TCP/IP Illustrated Episode 21

Loopback

Although it may seem surprising, in many cases clients may wish to communicate with servers on the same computer using Internet protocols such as TCP/IP. To enable this, most implementations support a network-layer loopback capability that typically takes the form of a virtual loopback network interface. It acts like a real network interface but is really a special piece of software provided by the operating system to enable TCP/IP and other communications on the same host computer. IPv4 addresses starting with 127 are reserved for this, as is the IPv6 address ::1 (see Chapter 2 for IPv4 and IPv6 addressing conventions). Traditionally, UNIXlike systems including Linux assign the IPv4 address of 127.0.0.1 (::1 for IPv6) to the loopback interface and assign it the name localhost. An IP datagram sent to the loopback interface must not appear on any network. Although we could imagine the transport layer detecting that the other end is a loopback address and shortcircuiting some of the transport-layer logic and all of the network-layer logic, most implementations perform complete processing of the data in the transport layer and network layer and loop the IP datagram back up in the network stack only when the datagram leaves the bottom of the network layer. This can be useful for performance measurement, for example, because the amount of time required to execute the stack software can be measured without any hardware overheads. In Linux, the loopback interface is called lo.

MTU and Path MTU

As we can see from Figure 3-3, there is a limit on the size of the frame available for carrying the PDUs of higher-layer protocols in many link-layer networks such as Ethernet. This usually limits the number of payload bytes to about 1500 for Ethernet and often the same amount for PPP in order to maintain compatibility with Ethernet. This characteristic of the link layer is called the maximum transmission unit (MTU). Most packet networks (like Ethernet) have a fixed upper limit. Most stream-type networks (serial links) have a configurable limit that is then used by framing protocols such as PPP. If IP has a datagram to send, and the datagram is larger than the link layer’s MTU, IP performs fragmentation, breaking the datagram up into smaller pieces (fragments), so that each fragment is smaller than the MTU.

[RFC1191] specifies the path MTU discovery (PMTUD) mechanism for IPv4, and [RFC1981] describes it for IPv6. A complementary approach that avoids some of the issues with these mechanisms is described in [RFC4821]. PMTU discovery is used to determine the path MTU at a point in time and is required of IPv6 implementations. In later chapters we shall see how this mechanism operates after we have described ICMP and IP fragmentation. We shall also see what effect it can have on transport performance when we discuss TCP and UDP.

Tunneling Basics

In some cases it is useful to establish a virtual link between one computer and another across the Internet or other network. VPNs, for example, offer this type of service. The method most commonly used to implement these types of services is called tunneling. Tunneling, generally speaking, is the idea of carrying lower-layer traffic in higher-layer (or equal-layer) packets. For example, IPv4 can be carried in an IPv4 or IPv6 packet; Ethernet can be carried in a UDP or IPv4 or IPv6 packet, and so on. Tunneling turns the idea of strict layering of protocols on its head and allows for the formation of overlay networks (i.e., networks where the “links” are really virtual links implemented in some other protocol instead of physical connections). It is a very powerful and useful technique. Here we discuss the basics of some of the tunneling options.

Unidirectional Links

An interesting issue arises when the link to be used operates in only one direction. Such links are called unidirectional links (UDLs), and many of the protocols described so far do not operate properly in such circumstances because they require exchanges of information (e.g., PPP’s configuration messages). To deal with this situation, a standard has been created whereby tunneling over a second Internet interface can be combined with operation of the UDL [RFC3077]. The typical situation where this arises is an Internet connection that uses a satellite for downstream traffic (headed to the user) and a dial-up modem link for upstream traffic. This setup can be useful in cases where the satellite-connected user’s usage is dominated by downloading as opposed to uploading and was commonly used in early satellite Internet installations. It operates by encapsulating link-layer upstream traffic in IP packets using a GRE encapsulation.