Internet Engineering Task Force V. Olteanu
Internet-Draft C. Raiciu
Intended status: Experimental University Politehnica of Bucharest
Expires: January 9, 2017 July 8, 2016

Layer 4 Loadbalancing for MPTCP


Layer 4 loadbalancers are widely used in the deployment of large-scale web services. A large number of servers accept incoming connections from clients, while multiple loadbalancers make sure that traffic is spread evenly across the servers.

Due to its use of multiple subflows, Multipath TCP poses several issues to the design of a scalable layer 4 loadbalancer that supports it. This document presents two ways in which MPTCP connections can be loadbalanced across a large pool of servers. Both approaches entail using a slightly modified server stack and work well with unmodified MPTCP clients.

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Table of Contents

1. Introduction

Layer 4 loadbalancing is widely used in datacenters. In order to ensure the smooth operation of a high-capacity web service, incoming requests from clients must be evenly spread across a large number of servers that service said requests.

Datacenter operators place a number of loadbalancers between the border routers and the servers. The loadbalancers are tasked with ensuring that all packets belonging to a connection reach the same server. This is typically achieved via a hashing-based scheme: a hash function is applied to each packet's 5-tuple and a destination server is chosen based on the result.

Multipath TCP [NSDI-12] is an extension to TCP designed to take advantage of the multiple paths that may exist between two hosts by using an arbitrary number of subflows. Current loadbalancer designs do not work with Multipath TCP, and this has hampered the adoption of the protocol, especially on the server side.

The main reason why loadbalancing MPTCP connections is not trivial is because a connection's individual subflows look like independent TCP connections. There is no discernible relationship between their 5-tuples, and loadbalancing them based on a hash of the 5-tuple will most likely result in them reaching different servers. Furthermore, connection-identifying information can only be extracted from the initial 3-way handshake of each flow.

This document proposes two mutually-exclusive solutions to these problems. They rely to varying degrees on getting the client to embed connection or server-identifying information in the packets that it sends out. This extra information can be used statelessly by the loadbalancers. Both solutions require modifications only to the server stack and work well with existing MPTCP clients.

2. Proposal 1

Our first proposal revolves around controlling the destination port that the client uses in all subflows aside from the initial one. It is possible for the server to advertise an additional port via the ADD_ADDR option [RFC6824]. This informs the client that it can send an MP_JOIN to this new port and initiate a new subflow.

To take advantage of this, each server is be assigned a unique 16-bit ID, which must be different from the port on which the service is being hosted (e.g. 80). As soon as a connection is initiated, the server sends an ADD_ADDR to the client advertising a new port equal to said ID.

Packets that arrive at the loadbalancer are treated as follows:

This approach has two drawbacks:

3. Proposal 2

Our second proposal is to loadbalance packets based on the server's token. The token's most significant 14 bits are treated as a hash value for the connection. They are embedded in all outgoing TCP timestamps, and subsequently echoed back by the client. Incoming packets that do not contain timestamps (such as FINs) are dealt with via redirection between the servers.

3.1. Connection Initiation

The client initiates an MPTCP connection by sending a SYN with the MP_CAPABLE option. Under normal operation, the server then picks a random 64-bit key for the connection, and uses it to compute its token.

To forward the packet appropriately, the loadbalancer must know the token before deciding what server to send it to. To accomplish this, we move the key generation to the loadbalancer. The connection's token can be computed based on the generated key.

The loadbalancer places the generated key, along with the IP of the server that would be responsible for the subflow under normal 5-tuple hashing (which we call the alternate server IP) in an IP option and forwards the SYN to the server.


                            1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |   Type = 96   |  Length = 16  |             Unused            |
      |                                                               |
      +                          Server Key                           +
      |                                                               |
      |                      Alternate Server IP                      |
             Figure 1: IP Option Used for MP_CAPABLE packets


The figure above depicts the IP option that is inserted into the MP_CAPABLE packet before it is sent to the server. We have chosen an IP option despite the fact that the data contained therein pertains to the transport layer, because TCP option space is very limited. IP option type 96 is currently classified as reserved [RFC0791].

Upon receipt of the packet, the server uses the key provided to compute the token for the connection. If no connection with the same token exists, the server uses the key provided. Otherwise, it takes a brute-force approach and randomly generates multiple keys and selects one that yields a token with the same 14 highest-order bits.

The use of the alternate server IP will be discussed in a later section.

3.2. Handling MP_JOIN packets

Additional subflows are initiated by the client by sending MP_JOIN packets. These packets contain the server's token.

Similarly to how MP_CAPABLE packets are treated, the loadbalancer uses an IP option to inform the server about which other server would be responsible for the subflow under normal 5-tuple hashing.


                            1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |   Type = 97   |   Length = 8  |             Unused            |
      |                      Alternate Server IP                      |
               Figure 2: IP Option Used for MP_JOIN packets


IP option type 97 is also classified as reserved [RFC0791].

3.3. Embedding the token in the timestamp

The TCP timestamp option [RFC7323] is present in most packets and is comprised of two fields: the TSval, which is set by the packet's sender, and TSecr, which contains a timestamp recently received from the other end.

Taking advantage of the fact that timestamps set by the server are echoed back by the client, the server shifts its timestamp clock left by 14 bits, and embeds the 14 highest-order bits of the token into the 14 lowest-order bits of the TSval. When a packet with the ACK flag set and with the TS option present arrives at the loadbalancer, it is forwarded based on the 14 least significant bits of the TSecr field.

3.3.1. Impact on PAWS

Timestamps supplied by the server are used by the client for protection against wrapped sequence numbers (PAWS).

We assume that the server uses a timestamp clock frequency of 1 tick per ms, which is the highest frequency recommended by [RFC7323]. The recycling time of the timestamp clock's sign bit is required to be greater than the Maximum Segment Lifetime of 255 seconds. Given that the clock ticks once every ms in increments of 2 ^ 14, its recycling time is roughly 262 s, which is within the bounds set by the standard.

While the quickly-increasing timestamp is benign to active subflows, PAWS will still cause segments to be dropped if the subflow in question had been idle for a period longer than the clock's recycling time. To solve this, the server periodically sends keepalive messages during idle periods.

3.4. Redirecting packets without timestamps

Some packets (most notably FINs) do not contain timestamps or any other connection-identifying information. As such, they are forwarded to a server based on a hash of the 5-tuple.

As seen in Section 3.1 and Section 3.2, whenever a new subflow is setup, the server responsible for it (A) also knows which other server (B) would be hit by the packets in case 5-tuple hashing is used.

A will use a simple peer-to-peer protocol to inform B to setup a redirection rule for the 5-tuple in question. The redirection rule will be deleted by B either at A's request, after the subflow has finished, or after a timeout. We do not discuss the specifics of the protocol in this document.

Redirection of a packet is performed using IP-in-IP encapsulation.

4. Conclusions

The ability to perform layer 4 loadbalancing in a scalable manner is crucial for the adoption of Multipath TCP. This document explored two ways in which this can be accomplished. We put forth that loadbalancing is feasible with the current version of MPTCP and that significant changes to the protocol for loadbalancing support are unnecessary.

5. References

5.1. Normative References

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981.
[RFC6824] Ford, A., Raiciu, C., Handley, M. and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013.
[RFC7323] Borman, D., Braden, B., Jacobson, V. and R. Scheffenegger, "TCP Extensions for High Performance", RFC 7323, DOI 10.17487/RFC7323, September 2014.

5.2. Informative References

[NSDI-12] Raiciu, C., Paasch, C., Barre, S., Ford, A., Honda, M., Duchene, F., Bonaventure, O. and M. Handley, "How hard can it be? designing and implementing a deployable multipath tcp", 2012.

Authors' Addresses

Vladimir Olteanu University Politehnica of Bucharest Splaiul Independentei 313 Bucharest, Romania EMail:
Costin Raiciu University Politehnica of Bucharest Splaiul Independentei 313 Bucharest, Romania EMail: