Homework 4 assignment for ECE671 Posted: 11/01/16 Due: 11/08/16

Transcription

1 ECE67: Homework 4 Homework 4 assignment for ECE67 Posted: //6 Due: /8/6 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 (2 Points): a. Consider a datagram network using 8-bit host addresses. Suppose a router uses longest prefix matching and has the following forwarding table: Prefix Match 2 otherwise 3 Interface For each of the four interfaces, give the associated range of destination host addresses and the number of addresses in the range. b. Given the following forwarding table, create a binary tree that allows longest prefix match lookup. Interface Prefix A /2 (F) B /4 (D) C /3 (E) D /2 (C) E / (A) F /4 (B) c. In that binary graph you created show how a search for the appropriate interface for address would be performed. Please indicate the individual steps in the correct order. d. Create a tree in which always two bits per step are looked up (Trie). e. Show how a TCAM realization for the following forwarding table would look like. Interface A ** B * C ** D Prefix

2 Solution: a. b. Destination Address Range through (32 addresses) through (64 addresses) through (32 addresses) through (28 addresses) Link Interface 2 3 st E 2 nd D A F X 5 th B C c. see b.) d. A F E E E D B C C

3 ECE67: Homework 4 3 e. 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 SDY? e. Describe in detail how QUIC achieves reliable transmission. f. QUIC makes use of an UUID. What s the benefit of employing this UUID Solution: a. UDP. Required changes like multipath and FEC are much easier to implement b..5;.5

4 c. Replaces part of TCP, TLS.2 (completely), and part of HTTP 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 flow. e. Reliable transport in QIUC is achieved through forward error correction (FEC). Currently, there is a % overhead, meaning that for every packets sent 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 clients IP address has changes. Problem 3 (2 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. Figure 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) Solution:

5 ECE67: Homework 4 5 A B C D E F G H 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 - A, 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 (2 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 2 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 3 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.

6 SYN, Port src =234,Port dst =8 +Option[Token=5678] ACK MyToken=5678 YourToken=6543 MyToken=6543 Figure 2 Dseq=,seq=23,"a" Dack=,ack= Dseq=, seq=,"c" Dack=, ack= DSeq=, seq=456,"b" Dack=,ack= Figure 3 Solution: 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.

7 ECE67: Homework 4 7 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. SYN, Port src =234,Port dst =8 SYN+ACK+Option[Token=6543] +Option[Token=5678] ACK MyToken=5678 YourToken=6543 SYN, Port src =235,Port dst =8 +Option[Token=6543] MyToken=6543 YourToken=5678 c. Gaps in sequence numbers are often not tolerated by middle boxes. Therefor, 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=,seq=23,"a" DAck=,ack=24 DSeq=2, seq=24,"c" DAck=3, ack=25 DSeq=, seq=456,"b" DAck=2,ack=457

8 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 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 (5 Points): Firewall (P25) Provide a filter table and connection table for a stateful firewall that is a restrictive as possible but accomplishes the following a. Allow all internal users to establish ssh sessions with external hosts b. Allow external users to surf the companies website at and access the companies mail server at c. Otherwise all inbound and outbound traffic should be blocked Solution: Filter table: Action Source Address Dest address Protocol Source port Dest port Flag bit allow * * TCP * 22 any allow * * TCP 22 > 23 ACK Allow * TCP * 8 Any Allow * TCP 8 * Any Allow * TCP * 25 All Allow * TCP 25 * All Deny All All All All All All