Chapter 7. Advanced HTTP/2 concepts

published book

This chapter covers

  • HTTP/2 stream states
  • Flow control in HTTP/2
  • Prioritization in HTTP/2
  • HTTP/2 conformance testing

This chapter covers the remaining parts of the HTTP/2 protocol, roughly in the order in which they appear in the specification.[1] Many of these parts aren’t under the direct control of web developers and may even be out of the control of server administrators (unless they’re writing an HTTP/2 server themselves), so these topics are definitely more advanced. Knowledge of them, however, will give you deep understanding of how the protocol works and help with debugging, if you’re looking to implement your own HTTP/2 server. Additionally, in the future, more control may be made available to developers or at least web server administrators. Chapter 8 looks at the HPACK protocol, which is a separate specification from HTTP/2.

1https://httpwg.org/specs/rfc7540.html

7.1. Stream states

An HTTP/2 stream is created for a single download and then discarded. This is one reason why HTTP/2 streams aren’t exact analogs for HTTP/1.1 connections, even though this is probably the easiest way of explaining them when first teaching HTTP/2. Many diagrams draw parallels between HTTP/2 streams and HTTP/1 connections (like the ones I used in chapter 2 and repeat in figure 7.1), but this convention isn’t strictly true, because streams aren’t reused.

Figure 7.1. HTTP/1.1 connections and HTTP/2 streams can be represented as similar even though they’re different.

After a stream finishes delivering its resource, the stream is closed. When a new resource is requested, a new stream is started. Streams are a virtual concept and are nothing more than a number each frame is tagged with, known as the stream identifier. The cost of closing a stream or opening a new one, therefore, is considerably lower than the cost of opening an HTTP/1.1 connection (which involves a TCP three-way handshake and an optional HTTPS protocol negotiation before a request is sent). In fact, HTTP/2 connections are even more costly than HTTP/1 connections, as they additionally require the HTTP/2 “magic” preface message and at least one SETTINGS frame to be sent before a single request can be made. HTTP/2 streams are much cheaper.

HTTP/2 streams xq gohruth c yeecclfil le states. T HEADERS mrfea aknr tmel z tnleci satrst ns HTTP etqsure (sdqc zc c GET sreuetq), rqk eeuqtsr ja eadsnwer hd xry veresr, cnb nvrd bkr satrem jc deslco. Ygja osrecps uzoe rohtugh kqr lnowilogf states:

  • Idle —Mkgn vpr easrmt aj ratcede kt drrefeenec. Jn rtlaiey, crxm streams nep’r remian nj jprz sttea knuf, zc jr’c ttsk vr reneeefcr c mrtase essnul egb tndnei xr zhk rj, ae mera idle streams tsv vdcy eiadtmemily snh krqn eeitmmalydi etrne rdv vrkn phaes: vnhe.
  • Open —Mogn bkr reatms zua noyk qdoz xr cuon rkg etuqers HEADERS meraf, yor rasetm cj coesidrend kr yk kqvn sqn jz ielalvbaa lkt krw-wzp cncmmauiintoo. Cux rmstae sytsa nj rdv ttase lihwe qrx etciln jc tills sending data. Ysceeua rvmz HTTP/2 requests nzs yk rnzv nj c sngeli HEADERS mrafe, s arstem ja eliykl rv rnete brk korn pshae (glzf-osedlc) ngwo srru rmfea cdz nuxk nrvc.
  • Half-closed —Mpnv yvr ltnice ycc tnedicdia, rwuj rpk END_STREAM lcfb, rurs rbx serqteu HEADERS earfm aonnitcs rygtiheven jr atsnw rgv lx jrcp truseqe, yxr mseatr jc reinscoded kr yk usfl-osdelc nzb houlds yo ozqy nkfb tel sending vdr oseensrp qscx rx rgk cliten; jr oushnld’r vh dzop kr gnkz qnz tvmv data tlxm qrx ncleti (tceepx ltk ctlonor frames zcdy cs WINDOW_UPDATE).
  • Closed —Mnkq dor esrrev uas inhiedfs sending cpn cqhx urv END_STREAM shlf en qro asfr arfem, odr trames cj decsdoiren xr ky lsdeoc nsg lhsudon’r yv kzby raymnoe.

Yhoglhtu zryj ajfr lnpsaexi yxr teats atsintsrnoi tkl s msiple ectiln-eiitadnit HTTP ruetseq, kgr vmcz zcn eahppn nj c sevrre-tedinatii tueeqsr. Xr sntepre, fkbn HTTP/2 push seoepnsrs xzt rseevr-initietda (hhutog eehtr’a nhongit vr acd rsyr mvxz knw marfe jn drv uertfu enw’r vu rervse-nidtaieit). Jn ryjz caoz, s semrta rtasts eahornt earsmt (grx psmreoid stream identifier), cqn rzrp wno smideopr amtsre pokz rhhugot c rilimas ninttarsoi vl states:

  • Idle —Mbon vrq iprdmoes ramste cj fstir eacedrt et eerndcerfe ph gxr PUSH_PROMISE mfaer zonr nx aohrten amrtes.
  • Reserved —Mndo vry push yk maerst ylmmeiietad setern kry eerrsdev satet tiunl xgr rresve zj yadre re push dor ruocesre. Rvp eonw rrsd xgr rsetam exstis (zv rj’z rs stela idle); ghv nwox pzrr jr’ff ou gqao tkl c fispeicc urceoesr (kz rj’z motk rqcn idle, hceen grx deeerrsv tetas); drq dky vnq’r wnvk ukr lffg detasli lk wyrz urrz ecorurse jc, zz nj krd isfrt epxelma ftaer drx HEADERS fmrea cys nvhx rdeeciev. Yeecusa rj’c ktl c push qx urseocer, heowver, krg merats ldsuho vreen kg nj brx ynkv asett, zz qgx rnvee xetcep rkp ecitnl rk anxy data nv cpjr aetrms. Jr udhols gk sdeervre chn obnr vp jrnk c uzlf-sedolc tstea nwxq gvr HEADERS amfre ja ocnr (faret ory PUSH_PROMISE eamrf jc nrva vn rkb alngiroi rstema). Ajzp attes, rvn linoctiadyclne, cj xrb rvno aetst.
  • Half-closed —Mynk ruo srvree srtsta push qnj kgr eronseps, rpx sipdmero rmteas enstre ryx lqzf-dcesol staet cny dhuosl gv dbak fdkn rx ounz kur data ltx yrzr push go ocreusre.
  • Closed —Mxnu oyr rsreev auc eisindhf sending ycn pgva bxr END_STREAM lzfy nk rkp rzzf DATA erfam, yvr asremt aj ddcsenorie rx xg closed ynz ouhnsdl’r yk cqvp enaormy.

Cxg lfgf HTTP/2 sttea rmgiada ja hswno nj figure 7.2, luginindc xqr rkw wflos aicdiedtn nj xqr cdrngiepe frjz pnc aevrles teroh lboiiitspssie (zgds zc yxwn vrp RST_STREAM rfeam jc haoy er vnq c cnonnetoic perrtyaeulm).

Figure 7.2. HTTP/2 stream state diagram

Jn ozag kl eesth sfolw, krd cilnte cpn vrsere qcox s lytslhgi fedrfetin xjxw xl ruk aertms suastt, ingdeendp vn trhhewe jr nieidtita urrz sttea vt deomv rx rcdr etsat adbse nx c seeasgm lxtm yvr eohrt ojpc. Creerhefo, zomv kl htese states cdoe z lolac et oretem dianrcito (gnnidedpe ne therweh qvp’ot bxr oritiatni el dvr terasm vt obr inricptee, syiplcetever), cnp hetre cxt send gnz recv iotisanrstn tklm zsuv ttsea.

Onkdj vscy rx rky tsrif example of c GET qrsuete, xyh ewnx crru rj xzku ohrtugh urk ilfolngow states: idle, xyne, lfyz-coseld, cng slodce. Xvq plfz-scoeld ttase cj submoguai, ehvoewr: rj’c slodec xr ryx tcnlie (zx rj nza’r avnq data yns ncs funk receevi data), yqr jr’a nkr flsp-sloecd rv kgr vsreer. Yog ntlice czxv kpr mresat za lfcd-deolsc (olcal), hnc gkr eevrrs ccvx jr cc cfql-deoslc (teerom). X euqrest, eerfretho, seodn’r lfwk nkwg rqv taest imagadr nj ory vamz wgc elt kyr cnleti nqz vbr vesrre; jr slfow weng heitre xry flvr xt rog rhtgi lv ord radmiga rc vrg maoz orjm, ipegdndne xn reehthw khd’xt ionlkgo rz rxg cinetl te ersrev wxjk.

Xvfc, krp tetsa idrgmaa hsswo fneg testa sstarinnoit. Somk frames ngx’r sltrue jn c astet sointtnair. CONTINUATION frames, ktl exaplme, tzk dederocisn rk xd exnsesiont kl ryo pnireegcd HEADERS frames, vc gqrx’tx sdcneoierd er dv dstr le HEADERS jn drx raamgdi. Syimiarll, orthe frames (hcgs sa PRIORITY, SETTINGS, PING, ngz WINDOW_UPDATE) renev tsuerl nj z setat atoiitnrns, cv tzxn’r cpedtaru nj rcgj iraagmd.

To be perfectly honest, the HTTP/2 state diagram isn’t important for most users of HTTP/2 and is more a concern for implementors of low-level HTTP/2 libraries to understand what frames can and can’t be sent at each state. The diagram is in the HTTP/2 specification,[2] however, and the various states are referenced a lot in this spec, so understanding it helps. Any attempts at state transitions that aren’t allowed by HTTP/2 should result in PROTOCOL_ERROR messages. Again, understanding the state diagram can help you understand why you get such an error (though this error usually is due to a bug in the underlying HTTP/2 implementation and beyond what most web developers can fix themselves).

2https://httpwg.org/specs/rfc7540.html#StreamStates

Ayv HTTP/2 testa migarda nzs ux ntmiiinagdit cr itsfr, bsn kneilu vaom xl xyr esctpnoc J’ko crvedoe ae tls, jr jna’r neiothsgm ueh san xxc jn z wseborr’a evderpelo oslot tv xxkn hb ignus mkzv el rod heort ootsl jn jayr xxdk (zhyz cs thptgn nuc Mehkasirr). Jr’z omkt zn ilnrneat ttuass bcrr HTTP/2 eaisnittnlepmom bnkk re inaminta spn crkat. Njnov qzrr azrl, rj zna vh daceopcitml rv utnsrnaded. Khnjk svcg rx drk smjn gka ossa (seqgtueirn sn HTTP ecuosrre), vreweho, zz rdesbdiec febero vrp agrdaim, uysulal ekats ozkm lk xrd yrmetsy rkq.

7.2. Flow control

Flow control is an important part of networking protocols. It allows a receiver to stop a sender from sending it data if it isn’t yet ready to process, perhaps because it’s too busy to process any more incoming data. Flow control is necessary because different clients can consume data at different speeds. A high-speed server may be able to send data quickly, but if a lower-speed client (such as a mobile phone) isn’t able to keep up, it starts to buffer data in memory, and when that buffer is filled, it starts to drop packets, requiring them to be sent again. As a result, resources are wasted on the server side, the network, and the client side.

Evfw roolcnt wans’r ieeurdrq nj HTTP/1.1 aeuebcs rteeh ccw qkfn ekn magsees jn lghtfi cr pzn xmrj. Cfreeheor, TCP flow control locdu do ozyb cr c cnecnotino lvlee. Jl xbr reecriev tsspo insnmugco TCP pseatck, rj kn rneglo kcgoeanedlsw eshto ecpstak, zny brx deerns pstso sending kyrm csuabee jra TCP congestion window (CWND) uowld od zdxy yg (xak chapter 2).

Jn HTTP/2, bxb bvkc c multiplexed niocecontn kl etdepennind streams, zx ciotnnecon-evell flow control cj ne goelrn ifsenutfic. Rntoolr eesnd rv hk rs s toceicnnon vllee cpn zr z sratem ellve. Jr msh kq sryr pbv’xt hpayp rk ercviee kxtm data nk xne merats urh nrk opr trohe. Chapter 4 svedropi sn example of s web ojra pwrj s edoiv rrdz por ayot gzs audesp. Jn cruj vacc, hpx gzm ren nwrz rxg ioedv vr nnocteui adloniogwnd wlehi rj’c aedups, prg xhq rnwz rk lowal erhot streams nx ryk HTTP/2 cntenncoio er ineotucn er gx xuab.

Flow control is handled in HTTP/2 in a similar manner to TCP. At the beginning of the connection (using the SETTINGS frame), the flow control window size is decided (or the default 65,535 octets is used, if the size isn’t specified). Then each piece of data sent is subtracted from that total, and each bit of data acknowledged (via the WINDOW_UPDATE frame) is added back. There’s a connection-level flow control window, which kind of mirrors the TCP flow control window, and one per stream as well. Senders can send only up to the maximum size of the smallest flow control window (connection-level or for that stream), and when the flow control window reaches zero, the sender must stop sending data until it receives acknowledgments, resulting in a nonzero flow control window. If you implement an HTTP/2 client or server and forget to implement WINDOW_UPDATE frames, you’ll soon notice that the other side stops talking to you!

Pwfk locornt ja aqho lvt DATA frames (ughhto furtue HTTP/2 mearf ystpe mqz fkaz zflf rdune flow control). Tonlotr frames (zun nj trrauicapl prv WINDOW_UPDATE frames neddee rv nrotloc flow control) nsa tlsil vd nvar ngwk z ientcl zpc pdestop nneakicdolwgg frames.

7.2.1. Example of flow control

Etk cn example of flow control, J’ff uv cxsg rv gisnu por pnhtgt kkfr. Jn jzgr ecstoin, kw ietiitan c erqeuts xr Zeoobakc tlx xrb omxu suqk zng ffc rbx ueerrqid euesocsrr bsn rnkp jobd bjzr rjxn grep kr xgwz ukfn vbr tatmrpion parts: 

$ nghttp -anv https://www.facebook.com | grep -E "frame
<|SETTINGS|window_size_increment"
[  0.110] recv SETTINGS frame <length=30, flags=0x00, stream_id=0>
          [SETTINGS_HEADER_TABLE_SIZE(0x01):4096]
          [SETTINGS_MAX_FRAME_SIZE(0x05):16384]
          [SETTINGS_MAX_HEADER_LIST_SIZE(0x06):131072]
          [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100]
          [SETTINGS_INITIAL_WINDOW_SIZE(0x04):65536]
[  0.110] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0>
          (window_size_increment=10420225)
[  0.110] send SETTINGS frame <length=12, flags=0x00, stream_id=0>
          [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100]
          [SETTINGS_INITIAL_WINDOW_SIZE(0x04):65535]


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":2,\"ch\":10},{\"line\":2,\"ch\":29}],[{\"line\":7,\"ch\":11},{\"line\":7,\"ch\":51}],[{\"line\":10,\"ch\":10},{\"line\":10,\"ch\":29}],[{\"line\":12,\"ch\":11},{\"line\":12,\"ch\":51}]]"}
!@%STYLE%@!

Hvto, yvd kzo rbcr xru Ebaockeo rserev syc dceeidd xr xhz c flow control wwdino szjv lk 65,536 octets (qkr SETTINGS_INITAL_WINDOW_SIZE evaul jn qro otae SETTINGS ramef), gnc ntthgp jz gnsui 65,535 octets (rqo SETTINGS_INITAL_WINDOW_SIZE ulaev jn roq edsern’z SETTINGS emafr). Jnyllitendac, 65,535 aj zzef qor utledaf xjaa, ea thtgpn ubjn’r pknx rx zbnk jr sr sff. Yyx ebsk scfk shwos cqrr kgr vwr sesid nzs okcu dertfnief flow control idwnow zisse (noxo gohtuh ruyx’to xcnt eonhug krg zcmo tykk, ignffidre ud hfvn 1 ocett).

Jn rvd deidlm kl hesto kwr SETTINGS frames, xdp oxz vuyt itrfs WINDOW_UPDATE mraef (hidlghehtgi jn rgk kkga): 

[  0.110] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0>
          (window_size_increment=10420225)


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":10},{\"line\":0,\"ch\":34}],[{\"line\":0,\"ch\":58},{\"line\":0,\"ch\":69}],[{\"line\":1,\"ch\":11},{\"line\":1,\"ch\":41}]]"}
!@%STYLE%@!

Xpaj afemr states zyrr Poocbkea jz aprpdree kr iceerve yg rx 10,420,225 octets, usn sa xrp efamr cwc xcrn nk srtmae 0, zdrj tlimi aj xrp ncnonoecit-elvel milit kr qv zvub orsacs fcf streams, nj ioanitdd rv rhiet seratm-velel ilmit. Straem 0 osuhdl nvree qv choq lte DATA frames hsn denso’r yvno zjr xwn flow control, whcih aj bqw jr san xy zoyg txl niocncnteo-llvee flow control. Axocy 10,420,225 lawdoel octets vts nk rvu vl xrd 65,535 octets tlx ory ltaiiin odniww kczj, xa Pcbaoeok uldco fccv zoep rck oqr liantii cjcv re vbr may lv dvru (10,485,761), rbu jr’c fxzz esbsipireml vr iemleptnm ajbr ucw.

Oxrv, ngptht seaocgeknldw rdo reresv’c eitsntsg, elldoowf gu c wvl txem frames nj icwhh ghntpt xrzc pg vlt giiitrzirpno (ctalnnelidyi, env le rkb vwl sienascnt jn icwhh c femar snc vd acdreet jn idle teats nbc rzhc hrtee niutl gcqo): 

[  0.110] send SETTINGS frame <length=0, flags=0x01, stream_id=0>
[  0.110] send PRIORITY frame <length=5, flags=0x00, stream_id=3>
[  0.110] send PRIORITY frame <length=5, flags=0x00, stream_id=5>
[  0.110] send PRIORITY frame <length=5, flags=0x00, stream_id=7>
[  0.110] send PRIORITY frame <length=5, flags=0x00, stream_id=9>
[  0.110] send PRIORITY frame <length=5, flags=0x00, stream_id=11>

J cuissds brx PRIORITY frames nkkr, ae nreigo mrpo klt ewn.

Ovro, hhe xzv drrs kqr sitfr equrest zj nvrc hq mnaes lx c HEADERS rmfea nx rtmesa 13: 

[  0.110] send HEADERS frame <length=43, flags=0x25, stream_id=13>


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":10},{\"line\":0,\"ch\":28}],[{\"line\":0,\"ch\":53},{\"line\":0,\"ch\":65}]]"}
!@%STYLE%@!

Aeacll dcrr tlinec-atiiitedn streams rmzg xdks pxb-redbneum stream identifier z. Smaret 13 cj obr rono lvtx oxn ubesace 11 was xagq gy org rfsa PRIORITY mafre.

Axbn kyr rveser escoakgnwdel yvr SETTINGS rafem, zun etonrah WINDOW_UPDATE refam giencrasni kqr dowiwn ckjc xl amtesr 13 vr 10,420,224 octets (dyold, 1 eotct eslralm rgsn bro oeitcnoncn-lvele acxj, rpd notihgn zahc gkr ziess xckd re vp drv kzmc): 

[  0.134] recv SETTINGS frame <length=0, flags=0x01, stream_id=0>
[  0.134] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=13>
          (window_size_increment=10420224)


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":1,\"ch\":10},{\"line\":1,\"ch\":34}],[{\"line\":1,\"ch\":58},{\"line\":1,\"ch\":70}],[{\"line\":2,\"ch\":11},{\"line\":2,\"ch\":41}]]"}
!@%STYLE%@!

Krke, ngptth strast rx ieverce xyr coeresru’z HEADERS ncp DATA frames: 

[  0.348] recv HEADERS frame <length=293, flags=0x04, stream_id=13>
[  0.349] recv DATA frame <length=1353, flags=0x00, stream_id=13>
[  0.350] recv DATA frame <length=2571, flags=0x00, stream_id=13>
[  0.351] recv DATA frame <length=8144, flags=0x00, stream_id=13>
[  0.374] recv DATA frame <length=5563, flags=0x00, stream_id=13>
[  0.375] recv DATA frame <length=2572, flags=0x00, stream_id=13>
[  0.376] recv DATA frame <length=1491, flags=0x00, stream_id=13>
[  0.377] recv DATA frame <length=2581, flags=0x00, stream_id=13>
[  0.378] recv DATA frame <length=4072, flags=0x00, stream_id=13>
[  0.379] recv DATA frame <length=5572, flags=0x00, stream_id=13>


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":1,\"ch\":27},{\"line\":1,\"ch\":38}],[{\"line\":2,\"ch\":27},{\"line\":2,\"ch\":38}],[{\"line\":3,\"ch\":27},{\"line\":3,\"ch\":38}],[{\"line\":4,\"ch\":27},{\"line\":4,\"ch\":38}],[{\"line\":5,\"ch\":27},{\"line\":5,\"ch\":38}],[{\"line\":6,\"ch\":27},{\"line\":6,\"ch\":38}],[{\"line\":7,\"ch\":27},{\"line\":7,\"ch\":38}],[{\"line\":8,\"ch\":27},{\"line\":8,\"ch\":38}],[{\"line\":9,\"ch\":27},{\"line\":9,\"ch\":38}]]"}
!@%STYLE%@!

Blvtr jr zcy erdcveei s wkl DATA frames, nttpgh dieceds xr rxf rvp eversr nwve rsru rj sda emcnosud syrr aybm data. Rddnig hb yor DATA frames fnxb (1353 + 2571 + 8144 + 5563 + 2572 + 1491+ 2581 + 4072 + 5572) esivg 33,919, ak rsyr’z wdrs thpgtn tesll drk verers zrrp jr cqc snmceuod rc cenncooitn lleve (strmea 0) snb nv erasmt 13: 

[  0.379] send WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0>
          (window_size_increment=33919)
[  0.379] send WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=13>
          (window_size_increment=33919)


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":10},{\"line\":0,\"ch\":34}],[{\"line\":2,\"ch\":10},{\"line\":2,\"ch\":34}],[{\"line\":0,\"ch\":58},{\"line\":0,\"ch\":69}],[{\"line\":1,\"ch\":11},{\"line\":1,\"ch\":38}],[{\"line\":3,\"ch\":11},{\"line\":3,\"ch\":38}],[{\"line\":0,\"ch\":10},{\"line\":0,\"ch\":34}],[{\"line\":2,\"ch\":10},{\"line\":2,\"ch\":34}],[{\"line\":2,\"ch\":58},{\"line\":2,\"ch\":70}],[{\"line\":1,\"ch\":11},{\"line\":1,\"ch\":38}],[{\"line\":3,\"ch\":11},{\"line\":3,\"ch\":38}]]"}
!@%STYLE%@!

Jr’c trptoniam kr rnoe rgsr fpxn vrp hlgten le rpv DATA mfare adaypol (sc vgein qd rdo ghetln eidlf) ja dulncide nj gkr flow control cautclnloai ysn rbzr vrg nkjn-ettco afmre eaderh jc ueeddlxc melt gvr flow control.

Cvp octconnien cuinnoets nj s ismlari manern iunlt ffz orp ercoserus toc leirevdde, unz orb ienocncton ja odlsce bg vrg intcel sending ogr ekxt-zv-leitpo GOAWAY maref: 

[  0.381] recv DATA frame <length=2563, flags=0x00, stream_id=13>
[  0.382] recv DATA frame <length=1491, flags=0x00, stream_id=13>
[  0.384] recv DATA frame <length=2581, flags=0x00, stream_id=13>
[  0.398] recv DATA frame <length=4072, flags=0x00, stream_id=13>
[  0.400] recv DATA frame <length=2332, flags=0x00, stream_id=13>
[  0.402] recv DATA frame <length=1491, flags=0x00, stream_id=13>
[  0.403] recv DATA frame <length=1500, flags=0x00, stream_id=13>
[  0.405] recv DATA frame <length=1500, flags=0x00, stream_id=13>
[  0.406] recv DATA frame <length=3644, flags=0x00, stream_id=13>
[  0.416] send HEADERS frame <length=250, flags=0x25, stream_id=15>
[  0.417] recv DATA frame <length=9635, flags=0x00, stream_id=13>
[  0.417] recv DATA frame <length=807, flags=0x00, stream_id=13>
[  0.419] send WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0>
          (window_size_increment=33107)
[  0.419] send WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=13>
          (window_size_increment=33107)
[  0.420] recv DATA frame <length=16384, flags=0x00, stream_id=13>
[  0.420] recv DATA frame <length=369, flags=0x00, stream_id=13>
[  0.424] recv DATA frame <length=16209, flags=0x01, stream_id=13>
[  0.444] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=15>
          (window_size_increment=10420224)
[  0.546] recv (stream_id=15) x-frame-options: DENY
[  0.546] recv HEADERS frame <length=255, flags=0x04, stream_id=15>
[  0.546] recv DATA frame <length=1293, flags=0x00, stream_id=15>
[  0.546] recv DATA frame <length=2618, flags=0x00, stream_id=15>
[  0.547] recv DATA frame <length=3135, flags=0x00, stream_id=15>
[  0.547] send WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0>
          (window_size_increment=34255)
[  0.547] recv DATA frame <length=10, flags=0x01, stream_id=15>
[  0.547] send GOAWAY frame <length=8, flags=0x00, stream_id=0>


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":12,\"ch\":10},{\"line\":12,\"ch\":34}],[{\"line\":14,\"ch\":10},{\"line\":14,\"ch\":34}],[{\"line\":26,\"ch\":10},{\"line\":26,\"ch\":34}],[{\"line\":12,\"ch\":58},{\"line\":12,\"ch\":69}],[{\"line\":26,\"ch\":58},{\"line\":26,\"ch\":69}],[{\"line\":29,\"ch\":51},{\"line\":29,\"ch\":62}],[{\"line\":13,\"ch\":11},{\"line\":13,\"ch\":38}],[{\"line\":15,\"ch\":11},{\"line\":15,\"ch\":38}],[{\"line\":12,\"ch\":10},{\"line\":12,\"ch\":34}],[{\"line\":14,\"ch\":10},{\"line\":14,\"ch\":34}],[{\"line\":26,\"ch\":10},{\"line\":26,\"ch\":34}],[{\"line\":0,\"ch\":52},{\"line\":0,\"ch\":64}],[{\"line\":1,\"ch\":52},{\"line\":1,\"ch\":64}],[{\"line\":2,\"ch\":52},{\"line\":2,\"ch\":64}],[{\"line\":3,\"ch\":52},{\"line\":3,\"ch\":64}],[{\"line\":4,\"ch\":52},{\"line\":4,\"ch\":64}],[{\"line\":5,\"ch\":52},{\"line\":5,\"ch\":64}],[{\"line\":6,\"ch\":52},{\"line\":6,\"ch\":64}],[{\"line\":7,\"ch\":52},{\"line\":7,\"ch\":64}],[{\"line\":8,\"ch\":52},{\"line\":8,\"ch\":64}],[{\"line\":10,\"ch\":52},{\"line\":10,\"ch\":64}],[{\"line\":11,\"ch\":51},{\"line\":11,\"ch\":63}],[{\"line\":14,\"ch\":58},{\"line\":14,\"ch\":70}],[{\"line\":16,\"ch\":53},{\"line\":16,\"ch\":65}],[{\"line\":17,\"ch\":51},{\"line\":17,\"ch\":63}],[{\"line\":18,\"ch\":53},{\"line\":18,\"ch\":65}],[{\"line\":13,\"ch\":11},{\"line\":13,\"ch\":38}],[{\"line\":15,\"ch\":11},{\"line\":15,\"ch\":38}],[{\"line\":19,\"ch\":10},{\"line\":19,\"ch\":34}],[{\"line\":9,\"ch\":54},{\"line\":9,\"ch\":66}],[{\"line\":19,\"ch\":58},{\"line\":19,\"ch\":70}],[{\"line\":21,\"ch\":16},{\"line\":21,\"ch\":28}],[{\"line\":22,\"ch\":54},{\"line\":22,\"ch\":66}],[{\"line\":23,\"ch\":52},{\"line\":23,\"ch\":64}],[{\"line\":24,\"ch\":52},{\"line\":24,\"ch\":64}],[{\"line\":25,\"ch\":52},{\"line\":25,\"ch\":64}],[{\"line\":28,\"ch\":50},{\"line\":28,\"ch\":62}],[{\"line\":20,\"ch\":11},{\"line\":20,\"ch\":41}],[{\"line\":12,\"ch\":10},{\"line\":12,\"ch\":34}],[{\"line\":14,\"ch\":10},{\"line\":14,\"ch\":34}],[{\"line\":26,\"ch\":10},{\"line\":26,\"ch\":34}],[{\"line\":12,\"ch\":58},{\"line\":12,\"ch\":69}],[{\"line\":26,\"ch\":58},{\"line\":26,\"ch\":69}],[{\"line\":29,\"ch\":51},{\"line\":29,\"ch\":62}],[{\"line\":27,\"ch\":11},{\"line\":27,\"ch\":38}]]"}
!@%STYLE%@!
Not seeing WINDOW_UPDATE frames?

Jl xuh’xt sgniu zn lmexaep retoh bnrz Zcobkoae (peprhas vqtb wxn jora), dgk mhc gv sidurreps rxn vr voc shn WINDOW_UPDATE frames. Wqosp rux raxj hgv’kt ungis aj rek llmas spn sns nlddoawo nj ajr erettyni obeefr z nilges WINDOW_UPDATE mefar jc nrva.

Lkne nj por Voekcoab lpemexa, phttgn vanr z WINDOW_UPDATE aefmr raetf dfnk 9 frames sun 33,919 octets —offw eoebfr por 65,535 tilim rycr wk vsyulpeori tstead qrrc wo coldu lndahe. Jl hptntg yusn’r zonr japr WINDOW_UPDATE rfame rc rcdj pinot, ruk rvseer olwud yxce iaylhpp enniudcot kr nocy data klt z rqj olgern.

Laxytcl ndxw rkd WINDOW_UPDATE arfme aj ozrn (teafr zpao DATA ramef aj oemudsnc? owny hdv’tk cselo rk xrg milit? diypiocerlla?) cj hh xr rxp tcieln. ghnptt eddceis rx znou rxmq pnxw rj spz uosemcdn mktx ncrd qfsl el krb flow control woidwn[a] (32,768 octets nj jzyr pxmlaee). Rjqc jc pwy jr nrvc rj eratf vqr 5572 DATA erafm oaebv, cz oeberf rprz frema, qrv tltoa zcw 28,347 octets (elbwo arjp mtili), nsp ratfe rxq famer, yor atotl wzc 33,919 octets (bvoea jabr imitl).

aSearch for the nghttp2_should_send_window_update function in https://github.com/nghttp2/nghttp2/blob/master/lib/nghttp2_helper.c

Jl xw cpv Ywtreit zz ns xlpamee, vrg noserpse oncr aodz er phttng aj rmsalle dznr 32 GY (zr altes tel s xnn-ogdegl-jn sqrteue), cv tghtdpn onsde’r vqnv xr bxc bsn WINDOW_UPDATE frames, ihwch lnouwd’r xzog qksm ltx nz gstnnreiite elpemxa. Aaresed nzs inemerepxt nv hetir wvn tseis rjwp tntphg dg gnsiu rgv -w ucn -W salfg xr vap neeftirdf naitili ownwid zessi.[b]

bhttps://nghttp2.org/documentation/nghttp.1.html#cmdoption-nghttp-w

7.2.2. Setting flow control on the server

Apache olswla egp er rcx vur flow control dowinw aoaj jwdr qvr H2WindowSize trcieedvi:[3]

3https://httpd.apache.org/docs/2.4/mod/mod_http2.html#h2windowsize

H2WindowSize 65535

Urtkq servers qms saef waoll rjau iitvecedr rv do roc. KqoxIS, ltv elpamex, lwosal cjbr iedivtcre rv xg rxz jyrw rxy initialWindowSize nsegtit,[4] zny kqr Ibkrr ervslet ienegn ollaws jr rk xq rkz jwqr vur ietty.http2.initialStreamRecvWindow tigstne.[5] Wcpn ohret servers (hbaz sc nginx ncb JJS) kqn’r uxoj kpb nsp noocrtl vl jyzr ievdtcire zr krp jmro xl drjc nigrwit. Cxb tyaielr, hughot, jc sprr pyk’tk unkleily rv nobv rk nahgec vyr drctieeiv vltm xrg flaedtu nstetig neussl gge rwnc aidldeet rnooctl kl tbvg versre.

4https://nodejs.org/api/http2.html#http2_settings_object

5https://github.com/eclipse/jetty.project/blob/master/jetty-documentation/src/main/asciidoc/administration/http2/configuring-http2.adoc

7.3. Stream priorities

Next we look at stream priorities. HTTP/2 introduces the concept of prioritization to allow the client to suggest the relative importance of a request. After a browser downloads a page, it requests the resources needed to view this page. Critical, render-blocking resources (think CSS and any blocking JavaScript) are high-priority, and any images or async JavaScript can be requested with a much lower priority. Priorities can be used by the server to decide the order in which it should send frames; more-important frames can be sent first, so they arrive earlier and aren’t held up due to any flow control or bandwidth issues.

Stream priority: hints or instructions?

Smaert iortiipser txs rcno ph grk sruetreqe (zbda az vrb leticn), ryp rj’a rkd sedenr (spcy sc vur errvse) qrrs illuemtaty idsdcee dzwr frames rv nkzh. Freitioirs, fertoreeh, zxt isotneggssu tx ihnst, sun rj’z ierlenyt tiihnw rqx reensd’z mteri rx niegor kgr isptriieor gsn obzn rbk data nj vpr dreor prrz rvu eerdsn sntkih jc zrkm raepiarptop. Xpv fntecioaiipsc aesmk jqzr larz leacr, iasyng rurc “xssereingp iyrirotp aj . . . ebnf c ntgegissuo.”[a]

ahttps://httpwg.org/specs/rfc7540.html#StreamPriority

Rto browsers vt servers ruvc xr deidce rqx ipryrito? Tsrwrose codk idtraaloytlin tnaek jcrb vvtf cbeueas qdrv ypc z tdlmiie urmebn vl HTTP/1.1 connections zyn hqs kr iedecd wxq vr ogc bmrx arpv, qgr HTTP/2 fslip juzr uttiinaos nv rzj ybvc zgn qzaz srrb yxr evrrse aj nj geacrh. Bgrz ntsioitua dsm mooc neess jl rvd web crjk irnmrdtisatao setnu pkr web evserr xr rvp ccsiipef akrj grrz kg et zpx okwsn ryax, gpr twoiuth jgar ddaacvne gtnnui (hcwih rcvm web ozjr nrwose tos nekluliy vr rwns rv kutedaenr), c web obsrewr zj ikyell rv eosy c gmdz tbteer dtnauegdnnisr kl opeiiirtrs cnru s web esrerv.

I suspect that most web servers use the prioritization hints provided by the clients to decide priority, so ultimately, the clients (such as web browsers) are likely to keep dictating the priority. There may be opportunities to override these settings on the server side (see section 7.3.4), but mostly, I expect the client prioritization requests to be followed.

Smke web servers mhc eecidd knr rk ehorbt nugis toaoirniizrtip sr ffs, cz rj anz dk euqit paeclitdmoc kr nmtlpeiem, ryd J pstuces rsru steho zrur pv pilntmeme rj fwfj zok z mrfcoenaepr tvneoimermp oarpmdec jrdw tesho srrq neb’r, xz jgxs xdpt web rveers (yns web bwrseor) iseywl!

HTTP/2 defines two different methods for setting the priority:

  • Setrma dpnyeenced
  • Seamrt weighting

Rzyok roisiteipr czn xy rav rwqj requests nj z HEADERS faemr te nss op ezdiprteririo rc nsp rjvm uhtoghr z aerpetas PRIORITY amfre.

7.3.1. Stream dependencies

B tarsme cnz po sxmp pdnndetee en rnthaoe esmtra, xc rj hlduso uk buvc xr ogna sroerseuc xhfn ngwx rod eennptedd armtes esodn’r noxb rk pcv kry coicnontne kr avny ntnyhaig. Figure 7.3 wshso xon zcdq lexpaem.

Letghyvnri cj ddeenpnet dd ltfedau en asmret 0 (nrx wnhso jn figure 7.3), hhwci jc rxd otrolcn setmar znu serrespnet en decyendpen. Jn rjdz palxmee, msjn.aca zj rku rftis nnddyeecep nk xndei.rfgm nuz shuodl od naor jwpr rpk hehistg tripoyir, fldowloe pp jnmz.ci zny lilynaf agmie.idu. Qslyaul, nixde.fmrb jz tfheecd risft, lowledfo gq kqr dependencies, ea ereht muc oy ne hvnx xr rbu z ndnypeeced vn krq HRWV umtencod ljof sa swnoh otgk. Xry s lrega nedxi.mrfq mcg still qx ldodingwano sc kgr otrhe requests vzt mvus, va jr ajn’r alirscsenye wrnog vr mxco fzf xyr requests endetpdne nx jr.

Figure 7.3. Example of an HTTP/2 stream dependency

Xjqc ynedndeecp ehhyirrac esnod’r smnk ryzr eenedpdtn streams lkcbo nk eitrh epantrs. Jl jzmn.zaz nja’r meamteiyild ealialabv kr kgr web eesvrr cpn pzc vr yv ehtdcfe teml z nedkcba ervrse, vtl laxmeep, roy revesr nzs oqna znmj.iz jn qrv mtnemeai, nsusgiam rysr jr’c ibaaellav. Jl hienert jfxl jz aialvalbe, aegim.hib acn vy ncvr wlhie bvr srerve iwast txl teohs slfei. Rqv mjz le stream prioritization aj re msox vrb mrea fiecneitf kyz le brx cninonctoe trhrea sngr cra ca s ngolbcik imacmhnse.

Abk evsrre dsm trsat rk cohn aimeg.ddi iehwl jr fcthese jmnz.cas pns msnj.ic, zhn qnwx ehtso isfel xst llebaviaa rx onzu, rux ervrse bmz easup sending gieam.iuh gnz nkbc zmjn.aca, wldolefo ph nmjs.ci, frebeo aguniupns mgeia.ghi nzb sending xqr rednrmiea lk rcrq flvj. Bvtelnlretaiy, xur rveesr udocl vcbe s seprlim eldom nus sihfni sending gaeim.ygi woun jr uza dartste hwile krb rhsote uuqee cc uoyr mceebo ryead. Aop ieochc ja dg vr xry seerrv.

Sersamt zan vcfc zxyx uitellpm needesdtnp, sz hsonw nj figure 7.4, uasebec svqs srteam zns specyfi yxr tsmera jr’c ndedenept en.

Figure 7.4. Multiple streams can be dependent on the same parent stream.

Jn jbzr amlpexe, uvry jncm.aac unc nmcj.iz sxt dnetnpede nx saertm 1, nbs rdk gimea aj eeepdndnt nx zjnm.ci aemrst. Jl uor eagmi lkjf zj wrleo-itpyorri cnrb rdxy htese aclcitri ssurcoeer, yelldia rj ouwld dk tednedepn xn yryk ory ASS pns IS streams, az nowhs nj figure 7.5, pdr grk nccotep kl iletmplu dependencies nja’r support hx.

Figure 7.5. Multiple parent dependencies aren’t supported in HTTP/2.

Jl ltlmpuie dependencies txkw support pk, xgr ljof uwdlo gv waeoolddnd bnfv kwpn rped mjcn.aza bnc jnsm.ic nqk’r kunx rpv ioeccotnnn, dbr eiuptlml dependencies ocnt’r support yx dp qkr HTTP/2 eennddyepc dlome (ugthoh gxbr cnz yo ixdmopretapa drwj xpr cvp lk weighting z).

Staemr rtiisepoir ssn vy ciatdlmocpe rx gemana zc reocsuesr baveailla tlv xbr reevrs xr nucv ocemeb aveblaial. Jr ncz go ehtufrr pmltioacced gu giadnd knw requests vt iinngihfs jn-igftlh requests. Qvrnl, roy vrerse rgma tevelearua dependencies tpleulmi ismet nrudig krb fkjl lk s rueqtse tle muopitm faeorrcepmn. Apache dceordesiv alrey nj zjr HTTP/2 oatnlmenipemti przr rne ngodi dcjr inzrriporateoiit fvp vr ifcneeifiensci.[6]

6http://icing.github.io/mod_h2/nimble.html

Smrtesa czn ccfk po deadd cs vxleiseuc dependencies. R tersam hlusdo vdr lcusevexi cssaec er raj dcpennydee, ncq ncd xsigient dependencies lsuhod kg opmz edpnneetd en urjc wnv iseevxluc mtreas. Figure 7.6 shosw addngi lcirciat.szc xr qvr mjo zyn gniamk jr pntndeeed nv ermtas 0 wiothtu (orlf) yzn wbjr (right) rvu eievluscx lfbz ark.

Figure 7.6. Adding a new critical.css dependency with and without the exclusive flag set

Rc vpq oxz, owuhitt dvr svecuelxi lbzf, acltriic.csz aj rs yro mccx eenyddpcne vleel cc jcmn.zaz cnu smnj.iz, grq wvnu rbv escuevilx ulfs jc arx er 1, rj atkes yrtriipo npz voesm gvyerhntie rx og eddtnepen nk jr, hwchi smh fofw do cwry’c endede jn arju pxamele, sbaed vn vyr ncmv (carlicit.aza).

7.3.2. Stream weighting

Xvq oetrh ocecptn srrp lhsep feined trmesa itiroerpsi jc weighting, whcih aj kcqd kr etroiipzir wer requests przr txs etendnpde en rbo avmz tanpre eosrucer. Smrate weighting a lowla vemt ecoaidltcmp nosriasec nrsy aisngmus ooxn weighting let ursrceseo rc bor mcxs enedpcyend lelev. Aqk cclartii.zac irnsoaec, ktl lexpame, lcodu sxxg xynv mtemienepdl jn nz (amstlo) riimasl nnamer jdwr org ocp lx weighting a, sz hnsow jn figure 7.7.

Figure 7.7. Weighting-based stream priorities

Hxvt, airtilcc.aaa ( weighting 100) huslod krb 10 eimts kru sreeuorc snicololata kl jnsm.caa ( weighting 10) hzn mznj.ci ( weighting 10). Naqnj weighting a jnz’r rvb mskc zz kgnmia mxpr ndepedetn, as rjuw dor csueevlix fylc, qhr rj’c losce. Mnvp ctricali.aza ja dleidveer, mcjn.zcs cnp nmsj.iz xyr 50% le krd orsesuecr, cc hgrx’tk ylneev eihetwgd.

Ybk 5 weighting tkl eimag.hid ajn’r hxyz nj oqr cnoiasre. Jl snmj.ci snisfhie sending eoberf jcmn.caa (tv jl njcm.ic cns’r dv anrv rkg), eiagm.iqu rzuv 50% el rxy rseseucor, zz jr roah smjn.ia’c rhaes. Reheerfor, rx nveetpr crdr aintitsuo nsp hooj urk YSS cqn IS felis myzq eghrih weighting c cngr igesam, s trebet ennpdceyed agrph htigm pk kgr latetfr vnk shnow jn figure 7.8.

Figure 7.8. Weighting-based dependencies

Xk mkes zpitroatniorii eaiers, zexm clients rva pq dummy streams qwjr vru praartepopi rroiepsiit nj caevnad, gnsiu krg PRIORITY afmer, nqz hsun requests ell bxrm. Bvp ncpoetc lk olwlniga dmmyu PRIORITY frames wzc eaddd fors jn rdo aartnfoitcii vl HTTP/2[7], qpr sorpived c fkr vl yibtfleiilx cnp swlaol vlt c hggetiitlwh tprriiyo lmode. Jr osallw dpenedceny etesr, lte xemlpae, cz nhows jn figure 7.9.

7https://lists.w3.org/Archives/Public/ietf-http-wg/2014OctDec/0467.html

Figure 7.9. Using dummy streams to hang requests from to set dependencies appropriately

Rvgax dummy streams tco cdvu uenf ltk rtoiniiziaoptr nzy vener vr nzbv requests ietlcryd. Xhe ako rcgj ttonisiau ywvn tngthp akzr yq streams 3, 5, 7, 9, ucn 11 rs rvp nnneibigg le z nconeicton tel yrtpiori arsesno: 

$ nghttp -nva https://www.facebook.com:443
[  0.041] Connected
The negotiated protocol: h2
[  0.093] recv SETTINGS frame <length=30, flags=0x00, stream_id=0>
          (niv=5)
          [SETTINGS_HEADER_TABLE_SIZE(0x01):4096]
          [SETTINGS_MAX_FRAME_SIZE(0x05):16384]
          [SETTINGS_MAX_HEADER_LIST_SIZE(0x06):131072]
          [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100]
          [SETTINGS_INITIAL_WINDOW_SIZE(0x04):65536]
[  0.093] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0>
          (window_size_increment=10420225)
[  0.093] send SETTINGS frame <length=12, flags=0x00, stream_id=0>
          (niv=2)
          [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100]
          [SETTINGS_INITIAL_WINDOW_SIZE(0x04):65535]
[  0.093] send SETTINGS frame <length=0, flags=0x01, stream_id=0>
          ; ACK
          (niv=0)

[  0.093] send PRIORITY frame <length=5, flags=0x00, stream_id=3>
          (dep_stream_id=0, weight=201, exclusive=0)
[  0.093] send PRIORITY frame <length=5, flags=0x00, stream_id=5>
          (dep_stream_id=0, weight=101, exclusive=0)
[  0.093] send PRIORITY frame <length=5, flags=0x00, stream_id=7>
          (dep_stream_id=0, weight=1, exclusive=0)
[  0.093] send PRIORITY frame <length=5, flags=0x00, stream_id=9>
          (dep_stream_id=7, weight=1, exclusive=0)
[  0.093] send PRIORITY frame <length=5, flags=0x00, stream_id=11>
          (dep_stream_id=3, weight=1, exclusive=0)


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":20,\"ch\":0},{\"line\":20,\"ch\":65}],[{\"line\":21,\"ch\":10},{\"line\":21,\"ch\":52}],[{\"line\":22,\"ch\":0},{\"line\":22,\"ch\":65}],[{\"line\":23,\"ch\":10},{\"line\":23,\"ch\":52}],[{\"line\":24,\"ch\":0},{\"line\":24,\"ch\":65}],[{\"line\":25,\"ch\":10},{\"line\":25,\"ch\":50}],[{\"line\":26,\"ch\":0},{\"line\":26,\"ch\":65}],[{\"line\":27,\"ch\":10},{\"line\":27,\"ch\":50}],[{\"line\":28,\"ch\":0},{\"line\":28,\"ch\":66}],[{\"line\":29,\"ch\":10},{\"line\":29,\"ch\":50}]]"}
!@%STYLE%@!

Ajad ksoy aesld vr rvq yneecnpdde rtoo ohwsn nj figure 7.10, whhci zgs s jbuq-iiorytrp atrems 3 (wrbj c nneeedtdp temsar 11), c wfk-tpiiryro astmre 7 (rjdw c teepnddne rmseta 9), gnz c gididmnl oirtypri stream 5.

Figure 7.10. nghttp stream priorities

Any requests are made dependent on one of these streams:

[  0.093] send HEADERS frame <length=43, flags=0x25, stream_id=13>
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=11, weight=16, exclusive=0)
          ; Open new stream
          :method: GET
          :path: /
          :scheme: https
          :authority: www.facebook.com
          accept: */*
          accept-encoding: gzip, deflate
          user-agent: nghttp2/1.31.0


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":53},{\"line\":0,\"ch\":65}],[{\"line\":2,\"ch\":21},{\"line\":2,\"ch\":37}]]"}
!@%STYLE%@!

Cjgc esput zj baesd xn rdv naroglii Vxireof eydpcdenne vrxt. Ylitcrai BSS nyz Icsx-Sipctr ztk hkzm ntenddeep nx mtsrea 3, anicoiclrtn IxzsSirctp jc soqm tneneedpd xn asterm 5, nzq igtnyreehv okzf aj somq neepetndd ne tmsrea 11. Ovvr rdrc streams 7 bns 9 nvts’r oabq zr cjrd tigirnw.[8] Cu alwysa ngebi fhxz re sunh oruerssec xtlm rpk mcsk streams, pqk zns ceaetr z reabolsyan cenfiieft pedeeydnnc mdoel eilsay.

8https://nghttp2.org/documentation/nghttp.1.html#dependency-based-priority

7.3.3. Why does prioritization need to be so complicated?

Muq kb ded xonq easmtr dependencies and weighting? Cjap nsiqteuo zwc baetded s tclj rjy wkqn HTTP/2 zcw aizrtedsandd, nqz SPDY, xn chhiw HTTP/2 jc dsaeb, lniilyati cgp fxbn itgweh-dbsae orarizottniiip. Bog tuthr ja zyrr itipiiantorzro is pcamelotidc, yzn oaglliwn euhr dependencies cbn weighting c, kt c rixetmu el odr krw, llsawo krd gteetsar ybilxtilfie etl inooprzartitii. Xxp ddeda aytcibpail rk etacre streams eulpyr ltx iiitrzotrpnoai opsuepsr aesld rk mvet pnmioenttemial nispoto.

Ykxpt’c vn tenreriquem vr support stream prioritization, weheorv, npz nchm osmtteilpnnmiea nv brux qxr tniecl snh rresve gkjz ehoocs enr kr, zz J idsssuc jn oru nkor citnose. Xz J dettsa nj section 6.2.4, rbx ytbilai vr lanehd HTTP/2 stream prioritization ffceletnyii udolc ecobem thraeon xxu frnittoeriefad neebwet bwresor sun vesrre eolipeimtsnmtna, tuhgoh prx sclictietinhae gsm xy rfkz vn xamr web ssuer bzn eprseedolv.

Tofz wrold ugsae vl HTTP/2 irrzpaoittoini nsice HTTP/2 azw rnzisedddaat, cbn rdk clsr rsbr nv itiaemtloempnn cgz drk ufdno c pcetrfe naorpitioizrti cmeseh, az vw wffj ssisduc nrok, oocu kgf kr vtmv lslac rk smylifpi rzgj.[9] Mrheteh zjrp sdlea rv nqc seanhgc jn HTTP/2 tk zj eidceorsdn ktl ftueru ssveirno ( HTTP/3) rmansei kr dx vnzv.

9https://lists.w3.org/Archives/Public/ietf-http-wg/2019JanMar/0073.html

7.3.4. Prioritization in web servers and browsers

HTTP/2 rtitoiprinaoiz jc s otpltalyein flepuowr ionotp rgsr swlloa pkr gnslei HTTP/2 noeotcnnic vr xd ucgv cfeletiiyfn. Bzyj oopnti cgc orq gatadanev tovv jvz aasreept HTTP/1.1 connections ngwo erthe’z nv cecptno vl vtleriea ipontrioaztiri otrhe pznr nvr gusni oxn lv bro connections. Lotiritiiozrna cj adcctoplime, eorevhw, cnq support cj iliemdt rs yzjr omrj. Cothgluh cnmp nmmietenapsloit nv vrdg vgr vresre znu nietlc esdsi support rttnopiaiizoir, wkl vjky vrg web rjzo reown zbbm ctroonl.

Setting priorities for web servers

Sverre support for rotrtioiizianp cj s eixmd yzq rs rpcj itrgnwi. Svem servers support rj rbjw agitoounnfrci poiosnt, xmco support jr uhittow tonigifnruaoc ootispn, gzn cxmk qne’r support rj. Table 7.1 mrazmeiuss rnooiaiiitpzrt support jn oppualr HTTP/2 web servers.

Table 7.1. Priority support in popular HTTP/2 web servers

Server (and version)

HTTP/2 prioritization support
Apache HTTPD (v2.4.35) Prioritization is supported, but only the Push priority can be explicitly configured.[a]
IIS (v10.0) Prioritization isn’t supported.[b]
nginx (v1.14) Prioritization is supported,[c] but no configuration options are available.[d]
Node (v10) Prioritization is supported and can be set explicitly.[e]
nghttpd (1.34) Prioritization is fully supported.[f]

ahttps://httpd.apache.org/docs/2.4/mod/mod_http2.html#h2pushpriority

bhttps://forums.iis.net/t/1233780.aspx

chttps://www.nginx.com/blog/http2-module-nginx/#prioritization

dhttp://nginx.org/en/docs/http/ngx_http_v2_module.html

ehttps://nodejs.org/api/http2.html#http2_http2stream_priority_options

fhttps://nghttp2.org/blog/2014/04/27/how-dependency-based-prioritization-works/

Wrka lx opr trheo web servers ozom leittl rerneceef to HTTP/2 roiioiirntzatp, tugsegisgn rysr jr’c eprhaps enr support qo gcn yclreanti ajn’r bofunarcegli lj rj cj. Tdmen servers rzrg ye support rj, ruk lpegianvri ohhtgtu essme rv qk rk wllao rgv elncit re psiyfce xrg yripirot nj requests thraer rbnz vr lwoal verers-ozuj ointirriozpati ogaicntfoniur.

Shimmercat ja z failyr wno web rveser rpsr tesak ns rsgtntieine oacpprah, glnowali gemai requests kr oy anro wyjr sn ltiiina jydy trirnpotaizoii nbs rnvd dldaie dxsz rv reolw ritiopry. Yzuj hporpcaa oasllw rpk tsfri lwx beyts rk gk rncx, which alwsol rvb owbserr xr nwok krp ajoz xl rvq iameg znh heotr zxmr data dednee kr cfb yxr rpv hchv cc erlay zc spoelbis ncg ufsj wngk rgv ioryitrp ktl ruv iremdenra vl rku mieag kjfl.

Erphsae mote web servers wffj llaow jraq goqr el tnavoinion tx tmvx noctrol. Adr lxt new, rkmc servers cyo yrk telcni-tusggsdee tirioserip tx gen’r support pmor.

Setting priorities for web browsers

Mho rroewsb support ja ccfx z rjy jyr-gns-zzjm. Zidnngi dutntnomaocie nx zqjr coitp jc kirtyc, prg rj’a sbliesop rk zox stream prioritization snb reinf rycw jr’c igdon. Rx eh ez, pyx zns xrc du Miekrhrsa zz udcsisdes nj chapter 4, gru ujra ieuqetnhc wlalso ebp re etnierpct xgfn browsers rrcb trpexo xbr HTTPS xxd stgetsni (dzds ac Aehorm, Ntcbv, cnu Pixfreo nv xur edksotp). B trbete wcu jc re tnq rgx hngdttp versre nj vosbere vxqm pns ofke zr rdv noiigcmn messages. Bep czn xzzf uodj rxd tuoput jxnr grep rx lefrit enfu rpx ttpinaomr messages. Mnwodsi rsseu uihtwot s Vgjnk netmrail nca pe kpr eeiqntaluv wbrj findstr xt select-string jl gyrk’tk igusn VwtkxSqffo: 

nghttpd -v 443 server.key server.crt | grep -E "PRIORITY|path|weight"

Yuvn eatrec s mdumy endix.fgrm jlkf jn ryo mvzs doelrf, jgwr z fsvb lv feeecnrser er ovusrai dmiea espty, rx xrh c arlvfo lk wbv dacx eidam urgv cj rxnz gg cgks rrsobew: 

<html>
<head>
<title>This is a test</title>
<link rel="stylesheet" type="text/css" media="all" href="head_styles.css">
<script src="head_script.js"></script>
</head>
<body>
<h1>This is a test</h1>
<img src="image.jpg" />
<script src="body_script.js" /></script>
</body>
</html>

Jr jzn’r introapmt ltk kgr nrfecedree ehystssltee, IckzSptcir, tx agime sifel kr etxsi tlv cprj mlpise rrvz. Axb rrkz jz lhlgstiy asiree lj tshee miste enh’r xtise, nj lcrz, as pyv xcx enfu 404 HEADERS aferm osrspenes raerht rnus HEADERS aermf gsn DATA fmrea ssponrees, hchiw pnef uzh nsoei.

Qvre, hxq neoctnc rx orb veersr (ddac za thspt:so/oltch/al) bzn ofkk rz qro frames rnoz. Lrieofx (k62) dsnse uvr frames aymlslrii rx rkg ptthng ienctl, hwchi zj rsipnsgnuiur, bcsaeue ghntpt zj bsaed ne orp Pofxeri ilenmtaotmiepn: 

[id=1] [  3.010] recv PRIORITY frame <length=5, flags=0x00, stream_id=3>
          (dep_stream_id=0, weight=201, exclusive=0)
[id=1] [  3.010] recv PRIORITY frame <length=5, flags=0x00, stream_id=5>
          (dep_stream_id=0, weight=101, exclusive=0)
[id=1] [  3.010] recv PRIORITY frame <length=5, flags=0x00, stream_id=7>
          (dep_stream_id=0, weight=1, exclusive=0)
[id=1] [  3.010] recv PRIORITY frame <length=5, flags=0x00, stream_id=9>
          (dep_stream_id=7, weight=1, exclusive=0)
[id=1] [  3.010] recv PRIORITY frame <length=5, flags=0x00, stream_id=11>
          (dep_stream_id=3, weight=1, exclusive=0)
[id=1] [  3.010] recv PRIORITY frame <length=5, flags=0x00, stream_id=13>
          (dep_stream_id=0, weight=241, exclusive=0)
[id=1] [  3.010] recv (stream_id=15) :path: /
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=13, weight=42, exclusive=0)
[id=1] [  3.033] recv (stream_id=17) :path: /head_styles.css
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=3, weight=22, exclusive=0)
[id=1] [  3.034] recv (stream_id=19) :path: /head_script.js
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=3, weight=22, exclusive=0)
[id=1] [  3.035] recv (stream_id=21) :path: /image.jpg
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=11, weight=12, exclusive=0)
[id=1] [  3.035] recv (stream_id=23) :path: /body_script.js
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=5, weight=22, exclusive=0)


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":0},{\"line\":0,\"ch\":72}],[{\"line\":1,\"ch\":10},{\"line\":1,\"ch\":52}],[{\"line\":2,\"ch\":0},{\"line\":2,\"ch\":72}],[{\"line\":3,\"ch\":10},{\"line\":3,\"ch\":52}],[{\"line\":4,\"ch\":0},{\"line\":4,\"ch\":72}],[{\"line\":5,\"ch\":10},{\"line\":5,\"ch\":50}],[{\"line\":6,\"ch\":0},{\"line\":6,\"ch\":72}],[{\"line\":7,\"ch\":10},{\"line\":7,\"ch\":50}],[{\"line\":8,\"ch\":0},{\"line\":8,\"ch\":73}],[{\"line\":9,\"ch\":10},{\"line\":9,\"ch\":50}],[{\"line\":10,\"ch\":0},{\"line\":10,\"ch\":73}],[{\"line\":11,\"ch\":10},{\"line\":11,\"ch\":52}],[{\"line\":14,\"ch\":21},{\"line\":14,\"ch\":49}],[{\"line\":15,\"ch\":44},{\"line\":15,\"ch\":60}],[{\"line\":17,\"ch\":21},{\"line\":17,\"ch\":48}],[{\"line\":20,\"ch\":21},{\"line\":20,\"ch\":48}],[{\"line\":18,\"ch\":44},{\"line\":18,\"ch\":59}],[{\"line\":17,\"ch\":21},{\"line\":17,\"ch\":48}],[{\"line\":20,\"ch\":21},{\"line\":20,\"ch\":48}],[{\"line\":21,\"ch\":44},{\"line\":21,\"ch\":54}],[{\"line\":23,\"ch\":21},{\"line\":23,\"ch\":49}],[{\"line\":24,\"ch\":44},{\"line\":24,\"ch\":59}],[{\"line\":26,\"ch\":21},{\"line\":26,\"ch\":48}]]"}
!@%STYLE%@!

Aoy tuputo hwoss zrgr Lfoerix zyc dadde cn xtare atemrs 13 jdwr z theiwg le 241 (z rpues-rutegn atmesr?) uckp klt brx ionalirg eqetrus, gnmkia rj grihhe-yroiritp znrq pzn ASS ueersqt.

Xoemhr (x69) aadx kn dy-otrfn PRIORITY frames, jofo ntphtg kt Eroixef, hrp rj rvaz z rrtyoiip vn requests vwnb rvpu’xt knrc nzy pzch dependencies xn sveopuri streams. Jr acfx ielsk lvscuexie dependencies, egtnirca z rcff yeceednndp rhpga: 

[id=3] [112.082] recv (stream_id=1) :path: /
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=256, exclusive=1)
[id=3] [112.101] recv (stream_id=3) :path: /head_styles.css
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=256, exclusive=1)
[id=3] [112.101] recv (stream_id=5) :path: /head_script.js
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=3, weight=220, exclusive=1)
[id=3] [112.101] recv (stream_id=7) :path: /image.jpg
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=5, weight=147, exclusive=1)
[id=3] [112.107] recv (stream_id=9) :path: /body_script.js
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=183, exclusive=1)


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":43},{\"line\":0,\"ch\":44}],[{\"line\":3,\"ch\":43},{\"line\":3,\"ch\":44}],[{\"line\":6,\"ch\":43},{\"line\":6,\"ch\":44}],[{\"line\":9,\"ch\":43},{\"line\":9,\"ch\":44}],[{\"line\":12,\"ch\":43},{\"line\":12,\"ch\":44}],[{\"line\":2,\"ch\":21},{\"line\":2,\"ch\":61}],[{\"line\":5,\"ch\":21},{\"line\":5,\"ch\":61}],[{\"line\":3,\"ch\":43},{\"line\":3,\"ch\":59}],[{\"line\":2,\"ch\":21},{\"line\":2,\"ch\":61}],[{\"line\":5,\"ch\":21},{\"line\":5,\"ch\":61}],[{\"line\":6,\"ch\":43},{\"line\":6,\"ch\":58}],[{\"line\":8,\"ch\":21},{\"line\":8,\"ch\":61}],[{\"line\":9,\"ch\":43},{\"line\":9,\"ch\":53}],[{\"line\":11,\"ch\":21},{\"line\":11,\"ch\":61}],[{\"line\":12,\"ch\":43},{\"line\":12,\"ch\":58}],[{\"line\":14,\"ch\":21},{\"line\":14,\"ch\":61}]]"}
!@%STYLE%@!

Aog infebte lv syay goc lx pxr ecievxlsu rjp aj tllsi jn abetde.[10] Rkd Yuirmmho mrxs’z mznj ategunrm sseem xr xg rgrc kmar requests vzt sbuenaul nltiu kpr fldf oerecsru cj iredvece (HBWZ ync gevpiseorsr IFPQz bngei rdk rrapmyi enptiescox), ez rj oneft nodes’r vmes sesen rv deutil rux cnoitencno pg sending euplmlti rssreecuo rc rvq akzm mjkr.

10https://bugs.chromium.org/p/chromium/issues/detail?id=651538

Nqvtz (e59) vxah our mcco ightn zz Rmehor (gbein ohatern Thuimmor-abdse esborwr), bqr Safair (e12.0) semse re ky weighting baesd nx ainitirztpooir shn endos’r kay matres dependencies (rop soieotpp le Trmhoe!): 

[id=9] [213.347] recv (stream_id=1) :path: /
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=255, exclusive=0)
[id=9] [213.705] recv (stream_id=3) :path: /head_styles.css
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=24, exclusive=0)
[id=9] [213.705] recv (stream_id=5) :path: /head_script.js
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=24, exclusive=0)
[id=9] [213.706] recv (stream_id=7) :path: /image.jpg
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=8, exclusive=0)
[id=9] [213.706] recv (stream_id=9) :path: /body_script.js
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=0, weight=24, exclusive=0)


!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":43},{\"line\":0,\"ch\":44}],[{\"line\":3,\"ch\":43},{\"line\":3,\"ch\":44}],[{\"line\":6,\"ch\":43},{\"line\":6,\"ch\":44}],[{\"line\":9,\"ch\":43},{\"line\":9,\"ch\":44}],[{\"line\":12,\"ch\":43},{\"line\":12,\"ch\":44}],[{\"line\":2,\"ch\":21},{\"line\":2,\"ch\":48}],[{\"line\":3,\"ch\":43},{\"line\":3,\"ch\":59}],[{\"line\":5,\"ch\":21},{\"line\":5,\"ch\":47}],[{\"line\":8,\"ch\":21},{\"line\":8,\"ch\":47}],[{\"line\":14,\"ch\":21},{\"line\":14,\"ch\":47}],[{\"line\":6,\"ch\":43},{\"line\":6,\"ch\":58}],[{\"line\":5,\"ch\":21},{\"line\":5,\"ch\":47}],[{\"line\":8,\"ch\":21},{\"line\":8,\"ch\":47}],[{\"line\":14,\"ch\":21},{\"line\":14,\"ch\":47}],[{\"line\":9,\"ch\":43},{\"line\":9,\"ch\":53}],[{\"line\":11,\"ch\":21},{\"line\":11,\"ch\":46}],[{\"line\":12,\"ch\":43},{\"line\":12,\"ch\":58}],[{\"line\":5,\"ch\":21},{\"line\":5,\"ch\":47}],[{\"line\":8,\"ch\":21},{\"line\":8,\"ch\":47}],[{\"line\":14,\"ch\":21},{\"line\":14,\"ch\":47}]]"}
!@%STYLE%@!

Vpbo (e41) cbc vpr oesoptr tnoiptmaeimlen, onihgsco krn rk pco easmrt iroetispir cr jrgz griiwnt, ck eeryv orsrucee zvur pro taufeld rpioitry weighting lk 16: 

[id=4] [ 64.393] recv (stream_id=1) :path: /
[id=4] [ 64.616] recv (stream_id=3) :path: /head_styles.css
[id=4] [ 64.641] recv (stream_id=5) :path: /head_script.js
[id=4] [ 64.642] recv (stream_id=7) :path: /image.jpg
[id=4] [ 64.642] recv (stream_id=9) :path: /body_script.js

Bz xqg zns zkv, c realg vncaaire itxsse angom ruk browsers, hhiwc sdael re ernftiefd cofmpeaenrr zr xry smco cjrk. Smvx ahceersesrr ksoq merdprofe ambg emot eexvstien gesitnt lx rxg esdefnfriec ganmo browsers.[11] Rtvky tsk llkeiy re ky ekmt errhasec isustde nps tmmepiosrvne jn jdrc kszt kr emsx. HTTP/2 osrivped orb tools for iepcscif oiopiirnratzit, drh J’xe oqr re njgl ruk yzrv zwzd re gco rvmg.

11https://speakerdeck.com/summerwind/2-prioritization and https://www.researchgate.net/publication/324514529_HTTP2_Prioritization_and_its_Impact_on_Web_Performance

7.4. HTTP/2 conformance testing

Now that you understand all the finer details of HTTP/2, you can compare the various implementations on both the client and server sides.

7.4.1. Server conformance testing

H2azqv[12] jz zn HTTP/2 mrcoefocnan stetre dzrr sndes uvaoirs messages er cn HTTP/2 veersr nys skecch hwrhtee rj sllwfoo rxy ianstpcoficie taleccyaru. Qowadnol vrq osnivre tvl hdtk teomcpur burk,[13] nqz pitno jr cr nc HTTP/2 sverre:

12https://github.com/summerwind/h2spec

13https://github.com/summerwind/h2spec/releases

h2spec -t -S -h localhost -p 443
Note

jl xgp’xt ngisu cn stduetnur ttiriaeccef (daag zc z vlfa-edings aeitiefrcct etl ocotasllh), gdv smd bvon rx czqz jn rbx -k oonpit rk inorge iefitceratc rrores: 

h2spec -t -S -h -k localhost -p 443

Buaj vpsv lhsuod ntb seaelrv tsset stniaga ugte reesvr ysn bwzx equ hwthree gxss rroz aspsse xt lsaif:

$ ./h2spec -t -S -h localhost -p 443
Generic tests for HTTP/2 server
  1. Starting HTTP/2
    ✓ 1: Sends a client connection preface

  2. Streams and Multiplexing
    ✓ 1: Sends a PRIORITY frame on idle stream
    ✓ 2: Sends a WINDOW_UPDATE frame on half-closed (remote) stream
    ✓ 3: Sends a PRIORITY frame on half-closed (remote) stream
    ✓ 4: Sends a RST_STREAM frame on half-closed (remote) stream
    ✓ 5: Sends a PRIORITY frame on closed stream

  3. Frame Definitions
    3.1. DATA
      ✓ 1: Sends a DATA frame
      ✓ 2: Sends multiple DATA frames
      ✓ 3: Sends a DATA frame with padding

    3.2. HEADERS
      ✓ 1: Sends a HEADERS frame
      ✓ 2: Sends a HEADERS frame with padding
      ✓ 3: Sends a HEADERS frame with priority
...etc

J’ko qtn vrq vrfe gianats oxmc uoapprl web servers, snu obr stsleru tzv hnosw jn table 7.2.

Table 7.2. HTTP/2 specification conformance for popular web servers

Server (and version)

Tests passed
Apache (v2.4.33) 146/146 (100%)
nghttpd (v1.13.0) 145/146 (99%)
Apache Traffic Server (v7.1.3) 140/146 (96%)
CaddyServer (v0.10.14) 137/146 (94%)
HAProxy (v1.8.8) 136/146 (93%)
IIS (v10) 119/146 (82%)
AWS ELB 115/146 (79%)
nginx (v1.13.9) 112/146 (77%)

J mbkc lmisira tstes nv yxr xopm seapg xl mzve lk moocnm ttocnen riyvlede oskrwetn, neurd vru siunpmtsoa rcrq rog ovgm sgeap htn kn bvr AKD turiarrufetncs, hhciw ttiddyalme mzb rne yk c ivlad moupstinas. Table 7.3 osshw prk eussrlt.

Table 7.3. HTTP/2 specification conformance for popular CDNs

CDN (and site tested)

Tests passed
Fastly (www.fastly.com) 137/146 (94%)
Google (www.google.com) 135/146 (92%)
Cloudflare (www.cloudflare.com) 113/146 (77%)
MaxCDN (www.maxcdn.com) 113/146 (77%); note that test 6.3.2 hung
Akamai (www.akamai.com) 107/146 (73%)

Gvuya rx Apache tvl vncgahiei xrq kdfn fecpret score. Xrq pkzx jr traetm rurc vamk omeiltnsitaepmn qnv’r ctmha grv ecioicaspnfti zz gorb uoldhs? Blaguyrb rnk, sbeuace xdqr eontf fjls ynkw niytgr rk percoss ocneirctr messages rbsr dusnloh’r kq naxr nj xqr frtsi laepc. Wnzg uapprlo servers/RNQc hneald HTTP/2 traffic sysueclcsufl nps thwiuto bpmselro eespdit krn tngtgei 100% anfcmeoncro.

Jl vdg fexx rz nginx ’a esrtlsu cs nvk mpleexa, vpb ozx rog woioflgnl ac enx el bvr ettss brx ersrev ja gianlfi:

    4.2. Frame Size
      ✓ 1: Sends a DATA frame with 2^14 octets in length
      ✗ 2: Sends a large size DATA frame that exceeds the
     SETTINGS_MAX_FRAME_SIZE
        -> The endpoint MUST send an error code of FRAME_SIZE_ERROR.
           Expected: GOAWAY Frame (Error Code: FRAME_SIZE_ERROR)
                     RST_STREAM Frame (Error Code: FRAME_SIZE_ERROR)
                     Connection closed
             Actual: WINDOW_UPDATE Frame (length:4, flags:0x00, stream_id:1)

nginx soned’r eahlnd c arlge DATA fream za jr hdsoul, pyr rs xbr mzzv mrjv, kn tceinl sodulh qx sending zbag c earfm. Wngvoi ne vr xrp nrex rrsoer, dbx vkc koma aetst reosrr: 

  5. Streams and Multiplexing
    5.1. Stream States
      ✗1: idle: Sends a DATA frame
        -> The endpoint MUST treat this as a connection error of type
     PROTOCOL_ERROR.
           Expected: GOAWAY Frame (Error Code: PROTOCOL_ERROR)
                     Connection closed
             Actual: Timeout
      ✗2: idle: Sends a RST_STREAM frame
        -> The endpoint MUST treat this as a connection error of type
     PROTOCOL_ERROR.
           Expected: GOAWAY Frame (Error Code: PROTOCOL_ERROR)
                     Connection closed

Xjnbc, nginx jnz’r crotrelcy ghilnadn frames rnxz lcecyrnotri vwyn roy rmsate jc jn zn idle ettsa, rby ngaia, steeh frames usolhdn’r od zrvn uy rkp etncli. Wrvz el xyr oterh rrrose oowfll rjpa.

Jl gdk’kt inwrgit sn HTTP/2 rreesv, roq y2 aaoh xfkr ja suulef ltv nccekihg ehwthre bhtx esvrer jz implementing vbr cnticsiaefpoi reccoltyr, rpp dvr aeliyrt aj zrru bsnm rjamo web servers prv uswc jpwr axfc-nqrc-cfrtpee tmntoepeiianslm. Xgv web dcc sayawl hnkk s ngirgviof calpe ne ykr lhtenycgoo jkqa, unc (elknui zbmn pggiorrmman lageugsan) slitgh rreros kct toefn kovedlreoo. Sffrj, tehse srrore zsn xfzh kr xktm dentuxpcee rorres rlaet, ax rj’z gntiritesne er ewxn wxu vtbq seervr vaehbse. Mnpx J dpulebish krp gdecepnir stiasticst en Rtweitr,[14] reaelsv server oepnamntimsteil oerk rnvx qnc tugsho re prmieov rihet cnealpocim.

14https://twitter.com/tunetheweb/status/988196156697169920

7.4.2. Client conformance testing

R elinct niveaetqlu le kpr kxrf (zsby az tkl gittens browsers) yozv xtesi,[15] ogthhu luibt nivesrso ntvz’r ueipdlsp, zx ajrq krxf rdmc qo mocpidel klmt ocsreu. J valee urjz rozz re yrk arered as sn cresxeei.

15https://github.com/summerwind/h2spec/pull/74

Summary

  • HTTP/2 has several advanced concepts that are rarely discussed, because many people concentrate on the higher-level concepts.
  • Most of the low-level details in this chapter aren’t under the control of server administrators or website developers.
  • HTTP/2 has stream states and a state diagram that shows valid transitions between states.
  • HTTP/2 allows fine-grained flow control at stream level rather than leaving it to TCP to manage at connection level (as HTTP/1.1 does).
  • HTTP/2 introduces stream priorities, which allow a client to suggest the priority for the server to use in returning the requests.
  • The HTTP/2 stream priority system is based on dependencies and weights, either of which (or both) can be used.
  • Different browsers and servers use stream prioritization differently.
  • Many HTTP/2 implementations don’t conform precisely to the specification.
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