tailieunhanh - The Complete IS-IS Routing Protocol- P23

The Complete IS-IS Routing Protocol- P23: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | Fragmentation and the OSI Reference Model 225 In the example the original KB stream is first split into 25 application segments that are 1460 bytes in size. Next the TCP header that holds the applications port numbers and other information is prepended which adds 20 bytes resulting in a frame that is called a TCP segment which is 1480 bytes in size. The TCP segment gets prepended by an IP header which gets prepended again by the Ethernet headers. Ultimately the Ethernet preamble and the CRC32 checksum gets added to the frame resulting in 1526 byte physical frame that is ready for transmission. How does the TCP stack know that it has to split the original email stream into chops of 1460 bytes Each layer in the OSI Reference Model has a constant called the maximum transmission unit MTU . There is an MTU for TCP there is one for IP and there is of course one for Ethernet as well as for any other physical circuit. What most networking stacks do is backtracking of the MTU from the underlying circuit. Going back to the example the first MTU that is determined is the one of the Ethernet circuit. The MTU here is defined as per the Ethernet specifications and is set to 1518 bytes. Meanwhile most Ethernet chipsets have support for frames larger than 1518 bytes up to 9 KB. 1518 bytes represents the lowest common denominator that each Ethernet device has to understand. The higher layer MTUs are derived from the Layer-2 MTU. They do vary dependend on the encoding size of the Layer-2 protocol. The IP MTU is the Ethernet MTU minus 18 bytes. The 18 bytes are necessary to store 6 bytes of source and destination MAC address and 2 bytes for the Ethernet type field plus 4 bytes for the CRC32 checksum that gets appended to the end of the frame. The TCP MTU is the Ethernet MTU minus 18 minus 20. The typical IP header without IP options that would make it longer is 20 bytes in size. The story goes on by deducting 20 bytes of the TCP header size to figure out what the .