Chapter 6 Neighbor Discovery 125 Neighbor Discovery Message Format ND messages use the ICMPv6 message structure and ICMPv6 types 133 through 137.ND messages consist of an ND message header,composed of an ICMPv6 header and ND message specific data,and zero or more ND options.Figure 6-1 shows the format of an ND message. IPv6 Header Neighbor Discovery Neighbor Discovery Next Header =58 Message Header Message Options (ICMPv6) -Neighbor Discovery Message Figure 6-1 The format of an ND message There are five different ND messages: Router Solicitation(ICMPv6 type 133) Router Advertisement (ICMPv6 type 134) Neighbor Solicitation (ICMPv6 type 135) Neighbor Advertisement(ICMPv6 type 136) Redirect(ICMPv6 type 137) ND message options provide additional information,indicating MAC addresses,on-link net- work prefixes,on-link MTU information,redirection data,mobility information,and specific routes. To ensure that ND messages that are received have originated from a node on the local link (either a physical link or a tunnel),all ND messages are sent with a hop limit of 255.When an ND message is received,the Hop Limit field in the IPv6 header is checked.If it is not set to 255,the message is silently discarded.Verifying that the ND message has a hop limit of 255 provides protection from ND-based network attacks that are launched from off-link nodes. With a hop limit of 255,a router could not have forwarded the ND message from an off-link node. Neighbor Discovery Options ND options are formatted in type-length-value(TLV)format.Figure 6-2 shows the TLV format Type Length W Vaive w.· Figure 6-2 The TLV format for ND options￾     !"#$ %&'%( )*+,--./,-0-,12,345678+,--./,-190:109,.;<3456781=>,-?@@129A0/2?@BC)* +,--./,-:A;-D-1AE.;)*+,--./,2,.<,9F:A+>A-,<AE.;3456782,.<,9.;<)*+,--./,G ->,:DED:<.1.F.;<H,9AA9+A9,)*A>1DA;-CID/09,8G?-2AJ-12,EA9+.1AE.;)*+,--./,C KL MN OPQRSTUVWSRVXYZUQ[[V\Q ]2,9,.9,ED7,<DEE,9,;1)*+,--./,-^ _ `A01,9aAbD:D1.1DA;c3456781=>,?@@d _ `A01,9e<7,91D-,+,;1c3456781=>,?@fd _ ),D/2gA9aAbD:D1.1DA;c3456781=>,?@hd _ ),D/2gA9e<7,91D-,+,;1c3456781=>,?@8d _ `,<D9,:1c3456781=>,?@Bd )*+,--./,A>1DA;->9A7D<,.<<D1DA;.bD;EA9+.1DA;FD;<D:.1D;/5e4.<<9,--,-FA;GbD;i;,1G JA9i>9,EDj,-FA;GbD;i5]kD;EA9+.1DA;F9,<D9,:1DA;<.1.F+AgDbD1=D;EA9+.1DA;F.;<->,:DED: 9A01,-C ]A,;-09,12.1)*+,--./,-12.1.9,9,:,D7,<2.7,A9D/D;.1,<E9A+.;A<,A;12,bA:.bbD;i c,D12,9.>2=-D:.bbD;iA9.10;;,bdF.bb)*+,--./,-.9,-,;1JD12.2A>bD+D1AElhhCm2,;.; )*+,--./,D-9,:,D7,<F12,nA>oD+D1ED,b<D;12,36782,.<,9D-:2,:i,<C3ED1D-;A1-,11A lhhF12,+,--./,D--Db,;1b=<D-:.9<,<Cp,9DE=D;/12.112,)*+,--./,2.-.2A>bD+D1AElhh >9A7D<,->9A1,:1DA;E9A+)*Gg.-,<;,1JA9i.11.:i-12.1.9,b.0;:2,<E9A+AEEGbD;i;A<,-C mD12.2A>bD+D1AElhhF.9A01,9:A0b<;A12.7,EA9J.9<,<12,)*+,--./,E9A+.;AEEGbD;i ;A<,C  !"#qr(s )*A>1DA;-.9,EA9+.11,<D;1=>,Gb,;/12G7.b0,c]opdEA9+.1CID/09,8Gl-2AJ-12,]opEA9+.1C KL Mt OPQOuvRSTUVWRSTYZSwWxSX[ yz{|}~€~ ‚ƒ„…†ƒ‡ˆƒ‰Š‹Œ Ž‘’ •~™š›œš{~ž Ÿ~œœ~}~€~ •~™š›œš{~ž Ÿ~œœ~ ¡¢š£œ ‚ƒ¤¥¦§¨‰©¤ª«¨’ƒ‰¬ƒªª‡¥ƒ ­¬®ƒ ¯ƒ°¥…¦ ±‡²³ƒ ´´´