COS-461 Assignments: Simple TCP

Assignment 3/4: Implementing a Reliable Transport Layer



In the take home assignment, you learned about the socket interface and how it is used by an application. By now, you're pretty much an expert in how to use the socket interface over a reliable transport layer, so now seems like a good time to implement your own socket layer and reliable transport layer! That's what you'll be doing in this assignment. You'll get to learn how the socket interface is implemented by the kernel and how a reliable transport protocol like TCP runs on top of an unreliable delivery mechanism. We're going to call your socket layer MYSOCK and it will contain all the features and calls that you used in Assignment #0. Your socket layer will implement a transport layer that we'll call STCP (Simple TCP), which is in essence a stripped down version of TCP. STCP is compatible with TCP, and provides a reliable, connection-oriented, in-order, full duplex end-to-end delivery mechanism. It is similar to early versions of TCP, which did not implement congestion control or optimizations such as selective ACKs or fast retransmit.

To help you get started, we're providing you with a skeleton system in which you will implement MYSOCK. In fact, the MYSOCK application socket layer has already been implemented for you; you get to add the functionality needed for the transport layer. The skeleton consists of a network layer, a bogus transport layer that you need to fill in, the MYSOCK socket interface, and also a dummy client and server application to help you debug your socket and transport layer.

There are two assignments: Assignment 3 is due April 22nd and Assignment 4 is due Dean's Date (May 10th). Assignment 4 is an extension to Assignment 3.

The assignments are split up as follows. Please also read the detailed information about each of the milestones here after you have read the basic STCP functionality on this web page.

Important: STCP is not TCP! While STCP is designed to be compatible with TCP, there are many distinct differences between the two protocols. When in doubt, the specifications in this assignment description should be used in your implementation.

The Structure of the Code

Network Layer

At the lowest layer is the network layer. We provide you with a fully functional network layer that emulates an unreliable datagram communication service with a peer application; i.e. it will send and receive data between a client and server, but does not guarantee that the data will arrive, or that it will arrive in order. As you'll see if you delve into our code, we actually implemented the so-called unreliable datagram service over a regular TCP connection. For the purposes of this assignment, it just appears to you and your code as a network layer.

You're going to find it helpful to force the network layer to be unreliable. To emulate the behavior of a congested multi-path network, setting the is_reliable parameter to false when creating a socket (use the -U flag when running the program) will cause the network layer to randomly reorder and drop packets. You'll see an example of how this is done in our dummy client/server code.

Transport Layer

The next layer up is the transport layer. We provide you with a bogus minimal transport layer in which some basic functions are already implemented. It is provided only so that the client and server will compile (but NOT run), and to give you an example of how to use the socket/transport/network layer calls. This is where you will implement the STCP functionality.

Application Layer

The application layers that we give you are the dummy client and dummy server. The dummy client and server are very simple and are provided to aid you with the debugging of your transport layer. When executed, the client prompts for a filename which it sends to the server. The server responds by sending back the contents of the file. The client stores this file locally under the filename "rcvd". The client can also ask for a file from the server on the command line in a non-interactive mode. The client and server work as expected if the file "rcvd" on the machine where the client is running is identical to the file asked for at the server machine. You may change the client and server as much as you like for debugging purposes. We will not use your versions of the dummy client and server for grading; in fact, we might grade your project with some other (simple and similar) application. Both client and server accept the -U flag to make the network layer unreliable. The client also accepts the -q option, which suppresses the output of the received data to the file.

Getting Started

Download the STCP tarball linked at the top of this document and extract it into a new directory in your Unix account. A Makefile is included for you in the tarball- if for some reason you need to do something different with make for testing purposes, please create your own Makefile and build with it by calling make -f yourMakefile during development. Your code must build with the standard Makefile when you submit!

MYSOCK Layer Definition

This section details the protocol your transport layer will implement. Be sure to also read RFC 793, which describes TCP in more detail.


STCP is a full duplex, connection oriented transport layer that guarantees in-order delivery. Full duplex means that data flows in both directions over the same connection. Guaranteed delivery means that your protocol ensures that, short of catastrophic network failure, data sent by one host will be delivered to its peer in the correct order. Connection oriented means that the packets you send to the peer are in the context of some pre-existing state maintained by the transport layer on each host.

STCP treats application data as a stream. This means that no artificial boundaries are imposed on the data by the transport layer. If a host calls mywrite() twice with 256 bytes each time, and then the peer calls myread() with a buffer of 512 bytes, it will receive all 512 bytes of available data, not just the first 256 bytes. It is STCP's job to break up the data into packets and reassemble the data on the other side.

STCP labels one side of a connection active and the other end passive. Typically, the client is the active end of the connection and server the passive end. But this is just an artificial labeling; the same process can be active on one connection and passive on another (e.g., the HTTP proxy of HW#2 that "actively" opens a connection to a web server and "passively" listens for client connections).

The networking terms we use in the protocol specification have precise meanings in terms of STCP. Please refer to the glossary.

STCP Packet Format

An STCP packet has a maximum segment size of 536 bytes. It has the same header format as TCP. The header format is defined in transport.h as follows:

typedef uint32_t tcp_seq;

struct tcphdr {
        uint16_t th_sport;              /* source port */
        uint16_t th_dport;              /* destination port */
        tcp_seq th_seq;                 /* sequence number */
        tcp_seq th_ack;                 /* acknowledgment number */
        u_int   th_x2:4,                /* (unused) */
                th_off:4;               /* data offset */
        u_int   th_off:4,               /* data offset */
                th_x2:4;                /* (unused) */
        uint8_t th_flags;
#define TH_FIN  0x01
#define TH_SYN  0x02
#define TH_RST  0x04
#define TH_PUSH 0x08
#define TH_ACK  0x10
#define TH_URG  0x20
        uint16_t th_win;                 /* window */
        uint16_t th_sum;                 /* checksum */
        uint16_t th_urp;                 /* urgent pointer */
        /* options follow */

typedef struct tcphdr STCPHeader;

For this assignment, you are not required to handle all fields in this header. Specifically, the provided network layer wrapper code sets th_sport, th_dport, and th_sum, while th_urp is unused; you may thus ignore these fields. Similarly, you are not required to handle all legal flags specified here: TH_RST, TH_PUSH, and TH_URG are ignored by STCP. The fields STCP uses are shown in the following table. Note that any relevant multi-byte fields of the STCP header will entail proper endianness handling with htonl/ntohl or htons/ntohs

The packet header field format (for the relevant fields) is as follows:

Field Type Description
th_seq tcp_seq Sequence number associated with this packet.
th_ack tcp_seq If this is an ACK packet, the sequence number being acknowledged by this packet. This may be included in any packet.
th_off 4 bits The offset at which data begins in the packet, in multiples of 32-bit words. (The TCP header may be padded, so as to always be some multiple of 32-bit words long). If there are no options in the header, this is equal to 5 (i.e. data begins twenty bytes into the packet).
th_flags uint8_t Zero or more of the flags (TH_FIN, TH_SYN, etc.), or'ed together.
th_win uint16_t Advertised receiver window in bytes, i.e. the amount of outstanding data the host sending the packet is willing to accept.

Sequence Numbers

STCP assigns sequence numbers to the streams of application data by numbering the bytes. The rules for sequence numbers are:

Data Packets

The following rules apply to STCP data packets:

ACK Packets

In order to guarantee reliable delivery, data must be acknowledged. The rules for acknowledging data in STCP are:

Sliding Windows

There are two windows that you will have to take care of: the receiver and sender windows.

The receiver window is the range of sequence numbers which the receiver is willing to accept at any given instant. The window ensures that the transmitter does not send more data than the receiver can handle.

Like TCP, STCP uses a sliding window protocol. The transmitter sends data with a given sequence number up to the window limit. The window "slides" (increments in the sequence number space) when data has been acknowledged. The size of the sender window, which is equal to the other side's receiver window, indicates the maximum amount of data that can be "in flight" and unacknowledged at any instant, i.e. the difference between the last byte sent and the last byte ack'd.

The rules for managing the windows are:

TCP Options

The following rules apply for handling TCP options:


It is an ugly fact of networking life that packets are lost. STCP detects this when no acknowledgment is received within a timeout period. The rules for timeouts are:

Network Initiation

Normal network initiation is always initiated by the active end. Network initiation uses a three-way SYN handshake exactly like TCP, and is used to exchange information about the initial sequence numbers. The order of operations for initiation is as follows:

For more details, be sure to read RFC 793. Pay special attention to each state in the connection setup, including the simultaneous open scenario

Network Termination

As in TCP, network termination is a four-way handshake between the two peers in a connection. The order of closing is independent of the network initialization order. Each side indicates to the other when it has finished sending data. This is done as follows:

RFC 793 includes more details on connection termination; pay special attention to the TCP state diagram as you will need to implement the majority of the FSM in the transport layer. Note that you are not required to support TIME_WAIT.


ACK packet
An acknowledgment packet; any segment with the ACK bit set in the flags field of the packet header.
The entire data path between two hosts, in both directions, from the time STCP obtains the data to the time it is delivered to the peer.
Data packet
Any segment which has a payload; i.e. the th_off field of the packet header corresponds to an offset less than the segment's total length.
FIN packet
A packet which is participating in the closing of the connection; any segment with the FIN bit set in the flags field of the packet header.
The optional part of the segment which follows the packet header and contains application data. Payload data is limited to a maximum size of 536 bytes in STCP.
Any packet sent by STCP. A segment consists of a required packet header and an optional payload.
Sequence Number
The uniquely identifying index of a byte within a stream.
The data path between two hosts provided by the network layer.
An ordered sequence of bytes with no other structure imposed. In an STCP connection, two streams are maintained: one in each direction.
Of a receiver's incoming stream, the set of sequence numbers for which the receiver is prepared to receive data. Defined by a starting sequence number and a length. In STCP, the length is fixed at 3072.

Transport Layer Interface

The interface to the transport layer is given in transport.h. The interface consists of only one function:

extern void transport_init(mysocket_t sd, bool_t is_active);
This initializes the transport layer, which runs in its own thread, one thread per connection. This function should not return until the connection ends. sd is the 'mysocket descriptor' associated with this end of the connection; is_active is TRUE if you should initiate the connection.

To implement the STCP transport layer, the only file you need to modify is transport.c. While STCP is a simplified version of TCP, it still implements the vast majority of the TCP FSM. Within transport.c, aside from transport_init, there is also a stub for a local function control_loop() where you should implement the majority of the "event-driven" STCP transport FSM. By event-driven we mean use of the stcp_wait_for_event() function to receive signals from the application layer for data or connection close, the network layer for incoming packets, and timeouts for implementing retransmission timers. Each iteration of the control_loop() should handle the current set of pending events and update the state of the transport FSM accordingly.

Network Layer Interface

The network layer provides an interface for the connectionless and unreliable datagram service delivery mechanism. Underpinning this interface are a pair of send/recv queues used for communciation between the transport and network layer threads. Your transport layer will build reliability on top of this layer using the functions implemented in the network layer. The interfaces are defined in stcp_api.h. Study it well. You are not required, but are highly recommended, to study the implementation of the functions in the network layer. Note that stcp_network_send() takes a variable number of arguments, but in general use, you will either use it with a single argument (full STCP packet buffer or just a STCP header buffer) or with two arguments (STCP header buffer, STCP data buffer). The last argument to stcp_network_send() must be NULL to demarcate the end of the vararg list.

Application Layer Interface

The application level socket interface is used by the client/server programs to establish STCP connections: myconnect(), mybind(), mylisten(), myaccept(), etc and to send/recv data. Underlying the interface between the application and transport layer are a pair of send/recv queues for communication between the two threads. All the transport layer needs to know is when there is data available on the recv queue and when the application has closed the connection, which is communicated via the stcp_wait_for_event() mechanism. Once again, study the interface functions defined in stcp_api.h well as they will be the essential interface for communciation and control between the transport layer and the application layer above and the network layer below.

Please note that you may only use the interfaces declared in stcp_api.h in your own code. You must not call any other (internal) functions used in the mysock implementation.

Assignment FAQ

A FAQ is also available. Please look over it before asking your question to your TA.

Descriptions of Milestones

Testing Your Code

The provided file transfer server and client should be used to test your code in both reliable and unreliable mode. You may modify the code for the client and server however you wish for testing purposes. We will be grading your submission using our own clients and servers, which will be similar to the provided client/server pair, and our STCP reference implementation.

Miscellaneous Notes and Hints

  1. The calls myconnect() and myaccept() block till a connection is established (or until an error is detected during the connection request). To cause them to unblock and return to the calling code, use the stcp_unblock_application() interface found in stcp_api.h.
  2. mybind() followed by mygetsockname() does not give the local IP address; mygetsockname() (like the real getsockname()) does not return the local address until a remote system connects to that mysocket.
  3. We will be testing your code on the Linux machines in the lab. Make sure your code compiles and runs correctly on those systems. Also make sure that you kill all your jobs when you are not running them. In the past we have had complaints from the sysadmins about many zombies.
  4. Correct endianness will be tested. Don't neglect to include your ntohs() and htons(), etc. calls where appropriate. If you forget them, your code may seem to work correctly while talking to other hosts of similar endianness, but break when talking to systems running on a different OS.

Extra Credit: SYN Cookies

For extra credit for assignment 4 (up to 20%), you can implement SYN cookies for your TCP connection setup equivalent. Recall that SYN cookies are designed to prevent denial-of-service attacks against the server-side of the TCP connection: they prevent a client from sending an initial SYN (possibly from a spoofed IP address) and therefore causing the server to allocate in-kernel connection contexts and buffers, without ever finishing the connection setup (or possibly being able to receive responses to the spoofed IP address).

The TCP specification allows each endpoints' ISN to be any value decided by that endpoint. SYN cookies ensure that ISNs are carefully constructure in the following fashion.

  1. Let ts = time() mod 32 (i.e., the time function logically right-shifted 6 positions, which gives a 64-second resolution).
  2. Let mss = some encoding representing possible choices for the server's MSS. In STCP, the MSS is given as 536, which you'll represent as the 3-bit value "001". (In typical SYN cookies for TCP, because the mss must later be encoded in 3 bits, the server is restricted to 8 unique values.)
  3. Let seq = func (client IP, client port, server IP, server port, ts);
where this cryptographic function also uses a secret known only to the server. For this assignment, you should implement this function using SHA-1, in a manner akin to the following pseudocode. A common SHA-1 implementation can be found in the openssl library, which is already installed on most linux machines. See here for a man page.
   server_init () {
      cookie_secret = random ();
   generate_syn_cookie (clientip, clientport, serverip, serverport, timestamp) {
      output = SHA1 (cookie_secret, clientip, clientport, serverip, serverport, timestamp);
      return output[last 24 bits];

Then, let the 32-bit ISN be encoded as:

   ISN = ts || mss || seq

That is, the top-5 bits encode the timestamp, the next 3 bits encode the MSS value (here, use "001"), and the bottom 24 bits include seq.

Recall that when a client response to a SYN request with an SYN+ACK, the client MUST use the server's ISN+1 in the packet's acknowledgement number. Then server then subtracts 1 from this acknowledgement number to reveal the SYN cookie sent the client.

The server then validates the SYN cookie as follows:

  1. Checks the SYN cookie's ts against the current time to see if the connection is expired.
  2. Recomputes seq' and validates it against the SYN cookie's seq to ensure they are the same.

From this point forward, the connection proceeds as normal. More information about SYN cookies can be found on Dan Bernstein's webpage.

Your modified STCP server implementation should not use SYN cookies by default, but you should supply the optional "-C" argument to turn on SYN cookies on the server.

If you do this extra credit, please briefly explain your design in the README file.


The deliverables for Assignment 3 and Assignment 4 of this assignment are:

  1. Your modified transport.c You are not allowed to modify any other .c or .h files found in the stub code download.
  2. README describing the design of your transport layer, and any design decisions/tradeoffs that you had to consider. One page is enough for the writeup.


To submit Assignment 4 run "make tar2" to create a .tgz file, run "make tar2" to create a .tgz file for Assignment 4. Submit the files via Blackboard.

Submit your assignment 4 here.


Last updated: Sun May 08 00:19:15 -0400 2011