Question

Consider the problem of implementing timers in your current practical. Selective Repeat does not resend all packets on timeout, so it must timeout packets individually. However, in our implementation we only have one timer. How might we solve this problem? What would happen if we re-start the timer every time a packet is sent? What would happen if we re-start the timer when the oldest packet is ACKed?

Answer 1

• A timeout event. The protocol's name, "Go-Back-N," is derived from the sender's behavior in the presence of lost or overly delayed packets. As in the stop-and-wait protocol, a timer will again be used to recover from lost data or acknowledgement packets. If a timeout occurs, the sender resends all packets that have been previously sent but that have not yet been acknowledged. Our sender in Figure 3.4-12 uses only a single timer, which can be thought of as a timer for the oldest tranmitted-but-not-yet-acknowledged packet. If an ACK is received but there are still additional transmitted-but-yet-to-be-acknowledged packets, the timer is restarted. If there are no outstanding unacknowledged packets, the timer is stopped.

The receiver's actions in GBN are also simple. If a packet with sequence number nis received correctly and is in-order (i.e., the data last delivered to the upper layer came from a packet with sequence number n−1), the receiver sends an ACK for packet n and delivers the data portion of the packet to the upper layer. In all other cases, the receiver discards the packet and resends an ACK for the most recently received in-order packet. Note that since packets are delivered one-at-a-time to the upper layer, if packet k has been received and delivered, then all packets with a sequence number lower than k have also been delivered. Thus, the use of cumulative acknowledgements is a natural choice for GBN.

In our GBN protocol, the receiver discards out-of-order packets. While it may seem silly and wasteful to discard a correctly received (but out-of-order) packet, there is some justification for doing so. Recall that the receiver must deliver data, in-order, to the upper layer. Suppose now that packet n is expected, but packet n+1 arrives. Since data must be delivered in order, the receiver couldbuffer (save) packet n+1 and then deliver this packet to the upper layer after it had later received and delivered packet n. However, if packet n is lost, both it and packet n+1 will eventually be retransmitted as a result of the GBN retransmission rule at the sender. Thus, the receiver can simply discard packet n+1.The advantage of this approach is the simplicity of receiver buffering – the receiver need not buffer any out-of-order packets. Thus, while the sender must maintain the upper and lower bounds of its window and the position of nextseqnum within this window, the only piece of information the receiver need maintain is the sequence number of the next in-order packet. This value is held in the variable expectedseqnum,shown in the receiver FSM in Figure 3.4-13. Of course, the disadvantage of throwing away a correctly received packet is that the subsequent retransmission of that packet might be lost or garbled and thus even more retransmissions would be required.

Figure 3.4-14: Go-Back-N in operation

Figure 3.4-14 shows the operation of the GBN protocol for the case of a window size of four packets. Because of this window size limitation, the sender sends packets 0 through 3 but then must wait for one or more of these packets to be acknowledged before proceeding. As each successive ACK (e.g.,ACK0 and ACK1) is received, the window slides forwards and the sender can transmit one new packet (pkt4 and pkt5, respectively). On the receiver side, packet 2 is lost and thus packets 3, 4, and 5 are found to be out-of-order and are discarded.

Before closing our discussion of GBN, it is worth noting that an implementation of this protocol in a protocol stack would likely be structured similar to that of the extendedFSM in Figure 3.4-12. The implementation would also likely be in the form of various procedures that implement the actions to be taken in response to the various events that can occur. In such event-based programming, the various procedures are called (invoked) either by other procedures in the protocol stack, or as the result of an interrupt. In the sender, these events would be (i) a call from the upper layer entity to invoke rdt_send(), (ii) a timer interrupt, and (iii) a call from the lower layer to invoke rdt_rcv() when a packet arrives. The programming exercises at the end of this chapter will give you a chance to actually implement these routines in a simulated, but realistic, network setting.

We note here that the GBN protocol incorporates almost all of the techniques that we will enounter when we study the reliable data transfer components of TCP in Section 3.5: the use of sequence numbers, cumulative acknowledgements, checksums, and a time-out/retransmit operation. Indeed, TCP is often referred to as a GBN style of protocol. There are, however, some differences. Many TCP implementations will buffer correctly-received but out-of-order segments [Stevens 1994]. A proposed modification to TCP, the so-called selective acknowledgment [RFC 2018], will also allow a TCP receiver to selectively acknowledge a single out-of-order packet rather than cumulatively acknowledge the last correctly received packet. The notion of a selective acknowledgment is at the heart of the second broad class of pipelined protocols: the so called selective repeat protocols.