Midterm 2 Flashcards

Question

What is a power boost?

Answer 1

Allows subscriber to send at higher rate for short period of time. Spare capacity for users who do not put a sustained load on the network. Time allowed for power boost = Beta / (r_boost - r_sustained)

Answer 2

When there is too much buffering and thus the latency (RTT) can significantly increase. Solutions: 1) smaller buffers (not realistic - lot of stuff currently deployed) 2) traffic shaping - prevent sending at a rate higher than the upload link provided by your ISP.

Answer 3

- Security - looking for rogue behavior (botnets, DOS) | - Billing - something like the 95th percentile

Answer 4

SNMP - simple network management protocol -- Interface byte and packet counts + Ubiquitous - Course

Answer 5

Can see full packet contents (tcpdump, ethereal, wireshark) + lots of detail (timing, header info) - high overhead - hard to keep up w/ high speed links (often requires additional hardware)

Answer 6

Monitors statistics per flow: -- a flow consists of packets that share common: src/dest IP, src/dest PORT, protocol type, TOS byte, interface -- sometimes also grouped if they are 'close' together in time + Less overhead - Much more course, no packets/payloads Could also just do 'sampling' instead of measuring all packets.

Answer 7

Request line: - Method (GET, POST, HEAD) - URL (relative, /index.html) - HTTP Version number Headers (not usually required): - Referer - what caused page to be requested - User Agent - client software used to fetch page - Accept: client will accept these types - Host: host location is being made to

Answer 8

Status Line: - HTTP Version - Response Code: - 100's - info - 200's - success - 300's - redirection - 400's - errors - 500's - server error - Location: could be used for redirection - server: server software (apache/2.2.16) - allow: http methods that are allowed - content-encoded: how content is encoded - content-length: length in bytes - expires: how long content cant be cached - modified: last time page was modified

Answer 9

multiple http requests/responses on a single TCP connection - delimiters indicate ends of requests - content-length Can be combined with 'pipelineing' - client sends requests as soon as it encounters a referenced object

Answer 10

1. Browser Config | 2. Origin sends a special reply telling the client to go to a cache to get the data

Answer 11

Content Delivery Network Overlay network of web caches designed to deliver content to client from the optimal location

Answer 12

``` Goal: Replicate content on many servers How? Where? How do you find it? How to choose server replicas? how to direct clients? ```

Answer 13

Could be: - lowest load - any 'alive' server - lowest latency (typically chosen)

Answer 14

1. Routing (eg, anycas) - simple but service providers have little control over which server client actually gets sent to 2. Application-Based - (http redirect) - effective but client must first go to origin to get redirect location 3. Naming-based (DNS) - when DNS lookup done, it returns a location that is best suited for client

Answer 15

CDNs with ISPs: + better throughput (lower latency) + redundancy +burtiness - lower transit costs ISPs with CDNs: + good performance for customers + lower transit costs

Answer 16

Example, a Chord - scaleable, distributed lookup service. A key - value map (DNS, directory service) + Scalability + Proven Correctness +

Answer 17

Keys and nodes are mapped to the same ID Space. Think of a ring with node ids distributed around it. The key ids are also distributed around the ring. In Chord, you key to the owning node by looking up the next higher node in the id space. So if there are nodes 1, 10, 32, and 43, then keys 33-42 are owned by node 43. 2 - 9 are owned by node 10, etc. + Load balancing + Flexibility - when node is added/removed, it's pretty simple to re-balance

Answer 18

``` Two options: * Every node knows location of every other node: + lookups = O(1) - Tables= O(n) * Node only knows about it's successor: - lookups = O(n) + tables = O(1) ```

Answer 19

* Every node knows M other nodes in Ring * Distance of nodes that it knows increases exponentially ``` So in ring w/ nodes: 1, 10, 32, 43 (out of 0-63 key space): * Node 10 maintains mappings from: ** 10+2^0, 10+2^1 ... 10+2^i Where Finger i points to the successor of 10 + 2^i, so: * 10+2^0 = 11 -> 32 ... * 10+2^4 = 23 ->32 *10+2^5 = 42 -> 43, etc ``` So it walks around the ring trying to find the predecessor of the data it's looking for and + Tables = O(log n) + Lookup = O(log n) Each node knows about the node half way around the ring, then half way to that node, and again half way to that node ... until it gets to it's successor. It's like a binary search. 1/2 -> 1/4 -> 1/8 ... etc.

Answer 20

* Much more simple algorithm * BIC can be too aggressive for short RTT systems and was designed for larger bandwidth/longer RTTs, making it unfriendly to other TCP competing flows * Based on Time b/t congestion events instead of doing something at each RTT. * Keeps BICS stability utilization strengths, but simpler algorithm with less to keep track of.

Answer 21

* Latency is critical * You don't mind dropped packets * Congestion Control can cause unacceptable delays

Answer 22

It takes a long time for the congestion window to reach its maximum. Short lived flows may never reach a congestion event and thus transmit unnecessarily slow.

Answer 23

* Starts out by linearly going toward Wmax (additive increase) * after meets specific threshold, transitions to binary search * Cuts distance b/t Wmax and Wmin in half, repeatedly for each RTT. This becomes new Wmin. * Keep doing this until increment is < Smin, then set to Wmax * Transitions to max probing which is symmetric view of the first half * When congestion event occurs, decrease by beta

Answer 24

Concave - Rapid growth back toward Wmax at the previous congestion event for quick recovery Plateau - AKA, TCP Friendly region. very slow, near linear growth as it approaches Wmax Convex - Max Probing - Grow quickly to find a new Wmax

Answer 25

If successive congestion events are encountered prior to Wmax, the window is further reduced [(2-Beta)/2] to free up additional bandwidth.

Answer 26

Short lived TCP connections - small data size on links with long RTTs. Performance of these flows is dominated by RTT so reducing one RTT can really help.

Answer 27

by using an encrypted cookie that must be requested by the requestor before initiating requests. Server uses this cookie to verify the requestor address is not a forgery.

Answer 28

* Strip out unrecognized TCP options/flags * Endpoints could both support MPTCP, but if middle box doesn't and strips the flags, it doesn't matter. * MPTCP will fallback to TCP if it doesn't get an ack containing the MPTCP flag.

Answer 29

* Buffer size is dictated by the largest RTT of the flow. so it's Link_Size*max(RTT). * Occurs because MPTCP allows out of order packets and allows other links to keep sending * Worst case is packet dropped early and must wait for it to be sent on slowest link, while faster link keeps sending data Controls: * Opportunistic Retransmission - retransmit un-ack'd data that was already sent on a slower flow (but now we send on the faster flow). * Subflows that induce opp. retransmission can be penalized and have their cwnd reduced. * Buffer itself dynamically grows over time as needed, so doesn't use worst case unless it must

Answer 30

* Connection starts normally, sending data until the application buffer is deemed full * Then data is sent in 'blocks' or bursts as the video is watched * The receive/send buffers are emptied during the pauses when no data is being sent, then when the large burst of data comes through, it is seen as congestion on the line and packet loss occurs reducing throughput

Answer 31

* Requiring the security cookie * Maintaining # of active new TFO requests and if threshold is hit, falls back to regular 3WHS while continuing to process existing ones. RSTs still count against this quota.

Answer 32

* The TFO cookie is basically enough. They would need cookies from multiple hosts and have to use them before they expire. In order to get the cookie the host may already be compromised so this is ultimately of little value.

Answer 33

Inject additional traffic into network to measure additional characteristics of the network. ping - delay to server, traceroute - network or ip level path b/t two hosts on the network.

Answer 34

* Assumed a single long lived flow * Currently there are thousands of flows in a backbone router, have varying RTTs and congestion windows are not synchronized.

Answer 35

Average queue length is only affected by the length of the TCP flows and the load on the link. It does not depend on: * Bandwidth * Propagation Delay of the flows * Number of flows

Answer 36

Controlled Delay Management * Assumes standing queue of target size is acceptable * At least one MTU worth of data must be in buffer before dropping packets * If min time in queue exceeds target value for at least a set interval, drop packets according to a control law until queue delay is back on target or data in buffer drops below one MTU. - Uses minimum rather than average as the queue measure - simplified single-state variable tracking of minimum - Use of queue-sojourn time

Answer 37

When there is a constant number of packets in the queue. Difference in the bottleneck link speed and link RTT result in some # of packets constantly occupying the queue. As queue is processed, the receiver sends ACKS back which are turned into more data by the sender.

Answer 38

Datetime of when document was last modified

Answer 39

The domain name - eg, www.w3.org/pub/WWW --> www.w3.org is domain

Answer 40

If set-cookie was set in a response field, future requests to the same domain would send the cookie in this field.

Answer 41

Routing (anycast) - give all replicas same IP and allow routing to take the user to the proper location - simple, but course at the mercy of routing - no control ``` Application Based (HTTP redirect) - simple, but long delays. slow because you have to visit the original page/location first ``` ``` DNS routing (Preferred) - Response when DNS lookup gives response of optimal cache. + fine grained control and fast ```

Answer 42

* Send to top N Peers (unchoked) + 1 optimistically unchoked peer * If optimistically unchoked peer sends fast enough to get into top N, then client boots slowest of top 5 and adds this one, and then will send back in return * If not, drops that peer and tries another

Answer 43

Choking! I won't send to you if you don't send to me

Answer 44

Lambda = k / (RTT * sqrt(p)) k = some constant Where p = loss rate RTT = round trip time Lamda = 3/4 * (Wmax/RTT) Loss Rate (1/2)(Wmax)*(Wmax/2+1)*k

Midterm 2 Flashcards

(68 cards)