Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Fast Retransmission


I had some problem with a WAN connection.

On the main building we had 2 cisco 26xx with

1eth 2serial each for a total of 4 dedicated line. Main building connect 2 remote building (A and B) with 2 line each for business continuity purpose.

On building A the cisco 2600 is configured with IP CEF load sharing, and each serials with ip load-sharing per-packet, A can balance the load over the 2 dedicated line. Same for B.

Sometimes TCP connections beetween the buldings fails unpredicately.

Sniffing packets, i notice "TCP DUP ACK" - Fast Retransmission. Retransmission required.

Too many retransmission.

Disabing load sharing all works correctly. It seems that some packets get lost over the trip.

Someone can help me to diagnose the situation and give me an hint?


  • Other Network Infrastructure Subjects

Re: Fast Retransmission

The following will give you an indepth knowledge on configuring CEF and load sharing and retransmission

1. The maximum theoretical TCP throughput is for any path is

T-RW = TCP-Receive-Window

T-SW = TCP-Send-Window

RTT = Round-Trip-Time

L-BW = minimum Bandwidth link in path

T-BW = maximum effective TCP bandwidth

T-PS = TCP MSS packet size.

T-BW = min(min(T-RW, T-SW)/RTT, L-BW)

So if you increase the mBW link it may not increase your throughput if the first factor is still


If we look at this for a T1 link (1.5 Mbs) and pick a reasonable window size (16KB = 128Kb), and

RTT (50ms - enough to get across US).

T-BW = min(128Kb/50ms, 1500Kbs)

= min(2560Kbs, 1500Kbs)

So this would mean that increasing the T1 to 2 T1s would help but 3 T1s wouldn't gain anything

more to speed up a single transfer. 3 T1s and would help by allowing more transfers to take place


In the above if you either increase the TCP window sizes or decrease the RTT then the first

factor becomes larger and you will get higher TCP

effective-BW as longs as the minimum Bandwidth link in path is larger.

It doesn't matter whether you are doing per-packet load-balancing over multiple links or Layer-2

MUXing of slower links together to increase the L-BW.

Per-destination (flow) load-balancing will not increase the throughput for a single TCP transfer

since you are only using one of the multiple slow path links.

2. There are other factors that can come into play when you do per-packet load-balancing over

multiple links and this caused by the chance of out-of-order packets. It does depend on how out of

order your packets are.

In general if your packets are never more than 2 packets out of order then the TCP sender should

ramp up its congestion window to be able to "fill the pipe" (L-BW*RTT)/T-PS. This will mean that the

TCP sender will be sending at a rate that matches the L-BW. The L-BW may be an aggregation of links

(per-packet-LB)or not.

If your packets are often more than 3 packets out of order, perhaps because of per-packet

load-balancing over 4 links, then TCP can erroneously trigger fast-retransmission.

Fast-retransmission is triggered by 3 duplicate-ACKs coming back from the receiver. Even if the

sender doesn't actually retransmit a packet it will still reduce its congestion window, and slow

down sending. This can result in slower throughput than if the transfer had been going through a

single link (per-destination load-balancing).

In general per-packet load-balancing may or may not improve performance of a single TCP transfer. If

the throughput for a single TCP transfer needs to be increased then it is better to get a single

higher speed line or MUX together multiple lower speed lines at layer-2 (for example MLPPP). Usually

for multiple links it is better to use per-destination (per-flow) load-balancing.

This widget could not be displayed.