Outline

• Switching and Forwarding - Source Routing
• Quiz
• Bridges and VLAN Switches
• Spanning Tree Algorithm

Introduction

• Last week, we had begun talking about switches: multi-input, multioutput devices used to transfer packets from an input to one or more outputs.
• We said where a link comes into a switch is called a port.
• The job of switch (called forwarding or switching) is to receive data on one port and figure out which port to send it out on.
• We talked about two common ways to do this: datagrams (a connectionless approach) and virtual circuits (a connection-oriented approach).
• We begin today by looking at a third approach: source routing.

Source Routing

• The idea in this strategy is that the source host provides all the information in the packet to switch the packet to the destination.
• One way to implement this is to put an ordered list of switch ports in the header of the packets.
• The first element in this list says which outgoing port the first switch should use, the second element says which outgoing port the second switch should use, and so on.
• For this to work:
  • The sender needs to know enough about the network topology to form the header for the path. This is somewhat similar to the problem of figuring out where to send the setup packet in a VC.
  • The header may be of variable length depending on its destination
  • At each switch, we need to modify the header. We could either strip off a port number or we could rotate the list of port numbers, or use a pointer to say which element in the list the next switch should use.
• Source routing can be used in both the datagram and VC network setting.
• The IP protocol provides a source routing option.
• In the VC setting, it is sometimes used to get the initial signalling message to its destination.
• One can either have "strict" or "loose" source routing. What we described above was strict. If we only specify certain "waypoint" switches that need to be traversed, than we are doing "loose" source routing.

Bridges and LAN Switches

• We now look at the class of switches which are used to forwards packets between shared media LANs such as Ethernets.
• Suppose you have a pair of Ethernets that you want to connect.
• If you use a repeater or hub, you might exceed the physical limitations of the Ethernet such as the maximum number of machines (1024) that can be on a single Ethernet.
• Another approach is to put a node between the two Ethernets and have the node forward frames transmitted on either side of the Ethernet.
• This node operates in promiscuous mode, accepting all frames transmitted on either side.
• This kind of node is called a bridge.
• The LANs connected by one or more bridges are together called an extended LAN.
• Nowadays, each LAN is often just a single machine connected by a point to point link to a switch in which case the term switched Ethernet is used for this kind of network.
• The technology to handle extended LANs and switched Ethernet though is the same.

Learning Bridges

• If A wants to send to B, there is no point for the bridge to forward the message it sees on port 1 to port 2. How does the bridge know when to forward and when not to?
• A human could create a bridge table with two columns, one for host, one for which port to use. If the frame comes in on the same port that it should also go out on, then the bridge wouldn't need to forward it.
• This would be using the datagram model of forwarding we talked about last day.
• One way to automate the process of building the bridge table is the following:
  1. When a bridge first turns on, it starts with an empty table.
  2. When the bridge gets a frame on a given interface, it checks its table to see if there is an entry which has the same source address. If not, it adds the entry (source address, interface) to its table and sets a timer. If yes, and the entry would be the same, the timer is reset.
  3. The bridge then checks: Does the destination MAC address have an entry in the table? If yes, and it is different from the incoming interface, forward the frame according to the entry. If there is no entry, then forward on all outgoing lines other than the one the frame came in on.
  4. If a timer goes to 0, then delete that entry. This guards against machines going down or changing the topology.

Spanning Tree Algorithm

• The algorithm of the last works back works provided there are no loops in the network graph. If there is a loop, and no one has an entry for a given frame destination, then a frame might cycle indefinitely.
• This is a problem because, loops occur reasonably naturally in bridged networks: They might arise because no one person building the network controls all the bridges; they also might arise because the builder wanted to ensure the reliability of the network against failures.
• In order to solve this problem, the bridges might run a distributed spanning tree algorithm. This algorithm was developed at DEC by Radia Perlman and is part of the 802.1. spec.
• Once a spanning tree is computed, a bridge can use the algorithm of the last slide with the change that it only forwards on ports which are part of the spanning tree.
• The spanning tree algorithm works as follows:
  1. Each bridge has its own unique ID (MAC address of lowest port + bridge priority, 8 bytes) which we denote B1, B2, B3 ...
  2. The bridges first elect a root bridge. This will be the bridge with the least ID.
  3. Each bridge then computes the shortest path to the root, and determines which of its ports is on this route. It also determines a designated forwarding bridge which will be responsible for forwarding messages to the route.

More on the Spanning Tree Algorithm

• In order to do each of these steps, bridges exchange configuration messages containing: (1) the ID of the bridge sending the message, (2) the ID for what the sending bridge thinks is the root bridge, (3) the distance in hops of the sending bridge from the root.
• If a bridge receives a configuration message, it checks if its own configuration message is better or worse than the one received.
• The new message is better if:
  • It identifies a root with a smaller ID.
  • It identifies a root with an equal ID, but 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, a bridge discards the old information and saves the new information. However, it adds 1 to the distance from the root to the bridge.
• When a bridge receives a message which tells it that it is not the root, it stops generating its own configuration messages, and instead only forwards messages from other bridges, adding 1 to the hop distance.
• When a bridge receives a message which tells it that it is not the designated bridge for the port of its neighbor, the bridge stops sending configuration messages over that port.
• The system stabilizes when only the root bridge is still generating messages. The root continues to do this periodically.
• If any node including the root goes down, downstream nodes will stop receiving configuration messages and then after a timeout period will once again claim to be root and run the above algorithm. This also happens if new nodes are added to the system.

Broadcast and Multicast

• To broadcast in a bridge network, a given bridge only needs to send out the frames it receives on each of its active ports other than the port on which it received the frame.
• On today's extended LANs, multicast is implemented in the same way as broadcast and it is up to each host to decide whether or not to accept a given frame.
• It might be the case that not all LANs in a given extended LAN have a host that needs to be multicast to. Another proposed way to implement multicast is for each host to periodically send a frame with the address for the multicast group in its source field and have the multicast address in its destination field. Bridges could then use the table algorithm to learn only those LANs that have hosts for that multicast address.

Limitations of Bridges

• The set-up of the previous few slides can be used to create networks of tens of bridges.
• The set-up does not scale to larger networks for a variety of reasons.
• One problem is that if a destination is unknown it has to be broadcast. As the network gets bigger there are more unknown hosts, and this becomes more and more of an issue.
• Another reason is that the spanning tree algorithm scales linearly in the number of nodes, so will take longer and longer to settle. It would be useful to have a hierarchy of bridges to solve this.

Virtual LANs

• One way to try to make extended LANs more scalable is to use virtual LANs (VLANs).
• Each VLAN is assigned an identifier (a color).
• Packets can only be sent from one segment to another if they have the same color.
• This makes it possible to do things like have a VLAN for the CS department as part of the extended LAN of the Science college. CS traffic on this VLAN wouldn't go to the Math department's VLAN (also on Science's LAN)and so they wouldn't see our private messages.
• The implementation of VLANs is specified in the 802.1Q spec. This spec changes the way Ethernet frames look to VLAN enabled bridges. It is backward compatible for non-such enabled bridges and hosts. Basically, before the length field in the frame 0x8100 is added, followed by a priority field, followed by a canonical format indicator, followed by the VLAN identifier.

Heterogeneity

• As bridges make use of the frame header, they are limited to connect networks which have the same format for addresses.
• So bridges could be used to interconnect Ethernets and Token rings. But they could not be used to connect to ATM networks.
• So there are limitations on the heterogeneity of the networks you can use with bridges.

Quiz

Which of the following is true?

1. The hidden nodes problems and the exposed nodes problem are the same thing.
2. OFDMA makes use of a scheduler (typically in a BBU or WiFi 6 router) that schedules resource elements.
3. Virtual circuit addresses used in Layer 2 Switching are typically larger than Ethernet Addresses.

What is an Internetwork?

• Recall from Chapter 1, an internetwork is an arbitrary collection of networks interconnected by some kind of host-to-host packet delivery service.
• The network technologies which are joined to the internetwork may be different: for example, FDDI, point-to-point links, and Ethernet.
• They also may involve more hosts than could be successfully dealt with by an extended LAN.
• The devices that are used to connect different networks are called routers.
• The Internet Protocol developed by Kahn-Cerf (1974) is the main protocol used in today's internetworks.
• There are other internetwork protocol that have been used to a greater or lesser extent. For example, Novell's IPX, Xerox- XNS, but IP is the dominant protocol.

Service Model

• A service model is the set of services we would like a given layer to provide.
• For IP, this consists of an addressing scheme that can be used to identify all hosts in the internetwork, and a datagram model (connectionless model) of data delivery.
• The service model is best-effort because it makes no guarantees of packet delivery.

IP Packet Format

• Version - is used to specify the version of IP. Currently, 4 and 6 are the most common corresponding to IPv4 and IPv6.
• HLen - specifies the length of the header in 32-bit words. When there are no options, there the header is 5 words longs.
• Length - is the length of the datagram in bytes. So the maximum is 65535.
• TTL - Time-to-Live is the number of seconds the packet still has to live. Nowadays, it pretty much counts the number of hops. The default start value is 64.
• Protocol - identifies the higher level protocol that is being used with IP. 6 = TCP, 17 = UDP. Here is a List of IP Protocol Numbers.
• Checksum - adds the header as a sequence of 16 bit words using 1's complement, and takes the 1's complement of the result
• Source and Destination address - say where the packet came from and where it is going.

Fragmentation and Reassembly

• Ethernet can accept packets up to 1500 bytes long. FDDI can accept packets up to 4500 bytes long.
• As IP packets can be much longer, we need some way to split packets so they can fit for a given network type.
• To do this a maximum transmission unit (MTU) is defined for each kind of network. This is the largest IP datagram that it can carry in a frame.
• Fragmentation of IP frames is then done in a router when it sees IP datagrams which exceed the MTU for the network the datagram is to be sent on.
• Note we can't The resulting fragment will still be in the format of a legal IP datagram.
• A receiving host needs to reassemble fragmented datagrams into a complete IP packet. (Notice Fragmentation occurs in routers, reassembly in hosts).
• The Ident field is used to identify all the fragments of a given packet. All fragments of the same packets have the same Ident field.
• The DF (don't fragment ) Flag is used to indicate whether datagram fit all in one fragment or is in more than one fragment.
• The MF (more fragment) Flag is used to indicate whether there are more fragments for a given datagram to follow.
• Fragmentation always occurs on 8 byte boundaries. The Offset is used to say which such 8 byte boundary in the whole datagram does the data in the given MTU begin with.
• Note we need to use offsets for this because fragmentation might occur at one router, and those fragments could be subsequently fragmented again at another router.
• So Ident tells us all members of a given datagram, offset allows us to reassemble the datagrams in order.
• If not all fragments arrive at the receiving host, the host gives up on the reassmbly process and discards the fragments that did arrive.
• For this reason, fragmentation is generally considered bad, so hosts are strongly encouraged to perform "path MTU discovery" to determine the smallest MTU from the sender to receiver. I.e., try to send a packet with a don't fragment bit set of a given size, if this fails an ICMPv4 message saying Fragmentation Needed (or ICMPv6 Packet Too Big) is returned, in which case reduce size and repeat.