HTTP/3 From A To Z: Core Ideas (Half 1)

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You’ll have learn some weblog posts or heard convention talks on this matter and assume you recognize the solutions. You’ve in all probability heard issues like: “HTTP/3 is way sooner than HTTP/2 when there’s packet loss”, or “HTTP/3 connections have much less latency and take much less time to arrange”, and doubtless “HTTP/3 can ship information extra rapidly and might ship extra sources in parallel”.

These statements and articles usually skip over some essential technical particulars, are missing in nuance, and often are solely partially right. Typically they make it appear as if HTTP/3 is a revolution in efficiency, whereas it’s actually a extra modest (but nonetheless helpful!) evolution. That is harmful, as a result of the brand new protocol will in all probability not be capable to stay as much as these excessive expectations in apply. I worry this may result in many individuals ending up dissatisfied and to newcomers being confused by heaps of blindly perpetuated misinformation.

I’m afraid of this as a result of we’ve seen precisely the identical occur with HTTP/2. It was heralded as an incredible efficiency revolution, with thrilling new options similar to server push, parallel streams, and prioritization. We’d have been capable of cease bundling sources, cease sharding our sources throughout a number of servers, and closely streamline the page-loading course of. Web sites would magically grow to be 50% sooner with the flip of a change!

5 years later, we all know that server push doesn’t actually work in apply, streams and prioritization are sometimes badly carried out, and, consequently, (decreased) useful resource bundling and even sharding are nonetheless good practices in some conditions.

Equally, different mechanisms that tweak protocol conduct, similar to preload hints, usually comprise hidden depths and bugs, making them troublesome to make use of accurately.

As such, I really feel you will need to stop the sort of misinformation and these unrealistic expectations from spreading for HTTP/3 as effectively.

On this article sequence, I’ll talk about the brand new protocol, particularly its efficiency options, with extra nuance. I’ll present that, whereas HTTP/3 certainly has some promising new ideas, sadly, their impression will doubtless be comparatively restricted for many internet pages and customers (but doubtlessly essential for a small subset). HTTP/3 can be fairly difficult to arrange and use (accurately), so take care when configuring the brand new protocol.

This sequence is split into three elements:

HTTP/3 historical past and core ideas
That is focused at folks new to HTTP/3 and protocols usually, and it primarily discusses the fundamentals.
HTTP/3 efficiency options (arising quickly!)
That is extra in depth and technical. Individuals who already know the fundamentals can begin right here.
Sensible HTTP/3 deployment choices (arising quickly!)
This explains the challenges concerned in deploying and testing HTTP/3 your self. It particulars how and should you ought to change your internet pages and sources as effectively.

This sequence is aimed primarily at internet builders who don’t essentially have a deep data of protocols and wish to study extra. Nevertheless, it does comprise sufficient technical particulars and plenty of hyperlinks to exterior sources to be of curiosity to extra superior readers as effectively.

Why Do We Want HTTP/3?

One query I’ve usually encountered is, “Why do we want HTTP/3 so quickly after HTTP/2, which was solely standardized in 2015?” That is certainly unusual, till you notice that we didn’t actually need a brand new HTTP model within the first place, however somewhat an improve of the underlying Transmission Management Protocol (TCP).

TCP is the primary protocol that gives essential companies similar to reliability and in-order supply to different protocols similar to HTTP. It’s additionally one of many causes we are able to hold utilizing the Web with many concurrent customers, as a result of it well limits every consumer’s bandwidth utilization to their justifiable share.

Did You Know?

When utilizing HTTP(S), you’re actually utilizing a number of protocols apart from HTTP on the similar time. Every of the protocols on this “stack” has its personal options and duties (see picture beneath). For instance, whereas HTTP offers with URLs and information interpretation, Transport Layer Safety (TLS) ensures safety by encryption, TCP allows dependable information transport by retransmitting misplaced packets, and Web Protocol (IP) routes packets from one endpoint to a different throughout totally different gadgets in between (middleboxes).

This “layering” of protocols on high of each other is completed to permit straightforward reuse of their options. Larger-layer protocols (similar to HTTP) don’t need to reimplement advanced options (similar to encryption) as a result of lower-layer protocols (similar to TLS) already do this for them. As one other instance, most functions on the Web use TCP internally to make sure that all of their information are transmitted in full. For that reason, TCP is without doubt one of the most generally used and deployed protocols on the Web.

TCP has been a cornerstone of the online for many years, nevertheless it began to indicate its age within the late 2000s. Its supposed alternative, a brand new transport protocol named QUIC, differs sufficient from TCP in a number of key ways in which working HTTP/2 immediately on high of it will be very troublesome. As such, HTTP/3 itself is a comparatively small adaptation of HTTP/2 to make it suitable with the brand new QUIC protocol, which incorporates many of the new options persons are enthusiastic about.

QUIC is required as a result of TCP, which has been round for the reason that early days of the Web, was probably not constructed with most effectivity in thoughts. For instance, TCP requires a “handshake” to arrange a brand new connection. That is executed to make sure that each shopper and server exist and that they’re keen and capable of change information. It additionally, nonetheless, takes a full community spherical journey to finish earlier than the rest might be executed on a connection. If the shopper and server are geographically distant, every round-trip time (RTT) can take over 100 milliseconds, incurring noticeable delays.

As a second instance, TCP sees the entire information it transports as a single “file” or byte stream, even when we’re really utilizing it to switch a number of recordsdata on the similar time (for instance, when downloading an online web page consisting of many sources). In apply, which means that if TCP packets containing information of a single file are misplaced, then all different recordsdata may even get delayed till these packets are recovered.

That is known as head-of-line (HoL) blocking. Whereas these inefficiencies are fairly manageable in apply (in any other case, we wouldn’t have been utilizing TCP for over 30 years), they do have an effect on higher-level protocols similar to HTTP in a noticeable manner.

Over time, we’ve tried to evolve and improve TCP to enhance a few of these points and even introduce new efficiency options. For instance, TCP Quick Open removes the handshake overhead by permitting higher-layer protocols to ship information alongside from the beginning. One other effort is named MultiPath TCP. Right here, the thought is that your cell phone usually has each Wi-Fi and a (4G) mobile connection, so why not use them each on the similar time for further throughput and robustness?

It’s not terribly troublesome to implement these TCP extensions. Nevertheless, this can be very difficult to really deploy them at Web scale. As a result of TCP is so widespread, nearly each related gadget has its personal implementation of the protocol on board. If these implementations are too previous, lack updates, or are buggy, then the extensions received’t be virtually usable. Put otherwise, all implementations have to know in regards to the extension to ensure that it to be helpful.

This wouldn’t be a lot of an issue if we had been solely speaking about end-user gadgets (similar to your laptop or internet server), as a result of these can comparatively simply be up to date manually. Nevertheless, many different gadgets are sitting between the shopper and the server that even have their very own TCP code on board (examples embody firewalls, load balancers, routers, caching servers, proxies, and many others.).

These middleboxes are sometimes tougher to replace and generally extra strict in what they settle for. For instance, if the gadget is a firewall, it may be configured to dam all site visitors containing (unknown) extensions. In apply, it seems that an unlimited variety of lively middleboxes make sure assumptions about TCP that now not maintain for the brand new extensions.

Consequently, it could possibly take years to even over a decade earlier than sufficient (middlebox) TCP implementations grow to be up to date to really use the extensions on a big scale. You can say that it has grow to be virtually inconceivable to evolve TCP.

In consequence, it was clear that we would wish a alternative protocol for TCP, somewhat than a direct improve, to resolve these points. Nevertheless, as a result of sheer complexity of TCP’s options and their numerous implementations, creating one thing new however higher from scratch could be a monumental enterprise. As such, within the early 2010s it was determined to postpone this work.

In any case, there have been points not solely with TCP, but in addition with HTTP/1.1. We selected to separate up the work and first “repair” HTTP/1.1, main to what’s now HTTP/2. When that was executed, the work may begin on the alternative for TCP, which is now QUIC. Initially, we had hoped to have the ability to run HTTP/2 on high of QUIC immediately, however in apply this could make implementations too inefficient (primarily on account of function duplication).

As a substitute, HTTP/2 was adjusted in a number of key areas to make it suitable with QUIC. This tweaked model was ultimately named HTTP/3 (as a substitute of HTTP/2-over-QUIC), primarily for advertising causes and readability. As such, the variations between HTTP/1.1 and HTTP/2 are rather more substantial than these between HTTP/2 and HTTP/3.


The important thing takeaway right here is that what we would have liked was probably not HTTP/3, however somewhat “TCP/2”, and we obtained HTTP/3 “without cost” within the course of. The primary options we’re enthusiastic about for HTTP/3 (sooner connection set-up, much less HoL blocking, connection migration, and so forth) are actually all coming from QUIC.

What Is QUIC?

You may be questioning why this issues? Who cares if these options are in HTTP/3 or QUIC? I really feel that is necessary, as a result of QUIC is a generic transport protocol which, very similar to TCP, can and might be used for a lot of use circumstances along with HTTP and internet web page loading. For instance, DNS, SSH, SMB, RTP, and so forth can all run over QUIC. As such, let’s take a look at QUIC a bit extra in depth, as a result of it’s right here the place many of the misconceptions about HTTP/3 that I’ve learn come from.

One factor you might need heard is that QUIC runs on high of yet one more protocol, known as the Person Datagram Protocol (UDP). That is true, however not for the (efficiency) causes many individuals declare. Ideally, QUIC would have been a completely impartial new transport protocol, working immediately on high of IP within the protocol stack proven within the picture I shared above.

Nevertheless, doing that will have led to the identical situation we encountered when attempting to evolve TCP: All gadgets on the Web would first need to be up to date with a purpose to acknowledge and permit QUIC. Fortunately, we are able to construct QUIC on high of the one different broadly supported transport-layer protocol on the Web: UDP.

Did You Know?

UDP is essentially the most bare-bones transport protocol potential. It actually doesn’t present any options, apart from so-called port numbers (for instance, HTTP makes use of port 80, HTTPS is on 443, and DNS employs port 53). It doesn’t arrange a reference to a handshake, neither is it dependable: If a UDP packet is misplaced, it’s not robotically retransmitted. UDP’s “finest effort” strategy thus signifies that it’s about as performant as you may get:

There’s no want to attend for the handshake and there’s no HoL blocking. In apply, UDP is usually used for stay site visitors that updates at a excessive fee and thus suffers little from packet loss as a result of lacking information is rapidly outdated anyway (examples embody stay video conferencing and gaming). It’s additionally helpful for circumstances that want low up-front delay; for instance, DNS area title lookups actually ought to solely take a single spherical journey to finish.

Many sources declare that HTTP/3 is constructed on high of UDP due to efficiency. They are saying that HTTP/3 is quicker as a result of, similar to UDP, it doesn’t arrange a connection and doesn’t watch for packet retransmissions. These claims are improper. As we’ve mentioned above, UDP is utilized by QUIC and, thus, HTTP/3 primarily as a result of the hope is that it’ll make them simpler to deploy, as a result of it’s already recognized to and carried out by (nearly) all gadgets on the Web.

On high of UDP, then, QUIC basically reimplements nearly all options that make TCP such a robust and widespread (but considerably slower) protocol. QUIC is totally dependable, utilizing acknowledgements for obtained packets and retransmissions to ensure misplaced ones nonetheless arrive. QUIC additionally nonetheless units up a connection and has a extremely advanced handshake.

Lastly, QUIC additionally makes use of so-called flow-control and congestion-control mechanisms that stop a sender from overloading the community or the receiver, however that additionally make TCP slower than what you might do with uncooked UDP. The important thing factor is that QUIC implements these options in a better, extra performant manner than TCP. It combines many years of deployment expertise and finest practices of TCP with some core new options. We’ll talk about these options in additional depth later on this article.


The important thing takeaway right here is that there isn’t any such factor as a free lunch. HTTP/3 isn’t magically sooner than HTTP/2 simply because we swapped TCP for UDP. As a substitute, we’ve reimagined and carried out a way more superior model of TCP and known as it QUIC. And since we need to make QUIC simpler to deploy, we run it over UDP.

The Large Adjustments

So, how precisely does QUIC enhance upon TCP, then? What’s so totally different? There are a number of new concrete options and alternatives in QUIC (0-RTT information, connection migration, extra resilience to packet loss and gradual networks) that we are going to talk about intimately within the subsequent a part of the sequence. Nevertheless, all of those new issues mainly boil all the way down to 4 primary adjustments:

QUIC deeply integrates with TLS.
QUIC helps a number of impartial byte streams.
QUIC makes use of connection IDs.
QUIC makes use of frames.

Let’s take a more in-depth take a look at every of those factors.

There Is No QUIC With out TLS

As talked about, TLS (the Transport Layer Safety protocol) is accountable for securing and encrypting information despatched over the Web. While you use HTTPS, your plaintext HTTP information is first encrypted by TLS, earlier than being transported by TCP.

Did You Know?

TLS’s technical particulars, fortunately, aren’t actually vital right here; you simply have to know that encryption is completed utilizing some fairly superior math and really giant (prime) numbers. These mathematical parameters are negotiated between the shopper and the server throughout a separate TLS-specific cryptographic handshake. Identical to the TCP handshake, this negotiation can take a while.

In older variations of TLS (say, model 1.2 and decrease), this usually takes two community spherical journeys. Fortunately, newer variations of TLS (1.3 is the most recent) scale back this to only one spherical journey. That is primarily as a result of TLS 1.3 severely limits the totally different mathematical algorithms that may be negotiated to only a handful (essentially the most safe ones). Which means that the shopper can simply instantly guess which of them the server will help, as a substitute of getting to attend for an express record, saving a spherical journey.

Within the early days of the Web, encrypting site visitors was fairly expensive when it comes to processing. Moreover, it was additionally not deemed vital for all use circumstances. Traditionally, TLS has thus been a completely separate protocol that may optionally be used on high of TCP. This is the reason we’ve got a distinction between HTTP (with out TLS) and HTTPS (with TLS).

Over time, our perspective in direction of safety on the Web has, in fact, modified to “safe by default”. As such, whereas HTTP/2 can, in idea, run immediately over TCP with out TLS (and that is even outlined within the RFC specification as cleartext HTTP/2), no (widespread) internet browser really helps this mode. In a manner, the browser distributors made a aware trade-off for extra safety at the price of efficiency.

Given this clear evolution in direction of always-on TLS (particularly for internet site visitors), it’s no shock that the designers of QUIC determined to take this pattern to the following stage. As a substitute of merely not defining a cleartext mode for HTTP/3, they elected to ingrain encryption deeply into QUIC itself. Whereas the primary Google-specific variations of QUIC used a customized set-up for this, standardized QUIC makes use of the present TLS 1.3 itself immediately.

For this, it kind of breaks the everyday clear separation between protocols within the protocol stack, as we are able to see within the earlier picture. Whereas TLS 1.3 can nonetheless run independently on high of TCP, QUIC as a substitute kind of encapsulates TLS 1.3. Put otherwise, there isn’t any manner to make use of QUIC with out TLS; QUIC (and, by extension, HTTP/3) is at all times absolutely encrypted. Moreover, QUIC encrypts nearly all of its packet header fields as effectively; transport-layer info (similar to packet numbers, that are by no means encrypted for TCP) is now not readable by intermediaries in QUIC (even among the packet header flags are encrypted).

For all this, QUIC first makes use of the TLS 1.3 handshake kind of as you’ll with TCP to determine the mathematical encryption parameters. After this, nonetheless, QUIC takes over and encrypts the packets itself, whereas with TLS-over-TCP, TLS does its personal encryption. This seemingly small distinction represents a elementary conceptual change in direction of always-on encryption that’s enforced at ever decrease protocol layers.

This strategy gives QUIC with a number of advantages:

QUIC is safer for its customers.
There isn’t a approach to run cleartext QUIC, so there are additionally fewer choices for attackers and eavesdroppers to pay attention to. (Current analysis has proven how harmful HTTP/2’s cleartext possibility might be.)
QUIC’s connection set-up is quicker.
Whereas for TLS-over-TCP, each protocols want their very own separate handshakes, QUIC as a substitute combines the transport and cryptographic handshake into one, saving a spherical journey (see picture above). We’ll talk about this in additional element partially 2 (coming quickly!).
QUIC can evolve extra simply.
As a result of it’s absolutely encrypted, middleboxes within the community can now not observe and interpret its interior workings like they will with TCP. Consequently, in addition they can now not break (unintentionally) in newer variations of QUIC as a result of they didn’t replace. If we need to add new options to QUIC sooner or later, we “solely” need to replace the tip gadgets, as a substitute of the entire middleboxes as effectively.

Subsequent to those advantages, nonetheless, there are additionally some potential downsides to intensive encryption:

Many networks will hesitate to permit QUIC.
Firms may need to block it on their firewalls, as a result of detecting undesirable site visitors turns into tougher. ISPs and intermediate networks may block it as a result of metrics similar to common delays and packet loss percentages are now not simply obtainable, making it tougher to detect and diagnose issues. This all signifies that QUIC will in all probability by no means be universally obtainable, which we’ll talk about extra partially 3 (coming quickly!).
QUIC has the next encryption overhead.
QUIC encrypts every particular person packet with TLS, whereas TLS-over-TCP can encrypt a number of packets on the similar time. This doubtlessly makes QUIC slower for high-throughput situations (as we’ll see partially 2 (coming quickly!)).
QUIC makes the online extra centralized.
A criticism I’ve encountered usually is one thing like, “QUIC is being pushed by Google as a result of it offers them full entry to the information whereas sharing none of it with others”. I principally disagree with this. First, QUIC doesn’t cover extra (or much less!) user-level info (for instance, which URLs you’re visiting) from exterior observers than TLS-over-TCP does (QUIC retains the established order).

Secondly, whereas Google initiated the QUIC mission, the ultimate protocols we’re speaking about at this time had been designed by a a lot wider workforce within the Web Engineering Activity Power (IETF). IETF’s QUIC is technically very totally different from Google’s QUIC. Nonetheless, it’s true that the folks within the IETF are principally from bigger corporations like Google and Fb and CDNs like Cloudflare and Fastly. As a consequence of QUIC’s complexity, will probably be primarily these corporations which have the required know-how to accurately and performantly deploy, for instance, HTTP/3 in apply. This can in all probability result in extra centralization in these corporations, which _is_ an actual concern.

On A Private Notice:

This is without doubt one of the causes I write a majority of these articles and do a number of technical talks: to ensure extra folks perceive the protocol’s particulars and might use them independently of those huge corporations.


The important thing takeaway right here is that QUIC is deeply encrypted by default. This not solely improves its safety and privateness traits, but in addition helps its deployability and evolvability. It makes the protocol a bit heavier to run however, in return, permits different optimizations, similar to sooner connection institution.

QUIC Is aware of About A number of Byte Streams

The second huge distinction between TCP and QUIC is a little more technical, and we are going to discover its repercussions in additional element partially 2 (coming quickly!). For now, although, we are able to perceive the primary points in a high-level manner.

Did You Know?

Think about first that even a easy internet web page is made up of a lot of impartial recordsdata and sources. There’s HTML, CSS, JavaScript, pictures, and so forth. Every of those recordsdata might be seen as a easy “binary blob” — a set of zeroes and ones which can be interpreted in a sure manner by the browser.

When sending these recordsdata over the community, we don’t switch them all of sudden. As a substitute, they’re subdivided into smaller chunks (usually, of about 1400 bytes every) and despatched in particular person packets. As such, we are able to view every useful resource as being a separate “byte stream”, as information is downloaded or “streamed” piecemeal over time.

For HTTP/1.1, the resource-loading course of is sort of easy, as a result of every file is given its personal TCP connection and downloaded in full. For instance, if we’ve got recordsdata A, B, and C, we might have three TCP connections. The primary would see a byte stream of AAAA, the second BBBB, the third CCCC (with every letter repetition being a TCP packet). This works however can be very inefficient as a result of every new connection has some overhead.

In apply, browsers impose limits on what number of concurrent connections could also be used (and thus what number of recordsdata could also be downloaded in parallel) — usually, between 6 and 30 per web page load. Connections are then reused to obtain a brand new file as soon as the earlier has absolutely transferred. These limits ultimately began to hinder internet efficiency on fashionable pages, which regularly load many greater than 30 sources.

Enhancing this case was one of many primary objectives for HTTP/2. The protocol does this by now not opening a brand new TCP connection for every file, however as a substitute downloading the totally different sources over a single TCP connection. That is achieved by “multiplexing” the totally different byte streams. That’s a flowery manner of claiming that we combine information of the totally different recordsdata when transporting it. For our three instance recordsdata, we might get a single TCP connection, and the incoming information may appear to be AABBCCAABBCC (though many different ordering schemes are potential). This appears easy sufficient and certainly works fairly effectively, making HTTP/2 usually simply as quick or a bit sooner than HTTP/1.1, however with a lot much less overhead.

Let’s take a more in-depth take a look at the distinction:

Nevertheless, there’s a downside on the TCP aspect. You see, as a result of TCP is a a lot older protocol and never made for simply loading internet pages, it doesn’t learn about A, B, or C. Internally, TCP thinks it’s transporting only a single file, X, and it doesn’t care that what it views as XXXXXXXXXXXX is definitely AABBCCAABBCC on the HTTP stage. In most conditions, this doesn’t matter (and it really makes TCP fairly versatile!), however that adjustments when there’s, for instance, packet loss on the community.

Suppose the third TCP packet is misplaced (the one containing the primary information for file B), however the entire different information are delivered. TCP offers with this loss by retransmitting a brand new copy of the misplaced information in a brand new packet. This retransmission can, nonetheless, take some time to reach (at the very least one RTT). You may assume that’s not a giant downside, as we see there isn’t any loss for sources A and C. As such, we are able to begin processing them whereas ready for the lacking information for B, proper?

Sadly, that’s not the case, as a result of the retransmission logic occurs on the TCP layer, and TCP doesn’t learn about A, B, and C! TCP as a substitute thinks that part of the only X file has been misplaced, and thus it feels it has to maintain the remainder of X’s information from being processed till the opening is stuffed. Put otherwise, whereas on the HTTP/2 stage, we all know that we may already course of A and C, TCP doesn’t know this, inflicting issues to be slower than they doubtlessly may very well be. This inefficiency is an instance of the “head-of-line (HoL) blocking” downside.

Fixing HoL blocking on the transport layer was one of many primary objectives of QUIC. In contrast to TCP, QUIC is intimately conscious that it’s multiplexing a number of, impartial byte streams. It, in fact, doesn’t know that it’s transporting CSS, JavaScript, and pictures; it simply is aware of that the streams are separate. As such, QUIC can carry out packet loss detection and restoration logic on a per-stream foundation.

Within the situation above, it will solely maintain again the information for stream B, and in contrast to TCP, it will ship any information for A and C to the HTTP/3 layer as quickly as potential. (That is illustrated beneath.) In idea, this might result in efficiency enhancements. In apply, nonetheless, the story is rather more nuanced, as we’ll talk about partially 2 (coming quickly!).

We will see that we now have a elementary distinction between TCP and QUIC. That is, by the way, additionally one of many primary explanation why we are able to’t simply run HTTP/2 as is over QUIC. As we mentioned, HTTP/2 additionally features a idea of working a number of streams over a single (TCP) connection. As such, HTTP/2-over-QUIC would have two totally different and competing stream abstractions on high of each other.

Making them work collectively properly could be very advanced and error-prone; so, one of many key variations between HTTP/2 and HTTP/3 is that the latter removes the HTTP stream logic and reuses QUIC streams as a substitute. As we’ll see partially 2 (coming quickly!), although, this has different repercussions in how options similar to server push, header compression, and prioritization are carried out.


The important thing takeaway right here is that TCP was by no means designed to move a number of, impartial recordsdata over a single connection. As a result of that’s precisely what internet looking requires, this has led to many inefficiencies through the years. QUIC solves this by making a number of byte streams a core idea on the transport layer and dealing with packet loss on a per-stream foundation.

QUIC Helps Connection Migration

The third main enchancment in QUIC is the truth that connections can keep alive longer.

Did You Know?

We regularly use the idea of a “connection” when speaking about internet protocols. Nevertheless, what precisely is a connection? Sometimes, folks converse of a TCP connection as soon as there was a handshake between two endpoints (say, the browser or shopper and the server). This is the reason UDP is usually (considerably misguidedly) mentioned to be “connectionless”, as a result of it doesn’t do such a handshake. Nevertheless, the handshake is admittedly nothing particular: It’s only a few packets with a particular type being despatched and obtained. It has a number of objectives, primary amongst them being to ensure there’s something on the opposite aspect and that it’s keen and capable of discuss to us. It’s price repeating right here that QUIC additionally performs a handshake, regardless that it runs over UDP, which by itself doesn’t.

So, the query turns into, how do these packets arrive on the right vacation spot? On the Web, IP addresses are used to route packets between two distinctive machines. Nevertheless, simply having the IPs on your cellphone and the server isn’t sufficient, as a result of each need to have the ability to run a number of networked packages at every finish concurrently.

This is the reason every particular person connection can be assigned a port quantity on each endpoints to distinguish connections and the functions they belong to. Server functions usually have a set port quantity relying on their operate (for instance ports 80 and 443 for HTTP(S), and port 53 for DNS), whereas shoppers often select their port numbers (semi-)randomly for every connection.

As such, to outline a novel connection throughout machines and functions, we want these 4 issues, the so-called 4-tuple: shopper IP tackle + shopper port + server IP tackle + server port.

In TCP, connections are recognized by simply the 4-tuple. So, if simply a type of 4 parameters adjustments, the connection turns into invalid and must be re-established (together with a brand new handshake). To grasp this, think about the parking-lot downside: You might be at present utilizing your smartphone within a constructing with Wi-Fi. As such, you’ve gotten an IP tackle on this Wi-Fi community.

If you happen to now transfer exterior, your cellphone may change to the mobile 4G community. As a result of this can be a new community, it can get a totally new IP tackle, as a result of these are network-specific. Now, the server will see TCP packets coming in from a shopper IP that it hasn’t seen earlier than (though the 2 ports and the server IP may, in fact, keep the identical). That is illustrated beneath.

However how can the server know that these packets from a brand new IP belong to the “connection”? How does it know these packets don’t belong to a new connection from one other shopper within the mobile community that selected the identical (random) shopper port (which may simply occur)? Sadly, it can’t know this.

As a result of TCP was invented earlier than we had been even dreaming of mobile networks and smartphones, there’s, for instance, no mechanism that enables the shopper to let the server comprehend it has modified IPs. There isn’t even a approach to “shut” the connection, as a result of a TCP reset or fin command despatched to the previous 4-tuple wouldn’t even attain the shopper anymore. As such, in apply, each community change signifies that present TCP connections can now not be used.

A brand new TCP (and probably TLS) handshake must be executed to arrange a brand new connection, and, relying on the application-level protocol, in-process actions would must be restarted. For instance, should you had been downloading a big file over HTTP, then that file might need to be re-requested from the beginning (for instance, if the server doesn’t help vary requests). One other instance is stay video conferencing, the place you might need a brief blackout when switching networks.

Notice that there are different explanation why the 4-tuple may change (for instance, NAT rebinding), which we’ll talk about extra partially 2 (coming quickly!).

Restarting the TCP connections can thus have a extreme impression (ready for brand new handshakes, restarting downloads, re-establishing context). To resolve this downside, QUIC introduces a brand new idea named the connection identifier (CID). Every connection is assigned one other quantity on high of the 4-tuple that uniquely identifies it between two endpoints.

Crucially, as a result of this CID is outlined on the transport layer in QUIC itself, it doesn’t change when transferring between networks! That is proven within the picture beneath. To make this potential, the CID is included on the entrance of every QUIC packet (very similar to how the IP addresses and ports are additionally current in every packet). (It’s really one of many few issues within the QUIC packet header that aren’t encrypted!)

With this set-up, even when one of many issues within the 4-tuple adjustments, the QUIC server and shopper solely want to have a look at the CID to know that it’s the identical previous connection, after which they will hold utilizing it. There isn’t a want for a brand new handshake, and the obtain state might be saved intact. This function is usually known as connection migration. That is, in idea, higher for efficiency, however, as we are going to talk about partially 2 (coming quickly!), it’s, in fact, a nuanced story once more.

There are different challenges to beat with the CID. For instance, if we might certainly use only a single CID, it will make it extraordinarily straightforward for hackers and eavesdroppers to observe a consumer throughout networks and, by extension, deduce their (approximate) bodily areas. To forestall this privateness nightmare, QUIC adjustments the CID each time a brand new community is used.

Which may confuse you, although: Didn’t I simply say that the CID is meant to be the identical throughout networks? Nicely, that was an oversimplification. What actually occurs internally is that the shopper and server agree on a frequent record of (randomly generated) CIDs that every one map to the identical conceptual “connection”.

For instance, they each know that CIDs Okay, C, and D in actuality all map to connection X. As such, whereas the shopper may tag packets with Okay on Wi-Fi, it could possibly change to utilizing C on 4G. These frequent lists are negotiated absolutely encrypted in QUIC, so potential attackers received’t know that Okay and C are actually X, however the shopper and server would know this, they usually can hold the connection alive.

It will get much more advanced, as a result of shoppers and servers may have totally different lists of CIDs that they select themselves (very similar to they’ve totally different port numbers). That is primarily to help with routing and cargo balancing in large-scale server set-ups, as we’ll see in additional element partially 3 (coming quickly!).


The important thing takeaway right here is that in TCP, connections are outlined by 4 parameters that may change when endpoints change networks. As such, these connections generally must be restarted, resulting in some downtime. QUIC provides one other parameter to the combo, known as the connection ID. Each the QUIC shopper and server know which connection IDs map to which connections and are thus extra sturdy towards community adjustments.

QUIC Is Versatile and Evolvable

A remaining side of QUIC is that it’s particularly made to be straightforward to evolve. That is completed in a number of alternative ways. First, as mentioned, the truth that QUIC is nearly absolutely encrypted signifies that we solely have to replace the endpoints (shoppers and servers), and never all middleboxes, if we need to deploy a more recent model of QUIC. That also takes time, however usually within the order of months, not years.

Secondly, not like TCP, QUIC doesn’t use a single fastened packet header to ship all protocol meta information. As a substitute, QUIC has quick packet headers and makes use of a number of “frames” (form of like miniature specialised packets) contained in the packet payload to speak further info. There may be, for instance, an ACK body (for acknowledgements), a NEW_CONNECTION_ID body (to assist arrange connection migration), and a STREAM body (to hold information), as proven within the picture beneath.

That is primarily executed as an optimization, as a result of not each packet carries all potential meta information (and so the TCP packet header often wastes fairly some bytes — see additionally the picture above). A really helpful aspect impact of utilizing frames, nonetheless, is that defining new body varieties as extensions to QUIC might be very straightforward sooner or later. An important one, for instance, is the DATAGRAM body, which permits unreliable information to be despatched over an encrypted QUIC connection.

Thirdly, QUIC makes use of a customized TLS extension to hold what are known as transport parameters. These permit the shopper and server to decide on a configuration for a QUIC connection. This implies they will negotiate which options are enabled (for instance, whether or not to permit connection migration, which extensions are supported, and many others.) and talk smart defaults for some mechanisms (for instance, most supported packet dimension, movement management limits). Whereas the QUIC customary defines a lengthy record of those, it additionally permits extensions to outline new ones, once more making the protocol extra versatile.

Lastly, whereas not an actual requirement of QUIC by itself, most implementations are at present executed in “consumer area” (versus TCP, which is often executed in “kernel area”). The small print are mentioned partially 2 (coming quickly!), however this primarily signifies that it’s a lot simpler to experiment with and deploy QUIC implementation variations and extensions than it’s for TCP.


Whereas QUIC has now been standardized, it ought to actually be considered QUIC model 1 (which can be clearly acknowledged within the Request For Feedback (RFC)), and there’s a clear intent to create model 2 and extra pretty rapidly. On high of that, QUIC permits for the simple definition of extensions, so much more use circumstances might be carried out.


Let’s summarize what we’ve realized on this half. We now have primarily talked in regards to the omnipresent TCP protocol and the way it was designed in a time when lots of at this time’s challenges had been unknown. As we tried to evolve TCP to maintain up, it turned clear this could be troublesome in apply, as a result of nearly each gadget has its personal TCP implementation on board that will must be up to date.

To bypass this situation whereas nonetheless bettering TCP, we created the new QUIC protocol (which is admittedly TCP 2.0 beneath the hood). To make QUIC simpler to deploy, it’s run on high of the UDP protocol (which most community gadgets additionally help), and to ensure it could possibly evolve sooner or later, it’s nearly solely encrypted by default and makes use of a versatile framing mechanism.

Apart from this, QUIC principally mirrors recognized TCP options, such because the handshake, reliability, and congestion management. The 2 primary adjustments apart from encryption and framing are the attention of a number of byte streams and the introduction of the connection ID. These adjustments had been, nonetheless, sufficient to forestall us from working HTTP/2 on high of QUIC immediately, necessitating the creation of HTTP/3 (which is admittedly HTTP/2-over-QUIC beneath the hood).

QUIC’s new strategy offers approach to a lot of efficiency enhancements, however their potential good points are extra nuanced than usually communicated in articles on QUIC and HTTP/3. Now that we all know the fundamentals, we are able to talk about these nuances in additional depth within the subsequent half of this sequence. Keep tuned!

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