How is the payload distributed across Multilink PPP connections

ppp

A site I support uses 3 T1s setup with multilink PPP. They are trying to use Jive which is a hosted VoIP provider, with horrible results. I want to know how packets are distributed between the individual T1s since I think this may help explain what is going on.

SNMP monitoring of the multilink interface shows that they have available capacity, but their VoIP test calls are horrible. It acts as if there is a huge amount of jitter and dropped packets. Though simple tests with PING/Iperf to not show jitter/latency that is as bad as you might expect given the call quality.

How are packets distributed between the multiple T1s. I assume that it isn't the same as Ethernet bonding.

If the multi-link PPP is an issue what can I look at on the routers that will show this? Can it be corrected? The routers are Cisco, I believe they are 2800s, but I would have to double check.

Best Answer

Oi...lemme dredge up those long-unused neurons to remember how Multilink PPP works.

During the LCP phase when the first link is brought up during a non-multilink PPP session, the two sides negotiate the MRU (Maximum Receive Unit) for each direction of the link...basically each side tells the other how big of a packet it can handle being sent to it.

With Multilink PPP, they negotiate that, but they also negotiate, when the first link is brought up, an MRRU, or Maximum Reconstructed Receive Unit.

Because Multilink PPP added extra frame header space, the creators were concerned about Path MTU problems, so they decided that Multilink would be able to fragment and reassemble packets on either end of the Multilink PPP session.

So, yes, unlike Ethernet bonding, its not just a matter of balancing frames across the multiple links...the frames can actually be fragmented and reassembled. This can cause a jump in latency and jitter.

On T-1's you should be able to configure each side with a significantly larger MRU and MRRU than any packets that would likely need to cross the link, and if the MRU is as large as, or larger than the MRRU, then you shouldn't see any fragmentation and reassembly occuring. Hopefully this will get the latency and jitter under control and help out your VoIP behavior. You will likely need to adjust this configuration on both sides of the links, though, as each direction is negotiated independently.

In general, I wouldn't expect VoIP packets to need to be fragmented, though...they tend to not be big enough to need that. Worth a shot to check your MRU and MRRU sizes on the links and the Multilink session, maybe they're way out of whack and causing problems.

Related Solutions

IPv4 Subnet – Why /30 Subnets are Used for PPP Connections

A /30 subnet is used for the two public IP addresses. Couldn't they just route our two IP addresses over our PPP link as isolated addresses that don't belong to any subnet? If my understanding is correct, this would allow them to effectively amount the number of "address couples" they can sell.

Well the way I see it is that they route 4 public IP addresses to you, and you decide whether to use them as one /30 subnet or as two /31 subnets or as 4 individual (/32) IP addresses - or even a combination of one /31 and two /32.

In other words if your ISP says they're routing the 10.0.0.0/30 subnet to you then they actually mean that they're routing the ip address range 10.0.0.0-10.0.0.3 to you.

Edit: adding an example to (hopefully) clarify further for those that do not understand. Assume this scenario:

ISProuter (a.b.c.1) ----- (a.b.c.2) MyRouter

ISProuter has a route for x.y.z.4/30 with next-hop a.b.c.2

Then on MyRouter I have roughly 4 options:

I configure x.y.z.5 on one of my interfaces, with a /30 subnet mask. In that case I can only use x.y.z.6 on another device (since .4 and .7 are the network and broadcast addresses in this scenario)
I configure 4 loopback interfaces with x.y.z.4, x.y.z.5, x.y.z.6 and x.y.z.7, each with a /32 subnet mask
I use x.y.z.4/31 and x.y.z.6/31 on 2 point-to-point links.
I route x.y.z.4/30 to another router, and on that router I have again the same 4 options.

Note: there are also combinations possible, e.g. I could use x.y.z.4/32 on a loopback, route x.y.x.5/32 to another router, and use x.y.z.6/31 on a p2p link. And depending on the features available on MyRouter (which could also be a load balancer, or a firewall, or...) the addresses could also be used without actually configuring them on an interface, but the result would be very similar to using them as /32's on a loopback.

A public subnet is used for the PPP link. Why is this the case? Since these IP addresses aren't publicly routed anyway, but only used by the two routers for the PPP link, isn't this a waste of the public address space?

You would need to ask the ISP, we can only guess. I guess most ISPs consider it best practice to use public IP addresses for public links. Private IP address might break stuff like PMTUD as ICMP from the 2 PPP endpoints would probably get dropped in a lot of places. But as @RonMaupin mentioned, they could have used a public /31 instead to save some address space.

Determining PPP Frame Length – Layer 2 Protocol Guide

The PPP header starts with a 16-bit protocol id. You need to look at that first in order to know how to interpret the rest.

If the protocol id is LCP/IPCP/IPv6CP then the header structure typically looks like this:

struct PPP {
    uint16_t protocolId;
    uint8_t code;
    uint8_t identifier;
    uint16_t length; // <= network-encoded length field
};

If the protocol id is IPv4 or IPv6 then the IP header comes immediately after the protocol id. Both IPv4 and IPv6 have a length field in their headers.

I'm not familiar with HDLC frames, but I assume you are receiving them from a socket. The socket read/receive function would provide you with the size of each incoming frame. So you can use that for validation purposes as well.

Best Answer

Related Solutions

IPv4 Subnet – Why /30 Subnets are Used for PPP Connections

Determining PPP Frame Length – Layer 2 Protocol Guide

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