Internet DRAFT - draft-iyengar-minion-protocol
draft-iyengar-minion-protocol
Network Working Group J. Iyengar
Internet-Draft Franklin and Marshall College
Intended status: Standards Track S. Cheshire
Expires: April 24, 2014 J. Graessley
Apple
October 21, 2013
Minion - Wire Protocol
draft-iyengar-minion-protocol-02
Abstract
Minion uses TCP-format packets on-the-wire, for compatibility with
existing NATs, Firewalls, and similar middleboxes, but provides a
richer set of facilities to the application, as described in the
Minion Service Model document. This document specifies the details
of the on-the-wire protocol used to provide those services.
Status of this Memo
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This Internet-Draft will expire on April 24, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Conventions and Terminology Used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
This document uses terminology like "kernel" and "user-level", as
those terms pertain to many of today's Unix-like operating systems.
Equivalent concepts apply to software that is built using a different
architectural model than may not include such an obvious kernel/user
split.
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2. Introduction
Minion uses TCP-format packets on-the-wire, to provide full
compatibility with existing NATs, Firewalls, and similar middleboxes,
but provides a richer set of facilities to the application, described
in the Minion Service Model and Conceptual API document [minserv].
This document specifies the details of the on-the-wire protocol used
to provide those services. Before reading this protocol
specification document, familiarity with the Minion Service Model
[minserv] is strongly recommended. That information is not repeated
here.
Minion runs over a standard TCP connection. Therefore, IP addresses
and TCP ports are used just as they are with TCP [RFC0793].
Minion is also designed to be able to use a modified TCP connection
which supports out-of-order delivery, giving better low-latency
performance on lossy networks, for use by the kinds of application
that today would use UDP [RFC0768] to achieve low-latency delivery.
The goal of providing low-latency delivery -- and consequently the
need to be able to handle a data stream that may have gaps -- is
reflected in various aspects of the Minion protocol design, such as
the use of DTLS instead of TLS, and the use of Consistent Overhead
Byte Stuffing [COBS] for reliably extracting messages from an
incomplete data stream. Minion is able to take advantage of out-of-
order delivery where the network stack offers that, but Minion does
not require it. Minion still works correctly when the performance
benefits of out-of-order delivery are not available.
Minion supports messages of arbitrary size. Large messages are
broken into chunks a little under 16 kilobytes each (the DTLS maximum
record size, minus a few bytes for Minion header). At the receiving
end the Minion chunks are reassembled into Minion messages and
delivered to the client application. Small messages are sent in a
single Minion chunk.
Normally messages are sent by the client as a single atomic unit, and
delivered to the receiving client as a single atomic unit. For
messages too large to fit conveniently in memory, the message may be
built incrementally by the sender, and delivered to the receiving
client incrementally, a chunk at a time.
When a Minion message is complete, or has at least one maximum Minion
chunk size of data accumulated, then if it is eligible to be sent
according to the message ordering facilities offered by the Minion
Service Model [minserv] (Sender Ordering, Receiver Ordering, and
Chaining) a Minion chunk is generated.
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Each Minion chunk contains a Minion chunk header followed by the
client's message data, as described in Section 3 "Minion Chunk
Format".
Each Minion chunk is encrypted using DTLS [RFC6347].
Each encrypted DTLS payload is then framed using RECOBS, as described
in Section 4 "Recursively Embeddable COBS", so that it begins with a
00 byte and ends with an FF byte.
The framed, encrypted chunk is then enqueued for transmission.
If the kernel networking code supports multiple priorities, then the
framed, encrypted chunk is placed in the transmission queue for the
stated priority level. Any time the TCP congestion window and/or
receive window rules allow more data to be sent, data is drawn from
the highest-priority non-empty transmit buffer, assigned the next
block of unused TCP sequence numbers, formed into a TCP segment, and
transmitted on the wire. This just-in-time TCP sequencing mechanism
has the effect of causing higher-priority data to be inserted right
at the front of the conceptual combined transmit buffer, at the
earliest possible byte boundary, unconstrained by message or chunk
boundaries in the lower-priority messages. This is possible because
the RECOBS framing is robust to pre-emption at any arbitrary byte
boundary.
Note that, when priorities are supported, chunks above the lowest
priority MUST be delivered to the kernel in such a way that they are
sent completely before the kernel resumes sending the lower-priority
traffic. The RECOBS framing supports interrupting a lower priority
stream with a higher-priority chunk, but not alternating back and
forth between two priority levels. Once a higher-priority chunk
interrupts lower-priority traffic, the higher-priority chunk must be
completed before the lower-priority traffic resumes. Typically this
is easily achieved by delivering the chunk to the kernel atomically
in a single write call.
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2.1. Comparison of TCP and UDP NAT Traversal
When connecting to a server with a globally routable address, TCP is
generally preferable to UDP. TCP includes the SYN and FIN bits which
tell a NAT gateway when a connection starts and ends. In particular,
the FIN bit tells the NAT gateway when it can discard state related
to that mapping. UDP has no defined connection start/end indicators,
which means that unused UDP mappings are much more likely to
accumulate, which means that NAT gateways tend to be more aggressive
about timing out UDP mappings [Study], which means that clients using
UDP need to be more aggressive about sending keepalive traffic, which
is bad both for network efficiency and for battery life. Port
Control Protocol (PCP) [RFC6887] offers some future hope of
alleviating this problem by allowing clients to explicitly negotiate
for longer mapping lifetimes, but PCP is not yet widely deployed. In
the meantime, if use of UDP increases, NAT gateways are likely to be
accumulating mappings even more rapidly, with no way to differentiate
which are still required and which may be safely discarded, with the
result that UDP mappings may have to be discarded even more
aggressively. While a discarded UDP mapping can be recreated by
another outgoing UDP packet, in the time between when the UDP mapping
is discarded and then recreated, the client is cut off an unable to
receive inbound communication from server or peer at the other end.
Therefore, we believe that it is preferable to use TCP where
possible.
However, when connecting to a peer which is itself also behind a NAT
gateway, in the absence of PCP support [RFC6887], techniques like
Interactive Connectivity Establishment (ICE) [RFC5245] are used, and
research has shown that there are cases where ICE works for UDP but
not for TCP [RFC5128].
To accomodate both usage scenarios, Minion is generally used with
standard TCP format packets, but for peer-to-peer scenarios where TCP
ICE is found not to work, Minion can be used encapsulated inside UDP
[TCPoUDP] instead.
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3. Minion Chunk Format
A Minion Chunk begins with an eight-byte header, followed by the
client's message data:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Code |Pri| This Minion Chunk ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |RCP| Referenced Minion Chunk ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Minion Chunk Data :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Minion Chunk Format
If the Complete ('C') bit is zero, this message is incomplete; the
receiver should expect to receive additional continuation chunks for
this message. If the Complete bit is one, this message is complete;
there will be no subsequent continuation chunks for this message.
The seven-bit chunk code identifies what type of chunk this is, as
described below.
The two-bit priority field indicates the priority level for this
message, with 0 being the highest priority and 3 being the default
(lowest-level) priority.
Every Minion chunk has a Chunk ID. This is a 22-bit value assigned
from a monotonically increasing 22-bit cyclic counter. This means
that Chunk IDs are reused every 2^22 chunks. At any given moment in
time though, only a small portion of the 22-bit ID space is actively
in use, so Chunk IDs are not ambiguous. Each of the four priority
levels has its own 22-bit Chunk ID space, i.e., Priority 1 Chunk 7
and Priority 2 Chunk 7 are different chunks. Also, the Chunk ID
spaces in opposite directions on a connection are separate. Each
sender is responsible for selecting the Chunk IDs for the chunks it
sends.
In some cases it is useful to refer to messages by ID, and the terms
"Message ID" and "Chunk ID" are sometimes used interchangeably. For
a message that is sent using a single chunk, the Message ID is the
same as the Chunk ID. For a message that is sent using multiple
chunks, the Message ID is the Chunk ID of the *final* chunk of the
message. One implication of this is that a message's ID is undefined
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until the message is complete.
Because Chunk IDs are eventually reused, issues of ID lifetime must
be carefully considered in the Minion protocol design. For example,
since a remote peer could, in principle, wait an arbitrary long
length of time before replying to a message, the Message ID of a
request that is awaiting a response MUST NOT be reused until the
response has been received, and the client has disposed of the
request message. Otherwise, a reply could be ambiguous, if there
were two outstanding request messages both using the same Message ID
at the same time. Likewise, the last Chunk ID of an incomplete
message MUST NOT be reused until some subsequent chunk has been added
to that message, referencing the previous Chunk ID.
The Reserved field MUST be set to zero on transmission, and MUST be
ignored on reception.
For chunk types that need to refer to some other chunk, the
Referenced Minion Chunk Priority (RCP) and Referenced Minion Chunk ID
fields identify the referenced chunk. Note that some chunk types
refer to chunks going in the same direction (e.g., a continuation
chunk) and some chunk types refer to chunks going in the reverse
direction (e.g., a reply chunk). For chunk types that do not to
refer to any other chunk, these two fields MUST be set to zero on
transmission, and MUST be ignored on reception.
The Minion Chunk payload data follows the Minion Chunk Header.
There is no explicit length field in the Minion Chunk Header, because
the chunk length is determined implicitly in the RECOBS decoding
step.
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3.1. Minion Chunk Codes
The seven-bit chunk code identifies what kind of chunk this is.
There are 128 chunk codes available. The following eight chunk codes
are currently defined:
00 Continuation. This is a continuation of a previously incomplete
message. The Referenced Minion Chunk ID identifies what
previous chunk this is adding to. (If the Complete bit is one
then this chunk is the final chunk and completes the message; no
further chunks for this message will be arriving.)
01 Cancellation. This is a cancellation of a previously incomplete
message. The Referenced Minion Chunk ID identifies what
previous chunk this is cancelling. In this case Complete bit is
unused; the Complete bit MUST be set to zero on transmission,
and MUST be ignored on reception.
02 Unordered Message. This chunk begins a new unordered message.
The Referenced Minion Chunk ID is unused, and MUST be set to
zero on transmission, and MUST be ignored on reception.
03 Sender Ordered Message. This chunk begins a new Sender Ordered
message. If received out-of-order, it should nonetheless be
delivered immediately to the receiving client. The Referenced
Minion Chunk ID is used to deduce the Sender Ordering that
should be applied if the receiving client generates a reply to
this message. If the received message identifed by the
Referenced Minion Chunk ID generated a reply A', then a reply to
this message should have an automatic Sender Ordering dependency
that it follow message A'.
04 Receiver Ordered Message. This chunk begins a new Receiver
Ordered message. This message is subject to Receiver Ordering;
it MUST NOT be delivered to the receiving client until the
message indicated by the Referenced Minion Chunk ID field has
been delivered. If the receiving client generates a reply to
this message, then the reply should have an automatic Receiver
Ordering dependency that it follow the reply to the message
indicated by the Referenced Minion Chunk ID field.
05 Superseding Messages. This chunk begins a new message that
supersedes a preceding message. The Referenced Minion Chunk ID
identifies the preceding message. If the preceding message has
not yet been received, then when it does arrive, it MUST be
silently discarded.
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06 Chained Message. This chunk begins a new message that chains on
after a preceding message. The Referenced Minion Chunk ID
identifies the preceding message. This message MUST NOT be
delivered to the receiving client until the previous message of
the chain as been delivered to the receiving client, and this
message MUST be delivered to the receiving client in a manner
that indicates to the client that it is related to the previous
message.
07 Reply/Acknowledge. This chunk begins a new message which is an
explicit reply to a previously received message. The Referenced
Minion Chunk ID identifies the received message to which this is
a reply. A reply may be empty, in which case it serves as a
simple acknowledgement that the request was received and
accepted, or it may contain data. It is anticipated that future
Minion protocol development will create additional Minion chunk
codes to negotiate future protocol features. For these
capability negotiation messages, an empty reply referencing the
request serves as an acknowledgement that the requested protocol
feature is supported.
08 Reject. A Minion Reject code indicates that the referenced
received message had an error or was not accepted for some other
reason. A Reject Message may be empty, or may contain data
giving information concerning the reason for the rejection. It
is possible to reject an incomplete message that is still
arriving, by sending a Reject referencing the most recent Chunk
ID for that partial message. The sender will respond by sending
a Cancellation for that message, confirming that no further
chunks will be sent. When used for Minion protocol capability
negotiation, a Reject message referencing the request indicates
that the requested protocol feature is not supported.
09 End Minion. It is anticipated that there will be existing
application protocols that initially add Minion as an optional
feature, which they use only when the remote peer indicates it
also has Minion support, and otherwise they will communicate
using the existing protocol without the Minion features. Such
application protocols typically will first connect using their
existing protocol, and then negotiate an "upgrade" to Minion
framing. For symmetry, it would be good if such an "upgrade"
were not an irreversible one-way path. We would like to offer
the ability for applications to connect over raw TCP, switch to
Minion for some message exchanges, and then drop back to raw TCP
for some subsequent communication. This Minion chunk code
exists to signal, "This is the final Minion-format message you
will receive in this particular Minion session; after this
you're on your own."
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4. Recursively Embeddable COBS
Consistent Overhead Byte Stuffing [COBS] allows complete messages to
be reliably located within an incomplete data stream that may contain
gaps.
COBS works by transforming the payload data to eliminate all
occurrences of zero bytes. This is like PPP byte stuffing, but more
efficient; COBS has a worst-case data size overhead below 0.5%.
Having created a zero-free payload, the payloads can then be
concatenated into a single byte stream, separated by single zero
bytes, and the zero bytes unambiguously mark the boundaries between
payloads, because we know the payloads themselves no longer contain
any zero bytes. At the receiving end the transformation is reversed
to recreate the original payload data.
The transformation process [COBS] is, in effect, a simple run length
encoding. An extremely simplified summary of the original 1997 COBS
encoding is as follows:
o If the payload begins with three nonzero bytes followed by a zero,
then the output is the byte value 4 (the run length) followed by
the three nonzero bytes, and the subsequent zero is skipped.
o If that is followed by fifty nonzero bytes followed by a zero,
then the output is the byte value 51 (the run length) followed by
the fifty nonzero bytes, and the subsequent zero is skipped.
o This process is repeated until the entire payload has been
replaced by its zero-free equivalent.
Recursively Embeddable COBS (RECOBS) is a derivative of the original
1997 COBS encoding. RECOBS code bytes have the following meanings:
00 New payload begins
01 Represents a single zero byte
02 Two bytes: a single nonzero byte, followed by a single zero byte
03 Three bytes: two nonzero bytes, followed by a single zero byte
n Represents n bytes: n-1 nonzero bytes, followed by a zero byte
FD 253 bytes: 252 nonzero bytes, followed by a single zero byte
FE 253 bytes: 253 nonzero bytes, with *no* following zero byte
FF Payload ends
This has the effect that, after encoding, every payload has
unambiguous bookends; every payload begins with a single 00, and ends
with a single FF. Using this encoding, recursive embedding becomes
possible. At *any* point in the encoded byte stream it is now
possible to interrupt the byte stream, insert a new RECOBS-encoded
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payload, and then resume the previous byte stream.
At the receiving end, the decoder is part-way through decoding a
payload when the interruption occurs. The decoder sees a 00, which
is not legal in RECOBS-encoded data, so the decoder knows a new
payload is beginning. Because the decoder has not yet seen the FF
end-marker for the previous payload, it knows that payload is
incomplete, so it saves its decoding state for later resumption. The
decoder then proceeds to decode the embedded payload. When the
decoder sees the FF end-marker for the embedded payload, it delivers
that fully decoded payload to the waiting client, and then resumes
its decoding of the previously interrupted payload.
In principle this recursive embedding could be nested arbitrarily
deeply, limited only by the amount of storage the decoder has
available for partially-received payloads and their associated
decoding state.
In practice, Minion limits RECOBS embedding to four levels (the base
level plus three levels of nested interruption) to establish a
defined upper bound on the amount of storage required by a decoder.
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5. Flow Control
TCP [RFC0793] implements flow control in the form of the advertised
receive window. This is to prevent a faster sender from overwhelming
a slower receiver. Minion requires similar protection to prevent a
slower receiver running out of memory trying to buffer messages
arriving faster than it can handle them.
For a pure user-level library implementation of Minion, this is
achieved by having the library set an upper bound on the amount of
memory it will use for storing received messages that have not yet
been handled by the client. Once this limit is met, the library
ceases reading TCP data from the kernel, which causes the TCP receive
window to fill up, which causes the sender to stop sending. Once the
client consumes some messages, the library then reads more data from
the kernel, the TCP receive window opens up, and the sender is
permitted to send more data.
However, this means that there is some duplication of buffering --
the TCP receive window in the kernel and additional buffering in the
user-level library. For this reason a kernel extension is proposed
where a client (the Minion library in this case) can read data from
the connection *without* raising the TCP receive window. In a sense
it is reading the data "secretly", without admitting to the sender at
the other end that it has been read. Those bytes, even though read
into user space, are still counted against the TCP receive window.
Later, after the client application has actually consumed the
message, another kernel call is made to acknowledge consumption of
those bytes, and the TCP receive window is raised.
This mechanism integrates message-level flow control with TCP's byte-
level flow control, rather than having two independent flow control
mechanisms happening concurrently at different levels, in ways that
might interact badly with each other.
Note that the Minion protocol design will have to consider possible
deadlock situations. For example, suppose one Minion host is
refusing to consume any more Minion Chunks because it wishes to send
a Reject message for them, but it cannot, because the peer's receive
window is closed. Suppose also that the reason the peer's receive
window is closed is because the peer also is sitting on a pile of
unwanted Minion Chunks that it refuses to consume until it can send a
Reject message for them. Possible deadlocks such as these need to be
considered, and mechanisms to avoid them created.
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6. Retransmission Policy
One of the main arguments that is often presented to justify why a
particular application protocol is built on UDP instead of TCP is
that, "UDP is better for 'real time' applications." The supporting
reasoning for this is often that, "TCP insists on continuing to
retransmit data long after the client doesn't need any more." In
truth the real problem is not retransmission; it is that the
conventional TCP APIs don't allow received data to be delivered out
of order. Suppose a TCP sender has 50 packets in flight at any given
time (e.g., the bandwidth x delay product is 75 kB) then the loss of
a single packet causes all 49 following packets to stall at the
receiver because the API doesn't allow for them to be delivered to
the client until the missing packet has been received.
Minion solves this problem by allowing data to be delivered as it
arrives, even if there are gaps. But the argument still remains that
even after removing the ordering requirement at the receiver, it may
still be a waste of bandwidth to retransmit data that will arrive too
late to be useful. And indeed, it is possible with TCP to
fraudulently acknowledge segments that were in fact not received, and
this will cause the sender to not retransmit those segments.
However, we chose not to use fraudulent acknowledgements to suppress
retransmissions, because certain NATs, Firewalls and other
middleboxes may block traffic if they observe implausible protocol
actions which they find suspicious. One of the important goals of
Minion is 100% compatibility with today's existing Internet devices,
not 99% compatibility.
We expect packet loss to be about 1% (at most a few percent) in a
functioning network, and the cost of retransmitting those lost
packets, even in the extreme case where *all* the retransmissions
turn out to be unnecessary, is an overhead of about 1%. We argue
that an overhead of about 1% is an acceptable price to pay in
exchange for 100% compatibility with existing NATs, Firewalls and
other middleboxes.
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7. Optional Kernel Extensions
While Minion can be implemented entirely as a user-level library
built on top of existing standard networking APIs like BSD sockets,
it can also benefit from some optional kernel extensions:
Send Priorities
Normal TCP APIs transmit data strictly in the order is is given to
the kernel. The addition of priority support allows a sendmsg()
call to be used in conjunction with cmsg ancillary data to
indicate the priority level of the data. For normal applications
this capability would be of little use because it would most
likely result in corruption of the data stream, but it is useful
with Minion because the RECOBS encoding is robust against message
insertion at arbitrary byte boundaries. An alternative way to
achieve a similar effect is, instead of buffering data in the
kernel, to keep the data in the user-space library for as long as
possible. When the TCP congestion window and/or receive window
rules allow more data to be sent, the kernel generates some kind
of upcall (e.g., a kevent notification) to the user-space library
informing it of the ability to transmit, and the user-space
library responds by selecting which particular block of data to
hand to the kernel next.
Just-In-Time Data Generation
Through operational experience, we have learned (not that this was
any great surprise) that excessive buffering in the kernel leads
to poor behaviors. For example, two messages at the same priority
level are not interleaved effectively if the first message is
swallowed whole by the kernel, and held in kernel buffers, before
the second message is even created. When that happens, the result
is that the first message is sent in its entirety, followed by the
second message in its entirety, with no interleaving.
To prevent this unintended serialization, we need to avoid
irrevocably handing off data to the kernel prematurely. We want
to give the kernel enough data to keep the pipeline full (an
amount equal to the connection's Bandwidth Delay Product) but no
more.
To this end, rather than having the kernel indicate that a socket
is writable any time the kernel has space available to buffer more
data, we'd like the kernel to indicate that a socket is writable
only when TCP (according its protocol rules, such as receive
window, congestion window, and Nagle's Algorithm) would be willing
to send data, but has no data available to send. When this
situation occurs, the socket becomes writable, and the client (the
user-level Minion library) is able to perform a just-in-time
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determination of what data ought to be sent next.
This just-in-time data generation could be achieved in the BSD
sockets API by adding a new socket option. When using this new
socket option, a socket will only be writable when TCP is actively
waiting for new data. If the context-switching latency or
software overhead is such that it takes the user-level code a
little too long to generate data strictly on demand, then a middle
ground can be achieved by modifying the new socket option such
that a socket will only be writable when the socket has less data
buffered than it expects to need imminently. For example, a TCP
connection in slow start expects it will need four TCP segments
when the next ack arrives. When used this way, if an incoming ACK
allows TCP to send out four segments then those four segments are
already buffered and ready in the kernel, and the socket then
becomes writable again to allow the user-level code to generate
the next four segments, so that they will be ready and waiting the
next time TCP is able to transmit additional segments.
We are currently experimenting with just-in-time data generation.
If it proves to be as effective as we hope, it might even work
well enough to provide effective priority support too, eliminating
the need for the "Send Priorities" kernel extension.
Immediate Receive
Normal TCP APIs deliver data only in TCP sequence number order.
The addition of support for new cmsg ancillary data in the
recvmsg() call allows the user-space library to request *any*
available data, not only in-order data. The cmsg ancillary data
returned from the recvmsg() call indicates to the user-space
library where in the TCP sequence space this particular block of
data lies. A setsockopt() option (or equivalent) is also required
to put the socket into this "Immediate Receive" mode, to inform
the kernel that the client will accept out-of-order data on this
socket, and therefore the client should be notified (via select(),
kevent(), etc.), not only when there is in-order data available to
be read, but also when there is out-of-order data available to be
read.
Integrated Receive Window
Normal TCP APIs raise the receive window any time data is read out
of the kernel into user space. The addition of new cmsg ancillary
data in the recvmsg() call allows the user-space library to
request that the kernel return received data *without* reflecting
this in its receive window calculation. After the client
application has consumed the message data from the user-space
Minion library, the Minion library makes a subsequent recvmsg()
call with appropriate cmsg ancillary data to inform the kernel how
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many bytes to add back into its receive window. In essence, the
receive window boundary is stretched outside the kernel to account
for data held by *both* the kernel *and* the user-space Minion
library.
These optional kernel extensions are a key part of what makes Minion
compelling. Minion can be adopted today by any application, using
Minion as a purely user-space library. Such an application performs
as well as any application can when it is built on top of standard
TCP. However, unlike an application built on top of standard TCP,
Minion offers the promise of future kernel support for even better
performance. Any given application with its own application-specific
protocol is unlikely to receive special kernel support to make just
that one application work better. But when many applications all use
the Minion protocol, it then becomes reasonable to add kernel support
to improve all of those applications.
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8. TCP Deviations
When implemented entirely as a user-level library, Minion naturally
adheres to the TCP specifications (insofar as the underlying
operating system adheres to the TCP specifications) because Minion is
merely using the operating system's networking APIs.
When optional kernel extensions are in use, they may allow Minion to
deviate from classical TCP protocol rules. One such instance of this
deviation has already been identified. The TCP protocol rules allow
a sender to send a FIN to end a connection, and then follow it with
additional data bytes (with higher TCP sequence numbers, so that they
fall later in the data stream) which the receiver is expected to
discard because it recognizes that they fall after the FIN in the
data stream. When out-of-order delivery is enabled, it's possible
that if the TCP segment containing the FIN is lost or delayed, then
subsequent TCP segments containing data bytes could be incorrectly
delivered to the client application, when the TCP protocol rules
dictate that they should have been discarded. The ability to send
data following the FIN that the receiver is expected to discard is
incompatible with out-of-order delivery. Note that this is referring
to data that follows the FIN in TCP sequence number space, not data
that follows the FIN in transmission order. If, after the FIN has
been sent, previously transmitted data is lost and needs to be
retransmitted, then this does not cause any problems; the bytes in
such retransmitted TCP segments fall *before* the FIN in TCP sequence
number space, not after. As a result of this observation, TCP's
protocol rules, when used with Minion traffic, are effectively
modified as follows:
o A client using Minion MUST NOT send new data on a connection after
that connection has been closed (i.e. a FIN indication has been
sequenced and sent).
In reality we do not expect this to be a major burden to TCP
implementations. We are not aware of TCP implementations that send
data after a connection is closed and then rely on the receiver to
discard that data.
9. IANA Considerations
No IANA actions are required by this document.
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10. Security Considerations
We take security seriously. As this work develops, this section will
contain details of any known security issues and possible
mitigations.
11. Acknowledgements
Many thanks to Bryan Ford, Padma Bhooma and Anumita Biswas for their
contributions to the development of Minion.
Thanks to Joe Touch for pointing out that Minion restricts TCP's
ability to send data, after a connection is closed, that will then be
ignored by the receiver.
12. References
12.1. Normative References
[COBS] Cheshire, S. and M. Baker, "Consistent Overhead Byte
Stuffing", September 1997,
<http://stuartcheshire.org/papers/COBSforToN.pdf>.
[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.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[minserv] Iyengar, J., "Minion - Service Model and Conceptual API",
draft-iyengar-minion-concept-02 (work in progress),
October 2013.
12.2. Informative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
Peer (P2P) Communication across Network Address
Translators (NATs)", RFC 5128, March 2008.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
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(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887,
April 2013.
[Study] Hatonen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
Sarolahti, P., and M. Kojo, "An Experimental Study of Home
Gateway Characteristics", September 1997,
<http://conferences.sigcomm.org/imc/2010/papers/p260.pdf>.
[TCPoUDP] Cheshire, S., Graessley, J., and S. Cheshire,
"Encapsulation of TCP and other Transport Protocols over
UDP", draft-cheshire-tcp-over-udp-00 (work in progress),
June 2013.
Authors' Addresses
Janardhan Iyengar
Franklin and Marshall College
Mathematics and Computer Science
PO Box 3003
Lancaster, Pennsylvania 17604-3003
USA
Phone: +1 717 358 4774
Email: janardhan.iyengar@fandm.edu
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
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Josh Graessley
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
USA
Phone: +1 408 974 5710
Email: jgraessley@apple.com
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