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In this series of exercises, you will explore in more details the operation of TCP and its congestion control scheme. TCP is a very important protocol in today's Internet since most applications use it to exchange data. We first look at TCP in more details by injecting segments in the Linux TCP stack and analyze how the stack reacts. Then we study the TCP congestion control scheme.
Injecting segments in the Linux TCP stack
Packet capture tools like tcpdump_ and Wireshark_ are very useful to observe the segments that transport protocols exchange. TCP is a complex protocol that has evolved a lot since its first specification :rfc:`793`. TCP includes a large number of heuristics that influence the reaction of a TCP implementation to various types of events. A TCP implementation interacts with the application through the ``socket`` API.
packetdrill_ is a TCP test suite that was designed to develop unit tests to verify the correct operation of a TCP implementation. A more detailed description of packetdrill_ may be found in [CCB+2013]_. packetdrill_ uses a syntax which is a mix between the C language and the tcpdump_ syntax. To understand the operation of packetdrill_, we first discuss several examples. The TCP implementation in the Linux kernel supports all the recent TCP extensions to improve its performance. For pedagogical reasons, we disable [#fsysctl]_ most of these extensions to use a simple TCP stack. packetdrill_ can be easily installed on recent Linux kernels [#finstall]_.
Let us start with a very simple example that uses packetdrill_ to open a TCP connection on a server running on the Linux kernel. A packetdrill_ script is a sequence of lines that are executed one after the other. There are three main types of lines in a packetdrill_ script.
packetdrill_ executes a system call and verifies its return value
packetdrill_ injects [#ftcpdump_pdrill]_ a packet in the instrumented Linux kernel as if it were received from the network
packetdrill_ compares a packet transmitted by the instrumented Linux kernel with the packet that the script expects
For our first packetdrill_ script, we aim at reproducing the simple connection shown in the figure below.
Let us start with the execution of a system call. A simple example is shown below.
The ``0`` indicates that the system call must be issued immediately. packetdrill_ then executes the system call and verifies that it returns ``3```. If yes, the processing continues. Otherwise the script stops and indicates an error.
For this first example, we program packetdrill_ to inject the segments that a client would send. The first step is thus to prepare a :manpage:`socket` that can be used to accept this connection. This socket can be created by using the four system calls below.
At this point, the socket is ready to accept incoming TCP connections. packetdrill_ needs to inject a TCP segment in the instrumented Linux stack. This can be done with the line below.
Each line of a packetdrill_ script starts with a `timing` parameter that indicates at what time the event specified on this line should happen. packetdrill_ supports absolute and relative timings. An absolute timing is simply a number that indicates the delay in seconds between the start of the script and the event. A relative timing is indicated by using ``+`` followed by a number. This number is then the delay in seconds between the previous event and the current line. Additional information may be found in [CCB+2013]_.
The description of TCP packets in packetdrill_ uses a syntax that is very close to the tcpdump_ one. The ``+0`` timing indicates that the line is executed immediately after the previous event. The ``<`` sign indicates that packetdrill_ injects a TCP segment and the ``S`` character indicates that the ``SYN`` flag must be set. Like tcpdump_, packetdrill_ uses sequence numbers that are relative to initial sequence number. The three numbers that follow are the sequence number of the first byte of the payload of the segment (``0``), the sequence number of the last byte of the payload of the segment (``0`` after the semi-column) and the length of the payload (``0`` between brackets) of the ``SYN`` segment. This segment does not contain a valid acknowledgment but advertises a window of 1000 bytes. All ``SYN`` segments must also include the ``MSS`` option. In this case, we set the MSS to 1000 bytes. The next line of the packetdrill_ script verifies the reply sent by the instrumented Linux kernel.
This TCP segment is sent immediately by the stack. The ``SYN`` flag is set and the dot next to the ``S`` character indicates that the ACK flag is also set. The SYN+ACK segment does not contain any data but its acknowledgment number is set to 1 (relative to the initial sequence number). For outgoing packets, packetdrill_ does not verify the value of the advertised window. In this line, it also accepts any TCP options (``<...>``).
The third segment of the three-way handshake is sent by packetdrill_ after a delay of 0.1 seconds. The connection is now established and the accept system call will succeed.
The :manpage:`accept` system call returns a new file descriptor, in this case value ``4``. At this point, packetdrill_ can write data on the socket or inject packets.
packetdrill_ writes 10 bytes of data through the :manpage:`write` system call. The stack immediately sends these 10 bytes inside a segment whose ``Push`` flag is set [#fpush]_. The payload starts at sequence number ``1`` and ends at sequence number ``10``. packetdrill_ replies by injecting an acknowledgment for the entire data after 100 milliseconds.
packetdrill_ can also inject data that will be read by the stack as shown by the lines below.
In the example above, packetdrill_ injects a segment containing two bytes. This segment is acknowledged and after that the :manpage:`read` system call succeeds and reads the available data with a buffer of 1000 bytes. It returns the amount of read bytes, i.e. ``2``.
We can now close the connection gracefully. Let us first issue inject a segment with the ``FIN`` flag set.
packetdrill_ injects the ``FIN`` segment and the instrumented kernel returns an acknowledgment. If packetdrill_ issues the :manpage:`close` system call, the kernel will send a ``FIN`` segment to terminate the connection. packetdrill_ injects an acknowledgment to confirm the end of the connection.
The complete packetdrill_ script is available from :download:`/exercises/packetdrill_scripts/connect.pkt`.
Another interesting features of packetdrill_ is that it is possible to inspect the state maintained by the Linux kernel for the underlying connection using the ``TCP_INFO`` socket option. This makes it possible to retrieve the value of some variables of the TCP control block.
Let us first explore how a TCP connection can be established. In the previous script, we have injected the segments that a client would send to a server. We can also use the Linux stack as a client and inject the segments that a server would return. Such a client process would first create its :manpage:`socket`` and then issue the :manpage:`connect` system call. At this point, the stack sends a ``SYN`` segment. To simplify the scripts, we have configured the stack to use a ``MSS`` of 1000 bytes and disabled the TCP extensions (the details of this configuration may be found at the beginning of the script). The server replies with a ``SYN+ACK`` and the stack sends acknowledges it to finish the three-way-handshake.
The ``tcpi_state`` variable used in this script is returned by ``TCP_INFO`` [#ftcpinfo]_. It tracks the state of the TCP connection according to TCP's finite state machine [#fstates]_. This script is available from :download:`/exercises/packetdrill_scripts/client.pkt`.
Another example is the simultaneous establishment of a TCP connection. The TCP stack sends a ``SYN`` and receives a ``SYN`` in response instead of a ``SYN+ACK``. It then acknowledges the received ``SYN`` and retransmits its own ``SYN``. The connection becomes established upon reception of the fourth segment. This script is available from :download:`/exercises/packetdrill_scripts/dual.pkt`.
Another usage of packetdrill_ is to explore how a TCP connection ends. The scripts below show how a TCP stack closes a TCP connection. The first example shows a local host that connects to a remote host and then closes the connection. The remote host acknowledges the ``FIN`` and later closes its direction of data transfer. This script is available from :download:`/exercises/packetdrill_scripts/local-close.pkt`.
As for the establishment of a connection, it is also possible for the two communicating hosts to close the connection at the same time. This is shown in the example below where the remote host sends its own ``FIN`` when acknowledging the first one. This script is available from :download:`/exercises/packetdrill_scripts/local-close2.pkt`.
A third scenario for the termination of a TCP connection is that the remote hosts sends its ``FIN`` first. This script is available from :download:`/exercises/packetdrill_scripts/remote-close.pkt`.

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cnp3-ebook/exercises/tcp-2
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Source string location
../../exercises/tcp-2.rst:95
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4 years ago
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Source string age
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locale/pot/exercises/tcp-2.pot, string 17