Source-Specific Routing in Babel

The Babel routing protocol is a distance vector routing protocol for next-hop routing. In next-hop routing, each node maintains a forwarding table which maps destination prefixes to next hops. The forwarding decision is a per-packet operation which depends on the destination address of the packets and on the entries of the forwarding table. When a packet is about to be routed, its destination address is compared to the prefixes of the routing table: the entry with the most specific prefix containing the destination address of the packet is chosen, and the packet is forwarded to the associated next-hop. Next-hop routing is a simple, well understood paradigm that works satisfactorily in a large number of cases. Source-specific routing , or Source Address Dependent Routing (SADR), is a modest extension to next-hop routing where the forwarding decision depends not only on the destination address but also on the source address of the packet being routed, which makes it possible for two packets with the same destination but different source addresses to be routed following different paths. The forwarding tables are extended to map pairs of prefixes (destination, source) to next hops. When multiple entries match a given packet, the one with the most specific destination prefix is chosen, and, in the case of equally specific destination prefixes, the one with the most specific source prefix. The main application of source-specific routing is a form of multihoming known as multihoming with multiple addresses. When using this technique in a network connected to multiple providers, every host is assigned multiple addresses, one per provider. When a host sources a packet, it picks one of its addresses as the source address, and source-specific routing is used to route the packet to an edge router that is connected to the corresponding provider, which is compatible with . Unlike classical multihoming, this technique is applicable to small networks, as it does not require the use of provider-independent addresses, or cause excessive growth of the global routing table. More details are given in . This document describes a source-specific routing extension for the Babel routing protocol . This involves minor changes to the data structures, which must include a source prefix in addition to the destination prefix already present, and some changes to the Update, Route Request and Seqno Request TLVs, which are extended with a source prefix. The source prefix is encoded using a mandatory sub-TLV ( Section 4.4).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

A number of the conceptual data structures described in Section 3.2 of contain a destination prefix. This specification extends these data structures with a source prefix. Data from the original protocol, which do not specify a source prefix, are stored with a zero length source prefix, which matches exactly the same set of packets as the original, non-source-specific data.

Every Babel node maintains a source table, as described in Section 3.2.5. A source-specific Babel node extends this table with the following field: The source prefix specifying the source address of packets to which this entry applies. The source table is now indexed by triples of the form (prefix, source prefix, router-id). Note that the route entry contains a source (see sections 2 and 3.2.5 of ) which itself contains a source prefix. These are two very different concepts that should not be confused.

Every Babel node maintains a route table, as described in Section 3.2.6. Each route table entry contains, among other data, a source, which this specification extends with a source prefix as described above. The route table is now indexed by triples of the form (prefix, source prefix, neighbour), where the prefix and source prefix are obtained from the source.

Every Babel node maintains a table of pending seqno requests, as described in , Section 3.2.7. A source-specific Babel node extends this table with the following entry: The source prefix being requested. The table of pending seqno requests is now indexed by triples of the form (prefix, source prefix, router-id).

In next-hop routing, if two routing table entries overlap, then one is necessarily more specific than the other; the "longest prefix rule" specifies that the most specific applicable routing table entry is chosen. With source-specific routing, there might no longer be a most specific applicable entry: two routing table entries might match a given packet without one necessarily being more specific than the other. Consider for example the following routing table:

This specifies that all packets with destination in 2001:DB8:0:1::/64 are to be routed through A, while all packets with source in 2001:DB8:0:2::/64 are to be routed through B. A packet with source 2001:DB8:0:2::42 and destination 2001:DB8:0:1::57 matches both entries, although neither is more specific than the other. A choice is necessary, and unless the choice being made is the same on all routers in a routing domain, persistent routing loops may occur. More details are given in Section IV.C of . A Babel implementation MUST choose routing table entries by using the so-called destination-first ordering, where a routing table entry R1 is preferred to a routing table entry R2 when either R1's destination prefix is more specific than R2's, or the destination prefixes are equal and R1's source prefix is more specific than R2's. (In other words, routing table entries are compared using the lexicographic product of the destination prefix ordering by the source prefix ordering.) In practice, this means that a source-specific Babel implementation must take care that any lower layer that performs packet forwarding obey this semantics. More precisely: If the lower layers implement the destination-first ordering, then the Babel implementation SHOULD use them directly; If the lower layers can hold source-specific routes, but not with the right semantics, then the Babel implementation MUST either silently ignore any source-specific routes, or disambiguate the routing table by using a suitable disambiguation algorithm (see Section V.B of for such an algorithm); If the lower layers cannot hold source-specific routes, then a Babel implementation MUST silently ignore any source-specific routes.

This extension does not fundamentally change the operation of the Babel protocol, and we therefore only describe differences between the original protocol and the extended protocol. In the original protocol, three TLVs carry a destination prefix: Updates, Route Requests and Seqno Requests. This specification extends these messages so that they may carry a Source Prefix sub-TLV, as described in below. The sub-TLV is marked as mandatory, so that an unextended implementation will silently ignore the whole enclosing TLV. A node obeying this specification MUST NOT send a TLV with a zero-length source prefix: instead, it sends a TLV with no Source Prefix sub-TLV. Conversely, an extended implementation MUST interpret an unextended TLV as carrying a source prefix of zero length. Taken together, these properties ensure interoperability between the original and extended protocols (see below).

This extension allows three TLVs of the original Babel protocol to carry a source prefix: Update TLVs, Route Request TLVs and Seqno Request TLVs. In order to advertise a route with a non-zero length source prefix, a node sends a source-specific Update, i.e., an Update with a Source Prefix sub-TLV. When a node receives a source-specific Update (prefix, source prefix, router-id, seqno, metric) from a neighbour neigh, it behaves as described in Section 3.5.4, except that the entry under consideration is indexed by (prefix, source prefix, neigh) rather than just (prefix, neigh). Similarly, when a node needs to send a Request of either kind that applies to a route with a non-zero length source prefix, it sends a source-specific Request, i.e., a Request with a Source Prefix sub-TLV. When a node receives a source-specific Request, it behaves as described in Section 3.8 of , except that the request applies to the Route Table entry carrying the source prefix indicated by the Source Prefix sub-TLV.

In the original protocol, the Address Encoding value 0 is used for wildcard messages: messages that apply to all routes, of any address family and with any destination prefix. Wildcard messages are allowed in two places in the protocol: wildcard retractions are used to retract all of the routes previously advertised by a node on a given interface, and wildcard Route Requests are used to request a full dump of the Route Table from a given node. Wildcard messages are intended to apply to all routes, including routes decorated with additional data and AE values to be defined by future extensions, and hence this specification extends wildcard operations to apply to all routes, whatever the value of the source prefix. More precisely, a node receiving an Update with the AE field set to 0 and the Metric field set to infinity (a wildcard retraction) MUST apply the route acquisition procedure described in Section 3.5.4 of to all of the routes that it has learned from the sending node, whatever the value of the source prefix. A node MUST NOT send a wildcard retraction with an attached source prefix, and a node that receives a wildcard retraction with a source prefix MUST ignore it. Similarly, a node that receives a route request with the AE field set to 0 (a wildcard route request) SHOULD send a full routing table dump, including routes with a non-zero length source prefix. A node MUST NOT send a wildcard request that carries a source prefix, and a node receiving a wildcard request with a source prefix MUST ignore it.

The protocol extension defined in this document is, to a great extent, interoperable with the base protocol defined in (and all previously standardised extensions). More precisely, if non-source-specific routers and source-specific routers are mixed in a single routing domain, Babel's loop-avoidance properties are preserved, and, in particular, no persistent routing loops will occur. However, this extension is encoded using mandatory sub-TLVs, introduced in , and therefore is not compatible with the older version of the Babel Routing Protocol which does not support such sub-TLVs. Consequently, this extension MUST NOT be used with routers implementing RFC 6126, otherwise persistent routing loops may occur.

The extension defined in this protocol uses a new Mandatory sub-TLV to carry the source prefix information. As discussed in Section 4.4 of , this encoding ensures that non-source-specific routers will silently ignore the whole TLV, which is necessary to avoid persistent routing loops in hybrid networks. Consider two nodes A and B, with A source-specific announcing a route to (D, S). Suppose that B (non-source-specific) merely ignores the source prefix information when it receives the update rather than ignoring the whole TLV, and re-announces the route as D. This re-announcement reaches A, which treats it as (D, ::/0). Packets destined to D but not sourced in S will be forwarded by A to B, and by B to A, causing a persistent routing loop:

(D,::/0) ]]>

In general, the discarding of source-specific routes by non-source-specific routers will cause route starvation. Intuitively, unless there are enough non-source-specific routes in the network, non-source-specific routers will suffer starvation, and discard packets for destinations that are only announced by source-specific routers. A simple yet sufficient condition for avoiding starvation is to build a connected source-specific backbone that includes all of the edge routers, and announce a (non-source-specific) default route towards the backbone.

This extension defines a new sub-TLV used to carry a source prefix: the Source Prefix sub-TLV. It can be used within an Update, a Route Request or a Seqno Request TLV to match a source-specific entry of the Route Table, in conjunction with the destination prefix natively carried by these TLVs. Since a source-specific routing entry is characterized by a single destination prefix and a single source prefix, a source-specific message contains exactly one Source Prefix sub-TLV. A node MUST NOT send more than one Source Prefix sub-TLV in a TLV, and a node receiving more than one Source Prefix sub-TLV in a single TLV MUST ignore this TLV. It MAY ignore the whole packet.

Fields: Set to 128 to indicate a Source Prefix sub-TLV. The length of the body, in octets, exclusive of the Type and Length fields. The length of the advertised source prefix. This MUST NOT be 0. The source prefix being advertised. This field's size is (Source Plen)/8 octets rounded upwards. The contents of the Source Prefix sub-TLV are interpreted according to the AE of the enclosing TLV. If a TLV with AE equal to 0 contains a Source Prefix sub-TLV, then the whole TLV MUST be ignored. Similarly, if a TLV contains multiple Source Prefix sub-TLVs, then the whole TLV MUST be ignored. Note that this sub-TLV is a mandatory sub-TLV. Therefore, as described in Section 4.4 of , the whole TLV MUST be ignored if that sub-TLV is not understood (or malformed). Otherwise, routing loops may occur (see ).

The source-specific Update is an Update TLV with a Source Prefix sub-TLV. It advertises or retracts source-specific routes in the same manner as routes with non-source-specific Updates (see ). A wildcard retraction (Update with AE equal to 0) MUST NOT carry a Source Prefix sub-TLV. Babel uses a stateful compression scheme to reduce the size taken by destination prefixes in update TLVs (see Section 4.5 of ). The source prefix defined by this extension is not compressed. On the other hand, compression is allowed for the destination prefixes carried by source-specific updates. As described in Section 4.5 of , unextended implementations will correctly update their parser state while otherwise ignoring the whole TLV.

A source-specific Route Request is a Route Request TLV with a Source Prefix sub-TLV. It prompts the receiver to send an update for a given pair of destination and source prefixes, as described in Section 3.8.1.1 of . A wildcard request (Route Request with AE equals to 0) MUST NOT carry a Source Prefix sub-TLV; if a wildcard request with a Source Prefix sub-TLV is received, then the request MUST be ignored.

A source-specific Seqno Request is a Seqno Request TLV with a Source Prefix sub-TLV. It requests the receiving node to perform the procedure described in Section 3.8.1.2 of , but applied to a pair of a destination and source prefix.

IANA has allocated sub-TLV number 128 for the Source Prefix sub-TLV in the Babel sub-TLV types registry.

The extension defined in this document adds a new sub-TLV to three TLVs already present in the original Babel protocol, and does not change the security properties of the protocol itself. However, the additional flexibility provided by source-specific routing might invalidate the assumptions made by some network administrators, which could conceivably lead to security issues. For example, a network administrator might be tempted to abuse route filtering (Appendix C of ) as a security mechanism. Unless the filtering rules are designed to take source-specific routing into account, they might be bypassed by a source-specific route, which might cause traffic to reach a portion of a network that was thought to be protected. Similarly, a network administrator might assume that no route is more specific than a host route, and use a host route in order to direct traffic for a given destination through a security device (e.g., a firewall); source-specific routing invalidates this assumption, and in some topologies announcing a source-specific route might conceivably be used to bypass the security device.

The authors are grateful to Donald Eastlake and Joel Halpern for their help with this document.