D2.2.4: Final IPv4 to IPv6 Transition Cookbook for ... - 6NET

IST-2001-32603Deliverable D 2.2.4technique [RFC3056], except that the latter needs special IPv6 addresses. However, 6PE can alsouse MPLS tunnels (LSPs) configured by other means, e.g., traffic engineering.3.4.2.2. Tunnel overheadTunnel encapsulations by definition add new protocol header and thus increase the transmissionoverhead. We can directly compare the overheads of the 6PE MPLS encapsulation and the two mostcommon tunnelling methods:• We saw previously that the 6PE encapsulation involves two labels, which are – at least inthe common cases of Ethernet or PoS link layers – contained in two “shim” headers, each 4bytes long. The structure of the entire packet is shown in Figure 3-4.4 bytes 4 bytesOuter MPLS shim header Inner MPLS shim header IPv6 datagramFigure 3-4: Structure of an MPLS packet• One option for configured IPv6 tunnels is the Generic Routing Encapsulation (GRE) definedin [RFC2784]. In this case, the overhead consists of the GRE header (4 bytes) and outerIPv4 header (mostly 20 bytes) as shown in Figure 3-5.20 bytes 4 bytesIPv4 header GRE header IPv6 datagramFigure 3-5: Structure of a GRE tunnel packet• In the IPv6 world, the most common tunnelling technique is the direct encapsulation of IPv6datagrams in IPv4 [RFC2893], which is used for both configured tunnels and automatic 6to4tunnels [RFC3056]. Figure 3-6 indicates that the overhead is just the IPv4 header of 20bytes (again assuming no IP header options).20 bytesIPv4 headerIPv6 datagramFigure 3-6: Structure of an IPv6 packet encapsulated in IPv4Table 3-1 shows overhead percentages for different lengths of the tunnelled IPv6 datagram. Forsmall packets the 6PE encapsulation is clearly superior whereas for packet sizes close to the MTUof common backbone media types (Gigabit Ethernet or PoS) the difference becomes essentiallynegligible.Datagram size [Bytes] 6PE/MPLS GRE IPv6 in IPv440 20.0% 60.0% 50.0%200 4.0% 12.0% 10.0%19