Homework 4 assignment for ECE671 Posted: 03/20/18 Due: 03/27/18

Transcription

1 ECE671: Homework 4 1 Homework 4 assignment for ECE671 Posted: 03/20/18 Due: 03/27/18 Note: In all written assignments, please show as much of your work as you can. Even if you get a wrong answer, you can get partial credit if you show your work. If you make a mistake, it will also help the grader show you where you made a mistake. Problem 1 (20 Points): Consider the network setup shown in Figure 1. Suppose that the ISP instead assigns the router the address and that the network address of the home network is /24. a. Assign addresses to all interfaces in the home network. b. Suppose each host has two ongoing TCP connections, all to port 80 at host Provide the six corresponding entries in the NAT translation table. Figure 1 a) Home addresses: , , with the router interface being b) NAT Translation Table WAN Side LAN Side , , , , , , , , 3446

2 , , , , 3546 Problem 2 (25 Points): QUIC a. What is QUIC s underlying transport layer protocol? Why did Google decide to make use of this transport protocol? b. How many RTTs does it take in the case of HTTP over TCP until the first bit of a get request reaches the server? How many does it take in the case of HTTP over QUIC? c. Which protocols does QUIC replace? d. Both SPDY and QUIC support multiplexing of multiple flows into one TCP session. Why is there still head of line blocking in the case of SPDY? e. Describe in detail how QUIC achieves reliable transmission. f. QUIC makes use of an UUID. What s the benefit of employing this UUID? a. UDP. Required changes like multipath, congestion control different from the standard TCP, and FEC are much easier to implement b. 1.5;.5 c. Replaces part of TCP, TLS 1.2 (completely), and part of HTTP(/2) d. SPDY still uses TCP and thus packets need to be processed in correct order. That s not the case for QUIC. Packet loss impacts only individual flows, there is HOL blocking. e. Reliable transport in QIUC is achieved through forward error correction (FEC). Currently, there is a 10% overhead, meaning that for every 10 packets sent 1 can be lost. f. In TCP a 5-tupel (Src IP, dest. IP, source port, dest port, proto) is required to identify a session. These parameters partly change if a mobile client, e.g., changes from WiFi to LTE and the session cannot be easily identified. This is different if a UUID is used. The flow can still be identified even if the client s IP address has changes. Problem 3 (20 Points): Consider the following network. With the indicated link costs, use Djikstra s shortest-path algorithm to compute the shortest path from A to all network nodes. Show how the algorithm works by computing a table below.

3 ECE671: Homework 4 3 Figure 2 St ep N D(A),p(A) D(B),p(B) D(C),p(C) D(D),p(D) D(E),p(E) D(F),p(F) D(G),p(G) D(H),p(H) A B C D E F G H St N D(A),p(A) D(B),p(B) D(C),p(C) D(D),p(D) D(E),p(E) D(F),p(F) D(G),p(G) D(H),p(H) ep 0 0, -

4 1 A - A, 1 A, 4 A, 8 2 AB - - B, 3 A, 4 B, 7 B, 7 3 ABC - - B, 3 C, 4 A, 4 B, 7 C, 5 4 ABCD C, 4 A, 4 B, 7 C, 5 D, 4 5 ABCDE A, 4 B, 7 C, 5 D, 4 6 ABCDEH B, 7 C, 5 D, 4 7 ABCDEH B, 7 C, 5 - G 8 ABCDEH GF B, Problem 4 (20 Points): MultiPath TCP a. Explain why there has been a recent effort to split a TCP session in one or more subflows that are routed over different paths? Why was this not considered in TCP s original design? b. Figure 3 illustrates the session setup for two subflows of a multipath TCP session. In the blank parts of this figure fill out the messages that are exchanged between the two end nodes. Also, briefly explain why this method for linking subflows is required. c. Fill out the blanks in Figure 4 and explain why multipath TCP makes use of two levels of sequence numbers. d. What is the shortcoming of multipath TCP in comparison to QUIC when it comes to packet losses? e. Explain what happens in the case of i) a fast retransmit, ii) timeout expiration, and iii) of loss of a subflow. SYN, Port src =1234,Port dst =80 MyToken=5678 YourToken=6543 ACK MyToken=6543 Figure 3

5 ECE671: Homework 4 5 Dseq=0,seq=123,"a" Dack=,ack= Dseq=, seq=,"c" Dack=, ack= DSeq=1, seq=456,"b" Dack=,ack= Figure 4 a. Mobile devices and servers in data centers have more than one network interface (multi-homing) and data can be routed via different path. In addition, ISPs tend to route data over multiple paths. This was not the case in the beginning of the Internet where the only devices with more than one interface were routers. b. End nodes need a way to identify to which multipath TCP session subflows belong. This is achieved by the usage of tokens. The token information is exchanged via options in the SYN messages.

6 SYN, Port src =1234,Port dst =80 SYN+ACK+Option[Token=6543] +Option[Token=5678] ACK MyToken=5678 YourToken=6543 SYN, Port src =1235,Port dst =80 +Option[Token=6543] c. Gaps in sequence numbers are often not tolerated by middle boxes. Therefore, each subflow uses two sequence numbers. Dseq is used as the subflow sequence number and Seq as the regular sequence number of the multipath TCP flow. Dseq=0,seq=123,"a" DAck=1,ack=124 DSeq=2, seq=124,"c" DAck=3, ack=125 MyToken=6543 YourToken=5678 DSeq=1, seq=456,"b" DAck=2,ack=457 d. Even multipath TCP has to deal with the head of line blocking issue. Data can only be passed to the application if all lost segments have been retransmitted. This is not the case for QUIC. In addition, multipath TCP does not provide a

7 ECE671: Homework 4 7 forward error correction mechanism, which is beneficial for video and audio transmissions. e. i) Fast retransmit is performed on the same subflow as the original transmission ; ii) upon timeout expiration, reevaluate whether the segment could be retransmitted over another subflow, iii) Upon loss of a subflow, all the unacknowledged data are retransmitted on other subflows. Problem 5 (15 Points): Firewall (P25) Complete the filter and connection table shown below for a stateful firewall that is as restrictive as possible but accomplishes the following: (Keep in mind that rules higher up in the table have higher priority! The first row shows an example for a rule that allows all internal users to establish http sessions with external hosts.) a. Allow all internal users to establish an smtp session to a mail server at b. Allow external users to surf the company s website at and access another server at via ssh c. Otherwise all inbound and outbound traffic should be blocked Action Source Address Dest address Protocol Source port Dest port allow * * TCP * 80 Filter table: Action Source Address Dest address Protocol Source port Dest port Flag bit allow * * TCP * 80 any allow * * TCP 22 > 1023 ACK Allow * TCP * 80 Any Allow * TCP 80 * Any Allow * TCP * 22 All

8 Allow * TCP 22 * All Deny All All All All All All