Asymmetric routing -SYN-SYN-ACK

[Maru R]

Hi ,

   How does the TCP handshake occurs in the case of asymmetric routing . Below topology we have traffic initiated from Source to R1  ,whereas R1 provides return path to Source via R2 .  R2 has a direct connection to R-WAN  (not mentioned in diagram ) In this case how the SYN-ACK is sent from R1 to the source ?

 Will the SYN-ACK be returned via different path ? Please enlighten me . Thanks in advance :)

Source
  "
  "
  "
R-WAN
  "
  "
  "
 R1 ========= R2 ( Consider R2 has better return path to R-WAN directly )

[Carlos Villagran]

Hi!

Well, the communication gets initiated with SYN/SYNACK AND ACK with SYN taking a different path than the SYNACK but at least the TCP handshake will be established. I am not really sure of the objective of the question though, there should be no problems.

This is assuming there are no filtering, degradation, lots of delay in any of the paths.

Hope it helps, best regards!

JC

[Maru R]

Hello Carlos ,

    I wanted to know will the SYN-ACK can be sent from a different path when it receives the 1st SYN packet . I'm asking this with asymmetric routing perspective , .

Thanks

[Jon Marshall]

The SYN ACK will be sent over whichever path is the best path according to the routers.

It makes no difference whether that path is the same path in both directions or a different one and it should work fine as long as there are no stateful devices in the path such as a firewall for example.

Jon

[Carlos Villagran]

Hi!

Also take in count the out-of-order behavior that this assymertic routing could cause multiple retransmission requests therefore causing a bad network performance. But as Jon said, there will be no problem with the SYN/ACK transmissions.

Hope it helps, best regards!

JC

[Pawan Raut]

Hi Asymmetric routing for router should work unless no Firewall coming in that traffic path. Router don't care about return path and it forwards packets according to routing table but Firewall cares about it as it verify the both Sync and Sync Ack should be symmetric else it drops packet but this is not case in routers.

[Maru R]

Thanks for the responses .

 What if there is multiple path for the return traffic ? Will this affect the TCP handshake process ?

In case of equal cost multiple paths is there any chance the latency would be affected between voice servers ,is there a chance TCP re-transmission occurring or will the TCP happen properly ? Please clarify me on this .