TCP/IP Tutorial and Technical Overview - IBM Redbooks

332 TCP/IP Tutorial and Technical Overview
46 Resource Reservation Protocol
58 IPv6 ICMP Packet
The following values are all extension headers:
0 Hop-by-Hop Options Header
43 IPv6 Routing Header
44 IPv6 Fragment Header
50 Encapsulating Security Payload
51 IPv6 Authentication Header
59 No Next Header
60 Destination Options Header
We discuss different types of extension headers in 9.2.1,
“Extension headers” on page 333.
Hop limit This is similar to the IPv4 TTL field but it is now measured
in hops and not seconds. It was changed for two reasons:
– IP normally forwards datagrams faster than one hop
per second and the TTL field is always decremented
on each hop, so, in practice, it is measured in hops
and not seconds.
– Many IP implementations do not expire outstanding
datagrams on the basis of elapsed time.
The packet is discarded after the hop limit is decremented
to zero.
Source address A 128-bit address. We discuss IPv6 addresses in 9.2.2,
“IPv6 addressing” on page 339.
Destination address A 128-bit address. We discuss IPv6 addresses in 9.2.2,
“IPv6 addressing” on page 339.
A comparison of the IPv4 and IPv6 header formats shows that a number of IPv4
header fields have no direct equivalents in the IPv6 header:
► Type of service
Type of service issues in IPv6 are handled using the flow concept, described
in 9.2.4, “Flow labels” on page 346.