Transcript fast ethernet

CMPT 371
Data Communications and Networking
Ethernet, ARP
0
© Janice Regan, CMPT 128, 2007-2012
Evolution of Ethernet: 1
 Early 1980’s: “DIX” Ethernet.
 1985: Ethernet 802.3: 10 Mbps
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Uses 1 persistent CSMA/CD on a bus.
Truncated exponential backoff
 1995: Fast Ethernet 802.3u: 100 Mbps
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Uses CSMA/CD on twisted pair star with central hub
OR switching on twisted pair star with switching hub
Frame structure same as 802.3, including maximum frame size
Faster transmission time reduces transmission time of frame
For CSMA/CD frame transmission time ≈ round trip travel time,
which is now shorter. Reduces maximum length of medium
Janice Regan © 2005-2013
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Evolution of Ethernet: 2
 1998: Gigabit Ethernet 802.3z: 1 Gbps
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Maintains same structure and procedures
New physical layers defined to support higher rate
Uses CSMA/CD or switching hubs
Increase minimum frame size to 512 bytes (from 64) to give
long enough frame for CSMA/CD at 1 Gbps.
Frame padded if it is shorter (Carrier Extension) or multiple
short frames are sent at once (Frame Bursting)
 2002: Gigabit Ethernet 802.3ae: 10 Gbps
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Maintains frame structure or supports SONET(optical)
Too fast for CSMA/CD, uses switching hubs only
Competes with ATM (developed for telephone system)
Originally optical fibre, now also works on twisted pair
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History of Ethernet growth
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MAC Frame
PREAMBLE
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Destination
address
Source
Address
Type
DATA
IP DATAGRAM
CR
C
Preamble: 8 octets of alternating 1’s and 0’s used to establish
synchronization. 7 octets of 10101010 a final octet of 10101011
Used to indicate the start of the frame and synchronize sender and
receiver clocks (variation in hardware)
Destination address (DA): address of the station for which the
frame is intended (MAC address of interface, 6 bytes)
Source address (SA): The address of the station that sent the
frame (6 bytes, 48 bits)
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MAC Frame
PREAMBLE
Destination
address
Source
Address
Type
DATA
IP DATAGRAM
CR
C
 Length/Type: Length of the LLC data field (<1500
octets), or the type of protocol (if not 802.3)
 Pad: added to make sure the transmission time of the
packet is at least as long as the propagation time
through the network (required for efficient use of
CSMA/CD). Minimum length is 46 bytes
 FCS: 32 bit CRC
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Framing
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Need to identify the beginning of a frame
Need to synchronize arriving packet and receivers clock
Preamble may be used. Look for a specific series of bits or bytes
 When that series of bits or bytes is found interpret it a the
beginning of a frame
A problem occurs if the series of bits occurs within the data
 How do we tell the difference between the start of the frame and
the data with the same pattern?
 The answer is bit stuffing or byte stuffing. Modify the data so
that it cannot be mistaken for the framing sequence but can be
reconstructed in its original form at the destination
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HDLC: bit stuffing
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(making flag pattern 01111110 unique to the flag), after any
sequence of five 1’s in the data insert a 0
Original Pattern
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111111111111011111101111110
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After Bit Stuffing
1111101111101101111101011111010
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Problem with bit Stuffing on a channel with errors
Flag
Flag
Flag
.
Flag
Flag
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Bit inverted
Flag
Flag
Bit inverted
Flag
Flag
Flag
transmitte
d
received
transmitte
d
received
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Scope of MAC addresses
 MAC addresses are used within a particular LAN
or LAN segment only
 The internet interface (network card) on each
machine has a MAC address
 For Ethernet the address is associated with a
unique Ethernet address (48 bit address) at
the time of manufacture.
 Sometimes can be set after manufacture
 Other types of LAN (using different MAC
protocols) have their own types of addresses
that may have different lengths from the
Ethernet address
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Ethernet interface + addresses
 Within an Ethernet the interface to each host is
assigned an Ethernet address (usually when
manufactured)
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The Ethernet address belongs to the interface, not
the host, if the interface is replaced the Ethernet
address will change
 48 bit Ethernet addresses can indicate one interface,
a network broadcast address or a multicast address.
 The interface to a host will extract from the network
packets with the Ethernet address of the interface
itself, the Ethernet network broadcast address of the
network to which the interface is attached, or any
Ethernet multicast addresses selected by the host.
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Sending an Ethernet frame
 The IP packet arrives in the MAC layer
 A MAC layer (Ethernet) header is added
 This header includes the source and destination
Ethernet address
 The host knows its own Ethernet address so it can
insert the source Ethernet address
 The host must determine the Ethernet destination
address corresponding to the next hop destination.
 Determining the Ethernet destination address based on
the next hop IP destination address is called address
resolution or address translation
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Address Resolution / Translation
 A router attached to the Ethernet will determine /
translate IP addresses to the Ethernet unicast
multicast or broadcast addresses, recognizable
by the interfaces
 For unicast determine Ethernet address using
ARP (Address Resolution Protocol)
 For multicast translate IP multicast address:
the lowest 23 bits of the IP multicast address
is placed in the special Ethernet address
01.00.5e.00.00.00 (Not a unique mapping as
IP has 28 significant bits)
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Address Translation (4)
225
125
22
5
1 1 1 0 0 0 0 1 0 1 1 1 1 1 0 1 0 0 0 1 0 1 1 0 0 0 0 0 0 1 0 1
0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0 0 1 1 1 1 1 0 1 0 0 0 1 0 1 1 0 0 0 0 0 0 1 0 1
01
00
5E
The first four bits of the IP address are fixed (they indicate a multicast address)
The next 5 bits (dark blue) are not used in the Ethernet address
Therefore there are 25 = 32 IP addresses with the same Ethernet broadcast address
© Janice Regan, 2006-2013
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ARP and sending a packet
 The IP packet arrives in the MAC layer
 A MAC layer (Ethernet) header is added
including the know source Ethernet address
 The host needs the Ethernet destination
address, use ARP to find that address
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If ARP already knows the Ethernet destination
address it can be inserted and the packet can be
sent
 If ARP does not already know the Ethernet
destination address ARP must first find the Ethernet
address
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Unicast Addresses: Data Link Layer
 If the packet is coming from outside the LAN it will arrive
through a router. Its source Ethernet address will be the
router’s Ethernet address
 If the packet is coming from another host on the LAN then
the source Ethernet address will be that host’s
 If we are delivering the packet on a LAN we will also need
the hardware address (data link layer address, MAC
address, Ethernet …), to use in the MAC header as the
destination address. Use ARP to find it!
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Entries in the ARP cache
 Entries in the ARP cache are static or dynamic.
 Static entries are entered by the administrator and stay
in the cache until removed
 Dynamic entries are added to the cache when the local
host tries to send a packet to a destination host on the
LAN
 When a dynamic entry is added to the cache a timer is
set to indicate the lifetime of that entry
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When the two hosts communicate, each host updates
(reinitializes) the timer corresponding to the entry for the other
host in its own ARP cache
When the timer expires the entry is removed
© Janice Regan, 2012
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Why use Dynamic Entries
 MAC addresses of hosts may change over time
 Replacement of a network card (interface) will result
in a changed MAC address
 Interface may no longer be reachable
 Host may be turned off or disconnected
 Interface may be disabled
 Interface may fail
 New hosts/interfaces may be added to the
network
 Cache should not grow forever as new hosts
are added
© Janice Regan, 2012
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Using ARP
 ARP (address resolution protocol) facilitates
mapping of physical addresses to IP addresses for
all hosts on the LAN (exceptions discussed later)
 The host will keep a table called the ARP cache
that will contain mappings from MAC to IP
addresses
 When the host wishes to send a packet it will look
in the ARP cache for an entry corresponding to the
desired destination host’s IP address.
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If there is an entry it will send to the indicated MAC
address
 If no entry exists an new entry must be added to the
ARP cache
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Adding new entries -ARP cache
 If no entry in the ARP cache corresponds to the IP
address of the destination of the packet to be sent
the packet will be queued or dropped and an ARP
request will be sent
 The ARP request is broadcast to the LAN
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All hosts on the LAN receive the request
 The host with IP address corresponding to the
destination IP address of the packet
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Will respond with a ARP reply unicast to the source
Will add/update the entry for the source to its ARP cache
© Janice Regan, 2012
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Sending a packet to an IP address
From
the
TCP/IP
Guide
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Regan
© 2005-2013
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