TCP Behavior draft-modadugu-nat-tcp-00

1
TCP Behaviordraft-modadugu-nat-tcp-00

• Nagendra Modadugu
• nagendra_at_cs.stanford.edu

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Incoming SYN

• Issue
• Binding created on observing outgoing SYN
• Other side tries TCP simultaneous-open
• Also sends a SYN
• Binding has already been created, does NAT allow
  incoming SYN?
• Recommendation
• Yes
• Also, NAT should be capable of tracking
  simultaneous-open handshake

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RST Behavior

• Issue
• External interface sees inbound SYN on unbound
  port
• Typical OS responds with RST
• NAT responding with RST means simultaneous-open
  doesnt work
• Recommendation
• Silent treatment of such inbound SYNs
• Many (gt 64) NATs already demonstrate this
  behavior

4
Premature Binding Expiry

• Issue
• TCP connection goes idle for extended period
• Releasing binding means that open connection gets
  dropped
• Thoughts?
• How many applications affected?
• Affected applications can implement SO_KEEPALIVE
• NAT implements TCP keep-alive
• Recommendation
• No active participation by NAT

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Timers

• Three timers
• Connecting (before handshake completion),
  Established (measures liveness), Closing
• Timer values
• Problems with mimicing standard TCP values?
• Pick max values across various OSs?
• Thoughts?

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Port Assignment (general behavior)

• Issue
• To Preserve or Not Preserve port numbers
• Desired port may be unavailable
• Discussion
• Secondary port assignment scheme required anyway
• Recommending one way or another not much value?
• Recommendation
• Leave unspecified
• Application developers cannot depend on port
  preservation anyway

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Bad Behavior

• Any bad behavior not covered by current
  recommendations?
• Other behavioral aspects needing consideration?