Duplex Mismatch
Historically, there have been some interoperability problems using autonegotiation, especially when a computer and its associated switch port are configured using different duplex configurations or when autonegotiation is disabled at one end of the link but not the other. In this case, a so-called duplex mismatch can occur. Perhaps surprisingly, when this happens the connection does not completely fail but instead may suffer significant performance degradation. When the network has moderate to heavy traffic in both directions (e.g., during a large data transfer), a half-duplex interface can detect incoming traffic as a collision, triggering the exponential backoff function of the CSMA/CD Ethernet MAC. At the same time, the data triggering the collision is lost and may require higher-layer protocols such as TCP to retransmit. Thus, the performance degradation may be noticed only when there is sufficient traffic for the half-duplex interface to be receiving data at the same time it is sending, a situation that does not generally occur under light load. Some researchers have attempted to build analysis tools to detect this unfortunate situation [SC05].
Wake-on LAN (WoL), Power Saving, and Magic Packets
In both the Linux and Windows examples, we saw some indication of power management capabilities. In Windows the Wake-Up Capabilities and in Linux the Wake-On options are used to bring the network interface and/or host computer out of a lower-power (sleep) state based on the arrival of certain kinds of packets. The kinds of packets used to trigger the change to full-power state can be configured. In Linux, the Wake-On values are zero or more bits indicating whether receiving the following types of frames trigger a wake-up from a low-power state: any physical-layer (PHY) activity §, unicast frames destined for the station (u), multicast frames (m), broadcast frames (b), ARP frames (a), magic packet frames (g), and magic packet frames including a password.
Link-Layer Flow Control
Operating an extended Ethernet LAN in full-duplex mode and across segments of different speeds may require the switches to buffer (store) frames for some period of time. This happens, for example, when multiple stations send to the same destination (called output port contention). If the aggregate traffic rate headed for a station exceeds the station’s link rate, frames start to be stored in the intermediate switches. If this situation persists for a long time, frames may be dropped.
One way to mitigate this situation is to apply flow control to senders (i.e., slow them down). Some Ethernet switches (and interfaces) implement flow control by sending special signal frames between switches and NICs. Flow control signals to the sender that it must slow down its transmission rate, although the specification leaves the details of this to the implementation. Ethernet uses an implementation of flow control called PAUSE messages (also called PAUSE frames), specified by 802.3x [802.3-2008].
PAUSE messages are contained in MAC control frames, identified by the Ethernet Length/Type field having the value 0x8808 and using the MAC control opcode of 0x0001. A receiving station seeing this is advised to slow its rate. PAUSE frames are always sent to the MAC address 01:80:C2:00:00:01 and are used only on full-duplex links. They include a hold-off time value (specified in quantas equal to 512 bit times), indicating how long the sender should pause before continuing to transmit.
The MAC control frame is a frame format using the regular encapsulation from Figure 3-3, but with a 2-byte opcode immediately following the Length/Type field. PAUSE frames are essentially the only type of frames that uses MAC control frames. They include a 2-byte quantity encoding the hold-off time. Implementation of the “entire” MAC control layer (basically, just 802.3x flow control) is optional.
Bridges and Switches
The IEEE 802.1d standard specifies the operation of bridges, and thus switches, which are essentially high-performance bridges. A bridge or switch is used to join multiple physical link-layer networks (e.g., a pair of physical Ethernet segments) or groups of stations.
Spanning Tree Protocol (STP)
Bridges may operate in isolation, or in combination with other bridges. When more than two bridges are in use (or in general when switch ports are cross-connected), the possibility exists for a cascading, looping set of frames to be formed.
Port States and Roles
To understand the basic operation of STP, we need to understand the operation of the state machine for each port at each bridge, as well as the contents of BPDUs. Each port in each bridge may be in one of five states: blocking, listening, learning, forwarding, and disabled. The relationship among them can be seen in the state transition diagram shown in Figure 3-14.