a
router or the destination host
The source must slow down the sending of datagrams
until the congestion is relieved
One source-quench message is sent for each datagram
that is discarded due to congestion
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Time exceeded ~ 11
Generated by a gateway to inform the source of
a datagram that the datagram has been discarded
due to the time to live field reaching zero
message may also be sent by a host if it fails to
reassemble a fragmented datagram
 http://en.wikipedia.org/wiki/ICMP_Time_Exceeded
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Parameter Problem ~ 12
Sent to the source host for any problem not
specifically covered by another ICMP message
– 0 ~ ambiguous
– 1 ~ missing option field
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Redirection ~ 5
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 http://www.tcpipguide.com/free/t_ICMPv4RedirectMessages.htm
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Redirection ~ 5
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Questions?
That’s all for today!
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