

Internet Engineering Task Force                          M. Sustrik, Ed.
Internet-Draft
Intended status: Informational                            March 27, 2014
Expires: September 28, 2014


                 TCP Mapping for Scalability Protocols
                           sp-tcp-mapping-01

Abstract

   This document defines the TCP mapping for scalability protocols.  The
   main purpose of the mapping is to turn the stream of bytes into
   stream of messages.  Additionaly, the mapping provides some
   additional checks during the connection establishment phase.

Status of This Memo

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   This Internet-Draft will expire on September 28, 2014.

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1.  Underlying protocol

   This mapping should be layered directly on the top of TCP.

   There's no fixed TCP port to use for the communication.  Instead,
   port numbers are assigned to individual services by the user.

2.  Connection initiation

   As soon as the underlying TCP connection is established, both parties
   MUST send the protocol header (described in detail below)
   immediately.  Both endpoints MUST then wait for the protocol header
   from the peer before proceeding on.

   The goal of this design is to keep connection establishment as fast
   as possible by avoiding any additional protocol handshakes, i.e.
   network round-trips.  Specifically, the protocol headers can be
   bundled directly with to the last packets of TCP handshake and thus
   have virtually zero performance impact.

   The protocol header is 8 bytes long and looks like this:

   +------+------+------+--------------+------------+----------------+
   | 0x00 | 0x53 | 0x50 | version (8b) | type (16b) | reserved (16b) |
   +------+------+------+--------------+------------+----------------+

   First four bytes of the protocol header are used to make sure that
   the peer's protocol is compatible with the protocol used by the local
   endpoint.  Keep in mind that this protocol is designed to run on an
   arbitrary TCP port, thus the standard compatibility check -- if it
   runs on port X and protocol Y is assigned to X by IANA, it speaks
   protocol Y -- does not apply.  We have to use an alternative
   mechanism.

   First four bytes of the protocol header MUST be set to 0x00, 0x53,
   0x50 and 0x00 respectively.  If the protocol header received from the
   peer differs, the TCP connection MUST be closed immediately.

   The fact that the first byte of the protocol header is binary zero
   eliminates any text-based protocols that were accidentally connected
   to the endpiont.  Subsequent two bytes make the check even more
   rigorous.  At the same time they can be used as a debugging hint to
   indicate that the connection is supposed to use one of the
   scalability protocols -- ASCII representation of these bytes is 'SP'
   that can be easily spotted in when capturing the network traffic.
   Finally, the fourth byte rules out any incompatible versions of this
   protocol.


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   Fifth and sixth bytes of the header form a 16-bit unsigned integer in
   network byte order representing the type of SP endpoint on the layer
   above.  The value SHOULD NOT be interpreted by the mapping, rather
   the interpretation should be delegated to the scalability protocol
   above the mapping.  For informational purposes, it should be noted
   that the field encodes information such as SP protocol ID, protocol
   version and the role of endpoint within the protocol.  Individual
   values are assigned by IANA.

   Finally, the last two bytes of the protocol header are reserved for
   future use and must be set to binary zeroes.  If the protocol header
   from the peer contains anything else than zeroes in this field, the
   implementation MUST close the underlying TCP connection.

3.  Message delimitation

   Once the protocol header is accepted, endpoint can send and receive
   messages.  Message is an arbitrarily large chunk of binary data.
   Every message starts with 64-bit unsigned integer in network byte
   order representing the size, in bytes, of the remaining part of the
   message.  Thus, the message payload can be from 0 to 2^64-1 bytes
   long.  The payload of the specified size follows directly after the
   size field:

   +------------+-----------------+
   | size (64b) |     payload     |
   +------------+-----------------+

   It may seem that 64 bit message size is excessive and consumes too
   much of valueable bandwidth, especially given that most scenarios
   call for relatively small messages, in order of bytes or kilobytes.

   Variable length field may seem like a better solution, however, our
   experience is that variable length size field doesn't provide any
   performance benefit in the real world.

   For large messages, 64 bits used by the field form a negligible
   portion of the message and the performance impact is not even
   measurable.

   For small messages, the overal throughput is heavily CPU-bound, never
   I/O-bound.  In other words, CPU processing associated with each
   individual message limits the message rate in such a way that network
   bandwidth limit is never reached.  In the future we expect it to be
   even more so: network bandwidth is going to grow faster than CPU
   speed.  All in all, some performance improvement could be achieved
   using variable length size field with huge streams of very small


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   messages on very slow networks.  We consider that scenario to be a
   corner case that's almost never seen in a real world.

   On the other hand, it may be argued that limiting the messages to
   2^64-1 bytes can prove insufficient in the future.  However,
   extrapolating the message size growth size seen in the past indicates
   that 64 bit size should be sufficient for the expected lifetime of
   the protocol (30-50 years).

   Finally, it may be argued that chaining arbitrary number of smaller
   data chunks can yield unlimited message size.  The downside of this
   approach is that the message payload cannot be continuous on the
   wire, it has to be interleaved with chunk headers.  That typically
   requires one more copy of the data in the receiving part of the stack
   which may be a problem for very large messages.

4.  Note on multiplexing

   Several modern general-purpose protocols built on top of TCP provide
   multiplexing capability, i.e. a way to transfer multiple independent
   message streams over a single TCP connection.  This mapping
   deliberately opts to provide no such functionality.  Instead,
   independent message streams should be implemented as different TCP
   connections.  This section provides the rationale for the design
   decision.

   First of all, multiplexing is typically added to protocols to avoid
   the overhead of establishing additional TCP connections.  This need
   arises in environments where the TCP connections are extremely short-
   lived, often used only for a single handshake between the peers.
   Scalability protocols, on the other hand, require long-lived
   connections which doesn't make the feature necessary.

   At the same time, multiplexing on top of TCP, while doable, is
   inferior to the real multiplexing done using multiple TCP
   connections.  Specifically, TCP's head-of-line blocking feature means
   that a single lost TCP packet will hinder delivery for all the
   streams on the top of the connection, not just the one the missing
   packets belonged to.

   At the same time, implementing multiplexing is a non-trivial matter
   and results in increased development cost, more bugs and larger
   attack surface.

   Finally, for multiplexing to work properly, large messages have to be
   split into smaller data chunks interleaved by chunk headers, which
   makes receiving stack less efficient, as already discussed above.


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5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   The mapping isn't intended to provide any additional security in
   addition to what TCP does.  DoS concerns are addressed within the
   specification.

Author's Address

   Martin Sustrik (editor)

   Email: sustrik@250bpm.com


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