P2P TCP behavior through NAT’s

Nagendra Modadugu

nagendra@cs.stanford.edu

The Problem

• Two peers, both behind NAT’s– Possibly multiply NAT’ed– Server available for assistance

• E.g iChat, Yahoo! chat, Net Meeting, BitTorrent, KaZaa

A BNAT_A

S

NAT_B

Solution 1: No changes to NAT

A BNAT_A

S

NAT_B

A & B send ISN’s to S

S “determines” external port numbers

A & B send SYN’s towards each other

S generates forged SYN/ACK’s

ISN 0xAD.. ISN 0xF5..

Control Channel

SYN (ISN:0xF5..;Port:5476)Forged

SYN/ACKForged

SYN/ACK

ACK

ACK

Port 3210 Port 5476

Port3210

Port5476 SYN (ISN:0xAD..;Port:3210)

Pros and Cons of Solution 1

Pros– Works without changing NAT’s

• Cons– Port # prediction may fail– Egress/ingress filtering may block forged packets– How portable is SO_REUSEPORT?

Solution 2: No forged packets

A BNAT_A

S

NAT_B

A & B send ISN’s to S

S “determines” external port numbers

A & B send SYN’s towards each other

Control Channel

SYN/ACK

ACK

Port 3211 Port 5477

Port3211

Port5477 SYN (Port:3211)

SYN(Port:5477)

Pros and Cons of Solution 2

• Pros– No changes needed to NAT devices

• Cons– Port # prediction may fail– Requires NAT’s to allow incoming SYN’s– How portable is SO_REUSEPORT?

Solution 3: Port Reservation

A BNAT_A

S

NAT_BReserve request: Port 3733,Auth info: Incoming Seq # 0xD2..

Port 3733 rsrvd Auth info: Seq # 0xD2..

SYN (Seq:0xD2..;Port 3733)SYN/ACK

ACK

Pros and Cons of Port Reservation

• Pros– No port number guessing--works reliably– Works even if NAT’s only on one side are upgraded– Client code simpler– Port reservation can be implemented as an ALG

• Cons– Need to define and deploy a new protocol for implementing

port reservation– ALL the NATs on one side must be upgraded

Summary

• Evaluated 3 possible solutions– Two require no changes to NAT devices– Port reservation will take time to adopt

• Deployment on client through an application proxy– No need to change application software

• Recommendations:– Port-restricted cone NAT’s are the way to go– Allow (address and port restricted) incoming SYN’s