An Overview of Rx

1
An Overview of Rx We like things that make us
go. Roman Mitz rmitz_at_cmu.edu AFS Kerberos Best
Practices Workshop 2005 http//rmitz.org/rx/rx-bpw
-2005.ppt
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Outline

• History, ancient and otherwise
• Why Rx?
• High level protocol overview
• Bottom-up tech

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In the beginning

• The Universe was created
• Well, perhaps well skip ahead just a bitto the
  ITC
• Information Technology Center, CMU/IBM

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In the beginning there was

• R and RFTP1. And it was okay.
• Actually, were skipping over some other bits
  here, like RPC2, RPC23, MultiRPC4ysee,
  Kazar
• R was an RPC protocol
• Built on the LWP5 and XDR6 packages
• Meant to provide a reliable datagram service
• Run on top of UDP/IP (whew!)
• Basic authentication via shared secret
• Data transfer limited to one packet
• RFTP was
• A simple addition to transfer files. Requires
  setting up the transfer then making an R call.

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And then the spawn, Rx

• Inherits the good things of the previous
  protocols
• Lightweight
• Security deeply embedded (but modular!)
• With some additions
• Arbitrary amounts of bulk data transfer as an
  ordinary call
• Adds more security hooks (encryption)
• Imports some TCP network adaptive behavior
  (increased over time 7)

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Rx

• Benefits
• Handles large numbers of connections (but..)
• Only requires one RPC for bulk data transfer
• Tight integration with AFS
• Very lightweight overall
• Drawbacks
• Not taking advantage of all the TCP research and
  enhancements over the years without explicit code
  (window sizes, security, etc.)
• Firewalls can sometimes cause issues
• Still a less-known factor

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Rx, High-level8

• An Rx-based server exports services
• ltIP address, UDP Port, service Idgt
• One UDP Port per server, 16bits of services
• Remember different server vs client distinctions
• Connection based
• Each connection support four simultaneous calls
  via a mechanism called channels
• Each call is half-duplex
• Calls can be aborted explicitly at any time, and
  via two different timeouts (max time, and max
  time with no activity)

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Rx, High-Level - Security

• Specified per-service and per-connection
• NOT per call
• A service may specify more than one security
  class, but a connection may not
• Work done via external security class (e.g.
  rxkad)
• Only needs to follow API.
• Pretty simple
• If a connection is to be authenticated, there is
  a challenge-response handshake

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Rx, the Header! 9
High-order bit of Epoch -- used for addressless
connections if set
Start time of the clients RPC Package
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
-------------------------
-------
Connection Epoch
-------------------------
-------
Connection ID
-------------------------
-------
Call Number
-------------------------
-------
Sequence Number
-------------------------
-------
Serial Number
-------------------------
------- Type Flags
Status Security
-------------------------
------- Checksum
Service ID
-------------------------
------- Payload .... -----
Used to determine channel number
DATA ACK BUSY ABORT ACKALL CHALLENGE RESPONSE DEBU
G PARAMS VERSION
CLIENT-INITIATED REQUEST-ACK LAST-PACKET
MORE-PACKETS SLOW-START-OK JUMBO-PACKET
Set by client. Could be random, but isnt
normally.
Incremented by client on each new call, beginning
with 1. Used for basic avoidance of call
duplication
Per-call
Per-connection
Defined for individual RPCs
Indicates type of security - none, rxkad, etc.
Used only by security plugins--rxkad calculates a
cryptographic header checksum solving Rx
hijacking issues10
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Greetings and Salutations

• Mahadev Satyanarayanan, Michael Kazar, Bob
  Sidebotham and the rest of the ITC
• Nickolai Zeldovich
• The OpenAFS Team

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References

• 1 - http//rmitz.org/rx/R.pdf
• 2 - http//reports-archive.adm.cs.cmu.edu/anon/i
  tc/CMU-ITC-011.pdf
• 3 - http//reports-archive.adm.cs.cmu.edu/anon/i
  tc/CMU-ITC-038.pdf
• 4 - http//reports-archive.adm.cs.cmu.edu/anon/i
  tc/CMU-ITC-047.pdf
• 5 - http//reports-archive.adm.cs.cmu.edu/anon/i
  tc/CMU-ITC-037.pdf
• 6 - ftp//ftp.rfc-editor.org/in-notes/rfc1014.tx
  t (later 1832)
• 7 - http//rmitz.org/rx/rxhex.pdf
• 8 - http//rmitz.org/rx/Rx.pdf
• 9 - http//web.mit.edu/kolya/afs/rx/rx-spec
• 10 - http//www.citi.umich.edu/techreports/repor
  ts/citi-tr-91-4.pdf
• 11 - OpenAFS src/rx/