3.1 Switching and Bridging

In the simplest terms, a switch is a mechanism that allows us to interconnect links to form a larger network. A switch is a multi-input, multi-output device that transfers packets from an input to one or more outputs. Thus, a switch adds the star topology (see Figure 1) to the set of possible network structures. A star topology has several attractive properties:

  • Even though a switch has a fixed number of inputs and outputs, which limits the number of hosts that can be connected to a single switch, large networks can be built by interconnecting a number of switches.

  • We can connect switches to each other and to hosts using point-to-point links, which typically means that we can build networks of large geographic scope.

  • Adding a new host to the network by connecting it to a switch does not necessarily reduce the performance of the network for other hosts already connected.

A switch provides a star topology.

This last claim cannot be made for the shared-media networks discussed in the last chapter. For example, it is impossible for two hosts on the same 10-Mbps Ethernet segment to transmit continuously at 10 Mbps because they share the same transmission medium. Every host on a switched network has its own link to the switch, so it may be entirely possible for many hosts to transmit at the full link speed (bandwidth), provided that the switch is designed with enough aggregate capacity. Providing high aggregate throughput is one of the design goals for a switch; we return to this topic later. In general, switched networks are considered more scalable (i.e., more capable of growing to large numbers of nodes) than shared-media networks because of this ability to support many hosts at full speed.

A switch is connected to a set of links and, for each of these links, runs the appropriate data link protocol to communicate with the node at the other end of the link. A switch's primary job is to receive incoming packets on one of its links and to transmit them on some other link. This function is sometimes referred to as either switching or forwarding, and in terms of the Open Systems Interconnection (OSI) architecture, it is the main function of the network layer, otherwise known as Layer 2.

The question, then, is how does the switch decide which output link to place each packet on? The general answer is that it looks at the header of the packet for an identifier that it uses to make the decision. The details of how it uses this identifier vary, but there are two common approaches. The first is the datagram or connectionless approach. The second is the virtual circuit or connection-oriented approach. A third approach, source routing, is less common than these other two, but it does have some useful applications.

One thing that is common to all networks is that we need to have a way to identify the end nodes. Such identifiers are usually called addresses. We have already seen examples of addresses, such as the 48-bit address used for Ethernet. The only requirement for Ethernet addresses is that no two nodes on a network have the same address. This is accomplished by making sure that all Ethernet cards are assigned a globally unique identifier. For the following discussions, we assume that each host has a globally unique address. Later on, we consider other useful properties that an address might have, but global uniqueness is adequate to get us started.

Another assumption that we need to make is that there is some way to identify the input and output ports of each switch. There are at least two sensible ways to identify ports: One is to number each port, and the other is to identify the port by the name of the node (switch or host) to which it leads. For now, we use numbering of the ports.

Datagrams

The idea behind datagrams is incredibly simple: You just include in every packet enough information to enable any switch to decide how to get it to its destination. That is, every packet contains the complete destination address. Consider the example network illustrated in Figure 2, in which the hosts have addresses A, B, C, and so on. To decide how to forward a packet, a switch consults a forwarding table (sometimes called a routing table), an example of which is depicted in Table 1. This particular table shows the forwarding information that switch 2 needs to forward datagrams in the example network. It is pretty easy to figure out such a table when you have a complete map of a simple network like that depicted here; we could imagine a network operator configuring the tables statically. It is a lot harder to create the forwarding tables in large, complex networks with dynamically changing topologies and multiple paths between destinations. That harder problem is known as routing and is the topic of a later section. We can think of routing as a process that takes place in the background so that, when a data packet turns up, we will have the right information in the forwarding table to be able to forward, or switch, the packet.

Datagram forwarding: an example network.

Destination Port
A 3
B 0
C 3
D 3
E 2
F 1
G 0
H 0

Table 1. Forwarding Table for Switch 2.

Datagram networks have the following characteristics:

  • A host can send a packet anywhere at any time, since any packet that turns up at a switch can be immediately forwarded (assuming a correctly populated forwarding table). For this reason, datagram networks are often called connectionless; this contrasts with the connection-oriented networks described below, in which some connection state needs to be established before the first data packet is sent.

  • When a host sends a packet, it has no way of knowing if the network is capable of delivering it or if the destination host is even up and running.

  • Each packet is forwarded independently of previous packets that might have been sent to the same destination. Thus, two successive packets from host A to host B may follow completely different paths (perhaps because of a change in the forwarding table at some switch in the network).

  • A switch or link failure might not have any serious effect on communication if it is possible to find an alternate route around the failure and to update the forwarding table accordingly.

This last fact is particularly important to the history of datagram networks. One of the important design goals of the Internet is robustness to failures, and history has shown it to be quite effective at meeting this goal.

Virtual Circuit Switching

A second technique for packet switching, which differs significantly from the datagram model, uses the concept of a virtual circuit (VC). This approach, which is also referred to as a connection-oriented model, requires setting up a virtual connection from the source host to the destination host before any data is sent. To understand how this works, consider Figure 3, where host A again wants to send packets to host B. We can think of this as a two-stage process. The first stage is "connection setup." The second is data transfer. We consider each in turn.

An example of a virtual circuit network.

In the connection setup phase, it is necessary to establish a "connection state" in each of the switches between the source and destination hosts. The connection state for a single connection consists of an entry in a "VC table" in each switch through which the connection passes. One entry in the VC table on a single switch contains:

  • A virtual circuit identifier (VCI) that uniquely identifies the connection at this switch and which will be carried inside the header of the packets that belong to this connection

  • An incoming interface on which packets for this VC arrive at the switch

  • An outgoing interface in which packets for this VC leave the switch

  • A potentially different VCI that will be used for outgoing packets

The semantics of one such entry is as follows: If a packet arrives on the designated incoming interface and that packet contains the designated VCI value in its header, then that packet should be sent out the specified outgoing interface with the specified outgoing VCI value having been first placed in its header.

Note that the combination of the VCI of packets as they are received at the switch and the interface on which they are received uniquely identifies the virtual connection. There may of course be many virtual connections established in the switch at one time. Also, we observe that the incoming and outgoing VCI values are generally not the same. Thus, the VCI is not a globally significant identifier for the connection; rather, it has significance only on a given link (i.e., it has link-local scope).

Whenever a new connection is created, we need to assign a new VCI for that connection on each link that the connection will traverse. We also need to ensure that the chosen VCI on a given link is not currently in use on that link by some existing connection.

There are two broad approaches to establishing connection state. One is to have a network administrator configure the state, in which case the virtual circuit is "permanent." Of course, it can also be deleted by the administrator, so a permanent virtual circuit (PVC) might best be thought of as a long-lived or administratively configured VC. Alternatively, a host can send messages into the network to cause the state to be established. This is referred to as signalling, and the resulting virtual circuits are said to be switched. The salient characteristic of a switched virtual circuit (SVC) is that a host may set up and delete such a VC dynamically without the involvement of a network administrator. Note that an SVC should more accurately be called a signalled VC, since it is the use of signalling (not switching) that distinguishes an SVC from a PVC.

Let's assume that a network administrator wants to manually create a new virtual connection from host A to host B. First, the administrator needs to identify a path through the network from A to B. In the example network of Figure 3, there is only one such path, but in general this may not be the case. The administrator then picks a VCI value that is currently unused on each link for the connection. For the purposes of our example, let's suppose that the VCI value 5 is chosen for the link from host A to switch 1, and that 11 is chosen for the link from switch 1 to switch 2. In that case, switch 1 needs to have an entry in its VC table configured as shown in Table 2.

Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
2 5 1 11

Table 2. Example Virtual Circuit Table Entry for Switch 1.

Similarly, suppose that the VCI of 7 is chosen to identify this connection on the link from switch 2 to switch 3 and that a VCI of 4 is chosen for the link from switch 3 to host B. In that case, switches 2 and 3 need to be configured with VC table entries as shown in Table 3. Note that the "outgoing" VCI value at one switch is the "incoming" VCI value at the next switch.

VC Table Entry at Switch 2:

Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
3 11 2 7

VC Table Entry at Switch 3:

Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
0 7 1 4

Table 3. Virtual Circuit Table Entries for Switches 2 and 3.

A packet is sent into a virtual circuit network.

Once the VC tables have been set up, the data transfer phase can proceed, as illustrated in Figure 4. For any packet that it wants to send to host B, A puts the VCI value of 5 in the header of the packet and sends it to switch 1. Switch 1 receives any such packet on interface 2, and it uses the combination of the interface and the VCI in the packet header to find the appropriate VC table entry. As shown in Table, the table entry in this case tells switch 1 to forward the packet out of interface 1 and to put the VCI value 11 in the header when the packet is sent. Thus, the packet will arrive at switch 2 on interface 3 bearing VCI 11. Switch 2 looks up interface 3 and VCI 11 in its VC table (as shown in Table) and sends the packet on to switch 3 after updating the VCI value in the packet header appropriately, as shown in Figure 5. This process continues until it arrives at host B with the VCI value of 4 in the packet. To host B, this identifies the packet as having come from host A.

In real networks of reasonable size, the burden of configuring VC tables correctly in a large number of switches would quickly become excessive using the above procedures. Thus, either a network management tool or some sort of signalling (or both) is almost always used, even when setting up "permanent" VCs. In the case of PVCs, signalling is initiated by the network administrator, while SVCs are usually set up using signalling by one of the hosts. We consider now how the same VC just described could be set up by signalling from the host.

A packet makes its way through a virtual circuit network.

To start the signalling process, host A sends a setup message into the network—that is, to switch 1. The setup message contains, among other things, the complete destination address of host B. The setup message needs to get all the way to B to create the necessary connection state in every switch along the way. We can see that getting the setup message to B is a lot like getting a datagram to B, in that the switches have to know which output to send the setup message to so that it eventually reaches B. For now, let's just assume that the switches know enough about the network topology to figure out how to do that, so that the setup message flows on to switches 2 and 3 before finally reaching host B.

When switch 1 receives the connection request, in addition to sending it on to switch 2, it creates a new entry in its virtual circuit table for this new connection. This entry is exactly the same as shown previously in Table 2. The main difference is that now the task of assigning an unused VCI value on the interface is performed by the switch for that port. In this example, the switch picks the value 5. The virtual circuit table now has the following information: "When packets arrive on port 2 with identifier 5, send them out on port 1." Another issue is that, somehow, host A will need to learn that it should put the VCI value of 5 in packets that it wants to send to B; we will see how that happens below.

When switch 2 receives the setup message, it performs a similar process; in this example, it picks the value 11 as the incoming VCI value. Similarly, switch 3 picks 7 as the value for its incoming VCI. Each switch can pick any number it likes, as long as that number is not currently in use for some other connection on that port of that switch. As noted above, VCIs have link-local scope; that is, they have no global significance.

Finally, the setup message arrives as host B. Assuming that B is healthy and willing to accept a connection from host A, it too allocates an incoming VCI value, in this case 4. This VCI value can be used by B to identify all packets coming from host A.

Now, to complete the connection, everyone needs to be told what their downstream neighbor is using as the VCI for this connection. Host B sends an acknowledgment of the connection setup to switch 3 and includes in that message the VCI that it chose (4). Now switch 3 can complete the virtual circuit table entry for this connection, since it knows the outgoing value must be 4. Switch 3 sends the acknowledgment on to switch 2, specifying a VCI of 7. Switch 2 sends the message on to switch 1, specifying a VCI of 11. Finally, switch 1 passes the acknowledgment on to host A, telling it to use the VCI of 5 for this connection.

At this point, everyone knows all that is necessary to allow traffic to flow from host A to host B. Each switch has a complete virtual circuit table entry for the connection. Furthermore, host A has a firm acknowledgment that everything is in place all the way to host B. At this point, the connection table entries are in place in all three switches just as in the administratively configured example above, but the whole process happened automatically in response to the signalling message sent from A. The data transfer phase can now begin and is identical to that used in the PVC case.

When host A no longer wants to send data to host B, it tears down the connection by sending a teardown message to switch 1. The switch removes the relevant entry from its table and forwards the message on to the other switches in the path, which similarly delete the appropriate table entries. At this point, if host A were to send a packet with a VCI of 5 to switch 1, it would be dropped as if the connection had never existed.

There are several things to note about virtual circuit switching:

  • Since host A has to wait for the connection request to reach the far side of the network and return before it can send its first data packet, there is at least one round-trip time (RTT) of delay before data is sent.

  • While the connection request contains the full address for host B (which might be quite large, being a global identifier on the network), each data packet contains only a small identifier, which is only unique on one link. Thus, the per-packet overhead caused by the header is reduced relative to the datagram model.

  • If a switch or a link in a connection fails, the connection is broken and a new one will need to be established. Also, the old one needs to be torn down to free up table storage space in the switches.

  • The issue of how a switch decides which link to forward the connection request on has been glossed over. In essence, this is the same problem as building up the forwarding table for datagram forwarding, which requires some sort of routing algorithm. Routing is described in a later section, and the algorithms described there are generally applicable to routing setup requests as well as datagrams.

One of the nice aspects of virtual circuits is that by the time the host gets the go-ahead to send data, it knows quite a lot about the network—for example, that there really is a route to the receiver and that the receiver is willing and able to receive data. It is also possible to allocate resources to the virtual circuit at the time it is established. For example, X.25 was an early (and now largely obsolete) virtual-circuit-based networking technology. X.25 networks employ the following three-part strategy:

  1. Buffers are allocated to each virtual circuit when the circuit is initialized.

  2. The sliding window protocol is run between each pair of nodes along the virtual circuit, and this protocol is augmented with flow control to keep the sending node from over-running the buffers allocated at the receiving node.

  3. The circuit is rejected by a given node if not enough buffers are available at that node when the connection request message is processed.

In doing these three things, each node is ensured of having the buffers it needs to queue the packets that arrive on that circuit. This basic strategy is usually called hop-by-hop flow control.

By comparison, a datagram network has no connection establishment phase, and each switch processes each packet independently, making it less obvious how a datagram network would allocate resources in a meaningful way. Instead, each arriving packet competes with all other packets for buffer space. If there are no free buffers, the incoming packet must be discarded. We observe, however, that even in a datagram-based network a source host often sends a sequence of packets to the same destination host. It is possible for each switch to distinguish among the set of packets it currently has queued, based on the source/destination pair, and thus for the switch to ensure that the packets belonging to each source/destination pair are receiving a fair share of the switch's buffers.

In the virtual circuit model, we could imagine providing each circuit with a different quality of service (QoS). In this setting, the term quality of service is usually taken to mean that the network gives the user some kind of performance-related guarantee, which in turn implies that switches set aside the resources they need to meet this guarantee. For example, the switches along a given virtual circuit might allocate a percentage of each outgoing link's bandwidth to that circuit. As another example, a sequence of switches might ensure that packets belonging to a particular circuit not be delayed (queued) for more than a certain amount of time.

There have been a number of successful examples of virtual circuit technologies over the years, notably X.25, Frame Relay, and Asynchronous Transfer Mode (ATM). With the success of the Internet's connectionless model, however, none of them enjoys great popularity today. One of the most common applications of virtual circuits for many years was the construction of virtual private networks (VPNs), a subject discussed in a later section. Even that application is now mostly supported using Internet-based technologies today.

Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM) is probably the most well-known virtual circuit-based networking technology, although it is now somewhat past its peak in terms of deployment. ATM became an important technology in the 1980s and early 1990s for a variety of reasons, not the least of which is that it was embraced by the telephone industry, which at that point in time been less than active in data communications (other than as a supplier of links from which other people built networks). ATM also happened to be in the right place at the right time, as a high-speed switching technology that appeared on the scene just when shared media like Ethernet and token rings were starting to look a bit too slow for many users of computer networks. In some ways ATM was a competing technology with Ethernet switching, and it was seen by many as a competitor to IP as well.

ATM cell format at the UNI.

The approach ATM takes has some interesting properties, which makes it worth examining a bit further. The picture of the ATM packet format—more commonly called an ATM cell—in Figure 6 will illustrate the main points. We'll skip the generic flow control (GFC) bits, which never saw much use, and start with the 24 bits that are labelled VPI (virtual path identifier—8 bits) and VCI (virtual circuit identifier—16 bits). If you consider these bits together as a single 24-bit field, they correspond to the virtual circuit identifier introduced above. The reason for breaking the field into two parts was to allow for a level of hierarchy: All the circuits with the same VPI could, in some cases, be treated as a group (a virtual path) and could all be switched together looking only at the VPI, simplifying the work of a switch that could ignore all the VCI bits and reducing the size of the VC table considerably.

Skipping to the last header byte we find an 8-bit cyclic redundancy check (CRC), known as the header error check (HEC). It uses CRC-8 and provides error detection and single-bit error correction capability on the cell header only. Protecting the cell header is particularly important because an error in the VCI will cause the cell to be misdelivered.

Probably the most significant thing to notice about the ATM cell, and the reason it is called a cell and not a packet, is that it comes in only one size: 53 bytes. What was the reason for this? One big reason was to facilitate the implementation of hardware switches. When ATM was being created in the mid- and late 1980s, 10-Mbps Ethernet was the cutting-edge technology in terms of speed. To go much faster, most people thought in terms of hardware. Also, in the telephone world, people think big when they think of switches—telephone switches often serve tens of thousands of customers. Fixed-length packets turn out to be a very helpful thing if you want to build fast, highly scalable switches. There are two main reasons for this:

  1. It is easier to build hardware to do simple jobs, and the job of processing packets is simpler when you already know how long each one will be.

  2. If all packets are the same length, then you can have lots of switching elements all doing much the same thing in parallel, each of them taking the same time to do its job.

This second reason, the enabling of parallelism, greatly improves the scalability of switch designs. It would be overstating the case to say that fast parallel hardware switches can only be built using fixed-length cells. However, it is certainly true that cells ease the task of building such hardware and that there was a lot of knowledge available about how to build cell switches in hardware at the time the ATM standards were being defined. As it turns out, this same principle is still applied in many switches and routers today, even if they deal in variable length packets—they cut those packets into some sort of cell in order to switch them, as we'll see in a later section.

There is another good argument in favor of small ATM cells, having to do with end-to-end latency. ATM was designed to carry both voice phone calls (the dominate use case at the time) and data. Because voice is low-bandwidth but has strict delay requirements, the last thing you want is for a small voice packet queued behind a large data packet at a switch. If you force all packets to be small (i.e., cell-sized), then large data packets can still be supported by reassembling a set of cells into a packet, and you get the benefit of being able to interleave the forwarding of voice cells and data cells at every switch along the path from source to destination. This idea of using small cells to improve end-to-end latency is alive and well today in the cellular network.

Having decided to use small, fixed-length packets, the next question is what is the right length to fix them at? If you make them too short, then the amount of header information that needs to be carried around relative to the amount of data that fits in one cell gets larger, so the percentage of link bandwidth that is actually used to carry data goes down. Even more seriously, if you build a device that processes cells at some maximum number of cells per second, then as cells get shorter the total data rate drops in direct proportion to cell size. An example of such a device might be a network adaptor that reassembles cells into larger units before handing them up to the host. The performance of such a device depends directly on cell size. On the other hand, if you make the cells too big, then there is a problem of wasted bandwidth caused by the need to pad transmitted data to fill a complete cell. If the cell payload size is 48 bytes and you want to send 1 byte, you'll need to send 47 bytes of padding. If this happens a lot, then the utilization of the link will be very low. The combination of relatively high header-to-payload ratio plus the frequency of sending partially filled cells did actually lead to some noticeable inefficiency in ATM networks that some detractors called the cell tax.

As it turns out, 48 bytes was picked for the ATM cell payload as a compromise. There were good arguments for both larger and smaller cells, and 48 made almost no one happy—a power of two would certainly have been better for computers to work with.

Source Routing

A third approach to switching that uses neither virtual circuits nor conventional datagrams is known as source routing. The name derives from the fact that all the information about network topology that is required to switch a packet across the network is provided by the source host.

There are various ways to implement source routing. One would be to assign a number to each output of each switch and to place that number in the header of the packet. The switching function is then very simple: For each packet that arrives on an input, the switch would read the port number in the header and transmit the packet on that output. However, since there will in general be more than one switch in the path between the sending and the receiving host, the header for the packet needs to contain enough information to allow every switch in the path to determine which output the packet needs to be placed on. One way to do this would be to put an ordered list of switch ports in the header and to rotate the list so that the next switch in the path is always at the front of the list. Figure 7 illustrates this idea.

Source routing in a switched network (where the switch reads the rightmost number).

In this example, the packet needs to traverse three switches to get from host A to host B. At switch 1, it needs to exit on port 1, at the next switch it needs to exit at port 0, and at the third switch it needs to exit at port 3. Thus, the original header when the packet leaves host A contains the list of ports (3, 0, 1), where we assume that each switch reads the rightmost element of the list. To make sure that the next switch gets the appropriate information, each switch rotates the list after it has read its own entry. Thus, the packet header as it leaves switch 1 enroute to switch 2 is now (1, 3, 0); switch 2 performs another rotation and sends out a packet with (0, 1, 3) in the header. Although not shown, switch 3 performs yet another rotation, restoring the header to what it was when host A sent it.

There are several things to note about this approach. First, it assumes that host A knows enough about the topology of the network to form a header that has all the right directions in it for every switch in the path. This is somewhat analogous to the problem of building the forwarding tables in a datagram network or figuring out where to send a setup packet in a virtual circuit network. Second, observe that we cannot predict how big the header needs to be, since it must be able to hold one word of information for every switch on the path. This implies that headers are probably of variable length with no upper bound, unless we can predict with absolute certainty the maximum number of switches through which a packet will ever need to pass. Third, there are some variations on this approach. For example, rather than rotate the header, each switch could just strip the first element as it uses it. Rotation has an advantage over stripping, however: Host B gets a copy of the complete header, which may help it figure out how to get back to host A. Yet another alternative is to have the header carry a pointer to the current "next port" entry, so that each switch just updates the pointer rather than rotating the header; this may be more efficient to implement. We show these three approaches in Figure 8. In each case, the entry that this switch needs to read is A, and the entry that the next switch needs to read is B.

Three ways to handle headers for source routing: (a) rotation; (b) stripping; (c) pointer. The labels are read right to left.

Source routing can be used in both datagram networks and virtual circuit networks. For example, the Internet Protocol, which is a datagram protocol, includes a source route option that allows selected packets to be source routed, while the majority are switched as conventional datagrams. Source routing is also used in some virtual circuit networks as the means to get the initial setup request along the path from source to destination.

Source routes are sometimes categorized as strict or loose. In a strict source route, every node along the path must be specified, whereas a loose source route only specifies a set of nodes to be traversed, without saying exactly how to get from one node to the next. A loose source route can be thought of as a set of waypoints rather than a completely specified route. The loose option can be helpful to limit the amount of information that a source must obtain to create a source route. In any reasonably large network, it is likely to be hard for a host to get the complete path information it needs to construct correct a strict source route to any destination. But both types of source routes do find application in certain scenarios, as we will see in later chapters.

Bridges and L2 Switches

Having discussed some of the basic ideas behind switching, we now focus more closely on some specific switching technologies. We begin by considering a class of switch that is used to forward packets between LANs (local area networks) such as Ethernets. Such switches are today known as L2 switches; historically, they have also been referred to as bridges, and they are very widely used in campus and enterprise networks.

Suppose you have a pair of Ethernets that you want to interconnect. One approach you might try is to put a repeater between them. This would not be a workable solution, however, if doing so exceeded the physical limitations of the Ethernet. (Recall that no more than two repeaters between any pair of hosts and no more than a total of 2500 m in length are allowed.) An alternative would be to put a node with a pair of Ethernet adaptors between the two Ethernets and have the node forward frames from one Ethernet to the other. This node would differ from a repeater, which operates on bits, not frames, and just blindly copies the bits received on one interface to another. Instead, this node would fully implement the Ethernet's collision detection and media access protocols on each interface. Hence, the length and number-of-host restrictions of the Ethernet, which are all about managing collisions, would not apply to the combined pair of Ethernets connected in this way. This device operates in promiscuous mode, accepting all frames transmitted on either of the Ethernets, and forwarding them to the other.

The node we have just described is typically called a bridge, and a collection of LANs connected by one or more bridges is usually said to form an extended LAN. In their simplest variants, bridges simply accept LAN frames on their inputs and forward them out on all other outputs. This simple strategy was used by early bridges but has some pretty serious limitations as we'll see below. A number of refinements have been added over the years to make bridges an effective mechanism for interconnecting a set of LANs. The rest of this section fills in the more interesting details.

Note that a bridge meets our definition of a switch from the previous section: a multi-input, multi-output device, which transfers packets from an input to one or more outputs. And recall that this provides a way to increase the total bandwidth of a network. For example, while a single Ethernet segment might carry only 100 Mbps of total traffic, an Ethernet bridge can carry as much as 100nn Mbps, where nn is the number of ports (inputs and outputs) on the bridge.

Learning Bridges

The first optimization we can make to a bridge is to observe that it need not forward all frames that it receives. Consider the bridge in Figure 9. Whenever a frame from host A that is addressed to host B arrives on port 1, there is no need for the bridge to forward the frame out over port 2. The question, then, is how does a bridge come to learn on which port the various hosts reside?

Illustration of a learning bridge.

One option would be to have a human download a table into the bridge similar to the one given in Table 4. Then, whenever the bridge receives a frame on port 1 that is addressed to host A, it would not forward the frame out on port 2; there would be no need because host A would have already directly received the frame on the LAN connected to port 1. Anytime a frame addressed to host A was received on port 2, the bridge would forward the frame out on port 1.

Host Port
A 1
B 1
C 1
X 2
Y 2
Z 2

Table 4. Forwarding Table Maintained by a Bridge.

No one actually builds bridges in which the table is configured by hand. Having a human maintain this table is too burdensome, and there is a simple trick by which a bridge can learn this information for itself. The idea is for each bridge to inspect the source address in all the frames it receives. Thus, when host A sends a frame to a host on either side of the bridge, the bridge receives this frame and records the fact that a frame from host A was just received on port 1. In this way, the bridge can build a table just like Table 4.

Note that a bridge using such a table implements a version of the datagram (or connectionless) model of forwarding described earlier. Each packet carries a global address, and the bridge decides which output to send a packet on by looking up that address in a table.

When a bridge first boots, this table is empty; entries are added over time. Also, a timeout is associated with each entry, and the bridge discards the entry after a specified period of time. This is to protect against the situation in which a host—and, as a consequence, its LAN address—is moved from one network to another. Thus, this table is not necessarily complete. Should the bridge receive a frame that is addressed to a host not currently in the table, it goes ahead and forwards the frame out on all the other ports. In other words, this table is simply an optimization that filters out some frames; it is not required for correctness.

Implementation

The code that implements the learning bridge algorithm is quite simple, and we sketch it here. Structure BridgeEntry defines a single entry in the bridge's forwarding table; these are stored in a Map structure (which supports mapCreate, mapBind, and mapResolve operations) to enable entries to be efficiently located when packets arrive from sources already in the table. The constant MAX_TTL specifies how long an entry is kept in the table before it is discarded.

#define BRIDGE_TAB_SIZE   1024  /* max size of bridging table */
#define MAX_TTL           120   /* time (in seconds) before an entry is flushed */

typedef struct {
    MacAddr     destination;    /* MAC address of a node */
    int         ifnumber;       /* interface to reach it */
    u_short     TTL;            /* time to live */
    Binding     binding;        /* binding in the Map */
} BridgeEntry;

int     numEntries = 0;
Map     bridgeMap = mapCreate(BRIDGE_TAB_SIZE, sizeof(BridgeEntry));

The routine that updates the forwarding table when a new packet arrives is given by updateTable. The arguments passed are the source media access control (MAC) address contained in the packet and the interface number on which it was received. Another routine, not shown here, is invoked at regular intervals, scans the entries in the forwarding table, and decrements the TTL (time to live) field of each entry, discarding any entries whose TTL has reached 0. Note that the TTL is reset to MAX_TTL every time a packet arrives to refresh an existing table entry and that the interface on which the destination can be reached is updated to reflect the most recently received packet.

void 
updateTable (MacAddr src, int inif) 
{
    BridgeEntry       *b;

    if (mapResolve(bridgeMap, &src, (void **)&b) == FALSE ) 
    {
        /* this address is not in the table, so try to add it */
        if (numEntries < BRIDGE_TAB_SIZE) 
        {
            b = NEW(BridgeEntry);
            b->binding = mapBind( bridgeMap, &src, b);
            /* use source address of packet as dest. address in table */
            b->destination = src;
            numEntries++;
        }
        else 
        {
            /* can`t fit this address in the table now, so give up */
            return;
        }
    }
    /* reset TTL and use most recent input interface */
    b->TTL = MAX_TTL;
    b->ifnumber = inif;
}

Note that this implementation adopts a simple strategy in the case where the bridge table has become full to capacity—it simply fails to add the new address. Recall that completeness of the bridge table is not necessary for correct forwarding; it just optimizes performance. If there is some entry in the table that is not currently being used, it will eventually time out and be removed, creating space for a new entry. An alternative approach would be to invoke some sort of cache replacement algorithm on finding the table full; for example, we might locate and remove the entry with the smallest TTL to accommodate the new entry.

Extended LAN with loops.

Spanning Tree Algorithm

The preceding strategy works just fine until the extended LAN has a loop in it, in which case it fails in a horrible way—frames potentially loop through the extended LAN forever. This is easy to see in the example depicted in Figure 10, where, for example, bridges B1, B4, and B6 form a loop. Suppose that a packet enters bridge B4 from Ethernet J and that the destination address is one not yet in any bridge's forwarding table: B4 sends a copy of the packet out to Ethernets H and I. Now bridge B6 forwards the packet to Ethernet G, where B1 would see it and forward it back to Ethernet H; B4 still doesn`t have this destination in its table, so it forwards the packet back to Ethernets I and J. There is nothing to stop this cycle from repeating endlessly, with packets looping in both directions among B1, B4, and B6.

Why would an extended LAN come to have a loop in it? One possibility is that the network is managed by more than one administrator, for example, because it spans multiple departments in an organization. In such a setting, it is possible that no single person knows the entire configuration of the network, meaning that a bridge that closes a loop might be added without anyone knowing. A second, more likely scenario is that loops are built into the network on purpose—to provide redundancy in case of failure. After all, a network with no loops needs only one link failure to become split into two separate partitions.

Whatever the cause, bridges must be able to correctly handle loops. This problem is addressed by having the bridges run a distributed spanning tree algorithm. If you think of the extended LAN as being represented by a graph that possibly has loops (cycles), then a spanning tree is a subgraph of this graph that covers (spans) all the vertices but contains no cycles. That is, a spanning tree keeps all of the vertices of the original graph but throws out some of the edges. For example, Figure 11 shows a cyclic graph on the left and one of possibly many spanning trees on the right.

Example of (a) a cyclic graph; (b) a corresponding spanning tree.

The idea of a spanning tree is simple enough: It's a subset of the actual network topology that has no loops and that reaches all the LANs in the extended LAN. The hard part is how all of the bridges coordinate their decisions to arrive at a single view of the spanning tree. After all, one topology is typically able to be covered by multiple spanning trees. The answer lies in the spanning tree protocol, which we'll describe now.

The spanning tree algorithm, which was developed by Radia Perlman at the Digital Equipment Corporation, is a protocol used by a set of bridges to agree upon a spanning tree for a particular extended LAN. (The IEEE 802.1 specification for LAN bridges is based on this algorithm.) In practice, this means that each bridge decides the ports over which it is and is not willing to forward frames. In a sense, it is by removing ports from the topology that the extended LAN is reduced to an acyclic tree. It is even possible that an entire bridge will not participate in forwarding frames, which seems kind of strange at first glance. The algorithm is dynamic, however, meaning that the bridges are always prepared to reconfigure themselves into a new spanning tree should some bridge fail, and so those unused ports and bridges provide the redundant capacity needed to recover from failures.

Representing an extended LAN as an abstract graph is a bit awkward. Basically, you let both the bridges and the LANs correspond to the vertices of the graph and the ports correspond to the graph's edges. However, the spanning tree we are going to compute for this graph needs to span only those nodes that correspond to networks. It is possible that nodes corresponding to bridges will be disconnected from the rest of the graph. This corresponds to a situation in which all the ports connecting a bridge to various networks get removed by the algorithm.

The main idea of the spanning tree is for the bridges to select the ports over which they will forward frames. The algorithm selects ports as follows. Each bridge has a unique identifier; for our purposes, we use the labels B1, B2, B3, and so on. The algorithm first elects the bridge with the smallest ID as the root of the spanning tree; exactly how this election takes place is described below. The root bridge always forwards frames out over all of its ports. Next, each bridge computes the shortest path to the root and notes which of its ports is on this path. This port is also selected as the bridge's preferred path to the root. Finally, all the bridges connected to a given LAN elect a single designated bridge that will be responsible for forwarding frames toward the root bridge. Each LAN's designated bridge is the one that is closest to the root. If two or more bridges are equally close to the root, then the bridges` identifiers are used to break ties, and the smallest ID wins. Of course, each bridge is connected to more than one LAN, so it participates in the election of a designated bridge for each LAN it is connected to. In effect, this means that each bridge decides if it is the designated bridge relative to each of its ports. The bridge forwards frames over those ports for which it is the designated bridge.

Spanning tree with some ports not selected.

Figure 12 shows the spanning tree that corresponds to the extended LAN shown in Figure 10. In this example, B1 is the root bridge, since it has the smallest ID. Notice that both B3 and B5 are connected to LAN A, but B5 is the designated bridge since it is closer to the root. Similarly, both B5 and B7 are connected to LAN B, but in this case B5 is the designated bridge since it has the smaller ID; both are an equal distance from B1.

While it is possible for a human to look at the extended LAN given in Figure 10 and to compute the spanning tree given in the Figure 12 according to the rules given above, the bridges in an extended LAN do not have the luxury of being able to see the topology of the entire network, let alone peek inside other bridges to see their ID. Instead, the bridges have to exchange configuration messages with each other and then decide whether or not they are the root or a designated bridge based on these messages.

Specifically, the configuration messages contain three pieces of information:

  1. The ID for the bridge that is sending the message.

  2. The ID for what the sending bridge believes to be the root bridge.

  3. The distance, measured in hops, from the sending bridge to the root bridge.

Each bridge records the current best configuration message it has seen on each of its ports ("best" is defined below), including both messages it has received from other bridges and messages that it has itself transmitted.

Initially, each bridge thinks it is the root, and so it sends a configuration message out on each of its ports identifying itself as the root and giving a distance to the root of 0. Upon receiving a configuration message over a particular port, the bridge checks to see if that new message is better than the current best configuration message recorded for that port. The new configuration message is considered better than the currently recorded information if any of the following is true:

  • It identifies a root with a smaller ID.

  • It identifies a root with an equal ID but with a shorter distance.

  • The root ID and distance are equal, but the sending bridge has a smaller ID

If the new message is better than the currently recorded information, the bridge discards the old information and saves the new information. However, it first adds 1 to the distance-to-root field since the bridge is one hop farther away from the root than the bridge that sent the message.

When a bridge receives a configuration message indicating that it is not the root bridge—that is, a message from a bridge with a smaller ID—the bridge stops generating configuration messages on its own and instead only forwards configuration messages from other bridges, after first adding 1 to the distance field. Likewise, when a bridge receives a configuration message that indicates it is not the designated bridge for that port—that is, a message from a bridge that is closer to the root or equally far from the root but with a smaller ID—the bridge stops sending configuration messages over that port. Thus, when the system stabilizes, only the root bridge is still generating configuration messages, and the other bridges are forwarding these messages only over ports for which they are the designated bridge. At this point, a spanning tree has been built, and all the bridges are in agreement on which ports are in use for the spanning tree. Only those ports may be used for forwarding data packets in the extended LAN.

Let's see how this works with an example. Consider what would happen in Figure 12 if the power had just been restored to the building housing this network, so that all the bridges boot at about the same time. All the bridges would start off by claiming to be the root. We denote a configuration message from node X in which it claims to be distance d from root node Y as (Y,d,X). Focusing on the activity at node B3, a sequence of events would unfold as follows:

  1. B3 receives (B2, 0, B2).

  2. Since 2 < 3, B3 accepts B2 as root.

  3. B3 adds one to the distance advertised by B2 (0) and thus sends (B2, 1, B3) toward B5.

  4. Meanwhile, B2 accepts B1 as root because it has the lower ID, and it sends (B1, 1, B2) toward B3.

  5. B5 accepts B1 as root and sends (B1, 1, B5) toward B3.

  6. B3 accepts B1 as root, and it notes that both B2 and B5 are closer to the root than it is; thus, B3 stops forwarding messages on both its interfaces.

This leaves B3 with both ports not selected, as shown in Figure 12.

Even after the system has stabilized, the root bridge continues to send configuration messages periodically, and the other bridges continue to forward these messages as described in the previous paragraph. Should a particular bridge fail, the downstream bridges will not receive these configuration messages, and after waiting a specified period of time they will once again claim to be the root, and the algorithm just described will kick in again to elect a new root and new designated bridges.

One important thing to notice is that although the algorithm is able to reconfigure the spanning tree whenever a bridge fails, it is not able to forward frames over alternative paths for the sake of routing around a congested bridge.

Broadcast and Multicast

The preceding discussion has focused on how bridges forward unicast frames from one LAN to another. Since the goal of a bridge is to transparently extend a LAN across multiple networks, and since most LANs support both broadcast and multicast, then bridges must also support these two features. Broadcast is simple—each bridge forwards a frame with a destination broadcast address out on each active (selected) port other than the one on which the frame was received.

Multicast can be implemented in exactly the same way, with each host deciding for itself whether or not to accept the message. This is exactly what is done in practice. Notice, however, that since not all the LANs in an extended LAN necessarily have a host that is a member of a particular multicast group, it is possible to do better. Specifically, the spanning tree algorithm can be extended to prune networks over which multicast frames need not be forwarded. Consider a frame sent to group M by a host on LAN A in Figure 12. If there is no host on LAN J that belongs to group M, then there is no need for bridge B4 to forward the frames over that network. On the other hand, not having a host on LAN H that belongs to group M does not necessarily mean that bridge B1 can avoid forwarding multicast frames onto LAN H. It all depends on whether or not there are members of group M on LANs I and J.

How does a given bridge learn whether it should forward a multicast frame over a given port? It learns exactly the same way that a bridge learns whether it should forward a unicast frame over a particular port—by observing the source addresses that it receives over that port. Of course, groups are not typically the source of frames, so we have to cheat a little. In particular, each host that is a member of group M must periodically send a frame with the address for group M in the source field of the frame header. This frame would have as its destination address the multicast address for the bridges.

Note that, although the multicast extension just described has been proposed, it is not widely adopted. Instead, multicast is implemented in exactly the same way as broadcast on today's extended LANs.

Limitations of Bridges

The bridge-based solution just described is meant to be used in only a fairly limited setting—to connect a handful of similar LANs. The main limitations of bridges become apparent when we consider the issues of scale and heterogeneity.

On the issue of scale, it is not realistic to connect more than a few LANs by means of bridges, where in practice few typically means "tens of." One reason for this is that the spanning tree algorithm scales linearly; that is, there is no provision for imposing a hierarchy on the extended LAN. A second reason is that bridges forward all broadcast frames. While it is reasonable for all hosts within a limited setting (say, a department) to see each other's broadcast messages, it is unlikely that all the hosts in a larger environment (say, a large company or university) would want to have to be bothered by each other's broadcast messages. Said another way, broadcast does not scale, and as a consequence extended LANs do not scale.

One approach to increasing the scalability of extended LANs is the virtual LAN (VLAN). VLANs allow a single extended LAN to be partitioned into several seemingly separate LANs. Each virtual LAN is assigned an identifier (sometimes called a color), and packets can only travel from one segment to another if both segments have the same identifier. This has the effect of limiting the number of segments in an extended LAN that will receive any given broadcast packet.

Two virtual LANs share a common backbone.

We can see how VLANs work with an example. Figure 13 shows four hosts on four different LAN segments. In the absence of VLANs, any broadcast packet from any host will reach all the other hosts. Now let's suppose that we define the segments connected to hosts W and X as being in one VLAN, which we'll call VLAN 100. We also define the segments that connect to hosts Y and Z as being in VLAN 200. To do this, we need to configure a VLAN ID on each port of bridges B1 and B2. The link between B1 and B2 is considered to be in both VLANs.

When a packet sent by host X arrives at bridge B2, the bridge observes that it came in a port that was configured as being in VLAN 100. It inserts a VLAN header between the Ethernet header and its payload. The interesting part of the VLAN header is the VLAN ID; in this case, that ID is set to 100. The bridge now applies its normal rules for forwarding to the packet, with the extra restriction that the packet may not be sent out an interface that is not part of VLAN 100. Thus, under no circumstances will the packet—even a broadcast packet—be sent out the interface to host Z, which is in VLAN 200. The packet is, however, forwarded on to bridge B1, which follows the same rules and thus may forward the packet to host W but not to host Y.

An attractive feature of VLANs is that it is possible to change the logical topology without moving any wires or changing any addresses. For example, if we wanted to make the segment that connects to host Z be part of VLAN 100 and thus enable X, W, and Z to be on the same virtual LAN, then we would just need to change one piece of configuration on bridge B2.

On the issue of heterogeneity, bridges are fairly limited in the kinds of networks they can interconnect. In particular, bridges make use of the network's frame header and so can support only networks that have exactly the same format for addresses. Thus, bridges can be used to connect Ethernets to Ethernets, token rings to token rings, and one 802.11 network to another. It's also possible to put a bridge between, say, an and an 802.11 network, since both networks support the same 48-bit address format. However, bridges do not readily generalize to other kinds of networks with different addressing formats, such as ATM.

Despite their limitations, bridges are a very important part of the complete networking picture. Their main advantage is that they allow multiple LANs to be transparently connected; that is, the networks can be connected without the end hosts having to run any additional protocols (or even be aware, for that matter). The one potential exception is when the hosts are expected to announce their membership in a multicast group.

Notice, however, that this transparency can be dangerous. If a host or, more precisely, the application and transport protocol running on that host is programmed under the assumption that it is running on a single LAN, then inserting bridges between the source and destination hosts can have unexpected consequences. For example, if a bridge becomes congested, it may have to drop frames; in contrast, it is rare that a single Ethernet ever drops a frame. As another example, the latency between any pair of hosts on an extended LAN becomes both larger and more highly variable; in contrast, the physical limitations of a single Ethernet make the latency both small and predictable. As a final example, it is possible (although unlikely) that frames will be reordered in an extended LAN; in contrast, frame order is never shuffled on a single Ethernet. The bottom line is that it is never safe to design network software under the assumption that it will run over a single Ethernet segment. Bridges happen.

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