cnp3-ebook/protocols/ipv6 — English @ Weblate

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89 ../../protocols/ipv6.rst:341

	English It is interesting to note that there is no checksum inside the IPv6 header. This is mainly because all datalink layers and transport protocols include a checksum or a CRC to protect their frames/segments against transmission errors. Adding a checksum in the IPv6 header would have forced each router to recompute the checksum of all packets, with limited benefit in detecting errors. In practice, an IP checksum allows for catching errors that occur inside routers (e.g. due to memory corruption) before the packet reaches its destination. However, this benefit was found to be too small given the reliability of current memories and the cost of computing the checksum on each router [#fipv4checksum]_. 702/10000 · 702
90 ../../protocols/ipv6.rst:343

	English When a host receives an IPv6 packet, it needs to determine which transport protocol (UDP, TCP, SCTP, ...) needs to handle the payload of the packet. This is the first role of the `Next header` field. The IANA_ which manages the allocation of Internet resources and protocol parameters, maintains an official list of transport protocols [#fianaprotocol]_. The following protocol numbers are reserved : 400/10000 · 400
91 ../../protocols/ipv6.rst:345

	English ``TCP`` uses `Next Header` number ``6`` 39/10000 · 39
92 ../../protocols/ipv6.rst:346

	English ``UDP`` uses `Next Header` number ``17`` 40/10000 · 40
93 ../../protocols/ipv6.rst:347

	English ``SCTP`` uses `Next Header` number ``132`` 42/10000 · 42
94 ../../protocols/ipv6.rst:349

	English For example, an IPv6 packet that contains an TCP segment would appear as shown in the figure below. 99/10000 · 99
95 ../../protocols/ipv6.rst:354

	English An IPv6 packet containing an TCP segment 40/10000 · 40
96 ../../protocols/ipv6.rst:358

	English However, the `Next header` has broader usages than simply indicating the transport protocol which is responsible for the packet payload. An IPv6 packet can contain a chain of headers and the last one indicates the transport protocol that is responsible for the packet payload. Supporting a chain of headers is a clever design from an extensibility viewpoint. As we will see, this chain of headers has several usages. 416/10000 · 416
97 ../../protocols/ipv6.rst:360

	English :rfc:`2460` defines several types of IPv6 extension headers that could be added to an IPv6 packet : 99/10000 · 99
98 ../../protocols/ipv6.rst:362

	English `Hop-by-Hop Options` header. This option is processed by routers and hosts. 75/10000 · 75
99 ../../protocols/ipv6.rst:363

	English `Destination Options` header. This option is processed only by hosts. 69/10000 · 69
100 ../../protocols/ipv6.rst:364

	English `Routing` header. This option is processed by some nodes. 57/10000 · 57
101 ../../protocols/ipv6.rst:365

	English `Fragment` header. This option is processed only by hosts. 58/10000 · 58
102 ../../protocols/ipv6.rst:366

	English `Authentication` header. This option is processed only by hosts. 64/10000 · 64
103 ../../protocols/ipv6.rst:367

	English `Encapsulating Security Payload`. This option is processed only by hosts. 73/10000 · 73
104 ../../protocols/ipv6.rst:369

	English The last two headers are used to add security above IPv6 and implement IPSec. They are described in :rfc:`2402` and :rfc:`2406` and are outside the scope of this document. 171/10000 · 171
105 ../../protocols/ipv6.rst:371

	English The `Hop-by-Hop Options` header was designed to make IPv6 easily extensible. In theory, this option could be used to define new fields that were not foreseen when IPv6 was designed. It is intended to be processed by both routers and hosts. Deploying an extension to a network protocol can be difficult in practice since some nodes already support the extensions while others still use the old version and do not understand the extension. To deal with this issue, the IPv6 designers opted for a Type-Length-Value encoding of these IPv6 options. The `Hop-by-Hop Options` header is encoded as shown below. 603/10000 · 603
106 ../../protocols/ipv6.rst:376

	English The IPv6 `Hop-by-Hop Options` header 36/10000 · 36
107 ../../protocols/ipv6.rst:378

	English In this optional header, the `Next Header` field is used to support the chain of headers. It indicates the type of the next header in the chain. IPv6 headers have different lengths. The `Hdr Ext Len` field indicates the total length of the option header in bytes. The `Opt. Type` field indicates the type of option. These types are encoded such that their high order bits specify how the header needs to be handled by nodes that do not recognize it. The following values are defined for the two high order bits : 512/10000 · 512
108 ../../protocols/ipv6.rst:380

	English ``00`` : if a node does not recognize this header, it can be safely skipped and the processing continues with the subsequent header 132/10000 · 132

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