Forwarding information base

A forwarding information base (FIB), also known as a forwarding table or CAM table, is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should forward a packet. It is a dynamic table that maps MAC addresses to ports. It is the essential mechanism that separates network switches from network hubs.

Operation

An Ethernet switch's role is to copy Ethernet frames from one port to another. The presence of a CAM table is one attribute that separates a switch from a hub. Without a functional CAM table, all frames received by a network switch would be echoed back out to all other ports, much like an Ethernet hub. A switch should only emit a frame on the port where the destination network device resides (unicast), unless the frame is for all nodes on the switch (broadcast) or multiple nodes (multicast).

Generally, the CAM table is a system memory construct used by Ethernet switch logic to map a station's MAC address to the switch port the station is connected to. This allows switches to facilitate communications between connected stations at high speed regardless of how many devices are connected to the switch. The CAM table is consulted to make the frame forwarding decision. Switches learn MAC addresses from the source address of Ethernet frames on the ports, such as Address Resolution Protocol response packets.

Applications at data link layer

A data link layer technology, such as media access control (MAC) protocols on local area networks, has an address that has no significance beyond a single medium.

Besides Ethernet bridging based on MAC layer addresses, other data-link-layer technologies using forwarding tables include frame relay and Asynchronous Transfer Mode (ATM) switches, and multiprotocol label switching (MPLS). ATM has both link-local addresses and addresses that have end-to-end significance in the ATM domain.

Bridging

MAC layer bridges learn the interface on which they first saw a particular source address, and associate that interface with that address. When the bridge subsequently receives a frame with a destination address in its forwarding table, it sends the frame out the interface stored in the forwarding table.

If the bridge has not seen the address yet, it treats the frame as if it were a broadcast and floods it out all active interfaces except for the interface on which it was received.

Frame Relay

While the exact mechanics of a forwarding table is implementation-specific, the general model is that frame relay switches have statically defined forwarding tables, one per interface. When a frame with a given data link connection identifier (DLCI) is received on one interface, the table associated with that interface gives the outgoing interface, and the new DLCI to insert into the frame's address field.

Asynchronous Transfer Mode

ATM switches have link-level forwarding tables much like those used in frame relay. Rather than a DLCI, however, interfaces have forwarding tables that specify the outgoing interface, virtual path identifier, and virtual circuit identifier. These tables may be configured statically, or they can be distributed by the private network-to-network interface (PNNI) protocol. When PNNI is in use, the ATM switches at the edges of the ATM "cloud" will map one of the standard ATM end-to-end identifiers, such as an NSAP, to the next-hop VPI/VCI.

Multiprotocol Label Switching

MPLS, which has been called "ATM without cells",[1] has many similarities, at the forwarding level, to ATM. The label edge routers (LER) at the edges of an MPLS cloud map between the end-to-end identifier, such as an IP address, and a link-local label. At each MPLS hop, there is a forwarding table that tells the label switched router (LSR) which outgoing interface is to receive the MPLS packet, and what label to use when sending the packet out that interface.

Applications at network layer

Network layer addresses, such as IP addresses, are conceptually similar on all media in the routing domain.

In forwarding

In contrast to routing tables, FIBs are optimized for fast lookup of destination addresses. Earlier implementations cached only a subset of the routes most frequently used in actual forwarding, and this worked reasonably well for enterprises where there is a meaningful most-frequently-used subset. Routers used for accessing the entire Internet, however, experienced severe performance degradation in refreshing a small cache, and various implementations moved to having FIBs in one-to-one correspondence with the RIB.[2] RIBs are optimized for efficient updating by routing protocols and other control plane methods, and contain the full set of routes learned by the router.

FIBs may also be implemented with fast hardware lookup mechanisms, such as ternary content addressable memory (TCAM). TCAM, however, is quite expensive, and tends to be used more in edge routers with relatively small numbers of routes than in routers that must carry full Internet routing tables, with supplementary internal routes.[3]

In ingress filtering against denial of service

FIBs can also play a role in an Internet best current practice (BCP) of ingress filtering. Though the simplest form of implementing ingress filtering is to use access lists to drop packets with improper source addresses, use of access lists becomes difficult on routers with a large number of adjacent networks, and traditional access lists are not used in high-performance router forwarding paths.

While the IETF document BCP 38 on ingress filtering[4] does not specify a method of implementing source address filtering, some router vendors have implemented a mechanism which employs lookups in the router's tables to perform this check. (See also reverse path filtering.) This is often implemented as a lookup in the FIB of the source address of the packet. If the interface has no route to the source address, the packet is assumed to be part of a denial of service attack, using a false or spoofed source address, and the router discards the packet.

When the router is multihomed, ingress filtering becomes more complex. There are perfectly reasonable operational scenarios in which a packet could arrive on one interface, but that specific interface might not have a route to the source address. For the routers near the edge of the Internet, packet filters can provide a simpler and more effective solution than methods which employ routing information lookup, though this approach can be challenging when managing routers which are reconfigured often. Ingress filtering for multihomed routers will accept the packet if there is a route back to its source address from any interface on the router. For this type of filtering, the router may also maintain an adjacency table, also organized for fast lookup, that keeps track of the router interface addresses that are on all directly connected routers.[5]

In quality of service

IP differentiated services provides an additional method to select outgoing interfaces, based on a field that indicates the forwarding priority of the packet, as well as the preference of the packet to be dropped in the presence of congestion.[6]

Routers that support differentiated service not only have to look up the output interface for the destination address, but need to send the packet to the interface that best matches the differentiated services requirements. In other words, as well as matching the destination address, the FIB has to match differentiated services code points (DSCP).

FIB information for additional processing

Specific router implementations may, when a destination address or other FIB criterion is matched, specify other action to be done before forwarding (e.g., accounting or encryption), or applying an access control list that may cause the packet to be dropped.

Attacks

CAM tables are often the target of layer 2 network attacks in a local area network to set up man-in-the-middle attacks. A threat agent which has control of a device connected to an Ethernet switch can attack the switch's CAM table. This attack usually involves exploiting a vulnerability in switch design that appears when the switch runs out of space to record all of the MAC address to port mappings it learns. If the table fills up due to MAC flooding, most switches are no longer able to reliably add new MAC addresses.

References

  1. ↑ Interview with the author (of an MPLS-based VPN article), G. Pildush
  2. ↑ Wire Speed Packet Classification Without TCAM: One More Register (And A Bit Of Logic) Is Enough Q. Dong et al., ACM SIGCOMM 2006
  3. ↑ RAM lookup FIB & prefixes > /24, R. Whittle, Internet Research Task Force (IRTF) Routing Research Group mailing list, 2007
  4. ↑ Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing, RFC2827, P. Ferguson & D. Senie, May 2000
  5. ↑ Ingress Filtering for Multihomed Networks, RFC 3704, F. Baker & P. Savola,March 2004
  6. ↑ Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers, RFC 2474, K. Nichols et al.,December 1998

External links

RIBs and FIBs (aka IP Routing Table and CEF Table), Ivan Pepelnjak. http://blog.ipspace.net/2010/09/ribs-and-fibs.html

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