6.033 | Spring 2018 | Undergraduate
Computer System Engineering
Week 7: Networking Part III

Lecture 13 Outline

  1. Introduction
    • “New" technologies on the Internet. How do they work? Are they overcoming any problems in the existing architecture? Do they invalidate any of our assumptions? Do they provide opportunities?
    • Today: File-sharing, VoIP, and video-streaming.
    • Commonalities: All deal with P2P networks, or related constructs (CDNs).
  2. File-Sharing: Getting a File from One Person (Machine) to Another
    • Can use client/server:
      • Client requests file, server responds with the data.
      • HTTP, FTP work this way.
    • Downsides: Single point of failure, expensive, doesn’t scale.
    • Could use CDNs:
      • Buy multiple servers, put them near clients to decrease latency.
      • No single point of failure, scales better.
      • See the next recitation for more discussion.
  3. Peer-To-Peer (P2P) Networks for File-Sharing
    • Distribute the architecture to the extreme.
    • Once a client downloads (part of) the file from the server, that client can upload (part of) the file to others. Put clients to work!
    • In theory: Infinitely scalable.
    • P2P networks create overlays on top of the underlying Internet (so do CDNs).
    • Problem: What if users aren’t willing to upload?
  4. BitTorrent: How to Incentivize Peers to Upload
    • Basics of original BitTorrent (BT) protocol:
      • Create a .torrent file, which contains meta-information about the file (file name, length, info about pieces that comprise the file, URL of tracker).
      • Have a tracker. A server that knows the identity of all the peers involved in your file transfer.
      • To download:
        • Peer contacts tracker.
        • Tracker responds with list of other peers involved in transfer.
        • Peer connects to these other peers, begins to transfer blocks (see below).
        • Some peers are seeders: Already have the entire file (maybe servers that host the file, or just nice peers who are sticking around).
    • In the actual download, peers request blocks: pieces of pieces.
      • Details/terminology doesn’t matter. Just know that blocks are small (~16KB) chunks of the file.
      • Request blocks in a random order (more or less).
    • What incentivizes users to upload (UL) rather than just download(DL)ing?
      • High-level: Users aren’t allowed to DL from a user unless they’re also ULing to that user.
        • So peers want mutual interest: A has to have blocks that B needs, and vice versa.
      • Protocol is divided into rounds. In round n, some number of peers upload blocks to Peer X. In round n+1, Peer X will send blocks to the peers that uploaded the most in round n. (Typically, to the top four peers.)
      • How do peers get started?  Each peer reserves some (small) amount of bandwidth to give away freely.
    • This method of incentivizing peers is part of what allowed P2P file-sharing to take off.
    • Lingering problem: tracker is central point of failure.
    • Most BT clients today are “trackerless”, and use Distributed Hash Tables (DHTs) instead.
  5. VoIP: Voice over IP
    • Talking specifically about Skype, a proprietary system.

    • Skype used to use a P2P network for two things: To improve performance, allow certain connections to work at all.

    • Recall the first networking lecture. Internet bred NATs: Network Address Translators.

      • Consider client A behind a NAT, who wants to initiate a connection to server S. A’s IP is private (can’t route to it); S’s and N’s are public.

      A — N —- S

      • A sends a packet: [to:S from:A].
      • N rewrites the header: [to:S from:N].
        • and stores some state.
      • S receives it, sends response back to N: [to:N from:S].
      • N uses stored state to figure out that this packet is really meant for A.
        • N will keep track of the port(s) that A is communicating on. Communication via those ports is then meant for A.
    • Now imagine two clients, both behind NATs:

      A — N1 —- N2 — S

      • Now A doesn’t even know S’s IP (private IPs aren’t routable). It also doesn’t know N2’s IP; it has no way to get that.
      • For Skype: Means that A and S can’t call each other.
      • Skype provides a directory service, so assume we can get N2’s public IP.  When N2 gets packet destined for S, it has no idea what to do with it.
      • (See Lecture 13 slides (PDF) for example.)
    • Skype will employ an additional node—a “supernode”—P, with a public IP, and route A and S’s calls through P: 

      Diagram showing connection between client A and client S through nodes.
      • P keeps a bunch of state to get this to work, and A and S must both be registered users of Skype. A and S will connect to P as part of starting up their Skype client (so private IP users initiate connections to public IPs).
        • In reality, there is not one supernode, but a network of supernodes. A, S are both connected to nodes in that network, and the overlay network routes data between them.
    • Seems like this will affect performance, so Skype only let you be a supernode if your memory/CPU is sufficient (and you have a public IP).

    • Good idea?

      • A/S might not want their (encrypted) call routed through someone else.
      • P might not want to pay to transit traffic for A and S.
    • Today: Microsoft owns all of the supernodes, making this less of a P2P network and more of a hierarchy.

  6. Video-Streaming (Briefly)
    • Can we just use BitTorrent to stream (live) video?
      • Streaming requires getting blocks (roughly) in order.
      • Also requires certain amount of bandwidth at all times.
    • Probably not:
      • BT works because peers can acquire blocks in any order.
      • Moreover, most BT peers are on residential links, which have underwhelming upload bandwidth.
    • What’s good for streaming? CDNs!
      • Thursday’s recitation: What CDNs bring to the table that P2P networks don’t.
      • Also think about whether you want to reconsider CDNs for file-sharing.
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Spring 2018
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