snaproute Go BGP Code Dive (10): Moving to Open Confirm

In the last post on this topic, we traced how snaproute’s BGP code moved to the open state. At the end of that post, the speaker encodes an open message using packet, _ := bgpOpenMsg.Encode(), and then sends it. What we should be expecting next is for an open message from the new peer to be received and processed. Receiving this open message will be an event, so what we’re going to need to look for is someplace in the code that processes the receipt of an open message. All the way back in the fifth post of this series, we actually unraveled this chain, and found this is the call chain we’re looking for—

• func (st *OpenSentState) processEvent()
• st.fsm.StopConnectRetryTimer()
• bgpMsg := data.(*packet.BGPMessage)
• if st.fsm.ProcessOpenMessage(bgpMsg) {
  • st.fsm.sendKeepAliveMessage()
  • st.fsm.StartHoldTimer()
  • st.fsm.ChangeState(NewOpenConfirmState(st.fsm)) }

I don’t want to retrace all those steps here, but the call to func (st *OpenSentState) processEvent() (around line 444 in fsm.go) looks correct. The call in question must be a call to a function that processes an event while the peer is in the open state. This call seems to satisfy both requirements. There is a large switch statement in this function; let’s see if we can sort out what a few of these do to get a general sense of what is in this switch.

• case BGPEventManualStop: this covers the case where the operator manually deconfigures or otherwise stops the BGP process, or the formation of this specific peer
• case BGPEventAutoStop: this covers the case where the BGP process is brought down for some automatically generated reason; for instance, this (probably) covers the case where the BGP process is shut down because the system itself is going down
• case BGPEventHoldTimerExp: when the peer was moved into the open state, the hold timer was configured and started running; if the hold timer expires before an open message is received from the peer, then a notification is sent and the peer is pushed back to idle state
• case BGPEventTcpConnFails: if the TCP socket reports that the connection has failed, the peer is cleared and set back to active state

The particular bit of code in this switch we’re interested in is—

case BGPEventBGPOpen:
  st.fsm.StopConnectRetryTimer()
  bgpMsg := data.(*packet.BGPMessage)
  if st.fsm.ProcessOpenMessage(bgpMsg) {
    st.fsm.sendKeepAliveMessage()
    st.fsm.StartHoldTimer()
    st.fsm.ChangeState(NewOpenConfirmState(st.fsm))
  }

Well, this doesn’t look so bad, right? Just a few short lines of code. 🙂

st.fsm.StopConnectRetryTimer() is pretty obvious, so I won’t spend a lot of time here. The peer is now connected, so there’s no reason to keep running the timer that causes events when the timer expires.

bgpMsg := data.(*packet.BGPMessage) might not be so obvious at first. In order to reach this state, the local peer has received a packet of some type. The contents of that packet must somehow be processed to actually form the peering relationship. This line of code just creates a new variable called bgpMsg and assigns the received packet to this variable. The := operator is specific to go, so it’s probably worth pausing for a second to explain.

Typing is a method a programming language uses to control memory usage, catch errors in the code during the compilation process, etc. If you define a new variable that is supposed to hold a whole number, or a number without a floating point component (the fractional part after the decimal point), and assign it the value 2, you might do something like this in C—

int a-number;
a-number = 2;

go does things a little differently, placing the name of the variable before the type, like this—

var a-number int
a-number = 2

The first line is consider the variable declaration, while the second is the variable assignment. These are normally two separate steps. But in go, there is a shortcut to this process. You can declare the variable and assign a value in one step, like this—

a-number := 2

How does the compiler know what kind or type of variable a-number is? By looking at the value assigned. In this case, the coder has declared a variable called bgpMsg, and assigned it the value of the contents of the open message just received in one step.

Next time, we’ll look at how this information is actually process. ’til then, happy coding.