132 Understanding IPv6,Second Edition Figure 6-8 shows a switched configuration where the MTU option is used to solve a mixed- media problem. Host A Ethernet Switch IPv6 Router FDDI Ethernet Backbone FDDI Rest of IPv6 Network Ethernet Switch Host B Figure 6-8 A mixed-media configuration Two IPv6 hosts,Host A and Host B,are connected to two different Ethernet (Layer 2)switches using Fiber Distributed Data Interface(FDDI)ports.The two switches are connected by an Ethernet backbone.When Host A and Host B negotiate a TCP connection,each reports a TCP maximum segment size of 4312(the FDDI IPv6 MTU of 4352,minus 40 bytes of the IPv6 header).However,when TCP data on the connection begins to flow,the switches silently dis- card IPv6 packets larger than 1500 bytes that are sent between Host A and Host B. With the MTU option,the IPv6 router for the subnet reports an IPv6 MTU of 1500 in the Router Advertisement message for all hosts on the link.When both Host A and Host B adjust their IPv6 MTU from 4352 to 1500,maximum-sized TCP segments sent between them are not discarded by the intermediate switches. Note FDDI is an older technology whose use has been made obsolete by 100-Mbps Ether- net.This configuration is unlikely to be used on modern networks and serves only as an example of a mixed-media subnet. Figure 6-9 shows the structure of the MTU option. Type =5 Length =1 Reserved Mwwww Figure 6-9 The structure of the MTU option The following list describes the fields in the MTU option: Type The value of this field is 5. Length The value of this field is 1.(There are 8 bytes in the entire option.)￾      !"#$!%!$&'"('#)*%&#)$"&"+,-#.&#)!!(&#!#/0%12( 1(%.#3/14 5 678 9:;<=>?:=>;@ABCD;EFG@H;BC ,$#IJ0"#!&!KL#!&M%)(L#!&NK%'#))'&(&#&$#(**)&O&")&PQ%RST!$&'"! !)3U!&3&(U%&%I)&*%'PUUIT.#&!4,"&$#!$&'"!%'#))'&(3R%) O&")&3%'V3#)4W")L#!&M%)(L#!&N)#&%&%,XJ'#))'&#)K%'".#&!%,XJ 1%211!1)&!Y#*Z[\SP&"UUIIJ0+,-#*Z[]SK1)!Z^3R&!#*&"IJ0 "%(T4L#$0K$"),XJ(%&%#)&"'#))'&#)3)!&#*/#$K&"!$&'"!!/)&/R(! '%(IJ0.%'V&!/%&"%)\]^^3R&!&"%&%!)&3&$)L#!&M%)(L#!&N4 W&"&"+,-#.&#)K&"IJ0#&*#&"!3)&.#&!%)IJ0+,-#*\]^^)&" _#&M(0&!1)&1!!%*#%//"#!&!#)&"/)V4W")3#&"L#!&M%)(L#!&N%(`!& &"IJ0+,-*#1Z[]S&#\]^^K1%211!Y(,XJ!1)&!!)&3&$)&"1%)#& (!'%((3R&")&1(%&!$&'"!4 a  bccdefghijklmnlophijiqrspifltflpgfullhvgkliufijlnlurwxxyzu{f|nplmy hln}~pefoiheqtmgneihefthje€ljrniultflkihviklmhhlnsim€fghkflmlfihjrgfgh l‚gv{jligve‚lkyvlkegftuhln} ƒ!"#$!&"!&'&#*&"+,-#.&#)4 5 67„ …†=‡HGFAHFG=BDH†=ˆ…‰BŠH;BC ,"*#//#$)/!&(!'3!&"*/(!)&"+,-#.&#)‹ Œ Ž ,"0%/#*&"!*/(!]4 Œ  ‘ ,"0%/#*&"!*/(!\4P,"% 3R&!)&")&#.&#)4T ’•™š› ¡¢¢£ šœ™’•žš™Ÿ›  ¡¢¢£ ¤™¥š›¦š™§Ÿ¨™©¥ ¤™¥š›¦š™ ª«© ¬¦š ­œ™® ­œ™ª ¤™¥š›¦š™§Ÿ¨™©¥ ¯°±š ²š¦³™¥ šœš›š´ µ¯¶ ·¸ ¹