Download May 11

CS335
Networking &
Network Administration
Tuesday, May 11, 2010
ARP – Address resolution
protocol
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Translates IP address into a hardware address
Physical network hardware does not know how to locate
a computer from its protocol address
Known as address resolution
ARP
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Can only resolve hardware addresses for
machines on the local physical network
Address Resolution
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Three techniques of address resolution
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Table lookup – stored in a table in memory
Table lookup
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For less than a dozen hosts sequential
search suffices
In larger networks this requires excessive
CPU cycles
Hashing – general purpose data structure
Direct indexing
Table lookup
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Direct indexing – uses the host address as an
index into the array
Address resolution
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Closed-form computation
Used when the network interface can be
assigned specific hardware addresses
Computed by a single Boolean and operation
Hardware_address = ip_address & 0xff
When a computer connects to a network that
uses this, resolution is trivial
Address resolution
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Message exchange
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Computers exchange messages across network
to resolve an address
3 types of address resolution
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Table lookup
Closed form computation
Dynamic message exchange
Address resolution
ARP
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ARP standard defines 2 basic message types
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Request – contains an IP address and requests
the hardware address
Response – has both the IP address and the
hardware address
ARP message delivery
ARP message format
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Although the ARP message format is
sufficiently general to allow arbitrary protocol
and hardware addresses, ARP is always
used to bind a 32 bit IP address to a 48 bit
Ethernet address
ARP is encapsulated directly in a hardware
frame
Identifying ARP frames
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The type field in the frame header specifies
that the frame contains an ARP message
ARP caching
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ARP software extracts and saves the
information
Uses small table of bindings in memory
Checks cache first before broadcasting an
ARP request
Improves the efficiency of network traffic
Higher levels use protocol
addressing
IP
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TCP/IP includes both connectionless and
connection-oriented services
Routers can connect heterogeneous
networks so they cannot transmit a copy of a
frame that arrives on one network across
another
IP is a hardware independent packet format
IP datagram
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Size of a datagram is determined by the application that
sends the data
Similar to format of a frame
Uses IP addresses in header
Can contain as little as a single octet of data or at most
64K octets
Forwarding IP datagrams
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Next hop – either the destination or the next
router
IP addresses and routing
tables
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Routing
Destination and Next-Hop
addresses
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The destination address in a datagram
header always refers to the ultimate
destination
When a router forwards the datagram to
another router the address of the next hop
does not appear in the datagram header.
Best effort delivery
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IP uses best-effort to describe the service
Doesn’t guarantee that it will handle:
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Datagram duplication
Delayed or out-of-order delivery
Corruption of data
Datagram loss
Additional layers of protocol software handle
these errors
IP Datagram Header
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Each field has a fixed size
Encapsulation
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Network hardware doesn’t understand
datagram format or IP addressing
Network understands its own frame format
and heterogeneous networks may have
different formats
IP datagram is encapsulated in a frame
Encapsulation
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Frame type field uses the value reserved for
IP
Receiver knows the data area contains IP
datagram
Uses a frame address for next hop obtained
by ARP
Transmission across an
internet
When a datagram arrives in a
network frame the receiver
extracts the datagram from the
frame data and discards the
frame header. Frame headers
don’t accumulate on the trip.
MTU – Maximum transmission
unit
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Each hardware technology has a limit to the
amount of data in a frame
Datagram must be smaller than the MTU or it
can’t be encapsulated for transmission
Fragmentation
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In a internet with heterogeneous networks,
MTU restrictions can be a problem
Routers fragment or divide a datagram into
smaller pieces to meet the MTU
Fragmentation
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Each fragment uses the IP datagram format
but carries only part of the data
Flags field of the header indicates whether it
is fragment or a complete datagram
Reassembly
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Process of creating a copy of the original datagram from
fragments
Fragment with the final data has an additional bit set in
header so receiver knows all fragments have arrived
Ultimate destination host reassembles fragments so the
routers
Identifying a datagram
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IP doesn’t guarantee delivery
Fragments can be lost or arrive out of order
Sender places a unique identification number
in the identification field of outgoing datagram
When a router fragments, the identification
number and source IP address determines to
which datagram a fragment belongs
Fragment offset field tells a receiver how to
order fragments
Fragment loss
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Fragments can be delayed or lost
IP specifies a maximum time to hold fragments if they are delayed
When a fragment arrives receiver starts a timer, if all arrive before
timer runs out, datagram is reassembled, otherwise they are
discarded
No way for receiver to tell sender what fragments didn’t arrive
Sender doesn’t know about fragmentation
Resent packets may take a different path with different
fragmentation
Fragments can be fragmented in case of an even smaller MTU
Future IP
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Let’s go to the lab and research IP v6!