2

Transport Media

The choice of transport media may, at first sight, appear to be a relatively arbitrary decision based more on issues of convenience and functionality than on scalability. However, the choice of transport media, and the architecture within which it is used, can have a significant impact on the scalability of the network.

This chapter will give a brief description of the features of each of the transport media and any scalability issues associated with carrying IP traffic. In some cases, these scalability issues are more generic, affecting Layer 3 protocols when using that Layer 2 technology. In other cases, the scalability issues are specific to the transport of IP.

There are a number of issues which are common to more than one transport media. We will take a look at these independently from the specific transport media.

2.1 MAXIMUM TRANSMISSION UNIT (MTU)

The MTU, as the name suggests, is the largest frame that can be transmitted across a particular link. In the original Internet, packets were transmitted with an MTU of 576 bytes or the MTU of the directly connected network, whichever was the lower. An MTU of 1500 bytes is now the default on Ethernet-based media with much higher MTUs possible on most media. If a router receives a frame on one interface, which has a frame size greater than the MTU of the outgoing interface, then it must split the packet into two or more packets so that each packet is small enough to be transmitted. This process of fragmentation is computationally expensive and may require that the packet being fragmented be sent to the central processor. Packets are not rebuilt until they arrive at the destination host.

Designing and Developing Scalable IP Networks G. Davies

2004 Guy Davies ISBN: 0-470-86739-6

8	TRANSPORT MEDIA

Sending packets with artificially low MTUs is inefficient and can lead to significantly lower throughput than could be achieved if the optimum MTU is used. This is because the IP and MAC headers are effectively a fixed size and as the MTU drops, the IP and MAC headers form a larger proportion of the total data transmitted over the wire.

This leads to the conclusion that fragmentation is a bad thing. As such, the ideal situation is to avoid fragmentation altogether. One means of assuring that fragmentation is not used is simply to ensure that the core of the network uses an MTU higher than that used at the edge. Almost all end hosts are attached to media with MTUs less than or equal to 1500 bytes. However, it is common to use technologies in the core of the network, which support MTUs in excess of 4400 bytes and some support MTUs of more than 9000 bytes.

2.1.1 PATH MTU DISCOVERY

A more elegant technical solution to this problem is the use of Path MTU Discovery. This mechanism works by transmitting packets with the MTU set to the MTU of the directly connected link and the Don’t Fragment (DF) bit set. If any link along the path has an MTU smaller than that used to transmit the packet, an ICMP Destination Unreachable message would be returned to the sender. This message includes a field, the Next-Hop MTU, that includes an indication of the MTU of the hop across which the entire packet could not be transmitted without fragmentation. Upon receipt of the ICMP Destination Unreachable packet, the source must reduce the MTU in line with the Next-Hop MTU. This process ends when either the packets start reaching their destination or if the source host decides to stop further reducing the MTU and clears the DF bit in subsequently transmitted packets.

This process is totally reliant upon the generation of ICMP Destination Unreachable messages. However, ICMP Destination Unreachables can be used as a means of generating a denial of service attack on routing devices because the generation of ICMP messages takes up processor cycles, which would otherwise be used for other functions. Because of the threat of denial of service attacks, some network operators configure their routers not to send ICMP Unreachables. This breaks Path MTU Discovery.

Path MTU Discovery is particularly important for IP security (IPsec), MPLS and other tunnel-based services. In many cases tunnels do not tolerate the fragmentation of packets so it is essential to ensure that excessively large packets are not injected into the tunnel.

2.1.2 PORT DENSITY

This could be viewed as part of the hardware chapter. However, since different transport media are presented via different copper and fibre presentations, the issues can be usefully discussed here.

There are a variety of connectors associated with both copper and fibre presentations. Copper is presented via RJ45, RJ48, various BNC type connectors, X.21 among others. Fibre has a similarly wide array of connectors including SC, ST, FC-PC, LC and MT-RJ.