This preview shows page 1. Sign up to view the full content.
Unformatted text preview: of the figure are all the same, and the areas of all the acknowledgments in the
bottom half are all the same.) Figure 20.13 Congestion caused by a bigger pipe feeding a smaller pipe.
In this figure we have labeled the router R1 as the "bottleneck," because it is the congestion
point. It can receive packets from the LAN on its left faster than they can be sent out the
WAN on its right. (Commonly R1 and R3 are the same router, as are R2 and R4, but that's
not required; asymmetrical paths can occur.) When router R2 puts the received packets onto
the LAN on its right, they maintain the same spacing as they did on the WAN on its left,
even though the bandwidth of the LAN is higher. Similarly, the spacing of the ACKs on
their way back is the same as the spacing of the slowest link in the path.
In Figure 20.13 we have assumed that the sender did not use slow start, and sent the
segments we've numbered 1-20 as fast as the LAN could take them. (This assumes the
receiving host advertised a window of at least 20 segments.) The spacing of the ACKs will
correspond to the bandwidth of the slowest link, as we show. We are assuming the
bottleneck router has adequate buffering for all 20 segments. This is not guaranteed, and can file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_bulk.htm (18 of 24) [12/09/2001 14.47.22] Chapter 20. TCP Bulk Data Flow lead to that router discarding packets. We'll see how to avoid this when we talk about
congestion avoidance in Section 21.6. 20.8 Urgent Mode
TCP provides what it calls urgent mode, allowing one end to tell the other end that "urgent
data" of some form has been placed into the normal stream of data. The other end is notified
that this urgent data has been placed into the data stream, and it's up to the receiving end to
decide what to do.
The notification from one end to the other that urgent data exists in the data stream is done
by setting two fields in the TCP header (Figure 17.2). The URG bit is turned on and the 16bit urgent pointer is set to a positive offset that must be added to the sequence number field
View Full Document
This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12