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rfc:rfc1263

Network Working Group S. O'Malley Request for Comments: 1263 L. Peterson

                                                 University of Arizona
                                                          October 1991
                 TCP EXTENSIONS CONSIDERED HARMFUL

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard.  Distribution of this document is
 unlimited.

Abstract

 This RFC comments on recent proposals to extend TCP.  It argues that
 the backward compatible extensions proposed in RFC's 1072 and 1185
 should not be pursued, and proposes an alternative way to evolve the
 Internet protocol suite.  Its purpose is to stimulate discussion in
 the Internet community.

1. Introduction

 The rapid growth of the size, capacity, and complexity of the
 Internet has led to the need to change the existing protocol suite.
 For example, the maximum TCP window size is no longer sufficient to
 efficiently support the high capacity links currently being planned
 and constructed. One is then faced with the choice of either leaving
 the protocol alone and accepting the fact that TCP will run no faster
 on high capacity links than on low capacity links, or changing TCP.
 This is not an isolated incident. We have counted at least eight
 other proposed changes to TCP (some to be taken more seriously than
 others), and the question is not whether to change the protocol
 suite, but what is the most cost effective way to change it.
 This RFC compares the costs and benefits of three approaches to
 making these changes: the creation of new protocols, backward
 compatible protocol extensions, and protocol evolution. The next
 section introduces these three approaches and enumerates the
 strengths and weaknesses of each.  The following section describes
 how we believe these three approaches are best applied to the many
 proposed changes to TCP. Note that we have not written this RFC as an
 academic exercise.  It is our intent to argue against acceptance of
 the various TCP extensions, most notably RFC's 1072 and 1185 [4,5],
 by describing a more palatable alternative.

O'Malley & Peterson [Page 1] RFC 1263 TCP Extensions Considered Harmful October 1991

2. Creation vs. Extension vs. Evolution

2.1. Protocol Creation

 Protocol creation involves the design, implementation,
 standardization, and distribution of an entirely new protocol. In
 this context, there are two basic reasons for creating a new
 protocol. The first is to replace an old protocol that is so outdated
 that it can no longer be effectively extended to perform its original
 function.  The second is to add a new protocol because users are
 making demands upon the original protocol that were not envisioned by
 the designer and cannot be efficiently handled in terms of the
 original protocol.  For example, TCP was designed as a reliable
 byte-stream protocol but is commonly used as both a reliable record-
 stream protocol and a reliable request-reply protocol due to the lack
 of such protocols in the Internet protocol suite.  The performance
 demands placed upon a byte-stream protocol in the new Internet
 environment makes it difficult to extend TCP to meet these new
 application demands.
 The advantage of creating a new protocol is the ability to start with
 a clean sheet of paper when attempting to solve a complex network
 problem.  The designer, free from the constraints of an existing
 protocol, can take maximum advantage of modern network research in
 the basic algorithms needed to solve the problem. Even more
 importantly, the implementor is free to steal from a large number of
 existing academic protocols that have been developed over the years.
 In some cases, if truly new functionality is desired, creating a new
 protocol is the only viable approach.
 The most obvious disadvantage of this approach is the high cost of
 standardizing and distributing an entirely new protocol.  Second,
 there is the issue of making the new protocol reliable. Since new
 protocols have not undergone years of network stress testing, they
 often contain bugs which require backward compatible fixes, and
 hence, the designer is back where he or she started.  A third
 disadvantage of introducing new protocols is that they generally have
 new interfaces which require significant effort on the part of the
 Internet community to use. This alone is often enough to kill a new
 protocol.
 Finally, there is a subtle problem introduced by the very freedom
 provided by this approach. Specifically, being able to introduce a
 new protocol often results in protocols that go far beyond the basic
 needs of the situation.  New protocols resemble Senate appropriations
 bills; they tend to accumulate many amendments that have nothing to
 do with the original problem. A good example of this phenomena is the
 attempt to standardize VMTP [1] as the Internet RPC protocol. While

O'Malley & Peterson [Page 2] RFC 1263 TCP Extensions Considered Harmful October 1991

 VMTP was a large protocol to begin with, the closer it got to
 standardization the more features were added until it essentially
 collapsed under its own weight. As we argue below, new protocols
 should initially be minimal, and then evolve as the situation
 dictates.

2.2. Backward Compatible Extensions

 In a backward compatible extension, the protocol is modified in such
 a fashion that the new version of the protocol can transparently
 inter-operate with existing versions of the protocol. This generally
 implies no changes to the protocol's header. TCP slow start [3] is an
 example of such a change. In a slightly more relaxed version of
 backward compatibility, no changes are made to the fixed part of a
 protocol's header. Instead, either some fields are added to the
 variable length options field found at the end of the header, or
 existing header fields are overloaded (i.e., used for multiple
 purposes). However, we can find no real advantage to this technique
 over simply changing the protocol.
 Backward compatible extensions are widely used to modify protocols
 because there is no need to synchronize the distribution of the new
 version of the protocol. The new version is essentially allowed to
 diffuse through the Internet at its own pace, and at least in theory,
 the Internet will continue to function as before. Thus, the explicit
 distribution costs are limited. Backward compatible extensions also
 avoid the bureaucratic costs of standardizing a new protocol. TCP is
 still TCP and the approval cost of a modification to an existing
 protocol is much less than that of a new protocol. Finally, the very
 difficulty of making such changes tends to restrict the changes to
 the minimal set needed to solve the current problem. Thus, it is rare
 to see unneeded changes made when using this technique.
 Unfortunately, this approach has several drawbacks. First, the time
 to distribute the new version of the protocol to all hosts can be
 quite long (forever in fact). This leaves the network in a
 heterogeneous state for long periods of time. If there is the
 slightest incompatibly between old and new versions, chaos can
 result. Thus, the implicit cost of this type of distribution can be
 quite high. Second, designing a backward compatible change to a new
 protocol is extremely difficult, and the implementations "tend toward
 complexity and ugliness" [5]. The need for backward compatibility
 ensures that no code can every really be eliminated from the
 protocol, and since such vestigial code is rarely executed, it is
 often wrong. Finally, most protocols have limits, based upon the
 design decisions of it inventors, that simply cannot be side-stepped
 in this fashion.

O'Malley & Peterson [Page 3] RFC 1263 TCP Extensions Considered Harmful October 1991

2.3. Protocol Evolution

 Protocol evolution is an approach to protocol change that attempts to
 escape the limits of backward compatibility without incurring all of
 the costs of creating new protocols. The basic idea is for the
 protocol designer to take an existing protocol that requires
 modification and make the desired changes without maintaining
 backward compatibility.  This drastically simplifies the job of the
 protocol designer. For example, the limited TCP window size could be
 fixed by changing the definition of the window size in the header
 from 16-bits to 32-bits, and re-compiling the protocol. The effect of
 backward compatibility would be ensured by simply keeping both the
 new and old version of the protocol running until most machines use
 the new version. Since the change is small and invisible to the user
 interface, it is a trivial problem to dynamically select the correct
 TCP version at runtime. How this is done is discussed in the next
 section.
 Protocol evolution has several advantages. First, it is by far the
 simplest type of modification to make to a protocol, and hence, the
 modifications can be made faster and are less likely to contain bugs.
 There is no need to worry about the effects of the change on all
 previous versions of the protocol. Also, most of the protocol is
 carried over into the new version unchanged, thus avoiding the design
 and debugging cost of creating an entirely new protocol. Second,
 there is no artificial limit to the amount of change that can be made
 to a protocol, and as a consequence, its useful lifetime can be
 extended indefinitely. In a series of evolutionary steps, it is
 possible to make fairly radical changes to a protocol without
 upsetting the Internet community greatly. Specifically, it is
 possible to both add new features and remove features that are no
 longer required for the current environment.  Thus, the protocol is
 not condemned to grow without bound. Finally, by keeping the old
 version of the protocol around, backward compatibility is guaranteed.
 The old code will work as well as it ever did.
 Assuming the infrastructure described in the following subsection,
 the only real disadvantage of protocol evolution is the amount of
 memory required to run several versions of the same protocol.
 Fortunately, memory is not the scarcest resource in modern
 workstations (it may, however, be at a premium in the BSD kernel and
 its derivatives). Since old versions may rarely if ever be executed,
 the old versions can be swapped out to disk with little performance
 loss. Finally, since this cost is explicit, there is a huge incentive
 to eliminate old protocol versions from the network.

O'Malley & Peterson [Page 4] RFC 1263 TCP Extensions Considered Harmful October 1991

2.4. Infrastructure Support for Protocol Evolution

 The effective use of protocol evolution implies that each protocol is
 considered a vector of implementations which share the same top level
 interface, and perhaps not much else.  TCP[0] is the current
 implementation of TCP and exists to provide backward compatibility
 with all existing machines. TCP[1] is a version of TCP that is
 optimized for high-speed networks.  TCP[0] is always present; TCP[1]
 may or may not be. Treating TCP as a vector of protocols requires
 only three changes to the way protocols are designed and implemented.
 First, each version of TCP is assigned a unique id, but this id is
 not given as an IP protocol number. (This is because IP's protocol
 number field is only 8 bits long and could easily be exhausted.)  The
 "obvious" solution to this limitation is to increase IP's protocol
 number field to 32 bits. In this case, however, the obvious solution
 is wrong, not because of the difficultly of changing IP, but simply
 because there is a better approach. The best way to deal with this
 problem is to increase the IP protocol number field to 32 bits and
 move it to the very end of the IP header (i.e., the first four bytes
 of the TCP header).  A backward compatible modification would be made
 to IP such that for all packets with a special protocol number, say
 77, IP would look into the four bytes following its header for its
 de-multiplexing information. On systems which do not support a
 modified IP, an actual protocol 77 would be used to perform the de-
 multiplexing to the correct TCP version.
 Second, a version control protocol, called VTCP, is used to select
 the appropriate version of TCP for a particular connection. VTCP is
 an example of a virtual protocol as introduced in [2]. Application
 programs access the various versions of TCP through VTCP. When a TCP
 connection is opened to a specific machine, VTCP checks its local
 cache to determine the highest common version shared by the two
 machines. If the target machine is in the cache, it opens that
 version of TCP and returns the connection to the protocol above and
 does not effect performance. If the target machine is not found in
 the cache, VTCP sends a UDP packet to the other machine asking what
 versions of TCP that machine supports. If it receives a response, it
 uses that information to select a version and puts the information in
 the cache.  If no reply is forthcoming, it assumes that the other
 machine does not support VTCP and attempts to open a TCP[0]
 connection. VTCP's cache is flushed occasionally to ensure that its
 information is current.
 Note that this is only one possible way for VTCP to decide the right
 version of TCP to use. Another possibility is for VTCP to learn the
 right version for a particular host when it resolves the host's name.
 That is, version information could be stored in the Domain Name

O'Malley & Peterson [Page 5] RFC 1263 TCP Extensions Considered Harmful October 1991

 System. It is also possible that VTCP might take the performance
 characteristics of the network into consideration when selecting a
 version; TCP[0] may in fact turn out to be the correct choice for a
 low-bandwidth network.
 Third, because our proposal would lead to a more dynamically changing
 network architecture, a mechanism for distributing new versions will
 need to be developed. This is clearly the hardest requirement of the
 infrastructure, but we believe that it can be addressed in stages.
 More importantly, we believe this problem can be addressed after the
 decision has been made to go the protocol evolution route.  In the
 short term, we are considering only a single new version of TCP---
 TCP[1]. This version can be distributed in the same ad hoc way, and
 at exactly the same cost, as the backward compatible changes
 suggested in RFC's 1072 and 1185.
 In the medium term, we envision the IAB approving new versions of TCP
 every year or so. Given this scenario, a simple distribution
 mechanism can be designed based on software distribution mechanisms
 that have be developed for other environments; e.g., Unix RDIST and
 Mach SUP.  Such a mechanism need not be available on all hosts.
 Instead, hosts will be divided into two sets, those that can quickly
 be updated with new protocols and those that cannot.  High
 performance machines that can use high performance networks will need
 the most current version of TCP as soon as it is available, thus they
 have incentive to change.  Old machines which are too slow to drive a
 high capacity lines can be ignored, and probably should be ignored.
 In the long term, we envision protocols being designed on an
 application by application basis, without the need for central
 approval. In such a world, a common protocol implementation
 environment---a protocol backplane---is the right way to go.  Given
 such a backplane, protocols can be automatically installed over the
 network. While we claim to know how to build such an environment,
 such a discussion is beyond the scope of this paper.

2.5. Remarks

 Each of these three methods has its advantages.  When used in
 combination, the result is better protocols at a lower overall cost.
 Backward compatible changes are best reserved for changes that do not
 affect the protocol's header, and do not require that the instance
 running on the other end of the connection also be changed.  Protocol
 evolution should be the primary way of dealing with header fields
 that are no longer large enough, or when one algorithm is substituted
 directly for another.  New protocols should be written to off load
 unexpected user demands on existing protocols, or better yet, to

O'Malley & Peterson [Page 6] RFC 1263 TCP Extensions Considered Harmful October 1991

 catch them before they start.
 There are also synergistic effects. First, since we know it is
 possible to evolve a newly created protocol once it has been put in
 place, the pressure to add unnecessary features should be reduced.
 Second, the ability to create new protocols removes the pressure to
 overextend a given protocol. Finally, the ability to evolve a
 protocol removes the pressure to maintain backward compatibility
 where it is really not possible.

3. TCP Extensions: A Case Study

 This section examines the effects of using our proposed methodology
 to implement changes to TCP. We will begin by analyzing the backward
 compatible extensions defined in RFC's 1072 and 1185, and proposing a
 set of much simpler evolutionary modifications. We also analyze
 several more problematical extensions to TCP, such as Transactional
 TCP. Finally, we point our some areas of TCP which may require
 changes in the future.
 The evolutionary modification to TCP that we propose includes all of
 the functionality described in RFC's 1072 and 1185, but does not
 preserve the header format.  At the risk of being misunderstood as
 believing backward compatibility is a good idea, we also show how our
 proposed changes to TCP can be folded into a backward compatible
 implementation of TCP.  We do this as a courtesy for those readers
 that cannot accept the possibility of multiple versions of TCP.

3.1. RFC's 1072 and 1185

 3.1.1.  Round Trip Timing
 In RFC 1072, a new ECHO option is proposed that allows each TCP
 packet to carry a timestamp in its header.  This timestamp is used to
 keep a more accurate estimate of the RTT (round trip time) used to
 decide when to re-transmit segments. In the original TCP algorithm,
 the sender manually times a small number of sends. The resulting
 algorithm was quite complex and does not produce an accurate enough
 RTT for high capacity networks. The inclusion of a timestamp in every
 header both simplifies the code needed to calculate the RTT and
 improves the accuracy and robustness of the algorithm.
 The new algorithm as proposed in RFC 1072 does not appear to have any
 serious problems. However, the authors of RFC 1072 go to great
 lengths in an attempt to keep this modification backward compatible
 with the previous version of TCP. They place an ECHO option in the

O'Malley & Peterson [Page 7] RFC 1263 TCP Extensions Considered Harmful October 1991

 SYN segment and state, "It is likely that most implementations will
 properly ignore any options in the SYN segment that they do not
 understand, so new initial options should not cause problems" [4].
 This statement does not exactly inspire confidence, and we consider
 the addition of an optional field to any protocol to be a de-facto,
 if not a de-jure, example of an evolutionary change. Optional fields
 simply attempt to hide the basic incompatibility inside the protocol,
 it does not eliminate it.  Therefore, since we are making an
 evolutionary change anyway, the only modification to the proposed
 algorithm is to move the fields into the header proper.  Thus, each
 header will contain 32-bit echo and echo reply fields. Two fields are
 needed to handle bi-directional data streams.
 3.1.2.  Window Size and Sequence Number Space
 Long Fat Networks (LFN's), networks which contain very high capacity
 lines with very high latency, introduce the possibility that the
 number of bits in transit (the bandwidth-delay product) could exceed
 the TCP window size, thus making TCP the limiting factor in network
 performance.  Worse yet, the time it takes the sequence numbers to
 wrap around could be reduced to a point below the MSL (maximum
 segment lifetime), introducing the possibility of old packets being
 mistakenly accepted as new.
 RFC 1072 extends the window size through the use of an implicit
 constant scaling factor. The window size in the TCP header is
 multiplied by this factor to get the true window size.  This
 algorithm has three problems. First, one must prove that at all times
 the implicit scaling factor used by the sender is the same as the
 receiver.  The proposed algorithm appears to do so, but the
 complexity of the algorithm creates the opportunity for poor
 implementations to affect the correctness of TCP.  Second, the use of
 a scaling factor complicates the TCP implementation in general, and
 can have serious effects on other parts of the protocol.
 A final problem is what we characterize as the "quantum window
 sizing" problem. Assuming that the scaling factors will be powers of
 two, the algorithm right shifts the receiver's window before sending
 it.  This effectively rounds the window size down to the nearest
 multiple of the scaling factor. For large scaling factors, say 64k,
 this implies that window values are all multiples of 64k and the
 minimum window size is 64k; advertising a smaller window is
 impossible. While this is not necessarily a problem (and it seems to
 be an extreme solution to the silly window syndrome) what effect this
 will have on the performance of high-speed network links is anyone's
 guess. We can imagine this extension leading to future papers
 entitled "A Quantum Mechanical Approach to Network Performance".

O'Malley & Peterson [Page 8] RFC 1263 TCP Extensions Considered Harmful October 1991

 RFC 1185 is an attempt to get around the problem of the window
 wrapping too quickly without explicitly increasing the sequence
 number space.  Instead, the RFC proposes to use the timestamp used in
 the ECHO option to weed out old duplicate messages. The algorithm
 presented in RFC 1185 is complex and has been shown to be seriously
 flawed at a recent End-to-End Research Group meeting.  Attempts are
 currently underway to fix the algorithm presented in the RFC. We
 believe that this is a serious mistake.
 We see two problems with this approach on a very fundamental level.
 First, we believe that making TCP depend on accurate clocks for
 correctness to be a mistake. The Internet community has NO experience
 with transport protocols that depend on clocks for correctness.
 Second, the proposal uses two distinct schemes to deal with old
 duplicate packets: the sliding window algorithm takes care of "new"
 old packets (packets from the current sequence number epoch) and the
 timestamp algorithm deals with "old" old packets (packets from
 previous sequence number epochs). It is hard enough getting one of
 these schemes to work much less to get two to work and ensure that
 they do not interfere with one another.
 In RFC 1185, the statement is made that "An obvious fix for the
 problem of cycling the sequence number space is to increase the size
 of the TCP sequence number field." Using protocol evolution, the
 obvious fix is also the correct one. The window size can be increased
 to 32 bits by simply changing a short to a long in the definition of
 the TCP header. At the same time, the sequence number and
 acknowledgment fields can be increased to 64 bits.  This change is
 the minimum complexity modification to get the job done and requires
 little or no analysis to be shown to work correctly.
 On machines that do not support 64-bit integers, increasing the
 sequence number size is not as trivial as increasing the window size.
 However, it is identical in cost to the modification proposed in RFC
 1185; the high order bits can be thought of as an optimal clock that
 ticks only when it has to.  Also, because we are not dealing with
 real time, the problems with unreliable system clocks is avoided.  On
 machines that support 64-bit integers, the original TCP code may be
 reused.  Since only very high performance machines can hope to drive
 a communications network at the rates this modification is designed
 to support, and the new generation of RISC microprocessors (e.g.,
 MIPS R4000 and PA-RISC) do support 64-bit integers, the assumption of
 64-bit arithmetic may be more of an advantage than a liability.

O'Malley & Peterson [Page 9] RFC 1263 TCP Extensions Considered Harmful October 1991

 3.1.3.  Selective Retransmission
 Another problem with TCP's support for LFN's is that the sliding
 window algorithm used by TCP does not support any form of selective
 acknowledgment. Thus, if a segment is lost, the total amount of data
 that must be re-transmitted is some constant times the bandwidth-
 delay product, despite the fact that most of the segments have in
 fact arrived at the receiver.  RFC 1072 proposes to extend TCP to
 allow the receiver to return partial acknowledgments to the sender in
 the hope that the sender will use that information to avoid
 unnecessary re-transmissions.
 It has been our experience on predictable local area networks that
 the performance of partial re-transmission strategies is highly non-
 obvious, and it generally requires more than one iteration to find a
 decent algorithm. It is therefore not surprising that the algorithm
 proposed in RFC 1072 has some problems.  The proposed TCP extension
 allows the receiver to include a short list of received fragments
 with every ACK.  The idea being that when the receiver sends back a
 normal ACK, it checks its queue of segments that have been received
 out of order and sends the relative sequence numbers of contiguous
 blocks of segments back to the sender. The sender then uses this
 information to re-transmit the segments transmitted but not listed in
 the ACK.
 As specified, this algorithm has two related problems: (1) it ignores
 the relative frequencies of delivered and dropped packets, and (2)
 the list provided in the option field is probably too short to do
 much good on networks with large bandwidth-delay products.  In every
 model of high bandwidth networks that we have seen, the packet loss
 rate is very low, and thus, the ratio of dropped packets to delivered
 packets is very low. An algorithm that returns ACKs as proposed is
 simply going to have to send more information than one in which the
 receiver returns NAKs.
 This problem is compounded by the short size of the TCP option field
 (44 bytes). In theory, since we are only worried about high bandwidth
 networks, returning ACKs instead of NAKs is not really a problem; the
 bandwidth is available to send any information that's needed. The
 problem comes when trying to compress the ACK information into the 44
 bytes allowed.  The proposed extensions effectively compresses the
 ACK information by allowing the receiver to ACK byte ranges rather
 than segments, and scaling the relative sequence numbers of the re-
 transmitted segments. This makes it much more difficult for the
 sender to tell which segments should be re-transmitted, and
 complicates the re-transmission code.  More importantly, one should
 never compress small amounts of data being sent over a high bandwidth
 network; it trades a scarce resource for an abundant resource.  On

O'Malley & Peterson [Page 10] RFC 1263 TCP Extensions Considered Harmful October 1991

 low bandwidth networks, selective retransmission is not needed and
 the SACK option should be disabled.
 We propose two solutions to this problem. First, the receiver can
 examine its list of out-of-order packets and guess which segments
 have been dropped, and NAK those segments back to the sender. The
 number of NAKs should be low enough that one per TCP packet should be
 sufficient. Note that the receiver has just as much information as
 the sender about what packets should be retransmitted, and in any
 case, the NAKs are simply suggestions which have no effect on
 correctness.
 Our second proposed modification is to increase the offset field in
 the TCP header from 4 bits to 16 bits.  This allows 64k-bytes of TCP
 header, which allows us to radically simplify the selective re-
 transmission algorithm proposed in RFC 1072.  The receiver can now
 simply send a list of 64-bit sequence numbers for the out-of-order
 segments to the sender. The sender can then use this information to
 do a partial retransmission without needing an ouji board to
 translate ACKs into segments.  With the new header size, it may be
 faster for the receiver to send a large list than to attempt to
 aggregate segments into larger blocks.
 3.1.4.  Header Modifications
 The modifications proposed above drastically change the size and
 structure of the TCP header. This makes it a good time to re-think
 the structure of the proposed TCP header. The primary goal of the
 current TCP header is to save bits in the output stream. When TCP was
 developed, a high bandwidth network was 56kbps, and the key use for
 TCP was terminal I/O.  In both situations, minimal header size was
 important.  Unfortunately, while the network has drastically
 increased in performance and the usage pattern of the network is now
 vastly different, most protocol designers still consider saving a few
 bits in the header to be worth almost any price. Our basic goal is
 different: to improve performance by eliminating the need to extract
 information packed into odd length bit fields in the header.  Below
 is our first cut at such a modification.
 The protocol id field is there to make further evolutionary
 modifications to TCP easier. This field basically subsumes the
 protocol number field contained in the IP header with a version
 number.  Each distinct TCP version has a different protocol id and
 this field ensures that the right code is looking at the right
 header.  The offset field has been increased to 16 bits to support
 the larger header size required, and to simplify header processing.
 The code field has been extended to 16 bits to support more options.

O'Malley & Peterson [Page 11] RFC 1263 TCP Extensions Considered Harmful October 1991

 The source port and destination port are unchanged. The size of both
 the sequence number and ACK fields have been increased to 64 bits.
 The open window field has been increased to 32 bits. The checksum and
 urgent data pointer fields are unchanged. The echo and echo reply
 fields are added.  The option field remains but can be much larger
 than in the old TCP.  All headers are padded out to 32 bit
 boundaries.  Note that these changes increase the minimum header size
 from 24 bytes (actually 36 bytes if the ECHO and ECHO reply options
 defined in RFC 1072 are included on every packet) to 48 bytes. The
 maximum header size has been increased to the maximum segment size.
 We do not believe that the the increased header size will have a
 measurable effect on protocol performance.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Protocol ID                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Offset           |              Code             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Source           |              Dest             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              Seq                              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              Ack                              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Window                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Checksum          |             Urgent            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             Echo                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Echo Reply                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Options                                      |     Pad       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 3.1.5.  Backward Compatibility
 The most likely objection to the proposed TCP extension is that it is
 not backward compatible with the current version of TCP, and most
 importantly, TCP's header. In this section we will present three
 versions of the proposed extension with increasing degrees of
 backward compatibility. The final version will combine the same
 degree of backward compatibility found in the protocol described in

O'Malley & Peterson [Page 12] RFC 1263 TCP Extensions Considered Harmful October 1991

 RFC's 1072/1185, with the much simpler semantics described in this
 RFC.
 We believe that the best way to preserve backward compatibility is to
 leave all of TCP alone and support the transparent use of a new
 protocol when and where it is needed. The basic scheme is the one
 described in section 2.4. Those machines and operating systems that
 need to support high speed connections should implement some general
 protocol infrastructure that allows them to rapidly evolve protocols.
 Machines that do not require such service simply keep using the
 existing version of TCP. A virtual protocol is used to manage the use
 of multiple TCP versions.
 This approach has several advantages. First, it guarantees backward
 compatibility with ALL existing TCP versions because such
 implementations will never see strange packets with new options.
 Second, it supports further modification of TCP with little
 additional costs. Finally, since our version of TCP will more closely
 resemble the existing TCP protocol than that proposed in RFC's
 1072/1185, the cost of maintaining two simple protocols will probably
 be lower than maintaining one complex protocol.  (Note that with high
 probability you still have to maintain two versions of TCP in any
 case.)  The only additional cost is the memory required for keeping
 around two copies of TCP.
 For those that insist that the only efficient way to implement TCP
 modifications is in a single monolithic protocol, or those that
 believe that the space requirements of two protocols would be too
 great, we simply migrate the virtual protocol into TCP. TCP is
 modified so that when opening a connection, the sender uses the TCP
 VERSION option attached to the SYN packet to request using the new
 version.  The receiver responds with a TCP VERSION ACK in the SYN ACK
 packet, after which point, the new header format described in Section
 3.1.4 is used. Thus, there is only one version of TCP, but that
 version supports multiple header formats. The complexity of such a
 protocol would be no worse than the protocol described in RFC
 1072/1185. It does, however, make it more difficult to make
 additional changes to TCP.
 Finally, for those that believe that the preservation of the TCP's
 header format has any intrinsic value (e.g., for those that don't
 want to re-program their ethernet monitors), a header compatible
 version of our proposal is possible.  One simply takes all of the
 additional information contained in the header given in Section 3.1.4
 and places it into a single optional field. Thus, one could define a
 new TCP option which consists of the top 32 bits of the sequence and
 ack fields, the echo and echo_reply fields, and the top 16 bits of
 the window field. This modification makes it more difficult to take

O'Malley & Peterson [Page 13] RFC 1263 TCP Extensions Considered Harmful October 1991

 advantage of machines with 64-bit address spaces, but at a minimum
 will be just as easy to process as the protocol described in RFC
 1072/1185.  The only restriction is that the size of the header
 option field is still limited to 44 bytes, and thus, selective
 retransmission using NAKs rather than ACKs will probably be required.
 The key observation is that one should make a protocol extension
 correct and simple before trying to make it backward compatible.  As
 far as we can tell, the only advantages possessed by the protocol
 described in RFC 1072/1185 is that its typical header, size including
 options, is 8 to 10 bytes shorter. The price for this "advantage" is
 a protocol of such complexity that it may prove impossible for normal
 humans to implement. Trying to maintain backward compatibility at
 every stage of the protocol design process is a serious mistake.

3.2. TCP Over Extension

 Another potential problem with TCP that has been discussed recently,
 but has not yet resulted in the generation of an RFC, is the
 potential for TCP to grab and hold all 2**16 port numbers on a given
 machine.  This problem is caused by short port numbers, long MSLs,
 and the misuse of TCP as a request-reply protocol. TCP must hold onto
 each port after a close until all possible messages to that port have
 died, about 240 seconds. Even worse, this time is not decreasing with
 increase network performance.  With new fast hardware, it is possible
 for an application to open a TCP connection, send data, get a reply,
 and close the connection at a rate fast enough to use up all the
 ports in less than 240 seconds. This usage pattern is generated by
 people using TCP for something it was never intended to do---
 guaranteeing at-most-once semantics for remote procedure calls.
 The proposed solution is to embed an RPC protocol into TCP while
 preserving backward compatibility. This is done by piggybacking the
 request message on the SYN packet and the reply message on the SYN-
 ACK packet. This approach suffers from one key problem: it reduces
 the probability of a correct TCP implementation to near 0. The basic
 problem has nothing to do with TCP, rather it is the lack of an
 Internet request-reply protocol that guarantees at-most-once
 semantics.
 We propose to solve this problem by the creation of a new protocol.
 This has already been attempted with VMTP, but the size and
 complexity of VMTP, coupled with the process currently required to
 standardize a new protocol doomed it from the start.  Instead of
 solving the general problem, we propose to use Sprite RPC [7], a much
 simpler protocol, as a means of off-loading inappropriate users from
 TCP.

O'Malley & Peterson [Page 14] RFC 1263 TCP Extensions Considered Harmful October 1991

 The basic design would attempt to preserve as much of the TCP
 interface as possible in order that current TCP (mis)users could be
 switched to Sprite RPC without requiring code modification on their
 part. A virtual protocol could be used to select the correct protocol
 TCP or Sprite RPC if it exists on the other machine. A backward
 compatible modification to TCP could be made which would simply
 prevent it from grabbing all of the ports by refusing connections.
 This would encourage TCP abusers to use the new protocol.
 Sprite RPC, which is designed for a local area network, has two
 problems when extended into the Internet. First, it does not have a
 usefully flow control algorithm. Second, it lacks the necessary
 semantics to reliably tear down connections. The lack of a tear down
 mechanism needs to be solved, but the flow control problem could be
 dealt with in later iterations of the protocol as Internet blast
 protocols are not yet well understood; for now, we could simple limit
 the size of each message to 16k or 32k bytes. This might also be a
 good place to use a decomposed version of Sprite RPC [2], which
 exposes each of these features as separate protocols. This would
 permit the quick change of algorithms, and once the protocol had
 stabilized, a monolithic version could be constructed and distributed
 to replace the decomposed version.
 In other words, the basic strategy is to introduce as simple of RPC
 protocol as possible today, and later evolve this protocol to address
 the known limitations.

3.3. Future Modifications

 The header prediction algorithm should be generalized so as to be
 less sensitive to changes in the protocols header and algorithm.
 There almost seems to be as much effort to make all modifications to
 TCP backward compatible with header prediction as there is to make
 them backward compatible with TCP.  The question that needs to be
 answered is: are there any changes we can made to TCP to make header
 prediction easier, including the addition of information into the
 header.  In [6], the authors showed how one might generalize
 optimistic blast from VMTP to almost any protocol that performs
 fragmentation and reassembly.  Generalizing header prediction so that
 it scales with TCP modification would be step in the right direction.
 It is clear that an evolutionary change to increase the size of the
 source and destination ports in the TCP header will eventually be
 necessary.  We also believe that TCP could be made significantly
 simpler and more flexible through the elimination of the pseudo-
 header. The solution to this problem is to simply add a length field
 and the IP address of the destination to the TCP header. It has also

O'Malley & Peterson [Page 15] RFC 1263 TCP Extensions Considered Harmful October 1991

 been mentioned that better and simpler TCP connection establishment
 algorithms would be useful.  Some form of reliable record stream
 protocol should be developed.  Performing sliding window and flow
 control over records rather than bytes would provide numerous
 opportunities for optimizations and allow TCP to return to its
 original purpose as a byte-stream protocol. Finally, it has become
 clear to us that the current Internet congestion control strategy is
 to use TCP for everything since it is the only protocol that supports
 congestion control. One of the primary reasons many "new protocols"
 are proposed as TCP options is that it is the only way to get at
 TCP's congestion control. At some point, a TCP-independent congestion
 control scheme must be implemented and one might then be able to
 remove the existing congestion control from TCP and radically
 simplify the protocol.

4. Discussion

 One obvious side effect of the changes we propose is to increase the
 size of the TCP header. In some sense, this is inevitable; just about
 every field in the header has been pushed to its limit by the radical
 growth of the network. However, we have made very little effort to
 make the minimal changes to solve the current problem. In fact, we
 have tended to sacrifice header size in order to defer future changes
 as long as possible. The problem with this is that one of TCP's
 claims to fame is its efficiency at sending small one byte packets
 over slow networks. Increasing the size of the TCP header will
 inevitably result in some increase in overhead on small packets on
 slow networks. Clark among others have stated that they see no
 fundamental performance limitations that would prevent TCP from
 supporting very high speed networks. This is true as far as it goes;
 there seems to be a direct trade-off between TCP performance on high
 speed networks and TCP performance on slow speed networks. The
 dynamic range is simply too great to be optimally supported by one
 protocol. Hence, in keeping around the old version of TCP we have
 effectively split TCP into two protocols, one for high bandwidth
 lines and the other for low bandwidth lines.
 Another potential argument is that all of the changes mentioned above
 should be packaged together as a new version of TCP. This version
 could be standardized and we could all go back to the status quo of
 stable unchanging protocols.  While to a certain extent this is
 inevitable---there is a backlog of necessary TCP changes because of
 the current logistical problems in modifying protocols---it is only
 begs the question. The status quo is simply unacceptably static;
 there will always be future changes to TCP.  Evolutionary change will
 also result in a better and more reliable TCP.  Making small changes
 and distributing them at regular intervals ensures that one change

O'Malley & Peterson [Page 16] RFC 1263 TCP Extensions Considered Harmful October 1991

 has actually been stabilized before the next has been made.  It also
 presents a more balanced workload to the protocol designer; rather
 than designing one new protocol every 10 years he makes annual
 protocol extensions. It will also eventually make protocol
 distribution easier: the basic problem with protocol distribution now
 is that it is done so rarely that no one knows how to do it and there
 is no incentive to develop the infrastructure needed to perform the
 task efficiently.  While the first protocol distribution is almost
 guaranteed to be a disaster, the problem will get easier with each
 additional one. Finally, such a new TCP would have the same problems
 as VMTP did; a radically new protocol presents a bigger target.
 The violation of backward compatibility in systems as complex as the
 Internet is always a serious step. However, backward compatibility is
 a technique, not a religion. Two facts are often overlooked when
 backward compatibility gets out of hand. First, violating backward
 compatibility is always a big win when you can get away with it.  One
 of the key advantages of RISC chips over CISC chips is simply that
 they were not backward compatible with anything. Thus, they were not
 bound by design decisions made when compilers were stupid and real
 men programmed in assembler. Second, one is going to have to break
 backward compatibility at some point anyway. Every system has some
 headroom limitations which result in either stagnation (IBM mainframe
 software) or even worse, accidental violations of backward
 compatibility.
 Of course, the biggest problem with our approach is that it is not
 compatible with the existing standardization process. We hope to be
 able to design and distribute protocols in less time than it takes a
 standards committee to agree on an acceptable meeting time.  This is
 inevitable because the basic problem with networking is the
 standardization process. Over the last several years, there has been
 a push in the research community for lightweight protocols, when in
 fact what is needed are lightweight standards.  Also note that we
 have not proposed to implement some entirely new set of "superior"
 communications protocols, we have simply proposed a system for making
 necessary changes to the existing protocol suites fast enough to keep
 up with the underlying change in the network.  In fact, the first
 standards organization that realizes that the primary impediment to
 standardization is poor logistical support will probably win.

5. Conclusions

 The most important conclusion of this RFC is that protocol change
 happens and is currently happening at a very respectable clip.  While
 all of the changes given as example in this document are from TCP,
 there are many other protocols that require modification.  In a more

O'Malley & Peterson [Page 17] RFC 1263 TCP Extensions Considered Harmful October 1991

 prosaic domain, the telephone company is running out of phone
 numbers; they are being overrun by fax machines, modems, and cars.
 The underlying cause of these problems seems to be an consistent
 exponential increase almost all network metrics: number of hosts,
 bandwidth, host performance, applications, and so on, combined with
 an attempt to run the network with a static set of unchanging network
 protocols.  This has been shown to be impossible and one can almost
 feel the pressure for protocol change building. We simply propose to
 explicitly deal with the changes rather keep trying to hold back the
 flood.
 Of almost equal importance is the observation that TCP is a protocol
 and not a platform for implementing other protocols. Because of a
 lack of any alternatives, TCP has become a de-facto platform for
 implementing other protocols. It provides a vague standard interface
 with the kernel, it runs on many machines, and has a well defined
 distribution path. Otherwise sane people have proposed Bounded Time
 TCP (an unreliable byte stream protocol), Simplex TCP (which supports
 data in only one direction) and Multi-cast TCP (too horrible to even
 consider).  All of these protocols probably have their uses, but not
 as TCP options. The fact that a large number of people are willing to
 use TCP as a protocol implementation platform points to the desperate
 need for a protocol independent platform.
 Finally, we point out that in our research we have found very little
 difference in the actual technical work involved with the three
 proposed methods of protocol modification. The amount of work
 involved in a backward compatible change is often more than that
 required for an evolutionary change or the creation of a new
 protocol.  Even the distribution costs seem to be identical.  The
 primary cost difference between the three approaches is the cost of
 getting the modification approved. A protocol modification, no matter
 how extensive or bizarre, seems to incur much less cost and risk. It
 is time to stop changing the protocols to fit our current way of
 thinking, and start changing our way of thinking to fit the
 protocols.

6. References

[1] Cheriton D., "VMTP: Versatile Message Transaction Protocol", RFC

   1045, Stanford University, February 1988.

[2] Hutchinson, N., Peterson, L., Abbott, M., and S. O'Malley, "RPC in

   the x-Kernel: Evaluating New Design Techniques", Proceedings of the
   12th Symposium on Operating System Principles, Pgs. 91-101,

O'Malley & Peterson [Page 18] RFC 1263 TCP Extensions Considered Harmful October 1991

   December 1989.

[3] Jacobson, V., "Congestion Avoidance and Control", SIGCOMM '88,

   August 1988.

[4] Jacobson, V., and R. Braden, "TCP Extensions for Long-Delay Paths",

   RFC 1072, LBL, ISI, October 1988.

[5] Jacobson, V., Braden, R., and L. Zhang, "TCP Extensions for High-

   Speed Paths", RFC 1185, LBL, ISI, PARC, October 1990.

[6] O'Malley, S., Abbott, M., Hutchinson, N., and L. Peterson, "A Tran-

   sparent Blast Facility", Journal of Internetworking, Vol. 1, No.
   2, Pgs. 57-75, December 1990.

[7] Welch, B., "The Sprite Remote Procedure Call System", UCB/CSD

   86/302, University of California at Berkeley, June 1988.

7. Security Considerations

 Security issues are not discussed in this memo.

8. Authors' Addresses

 Larry L. Peterson
 University of Arizona
 Department of Computer Sciences
 Tucson, AZ 85721
 Phone: (602) 621-4231
 EMail: llp@cs.arizona.edu
 Sean O'Malley
 University of Arizona
 Department of Computer Sciences
 Tucson, AZ 85721
 Phone: 602-621-8373
 EMail: sean@cs.arizona.edu

O'Malley & Peterson [Page 19]

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