## Routing

This section is a short introduction to a complex topic. IPv6 packets
are routed individually and statelessly, like any datagram protocol.
Consecutive packets may follow different routes, may be lost on the way,
may arrive out of order, and transit times are variable. In practice,
operators attempt to minimize these effects but upper layer protocols
cannot rely on this. In some cases, quality of service mechanisms such
as differentiated services
\[[2. Traffic class and flow label](../2.%20IPv6%20Basic%20Technology/Traffic%20class%20and%20flow%20label.md)\]
may help, but packet delivery remains statistical.

IPv6 routing in general operates by longest-match, i.e. each router
forwards each packet to another router known to handle an address prefix
that is the longest one (up to 128 bits) that matches the packet's
destination address
\[[BCP198](https://www.rfc-editor.org/info/bcp198)\]. Routers use
various routing protocols among themselves to distribute information
about which prefixes they handle. Common routing protocols are:

*For site and enterprise networks:*

- OSPFv3 \[[RFC5340](https://www.rfc-editor.org/info/rfc5340)\] is most
  common.

- IS-IS \[[RFC5308](https://www.rfc-editor.org/info/rfc5308),
  [RFC7775](https://www.rfc-editor.org/info/rfc7775)\].

- RIPng \[[RFC2080](https://www.rfc-editor.org/info/rfc2080),
  [RFC2081](https://www.rfc-editor.org/info/rfc2081)\] is defined but
  seems to be little used.

*Small enterprise and home networks*

- The Babel Routing Protocol
  \[[RFC8966](https://www.rfc-editor.org/info/rfc8966)\].

*Inside carrier (ISP) networks or very large enterprise networks:*

- IBGP (internal use of BGP-4) optimized by route reflection
  \[[RFC4456](https://www.rfc-editor.org/info/rfc4456)\].

- IS-IS \[[RFC5308](https://www.rfc-editor.org/info/rfc5308),
  [RFC7775](https://www.rfc-editor.org/info/rfc7775)\]

- OSPFv3 \[[RFC5340](https://www.rfc-editor.org/info/rfc5340)\].

*Between carrier (ISP) networks (inter-domain routing):*

- Border Gateway Protocol 4 (BGP-4) in its multiprotocol form
  \[[RFC2545](https://www.rfc-editor.org/info/rfc2545),
  [RFC4271](https://www.rfc-editor.org/info/rfc4271),
  [RFC4760](https://www.rfc-editor.org/info/rfc4760)\]. Autonomous
  System numbers work the same way for IPv6 and IPv4.

*For emerging mesh networks:*

- RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks)
  \[[RFC6550](https://www.rfc-editor.org/info/rfc6550),
  [RFC9008](https://www.rfc-editor.org/info/rfc9008),
  [RFC9010](https://www.rfc-editor.org/info/rfc9010)\].

- The Babel Routing Protocol
  \[[RFC8966](https://www.rfc-editor.org/info/rfc8966)\].

IPv6 routers can be placed in various categories, each of which requires
different features to be active:

- Customer Edge (CE) routers (enterprise): These are routers that
  connect an enterprise network to one or more ISPs
  \[[RFC7084](https://www.rfc-editor.org/info/rfc7084)\],
  {{RFC8585}},
  \[[RFC9096](https://www.rfc-editor.org/info/rfc9096)\].

- Enterprise routers: Internal routers within a large enterprise
  network.

- Subnet routers: Internal routers that support one or more links
  connecting end hosts (typically Ethernet or WiFi). Such a router will
  be the last-hop router for incoming traffic and the first-hop router
  for outgoing traffic. It must also provide Router Advertisement
  services for the end hosts, and either SLAAC or DHCPv6 or both \[See
  [2. Address resolution](../2.%20IPv6%20Basic%20Technology/Address%20resolution.md)
  etc.\].

- Customer Edge (CE) routers (domestic): These are cheap routers
  connecting home or small office networks to an ISP. They typically act
  as subnet routers too, but are unlikely to provide the full set of
  enterprise CE router services.

- Provider Edge routers. These are routers within ISP networks that
  directly connect to CE routers.

- Transit routers within ISPs.

- Inter-domain routers connecting ISPs to peer ISPs and/or Internet
  Exchange Points.

A general comment is that IPv6 prefixes being longer than IPv4 prefixes
(up to 64 bits instead of, say, 24 bits), one might expect routing
tables to require much more memory space. While this is true, IPv6 was
designed for classless route aggregation from the beginning, which
generally permits there to be fewer IPv6 prefixes, mitigating the table
size issue. (Nevertheless, the BGP-4 table for IPv6 continues to grow,
as discussed in [this CCR paper](https://dl.acm.org/doi/10.1145/3477482.3477490).)
Interested readers can find exhaustive data on BGP-4 table
sizes at [Geoff Huston's site](https://bgp.potaroo.net/index-bgp.html).
For a deep dive on BGP-4 itself, with much focus on IPv6, see the e-book
by Iljitsch van Beijnum:
[Internet Routing with BGP](https://www.iljitsch.com/2022/11-18-new-e-book-internet-routing-with-bgp.html)
(2022).

Finally, as explained in
[3. Dual stack scenarios](../3.%20Coexistence%20with%20Legacy%20IPv4/Dual%20stack%20scenarios.md),
IPv6 routing generally works independently of IPv4 routing, which was
indeed a fundamental design choice. However, if necessary, encapsulated
IPv4 traffic can be carried over an IPv6-only path. To enable this,
multiprotocol BGP-4 has provision to advertise IPv4 reachability over an
IPv6-only path \[[RFC8950](https://www.rfc-editor.org/info/rfc8950)\].

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