2019计算机网络实验指南：协议层至Socket编程

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实验一 Protocol Layer

- 9 -

“demultiplexing key” to determine the higher layer.

Look at the Ethernet and IP headers of a download packet in detail to answer the following

questions:

1. Which Ethernet header field is the demultiplexing key that tells it the next higher

layer is IP? What value is used in this field to indicate “IP”?

2. Which IP header field is the demultiplexing key that tells it the next higher layer is

TCP? What value is used in this field to indicate “TCP”?

Turn-in: Hand in your answers to the above questions.

Explore on your own

We encourage you to explore protocols and layering once you have completed this lab. Some

ideas:

• Look at a short TCP packet that carries no higher-layer data. To what entity is this

packet destined? After all, if it carries no higher-layer data then it does not seem

very useful to a higher layer protocol such as HTTP!

• In a classic layered model, one message from a higher layer has a header appended

by the lower layer and becomes one new message. But this is not always the case.

Above, we saw a trace in which the web response (one HTTP message comprised of

an HTTP header and an HTTP payload) was converted into multiple lower layer

messages (being multiple TCP packets). Imagine that you have drawn the packet

structure (as in step 2) for the first and last TCP packet carrying the web response.

How will the drawings differ?

• In the classic layered model described above, lower layers append headers to the

messages passed down from higher layers. How will this model change if a lower

layer adds encryption?

• In the classic layered model described above, lower layers append headers to the

messages passed down from higher layers. How will this model change if a lower

layer adds compression?

[END]

实验二 Ethernet

- 10 -

实验二 Ethernet

Objective

To explore the details of Ethernet frames. Ethernet is a popular link layer protocol that is covered

in §4.3 of your text; modern computers connect to Ethernet switches (§4.3.4) rather than use

classic Ethernet (§4.3.2). Review section §4.3 before doing this lab.

Requirements

Wireshark: This lab uses the Wireshark software tool to capture and examine a packet trace. A

packet trace is a record of traffic at a location on the network, as if a snapshot was taken of all the

bits that passed across a particular wire. The packet trace records a timestamp for each packet,

along with the bits that make up the packet, from the lower-layer headers to the higher-layer

contents. Wireshark runs on most operating systems, including Windows, Mac and Linux. It

provides a graphical UI that shows the sequence of packets and the meaning of the bits when

interpreted as protocol headers and data. It color-codes packets by their type, and has various ways

to filter and analyze packets to let you investigate the behavior of network protocols. Wireshark is

widely used to troubleshoot networks. You can download it from www.wireshark.org

if it is not

already installed on your computer. We highly recommend that you watch the short, 5 minute

video “Introduction to Wireshark” that is on the site.

ping: This lab uses “ping” to send and receive messages. ping is a standard command-line

utility for checking that another computer is responsive. It is widely used for network

troubleshooting and comes pre-installed on Window, Linux, and Mac. While ping has various

options, simply issuing the command “ping www.bing.com” will cause your computer to send

a small number of ICMP ping requests to the remote computer (here www.bing.com), each of

which should elicit an ICMP ping response.

Step 1: Capture a Trace

Proceed as follows to capture a trace of ping packets; alternatively you may use a supplied trace.

We will use ping simply as an easy way to collect a small trace. Perhaps surprisingly, you can

capture a trace for this lab from a computer connected to the Internet using either wired Ethernet

or wireless 802.11.

1. Pick a remote web server or other publicly reachable Internet host and use ping to

send some ping messages and check that it sends replies. For example, “ping

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