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Discussion 2

CS 168, Spring 2026 @ UC Berkeley

Slides credit: Sylvia Ratnasamy, Rob Shakir, Peyrin Kao, Iuniana Oprescu

Packets and Sockets 📦

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Logistics

Project 1B: Traceroute Error Handling

Deadline: Friday, Feb 13

Still far ahead: Midterm on Friday, Mar 20 7–9pm (put it in your calendar)!

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Delays

Transmission Delay
Propagation Delay
Bandwidth Delay
Queueing Delay
Burstiness and Queues
End-to-End Delay
Round Trip Time (RTT)

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Delays

How long does it take for your packet to travel through the network?
It depends on…

how much data you’re sending and the link speed�→ transmission delay
your distance from the destination�→ propagation delay
the traffic pattern�→ queuing delay

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Transmission Delay

How long it takes for the all bits in the packet to get on the wire

The time between when the first and last bits enter the link

Limited by the bandwidth

Bandwidth: Number of bits you can send through a wire per unit of time

Capacity = Delay × Bandwidth

Bandwidth

Propagation Delay

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Transmission Delay: Formula

Usually bits/second

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Transmission Delay: Example

Bandwidth = 4Mbps

Propagation Delay

Capacity = Delay × Bandwidth

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Propagation Delay (latency)

End-to-end transmission time of one bit
Depends on the length of the link
Limited by the speed of light (propagation speed of link)
Does NOT depend on the size of the packet

Capacity = Delay × Bandwidth

Bandwidth

Propagation Delay

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Propagation Delay: Formula

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Propagation Delay: Example

Capacity = Delay × Bandwidth

Bandwidth

Propagation Delay

speed of light

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Measuring Packet Delay with Timing Diagrams

Packet Delay = Transmission Delay + Propagation Delay

Packet Delay = (Packet Size / Bandwidth) + Propagation Delay

Bandwidth: 1,000,000 bps

Delay: 0.001s

t = 0s

t = 0.000001s

t = 0.001001s

t = 0.000800s

t = 0.001800s

800-bit packet

Transmission Delay

Propagation Delay

f you're

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really looking for the formula roughly speaking this is how we computed it so this is the math that we did so we took

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the packet size that was 800 bits and we divided it by the bandwidth so this

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number in red gave us 800 millionth of a second basically tells us when the last

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bite shows up so maybe another way of thinking it of it is one over bandwidth is how long it takes to send one bit and

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you have packets size number of bits so in total if each one takes one over bandwidth to send and you have packet

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size number of bits the total amount of time it will take to just get everyone in the pipe is Packet size over

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bandwidth and that's this time right here this is the amount of time it takes to just get everyone into the pipe

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regardless of what happens on the other side so the sender has to get until T equals 800 millionth of a second just to

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get everyone into the pipe and this term actually also has a special term we call it transmission delay it's the amount of

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time it takes just to shove everyone in the pipe regardless of what time they actually show up on the other side and

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because this is the time it takes to just get everyone in the pipe by the time the last person enters the pipe or

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the last bit we also have to wait one more propagation delay for that last bit to exit so at the time the last bit

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shows up wait one more propagation delay the last bit exits the pipe and that gives us our final answer so in total

delay comes from two sources one is the transmission delay and it's affected by packet size and bandwidth the more

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packets you have the longer it takes to shove everyone in the pipe and the narrower the packet the pipe is the

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longer it takes to shove everyone into the pipe that's the bandwidth and the second term is propagation delay the

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longer the pipe is the longer it takes for packets to show up and we have one term of propagation delay at the end for

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the last packet to show up all the other packets show up earlier so we don't have to add them in this

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calculation that is is to show up up here so if you really needed the magic

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formula it's this but be ready to draw the timing diagrams to really apply your

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knowledge

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Pipe Diagrams

Pipe diagram shows the bits on the link at a frozen moment in time.

Height = bandwidth. How many bits we can put in the pipe per unit time.
Width = propagation delay. How long it takes for bits to travel through the pipe.
Area = bandwidth-delay product. How many bits fit in the pipe at a given instant.

Now that we know the propagation delay, we can tell how many bits are “in flight” (on the link) at any time

Bandwidth: 5 bps

Delay: 7s

Bandwidth

Propagation Delay

in real life what they really care about is the

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rectangle itself they want to draw this rectangle as a visual representation of

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the pipe so that they can analyze what fits in the pipe at any given time and the way that they draw this pipe is or

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sorry this pipe yeah H the way that they draw this pipe is based on the rectangle

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and the rectangle is based on the two properties that we already talked about so they're going to draw this pipe using

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the bandwidth to determine the height and remember we talked about this earlier the bandwidth tells us how many

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bits we can fit in the pipe so the reason why I added this crazy animation is because I wanted to show you that if

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you increase bandwidth what happens the rectangle gets taller and if the rectangle gets taller what happens it

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means more of these bits can line up to enter the pipe at any given unit of time

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that's why bandwidth represents the height of this rectangle so the thing that always confuses people is they say

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why is bandwidth the height well one intuitive way of saying it is we talked about the water pipe the bandwidth tells

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us how wide the pipe is another way of seeing it is if you look at this diagram it tells us that if the bandwidth is

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increased it means that you can stack more of these dots which represent bits and more of the bits can enter at any

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given time that is exactly what it means to have higher bandwidth that's why the height is determined by the

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bandwidth and the width tells us the propagation delay again it's kind of like the water example the width tells

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us how long the pipe is and how long it takes for a single drop of water to travel through the pipe but we can see

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that through our little animation as well that if you make this rectangle longer these bits are going to have to

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March for more time steps before they can exit on the other side and just like we saw earlier bandwidth times

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propagation delay if you multiply these two terms together you get the area of this rectangle which is the capacity of

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this pipe and again the animation shows us that this is indeed the capacity of the pipe the area of this rectangle

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because the bigger this rectangle is whether you make it taller or wider the bigger this rectangle is the more dots

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or bits you can fit into this pipe so hopefully with these dots I've convinced you that the pipe diagram is really just

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a rectangle where the bandwidth is the height and the propagation delay is the width and the dots are really just there

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for clarity but the rectangle is what people really care about when they draw

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these and

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Packets might have to wait before they can be transmitted…

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Transient Overload

What happens if two packets arrive at the router simultaneously?

Can't process both at the same time! Router must queue one for later.
When there are no incoming packets, router can drain the queue.
This is called transient overload, and it's fairly common.

Router

Need a queue!

so take a look at

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this picture the router has two packets arriving at the exact same time so in this pipe diagram Frozen in Time A1 and

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B1 are both about to arrive at the router at the same time they're right there next to the router and on the next

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time step they're both going to arrive well that's a problem because this router can't handle two packets at once

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can only handle one packet at a time it looks at the packet thinks about where it needs to go forwards the packet so if

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this router can only handle one packet at a time what happens if it suddenly gets two at once well it's a problem and

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the solution is the router is going to keep a que and the que basically says here are some packets that I've received

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but I have not processed so I'm going to save them for processing later and that's actually totally fine happens all

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the time in the internet

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Queuing Delay

How long the packet waits to get transmitted on the wire
Happens only when arrival rate is greater than transmission rate

More packets are arriving than are getting transmitted

Router

Need a queue!

so take a look at

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this picture the router has two packets arriving at the exact same time so in this pipe diagram Frozen in Time A1 and

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B1 are both about to arrive at the router at the same time they're right there next to the router and on the next

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time step they're both going to arrive well that's a problem because this router can't handle two packets at once

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can only handle one packet at a time it looks at the packet thinks about where it needs to go forwards the packet so if

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this router can only handle one packet at a time what happens if it suddenly gets two at once well it's a problem and

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the solution is the router is going to keep a que and the que basically says here are some packets that I've received

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but I have not processed so I'm going to save them for processing later and that's actually totally fine happens all

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the time in the internet

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Persistent overload

Persistent overload: Not enough capacity to handle the incoming packets!

Queue won't help us. If the queue fills up, the router must drop packets.

How do we solve persistent overload?

Operators can detect the overload and (manually) upgrade the link.
Routers can tell the senders to slow down.

Router

Uh oh...

but sometimes the router cannot handle

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all the incoming load so if the two pipes that are incoming look like this well this router is doomed it's not

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going to be able to handle all of these packets no matter how much it tries to

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put packets in a queue it's just not going to have enough bandwidth to send out this many packets over a single link

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so this router is in trouble the que is going to build up and build up and eventually the queue itself is going to

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exceed some limit and become to full and at that point this router simply has to give up and it's going to start dropping

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packets which means it receives a packet and just ignores it doesn't get forwarded the packet just gets lost and

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stuff like this by the way is why we said that the internet has best effort at layer three it's because we design

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routers like this and the way that we design routers which is that we allow them to drop packets in cases like this

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is consistent with our design that we said routers only have to offer best effort service so it's a nice case of

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the design matching the implementation because we said that the Internet only provides best effort well then this

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router is allowed to drop packets if it can't handle them because the internet is best effort so you could have a que

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but the que is not going to help you if you try drawing this out with a que the que itself is going to get packed and to

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full and eventually some of these later packets are just going to get dropped so here we call this persistent overload

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because it's not the case that the router temporarily had a little bit too much to do but later it was able to

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clean out the queue this is the case where the queue is never getting clean we just don't have enough capacity this

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router is just not powerful enough or doesn't have enough processing power doesn't have enough outgoing bandwidth

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to process all the packets that are incoming so this is a problem of not enough capacity it's not a problem of a

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temporary overload that gets solved this is a persistent problem that's why people call persistent overload so how

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do you fix this turns out there's not really a great answer one possible answer is the router itself logs

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somewhere for its operator the person operating this router hey I am not able to send out all the packets I have been

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dropping a lot of things lately you might want to consider upgrading me to a better model give me some more

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processing power connect me to some more links so that I can actually handle all the packets but that's a manual process

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it's not something that the router can just automatically do routers don't upgrade themselves automatically someone

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would actually have to come in and rewire this router or install a better one to handle this kind of load and

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something else we'll see later in the semester is this router might actually be able to send a packet backwards to

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the sender and tell the sender you are sending me too much you need to slow down because I can't handle this kind of

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load and that's something we'll spend a lot of time on when we talk about congestion control but for now I just

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wanted to get the point across that routers take in incoming packets they forward them sometimes there is

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transient overload and sometimes there is persistent overload which we will try and solve a lot

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later any thoughts on this stuff

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Burstiness and Queues

How does burstiness affect queuing delays?

Bursty flows tend to increase queuing delay

What happens when the queue is full?

Packets are dropped

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Packet Queuing and Life of a Packet

Queues introduce extra delay.

Packet delay = Transmission Delay + Propagation Delay + Queuing Delay.

Life of a packet:

Sender puts payload in a packet, adding headers.
Packet travels along a link.
Packet arrives at a router. Router forwards packet to the next hop.

Packet might be queued or dropped.

Repeat the last step until:

Packet reaches destination.
Packet is dropped.

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okay so we actually have to go back now and revisit what a packet does when it

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travels from its source to its destination because remember earlier we said that you send a packet the delay of

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the packet comes from two sources one is the transmission delay that is how long does it take for all of the bits to

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March into the pipe and the propagation delay is how long the pipe is how long does it take for the bits to travel

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across the pipe but there's actually a third term that we've now introduced that we previous previously had not

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thought of which is queuing delay because it could be the case that this packet travels multiple hops to its

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destination and some of the intermediate routers trap the packet in a queue for some time before the packet can get

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forwarded so that actually could introduce additional delay into the time

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it takes to send a packet from a source to a destination so we already saw these

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two terms those are in terms of the link that you have the bandwidth the propagation delay and now we have a

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third term which is the queuing delay and that comes from routers that might have to put this packet in a que so in

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total if you think about what does a packet do from the moment it's created to the moment it's received well the

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sender is the one that creates the packet it wraps the packet around a bunch of headers like we've seen before

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and then it sends the bits of the packet along a link and how long it takes to send the packet along the link depends

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on those two terms eventually the packet arrives at a router and the router has

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to forward the packet closer to its destination so the router says I'm not the Final Destination but the final

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Destin is that way so send it that way so the router is going to forward the packet

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and in doing so the packet might have to wait a little bit in the queue or if this router is totally overwhelmed maybe

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that packet just gets dropped that's also an option and then this just repeats over and over again we get to

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the next router there might be some queuing time the router might drop it and if it doesn't drop then that packet

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gets forwarded to the next router that router cues it for a little bit maybe maybe drops the pack pet forwards it

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again and this repeats over and over again until either the packet reaches its destination because all the routers

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along the path correctly forwarded the packet or the packet got dropped along the way in which case it's not reaching

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its destination the end host will have to do something about that so roughly speaking that is what a packet does from

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the moment it's created to either the moment it drops or the moment it reaches its

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destination any thoughts on this

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Round Trip Time (RTT)

The time it for the packet to reach its destination and receive a response

RTT = 2 * (End-to-End Delay)

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Visualizing end-to-end-delay…

Two packets, back-to-back

Transmission delay

Propagation delay

Transmission delay

Propagation delay

Queueing Delay

Time

End-to-end delay of brown packet

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Statistical Multiplexing

Statistical Multiplexing
Layers
Sockets

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Statistical Multiplexing

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Statistical Multiplexing

Sum of the peaks is always greater than the peak of the sums (peak of the aggregate). Usually much greater.

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Layering in the Open Systems Interconnected (OSI) Model

L7: Application

do the thing

L6: Presentation

(ignored here)

L5: Session

(ignored here)

L4: Transport

beyond delivery: (un)reliability, packet assembly, congestion control, ...

L3: Network

global delivery, best-effort

L2: Data link

local delivery, best-effort

L1: Physical

physical transfer of bits

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Layering in practice

HTTP(S)

TCP

Ethernet

802.11 Wi-Fi

CAN bus

UDP

USB

SSH

Email (IMAP/POP)

...

L7: Application

L6: Presentation

L5: Session

L4: Transport

L3: Network

L2: Data link

L1: Physical

L3.5: research!

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Sockets

Endpoint for sending or receiving data across a network
OS abstraction for connections
Allow L7 applications to operate on data streams (not packets)

Connect, listen, accept, send, receive

Open a socket between:

Source IP address : port
Destination IP address : port

Application

Transport

Network

Datalink

Physical

app

(networking stack)

app

NIC

Port

(Network Interface Card)

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Connection (the basic abstraction)

Pipes data between two processes (on different hosts)
Data flows both ways!

The Internet

Connection

Process

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Connection (the basic abstraction)

Data is sent simply as a stream of bits
Reconstruction of bits only at the endpoints
The Internet knows nothing* about what it’s transmitting!

* (unless you’re implementing security)

The Internet

Connection

Process

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Connections

Two types of sockets

Server and Client

Servers listen for clients to connect to them

Wait until a connection is attempted

Accept and dispatch connection

Usually serving many clients at once

Clients initiate new connections to servers
Example

Server: berkeley.edu
Client: Your internet browser

Client

Server

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Connections

Hosts have addresses

Unique identifier (just like a street address)

Clients (different users) find servers with their addresses

Servers send data back with the client address

Example addresses ➔

Client

Server

Are addresses enough to make this work?

1.2.3.4

5.6.7.8

10.20.30.40

9.10.11.12

13.14.15.16

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Addresses aren’t enough

Server 1 Machine (5.6.7.8)

Server Process 1

Client 1 Machine (1.2.3.4)

Process 1

Process 2

How does the client computer know which process (i.e. web browser) to deliver data to?

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Ports

Server Machine

Client Machine (1.2.3.4)

Process 1

(Port 10000)

Process 2

(Port 20000)

Sockets are identified by unique IP:port pairs
A port is a number that the OS associates with a socket when it is created

i.e. sending to address “1.2.3.4:10000” would send data to the socket owned by Process 1

Process 1

(Port 80)

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Ports

Packets carry port number
Servers listen on a port

Which one depends on application
HTTP: 80
SSH: 22

Client process connects to

well known port

Client also has a port

Randomly assigned by OS
Used by OS to send data to correct process

Server Machine

Client Machine (1.2.3.4)

Process 1

(Port 10000)

Process 2

(Port 20000)

Process 1

(Port 80)

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Worksheet

True or False
End-to-End Delay
Statistical Multi-What?

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Question 1: True or False

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Worksheet

True or False
End-to-End Delay
Statistical Multi-What?

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Question 2: End-to-End Delay

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Question 2: End-to-End Delay

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Question 2: End-to-End Delay

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Question 2: End-to-End Delay

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Question 2: End-to-End Delay

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Question 2: End-to-End Delay

‼️

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Question 2: End-to-End Delay

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Worksheet

True or False
End-to-End Delay
Statistical Multi-What?

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Question 3: Statistical Multi-What?

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Question 3: Statistical Multi-What?

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Question 3: Statistical Multi-What?

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Questions?

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