rfc2185.txt

<VC++网络游戏建摸与实现>源代码

Network Working Group                                          R. Callon
Request for Comments: 2185                    Cascade Communications Co.
Category: Informational                                        D. Haskin
                                                       Bay Networks Inc.
                                                          September 1997


                   Routing Aspects Of IPv6 Transition

Status of this memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document gives an overview of the routing aspects of the IPv6
   transition.  It is based on the protocols defined in the document
   "Transition Mechanisms for IPv6 Hosts and Routers" [1].  Readers
   should be familiar with the transition mechanisms before reading this
   document.

   The proposals contained in this document are based on the work of the
   Ngtrans working group.

1. TERMINOLOGY

   This paper uses the following terminology:

   node      - a protocol module that implements IPv4 or IPv6.

   router    - a node that forwards packets not explicitly
               addressed to itself.

   host      - any node that is not a router.

   border router - a router that forwards packets across
               routing domain boundaries.

   link      - a communication facility or medium over which
               nodes can communicate at the link layer, i.e., the layer
               immediately below internet layer.

   interface - a node's attachment to a link.

   address   - an network layer identifier for an interface or
               a group of interfaces.



Callon & Haskin              Informational                      [Page 1]

RFC 2185           Routing Aspects Of IPv6 Transition     September 1997


   neighbors - nodes attached to the same link.

   routing domain - a collection of routers which coordinate
               routing knowledge using a single routing protocol.

   routing region (or just "region")  - a collection of routers
               interconnected by a single internet protocol (e.g. IPv6)
               and coordinating their routing knowledge using routing
               protocols from a single internet protocol stack. A
               routing region may be a superset of a routing domain.

   tunneling  - encapsulation of protocol A within protocol B,
               such that A treats B as though it were a datalink layer.

   reachability information - information describing the set of
               reachable destinations that can be used for packet
               forwarding decisions.

   routing information - same as reachability information.

   address prefix - the high-order bits in an address.

   routing prefix - address prefix that expresses destinations
               which have addresses with the matching address prefixes.
               It is used by routers to advertise what systems they are
               capable of reaching.

   route leaking - advertisement of network layer reachability
               information across routing region boundaries.

2. ISSUES AND OUTLINE

   This document gives an overview of the routing aspects of IPv4 to
   IPv6 transition. The approach outlined here is designed to be
   compatible with the existing mechanisms for IPv6 transition [1].

   During an extended IPv4-to-IPv6 transition period, IPv6-based systems
   must coexist with the installed base of IPv4 systems. In such a dual
   internetworking protocol environment, both IPv4 and IPv6 routing
   infrastructure will be present. Initially, deployed IPv6-capable
   domains might not be globally interconnected via IPv6-capable
   internet infrastructure and therefore may need to communicate across
   IPv4-only routing regions. In order to achieve dynamic routing in
   such a mixed environment, there need to be mechanisms to globally
   distribute IPv6 network layer reachability information between
   dispersed IPv6 routing regions. The same techniques can be used in
   later stages of IPv4-to-IPv6 transition to route IPv4 packets between
   isolated IPv4-only routing region over IPv6 infrastructure.



Callon & Haskin              Informational                      [Page 2]

RFC 2185           Routing Aspects Of IPv6 Transition     September 1997


   The IPng transition provides a dual-IP-layer transition, augmented by
   use of encapsulation where necessary and appropriate. Routing issues
   related to this transition include:

   (1) Routing for IPv4 packets

   (2) Routing for IPv6 packets
           (2a) IPv6 packets with IPv6-native addresses
           (2b) IPv6 packets with IPv4-compatible addresses

   (3) Operation of manually configured static tunnels

   (4) Operation of automatic encapsulation
           (4a) Locating encapsulators
           (4b) Ensuring that routing is consist with
               encapsulation

   Basic mechanisms required to accomplish these goals include: (i)
   Dual-IP-layer Route Computation; (ii) Manual configuration of point-
   to-point tunnels; and (iii) Route leaking to support automatic
   encapsulation.

   The basic mechanism for routing of IPv4 and IPv6 involves dual-IP-
   layer routing. This implies that routes are separately calculated for
   IPv4 addresses and for IPv6 addressing. This is discussed in more
   detail in section 3.1.

   Tunnels (either IPv4 over IPv6, or IPv6 over IPv4) may be manually
   configured. For example, in the early stages of transition this may
   be used to allow two IPv6 domains to interact over an IPv4
   infrastructure. Manually configured static tunnels are treated as if
   they were a normal data link. This is discussed in more detail in
   section 3.2.

   Use of automatic encapsulation, where the IPv4 tunnel endpoint
   address is determined from the IPv4 address embedded in the IPv4-
   compatible destination address of IPv6 packet, requires consistency
   of routes between IPv4 and IPv6 routing domains for destinations
   using IPv4-compatible addresses. For example, consider a packet which
   starts off as an IPv6 packet, but then is encapsulated in an IPv4
   packet in the middle of its path from source to destination. This
   packet must locate an encapsulator at the correct part of its path.
   Also, this packet has to follow a consistent route for the entire
   path from source to destination. This is discussed in more detail in
   section 3.3.

   The mechanisms for tunneling IPv6 over IPv4 are defined in the
   transition mechanisms specification [1].



Callon & Haskin              Informational                      [Page 3]

RFC 2185           Routing Aspects Of IPv6 Transition     September 1997


3. MORE DETAIL OF BASIC APPROACHES

3.1 Basic Dual-IP-layer Operation

   In the basic dual-IP-layer transition scheme, routers may
   independently support IPv4 and IPv6 routing. Other parts of the
   transition, such as DNS support, and selection by the source host of
   which packet format to transmit (IPv4 or IPv6) are discussed in [1].
   Forwarding of IPv4 packets is based on routes learned through running
   IPv4-specific routing protocols. Similarly, forwarding of IPv6
   packets (including IPv6-packets with IPv4-compatible addresses) is
   based on routes learned through running IPv6-specific routing
   protocols. This implies that separate instances of routing protocols
   are used for IPv4 and for IPv6 (although note that this could consist
   of two instances of OSPF and/or two instances of RIP, since both OSPF
   and RIP are capable of supporting both IPv4 and IPv6 routing).

   A minor enhancement would be to use an single instance of an
   integrated routing protocol to support routing for both IPv4 and
   IPv6.  At the time that this is written there is no protocol which
   has yet been enhanced to support this. This minor enhancement does
   not change the basic dual-IP-layer nature of the transition.

   For initial testing of IPv6 with IPv4-compatible addresses, it may be
   useful to allow forwarding of IPv6 packets without running any IPv6-
   compatible routing protocol. In this case, a dual (IPv4 and IPv6)
   router could run routing protocols for IPv4 only. It then forwards
   IPv4 packets based on routes learned from IPv4 routing protocols.
   Also, it forwards IPv6 packets with an IPv4-compatible destination
   address based on the route for the associated IPv4 address. There are
   a couple of drawbacks with this approach: (i) It does not
   specifically allow for routing of IPv6 packets via IPv6-capable
   routers while avoiding and routing around IPv4-only routers; (ii) It
   does not produce routes for "non-compatible" IPv6 addresses. With
   this method the routing protocol does not tell the router whether
   neighboring routers are IPv6-compatible. However, neighbor discovery
   may be used to determine this. Then if an IPv6 packet needs to be
   forwarded to an IPv4-only router it can be encapsulated to the
   destination host.

3.2 Manually Configured Static Tunnels

   Tunneling techniques are already widely deployed for bridging non-IP
   network layer protocols (e.g. AppleTalk, CLNP, IPX) over IPv4 routed
   infrastructure. IPv4 tunneling is an encapsulation of arbitrary
   packets inside IPv4 datagrams that are forwarded over IPv4
   infrastructure between tunnel endpoints. For a tunneled protocol, a
   tunnel appears as a single-hop link (i.e. routers that establish a



Callon & Haskin              Informational                      [Page 4]

RFC 2185           Routing Aspects Of IPv6 Transition     September 1997


   tunnel over a network layer infrastructure can inter-operate over the
   tunnel as if it were a one-hop, point-to-point link). Once a tunnel
   is established, routers at the tunnel endpoints can establish routing
   adjacencies and exchange routing information.  Describing the
   protocols for performing encapsulation is outside the scope of this
   paper (see [1]).  Static point-to-point tunnels may also be
   established between a host and a router, or between two hosts. Again,
   each manually configured point-to-point tunnel is treated as if it
   was a simple point-to-point link.

3.3  Automatic Tunnels

   Automatic tunneling may be used when both the sending and destination
   nodes are connected by IPv4 routing.  In order for automatic