Internet Draft Routing Aspects Of IPv 6 Transition November 1994

This paper discusses routing aspects associated with the transition from IPv4 to IPv6. The approach outlined here is designed to be compatible with the Simple Internet Transition (SIT) mechanism. The proposals contained in this document are the opinions of the authors, and have not yet been discussed in detail by the working group. This document is intended as input to the IPNG, Tacit, and Ngtrans working groups. Internet Draft Routing Aspects Of IPv6 Transition November 1994 Expires May 1995 [Page 2] 1 Terminology This paper uses the following terminology: Node A protocol module that implements IPv6. Router A node that forwards packets not explicitly addressed to itself. Host Any node that is not a router. Link A communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below network layer. Interface A node’s attachment to a link. Address An network layer identifier for an interface or a group of interfaces. Neighbors Nodes attached to the same link. Routing Domain A collection of routers which coordinate the 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 can be a subset or a superset of a routing domain. Border Router A router that forwards packets across routing region boundaries. Tunneling Encapsulation of protocol A within protocol B, such that A treats B as though it were a datalink layer. Internet Draft Routing Aspects Of IPv6 Transition November 1994 Expires May 1995 [Page 3] 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 locate a link for delivery a datagram. Route Leaking Advertisement of network layer reachability information across routing region boundaries. Translating Router A dual (IPv4/IPv6) protocol router that is capable translate IPv4 packet headers into IPv6 headers and vise versa. 2 Issues And Outline This internet draft gives an initial overview of the routing aspects of IPv4 to IPv6 transition. The approach outlined here is designed to be compatible with the Simple IPv6 Transition (SIT) [4][5]. During an extended IPv4-to-IPv6 migration 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 an 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 an IPv6 infrastructure. Internet Draft Routing Aspects Of IPv6 Transition November 1994 Expires May 1995 [Page 4] Similarly, at some stages of transition, translation between IPv4 and IPv6 packet formats may be necessary in order to allow IPv6-only systems to talk with IPv4-only systems. The SIT transition provides a dual-stack transition, augmented by use of encapsulation and translation where necessary and appropriate. Routing issues related to this transition include: (1) Routing for IPv4 packets (2) Routing for IPv6 packet (2a) IPv6 packets with IPv4-incompatible addresses (2b) IPv6 packets with IPv4-compatible addresses (3) Operation of manually configured static tunnels (4) Operation of automatic encapsulation and translation (4a) Locating encapsulators and translators (4b) Ensuring that routing is consist with encapsulation and translation Basic mechanisms required to accomplish these goals include: (i) Dual Stack Route Computation; (ii) Manual configuration of tunnels; and (iii) Route Leaking to support translation and automatic encapsulation. The basic mechanism for routing of IPv4 and IPv6 involves dual-stack routing. This implies that routes are separately calculated for IPv4 addresses and for IPv6 addresses. This may be done either by running a separate routing protocol for each, or for running one “integrated” routing protocol which calculates routes for both IPv4 and IPv6. For example, one instance of OSPF might be used, with a single topology (a single area structure, a single router ID for each router, a single DR per LAN, etc.) to calculate routes for both IPv4 and IPv6. 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 regions 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 3.1. Use of automatic encapsulation and translation requires consistency of routes between IPv4 routes and IPv6 routes for destinations using IPv4-compatible addresses. For Internet Draft Routing Aspects Of IPv6 Transition November 1994 Expires May 1995 [Page 5] example, consider a packet which starts off as an IPv6 packet, but then is translated into an IPv4 packet in the middle of its path from source to destination. This packet must locate a translator 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 sections 3.2 and 4. 3 Route Dissemination Techniques 3.1 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. IP tunneling is an encapsulation of arbitrary packets inside IP datagrams that are forwarded over IP infrastructure between tunnel endpoints. For a tunneled protocol, a tunnel appears as a single-hop link (i.e. routers that establish a 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. In order for a IPv6 router to be able to encapsulate IPv6 packets into IPv4 datagrams and to forward encapsulated packet over a IPv4 tunnel, such a router must also be able to speak IPv4 (i.e. be a dual protocol router). The route tunnelling techniques call for dual protocol routers to be deployed at demarcation points between adjacent IPv6 and IPv4-only routing regions: Forwarding of IPv6 packets between IPv6 routing regions is straightforward -when a packet reaches a border router, the border router examines it’s routing database to find an interface to the next-hop router. If the forwarding interface is connected to a tunneled link, the packet must be encapsulated into an IPv4 datagram according to the adopted encapsulation scheme, and the resulting datagram is forwarded over the tunnel. Intermediary IPv4 routers between the tunnel endpoints forward the datagram IPv6 region IPv6 region tunnel IPv4 region dual routers Internet Draft Routing Aspects Of IPv6 Transition November 1994 Expires May 1995 [Page 6] as they would any other IPv4 datagram, using information in the encapsulating header. The recipient border router, in an adjacent IPv6 region, strips off the encapsulating header and forwards the original IPv6 packets toward its ultimate destination. Note that the destination and source addresses in the encapsulating header are the IPv4 addresses of the tunnel endpoints, not derivatives of the destination and source addresses of the encapsulated IPv6 packet. In the route tunneling scheme described here, there is a complete separation of the IPv6 network reachability information from the IPv4 routing information; i.e. this is a dual stack approach. Nevertheless, an IPv4 routing region and an IPv6 routing region can overlap. Each such region can share routers which support both IPv4 and IPv6 routing. The downside of the dual stack approach is that it does not support a dynamic end-to-end routing of data sent between an IPv4-only node and an IPv6-only node with an IPv4-compatible address. [It is already accepted that IPv4-only nodes will not be able to interoperate with IPv6-only nodes with IPv4-incompatible addresses.] When sending packets between IPv4-only and IPv6-only nodes, due to the routing information separation, the route to the destination node is not available to the routing protocol at the source node. Therefore, packets first have to be sent to a default translating dual protocol router which then can dynamically route converted packets to their ultimate destinations. There are the following advantages to employ encapsulation for exchanging routing information: • The underline infrastructure which furnishes a tunnel link is transparent to protocols that are bridged with the tunnel (i.e. there is no changes to bridged protocols). • All types of IPv6 route prefixes without exception can be advertised with routing protocols. Therefore, no restriction need to be imposed on formats of the addresses in IPv6 packets that can be routed with this scheme. • If a connectivity between IPv6 nodes is all that needed, only border routers at boundaries with IPv4-only routing regions need to be dual protocol routers. • Since IPv6 packets are encapsulated only when they travel over network segments that don’t support IPv6, and are forwarded according to their native headers anywhere else, all kind of policy routing can be employed over the entire IPv6 portion of data path. • Routers from major vendors already support the multiprotocol operation that is needed at tunnel endpoints. Internet Draft Routing Aspects Of IPv6 Transition November 1994 Expires May 1995 [Page 7] The disadvantages of tunneling are that: • They need to be manually configured. • They may circumvent security firewalls of the encapsulating infrastructure -only addresses of the tunnel endpoints are normally visible to such firewalls, not actual attributes of encapsulated packets. Note that the same stands true for “automatic” tunneling which is used in the route leaking scheme. • Since a tunnel may appear as a one-hop link, some routing protocols may prefer a tunnel over a real multi-hop link. Therefore, IPv6 packets may be routed across an IPv4 routing region even when an alternative homogeneous IPv6 path is available. Note that this disadvantage may be eliminated by using a routing protocol such as OSPF which allows a considerable dynamic range of metric values to be assigned to links. Though this section describes tunneling of IPv6 routing information and IPv6 packets over an IPv4 infrastructure, when a need arises for IPv4 routing regions to communicate via an IPv6 routing infrastructure, the similar tunneling technique can be used -except IPv4 packets will be encapsulated within IPv6 datagrams.