Princeton University Computer Science Department

Computer Science 461 Computer Networks Jennifer Rexford

Spring 2006

Contents

• Overview
• Getting Started
• MYSOCK Layer Definition
• STCP Protocol Definition
• Overview
• STCP Packet Format
• Sequence Numbers
• Data Packets
• ACK Packets
• Sliding Windows
• Network Initiation
Network Termination
• Glossary
• Transport Layer
• Network Layer
• Assignment FAQ
• Description of Requirements
• Testing of the code
• Miscellaneous Notes
• Deliverables
• Support Code

Overview

To help you get started, we're providing you with a skeleton system in which you will implement the MYSOCK and STCP layers. In fact, the MYSOCK layer has already been implemented for you;  you get to add the the functionality needed for the transport layer. The skeleton consists of a network layer, a bogus transport layer, and also a dummy client and server application to help you debug your socket and transport layer. When you have debugged your transport layer and had it work with the dummy client and server, you'll be compiling it with your ftp client code from the first assignment. We will provide an ftp server against which you can then test your new ftp client code.

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 reliable datagram communication service with a peer application; i.e. it will send and receive data between a client and server. As you'll see if you delve into our code, we actually implemented the so-called 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.

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.

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 this file into a suitable directory and then run the command:

tar zxvf tcp.tgz

This will create a directory "tcp" into which all the stub files will be put. The only file that you should edit is transport.c, and the only API that you should call are in stcp_api.h. Note that there are a few structures and macros in transport.h that you might be interested in using.

We provide a Makefile which will build a version of the dummy client and server suitable for use under gdb. The Makefile expects all of your code to be put into the file transport.c. Please do not change the Makefile for the first two milestones. If you need to modify it to test something, create a new Makefile and use "make -f NewMakefile". This new makefile will not be used in the grading.

MYSOCK layer interface definition

STCP Protocol Definition

Overview

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 FTP client of HW#1 that "actively" opens a control connection and "passively" listens on a data connection).

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 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 */
#ifdef _BIT_FIELDS_LTOH
        u_int   th_x2:4,                /* (unused) */
                th_off:4;               /* data offset */
#else
        u_int   th_off:4,               /* data offset */
                th_x2:4;                /* (unused) */
#endif
        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 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.

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

• Sequence numbers sequentially number the SYN flag, FIN flag, and bytes of data, starting with a randomly chosen sequence number between 0 and 255, both inclusive. (The sequence numbers are exchanged using a 3-way SYN handshake in STCP as explained later).
• The transport layer manages two streams for each connection: incoming and outgoing data. The sequence numbers of these streams are independent of each other.
• Both SYN and FIN indicators are associated with one byte of the sequence space.
• The sequence number should always be set in every packet. If the packet is a pure ACK packet (i.e., no data, and the SYN/FIN flags are unset), the sequence number should be set to the next unsent sequence number. (This is purely for compatibility with real TCP implementations in HW4; they discard packets with sequence numbers that fall well outside the sender's window. STCP itself doesn't care about sequence numbers in pure ACK packets).

Data Packets

• The maximum payload size is 536 bytes.
• The th_seq field in the packet header contains the sequence number of the first byte in the payload.
• Data packets are sent as soon as data is available from the application. STCP performs no optimizations for grouping small amounts of data on the first transmission.

ACK Packets

• Data is acknowledged by setting the ACK bit in the flags field in the packet header. If this bit is set, then the th_ack field contains the sequence number of the next byte of data the receiver expects (i.e. one past the last byte of data already received). This is true no matter what data is being acknowledged. The th_seq field should be set to the sequence number that would be associated with the next byte of data sent to the peer. The th_ack field should be set whenever possible (this again isn't required by STCP, but is done for compatibility with standard TCP in HW4).
• Data may accompany an acknowledgement. STCP is not required to generate such packets (i.e. if you have outstanding data and acknowledgements to send, you may send these separately), but it is required to be capable of handling such packets.
• Acknowledgements are not delayed in STCP as they are in TCP. An acknowledgment should be sent as soon as data is received.
• If a packet is received that contains duplicate data, a new acknowledgment should be sent.
• Acknowledgements are only sent for the last contiguous received piece of data. For example, assume the last acknowledgment sent was 10. A packet containing data with sequence number 15 of length 5 is received. An acknowledgment for this packet cannot be sent because of the gap at sequence number 10 of length 5. Instead, an ACK should be sent for 10 again. However, once all the data in this range has been received, the correct acknowledgment to send is for sequence number 20.

Sliding 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 as and when data has been acknowledged. The size of the sender window indicates the maximum amount of data that can be unacknowledged at any instant. Its size is equal to the other side's receiver window.

The rules for managing the windows are:

• The local receiver window has a fixed size of 3072 bytes.
• The sender window is the minimum of the other side's advertised receiver window, and the congestion window.
• The congestion window has a fixed size of 3072 bytes.
• As stated in the acknowledgment section, data which is received with sequence numbers lower than the start of the receiver window generate an appropriate acknowledgment, but the data is discarded.
• Received data which has a sequence number higher than that of the last byte of the receiver window is discarded and remains unacknowledged.
• Note that received data may cross either end of the current receiver window; in this case, the data is split into two parts and each part is processed appropriately.
• Do not send any data outside of your sender window.
• The first byte of all windows is always the last acknowledged byte of data. For example, for the receiver window, if the last acknowledgment was sequence number 8192, then the receiver is willing to accept data with sequence numbers of 8192 through 11263 (=8192+3072-1), inclusive.

Options

The following rules apply for handling TCP options:

• STCP does not generate options in the packets it sends.
• STCP ignores options in any packets sent by a peer. It must be capable of correctly handling packets that include options.

Network Initiation

• The requesting active end sends a SYN packet to the other end with the appropriate seq number (seqx) in the header. The active end then sits waiting for a SYN_ACK.
• The passive end sends a SYN_ACK with its own sequence number (seqy) and acks with the number (seqx+1). The passive end waits for an ACK.
• The active end records seqy so that it will expect the next byte from the passive end to start at (seqy+1). It ACKs the passive end's SYN request, and changes its state to the established state.

For more details, be sure to read RFC 793.

Network Termination

• When one of the peers has finished sending data, it transmits a FIN segment to the other side. At this point, no more data will be sent from that peer. The FIN flag may be set in the last data segment transmitted by the peer. (You are not responsible for generating such packets, but your receiver must be capable of handling them). The usual rules for sequence numbers apply for the FIN segment.
• The peer waits for an ACK of its FIN segment, as usual. If both sides have sent FINs and received acknowledgements for them, the connection is closed once the FIN is acknowledged. Otherwise, the peer continues receiving data until the other side also initiates a connection close request. If no ACK for the FIN is ever received it terminates its end of the connection anyway.

RFC 793 includes more details on connection termination; see in particular the state diagram. Note that you are not required to support TIME_WAIT.

Glossary

ACK packet

An acknowledgment packet; any segment with the ACK bit set in the flags field of the packet header.

Connection

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.

Payload

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.

Segment

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.

Network

The data path between two hosts provided by the network layer.

Stream

An ordered sequence of bytes with no other structure imposed. In an STCP connection, two streams are maintained: one in each direction.

Window

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

extern void transport_init(mysocket_t sd, bool_t is_active);

This initialises 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.

Network Layer

The network layer provides an interface for the connectionless and unreliable datagram service delivery mechanism. 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. You are not required, but are highly recommended, to study the implementation of the functions in the network layer.

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

Description of Requirements

• You need to do everything that is required to make the client and server programs work assuming no packet drops. Thus, you need not worry about dropped packets, reordering, retransmissions, timeouts in the code. To grade your implementation, we will first make sure that no packets are dropped or appear out of order and then test whether your code meets the remaining functionality.
We have supplied you the code for an ftp server daemon which is already in the tarball that you should have downloaded. You can compile it with: make ftpd_tcp or just run make and it should be there.

This ftp server talks STCP over our simulated network layer, and you are required to modify your ftp client from the first assignment to work with this server when your ftp client uses the mysock/stcp implementation you have completed. You need to make a few changes to your ftpclient code. These changes are described here. Alternatively, you could use our solution to Assignment#1 already modified appropriately for this milestone. However these object files do not contain symbol information, so you cannot step into them when debugging your code.

To add your code into the build, edit the Makefile line: FTPCOPY_SRCS = to point to your .c files, and then run make your.ftpcopy_tcp or just run make and it should compile automatically.

Testing your code

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.

Deliverables

1. Your modified transport.c (You are not allowed to modify or submit any other .c or .h file)
2. README describing the design of your transport layer. One page is enough for the writeup.
3. *.c - Source code for ftpclient (only if using your ftpcopy)
4. Makefile

Links

• RFC 793 - TCP
• RFC 1122 - Requirements for Internet Hosts (Communication Layers)