By Arjuna Sathiaseelan Tomasz Radzik Department of Computer Science King’s College London

1. EPDN: Explicit Packet Drop Notification and its uses ByArjuna Sathiaseelan Tomasz RadzikDepartment of Computer ScienceKing’s College London

2. Motivation • Reordering or Corruption of packets leads to overestimation of the congestion of the network. • Decreases the TCP performance of a network • Imperative to propose a mechanism that allows the TCP sender to know the exact cause of the out of order data

3. Problem of Packet Reordering • Packet reordering is common : due to parallelism in networks. • Parallelism reduces cost of equipments and trunks. Types of Parallelism: • Local parallelism : Multiple paths within a device. • Multi-path routing. Effects of Parallelism: • Decreases the TCP performance of the network.

4. Detecting packet loss • Retransmission Timer • Fast Retransmit

5. Implications of Reordering • Unnecessary retransmission of data segments means that some of the bandwidth is wasted. • Unnecessary reduction of the congestion window. • TCP ensures that the receiving application receives data in order. Burden on the TCP receiver since the receiver must buffer the out-of-order data until the missing data arrive to fill the gaps.

6. Impact of Reordering

7. Related Work • [Floyd, Mahdavi, Mathis, Podolsky; 2000]: DSACK option allows the TCP receiver to report to the sender when duplicate segments arrive at the receiver. Using this information, the sender can determine when a retransmission is spurious. If spurious, the cwnd can be restored to the previous value. • RR-TCP [Zhang, Karp, Floyd, Peterson; 2003]: RR-TCP: mechanisms to detect and recover from false retransmits using the DSACK information. They propose several algorithms for proactively avoiding false retransmits by adaptively varying DUPTHRESH.

8. EPDN: Explicit Packet Drop Notification • Each gateway has a hashtable storing entries: <i,max:PNO,min:PNO> • i is the flow id (also the key usedto index the hashtable). • max:PNO is the maximum sequence number of the packet dropped in the gateway. • min:PNO is the minimum sequence number of the packet dropped in the gateway.

9. Packet Drop P2 P3 P4 P4 3,2 P1 Hash Table Flow Id: Min: Max: Hash Table Flow Id: 1 Min: P2 Max: P2 Hash Table Flow Id: 1 Min: P2 Max: P3 Hash Table Flow Id: Min: Max: Received Packets Received Packets P1 Received Packets P1 P4 Droplist: Droplist: 2 3 Reorderedlist:

10. Packet Reorder P1 P3 P2 P3 Hash Table Flow Id: Min: Max: Received Packets Received Packets P1 Received Packets P1 P3 Droplist: Reorderedlist: Reorderedlist: P2

11. RN-TCP: Reorder Notifying TCP • Receiver uses the following information to identify whether a packet has been dropped or reordered. Packet Notation: P<fid,n,max,min> • P:n - Sequence number of the current received packet • P:max - Maximum dropped entry • P:min - Minimum dropped entry • Q:n - Last received packet in the receiver buffer queue

12. RN-TCP: Reorder Notifying TCP (contd.) • Receiver maintains two lists: - Reorder List - Drop List • Drop is notified by not setting ‘drop-negative’ bit in the ACK. • Reorder is notified by setting ‘drop-negative’ bit in the ACK.

13. Storage and Computational Costs • The IP options field has 40 bytes. • We use 4 bytes for each minimum and maximum dropped entries to be inserted into the option field of the IP segment. • We use one bit from the reserved bits in TCP to denote the ‘drop-negative' bit. • Our monitoring process records only flows whose packets have been dropped. • When the dropped information is inserted into the corresponding packet that leaves the gateway successfully, the entry is deleted. • Rough estimate: 500KB SRAM would be sufficient for storing infoirmation about dropped packets ?

14. Storage and Computational Costs(contd..) • For each packet, the computational cost in each gateway is constant, assuming a constant time access/update of the hashtable. • Computational costs at the receiver are as follows: - Insertion O(n) where n is the number of packets the receiver has assumed to be dropped or reordered. - Deletion and comparison costs O(m) where m is the length of the list. • Computational cost O(log n) and O(log m) respectively if we use balanced trees.

15. Simulation Details • Used Network Simulator ns-2. • Segment size : 1500 bytes. • Conducted experiments for Drop-tail. • Queue size used : 65 segments. • Used FTP traffic flows.

16. Throughput – Packet Delays (Delays: Normally distributed with mean = 25ms, std.dev = 8ms)

17. Throughput – Multipath Routing(Delays: Modal Distribution)

18. Link Utilization(30% Packets Delayed)

19. Throughput: Delays and Drops(5% Packets Delayed)

20. TCP-R : Robust TCP • RN-TCP: proposed forterrestrial networks. • Terrestrial networks: - the out of order packets are caused mainly due to losses related to congestion and reordering of packets. - Losses due to corruption is negligible. • Satellite links have high RTTs, typically on the order of several hundred milliseconds. • Losses mainly due to corruption than congestion. • If a packet had been actually dropped due to corruption, having an increased value of dupthresh requires a RTO to detect the packet loss. • Imperative to propose a mechanism that improves the performance when packets get corrupted, reordered and dropped for networks with large RTT.

21. TCP-R : Robust TCP • TCP-R uses EPDN. • The TCP-R receiver informs the TCP-R sender about its assumption. • If the packets had been dropped in the network, the lost packets are retransmitted after waiting for 3 DUPACKS (fast retransmit) and reduces the congestion window by half (fast recovery). • If the packets are assumed to be reordered or corrupted in the network, - TCP-R sender retransmits the packet after receiving three DUPACKS - Enters our modified fast recovery mechanism where the procedure of reducing the slow start threshold (ssthresh) and the congestion window (cwnd) are bypassed.

22. Simulations • Segment size : 1000 bytes • Queue size used : 100 segments. • Used FTP traffic flows.

23. Throughput – Packet Delays (Delays: Normally distributed with mean = 200ms, std.dev = 80ms)

24. Throughput: Delays and Corruption(5% Packets Delayed)ANDTimeout Avoidance

25. Throughput – Delays and Drops(3% Packets Delayed)

26. Future Work • Scalability of EPDN in wired networks – is it possible just to use EPDN in hot spots where packets get dropped? • In case the routing changes and if the dropped information cannot be propagated to the receiver - send the dropped information in an ICMP message to the sender ? • Use the RED packet drop history for EPDN? • Verify the size requirements needed for EPDN?