Algorithm at the control plane

6.3 Algorithm for concurrent consistent per-flow updates

6.3.4 Algorithm at the control plane

The control plane (CP) of the switch exchanges messages with the controller and config- ures the data plane (DP) of the switch, as shown in Figure 6.1. The stateful lists that are used by the CP and the DP to realize Proflow, the indices used for those lists, the values each item may take and their purpose, are shown in Table 6.2. In the description of algorithms, these lists are not always shown with their indices. The table also shows which entity writes values into each item and which entity reads the values. The value of T associated with each rule is used by the rule to compare packet time stamp values. It is initialized to Tmax, for every rule. The value of rule type associated with each rule is initialized toU, indicating that the rule is unaffected. This section specifies the algorithm for the control plane, at the controller and at each si∈S in Figure 6.1. The message ex- changes below are the same as in the algorithms in previous chapters ; the parameters in

Table 6.2: Stateful lists used by the affected and ingress switches Variable

[index]

Used by

Written by

Read by

Values Initial value

Purpose

T [n] si Controller DP 0 to Tmax Tmax To compare with the packet time stamp to decide label of a packet

rule type [n]

s_i Controller DP N EW, OLD

or U

U Indicates the type of rule

state [d hash]

s_i DP DP N EW F L,

OLD F L

OLD F L Indicates the type of flow

prev t [d hash]

s_i DP DP 0 to T_max 0 Stores the time stamp of a packet as it exits

a switch live f l

[V]

s_i DP CP 0 to T_max 0 Indicates if there are any old flows alive, us- ing a packet time stamp.

ev [V] si CP DP ST ART,

ST OP

ST OP Indicates if DP must update live f l

T_p [V] s_i Controller CP 0 to T_max T_max If the inter-packet delay is less than this value, an old flow is considered alive

T_RC [V] s_i Controller CP 0 to T_max T_i Time at which Ready To Commit was sent.

Used to distinguish new and old flows

V [n] s_i Controller CP 0 to K 0 Version number of an RU

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istate [hash]

ingress DP DP N EW F L,

U F L

U F L Indicates type of flow

itstamp [hash]

ingress DP DP 0 to T_max 0 The time stamp of SYN

DP: Switch Data Plane, CP: Switch Control Plane,n: rule number, v: RU identifier, T_max: 1 less than the maximum value the item can hold,T_i: Current time at switchi,hash: Hash of the 5-tuple (with the same value for forward and reverse flows),d hash: Hash of the 5-tuple (with a different value for forward and reverse flows),si: an affected switch,K: Maximum number of simultaneous RUs

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them and the actions upon receiving them have been modified for a PFC update.

1. The Controller, upon receiving an RU from the application, with S, and R_oi and R_ni for every s_i∈S, sends a “Commit” to every s_i∈S with v, R_oi and R_ni. v is a unique RU identifier, used only between the controller and the switches.

2. The CP of every switch s_i∈S receives “Commit”, a) installs the R_ni rules with a higher priority thanR_oi rules, b) changes the installedR_oi rules to check if a packet is labelledOLD_porU_p by changing their match fields (R_nirules check if an incoming packet is labelledN EW_p orU_p), c) sets rule type of the R_oi rules as OLD and R_ni rules as N EW, d) sets V of each affected rule to v, denoted by V[n]=v. V[n] is henceforth referred to asV. e) sets T of each affected rule toT_max, f) sends “Ready to Commit” with T_i and g) sets T_RC[V]=T_i. The above actions, per changed rule, must be atomic.

3. The controller, upon receiving “Ready to Commit” from all s_i∈S, updates T_last to reflect the largestT_ireceived and sends “Commit OK” to alls_i∈S, withT_last andt_p. As long as the inter-packet delay of an old flow is less thant_p, the flow is considered alive.

4. Upon receiving “Commit OK”, the switch CP a) sets T_p[V]=t_p b) sets T=T_last in R_oi and R_ni c) sets ev to ST ART, to inform the DP that it has to begin updating live f l[V] (explained subsequently) to indicate that there are live old flows and d) starts a timer of value 2∗ T_p. All the above actions after receiving Commit OK are atomic, per rule. Upon expiry of the timer, if live f l[V] has increased from the previous time it was checked, there are live old flows, in which case the timer is restarted. Otherwise, it considers all the old flows complete, sets ev to ST OP and sends “Ack Commit OK” to the controller, with T_i.

5. Upon receiving “Ack Commit OK” from all s_i∈S, the controller sets the largest of T_i received in “Ack Commit OK” to T_del. It sends “Discard Old” to all s_i∈S with T_del.

6. Upon receiving “Discard Old”, the switch CP starts a timerT_del+M−T_i, whereM is the maximum lifetime of a packet within the network. When the timer expires, as

all the packets that were switched using R_oi are no longer in the network, the switch deletes Roi. It marks all the rules inRni as unaffected by setting their rule type to U and modifies their match fields such that they cease to check for the packet label N EWp. T need not be initialised to Tmax as packets matching unaffected rules do not check for T. Next, it sends “Discard Old Ack” to the controller. the update at the switch is complete.

7. After the controller receives “Discard Old Ack” from alls_i∈S, the RU is complete at the controller. After M units after timer expiry at the lastsi∈S, the last of packets that have label set to N EW_p are no longer in the network. Now the next update not disjoint with the current one may begin.