"Linux Gazette...making Linux just a little more fun!"

T/TCP: TCP for Transactions

By Mark Stacey, John Nelson and Ivan Griffin

T/TCP is an experimental extension for the TCP protocol. It was designed to address the need for a transaction-based transport protocol in the TCP/IP stack. TCP and UDP are the current choices available for transaction-based applications. Both of these protocols have their advantages and disadvantages. TCP is reliable but inefficient for transactions whereas UDP is unreliable but highly efficient. T/TCP sits between these two protocols making it an alternative for certain applications.

Currently, a number of flavours of UNIX support T/TCP. SunOS 4.1.3 (a Berkeley-derived kernel) was the very first implementation of T/TCP and made available in September 1994. The next implementation was for FreeBSD 2.0 and released in March 1995. For my final year project, I implemented T/TCP for Linux for the University of Limerick in April 1998. The source code is available at http://www.csn.ul.ie/~heathclf/fyp/.

In this article, I discuss the operation, advantages and flaws of T/TCP. This will allow application developers to decide when T/TCP is appropriate for networking applications. I present my results of a comparative analysis between T/TCP and TCP based on the number of packets per session for each transaction, as well as my conclusions on a case study into the possible impact of T/TCP on the World Wide Web.

1 Introduction

The TCP/IP reference model is a specification for a networking stack on a computer. It exists to provide a common ground for network developers. This allows easier interconnection of the different vendor supplied networks, reducing the cost of installing completely new networks in order for one to work with another.

The most popular implementation of the transport layer in the reference model is the Transmission Control Protocol (TCP). This is a connection-oriented protocol. Another popular implementation is the User Datagram Protocol (UDP), which is a connectionless protocol.

Both of these protocols have advantages and disadvantages. The two main aspects of the protocols make them useful in different areas. UDP is a connectionless protocol. UDP always assumes that the destination host received the data correctly. The application layer above it looks after error detection and recovery. Even though UDP is unreliable, it is quite fast and useful for applications, such as DNS (Domain Name System) where speed is preferred over reliability. TCP, on the other hand, is a reliable, connection-oriented protocol. It looks after error detection and recovery. Data is retransmitted automatically if a problem is detected. As a result of being more reliable, TCP is a slower protocol than UDP.

In recent years, with the explosion of the Internet, a need for a new specification arose. The current transport protocols were either too verbose or not reliable enough. A protocol was needed that was faster than TCP but more reliable than UDP. These protocols lie at either end of the scale in taking into account speed and reliability. TCP has reliability at the cost of speed, whereas UDP has speed at the cost of reliability. A standard was needed, that would allow the reliable transmission of data at a faster rate than the current TCP standard. This new protocol could reduce bandwidth and increase the transmission speed of data.

TCP for Transactions (T/TCP) is envisioned as the successor to both TCP and UDP in certain applications. T/TCP is a transaction-oriented protocol based on a minimum transfer of segments, so it does not have the speed problems associated with TCP. By building on TCP, it does not have the unreliability problems associated with UDP. With this in mind, RFC1379 was published in November 1992. It discussed the concepts involved in extending the TCP protocol to allow for a transaction-oriented service, as opposed to a connection-oriented service for TCP and a connectionless service for UDP. Some of the main points that the RFC discussed were the bypassing of the 3-way handshake and the shortening of the TIME-WAIT state from 240 seconds to 12 seconds. T/TCP cuts out much of the unnecessary handshaking and error detection of the current TCP protocol and as a result increases the speed of connection and reduces the necessary bandwidth. Eighteen months later, RFC1644 was published, with the specification for Transaction TCP.

2 Transaction Transmission Control Protocol

T/TCP can be considered a superset of the TCP protocol. The reason for this is that T/TCP is designed to work with current TCP machines seamlessly. If a TCP host tries to connect to a T/TCP host, the T/TCP host will respond with the original TCP 3-way handshake. What follows is a brief description of T/TCP and how it differs to the current TCP standard in operation.

2.1 What is a Transaction?

The term transaction refers to the request sent by a client to a server along with the server's reply. RFC955 lists some of the common characteristics of transaction processing applications:

2.2 Background to T/TCP

The growth of the Internet has put a strain on the bandwidth and speed of networks. With more users than ever, the need for a more efficient form of data transfer is needed.

The absolute minimum number of packets required for a transaction is two: one request followed by one response. UDP is the one protocol in the TCP/IP protocol stack that allows this, the problem here being the unreliability of the transmission.

T/TCP solves these problems to a large degree. It has the reliability of TCP and comes very close to realizing the two-packet exchange (three in fact). T/TCP uses the TCP state model for its timing and retransmission of data, but introduces a new concept to allow the reduction in packets.

Even though three packets are sent using T/TCP, the data is carried on the first two; thus, the applications can see the data with the same speed as UDP. The third packet is the acknowledgment to the server by the client that it has received the data, which incorporates the TCP reliability.

2.3 Basic Operation


Figure 1. Time Line of T/TCP/Client-Server Transaction

Consider a DNS system, one where a client sends a request to a server and expects a small amount of data in return. A diagram of the transaction can be seen in Figure 1. This diagram is very similar to a UDP request. In comparison to the TCP 3-way handshake in Figure 2 it can be seen that an equal number of packets are required for this transaction and the 3-way handshake. Whereas with TCP three packet transmissions are associated with the establishment of a connection alone (with nine altogether), a total of three packet transmissions are associated with the whole process--a savings of 66% in packets transferred compared to TCP. Obviously, in cases where a large amount of data is being transferred, more packets will be transmitted, resulting in a decrease in the percentage saving. Timing experiments have shown a slightly longer time is required for T/TCP than for UDP, but this is a result of the speed of the computer and not the network. As computers get more powerful, the performance of T/TCP will approach that of UDP.


Figure 2. TCP 3-way handshake

2.4 TCP Accelerated Open

The TCP Accelerated Open (TAO) is a mechanism introduced by T/TCP designed to cut down the number of packets needed to establish connection with a host.

T/TCP introduces a number of new options. These options allow the establishment of a connection with a host using the TAO. T/TCP uses a 32-bit incarnation number, called a connection count (CC). This option is carried in the options part of a T/TCP segment, Figure 3. A distinct CC value is assigned to each direction of an open connection. Incremental CC values are assigned to each connection that a host establishes, either actively or passively.


Figure 3. TCP Header

The 3-way handshake is bypassed using the CC value. Each server host caches in memory (or in a file) the last valid CC value it received from each different client host. This CC value is sent with the initial SYN segment to the server. If the initial CC value for a particular client host is larger than the corresponding cached value, the property of the CC options (the increasing numbers) ensures the SYN segment is new and can be accepted immediately.

The TAO test fails if the CC option that arrives in the SYN segment is smaller than the last CC value received and cached by the host or if a CCnew option is sent. The server then initiates a 3-way handshake in the normal TCP/IP fashion. Thus, the TAO test is an enhancement to TCP, with the normal 3-way handshake to fall back on for reliability and backward compatibility.

2.5 Truncation of TIME-WAIT

The TIME-WAIT state is a state entered by all TCP connections when the connection has been closed. The length of time for this state is 240 seconds to allow any duplicate segments still in the network from the previous connection to expire. The introduction of the CC option in T/TCP allows for the truncation of the TIME-WAIT state. The CC option provides protection against old duplicates being delivered to the wrong incarnation of a given connection.

Time constraints are placed on this truncation, however. Because the CC value from the host is monotonically increasing, the numbers may wrap around from the client host. A CC value that is the same as some duplicate segments from the previous incarnation can be encountered. As a rule, the truncation can only be performed whenever the duration of the connection is less than the maximum segment lifetime (MSL). The recommended value for the MSL is 120 seconds. As with the original TCP, the host that sends the first FIN is required to remain in the TIME-WAIT state for twice the MSL once the connection is completely closed at both ends. This implies that the TIME-WAIT state with the original TCP is 240 seconds, even though some implementations of TCP have the TIME-WAIT set to 60 seconds. Stevens shows how the TIME-WAIT state for T/TCP may be shortened to 12 seconds.

CC options do have problems when used on networks with high-speed connections. This is rarely a problem on older networks, but with FDDI and gigabit Ethernets becoming more frequent, the wrapping of the CC value will become more frequent. In this situation, the CC value may wrap around fast enough for problems to occur. Whereas CC options are not sufficient in certain conditions, the PAWS (protection against wrapped sequences) option adds another layer of security against this problem.

2.6 Examples

T/TCP can be beneficial to some of the applications which currently use TCP or UDP. At the moment, many applications are transaction-based rather than connection-based, but still must rely on TCP along with the overhead. UDP is the other alternative, but not having time-outs and retransmissions built into the protocol means the application programmers must supply the time outs and reliability checking. Since T/TCP is transaction-based, there is no set-up and shutdown time, so the data can be passed to the process with minimal delay.

2.6.1 HTTP and RPC

Hypertext Transfer Protocol is the protocol used by the World Wide Web to access web pages. The number of round trips used by this protocol is more than necessary. T/TCP can be used to reduce the number of packets required.

HTTP is the classic transaction style application. The client sends a short request to the server requesting a document or an image and then closes connection. The server then sends on the information to the client. T/TCP can be used to improve this process and reduce the number of packets on the network.

With TCP, the transaction is accomplished by connecting to the server (3-way handshake), requesting the file (GET file), then closing the connection (sending a FIN segment). T/TCP operates by connecting to the server, requesting the document and closing the connection all in one segment (TAO). It is obvious that bandwidth has been saved.

Remote Procedure Calls also adhere to the transaction style paradigm. A client sends a request to a server for the server to run a function. The results of the function are then returned in the reply to the client. Only a tiny amount of data is transferred with RPCs.

2.6.2 DNS

The Domain Name System is used to resolve host names into the IP addresses that are used to locate the host. To resolve a domain name, the client sends a request with the IP address or a host name to the server. The server responds with the host name or IP address where appropriate. This protocol uses UDP as its underlying process.

As a result of using UDP, the process is fast but not reliable. Furthermore, if the response by the server exceeds 512 bytes of data, it sends the data back to the client with the first 512 bytes and a truncated flag. The client has to resubmit the request using TCP, since there is no guarantee that the receiving host will be able to reassemble an IP datagram exceeding 576 bytes. For safety, many protocols limit the user data to 512 bytes.

T/TCP is the perfect candidate for the DNS protocol, because of its speed and reliability.

2.7 Summary

T/TCP provides a simple mechanism that allows the number of segments involved in a data transmission to be reduced--the TAO. This test allows a client to open a connection, send data and close a connection all in one segment. With TCP, opening a connection, transmission of data and the closing of the connection are all completely separate processes.

The highest savings result with small data transfers. This leads to the conclusion that T/TCP favors situations with small amounts of data to be transferred. HTTP, RPCs and DNS are protocols that require the exchange of small amounts of data.

3. Testing and Analysis

In order to investigate the benefits or drawbacks of this implementation of T/TCP, it is important to both test its operation and compare it to the original TCP/IP operation. I performed these tests using the Linux 2.0.32 kernel with T/TCP modifications and FreeBSD version 2.2.5 that already implements T/TCP.

3.1 Operation Examples

This section demonstrates the operation of the protocol under various conditions.

3.1.1 Client Reboot

In this scenario, I rebooted the client and the TAO cache has been reinitialized.

When the client attempts a connection with a server, it finds that the latest CC value received from the server is undefined. Hence it sends a CCnew option to indicate that a 3-way handshake is needed.

The sequence of segments below conforms to the protocol implementation.

elendil.ul.ie.2177 > devilwood.ece.ul.ie.8888: SFP 3066875000:3066875019(19) win 15928 <mss 1460,nop,nop,ccnew 10> (DF)

devilwood.ece.ul.ie.8888 > elendil.ul.ie.2177: S 139872882:139872882(0) ack 3066875001 win 17424 <mss 1460,nop,nop,cc 3, nop,nop,ccecho 10> (DF)

elendil.ul.ie.2177 > devilwood.ece.ul.ie.8888: F 20:20(0) ack 1 win 15928 <nop,nop,cc 10> (DF)

devilwood.ece.ul.ie.8888 > elendil.ul.ie.2177: . ack 21 win 17405 <nop,nop,cc 3> (DF)

devilwood.ece.ul.ie.8888 > elendil.ul.ie.2177: FP 1:31(30) ack 21 win 17424 <nop,nop,cc 3> (DF)

elendil.ul.ie.2177 > devilwood.ece.ul.ie.8888: . ack 32 win 15928 <nop,nop,cc 10> (DF) 3.1.2 Normal T/TCP Transaction
Once the client has completed its first transaction with the server, the CC value in the TAO cache will contain a number. This allows the client to send a normal CC option, indicating to the server that it may bypass the 3-way handshake if possible.

The client and the server hold state information about the other host, so the TAO test succeeds and the minimal 3-segment exchange is possible.

elendil.ul.ie.2178 > devilwood.ece.ul.ie.8888: SFP 2021229800:2021229819(19) win 15928 <mss 1460,nop,nop,cc 11> (DF)

devilwood.ece.ul.ie.8888 > elendil.ul.ie.2178: SFP 164103774:164103804(30) ack 2021229821 win 17424 <mss 1460,nop,nop,cc 4, nop,nop,ccecho 11>

elendil.ul.ie.2178 > devilwood.ece.ul.ie.8888: . ack 32 win 15928 <nop,nop,cc 11> (DF)

3.1.3 Server Reboot

If the server is rebooted after the previous two tests, all the state information about the host will be lost.

When the client request arrives with a normal CC option, the server forces a 3-way handshake since the CC value received from the client is undefined. The SYNACK segment forces the 3-way handshake when only the client SYN and not the data are acknowledged.

elendil.ul.ie.2141 > devilwood.ece.ul.ie.8888: SFP 2623134527:2623134546(19) win 15928 <mss 1460,nop,nop,cc 9> (DF)

arp who-has elendil.ul.ie tell devilwood.ece.ul.ie

arp reply elendil.ul.ie is-at 0:20:af:e1:41:4e

devilwood.ece.ul.ie.8888 > elendil.ul.ie.2141: S 25337815:25337815(0) ack 2623134528 win 17424 <mss 1460,nop,nop,cc 2, nop,nop,ccecho 9> (DF)

elendil.ul.ie.2141 > devilwood.ece.ul.ie.8888: F 20:20(0) ack 1 win 15928 <nop,nop,cc 9> (DF)

devilwood.ece.ul.ie.8888 > elendil.ul.ie.2141: . ack 21 win 17405 <nop,nop,cc 2> (DF)

devilwood.ece.ul.ie.8888 > elendil.ul.ie.2141: FP 1:31(30) ack 21 win 17424 <nop,nop,cc 2> (DF)

elendil.ul.ie.2141 > devilwood.ece.ul.ie.8888: . ack 32 win 15928 <nop,nop,cc 9> (DF)

3.1.4 Request or Reply Exceeds MSS

If the initial request exceeds the maximum segment size allowed, the request will have to be fragmented.

When the server receives the initial SYN with just the data and no FIN, depending on the time outs, it either responds with a SYNACK immediately or waits for the FIN bit to arrive before responding with the SYNACK that acknowledges all of the data. The server then proceeds to send the multi-packet response if required.

localhost.2123 > localhost.8888: S 2184275328:2184278860(3532) win 14128 <mss 3544,nop,nop,cc 5> (DF)

localhost.2123 > localhost.8888: FP 2184278861:2184279329(468) win 14128 <nop,nop,cc 5>: (DF)

localhost.8888 > localhost.2123: S 1279030185:1279030185(0) ack 2184278861 win 14096 <mss 3544,nop,nop,cc 6,nop,nop,ccecho 5>

localhost.2123 > localhost.8888: F 469:469(0) ack 1 win 14128 <nop,nop,cc 5> (DF)

localhost.8888 > localhost.2123: . ack 470 win 13627 <nop,nop,cc 6> (DF)

localhost.8888 > localhost.2123: FP 1:31(30) ack 470 win 13627 <nop,nop,cc 6> (DF)

localhost.2123 > localhost.8888: . ack 32 win 14128 <nop,nop,cc 5> (DF)

3.1.5 Backward Compatibility

Because T/TCP is a superset of TCP, it must be able to communicate seamlessly with other hosts not running T/TCP.

There are a couple of different scenarios in this situation. Some implementations hold the data in the SYN until the 3-way handshake has passed. In this situation the client only needs to resend the FIN segment to let the server know that all the data has been sent. The server then responds with normal TCP semantics.

In other implementations, the SYN segment is dumped once it has been processed, including the data sent in the initial SYN. The server sends a SYNACK acknowledging only the SYN sent. The client times out after a period and resends the data and FIN. The server then proceeds as normal.

When testing the implementation for backward compatibility, I found an unusual feature (bug?) of Linux. When a SYN is sent with the FIN bit set, the Linux host responds with the SYNACK segment but also with the FIN bit turned on. This causes the client to mistakenly believe the server has sent the reply back to the client.

This problem was traced to the way Linux constructs its SYNACK segment. It copies the header of the original SYN (and so all the flags), then sets all the flags except the FIN flag. This results in the Linux host sending a FIN without knowing it. I pointed this out to the developers of the Linux kernel. Their reasoning was that T/TCP leaves hosts open to a SYN flood attack and as such should not be allowed into main stream protocols. As it turned out, it was only a small check that was needed to solve this problem.

elendil.ul.ie.2127 > skynet.csn.ul.ie.http: SFP 520369398:520369417(19) win 15928 <mss 1460,nop,nop,ccnew 7> (DF)

skynet.csn.ul.ie.http > elendil.ul.ie.2127: SF 2735307581:2735307581(0) ack 520369399 win 32736 <mss 1460>

elendil.ul.ie.2127 > skynet.csn.ul.ie.http:  F 20:20(0) ack 1 win 15928 (DF)<\n>

skynet.csn.ul.ie.http > elendil.ul.ie.2127: . ack 1 win 32736 (DF)

elendil.ul.ie.2127 > skynet.csn.ul.ie.http: FP 520369399:520369418(19) win 15928 <mss 1460,nop,nop,ccnew 7> (DF)<\n>

skynet.csn.ul.ie.http > elendil.ul.ie.2127: . ack 21 win 32716 (DF)

skynet.csn.ul.ie.http > elendil.ul.ie.2127: P 1:242(241) ack 21 win 32736 (DF)

skynet.csn.ul.ie.http > elendil.ul.ie.2127: F 242:242(0) ack 21 win 32736

elendil.ul.ie.2127 > skynet.csn.ul.ie.http: . ack 243 win 15928 (DF)

3.2 Performance Analysis

To investigate the performance of T/TCP in comparison to the original TCP/IP, I compiled a number of executables that returned different sized data to the client. The two hosts involved were elendil.ul.ie (running Linux) and devilwood.ece.ul.ie (running FreeBSD 2.2.5). The tests were performed for 10 different response sizes to vary the number of segments required to return the full response. Each request was sent 50 times and the results averaged. The maximum segment size in each case is 1460 bytes.

The metric measured used for performance evaluation was the average number of segments per transaction. I used Tcpdump to examine the packets exchanged. Note that Tcpdump is not entirely accurate. During fast packet exchanges, it tends to drop some packets to keep up. This accounts for some discrepancies in the results.

3.2.1 Number of Packets per Transaction


Figure 4. Number of Segments versus Size of Data Transfer

Figure 4 shows the testing results for the number of segments for T/TCP versus number of segments for normal TCP/IP. It is immediately obvious that there is a saving of an average five packets. These five packets are accounted for in the 3-way handshake and the packets sent to close a connection. Lost packets and retransmissions cause discrepancies in the path of the graph.

When using a TCP client and a T/TCP server, there is still a saving of one segment. A normal TCP transaction requires nine segments, but because the server was using T/TCP, the FIN segment was piggybacked on the final data segment, reducing the number of segments by one. Thus, a reduction in segments results even if just one side is T/TCP aware.


Figure 5. Percentage Savings per Size of Data Transfer

Figure 5 shows the percentage savings for the different packet sizes. The number of packets saved remains fairly constant, but because the number of packets being exchanged increases, the overall savings decreases. This indicates T/TCP is more beneficial to small data exchanges. These test results were obtained from two hosts on the same intranet. For comparison purposes, the tests were repeated for a host on the Internet; www.elite.net was chosen as the host. Requests were sent to the web server for similar sized data. Figure 6 shows these results. This graph is not as smooth as the graph seen in Figure 4 due to a higher percentage of packets being lost and retransmitted.


Figure 6. Number of Segments versus Size of Data Transfer for Internet Host

3.3 Memory Issues

The main memory drain in the implementation is in the routing table. In Linux, for every computer that the host comes into contact with, an entry for the foreign host is made in the routing table. This applies to a direct connection or along a multi-hop route. This routing table is accessed through the rtable structure. The implementation of T/TCP adds in two new fields to this structure, CCrecv and CCsent.

The entire size of this structure is 56 bytes. This isn't a major memory hog on a small stand-alone host. On a busy server though, where the host communicates with perhaps thousands of other hosts an hour, it can be a major strain on memory. Linux has a mechanism where a route that is no longer in use can be removed from memory. A check is run periodically to clean out unused routes and those that have been idle for a time.

The problem here is the routing table holds the TAO cache. Thus, any time a route containing the last CC value from a host is deleted, the local host has to re-initiate the 3-way handshake with a CCnew segment.

A separate cache can be created to hold the TAO values, but the route table is the handiest solution. Also, a check can be added when cleaning out the routing entries for a CC value other than zero (undefined). In this case, the route could either be left for a longer time span or permanently.

The benefits of leaving the routing entries up permanently are clear. The most likely use of this option would be a situation where a host only talks to a certain set of foreign hosts and denies access to unknown hosts. In this case, it is advantageous to keep a permanent record in memory so that the 3-way handshake can be bypassed more often.

3.4 Protocol Analysis

The original protocol specification (RFC1644) labeled T/TCP as an experimental protocol. Since the RFC was published no updates have been made to the protocol to fix some of the problems. The benefits are obvious compared to the original TCP protocol, but is it a case of the disadvantages out-weighing the advantages?

One of the more serious problems with T/TCP is that it opens the host to certain denial-of-service attacks. SYN flooding (see http://www.sun.ch/SunService/technology/bulletin/bulletin963.html for more information) is the term given to a form of denial of service attack where the attacker continually sends SYN packets to a host. The host creates a sock structure for each of the SYNs, thus reducing the number of sock structures that can be made available to legitimate users. This can eventually result in the host crashing if enough memory is used up. SYN cookies were implemented in the Linux kernel to combat this attack. It involves sending a cookie to the sender to verify the connection is valid. SYN cookies cause problems with T/TCP as no TCP options are sent in the cookie and any data arriving in the initial SYN can't be used immediately. The CC option in T/TCP does provide some protection on its own, but it is not secure enough.

Another serious problem was discovered during research was that attackers can by-pass rlogin authentication. An attacker creates a packet with a false IP address in it, one that is known to the destination host. When the packet is sent, the CC options allow the packet to be accepted immediately, and the data passed on. The destination host then sends a SYNACK to the original IP address. When this SYNACK arrives, the original host sends a reset, as it is not in a SYN-SENT state. This happens too late, as the command will already have been executed on the destination host. Any protocol that uses an IP address as authentication is open to this sort of attack. (See http://geek-girl.com/bugtraq/1998_2/0020.html.) There are methods of avoiding this security hole.

Kerberos is a third-party authentication protocol but requires the presence of a certification authority and an increase in the number of packets transferred. The IP layer has security and authentication built into it. With the new IP version being standardized, IPv6, the authentication of IP packets will be possible without third-party intervention. This is accomplished through the use of an authentication header that provides integrity and authentication without confidentiality.

RFC1644 also has a duplicate transaction problem. This can be serious for non-idempotent applications (repeat transactions are very undesirable). Requesting time from a timeserver can be considered idempotent because there is no adverse effect results on either the client or the server if the transaction is repeated. In the case of a banking system however, if an account transaction were repeated accidentally, the owner would either gain or lose twice as much as anticipated. This error can occur in T/TCP if a request is sent to a server and the server processes the transaction, but before it sends back an acknowledgment the process crashes. The client side times out and retransmits the request, if the server process recovers in time, it can repeat the same transaction. This problem occurs because the data in a SYN can be immediately passed onto the process, rather then in TCP where the 3-way handshake has to be completed before data can be used. The use of two-phase commits and transaction logging can keep this problem from occurring.

3.5 Summary

This chapter illustrates the required functionality of T/TCP for Linux. It also displays the advantages in speed and efficiency T/TCP has over normal TCP.

T/TCP admittedly has some serious problems, but these problems are not relevant to all situations. Where hosts have some form of protection (other than pure T/TCP semantics) and basic security precautions are taken, T/TCP can be used without any worries.

4. Case Study: T/TCP Performance over Suggested HTTP Improvements

With the World Wide Web being the prime example of a client-server transaction processing nowadays, this section will focus on the benefits of T/TCP to the performance of the Web.

Currently, the HTTP protocol sits in the application layer of the TCP/IP reference model. It uses the TCP protocol to carry out all its operations, UDP being too unreliable. There is a lot of latency involved in the transfer of information, the 3-way handshake and the explicit shutdown exchanges being the examples. Using the criteria specified in section 2.1 it is apparent that the World Wide Web's operation is one of transactions.

4.1 Web Document Characteristics

In a survey of 2.6 million web documents searched by the Inktomi web crawler search engine (see: http://inktomi.berkeley.edu) it was found that the mean document size on the world wide web was 4.4KB, the median size was 2.0KB and the maximum size that was encountered was 1.6MB.

Referring to figure 3.2 it can be seen that the lower the segment size, the better the performance of T/TCP over normal TCP/IP. With a mean document size of 4.4KB, this results in an average saving of just over 55% in the number of packets. When taking the median size into account, there is a saving of approximately 60%.

Time-wise there will be an improvement in speed, depending of course on the reliability of the network.

4.2 Suggested Performance Improvements for HTTP

There have been a number of suggestions put forward to improve the operation of HTTP and reduce the time and bandwidth required downloading information. Most of these suggestions have as their basis compression and/or delta encoding.

4.2.1 Compression

At the moment, all web pages are transferred in plaintext form, requiring little work from either the server side or the client side to display the pages.

In order to introduce compression into the HTTP protocol, a number of issues would have to be resolved.

First and foremost would be the issue of backward compatibility, with the web having reached so far across the world, switching to compression would take a long time. Browsers need to be programmed to handle compressed web pages and web servers also need to be configured to compress the information requested before sending it onto the user. It would be a straightforward task for the IETF (Internet Engineering Task Force) to introduce a compression standard. It would then be up to the vendors and application writers to modify the browsers and servers for the new standard.

Another issue would be the load placed on the server when it is requested to compress the information. Many busy servers would not have the power to handle the extra workload. This holds to a lesser extent on the client side, with a minimal overhead involved in decompressing a few pages at a time.

In their paper ˘Network Performance Effects of HTTP/1.1, CSSI and PNG÷, the authors investigated the effect of introducing compression to the HTTP protocol. They found that the compression resulted in a 64% saving in the speed of downloading with a 68% decrease in the number of packets required. Over normal TCP/IP, this brings the packet exchanges and size of data down to the level where T/TCP becomes beneficial. Thus a strategy involving both compression and T/TCP can result in enormous savings in time and bandwidth.

4.2.2 Delta Encoding

In this situation, a delta refers to the difference between two files. On UNIX systems, the diff command can be used to generate the delta between two files. Using the changed file, and the delta, the original file can be regenerated again, and vice-versa.

For delta encoding on the web, the client initially requests a document and the complete document is downloaded. This will result in about a 55% benefit if using T/TCP and taking into account the mean size of a document. Once the client has the page, it can be cached and stored indeterminately. When the client requests the document the next time, the browser will already have the original document cached. Using delta encoding, the browser would present the web server with the last date the cached document was modified. The server determines if the document has been updated since the cached copy was stored, and if so, a delta of the server side document is created. The delta, rather than the original document are transferred.

Of course, there are quite a few difficulties that need to be considered.

  1. The client needs to retain a cached copy of the document. This is not so much a hassle with more modern browsers, as this is already done. In fact the HTTP protocol defines a command that can be used to request the last modified date from a document on a server. This is then compared to the cached document and a decision made whether to download the new file, or display the original.
  2. From the server side, multiple versions of the document have to be cached to allow the server to create deltas. A decision has to be made of how many changed versions are allowed. Should the older versions be kept in the user side, or should a separate database of old versions be kept? A more detailed study of the impact of caching documents can be found in Braun & Claffy's book (see Resources).
  3. In the case where there have been a number of updates to the server side document since the client side was cached, it should be decided how many updates are allowed before the new document is sent, as opposed to sending a delta. The more changes applied to a document, the larger a delta is, hence, a loss in the savings by using delta encoding.
  4. Again there is the question of the load placed on the server by generating a delta for each document requested, similar to the compression method.
Mogul, et al. (see Resources) investigated the effect that delta encoding has on the web. In their testing, they not only used delta encoding; they also compressed the delta generated to further reduce the amount of information transferred. They discovered that using the ˘vdelta÷ delta generator and compression they could achieve up to 83% savings in the transmission of data.

If this method was used with T/TCP, there could be as much as a further 66% saving in packets transferred. This is a total of 94% reduction in packet transfer.

It should be noted however that this is a best case scenario. In this situation, the document will already have been cached on both the server and the client side, and the client and server will previously have completed the 3-way handshake in order to facilitate the TAO tests.

4.2.3 Persistent HTTP

RFC2068 describes a modification to HTTP that maintains a continuous connection to an HTTP server for multiple requests, P-HTTP. This removes the inefficiency of continually reconnecting to a web server to download multiple images from the same page. The constant connection and reconnection results in a lot of unnecessary overhead.

Some advantages over the original HTTP protocol are:

  1. Opening and closing fewer TCP connections save CPU time and memory.
  2. Multiple HTTP requests and responses can be sent without waiting for a response that would otherwise be necessary when opening and closing multiple connections.
  3. Network congestion is reduced since there are fewer packets.
This technique is one step away from T/TCP. Instead of using transactions, it uses persistent connections much like the TELNET protocol. In this situation T/TCP would not be of much benefit, the connection will remain open for a length of time, with multiple requests being exchanged. This violates the transaction characteristics discussed in section 2.1.

4.3 Summary

Using the results obtained in section 3 and the characteristics of documents available on the World Wide Web, a study is presented on how T/TCP can benefit, or otherwise, some of the suggestions for improving the HTTP protocol.

The main case for the introduction of compression and delta encoding is the reduction in the size of the data that needs to be transferred. The results obtained from the performance analysis of T/TCP suggest that a greater benefit be obtained on small data transfers. The compression and delta encoding ideas result in data small enough that can be sent in one packet. Under these conditions, T/TCP operates best.

P-HTTP puts forward the idea that a connection should be semi-permanent, unlike the current open-close operation HTTP currently employs. In this scenario, T/TCP will not work at all because of its transaction-oriented style.

5. Socket Programming Under T/TCP

Programming for T/TCP is slightly different using socket programming.

As an example, the chain of system calls to implement a TCP client would be as follows:

Whereas with T/TCP the chain of commands would be:

The sendto function has to be able to use a new flag MSG_EOF, to indicate to the kernel that it has no more data to send on this connection. This is the transaction-processing coming into effect.

Programming under T/TCP is much like programming under UDP.

6. Conclusion

T/TCP was originally designed to address the need for a more efficient protocol for transaction style applications. The original protocols defined in the TCP/IP reference model were either too verbose or not reliable enough.

T/TCP works by building on the TCP protocol and introducing a number of new options that allow the 3-way handshake to be bypassed in certain situations. When this occurs, the transaction can almost realize the minimum number of segments that are required for a data transfer. T/TCP can reduce the average number of segments involved in a transaction from 9 (TCP) to 3 using the TAO test. This has potential benefits to overloaded networks where there is a need to introduce a more efficient protocol.

Analysis of T/TCP shows that it benefits small transaction-oriented transfers more than large-scale information transfer. Aspects of transactions can be seen in such cases as the World Wide Web, Remote Procedure Calls and DNS. These applications can benefit from the use of T/TCP in efficiency and speed. T/TCP reduces on average both the numbers of segments involved in a transaction and the time taken.

As T/TCP is still an experimental protocol, there are problems that need to be addressed. Security problems encountered include the vulnerability to SYN flood attacks and rlogin authentication bypassing. Operational problems include the possibility of duplicate transactions occurring. Problems that occur less frequently would be the wrapping of the CC values on high-speed connections and thus opening up a destination host to accepting segments on the wrong connection.

Many people recognize the need for a protocol that favors transaction style processing and are willing to accept T/TCP as the answer. The security considerations lead to the conclusion that T/TCP would be more useful in a controlled environment, one where there is little danger from a would-be attacker who can exploit the weaknesses of the standard. Examples of enclosed environments would be company Intranets and networks protected by firewalls. With a lot of companies seeing the web as the future of doing business, internal and external, a system employing T/TCP and some of the improvements to HTTP, such as compression and delta encoding, would result in a dramatic improvement in speed within a company Intranet.

Where programmers are willing to accept T/TCP as a solution to their applications, there are only minor modifications needed for the application to become T/TCP aware. For client side programming, it involves the elimination of the connect() and shutdown() function calls, which can be replaced by adding the MSG_EOF flag to the sendto() command. Server side modifications involve simply adding the MSG_EOF flag to the send() function.

In conclusion, researches into T/TCP suggest that it is a protocol that is nearly, but not quite, ready to take over transaction processing for general usage. For T/TCP alone, more work needs to be done to develop it further and solve the security and operational problems. Security problems can be solved using other authentication protocols such as Kerberos and the authentication facilities of IPv6. Operational problems can be dealt with using greater transaction reliability built into the applications that will use T/TCP, such as two phase commits and transaction logs.

Future work in this area could involve the promotion of T/TCP as an alternative to the TCP and UDP protocols for certain applications. T/TCP has been slow to take off. FreeBSD is the most widespread implementation of T/TCP for PC computers. Now that Linux is T/TCP aware, it can push the use of the protocol more. Applications can be easily modified to use T/TCP when available, any applications that involve an open-close connection can use T/TCP efficiently, and the more prominent examples would be web browsers, web servers and DNS client-server applications. To a smaller extent, applications such as time, finger and whois daemons can benefit from T/TCP as well. There are many networking utilities available that can take advantage of the efficiency of the protocol, all that is needed is the incentive to do it. Perhaps a more immediate task though, is to port the T/TCP code to the new Linux kernel series, 2.1.x.


Braun H W, Claffy K C, "Web Traffic Characterization: An Assessment of the Impact of Caching Documents from NCSA's Web Server", Proceedings of the Second World Wide Web Conference '94: Mosaic and the Web, October 1994

Mogul J C, Douglis F, Feldmann A, Krishnamurthy B, "Potential Benefits of Delta Encoding and Data Compression for HTTP", ACM SIGCOMM, September 1997

Prud'Hommeaux E, Lie H W, Lilley C, "Network Performance Effects of HTTP/1.1, CSSI and PNG", ACM SIGCOMM, September 1997

Stevens W R, TCP/IP Illustrated, Volume 3, TCP for Transactions, HTTP, NNTP, and the UNIX Domain Protocols, Addison-Wesley, 1996

Copyright © 1999, Mark Stacey
Published in Issue 47 of Linux Gazette, November 1999

[ TABLE OF CONTENTS ] [ FRONT PAGE ]  Back [ Linux Gazette FAQ ]  Next