CS65-Computer Networks Dept. of EEE S.Muralidharan 1 3/2/2011 1 INTERNETWORKING INTERNETWORKING 3/2/2011 2 AN INTERNET 3/2/2011 3 PRINCIPLES OF INTERNETWORKING REQUIREMENTS ON INTERNETWORKING FACILITY: • Provide a link between networks. • Provide for the routing and delivery of data between processes on different networks • Provide an accounting service that keeps track of the use of the various networks and gateways and maintains status information. • Internetworking facility must accommodate a number ofdifferences among networks. 3/2/2011 4 DIFFERENCES AMONG NETWORKS • Different addressing schemes • Different maximum packet size • Different network access mechanisms • Different timeouts • Error recovery • Status reporting • Routing techniques • User access control • Connection, connectionless.
– Bridges are simplified gateway used to connect homogenous
networks. Such networks exhibit the same interface to attached
stations and use the same internal protocols.
– Bridges can be used to connect two or more LAN segments of the
same type (e.g. Ethernet to Ethernet, or Token-Ring to Token-
Ring).• Bridges Vs Repeaters
– Example is the “repeater” used in base band networks. However,
this is not a true multiple network system. The repeater is merely
used to extend the length of the base band cable. I t amplifies and
retransmits all signals, including collisions. Thus the system
behaves like a single network. Moreover each port on a bridge has
its own MAC address which is not the case of a repeater
• When bridges are powered on in an Ethernet network, they start tolearn the network's topology by analysing the source addresses of incoming frames from all attached network segments (a processcalled backward learning ). Over a period of time, they build up arouting table .– Unless the source and the destination are on different network segments,
there is no need for the bridge to transfer an incoming frame to another network segment. If the source and the destination are on differentsegments, the bridge needs to be able to determine which segment thedestination device belongs to.
– The bridge monitors all traffic on the segments it connects, and checks thesource and destination address of each frame against its routing table. Whenthe bridge first becomes operational, the routing table is blank, but as data istransmitted back and forth, the bridge adds the source MAC address of any incoming frame to the routing table and associates the address with the port on which the frame arrives. In this way, the bridge quickly builds up acomplete picture of the network topology. If the bridge does not know thedestination segment for an incoming frame, it will forward the frame to all attached segments except the segment on which the frame was transmitted.Bridges reduce the amount of traffic on individual segments by acting as afilter, isolating intra-segment traffic. This can greatly improve response times.
• Because Ethernet bridges determine whether or not toforward frames on the basis of the desination MACaddress, they are said to operate at the data link layer of
the OSI Reference Model. Etherenet bridges are
sometimes referred to as transparent , because their
presence and operation are transparent to network users,
although they successfully isolate intrasegment traffic,
reducing network traffic overall and improving networkresponse times.
• In case 4, the gateway returns an error message to thesource of the datagram.
• For case 1 to 3, the gateway must select appropriate route
for the data, and insert them into the appropriate network
with the appropriate address.
• For case 1, the address is the destination address.
• For case 2 and 3, the address is a gateway address
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• Before actually sending data, the gateway must need tosegment the datagram to accommodate a smaller packetsize limitation on the outgoing network.
• Each segment becomes an independent IP datagram.
• The gateway then queues each packet for transmission. Itmay also enforce a maximum queue length. Once that limitis reached, additional datagrams are simply dropped.
• The process described above continues through zero or more gateways until the datagram reaches the destinationstation.
• The destination station recovers the IP datagram from itsnetwork wrapping.
• If segmentation has occurred, the IP module in thedestination station buffers the incoming data until theoriginal data field is reassembled.
IPv4 FRAME FORMAT
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• Protocol format includes
– Version (4-bits):
• The current protocol version is 6.
• Including a version number allows a future version of IP be
used along side the current version, facilitating migration to
new protocols.
– Internet header length (4-bits):
• Length of the datagram header (excluding data) in 32-bitwords.
• The minimum length is 5 words = 20 bytes, but can be up to
15 words if options are used.
• In practice, the length field is used to locate the start of the
data portion of the datagram
– Service
• This is an 8-bit field.
• Previously it was called as “SERVICE TYPE”, now it is called
• Type of service(8-bits):Precedence (3-bits): A priority indication, where 0 is the lowest andmeans normal service, while 7 is highest and is intended for networkcontrolmessages (e.g., routing,congestioncontrol).
Delay (1-bit): An Application can request low delay service (e.g., for interactive use).
Throughput(1-bit): Applicationrequests high throughput.
Reliability (1-bit):Applicationrequests high reliability
– Time to live (8 bits) :• A counter that is decremented by each gateway.
• Shouldthis hopcount reach 0, discard the datagram.
• Originally, the time-to-live field was intended to reflect real time.
• In practice, it is now a hopcount.
• The time-to-livefield squashes looping packets.
• It also guarantees that packets don't stay in the network for longer than 255 seconds, a property needed by higher layer protocols that reuse sequencenumbers.
– Protocol(8 bits):
• What type of data the IP datagramcarries (e.g., TCP, UDP, etc.).
• Needed by the receiving IP to know the higher level service that
will next handle the data.– Header checksum (16 bits) :
• Frame check sequence on the header only
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– Source address (32 bits) :• Coded to allow a variable allocation of bits to specify the network
and the station within the specified network
– Destination address (32 bits) :
• As above
– Options
IP datagrams allow the inclusion of optional, varying length fields that
need not appear in every datagram. We may sometimes want to
send special information, but we don't want to dedicate a field in the
packet header for this purpose.
Options start with a 1-byte option code, followed by zero or morebytes of option data.
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The option code byte contains three parts:
copy flag (1 bit): If 1, replicate option in each fragment of a fragmenteddatagram. That is, this option should appear in every fragment as well. If 0, optionneed only appear in first fragment.
optionclass (2 bits): Purpose of option:
0 = network control1 = reserved
2 = debuggingand measurement
3 = reserved
optionnumber(5 bits): A code indicating the option's type.
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– Padding
• Used to ensure that the internet header ends on a 32 bit
boundary
– Data
• The field must be a multiple of eight bits in length. Total length of
the data field plus header is a maximum of 65,535 octets
• The network address required by gateway can bespecified by several ways.– The application can refer to a network by a unique number
– The internet logic in the station can translate a network nameinto a network address.
– A global station addressing scheme can be used. ie. Uniqueidentifier for each station.
• The third approach was proposed by Ethernetdevelopers. It recommends a 48 bit address which canbe used for 1014 unique referents– Advantage : it permits stations to move from one network to
another.
– Disadvantage : some central facility must manage theassignment of names
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•
So gate way will receive an internet packet with areferent in the form net.stationnet.station where net is the networkaddress and station is the address of the individualsystem.– Since each station has different processes running, identifying
the process becomes difficult
• So net.station.SAP net.station.SAP could solve this problem. Thismakes the internet protocol as process to processrather than station to station.– It is the responsibility of the internet layer to multiplex and de-
multiplex between various SAPs. This simplify the higher
layer’s task. This allows the usage of microprocessor hardwaredevices.
• However one or more directory servers are needed toidentify the net.stationnet.station address.
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ROUTING
• Routing is accomplished by maintaining a routing tablein each station and gateways.
• Routing table may be static or dynamic– A static table could contain alternate routes if a gateway is
unavailable
– A dynamic table is more flexible in responding both to error andcongestion situations.
• Routing table may also be used to support other internetservices such as security and priority
• Source routing – source station specifies the route byincluding a sequential list of gateways in the data grams
• Route recording - To record a route, each gatewayappends its address to a list of addresses in the datagram
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DATAGRAM LIFETIME
• If dynamic or alternate routing is used, the potential
exists for a data gram or some of its fragments to
loop indefinitely through the internet.
• This is undesirable because
– An endlessly circulating datagram consumes resources
• During reassembly, as the segments with the same IDarrive, their data fields are inserted in the proper position in the buffer until the entire data gram isreassembled. This is identified by the sequence of datapacket having ‘Offset’ of zero and ends with packethaving false ‘More Flag’.
• Since connectionless service does not guarantee the
delivery, some means is needed to decide to abandon areassembly effort.– This can be done with timer. Once the timer expires, all
received segments are discarded.
– The destination IP can make use of the datagram lifetime
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ERROR CONTROL• When the data gram is discarded in the gateway, the
gateway should attempt to return some information to
the source. Based on this source may modify the
transmission strategy
• Data grams may be discarded because of
– Lifetime expires
– Congestion
– Frame Check Sequence (FCS) error – here notification is not
possible because source address field may have been
