Basic redundant internet link LB question

[rpastor]

Hi everybody,

I am looking into load balancing internet usage between our two WAN providers, and am reasonably comfortable with the idea of redistributing BGP into OSPF so our inside device can make decisions as to which link to choose for outgoing traffic. The question I have, however, is a more fundamental one.  As with many institutions these days, most of our traffic consists of downloads, specifically video streams.  For the moment, we'll disregard the difficulty of identifying video streams, which is a separate discussion.  My question is whether  there is any way of controlling how traffic will come into your network (outside of manipulating the BGP MED value), even when the original session is initiated on the inside.  In other words, if a user starts a YouTube download from inside the LAN, and the traffic heads out one ISP, will the return traffic come over the same ISP, or  is a separate session started by the YouTube server, which picks whatever link it wants to to reach the requesting user?   Note that we have our own address space, with DNS managed by a third party, so there are two ways to reach the same internal user.

My thinking is that if the user requests a download from a particular website, and the session remains intact for the entire download, then I have a way of  controlling what links are utilized for what traffic, since it's outgoing traffic I'm trying to control.  But if the user is requesting service from a website, and a separate session is started by the website to serve the pages, then I know I have no real way of controlling our link usage in this way (not including caching, acceleration, shaping, and  other methods).

Thanks very much,

Richard Pastor

[Giuseppe Larosa · Accepted solution]

Hello Richard,

>> Note that we have our own address space, with DNS managed by a third party, so there are two ways to reach the same internal user.

you are multihomed with two ISPs

>> if a user starts a YouTube download from inside the LAN, and the traffic heads out one ISP, will the return traffic come over the same ISP, or  is a separate session started by the YouTube server, which picks whatever link it wants to to reach the requesting user? 

Return traffic may follow the other path depending on routing decisions made by network devices nearby the website server.

The TCP session is built in any case even if routing of packets is asymmetric as the socket of the sessions include the IP addresses and the associated TCP ports.

Actually in many cases there are multiple servers involved one is the website and when the user click on the play button on the web page a new session is created with another server that provides the video content.

Again how return traffic comes back is not related to the path taken by upstream direction.

Hope to help

Giuseppe

[rpastor]

Thank you.  This answers my question.  In this case, since most of our (school's) traffic is  downloaded, do you think that allowing all traffic to exit one link while also keeping the other link active for incoming (downloaded) traffic is  a good idea?  In other words, if both links are the same speed, and the amount of outbound (upstream) traffic is minimal, is it customary to allow that outbound traffic to head out just one of the links, but allow incoming traffic to enter both links?  (auto-failover would still be configured, which is a different issue)