Point-toPoint Routing in the Internet in Java

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Point-toPoint Routing in the Internet
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Fragments need to be reassembled before they reach the transport layer at the destination Indeed, both TCP and UDP are expecting to receive from the network layer complete, un-fragmented segments The designers of IPv4 felt that reassembling (and possibly re-fragmenting) datagrams in the routers would introduce significant complication into the protocol and put a damper on router performance (If you were a router, would you want to be reassembling fragments on top of everything else you have to do ) Sticking to end-to-end principle for the Internet, the designers of IPv4 decided to put the job of datagram reassembly in the end systems rather than in the network interior When a destination host receives a series of datagrams from the same source, it needs to determine if any of these datagrams are fragments of some "original" bigger datagram If it does determine that some datagrams are fragments, it must further determine when it has received the last fragment and how the fragments it has received should be pieced back together to form the original datagram To allow the destination host to perform these reassembly tasks, the designers of of IP (version 4) put identification, flag and fragmentation fields in the IP datagram When a datagram is created, the sending host stamps the datagram with an identification number as well as a source and destination address The sending host increments the identification number for each datagram it sends When a router needs to fragment a datagram, each resulting datagram (ie, "fragment") is stamped with the source address, destination address and identification number of the original datagram When the destination receives a series of datagrams from the same sending host, it can examine the identification numbers of the datagrams to determine which of the datagrams are actually fragments of the same bigger datagram Because IP is an unreliable service, one or more of the fragments may never arrive at the destination For this reason, in order for the destination host to be absolutely sure it has received the last fragment of the original datagram, the last fragment has a flag bit set to 0 whereas all the other fragments have this flag bit set to 1 Also, in order for the destination host to determine if a fragment is missing (and also to be able to reassemble the fragments in the proper order), the offset field is used to specify where the fragment fits within the original IP datagram This bit is set to 1 in all except the last fragment
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Figure 44-9: IP Fragmentation Figure 44-9 illustrates an example A datagram 4,000 bytes arrives to a router, and this datagram must be forwarded to a link with a MTU of 1500 bytes These implies that the 3,980 data bytes in the original datagram must be allocated to three separate fragments (each of which are also IP datagrams) Suppose that the original datagram is stamped with an identification number of 777 Then the characteristics of the three fragments are as follows: 1st fragment r 1480 bytes in the data field of the IP datagram r identification = 777 r offset = 0 (meaning the data should be inserted beginning at byte 0) r flag = 1 (meaning there is more) 2nd fragment r 1480 byte information field r identification = 777 r offset = 1,480 (meaning the data should be inserted beginning at btye 1,480 r flag = 1 (meaning there is more) 3rd fragment r 1020 byte (=3980-1480-1480) information field
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Point-toPoint Routing in the Internet
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identification = 777 offset = 2,960 (meaning the data should be inserted beginning at byte 2,960) flag = 0 (meaning this is the last fragment)
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The payload of the datagram is only passed to the transport layer once the IP layer has fully reconstructed the original IP datagram If one or more of the fragments does not arrive to the destination, the datagram is "lost" and not passed to the transport layer But, as we learned in the previous chapter, if TCP is being used at the transport layer, then TCP will recover from this loss by having the source retransmit the data in the original datagram Fragmentation and reassembly puts an additional burden on Internet routers (the additional effort to create fragments out of a datagram) and on the destination hosts (the additional effort to reassembly fragments) For this reason it is desirable to keep fragmentation to a minimum This is often done by limiting the TCP and UDP segments to a relatively small size, so that the fragmentation of the corresponding datagrams is unlikely Because all data link protocols supported by IP are supposed to have MTUs of at least 576 bytes, fragmentation can be entirely eliminated by using a MSS of 536 bytes, 20 bytes of TCP segment header and 20 bytes of IP datagram header This is why most TCP segments for bulk data transfer (such as with HTTP) are 512536 bytes long (You may have noticed while surfing the Web that 500 or so bytes of data often arrive at a time) Following this section we provide a Java applet that generates fragments You provide the incoming datagram size, the MTU and the incoming datagram identification It automatically generates the fragments for you
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