CS402: Computer Communications and Networks

In a transport protocol such as TCP that offers a bytestream, a practical issue that was left as an implementation choice in RFC 793 is to decide when a new TCP segment containing data must be sent. There are two simple and extreme implementation choices. The first implementation choice is to send a TCP segment as soon as the user has requested the transmission of some data. This allows TCP to provide a low delay service. However, if the user is sending data one byte at a time, TCP would place each user byte in a segment containing 20 bytes of TCP header 17. This is a huge overhead that is not acceptable in wide area networks. A second simple solution would be to only transmit a new TCP segment once the user has produced MSS bytes of data. This solution reduces the overhead, but at the cost of a potentially very high delay.

An elegant solution to this problem was proposed by John Nagle in RFC 896. John Nagle observed that the overhead caused by the TCP header was a problem in wide area connections, but less in local area connections where the available bandwidth is usually higher. He proposed the following rules to decide to send a new data segment when a new data has been produced by the user or a new ack segment has been received

if rcv.wnd>= MSS and len(data) >= MSS:
 send one MSS-sized segment
else
 if there are unacknowledged data:
 place data in buffer until acknowledgement has been received
 else
 send one TCP segment containing all buffered data

The first rule ensures that a TCP connection used for bulk data transfer always sends full TCP segments. The second rule sends one partially filled TCP segment every round-trip-time.

This algorithm, called the Nagle algorithm, takes a few lines of code in all TCP implementations. These lines of code have a huge impact on the packets that are exchanged in TCP/IP networks. Researchers have analyzed the distribution of the packet sizes by capturing and analyzing all the packets passing through a given link. These studies have shown several important results:

• in TCP/IPv4 networks, a large fraction of the packets are TCP segments that contain only an acknowledgment. These packets usually account for 40-50% of the packets passing through the studied link
• in TCP/IPv4 networks, most of the bytes are exchanged in long packets, usually packets containing up to 1460 bytes of payload which is the default MSS for hosts attached to an Ethernet network, the most popular type of LAN

The figure below provides a distribution of the packet sizes measured on a link. It shows a three-modal distribution of the packet size. 50% of the packets contain pure TCP acknowledgments and occupy 40 bytes. About 20% of the packets contain about 500 bytes 18 of user data and 12% of the packets contain 1460 bytes of user data. However, most of the user data is transported in large packets. This packet size distribution has implications on the design of routers as we discuss in the next chapter.

Figure 4.43: Packet size distribution in the Internet

Recent measurements indicate that these packet size distributions are still valid in today’s Internet, although the packet distribution tends to become bimodal with small packets corresponding to TCP pure acks (40-64 bytes depending on the utilization of TCP options) and large 1460-bytes packets carrying most of the user data.

Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
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