Taking a Pass on Routing
Last class, we talked about many of the details associated with the IP protocol and how packets find their way across a network. We talked about the role of routers. We talked about the routing table, the table held by each router that describes the global state of the network. And, we talked about the forwarding table, the table derived from the routing table, also present on each router, that contains the simple mapping from destination network to port number. And, we said that the routers talk to each other in order to collect routing information and then build their own forwarding tables.
But, we did not talk about the details of this router-router conversation. We're going to leave that for 15-441 (Networks). It isn't that it is hard. It isn't. But, there are actually several different protocols that might be in use. And, they all of havey interesting, and sometimes subtle, behaviors. And, some of them dovetail really nicely with discussions about non-routing (circuit or virtual circuit) techniques for selecting a message's path.
So, in the end, the details of routing protocols are a long and interesting discussion -- and an unnecessary one for us at this time. It is really a discussion about disributed state -- and we'll get into that later. For now, we can just assume that we're old fashioned and that a sysadmin builds the tables. The model still works and we get to keep focus.
Network Layers, A Reference Model
As we work our way up from network hardware to the application programmer, we are beginning to see the overal organization of a network. This architecture is sometimes described using the following model:
Application Layer: The details of the messages and structures used by a particular application Transport Layer: Establishment of endpoints and other services commonly used by programmers Network Layer: Movement of packets from network to network across an intern-network Link Layer: Management of stations sharing the same channel Physical layer: Voltages, connector shapes, power levels, light colors, &c
Thus far, we've worked our way up, talking a little about the physical layer, which is really the domain of various engineering disciplines, and a lot about the link and network layers. Today, we are going to begin our discussion of the transport layer.
The Transport Layer
The transport layer establishes an end-to-end abstraction that is useful to the programmer. Part of that is that it needs to hide the hop-by-hop nature of the network layer's routing process. And, part of that is that it needs to establish program-to-program communication, since multiple programs might be running on the same host -- and the network layer just goes hop-to-hop.
In addition to these basic requirements, it must somehow answer the question, "What is a message, and how do we know when we have one?". For example, we often classify transport layers as being either:
- Message-oriented: Messages are sent like the mail. When you get a message, it comes in a discrete chunk. Whatever is placed into the envelope when sent is exactly what is in the envelope when it is read. Envelopes, even those sent in series from the same sender to the same receiver are never merged.
- Stream-oriented: There really isn't the concept of a discrete message -- there is the flow of data. Consider a phone conversation or radio broadcast. These don't come in envelopes. There can be periods of quiet, but there is no packaging or dividing line.
Protocols are often also classified in terms of their quality of service:
- Best-effort: Also like the post office, the protocol does its best, but makes no guarantees. Messages may be lost or delivered out of order
- Reliable: The protocol will try diligently to resent anything that is not confirmed to be delivered.
As we'll talk about soon, unreliable protocols may, or may not, be session-oriented. A session-oriented protocol establishes a relationship between the sender and receiver before any data is exchanged. this session remains in place until it is closed. So, in some sense, the recipient knows to be waiting for communication. Unreliable protocols need not be session-oriented. But, reliable protocols need to be session oriented so that the sender and receiver can coordinate what has, and what has not, been successfully received.
In the context of Internet protocols, the TCP/IP protocol suite, there are two general-purpose transport protocols:
- User Datagram Protocol (UDP): Unreliable, message-oriented
- Transport Control Protocol (TCP): Reliable, stream-oriented
UDP adds very little value of IP, itself -- basically, it adds port numbers. It allows applications to be identified with ports so that messages, upon arriving at the destination, can be sent to the right program.
We'll talk about TCP tomorrow. It adds a lot of value. It adds streams and reliability. But, for today, I'd like to examine what it means --and does not mean-- to be a reliable protocol.
It is easy to see how we could create a reliable protocol above UDP. We add a sequence number to each message. We send a message and wait for an ACKnowledgement. We know the maximum round-trip time, and wait at least that long. If we don't get an ACK within that time, we assume that the message got lost en route to the recipient -- even though maybe only the ACK got lost. We resend. When the sender gets it, it'll ACK, possibly again. There won't be any confusion, even if it is received twice, because the sequence number will enable the duplicate to be detected and discarded. The same is true of a duplicate ACK. If we send more than once copy, and more than one ACK eventually makes its way to the sender, the sender just ignores the duplicates -- it ignores any ACK that is not associated with the present message number.
To this end, it is important to note that only one message is in flight at a time. The time between the sending of a message and when its ACK is received is dead air. For this reason, this type of reliable protocol is often known as a stop-and-wait protocol.
The Two Army Story
Consider two waring armies, the Red Army, and the Blue Army. The Blue Blue Army is camping in a mountain valley. The Red Army, while larger and more powerful, is divided into two groups hiding in the surrounding mountains.
If the two Red Army platoons attack the Blue Army together and at exactly the right time, they will prevail. The longer they wait, the more surpised the Blue Army will be by the attack. But if they wait too long, they will run out of supplies, grow weak, and starve. The timing is critical. But if they are not coordinated, they will surely lose. They must attack at exactly the same time.
When the time is right R1 will send a messenger to R2 that says, "Attack at dawn!" But R1 may become concerned that R2 did not get the message and consequently not attack. If that happens, R1 will be defeated. Alternately, R2 may become concerned that R1 will beocme concerned, so they may not attack, leaving R1 to be defeated in solitude.
So what if they agree that the recipient of the message, R2, will return an ACK to the sender, R1? This is just one level of indirection. The R2 may become concerned that the ACK was lost and not attack. Or R1 may become concerned that R2 became concerned and did not attack. This problem can't be solved by ACK-ACK, or even ACK-ACK-ACK-ACK -- more ACKs just add more levels of indirection, but the same problem remains.
Another issue might be fake messages. What if the Blue Army sent an imposter to deliver a message to R2 telling them to attack too early. They would be defeated if they followed it. But if they did not obey messages for fear that they were fraudulant, R1 would be defeated when they did attack, even after advising R2. This fear might also prompt an army from acting upon a perfectly valid message.
The moral of this story is that there is no solution to this probem if the communications medium is unreliable. Please note that I said medium, not protocol. This is an important distinction. A reliable protocol above an unreliable medium can guarantee that a message will eventually be sent, provided of course that the recipient eventually becomes ready and accessible. But no protocol can guarantee that a message will be delivered within a finite amount of time -- error conditions may persist for long and indeterminate amounts of time.
Reliable vs Unreliable
A reliable media certainly beats one that is not. But, as we now know, a reliable protocol is really just a diligent protocol. It tries, and tries, and tries some more.
But, this is not always desirable. In some cases, a late packet is worthless -- and resending it just wastes network time. This is the case for many types of real-tiem communication, such as live video or audio, e.g. telephone calls or web cams.
What does one do with a 10 minute old syllabal? If we delay the subsequent syllabals by 10 minutes, the call is worthless. And, if we charge forward, we can't exactly introduce a stray word later. It is best to just let it go an hear a brief pause or pop. The same is true of video. We'd rather see a brief freeze and a jump in one part of the frame than have th4e whole thing delayed.