| TCP Maintenance and Minor Extensions (tcpm) | B. Briscoe |
| Internet-Draft | BT |
| Updates: 793 (if approved) | October 19, 2014 |
| Intended status: Experimental | |
| Expires: April 22, 2015 |
Inner Space for TCP Options
draft-briscoe-tcpm-inner-space-00
This document describes an experimental method to extend the limited space for control options in every segment of a TCP connection. It can use a dual handshake so that, from the very first SYN segment, extra option space can immediately start to be used optimistically. At the same time the dual handshake prevents a legacy server from getting confused and sending the control options to the application as user-data. The dual handshake is only one strategy - a single handshake will usually suffice once deployment has got started. The protocol is designed to traverse most known middleboxes including connection splitters, because it sits wholly within the TCP Data. It also provides reliable ordered delivery for control options. Therefore, it should allow new TCP options to be introduced i) with minimal middlebox traversal problems; ii) avoiding incremental deployment problems with pre-existing servers; iii) without an extra round of handshaking delay iv) without having to provide its own loss recovery and ordering mechanism and v) without arbitrary limits on available space.
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TCP has become hard to extend, partly because the option space was limited to 40B when TCP was first defined [RFC0793] and partly because many middleboxes only forward TCP headers that conform to the stereotype they expect.
This specification ensures new TCP capabilities can traverse most middleboxes by tunnelling TCP options within the TCP Data as 'Inner Options' (Figure 1). Then the TCP receiver can reconstruct the Inner Options sent by the sender, even if the middlebox resegments the data stream and even if it strips 'Outer' options from the TCP header that it does not recognise. The two words 'Inner Space' are appropriate as a name for the scheme; 'Inner' because it encapsulates options within the TCP Data and 'Space' because the space within the TCP Data is virtually unlimited—constrained only by the maximum segment size.
,-----. TCP Payload ,-----. | App |<----------------------------------------->| App | |-----| |-----| | | Inner Options within TCP Data | | | |<----------------------------------------->| | | | | | | TCP | TCP Header and TCP header and | TCP | | | Outer Options ,---------. Outer Options | | | |<-------------->|Middlebox|<-------------->| | |-----| |---------| |-----| | IP | | IP | | IP | : : : : : :
Figure 1: Encapsulation Approach
TCP options fall into three main categories:
Pressure of space is most acute in the initial segments of each half-connection, i.e. the SYN and SYN/ACK, and particularly the SYN. Even though Inner Space is not suitable for category (a) options, moving all of categories (b) and (c) into Inner Space frees up plenty of outer space in the header for category (a).
The following list of options that might be required on a SYN illustrates how acute the problem is:[I-D.bittau-tcpinc]).
There is probably potential for compressing together multiple options in order to mitigate the option space problem. However, the option space problem has to be faced, because complex special placement is already being contemplated for options that can be larger than 40B on their own (e.g. the key agreement options of tcpcrypt
Given the Inner Space protocol places control options within TCP Data, it is critical that a legacy TCP receiver is never confused into passing this mix to an application as if it were pure data. Naïvely, both ends could handshake to check they understand the protocol, but this would introduce a round of delay and it would not solve the shortage of space in a SYN. Instead, the client uses a dual handshake; one suitable for an upgraded server, and the other for an ordinary server. Then, if the client discovers that the server does not understand the new protocol, it can abort the upgraded handshake before the server passes corrupt data to the application. Otherwise, if the server does understand the new protocol, the client can abort the ordinary handshake. Either way, it has added zero extra delay. Interworking of the dual handshake with TCP Fast Open [I-D.ietf-tcpm-fastopen] is carefully defined so that the server can even pass data to the application as soon as the first SYN arrives.
When control options are placed within the TCP Data they inherently get delivered reliably and in order. Although this was not originally recognised as part of the design brief, it offers the significant benefit of simplifying the design of new TCP options. Reliable ordered delivery no longer has to be individually crafted into the design of each new TCP option.
Solving the five problems of i) option-space exhaustion; ii) middlebox traversal; iii) legacy server confusion; iv) reliable ordered control message delivery; and v) handshake latency; does not come without cost:
Finally, it should be noted that the ambition of this work is more than just an incrementally deployable, low latency way to extend TCP option space. The aim is to move towards a more structured way for middleboxes to interact transparently with, rather than arbitrarily interfere with, end-system TCP stacks. This has been achieved for connection and stream control options, but it will still be hard to introduce new per-segment control options, which will still have to be located with the traditional Outer TCP Options.
It seems inevitable that ultimately more option space will be needed, particularly given that many of the TCP options introduced recently consume large numbers of bits in order to provide sufficient information entropy, which is not amenable to compression.
Extension of TCP option space requires support from both ends. This means it will take many years before the facility is functional for most pairs of end-points. Therefore, given the problem is already becoming pressing, a solution needs to start being deployed now.
This experimental specification extends the TCP wire protocol. It is independent of the dynamic behaviour of TCP and it is independent of (and thus compatible with) any protocol that encapsulates TCP, including IPv4 and IPv6.
TCP is critical to the robust functioning of the Internet, therefore any proposed modifications to TCP need to be thoroughly tested.
The body of the document starts with a full specification of the Inner Space extension to TCP (Section 2). It is rather terse, answering 'What?' and 'How?' questions, but deferring 'Why?' to Section 3. The careful design choices made are not necessarily apparent from a superficial read of the specification, so the Design Rationale section is fairly extensive. The body of the document ends with Section 4 that checks possible interactions between the new scheme and pre-existing variants of TCP, including interaction with partial implementations of TCP in known middleboxes.
Appendix A specifies optional extensions to the protocol that will need to be implemented experimentally to determine whether they are useful. And Appendix B discusses the merits of the chosen design against alternative schemes.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying RFC-2119 significance.
Note that the term 'Ordinary' is used for segments and connections, but the term 'Legacy' is used for hosts. This is because, if the Inner Space protocol were widely used in future, a host that could not open an Upgraded Connection would be considered deficient and therefore 'Legacy', whereas an Ordinary Connection would not be considered deficient in the future; because it will always be legitimate to open an Ordinary Connection if extra option space is not needed.
During initial deployment, an Upgraded TCP Client sends two alternative SYNs: an Ordinary SYN in case the server is legacy and a SYN-U in case the server is upgraded. The two SYNs MUST have the same network addresses and the same destination port, but different source ports. Once the client establishes which type of server has responded, it continues the connection appropriate to that server type and aborts the other without completing the 3-way handshake.
The format of the SYN-U will be described later (Section 2.2.2). At this stage it is only necessary to know that the client can put either TCP options or payload (or both) in a SYN-U, in the space traditionally intended only for payload. So if the server's response shows that it does not recognise the Upgraded SYN-U, the client is responsible for aborting the Upgraded Connection. This ensures that a Legacy TCP Server will never erroneously confuse the application by passing it TCP options as if they were user-data.
Section 3.1 explains various strategies the client can use to send the SYN-U first and defer or avoid sending the Ordinary SYN. However, such strategies are local optimizations that do not need to be standardized. The rules below cover the most aggressive case, in which the client sends the SYN-U then the Ordinary SYN back-to-back to avoid any extra delay. Nonetheless, the rules are just as applicable if the client defers or avoids sending the Ordinary SYN.
Table 1 summarises the TCP 3-way handshake exchange for each of the two SYNs in the two right-hand columns, between an Upgraded TCP Client (the active opener) and either:
Because the two SYNs come from different source ports, the server will treat them as separate connections, probably using separate threads (assuming a threaded server). A load balancer might forward each SYN to separate replicas of the same logical server. Each replica will deal with each incoming SYN independently - it does not need to co-ordinate with the other replica.
| Ordinary Connection | Upgraded Connection | ||
|---|---|---|---|
| 1 | Upgraded Client | >SYN | >SYN-U |
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Legacy Server | <SYN/ACK | <SYN/ACK |
| 3a | Upgraded Client | Waits for response to both SYNs | |
| 3b | " | >ACK | >RST |
| 4 | Cont... | ||
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Upgraded Server | <SYN/ACK | <SYN/ACK-U |
| 3a | Upgraded Client | Waits for response to SYN-U | |
| 3b | " | >RST | >ACK |
| 4 | Cont... |
Each column of the table shows the required 3-way handshake exchange within each connection, using the following symbols:
The connection that starts with an Ordinary SYN is called the 'Ordinary Connection' and the one that starts with a SYN-U is called the 'Upgraded Connection'. An Upgraded Server MUST respond to a SYN-U with an Upgraded SYN/ACK (termed a SYN/ACK-U and defined in Section 2.2.2). Then the client recognises that it is talking to an Upgraded Server. The client's behaviour depends on which response it receives first, as follows:
If the client receives a response to the SYN, but a short while after that {ToDo: duration TBA} the response to the SYN-U has not arrived, it SHOULD retransmit the SYN-U. If latency is more important than the extra TCP option space, in parallel to any retransmission, or instead of any retransmission, the client MAY give up on the Upgraded (SYN-U) Connection by sending a reset (RST) and completing the 3-way handshake of the Ordinary Connection.
If the client receives no response at all to either the SYN or the SYN-U, it SHOULD solely retransmit one or the other, not both. If latency is more important than the extra TCP option space, it will retransmit the SYN. Otherwise it will retransmit the SYN-U. It MUST NOT retransmit both segments, because the lack of response could be due to severe congestion.
Once an Upgraded Connection has been successfully negotiated in the SYN, SYN/ACK exchange, either host can allocate any amount of the TCP Data space in any subsequent segment for extra TCP options. In fact, the sender has to use the upgraded segment structure in every subsequent segment of the connection that contains non-zero TCP Payload. The sender can use the upgraded structure in a segment carrying no user-data (e.g. a pure ACK), but it does not have to.
As well as extra option space, the facility offers other advantages, such as reliable ordered delivery of Inner TCP Options on empty segments and more robust middlebox traversal. If none of these features is needed, at any point the facility can be disabled for the rest of the connection, using the ModeSwitch TCP option in Appendix A.1. Interestingly, the ModeSwitch options itself can be very simple because it uses the reliable ordered delivery property of Inner Options, rather than having to cater for the possibility that a message to switch to disabled mode might be lost or reordered.
An Upgraded Segment is structured as shown in Figure 2. Up to the TCP Data Offset, the structure is identical to an Ordinary TCP Segment, with a base TCP Header (BaseHdr) and the usual facility to set the Data Offset (DO) to allow space for TCP options. These regular TCP options are renamed by this specification to Outer TCP Options or just Outer Options, and labelled as OuterOpts in the figure.
The first segment in each direction (i.e. the SYN or the SYN/ACK) is identifiable as upgraded by the presence of the 4-octet Magic Number A (MagicA) at the start of the TCP Data. The probability that an Upgraded Server will mistake arbitrary data at the beginning of the payload of an Ordinary Segment for the Magic Number has to be allowed for, but it is vanishingly small (see Section 3.2.1). Once an Upgraded Connection has been negotiated during the SYN - SYN/ACK exchange, a magic number is not needed to identify Upgraded Segments, because both ends know that the protocol requires the sender to use the upgraded format on all subsequent segments with non-zero TCP Data. Aside from the magic number, the structure of the rest of an Upgraded Segment is the same whether a) SYN=1 or b) SYN=0.
| SOO |
a) SYN=1 ,--------->|
| DO | 1 | Len | InOO | SPS |
,------------------>,------>,------->,-------------------->,------->|
+--------+----------+-------+--------+----------+----------+--------+
| BaseHdr| OuterOpts| MagicA| InSpace|PrefixOpts|SuffixOpts| Payload|
+--------+----------+-------+--------+----------+----------+--------+
| '----------.----------' |
| Inner Options |
`-----------------------.-----------------------'
TCP Data
b) SYN=0
| DO | Len | InOO | SPS |
,------------------>,------->,---------------------->,------->|
+--------+----------+--------+-----------------------+--------+
| BaseHdr| OuterOpts| InSpace| Inner Options | Payload|
+--------+----------+--------+-----------------------+--------+
`----------------.------------------------'
TCP Data
All offsets are specified in 4-octet (32-bit) words, except SPS, which is in octets.
Figure 2: The Structure of an Upgraded Segment (not to scale)
Unlike an Ordinary TCP Segment, the Payload of an Upgraded Segment does not start straight after the TCP Data Offset. Instead, Figure 2 shows that space is provided for additional Inner TCP Options before the TCP Payload. The size of this space is termed the Inner Options Offset (InOO). The TCP receiver reads the InOO field from the Inner Option Space (InSpace) option defined in Section 2.2.2.
The InSpace Option is located in a standardized location so that the receiver can find it:
Because the InSpace Option is only ever located in a standardized location it does not need to follow the RFC 793 format of a TCP option. Therefore, although we call InSpace an 'option', we do not describe it as a 'TCP option'.
The Sent Payload Size (SPS) is also read from within the InSpace Option. If the byte-stream has been resegmented, it allows the receiver to step from one InSpace Option to the next even if the InSpace Options are no longer at the start of each segment (see Section 2.3).
On a segment with SYN=1 (i.e. a SYN or SYN/ACK) the Suffix Options Offset (SOO) is also read from within the InSpace Option. It delineates the end of the Prefix TCP Options (PrefixOpts in the figure) and the start of the Suffix TCP Options (SuffixOpts). When SYN=1, the receiver processes PrefixOpts before OuterOpts, then SuffixOpts afterwards. When SYN=0, the receiver processes the Outer Options before the Inner Options. Full details of option processing are given in Section 2.3.
The internal structure of the InSpace Option for an Upgraded SYN or SYN/ACK segment (SYN=1) is defined in Figure 3a) and for a segment with SYN=0 in Figure 3b).
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 a) SYN = 1 +-------------------------------+---------------------------+---+ | Sent Payload Size (SPS) |Inner Options Offset (InOO)|Len| +-------------------------------+---------------------------+---+ | Magic Number B |Suffix Options Offset (SOO)|CU | +-------------------------------+---------------------------+---+ b) SYN = 0 +-------------------------------+---------------------------+---+ | Sent Payload Size (SPS) |Inner Options Offset (InOO)|Len| +-------------------------------+---------------------------+---+
Figure 3: InSpace Option Format
The fields are defined as follows (see Section 3.3 for the rationale behind these format choices):
The following fields are only defined on a segment with SYN=1 (i.e. a SYN or SYN/ACK):
If an Upgraded TCP Client uses a TCP Fast Open (TFO) cookie [I-D.ietf-tcpm-fastopen] in an Upgraded SYN-U, it MUST place the TFO option within the Inner TCP Options, beyond the Data Offset.
This rule is specific to TFO, but it can be generalised to any capability similar to TFO as follows: An Upgraded TCP Client MUST NOT place any TCP option in the Outer TCP Options of a SYN if it might cause a TCP server to pass user-data directly to the application before its own 3-way handshake completes.
If a client uses TCP Fast Open cookies on both the parallel connection attempts of a dual handshake, an Upgraded Server will deliver the TCP Payload to the application twice before the client aborts the Ordinary Connection. This is not a problem, because [I-D.ietf-tcpm-fastopen] requires that TFO is only used for applications that are robust to duplicate requests.
If the end of the last Prefix TCP Option does not align on a 4-octet boundary, the sender MUST append sufficient no-op TCP options. The end of the Suffix TCP Options MUST be similarly aligned.
If a block-mode transformation (e.g. compression or encryption) is being used, the sender might have to add some padding options to align the end of the Inner Options with the end of a block. Any future encryption specification will need to carefully define this padding in order not to weaken the cipher.
TCP's sequence number and acknowledgement number space MUST include all the TCP Data, i.e. the InSpace Option, any Inner Options, and any magic number as well as the TCP Payload. This rule has significant implications, which are discussed in Section 3.2.4.
Whenever the sender includes non-zero user-data payload in a segment, it MUST also include an InSpace Option, whether or not there are any Inner Options.
If the sender includes no user-data in a segment (e.g. pure ACKs, RSTs) it MAY include an InSpace Option but it does not have to. {ToDo: Consider whether there is any reason to preclude Inner Options on a RST, FIN or FIN-ACK.}
Once a sender has included the InSpace Option and possibly other Inner Options on a segment with no TCP Payload, while it has no further user-data to send it SHOULD NOT repeat the same set of control options on subsequent segments. Thus, in a sequence of pure ACKs, any particular set of Inner Options will only appear once, and other pure ACKs will be empty. The only envisaged exception to this rule would be infrequent repetition (i.e. tens of minutes to hours) of the same control options, which might be necessary to provide a heartbeat or keep-alive capability.
The rules for reading Inner TCP Options are divided between the following two subsections, depending on whether SYN=1, SYN=0.
This subsection applies when TCP receives a segment with SYN=1, i.e. when the server receives a SYN or the client receives a SYN/ACK.
Before processing any TCP options, unless the size of the TCP Data is less than 8 octets, an Upgraded Receiver MUST determine whether the segment is an Upgraded Segment by checking that all the following conditions apply:
If all these conditions pass, the receiver MAY walk the sequence of Inner TCP Options, using the length of each to check that the sum of their lengths equals InOO. The receiver then concludes that the received segment is an Upgraded Segment.
The receiver then processes the TCP Options in the following order:
The receiver removes the magic number, the InSpace Option and each TCP Option from the TCP Data as it processes each, until only the TCP Payload remains, which it holds ready to pass to the application. It then returns the appropriate Upgraded Acknowledgement to progress the dual handshake (see Section 2.1.1).
If any of the above tests to find the InSpace Option fails:
For the avoidance of doubt the above rules imply that, as long as an InSpace Option has not been found in the segment, the receiver might rerun the tests for it multiple times if multiple Outer TCP Options alter the TCP Data. However, once the receiver has found an InSpace Option, it MUST NOT rerun the tests for an Upgraded Segment in the same segment.
If the receiver has not found an InSpace Option after processing all the Outer Options, it returns the appropriate Ordinary Acknowledgement to progress the dual handshake (see Section 2.1.1). As normal, it holds any TCP Payload ready to pass to the application.
This subsection applies once the TCP connection has successfully negotiated to use the upgraded InSpace structure.
As each segment with SYN=0 arrives, the receiver immediately processes any Outer TCP options.
As the receiver buffers TCP Data, it uses TCP's regular mechanisms to fill any gaps due to reordering or loss so that it can work its way along the ordered byte-stream. As the receiver encounters each set of Inner Options, it MUST process them in the order they were sent, as illustrated in Figure 5a) in Section 3.2.4. The receiver MUST remove the InSpace Option and TCP Options from the TCP Data as it processes them, until only the TCP Payload remains, which it passes to the application. It uses each InSpace Option to calculate the extent of the associated Inner Options (using InOO), and the amount of payload data before the next InSpace Option (using Sent Payload Size).
The receiver MUST NOT locate InSpace Options by assuming there is one at the start of the TCP Data in every segment, because resegmentation might invalidate this assumption.
Therefore, the receiver processes the Inner Options in the order they were sent, which is not necessarily the order in which they are received. And if an Inner Option applies to the data stream, the receiver applies it at the point in the data stream where the sender inserted it. As a consequence, the receiver always processes the Inner Options after the Outer Options.
The Inner Options are deliberately placed within the byte-stream so that the sender can transform them along with the payload data, e.g. to compress or encrypt them. Therefore, a previous control message might have required the TCP receiver to alter the byte-stream before passing it to the application, e.g. decompression or decryption. If so, the TCP receiver applies transformations progressively, to one sent segment at a time, in the following order:
Middleboxes exist that process some aspects of the TCP Header. Although the present specification defines a new location for Inner TCP Options beyond the Data Offset, this is intended for the exclusive use of the destination TCP implementation. A middlebox MUST treat any octets beyond the Data Offset as immutable user-data. Legacy Middleboxes already do not expect to find options beyond the Data Offset anyway.
A TCP implementation is not necessarily aware whether it is deployed in a middlebox or in a destination, e.g. a split TCP connection might use a regular off-the-shelf TCP implementation. Therefore, a general-purpose TCP that implements the present specification will need a configuration switch to disable any search for options beyond the Data Offset.
{ToDo: Define behaviour of forwarding or receiving nodes if the structure or format of an Upgraded Segment is not as specified.}
If an Upgraded TCP Receiver receives an InSpace Option with a Length it does not recognise as valid, it MUST drop the packet and acknowledge the octets up to the start of the unrecognised option.
Values of Sent Payload Size greater than 2^16 - 25 (=65,511) octets MUST be treated as the distance to the next InSpace option, but they MUST NOT be taken as indicative of the size of the TCP Payload when it was sent. This is because the TCP Payload in even an IPv6 (non-jumbogram) packet cannot be greater than (2^16 -1 - 20 - 4) octets (given the minimum TCP header is 20 octets and the minimum InSpace Option is 4 octets). A Sent Payload Size of 0xFFFF octets MAY be used to minimise the occurrence of empty InSpace options without permanently disabling the Inner Space protocol for the rest of the connection.
{Temporary note: This section will be taken out into a stand-alone Draft. It is initially included here for convenience.}
The Inner Space protocol allows a potentially large amount of control state to be negotiated during the SYN exchange. The SYN Cookie mechanism [RFC4987] embeds the cookie within part of the TCP Initial Sequence Number, so a server can only store a limited amount of state in the cookie. The only reason that space is limited is that servers cannot assume all clients will support a cookie mechanism that could hold more information. The simple solution to this problem is to make a generic cookie-echo mechanism a prerequisite for all client InSpace implementations.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 /| ...
+---------------+---------------+-----------------/ | /---------+
| Echo | Length | OpaqueCookie /||/ ... |
+---------------+---------------+----------------/ | /----------+
|/
Figure 4: The Echo TCP Option
The Echo TCP option is defined in Figure 4. The option Kind is Echo with value {ToDo: Value TBA}. The Length in octets can be any value greater than 1. The OpaqueCookie field is available for the server to fill with any amount of any type of data it wishes to store in the cookie, only constrained in size to an integer number of octets. The server can locate the option in a SYN/ACK, either as an Outer or an Inner TCP Option.
If a TCP receiver receives an Echo TCP option, it MUST echo an identical Echo TCP option in the following ACK.
A client MAY send an empty Echo TCP option on the SYN to indicate that it supports the Echo facility. A client implementation of the Inner Space protocol MUST implement the Echo TCP option. Therefore a SYN-U is sufficient to imply support for the Echo facility without having to send an empty Echo TCP option on a SYN as well.
The mechanism a server uses to determine whether the echoed contents of the cookie are the same as the contents it sent are implementation dependent and do not need to be standardised.
This section is informative, not normative.
In traditional [RFC0793] TCP, the space for options is limited to 40B by the maximum possible Data Offset. Before a TCP sender places options beyond that, it has to be sure that the receiver will understand the upgraded protocol, otherwise it will confuse and potentially crash the application by passing it TCP options as if they were payload data.
The Dual Handshake (Section 2.1.1) ensures that a Legacy TCP Server will never pass on TCP options as if they were user-data. If a SYN carries TCP Data, a TCP server typically holds it back from the application until the 3-way handshake completes. This gives the client the opportunity to abort the Upgraded Connection if the response from the server shows it does not recognise an Upgraded SYN.
The strategy of sending two SYNs in parallel is not essential to the Alternative SYN approach. It is merely an initial strategy that minimises latency when the client does not know whether the server has been upgraded. Evolution to a single SYN with greater option space could proceed as follows:
There is concern that, although dual handshake approaches might well eventually migrate to a single handshake, they do not scale when there are numerous choices to be made simultaneously. For instance:
Nonetheless, it is not necessary to try every possible combination of N choices, which would otherwise require 2^N handshakes (assuming each choice is between two options). Instead, a selection of the choices could be attempted together. At the extreme, two handshakes could be attempted, one with all the new features, and one without all the new features.
This section justifies the magic number approach by contrasting it with a more 'conventional' approach. A conventional approach would use a regular (Outer) TCP option to point to the dividing line within the TCP Data between the extra Inner Options and the TCP Payload.
This 'conventional' approach cannot provide extra option space over a path on which a middlebox strips TCP options that it does not recognise. [Honda11] quantifies the prevalence of such paths. It reports on experiments conducted in 2010-2011 that found unknown options were stripped from the SYN-SYN/ACK exchange on 14% of paths to port 80 (HTTP), 6% of paths to port 443 (HTTPS) and 4% of paths to port 34343 (unassigned). Further analysis found that the option-stripping middleboxes fell into two main categories:
In contrast, the magic number approach ensures that not only are the Inner Options tucked away beyond the Data Offset, but the option that gives the extent of the Inner Options is also beyond the Data Offset (see Section 2.2.1). This ensures that all the TCP Headers and options up to the Data Offset are completely indistinguishable from an Ordinary Segment. It is very unusual for a middlebox not to forward TCP Data unchanged, so it will be highly likely (but not certain—see Appendix A.2.4) to forward the extra Inner Options.
The downside of the magic number approach is that it is slightly non-deterministic, quantified as follows:
The above probability is based on the assumptions that:
Therefore even though 2^{-66} is a vanishingly small probability, the actual probability of a collision will be much lower.
If a collision does occur, it will result in TCP removing a number of 32-bit words of data from the start of a byte-stream before passing it to the application.
The purpose of locating control options within the TCP Data is not to evade security. Security middleboxes can be expected to evolve to examine control options in the new inner location. Instead, the purpose is to traverse middleboxes that block new TCP options unintentionally—as a side effect of their main purpose—merely because their designers were too careless to consider that TCP might evolve. This category of middleboxes tends to forward the TCP Payload unaltered.
By sitting within the TCP Data, the Inner Space protocol should traverse enough existing middleboxes to reach critical mass and prove itself useful. In turn, this will open an opportunity to introduce integrity protection for the TCP Data, whereas today, no operating system would introduce integrity protection of Outer TCP options, because it would fail in too many cases. Once the integrity of Inner Options is protected, it will raise the stakes. Any attempt to meddle with control options within the TCP Data will not just close off the theoretical potential benefit of a protocol advance that no-one knows they want yet; it will fail integrity checks and therefore completely break any communication. No network operator will buy a middlebox that does that.
Then middlebox designers will be on the back foot. To completely block communications they will need a sound justification. If they block an attack, that will be fine. But if they block everything abnormal they will have to block the whole communication, or nothing. So the operator will want to choose middlebox vendors who take much more care to ensure their policies track the latest advances—to avoid costly support calls.
Some middleboxes discard a segment sent to a well-known port (particularly port 80) if the TCP Data does not conform to the expected app-layer protocol (particularly HTTP). Often such middleboxes only parse the start of the app-layer header (e.g. Web filters only continue until they find the URL being accessed, or DPI boxes only continue until they have identified the application-layer protocol).
The segment structure defined in Section 2.2.1 would not traverse such middleboxes. An alternative segment structure that avoids the start of the first two segments in each direction is defined in Appendix A.4. It is not mandatory to implement in the present specification. However, it is hoped that it will be included in some experimental implementations so that it can be decided whether it is worth making mandatory.
Including Inner Options within TCP's sequence space gives the sender a simple way to ensure that control options will be delivered reliably and in order to the remote TCP, even if the control options are on segments without user-data. By using TCP's existing stream delivery mechanisms, it adds no extra protocol processing, no extra packets and no extra bits.
The sender can even choose to place control options on a segment without user-data, e.g. to reliably re-key TCP-level encryption on a connection currently sending no data in one direction. The sender can even add an InSpace Option without further Inner Options. Then it can ensure that the segment will automatically be delivered reliably and in order to the remote TCP, even though it carries no user-data or other TCP control options, e.g. for a test probe, a tail-loss probe or a keep-alive.
Figure 5a) illustrates control options arriving reliably and in order at the receiving TCP stack in comparison with the traditional approach shown in Figure 5b), in which control options are outside the sequence space. In the traditional approach, during a period when the remote TCP is sending no user-data, the local TCP may receive control options E, B and D without ever knowing that they are out of order, and without ever knowing that C is missing.
a) __ ____ _______ _ __
|__|____|_______|_| |__| control
:E : D : C :B: :A :
________________: : : : :__________________: :
|________________| |__________________| data
b) __
|__| E
|_|__ B __
|____|D |__|A control
\ / \ /
________________\/__________________\/
|________________||__________________| data
!
!drop
____!__
|_______|C
Figure 5: Control options a) inside vs. b) outside TCP sequence space`
By including Inner Options within the sequence space, each control option is automatically bound to the start of a particular byte in the data stream, which makes it easy to switch behaviour at a specific point mid-stream (e.g. re-keying or switching to a different control mode). With traditional TCP options, a bespoke reliable and ordered binding to the data stream would have to be developed for each TCP option that needs this capability (e.g. co-ordinating use of new keys in TCP-AO [RFC5925] or tcpcrypt [I-D.bittau-tcpinc]).
Including Inner Options in sequence also allows the receiver to tell the sender the exact point at which it encountered an unrecognised TCP option using only TCP's pre-existing byte-granularity acknowledgement scheme.
Middleboxes exist that rewrite TCP sequence and acknowledgement numbers, and they also rewrite options that refer to sequence numbers (at least those known when the middlebox was produced, such as SACK, but not any introduced afterwards). If Inner Options were not included in sequence, the number of bytes beyond the TCP Data Offset in each segment would not match the sequence number increment between segments. Then, such middleboxes could unintentionally corrupt the user-data and options by 'normalising' sequence or acknowledgement numbering. Fortunately, including Inner Options in sequence improves robustness against such middleboxes.
A middlebox that splits a TCP connection can coalesce and/or divide the original segments. Segmentation offload hardware introduces similar resegmentation. Inclusion of the Sent Payload Size field in the InSpace Option makes the scheme robust against such resegmentation.
The Sent Payload Size is not strictly necessary on a SYN (SYN=1, ACK=0) because a SYN is never resegmented. However, for simplicity, the layout for a SYN is made the same as for a SYN/ACK. This future-proofs the protocol against the possibility that SYNs might be resegmented in future. And it makes it easy to introduce the alternative segment structure of Appendix A.4 if it is needed.
The format of the InSpace Option (Figure 3) does not necessarily have to comply with the RFC 793 format for TCP options, because it is not intended to ever appear in a sequence of TCP options. In particular, it does not need an Option Kind, because the option is always in a known location. In effect the magic number serves as a multi-octet Option Kind for the first InSpace Option, and the location of each subsequent options is always known as an offset from the previous one, using InOO and Sent Payload Size fields.
Other aspects of the layout are justified as follows:
When SYN=1 the layout of the InSpace Option is extended to include:
The overhead of the Inner Space protocol is quantified as follows:
For example, if P=80% and D=10%, the connection rate will inflate by 8%. P is difficult to predict. D is likely to be small, and in the longer term it should reduce to the proportion of connections to remaining legacy servers, which are likely to be the less frequently accessed ones. In the worst case if both P & D are 100%, the maximum that the connection rate can inflate by is 100% (i.e. to twice present levels).
This is because a server or middlebox only holds dual connection state for one round trip, until the RST on one of the two connections. For example, keeping P & D as they were in the above example, if R = 3 round trips {ToDo: TBA}, connection state would inflate by 2.7%. In the longer term, any extra connection state would be focused on legacy servers, with none on upgraded servers. Therefore, if memory for dual handshake flow state was a problem, upgrading the server to support the Inner Space protocol would solve the problem.
For example, keeping and P & D as they were in the above example, if J = 50KiB for IPv4 and K = 70 packets (ToDo: TBA), traffic overhead would be 0.03% counting in bytes or 0.2% counting in packets.
This is because the InSpace option adds 4B per segment. For example, keeping P as it was in the above example and taking Q=10% and F=750B, the traffic overhead is 0.04%. It is as difficult to predict Q as it is to predict P.
A TCP option MUST by default only be used as an Outer Option, unless it is explicitly specified that it can (or must) be used as an Inner Option. The following list of pre-existing TCP options can be located as Inner Options:
The following MUST NOT be located as Inner Options:
{ToDo: The above list is not authoritative. Many of the above schemes involve multiple different types of TCP option, and all the types need to be separately assessed.}
The Inner Space protocol supports TCP Fast Open, by constraining the client to obey the rules in Section 2.3.1.1).
An Upgraded Server can support SYN Cookies [RFC4987] for Ordinary Connections. For Upgraded Connections Section 2.5 defines a new SYNCookie TCP option that is a prerequisite for InSpace implementations, and provides sufficient space for the more extensive connection state requirements of an InSpace server.
The interaction with the assumptions about TCP made by middleboxes is covered extensively in Section 2.3.3, Section 3.2.1, Section 3.2.2, Section 3.2.4 and Section 3.2.5).
{ToDo: TCP API, TCP States and Transitions, Connectionless Resets, ICMP Handling, Interactions with Other TCP Variants, Forward-Compatibility.}
This specification requires IANA to allocate four values from the TCP option Kind name-space against the names:
Early implementation before the IANA allocation MUST follow [RFC6994] and use experimental option 254 and respective Experiment IDs:
{ToDo: Values TBA and register them with IANA} then migrate to the assigned option after allocation.
Certain cryptographic functions have different coverage rules for the TCP Header and TCP Payload. Placing some TCP options beyond the Data Offset could mean that they are treated differently from regular TCP options. This is a deliberate feature of the protocol, but application developers will need to be aware that this is the case.
A malicious host can send bogus SYN segments with a spoofed source IP address (a SYN flood attack). The Inner Space protocol does not alter the feasibility of this attack. However, the extra space for TCP options on a SYN allows the attacker to include more TCP options on a SYN than before, so it can make a server do more option processing before replying with a SYN/ACK. A server under stress could deprioritise SYNs with longer option fields to focus its resources on SYNs that require less processing.
{ToDo: More?}
The idea of this approach grew out of discussions with Joe Touch while developing draft-touch-tcpm-syn-ext-opt, and with Jana Iyengar and Olivier Bonaventure. The idea that it is architecturally preferable to place a protocol extension within a higher layer, and code its location into upgraded implementations of the lower layer, was originally articulated by Rob Hancock. {ToDo: Ref?} The following people provided useful review comments: Joe Touch, Yuchung Cheng, John Leslie, Mirja Kuehlewind, Andrew Yourtchenko.
Bob Briscoe's contribution is part-funded by the European Community under its Seventh Framework Programme through the Trilogy 2 project (ICT-317756) and the Reducing Internet Transport Latency (RITE) project (ICT-317700). The views expressed here are solely those of the author.
| [I-D.ietf-tcpm-fastopen] | Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", Internet-Draft draft-ietf-tcpm-fastopen-10, September 2014. |
| [RFC0793] | Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC6994] | Touch, J., "Shared Use of Experimental TCP Options", RFC 6994, August 2013. |
This appendix specifies protocol extensions that are OPTIONAL while the specification is experimental. If an implementation includes an extension, this section gives normative specification requirements. However, if the extension is not implemented, the normative requirements can be ignored.
{Temporary note: The IETF may wish to consider making some of these extensions mandatory to implement if early testing shows they are useful or even necessary. Or it may wish to make at least the receiving side mandatory to implement to ensure that two-ended experiments are more feasible.}
This appendix is normative. It is separated from the body of the specification because it is OPTIONAL to implement while the Inner Space protocol is experimental. It defines the new ModeSwitch TCP option illustrated in Figure 6. This option provides a facility to disable the Inner Space protocol for the remainder of a connection. It also provides a general-purpose facility for a TCP connection to co-ordinate between the endpoints before switching into a yet-to-be-defined mode.
0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +---------------+---------------+-----------+-+-+ | ModeSwitch | Length=3 |Flags (CU) |I|R| +---------------+---------------+-----------+-+-+
Figure 6: The ModeSwitch TCP Option
The option Kind is ModeSwitch, the value of which is to be allocated by IANA {ToDo: Value TBA}. ModeSwitch MUST be used only as an Inner Option, because it uses the reliable ordered delivery property of Inner Options. Therefore implementation of the Inner Space protocol is REQUIRED for an implementation of ModeSwitch. Nonetheless, ModeSwitch is a generic facility for switching a connection between yet-to-be-defined modes that do not have to relate to extra option space.
The sender MUST set the option Length to 3 (octets). The Length field MUST be forwarded unchanged by other nodes, even if its value is different.
The Flags field is available for defining modes of the connection. Only two connection modes are currently defined. The first 6 bits of the Flags field are Currently Unused (CU) and the sender MUST set them to zero. The CU flags MUST be ignored and forwarded unchanged by other nodes, even if their value is non-zero.
The two 1-bit connection mode flags that are currently defined have the following meanings:
The default Inner Space mode at the start of a connection is I=1, meaning Inner Space is in enabled mode.
The procedure for changing a mode or modes is as follows:
The regular TCP sequence numbers and acknowledgement numbers of requests or confirmations can be used to disambiguate overlapping requests or responses.
Once a host switches to Disabled mode, it MUST NOT send any further InSpace Options. Therefore it can send no further Inner Options and it cannot switch back to Enabled mode for the rest of the connection.
To temporarily reduce InSpace overhead without permanently disabling the protocol, the sender can use a value of 0xFFFF in the Sent Payload Size (see Section 2.4).
This appendix is normative. It is separated from the body of the specification because it is OPTIONAL to implement while the Inner Space protocol is experimental. It is not mandatory to implement because it will be more useful once the Inner Space protocol has become accepted widely enough that fewer middleboxes will discard SYN segments carrying this option (see Appendix B for when best to deploy it). It only works if both ends support it, but it can be deployed one end at a time, so there is no need for support in early experimental implementations.
{Temporary note: The choice between the explicit handshake in the present section or the handshake in Section 2.1.1 is a tradeoff between robustness against middlebox interference and minimal server state. During the IETF review process, one might be chosen as the only variant to go forward, at which point the other will be deleted. Alternatively, the IETF could require a server to understand both variants and a client could be implemented with either, or both. If both, the application could choose which to use at run-time. Then we will need a section describing the necessary API.}
This explicit dual handshake is similar to that in Section 2.1.1, except the SYN that the Upgraded Client sends on the Ordinary Connection is explicitly distinguishable from the SYN that would be sent by a Legacy Client (using a simple Outer TCP Option. Then, if the server actually is an Upgraded Server, it can reset the Ordinary Connection itself, rather than creating connection state for at least a round trip until the client resets the connection.
For an explicit dual handshake, the TCP client still sends two alternative SYNs: a SYN-O intended for Legacy Servers and a SYN-U intended for Upgraded Servers. The two SYNs MUST have the same network addresses and the same destination port, but different source ports. Once the client establishes which type of server has responded, it continues the connection appropriate to that server type and aborts the other. The SYN intended for Upgraded Servers includes additional options within the TCP Data (the SYN-U defined as before in Section 2.2.1).
Table 2 summarises the TCP 3-way handshake exchange for each of the two SYNs in the two right-hand columns, between an Upgraded TCP Client (the active opener) and either:Table 1 that have already been explained in Section 2.1.1.
The table uses the same layout and symbols as
| Ordinary Connection | Upgraded Connection | ||
|---|---|---|---|
| 1 | Upgraded Client | >SYN-O | >SYN-U |
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Legacy Server | <SYN/ACK | <SYN/ACK |
| 3a | Upgraded Client | Waits for response to both SYNs | |
| 3b | " | >ACK | >RST |
| 4 | Cont... | ||
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Upgraded Server | <RST | <SYN/ACK-U |
| 3 | Upgraded Client | >ACK | |
| 4 | Cont... |
As before, an Upgraded Server MUST respond to a SYN-U with a SYN/ACK-U. Then, the client recognises that it is talking to an Upgraded Server.
Unlike before, an Upgraded Server MUST respond to a SYN-O with a RST. However, the client cannot rely on this behaviour, because a middlebox might strip the Outer TCP Option from the SYN-O, turning it into a regular SYN before it reaches the server. Then the handshake would effectively revert to the implicit variant. Therefore the client's behaviour still depends on which SYN-ACK arrives first, so its response to SYN-ACKs has to follow the rules specified for the implicit handshake variant in Section 2.1.1.
The rules for processing TCP options are also unchanged from those in Section 2.3.
The SYN-O is merely a SYN with an extra InSpaceO Outer TCP Option as shown in Figure 7. It merely identifies that the SYN is opening an Ordinary Connection, but explicitly identifies that the client supports the Inner Space protocol.
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---------------+---------------+ | Kind=InSpaceO | Length=2 | +---------------+---------------+
Figure 7: An InSpaceO TCP Option Flag
An InSpaceO TCP Option has option Kind InSpaceO with value {ToDo: Value TBA} and MUST have Length = 2 octets.
To use this option, the client MUST place it with the Outer TCP Options. A Legacy Server will just ignore this TCP option, which is the normal behaviour for an option that TCP does not recognise [RFC0793].
If the client receives a RST on one connection, but a short while after that {ToDo: duration TBA} the response to the SYN-U has not arrived, it SHOULD retransmit the SYN-U. If latency is more important than the extra TCP option space, in parallel to any retransmission, or instead of any retransmission, the client MAY send a SYN without any InSpace TCP Option, in case this is the cause of the black-hole. However, the presence of the RST implies that the SYN with the InSpaceO TCP Option (the SYN-O) probably reached the server, therefore it is more likely (but not certain) that the lack of response on the other connection is due to transmission loss or congestion loss.
If the client receives no response at all to either the SYN-O or the SYN-U, it SHOULD solely retransmit one or the other, not both. If latency is more important than the extra TCP option space, it SHOULD send a SYN without an InSpaceO TCP Option. Otherwise it SHOULD retransmit the SYN-U. It MUST NOT retransmit both segments, because the lack of response could be due to severe congestion.
There is a small but finite possibility that the Explicit Dual Handshake might encounter the cases below. The Implicit Handshake (Section 2.1.1) is robust to these possibilities, but the Explicit Handshake is not, unless the following additional rules are followed:
If a path either holds back or discards data in a SYN-U, but there is evidence that the server is upgraded from a RST response to the SYN-O, the strategy below might at least allow a connection to use extra option space on all the segments except the SYN.
It is assumed that the symptoms described in the 'both aborted' case (Appendix A.2.3) have occurred, i.e. the server has responded to the SYN-O with a RST, but it has responded to the SYN-U with an Ordinary SYN/ACK not a SYN/ACK-U, so the client has had to RST the Upgraded Connection as well. In this case, the client SHOULD attempt the following (alternatively it MAY give up and fall back to opening an Ordinary TCP connection).
The client sends an 'Alternative SYN-U' by including an InSpaceU Outer TCP Option (Figure 8). This Alternative SYN-U merely flags that the client is attempting to open an Upgraded Connection. The client MUST NOT include any Inner Options or InSpace Option or Magic Number. If the previous aborted SYN/ACK-U acknowledged the data that the client sent within the original SYN-U, the client SHOULD resend the TCP Payload data in the Alternative SYN-U, otherwise it might as well defer it to the first data segment.
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---------------+---------------+ | Kind=InSpaceU | Length=2 | +---------------+---------------+
Figure 8: An InSpaceU Flag TCP option
An InSpaceU Flag TCP Option has option Kind InSpaceU with value {ToDo: Value TBA} and MUST have Length = 2 octets.
To use this option, the client MUST place it with the Outer TCP Options. A Legacy Server will just ignore this TCP option, which is the normal behaviour for an option that TCP does not recognise [RFC0793]. Because the client has received a RST from the server in response to the SYN-O it can assume that the server is upgraded. So the client probably only needs to send a single Alternative SYN-U in this repeat attempt. Nonetheless, the RST might have been spurious. Therefore the client MAY also send an Ordinary SYN in parallel, i.e. using the Implicit Dual Handshake (Section 2.1.1).
If an Upgraded Server receives a SYN carrying the InSpaceU option, it MUST continue the rest of the connection as if it had received a full SYN-U (Section 2.2), i.e. by processing any Outer Options in the SYN-U and responding with a SYN/ACK-U.
This appendix is normative. It is separated from the body of the specification because it is OPTIONAL to implement while the Inner Space protocol is experimental. In experimental implementations, it will be sufficient to implement the required behaviour for when the Length of a received InSpace Option is not recognised (Section 2.4).
If the IPv6 Jumbo extension header is used, the SentPayloadSize field will need to be 4 octets wide, not 2 octets. This section defines the format of the InSpace Option necessary to support jumbograms.
If sending a jumbogram, a sender MUST use the InSpace Option format defined in Figure 9. All the fields have the same meanings as defined in Section 2.2.2, except InOO and SentPayloadSize use more bits.
When reading a segment, the Jumbo InSpace Option could be present in a packet that is not a jumbogram (e.g. due to resegmentation). Therefore a receiver MUST use the Jumbo InSpace Option to work along the stream irrespective of whether arriving packets are jumbo sized or not.
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 +-----------------------------------------------------------+---+ | Inner Options Offset (InOO) |Len| +-----------------------------------------------------------+---+ | Sent Payload Size (SPS) | +---------------------------------------------------------------+
Figure 9: InSpace Option for a Jumbo Data-UNJH
This appendix is normative. It is separated from the body of the specification because it is OPTIONAL to implement while the Inner Space protocol is experimental. An Upgraded Receiver implementation that has implemented this extension will not be able to use the base Inner Space protocol if it is sent a segment with this structure. However, it will at least fall-back naturally to regular TCP behaviour.
In experiments conducted between 2010 and 2011, [Honda11] reported that 7 of 142 paths (about 5%) blocked access to port 80 if the payload was not parsable as valid HTTP. This variant of the specification has been defined in case experiments prove that it significantly improves traversal of such deep packet inspection (DPI) boxes.
This variant starts the TCP Data with the expected app-layer headers on the first two segments in each direction:
From the second InSpace Option onwards, the structure of the stream reverts to that already defined in Section 2.2.1. So the value of Sent Payload Size (SPS#2) in the second InSpace Option (InSpace #2) defines the length of any remaining TCP Payload before the end of the first data segment, as shown.
TCP Data
.------------------------'----------------------.
| Inner Options |
a) SYN=1 | .----------'----------. |
+--------+----------+--------+----------+----------+---------+------+
| BaseHdr| OuterOpts| Payload|PrefixOpts|SuffixOpts|InSpace#1|MagicA|
+--------+----------+--------+----------+----------+---------+------+
| DO | | SOO | | | 1 |
`------------------>| `--------->| | Len |<-----'
| | | InOO |<--------' |
|<--------------------' |
b) First SYN=0 segment in either direction
+--------+----------+----------+---------+---------------+----------+
| BaseHdr| OuterOpts| Payload |InSpace#2| Inner Options | Payload |
+--------+----------+----------+---------+---------------+----------+
| DO | SPS#1 | Len | InOO | SPS#2 |
`------------------>`--------->`-------->`-------------->`--------->|
All offsets are specified in 4-octet (32-bit) words, except SPS, which is in octets.
Figure 10: Segment Structures to Traverse DPI boxes (not to scale)
It is recognised that having to work from the end of the first segment makes processing more involved. Experimental implementation of this approach will determine whether the extra complexity improves DPI box traversal sufficiently to make it worthwhile.
In the body of this specification, two variants of the dual handshake are defined:
Both schemes double up connection state (for a round trip) on the Legacy Server. But only the implicit scheme doubles up connection state (for a round trip) on the Upgraded Server as well. On the other hand, the explicit scheme risks delay accessing a Legacy Server if a middlebox discards the SYN-O (it is possible that some firewalls will discard packets with unrecognised TCP options {ToDo: ref?}). Table 3 summarises these points.
| SYN (Implicit) | SYN-L (Explicit) | |
|---|---|---|
| Minimum state on Upgraded Server | - | + |
| Minimum risk of delay to Legacy Server | + | - |
There is no need for the IETF to choose between these. If the specification allows either or both, the tradeoff can be left to implementers at build-time, or to the application at run-time.
Initially clients might choose the Implicit Dual Handshake to minimise delays due to middlebox interference. But later, perhaps once more middleboxes support the scheme, clients might choose the Explicit scheme, to minimise state on Upgraded Servers.
This appendix is informative, not normative. It records outstanding issues with the protocol design that will need to be resolved before publication.
A detailed version history can be accessed at <http://datatracker.ietf.org/doc/draft-briscoe-tcpm-inner-space/history/>